Cholinergic mechanisms of lordotic behavior in rats

Cholinergic mechanisms of lordotic behavior were studied in hooded rats using behavioral techniques and autoradiographic analysis of the diffusion of [3H]N methyl scopolamine ([3H]NMS) applied to the hypothalamus. Bilateral cannulae were implanted chronically in the region of the ventromedial nuclei (VMN). A series of cholinergic drugs and estradiol (E2) were administered to each rat during successively induced periods of estrus. Robust inhibition of lordosis resulted from administration of atropine and scopolamine. Robust facilitation followed carbachol administration. Pirenzepine, hexamethonium, and tetraethylammonium did not inhibit lordosis. [3H]NMS, the last drug in the series to be administered, inhibited lordosis in two of three rats. Autoradiographic analysis of [3H]NMS diffusion in these rats revealed that radioactivity was confined entirely to the hypothalamus and appeared in the region of the VMN. In an additional experiment, the in vitro binding characteristics of [3H]NMS in the basomedial hypothalamus were examined. The VMN appeared lightly labeled and were surrounded by regions of darker labeling. We conclude that cholinergic drugs influence lordotic behavior when applied in crystalline form in the vicinity of the VMN of the female rat hypothalamus.