Classical conditioning of feeding in Aplysia: II. Neurophysiological correlates

Dopamine as a Multifunctional Neurotransmitter in Gastropod Molluscs: An Evolutionary Hypothesis

AbstractThe catecholamine 3,4-dihydroxyphenethylamine, or dopamine, acts as a neurotransmitter across a broad phylogenetic spectrum. Functions attributed to dopamine in the mammalian brain include regulation of motor circuits, valuation of sensory stimuli, and mediation of reward or reinforcement signals. Considerable evidence also supports a neurotransmitter role for dopamine in gastropod molluscs, and there is growing appreciation for its potential common functions across phylogeny. This article reviews evidence for dopamine's transmitter role in the nervous systems of gastropods. The functional properties of identified dopaminergic neurons in well-characterized neural circuits suggest a hypothetical incremental sequence by which dopamine accumulated its diverse roles. The successive acquisition of dopamine functions is proposed in the context of gastropod feeding behavior: (1) sensation of potential nutrients, (2) activation of motor circuits, (3) selection of motor patterns from multifunctional circuits, (4) valuation of sensory stimuli with reference to internal state, (5) association of motor programs with their outcomes, and (6) coincidence detection between sensory stimuli and their consequences. At each stage of this sequence, it is proposed that existing functions of dopaminergic neurons favored their recruitment to fulfill additional information processing demands. Common functions of dopamine in other intensively studied groups, ranging from mammals and insects to nematodes, suggest an ancient origin for this progression.

Latency to Reward Predicts Social Dominance in Rats: A Causal Role for the Dopaminergic Mesolimbic System

Reward signals encoded in the mesolimbic dopaminergic system guide approach/seeking behaviors to all varieties of life-supporting stimuli (rewards). Differences in dopamine (DA) levels have been found between dominant and submissive animals. However, it is still unclear whether these differences arise as a consequence of the rewarding nature of the acquisition of a dominant rank, or whether they preexist and favor dominance by promoting reward-seeking behavior. Given that acquisition of a social rank determines animals' priority access to resources, we hypothesized that differences in reward-seeking behavior might affect hierarchy establishment and that modulation of the dopaminergic system could affect the outcome of a social competition. We characterized reward-seeking behaviors based on rats' latency to get a palatable-reward when given temporary access to it. Subsequently, rats exhibiting short (SL) and long (LL) latency to get the rewards cohabitated for more than 2 weeks, in order to establish a stable hierarchy. We found that SL animals exhibited dominant behavior consistently in social competition tests [for palatable-rewards and two water competition tests (WCTs)] after hierarchy was established, indicating that individual latency to rewards predicted dominance. Moreover, because SL animals showed higher mesolimbic levels of DA than LL rats, we tested whether stimulation of mesolimbic DA neurons could affect the outcome of a social competition. Indeed, a combination of optical stimulation of mesolimbic DA neurons during individual training and during a social competition test for palatable rewards resulted in improved performance on this test.

A role for dopamine in the peripheral sensory processing of a gastropod mollusc

Histological evidence points to the presence of dopamine (DA) in the cephalic sensory organs of multiple gastropod molluscs, suggesting a possible sensory role for the neurotransmitter. We investigated the sensory function of DA in the nudipleuran Pleurobranchaea californica, in which the central neural correlates of sensation and foraging behavior have been well characterized. Tyrosine hydroxylase-like immunoreactivity (THli), a signature of the dopamine synthetic pathway, was similar to that found in two other opisthobranchs and two pulmonates previously studied: 1) relatively few (<100) THli neuronal somata were observed in the central ganglia, with those observed found in locations similar to those documented in the other snails but varying in number, and 2) the vast majority of THli somata were located in the peripheral nervous system, were associated with ciliated, putative primary sensory cells, and were highly concentrated in chemotactile sensory organs, giving rise to afferent axons projecting to the central nervous system. We extended these findings by observing that applying a selective D₂/D₃ receptor antagonist to the chemo- and mechanosensory oral veil-tentacle complex of behaving animals significantly delayed feeding behavior in response to an appetitive stimulus. A D₁ blocker had no effect. Recordings of the two major cephalic sensory nerves, the tentacle and large oral veil nerves, in a deganglionated head preparation revealed a decrease of stimulus-evoked activity in the former nerve following application of the same D₂/D₃ antagonist. Broadly, our results implicate DA in sensation and engender speculation regarding the foraging-based decisions the neurotransmitter may serve in the nervous system of Pleurobranchaea and, by extension, other gastropods.

Toward a multiscale modeling framework for understanding serotonergic function

Despite its importance in regulating emotion and mental wellbeing, the complex structure and function of the serotonergic system present formidable challenges toward understanding its mechanisms. In this paper, we review studies investigating the interactions between serotonergic and related brain systems and their behavior at multiple scales, with a focus on biologically-based computational modeling. We first discuss serotonergic intracellular signaling and neuronal excitability, followed by neuronal circuit and systems levels. At each level of organization, we will discuss the experimental work accompanied by related computational modeling work. We then suggest that a multiscale modeling approach that integrates the various levels of neurobiological organization could potentially transform the way we understand the complex functions associated with serotonin.

A novel in vitro analog expressing learning-induced cellular correlates in distinct neural circuits

When presented with noxious stimuli, Aplysia exhibits concurrent sensitization of defensive responses, such as the tail-induced siphon withdrawal reflex (TSWR) and suppression of feeding. At the cellular level, sensitization of the TSWR is accompanied by an increase in the excitability of the tail sensory neurons (TSNs) that elicit the reflex, whereas feeding suppression is accompanied by decreased excitability of B51, a decision-making neuron in the feeding neural circuit. The goal of this study was to develop an in vitro analog coexpressing the above cellular correlates. We used a reduced preparation consisting of buccal, cerebral, and pleural-pedal ganglia, which contain the neural circuits controlling feeding and the TSWR, respectively. Sensitizing stimuli were delivered in vitro by electrical stimulation of afferent nerves. When trained with sensitizing stimuli, the in vitro analog expressed concomitant increased excitability in TSNs and decreased excitability in B51, which are consistent with the occurrence of sensitization and feeding suppression induced by in vivo training. This in vitro analog expressed both short-term (15 min) and long-term (24 h) excitability changes in TSNs and B51, depending on the amount of training administered. Finally, in vitro application of serotonin increased TSN excitability without altering B51 excitability, mirroring the in vivo application of the monoamine that induces sensitization, but not feeding suppression.

pT305-CaMKII stabilizes a learning-induced increase in AMPA receptors for ongoing memory consolidation after classical conditioning

The role of CaMKII in learning-induced activation and trafficking of AMPA receptors (AMPARs) is well established. However, the link between the phosphorylation state of CaMKII and the agonist-triggered proteasomal degradation of AMPARs during memory consolidation remains unknown. Here we describe a novel CaMKII-dependent mechanism by which a learning-induced increase in AMPAR levels is stabilized for consolidation of associative long-term memory. Six hours after classical conditioning the levels of both autophosphorylated pT305-CaMKII and GluA1 type AMPAR subunits are significantly elevated in the ganglia containing the learning circuits of the snail Lymnaea stagnalis. CaMKIINtide treatment significantly reduces the learning-induced elevation of both pT305-CaMKII and GluA1 levels and impairs associative long-term memory. Inhibition of proteasomal activity offsets the deleterious effects of CaMKIINtide on both GluA1 levels and long-term memory. These findings suggest that increased levels of pT305-CaMKII play a role in AMPAR dependent memory consolidation by reducing proteasomal degradation of GluA1 receptor subunits.

PKG-mediated MAPK signaling is necessary for long-term operant memory in Aplysia

Signaling pathways necessary for memory formation, such as the mitogen-activated protein kinase (MAPK) pathway, appear highly conserved across species and paradigms. Learning that food is inedible (LFI) represents a robust form of associative, operant learning that induces short- (STM) and long-term memory (LTM) in Aplysia. We investigated the role of MAPK signaling in LFI memory in vivo. Inhibition of MAPK activation in animals prior to training blocked STM and LTM. Discontinuing MAPK signaling immediately after training inhibited LTM with no impact on STM. Therefore, MAPK signaling appears necessary early in memory formation for STM and LTM, with prolonged MAPK activity required for LTM. We found that LFI training significantly increased phospho-MAPK levels in the buccal ganglia. Increased MAPK activation was apparent immediately after training with greater than basal levels persisting for 2 h. We examined the mechanisms underlying training-induced MAPK activation and found that PKG activity was necessary for the prolonged phase of MAPK activation, but not for the early MAPK phase required for STM. Furthermore, we found that neither the immediate nor the prolonged phase of MAPK activation was dependent upon nitric oxide (NO) signaling, although expression of memory was dependent on NO as previously reported. These studies emphasize the role of MAPK and PKG in negatively reinforced operant memory and demonstrate a role for PKG-dependent MAPK signaling in invertebrate associative memory.

Octopamine promotes rhythmicity but not synchrony in a bilateral pair of bursting motor neurons in the feeding circuit of Aplysia

Octopamine-like immunoreactivity was localized to a limited number (<40) of neurons in the Aplysia central nervous system, including three neurons in the paired buccal ganglia (BG) that control feeding movements. Application of octopamine (OA) to the BG circuit produced concentration-dependent (10(-8)-10(-4) mol l(-1)) modulatory actions on the spontaneous burst activity of the bilaterally paired B67 pharyngeal motor neurons (MNs). OA increased B67's burst duration and the number of impulses per burst. These effects reflected actions of OA on the intrinsic tetrodotoxin-resistant driver potential (DP) that underlies B67 bursting. In addition to its effects on B67's burst parameters, OA also increased the rate and regularity of burst timing. Although the bilaterally paired B67 MNs both exhibited rhythmic bursting in the presence of OA, they did not become synchronized. In this respect, the response to OA differed from that of dopamine, another modulator of the feeding motor network, which produces both rhythmicity and synchrony of bursting in the paired B67 neurons. It is proposed that modulators can regulate burst synchrony of MNs by exerting a dual control over their intrinsic rhythmicity and their reciprocal capacity to generate membrane potential perturbations. In this simple system, dopaminergic and octopaminergic modulation could influence whether pharyngeal contractions occur in a bilaterally synchronous or asynchronous fashion.

Endogenous motor neuron properties contribute to a program-specific phase of activity in the multifunctional feeding central pattern generator of Aplysia

Multifunctional central pattern generators (CPGs) are circuits of neurons that can generate manifold actions from a single effector system. This study examined a bilateral pair of pharyngeal motor neurons, designated B67, that participate in the multifunctional feeding network of Aplysia californica. Fictive buccal motor programs (BMPs) were elicited with four distinct stimulus paradigms to assess the activity of B67 during ingestive versus egestive patterns. In both classes of programs, B67 fired during the phase of radula protraction and received a potent inhibitory postsynaptic potential (IPSP) during fictive radula retraction. When programs were ingestive, the retraction phase IPSP exhibited a depolarizing sag and was followed by a postinhibitory rebound (PIR) that could generate a postretraction phase of impulse activity. When programs were egestive, the depolarizing sag potential and PIR were both diminished or were not present. Examination of the membrane properties of B67 disclosed a cesium-sensitive depolarizing sag, a corresponding I(h)-like current, and PIR in its responses to hyperpolarizing pulses. Direct IPSPs originating from the influential CPG retraction phase interneuron B64 were also found to activate the sag potential and PIR of B67. Dopamine, a modulator that can promote ingestive behavior in this system, enhanced the sag potential, I(h)-like current, and PIR of B67. Finally, a pharyngeal muscle contraction followed the radula retraction phase of ingestive, but not egestive motor patterns. It is proposed that regulation of the intrinsic properties of this motor neuron can contribute to generating a program-specific phase of motor activity.

Feeding behavior of Aplysia: a model system for comparing cellular mechanisms of classical and operant conditioning

Feeding behavior of Aplysia provides an excellent model system for analyzing and comparing mechanisms underlying appetitive classical conditioning and reward operant conditioning. Behavioral protocols have been developed for both forms of associative learning, both of which increase the occurrence of biting following training. Because the neural circuitry that mediates the behavior is well characterized and amenable to detailed cellular analyses, substantial progress has been made toward a comparative analysis of the cellular mechanisms underlying these two forms of associative learning. Both forms of associative learning use the same reinforcement pathway (the esophageal nerve, En) and the same reinforcement transmitter (dopamine, DA). In addition, at least one cellular locus of plasticity (cell B51) is modified by both forms of associative learning. However, the two forms of associative learning have opposite effects on B51. Classical conditioning decreases the excitability of B51, whereas operant conditioning increases the excitability of B51. Thus, the approach of using two forms of associative learning to modify a single behavior, which is mediated by an analytically tractable neural circuit, is revealing similarities and differences in the mechanisms that underlie classical and operant conditioning.

Target-specific regulation of synaptic efficacy in the feeding central pattern generator of Aplysia: potential substrates for behavioral plasticity?

The contributions to this symposium are unified by their focus on the role of synaptic plasticity in sensorimotor learning. Synaptic plasticities are also known to operate within the central pattern generator (CPG) circuits that produce repetitive motor programs, where their relation to adaptive behavior is less well understood. This study examined divergent synaptic plasticity in the signaling of an influential interneuron, B20, located within the CPG that controls consummatory feeding-related behaviors in Aplysia. Previously, B20 was shown to contain markers for catecholamines and GABA (Díaz-Ríos et al., 2002), and its rapid synaptic signaling to two follower motor neurons, B16 and B8, was found to be mediated by dopamine (Díaz-Ríos and Miller, 2005). In this investigation, two incremental forms of increased synaptic efficacy, facilitation and summation, were both greater in the signaling from B20 to B8 than in the signaling from B20 to B16. Manipulation of the membrane potentials of the two postsynaptic motor neurons did not affect facilitation of excitatory postsynaptic potentials (EPSPs) to either follower cell. Striking levels of summation in B8, however, were eliminated at hyperpolarized membrane potentials and could be attributed to distinctive membrane properties of this postsynaptic cell. GABA and the GABAB agonist baclofen increased facilitation and summation of EPSPs from B20 to B8, but not to B16. The enhanced facilitation was not affected when the membrane potential of B8 was pre-set to hyperpolarized levels, but GABAergic effects on summation were eliminated by this manipulation. These observations demonstrate a target-specific amplification of synaptic efficacy that can contribute to channeling the flow of divergent information from an intrinsic interneuron within the buccal CPG. They further suggest that GABA, acting as a cotransmitter in B20, could induce coordinated and target-specific pre- and postsynaptic modulation of these signals. Finally, we speculate that target-specific plasticity and its modulation could be efficient, specific, and flexible substrates for learning-related modifications of CPG function.

Operant conditioning of an in vitro CNS-pneumostome preparation of Lymnaea

Operant conditioning of aerial respiratory behaviour and its consolidation into long-term memory in Lymnaea has been previously studied in both intact, freely moving snails and in in vitro preparations made from previously trained snails. Here, we show in previously untrained semi-intact in vitro Lymnaea preparations that aerial respiratory behaviour can also be operantly conditioned. Neither yoked control nor 'run-down' control procedures in these in vitro preparations result in an alteration of aerial respiratory behaviour. Memory in the operantly trained semi-intact preparations persists for at least 1h after training. Intracellular recordings made from RPeD1, one of the 3-CPG neurons and the neuron that initiates CPG activity; show that there are specific changes in central excitatory input to this neuron concurrent with learning and its consolidation into memory. In addition following the acquisition of learning and its consolidation into memory the ability of RPeD1 and VI/J neurons when depolarized to cause a pneumostome opening is significantly decreased. Thus, previously untrained in vitro semi-intact preparations can be used to study changes in neuronal activity in a neuron known to be both necessary for the behaviour and for memory formation.

Reinforcement in an in vitro analog of appetitive classical conditioning of feeding behavior in Aplysia: blockade by a dopamine antagonist

In a recently developed in vitro analog of appetitive classical conditioning of feeding in Aplysia, the unconditioned stimulus (US) was electrical stimulation of the esophageal nerve (En). This nerve is rich in dopamine (DA)-containing processes, which suggests that DA mediates reinforcement during appetitive conditioning. To test this possibility, methylergonovine was used to antagonize DA receptors. Methylergonovine (1 nM) blocked the pairing-specific increase in fictive feeding that is usually induced by in vitro classical conditioning. The present results and previous observation that methylergonovine also blocks the effects of contingent reinforcement in an in vitro analog of appetitive operant conditioning suggest that DA mediates reinforcement for appetitive associative conditioning of feeding in Aplysia.

Rapid Dopaminergic Signaling by Interneurons That Contain Markers for Catecholamines and GABA in the Feeding Circuitry of Aplysia

Consummatory feeding behaviors in Aplysia californica are controlled by a polymorphic central pattern generator (CPG) circuit. Previous investigations have demonstrated colocalization of markers for GABA and catecholamines within two interneurons, B20 and B65, that participate in configuring the functional output of this CPG. This study examined the contributions of GABA and dopamine (DA) to rapid synaptic signaling from B20 and B65 to follower cells that implement their specification of motor programs. Pharmacological tests did not substantiate the participation of GABA in the mediation of the excitatory postsynaptic potentials (EPSPs) from either B20 or B65. However, GABA and the GABAB receptor agonist baclofen were found to modify these signals in a target-specific manner. Several observations indicated that DA acts as the neurotransmitter mediating fast EPSPs from B20 to two radula closer motor neurons B8 and B16. In both motor neurons, application of DA produced depolarizing responses associated with decreased input resistance and increased excitation. B20-evoked EPSPs in both follower cells were occluded by exogenous dopamine and blocked by the DA antagonist sulpiride. While dopamine occlusion and sulpiride block of convergent signaling to B8 from B65 resembled that of B20, both of these actions were less potent on the rapid signaling from B65 to the multifunctional and widely acting interneuron B4/5. These findings indicate that dopamine mediates divergent (B20 to B16 and B8) and convergent (B20 and B65 to B8) rapid EPSPs from two influential CPG interneurons in which it is colocalized with GABA-like immunoreactivity.

Extending in vitro conditioning in Aplysia to analyze operant and classical processes in the same preparation

Operant and classical conditioning are major processes shaping behavioral responses in all animals. Although the understanding of the mechanisms of classical conditioning has expanded significantly, the understanding of the mechanisms of operant conditioning is more limited. Recent developments in Aplysia are helping to narrow the gap in the level of understanding between operant and classical conditioning, and have raised the possibility of studying the neuronal processes underlying the interaction of operant and classical components in a relatively complex learning task. In the present study, we describe a first step toward realizing this goal, by developing a single in vitro preparation in which both operant and classical conditioning can be studied concurrently. The new paradigm reproduced previously published results, even under more conservative and homogenous selection criteria and tonic stimulation regime. Moreover, the observed learning was resistant to delay, shortening, and signaling of reinforcement.

In vitro analog of classical conditioning of feeding behavior in aplysia

The feeding behavior of Aplysia californica can be classically conditioned using tactile stimulation of the lips as a conditioned stimulus (CS) and food as an unconditioned stimulus (US). Moreover, several neural correlates of classical conditioning have been identified. The present study extended previous work by developing an in vitro analog of classical conditioning and by investigating pairing-specific changes in neuronal and synaptic properties. The preparation consisted of the isolated cerebral and buccal ganglia. Electrical stimulation of a lip nerve (AT4) and a branch of the esophageal nerve (En2) served as the CS and US, respectively. Three protocols were used: paired, unpaired, and US alone. Only the paired protocol produced a significant increase in CS-evoked fictive feeding. At the cellular level, classical conditioning enhanced the magnitude of the CS-evoked synaptic input to pattern-initiating neuron B31/32. In addition, paired training enhanced both the magnitude of the CS-evoked synaptic input and the CS-evoked spike activity in command-like neuron CBI-2. The in vitro analog of classical conditioning reproduced all of the cellular changes that previously were identified following behavioral conditioning and has led to the identification of several new learning-related neural changes. In addition, the pairing-specific enhancement of the CS response in CBI-2 indicates that some aspects of associative plasticity may occur at the level of the cerebral sensory neurons.

Multiple serotonergic mechanisms contributing to sensitization in aplysia: evidence of diverse serotonin receptor subtypes

The neurotransmitter serotonin (5-HT) plays an important role in memory encoding in Aplysia. Early evidence showed that during sensitization, 5-HT activates a cyclic AMP-protein kinase A (cAMP-PKA)-dependent pathway within specific sensory neurons (SNs), which increases their excitability and facilitates synaptic transmission onto their follower motor neurons (MNs). However, recent data suggest that serotonergic modulation during sensitization is more complex and diverse. The neuronal circuits mediating defensive reflexes contain a number of interneurons that respond to 5-HT in ways opposite to those of the SNs, showing a decrease in excitability and/or synaptic depression. Moreover, in addition to acting through a cAMP-PKA pathway within SNs, 5-HT is also capable of activating a variety of other protein kinases such as protein kinase C, extracellular signal-regulated kinases, and tyrosine kinases. This diversity of 5-HT responses during sensitization suggests the presence of multiple 5-HT receptor subtypes within the Aplysia central nervous system. Four 5-HT receptors have been cloned and characterized to date. Although several others probably remain to be characterized in molecular terms, especially the Gs-coupled 5-HT receptor capable of activating cAMP-PKA pathways, the multiplicity of serotonergic mechanisms recruited into action during learning in Aplysia can now be addressed from a molecular point of view.

Operant conditioning in invertebrates

Learning to anticipate future events on the basis of past experience with the consequences of one's own behavior (operant conditioning) is a simple form of learning that humans share with most other animals, including invertebrates. Three model organisms have recently made significant contributions towards a mechanistic model of operant conditioning, because of their special technical advantages. Research using the fruit fly Drosophila melanogaster implicated the ignorant gene in operant conditioning in the heat-box, research on the sea slug Aplysia californica contributed a cellular mechanism of behavior selection at a convergence point of operant behavior and reward, and research on the pond snail Lymnaea stagnalis elucidated the role of a behavior-initiating neuron in operant conditioning. These insights demonstrate the usefulness of a variety of invertebrate model systems to complement and stimulate research in vertebrates.

Changes in the activity of a CpG neuron after the reinforcement of an operantly conditioned behavior in Lymnaea

We have previously shown that the aerial respiratory behavior of the mollusk Lymnaea stagnalis can be operantly conditioned, and the central pattern generating (CPG) neurons underlying this behavior have been identified. As neural correlates of operant conditioning remain poorly defined in both vertebrates and invertebrates, we have used the Lymnaea respiratory CPG to investigate neuronal changes associated with the change in behavior after conditioning. After operant conditioning of the intact animals, semi-intact preparations were dissected, so that changes in the respiratory behavior (pneumostome openings) and underlying activity of the identified CPG neuron, right pedal dorsal 1 (RPeD1), could be monitored simultaneously. RPeD1 was studied because it initiates the rhythmic activity of the CPG and receives chemo-sensory input from the pneumostome area. Pneumostome openings and RPeD1 activity were monitored both before and after a reinforcing training stimulus applied to the open pneumostome of operantly conditioned and yoked control preparations. After presentation of the reinforcing stimulus, there was a significant reduction in both breathing behavior and RPeD1 activity in operant preparations but not in yoked and naïve controls. Furthermore these changes were only significant in the subgroup of operantly conditioned animals described as good learners and not in poor learners. These data strongly suggest that changes in RPeD1 activity may underlie the behavioral changes associated with the reinforcement of operant conditioning of the respiratory behavior.

Operant reward learning in Aplysia: neuronal correlates and mechanisms

Operant conditioning is a form of associative learning through which an animal learns about the consequences of its behavior. Here, we report an appetitive operant conditioning procedure in Aplysia that induces long-term memory. Biophysical changes that accompanied the memory were found in an identified neuron (cell B51) that is considered critical for the expression of behavior that was rewarded. Similar cellular changes in B51 were produced by contingent reinforcement of B51 with dopamine in a single-cell analog of the operant procedure. These findings allow for the detailed analysis of the cellular and molecular processes underlying operant conditioning.

Improved electrochemical detection of biogenic amines in Aplysia using base-hydrolyzed cellulose-coated carbon fiber microelectrodes

A major challenge with electrochemical techniques in vivo, using carbon-fiber microelectrodes, is to achieve sufficient sensitivity to detect the low concentrations of transmitters released by neurons. In particular, when an electrode is inserted into living tissue, its sensitivity is usually substantially decreased as a result of the degradation of the carbon surface by proteins. Here, we show that this decrease can be significantly attenuated by coating the electrode with cellulose acetate. The cellulose film offers a steric barrier that prevents macromolecules from diffusing to the carbon surface and its porosity can be progressively increased by controlled hydrolysis. We compared different cellulose-coated electrodes, either non-hydrolysed or hydrolyzed, in 0.08 N KOH for 10-30 min. We found that dopamine and serotonin detection was blocked by non-hydrolysed cellulose films, but that hydrolysis restored optimal detection similar to uncoated electrodes. Moreover, cellulose films (hydrolyzed for 20 min) significantly diminished electrode degradation in vivo and allowed reliable detection of fast concentration changes with <0.5 s delay, compared to uncoated electrodes. Finally, the sensitivity to endogenous 5-HT release in Aplysia central nervous system was more than doubled with these electrodes. We conclude that the optimal hydrolysis time of cellulose-coated electrodes is approximately 20 min with our protocol and carbon fiber electrodes prepared with this method offer improved sensitivity for the detection of biogenic amines.

Colocalization of gamma-aminobutyric acid-like immunoreactivity and catecholamines in the feeding network of Aplysia californica

Functional consequences of neurotransmitter coexistence and cotransmission can be readily studied in certain experimentally favorable invertebrate motor systems. In this study, whole-mount histochemical methods were used to identify neurons in which gamma-aminobutyric acid (GABA)-like immunoreactivity (GABAli) was colocalized with catecholamine histofluorescence (CAh; FaGlu method) and tyrosine hydroxylase (TH)-like immunoreactivity (THli) in the feeding motor circuitry (buccal and cerebral ganglia) of the marine mollusc Aplysia californica. In agreement with previous reports, five neurons in the buccal ganglia were found to exhibit CAh. These included the paired B20 buccal-cerebral interneurons (BCIs), the paired B65 buccal interneurons, and an unpaired cell with projections to both cerebral-buccal connectives (CBCs). Experiments in which the FaGlu method was combined with the immunohistochemical detection of GABA revealed double labeling of all five of these neurons. An antibody generated against TH, the rate-limiting enzyme in the biosynthesis of catecholamines, was used to obtain an independent determination of GABA-CA colocalization. Biocytin backfills of the CBC performed in conjunction with TH immunohistochemistry revealed labeling of the rostral B20 cell pair and the unpaired CBI near the caudal surface of the right hemiganglion. THli was also present in a prominent bilateral pair of caudal neurons that were not stained with CBC backfills. On the basis of their position, size, shape, and lack of CBC projections, the lateral THli neurons were identified as B65. Double-labeling immunohistochemical experiments revealed GABAli in all five buccal THli neurons. Finally, GABAli was observed in individual B20 and B65 neurons that were identified using electrophysiological criteria and injected with a marker (neurobiotin). Similar methods were used to demonstrate that a previously identified catecholaminergic cerebral-buccal interneuron (CBI) designated CBI-1 contained THli but did not contain GABAli. Although numerous THli and GABAli neurons and fibers were present in the cerebral and buccal ganglia, additional instances of their colocalization were not observed. These findings indicate that GABA and a catecholamine (probably dopamine) are colocalized in a limited number of interneurons within the central pattern generator circuits that control feeding-related behaviors in Aplysia.

Voltage-gated ionic currents in an identified modulatory cell type controlling molluscan feeding

An important modulatory cell type, found in all molluscan feeding networks, was investigated using two-electrode voltage- and current-clamp methods. In the cerebral giant cells of Lymnaea, a transient inward Na+ current was identified with activation at -58 +/- 2 mV. It was sensitive to tetrodotoxin only in high concentrations (approximately 50% block at 100 microm), a characteristic of Na+ channels in many molluscan neurons. A much smaller low-threshold persistent Na+ current (activation at < -90 mV) was also identified. Two purely voltage-sensitive outward K+ currents were also found: (i) a transient A-current type which was activated at -59 +/- 4 mV and blocked by 4-aminopyridine; (ii) a sustained tetraethylammonium-sensitive delayed rectifier current which was activated at -47 +/- 2 mV. There was also evidence that a third, Ca2+-activated, K+ channel made a contribution to the total outward current. No inwardly rectifying currents were found. Two Ca2+ currents were characterized: (i) a transient low-voltage (-65 +/- 2 mV) activated T-type current, which was blocked in NiCl2 (2 mm) and was completely inactivated at approximately -50 mV; (ii) A sustained high voltage (-40 +/- 1 mV) activated current, which was blocked in CdCl2 (100 microm) but not in omega-conotoxin GVIA (10 microm), omega-agatoxin IVA (500 nm) or nifedipine (10 microm). This current was enhanced in Ba2+ saline. Current-clamp experiments revealed how these different current types could define the membrane potential and firing properties of the cerebral giant cells, which are important in shaping the wide-acting modulatory influence of this neuron on the rest of the feeding network.

Could the 5-HT1B receptor inverse agonism affect learning consolidation?

Diverse evidence indicates that, the 5-HT system might play a role in learning and memory, since it occurs in brain areas mediating such processes and 5-HT drugs modulate them. Hence in this work, in order to explore further 5-HT involvement on learning and memory 5-HT1B receptors' role is investigated. Evidence indicates that SB-224289 (a 5-HT1B receptor inverse agonist) post-training injection facilitated learning consolidation in an associative autoshaping learning task, this effect was partially reversed by GR 127935 (a 5-HT1B/1D receptor antagonist), but unaffected by MDL 100907 (a 5-HT2A receptor antagonist) or ketanserin (a 5-HT1D/2A/7 receptor antagonist) at low doses. Moreover, SB-224289 antagonized the learning deficit produced by TFMPP (a 5-HT1A/1B/1D/2A/2C receptor agonist), GR 46611 (a 5-HT1A/1B/1D receptor agonist), mCPP (a 5-HT2A/2C/3/7 receptor agonist/antagonist) or GR 127935 (at low dose). SB-224289 did not alter the 8-OH-DPAT (a 5-HT1A/7 receptor agonist) learning facilitatory effect. SB-224289 eliminated the deficit learning produced by the anticholinergic muscarinic scopolamine or the glutamatergic antagonist dizocilpine. Administration of both, GR 127935 (5mg/kg) plus ketanserin (0.01 mg/kg) did not modify learning consolidation; nevertheless, when ketanserin dose was increased (0.1-1.0mg/kg) and SB-224289 dose was maintained constant, a learning facilitation effect was observed. Notably, SB-224289 at 1.0mg/kg potentiated a subeffective dose of the 5-HT1B/1D receptor agonist/antagonist mixed GR 127935, which facilitated learning consolidation and this effect was abolished by ketanserin at a higher dose. Collectively, the data confirm and extend the earlier findings with GR 127935 and the effects of non-selective 5-HT(1B) receptor agonists. Clearly 5-HT1B agonists induced a learning deficit which can be reversed with SB-224289. Perhaps more importantly, SB-224289 enhances learning consolidation when given alone and can reverse the deficits induced by both cholinergic and glutamatergic antagonist. Hence, 5-HT1B receptor inverse agonists or antagonists could represent drugs for the treatment of learning and memory dysfunctions.

Selective expression of electrical correlates of differential appetitive classical conditioning in a feeding network

Electrical correlates of differential appetitive classical conditioning were recorded in the neural network that underlies feeding in the snail Lymnaea stagnalis. In spaced training (15 trials over 3 days), the lips and the tentacle were used as CS+ (reinforced conditioned stimulus) or CS- (nonreinforced conditioned stimulus) sites for behavioral tactile conditioning. In one group of experimental animals, touch to the lips (the CS+ site) was followed by sucrose (the unconditioned stimulus, US), but touch to the tentacle (the CS- site) was not reinforced. In a second experimental group the CS+/CS- sites were reversed. Semi-intact lip-tentacle-CNS preparations were made from both experimental groups and a naive control group. Intracellular recordings were made from the B3 motor neuron of the feeding network, which allowed the monitoring of activity in the feeding central pattern generator (CPG) interneurons as well as early synaptic inputs evoked by the touch stimulus. Following successful behavioral conditioning, the touch stimulus evoked CPG-driven fictive feeding activity at the CS+ but not the CS- sites in both experimental groups. Naive snails/preparations showed no touch responses. A weak asymmetrical stimulus generalization of conditioned feeding was not retained at the electrophysiological level. An early excitatory postsynaptic potential (EPSP) response to touch was only enhanced following conditioning in the Lip CS+/tentacle CS- group but not in the Tentacle CS+/lip CS- group. The results show that the main features of differential appetitive classical conditioning can be recorded at the electrophysiological level, but some characteristics of the conditioned response are selectively expressed in the reduced preparation.

Classical conditioning of feeding in Aplysia: I. Behavioral analysis

A training protocol was developed to classically condition feeding behavior in Aplysia californica using tactile stimulation of the lips as the conditional stimulus (CS) and food as the unconditional stimulus (US). Paired training induced a greater increase in the number of bites to the CS than unpaired training or US-only stimulation. Memory for classical conditioning was retained for at least 24 hr. The organization of the reinforcement pathway that supports classical conditioning was analyzed in additional behavioral experiments. No evidence was found for the contribution to appetitive reinforcement of US-mediating pathways originating in the lips of the animals. Bilateral lesions of the anterior branch of the esophageal nerve, which innervates parts of the foregut, however, were found to attenuate classical conditioning. Thus, it appears likely that reinforcement during appetitive classical conditioning of feeding was mediated by afferent pathways that originate in the foregut. The companion paper () describes two neurophysiological correlates of the classical conditioning.

A systems approach to the cellular analysis of associative learning in the pond snail Lymnaea

We show that appetitive and aversive conditioning can be analyzed at the cellular level in the well-described neural circuitries underlying rhythmic feeding and respiration in the pond snail, Lymnaea stagnalis. To relate electrical changes directly to behavior, the snails were first trained and the neural changes recorded at multiple sites in reduced preparations made from the same animals. Changes in neural activity following conditioning could be recorded at the level of motoneurons, central pattern generator interneurons and modulatory neurons. Of significant interest was recent work showing that neural correlates of long-term memory could be recorded in the feeding network following single-trial appetitive chemical conditioning. Available information on the synaptic connectivity and transmitter content of identified neurons within the Lymnaea circuits will allow further work on the synaptic and molecular mechanisms of learning and memory.