In vivo assessment of opioid agonists and antagonists on ovarian maturation in the red swamp crayfish, Procambarus clarkii.

The possible involvement of an endogenous opioid system in the regulation of ovarian development in the red swamp crayfish, Procambarus clarkii, was investigated in vivo. Injections of the opioid, methionine (Met) enkephalin, into females significantly slowed ovarian maturation in a dose-dependent manner. In contrast, injection of the general opioid antagonist, naloxone, produced dose-dependent ovarian maturation. Furthermore, the highly selective delta opioid agonist, DADLE, also inhibited ovarian maturation, whereas a highly selective delta receptor antagonist, ICI-174,864, stimulated ovarian maturation. In view of these results and the fact that Met-enkephalin is a relatively selective delta receptor agonist, we hypothesize that in the crayfish the inhibitory effect of Met-enkephalin involves this type of binding site. Furthermore, it is hypothesized that the inhibitory action of these opioids is due to either (a) stimulation of release of the gonad-inhibiting hormone, (b) inhibition of release of the gonad-stimulating hormone or (c) both (a) and (b).