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.

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