Affective defense behavior elicited from the feline midbrain periqueductal gray is regulated by mu and delta opioid receptors.

The present study sought to identify specific opioid receptor subtypes involved in the modulation of affective defense behavior (AD) at the level of the midbrain periaqueductal gray (PAG). Cannula electrodes were utilized for eliciting AD from the PAG as well as for microinjecting mu, delta and kappa agonists and antagonists into these sites. Following microinjections of morphiceptin, D-Pen2,D-Pen5 enkephalin (DPDPE), or U-488H into sites from which AD was elicited, threshold values were determined. The results indicated that morphiceptin and DPDPE significantly suppressed AD in a dose- and time-dependent manner. Pretreatment with mu and delta opioid antagonists, B-FNA and ICI 174,864, completely blocked the suppressive effects of morphiceptin and DPDPE, respectively. Microinjections of morphiceptin and DPDPE failed to alter response thresholds for circling behavior also elicited from electrical stimulation of dorsal PAG. Administration of the selective kappa agonist, U-488H, or vehicle alone, did not alter the threshold for AD. The results of this study indicate that opioid peptides interact with mu and delta receptors within the midbrain PAG to powerfully suppress AD.