Showing papers in "Journal of Neurobiology in 1974"

Activity patterns of catecholamine-containing pontine neurons in the dorso-lateral tegmentum of unrestrained cats

Abstract: Single catecholamine-containing neurons in the dorso-lateral pontine tegmentum were recorded in unanesthetized, freely behaving cats. Microscopic lesions were made on each recorded unit and subsequently identified for catecholamines by fluorescence histochemical analysis. The majority of the NE-containing neurons exhibited low spontaneous discharge rates in quiet wakefulness and in slow wave sleep. In paradoxical sleep, the medially located NE-containing neurons tend to exhibit ursting discharges which are closely related to individual PGO waves. The laterally or dorsally situated NE units fire more slowly and tend to derease their firing activity during paradoxical sleep. Tonically active, relatively fast NE-containing units were also found. Often, those neurons showed greatly increased activity before the cat awoke from either slow wave sleep or paradoxical sleep, with activity persisting during the early moments of waking. The combination of histochemical and electrophysiologic methods proved essential for correlating neurochemistry and function within the heterogeneous NE-containing nuclei of the cat pons.

Physiological and pharmacological properties of Purkinje cells in rat cerebellum degranulated by postnatal x-irradiation.

Abstract: Elimination of most granule, basket, and stellate interneurons in the rat cerebellum was achieved by repeated doses of low level x-irradiation applied during the first two weeks of postnatal life. Purkinje neurons in these rats, studied when adults, exhibited sustained spiking activity in Halothane anesthetized preparations. Mean firing rates were 35--40/sec, no different from normal. Spontaneous bursts presumed to be generated by climbing fiber synaptic activity differed from normal by often consisting of full sized spikes rather than characteristic inactivation responses. Intracellularly observed correlates of bursts consisted of epsp's of several discretely different amplitudes appearing independently in time. Stimulation of white matter revealed evidence for, (a) graded synaptic excitation of Purkinje cells indicating more than one converging excitatory synapse, and (b) inhibitory actions on Purkinje cells either through a few remaining inhibitory interneurons or through Purkinje cell recurrent collaterals. Iontophoretic drug application studies showed normal chemosensitivity of the Purkinje cell membrane, i.e., excitation by glutamate and inhibition by gamma-amino butyric acid, serotonin, norepinephrine, and 3′5′ cyclic AMP. These studies indicate considerable autonomy of Purkinje cell ontogenesis in the absence of normal interneuronal input. A unique synaptic relation only rarely found in normal cerebellum is the innervation of single Purkinje cells by more than one climbing fiber.

The protein subunit of calf brain neurofilament.

Abstract: A preparation of the protein subunits from calf neurofilaments has been obtained. Axon segments from an homogenate of calf brain white matter have been concentrated by a subcellular fractionation procedure. After further steps to achieve a complete elimination of unbound lipid, the neurofilament protein was dissolved in 4M guanidine hydrochloride solutions. The neurofilalment subunit was the major constituent of the guanidine hydrochloride extracts; other proteins of higher molecular weight were eliminated by agarose gel filtration in urea solutions. Dialysis to solutions of low ionic strength allowed the reformation of fibril aggregates, some of which, under electron microscopy, resembled the original neurofilaments in morphology. The physical characteristics of the neurofilament subunit protein are reported and a preliminary amino acid analysis is given. The neurofilament protein is clearly distinguishable from actin and tubulin on physical and chemical grounds.

The morphology of cricket giant interneurons.

Abstract: Axonal iontophoresis of cobalt chloride was used to describe the morphology of the seven largest interneurons in the abdominal nervous system of the cricket Acheta domesticus. The somata of these neurons are all found in the terminal (5th) abdominal ganglion. This ganglion is derived from four primitive segmental ganglia (the 7th–10th primitive segments) and the morphology of the interneurons within this ganglion suggests that the neurons are derived from different primitive segments. Furthermore, some of the neurons appear to be serially homologous elements. Three aspects of the morphology support these suggestions: (1) soma position, (2) relative positions of commissural processes, and (3) the general shape and extent of dendritric fields.

Biochemical and physiological properties of a purified snake venom neurotoxin which acts presynaptically.

Abstract: A highly toxic protein has been isolated from the venom of the snake Bungarus multicinctus. No major contaminants were detected on isoelectric focusing, gel filtration, or gel electrophoresis. The molecular weight calculated from its amino acid composition (21,800) agreed well with that calculated from gel filtration and equilibrium sedimentation data. Gel electrophoresis showed that the toxin consisted of two subunits of molecular weight 8,800 and 12,400 held together by disulfide bonds. Both the structure, determined by circular dichroism, and the biological activity, determined by toxicity assay, were remarkably resistant to heating. In the rat phrenic nerve-diaphragm preparation, the toxin caused a failure of neuromuscular transmission, after which subthreshold endplate potentials could be detected. The frequency of spontaneous miniature potentials was altered only to a very slight extent. The toxin appeared to have no postsynaptic activity. Similarities between the properties of this neurotoxin and that described by Lee and Chang (1966) suggest that it is identical to β-bungarotoxin.

Histology and histochemistry of the peripheral neural plexus in the Aplysia gill.

Abstract: The neural plexus in the Aplysia gill is comprised of monopolar, bipolar, and multipolar neurons Histochemical procedures reveal acetycholinesterase (AChE), serotonin (5HT), and possibly dopamine present in plexus neurons The three cell morphologies in the plexus are found in specific regions of the gill Monopolar cells singly and in clusters lie along branches of the branchial and ctenidial nerves The largest cluster, the gill ganglion, is located on the branchial nerve; ganglion neurons show a marked similarity to neurons in the central nervous system AChE is present in monopolar neurons; its presence was demonstrated using acetylthiocholine as a substrate with inhibitors specific for esterases other than AChE Monoamines are also contained in monopolar cells; yellow and green fluorescence, resulting from exposure to paraformaldehyde vapor, indicated that 5HT and catecholamines, respectively, are present in the cells Bipolar neurons, containing AChE, are located beneath the ciliated epithelium in the pinnule Multipolar, monopolar, and bipolar cells are found in the gill musculature Single neurons appear to innervate at least two muscle bundles Evidence is also presented suggesting that cells containing different neurotransmitters can terminate in the same muscle Acetylcholine, 5HT, and dopamine, when infused in the gill, elicited discernable movements for each agent The plexus, with its apparent hierarchy of neurons, has a number of properties ascribed to the central nervous system The hierarchy is discussed in relation to the physiology of plexus neurons and to the behavior manifested by the gill

The properties and connections of supernumerary sensory and motor nerve cells in the central nervous system of an abnormal leech.

Abstract: The characteristic features of individual neurons in leech ganglia are highly consistent from animal to animal. Hy chance a leech was dis- covered that had more cells to be seen than usual in many of its ganglia. In some segmental ganglia of the abnormal animal there were four or even five sensory cells responding independently to touch (T cells) instead of the usual complement of three on each side. Ganglia also contained three or four pressure sensitive cells (P cells) and three or four nociceptive cells (N cells) instead of two of each. The membrane properties, as well as the shapes, sizes, and positions, of all these cells were normal. Their axons reached the skin by the appropriate nerve bundles. Two sensory neurons of one modality, often independently innervated, overlay regions of skin that normally would be supplied by one cell. On the other hand, the areas innervated by some supernumerary cells were abnormal in their shape and position. In some ganglia instead of one motor neuron that raised the skin into bumps (the annulus erector, or AE cell) there were two, each of which could independently erect annuli. The regions innervated by the extra AE cells appeared normal and overlapped extensively. Normal synaptic interactions were found between supernumerary sensory cells and motor cells. The results with this animal indicate that two or more nerve cells can form effective connections with other neurons and with skin that normally would be innervated by a single cell.

The in vitro polymerization of tubulin from beef brain.

Abstract: The conditions for the in vitro polymerization of beef brain tubulin have been examined using turbidity and electron microscopic examination as methods of following the reaction. The optimal conditions are 37°, pH 6.0--6.4, and an ionic strength of approximately 0.08 to 0.1 μ. The ionic strength is particularly critical, at 0.25 μ the polymerization does not occur and increasing the ionic strength of a solution contaning microtubules to this value causes the tubules to depolymerize. GTP and Mg2+ are required. After an initial lag period the rate of polymerization appears to follow first-order kinetics. Although colchicine inhibits the polymerization reaction it does not cause the tubules formed in vitro to depolymerize. Binding studies show that colchicine binds to free tubulin better than to tubules. Calcium also inhibits polymerization and causes the depolymerization of tubules.

Levels of glutamate decarboxylase, choline acetyltransferase and acetylcholinesterase in identified motorneurons of the locust.

Abstract: Isolated somata of identified motorneurons in the metathoracic ganglion of the locust were analyzed for glutamate decarboxylase (GAD), choline acetyltransferase (ChAc), acetylcholinesterase (AChE), and aromatic amino acid decarboxylase (AAD). Isolated inhibitory motorneuron somata have GAD activity but those of excitatory motorneurons do not. Since this enzyme is responsible for the synthesis of gamma amino butyric acid there is a metabolic basis for these motorneurons to accumulate GABA. In contrast to the specific localization of GAD in inhibitory motorneurons, AChE was found in large amounts in all motorneurons tested. The specific activity of ChAc and AAD in both inhibitory and excitatory motorneurons were similar or lower than the specific activity of the whole ganglion. Degeneration experiments suggest that ChAc is mainly localized in the sensory neurons.

Specific protein metabolism in identifiable neurons of Aplysia californica

Abstract: The patterns of protein metabolism of identified cholinergic (R2, L10, and L11) and neurosecretory (R3–13, R14, and Bag cells) cells were analyzed with the use of SDS-polyacrylamide gel electrophoresis and isoelectric focusing. L11 was unique amongst the three cholinergic neurons in its relatively high rate of incorporation of labeled leucine into proteins at 12,000 daltons. All the neurosecretory cells incorporated relatively large quantities of 3H-leucine into small proteins (<12,000 daltons), which were concentrated in putative neurosecretory granule fractions of these cells. R3–13 and R14 synthesized a distinct basic protein which appeared specifically in the branchial nerve which contains the axons of these cells. The combination of these two analytical electrophoretic techniques allowed for the characterization of protein specificity in single nerve cells.

Cyclic AMP and amine effects on phosphorylation of specific protein in abdominal ganglion of Aplysia californica; localization and kinetic analysis

Abstract: Incubation of abdominal ganglia, obtained from Aplysia californica weighing 120--260 g, with octopamine or serotonin led to marked increases in cAMP (cyclic adenosine 3′,5′-monophosphate) levels after 10 min of incubation; these increases were blocked by phentolamine and methysergide, respectively. Incubation for 21 hr with octopamine also produced an elevation in cAMP levels but prolonged incubation with serotonin did not. Both these amines increased incorporation of 32P or 33P into a specific protein with apparent molecular weight of 118,000, upon incubation with the ganglion for 21 hr. The selective effect on this phosphoprotein was previously shown to be blocked by phentolamine (for octopamine effect) or methysergide (for serotonin effect) and could be mimicked by addition of dibutyryl cAMP to the medium. Incubation for more than 15 hr was required to generate a stable phosphoprotein pattern in which 5 phosphoprotein peaks could be reliably identified on polyacrylamide gels. With shorter incubations there was marked variability between ganglia with respect to the proteins which incorporated radioactive phosphate. The dibutyryl cAMP effect on phosphorylation of specific protein could not be demonstrated with incubations shorter than 15 hr. The specific effect on the phosphoprotein with molecular weight 118,000 was not observed in a number of large identified cells removed from the ganglion after 21 hr of incubation with radioactive phosphate and dibutyryl cAMP. The effect was also not observed in connective nerves and the bag cell clusters, but was consistently present in that portion of the ganglion remaining after these dissections. Upon subcellular fractionation the phenomenon was not observed in a crude nuclear fraction but was consistently present in a crude mitochondrial fraction. All these results are consistent with the possibility that the change in phosphoprotein metabolism produced by dibutryl cAMP may be localized in the neuropil, which contains all the synaptic contacts in this ganglion.

Cerebral energy reserves and glycolysis in neural tissue of 6-aminonicotinamide-treated mice.

Abstract: Cerebral energy metabolism was studied in brains of mice treated with the nicotinamide analog, 6-aminonicotinamide (6-AN), which is converted to a potent inhibitor of 6-phosphogluconate dehydrogenase in viro. Six hours after administration of 6-AN, the level of cerebral 6-phosphogluconate was increased at least 170-fold, whereas substrates for other NADP +-dependent dehydrogenases either did not change or increased only slightly. Large increases in 6-phosphogluconate were observed in the cerebral cortex, three layers of the cerebellum, and the dorsal column and anterior horn regions of the spinal cord after administration of 6-AN. Thus, glucose was metabolized via the oxidative enrymes of the pentose phosphate pathway in a wide variety of neuronal structures in vivo. Highest concentrations of 6-phosphogluconate were observed in the cerebellar molecular layer. Energy use rates and glycolysis were studied by measuring the disappearance of the major energy reserves during periods of anoxic-ischemia imposed on cerebral tissue of 6-AN-treated mice. Production of lactate and utilization of glycogen during anoxic-ischemia were reduced in brain of 6-AN-treated mice, which is in accord with the suggestion that glycolysis is inhibited in cerebral tissue of 6-AN-treated animals. Levels and ratios of pyridine nucleotides in cerebral tissue were not altered by 6-AN treatment. From changes in levels of ATP, phosphocreatine, glucose, and glycogen in brain during ischemia, the rate of energy use was calculated to be 21.7 mmol · kg wet tissue−1 · min−1, which was not significantly different from that observed in normal adult mouse brain. Therefore, despite an apparent slowing of hexose utilization in cerebral tissue 6-AN-treated mice, normal cerebral energy use was maintained during the initial period of anoxic-ischemia.

Comparative effects of nerve growth factor and ganglionic nonneuronal cells on purified mouse ganglionic neurons in culture

Abstract: Purified neuronal fractions from dissociated mouse dorsal root ganglia were cultured in the presence of variable amounts of homologous ganglionic nonneuronal cells, and with or without addition of variable amounts of exogenous Nerve Growth Factor in the culture medium. The ability of the nonneurons to support long-term neuronal survival in the absence of exogenous Nerve Growth Factor increased linearly with their numbers and maximal neuronal performance was achieved with a 4:1 ratio of non-neurons to neurons; however, neuronal attachment was elicited maximally as soon as that ratio reached or slightly exceeded a value of one. Besides this Nerve Growth Factor-like function, the nonneurons were also involved in determining whether and to what extent the factor was capable of supporting neuronal performance. In the absence of non-neurons, even high concentrations of the factor failed to elicit neuronal attachment. As more nonneuronal cells were made available, Nerve Growth Factor progressively raised neuronal performance and did so at sharply declining requirement levels: with 12,000 nonneurons/dish maximal performance required only 10−2 biological unit/ml of the factor. Further increases of the nonneurons reduced the scope of a neuronal response to Nerve Growth Factor between the maximal performance of the culture system and the increasing performance elicited without the factor. The results indicate that, given a minimal level of nonneurons, the performance in culture of these ganglionic neurons could be promoted in very similar patterns by either increasing the exogenous Nerve Growth Factor in the medium or increasing the ganglionic nonneurons on the culture floors, with an approximate and fixed ratio of 10−7 biological unit per nonneuron. Possibilities are discussed of a common mechanism underlying the supportive properties of the two agents.

Histofluorescence study of chromaffin cells in dissociated cell cultures of chick embryo sympathetic ganglia

Abstract: Chromaffin cells were studied by fluorescence microscopy in long-term cell cultures of dissociated chick embryo sympathetic ganglia. These cells were distinguishable in culture from the sympathetic ganglionic neurons by their small size, relative infrequency of occurrence, intense fluorescence and survival in the absence of nerve growth factor (NGF). By several weeks in culture, with or without NGF in the medium, the chromaffin cells produced extensive ramifications of fluorescent varicose processes. It is suggested that at least part of the intraganglionic system of adrenergic varicose terminals arises from chromaffin cells. This capability with regard to fiber production lends credence to the suggestion that chromaffin cells may play an active role in ganglionic function.

Cephalic mechanism for social control of fissioning in planarians. II. Localization and identification of the receptors by electron micrographic and ablation studies.

Abstract: Asexual reproduction (fissioning) in the planarian Dugesia dorotocephala is socially controlled through a cephalic mechanism; isolation releases fissioning, grouping inhibits it; decapitation releases it even in grouped subjects. Experiments were performed to confirm previous indications that direct contact is involved in the sensing of cohorts; the partial release of fissioning in grouped planarians by introduction of a chemically inert lubricant (0.1% purified agar) into the habitat water further confirms the necessity of intimate contact between the sensors and bodily surfaces of the sensed cohorts. Further experiments, correlating electron microscopic observations with the release of fissioning by various selected surgical ablations, reveal the pertinent sensors to be clumps of neural cilia concentrated in the cephalic margins. Comparable structures occur in the chemoreceptors of catfish, lamprey, and honey bee as well as in the olfactory epithelium of mammals.

Optimum topographical conditions for reverberating cortical spreading depression in rats.

Abstract: Reverberation of cortical spreading depression (RCSD) around circular cortical lesions was studied in anesthetized rats. Reverberation was started by KCl injection into an area recovering from a first CSD wave in a narrow region between the lesion and the sagittal sulcus. Frontal lesions 3.3 mm in diameter yielded less reliable reverberation than lesions 5.2 mm in diameter. With continuing reverberation the spreading rate of CSD gradually decreased. Reverberation stopped when the CSD relocity dropped below 2.2 mm/min. With large lesions (5.2 mm in diameter) RCSD lasted longer with frontal (12.2 ± 2.0 cycles) than with parietal (4.0 ± 0.4) and occipital (4.4 ± 0.9) obstacles. Application of 10± KCl on the cortical area between the lesion and the sagittal sulcus could also elicit RCSD because the CSD waves spreading in the rostral and caudal directions were not generated simultaneously and some of them went round the obstacle without being intercepted by the opposite wave. The experimental data agree with the mathematical model of excitation spreading in simply connected sheets of excitable tissue and indicate regional differences in CSD susceptibility in the rat.

Distribution of putrescine, spermidine, and spermine in rhesus monkey brain: decrease in spermidine and spermine concentrations in motor cortex after electrical stimulation.

Abstract: Polyamine levels vary markedly throughout the brain of the rhesus monkey. In general, there is an inverse relationship between spermine and spermidine concentrations; i.e., areas with high spermine levels have low spermidine levels and vice versa. The ratio of spermidine to spermine is as high as 13 in the optic nerve, and as low as 1.2 in the vermis. The variability of this ratio suggests that the functions of the polyamines in the adult brain may be different from those in other tissues. For instance, the ratio of spermidine to spermine in adult rat liver and most other organs is approximately 1, and this ratio increases in growth-stimulated systems. The precursor of spermidine, diaminobutane, is present in very low quantities in the brain. Electrical stimulation of the precentral gyrus was found to decrease the content of both spermidine and spermine, and these data provide further evidence for a separate brain function of the polyamines. Depletion of spermidine and spermine in the motor cortex by electrical stimulation suggests that these amines may be involved in transmission phenomena.

Motor and sensory mechanisms of feeding in Pleurobranchaea.

Abstract: Nerves of the cerebropleural ganglion play a major role in the sensory and motor aspects of feeding in the marine mollusk, Pleurobranchaea. Investigations of these nerves were carried out using whole-body preparations capable of essentially normal feeding behavior. Chemosensory nerves which function in food arousal are the rhinophore, tentacle (large branch) and large oral veil nerves, and a nerve of the pedal ganglion (P3). Recordings from these nerves are consistent with previous behavioral findings concerned with the location of receptor regions. The feeding cycle of proboscis extension, thrusting, and withdrawal, is controlled primarilly by cerebral nerves. The mouth nerve and small oral veil nerve supply muscles which control the oral tube. When these muscles contract, the oral tube and mouth are pulled open, and the buccal mass, which forms the core of the proboscis, advances part way. At his time, the body wall musculature, innervated by the body wall nerve and a pedal nerve (P4), contracts behind the buccal mass, forcing it out of the open mouth. Proboscis thrusting is accomplished by rhythmic bursts of the mouth and small oral veil nerves. Withdrawal of the buccal mass is accomplished essentially by the reverse process. This conception of the feeding cycle was developed by considering the muscular anatomy and by behavior simulation. It was confirmed by recording motor nerve output and muscle activity during food stimulation.

Regeneration of the antennules in the Australian freshwater crayfish, Cherax destructor

Abstract: Removal of one eye from the Australian freshwater crayfish, Cherax destructor, results in the growth of a heteromorph appendage which re- sembles the outer flagellum of the antennule. The heteromorph is con- sistently uniramous. Transplanting part of the animal's own antennule into the eye stalk after removal of the ipsilateral eye results in a significant number of biramous appendages being developed in the eye socket. Biramous appendages when they possess fully differentiated hairs, can be identified as being the external and internal flagella of the antennule. Triramous appendages were never observed. The specificity of the antennular sensilla was tested in relation to the cleaning behavior released by stimulating them electrically. Sensilla on the external flagellum of the antennule produce depression of the antennule and cleaning using the maxillipeds. This behavior is also produced when the heteromorph appendage is stimulated although in this case it is misdirected. Stimulation of the hairs in the eye socket releases cleaning of the eye socket with the 2nd and 3rd walking legs. Eye socket cleaning is not released by stimulation of the heteromorph indicating that the antennular sensilla retain their specificity in relation to the cleaning behavior released. Removal of the antennule results in the ipsilateral olfactory lobe becoming smaller than the contralateral lobe. No change was observed in the accessory lobes.

Control of habituation of the withdrawal reflex by the gill ganglion in Aplysia

Abstract: The regions of the gill neural plexus responsible for habituation of the withdrawal reflex were studied using surgical and electrophysiological techniques. Waterdrops were applied to the gill pinnule, and testing for habituation was carried out in each preparation before and after ganglionectomy of the large ganglion in the gill and pinnule isolation. Three parameters of habituation, response decrement, spontaneous recovery, and dishabituation, were used to evaluate the function of the ganglion and the plexus in the pinnule in habituation of the reflex. Disruption of response decrement, of spontaneous recovery, and of dishabituation were caused by either isolating the stimulated pinnule from the gill ganglion by severing the branchial nerve, or by extirpating the gill ganglion. We conclude that: (a) the gill ganglion is necessary for the withdrawal reflex to exhibit the three parameters which characterize hibituation; and (b) the plexus in the pinnule mediates the withdrawal reflex. Extracellular recordings from the pinnule showed that sensory adaptation cannot be contributing to habituation of the reflex since the apparent sensory activity evoked by waterdrops in the pinnule was unaltered by repeated stimulation. In gill ganglion neurons, biphasic potentials were evoked by waterdrops and consisted of an initial depolarization followed by a longer lasting hyperpolarization whose duration was increased with repeated stimulation. In three spontaneously active neurons the rate of activity decreased with repeated stimulation. It is felt that the decreased rate is brought about by potentiation of the hyperpolarization. The rate of spike activity is considered to control the reflex amplitude since the increased rate with intracellular stimulation, in one neuron, resulted in an increased amplitude and since the absence of spontaneous discharge, after ganglionectomy, resulted in a reduced amplitude. A neural network mediating habituation of the withdrawal reflex is proposed.

Habituation of the motion detectors of the crayfish optic nerve: their relationship to the visually evoked defense reflex.

Abstract: Examination of sustaining units, dimming units, and motion detectors under stimulus conditions associated with defense reflex habituation revealed that only the motion detectors exhibit a response decrement with stimulus repetition. Motion detector habituation was specific to the retinal locus of the repetitive stimulus and thus can provide a basis for the spatially specific component of defense reflex habituation. When stimulated at low repetition rates (2--4 min) the motion detectors exhibited a time course of habituation and spontaneous recovery similar to that observed behaviorally. At high repetition rates (20/mm) the motion detectors exhibited a more rapid rate of habituation than was observed for the defense reflex. Simultaneous recordings of motion detector and cheliped levatory muscle activity revealed consistently significant and frequently substantial positive correlations. Both defense reflex and motion detector habituation were transiently reversed by treatment with the GABA antagonist, picrotoxin (1.8 2.8 mg kg body weight). The action of picrotoxin is likely to be mediated via mechanisms controlling the general level of arousal. It was concluded that the response variations of motion detectors subjected to repetitive stimuli contribute to the habituation of the visually evoked defense refle.

Effects of fluorodeoxyuridine on growth and choline acetyltransferase activity in fetal rat brain cells in surface culture

Abstract: Inhibition of DNA synthesis by 5-fluoro-2-deoxyuridine in cultures of rapidly growing fetal rat brain cells resulted in a marked decrease in the development of total activity and a stimulation of specific activity (pmoles\min\μg DNA) of the enzyme choline acetyltransferase, a putative neuronal marker. Similar treatment of higher density, less rapidly dividing cultures of the same age resulted in no change in total activity development, although an increment in specific activity was again found. The findings appeared to be the result of inhibition of DNA synthesis by fluorodeoxyuridine, rather than secondary effects of the drug, and indicate either that the nucleotide analog specifically inhibited choline acetyltransferase gene expression in rapidly growing, but not in stationary cultures, or that cells capable of producing the enzyme activity were able to proliferate in culture. If the latter were true and if cells producing the enzyme were, in fact, neurons, it may be possible to clone untransformed neurons for in-depth study in a culture system.

Feedback cue and switching characteristics

Abstract: Asexual reproduction (fissioning) in planarians is controlled by group size; increased group size suppresses fissioning, isolation releases it. This effect is mediated through the brain; social stimulation from cohorts causes the brain to inhibit the segmental plexus fissioning system from initiating the events of fissioning. Experiments were conducted to elucidate (1) the stimulus cue conveying information regarding cohort number and (2) the switching characteristics of the control system. The results of these show that the social stimulus responsible for the effect involves neither vision nor discharge of pheromonal substances into the habitat; the sensory event apparently involves direct contact and chemoreceptive, as well as tactile, modalities. The switching characteristics (graduated response, long release time, short reinstatement time) found for the brain inhibitory system agree with those expected from the mathematical model derived for a simple neurohormonal control system in which it is postulated that social stimulation causes neurosecretory cells of the brain to release a fissioning inhibitory hormone (FIH) that inhibits the segmental plexus fissioning (SPF) system from initiating fissioning.

The visually evoked defense reflex of the crayfish: Habituation, facilitation, and the influence of picrotoxin

Abstract: When stimulated with rapidly approaching targets, the crayfish defense reflex exhibited habituation, spontaneous recovery, dishabituation, and stimulus generalization. The rate of habituation increased with stimulus repetition rate over a narrow range of stimulus rates. At the highest repetition rate examined, 20 min, the rate of habituation diminished. After 150 trials at 20 min, responsiveness to subsequent stimuli at lower repetition rates was enhanced. Dishabituation was elicited with mechanical prods to the dorsal thoracic carapace or a 1-min session of 20\min stimulation. After completion of habituation a 5° shift in the axis of a 10° target resulted in substantial response recovery. Evidence for weak interocular generalization of habituation was also obtained. These results suggest both retinal-dependent and retinal-independent sites of habituation. Picrotoxin (2.4 mg\kg body weight) reversed a previously established response habituation and delayed the onset of rehabituation to repetitive stimuli. The effects of picrotoxin suggest the participation an inhibitory mechanism in defense reflex habituation.

Incorporation of (14-C) ethanolamine and (3-H) methionine into phospholipids of rat brain and liver in vivo and in vitro.

Abstract: Comparative studies were undertaken on the in vivo and in vitro incorporation of [14C] ethanolamine, [3H] methionine and [14C] S-adenosyl-methionine into phosphatidylethanolamine (PhE) and phosphatidylcholine (PhC) of rat liver and brain. It was observed that brain can synthesize de novo PhC from PhE via the transmethylation pathway, however synthesis rates were (1) markedly lower than those of liver and (2) decreased significantly with age. In the choline-containing lipids more than 95% of the radioactivity was found in PhC. Studies on the localization of the radioactivity in PhC following the intracranial injection of [3H] methionine or [14C] ethanolamine revealed that both precursors are incorporated almost exclusively into the choline moiety of this phospholipid. There was significant labeling of PhC only when the precursors were administered intracranially and much less incorporation was observed with the systemic routes. Thus following the intravenous administration of [14C] ethanolamine, the specific radioactivities of liver PhE and PhC were up to 75 times as high as those of brain and 4 to 5 times as high in the organs of the 20-day old as those of the adult. In contrast, when this precursor was administered intracranially the specific radioactivities of both phospholipids in liver were only twice as high as those of brain. Although the short-and long-term time-course studies on the in vivo incorporation of [14C] ethanolamine and [3H] methionine into PhC of both organs could suggest a precursor-product relationship between the biosynthesis of this phospholipid in liver and brain, this apparent relationship could also be due to the high turnover of PhE in liver, with half-life of 2.87 hr, and its low turnover in brain, with half-life of 10.7 days. The present findings on the low rate of formation of PhC from PhE in brain coupled with the fact that this conversion declines sharply with age, especially when the isotopes are administered systemically, could explain the observation of previous investigators that the brain cannot synthesize its own choline and thus it must derive its choline from exogenous sources such as lipid-choline. It was concluded that the brain can synthesize its own choline; however it remains also dependent on liver and dietary choline which are probably transported into the brain as free choline.

Memory evoked optomotor responses in crustaceans.

Abstract: Optomotor memory in crustaceans was studied by recording the activity of optomotor neurons. Animals were placed in a stationary striped drum, which was subsequently turned in the dark to a new position. Upon reillumination the eye movements of the animals follow the apparent movement of the stripes. It can be inferred that the position of the stripes after movement is compared with the remembered position of the stripes before movement. The size and sign of the memory response of the optomotor neurons are functions of the apparent movement. Changes as small as 0.1° can be detected. Memory starts forming almost immediately upon exposure of a position and develops as a function of viewing time. While a new memory is forming, the old one, though declining, is still present for 30 sec. In the dark a memory declines gradually, lasting for more than 8 min. In certain circumstances the set point of memory is adjusted for the effects of eye movements. The possibilities that memory involves a peripheral afterimage or stabilization of a particular part of the eye upon a particular feature of the surroundings are considered.

Changes in conduction velocity within a nerve net.

Abstract: Conduction velocity within the through-conducting nerve net of the sea anemone Calamactis praelongus is altered as a result of the passage of one or more impulses. Facilitation of conduction velocity occurs when the second evoked impulse follows the first by less than 2 sec. The facilitating effect increases with decreasing interval between shocks until impulses are less than 0.5 sec apart and then it declines. Antifacilitation occurs when the nerve net is shocked at frequencies between one per 2 sec and one per 20 sec. It is seen also after the first two or three responses when shocks are applied at higher frequencies. Facilitation and antifacilitation increase with spread of excitation. Although the relationship of velocity changes to the behavior of Calamactis has not yet been determined, facilitation of conduction velocity in the net may produce greater synchronization of muscle contraction.