The stimulatory effect of 5HT and the role of the paraventricular nucleus on PMS induced ovulation in the immature rat.

Immature rats can be induced to ovulate with pregnant mare serum (PMS) as long as they weigh over 60 g. In the rats weighing less than 60 g 5-hydroxytryptamine (5HT)-60 micrograms/rat intraventricularly or 2 micrograms/rat in the paraventricular nucleus (PVN)-stimulated ovulation. Injections into several other hypothalamic nuclei were ineffective. In rats weighing greater than 60 g, lesions of the PVN or injection of 10 micrograms/rat p-chlorophenylalanine into the PVN inhibited ovulation. The hypothalamic levels of 5HT and 5-hydroxy indole acetic acid (5HIAA) were generally lower in the afternoon and evening in the less than 60 g when compared with the greater than 60 g rats. It is possible that increased 5HT activity is required to stimulate ovulation and that its site of action is the PVN.