Ovariectomy in the rat increases brain norepinephrine synthesis and turnover. The present study investigated the effect of ovariectomy and subsequent replacement of ovarian steroids on tyrosine hydroxylase activity (TH), which is purported to be the rate-limiting step in catecholamine (CA) synthesis. Animals were ovariectomized and tyrosine and tyrosine hydroxylase activity assayed 2, 4, 26 and 60 days postoperatively. Estrogen (1.5 M*g/rat) or vehicle was injected for one week beginning on day 21. Progesterone (0.5 mg or 2 mg) or vehicle was administered at 1000 hours on day 26 and the animals sacrificed 3 or 6 hours later. Ovariectomy caused a 2-3 fold increase in T H having an onset of 4 days and a duration of at least 60 days. Daily injection of estrogen for 1 week caused a 30% increase in TH. TH was significantly reduced 6 hours after injection of 0.S mg progesterone in estrogen-treated ovariectomized controls. After injection of 2 mg progesterone, TH of ovariectomized and ovariectomized estrogen-treated rats was unchanged after 3 hours, but significantly decreased at 6 hours. Thus, CA synthesis in the rat hypothalamus may be altered by ovarian steroids, with progesterone having an inhibitory effect and estrogen a stimulatory effect. The inhibitory effect of progesterone on T H was facilitated by the presence of small amounts of estrogen. (Endocrinology 9 1 : 276, 1972) M ONOAMINERGIC participation in the control of ovulation and gonadal function has been documented (1). Recent investigations suggest that hypothalamic norepinephrine (NE) synthesis and turnover are accelerated after ovariectomy (2,3), but not hypophysectomy (4), suggesting that gonadal steroids may influence catecholamine (CA) synthesis in the brain. Simultaneous replacement of estrogen and progesterone to ovariectomized rats decreases the formation of 3 H-norepinephrine from 3 H-tyrosine (2, 3).These observations prompted an investigation of the effects of ovariectomy and ovarian steroid replacement on rat hypothalamic tyrosine hydroxylase activity (TH) as a possible mechanism for gonadal steroid influence on NE synthesis. If ovariectomy increases NE synthesis, then the activity of

Apparently normal sex behavior was produced in spayed rats by daily injections of 0.8 or 3.2 /*g estradiol benzoate. Adrenalectomy had no deleterious effect on this behavior. Uterine weight data suggested that the lower dose produced circulating estrogen levels within or below those occurring in physiological conditions. Since, in the combined ovariectomized-adrenalectomized females, all known sources of steroid hormones were removed, it is concluded that the expression of sexual receptivity in female rats does not require the presence of progesterone or adrenal androgens. (Endocrinology 82: 193, 1968) I T IS generally assumed that the expression of sexual behavior in the female rat depends upon adequate secretion of both estrogenic and progestational hormones by the ovary (1). To produce experimentally the characteristic lordosis response denoting sexual receptivity, most investigators use a regimen of 1 or 2 injections of an estrogen, followed by a larger quantity of progesterone about 6 hr before testing. While it is known that administration of large doses of estrogen alone can elicit this response (2), it has been suggested that the behavior of these animals is not normal (2), or that under such circumstances progesterone was supplied by the adrenal cortex (3). The fact that estrogen stimulates adrenocortical activity in the rat (4-6) lends some credence to the latter notion. Since ovariectomy or menopause does not seem to reduce sexual desire in women, it has been proposed that the hormonal determinants of their sexual behavior are of adrenocortical rather than ovarian origin (7). It was, therefore, of interest to determine whether the induction of sexual behavior in ovariectomized female rats by relatively low doses of estrogen was affected by the presence of the adrenal glands. Materials and MethodsFifteen 60-day-old female Long-Evans rats were ovariectomized and 19 others were simultaneously ovariectomized and adrenalectomized. Seven others Received July 5, 1967. This work was supported by USPHS Grant HD 00778.1 USPHS postdoctoral fellow. 2 USPHS predoctoral fellow.of similar age were not operated. The adrenalectomized rats received 0.9 % NaCl in their drinking bottles, but no corticosteroid replacement. Three weeks postoperatively, approximately half of the animals in each operated group received either 0.8 or 3.2 ng of estradiol benzoate in daily subcutaneous injections. Behavioral tests were conducted on days 1, 2, 3, 4, 5, 8, 10 and 12 following the onset of injections. The unoperated rats were not injected but were tested behaviorally at different stages of the estrous cycle. Illumination was supplied from 12 PM to 12 AM and behavioral tests were commenced at approximately 2 PM. Females were placed in semicircular testing cages with sexually experienced males. Each female received a minimum of 10 mounts, and all mounts by the male and lordoses by the female were recorded. Two observers were present during all testing sessions. On the day of the last test, all animals were autopsied, the complete...

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