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The acute effects of tobacco smoke inhalation on the spontaneous proestrous rise in serum luteinizing hormone and prolactin have been investigated in the female Wistar rat. It was found that the surge of LH normally seen throughout the afternoon of proestrus was delayed by inhalation of tobacco smoke and that the delay was dose-related to the nicotine content of the cigarettes used in the experiment. In the case of prolactin neither the timing nor the magnitude of the surge was altered when compared with controls. These results suggest that under certain well-defined conditions inhalation of tobacco smoke of known nicotine content is capable of exerting profound influences on the hypothalamic-pituitary-gonadal axis.

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The phenomenon of diurnal variation in secretion of FSH, LH and prolactin (Prl) has been investigated in adult male Wistar rats. When these animals were decapitated at 2 h intervals throughout the 24 h day, a highly significant elevation of serum FSH was observed at 12.30 and 14.30 h. No definite diurnal pattern was observed for LH and Prl. The basic FSH rhythm was found to persist in animals bled via an indwelling atrial cannula or by cardiac puncture. However, these techniques of blood collection were found to produce some variation in the timing of the surge. These observations suggest the existence of diurnal variation in the secretion of FSH in the adult male rat.

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The luteotropic stimuli necessary to transform the corpus luteum of the estrous cycle into a corpus luteum of psuedopregnancy on the morning of diestrus-2 (Day 2), as reflected by a dramatic divergence in progesterone secretion, were studied (Day 1 was taken as the first day of diestrus of pseudopregnancy). The requirement of prolactin (PRL) as a luteotropic stimulus was determined by inhibiting the diurnal and nocturnal PRL surges that occur immediately before and during the divergence in progesterone. Following cervical stimulation, 1 mg of 2-Br-alpha-ergocryptine (EC) was injected at 1100 and 2300 h on Day 1 (lights on 0600-1800 h), and the animals were decapitated at 2-4 h intervals from 1100 h on Day 1 to 1700 h on Day 2. In the control animals, the PRL surges on Day 1 and Day 2 were associated with an increase in progesterone secretion on Day 2. However, the regimen of EC treatment resulted in an inhibition of PRL surges, prolactin remaining at baseline values from 1100 h on Day 1 to 1700 h on Day 2. The inhibition of PRL secretion was associated with a fall in progesterone concentration to reach baseline values by 1700h on Day 2. Furthermore, a group of animals similarly treated with EC returned to vaginal estrus 2 days later. LH concentrations did not differ in control and EC-treated animals. The effect of EC on corpus luteum function could be completely reversed by the simultaneous administration of PRL. In addition, if PRL was administered at 1100 h and 2300 h on diestrus-1 of the estrous cycle, in an attempt to mimic the surges os pseudopregnancy, regression of the corpora lutea did not occur. Progesterone levels increased to reach values comparable to those observed in pseudopregnancy on diestrus-2. The role of LH was studied by administering a dose of LH antiserum at 110 and 2300 h on Day 1 of pseudopregnancy. This treatment failed to inhibit the increase in progesterone observed on Day 2. These results demonstrate that the surges of plasma PRL initiated by cervical stimulation are responsible for transforming a corpus luteum of the estrous cycle into a corpus luteum of pseudopregnancy, as reflected by an increase in progesterone secretion of Day 2. LH seems to have a minor role in maintaining corpus luteum function beyond that observed during the estrous cycle.

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The direct effects of median eminence extract (MEE), estradiol (E) or progesterone (P) or combinations of these hormone preparations on the structure of the adenohypophysis and its secretion of gonadotrophins were examined in female rats bearing pituitary homografts obtained from long-term ovariectomized (LTO) donors. Preliminary investigation had shown that the pars distalis of LTO animals retained large numbers of hypertrophied gonadotrophs for several weeks even when homografted under the kidnay capsule of hypophysectomized recipients. Homografts from vehicle-treated controls contained many gonadotrophs at 24 days. Treatment with E or P resulted in complete disappearance of gonadotrophs at 24 days, but not at 10 days. E produced depletion of gonadotrophs even in the presence of MEE, while P + MEE treatment resulted in retention of gonadotrophs. In general, ovarian morphology correlated well with changes in pituitary cytology. These results suggest that the ability of MEE to stimulate release (and probably synthesis) of gonadotrophic hormones depends on the steroid sensitization of pituitary tissue and the differential effect of E and P on gonadotrophin release in the presence of MEE.

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In this presentation experimental evidence is reviewed and new information is presented concerning the question of cyclic fluctuations in pituitary responsiveness to endogenous and exogenous hypothalamic-releasing hormones as well as the direct role of steroids in modulating pituitary sensitivity. Experiments are described which show that four-day and five-day cyclic rats respond differently to LH-RH, presumably because of differences in secretion patterns of ovarian steroids. The direct role of these steroids in this process was investigated both from the morphological (light and electron microscopy) and physiological (radioimmunoassay measurement of LH) viewpoint in hypophysectomized rats bearing pituitary homografts. It was found that steroids may directly alter both the morphology and the secretory function of pituitary gonadotrophs. Estradiol was found to depress the amount of LH that was secreted while progesterone was seen to influence primarily the time of LH release following LH-RH injection. Thus, it appears that direct feedback effects of ovarian steroids at the level of the pituitary are important regulators of the sensitivity of pituitary gonadotrophs to the LH- releaseing action of hypothalamic LH-RH.

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The possible involvement of cholinergic mechanisms in the control of rhythmic secretion of prolactin has been examined in the pseudopregnant rat. Baseline data were obtained in decapitated animals in which the diuranl surge was observed in the 1430--2030 h range and the nocturnal surge during the 2330-0530 h interval. Atrial cannulation permitted a faithful reproduction of the prolactin pattern seen in decapitated rats if at least 3 days elapsed between the cannulation operation and bleeding, while cardiac puncture, under light ether anesthesia, appeared to suppress the diurnal surge. This latter observation appeared to depend on the time of sampling in relation to the onset of the dark phase of the daily lighting cycle. Atropine (35 or 70 mg/100 g BW) was found to inhibit the nocturnal surge in animals bled by either the cardiac puncture or cannulation techniques --an effect which was reversed by pretreatment with eserine (50 or 100 mug/100 g BW). Nicotine (0.75 mg/100 g BW) was found to delay, but not completely to inhibit, the nocturnal surge. When evaluated in the light of other available information, these observations suggest that a complex cholinergic mechanism is involved in control of the nocturnal rhythm of prolactin in pseudopregnancy and that the two surges, diurnal and nocturnal, are differentially controlled.

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The direct role of estradiol and progesterone in sensitizing the adenohypophysis to the releasing action of LHRH (luteinizing hormone-releasing hormone) has been examined in rats bearing pituitary homografts. Groups (10-14/group) of virgin female rats (CDF) strain) were hypophysectomized, ovariectomized, and pituitaries obtained from long-term ovariectomized donors were implanted under the kidney capsule. The animals were treated on the sixth day after transplantation with sesame oil vehicle, estradiol (1.6 mug/100 g BW), progesterone (0.6 mg/100 g BW), or estradiol plus progesterone. This injection regimen was repeated 12 h later. Ninety minutes after the steroid injection on day 7 half of the animals in each group received LHRH (ip) and half received saline. Basal levels of LH in the steroid-treated groups not injected with LHRH were, in general, elevated as compared to the oil-injected group. Following LHRH, blood samples were collected by cardiac puncture under light ether anesthesia at 5, 15, 30, 60, and 90 min postinjection. Animals receiving LHRH responded with significant elevations of LH both in the presence and absence of steroids while saline was without effect. However, the magnitude of the LH peak was found to be significantly blunted by estrogen treatment. Moreover, the peak LH response to LHRH occurred 30 min following injection in all groups except the progesterone primed animals. In this group the peak LH was delayed by 30 min. These results suggest that estradiol directly inhibits the release of LH at the level of the pituitary and that the ovarian steroids, estradiol and progesterone probably interact to determine the time of optimal pituitary sensitivity to LHRH.

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