Intersensory interactions 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.

Associative behavioral modification in hermissenda: cellular correlates

Three days of training consisting of trials of light paired with rotation produces a long-term modification of photopositive behavior in Hermissenda crassicornis. The behavioral modification depends on the temporal association of light and rotation. For animals that received light paired with rotation, significant increases in the spontaneous activity of type B photoreceptors were correlated with changes in photopositive behavior after training. A persistent tonic depolarization of type B photoreceptors can explain the cellular changes correlated with the long-term behavioral modification produced by the temporal association of light and rotation.

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.

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.

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.

Network interactions among sensory neurons in the leech

Interactions among mechanosensory neurons, sensitive to touch, pressure and nociceptive stimuli in the leech nervous system were studied in isolated ganglia and in body-wall preparations. Pairs of touch-pressure, touch-nociceptive and pressure-nociceptive neurons were tested by suprathreshold stimulation of one neuron while recording the response of the other, in both directions. Pressure and nociceptive stimulation evoked depolarizing and hyperpolarizing responses in touch cells, mediated by interneurons. The relative expression of these responses depended on the stimulus duration. One or two pressure cell spikes produced, predominantly, a depolarization of the touch cells, and increasing number of spikes evoked a hyperpolarization. Nociceptive cells produced primarily the hyperpolarization of touch cells at any stimulus duration. When touch cells were induced to fire by injection of positive current into the soma, stimulation of pressure cells inhibited touch cell activity. However, when touch cells were induced to fire by peripheral stimulation, pressure cell activation failed to inhibit touch cell firing. The results suggest that excitation of pressure and nociceptive cells would not limit the responses of touch cells to peripheral stimuli, but would inhibit the firing of touch cells evoked by their central connectivity network.

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.

Neurophysiological substrates of context conditioning in Hermissenda suggest a temporally invariant form of activity-dependent neuronal facilitation

The neurophysiological basis for context conditioning is conceptually problematic because neurophysiological descriptions of activity-dependent (associative) forms of neuronal plasticity uniformly assume that a specific temporal relationship between signals is necessary for memory induction. In the present experiments, this problem is addressed empirically by presenting, as a temporally diffuse contextual signal, a stimulus that results in known neural modifications following punctate (temporally contiguous) pairings with an aversive unconditioned stimulus. Hermissenda were trained to discriminate between adjoining contexts that were distinguished only in that one was lit and one was dark. Thirty unsignaled rotations were presented during each of three 15-min sessions in one of the two (lit or dark) contexts. Prior to training, animals displayed a slight preference for the lit context. After exposure to unsignaled rotation, animal's preferences shifted strongly to the dark context if unsignaled rotations were presented in the light, and tended (nonsignificantly) to the lit context if unsignaled rotations were presented in the dark. The B photoreceptors of the Hermissenda eye undergo several forms of activity-dependent facilitation (e.g., an increase in neuronal input resistance and evoked spike frequency) following pairings of punctate light (CS) and presynaptic vestibular stimulation (US). Similar facilitation in the B photoreceptor was observed following in vitro training that mimicked context conditioning in which presynaptic vestibular stimulation was presented repetitively during a continuous 7.5-min light. Subsequently, Ca(2+)-imaging experiments were conducted with Fura-2AM. It was determined that intracellular Ca(2+), the CS-induced second messenger critical for the induction of activity-dependent facilitation, was elevated in the B photoreceptor throughout the 7.5-min light presentation. These results indicate that activity-dependent facilitation within similar neural structures can underlie learning about both temporally diffuse contextual stimuli and temporally punctate CS-US pairings. These results suggest that a common mechanism may underlie learning about diffuse contextual stimuli as well as punctate-conditioned stimuli, provided that the stimuli are processed similarly in each type of conditioning arrangement. Consequently, the expression of different responses to contextual and discrete stimuli are likely to reflect a higher property of the neural network, and do not necessarily arise from unique underlying mechanisms.

Phospholipases and arachidonic acid contribute independently to sensory transduction and associative neuronal facilitation in Hermissenda type B photoreceptors

During contiguous pairings of light and rotation, B photoreceptors in the Hermissenda eye undergo an increase in excitability that contributes to a modification of several light-elicited behaviors. This excitability increase requires a light-induced rise in intracellular Ca2+ in the photoreceptor concomitant with transmitter binding to G protein-coupled receptors as a result of presynaptic vestibular hair cell stimulation. Phospholipases and arachidonic acid (ArA) are here reported to be involved in independent signal transduction pathways that underlie both receptor function and activity-dependent facilitation of the B photoreceptor. 4-Bromophenacyl bromide (BPB), an inhibitor of phospholipases A2 (PLA2) and C (PLC), blocked the generation of light-induced depolarizing generator potentials, but had no affect on the inhibitory postsynaptic potential (IPSP) in the B cell that results from hair cell stimulation. Quinacrine, which predominantly blocks the activity of PLA2 in neurons, had no affect on either the light response or the IPSP, but did block increases in excitability (i.e. increased input resistance and elicited spike rate) of the B cell that results from pairings of light and presynaptic vestibular stimulation (i.e., in vitro associative conditioning). Neither nordihydroquararetic acid (NDGA), which inhibits metabolism of ArA by cyclooxygenase, nor indomethacin, which inhibits lipoxygenase metabolism of ArA, affected the light response or IPSP, but both blocked the increases in excitability in the B cell that accompanied in vitro conditioning. In combination with earlier results, these data suggest that ArA activates PKC in a synergistic fashion with Ca2+ and diacylglycerol in the B cell, and suggest that PLA2-induced ArA release, though not necessary for transduction of light or the hair cell-induced IPSP in the B cell, is a critical component of the convergence of signals that precipitates associative facilitation in this system.

GABA-induced synaptic facilitation at type B to A photoreceptor connections in Hermissenda

Gamma-aminobutyric acid (GABA) is a prevalent neurotransmitter in both vertebrate and invertebrate systems. Here we report that, in addition to its usual inhibitory actions, GABA induced synaptic facilitation at type B to A photoreceptor connections of the marine mollusk Hermissenda when applied transiently to the isolated nervous system. Synaptic facilitation also occurred in response to mechanical stimulation of the GABAergic hair cells, which are normally activated by rotational unconditioned stimuli during behavioral training of the intact animal. This synaptic facilitation represents a novel form of GABA-induced neuromodulation which may contribute to learning-dependent suppression of phototaxis in Hermissenda.

Current, voltage and pharmacological substrates of a novel GABA receptor in the visual-vestibular system of Hermissenda

In the marine mollusc, Hermissenda crassicornis, Type B photoreceptors exhibit an IPSP to both presynaptic hair cell stimulation and microapplication of gamma-aminobutyric acid (GABA) to the terminal branches. It was found that both the endogenous IPSP and the response to exogenously applied GABA were mediated to a large part by an outward current which reversed at approximately -80 mV. Additionally, these hyperpolarizing responses were found to mask a smaller depolarization that was mediated by the reduction of a basal outward current. Both the IPSP and the hyperpolarizing response to GABA, as well as the sublimated depolarizing response to GABA, were attenuated by the K+ channel blocker tetraethylammonium chloride (TEA) and displayed a strong sensitivity to [K+]o, while showing no sensitivity to [Cl-]o or the Cl- channel blocker picrotoxin. Moreover, iontophoretic injections of stable guanine analogues, GTP[gamma S] and GDP[beta S], into B photoreceptors eliminated both the IPSP and the GABA-induced hyperpolarization, while cholinergically mediated, interphotoreceptor interactions were unaffected. These results suggest that the endogenous receptor is at least partially homologous to the mammalian GABAB class receptor. Consistent with this classification, microapplication of selective GABAB receptor agonist baclofen onto the terminal region of the B photoreceptor resulted in a hyperpolarizing response that was qualitatively similar to that of GABA, although the GABAA agonist muscimol was also active, but less so than either GABA or baclofen. Attempts to block the endogenous IPSP or GABA-induced hyperpolarization by bath application of the GABAA receptor subtype antagonist bicuculline was ineffective and the GABAB receptor subtype antagonist saclofen was only weakly effective. These data demonstrate that the presynaptic hair cell's influence on postsynaptic B photoreceptors is in many respects similar to GABAB mediated responses in the mammalian CNS. This receptor is in some respects unique, however, in terms of its cross-sensitivity to both GABAA and GABAB agonists, its weak sensitivity to saclofen, and its apparent anomalous modulation of multiple K+ conductances.

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.

Singleness of action in the interactions of feeding with other behaviors in Hermissenda crassicornis

Singleness of action in the interactions of feeding behavior with other behaviors was studied in Hermissenda crassicornis. In Experiment 1 withdrawal of the oral veil from a tactile stimulus was inhibited during feeding elicited by application of a mussel homogenate to hungry animals. Rolling-over behavior was also inhibited during feeding. When animals were satiated, thereby abolishing the feeding response to mussel homogenate, withdrawal occurred in the presence of the food stimulus; however, rolling-over behavior was still inhibited. In Experiments 2 and 3 it was demonstrated that spontaneous locomotion was also inhibited in hungry animals by food stimuli; however, satiated animals ignored the food stimulus and continued to locomote. Both biting behavior and the suppressive effect of food stimuli on locomotion were also observed in dissected anterior ends of Hermissenda, suggesting that the mechanisms underlying the interaction of feeding with other behaviors may be accessible to study in the nervous system of reduced preparations.

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.

Calcium-mediated reduction of ionic currents: a biophysical memory trace

Learning behavior similar to vertebrate classical conditioning was demonstrated for the mollusc Hermissenda crassicornis. Postsynaptic membrane changes within well-defined neural systems that mediate the learning play a casual role in recording the learned association for later recall. Specific ionic currents in neural tissue undergo transformations lasting days after associative training with physiologic stimuli. During acquisition the intracellular calcium increases; this increase is accompanied by specific potassium current reduction that lasts for days after conditioning. The increase of calcium enhances calmodulin-dependent phosphorylation of proteins that either regulate or are part of ion channels. These currents and the conditions that precede their transformation occur in many types of vertebrate neurons, and hence this biophysical basis of Hermissenda learning could have relevance for species other than the gastropod studied.

Modification of the initiation of locomotion in Hermissenda: behavioral analysis

We have examined the effect of a conditioning procedure on various components of visually guided locomotion in Hermissenda. Temporally specific stimulation of the visual system and gravity detecting system (statocysts) with light and rotation produced long-term changes in locomotor behavior. We found that the latency to initiate locomotion in the presence of light was significantly increased for the conditioned group as compared to baseline pre-test latencies and groups that received random presentations of the conditioning stimuli. The variability in the time taken by animals to enter a central illuminated area as reported in earlier studies can be accounted for by the increase in the latency to initiate locomotion. The modifications of visually influenced locomotion exhibits stimulus (CS) specificity since locomotor behavior is not changed following conditioning in the absence of light. In addition, conditioned animals remained in the brightest part of the light gradient significantly less than pre-test measurements. Since this response (initiation of locomotion) can be studied in a semi-intact preparation, it should be possible to investigate how this example of learning generates changes in behavior.

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.

Membrane depolarization accumulates during acquisition of an associative behavioral change

Long-lasting electrical changes of identified Hermissenda neurons, the type B photoreceptors, can account for concomitant associative behavioral changes. Depolarization of the type B cells after paired light and rotation accumulates (as monitored with intracellular electrodes) with reptition. This accumulation was specific to stimulus pairing (versus light alone or explicitly unpaired stimuli) and to the orientation of the nervous system with respect to the center of rotation; it provides a neural step in the acquisition of associative behavioral changes for gastropod mollusks and possibly other species.

Voltage-dependent calcium and potassium ion conductances: a contingency mechanism for an associative learning model

Persistent light-induced depolarization results from Ca2+ influx across a photoreceptor membrane. The marked dependence on potential of this Ca2+ influx and a Ca+-dependent K+ efflux accounts for enhancement of the light-induced depolarization when light is paired with rotation. A positive feedback cycle between light-induced depolarization and synaptic depolarization due to stimulus pairing can explain long-lasting behavioral changes produced by associative training but not control paradigms. The sensitivity of this Ca2+ influx to intracellular levels of adenosine 3'-5'-monophosphate suggests biochemical steps for this model of associative learning.

Ultrastructure of photoreceptors in the eye of Hermissenda labelled with intracellular injections of horseradish peroxidase

The terminal processes of single and of pairs of identified photoreceptors in the eyes of the nudibranch mollusc Hermissenda crassicornis were studied by light and transmission electron microscopy after their somata were labelled by intracellular iontophoresis of horseradish peroxidase (HRP). The HRP spread from the somata into the axons and fine terminal processes within the neuropil of the cerebropleural ganglia. The photoreceptors ended in extensive secondary branches in the neuropil where previous electrophysiological studies had indicated the probable site of synaptic interactions between photoreceptors. Clear round vesicles (54-126 nm diameter) within labelled processes were similar to the vesicles found in the somata and axon hillocks. The terminal processes of pairs of type B photoreceptors contained different intensities of the HRP label. Uniform intensities of the HRP label were found in the terminal processes of single type B photoreceptors. These differences in intensity suggested that the terminal processes were from different type B photoreceptors. This finding suggests that the connections between type B photoreceptors are probably monosynaptic.

Interaction of chemosensory, visual, and statocyst pathways in Hermissenda crassicornis

Neurons in the cerebropleural ganglia (CPG), photoreceptors in the eye, optic ganglion cells, and statocyst hair cells of the nudibranch mollusk Hermissenda crassicornis responded in specific ways, as recorded intracellularly, to stimulation of the chemosensory pathway originating at the tentacular chemoreceptors as well as to stimulation of the visual pathway originating at the photoreceptors. Synaptic inhibition of photoreceptors occurs via the chemosensory pathway. The possible significance of such intersensory interaction is discussed with reference to preliminary investigation of the animal's gustatory behavior and possible neural mechanisms of behavioral choice.

Signal transformation with pairing of sensory stimuli

Rotation of the isolated nervous system of Hermissenda in a caudal orientation causes a synaptic hyperpolarization accompanied by elimination of impulse activity during the steady-state phase of type A but not type B photoreceptors' responses to light. Rotation of the isolated nervous system in a cephalic orientation causes a synaptic depolarization with increase of impulse activity during the steady-state phase of both type A and type B photoreceptors' responses to light. These effects of rotation on photorecptors are explained by known synaptic interactions. Sufficient redundancy is found to be provided by the neural organization of the visual system and its interaction with the statocyst to preserve much of the visual information in spite of signal transformation in specific photorecptors resulting from pairing of rotation with light.

Responses of hair cells to statocyst rotation

A new technique is described for stimulating hair cells of the Hermissenda statocyst. The preparation and recording apparatus can be rotated at up to 78 rpm while recording intracellular potentials. Hair cells in front of the centrifugal force vector depolarize in response to rotation. Hair cells in back of the centrifugal force vector hypoerpolarize in response to rotation. Mechanisms by which the hair cell generator potential might arise are examined.

A dual synaptic effect on hair cells in Hermissenda

Type A photorecptors can produce an initial hyperpolarizing wave followed by a delayed long-lasting increase in firing which is usually accompanied by a small depolarizing wave. The initial hyperpolarizing wave arises from an increase in conductance while the depolarizing wave was shown to arise from a decrease in conductance. The data presented indicate that both effects produced by the type A photoreceptors in ipsilateral hair cells are synaptic.

Neural correlates of associative training in Hermissenda

Hair cells in Hermissenda respond to illumination of the ipsilateral and contralateral eyes. These responses are modified by associative training of the animal. The observed electrophysiological changes appear to result from changes in the photoreceptors' synaptic input to the hair cells.

Associative training of Hermissenda

Reflex behavior of Hermissenda in response to visual and rotational stimuli is described. It is shown that repeated association of light with rotation modifies the subsequent responses of the animals to light. This modification does not occur after the same period of light or rotation alone. The effect of the associative training is strongly dependent on the amount of daily light with which the animals are maintained.

Hair cell interactions in the statocyst of Hermissenda

Hair cells in the statocyst of Hermissenda crassicornis respond to mechanical stimulation with a short latency (<2 ms) depolarizing generator potential that is followed by hyperpolarization and inhibition of spike activity. Mechanically evoked hyperpolarization and spike inhibition were abolished by cutting the static nerve, repetitive mechanical stimulation, tetrodotoxin (TTX), and Co(++). Since none of these procedures markedly altered the generator potential it was concluded that the hyperpolarization is an inhibitory synaptic potential and not a component of the mechanotransduction process. Intracellular recordings from pairs of hair cells in the same statocyst and in statocysts on opposite sides of the brain revealed that hair cells are connected by chemical and/or electrical synapses. All chemical interactions were inhibitory. Hyperpolarization and spike inhibition result from inhibitory interactions between hair cells in the same and in opposite statocysts.