Nudibranch Mollusc Hermissenda Research Articles

6-bromohypaphorine from marine nudibranch mollusk Hermissenda crassicornis is an agonist of human α7 nicotinic acetylcholine receptor.

6-Bromohypaphorine (6-BHP) has been isolated from the marine sponges Pachymatisma johnstoni, Aplysina sp., and the tunicate Aplidium conicum, but data on its biological activity were not available. For the nudibranch mollusk Hermissenda crassicornis no endogenous compounds were known, and here we describe the isolation of 6-BHP from this mollusk and its effects on different nicotinic acetylcholine receptors (nAChR). Two-electrode voltage-clamp experiments on the chimeric α7 nAChR (built of chicken α7 ligand-binding and glycine receptor transmembrane domains) or on rat α4β2 nAChR expressed in Xenopus oocytes revealed no action of 6-BHP. However, in radioligand analysis, 6-BHP competed with radioiodinated α-bungarotoxin for binding to human α7 nAChR expressed in GH4C1 cells (IC50 23 ± 1 μM), but showed no competition on muscle-type nAChR from Torpedo californica. In Ca2+-imaging experiments on the human α7 nAChR expressed in the Neuro2a cells, 6-BHP in the presence of PNU120596 behaved as an agonist (EC50 ~80 μM). To the best of our knowledge, 6-BHP is the first low-molecular weight compound from marine source which is an agonist of the nAChR subtype. This may have physiological importance because H. crassicornis, with its simple and tractable nervous system, is a convenient model system for studying the learning and memory processes.

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.

Network model for visually mediated ciliary locomotion in Hermissenda

The overall goal of this study is to investigate the ways in which learning modifies behavior. A combination of computational and empirical studies is being used to address this issue. Empirical studies investigate learning from a cellular and synaptic perspective in the relatively simple nervous system of the nudibranch mollusk Hermissenda [1-3]. Pavlovian conditioning produces light-elicited inhibition of normal positive phototaxis in Hermissenda. Learning changes both cellular excitability and synaptic strength in the neural circuit that supports phototaxis. In the present study, a model of the circuit that supports visually mediated locomotion (Fig. ​(Fig.1A)1A) was developed. Consistent with empirical observations, simulated responses to light increased the level of VP1 spike activity (Fig. 1B1), which is equivalent to positive phototaxis. Simulations indicated that phototaxis resulted from disinhibition of VP1. Light increased activity in Ie and decreased activity in Ii (Fig. 1B2). The net result was less activity in IIIi and disinhibition of VP1 (Fig. 1B2). Simulations also indicated that disinhibition produced phototaxis only if VP1 had a high level of tonic firing. The model is being refined and expanded, and will be used to investigate the generation of other behaviors (e.g., foot contraction), the responses to other sensory inputs (e.g., gravity), and the influence of learning-induced plasticity (e.g., increased Ie excitability and decreased VP1 tonic firing). Simulations also will help identify features of the model that warrant further empirical investigation. Figure 1 Network model for visually mediated ciliary locomotion. A: The model had six cells: LB, lateral B-type photoreceptor; Ib, Ie, Ii and IIIi, interneurons; VP1, ciliary motor neuron. The cells were Hodgkin-Huxley-like neurons. Membrane conductances also ...

Sensory Regulation of Network Components Underlying Ciliary Locomotion inHermissenda

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 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.

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.

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.

PP1 inhibitors depolarize Hermissenda photoreceptors and reduce K+ currents.

Previous research indicates that activation of protein kinase C (PKC) plays a critical role in the induction and maintenance of memory-related changes in neural excitability of Type B photoreceptors in the eyes of nudibranch mollusk Hermissenda crassicornis (H.c.). The enhanced excitability of B cells is due in part to PKC-mediated reduction in somatic K+ currents. Here we examined the effects of protein phosphatase inhibitors on Type B photoreceptor excitability and K+ currents to determine the role(s) of protein phosphatases on memory formation in Hermissenda. Using electrophysiological and pharmacological methods, we found that the PP1 inhibitors calyculin A and inhibitor-2 depolarized Type B photoreceptors by 20-30 mV. A broad-spectrum kinase inhibitor, H7, blocked this effect. The depolarization induced by PP1 inhibition occluded that produced by an in vitro associative conditioning procedure. Calyculin and inhibitor-2 reduced the same B cell K+ currents (I(A) and I(delayed)) that are reduced by in vitro and behavioral conditioning. H7 blocked the reductions. Cantharidic acid (PP2A inhibitor) and cyclosporin (PP2B inhibitor) had negligible effects on B cell resting membrane potential, K+ currents, and in vitro conditioning-produced cumulative depolarization of B cells. These results suggest that the functional activity of K+ channels in B cells is sustained by basal activity of PP1. Inhibiting PP1 appears to allow one or more constitutively active kinase(s) to reduce K+ channel activity and thus mimic the effects of conditioning. Our results suggest that PP1 may oppose and/or constrain the extent of learning-produced changes in B cell excitability.

Serotonin activation of the ERK pathway in Hermissenda: contribution of calcium-dependent protein kinase C.

The mitogen-activated protein kinase (MAPK) cascade is an important contributor to synaptic plasticity and learning in both vertebrates and invertebrates. In the nudibranch mollusk Hermissenda, phosphorylation and activation of the extracellular signal-regulated protein kinase (ERK), a key member of a MAPK cascade, is produced by one-trial and multitrial Pavlovian conditioning. Several signal transduction pathways that are activated by 5-hydroxytryptamine (5-HT) and may contribute to conditioning have been identified in type B photoreceptors. However, the regulation of ERK activity by 'upstream' signaling molecules has not been previously investigated in Hermissenda. In the present study we examined the role of protein kinase C (PKC) in the serotonin (5-HT) activation of the ERK pathway. The phorbol ester TPA produced an increase in ERK phosphorylation that was blocked by the PKC inhibitors GF109203X or Gö6976. TPA-dependent ERK phosphorylation was also blocked by the MEK1 inhibitors PD098059 or U0126. The increased phosphorylation of ERK by 5-HT was reduced but not blocked by pretreatment with the calcium chelator BAPTA-AM or pretreatment with Gö6976 or GF109203X. These results indicate that Ca(2+)-dependent PKC activation contributes to ERK phosphorylation, although a PKC-independent pathway is also involved in 5-HT-dependent ERK phosphorylation and activation.

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.

Behavioral and Neural Bases of Noncoincidence Learning inHermissenda

Neurobiological studies of associative learning and memory have focused nearly exclusively on the analysis of neural plasticity resulting from paired stimuli. A second major category of associative-learning processes, one that has been conspicuously neglected in cellular studies, is that of conditioned inhibition (CI), learning that one stimulus signals the absence of another. The physiological bases of CI are obscure and unexplored. To study the behavioral and neural bases of CI, we exposed the nudibranch mollusc Hermissenda crassicornis to explicitly unpaired (EU) presentations of light and rotation. We report here that Hermissenda exhibited persistent increases in phototactic behavior after EU training. Retardation-of-learning test results provided further evidence that EU animals learned that light signaled the absence of rotation. The increased phototactic behavior of EU animals was paralleled by selective decreases in the magnitude of ocular type B cell photoresponses and the frequency of light-elicited action potentials: the first report of a neural correlate of noncoincidence learning. Plasticity arising from explicitly unpaired stimulus presentations raises provocative questions as to how noncoincidence is detected and represented within the nervous system.

Competence and metamorphosis in the long-term planktotrophic larvae of the nudibranch mollusc Hermissenda crassicornis (Eschscholtz, 1831)

Hermissenda crassicornis is a eurytrophic carnivorous opisthobranch mollusc that possesses long-term planktotrophic larvae. No morphological criteria for determining larval competence in this slug are reliable; thus competence has been established by inducing metamorphosis with the hydroid Tubularia crocea. The competence period lasts approximately three weeks at 12–17°C, the best time for inducing metamorphosis being from days 31–50 within a given egg mass, and more precisely from day 41 to 50, as concluded from several egg masses. Within-clutch and between-clutch variability exists in both Tubularia-induced and spontaneous metamorphosis. Variability within egg masses in Hermissenda is probably due to the gradual acquisition of metamorphic competence. Spontaneous metamorphosis increases significantly as larvae age within a given egg mass. Metamorphosis is non-specific, since the larvae are able to metamorphose with several natural inducers, such as other hydroids ( Pennaria sp.) and anemones ( Metridium senile and Haliplanella luciae). Combinations of Tubularia with glutamate at 10 −4 M or with Mg 2+ at 20 mM significantly increased the metamorphic yields, while NH 4 + was ineffective. K + at 30 mM and Cs + at 1 mM also increased the metamorphic yields, although Cs + at 30 mM was toxic. Several combinations of the natural inducers and some cations also produced high metamorphic yields, up to 25%. The effect of epinephrine at 10 −5 M in increasing the metamorphic yields, when used in combination with Tubularia, was equal to that obtained when using aged epinephrine, indicating that the oxidized products of epinephrine are probably responsible for this activity. Increasing the temperature from 12°C up to 17°C also had a significant effect in increasing the metamorphic yields with several inducers. The extracts of Tubularia with several organic solvents did not have an inducing effect. The natural inducer from Tubularia seems to be a water-soluble compound. Aqueous extracts of Tubularia at high concentrations (about eight-times the normal amounts) produced yields up to 50±8.3% metamorphosis, i.e., five-times the maximum yields ever reported for Hermissenda.

CHEMOTAXIS IN THE NUDIBRANCH HERMISSENDA CRASSICORNIS: DOES INGESTIVE CONDITIONING INFLUENCE ITS BEHAVIOUR IN A Y-MAZE?

The nudibranch mollusc Hermissenda crassicornis can be cultured easily in the laboratory and has been used as a biomedical model for learning and memory studies. Cultured animals whose only prey was Tubularia crocea were tested for responses to other prey. Naive slugs were conditioned with six different diets and tested for behaviour in a Y-maze. H. crassicornis was able to detect chemotactically food items which it had never been in contact with (such as Pennaria), but it did not detect some of the conditioning diets (e.g. Metridium). At least three hydroid species induce a chemotactic behaviour in the slug. It is shown that ingestive conditioning does affect their response in single-choice and double-choice experiments. As food items other than cnidarians are also detected, it is likely that different Wairomones are responsible for the nudibranch's behaviour.

Feeding behavior and growth of Hermissenda crassicornis (mollusca: Nudibranchia) in the laboratory

The feeding behavior and growth of the nudibranch mollusc Hermissenda crassicomis, a gener‐alist carnivore, has been studied; this species is widely used as a biomedical and neurobiological model. Relative to the stalks, polyps of the hydroid prey, Tubularia crocea, were preferentially consumed by adults of H. crassicomis at a fast rate. Starved slugs did not distinguish between reproductive and unreproductive polyps, but unstarved slugs did select unreproductive polyps of T. ‘crocea. Experiments with diets based on different parts of T. crocea (polyps, stalks, or whole) showed no obvious differences in juvenile growth among treatments, but survival of juveniles was low. Juveniles of H. crassicomis .grew better on a diet of the small anemone Haliplanella luciae than on T. crocea or the anemone Metridium senile. The relationship between body length and wet weight of H. crassicomis was similar for animals grown in the laboratory on a T. crocea diet and specimens from the field.

Biological factors affecting larval growth in the nudibranch mollusc Hermissenda crassicornis (Eschscholtz, 1831)

The nudibranch mollusc Hermissenda crassicornis is used as a biomedical model for studying learning and memory. It is an eurytrophic benthic species with long-term, planktotrophic larvae, and it has been cultured in our laboratory for several years. This paper reports the effects of some dietary factors on H. crassicornis larvae that were investigated in order to establish the conditions for optimal larval growth. Of the several treatments tested, densities of 1–4 larvae ml −1, a diet of Isochrysis galbana Parke and Rhodomonas salina (Wislouch) Butcher at a 1:1 mixture, and an algal density of 10–25×10 3 cells ml −1 yielded the largest, healthiest larvae, that underwent metamorphosis. Both diet quantity and quality, as well as larval density, had an effect on larval growth and metamorphosis of Hermissenda crassicornis in the laboratory.

Time‐Dependent Increase in Protein Phosphorylation Following One‐Trial Enhancement in Hermissenda

One-trial conditioning of the nudibranch mollusk Hermissenda produces short- and long-term changes in excitability (enhancement) of identified sensory neurons. To investigate the biochemical mechanisms underlying this example of plasticity, we have examined changes in protein phosphorylation at different times following the in vitro conditioning trial. Changes in the incorporation of 32 PO4 into proteins were determined using two-dimensional polyacrylamide gel electrophoresis, autoradiography, and densitometry. Conditioning resulted in increases in levels of several phosphoproteins, five of which, ranging in apparent molecular mass from 22 to 55 kDa, were chosen for analysis. The increased phosphorylation of the 46- and 55-kDa phosphoproteins, detected 2 h postconditioning was significantly greater than the level of phosphorylation detected in an unpaired control group, indicating that long-term enhancement is pairing specific. Statistically significant increases in phosphorylation as compared with the control group that received only light were detected immediately after conditioning (5 min) for the 55-, 46-, and 22-kDa phosphoproteins, at 1 h for the 55- and 46-kDa phosphoproteins, and at 2 h for the 55-, 46-, and 22-kDa phosphoproteins. The 46- and 55-kDa phosphoproteins are putative structural proteins, and the 22-kDa phosphoprotein is proposed to be a protein kinase C substrate previously identified in Hermissenda following multitrial classical conditioning. Time-dependent increases in protein phosphorylation may contribute to the induction and maintenance of different memory stages expressed in sensory neurons after one-trial conditioning.

Induction of metamorphosis inHermissenda crassicornislarvae (Molluscs: Nudibranchia) by GABA, choline and serotonin

Summary The nudibranch mollusc Hermissenda crassicornis is currently used as a biomedical model in neurobiological studies. It possesses planktotrophic larvae which metamorphose in the laboratory in the presence of the hydroid Tubularia crocea in yields of about 2–5% (Tamse et al., 1990). This study presents evidence that artificial metamorphic inducers such as GABA (γ-aminobutyric acid), choline, and serotonin (alone or combined with the natural inducer T. crocea), can be used to improve the metamorphic success in H. crassicornis larvae. GABA at 10−5M and 10−4M, choline at 10−3M and 10−4M, and serotonin at 10−5 M and 10−4M, were most effective, while serotonin at 10−3 M was toxic. Larvae 47 and 63 days old metamorphosed at different rates when exposed to the same concentrations of chemicals, but these had no positive effect on young larvae, i.e., 28 and 39 days old. On the other hand, when young larvae were exposed to the natural inducer, an habituation phenomenon was observed, i.e., the larvae did not m...

Food Detection and Preferences of the Nudibranch Mollusc Hermissenda crassicornis: Experiments in a Y-Maze

Previous articleNext article No AccessBEHAVIORFood Detection and Preferences of the Nudibranch Mollusc Hermissenda crassicornis: Experiments in a Y-MazeE. Tyndale, C. Avila, and A. M. KuzirianE. Tyndale Search for more articles by this author , C. Avila Search for more articles by this author , and A. M. Kuzirian Search for more articles by this author PDFPDF PLUS Add to favoritesDownload CitationTrack CitationsPermissionsReprints Share onFacebookTwitterLinkedInRedditEmail SectionsMoreDetailsFiguresReferencesCited by The Biological Bulletin Volume 187, Number 2October 1994 Published in association with the Marine Biological Laboratory Article DOIhttps://doi.org/10.1086/BBLv187n2p274 Views: 93Total views on this site Copyright © 1994 by Marine Biological LaboratoryPDF download Crossref reports no articles citing this article.

Enhancement of type B and A photoreceptor inhibitory synaptic connections in conditioned Hermissenda

Intrinsic changes have been identified in isolated and intact type A and type B photoreceptors following classical conditioning of the nudibranch mollusk Hermissenda. Aspects of various intrinsic, nonsynaptic modifications are expressed by alterations in the excitability of identified photoreceptors in response to the conditioned stimulus. In addition to changes in cellular excitability, changes in synaptic strength between identified neurons have been proposed as a possible mechanism of associative learning in several invertebrate preparations. Here we report that classical conditioning produces differential effects upon the strength of inhibitory monosynaptic connections between identified pairs of type B and type A photoreceptors. The amplitude of IPSPs elicited by an action potential in the medial type B photoreceptor and recorded from medical type A photoreceptors was significantly enhanced in conditioned animals as compared to pseudorandom controls. In contrast, the amplitude of IPSPs elicited by an action potential in the lateral type B photoreceptor and recorded from lateral type A photoreceptors did not show significant synaptic enhancement following conditioning. These results provide additional evidence for differential effects of conditioning upon cellular modifications in identified type A and type B photoreceptors, and further indicate that multiple sites of cellular plasticity exist in the visual system of conditioned Hermissenda.

GABA-mediated synaptic interaction between the visual and vestibular pathways of Hermissenda.

The synaptic convergence of the eyes and the vestibular hair cells in the nudibranch mollusc Hermissenda has been shown previously to mediate the learning of simple visual-vestibular associations. The neurotransmitter mediating this interaction between the visual and vestibular organs was characterized. HPLC chromatography, confirmed by mass spectroscopic analysis, demonstrated endogenous GABA in the statocysts, in a concentration approximately 150 times greater than in the whole CNS. Additional confirmation was provided by immunocytochemical localization of GABA in hair cell axons and branches that converge with photoreceptor terminal branches. Depolarization of the hair cells in the caudal region of the statocyst in response to positive current injection or vibratory stimulation caused a hyperpolarization and a cessation of the type B photoreceptor impulse activity. The inhibition of the B cell was unaffected by addition to the artificial sea water bath of the adrenergic antagonist yohimbine (250 microM), the cholinergic antagonist atropine (250 microM), and the serotonergic antagonist imipramine (50 microM). In contrast, the GABAA antagonist bicuculline (250 microM) significantly reduced the inhibitory interaction. Moreover, the GABA reuptake inhibitor guvisine (250 microM) increased the hyperpolarization. Pressure microapplication of GABA (12.5 or 25 microM) onto the terminal branches of the B cell resulted in a concentration-dependent hyperpolarization and cessation of spikes in the B cell. Depolarization of the caudal hair cell, or direct GABA application, decreased input resistance across the B cell soma membrane. Moreover, removal of chloride from the extracellular solution reduced inhibition of the B cell induced by GABA application or hair cell stimulation. Furthermore, application of the GABAB agonist baclofen hyperpolarized the type B cell and reduced or eliminated spontaneous impulse activity at the resting membrane potential. The reversal potentials for inhibition induced in all three procedures ranged from -70 to -80 mV and were consistent with mixed Cl- and K+ conductances. These results implicate GABA as the endogenous neurotransmitter mediating visual-vestibular interactions in this animal, and suggest a possible role of GABA in visual-vestibular associative learning.