Opioids Act Centrally to Modulate Stress-Induced Decrease in Luteinizing Hormone in the Rat*

Because endogenous opioid peptides (EOP) and CRF are activated during stress and decrease LH levels when injected centrally, we have explored the roles of these peptides in the stress-induced inhibition of LH secretion. Three opioid peptide systems i.e. endorphin (END), enkephalin, dynorphin (DYN), are present in the hypothalamus, acting on different opiate receptor subtypes (i.e. mu, delta, kappa). We first evaluated which EOP might be involved in the stress-induced decrease of LH levels. Immunoneutralization of EOP and pharmacological blockade of opiate receptors were used to reverse the decrease in LH induced by inescapable intermittent footshock in castrated male rats. Anti-beta-END and anti-DYN-A serum (intracerebroventricularly icv) or pretreatment with beta-END antagonist, beta-human END-(6-31) (2 and 5 nmol, icv), or with kappa-antagonists, Mr1452 MS and Mr2266 BS (10 mg/kg BW, ip), reversed electroshock-induced decrease in LH concentrations. beta-funaltrexamine (beta-FNA), an irreversible mu 1-opiate receptor antagonist, was partially effective in blocking the inhibitory effect of stress on LH levels. Neither passive immunization with anti-enkephalin nor the pretreatment with the delta-opiate receptor antagonist, ICI 154,129, (10 and 100 nmol, icv) modified the effect of stress. We then evaluated which endogenous opioid ligands and/or receptors might mediate the inhibitory effect on LH levels of 2 nmol ovine CRF injected icv. Anti-beta-END serum and beta-human END-(6-31), reversed the CRF-induced decrease of LH concentrations, whereas beta-FNA was only partially active. Anti-DYN-A and anti-ENK serum, kappa- and delta-antagonists did not prevent the decline of LH levels in rats receiving CRF centrally. These data suggest that stress-induced inhibition of LH secretion involves the stimulation of beta-END and DYN-A systems via mu/epsilon- or kappa-opiate receptors and that the decrease in circulating LH levels induced by centrally administered CRF may be mediated by the activation of beta-END system. Therefore, it is possible that the activation of the central CRF/beta-END pathway may play an important role in the stress-induced inhibition of reproductive functions.