Sensory convergence on central visual neurons in Hermissenda

Network interneurons underlying ciliary locomotion in Hermissenda

In the nudibranch mollusk Hermissenda, ciliary locomotion contributes to the generation of two tactic behaviors. Light elicits a positive phototaxis, and graviceptive stimulation evokes a negative gravitaxis. Two classes of light-responsive premotor interneurons in the network contributing to ciliary locomotion have been recently identified in the cerebropleural ganglia. Aggregates of type I interneurons receive monosynaptic excitatory (I(e)) or inhibitory (I(i)) input from identified photoreceptors. Type II interneurons receive polysynaptic excitatory (II(e)) or inhibitory (II(i)) input from photoreceptors. The ciliary network also includes type III inhibitory (III(i)) interneurons, which form monosynaptic inhibitory connections with ciliary efferent neurons (CENs). Illumination of the eyes evokes a complex inhibitory postsynaptic potential, a decrease of I(i) spike activity, a complex excitatory postsynaptic potential, and an increase of I(e) spike activity. Here, we characterized the contribution of identified I, II, and III(i) interneurons to the neural network supporting visually guided locomotion. In dark-adapted preparations, light elicited an increase in the tonic spike activity of II(e) interneurons and a decrease in the tonic spike activity of II(i) interneurons. Fluorescent dye-labeled type II interneurons exhibited diverse projections within the circumesophageal nervous system. However, a subclass of type II interneurons, II(e(cp)) and II(i(cp)) interneurons, were shown to terminate within the ipsilateral cerebropleural ganglia and indirectly modulate the activity of CENs. Type II interneurons form monosynaptic or polysynaptic connections with previously identified components of the ciliary network. The identification of a monosynaptic connection between I(e) and III(i) interneurons shown here suggest that they provide a major role in the light-dependent modulation of CEN spike activity underlying ciliary locomotion.

Serotonin regulates voltage-dependent currents in type I(e(A)) and I(i) interneurons of Hermissenda

Serotonin (5-HT) has both direct and modulatory actions on central neurons contributing to behavioral arousal and cellular-synaptic plasticity in diverse species. In Hermissenda, 5-HT produces changes in intrinsic excitability of different types of identified interneurons in the circumesophageal nervous system. Using whole cell patch-clamp techniques we have examined membrane conductance changes produced by 5-HT that contribute to intrinsic excitability in two identified classes of interneurons, types I(i) and I(eA). Whole cell currents were examined before and after 5-HT application to the isolated nervous system. A 4-aminopyridine-sensitive transient outward K(+) current [I(K(A))], a tetraethylammonium-sensitive delayed rectifier K(+) current [I(K(V))], an inward rectifier K(+) current [I(K(IR))], and a hyperpolarization-activated current (I(h)) were characterized. 5-HT decreased the amplitude of I(K(A)) and I(K(V)) in both type I(i) and I(eA) interneurons. However, differences in 5-HT's effects on the activation-inactivation kinetics were observed in different types of interneurons. 5-HT produced a depolarizing shift in the activation curve of I(K(V)) and a hyperpolarizing shift in the inactivation curve of I(K(A)) in type I(i) interneurons. In contrast, 5-HT produced a depolarizing shift in the activation curve and a hyperpolarizing shift in the inactivation curve of both I(K(V)) and I(K(A)) in type I(eA) interneurons. In addition, 5-HT decreased the amplitude of I(K(IR)) in type I(i) interneurons and increased the amplitude of I(h) in type I(eA) interneurons. These results indicate that 5-HT-dependent changes in I(K(A)), I(K(V)), I(K(IR)), and I(h) contribute to multiple mechanisms that synergistically support modulation of increased intrinsic excitability associated with different functional classes of identified type I interneurons.

5-HT and GABA modulate intrinsic excitability of type I interneurons in Hermissenda

The sensory neurons (photoreceptors) in the visual system of Hermissenda are one site of plasticity produced by Pavlovian conditioning. A second site of plasticity produced by conditioning is the type I interneurons in the cerebropleural ganglia. Both photoreceptors and statocyst hair cells of the graviceptive system form monosynaptic connections with identified type I interneurons. Two proposed neurotransmitters in the graviceptive system, serotonin (5-HT) and gamma-aminobutyric acid (GABA), have been shown to modify synaptic strength and intrinsic neuronal excitability in identified photoreceptors. However, the potential role of 5-HT and GABA in plasticity of type I interneurons has not been investigated. Here we show that 5-HT increased the peak amplitude of light-evoked complex excitatory postsynaptic potentials (EPSPs), enhanced intrinsic excitability, and increased spike activity of identified type I(e(A)) interneurons. In contrast, 5-HT decreased spike activity and intrinsic excitability of type I(e(B)) interneurons. The classification of two categories of type I(e) interneurons was also supported by the observation that 5-HT produced opposite effects on whole cell steady-state outward currents in type I(e) interneurons. Serotonin produced a reduction in the amplitude of light-evoked complex inhibitory PSPs (IPSPs), increased spontaneous spike activity, decreased intrinsic excitability, and depolarized the resting membrane potential of identified type I(i) interneurons. In contrast to the effects of 5-HT, GABA produced inhibition in both types of I(e) interneurons and type I(i) interneurons. These results show that 5-HT and GABA can modulate the intrinsic excitability of type I interneurons independent of the presynaptic effects of the same transmitters on excitability and synaptic efficacy of photoreceptors.

Polysensory interneuronal projections to foot contractile pedal neurons in Hermissenda

A Pavlovian-conditioning procedure may produce modifications in multiple behavioral responses. As an example, conditioning may result in the elicitation of a specific somatomotor conditioned response (CR) and, in addition, other motor and visceral CRs. In the mollusk Hermissenda conditioning produces two conditioned responses: foot-shortening and decreased locomotion. The neural circuitry supporting ciliary locomotion is well characterized, although the neural circuit underlying foot-shortening is poorly understood. Here we describe efferent neurons in the pedal ganglion that produce contraction or extension of specific regions of the foot in semi-intact preparations. Synaptic connections between polysensory type Ib and type Is interneurons and identified foot contractile efferent neurons were examined. Type Ib and type Is interneurons receive synaptic input from the visual, graviceptive, and somatosensory systems. Depolarization of type Ib interneurons evoked spikes in identified tail and lateral foot contractile efferent neurons. Mechanical displacement of the statocyst evoked complex excitatory postsynaptic potentials (EPSPs) and spikes recorded from type Ib and type Is interneurons and complex EPSPs and spikes in identified foot contractile efferent neurons. Depolarization of type Ib interneurons in semi-intact preparations produced contraction and shortening along the rostrocaudal axis of the foot. Depolarization of Is interneurons in semi-intact preparations produced contraction of the anterior region of the foot. Taken collectively, the results suggest that type Ib and type Is polysensory interneurons may contribute to the neural circuit underlying the foot-shortening CR in Hermissenda.

Sensory regulation of network components underlying ciliary locomotion in Hermissenda

Ciliary locomotion in the nudibranch mollusk Hermissenda is modulated by the visual and graviceptive systems. Components of the neural network mediating ciliary locomotion have been identified including aggregates of polysensory interneurons that receive monosynaptic input from identified photoreceptors and efferent neurons that activate cilia. Illumination produces an inhibition of type I(i) (off-cell) spike activity, excitation of type I(e) (on-cell) spike activity, decreased spike activity in type III(i) inhibitory interneurons, and increased spike activity of ciliary efferent neurons. Here we show that pairs of type I(i) interneurons and pairs of type I(e) interneurons are electrically coupled. Neither electrical coupling or synaptic connections were observed between I(e) and I(i) interneurons. Coupling is effective in synchronizing dark-adapted spontaneous firing between pairs of I(e) and pairs of I(i) interneurons. Out-of-phase burst activity, occasionally observed in dark-adapted and light-adapted pairs of I(e) and I(i) interneurons, suggests that they receive synaptic input from a common presynaptic source or sources. Rhythmic activity is typically not a characteristic of dark-adapted, light-adapted, or light-evoked firing of type I interneurons. However, burst activity in I(e) and I(i) interneurons may be elicited by electrical stimulation of pedal nerves or generated at the offset of light. Our results indicate that type I interneurons can support the generation of both rhythmic activity and changes in tonic firing depending on sensory input. This suggests that the neural network supporting ciliary locomotion may be multifunctional. However, consistent with the nonmuscular and nonrhythmic characteristics of visually modulated ciliary locomotion, type I interneurons exhibit changes in tonic activity evoked by illumination.

Random Noise Paradoxically Improves Light-Intensity Encoding in Hermissenda Photoreceptor Network

Neurons are notoriously noisy devices. Although the traditional view posits that noise degrades system performance, recent evidence suggests that noise may instead enhance neural information processing under certain conditions. Here we report that random channel and synaptic noise improve the ability of a biologically realistic computational model of the Hermissenda eye to encode light intensity. The model was created in GENESIS and is based on a previous model used to examine effects of changes in type B photoreceptor excitability, synaptic strength, and network architecture. The network consists of two type A and three type B multicompartmental photoreceptors. Each compartment contains a population of Hodgkin–Huxley-type ion channels and each cell is stimulated via artificial light currents. We found that the addition of random channel and synaptic noise yielded a significant improvement in the accuracy of the network's encoding of light intensity across eight light levels spanning 3.5 log units ( P < 0.001, modified Levene test). The benefits of noise remained after controlling for several consequences of randomness in the model. Additionally, improvements were not confined to perithreshold stimulus intensities. Finally, the effects of noise are not present in individual neurons, but rather are an emergent property of the synaptically connected network that is independent of stochastic resonance. These results suggest that noise plays a constructive role in neural information processing, a concept that could have important implications for understanding neural information processing or designing neural interface devices.

Comparative study of visuo-vestibular conditioning in Lymnaea stagnalis

In this review, we compare the current understanding of visuo-vestibular conditioning in Hermissenda crassicornis and Lymnaea stagnalis on the basis of behavioral, electrophysiologic, and morphologic studies. Paired presentation of a photic conditioned stimulus (CS) and an orbital rotation unconditioned stimulus (US) results in conditioned escape behavior in both species. In Hermissenda, changes in excitability of type B photoreceptors and morphologic modifications at the axon terminals follow conditioning. Caudal hair cells, which detect mechanical turbulence, have reciprocal inhibition with type B photoreceptors. In Lymnaea, the interaction between photoreceptors and hair cells is dependent on statocyst location. Furthermore, the organization of the Lymnaea eye is complex, with more than 100 photoreceptors distributed in a uniquely folded retina. Although the optimal conditions to produce long-term memory (memory persistent for >1 week) are almost identical in Hermissenda and Lymnaea, physiologic and morphologic differences suggest that the neuronal mechanisms underlying learning and memory are distinct.

Pavlovian conditioning in Hermissenda: a circuit analysis

An understanding of associative learning requires (1) an adequate description of the experimental conditions under which learning is produced, (2) a knowledge of what is learned or the determination of the content of learning, and (3) an explanation of how learning generates changes in behavior (Rescorla, 1980). These basic issues are being addressed at both the behavioral and cellular/molecular levels by the analysis of associative learning in animals with relatively uncomplex nervous systems. Use of Pavlovian conditioning of invertebrates as a model for associative learning has led to the identification of cellular and synaptic mechanisms underlying the formation of basic associations. However, an understanding of the associative processes that form the basis for Pavlovian conditioning requires an explanation not only of the mechanisms of temporal contiguity or predictability between the conditioned stimulus (CS) and the unconditioned stimulus (US), but also of how changes produced in the nervous system by conditioning are expressed in behavior. Studies with invertebrates have provided the opportunity to examine how associative learning is expressed in the neural circuitry that supports the generation of learned behavior.

Electrophysiological Responses to Light of Neurons in the Eye and Statocyst of Lymnaea stagnalis

Lymnaea can be classically conditioned by pairing photic stimulation with a rotational stimulus. The electrophysiological properties of the Lymnaea photoreceptors and statocyst neurons are incompletely known. There are 2 types of ocular photoreceptors and 3 types of statocyst “hair cells.” Type A photoreceptors had a response latency from 200 to 400 ms, with a graded depolarizing response having maximum action spectra at 480–500 nm, corresponding to the βmax of rhodopsin. Additionally they extend their axons in the direction of the other type of photoreceptor neuron, the type T cell. These neurons have a 2-component response to light: a response reversibly reduced in Ca2+-free saline, and a component persisting in Ca2+-free saline. Type T cells send processes into the cerebral ganglion and terminate close to the ending of the statocyst hair cells. Hair cells send their terminal branches to the cerebral ganglia close to the terminations of the type T cells. Caudal hair cells respond to a light flash with a depolarization, whereas the rostral cells respond with a hyperpolarization. The response latency in all hair cells was dependent on the stimulus intensity; the brightest light tested had a latency of 200 ms. The photo-induced response was abolished in Ca2+-free saline, whereas it was still present in high Ca2+–high Mg2+ saline, consistent with the hypothesis that the connection between the photoreceptors and hair cells is monosynaptic. Thus the sensory information necessary for forming an association between photic and rotational stimuli converges on the statocyst neurons.

Pavlovian conditioning of Hermissenda: current cellular, molecular, and circuit perspectives

The less-complex central nervous system of many invertebrates make them attractive for not only the molecular analysis of the associative learning and memory, but also in determining how neural circuits are modified by learning to generate changes in behavior. The nudibranch mollusk Hermissenda crassicornis is a preparation that has contributed to an understanding of cellular and molecular mechanisms of Pavlovian conditioning. Identified neurons in the conditioned stimulus (CS) pathway have been studied in detail using biophysical, biochemical, and molecular techniques. These studies have resulted in the identification and characterization of specific membrane conductances contributing to enhanced excitability and synaptic facilitation in the CS pathway of conditioned animals. Second-messenger systems activated by the CS and US have been examined, and proteins that are regulated by one-trial and multi-trial Pavlovian conditioning have been identified in the CS pathway. The recent progress that has been made in the identification of the neural circuitry supporting the unconditioned response (UR) and conditioned response (CR) now provides for the opportunity to understand how Pavlovian conditioning is expressed in behavior.

Statocyst Hair Cell Activation of Identified Interneurons and Foot Contraction Motor Neurons in Hermissenda

Pavlovian conditioning of Hermissenda produces both light-elicited inhibition of normal positive phototactic behavior and conditioned stimulus (CS)-elicited foot-shortening. Rotation, the unconditioned stimulus (US) elicits foot-shortening and reduced forward ciliary locomotion. The neural circuit supporting ciliary locomotion and its modulation by light is known in some detail. However, the neural circuits responsible for rotation-elicited foot-shortening and reduced forward ciliary locomotion are not known. Here we describe components of the neural circuit in Hermissenda that produce anterior foot contraction and ciliary activation mediated by statocyst hair cells. We have characterized in semi-intact preparations newly identified pedal ventral contraction motor neurons (VCMNs) and interneurons (Ib). Type Ib interneurons receive polysynaptic input from statocyst hair cells and project directly to VCMNs and cilia-activating motor neurons. Depolarization of VCMNs with extrinsic current in normal artificial seawater (ASW) and high-divalent cation ASW, and under conditions where central synaptic transmission was suppressed with 5 mM Ni2+ ASW, elicited a contraction of the ipsilateral anterior foot measured from videotape recordings. Mechanical displacement of the statocyst or depolarization of identified statocyst hair cells with extrinsic current elicited spikes and complex excitatory postsynaptic potentials (EPSPs) in type Ib interneurons and complex EPSPs and spikes recorded in VCMNs. Type Ib interneurons are electrically coupled and project to VCMNs and VP1 cilia-activating motor neurons located in the contralateral pedal ganglia. The results indicate that statocyst hair-cell-mediated anterior foot contraction and graviceptive ciliary locomotion involve different interneuronal circuit components from the circuit previously identified as supporting light modulated ciliary locomotion.

Neural correlates of Pavlovian conditioning in components of the neural network supporting ciliary locomotion in Hermissenda

Pavlovian conditioning in Hermissenda consists of pairing light, the conditioned stimulus (CS) with activation of statocyst hair cells, the unconditioned stimulus (US). Conditioning produces CS-elicited foot shortening and inhibition of light-elicited locomotion, the two conditioned responses (CRs). Conditioning correlates have been identified in the primary sensory neurons (photoreceptors) of the CS pathway, interneurons that receive monosynaptic input from identified photoreceptors, and putative pedal motor neurons. While cellular mechanisms of acquisition produced by the synaptic interaction between the CS and US pathways are well-documented, little is known about the mechanisms responsible for the generation or expression of the CR. Here we show that in conditioned animals light reduced tonic firing of ciliary activating pedal neurons (VP1) below their pre-CS baseline levels. In contrast, pseudorandom controls expressed a significant increase in CS-elicited tonic firing of VP1 as compared to pre-CS baseline activity. Identified interneurons in the visual pathway that have established polysynaptic connections with VP1 were examined in conditioned animals and pseudorandom controls. Depolarization of identified type Ie interneurons with extrinsic current elicited a significant increase in IPSPs recorded in VP1 pedal neurons of conditioned animals as compared with pseudorandom controls. Conditioning also enhanced intrinsic excitability of type Ie interneurons of conditioned animals as compared to pseudorandom controls. Light evoked a modest increase in IPSP frequency in VP1 of conditioned preparations and a significant decrease in IPSP frequency in VP1 of pseudorandom controls. Our results show that a combination of synaptic facilitation and intrinsic enhanced excitability in identified components of the CS pathway may explain light-elicited inhibition of locomotion in conditioned animals.

Interneuronal projections to identified cilia-activating pedal neurons in Hermissenda

Neural networks have been shown to support the generation of more than one behavioral motor act. In the nudibranch mollusk Hermissenda, Pavlovian conditioning results in light, the conditioned stimulus (CS), evoking both inhibition of locomotion and foot contraction. The synaptic organization of the eyes and optic ganglion is well documented; however, the characterization of the neural network mediating visually modulated behaviors is incomplete. We have now characterized synaptic connections between identified photoreceptors and a newly identified interneuron (II(b)), identified synaptic projections from type I and type II interneurons to an inhibitory interneuron (III(i)) and to two newly identified pedal neurons, VP1 and VP2. Here we show that VP1 activates ciliary movement on the anterior foot and VP2 innervates the anterior foot and ventral tentacle. Stimulation of the photoreceptors with light produced two effects on the activity of VP1 and VP2. First, light inhibits type I(i) and II(i) interneurons and disinhibits VP1 and VP2. Depolarization of type II(e) interneurons also disinhibits VP1 and VP2. Second, the light-elicited depolarization and increased tonic activity of VP1 and VP2 is produced by excitatory synaptic input from ipsilateral and contralateral type II(b) interneurons. Pedal neurons VP1 and VP2 receive similar synaptic input from type I, II, and III(i) interneurons; this is in agreement with previous research showing that the visual pathway influences both ciliary locomotion and foot movement. The organization of the visual system in Hermissenda provides for the expression of cellular and synaptic plasticity supporting learning without altering the networks ability to carry out the requirements for normal visual processing.

Facilitation of monosynaptic and complex PSPs in type I interneurons of conditioned Hermissenda

Synaptic plasticity and intrinsic changes in neuronal excitability are two mechanisms for Pavlovian conditioning. Pavlovian conditioning of Hermissenda produces synaptic facilitation of monosynaptic medial B-medial A IPSPs and intrinsic changes in excitability of type A and B cells in isolated and intact sensory neurons of the conditioned stimulus (CS) pathway. Recently two types of interneurons that receive either excitatory or inhibitory monosynaptic or polysynaptic input from photoreceptors have been identified. On the basis of morphological and electrophysiological criteria, the interneurons have been classified as type I(e), I(i) (direct), and type II(e), II(i) (indirect). We have now examined synaptic facilitation of monosynaptic PSPs in type I(e) and I(i) interneurons after conditioning and pseudorandom control procedures. Here we report that CS-elicited spike activity is increased in type I(e) interneurons and decreased in type I(i) interneurons of conditioned animals relative to their respective baseline activity and pseudorandom control groups. Classical conditioning resulted in synaptic facilitation of type I(e) and I(i) monosynaptic PSPs elicited by lateral B spikes and enhancement of the amplitude of complex PSPs elicited by the CS. These results provide additional sites of plasticity in the neural circuit involved with the expression of learned behavior produced by Pavlovian conditioning of Hermissenda.

Morphological characteristics and central projections of two types of interneurons in the visual pathway of Hermissenda

The synaptic interactions between photoreceptors in the eye and second-order neurons in the optic ganglion of the nudibranch mollusk Hermissenda are well characterized. However, the higher-order neural circuitry of the visual system, consisting of cerebropleural interneurons that receive synaptic input from photoreceptors and project to pedal motor neurons that mediate visually guided behaviors, is only partially understood. In this report we have examined the central projections of two identified classes of cerebropleural interneurons that receive excitatory or inhibitory synaptic input from identified photoreceptors. The classification of the interneurons was based on both morphological and electrophysiological criteria. Type I interneurons received monosynaptic excitatory or inhibitory synaptic input from identified photoreceptors and projected to postsynaptic targets within the cerebropleural ganglion. Type II interneurons, characterized here for the first time, received polysynaptic excitatory or inhibitory synaptic input from identified photoreceptors and projected to postsynaptic targets in either the ipsilateral pedal ganglion or the contralateral cerebropleural ganglion. Type I interneurons exhibited unique intraganglionic projections to different regions of the cerebropleural ganglion, depending on whether they received excitatory or inhibitory synaptic input from identified photoreceptors. Type I interneurons that received monosynaptic excitatory input from identified B photoreceptors terminated near the cerebropleural commissure and had multiple regions of varicosities located at branches that projected from the primary axon. Type I interneurons that received monosynaptic inhibitory input from identified B photoreceptors projected to the anterior cerebropleural ganglion and exhibited varicosities localized to the terminal region of the primary axonal process. Type II interneurons that received polysynaptic inhibitory input from identified photoreceptors projected to the contralateral cerebropleural ganglion. Most type II interneurons that projected to the pedal ganglia received polysynaptic excitatory input from identified photoreceptors. These results indicate that there is at least one additional interneuron in the higher-order visual circuit between type I interneurons and pedal motor neurons responsible for the generation of phototactic locomotion in Hermissenda.

Monosynaptic connections between identified A and B photoreceptors and interneurons in Hermissenda: evidence for labeled-lines

The cellular and synaptic organization of the eye of the nudibranch mollusk Hermissenda is well-documented. The five photoreceptors within each eye are mutally inhibitory and can be classified into two types: A and B based on electrophysiological and anatomical criteria. Two of the three type B and two type A photoreceptors can be further identified according to their medial or lateral positions within each eye. In addition to reciprocal synaptic connections between photoreceptors, photoreceptors also project to second-order neurons in the cerebropleural ganglion. The second-order neurons receive convergent synaptic input from two additional sensory pathways; however, it has not been previously established if lateral A, lateral B, or medial B photoreceptors converge onto the same second-order neurons. To determine the specific synaptic organization of these components of the visual system, we have examined monosynaptic connections between identified lateral and medial type A and B photoreceptors and second-order cerebropleural (CP) interneurons. We found that monosynaptic connections between identified lateral A and lateral and medial B photoreceptors and CP interneurons follow a labeled-line principle. Illumination of the eyes or extrinsic depolarizing current applied to identified photoreceptors evoked excitatory and inhibitory postsynaptic potentials (EPSPs and IPSPs, respectively) in different CP interneurons. The PSPs in CP interneurons followed one-for-one spikes in the photoreceptors and could be elicited in artificial seawater solutions containing high divalent cations. Identified photoreceptors projected to more than one CP interneuron and expressed both excitatory and inhibitory connections with the different CP interneurons. In examples where a monosynaptic connection between a lateral B photoreceptor and a CP interneuron was identified, lateral A, medial A, or medial B photoreceptors did not project to the same CP interneuron. Moreover, when connections between medial B and CP interneurons were identified, lateral A, medial A, and lateral B connections were not found to project to the same CP interneuron. Similar results were obtained for a lateral A and CP interneuron connection. These results indicate that divergent labeled-lines exist between specific photoreceptors and second-order CP interneurons and potential convergence of synaptic input from primary and secondary elements of the visual system must occur at sites that are postsynaptic to the CP interneurons.

Ubiquitous molecular substrates for associative learning and activity-dependent neuronal facilitation

Recent evidence suggests that many of the molecular cascades and substrates that contribute to learning-related forms of neuronal plasticity may be conserved across ostensibly disparate model systems. Notably, the facilitation of neuronal excitability and synaptic transmission that contribute to associative learning in Aplysia and Hermissenda, as well as associative LTP in hippocampal CA1 cells, all require (or are enhanced by) the convergence of a transient elevation in intracellular Ca2+ with transmitter binding to metabotropic cell-surface receptors. This temporal convergence of Ca2+ and G-protein-stimulated second-messenger cascades synergistically stimulates several classes of serine/threonine protein kinases, which in turn modulate receptor function or cell excitability through the phosphorylation of ion channels. We present a summary of the biophysical and molecular constituents of neuronal and synaptic facilitation in each of these three model systems. Although specific components of the underlying molecular cascades differ across these three systems, fundamental aspects of these cascades are widely conserved, leading to the conclusion that the conceptual semblance of these superficially disparate systems is far greater than is generally acknowledged. We suggest that the elucidation of mechanistic similarities between different systems will ultimately fulfill the goal of the model systems approach, that is, the description of critical and ubiquitous features of neuronal and synaptic events that contribute to memory induction.

Modeling Hermissenda: I. Differential contributions of IA and IC to type-B cell plasticity

We developed a multicompartmental Hodgkin-Huxley model of the Hermissenda type-B photoreceptor and used it to address the relative contributions of reductions of two K+ currents, IA and IC, to changes in cellular excitability and synaptic strength that occur in these cells after associative learning. We found that reductions of [symbol: see text] C, the peak conductance of IC, substantially increased the firing frequency of the type-B cell during the plateau phase of a simulated light response, whereas reductions of [symbol: see text] A had only a modest contribution to the plateau frequency. This can be understood at least in part by the contributions of these currents to the light-induced (nonspiking) generator potential, the plateau of which was enhanced by [symbol: see text] C reductions, but not by [symbol: see text] A reductions. In contrast, however, reductions of [symbol: see text] A broadened the type-B cell action potential, increased Ca2+ influx, and increased the size of the postsynaptic potential produced in a type-A cell, whereas similar reductions of [symbol: see text] C had only negligible contributions to these measures. These results suggest that reductions of IA and IC play important but different roles in type-B cell plasticity.

Structural organization of the sensory systems of the snail

The author's data on the structural organization of the nervous system of the body wall, the gravitational organs (the statocysts, the eyes, and the chemosensory organs), and the head tentacles of the snails Helix vulgaris and Helix pomatia are correlated in this paper. The localization in the CNS and some structural characteristics of the central divisions of the sensory systems are shown. Information is presented on the structure of the procerebrum, an associative center participating in the processing of chemosensory information. The data were obtained using the Golgi method, the retro- and anterograde transport of CoCl2, horseradish peroxidase, and lucifer yellow, as well as a number of other classical histological methods.

Cellular correlates of classical conditioning in identified light responsive pedal neurons of Hermissenda crassicornis

Cellular correlates of classical conditioning were examined in two recently identified light responsive pedal neurons. The correlates of conditioning consisted of significant decreases in the pedal cells' responses to light (conditioned stimulus) recorded from conditioned animals compared to random controls. Pedal cell P7, which exhibits an inhibitory response to light in naive animals, showed significantly less inhibition during a 5 min light step in conditioned animals as compared to random controls. Pedal neuron P9, which exhibits an excitatory response to light in naive animals, showed significantly less excitation during a 10 s light step in conditioned animals as compared to random controls. The changes in the response to light recorded from pedal neurons P7 and P9 in conditioned animals were not accompanied by any significant changes in membrane potential, action potential amplitude or dark-adapted spike frequency.

Characterization of 4 light-responsive putative motor neurons in the pedal ganglia of Hermissenda crassicornis

As part of the analysis of the circuitry underlying phototaxis, 4 light-responsive pedal neurons were identified and characterized. The 4 newly identified neurons have been designated as pedal neurons P7, P8, P9 and P10. Pedal cell P7 has an inhibitory response to light, lasting several minutes. Pedal cells P8, P9 and P10 exhibit excitatory 'on' responses to light that last for a few seconds after light onset. Lucifer yellow fills showed that each identified pedal cell has only one process which exits the nervous system through one of the pedal nerves. Various procedures were used to investigate the responses to illumination expressed by the 4 identified pedal neurons. The results indicate that: (1) the light responses are not intrinsic, but are due to synaptic input from other light-responsive cell(s), and (2) the sources of the synaptic input to the pedal cells are the photoreceptors of the eye, and not extraocular photoreceptors or light sensitive neurons within the circumesophageal nervous system.

Motor correlates of phototaxis and associative learning in Hermissenda crassicornis

Motor correlates of Hermissenda phototactic behavior and their modification by associative training were examined. Anatomical studies indicated the posterior three-fourths of the foot to be innervated by nerve P1 while the anterior portion was innervated by nerve P2. Bilateral lesions of either P1 or P2 pedal nerves in untrained animals reduced phototactic behavior. Extracellular recordings of pedal nerves from untrained animals revealed significant increases in total multi-unit activity (MUA) during a light presentation. Prominent components of nerve P2 activity were synchronous bursts apparent in peri-stimulus time (PST) histograms. Burst frequency was increased by light. Associative training resulted in significant decreases in light-evoked MUA frequency in P1 and light-evoked burst frequency in P2. Intracellular recordings were obtained from three classes of putative motoneurons with axons in P2. These were located using cobalt backfills and verified for each preparation by simultaneous extracellular recording from P2. The characteristic pattern of activity of these cells in the dark and its modulation by light was established.

Associatively reduced withdrawal from shadows in Hermissenda: a direct behavioral analog of photoreceptor responses to brief light steps

When the nudibranch Hermissenda crassicornis encounters a shadow in an otherwise uniformly illuminated field, it stops and turns back into the light within seconds. Associative conditioning, with paired light and rotation stimuli, produces learned modifications of phototaxis in illumination gradients. This same training procedure significantly reduced the ability of paired, but not random or naive control animals, to withdraw from shadows. In naive animals, after 13 min of dark adaptation, withdrawal from shadows was less apparent when animals encountered this stimulus the first time than after the second encounter. This difference in responsiveness to the first and second edge stimulus paralleled differences in type B photoreceptor impulse frequencies recorded during and after first and second steps of light. Earlier studies have shown that associative training of Hermissenda increases a long-lasting depolarization (LLD) which follows a light step. Our present findings suggest a functional relationship between the LLD of the type B photoreceptor and the behavioral response to light-dark differences. This supports the view that membrane changes which cause modifications of LLD magnitude store the learned association for later recall.

Light paired with serotonin mimics the effect of conditioning on phototactic behavior of Hermissenda

A conditioning procedure consisting of pairing-specific stimulation of the eyes and gravity-detecting statocysts in Hermissenda results in a long-term modification of normal positive phototactic behavior. The learning is expressed by a significant suppression of the initiation of locomotion in the presence of light. We now report that an analogue of the classical conditioning procedure, consisting of light paired with serotonin (5-HT) applied directly to the exposed circumesophageal nervous system of otherwise intact animals, mimics the effect of conditioning on long-term changes in phototactic behavior. The effect of the conditioning analogue on behavior shows some specificity with 5-HT since light paired with dopamine or octopamine does not significantly affect phototactic behavior. The conditioning analogue exhibits pairing specificity since unpaired light and 5-HT and 5-HT applied in the dark do not produce behavioral suppression. Animals that initially received unpaired light and 5-HT do show behavioral suppression after receiving paired light and 5-HT. These results indicate that light (the conditioned stimulus) paired with the putative transmitter of the unconditioned stimulus pathway (5-HT) is sufficient to produce long-term phototactic suppression.

Autoradiographic measurement of tritiated agmatine as an indicator of physiologic activity in Hermissenda visual and vestibular neurons

[3H]Agmatine (amino-4-guanidobutane) has been shown to be potentially useful for identifying and assessing the ACh sensitivity of specific neurons. Small cationic amines are able to permeate ACh-activated ion channels in sympathetic neurons and vertebrate endplates. Sensory neurons of the photic pathway in the nudibranch mollusc Hermissenda crassicornis are cholinergic and the synaptic interactions between the photic and vestibular systems have been well characterized electrophysiologically. We have therefore tested the feasibility of using autoradiography with [3H]agmatine, (a) to identify known ACh-responsive postsynaptic cells and (b) to examine its ability to serve as an indicator of physiologic activity within the photic and vestibular pathways under conditions of darkness and light stimulation. Scintillation counting revealed that approximately 70% of the radioactivity was associated with the CNS while approximately 30% was found in the processing fluids, indicating that routine glutaraldehyde-osmium fixation and subsequent processing for epoxy embedding allows retention of substantial amounts of the radiolabel. The autoradiographic results consistently demonstrated that the uptake patterns for [3H]agmatine did reflect some of the known neuronal interactions under the experimental conditions of light and dark. The accuracy extended to the second order cells of the optic ganglion and to putative interneurons along the photic tract in the cerebropleural ganglion. Since all the neurons in these pathways are unipolar with their synaptic interactions occurring only at the terminal endings, the radiolabel accumulated in the somata resulted from retrograde axonal transport. In the photic-vestibular pathways, the highest silver grain densities were found over structures (cell bodies or axon tracts) with increased synaptic activity coupled with higher levels of cellular activity (i.e. increased excitatory postsynaptic potentials or increased spontaneous impulse activity). Slightly less label was found in cells which received increased numbers of inhibitory postsynaptic potentials that produced hyperpolarization and a transient cessation of impulse activity under conditions of illumination. Therefore, the uptake levels of [3H]agmatine as revealed by autoradiography appear to reflect not only changes in sensitivity or density of ACh-activated channels but also changes in cellular activity as indicated by increased amounts of retrograde transport. These results represent the first example of the effective use of this radiolabel as an indicator of synaptic activity in invertebrates and in sensory systems.

Reduction of two voltage-dependent K+ currents mediates retention of a learned association

A single identified neuron, the medial type B photoreceptor, was isolated by axotomy from the nervous systems of nudibranch molluscs (Hermissenda) which had been exposed to three different training experiences. Paired animals had been trained with repeated paired presentations of light and rotation and random animals with randomized light and rotation; naive animals had no training. A two-microelectrode voltage clamp of axotomized type B somata (separated from all synaptic interactions and impulse activity) was used to measure, with a blind procedure, three distinct ionic currents at least 24 h after the training experience. An early K+ current, IA, and a Ca2+-dependent K+ current, ICa2+-K+, but not a light-induced inward Na+ current, were significantly reduced for the paired as compared to the random and naive animals. The magnitude of ICa2+-K+ reduction was related (again measured blindly) to the degree of training-induced suppression of phototaxis (a measure of the learned behavior) for the paired animals. These data are consistent with previous observations indicating that changes of intrinsic type B membrane properties are an important means for encoding the acquisition and retention of Hermissenda associative learning.

Correlated receptor and motorneuron changes during retention of associative learning of Hermissenda crassicornis

1. Responses to light of an identified motorneuron (LP1) were recorded simultaneously with those of an identified Hermissenda photoreceptor (the lateral Type B) following three days of training with paired light and rotation. 2. These responses were significantly different when compared to responses of cells from animals trained with unpaired stimuli and from naive animals. 3. The differences of the LP1 responses can be explained as a consequence of the photoreceptor response changes. 4. The same training with paired stimuli has been shown to produce behavioural changes which satisfy criteria for vertebrate associative learning. 5. The observed neural correlates are consistent with previous findings which indicate that membrane changes within the Type B cell bodies play a causal role in associative learning of the nudibranch mollusc, Hermissenda crassicornis.