Vincent F. Castellucci

Facilitatory transmitter causes a selective and prolonged increase in adenosine 3':5'-monophosphate in sensory neurons mediating the gill and siphon withdrawal reflex in Aplysia

Sensitization of the gill and siphon withdrawal reflex in the marine mollusc, Aplysia california, is a simple form of learning Underlying this behavioral changes is a cascade of biochemical events. The first step in this cascade is postulated to be an increase in cAMP within the sensory neurons of the abdominal ganglion. We have developed a labeling protocol with 32Pi which permits us to measure the synthesis of cAMP within a single sensory neurons. Application of serotonin for 5 min was found to triple the content of [32P]cAMP in sensory neurons. The response is specific to serotonin: dopamine, a transmitter that does not produce sensitization, did not increase cAMP. Physiological stimulation of facilitator neurons also resulted in a 3.5-fold increase of cAMP in sensory neurons but not in other cells of the ganglion. We studied the time course of the increase of cAMP in sensory cells stimulated with serotonin and found that it parallels closely the time course of the short term form of presynaptic facilitation. We also have determined the effects of transmitters on the synthesis of cAMP in other identified neurons of the ganglion. The bag cells responded specifically to serotonin. R15, which has been shown to be hyperpolarized both the serotonin and by dopamine, responded to both transmitters by increased synthesis synthesis of cAMP. Thus, the dopamine- and serotonin-sensitive cyclase can be localized to both the same and different cells. Other cells did not respond to serotonin or to dopamine, indicating that a transmitter-sensitive adenylate cyclase is a specific property and is not present in all neurons.

An Analysis of Dishabituation and Sensitization of The Gill-Withdrawal Reflex In Aplysia

We have used a combined behavioral and cellular neurophysiological analysis to examine the relationship of sensitization to dishabituation of the gill-withdrawal reflex in Aplysia. The reflex withdrawal of the gill to tactile stimulation of the siphon or the purple gland (at the edge of the mantle shelf) shows habituation, dishabituation and sensitization. We have found that the purple gland and siphon provide independent afferent pathways each capable of eliciting the gill-withdrawal reflex. Habituation of one pathway did not affect the other, but a common ‘dishabituatory’ stimulus produced dishabituation of the habituated pathway as well as sensitization of the non-habituated pathway. These findings support the idea that dishabituation is not due to the removal of habituation but is an independent facilitation superimposed upon habituation. Our neurophysiological analysis showed that, on the cellular level, the neural correlates of sensitization and dishabituation are different reflections of a common h...

Ultrastructure of the synapses of sensory neurons that mediate the gill-withdrawal reflex inAplysia

SummaryWe have identified the processes of mechanoreceptor sensory neurons by intracellular injection of horseradish peroxidase in order to study the structure of synapses which exhibit profound, behaviourally-relevant plasticity. These synapses are located at small, varicose expansions along or at the end of the fine, microtubule-containing neurites, and they are crowded with vesicles some of which are associated with the varicosity membrane at regions of membrane specialization morphologically equivalent to active zones described in other species.These active zones occur between pre- and postsynaptic processes at two varieties of apposition: a conventional flat one, and a more elaborate indented one. At indented appositions, the presynaptic varicosity is invaginated by a thin (less than 0.25 μm diameter) spine of variable length. The active zones of indented synapses have approximately twice the vesicle frequency of flat synapses, suggesting that indented synapses are more effective.Sensory neuron terminals are relatively uniform in their structure, having similar concentrations of vesicles and numbers of active zones, and the majority of the processes postsynaptic to them are less than 0.5 μm in diameter. These regularities, and the presence of two strikingly-different types of synaptic apposition, flat and indented, should facilitate structural comparisons of neurons from naive and behaviourally-modified animals. The possible dynamic interconversion of indented and flat appositions at the synaptic terminals of sensory neurons and its behavioural relevance are discussed.

Contribution of polysynaptic pathways in the mediation and plasticity of Aplysia gill and siphon withdrawal reflex: evidence for differential modulation

The gill and siphon withdrawal (GSW) reflex of Aplysia is centrally mediated by a monosynaptic and a polysynaptic pathway between sensory and motor neurons. The first objective of this article was to evaluate quantitatively the relative importance of these two components in the mediation of the GSW reflex. We have used an artificial sea water (ASW) solution containing a high concentration of divalent cations to raise the action potential threshold of the interneurons without affecting the monosynaptic component of the reflex (2:1 ASW). Compound EPSPs induced in gill or siphon motor neurons by direct stimulation of the siphon nerve or by tactile stimulation of the siphon skin were reduced by more than 75% in 2:1 ASW. These results indicate that interneurons intercalated between sensory and motor neurons are responsible for a considerable proportion of the afferent input to the motor neurons of the reflex. The second objective of this article was to compare the modulation of the monosynaptic and polysynaptic pathways. We have evaluated their respective contribution in sensitization of the GSW reflex by testing the effects of two neuromodulators of the reflex, 5- HT and small cardioactive peptide B (SCPB). We found that these two neuromodulators have a differential action on the two components of the GSW neuronal network. The polysynaptic pathway was more facilitated than the monosynaptic pathway by the neuropeptide SCPB. By contrast, 5- HT displayed an opposite selectivity. These results suggest that the polysynaptic component of the neuronal network underlying the GSW reflex is very important for its mediation. The data also indicate that the monosynaptic and polysynaptic components of the reflex can be differentially modulated. The diversity of modulatory actions at various sites of the GSW network should be relevant for learning- associated modifications in the intact animal.

Some principles emerging from the study of short- and long-term memory

Recent studies indicate that in invertebrates short-term memory for various forms of learning involves covalent modifications of pre-existing proteins. By contrast, long-term memory utilizes genes and proteins not required for short-term memory.

Chapter 9 Synaptic plasticity and behavioral modifications in the marine mollusk Aplysia

Publisher Summary An important set of questions in the study of long-term memory is to find the signals involved in triggering the transition between short-term and longer term forms of memory. A second set of questions is to ask if there are many intermediate forms of memory and what are the mechanisms that maintain them. This chapter investigates these issues in Aplysia . In one approach, it analyzes short lasting behavioral modifications and longer lasting ones in adult animals; in the second approach, the chapter compares young Aplysias in which good mnemonic traces can be established with older ones with various types of memory impairments. The chapter reviews some studies that have done on the relationships of short-term and long-term depression and facilitation of the gill and siphon withdrawal reflex in intact adult animals, in isolated reflex preparations, and in dissociated cell culture systems. These studies indicate that the transition between short-term and long-term effects may be dependent on synthesis of new proteins.

Behavioral changes in aging Aplysia: A model system for studying the cellular basis of age-impaired learning, memory, and arousal

The marine mollusc Aplysia californica was used to examine the effects of age on simple forms of learning, memory, and arousal. We have found that aging impairs the long-term retention of habituation and prevents the acquisition of sensitization in the siphon withdrawal reflex. In addition, aging reduces arousal as evident in the heart rate component of the response to food stimuli. Our results are similar to the age-dependent alterations in the capacity for behavioral plasticity that have been reported in a variety of vertebrates, including man. These similarities suggest that the mechanisms underlying the effects of age on behavior and its modification may share common features across phyla and therefore might be studied to advantage in Aplysia whose central nervous system is especially accessible to cell biological approaches.

Chapter 6 Convergence of small molecule and peptide transmitters on a common molecular cascade

Publisher Summary This chapter summarizes the evidence that a conventional transmitter, serotonin, and the small cardioactive peptides—SCP A and SCP B —can modulate a specific K + channel by means of cyclic adenosine monophosphate. Although the information is still fragmentary, there is the possibility that a third class of transmitter, yet unidentified, works by the same mechanism. In modulating this K + channel, each of these transmitters also regulates transmitter release at a specific set of synapses involved in two short-term forms of memory, each lasting minutes to hours: the memory for short-term behavioral sensitization and for classical conditioning. The specific set of connections modulated during short-term sensitization also undergoes a prolonged modulation lasting days and weeks, which contributes to long-term memory for sensitization. The ion channel gated by transmitters consists of at least three functional components: (1) a receptor or recognition site, (2) a channel, and (3) a gate. The receptor recognizes the transmitter and instructs the gate to open or close the channel. The channel conducts ions only when it is gated open.