REGULATION OF FEEDING BY MULTIPLE OPIOID RECEPTORS IN CINGULATE CORTEX - FOLLOW-UP TO AN IN-VIVO AUTORADIOGRAPHIC STUDY

A previous in vivo autoradiographic study demonstrated reduced H-3-dip renorphine binding in anterior cingulate cortex of rats that were inje cted (i.v.) with the radiolabeled opiate during lateral hypothalamic s timulation-induced feeding (SIF). This suggests that an opioid peptide is released in cingulate cortex during feeding and excludes binding o f the tracer. The aim of the present study was to determine whether op ioid activity in cingulate cortex contributes to the expression of SIF . Agonists and antagonists for multiple opioid receptors were microinj ected into cingulate cortex and effects on stimulation frequency thres hold for SIF were determined. Although the universal opioid antagonist naloxone (20.0 mu g) increased threshold, high doses of selective ant agonists for mu, delta, and kappa receptors - D-Tic-CTAP, natrindole a nd norbinaltorphimine, respectively - had no effect. The unique effica cy of naloxone may be due to this lipophilic compound's rapid diffusio n throughout an extensive volume of anterior cingulate tissue. While h igh doses of the Ic agonist U50,488 and the delta agonist DPDPE had no effect, the mu agonist, DAGO (1.0 mu g), decreased the SIF threshold. Moreover, the threshold-lowering effect of DAGO was blocked by pretre atment with the irreversible mu antagonist beta-FNA. These results sug gest that mu opioid activity in cingulate cortex can facilitate SIF bu t that under basal conditions endogenous opioid activity in this brain region makes only a small positive contribution, if any, to the expre ssion of SIF.