Medial Basal Hypothalamic LHRH Research Articles

The effect of hyperprolactinemia produced by transplantable pituitary MtTW15 tumor cells in male rats on hypothalamic luteinizing hormone-releasing hormone release in vitro: effects of naloxone and K+.

Although hyperprolactinemia has been reported to decrease reproductive function in male rats, the mechanism of these effects is not fully understood. We examined the effects of chronic hyperprolactinemia and castration on the LHRH content of the medial basal hypothalamus (MBH) and on the basal and evoked in vitro release of LHRH from the MBH-preoptic area (POA). Adult Wistar-Furth male rats were inoculated with MtTW15 tumor fragments; 3 weeks later half of the rats were castrated. Hyperprolactinemic (H) and normoprolactinemic (N) rats were decapitated 2 weeks later to measure MBH LHRH and serum PRL and LH levels. Elevated PRL levels (greater than 2 micrograms/ml) resulted in significantly increased MBH LHRH stores. Castration caused a 57% depletion of MBH LHRH in N rats; in castrated H rats the MBH LHRH content was also reduced by 40%, a significantly lesser extent. Although serum LH levels in intact H rats were only slightly reduced, postcastration LH hypersecretion was significantly attenuated. In a parallel study, the LHRH release rate was assessed by in vitro perfusion of the MBH-POA. The basal LHRH release rates of intact N and H rats were similar. Castrated N rats released LHRH at a reduced rate (50%; P less than 0.01), whereas in castrated H rats the LHRH release rate was reduced by 20%, which corresponded with the partial depletion of the MBH LHRH content in these rats. To examine the possibility of opiate involvement, LHRH release evoked by two consecutive naloxone (NAL) infusions (1 mg/ml for 30 min) was studied. The two NAL infusions resulted in two similar significant increments of LHRH in the MBH-POA of intact N and H rats. However, castration produced different effects on the NAL-induced LHRH release. First, the second NAL pulse was not effective in stimulating LHRH release from the MBH-POA of N and H castrated rats. Further, the first NAL infusion elicited a significant increase in LHRH output from the MBH-POA of N and H castrated rats, but it was significantly lower in comparison with that in their respective intact counterparts. In addition, the NAL-induced LHRH response was higher from the MBH-POA of castrated H than that in castrated N rats. These studies show that neither basal nor evoked LHRH output in vitro is affected by severe and chronic hyperprolactinemia produced by MtTW15 pituitary tumor cells in intact male rats.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of short photoperiod on hypothalamic methionine-enkephalin and LHRH content and serum beta-endorphin-like immunoreactivity (beta-end LI) levels in golden hamsters.

Adult female golden hamsters were used to study the effect of short photoperiod on the endogenous opioid system and the effect of pinealectomy on the serum beta-endorphin-like immunoreactivity (beta-end LI) levels. Hamsters were housed under either long photoperiod (14L:10D) or short photoperiod (2L:22D) and the regularity of the estrous cycles was determined by daily vaginal exfoliative cytology. Hamsters under short photoperiod became acyclic after about 7 wk. At the end of 8 wk, all the hamsters were decapitated and medial basal hypothalamic (MBH) content of LHRH and methionine-enkephalin (met-enkephalin) were measured by specific radioimmunoassays (RIA). Both LHRH and met-enkephalin levels of the MBH were significantly elevated in the short-photoperiod hamsters as compared to the normally cycling control animals under long photoperiod. In a second experiment, the effect of pinealectomy (PNX) on the serum levels of beta-end LI in the short-photoperiod hamsters was determined. The serum beta-end LI levels were increased approximately threefold in the noncyclic hamsters housed under 8 wk of short-photoperiod conditions. Pinealectomized hamsters kept under 8 wk of short-photoperiod exhibited lower serum beta-end LI levels similar to those of normally cycling hamsters kept under long photoperiod. These results indicate a possible functional relationship between increased pineal activity (as a result of short photoperiod) and increased MBH met-enkephalin, LHRH, and serum beta-end LI levels.

Discriminative effects of testosterone on hypothalamic luteinizing hormone-releasing hormone levels and luteinizing hormone secretion in castrated male rats: analyses of dose and duration characteristics.

We have previously reported that in castrated male rats gonadal steroids can raise LHRH levels in the medial basal hypothalamus (MBH). However, there was an important dissociation between episodic LH release and the MBH LHRH response to 17 beta-estradiol. In the present study we have examined the effects of varying the dose of testosterone (T) on the MBH LHRH levels, episodic LH release pattern, and pituitary responsiveness to LHRH; in addition we have determined the duration of T exposure required to elicit the MBH LHRH response. Results show that low serum levels of T (413-638 pg/ml) were just as effective as higher concentrations (1337-1776 pg/ml) in raising the MBH LHRH levels at 96 h; the minimum duration of exposure to T was 72 h. However, whereas higher T levels (greater than 1 ng/ml) suppressed LH release coincident with elevated LHRH levels, low T concentrations did not alter LH release contemporaneous with the MBH LHRH elevations. Analysis of the episodic LH secretion pattern disclosed that these low concentrations of T changed neither the number of pulses per 3 h per rat, pulse amplitude, nor the interpulse interval. On the other hand, as T concentrations were increased, pulse amplitude fell in a dose-related manner while other components of episodic LH secretion remained unchanged. Despite these differential effects of T on the LH release pattern, pituitary sensitivity to exogenous LHRH was reduced by the low as well as high levels of T. These studies show that 1) T can activate those intracellular neurosecretory events that are involved in augmented LHRH supply to the nerve terminals in the median eminence; 2) T can stimulate LHRH accumulation in the MBH by mechanisms that may not involve inhibition of LHRH release; and 3) higher T levels do not alter LH pulse frequency but depress only the pulse amplitude that may result from decrements in the amounts of LHRH released with each neural episode and depressed pituitary sensitivity to LHRH.

Progesterone-induced changes in hypothalamic luteinizing hormone-releasing hormone and catecholamines: differential effects of pentobarbital.

The effects of pentobarbital (Pnt) treatment on the progesterone (P)-induced afternoon increase in the medial basal hypothalamic (MBH) LHRH and serum LH and FSH levels in ovariectomized estradiol benzoate-primed rats were studied. Pnt injection before P blocked the afternoon rise in serum gonadotropins but failed to alter the increase in the MBH LHRH levels. Moreover, when Pnt was injected 150 min after P, the MBH LHRH content continued to rise to levels 25-37% above those seen in control rats. Analyses of LHRH concentrations in discrete hypothalamic nuclei revealed that the Pnt-induced accumulation was confined mainly to the median eminence, with a small increase in the suprachiasmatic nuclei region. P administration increased the MBH norepinephrine activity and concurrently decreased dopamine activity. Pnt was ineffective in suppressing the MBH LHRH response in these rats, but drastically reduced norepinephrine and accelerated dopamine turnovers in the MBH. These studies show 1) no definitive cause and effect relationship of the increments in MBH LHRH either with LH release (or LHRH release) or with changes in hypothalamic catecholamines induced by P treatment, and 2) that the striking rise in the MBH LHRH levels in estradiol benzoate-primed rats may represent formation of new immunoreactive LHRH predominantly in the median eminence region.

Differential effects of low serum levels of estradiol-17 beta on hypothalamic LHRH levels and LH secretion in castrated male rats.

In castrated male rats, gonadal steroid treatment raises the medial basal hypothalamic (MBH) LHRH levels and suppresses LH release. We have now examined whether the steroid-induced MBH LHRH response could occur concomitantly with normal pituitary LH secretion. Of the various concentrations tested, serum estradiol-17 beta (E2) levels in the range of 9-16 pg/ml consistently and selectively stimulated the accumulation of LHRH in the MBH after a long latency period of 60-72 h. These low E2 levels did not suppress LH release at any time interval. Serum LH levels and the episodic pattern of LH secretion were similar to those of castrated control rats. It is unlikely that enhanced pituitary sensitivity in E2-treated rats may have compensated for a decrease in LHRH release since the LH response to exogenous LHRH was similar in E2-treated and control rats. These studies show that dissociation of the feedback effects on the MBH LHRH and pituitary gland LH release may be obtained with low serum concentrations of E2 in male rats; low E2 levels stimulate LHRH accumulation in the MBH by mechanisms which may not involve inhibition of LHRH release.

Modulation of Hypothalamic Luteinizing Hormone-Releasing Hormone Levels by Intracranial and Subcutaneous Implants of Gonadal Steroids in Castrated Rats: Effects of Androgen and Estrogen Antagonists*

Castration-induced hypersecretion of pituitary LH was associated with a marked reduction in the medial basal hypothalamic (MBH) LHRH contents. Testosterone (T) therapy promptly reduced LH to intact serum levels within 24 h, while the MBH LHRH levels were restored only after 7 days. Withdrawal of T replacement caused hypersecretion of LH within 8 h, although the first significant decrease in the MBH LHRH contents occurred between days 4–7. The two metabolites of T, 5α-dihydrotestosterone (DHT) and 17β-estradiol (E2), were as effective as T in raising the MBH LHRH content; however, their effects on serum LH were variable. The antiandrogen flutamide, but not the antiestrogen nafoxidine hydrochloride, blocked the stimulatory effects of T and DHT on MBH LHRH levels. Nafoxidine treatment was also ineffective in reversing this central effect of E2. None of these treatments significantly altered LHRH content in the preoptic area (POA). In view of the wide distribution of LHRH-containing neurons in the septal POA ...

DYNAMIC CHANGES IN HYPOTHALAMIC LH-RH LEVELS ASSOCIATED WITH THE OVARIAN STEROID-INDUCED GONADOTROPHIN SURGE

Pro-oestrous-type LH surges were induced by treating ovariectomized rats (day 0) with either oestradiol benzoate (OeB, 40 micrograms/rat) or oestrogen (10 micrograms/rat followed by progesterone (4 mg/rat, day 2, OeBP). To characterize the temporal changes associated with the LH surge on day 2, LH-RH levels in the medial basal hypothalamus (MBH) and the pre-optic area (POA) were analyzed by RIA in rats sacrified at hourly intervals between 10.00--19.00 h. Following these treatments the LH-RH content of POA fluctuated somewhat but no definite pattern was observed. In OeB rats, the MBH LH-RH levels were unchanged through the initial LH surge period (13.00--15.00 h) declining significantly, as in the OeBP group, near the end of the surge period (19:00 h). However, in OeBP rats, the MBH LH-RH contents decreased at 14.00 h and these were followed by markedly elevated levels between 15.00--17.00 h coincident with maximal increments in serum LH. The OeBP group alone showed a surge of serum FSH.

Effect of norepinephrine synthesis inhibitors and a dopamine agonist on hypothalamic LH-RH serum gonadotrophin and prolactin levels in gonadal steroid treated rats.

ABSTRACT The relationship between the medial basal hypothalamus (MBH) LH-RH activity and LH release was studied following progesterone (P) treatment of oestrogen-primed ovariectomized rats (day 0). Following P administration at 10.00 h (day 2) serum LH levels increased rapidly after 13.00 h to peak levels attained at 15.00 h and maintained until 18.00 h. Coincident with the onset of augmented release and peak serum LH concentrations at 15.00 h there was a significant enhancement in the MBH LH-RH activity. Thereafter, the MBH LH-RH stores promptly fell and remained at morning low levels through the rest of the LH surge period. P treatment also stimulated release of FSH and prolactin in the afternoon. Administration of norepinephrine (NE) synthesis inhibitors, diethyldithiocarbamate (DDC) and U-14,624 before P blocked the afternoon increments in serum gonadotrophins and the MBH LH-RH levels; prolactin release was also suppressed in DDC treated rats. In contrast, lergotrile mesylate (dopamine agonist) treatment prior to P administration suppressed only the afternoon increase in prolactin release. These studies show that (1) P can stimulate MBH LH-RH activity in oestrogen-primed rats and these effects are transmitted to the LH-RH peptidergic neurons via NE synapses in the preoptic area and (2) a common central NE system may mediate the stimulatory feedback effects of P on gonadotrophin and prolactin release.

TEMPORAL CHANGES IN THE HYPOTHALAMIC AND SERUM LUTEINIZING HORMONE-RELEASING HORMONE (LH-RH) LEVELS AND THE CIRCULATING OVARIAN STEROIDS DURING THE RAT OESTROUS CYCLE

Cycling female rats were sacrificed at various times during the 4-day oestrous cycle. LH-RH in the medial basal hypothalamus (MBH) and serum LH-RH, LH, oestradiol (Oe2), and progesterone were analyzed by radioimmunoassays. The MBH LH-RH content was lowest at 19.55-20.15 h during pro-oestrus but increased gradually through oestrus and dioestrus I to significantly higher values at noon of dioestrus II, and then decreased precipitously at 18.00 h. Although serum LH levels remained basal from midnight of pro-oestrus through dioestrus II, serum LH-RH levels were significantly elevated at 12.00 and 21.00 h on oestrus (vs. the midnight pro-oestrus levels) and declined between 15.00 and 22.00 h of dioestrus I. In conjunction with high Oe2 titres, LH-RH in the MBH and serum declined initially during pro-oestrus between 14.07-14.45 h and then increased abruptly between 14.55-15.55 h; the pre-ovulatory rise in serum LH was observed from 16.05 h onwards. LH-RH activity in the MBH and serum receded gradually to the early afternoon levels by 19.55-24.00 h while the peak in serum LH was observed at 17.05-18.05 h. These studies suggested that the hypersecretion of the MBH LH-RH (synthesis + release) may be responsible for the pre-ovulatory discharge of LH.

Tissue levels of luteinizing hormone-releasing hormone in the preoptic area and hypothalamus, and serum concentrations of gonadotropins following anterior hypothalamic deafferentation and estrogen treatment of the female rat.

Female rats were ovariectomized prior to anterior hypothalamic deafferentation (AHD) or sham AHD of the medial basal hypothalamus (MBH). LHRH in the MBH and the preoptic area (POA), and the blood concentrations of LHRH, FSH, and LH were measured following decapitation at the end of 30 days in experiment 1 and 28 days in experiment 2. Interruption of anterior neural links of the MBH resulted in a drastic reduction in the amounts of LHRH in the MBH and a significant increment in the POA. The circulating levels of FSH (experiments 1 and 2) and LH (experiment 2) were significantly depressed whereas serum LHRH was unaltered following deafferentation. In experiment 3, rats were ovariectomized 25 days following either AHD or sham AHD, and were given estradiol benzoate (EB, 10 mug/rat in oil, SC) or oil alone on day 7 postovariectomy and sacrificed 2 days later. Relative to sham AHD, a reduction in the MBH LHRH and a pronounced elevation in the POA LHRH was observed in AHD rats. Serum LH and FSH concentrations were also significantly decreased. EB treatment significantly lowered serum gonadotropins, whereas LHRH levels in the MBH of both AHD and sham AHD rats increased two-fold. Serum LHRH concentrations were also significantly reduced in AHD rats following EB injection. These studies indicate that 1) since a substantial portion of the LHRH activity normally detected in the MBH of female rats appears to be derived from that synthesized in the rostral regions, it is logical to infer that POA LHRH may be involved in the tonic as well as the cyclic discharge of LH, and 2) an increase in the LHRH content of the MBH after estrogen treatment may be due to a partial inhibition in release and/or an increased rate of synthesis of LHRH in the MBH.