, 2008b) with studies of an elementary neural circuit that underlies a simple form of learned fear in Aplysia—sensitization of the gill-withdrawal reflex. A critical component of this reflex that

contributes importantly to the behavior is a direct monosynaptic connection from the siphon sensory neurons to the gill motor neurons. The sensory-to-motor neuron synapse can be reconstituted in dissociated cell culture where it is modulated, as in the intact animal, by serotonin (5-HT), a modulatory transmitter released http://www.selleckchem.com/products/kpt-330.html during the learning of fear ( Marinesco et al., 2006). Five applications of 5-HT over a period of 1.5 hr—designed to simulate five shocks to the tail that produce behavioral sensitization—produce both a long-term learn more increase in the strength of the sensory-to-motor neuron synaptic connection lasting several days (long-term facilitation: LTF, Montarolo et al., 1986) and structural remodeling and growth of new sensory-to-motor neuron synapses ( Glanzman et al., 1990, Bailey

et al., 1992 and Kim et al., 2003). Our recent finding that neurexin and neuroligin are cargo of kinesin transport and are upregulated by 5-HT at the Aplysia sensory-to-motor neuron synapse ( Puthanveettil et al., 2008) further indicates the utility of this model system to study directly the role of the neurexin-neuroligin transsynaptic interaction in learning and memory. We report here that the Aplysia homologs of neurexin (ApNRX) and neuroligin (ApNLG) exhibit strong similarities with their vertebrate counterparts in both domain structure and subcellular localization. Next, we find that depleting

ApNRX in the presynaptic sensory neuron or ApNLG in the postsynaptic motor neuron abolishes both LTF and the associated 5-HT-induced presynaptic structural changes. In addition to their Tryptophan synthase role in the initiation of LTF, we find that ApNRX and ApNLG also play a critical role in the stabilization and persistence of LTF. Finally, we find that overexpression of the ApNLG autism-linked mutant in the postsynaptic motor neuron blocks both intermediate-term and long-term facilitation. We used a primer design strategy for PCR amplification of distantly related gene sequences based on consensus-degenerate hybrid oligonucleotide primers (CODEHOPs, http://blocks.fhcrc.org/codehop.html; Rose et al., 2003) to clone a single Aplysia homolog of neuroligin (ApNLG). Comparison of the deduced amino acid sequence of ApNLG with vertebrate and invertebrate homologs shows that ApNLG shares 39% identities with human neuroligin-3 (NLG-3) ( Figure S1 available online). Despite repeated and rigorous efforts utilizing PCR-based cloning as well as screening cDNA libraries prepared from the Aplysia CNS extracts, we did not find any splice isoforms of ApNLG. The domain structure of ApNLG is similar to vertebrate neuroligins.