Behavioral effects of a LHRH antagonist in intact and ovariectomized rats.

The effects of the LHRH antagonism on the acquisition of conditioned responses (CARs) and spontaneous motility were studied in intact and ovariectomized rats. A synthetic antagonist of LHRH, [N-acetyl-D-p-chloro-Phe1,2,D-Trp3,D-Arg6,D-Ala10]-LHRH, was injected in a single dose (10 micrograms/rat, SC) at noon on the day of proestrus in the normally cycling rat, and behavioral experiments were carried out on the morning of estrus or metestrus. Two procedures were followed in the ovariectomized rats: in the first, the antagonist was injected 1 h before estradiol, and in the second, at noon on the day after estradiol replacement. The experiments were carried out 24 and 48 h after estradiol, respectively. The LHRH antagonist facilitated the acquisition of CARs in both experimental groups, thus reversing the impairments observed during estrus and metestrus and those induced by estradiol replacement. The antagonist decreased the number of head shakes during estrus, whereas it induced an increase in total motility and rears in ovariectomized control animals. On the other hand, the antagonist increased the number of rears and reversed the decrease in grooming behavior induced by estradiol. The results led to the idea of a role of LHRH in behaviors not apparently related to sex, which could explain the behavioral changes observed across the estrous cycle and those induced by estradiol replacement in ovariectomized rats.