Serotonin facilitates AMPA-type responses in isolated siphon motor neurons of Aplysia in culture.

Serotonin (5-HT) facilitates the connections between sensory and motor neurons in Aplysia during behavioural sensitization. The effect of 5-HT on sensorimotor synapses is believed to be primarily presynaptic. Here we tested whether 5-HT can have an exclusively postsynaptic facilitatory effect. Siphon motor neurons were individually dissociated from the abdominal ganglion of Aplysia and placed into cell culture. Brief pulses of glutamate, the putative sensory neuron transmitter, were focally applied (0.1 Hz) to solitary motor neurons in culture, and the glutamate-evoked postsynaptic potentials (Glu-PSPs) were recorded. When 5-HT was perfused over the motor neuron for 10 min, the amplitude of the Glu-PSPs was significantly increased. The 5-HT-induced enhancement of the Glu-PSPs persisted for at least 40 min after washout. Prior injection into the motor neuron of the calcium chelator BAPTA, GDP-β-S or GTP-γ-S blocked the 5-HT-induced facilitation of the Glu-PSPs. However, the facilitation was not blocked when APV, an NMDA receptor antagonist, was applied together with the 5-HT. The enhancement of the Glu-PSPs by 5-HT was reversed by the AMPA receptor antagonist DNQX, indicating that 5-HT increased the functional expression of AMPA-type receptors in the motor neuron. The presence of botulinum toxin in the motor neuron blocked the 5-HT-induced enhancement of the Glu-PSPs. As botulinum toxin prevents exocytosis we hypothesize that during sensitization 5-HT causes the insertion of additional AMPA-type receptors into the postsynaptic membrane of sensorimotor synapses via exocytosis. This postsynaptic mechanism may contribute to facilitation of the synapses. The endogenous neurotransmitter serotonin (5-HT) plays a critical role in sensitization of the siphon-withdrawal reflex in Aplysia. Stimuli that induce behavioural sensitization in Aplysia cause the release of 5-HT from interneurons; 5-HT, in turn, facilitates synapses between sensory and motor neurons in the abdominal ganglion (Mackey et al. 1989). 5-HT-induced facilitation of these sensorimotor connections mediates, in part, sensitization of the siphon-withdrawal reflex (Glanzman et al. 1989). Several presynaptic mechanisms appear to contribute to facilitation of sensorimotor synapses during sensitization (Byrne & Kandel, 1996). However, previous results from our laboratory suggest that postsynaptic mechanisms may also contribute. Postsynaptic application of the rapid Ca2+ chelator 1,2-bis(O-aminophenoxy)ethane-N,N,N‘,N’-acid (BAPTA) was found to block all enhancement of the sensorimotor synapse in a cellular analogue of classical conditioning (Murphy & Glanzman, 1996). By contrast, dl-2-amino-5-phosphonovalerate (APV), an antagonist of N-methyl-d-aspartate (NMDA)-type receptors, selectively blocked the associative component of conditioning-related synaptic enhancement (Murphy & Glanzman, 1997). Taken together, these results imply that postsynaptic Ca2+ may play a critical role in non-associative, sensitization-related enhancement of the sensorimotor synapse, as well as in associative enhancement. Siphon motor neurons appear to possess 5-HT receptors, because 5-HT causes a slow depolarization of these neurons (Frost et al. 1988). To isolate a potential postsynaptic modulatory action of 5-HT, we utilized solitary siphon motor neurons in dissociated cell culture. We found that 5-HT enhances the response of solitary siphon motor neurons to glutamate, the putative transmitter of Aplysia sensory neurons (Dale & Kandel, 1993; also see Trudeau & Castellucci, 1993). This postsynaptic action of 5-HT appears to involve increased functional expression of AMPA-type receptors in the motor neurons. Some of our results have been published previously in abstract form (Chitwood et al. 2000; Li et al. 2000).