Izja Lederhendler

Acquisition of conditioned associations in Hermissenda : additive effects of contiguity and the forward interstimulus interval

Conditioned suppression of photokinesis by the marine mollusc Hermissenda was examined in 3 experiments. In each experiment, groups of animals received light (the conditioned stimulus, CS) that was paired with high-speed orbital rotation (the unconditioned stimulus, UCS), light and rotation explicitly unpaired, or no exposure to these stimuli. Twenty-four hours after training, all animals were tested for suppression of photokinesis in the presence of the light. To establish the effectiveness of our conditioning procedure, in Experiment 1 individual groups of animals received either 50, 100, or 150 CS-UCS pairings. Fifty pairings resulted in a marginal suppression of photokinesis, whereas 100 and 150 pairings produced strong suppression. In Experiment 2, the delay between CS onset and UCS onset was varied between 1 and 10 s. The 10-s interstimulus interval (ISI) did not support conditioning, whereas 1-s and 2-s ISIs were effective. As predicted by the current understanding of Hermissendas neural network, in Experiment 3 it was found that CS-UCS pairings in which the CS preceded the onset of the UCS and terminated with the offset of the UCS evoked stronger conditioned suppression than either a CS that preceded the UCS and terminated with its onset or a CS that was paired in simultaneous compound with the UCS. This result indicates that CS-UCS contiguity as well as the forward ISI act additively to establish the CS-UCS association. In none of the 3 experiments were any differences observed between groups that were untreated and that received the CS and UCS unpaired. In total, these experiments suggest strong similarities in the temporal characteristics of associative learning in Hermissenda and vertebrate species.

The interstimulus interval and classical conditioning in the marine snail Hermissenda crassicornis.

We examined the role of the interstimulus interval for the conditioned association between light and rotation stimuli in the marine snail Hermissenda. This interval between the conditioned stimulus (CS) and the unconditioned stimulus (US) is an important and widespread property of vertebrate associative learning. We demonstrated that with a forward CS-US delay of 1.0 s we were able to produce significant 24-h retention of an associative memory after 50 training trials. Other paired treatments providing intervals of 1.5 s, 0.5 s, simultaneous, and backward arrangements did not support retention at 24 h.

Outgrowths fromHermissenda photoreceptor somata are associated with activation of protein kinase C

We have found changes in the morphology of photoreceptor somata from the molluscHermissenda that are produced by application of 12,13-phorbol dibutyrate (PDBU), an activator of PKC, in combination with elevated intracellular Ca2+ levels. The changes in morphology were expressed as rapid and repetitive outgrowths and additionally as more general changes in shape of the soma. Application of4a-PMA< a phorbol ester which does not activate PKC, did not produce these changes. The functional integrity of the photoreceptors in these dissociated eye prepaations was maintained throughout the period of incubation with PDBU according to standard electrophysiological criteria. It has previously been shown that classical conditioning produced a reduction of dendritic volume in the type B photoreceptor ofHermissenda, a specific locus for associative memory storage. These changes in dendritic morphology were correlated with increased resistance across the cell membrane caused by learning-induced reductions of outward somatic K+ currents. Such conditioning-specific reductions of somatic K+ currents appear to depend on the phosphorylation of a 20-kDa G-protein (CP20) mediated by the Ca2+ and phospholipid-dependent kinase, protein kinase C (PKC). Thus PKC activity may be important in structural changes of the synaptic region of specific neurons involved in associative memory. The results of the present study suggest that the effects of PKC activation may also include structural changes in the soma of these same neurons.

Effects of α2‐Adrenergic Agonists and Antagonists on Photoreceptor Membrane Currents

Abstract: Type B photoreceptors of the nudibranch mollusc Hermissenda crassicornis receive excitatory synaptic potentials (EPSPs) whose frequency is controlled by potential changes of a neighboring cell known as the S optic ganglion cell which is thought to be electrically coupled to the presyn‐aptic source of these EPSPs, the E optic ganglion cell. The frequency of the EPSPs increases when a conditioned stimulus (light) is paired with an unconditioned stimulus (rotation) during acquisition of a Pavlovian conditioned response. The results of the present study are consistent with an adrenergic origin for these EPSPs. Noradrenergic agonists (≥ 100 μM), norepinephrine and clonidine, only slightly depolarize the type B cell but clearly prolong its depolarizing response to light. Serotonin, by contrast, causes hyperpolarization of the type B cells resting potential as well as after a light step. Clonidine reduces voltage‐dependent outward K+ currents (IA, an early current, Ica2+‐K+, a late Ca2+‐dependent current) that control the type B cells excitability (and thus its light response and membrane potential). These effects of clonidine are reduced or blocked by the α2‐receptor antagonist, yohimbine (0.5 μM), but not the α1‐blocker, prazosin. The same yohimbine concentration also blocked depolarizing synaptic excitation of the type B cell in response to depolarization of a simultaneously impaled S optic ganglion cell. Histochemical techniques (both the glyoxylic acid method of de la Torre and Surgeon and the formaldehyde‐induced fluorescence or Falck‐Hillarp method) demonstrated the presence of a biogenic amine(s) within a single neuron in each optic ganglion as well as three or four cells within the vicinity of previously identified visual interneurons. No serotonergic neurons were found within the optic ganglion or in proximity to visual interneurons. A clonidine‐like synaptic effect on type B cells, therefore, could amplify conditioning‐specific changes of membrane currents by increasing type B depolarization and possibly, as well, by elevating intracellular second messengers.

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

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

Implicating causal relations between cellular function and learning behavior

Learning in the nudibranch mollusc Hermissenda shows many features of vertebrate associative conditioning. Pairings of light and rotation produce conditioned suppression of phototaxis, which is retained for days, shows savings, extinction, contingency sensitivity, and, recently, temporal specificity. In addition, specific features of the behavior have been shown to undergo classical Pavlovian conditioning. Extensive analysis of the neural networks mediating the flow of visual and graviceptive information have demonstrated convergent pathways at specific cellular loci. These cells are critically implicated for a primary role in the conditioned modifications of behavior. A variety of experimental approaches consistently support the proposal that reductions of specific K+ currents in the Type B photoreceptor soma play a causal role for several different behavioral expressions of the conditioning. In this article, we review several of these behaviors to show how the demonstrated close temporal correspondence of cellular and behavioral functions further implicates certain causal relations. For example, studies of the shadow withdrawal behavior of Hermissenda suggest a causal relation between the long-lasting depolarization of the Type B photoreceptor and the animals reduced ability to turn towards the light at light/dark boundaries. Whereas the shadow response corresponded to cellular events at the end of a light step, responses to the onset of light or rotation were largely unexplored. By using a different approach, we identified behavioral responses during the first few seconds of stimulation with light and rotation. These responses, for which Pavlovian conditioning was demonstrated, correspond closely in time to known cellular correlates.(ABSTRACT TRUNCATED AT 250 WORDS)

Sequential changes of potassium currents in Hermissenda Type B photoreceptor during early stages of classical conditioning

Classical conditioning of the marine snail Hermissenda can be produced in a single session of 50 pairings of light and rotation stimuli. Voltage clamp measurements of two outward K+ currents, IA and ICa2(+)-K+ were obtained from medial Type B photoreceptors that were isolated from the nervous system 1 day after animals were exposed to paired light and rotation stimuli or control procedures (Unpaired, or no exposure to light and rotation), ICa2(+)-K+ was found to be unchanged 18-30 h after 50 training trials. This result is consistent with a previous study where ICa2(+)-K+ was found to be unchanged after 50 light and rotation trials, although significantly reduced by 100 trials. In the present study 50 pairings of light and rotation produced a significant reduction in IA, suggesting an important role for this current in the earliest stages of classical conditioning.