Discussions 179 



becomes anestrus after hysterectomy, and Dr. Hansel pointed out that even when 

 ovulation is induced by administration of exogenous gonadotropins, the hysterectom- 

 ized cow does not show behavioral estrus. This apparent species difference certainly 

 invites further investigation. 



Our studies in the sheep therefore add this species to the list of animals in which a 

 hypothalamic centre must be intact for heat to occur. This centre is probably stimulated 

 directly by estrogens, and it is an inviting hypothesis that the aaion of progesterone 

 in potentiating the heat-producing action of estrogen rests in some sort of a p.nming 

 action on this brain centre. Certainly, there is ample precedent for an action of 

 progesterone on the brain in the data presented by Dr. Sawyer this morning, and 



SITE OF HYPOTHALAMIC LESOHS W 9 EWES WITH NO HEAT 

 PERIODS AND ACYCUC O^RIES 



Fig. 3. 



progesterone therapy not only lowers the heat-producing threshold for estrogen, but 

 is essential if heat periods are to recur c>'clically (Robinson, Endocrinology 55, 403, 

 1955). 



The brain also appears to be involved in the control of anterior pituitary secretion 

 of gonadotropins in the ewe, as in other species. As indicated above, nine of the ewes 

 in our series showed in addition to absence of beha\ ioral estrus, only small follicles 

 and no corpora lutea in their ovaries. This fact indicates that periodic stimulation of 

 the ovary was no longer present after the lesions were made. The sites of these lesions 

 are shown in Fig. 3. These animals also had ventral hypothalamic lesions, but the 

 common area of destruction in these sheep was more ventrally and caudally located 

 than the common area in the sheep with the absent heat only. All these animals had 

 some pituitary stalk damage, but the lesions probably did not produce their effect 

 by damaging the pituitary blood supply because in those animals in which they were 

 studied, thyroid function and adrenal size and morphology were normal. In two of 

 the animals, 17-hydroxyconicoid levels in the peripheral blood following surgical 

 stress were abnormally low, but in the remaining animals they were normal (Clegg 

 and Ganong, Endocrinology, in press, I960). 



We have been interested in correlating the physiological effects of these hypothalamic 

 lesions with changes in the gonadotropic potency of the anterior pituitary. The LH 

 potencv' of the pituitaries from the animals with lesions has been measured by the 

 ventral prostate response in h\'pophysectomized assay rats, and the follicle-stimulating 

 potency by the effect on ovarian weight in immature rats receiving chorionic gonado- 

 tropin (Clegg et ai, Endocrinology 62, 790, 1958). The values found in normal ewes, 

 ewes with lesions that did not affect the sexual c>cle, and ewes with c>'clic ovaries 

 but absent heat periods are summarized in Table 1. The differences between the latter 

 two groups of animals and the normal controls are not statistically significant, an 

 additional piece of evidence in favour of the concept of a hypothalamic centre concerned 

 with sexual behasiour which is independent of the areas concerned with regulation of 

 gonadotropin secretion by the pituitary. We do not as yet ha%e sufficient data for 

 statistical analysis of pituitary gonadotropin content in sheep with lesions and acvclic 

 ovaries. However, it is interesting that in the two ewes assayed to date, a slight 



