In the rat, exposure to stress increases prolactin (Prl) secretion, and endogenous opioid peptides (EOP) are believed to play a role in this response. The aim of the present study was to evaluate the specific involvement of the different EOP (i.e. β-endorphin [β-END], dynorphin A [DYN-A], methionine-enkephalin [Met-ENK], and/or opiate receptors (i.e., µ/Ε, ĸ, δ) in the stress-related increase in circulating Prl. Rats were subjected to inescapable intermittent footshock (60 Hz, 2.5 mA, 1 s duration, 2 h) 2 h after the intracerebroventricular (i.c.v.) injection of specific antisera raised against β-END, DYN-A or Met-ENK. In addition, selective opiate antagonists (βh-END-[6–31], a peptide β-END antagonist [5 nmol, i.c.v.], β-funaltrexamine [β-FNA], an µi receptor antagonist [4.8 nmol, i.c.v.], Mr 1,452 MS and Mr 2,266 BS, two K-receptor antagonists [10 mg/kg body weight, i.p.], ICI 154,129, a δ-receptor antagonist [100 nmol, i.c.v.]) were administered prior to footshock stress. Blood samples were collected through an indwelling jugular cannula. Exposure to footshock rapidly and significantly increased plasma Prl levels. This stress-induced release of Prl was reduced by both antisera against β-END or DYN-A, as well as by pretreatment with βh-END-(6–31), β-FNA and K-receptor antagonists. Antiserum against Met-ENK and δ-antagonist were inactive. These results suggest that the activation of the two endogenous opioid systems, β-END and DYN-A, centrally modulate the release of Prl induced by footshock stress.

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