Ronald F. Rogers

Trial spacing effects in Hermissenda suggest contributions of associative and nonassociative cellular mechanisms.

In behaving Hermissenda, a preparatory conditioned response developed across repeated pairings of light (conditioned stimulus; CS) and rotation (unconditioned stimulus; US) with intertrial intervals (ITIs) of 60 and 120 s, but not 30 s. Likewise, contiguous in vitro stimulation of the visual and vestibular receptors, an analog of behavioral conditioning, resulted in an increase in the input resistance (i.e., excitability, a correlate of conditioning) of the B photoreceptors of the Hermissendas eye, but only with ITIs greater than 60 s. Calcium signaling in the B cell, critical to the induction of this neuronal plasticity, was attenuated with shorter ITIs owing to (a) a reduction of the light-induced generator potential and hence voltage-dependent Ca2+ influx during the light CS, (b) a depression of the Ca2+ current that persisted throughout shorter ITIs, and (c) a steady-state inactivation of the Ca2+ current as a result of a sustained depolarization persisting from the previous trial. These results are consistent with a 2-process theory of associative learning in which a primary process (Ca2+ influx) may be opposed by a secondary process (depression of the Ca2+ current) during short ITIs.

Ubiquitous Molecular Substrates for Associative Learning and Activity-Dependent Neuronal Facilitation

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

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 cells 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.

Novel factors contributing to the expression of latent inhibition.

Behavioral and neural correlates of latent inhibition (LI) during eyeblink conditioning were studied in 2 experiments. In Experiment 1, rabbits (Oryctolagus cuniculus) were conditioned after 8 days of tone conditioned stimulus (CS) presentations or 8 days of context-alone experience. LI was seen in the CS-preexposed rabbits when a relatively intense (5 psi) airpuff unconditioned stimulus was paired with the CS. In Experiment 2, rabbits were given 0, 4, or 8 days of CS preexposures or context-alone experience. Hippocampal activity was monitored from the 8-day CS- or context-exposure rabbits. The LI effect was seen only in rabbits given 4 days of CS preexposure, thus suggesting that LI depended largely on the rate of acquisition in the context-preexposed control group. The neural recordings showed that the hippocampus was sensitive to the relative novelty of the stimuli and the overall context, regardless of whether exposure to stimuli and context promoted LI.

Contextually Based Conditional Discrimination of the Rabbit Eyeblink Response

Rabbits received conditional discrimination training using contextual stimuli to set the occasion for stimulus pairings during eyelid conditioning. Specifically, animals were exposed to either the presence or the absence of an oscillating chamber light throughout the intertrial interval (50 +/- 10 s). For half the animals, this light signaled paired presentations of a discrete tone conditioned stimulus (CS) and air puff unconditioned stimulus (US) while darkness signaled presentations of only the tone CS. The remaining animals experienced the opposite contextual relationship to the conditioning stimuli. These trial types occurred pseudo-randomly across a session, with all transitions between contextual settings (i.e., light or dark) taking place immediately at the CS-US offset. Under these conditions, animals successfully utilized the contextual stimuli as conditional cues for differential responding to the shared CS. Moreover, both light and dark were equally effective as discriminative stimuli. A subset of animals received further training in which the contextual contingency was removed by restricting all conditioning to the CS-alone context. Without the contingency in place, subsequent CS presentations (paired and CS-alone) evoked equivalent conditioned responding across three sessions of training. Following the reinstatement of the contextual contingencies, discriminatory responding was immediately observed and returned to previous levels within three sessions. Finally, animals appeared to use the static representation of the conditional cue, rather than the phasic transition between cues, for discriminatory responding. These findings are discussed in terms of current neurobiological models of eyelid conditioning.

G-protein mediated responses to localized serotonin application in an invertebrate photoreceptor.

A G-protein dependent serotonin (5-HT) receptor on B photoreceptors of Hermissenda crassicornis was investigated. Microapplication of 5-HT to the soma region, but not to the terminal branches, resulted in a rapid, biphasic depolarization with a slow time course of dissipation. The 5-HT-induced depolarization increased at hyperpolarized potentials, and exhibited a strong and complex sensitivity to external K+, but not Na+ or Ca2+. The 5-HT response, but not a cholinergically mediated intraphotoreceptor interaction, was abolished by intracellular injection of the G protein antagonist, GDP[beta S], although the response was unaffected by pretreatment with pertussis toxin. These results are discussed in terms of known 5-HT receptor subtypes, and the potential role of this receptor in activity-dependent forms of plasticity exhibited by these cells.

Chemosensory-based contextual conditioning in Hermissenda crassicornis

In two experiments, the marine molluskHermissenda crassicornis was exposed to context discrimination training. In one context, defined by the presence of a diffuse chemosensory stimulus (shellfish extract A), brief, unsignaled, unconditioned stimuli (USs; high-speed rotation) were presented; in a second context, defined by the presence of shellfish extract B, no USs were presented. Animals were then tested (at both 1.5 and 24 h) by exposing them to small pieces of the shellfish meat used to define the two contexts. The latency to strike at the meat served as an index of the context-US association. In Experiment 1, the latency to strike at the cue associated with rotation was reduced relative to both preconditioning strike latencies and the associatively neutral cue. However, in a two-choice test where the animals could approach the conditioned or neutral stimulus, the animals regularly avoided the stimulus paired with rotation. Moreover, if, following conditioning, the animals were presented with an unsignaled rotation in the conditioned context or the neutral context, the animals exhibited more effective defensive clinging (an unconditioned reflex normally elicited by rotation) in the conditioned context, suggesting that it “prepared” the animal for the aversive US. In total, these results demonstrate thatHermissenda is capable of making associations to diffuse background (contextual) stimuli. Moreover, the results suggest that pairing the chemosensory cue with an aversive US elicits a strike response inHermissenda when the animal is placed in forced contact with the cue and an active avoidance response when the animal can choose between that cue and a neutral cue.

Bidirectional regulation of neuronal potassium currents by the G-protein activator aluminum fluoride as a function of intracellular calcium concentration

Hydrolysis-resistant activation of G-proteins by extracellular perfusion of fluoride ions was examined in Type B cells isolated from the cerebral ganglion of the marine mollusc Hermissenda. Under single-electrode voltage-clamp, modulation by aluminum fluoride ions of several classes of outward K+ currents as well as an inward Ca2+ current was observed. Following injection of the Ca2+ chelator EGTA, aluminum fluoride ions selectively increased a slow, voltage-dependent K+ current (IK) within 5 min of application, while in the absence of EGTA, aluminum fluoride ions induced a small, transient reduction of IK. Neither the magnitude nor steady-state inactivation of a fast, voltage-dependent K+ current (IA), nor a slow, Ca2+-dependent K+ current (IK-Ca), were affected by aluminum fluoride ions. In contrast, when perfusion of aluminum fluoride ions was accompanied by a repetitive depolarization and a concomitant increase in intracellular Ca2+, both IA and the combined late currents (IK and IK-Ca) were markedly reduced, a reduction which was not observed following depolarization alone or if the pairing of aluminum fluoride ions and depolarization was preceded by an injection of EGTA. The reduction of membrane conductance by the pairing of aluminum fluoride ions with depolarization could not be accounted for by an increased Ca2+ conductance, as aluminum fluoride ions produced only a small decrease in the voltage-dependent Ca2+ current. In total, these results indicate that regulatory G-proteins may bidirectionally modulate neuronal K+ currents, the direction of which is dependent on intracellular Ca2+ concentration. Such a dual regulatory mechanism may contribute to the modulation of membrane excitability observed when presynaptic activity is paired with postsynaptic depolarization, and thus may contribute to some forms of activity-dependent plasticity involving metabatropic receptors.