Although there are several different types of opiate receptors in mammals (Goldstein and James, 1984) and a variety of different ionic responses can be elicited by each of the receptor types on different neurones (Crain and Shen, 1990), relatively little attention has been given to the role of opiate receptors in invertebrates. We have previously demonstrated that the nervous system of the marine mollusc Aplysia californica contains both leucine-enkephalin-like and methionine-enkephalin- like opiate peptides (Leung et al. 1986) and that receptors for these substances exist on many neurones (Carpenter and Hall, 1986; S.-Rozsa et al. 1991; Kemenes et al. 1992). Although these responses are not identical to those observed in mammalian neurones, there are several common features.
1. Monosynaptic connections between a giant serotonin-containing neurone and its serotonin-containing followers in the snail Helix pomatia were studied in isolated preparations of the central nervous system. The presynaptic cell was the LP3 cell in the left pedal ganglion and the followers were the RPas cells in the right parietal ganglion. 2. The light microscopical morphology of the pre- and postsynaptic cells was investigated in whole-mount preparations following intracellular injection with a nickel-lysine complex. Axons from LP3 project towards the cerebral and suboesophageal ganglia or run in peripheral nerves which innervate feeding muscles and the foot. The follower neurones (RPas) project into nerves which innervate the heart and other visceral organs. The axons of LP3 and the RPas cells run in close proximity in the visceral ganglia. 3. Ionophoretic application of serotonin onto the membrane of the postsynaptic RPas neurones mimicked the excitatory effect of the stimulation of the presynaptic LP3. Both the synaptic transmission between LP3 and its followers and the excitatory effect of exogenously applied serotonin on the RPas neurones decreased or were blocked in the presence of serotonin or the serotonin antagonists tryptamine, bufotenine, 7- methyltryptamine and MDL 72222-EFO2 in the bath. From this, we conclude that the excitatory neurotransmitter between LP3 and followers is serotonin and not some other neurotransmitter which might coexist with serotonin in LP3. 4. The serotonergic monosynaptic connections between LP3 and its right parietal followers may play a role in a variety of serotonin-mediated physiological and behavioural responses, forming a link between feeding, locomotion and visceral functions.