The synaptic junctions of LE and RF cluster sensory neurones of Aplysia californica are differentially modulated by serotonin.

The monosynaptic component of withdrawal reflexes in Aplysia californica, from sensory neurones to motor neurones, is a critical site of the synaptic modulation occurring during short-term and long-term behavioural changes. There are four clusters of sensory neurones (LE, rLE, RE, RF) innervating the receptive field for the gill and siphon withdrawal reflex. The receptive fields of these cells are located on the siphon, the mantle, the branchial cavity and the gill itself. In most studies, the synapses made by the sensory neurones of the LE cluster of the abdominal ganglion or the VC cluster of the pleural ganglion have been investigated and shown to be facilitated by the neuromodulator serotonin (5-hydroxytryptamine, 5-HT). In this report, we have examined the effect of 5-HT on the synaptic junctions of the RF cluster neurones. The duration of action potentials in these cells, unlike those of the other clusters, is barely affected by serotonin. We found that while the LE synapses are facilitated by 5-HT (10 micromol l-1), the RF synapses are not. In fact, the RF-L14 connections are actually depressed by 5-HT; this effect is not due to shunting in the postsynaptic neurone. The RF-L7 connections are also depressed by 5-HT, although the effect is smaller. The RF-L14 connections are blocked by the non-NMDA receptor agonist CNQX (100 micromol l-1), suggesting that the transmitter and the postsynaptic receptors involved are similar to those present on the LE or VC cluster cells. The absence of serotonin-induced facilitation of the RF cluster cells may provide the animal with a means of reducing the nonspecific effects of aversive sensitization and therefore of allowing a greater specificity and more flexibility in plastic behavioural changes.