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.The marine mollusk Aplysia has proven to be a powerful model system for the study of learning and memory. This animal displays several simple forms of nonassociative learning, such as habituation, dishabituation, and sensitization (Pinsker et al. 1970(Pinsker et al. , 1973Carew et al. 1971), but also more complex forms of associative learning such as classical, operant, and fear conditioning (Carew et al. , 1983Walters et al. 1981; Lechner et al. 2000a,b;Brembs et al. 2002). The strength of this model system arises from the relative simplicity of its central nervous system (CNS), which contains a small number of neurons, some of which are well characterized both morphologically and electrophysiologically. Thus, one can study a specific synaptic connection in different animals subjected to a wide variety of behavioral training protocols. For this reason, it has been possible to discover many of the mechanisms of learning and memory in this animal at the behavioral, cellular, and molecular levels, and provide direct evidence that certain forms of learning rely on the plasticity of individual synaptic connections (for review, see Kandel 2001).One of the best characterized forms of learning in Aplysia is sensitization, in which a noxious stimulus facilitates an animal's pre-existing response to the presentation of another innocuous stimulus. It has been most thoroughly studied in the defensive reflex responses of Aplysia. For example, a mild tactile stimulus applied to the tail evokes the retraction of respiratory organs (gill and siphon) inside the mantle cavity situated on the back of the animal. The stre...

Serotonin has been shown to be a neuromodulator in the Aplysia californica CNS. The diversity of serotonin actions is due to the existence of several different receptor subtypes. In this study we report the cloning of a full-length cDNA, coding for a novel serotonin receptor (5-HT ap2 ). The receptor protein bears the characteristics of G protein-coupled receptors. It shares 68% and 34% of its amino acid sequence identity with the 5-HT lym receptor from Lymnaea stagnalis and the mam- The expression of 5-HT ap2 restricted to the CNS suggests an important role for this receptor in the modulation of neuronal functions in Aplysia. Moreover, the high expression of 5-HT ap2 in the bag cells, associated with its pharmacological pro®le, suggests that this receptor may be implicated in modulating the afterdischarge during the egg-laying behavior.

In Aplysia, the neurotransmitter dopamine is involved in the regulation of various physiological processes and motor functions, like feeding behaviour, and in the siphon-gill withdrawal reflex. In this paper, we report the characterization of the first Aplysia D1-like dopamine receptor (Apdop1) mainly expressed in the CNS, heart and buccal mass. Following expression of the Apdop1 receptor in HEK293 cells, a higher level of cAMP was observed in the absence of the receptor ligand, showing that Apdop1 is constitutively active. This activity was blocked by the inverse agonist flupentixol. Application of dopamine (EC50 of 35 nm) or serotonin (EC50 of 36 microm) to Apdop1-transfected HEK293 cells further increased the level of cAMP, suggesting that the receptor is linked to the stimulatory Gs protein pathway. When expressed in cultured sensory neurons, Apdop1 immunoreactivity was observed in the cell body and neurites. Control sensory neurons responded to dopamine with a decrease in excitability mediated by a pertusis toxin-sensitive G protein. Expression of Apdop1 produced an increase in hyperpolarization in the absence of agonist and an increase in membrane excitability following stimulation by dopamine. In the presence of pertussis toxin to inhibit the Gi protein inhibitory pathway responsible for decrease in excitability mechanism, Stimulation of membrane excitability was observed. Apdop1 sensitivity to dopamine makes it a potential modulator of operant conditioning procedure.

Aplysia californica is a powerful model for understanding the cellular and molecular mechanisms underlying modulation of neuronal plasticity and learning. In the central nervous system of Aplysia, serotonin is associated with various behaviors. For example, it induces short-, intermediate-, and long-term synaptic changes in sensory neurons during learning and inhibits the afterdischarge of the bag cells that initiate egg-laying behavior. Little is known about the nature and contribution of serotonin receptors involved in the numerous serotonin-mediated physiological responses in Aplysia. Recently, two G(i)-coupled serotonin receptors (5-HT(ap1) and 5-HT(ap2)) were cloned. We now report that, by using in situ hybridization to express the profile of these receptors, we are able to gain critical insight into their roles in the behavior of Aplysia. We compared their distribution to that of sensorin-A, a peptide specifically found in sensory neurons. We wished to determine their involvement in some simple forms of behavioral modifications. 5-HT(ap1) and 5-HT(ap2) mRNAs are expressed in all ganglia of the Aplysia central nervous system. Stronger signal was observed with the 5-HT(ap2) antisense probe than with the 5-HT(ap1) antisense probe. Notably, mRNA coding for the receptors was found in several identified neurons, in the bag cells, in characterized serotonergic neurons, and in neurons of the mechanosensory clusters that expressed sensorin. We also observed heterogeneity of receptor expression between R2 and LPl1 and among neurons of a single cluster of sensory neurons. These results suggest that 5-HT(ap1) and 5-HT(ap2) receptors may regulate the response to serotonin and/or its release in several neurons.

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