Short Communication Cyclic-amp-mediated Excitatory Responses to Leucine Enkephalin in Aplysia Neurones

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.-Rózsa et al. 1991; Kemenes et al. 1992). Although these responses are not identical to those observed in mammalian neurones, there are several common features. A common response to leucine enkephalin and the stable analogue D-Ala2-leucine enkephalin (DALEU) is a slow depolarization which is not associated with a significant conductance increase. This response has been characterized using an identified giant neurone, the metacerebral giant cell (MGC) or neurone C-1 (Kemenes et al. 1992), but similar responses are found in many neurones in the B cluster of the cerebral ganglion (S.-Rózsa et al. 1991). We have explored the effects of cyclic AMP derivatives and phosphodiesterase inhibitors on these responses in order to test the possibility that the receptors activated by DALEU and leucine enkephalin act through stimulation of cyclic AMP synthesis. Such second-messenger-mediated slow responses to endogenous opiates may contribute to the modulatory action of these peptides on a variety of physiological and behavioural processes (for a review, see Crain and Shen, 1990). Most of the methods used here have previously been described in detail (Kemenes et al. 1992). Experiments were performed on the cerebral ganglia of adult Aplysia californica. The connective tissue sheath was removed by dissection above the MGCs or the B cluster cells and the neurones were penetrated with two independent glass microelectrodes filled with 3mol l21 potassium acetate and connected to a Dagan model