The effect of subcutaneous injections of melatonin, arginine vasotocin, and related peptides on pituitary and plasma levels of luteinizing hormone, follicle-stimulating hormone, and prolactin in castrated adult male rats

A sc injection of 2 μg arginine vasotocin (AVT) administered every 2 h for a 25-h period starting 1 h after castration of adult male rats caused a 43% reduction (P < 0.05) in plasma levies of LH compared to diluent-treated castrated controls. In Exp 2, a sc injection of 5 μg AVT or diluent was administered to intact or acutely castrated male rats every 3 h for 48 h. After AVT administration, both plasma LH (P < 0.001) and FSH (P < 0.05) were significantly reduced, with a concomitant increase in pituitary levels of these gonadotropins. Using the same injection regimen in Exp 3, 250 μg melatonin had no effect on plasma or pituitary levels of LH in castrated male rats. In the fourth experiment, neither 1IU arginine vasopressin nor 1 IU oxytocin had an effect on plasma or pituitary levels of LH, whereas 1 IU of AVT significantly lowered plasma levels of LH (P < 0.05) and prevented the fall in the pituitary level of this gonadotropin (P < 0.01) when compared to diluent-treated castrated control rats. Oxytocin (1 IU) significantly inhibited (P < 0.01) plasma levels of FSH and raised plasma levels of PRL (P < 0.01) in castrated male rats. No effect on plasma titers of PRL were observed after treatment of castrated rats with AVT in Exp 2 or 4. It is concluded that in acutely castrated male rats, AVT could possibly act either on the hypothalamus and/or the pituitary to affect pituitary and plasma levels of gonadotropins.