Showing papers in "Synapse in 2001"

Amphetamine-type central nervous system stimulants release norepinephrine more potently than they release dopamine and serotonin.

Abstract: A large body of evidence supports the hypothesis that mesolimbic dopamine (DA) mediates, in animal models, the reinforcing effects of central nervous system stimulants such as cocaine and amphetamine. The role DA plays in mediating amphetamine-type subjective effects of stimulants in humans remains to be established. Both amphetamine and cocaine increase norepinephrine (NE) via stimulation of release and inhibition of reuptake, respectively. If increases in NE mediate amphetamine-type subjective effects of stimulants in humans, then one would predict that stimulant medications that produce amphetamine-type subjective effects in humans should share the ability to increase NE. To test this hypothesis, we determined, using in vitro methods, the neurochemical mechanism of action of amphetamine, 3,4-methylenedioxymethamphetamine (MDMA), (+)-methamphetamine, ephedrine, phentermine, and aminorex. As expected, their rank order of potency for DA release was similar to their rank order of potency in published self-administration studies. Interestingly, the results demonstrated that the most potent effect of these stimulants is to release NE. Importantly, the oral dose of these stimulants, which produce amphetamine-type subjective effects in humans, correlated with the their potency in releasing NE, not DA, and did not decrease plasma prolactin, an effect mediated by DA release. These results suggest that NE may contribute to the amphetamine-type subjective effects of stimulants in humans.

Cocaine self-administration alters the morphology of dendrites and dendritic spines in the nucleus accumbens and neocortex.

Abstract: We studied the influence of cocaine use on the structure of neurons in brain regions that contribute to its rewarding effects by allowing rats to self-administer cocaine (0.33 mg/infusion) fo r1had ay for 1month. Control animals were left undis- turbed or allowed to work for food for the same period of time. After an additional 1 month drug-free period the brains were processed for Golgi-Cox staining. In rats that self-administered cocaine, but not rats that worked for food, there was a significant increase in dendritic branching and in the density of dendritic spines on medium spiny neurons in the shell of the nucleus accumbens and on pyramidal cells in the prefrontal and parietal (but not occipital) cortex. There was also a 2.6-fold increase in the incidence of spines with multiple heads (branched spines) on medium spiny neurons. Finally, in the prefrontal cortex some of the apical dendrites of pyramidal cells appeared mis- shaped, having large bulbous structures on their terminal tips. We speculate that cocaine self-administration experience alters patterns of synaptic connectivity within limbocortical circuitry that is thought to contribute to cocaine's incentive motivational effects and may have neuropathological effects in frontal areas involved in decision making and judgment. Together, these two classes of drug-induced neuroadaptations may contribute to the development of addiction. Synapse 39:257-266, 2001. © 2001 Wiley-Liss, Inc.

Inhibition of the cyclooxygenase isoenzymes COX-1 and COX-2 provide neuroprotection in the MPTP-mouse model of Parkinson's disease.

Abstract: To study the possible role of the isoenzymes of cyclooxygenase COX-1 and COX-2 in the MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) mouse model of Parkinson's disease we used acetylsalicylic acid, a COX-1/COX-2 inhibitor, in comparison with meloxicam, a preferential COX-2 inhibitor. As markers of protection we determined the effects on MPTP-induced striatal dopamine depletion, locomotor activity, cell loss, and tyrosine hydroxylase immunoreactivity (TH-IR) in the substantia nigra pars compacta. Male C57BL/6 mice (n = 82) were treated with a single dose of acetylsalicylic acid (10, 50, 100 mg/kg i.p.) or meloxicam (2, 7.5, 50 mg/kg i.p.) immediately prior to administration of MPTP (30 mg/kg s.c.) or saline. After 7 days the mice were sacrificed to analyze striatal dopamine and metabolite levels. Nigral sections were processed for Nissl-staining and TH-IR. In the saline-treated MPTP control group striatal dopamine levels were reduced to 15.9% of control values. Dopamine depletion was significantly attenuated to values of 37.1 and 38.6% of saline control values by acetylsalicylic acid (50 and 100 mg/kg) and to values of 36 and 40% by meloxicam (7.5 and 50 mg/kg), respectively. MPTP-induced decrease of locomotor activity was significantly attenuated by acetylsalicylic acid and meloxicam. Remarkably, the MPTP-induced decrease of TH-IR as well as the loss of nigral neurons was nearly completely prevented by acetylsalicylic acid (100 mg/kg) and meloxicam (7.5 and 50 mg/kg). In conclusion, the inhibition of either COX-1/COX-2 by acetylsalicylic acid or preferentially COX-2 by meloxicam provided a clear neuroprotection against MPTP-toxicity on the striatal and nigral levels.

Sex differences in [123I]β‐CIT SPECT measures of dopamine and serotonin transporter availability in healthy smokers and nonsmokers

Abstract: Nicotine and other constituents of tobacco smoke elevate dopamine (DA) and serotonin (5-HT) levels in brain and may cause homeostatic adaptations in DA and 5-HT transporters. Since sex steroids alter DA and 5-HT transporter expression, the effects of smoking on DA and 5-HT transporter availability may differ between sexes. In the present study, DA and 5-HT transporter availabilities were quantitated using single photon emission computed tomography (SPECT) imaging approximately 22 h after bolus administration of [123I]β-CIT, an analog of cocaine which labels DA and 5-HT transporters. Forty-two subjects including 21 pairs of age-, race-, and gender-matched healthy smokers and nonsmokers (12 female and 9 male pairs) were imaged. Regional uptake was assessed by the outcome measures, V3″, which is the ratio of specific (i.e., ROI-cerebellar activity) to nondisplaceable (cerebellar) activity, and V3, the ratio of specific to free plasma parent. Overall, striatal and diencephalic [123I]β-CIT uptake was not altered by smoking, whereas brainstem [123I]β-CIT uptake was modestly higher (10%) in smokers vs. nonsmokers. When subgrouped by sex, regardless of smoking status, [123I]β-CIT uptake was higher in the striatum (10%), diencephalon (15%), and brainstem (15%) in females vs. males. The sex*smoking interaction was not significant in the striatum, diencephalon, or brainstem, despite the observation of 20% higher brainstem [123I]β-CIT uptake in male smokers vs. nonsmokers and less than a 5% difference between female smokers and nonsmokers. The results demonstrate higher DA and 5-HT transporter availability in females vs. males and no overall effect of smoking with the exception of a modest elevation in brainstem 5-HT transporters in male smokers. Although these findings are preliminary and need validation with a more selective 5-HT transporter radiotracer, the results suggest that brainstem 5-HT transporters may be regulated by smoking in a sex-specific manner. Synapse 41:275–284, 2001. © 2001 Wiley-Liss, Inc.

Periodic maternal separation of neonatal rats produces region- and gender-specific effects on biogenic amine content in postmortem adult brain.

Abstract: Early environment exerts profound effects on mammalian behavioral and neural development. The aim of this study was to describe changes in adult neurochemistry in the rat following repeated neonatal maternal separation (RMS) during the preweaning period, a procedure known to induce enduring behavioral effects. Following RMS, rats show an attenuated locomotor response to novelty, to D-amphetamine, and attenuated behavioral responses for conditioned incentives as adults. These behavioral effects are broadly opposite in direction to those found following postweaning isolation rearing. Isolation rearing-induced behavioral changes are associated with profound changes in central monoamine function. Following RMS, adult rats had increased tissue levels of dopamine in both dorsal and ventral striatum. The turnover of dopamine, as determined by the ratio of DOPAC to dopamine, was decreased in the mPFC of RMS subjects. Serotonin levels were reduced in dorsal hippocampus of RMS rats of both sexes and in the mPFC of male RMS rats. Noradrenaline levels were increased in the dorsal hippocampus in female, but not in male, RMS rats. These data provide evidence that, in addition to the adult behavioral consequences, RMS leads to profound, region-, and gender-specific changes in brain monoamine content. The developmental specificity of these results is discussed with respect to their possible role in altered behavioral development and psychopathology.

Neuroprotective properties of 17beta-estradiol, progesterone, and raloxifene in MPTP C57Bl/6 mice.

Abstract: Previous work from our laboratory showed prevention of 1-methyl-4-phenyl-1,2,3,6 tetrahydropyridine (MPTP) induced dopamine depletion in striatum of C57Bl/6 mice by 17β-estradiol, progesterone, and raloxifene, whereas 17α-estradiol had no effect. The present study investigated the mechanism by which these compounds exert their neuroprotective activity. The hormonal effect on the dopamine transporter (DAT) was examined to probe the integrity of dopamine neurons and glutamate receptors in order to find a possible excitotoxic mechanism. Drugs were injected daily for 5 days before MPTP (four injections, 15 mg/kg ip at 2-h intervals) and drug treatment continued for 5 more days. MPTP induced a decrease of striatal DAT-specific binding (50% of control) and DAT mRNA in the substantia nigra (20% of control), suggesting that loss of neuronal nerve terminals was more extensive than cell bodies. This MPTP-induced decrease of striatal [125I]RTI-121 specific binding was prevented by 17β-estradiol (2 μg/day), progesterone (2 μg/day), or raloxifene (5 mg/kg/day) but not by 17α-estradiol (2 μg/day) or raloxifene (1 mg/kg/day). No treatment completely reversed the decreased levels of DAT mRNA in the substantia nigra. Striatal [125I]RTI-121 specific binding was positively correlated with dopamine concentrations in intact, saline, or hormone-treated MPTP mice. Striatal NMDA-sensitive [3H]glutamate or [3H]AMPA specific binding remained unchanged in intact, saline, or hormone-treated MPTP mice, suggesting the unlikely implication of changes of glutamate receptors in an excitotoxic mechanism. These results show a stereospecific neuroprotection by 17β-estradiol of MPTP neurotoxicity, which is also observed with progesterone or raloxifene treatment. The present paradigm modeled early DA nerve cell damage and was responsive to hormones. Synapse 41:131–138, 2001. © 2001 Wiley-Liss, Inc.

Altered dendritic spine density in animal models of depression and in response to antidepressant treatment.

Abstract: Olfactory bulbectomy, neonatal clomipramine administration, and maternal deprivation have been employed as animal models of depression. Each model is unique with respect to the experimental manipulations required to produce "depressive" signs, expression and duration of these signs, and response to antidepressant treatments. Dendritic spines represent a possible anatomical substrate for the enduring changes seen with depression and we have previously shown that chronic antidepressant drug exposure alters the density of hippocampal dendritic spines in an enduring fashion. The purpose of the present study was to determine whether persistent alteration of hippocampal spine density is a common element in each of these different models of depression and whether such alterations could be reversed with chronic antidepressant treatment. The results show that olfactory bulbectomy reduced spine density in CA1, CA3, and dentate gyrus compared to sham-operated controls. Chronic treatment with amitriptyline, a tricyclic antidepressant, reversed the bulbectomy- induced reduction in dendritic spine density in CA1, CA3, and dentate gyrus, whereas treatment with mianserin, an atypical antidepressant, reversed this reduction only in dentate gyrus. On the other hand, neither neonatal clomipramine administration nor maternal deprivation affected hippocampal dendritic spine density. Repeated neonatal handling, however, as a control or as part of the maternal deprivation procedure, elevated spine density in dentate gyrus. These data suggest that long-lasting alterations in hippocampal dendritic spine density contribute to the neural mechanism underlying the olfactory bulbectomy model of depression, but not the neonatal clomipramine or maternal deprivation models.

Total regional and global number of synapses in the human brain neocortex

Abstract: An estimator of the total number of synapses in neocortex of human autopsy brains based on unbiased stereological principles is described. Each randomly chosen cerebral hemisphere was stratified into the four major neocortical regions. Uniform sampling with a varying sampling fraction in each region of neocortex was performed. The total volume of each neocortical region was estimated using point counting according to Cavalieri's principle. The ethanolic phosphotungstic acid staining technique was modified for synapses in human autopsy brains. The numerical density of synapses in each neocortical region studied was estimated using the disector at the electron microscopical level. The total number of neocortical synapses in each region was estimated as the product of the total volume of neocortex and the numerical density of synapses. The influence of the postmortem fixation delay on the number of synapses was investigated in five large mammals (one dog, one cow, and three pigs), the brains of which were kept under conditions similar to those under which human corpses are normally kept. The apparent decrease of 3.9% in the numerical density of synapses in the large mammals following a 2-day fixation delay was not significant. The average total number of synapses in the neocortex of five young male brains was 164 × 1012 (CV = 0.17). An analysis of the precision of the estimate of the total number of synapses in neocortex indicates that blocks represent both the major source of variation and the largest workload. Using eight blocks per brain the imprecision of the estimate is, however, only 66% of the total variance. © 2001 Wiley-Liss, Inc.

Relevance of pharmacokinetic parameters in animal models of methamphetamine abuse.

Abstract: Although the behavioral consequences of methamphetamine (METH) abuse have been extensively documented, a more precise and thorough understanding of underlying neurobiological mechanisms still requires the use of animal models. To study these biochemical processes in experimental animals requires consideration for the broad range of human METH abuse patterns and the many factors that have been identified to profoundly influence the behavioral and neurochemical effects of exposure to METH-like stimulants. One potentially critical issue relates to pharmacokinetic differences between the species. In this review, METH plasma pharmacokinetic profiles after single and multiple dose intravenous METH administration are compared for the rat and human. Significant differences in elimination half-life between the two species (t1/2: rat-70 min, human-12 h) result in markedly dissimilar profiles of METH exposure. However, the plasma profile of a human METH binge pattern can be approximated in the rat by increasing METH dose frequency. Consideration of METH pharmacokinetics in animal models should permit a closer simulation of the temporal profile of METH exposure in the human CNS and should provide further insight into the mechanisms contributing to the addiciton and psychopathology associated with METH abuse.

Electron microscopic immunocytochemical detection of PSD-95, PSD-93, SAP-102, and SAP-97 at postsynaptic, presynaptic, and nonsynaptic sites of adult and neonatal rat visual cortex

Abstract: Membrane-associated guanylate kinases (MAGUKs) assemble protein complexes at sites of cell-cell contact. At excitatory synapses in brain, MAGUKs localize to the postsynaptic density (PSD) and interact with N-methyl-D-aspartate (NMDA) glutamate receptors and downstream signaling proteins. However, NMDA receptors are not restricted to the PSDs, as electron microscopic immunocytochemical (EM-ICC) results indicate that NMDA receptors also occur at nonsynaptic portions of dendrites, perhaps functioning as reserves for rapid insertion into synaptic membranes in response to appropriate synaptic activity. NMDA receptors also occur in axons, at least in part to support glutamate-dependent enhancement of transmitter release. In this study, a systematic EM-ICC survey was performed to determine whether the distributions of four neuronal MAGUKs-PSD-95, PSD-93, SAP-102, and SAP-97-resemble that of NMDA receptors. Quantitative analysis revealed that the density of PSD-95 over thick PSDs of asymmetric axo-spinous synaptic junctions is 2-3-fold the level in the immediately adjacent cytoplasm of spines and terminals, while symmetric synapses show no association with PSD-95. Similarly, all four MAGUKs occur over PSDs of spines. However, we also detected MAGUK immunoreactivity, albeit more diffusely, along presynaptic membranes and in the cytoplasm of axons and dendritic shafts. In fact, the overall distribution of PSD-95 within the neuropil is equally prevalent along plasma membranes (including synaptic portions) as in the cytoplasm, away from plasma membranes. These results suggest that MAGUKs have dual roles: to maintain receptors at synapses and to regulate shuttling of receptors between nonsynaptic and synaptic sites.

Changes in neuronal activation with increasing attention demand in healthy volunteers: an fMRI study.

Abstract: Several lines of evidence suggest that structures involved in mediating attention differentially respond to increasing processing demand. Investigation of differences in neuronal activation, however, has been complicated by methodological inconsistencies and concomitant discrepancies in degree of difficulty and subject effort between disparate tasks. In this study, we utilized fMRI to compare neural activation patterns associated with two related attention tasks associated with different degrees of processing load while controlling for degree of performance difficulty. Healthy volunteers performed two continuous performance tasks, utilizing an identical pairs paradigm (CPT-IP) and a matched simple number recognition paradigm with degraded stimuli (CPT-DS) during a single fMRI scan. Degree of stimulus resolution degradation in the latter CPT was designed to equalize degree of performance difficulty between the two tasks. CPT-IP and CPT-DS were both associated with activation of frontal, limbic, subcortical, and sensory integratory structures. CPT-IP administration was associated with significantly greater activation of left dorsolateral prefrontal cortex, bilateral posterior temporal cortex, bilateral putamen, and thalamus. This study demonstrates both that differing attention tasks are associated with a high degree of functional overlap and that increasing processing demand is associated with increased activation of specific portions of attentional networks. Synapse 42:266–272, 2001. © 2001 Wiley-Liss, Inc.

Major coexpression of kappa-opioid receptors and the dopamine transporter in nucleus accumbens axonal profiles.

Abstract: The behavioral effects of psychostimulants, which are produced at least in part through inhibition of the dopamine transporter (DAT), are modulated by κ-opioid receptors (KOR) in the nucleus accumbens (Acb) Using electron microscopic immunocytochemistry, we reveal that in the Acb KOR labeling is mainly, and DAT immunoreactivity is exclusively, presynaptic From 400 KOR-labeled presynaptic structures, including axon terminals, intervaricosities, and small axons, 51% expressed DAT and 29% contacted another population of terminals exclusively labeled for DAT Within axonal profiles that contained both antigens, DAT and KOR were prominently localized to plasma membrane segments that showed overlapping distributions of the respective immunogold-silver and immunoperoxidase markers KOR labeling was also localized to membranes of small synaptic vesicles in terminals with or without DAT immunoreactivity In addition, from 24 KOR-immunoreactive dendritic spines 42% received convergent input from DAT-containing varicosities and unlabeled terminals forming asymmetric, excitatory-type synapses Our results provide the first ultrastructural evidence that in the Acb, KOR is localized to strategic sites for involvement in the direct presynaptic release and/or reuptake of dopamine These data also suggest a role for KOR in the presynaptic modulation of other neurotransmitters and in the postsynaptic excitatory responses of single spiny neurons in the Acb Dual actions on dopamine terminals and their targets in the Acb may account for KOR-mediated attenuation of drug reinforcement and sensitization Synapse 42:185–192, 2001 © 2001 Wiley-Liss, Inc

Reduced dopamine D1 receptor binding in the ventral striatum of cigarette smokers.

Abstract: Several drugs of abuse, including nicotine, are thought to exert their reinforcing effects through actions on the mesolimbic dopamine system. Animal and human studies suggest that chronic administration of addictive drugs may lead to impaired dopamine neurotransmission in the nucleus accumbens. We measured D1 receptor density in 11 smokers and 18 nonsmokers using positron emission tomography and the D1 receptor ligand [11C]SCH 23390. Ten of the smokers were scanned twice, once after overnight abstinence from cigarettes, and once while smoking at their usual rate, to account for possible acute effects of cigarette smoking on D1 receptor binding. In addition, eight control subjects were scanned twice to assess the reproducibility of the method. We used compartmental modeling to measure [11C]SCH 23390 binding potential, a measure of D1 receptor density. There were no differences in binding between abstinent and nonabstinent scans in smokers or in the two scans in controls. However, there was a significant reduction in [11C]SCH 23390 binding potential in smokers compared to nonsmokers in the striatum, most prominently in the ventral striatum. This suggests that there is a reduction in dopamine D1 receptor density in the ventral striatum of human cigarette smokers relative to nonsmokers, which implies that the postsynaptic mesolimbic dopamine system may be chronically underactive in smokers, either as an antecedent or consequence of addiction to cigarettes. Such a hypodopaminergic state may play an important role in sustaining nicotine-seeking behavior. Alternatively, an inherited reduction in dopamine receptors in the striatum may be associated with an increased risk of addictive behavior.

SCH 58261, an A(2A) adenosine receptor antagonist, counteracts parkinsonian-like muscle rigidity in rats.

Abstract: The aim of the present study was to find out whether blockade of adenosine A(2A) receptors by a selective antagonist, SCH 58261, influenced parkinsonian-like muscle rigidity. Muscle tone was examined using a combined mechano- and electromyographic method which simultaneously measured muscle resistance (MMG) of a rat hindfoot to passive extension and flexion in the ankle joint and electromyographic activity (EMG) of the antagonistic muscles of that joint: gastrocnemius and tibialis anterior. Muscle rigidity produced by reserpine (5 mg/kg + alpha-methyl-p-tyrosine, 250 mg/kg) was antagonized by SCH 58261 (0.1-5 mg/kg). SCH 58261 (5 mg/kg) also reduced reserpine-enhanced tonic and reflex EMG activities in both the gastrocnemius and the tibialis muscles. Moreover, SCH 58261 in doses of 1 and 5 mg/kg abolished muscle resistance induced by haloperidol (0.5 mg/kg). However, only the highest dose of SCH 58261 (5 mg/kg) decreased tonic EMG activity enhanced by haloperidol. Administration of L-DOPA (75 and 100 mg/kg) dose-dependently decreased the muscle resistance as well as tonic EMG activity evoked by haloperidol. Combined administration of SCH 58261 (0.1 mg/kg) and L-DOPA (50 mg/kg) in doses which did not affect the haloperidol-induced muscle rigidity produced a pronounced synergistic effect. The ability of SCH 58261 to diminish the parkinsonian-like muscle rigidity and to potentiate the effect of L-DOPA in this model seems to indicate a therapeutic value of this compound in the treatment of Parkinson's disease.

Motor stimulant effects of the adenosine A2A receptor antagonist SCH 58261 do not develop tolerance after repeated treatments in 6-hydroxydopamine-lesioned rats.

Abstract: Several evidences indicate that the selective blockade of adenosine A(2A) receptors counteracts the motor activity impairment in experimental models of Parkinson's disease. In the present study, the effects of the adenosine A(2A) receptor antagonist, SCH 58261 (5-amino-7-(beta-phenylethyl)-2-(8-furyl)pyrazolo(4,3-e)-1,2,4-triazolo(1,5-c)pyrimidine, were assessed following a repeated treatment schedule in the contralateral turning behavior rat model of Parkinson's disease. Unilateral lesions of the nigrostriatal pathway were induced by injecting 6-hydroxydopamine (6-OHDA) in medial forebrain bundle. Repeated administration of SCH 58261 was performed either alone (7 and 14 days repeated SCH 58261) or together with L-dopa (19 days repeated SCH 58261 plus L-dopa or L-dopa alone). After a 7- and 14-day repeated administration schedule, SCH 58261 (5 mg/kg) maintained its ability to potentiate the contralateral turning behavior induced by a subthreshold dose of L-dopa (2 mg/kg i.p.), showing no tolerance to its stimulant effects. SCH 58261 (5 mg/kg) plus L-dopa (3 mg/kg) or L-dopa (6 mg/kg) alone induced, at these dosages, the same number of contralateral turnings after the first administration. While chronic intermittent SCH 58261 plus L-dopa did not lead to a modified turning behavior during treatment, L-dopa alone produced a progressive increase in turning behavior intensity and duration. These results provide evidence that SCH 58261 retains its ability to potentiate L-dopa effects in a validated rat model of Parkinson's disease even after repeated treatments. Moreover, these results suggest that adenosine A(2A) blockade prevents the appearance of motor response alterations in L-dopa-treated rats, supporting the concept that A(2A) receptor antagonists have a therapeutic potential for the treatment of Parkinson's disease. Copyright 2001 Wiley-Liss, Inc.

Plasmalemmal mu-opioid receptor distribution mainly in nondopaminergic neurons in the rat ventral tegmental area.

Abstract: Opiate-evoked reward and motivated behaviors reflect, in part, the enhanced release of dopamine produced by activation of the μ-opioid receptor (μOR) in the ventral tegmental area (VTA). We examined the functional sites for μOR activation and potential interactions with dopaminergic neurons within the rat VTA by using electron microscopy for the immunocytochemical localization of antipeptide antisera raised against μOR and tyrosine hydroxylase (TH), the synthesizing enzyme for catecholamines. The cellular and subcellular distribution of μOR was remarkably similar in the two major VTA subdivisions, the paranigral (VTApn) and parabrachial (VTApb) nuclei. In each region, somatodendritic profiles comprised over 50% of the labeled structures. μOR immunolabeling was often seen at extrasynaptic/perisynaptic sites on dendritic plasma membranes, and 10% of these dendrites contained TH. μOR-immunoreactivity was also localized to plasma membranes of axon terminals and small unmyelinated axons, none of which contained TH. The μOR-immunoreactive axon terminals formed either symmetric or asymmetric synapses that are typically associated with inhibitory and excitatory amino acid transmitters. Their targets included unlabeled (30%), μOR-labeled (25%), and TH-labeled (45%) dendrites. Our results suggest that μOR agonists in the VTA affect dopaminergic transmission mainly indirectly through changes in the postsynaptic responsivity and/or presynaptic release from neurons containing other neurotransmitters. They also indicate, however, that μOR agonists directly affect a small population of dopaminergic neurons expressing μOR on their dendrites in VTA and/or terminals in target regions. Synapse 41:311–328, 2001. © 2001 Wiley-Liss, Inc.

Serotonin 5-HT1A receptor expression is selectively enhanced in the striosomal compartment of chronic Parkinsonian monkeys

Abstract: Cynomolgus monkeys (Macaca fascicularis) were chronically treated with the dopaminergic neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) until stable parkinsonism was reached. Two months later, monkeys were sacrificed and monoamine content was measured in different brain regions of the lesioned monkeys and of age-matched controls. 5-HT1A serotonin receptor density was measured in coronal sections labeled with [3H]8-OH-DPAT. As expected, dopamine was virtually nonexistent in the caudate nucleus and putamen of MPTP-treated monkeys. Serotonin levels were significantly reduced in different brain regions, particularly in the raphe nuclei. 5-HT1A receptor density of control animals was high in the hippocampus, notably in the CA1 field and also in the raphe nuclei, and much lower in the striatum, where 5-HT1A receptors showed a patchy distribution which corresponded to striosomes with poor calbindin immunostaining. 5-HT1A receptor density was reduced in hippocampal fields and in the raphe nuclei of parkinsonian monkeys. Conversely, in the severely lesioned striatal nuclei 5-HT1A receptor density was increased at caudal levels of the striatum, particularly in the putamen. The results tend to support the possibility of an increased synthesis of 5-HT1A receptors in brain regions with higher neuronal cell death. Upregulation of this 5-HT receptor subtype in the limbic compartment of the striatum may represent a compensatory event for the serotonergic dysfunction and associated mental disorders in neurodegenerative diseases such as Parkinson disease. Synapse 39:288–296, 2001. © 2001 Wiley-Liss, Inc.

Regional distribution and cellular localization of 5-HT2C receptor mRNA in monkey brain: comparison with [3H]mesulergine binding sites and choline acetyltransferase mRNA.

Abstract: The distribution of serotonin 5-HT(2C) receptor mRNA in monkey brain was studied by in situ hybridization and compared with the distribution of [3H]mesulergine binding sites as visualized by receptor autoradiography. 5-HT(2C) receptor transcripts showed a widespread and heterogeneous distribution. The strongest hybridization signal was detected in choroid plexus. In neocortex, 5-HT(2C) mRNA was detected in layer V of all cortical regions examined except in the calcarine sulcus, which was devoid of signal. Several structures within the striatum and basal forebrain were strongly labeled: nucleus accumbens, ventral aspects of anterior caudate and putamen, septal nuclei, diagonal band, ventral striatum, and extended amygdala. Several thalamic, midbrain, and brainstem nuclei also contained 5-HT(2C) mRNA. Comparison of the distributions of 5-HT(2C) mRNA and specific [3H]mesulergine binding sites showed a good agreement in the majority of brain regions, suggesting a predominant somatodendritic localization of 5-HT(2C) receptors. A possible localization to axon terminals of 5-HT(2C) receptors is suggested by the disagreement observed in some regions such as septal nuclei and horizontal limb of the diagonal band (presence of mRNA with apparent absence of binding sites) and interpeduncular nucleus (presence of binding sites with apparent absence of mRNA). Comparison of 5-HT(2C) receptor and choline acetyltransferase mRNA distributions indicate that some regions where cholinergic cells are located are also enriched in cells containing 5-HT(2C) mRNA. Although the present methodology does not allow strict colocalization of both mRNA species to the same cells, the codistribution observed in several regions provides a possible anatomical substrate for the described modulation of acetylcholine release by 5-HT(2C) receptors.

Ascorbic acid prevents 3,4-methylenedioxymethamphetamine (MDMA)-induced hydroxyl radical formation and the behavioral and neurochemical consequences of the depletion of brain 5-HT.

Abstract: MDMA-induced 5-HT neurotoxicity has been proposed to involve oxidative stress due to increased formation of hydroxyl radicals. Recently, MDMA-induced 5-HT neurotoxicity has been shown to be accompanied by a suppression of behavioral and neurochemical responses to a subsequent injection of MDMA. The intent of the present study was to examine whether suppression of the MDMA-induced formation of hydroxyl radicals by an antioxidant, ascorbic acid, attenuates both the MDMA-induced depletion of 5-HT and the functional consequences associated with this depletion. Treatment of rats with ascorbic acid suppressed the generation of hydroxyl radicals, as evidenced by the production of 2,3-dihydroxybenzoic acid from salicylic acid, in the striatum during the administration of a neurotoxic regimen of MDMA. Ascorbic acid also attenuated the MDMA-induced depletion of striatal 5-HT content. In rats treated with a neurotoxic regimen of MDMA, the ability of a subsequent injection of MDMA to increase the extracellular concentration of 5-HT in the striatum, elicit the 5-HT behavioral syndrome, and produce hyperthermia was markedly reduced compared to the responses in control rats. The concomitant administration of ascorbic acid with the neurotoxic regimen of MDMA prevented the diminished neurochemical and behavioral responses to a subsequent injection of MDMA. Finally, a neurotoxic regimen of MDMA produced significant reductions in the concentrations of vitamin E and ascorbic acid in the striatum and hippocampus. Thus, the MDMA-induced depletion of brain 5-HT and the functional consequences thereof appear to involve the induction of oxidative stress resulting from an increased generation of free radicals and diminished antioxidant capacity of the brain.

Temporal profiling of methamphetamine-induced changes in gene expression in the mouse brain: evidence from cDNA array.

Abstract: Methamphetamine (METH) is a neurodegenerative drug of abuse. Its toxicity is characterized by destruction of monoaminergic terminals and by apoptosis in cortical and striatal cell bodies. Multiple factors appear to control METH neurotoxicity, including free radicals and transcription factors. Here, using cDNA arrays, we show the temporal profile of gene expression patterns in the cortex of mice treated with this drug. We obtained two patterns of changes from 588 genes surveyed. First, an early pattern is characterized by upregulation of transcription factors, including members of the jun family. Second, a delayed pattern includes genes related to cell death and to DNA repair. A number of trophic factors were also activated at the later timepoint. These observations suggest that METH can activate a multigene machinery that participates in the production of its toxic effects. The resulting degenerative effects of the drug are thus the result of a balance between protoxic and antiapoptotic mechanisms triggered by its administration to these animals. These observations are of clinical relevance because of the recent identification of degenerative changes in the brains of METH abusers.

Differential distribution of cAMP-specific phosphodiesterase 7A mRNA in rat brain and peripheral organs.

Abstract: This work was supported by grants from Fundacio La Marato de TV3 (1017/97), and CICYT (SAF1999-0123 and 2FD97-0395). X.M. is a recipient of a fellowship from CIRIT (Centre de Referencia of the Generalitat de Catalunya) and S.P.-T. from CIRIT (Generalitat de Catalunya)

Stress, hippocampal plasticity, and spatial learning.

Abstract: During the last two decades numerous studies have been conducted in an attempt to correlate the mechanisms of long-term potentiation (LTP) of hippocampal synaptic transmission with those required for spatial memory formation in the hippocampus. Because stressful events block the induction of hippocampal LTP, it has been suggested that deficits in spatial learning following stress may be related to suppression of LTP-like phenomena in the hippocampus. Here I review these studies and discuss them in light of the emerging view that stress may induce changes in thresholds for synaptic plasticity necessary for both LTP induction and spatial memory formation. This phenomenon, known as metaplasticity, may involve a glucocorticoid modulation of calcium homeostasis. Synapse 40:180–183, 2001. © 2001 Wiley-Liss, Inc.

Effect of 5-HT on binding of [11C] WAY 100635 to 5-HT1A receptors in rat brain, assessed using in vivo microdialysis and PET after fenfluramine

Abstract: By using a combination of positron emission tomography (PET) and postmortem tissue dissection, the effect of increased endogenous serotonin on specific binding of [(11)C]WAY 100635 to the 5-HT(1A) receptor was investigated in rat brain in vivo. The binding studies were complemented by in vivo microdialysis to monitor 5-HT levels in similarly treated isoflurane-anaesthetised rats, with the dialysis probe locations corresponding to two of the tissues sampled for specific binding of the radioligand. Fenfluramine treatment (10 mg/kg i.p.) resulted in a approximately 5-fold increase in extracellular 5-HT in medial prefrontal cortex and a approximately 15-fold increase in lateral hippocampus, maximal at approximately 40 min after injection. PET scan duration was either 60 or 90 min, beginning 30 min after fenfluramine injection. The specific binding of [(11)C]WAY 100635 was reduced by 10-20% in hippocampus, which showed highest binding in control animals. Specific binding, however, was unaffected in both prefrontal cortex and midbrain raphe, each additional high binding regions. The minimal effects are consistent with a low baseline occupancy of the 5-HT(1A) receptor by 5-HT in vivo, so that only a large change in endogenous agonist concentration will affect radioligand binding. This implies that utilisation of [(11)C]WAY 100635 in human PET to quantify 5-HT(1A) receptor expression can be extended to pathology where synaptic 5-HT levels are altered as a consequence of the disease state.

Differential behavioral responses to cocaine are associated with dynamics of mesolimbic dopamine proteins in Lewis and Fischer 344 rats.

Abstract: Differential behavioral and biochemical responses to drugs of abuse may reflect genetic makeup as suggested by studies of inbred Lewis (LEW) and Fischer 344 (F344) rats. We investigated locomotor activity, stereotypy signs, and levels of specific proteins in the nucleus accumbens (NAc) and ventral tegmental area (VTA) in these strains at baseline and following chronic administration of cocaine (30 mg/kg/day for 14 days). Using Western blot analysis, we replicated our previous findings of baseline strain differences and found lower levels of ΔFosB immunoreactivity in NAc of F344 vs. LEW rats. F344 rats showed greater baseline locomotor activity, sniffing, and grooming compared to LEW rats. Chronic cocaine increased ΔFosB levels in NAc in both strains, whereas adaptations in other proteins were induced in F344 rats only. These included reduced levels of tyrosine hydroxylase (TH) in NAc and increased TH and glial fibrillary acidic protein (GFAP) immunoreactivity in VTA. Chronic cocaine led to greater increases in overall stereotypy in F344 vs. LEW rats and decreased exploratory behaviors in LEW rats. Opposing effects by strain were seen in locomotor activity. Whereas F344 rats showed higher initial activity levels that decreased with cocaine exposure (tolerance), LEW rats showed increased activity over days (sensitization) with no strain differences seen at 14 days. Further, conditioned locomotor activation to vehicle injections was greater in F344 vs. LEW rats. These results suggest that behavioral responsiveness to chronic cocaine exposure may reflect dynamics of mesolimbic dopamine protein levels and demonstrate the role of genetic background in responsiveness to cocaine. Synapse 41:179–190, 2001. © 2001 Wiley-Liss, Inc.

Vesicular neurotransmitter transporters in Huntington's disease: initial observations and comparison with traditional synaptic markers.

Abstract: Markers of identified neuronal populations have previously suggested selective degeneration of projection neurons in Huntington's disease (HD) striatum. Interpretations are, however, limited by effects of compensatory regulation and atrophy. Studies of the vesicular monoamine transporter type-2 (VMAT2) and of the vesicular acetylcholine transporter (VAChT) in experimental animals indicate that they are robust markers of presynaptic integrity and are not subject to regulation. We measured dopamine and acetylcholine vesicular transporters to characterize the selectivity of degeneration in HD striatum. Brains were obtained at autopsy from four HD patients and five controls. Autoradiography was used to quantify radioligand binding to VMAT2, VAChT, the dopamine plasmalemmal transporter (DAT), benzodiazepine (BZ) binding sites, and D2-type dopamine receptors. The activity of choline acetyltransferase (ChAT) was determined as an additional marker of cholinergic neurons. Autoradiograms were analyzed by video-assisted densitometry and assessment of atrophy was made from regional structural areas in the coronal projection. Striatal VMAT2, DAT, and VAChT concentrations were unchanged or increased, while D2 and BZ binding and ChAT activity were decreased in HD. After atrophy correction, all striatal binding sites were decreased. However, the decrease in ChAT activity was 3-fold greater than that of VAChT binding. In addition to degeneration of striatal projection neurons, there are losses of extrinsic nigrostriatal projections and of striatal cholinergic interneurons in HD on the basis of vesicular transporter measures. There is also markedly reduced expression of ChAT by surviving cholinergic striatal interneurons.

Serotonin (1A) receptor ligands act on norepinephrine neuron firing through excitatory amino acid and GABA(A) receptors: a microiontophoretic study in the rat locus coeruleus.

Abstract: It was previously shown that the excitatory effect of the 5-HT1A agonist 8-OH-DPAT on firing activity of locus coeruleus (LC) norepinephrine (NE) neurons and the inhibitory action of the 5-HT1A antagonist WAY 100,635 are dependent on the presence of 5-HT neurons, whereas the inhibitory action of the 5-HT2 agonist DOI is not. Using in vivo extracellular unitary recordings performed in anesthetized rats, iontophoretic applications of the excitatory amino acid antagonist kynurenate attenuated the enhancement in firing produced by glutamate and kainate. In contrast, GABA applications decreased the firing activity of NE neurons which was attenuated by the enhancement produced by glutamate and kainate. In contrast, GABA applications decreased the firing activity of NE neurons which was attenuated by the GABAA receptor antagonist bicuculline. 8-OH-DPAT (10–60 μg kg−1, i.v.) produced a dose-dependent enhancement in the firing activity of NE neurons that was abolished in the presence of kynurenate application. The selective 5-HT1A receptor antagonist WAY 100,635 (100 μg kg−1, i.v.) suppressed NE firing which was reversed by the selective 5-HT2A antagonist MDL 100,907 (200 μg kg−1, i.v.). In the presence of bicuculline, the inhibitory effect of WAY 100,635 was blunted. These results suggest that WAY 100,635 mainly attenuates NE neuron firing by blocking inhibitory 5-HT1A receptors on glutamatergic neurons, thereby enhancing glutamate release and activating excitatory amino acid receptors, possibly of the kainate subtype, on 5-HT terminals. The ensuing increased 5-HT release would then act on excitatory 5-HT2A receptors on GABA neurons that would ultimately mediate the inhibition of NE neurons. The prevention of the excitatory action of 8-OH-DPAT on NE neuron firing by kynurenate is also consistent with this neurocircuitry. Synapse 42:203–212, 2001. © 2001 Wiley-Liss, Inc.

Expression of serotonin 5-HT(2A) receptors in the human cerebellum and alterations in schizophrenia.

Abstract: The occurrence of human cerebellar serotonin 5-HT2A receptors (5-HT2AR) is equivocal and their status in schizophrenia unknown. Using a range of techniques, we investigated cerebellar 5-HT2AR expression in 16 healthy subjects and 16 subjects with schizophrenia. Immunocytochemistry with a monoclonal antibody showed labelling of Purkinje cell bodies and dendrites, as well as putative astrocytes. Western blots showed a major band at ∼45 kDa. Receptor autoradiography and homogenate binding with [3H]ketanserin revealed cerebellar 5-HT2AR binding sites present at levels approximately a third of that in prefrontal cortex. 5-HT2AR mRNA was detected by reverse transcriptase-polymerase chain reaction, with higher relative levels in men than women. Several aspects of 5-HT2AR expression were altered in schizophrenia. 5-HT2AR immunoreactivity in Purkinje cells was partially redistributed from soma to dendrites and was increased in white matter. 5-HT2AR mRNA was decreased in the male patients. 5-HT2AR measured by dot blots and [3H]ketanserin binding (Bmax and Kd) were not significantly altered in schizophrenia. These data show that 5-HT2AR gene products (mRNA, protein, binding sites) are expressed in the human cerebellum at nonnegligible levels; this bears upon 5-HT2AR imaging studies which use the cerebellum as a reference region. 5-HT2AR expression is altered in schizophrenia; the shift of 5-HT2AR from soma to dendrites is noteworthy since atypical antipsychotics have the opposite effect. Finally, the results emphasise that expression of a receptor gene is a mutifaceted process. Measurement of multiple parameters is necessary to give a clear picture of the normal situation and to show the profile of alterations in a disease. Synapse 42:104–114, 2001. © 2001 Wiley-Liss, Inc.

Imaging of dopamine transporters with [123I]FP-CIT SPECT does not suggest a significant effect of age on the symptomatic threshold of disease in Parkinson's disease.

Abstract: Parkinson's disease (PD) is characterized neuropathologically by degeneration of the nigrostriatal dopaminergic pathway. With natural aging there is loss of dopaminergic cells in the substantia nigra and, consequently, loss of dopamine transporters in the striatum. It has been suggested that PD is caused by an accelerated rate of cell death. Conceptually, symptoms in idiopathic PD become apparent after a critical level of cell loss, the "symptom threshold." It has been suggested that this symptom threshold is independent of age. In this study, [123I]FP-CIT SPECT was used to assess the effect of aging on the density of striatal dopamine transporters in vivo in controls (n = 36) and early, drug-naive, patients with PD (n = 32). We found a significant age-associated decline of [123I]FP-CIT binding to striatal dopamine transporters in controls, but not in parkinsonian patients. This finding might give further support for the existence of an age-independent threshold in PD. In a subgroup of patients with hemi-PD, we found a significant loss of dopamine transporters bilaterally in the caudate nucleus and putamen. This loss was more pronounced in the putamen than in the caudate nucleus and the contralateral binding was significantly lower than the ipsilateral binding. By using age-corrected data, we estimated that in our particular patient group motor signs started when the loss of [123I]FP-CIT binding ratios in the putamen was 46-64%.

Assessment of muscarinic receptor concentrations in aging and Alzheimer disease with [11C]NMPB and PET.

Abstract: Cerebral cholinergic deficits have been described in Alzheimer disease (AD) and as a result of normal aging. At the present time, there are very limited options for the quantification of cholinergic receptors with in vivo imaging techniques such as PET. In the present study, we examined the feasibility of utilizing ( 11 C)N-methyl-4- piperidyl benzilate (NMPB), a nonselective muscarinic receptor ligand, in the study of aging and neurodegenerative processes associated with cholinergic dysfunction. Based on prior data describing the accuracy of various kinetic methods, we examined the concentration of muscarinic receptors with ( 11 C)NMPB and PET using two- and three- compartment kinetic models. Eighteen healthy subjects and six patients diagnosed with probable AD were studied. Pixel-by-pixel two-compartment model fits showed acceptable precision in the study of normal aging, with comparable results to those obtained with a more complex and less precise three-compartment model. Normal aging was associ- ated with a reduction in muscarinic receptor binding in neocortical regions and thala- mus. In AD patients, the three-compartment model appeared capable of dissociating changes in tracer transport from changes in receptor binding, but suffered from statis- tical uncertainty, requiring normalization to a reference region, and therefore limiting its potential use in the study of neurodegenerative processes. After normalization, no regional changes in muscarinic receptor concentrations were observed in AD. Synapse 39:275-287, 2001. © 2001 Wiley-Liss, Inc.