Evidence for an absence of estrogen-concentration by CCK-immunoreactive neurons in the hypothalamus of the female rat.

It is becoming increasingly clear that the neuropeptide cholecystokinin (CCK), widely distributed in the rat hypothalamus and limbic system, is subject to both organizational and activational influences of steroid hormones. Sex differences in numbers of CCK-immunoreactive elements have been demonstrated in sexually dimorphic structures such as the bed nucleus of the stria terminalis, medial preoptic nucleus, and ventromedial nucleus of the hypothalamus. Steroid activation of CCK has been indicated by findings that hypothalamic CCK levels and binding capacity vary over the estrous cycle. These studies, in combination with evidence of CCK mediation of sexually differentiated functions, prompted us to test for estrogen concentration among CCK-containing cells of the female rat hypothalamus by combining the techniques of immunohistochemistry and autoradiography. A method employing 2-week ovariectomies and perfusion fixation with 4% paraformaldehyde was compatible with the localization of both estrogen-accumulating and CCK-immunoreactive cell bodies. The maintenance of numbers of CCK-positive cells after gonadectomy suggested that expression of this peptide may not be directly regulated by ovarian steroids in female rats. This suggestion was substantiated by the finding that, with rare exceptions, CCK-immunoreactive cells did not concentrate estrogen in tissues collected from the anterior-posterior extent of the bed nucleus of the stria terminalis, medial preoptic nucleus, anterior hypothalamic area, and paraventricular nucleus.