Sex differences in locomotor activity following beta-endorphin in the ventrolateral periaqueductal gray

Both morphine and selective opioid receptor agonists produce typical biphasic effects upon activity in male rats with initial reductions in activity followed by subsequent increases in activity. Sex differences have been observed in a number of opioid-mediated responses, including the observation that microinjection of the opioid peptide, beta-endorphin into the ventrolateral periaqueductal gray (vlPAG) produces more pronounced analgesia in male rats relative to female rats. The present study evaluated whether beta-endorphin (5.2-26 microg) in the vlPAG produced biphasic activity effects in male and female rats. Both male and female rats displayed initial reductions in total, ambulatory, and stereotypic activity following beta-endorphin in the vlPAG. Whereas male rats displayed subsequent (90-120 min) increases in total and ambulatory activity following beta-endorphin in the vlpAG, female rats tested during the estrous stage of their estrous cycle failed to display any changes. These sex differences in the opioid modulation in the pattern of activity measures in the vlPAG are discussed in terms of the roles of sex hormones in opioid peptide processing within the vlPAG.