An infraciliary network in statocyst hair cells

Ultrastructural analysis of the statocyst, a primitive vestibular organ, of the nudibranch mollusc Hermissenda crassicornis, indicates that in addition to the basal foot, there is an infraciliary rootlet system between basal bodies of adjacent sensory cilia. These rootlets project perpendicularly from the basal bodies and parallel to the cell surface in an astral array. A polarity within the network also appears to exist; the array is longest and most extensive on the side of the basal body directed away from the cell centre, but the overall arrangement of the basal bodies indicates a multidirectional sensitivity for each of the 13 sensory cells. This rootlet system, in conjunction with the attachment system of the basal bodies to the cell membrane (button anchors), may serve an integrative function for the mechanical stimuli experienced by sensory cells and/or be involved with their transductive processes by maximizing the stress to, and membrane distortion of, the transductive site caused by weighting of the cilia. Evidence was also obtained for the intracellular synthesis of statoconia by the nonsensory supporting cells.

Neural organization predicts stimulus specificity for a retained associative behavioral change

Paired, but not random, presentations of light and rotation produced long-term changes in Hermissenda's response to light. The nature of this change depended on the orientation of the animals with respect to the center of rotation and was predicted by known organizational features of Hermissenda's nervous system. When rotation that excited caudal hair cells was paired with light, a significant increase in response latency to test lights resulted. Rotation exciting cephalic hair cells when paired with light decreased the response latencies compared with latencies produced by random presentation of light and rotation.

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.

Motile statocyst cilia transmit rather than directly transduce mechanical stimuli

We have investigated the role of motile cilia in mechanotransduction by statocysts of the nudibranch mollusk Hermissenda crassicornis. Movement of the cilia that experience the weight of statoconia causes increased variance of voltage noise and membrane depolarization of the statocyst hair cell. Two complementary approaches were used to immobilize the cilia. Vanadate anion was iontophoretically injected into hair cells. This reversible inhibitor of vibratile form and to assume a more classic, pliable beat pattern. Voltage noise decreased as the cilia slowed and bent more extremely, nearly disappearing as motility was lost. When the intracellular vanadate concentration approached 10(-5) M, the cilia were arrested in an effective stroke against the cell membrane. The cell no longer depolarized upon gravitational or local mechanical stimulation. Rapid reversal of ciliary inhibition by norepinephrine or slow reversal with time restored both the voltage noise and depolarization response. Cilia were rendered rigid and upright by covalent cross-linkage of their membrane "sleeve" to the 9 + 2 axoneme, using the photoactivated, lipophilic, bifunctional agent 4,4'-dithiobisphenyl azide. In the initial stages of cross-linkage, the cilia remained vibratile but slowed and moved through wider excursions. Voltage noise decreased in frequency but increased in amplitude. When the cilia were fully arrested, voltage noise was minimized while the resting potential and membrane resistance remained essentially constant. Mechanical stimulation of the rigid cilia, normal to the cell membrane, elicited a generator potential of the same amplitude but of greater duration than before treatment. Because cilia that are partially arrested by vanadate undergo increased bending, although the hair cell shows decreased noise, neither the axoneme nor the ciliary membrane proper would appear to be sites of direct transduction. In cells with beating but stiffened cilia, however, the voltage noise becomes amplified, implying an increased efficiency of transduction. We suggest that active but rigid flexure of the axoneme is involved in amplification and continuous signal detection. The basal insertion area is the most likely transduction site, being the terminal leverage point through which force is applied to the plasma membrane via the flexing ciliary shaft.

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.

A common origin of voltage noise and generator potentials in statocyst hair cells

Voltage noise, generator potentials, and hair movements in the Hermissenda statocyst were analyzed. Motile hairs on the cyst's luminal surface moved as rods through +/- 10 degrees Hz when free and at 7 Hz when loaded with the weight of the statoconia (at 120 degrees C). For hair cells oriented opposite to a centrifugal force vector, rotation caused depolarization and increase of voltage noise variance. The depolarizing generator potential and the increase in voltage noise variance were similarly reduced by perfusion with zero external sodium or chloral hydrate. Cooling, perfusion with zero external sodium or chloral hydrate reduced the movement frequencies of the hairs but increased their range of motion. The same treatments reduced voltage noise variance and increased input resistance of the hair cell membrane. The results indicate that voltage noise and hair cell generator potential have a common origin: exertion of force on statocyst hairs by the weight of statoconia. The collision of statoconia with the motile hairs, not the hairs' bending, produces most of the voltage noise.

Cholinergic features of photoreceptor synapses in Hermissenda

1. A number of observations, as listed below, suggested a cholinergic basis for inhibitory interactions between photoreceptors of the eye in the nudibranch mollusk Hermissenda crassicornis. 2. The isolated eyes synthesized and accumulated acetylcholine but not other putative neurotransmitter substances. Synthesis and accumulation were determined by electrophoretic separation of products that incorporated radioactive label. Electron microscopic visualization of clear round vesicles within the photoreceptors' somata and axon hillocks was consistent with synthesis and storage of acetylcholine within these cells. 3. Pharmacologic experiments indicated the presence of cholinergic receptors on the terminal branches of the photoreceptors, which are pre- and postsynaptic to each other. Carbachol or nicotine produced hyperpolarization of the photoreceptors' membrane accompanied by a reduction of the input resistance. The reversal potential of carbachol-induced hyperpolarization coincided with the reversal potentials of the IPSPs that followed, one for one, impulses of neighboring photoreceptors. Eserine often caused blockade of the IPSPs. This blockade was associated with substantial membrane hyperpolarization and reduction of membrane resistance. 4. Neuronal endings within the optic tract in the area of the photoreceptor's terminal branches stained for acetylcholinesterase. 5. The results of these different experiments, especially when considered together, strongly suggest, although by no means unequivocally demonstrate, that the neurotransmitter of the photoreceptors is acetylcholine.

Retention of an associative behavioral change in Hermissenda

The nudibranch mollusk Hermissenda crassicornis is normally attracted to a test light. Three days of training consisting of 50 trials per day of light paired with a rotational stimulus led to a significant increase, lasting for days, in the animal's response latency to enter a test light. The group that received light associated with rotation was significantly different from groups subjected to nonassociative control procedures. Modifications of well-known sensory networks may be related to a behavioral change that shares several operational features with associative learning.

Larval rearing, metamorphosis, growth and reproduction of the eolid nudibranch hermissenda crassicornis (eschscholtz, 1831) (gastropoda: opisthobranchia)

1. Hermissenda crassicornis is a subannual nudibranch species that reproduces year-round. 2. There is a significant positive relationship between adult weight, diameter of the egg mass, estimated number of eggs per egg mass, and average number of eggs per capsule. 3. There is a planktonic veliger stage of 34 days minimum at 13 degrees -15 degrees C. 4. Larvae metamorphose on at least three species of hydroids. 5. To develop in reasonable numbers to a state competent to metamorphose veligers require a diet that includes phytoplankton of larger cell size (10-11 microm) than the commonly used Isochrysis and Monochrysis (5 microm). 6. Although Hermissenda feeds on a wide variety of sessile invertebrate species in the ocean, a diet of tunicate alone (Ciona intestinalis) promotes good growth and survival in the laboratory. 7. Egg mass deposition is initiated only after first copulation, except in the last month of life, and continues from about one-month post-metamorphosis to death, at about four months post-metamorphosis. Generation time (egg-to-egg) may be as short as 2.5 months. 8. A laboratory strain of Hermissenda is being established to provide animals of known history for research on the neural correlates of behavior. Animals, at least initially, are being selected for fast growth rate.

Evidence for nonsynaptic neuronal interaction

1. Evidence is presented that synaptic interactions within and between the statocyst and visual pathways of Hermissenda are eliminated after 0.5-4 min exposure to 20-40 mM Co2+. 2. Synaptic blockade was also produced by perfusion with low Ca2+ (5mM) plus 10-20 mM Co2+. 3. Depolarization of hair cells by impulses of type A photoreceptors remains after the same exposure to Co2+, or low Ca2+ plus Co2+. 4. The increased resistance previously observed during this depolarization of hair cells cannot be observed after exposure to Co2+. 5. The depolarization which remains after exposure to Co2+ did not change with different levels of membrane potential from -20 mV below to +10 mV above the resting level. 6. The time course of potassium accumulation, monitored by the amplitude of the type A impulse afterpotential, closely followed the time course of hair cell depolarization and also of changes in the amplitude of the hair cell afterpotential. 7. The depolarization of hair cells by type A impulses decreased with increased extracellular potassium, but was only slightly reduced by lowered extracellular potassium. 8. The amount of potassium accumulation following a type A impulse train could be estimated from the effects of changes in extracellular potassium in the perfusate on the type A impulse afterpotential. From this extimated increase of extracellular potassium it was possible to predict with some accuracy, the observed hair cell depolarization. 9. Although type A cells are not electrically coupled to ipsilateral hair cells, firing of these hair cells slightly depolarized the type A photoreceptor which excites them. 10. Strophanthidin (10-4 M) did not block the depolarization of hair cells by type A impulses. 11. The data are evidence for nonsynaptic excitation of hair cells by type A photoreceptor impulses. The data are also consistent with the interpretation that the excitation arises from potassium accumulation around the type A and hair cell axonal membranes.

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.

Neural modification by paired sensory stimuli

With repetitive stimulation of two sensory pathways which are intact within the isolated nervous system of Hermissenda, features of a cellular conditioning paradigm were identified. Type A photoreceptors, unlike type B photoreceptors, produce fewer impulses in response to light following temporally specific pairing of light stimuli with rotation stimuli. Type A photoreceptor impulse wave-forms are also specifically changed by such stimulus regimens. These findings can be explained, at least in part, by increased inhibition of type A cells by type B cells after stimulus pairing.

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.