Showing papers in "Psychobiology in 1985"

Behavioral functions of locus coeruleus derived from cellular attributes

Abstract: The electrophysiological activity of noradrenergic neurons in the locus coeruleus (LC) was examined in unanesthetized rats during spontaneously occurring behavior and sensory stimulation. The pattern of spontaneous and evoked discharge during sleep, grooming, drinking, and orienting behaviors, considered in light of other cellular anatomic and physiologic attributes, implicates the LC system in the control of vigilance and initiation of adaptive behavioral responses.

Real-time attentional models for classical conditioning and the hippocampus

Abstract: Moore and Stickney (1980) described a real-time computational version of Mackintosh’s (1975) attentional model of associative learning. By assuming that hippocampal lesions affect computations that control the rate of learning, they were able to simulate impairments of latent inhibition and blocking, as reported in studies of classical conditioning. Schmajuk (1984a) proposed that hippocampal lesions affect computations of stimulus associabilities, as defined in Pearce and Hall’s (1980) model of learning. A revised version of the Moore-Stickney model and a real-time version of Pearce and Hall’s (1980) model, both incorporating the proposed modifications for the effect of hippocampal lesions, were applied to different classical conditioning paradigms. Simulation experiments with both models were carried out for the following protocols: acquisition under simultaneous, delay, and trace conditioning; partial reinforcement; noncontingent training; conditioned inhibition; differential conditioning; extinction; latent inhibition; blocking; overshadowing; and discrimination reversal. Although some discrepancies between simulation experiments and relevant literature were noted, both models proved capable of simulating most hippocampal lesion effects.

The locus coeruleus and cerebral metabolism: Recovery of function after cortical injury

Abstract: Cerebral metabolic effects of locus coeruleus (LC) lesion or drugs affecting LC were investigated after unilateral injury of sensorimotor cortex in rats. Sensoriomotor cortex ablation produced a widespread depression of cerebral 14C-2-deoxyglucose utilization which was reversed by amphetamine (AMP, 2 mg/kg) and worsened by haloperidol (HAL, 0.4 mg/kg). Lesion of LC alone did not affect cerebral oxidative metabolism, measured by a stain for the enzyme alpha-glycerophosphate dehydrogenase (α-GPDH). Lesion of LC prior to undercut laceration of motor cortex shortened time to onset of α-GPDH cortical paling. Treatment with AMP (2 mg/kg) blocked cortical paling of the enzyme stain at 4 days postinjury, an effect prevented by concomitant HAL (0.3 or 0.6 mg/kg). Apomorphine (1 mg/kg) did not block cortical paling. These data parallel effects of these drugs on recovery of function. The results suggest that a metabolic “remote functional depression” (RFD) is alleviated by catecholamine activation after cortical injury, whereas onset of RFD is accelerated by LC lesions and exacerbated by catecholamine blockade.

The locus coeruleus and cognitive function: Attempts to relate noradrenergic enhancement of signal/noise in the brain to behavior

Abstract: Behavioral manipulations that alleviate forgetting in a complex maze task are paralleled by pharmacological manipulations which modulate noradrenergic activity. Increased noradrenergic activity is observed following prior exposure to contextual reminder cues. It is proposed that the facilitated retrieval reflects a conditioned arousal and an enhanced attention to discriminative stimuli mediated by the noradrenergic response. Failure to find learning and memory retrieval deficits in rats that are depleted of noradrenaline is discussed in terms of functional recovery after lesions.

Functional hypotheses of the coeruleocortical noradrenergic projection: A review of recent experimentation and theory

Abstract: The behavioral function of the coeruleocortical system and the hypotheses implicating its role in mediating selective attention and behavioral inhibition to anxiogenic cues are evaluated. The diverse pattern of results produced by lesions induced by 6-hydroxydopamine on a variety of tasks (appetitive conditional discrimination, nonreversal shift, blocking, latent inhibition, distraction, taste aversion, neophobia, and conditioned emotional response) are integrated to achieve a tentative hypothesis of coeruleocortical noradrenergic function.

Habituation and dishabituation of the human orienting reflex under instruction-induced stress

Abstract: Twenty subjects received habituation trials with a 1000-Hz tone until two consecutive non-responses (skin conductance response) occurred. Then a single, novel 1400-Hz tone was administered, followed by another presentation of the 1000-Hz tone. Half the subjects were told that an intelligence test would follow. All of the subjects were initially given the STAI trait anxiety test. The 10 subjects told to expect an intelligence test needed significantly more trials to habituate and made significantly larger SCRs to the novel stimulus than did the other 10 subjects. Trait anxiety neither interacted with these effects nor had any effects of its own. The absence of trait anxiety effects was interpreted as being in disagreement with Spielberger’s assertions regarding trait and state anxiety.

Regulation of noradrenergic neuronal activity in the rat locus coeruleus by serotoninergic afferents

Abstract: From an anatomical point of view, the serotonin axonal varicosities in the locus coeruleus have been defined as nonsynaptic terminals originating from the raphe dorsalis, raphe centralis, and raphe pontis. Electrolytic or chemical destruction of these serotoninergic afferents has produced pronounced increases in the activity of the noradrenaline biosynthetic enzyme, tyrosine hydroxylase, in the locus coeruleus and in the noradrenaline metabolite DOPEG in the terminal field of the hippocampus and cerebral cortex. A good correlation has been observed between the depletion of serotonin and the increase in tyrosine hydroxylase activity in the locus coeruleus after chemical denervation of serotoninergic terminals. Electrical stimulation of serotonin cell bodies has increased noradrenaline levels in the locus coeruleus and cerebral cortex. Altogether these data suggest that the function of the ascending noradrenergic system originating from the locus coeruleus might be largely dependent on its interaction with serotonin-containing neurons in the raphe system.

Cooling orbital frontal cortex disrupts matching-to-sample and visual discrimination learning in monkeys

Abstract: Matching-to-sample and visual discrimination performance was examined during cooling of the, orbital cortex in monkeys. Cooling produced severe deficits in simultaneous match-to-sample and delayed match-to-sample performance across all delays, 0–60 sec, and resulted in substantial difficulties in learning visual discriminations. Retention of discriminations learned during cooling of the orbital cortex was unaffected when tested under cooling or noncooling conditions. Discriminations learned prior to cooling were also not affected by orbital cooling. These results suggest that the orbital cortex is important in visual learning but not in retention, and are similar to findings on these tasks with anterior temporal lobe cooling.

Mechanisms of action of noradrenaline in the brain

Abstract: The noradrenaline (NA) innervation of the brain has been studied extensively over the past decade. Early views suggested that the nucleus locus coeruleus (LC) has a widespread, diffuse, slow conducting projection system with little anatomical or functional specificity. These views were supported by the observations that (a) the LC projects into nearly every brain structure in a rather diffuse manner, (b) LC fibers are small, unmyelinated and slowly conducting and therefore unlikely to convey specific information (c) action potential discharge rates of LC neurons vary considerably with sleep-wakefulness cycles being nearly silent in REM sleep and (d) iontophoretic application of NA produces in nearly every brain area a long lasting beta receptor-mediated suppression of spontaneous firing rates. It is now clear, however, that these conclusions represent an oversimplification of the actual evidence and that LC functions might be more specific and complex. LC innervation of the for-ebrain is actually quite heterogeneous with some structures being heavily invested with NA fibers while others are relatively devoid of them. Furthermore, within a given structure, e.g. the neocortex, there is a distinct laminar distribution of NA fibers and receptors.

Performance of hippocampectomized rats in a reference/working-memory task: Effects of preoperative versus postoperative training

Abstract: Hippocampectomized and sham-operated male rats were pre- or postoperatively trained to perform a reference/working-memory task in a standard radial eight-arm maze. On each trial, three arms were baited. These “correct” arms formed one of two configurations, which changed between conditions. The remaining arms were never baited. The rats’ task was to enter each of the baited arms only once, and to refrain from entering any of the unbaited arms. Incorrect choices were punished with a brief period of confinement. Preoperatively trained groups were tested postoperatively for retention, and then trained on a new reference-memory task in which the baited arms formed the second configuration. Postoperatively trained groups were also trained on two such tasks in succession. Order of training on the two configurations was counterbalanced between groups. Postoperatively trained hippocampal groups were severely impaired in acquisition of the tasks. Preoperatively trained hippocampal groups were unimpaired, relative to sham-operated groups, in all measures of performance.

Safety signals can mimic responses in reducing the ulcerogenic effects of prior shock.

Abstract: The severity of restraint-induced gastric ulcerations in rats may be influenced by proactive effects of earlier shock experience. Earlier experience with escapable shock offers some protection against the ulcerogenic effects of restraint stress. In this experiment, we tested the hypothesis that providing animals with safety signals during a Pavlovian conditioning session would also provide a proactive protection against restraint ulceration similar to that provided by escape responses. Animals were subjected to five daily sessions of 20 shocks before they were subjected to a single 23-h restraint stress procedure. The animals given safety signals during the conditioning sessions developed less ulceration than those subjected to random tone-shock pairings and those that were not shocked. This complements other reports of the similar properties shared by escape conditioning and safety-signal (backward) conditioning. In contrast, postrestraint corticosterone levels were higher in animals provided earlier with safety signals than they were in other groups.

Serial-pattern-learning processes dissociated by trimethyltin exposure in rats

Abstract: Trimethyltin (TMT) is a neurotoxic organometal which produces a variety of learning and memory impairments in laboratory animals and humans, including impairments of avoidance learning, maze learning, and problem solving. Two studies investigated the effects of TMT exposure on serial-pattern learning in rats. Rats in both experiments were intubated once with either 0 or 7.0 mg/kg TMT 1 week prior to the pattern-learning procedure. Rats learned serial patterns composed of various quantities of brain-stimulation reward (BSR) pulses; they received BSR quantities in a predetermined order for leverpresses in a discrete-trial operant task. In Experiment 1, all rats received two serial patterns (20-10-0 vs. 1-29-0 pulses of BSR) that alternated within each daily session of 100 patterns. In Experiment 2, all rats received two serial patterns (18-10-6-3-1-0 vs. 18-1-3-6-10-0 pulses of BSR) that alternated within each daily session. In Experiment 1, TMT-exposed rats learned both their patterns more slowly than did controls. In Experiment 2, TMT-exposed rats learned the formally simple 18-10-6-3-1-0 pattern of BSR quantities faster than did controls, but were significantly slower than controls in learning the formally more complex 18-1-3-6-10-0 pattern. In both experiments, however, TMT exposure did not affect either the reinforcing properties of BSR or rats’ asymptotic performance. These results support the notion that TMT exposure impaired some aspects of the rote processes involved in serial-pattern learning in rats, yet spared the rats’ ability to encode some representation of the formal rule-based structure of the pattern. The results thus suggest that the processes involved in learning simple versus complex serial patterns may be mediated by different systems in the brain.

Electrophysiological perspectives on locus coeruleus: Its role in cognitive versus vegetative functions

Abstract: The hypothesis that the locus coeruleus (LC) may be involved in the control of cognitive and/or vegetative processes is reviewed, primarily focusing on electrophysiological experiments. With the impact of this nucleus on target neurons in different brain areas and with the activity pattern of LC neurons in conscious animals, the electrophysiological data are, to a large extent, in keeping with the postulated role of the LC in cognitive processes. A considerable body of evidence from lesion and stimulation experiments points to a possible function of the LC in the control of autonomic processes. In particular, many studies suggest a role in the central control of blood pressure. The electrophysiological data, although not entirely ruling out such a function, are not in keeping with this hypothesis. In conclusion, although there is a case for the LC’s having a role both in the cognitive sphere and in the vegetative sphere, the exact role of the LC in both of them remains to be elucidated.

Effects of naloxone and naltrexone on morphine-elicited changes in hamster locomotor activity

Abstract: Four experiments investigated the effects of naloxone (0.4 mg/kg) and naltrexone (1.0 mg/kg) on morphine(15 mg/kg)-elicited sedation and hyperactivity in the hamster. Experiments 1 and 2 demonstrated that naloxone blocks morphine sedation and hyperactivity, respectively. Experiments 3 and 4 demonstrated that naltrexone blocks morphine sedation and hyperactivity, respectively. Moreover, in Experiments 2 and 4, morphine-elicited hyperactivity was replaced by sedation following administration of the antagonists. In Experiment 1, morphine-elicited sedation was replaced by hyperactivity following administration of naloxone. These results are consistent with the hypothesis that morphine’s biphasic time-effect curve is the behavioral resultant of two underlying processes.

Spatial problem solving in the rat following medial frontal lesions

Abstract: Rats with medial frontal lesions were unable to perform as successfully as normal rats on the Maier 3-table spatial reasoning task. Performance did not improve with repeated testing. Instead, the test-trial performance of medial frontal animals was characterized by the use of a directional response (i.e., body turn) strategy at the choice point. Normal rats habituated (i.e., decreased the number of table entries) within the first few minutes of the daily exploratory period. Although medial frontal rats initially did not display this habituation pattern, they did develop this habituation pattern over blocks of trials. The presence of the habituation pattern has been used as evidence to indicate that the table-entry behavior reflects exploratory rather than general activity levels. The fact that medial frontal animals do show a habituation pattern but are unable to solve the problem suggests that such animals are able to acquire spatial information but are unable to use it.

The effects of housing and preshock on activity-stress ulcer

Abstract: Rats were either group-housed, four rats per cage, or housed singly for 6 weeks. Subgroups were subsequently exposed to a daily 3-min 1.25-mA grid shock for 5 days. The rats were then housed in running-wheel activity cages and, following a 4-day free-feeding period, were fed only 1 h daily for 20 days. Group-housed rats had more ulcers, but the previously reported protective effect of preshock was not observed in this study.

Age differences in the potentiation of taste aversion by odor cues

Abstract: The potentiation of a conditioned aversion to saccharin by maple odor was examined in a latent inhibition paradigm using weanling, young-adult, and old-age rats Potentiation of the flavor aversion was age-related There was a potentiation effect only in young-adult rats These results are discussed in terms of current explanations of potentiation It is suggested that when the components of a compound stimulus are in different systems, that is, interoceptive-exteroceptive, conditioning occurs in both systems and one system enhances conditioning in the other This could explain the apparent reciprocal potentiation in odor/taste pairings as well as age differences in potentiation

The effect of previous experience upon operant performance following cerebellar lesions in the rat

Abstract: Lesions of the fastigial nuclei and cerebellar vermis, but not lesions of the dentate nuclei, were found to produce marked performance deficits on a differential reinforcement of low rates (DRL) schedule of reinforcement. This deficit was characterized by an abnormal number and distribution of responses within the schedule interval. Lesions, however, did not produce a deficit following preoperative training or when subjects were tested on a fixed-interval (FI) schedule. In addition, when DRL and FI performance was contrasted, all subjects were responsive to schedule contingencies. Results suggest that the DRL deficit following cerebellar lesions is due to a tendency to perseverate in response strategies, and is not related to a global disruption of timing or a pervasive inability to suppress responding.

Hippocampal unit response during temporal single alternation of classical conditioning with rewarding brain stimulation in the rat

Abstract: To evaluate hippocampal function on working memory, an experiment of rewarding classical conditioning with a temporal single-alternation (SA) paradigm was conducted. White rats that showed positive self-stimulation to the lateral hypothalamus were trained on an SA schedule with 60-sec intervals, in which reinforced trials (R; CS-US) were regularly alternated with non-reinforced trials (N; CS alone). Unit activity from the hippocampus and the animals’ movements were recorded during the pairing of auditory CS and lateral hypothalamic stimulation (US). When the experimental schedule was shifted to SA from pseudoconditioning, the hippocampal unit activity increased in the CS periods on both R and N trials in the absence of behavioral response alternation. During SA training, hippocampal unit activity in the CS period significantly increased on R trials relative to N trials, when a differential behavioral response also appeared. No detectable differences in the hippocampal unit background pre-CS levels were found between R and N trials. The results are discussed in terms of the interaction between working memory and reference memory in the hippocampal function.

Properties of mapping induced by fornix damages: Learning and memorizing the radial maze task

Abstract: A correlational analysis of learning scores and degrees of divergence of the patterns of choice was performed in fornix-damaged, sham-operated, and control rats during the acquisition stage of the radial maze task. The results show that dorsal fornix sections, which initially reduce choice accuracy, induce the parallel adoption of weakly divergent patterns of choice. The positive correlation observed during the course of training between these two dependent variables suggests that fornix-damaged animals learn the task on the basis of specific gathering and processing of information: initial choice of few proximal paths with progressive increases in the number of visited paths under the control of reinforcement determines a correlative increase in the degrees of divergence of the patterns of choice. This observation indicates that mapping in lesioned animals occurs through the progressive introduction of new spatial elements in an initial sectorial exploratory system, whereas sham-operated and control animals immediately process more information on the total configuration of the apparatus. This particular gathering and processing of spatial information could determine specific reference and working memory processes in lesioned animals.

Prenatal alcohol exposure and offspring hyperactivity: The effects of home-cage shavings and test chamber temperature

Abstract: Rats were fed a liquid diet containing alcohol from Days 6–19 of gestation. Controls were pairfed the same diet with sucrose substituted for ethanol, or were allowed ad-lib chow and water. The activity of the offspring was observed in isolation at 10, 16, 22, or 28 days of age. Offspring exposed to alcohol prenatally were hyperactive relative to controls at 16 and 22 days, but not at 10 or 28 days. The presence of home-cage shavings in the test chamber affected the activity of the three groups similarly. In a second study, isolation-induced activity was observed at a test room temperature of 23° or 33° C. Offspring exposed to alcohol were hyperactive in comparison with pair-fed controls, at both 16 and 22 days, when tested at 23°C. However, the difference in activity was attenuated at 16 days and abolished at 22 days if pups were tested at 33° C. These findings indicate that the hyperactivity displayed by alcohol-treated offspring may be due in part to their experiencing greater thermal stress when removed from their dam and littermates.

Classically conditioned bradycardia and skeletal-motor activity in restrained rats

Abstract: Classical conditioning of heart rate (HR) and skeletal-motor activity in restrained rats was ex­ amined using a single, brief (5-msec) pulse of shock to the rat's tail as both the CS and US Elec­ tric shock was selected as a CS to discourage behavioral freezing and to provide an above-zero level of movement activity for evaluating the presence of conditioning Three CS intensities and two US intensities were combined in a factorial design creating six separate experimental groups which received 50 paired trace conditioning trials at a CS-US interval of 81 sec Three control groups were given 50 unpaired presentations of one of the CSs and the highest intensity US All of the experimental groups showed the development of a bradycardia CR in the absence of a conditioned change in movement During conditioning, HR and movement to CS onset varied together in both the experimental and control groups Other examples of a close correspondence between HR and movement included some, but not all, of the preconditioning CS-alone responses, the URs, and shifts in baselines The results suggested that although some nonassociatively based HR changes may have been tied to movement, the conditioned bradycardias in the experimental groups appeared to be relatively independent of movement Studies showing that changes in heart rate (HR) may covary directly with increases or decreases in skeletal­ motor activity (Obrist et al, 1974) have led to the proposal (Obrist, 1981) that HR responses in a variety of behavioral situations may be tightly tied to movement through well-known (Rushmer, 1976) cardiac­ somatomotor pathways within the central nervous system Many of the studies showing a close correspondence be­ tween HR and movement have involved instrumental paradigms in which HR was measured while the subject received contingent reinforcement for changes in overt responding The complexity of these paradigms and the absence of control procedures to evaluate the possible in­ fluences of nonassociative factors, such as sensitization and pseudoconditioning, have made it difficult to deter­ mine whether the HR changes accompanying movement were learned or nonspecific in nature That the relationship between learned HR responses and gross body movement may not be a simple one is sug­ gested by the outcomes of aversive classical condition­ ing studies A number of these studies have shown that although HR CRs (tachycardias as well as bradycardias) may at times covary with and be augmented in the presence of directionally similar changes in overt move­ ment, the HR CRs are also seen in the absence of any

Neural and endocrine effects on classical conditioning: A comparison of ACTH and hippocampectomy

Abstract: The effects of bilateral hippocampal lesions and systemic elevation of ACTH were compared using the classically conditioned rabbit nictitating membrane response Animals with hippocampal lesions and unoperated animals that received ACTH (5 IU/kg) exhibited a facilitated rate of acquisition and decreased conditioned response onset latency relative to normal control groups or animals with cortical lesions The administration of ACTH did not alter performance in animals with hippocampal damage, indicating that the effects of these treatments are not additive The performance of a second group of animals that was treated with a smaller dosage of ACTH (25 IU/kg) was no different from that of any other group, suggesting a dose-dependent effect of the hormone The selective replication of hippocampal deficits ascribed to a loss of response modulation suggests that ACTH may influence learning by stimulating hippocampal neurons

Effect of posterior cortical lesion on two versions of the brightness-discrimination task

Abstract: The brightness-discrimination performance of normal and posterior decorticate rats was measured in two test situations designed to provide different spatial task characteristics. A Y-maze and an open field with otherwise matched stimulus properties provided tests with many or few spatial characteristics, respectively. In contrast to predictions from visuospatial integrative interpretations of lesion consequences, the present results indicated no attenuation of the typical operative impairment when the reduced spatial-cue tasks were employed. Furthermore, this outcome was common to both simple acquisition/retention (Experiment 1) and successive discrimination reversal (Experiment 2) versions of the tasks. Interpretations of the relative contribution of memory or visuospatial integrative deficits to the measured behavioral consequences are discussed.

Relation between the severity of early malnutrition and the effects of later frontal cortical lesions in rats

Abstract: Rats with an early history of severe malnutrition received frontal cortical (FC) lesions or control operations at 90 days of age. These severely malnourished groups were compared with each other and with moderately malnourished and well-fed groups (FC and control) for acquisition and three reversals of a tactile discrimination (rough-smooth). Main effects of lesion and nutrition were found for acquisition and reversals. The groups with malnutrition plus frontal lesions displayed the poorest performance. Although the severely and moderately malnourished groups did not differ in acquisition of the discrimination, the severely malnourished group made more reversal errors. The severely malnourished rats also showed greater brain growth retardation (size and weight) than the moderately malnourished rats. These results indicate that the effects of early dietary history and the reaction to a later, acute brain injury may be additive under certain circumstances.

Heart rate reactivity, habituation, and associative learning in developmentally disabled preschool children

Abstract: The relationship between adaptive behavior and cardiac indices of orienting, habituation, and associative learning were evaluated in a heterogeneous population of developmentally disabled infants and young children. Cardiac reactivity and habituation were examined through responses to simple nonsignal stimuli, and associative learning was evaluated by the cardiac response to stimulus omission in a conditioned expectancy paradigm. Results revealed a significant negative correlation between the magnitude of the deceleratory orienting response and adaptive competence, as measured by the Adaptive Behavior Scale for Infants and Early Childhood. This correlation was independent of both age and baseline heart rate. In contrast, habituation and simple associative learning were not related to the level of adaptive behavior. These findings suggest that cardiac reactivity may provide a sensitive measure for the evaluation of functional capacities of impaired populations.

Preexposure to contextual stimuli: Effects on startle responding in humans

Abstract: Previous research with rats has shown that preexposure to the contextual stimuli of the experimental situation results in facilitated habituation of the startle reflex. These findings support the predictions of nonrepresentational theories of habituation (e.g., Groves & Thompson, 1970), and oppose the predictions of representational theories (e.g., Wagner, 1976). The current experiment was designed to test whether the same results could be obtained with humans. Twentyfour hours prior to the habituation test, one group of subjects (E-P) was exposed to the contextual stimuli of the experimental situation, a second group (C-E) was exposed to different contextual stimuli, and a third group (C-N) received no pretreatment. The habituation test consisted of two sessions of 15 startle-eliciting noise bursts. The P-E group responded less on the first trial of Session 2. This finding is suggestive of greater long-term response decrements in this group, and thus supports the nonrepresentational theories of habituation.

An equipotential function of the cerebral cortex

Abstract: Rats were prepared with serial injuries to the neocortex. Their postoperative performance of a learned task, when measured after each procedure, confirmed two predictions from a quantitative theory that the neocortex is equipotentially involved in remembering of memories.

Theoretical Note Parsimony in neural representations: Generalization of a model of spatial orientation ability

Abstract: Brains must be adequate to process complex information, but they must also have a simple enough underlying organization to have evolved by natural selection. Beginning with a network model, originally developed to show how spatial orientational behavior might be organized with a modest number of neural connections, the following hypothesis is offered about a pattern of connections recurring within the brain: All significant activation patterns of a large number of input elements are transformed to output patterns by small numbers of associative subsystems, or modules, which do not require computerlike algorithms. They are single neurons or other functional units, which individually merely summate inputs; together, they discriminate among probable inputs without requiring a complex representation to do so. They may be thought of as registering (1) values of input dimensions, (2) combinations of activated input elements, or (3) numerical labels for distinct inputs. The informational capacity of a set of modules is a function of its number of modules and their dynamic range. The set works most efficiently if all its modules have the same range. Elementary combinatorial considerations suggest that besides receiving patterns of connections appropriate to the information being processed, sets of associative modules probably receive inputs systematically restricted in more general ways, for example by lateral inhibition or by connections from a small or large, but not intermediate, proportion of the set of input modules.

Long-term modulation of learning following locus coeruleus stimulation: Behavioral and neurochemical data

Abstract: This paper summarizes some behavioral and neurochemical data relating to the delayed effect of locus coeruleus stimulation on the learning capabilities of the rat. The first observation showed that electrical stimulation of the locus coeruleus of a 15-day-old rat improved the acquisition and extinction of a food-reinforced operant task performed 4 weeks later. The neurochemical lesion of the dorsal noradrenergic bundle, 10 days before the stimulation, did not decrease the behavioral effect, whereas the neurochemical lesion of the locus coeruleus proper suppressed the improvement. Furthermore, we studied the long-term effect of the stimulation on some biochemical parameters of the coerulean system: First, the turnover of noradrenaline increased in the locus coeruleus 4 days after the stimulation, whereas it decreased 4 weeks after the treatment in the nucleus proper and in the hippocampus. Second, 4 weeks after the stimulation, the characteristics of some adrenoceptor populations were significantly changed, in particular regions of the brain. The number of α2-adrenoceptors was increased in the cortex, the hippocampus, and particularly the hypothalamus. The number of α1-adrenoceptors was increased in the cortex. Neither the number nor the affinity of β receptors was altered by the stimulation. Likewise, the number of α1-, α2-, and β-adrenoceptors in the brainstem was not changed. These results are discussed. To explain our behavioral data, we assume that the performance improvement in prestimulated rats is due, for the most part, to a reduced reaction to stressful situations. On this basis, the stress-reduction hypothesis is tentatively related to the binding data showing that the most significant increase of the α2-receptors was observed in the hypothalamus.