Naloxone, but not Estradiol, Affects the Gonadectomy-Induced Increase in Hippocampal Cholineacetyltransferase Activity in Male Rats

Cholinergic neurons located in the medial septum and the diagonal band of Broca are a major source of cholinergic innervation to the hippocampal formation. These neurons play an important role in learning and memory processes [1]. Gonadal hormones can significantly affect the function of these cholinergic neurons [2]. Estrogens were found to increase cholineacetyltransferase (ChAT) activity and the number of ChAT neurons in the medial septum in ovariectomized female rats [3]. Moreover, estrogens significantly improve cognition capacities in menopausal women treated with estrogen-replacement therapy [4]. Although not a limiting factor in the synthesis of ACh, ChAT activity is considered an index of cholinergic activation [5]. We have previously shown that a persistent, painful stimulus (for-malin test) decreases ChAT activity in male rats, an effect not present in females [6], and induces higher c-Fos expression levels in the hippocampus of female rats than in males [7].