Inhibition of sexual behavior in female rats by intracerebral injections of Met-enkephalin in combination with an inhibitor of enkephalin degrading enzymes.

Injection of Met-enkephalin, either subcutaneously (10 or 100 microgram, s.c.), intracerebroventricularly (100 ng or 1 microgram, i.c.v.) or into the mesencephalic central gray (100 ng), had no inhibitory effect on sexual behavior in female rats. If combined with [(R)-3-(N-hydroxy)-carboxamido-2-benzylpropanoyl]-L-alanine (kelatorphan, 5 micrograms i.c.v.; 1.5 micrograms in the central gray; 200 micrograms s.c.), an inhibitor of enkephalin degrading enzymes, however, i.c.v. or central gray (100 ng), but not s.c. (100 micrograms), injection of Met-enkephalin suppressed the behavior. Injection of Leu-enkephalin, s.c. or i.c.v. alone or in combination with kelatorphan, had no inhibitory effect. Peptidases may rapidly inactivate Met-enkephalin after intracerebral injection and prevent the behavioral effect. Inhibition of sexual behavior by Met-enkephalin may occur during lactation, a physiological state when the behavior is suppressed by an opioid peptide.