Transformation of siphon responses during conditioning of Aplysia suggests a model of primitive stimulus-response association

Associative learning in invertebrates

This work reviews research on neural mechanisms of two types of associative learning in the marine mollusk Aplysia, classical conditioning of the gill- and siphon-withdrawal reflex and operant conditioning of feeding behavior. Basic classical conditioning is caused in part by activity-dependent facilitation at sensory neuron-motor neuron (SN-MN) synapses and involves a hybrid combination of activity-dependent presynaptic facilitation and Hebbian potentiation, which are coordinated by trans-synaptic signaling. Classical conditioning also shows several higher-order features, which might be explained by the known circuit connections in Aplysia. Operant conditioning is caused in part by a different type of mechanism, an intrinsic increase in excitability of an identified neuron in the central pattern generator (CPG) for feeding. However, for both classical and operant conditioning, adenylyl cyclase is a molecular site of convergence of the two signals that are associated. Learning in other invertebrate preparations also involves many of the same mechanisms, which may contribute to learning in vertebrates as well.

Molecular mechanisms of memory storage in Aplysia

Cellular studies of implicit and explicit memory suggest that experience-dependent modulation of synaptic strength and structure is a fundamental mechanism by which these memories are encoded, processed, and stored within the brain. In this review, we focus on recent advances in our understanding of the molecular mechanisms that underlie short-term, intermediate-term, and long-term forms of implicit memory in the marine invertebrate Aplysia californica, and consider how the conservation of common elements in each form may contribute to the different temporal phases of memory storage.

Neuroscience of Pavlovian conditioning: a brief review

Current knowledge on the neuronal substrates of Pavlovian conditioning in animals and man is briefly reviewed. First, work on conditioning in aplysia, that has showed amplified pre-synaptic facilitation as the basic mechanism of associative learning, is summarized. Then, two exemplars of associative learning in vertebrates, fear conditioning in rodents and eyelid conditioning in rabbits, are described and research into its neuronal substrates discussed. Research showing the role of the amygdala in fear conditioning and of the cerebellum in eyelid conditioning is reviewed, both at the circuit and cellular plasticity levels. Special attention is given to the parallelism suggested by this research between the neuronal mechanisms of conditioning and the principles of formal learning theory. Finally, recent evidence showing a similar role of the amygdala and of the cerebellum in human Pavlovian conditioning is discussed.

Activity-dependent presynaptic facilitation and hebbian LTP are both required and interact during classical conditioning in Aplysia

Using a simplified preparation of the Aplysia siphon-withdrawal reflex, we previously found that associative plasticity at synapses between sensory neurons and motor neurons contributes importantly to classical conditioning of the reflex. We have now tested the roles in that plasticity of two associative cellular mechanisms: activity-dependent enhancement of presynaptic facilitation and postsynaptically induced long-term potentiation. By perturbing molecular signaling pathways in individual neurons, we have provided the most direct evidence to date that each of these mechanisms contributes to behavioral learning. In addition, our results suggest that the two mechanisms are not independent but rather interact through retrograde signaling.

The contribution of activity-dependent synaptic plasticity to classical conditioning in Aplysia

Plasticity at central synapses has long been thought to be the most likely mechanism for learning and memory, but testing that idea experimentally has proven to be difficult. For this reason, we have developed a simplified preparation of the Aplysia siphon withdrawal reflex that allows one to examine behavioral learning and memory while simultaneously monitoring synaptic connections between individual identified neurons in the CNS. We previously found that monosynaptic connections from LE siphon sensory neurons to LFS siphon motor neurons make a substantial contribution to the reflex in the siphon withdrawal preparation (Antonov et al., 1999a). We have now used that preparation to assess the contribution of various cellular mechanisms to classical conditioning of the reflex with a siphon tap conditioned stimulus (CS) and tail shock unconditioned stimulus (US). We find that, compared with unpaired training, paired training with the CS and US produces greater enhancement of siphon withdrawal and evoked firing of LFS neurons, greater facilitation of the complex PSP elicited in an LFS neuron by the siphon tap, and greater facilitation of the monosynaptic PSP elicited by stimulation of a single LE neuron. Moreover, the enhanced facilitation of monosynaptic LE-LFS PSPs is greater for LE neurons that fire during the siphon tap and correlates significantly with the enhancement of siphon withdrawal and evoked firing of the LFS neurons. These results provide the most direct evidence to date that activity-dependent plasticity at specific central synapses contributes to behavioral conditioning and support the idea that synaptic plasticity is a mechanism of learning and memory more generally.

The contribution of activity-dependent synaptic plasticity to classical conditioning in Aplysia

Plasticity at central synapses has long been thought to be the most likely mechanism for learning and memory, but testing that idea experimentally has proven to be difficult. For this reason, we have developed a simplified preparation of the Aplysia siphon withdrawal reflex that allows one to examine behavioral learning and memory while simultaneously monitoring synaptic connections between individual identified neurons in the CNS. We previously found that monosynaptic connections from LE siphon sensory neurons to LFS siphon motor neurons make a substantial contribution to the reflex in the siphon withdrawal preparation (Antonov et al., 1999a). We have now used that preparation to assess the contribution of various cellular mechanisms to classical conditioning of the reflex with a siphon tap conditioned stimulus (CS) and tail shock unconditioned stimulus (US). We find that, compared with unpaired training, paired training with the CS and US produces greater enhancement of siphon withdrawal and evoked firing of LFS neurons, greater facilitation of the complex PSP elicited in an LFS neuron by the siphon tap, and greater facilitation of the monosynaptic PSP elicited by stimulation of a single LE neuron. Moreover, the enhanced facilitation of monosynaptic LE-LFS PSPs is greater for LE neurons that fire during the siphon tap and correlates significantly with the enhancement of siphon withdrawal and evoked firing of the LFS neurons. These results provide the most direct evidence to date that activity-dependent plasticity at specific central synapses contributes to behavioral conditioning and support the idea that synaptic plasticity is a mechanism of learning and memory more generally.

Dynamic regulation of the siphon withdrawal reflex of Aplysia californica in response to changes in the ambient tactile environment

The state of an animal's environment can be viewed as a source of information that can be used to regulate both ongoing and future behavior. The present work examined how the ambient environment can regulate the Aplysia siphon withdrawal reflex (SWR) by changing the environment between calm and turbulent. Results indicate that the SWR is dynamically regulated on the basis of variations in external conditions, so that responsiveness (measured as both reflex duration and threshold) is matched to the state of the environment. Prior exposure to a noxious stimulus (tailshock) has selective effects on this regulation, suggesting the existence of multiple regulatory mechanisms. Further, neurophysiological correlates to behavioral observations were measured in sensory and motor neurons. This will allow for a detailed cellular analysis of environmental information-processing in this system.

Separate Effects of a Classical Conditioning Procedure on Respiratory Pumping, Swimming, and Inking in Aplysia fasciata

We examined whether swimming and inking, two defensive responses in Aplysia fasciata, are facilitated by a classical conditioning procedure that has been shown to facilitate a third defensive response, respiratory pumping. Training consisted of pairing a head shock (UCS) with a modified seawater (85%, 120%, or pH 7.0 seawater—CSs). Animals were tested by re-exposing them to the same altered seawater 1 hr after the training. For all three altered seawaters, only respiratory pumping is specifically increased by conditioning. Swimming is sensitized by shock, and inking is unaffected by training, indicating that the conditioning procedure is likely to affect a neural site that differentially controls respiratory pumping. Additional observations also indicate that the three defensive responses are differentially regulated. First, different noxious stimuli preferentially elicit different defensive responses. Second, the three defensive responses are differentially affected by shock. Inking is elicited only immediately following shock, whereas swimming and respiratory pumping are facilitated for a period of time following the shock. Third, swimming and respiratory pumping are differentially affected by noxious stimuli that are delivered in open versus closed environments. These data confirm that neural pathways exist that allowAplysia to modulate separately each of the three defensive behaviors that were examined.

Separate effects of a classical conditioning procedure on respiratory pumping, swimming, and inking in Aplysia fasciata

We examined whether swimming and inking, two defensive responses in Aplysia fasciata, are facilitated by a classical conditioning procedure that has been shown to facilitate a third defensive response, respiratory pumping. Training consisted of pairing a head shock (UCS) with a modified seawater (85%, 120%, or pH 7.0 seawater--CSs). Animals were tested by re-exposing them to the same altered seawater 1 hr after the training. For all three altered seawaters, only respiratory pumping is specifically increased by conditioning. Swimming is sensitized by shock, and inking is unaffected by training, indicating that the conditioning procedure is likely to affect a neural site that differentially controls respiratory pumping. Additional observations also indicate that the three defensive responses are differentially regulated. First, different noxious stimuli preferentially elicit different defensive responses. Second, the three defensive responses are differentially affected by shock. Inking is elicited only immediately following shock, whereas swimming and respiratory pumping are facilitated for a period of time following the shock. Third, swimming and respiratory pumping are differentially affected by noxious stimuli that are delivered in open versus closed environments. These data confirm that neural pathways exist that allow Aplysia to modulate separately each of the three defensive behaviors that were examined.

Generalised and signal-specific long-term nociceptive sensitization in the common snail

It was found in experiments carried out in common snails that applications of a concentrated solution of quinine (10%) to the animal's head lead to long-term (more than 24 h) facilitation of defense reactions. Facilitation of synaptic components of the responses to test stimulation, an increase in the excitability of the membrane, and its depolarization correspond to the behavioral manifestations of long-term sensitization in the command neurons of defense behavior. The degree of manifestation of the sensitization effects depends on the duration of its development. After a one-day development of sensitization, manifestations of signal-specific sensitization, including site-specific sensitization (more pronounced synaptic facilitation in the responses to test stimulation of the segment of the body which had been subjected to the sensitizing influence, as compared with facilitation of responses to stimulation of other points) and modality-specific sensitization (more pronounced facilitation of responses to test stimulation which coincides in modality with the sensitizing stimulus, by comparison with the degree of manifestation of responses to a test stimulation of a different sensory modality), predominate. After a three-day development, signs of generalized sensitization predominated in the command neurons: marked facilitation of responses in all synaptic inputs of the command neurons, an increase in excitability, and depolarization of the membrane.

Hebbian induction of long-term potentiation of Aplysia sensorimotor synapses: partial requirement for activation of an NMDA-related receptor

Long-term potentiation (LTP) of Aplysia sensorimotor synapses in cell culture can be induced by pairing sensory neuron activity with depolarization of the motorneuron. This pairing-induced LTP is prevented by perfusion with D,L-2-amino-5-phosphononovalerate (APV), a selective antagonist for the N-methyl-D-asparate (NMDA) subclass of glutamate receptors. Repeated pairing of presynaptic activity with postsynaptic depolarization induces LTP comprising both APV-sensitive and APV-insensitive components. Infusing BAPTA, a selective Ca2+ chelator, into the postsynaptic motoneuron completely blocks pairing-induced LTP. These results demonstrate that Aplysia sensorimotor synapses are capable of hebbian LTP-similar to that exhibited by synapses in the mammalian hippocampus - and suggest a role for this type of synaptic plasticity in classical conditioning of the defensive withdrawal reflex of Aplysia.

A Neural Network Model of Inhibitory Information Processing in Aplysia

Recent cellular studies have revealed a novel form of inhibitory information processing in the siphon withdrawal reflex of the marine mollusc Aplysia: Motorneuronal output is significantly reduced by activity-dependent potentiation of recurrent inhibition within the siphon withdrawal network (Fischer and Carew 1991, 1993). This inhibitory modulation is mediated by two types of identified interneurons, L29s and L30s. In an effort to describe and analyze this and other forms of inhibitory information processing in Aplysia, and to compare it with similar processing in other nervous systems, we have constructed a neural network model that incorporates many empirically observed features of these interneurons. The model generates important aspects of the interactions of cells L29 and L30, and with no further modification, exhibits many network level phenomena that were not explicitly incorporated into the model.

Classical conditioning of the Aplysia siphon-withdrawal reflex exhibits response specificity

The gill- and siphon-withdrawal reflex of Aplysia undergoes classical conditioning of its amplitude and duration when siphon stimulation (the conditioned stimulus, CS) is paired with tail or mantle shock (the unconditioned stimulus, US). This conditioning of a preexisting response exhibits both temporal and stimulus specificities, which can be accounted for by activity-dependent enhancement of presynaptic facilitation of the siphon sensory neurons. To test whether conditioning of the reflex also exhibits response specificity (development of a new type of response to the CS that often resembles the response to the US), we measured the direction of siphon withdrawal in response to siphon stimulation (the CS) with tail or mantle shock as the US. The unlearned response to siphon stimulation is straight contraction, the response to tail shock is backward bending, and the response to mantle shock is forward bending. In the first experiment, we trained different animals with the tail or mantle US paired or unpaired with the CS; in a second experiment, we trained each animal with two CSs, one of which was paired with the US; in a third experiment, we varied US intensity; and in a fourth experiment, we trained each animal with two USs, one of which was paired with the CS. There was a significant, pairing-specific tendency for the direction of the response to the CS to resemble the response to the US after training in each experiment, demonstrating response specificity in conditioning of the withdrawal reflex. This feature of conditioning could in principle be accounted for by an elaboration of activity-dependent facilitation.