SUMMARY

Unit activity in the hypothalamus and other diencephalic regions was recorded with stereotaxically oriented steel microelectrodes in adult female rats under light urethane anaesthesia.

Spontaneous firing rates of neurones varied from < 1/10 sec. to > 50/sec., but the majority fired at 1–10/sec. Some variations in the pattern of firing are described.

Acceleration of firing rate was most readily induced by pain stimuli (64% of neurones) and then by cold (60%), probing the cervix (47%), smell (20%), light (5%) and noise (3%) in that order. A minority of neurones were inhibited by the stimuli.

Many neurones responded to several different stimuli, most commonly by accelerating to cervical probing, cold and pain. Inhibitory convergence was also observed, e.g. blockade of the response to cervical probing by an olfactory stimulus, and inhibition by cervical probing of the response to cold or pain.

The proportion of neurones excited by smell in prooestrous rats was more than double that in oestrous or dioestrous rats. Oestrous rats had relatively more neurones which were unresponsive or inhibited by the test stimuli.

Slow intravenous injection of 400 μg. progesterone induced a selective depression of the response of lateral hypothalamic neurones to cervical probing. The effect was maximal at about 30 min. and full recovery occurred in 1 hr.

The possible significance of these observations is discussed with particular reference to the neural control of luteotrophin secretion.

SUMMARY

In the present investigation the mating behaviour of the androgensterilized rat has been studied. It was observed that rats sterilized by prepubertal administration of 1·25 mg. of testosterone propionate were sexually unreceptive to the male. Progesterone, when given as either spaced or daily injections, did not restore mating behaviour. Furthermore, although mating behaviour could be restored in normal ovariectomized rats by combinations of oestrogen and progesterone, identical treatment of spayed sterile rats was ineffective. Rats pretreated with 10 μg. of testosterone propionate at 5 days of age were found to be sexually receptive to the male even though they exhibited the same anovulatory persistentoestrous syndrome as did rats which received the higher dose. One rat accepted the male for nine consecutive days whilst others exhibited more bizarre patterns of sexual receptivity. It is suggested from these and other studies that a ' mating centre' exists in the hypothalamus of the rat and that it is located in the anterior hypothalamus.

ABSTRACT

Considerable differences have previously been found in the hypothalamo-hypophysial responsiveness to oestrogen, depending upon the time between gonad removal and exposure to oestrogen. In the present study a detailed analysis was made of some of the differences which may exist in pituitary LH-releasing hormone (LHRH) receptors and the amount of LH released in response to electrochemical depolarization of the medial preoptic area after 2 or 7 days of oestradiol treatment of long- and short-term gonadectomized male and female rats. The pituitary glands of long-term gonadectomized males and females secreted more LH in response to two pulse injections of LHRH than did short-term gonadectomized rats. The amount of LH released on day 2, however, was equivalent to that secreted after 7 days of oestradiol treatment. Moreover, long-term gonadectomized males and females had equivalent LHRH receptor concentrations, which were greater than those of short-term gonadectomized animals. Peak serum LH concentrations observed after preoptic stimulation were equivalent in short- and long-term castrated rats after 2 days of oestrogen exposure. Serum LH concentrations following preoptic stimulation in short-term gonadectomized males and females were significantly greater on day 7 than on day 2 of oestradiol treatment, whereas in long-term gonadectomized animals the stimulated release of LH was equivalent both in magnitude and time of peak release on both days.

These studies demonstrate that the differential effects of oestradiol on LH release on day 2 (no negative feedback) compared with day 7 (both negative and positive feedback exist) are not due to differences in the ability of the pituitary gland to release LH in response to LHRH, nor in the releasable pools of hypothalamic LHRH in long-term gonadectomized rats. Rather, they seem to be due to a refractoriness in some unidentified central nervous process which regulates tonic LH release in gonadectomized rats.

J. Endocr. (1986) 110, 367–373

In this study, we have examined whether the suppression of raised plasma FSH concentrations at pro-oestrus and/or oestrus by porcine follicular fluid (PFF) affected the development of follicles for ovulation in the next cycle. Adult, 4-day-cyclic rats were injected with PFF or pig serum at various hours of pro-oestrus and oestrus. Plasma FSH levels were suppressed following PFF treatment at any time of pro-oestrus and oestrus. Furthermore, this suppression was always followed by a 'rebound' increase in plasma FSH. In contrast, plasma LH concentrations were unaffected by PFF treatment and neither gonadotrophin was altered by treatment with pig serum.

When rats treated with PFF or pig serum were allowed to complete one additional cycle, plasma LH and FSH concentrations at the pro-oestrus and oestrus after treatment were not significantly different among groups regardless of treatment or time of treatment. All ovaries of rats treated with PFF or pig serum on the next pro-oestrus morning before ovulation were histologically similar. Furthermore, all animals ovulated a normal complement of ova at the next oestrus regardless of whether preovulatory, secondary or both increases of plasma FSH had been blocked by PFF treatment during the previous cycle. However, in animals given PFF during the preceding cycle, plasma oestradiol and progesterone concentrations were significantly altered on the morning and afternoon of pro-oestrus respectively.

These results suggest that increased plasma FSH concentrations at pro-oestrus and oestrus may not be essential for folliculogenesis and ovulation in the subsequent cycle. Alternatively, the 'rebound' of FSH on day 1 of dioestrus after the suppression of both phases of FSH secretion at pro-oestrus and oestrus may be sufficient to provide ovulatory follicles for the next pro-oestrous day.