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C S Cohan

Publications and source records attributed to C S Cohan.

34 records · Page 2Linked to original sources
PubMed1988

Interactive effects of serotonin and acetylcholine on neurite elongation.

Serotonin (5-HT) inhibits elongation of neurites of specific identified neurons. Here we report a novel, growth-enabling action of another neurotransmitter, acetylcholine (ACh). When applied simultaneously with serotonin, ACh prevents the inhibition of Helisoma neuron B19 neurite elongation that would occur in response to application of 5-HT alone. We also report that ACh prevents the rise in growth cone Ca2+ that would occur in response to application of 5-HT alone and that ACh blocks the electrical excitatory effect of 5-HT on neuron B19. These results support the hypothesis that growth cone motility and neurite elongation can be regulated by voltage-gated Ca2+ fluxes and suggest that the dynamics of neurite morphology may be complexly regulated by an array of neurotransmitters, as is functional electrical activity.

Acetylcholine↗
PubMed1987

Formation, maintenance, and functional uncoupling of connections between identified Helisoma neurons in situ.

Previous work with identified Helisoma neurons has characterized an array of neuroplastic responses to axotomy that include the generation of new neuritic outgrowth, the reinnervation of target organs, and the formation of new electrical synapses. These responses are not random, but rather occur in a precise, predictable manner under a variety of culture conditions. The present investigation demonstrates that specific identified neurons display similar neuroplastic "behavior" within the living animal. In response to in situ nerve crushes, neurons B4 and B5 generate new neuritic outgrowth, neuron B4 functionally reinnervates the salivary glands, and new electrical synapses form between the left and right neurons B5. The in situ paradigm employed in the present experiments made it possible to examine responses to axotomy over longer periods than in earlier studies with organ cultures. New B5R-B5L connections, previously found to be stable over the short term in culture, gradually decreased in strength in situ, and the cells effectively uncoupled by 8 weeks after axotomy. This uncoupling did not depend upon target reinnervation and occurred in the continued presence of neurites in the buccal commissure. It is suggested that the stability of new connections is related to whether the connection previously existed in the unperturbed nervous system. The similarities between the ability of identified neurons to grow and to form synaptic connections in situ and in culture suggests that neurons are endowed with a specific program of regenerative responses that can be expressed reliably in a wide variety of environmental conditions.

Animals↗
PubMed1987

Electrically and chemically mediated increases in intracellular calcium in neuronal growth cones.

In the present report we used the calcium indicator fura-2 to compare intracellular levels of free calcium in growth cones of isolated Helisoma neurons under a variety of experimental conditions. We tested whether 2 different signals that inhibit growth cone motility--action potentials and serotonin--changed calcium levels in growth cones. Electrical stimulation of the cell body caused a rise in calcium levels at the growth cone. After brief stimulation, calcium levels quickly recovered to normal values, whereas longer stimulation periods required longer recovery times. The application of serotonin to growth cones caused an increase in calcium levels that was selective for growth cones of neurons whose outgrowth was inhibited by serotonin, but not for neurons whose outgrowth was not affected. We also found that motile growth cones had higher free calcium levels than growth cones that had spontaneously stopped growing. Furthermore, the distribution of calcium in neurons that contained motile growth cones was heterogeneous; calcium levels were always higher in the growth cone than in the neurite or soma. These data indicate that calcium levels in growth cones vary in different states of outgrowth and that calcium levels can be modulated by both electrical and chemical signals.

Action Potentials↗
PubMed1986

Suppression of neurite elongation and growth cone motility by electrical activity.

Electrical activity may regulate a number of neuronal functions in addition to its role in transmitting signals along nerve cells. The hypothesis that electrical activity affects neurite elongation in sprouting neurons was tested by stimulating individual snail neurons isolated in cell culture. The findings demonstrated that growth cone advance, and thus neurite elongation, is reversibly stopped during periods when action potentials are experimentally evoked. A decrease in filopodial number and growth cone area was also observed. Thus, action potentials can mediate the cessation of neurite outgrowth and thereby may influence structure and connectivity within the nervous system.

Action Potentials↗
PubMed1986

Discriminative behavior and Pavlovian conditioning in the mollusc Pleurobranchaea.

The buccal motor system in the sea slug Pleurobranchaea californica is multifunctional; similar sets of neurons and muscles generate different behaviors through similar electrophysiological motor patterns. Such multifunctional systems compromise the traditional practice of identifying a motor pattern and then using that pattern to indicate the behavior in reduced preparations. We address this issue in a series of experiments leading to the comparison of differential Pavlovian conditioning in whole animals with the conditioned behavior of the same animals during electrophysiological recording. Because differential conditioning requires two conditioned stimuli (CSs), we show here that each of two CSs activated the conditioned response from animals after they received the stimulus (CS+) paired with an unconditioned stimulus (UCS). Conditioning sessions consisted of 5 training trials with a 2-h intertrial interval. In one study, experimental animals received a 60-s CS+, derived from beer (Sbr), paired with a 50-s electrical shock UCS whose onset occurred 10 s after the CS+ onset; control animals received the Sbr and UCS explicitly unpaired. In a second study, animals received similar procedures as in the first but with a CS+ consisting of squid homogenate (Ssq). Tests with both CSs showed that animals did not discriminate between Sbr and Ssq before beginning conditioning, but did so afterward. Experimental animals exhibited robust food aversion (withdrawal and suppressed feeding) to the CS+, but retained strong appetitive responses to the CS they did not receive in training; response thresholds to the CS+ changed as much as 1000-fold by comparison to the preconditioning values. Control animals exhibited similar though significantly smaller behavioral changes as the experimental animals. Both stimuli evoked associatively learned responses, but Sbr produced greater experimental-control differences than Ssq did. Two accompanying papers show the results of using both CSs in differential conditioning, and describe the behavioral/electrophysiological comparisons.

Animals↗
PubMed1986

Differential Pavlovian conditioning in the mollusc Pleurobranchaea.

The present differential Pavlovian conditioning experiments on the sea slug Pleurobranchaea californica extend conditioning described in a preceding paper and provide the conditioning foundation for studies reported in another accompanying paper comparing learned behavior in whole animals with the behavior and motor patterns of electrophysiological preparations. All animals received two appetitive-conditioned stimuli (CSs), one derived from beer (Sbr) and the other derived from squid muscle (Ssq), in different temporal relationships to an electric shock unconditioned stimulus (UCS). Two groups of animals were run concurrently. One group (n = 19) received Sbr as the CS+ in close temporal pairing with the UCS, and Ssq as the CS- explicitly unpaired with the UCS (Sbr +/Ssq-). The second group (n = 20) received the opposite contingencies (Sbr-/Ssq+). All animals received only one day of conditioning involving 5 trials with an intertrial interval of 2 h. There were two replicate experiments, each involving about half of the total n, and each yielding similar results as the sum we report here. Before conditioning, animals exhibited feeding behavior (extension of the proboscis and bite-strike responses) to both stimuli at similar low thresholds. Conditioning produced long-term behavioral changes in all animals throughout the 4.5-day postconditioning observation period. However, only the Sbr+/Ssq- animals consistently exhibited the appropriate differentially conditioned food-aversion behavior which consisted of strong withdrawal and high-threshold feeding responses to Sbr, and low-threshold feeding responses to Ssq. We discuss the possibility that such differences between Sbr+/Ssq- and Sbr-/Ssq+ conditioning may arise either from inherent differences in the responses of the animals to Sbr and Ssq, or, as seems more likely to us, from training and testing effects produced by differences in the compositions of the two stimuli.

Animals↗
PubMed1986

Comparison of differential Pavlovian conditioning in whole animals and physiological preparations of Pleurobranchaea: implications of motor pattern variability.

The present study compares differential Pavlovian conditioning in whole animals with the behavior of the same animals during electrophysiological recording. Untrained specimens of the sea slug Pleurobranchaea did not discriminate between two appetitive stimuli, one derived from an extract of beer (Sbr) and the other from a homogenate of squid muscle (Ssq). When animals received Sbr as the CS+ and Ssq as the CS- in a single day of five-trial, differential Pavlovian conditioning they learned to avoid selectively the Sbr but continued to exhibit appetitive responses to Ssq. Quantitative measures show that there was over a 1000-fold increase in the thresholds of the proboscis extension and bite-strike responses, many animals ceased all feeding behavior, and exhibited withdrawal responses to Sbr. We examined the behavior of the same trained animals immediately before preparing them for physiological recording and during the recording session. There was a close one-to-one relationship between these behavioral observations, showing that the qualitative and quantitative features of whole-animal Pavlovian conditioning persist into the physiological preparations. Unexpectedly, motor patterns from untrained preparations showed considerable variability both within the same preparation at different times and between preparations; conditioning appeared to increase such variability. Thus, it was not possible to state unequivocally the behavior of the animal by examining the electromyogram recording alone. Many of the trained preparations not only exhibited suppressed feeding behavior and withdrawal responses to Sbr, but, as a consequence of the multifunctional nature of the Pleurobranchaea buccal-oral system, also regurgitated previously ingested Ssq or squid meat when they were stimulated with Sbr. We discuss the findings with respect to self-organizing mechanisms that may establish motor patterns in multifunctional systems, and suggest that such mechanisms may lead to the generation of behaviors that are not specifically encoded by the conditioned cellular changes.

Animals↗
PubMed1986

Convergence in a distributed nervous system: parallel processing and self-organization.

The present findings show that the motor system of the carnivorous sea slug Pleurobranchaea californica consists of parallel, distributed, and interconnected neuronal channels by which motor activity may emerge from the dynamics of the system rather than from "switchboard" circuitry. The findings are shown primarily through the properties of the buccal-cerebral neurons (BCNs) that extensively converge and diverge monosynaptically and polysynaptically onto brain motoneurons, providing them with drive and patterned activity. The motoneurons, some of which are electrically coupled, feed back onto the BCNs. The BCNs are functionally heterogeneous both as a group and individually. Many are multifunctional in that they take part in the generation of different behaviors, and some also appear to change their timing with respect to the phase of the pattern generator in the different motor patterns. In the buccal ganglion, the BCNs affect the characteristics of the pattern generator and may be part of the pattern generator itself. By sending axons to buccal roots and to the brain, some BCNs may act as motoneurons and also integrate the activity of brain motoneurons. Because of the effects produced by the extensive interconnections among such functionally heterogeneous and nonlinear elements, and because the "history" of activity in the system can bias subsequent activity, there is ambiguity in assessing the response properties of neurons by examining them individually or in pairs. Such an assessment requires, first, an understanding of the context of activity in which a neuron becomes coactive, and, second, because of inherent variability in the system, it is necessary to consider the temporal, nonlinear computations of the system as a whole. We discuss the findings with regard to the attractor theory that has been used to study complex mammalian systems but that does not rely on modeling of any neuronal activity. The Pleurobranchaea nervous system may provide the means for studying individual neurons within such analyses of global activity.

Animals↗
PubMed1985

Neuron-specific growth cone properties as seen in identified neurons of Helisoma.

By using cell-cultured identified neurons of the snail Helisoma, we demonstrate that the growth cones of different neurons are intrinsically different from one another in terms of their structure, behavior, and response to environmental signals. Structurally neuron 5 has a greater number of filopodia per growth cone, has shorter filopodia, and has a smaller interfilopodial distance than neuron 19. Behaviorally, the growth cones of neuron 5 advance over the substratum at a faster rate than those of neuron 19; and the growth cones of neuron 19, but not of neuron 5, respond to the presence of serotonin in their environment by retracting their filopodia. In addition to such intrinsic differences between the growth cones of different neurons, we also demonstrate that the separate growth cones of a single neuron, while having identical properties, can act independently of one another. Focal application of serotonin to a growth cone causes only a localized retraction of that growth cones' filopodia. Other growth cones that are attached to the same neuron but that are not exposed to serotonin retain their normal structural features.

Animals↗
PubMed1985

Single channel activity differs in growing and nongrowing growth cones of isolated identified neurons of Helisoma.

When plated in cell culture, Helisoma neurons extend new neurites with large growth cones. These growth cones progress from actively growing organelles to stable structures over a period of several days. The ability to distinguish morphologically between these growth states has provided the opportunity to test whether changes in growth status are correlated with changes in the ionic properties of the growth cone membrane. This investigation has begun to study these changes by examining single channel ionic currents using patch clamp recording techniques and has demonstrated the presence of at least one ion channel having a conductance of 70 pS in the membrane of Helisoma growth cones. Recordings from cell-attached patches show that this channel is normally active in growing growth cones, whereas channel activity is absent in recordings from stable growth cones. The absence of channel activity in growth cones that have reached the stable state is not due to the loss of channels from these growth cones. Rather, the channel is present in an inactivated state. The activity of this channel is not dependent on the presence of Ca, as channel activity persists when Ca is removed from the solution that bathes the internal surface of the membrane. These data demonstrate a correlation between channel activity and the growth state of neuronal growth cones.

Action Potentials↗
PubMed1983

Electrical synapse formation depends on interaction of mutually growing neurites.

A neuron's competence to form electrical synapses depends on its growth status. Experiments in situ and in cell culture with identified neurons of the snail Helisoma demonstrate that active neurite outgrowth from both potential partners must be spatially and temporally coincident for electrical synapse formation to occur.

Action Potentials↗
PubMed1983

'Zap axotomy': localized fluorescent excitation of single dye-filled neurons induces growth by selective axotomy.

The response of populations of neurons to axotomy has traditionally been studied by crushing or sectioning whole nerve trunks. The present communication describes a technique by which single neurons can be reliably and selectively axotomized in the absence of damage to other axons and non-neuronal cells within the nerve. To obtain selective axotomy, identified neurons of the buccal ganglia of the snail, Helisoma, were first filled with fluorescent dye. Next, the preparation was positioned in a restricted beam of blue light using low light video fluorescence microscopy. Finally, the selected region of axon was briefly exposed to light levels normally employed for fluorescence microscopy. Shortly after irradiation of the identified neuron 5, antidromic action potentials no longer propagated past the region of exposure in the dye-filled cell, whereas adjacent axons were physiologically intact. Several days after exposure, profuse neurite outgrowth was observed from the proximal region of axon of neuron 5, but never in neighboring axons which were not filled with dye at the time of irradiation. When the axons of both neurons 5R and 5L were spot irradiated neurite outgrowth resulted in the formation of a novel electrical connection between these cells. These changes in growth and connectivity which were induced by selective axotomy of single axons were indistinguishable from the changes which are produced by crushing entire nerve trunks.

Animals↗
PubMed1983

The generation of rhythmic activity in a distributed motor system.

Rhythmic activity that is distributed to the brain and buccal ganglia and which underlies several types of behaviour, can be evoked from isolated nervous systems of Pleurobranchaea californica by tonic nerve stimulation. The experiments presented here were designed to test whether this rhythmic activity is produced by independent neuronal oscillators located in each ganglion or whether the rhythmic activity arises from a single oscillatory locus in the buccal ganglion and is transmitted passively to the brain. By interrupting the conduction of activity in the cerebrobuccal connectives (CBC) between brain and buccal ganglia we show that motor output from the brain depends on sustained, cycle to cycle input from the buccal ganglion and cannot be reset with respect to the buccal activity. The production of rhythmic activity in the brain depends on the generation of rhythmic activity in the buccal ganglia whether the rhythms are activated by stimulation of buccal roots or paracerebral command cells in the brain. Simultaneous intracellular recordings from brain motoneurones and buccal interneurones which project to the brain indicate that these interneurones provide both the drive and the pattern for rhythmic motor output in the brain. Tonic stimulation of the CBC can produce rhythmic activity in isolated brains in which all nerve roots and connectives have been cut. This can be explained by the fact that tonic stimulation of the connectives is transformed into phasic activity by the axons within the connective. We conclude therefore, that rhythmic, coordinated activity in the brain and buccal ganglia of Pleurobranchaea arises from oscillatory circuits that are located only in the buccal ganglia.

Animals↗