The enkephalin system in the rat anterior pituitary: regulation by gonadal steroid hormones and psychotropic drugs.

The regulation of pituitary enkephalin system by gonadal steroid hormones and psychotropic drugs was studied. A distinct sex-related difference of (Met5)- and (Leu5)-enkephalin-like immunoreactivity (ME-LI and LE-LI, respectively) was found in the anterior lobe but not in the neurointermediate lobe. Levels of ME-LI and LE-LI in the anterior pituitary of male rats were 4.8- and 6.2-fold as much as those of female rats, respectively. Estradiol administration caused a marked decrease of ME-LI and LE-LI only in the anterior lobe of male rats whereas dihydrotestosterone treatment significantly increased the level of ME-LI in the anterior lobe of female rats. Castration of male rats decreased both ME-LI and LE-LI only in the anterior lobe, and the diminished levels of both peptides were partially restored by the administration of dihydrotestosterone. On the other hand, ovariectomy caused a significant increase in both ME-LI and LE-LI in the anterior lobe of female rats, and this increase was completely prevented by the administration of estradiol. These results indicate that the enkephalin system in rat anterior pituitary is regulated by circulating gonadal steroid hormones. The anterior pituitary levels of ME-LI and LE-LI were significantly reduced by the repeated administration of haloperidol or lithium chloride, whereas hypertonic NaCl loading elicited no effect. On the other hand, ME-LI and LE-LI levels in the neurointermediate lobe were significantly decreased by repeated administration of hypertonic NaCl solution or lithium chloride; both treatments caused a marked reduction of antidiuretic hormone level in the same lobe. These results suggest that the regulation of the enkephalin system in the anterior lobe may be mediated, at least in part, by the dopamine system, whereas the enkephalin system in the neurointermediate lobe may be regulated mainly through a mechanism similar to that regulating antidiuretic hormone release.