Asymmetric ovulatory response induced by a unilateral implant of atropine in the anterior hypothalamus of the cyclic rat.

The effects were analysed, on ovulation at the next oestrus, of unilaterally implanting atropine in the anterior hypothalamus of rats on each day of the oestrous cycle. Implantation on day 1 of dioestrus on either side of the anterior hypothalamus blocked ovulation. Implantation on the left side of the hypothalamus on day 2 of dioestrus blocked ovulation in all animals, whereas implantation on the right side allowed 71% of the animals to ovulate (0/4 vs 5/7; P less than 0.05). Implantation at pro-oestrus on either side of the hypothalamus did not modify the rate of ovulation. When implantation was carried out on the day of oestrus on the right, none of the animals ovulated, but all ovulated when the implant was on the left (0/8 vs 8/8; P less than 0.01). The results suggest the existence of a cholinergic hypothalamic lateralization in the mechanism regulating ovulation which depends on the day of the oestrous cycle.