Biomedical subjects

M Selmanoff

Publications and source records attributed to M Selmanoff.

At least 19 recordsLinked to original sources

PubMed1996

GABAergic neuronal activity and mRNA levels for both forms of glutamic acid decarboxylase (GAD65 and GAD67) are reduced in the diagonal band of Broca during the afternoon of proestrus.

There is considerable evidence that GABAergic neurons play an important role in the regulation of gonadotropin-releasing hormone (GnRH) secretion, and that these neurons may mediate the feedback actions of gonadal steroids on GnRH neurons. The aim of the present study was to investigate whether endogenous changes in ovarian steroid secretion during the estrous cycle influenced GABAergic neuronal activity in the preoptic region of the hypothalamus, and in other steroid-sensitive brain regions. Intact, adult female rats were sacrificed at various times during the days of metestrus or proestrus. GABAergic neuronal activity was estimated by measuring the rate of accumulation of GABA in microdissected brain regions after pharmacological inhibition of GABA degradation. Concentrations of mRNA for both forms of glutamic acid decarboxylase (GAD65 and GAD67) were quantified in microdissected brain regions by a microlysate ribonuclease protection assay. In the diagonal band of Broca at the level of the organum vasculosum of the lamina terminalis (DBB(ovlt)), GABAergic neuronal activity was significantly reduced during the afternoon of proestrus compared with the morning of either proestrus or metestrus. In the lateral septal nucleus, GABAergic neuronal activity was significantly increased in the afternoon of proestrus compared with the morning. There were no significant effects of time of day or day of estrous cycle in the medial preoptic nucleus, median eminence, ventromedial nucleus, suprachiasmatic nucleus, medial septal nucleus, hippocampus (CA1 region), or cingulate cortex. In the DBB(ovlt), mRNA levels for both GAD65 and GAD67 were significantly reduced in the afternoon of proestrus compared with the afternoon of metestrus. By contrast, there was no change in GAD65 and GAD67 mRNA levels in the cingulate cortex at any of the times examined. These results demonstrate that GABAergic neuronal activity, and mRNA levels for both GAD65 and GAD67, are reduced in the DBB(ovlt) during the afternoon of proestrus. These results support the hypothesis that decreased GABAergic neuronal activity in this region plays a major permissive role in the generation and maintenance of the estrogen-induced LH surge.

Animals↗

PubMed1996

Hormonal and neurotransmitter regulation of GnRH gene expression and related reproductive behaviors.

Gonadotropin-releasing hormone (GnRH), having a highly conserved structure across mammalian species, plays a pivotal role in the control of the neuroendocrine events and the inherent sexual behaviors essential for reproductive function. Recent advances in molecular genetic technology have contributed greatly to the investigation of several aspects of GnRH physiology, particularly steroid hormone and neurotransmitter regulation of GnRH gene expression. Behavioral studies have focused on the actions of GnRH in steroid-sensitive brain regions to understand better its role in the facilitation of mating behavior. To date, however, there are no published reports which directly correlate GnRH gene expression and reproductive behavior. The intent of this article is to review the current understanding of the way in which changes in GnRH gene expression, and modifications of GnRH neuronal activity, may ultimately influence reproductive behavior.

Animals↗

PubMed1996

Three forms of gonadotropin-releasing hormone in a perciform fish (Sparus aurata): complementary deoxyribonucleic acid characterization and brain localization.

Three forms of GnRH-salmon (sGnRH), seabream (sb-GnRH), and chicken (cGnRH-II)-have been described in the gilthead seabream (Sparus aurata) brain, and the cDNA encoding the sbGnRH precursor was recently isolated. In the present study, the cDNAs encoding the sGnRH and cGnRH-II were isolated and characterized, and the neurons producing the three GnRHs were localized in the seabream brain. Fragments of sGnRH and cGnRH-II cDNAs were amplified by polymerase chain reaction and used as probes to isolate the full-length cDNAs from a brain cDNA library. The cDNA encoding the cGnRH-II precursor is 573 nucleotides (nt) long, and the cDNA encoding the sGnRH precursor is 1971 nt in length with an unusually long 5' untranslated region. Specific single-strand DNA probes for in situ detection of mRNA were designed according to nonconserved regions among the three GnRH c-DNAs. Localization of GnRH mRNA-producing cells in the brain revealed five distinct populations of cells: sGnRH-producing cells in the ventromedial olfactory bulbs and the terminal nerve, sbGnRH-producing cells in the preoptic area and the ventral thalamus, and cGnRH-II-producing cells in the midbrain tegmentum. The discrete sites of expression of the three forms of GnRH indicate that only sbGnRH is directly involved in the control of gonadotropin secretion.

Amino Acid Sequence↗

PubMed1996

Antiandrogen microimplants into the rostral medial preoptic area decrease gamma-aminobutyric acidergic neuronal activity and increase luteinizing hormone secretion in the intact male rat.

gamma-Aminobutyric acid (GABA)ergic neurons terminating in the rostral hypothalamus are stimulated by testosterone. To investigate whether this action is mediated locally through androgen receptors in the rostral hypothalamus, bilateral microcannulas (28 gauge) containing the androgen receptor antagonist, hydroxyflutamide (HF), were stereotaxically implanted into the rostral medial preoptic area (rMPA) just dorsal to the major population of GnRH cell bodies. Two days later, blood samples were collected for assay of LH, and animals were killed for determination of GABAergic neuronal activity in tissue dissected from the site of the implanted cannulas. Animals were decapitated either without treatment or 60 min after inhibition of GABA degradation by aminooxyacetic acid (100 mg/kg, ip). The rate of GABA accumulation in the tissue after aminooxyacetic acid treatment was used as a measure of GABA turnover. Levels of messenger RNA for both forms of glutamic acid decarboxylase (GAD65 and GAD67), the rate-limiting enzyme responsible for GABA synthesis also were measured by a microlysate ribonuclease protection assay. LH levels were significantly increased (1.8-fold) in HF-treated animals compared with controls. In the MPA, beneath the implant cannulas, GABA turnover was significantly reduced in HF-treated rats. There was no effect of treatment in the frontal cortex, which was used as a control region. Surprisingly, levels of messenger RNA for both GAD65 and GAD67 were significantly increased in HF-treated rats. The results indicate that GABAergic neurons terminating in the rostral hypothalamus are tonically stimulated by testosterone acting by means of androgen receptors localized in this region. These findings support the working hypothesis that androgen-sensitive GABAergic neurons in the rMPA mediate the negative feedback action of testosterone on GnRH secretion in the male rat.

Androgen Antagonists↗

PubMed1996

Prolactin- and testosterone-induced inhibition of LH secretion after orchidectomy: role of catecholaminergic neurones terminating in the diagonal band of Broca, medial preoptic nucleus and median eminence.

Central catecholaminergic neurones projecting to specific hypothalamic structures are involved in stimulating and inhibiting the activity of the GnRH-containing neurosecretory neurones. Both testosterone and elevated circulating prolactin (PRL) levels inhibit postcastration LH release. Three groups of adult male rats were orchidectomized and adrenalectomized, received corticosterone replacement and were: (i) administered purified ovine PRL (oPRL; 2400 microgram/s.c. injection) or (ii) its diluent, polyvinylpyrrolidone (PVP), every 12 h, or (iii) received physiological testosterone replacement for 2 days. At 0, 2 and 6 days postcastration, norepinephrine (NE), epinephrine (E) and dopamine (DA) turnover were estimated by the alpha-methyl-p-tyrosine method in three micro-dissected hypothalamic structures: the diagonal band of Broca at the level of the organum vasculosum of the lamina terminalis (DBB(ovlt)), the medial preoptic nucleus (MPN) and the median eminence (ME). In control (PVP-treated) rats, serum LH concentrations increased eightfold at 2 and 6 days postcastration and this rise was prevented by testosterone. oPRL treatment transiently suppressed LH secretion at 2 but not 6 days postcastration. Castration significantly decreased basal rat PRL (rPRL) levels at 2 and 6 days and testosterone administration partially prevented this effect. NE turnover in the ME and E turnover in the MPN increased markedly at 2 and 6 days postcastration, and testosterone replacement for 2 days prevented these increases. Thus, noradrenergic neurones innervating the ME and adrenergic neurones innvervating the MPN may drive postcastration LH secretion by providing stimulatory afferent input to the GnRH neurones. It was striking to observe that oPRL blocked the increases in both ME NE and MPN E turnover at 2 but not 6 days postcastration. Hence, oPRL may transiently suppress LH release by an inhibitory action on these NE and E neurones. DA turnover in the DBB(ovlt) was significantly decreased by 6 days postcastration. Testosterone-treated (2 days postcastration) and oPRL-treated (2 and 6 days postcastration) rats exhibited turnover values indistinguishable from day 0 controls. Hence, the A14 dopaminergic neurones, which synapse on GnRH neurones in the rostral preoptic area and may exert an inhibitory effect on them, are positively regulated by PRL and perhaps by testosterone as well. Autoregulatory feedback suppression of endogenous rPRL secretion by oPRL was observed both 2 and 6 days postcastration. In contrast to the A14 dopaminergic neurones, turnover in the A12 tuberoinfundibular dopaminergic (TIDA) neurones innervating the ME increased significantly by 6 days postcastration in control rats while oPRL administration further increased ME DA turnover at both 2 and 6 days. Hence, autofeedback regulation of rPRL secretion persists through at least 6 days of oPRL exposure temporally associated with markedly increased turnover in the TIDA neurones. In summary, our results support the hypothesis that the inhibitory effect of PRL on postcastration LH release is mediated by suppression of the activity of NE neurones innervating the ME and E neurones terminating in the MPN which, with time, become refractory to continued PRL exposure.

Adrenalectomy↗

PubMed1995

Orchidectomy and NMDA increase GnRH secretion as measured by push-pull perfusion of rat anterior pituitary.

Using push-pull perfusion to measure concentrations of gonadotropin-releasing hormone (GnRH) in the extracellular fluid of the anterior pituitary gland of the male rat, we have measured GnRH release at specific times before and after castration and in response to acute administration of N-methyl-D-aspartate (NMDA). After castration (7 days), mean GnRH levels were substantially increased (4.3-fold) compared with intact controls (0.94 +/- 0.16 vs. 0.22 +/- 0.08 pg/10 min, respectively, P < 0.05) due to an increase in both the frequency and amplitude of GnRH pulses. Testosterone partially reduced GnRH release (0.62 +/- 0.10 pg/10 min). NMDA induced a rapid increase in plasma luteinizing hormone (LH) in both intact and castrated rats and increased GnRH concentrations in the perfusion samples (P < 0.05). There was no change in LH release induced by two doses of injected GnRH (5 and 25 ng/100 g body wt) 2 days after castration, but by 6 days after castration the response to both doses was significantly increased. These results demonstrate that GnRH release in the male rat is acutely increased by NMDA and is chronically increased after orchidectomy. Increased pituitary sensitivity to GnRH also contributes to the hypersecretion of LH after castration, particularly at longer times after removal of testosterone negative feedback.

Animals↗

PubMed1994

Castration-induced decrease in the activity of medial preoptic and tuberoinfundibular GABAergic neurons is prevented by testosterone.

We recently determined that castration specifically decreased GABA turnover in discrete rostral and mediobasal hypothalamic structures. This study aimed to investigate whether testosterone could stimulate GABAergic neuronal activity in these hypothalamic GABAergic neurons in the castrate rat, and to compare the effects of episodic testosterone replacement with the constant levels provided by subcutaneous testosterone implants. Animals were divided into 4 experimental groups: intact, 48 h castrate, 48 h castrate+testosterone capsules (2 x 30 mm Silastic implants, 1.57 mm ID, 3.18 mm OD) and 48 h castrate+testosterone injections (100 micrograms/injection s.c., every 8 h). GABA concentrations were measured in 4 microdissected brain regions either before or 60 min after inhibition of the GABA degrading enzyme, GABA transaminase, by injection of aminooxyacetic acid (AOAA, 100 mg/kg i.p.). The rate of GABA accumulation in the tissue following injection of AOAA was used as an index of GABAergic neuronal activity. Castration resulted in a 10-fold increase in serum LH concentrations compared with intact rats. Either mode of testosterone administration completely prevented this castration-induced LH rise. In the diagonal band of Broca at the level of the organum vasculosum of the lamina terminalis, the medial preoptic nucleus and in the median eminence, GABA turnover was significantly reduced by castration to approximately 50% that of intact rats. Either testosterone implants or testosterone injections prevented this castration-induced decrease in GABA turnover, such that the turnover rates were not significantly different from intact rats. There was no effect of castration with or without testosterone replacement in the cingulate cortex.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1994

Prolactin- and testosterone-induced inhibition of LH secretion after orchidectomy: role of preoptic and tuberoinfundibular gamma-aminobutyric acidergic neurones.

The inhibitory amino acid neurotransmitter gamma-aminobutyric acid (GABA) may play an important role in the regulation of LH-releasing hormone secretion. The present study examined the effect of prolactin on GABAergic neuronal activity in microdissected brain regions of the orchidectomized rat, to determine whether inhibition of LH secretion after castration by acute hyperprolactinaemia was associated with prolactin-induced changes in GABAergic neuronal activity. The effects of prolactin were contrasted with the effects of testosterone on hypothalamic GABAergic neurones after orchidectomy. GABA concentrations were measured by high pressure liquid chromatography in eight microdissected brain regions in untreated rats and 60 min after inhibition of the GABA catabolic enzyme GABA transaminase by injection of amino-oxyacetic acid (AOAA). The rate of GABA accumulation in microdissected brain regions following injection of AOAA was taken as an index of GABAergic neuronal activity. Rats were divided into seven experimental groups: intact controls, 2 days after castration, 2 days after castration with prolactin treatment (2.5 mg ovine prolactin injected s.c. every 12 h, starting at the time of castration), 2 days after castration with testosterone replacement (30 mm silicone elastomer implant containing crystalline testosterone), 6 days after castration, 6 days after castration with prolactin treatment, and 6 days after castration with testosterone replacement. Both 2 and 6 days after castration, plasma LH was markedly elevated above levels in intact rats, and AOAA-induced GABA accumulation was significantly decreased in the diagonal band of Broca at the level of the organum vasculosum of the lamina terminalis, in the medial preoptic nucleus and in the median eminence. Hyperprolactinaemia significantly reduced LH levels 2 days but not 6 days after castration. GABAergic neuronal activity, however, was not significantly affected by prolactin at either time. Testosterone replacement blocked the postcastration elevation in plasma LH and prevented the castration-induced suppression of GABAergic neuronal activity both 2 and 6 days after castration. There were no castration- or hormone-induced changes in GABAergic neurones observed in the medial or lateral septum, caudate nucleus, cingulate cortex or arcuate nucleus. These results demonstrate that the activity of GABAergic neurones terminating in the rostral hypothalamus and the median eminence is positively regulated by testosterone, and that these steroid-sensitive GABAergic neurones may be important in the negative-feedback control of LH secretion. Inhibition of LH secretion by hyperprolactinaemia, however, may not be mediated by changes in GABAergic neuronal activity.

Animals↗

PubMed1993

Regional variation in gamma-aminobutyric acid turnover: effect of castration on gamma-aminobutyric acid turnover in microdissected brain regions of the male rat.

This study compared the turnover of GABA neurons in different brain areas of the male rat and examined the effect of castration on GABA turnover in regions of the brain associated with the control of gonadotropin secretion. To estimate GABA turnover, GABA was quantified by HPLC in microdissected brain regions 0, 30, 60, 90, and 120 min after inhibition of GABA degradation by aminooxyacetic acid (100 mg/kg, i.p.). GABA accumulation was linear in all areas for 90 min (p < 0.01), and GABA turnover was estimated as the slope of the line formed by increased GABA concentration versus time, determined by linear regression. There was considerable regional variation both in the initial steady-state concentrations of GABA and in the rates of GABA turnover. Of 10 discrete brain structures, GABA turnover was highest in the medial preoptic nucleus and lowest in the caudate nucleus. Turnover times in the terminal fields of known GABAergic projection neurons ranged sevenfold, from 2.6 h in the substantia nigra to 0.4 h in the lateral vestibular nucleus. The effect of castration on GABA turnover in 13 microdissected brain regions was investigated by measuring regional GABA concentrations before and 30 min after injection of aminooxyacetic acid in intact rats or 2 or 6 days postcastration. Following castration, steady-state GABA concentrations were increased, and GABA turnover decreased in the diagonal band of Broca, the medial preoptic area, and the median eminence. GABA turnover increased in the medial septal nucleus and was unaffected in the cortex, striatum, and hindbrain. These results are consistent with the hypothesis that testosterone negative-feedback control of luteinizing hormone-releasing hormone involves steroid-sensitive GABAergic neurons in the rostral and medial basal hypothalamus.

Aminooxyacetic Acid↗

PubMed1993

Graded hyperprolactinemia first suppresses LH pulse frequency and then pulse amplitude in castrated male rats.

We recently demonstrated that the ability of administered ovine prolactin (oPRL) to suppress postcastration LH secretion exhibited a clear dose dependency. In the present study, we determined whether this dose-related suppression of mean LH levels resulted from differential, dose-related effects of oPRL on LH pulse amplitude and pulse frequency. Adult male rats were orchiectomized and adrenalectomized, implanted with an atrial cannula and a 50% corticosterone pellet, and injected every 12 h with oPRL or its polyvinylpyrrolidone (PVP) vehicle beginning at time 0. Increasing doses of oPRL (600, 2,400 and 9,600 micrograms/injection) suppressed mean LH titers in a dose-dependent manner at 48 h postcastration. The mean maximal LH increments (delta LH) to two LHRH challenges at two doses (5 and 25 ng LHRH/100 g body weight) were unaffected by oPRL administration. The 600 micrograms oPRL dose significantly suppressed mean LH values by markedly increasing the inter-peak interval (42.6 +/- 6.7 min) compared with controls (26.6 +/- 0.2 min) since the pulse amplitude was unaffected (2.8 +/- 0.4 vs. 2.6 +/- 0.4 ng/ml, respectively). The two higher oPRL doses suppressed both LH pulse frequency and pulse amplitude. Hence, elevated PRL levels first suppress LH pulse frequency and then, at higher concentrations, pulse amplitude as well. Presuming that LHRH pulses result from ensemble firing of all or a significant proportion of the LHRH neurons projecting to the median eminence, the present data suggest that the neurons first affected by elevated PRL levels are the ones responsible for this frequency of this coordinated firing.(ABSTRACT TRUNCATED AT 250 WORDS)

Algorithms↗

PubMed1993

Hyperprolactinemia suppresses the luteinizing hormone responses to N-methyl-D-aspartate, epinephrine, and neuropeptide-Y in male rats.

This study characterizes the responses of LHRH neurons to N-methyl-D-aspartate (NMDA), norepinephrine, epinephrine (E), and neuropeptide-Y (NPY), as evidenced indirectly by the measurement of circulating LH titers, and investigates whether neurons using these compounds as neurotransmitters might be involved in mediating hyperprolactinemic (HP) suppression of LH release. Male rats were orchidectomized, adrenalectomized, and implanted with a testosterone-containing Silastic capsule, a 50% corticosterone pellet, and third cerebroventricular and right atrial cannulae at time zero. Rats received sc injections of ovine PRL (2400 micrograms/250 microliters) in a polyvinylpyrrolidone depot or vehicle every 12 h for 48 h when experiments were performed. The mean maximal LH increments (delta LH) in response to two doses of LHRH (0.4 and 0.8 ng/100 g BW) were not altered in HP rats, indicating that ovine PRL did not cause a change in pituitary responsiveness. NMDA (20 mg/kg BW, iv)-induced LH release peaked 5 min after injection. The delta LH (0-5 min) in HP rats was suppressed by 53% compared with the control value. Epinephrine [5, 10, and 15 micrograms/2 microliters, intracerebroventricularly (icv)], but not norepinephrine (20 and 40 micrograms/2 microliters, icv), produced dose-dependent LH responses that peaked at 10 min. The delta LH (0-10 min) in HP rats in response to 10 micrograms/2 microliters E was suppressed by 68% compared with the control value. Two doses of NPY (2 and 10 micrograms/2 microliters, icv) produced dose-dependent LH increments that peaked at 10 min. In HP rats, the delta LH (0-10 min) in response to 10 micrograms/2 microliters NPY was suppressed 52% compared with the control value. The combined administration of E (10 or 16 micrograms) and NPY (5 or 10 micrograms) produced mean maximal LH responses that significantly exceeded the additive responses of these compounds individually. This synergistic effect may be mediated by separate adrenergic and NPYergic afferents to the LHRH neurons or may, in fact, reflect corelease of these two neurotransmitters from the same neurons. The LH responses to NMDA, E, and NPY were all inhibited in HP rats. This suggests that elevated PRL levels act on the LHRH neurons, either directly or indirectly through an inhibitory afferent neuronal system, to decrease their responsivity to all stimuli.

Adrenalectomy↗

PubMed1993

Differential effects of adrenalectomy on the prolactin-induced suppression of LH and FSH secretion after castration in male rats.

Hyperprolactinaemia inhibits gonadotrophin secretion in males and females of many species. The aim of this study was to determine the role of the adrenal gland in mediating the inhibitory effects of prolactin by contrasting the effects of acute hyperprolactinaemia on LH and FSH secretion in adrenal-intact and adrenalectomized rats with and without physiological corticosterone replacement. Adult male rats were administered purified ovine prolactin every 12 h (2.4 mg per injection s.c.) beginning at the time of castration. Blood samples were collected every 3 h for 36 h, then every 12 h until 10 days after castration. Ovine prolactin significantly reduced LH secretion in all groups from approximately 15 to 48 h after castration. In contrast, plasma FSH concentrations were reduced by ovine prolactin from 21 to 48 h only in the adrenal-intact rats and not in the adrenalectomized or adrenalectomized plus corticosterone groups. In all groups, ovine prolactin inhibited endogenous prolactin secretion in rats by short-loop autofeedback as soon as 3 h after the first ovine prolactin injection and throughout the 10 days of the study. Adrenalectomy per se, with or without corticosterone replacement, also had a differential effect on LH and FSH secretion after castration, causing only a transient delay in the rise in LH after castration, but inducing a significant and long-lasting inhibition of FSH secretion. The results demonstrate that ovine prolactin-induced suppression of LH secretion after castration occurs with or without the adrenal glands. Suppression of FSH secretion after castration by ovine prolactin, however, may involve an adrenal component.(ABSTRACT TRUNCATED AT 250 WORDS)

Adrenal Glands↗

PubMed1991

Tyrosine hydroxylase and POMC mRNA in the arcuate region are increased by castration and hyperprolactinemia.

We have examined the changes which occur in neuronal expression of tyrosine hydroxylase (TH) and proopiomelanocortin (POMC) mRNA in response to castration and hyperprolactinemia (HP) in male rats. Steady-state mRNA levels were determined by quantitative in situ hybridization histochemistry (ISHH) using 35S-labeled synthetic 48-base oligodeoxynucleotide probes. Castration produced a 27% increase in TH mRNA in the periventricular and arcuate nuclei. PRL-exposed rats exhibited a further 27% increase in the level of TH mRNA and a striking 48% increase in POMC mRNA in periarcuate region cell bodies. These results indicate that gonadal steroids and PRL are involved, either directly or indirectly, in regulating the biosynthesis of TH and POMC in the hypothalamus.

Analysis of Variance↗

PubMed1991

Dose-dependent suppression of postcastration luteinizing hormone secretion exerted by exogenous prolactin administration in male rats: a model for studying hyperprolactinemic hypogonadism.

We examined the inhibitory effects of acute hyperprolactinemia on the postcastration rise in mean luteinizing hormone (LH) levels in adult male rats. The animals were administered purified ovine prolactin (oPRL) subcutaneously in a polyvinyl-pyrrolidone depot every 12 h for 96 h, beginning at the time of castration. oPRL suppressed postcastration LH secretion from 24 to 72 h when the effect spontaneously reversed in the face of elevated oPRL levels. oPRL suppressed postcastration LH secretion in a graded, dose-dependent fashion. The rats were administered increasing doses of oPRL and studied 48 h later, a time of maximal LH suppression. The first significant inhibition began in the high physiological range (about 200 ng/ml) and continued into the pathophysiological tumor range (about 2,500 ng/ml) of circulating oPRL levels. The highest oPRL levels markedly suppressed postcastration LH release. Autoregulatory feedback of oPRL on endogenous rat PRL secretion was examined in the dose-response study. It was striking to discover that PRL autofeedback appeared regulated as a threshold instead of a graded dose response and, more importantly, that the oPRL dose which produced the first significant suppression of LH secretion was the same dose which exerted PRL autofeedback. These findings indicate that postcastration LH secretion is inhibited by circulating PRL titers (about 200 ng/ml) which are above basal and stress-induced levels, but are within the range encountered during pregnancy, pseudopregnancy, and lactation. In addition, a common hypothalamic mechanism (perhaps dopaminergic), activated by elevated oPRL levels in this range, may inhibit both LH and rat PRL secretion.

Adrenalectomy↗

PubMed1991

Effects of tumor-induced hyperprolactinemia on LH secretion following stimulation of the medial preoptic area, pituitary responsiveness and the estrogen-induced LH surge.

In the present study we utilized the 7315a PRL- and ACTH-secreting tumor to induce a hyperprolactinemic (HP) state sufficient to profoundly suppress the postcastration LH rise in female rats. Tumor-induced prolactin levels which ranged 2,000-3,000 ng/ml substantially reduced the LH rise in both ovariectomized (OVX) and OVX + estradiol-17 beta (E2)-treated rats. Bilateral electrochemical stimulation (ECS, 100 microA DC for 60 s) of the ventral diagonal band of Broca-medial preoptic area (DBB-MPOA) resulted in comparable LH responses in control and HP rats in the presence of absence of estradiol. Transient decreases in PRL release occurred following ECS of the DBB-MPOA. Pituitary responsiveness was assessed with two LHRH challenges spaced 60 min apart at doses of 25 and 50 ng LHRH/100 g body weight. The mean maximal LH increments (delta LH) to some of these LHRH challenges were decreased in HP rats. Finally, the LH surge induced in the afternoon in OVX + E2-treated rats was diminished 71% by the presence of the PRL-secreting tumor.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1991

Single cell levels of hypothalamic messenger ribonucleic acid encoding luteinizing hormone-releasing hormone in intact, castrated, and hyperprolactinemic male rats.

We have examined the changes that occur in neuronal expression of LHRH mRNA in response to castration and hyperprolactinemia in male rats. Single cell levels of LHRH mRNA were determined by quantitative in situ hybridization histochemistry using an 35S-labeled synthetic 48-base oligodeoxynucleotide probe and quantitative autoradiography. Nine days postcastration, a 10.4-fold increase in mean plasma LH titers was observed which was associated with significantly increased LHRH mRNA in rostral hypothalamic neuronal cell bodies. Both increases were blocked in rats rendered hyperprolactinemic by the presence of the 7315a PRL-secreting pituitary tumor. The location and number of neurons expressing LHRH mRNA were unchanged, indicating that these differences were attributable to amounts of mRNA expressed per neuron. Experimental differences occurred in LHRH perikarya situated throughout the rostral hypothalamus from the organum vasculosum of the lamina terminalis to the caudal extent of the medial preoptic nucleus. These results suggest that gonadal steroids and PRL are involved, either directly or indirectly, in regulating the biosynthesis of LHRH in the rostral hypothalamus.

Animals↗

PubMed1990

Changes in the kinetics of [3H]dopamine release from median eminence and striatal synaptosomes during aging.

The release of preaccumulated tritium-labeled dopamine [( 3H]DA) was examined in isolated nerve terminals (synaptosomes) prepared from the median eminence (ME) and corpus striatum (CS) of young (2-3 months), middle-aged (11-12 months), and old (19-21 months) male rats. Fractional release of [3H]DA was measured over 1- to 10-sec time intervals under basal (5 mM K+) and depolarizing (75 mM K+) conditions in the presence of calcium. No differences in the rate of basal efflux between the age groups were observed in either ME or CS preparations. Fast-phase evoked [3H]DA release (0-1 sec) from CS synaptosomes was unchanged from young to middle-aged, but was decreased in old preparations. These data demonstrate that the nigrostriatal nerve terminal has a diminished ability to respond fully to depolarizing stimuli in advanced age. Mean serum PRL levels in old rats were 2.3-fold greater than those in both young and middle-aged rats, while serum LH levels were decreased 2.0-fold in middle-aged and old compared with those in young rats. The fact that LH levels were already decreased in middle-aged rats while PRL levels had not yet increased suggests that decreased gonadotropin titers in old rats do not result from the coincident hyperprolactinemia. In ME synaptosomes, depolarization-induced [3H]DA release was decreased at all time points in middle-aged preparations compared to that in young preparations. The reduced fractional release from the middle-aged ME synaptosomes was due to a depressed rate of release during the initial second of depolarization. Evoked release from ME terminals of old rats was comparable to that measured in the young group. Thus, there occurred an age-related biphasic change in the initial rate of evoked DA release from ME synaptosomes. Diminished response of ME dopaminergic terminals to depolarizing stimuli during middle age may be important in the later development of hyperprolactinemia in aging male rats. The increased PRL available for feedback on the tuberoinfundlbular dopaminergic neurons may, in turn, be associated with the apparent recovery of evoked [3H]DA release from ME synaptosomes of old rats.

Aging↗

PubMed1988

Phorbol esters potentiate rapid dopamine release from median eminence and striatal synaptosomes.

In the present study, we investigated the ability of phorbol esters to potentiate Ca2+-dependent depolarization-induced release of tritium-labeled dopamine ([3H]DA) from median eminence and striatal synaptosomes. Phorbol esters potentiated [3H]DA release in a concentration-dependent manner in both kinds of dopaminergic nerve terminals and with a potency series similar to that reported for stimulation of protein kinase-C (PKC) activity in other cell systems. Evoked [3H]DA release was increased by 12-O-tetradecanoylphorbol-13-acetate (TPA; 10(-7) M) after 1, 3, 5, and 10 sec of depolarization. The effect of TPA was suppressed by sphingosine, a PKC inhibitor. TPA enhanced [3H]DA release evoked by high K+, veratridine or the Ca2+ ionophore A23187. Phorbol ester potentiation was found to be depolarization dependent, as it was present from 30-75 mM, but not at 5-20 mM external K+. Potentiation was seen at all external Ca2+ concentrations studied between 0.01-3 mM. However, in the absence of external free Ca2+ (i.e. with 0.1 mM EGTA), the phorbol effect was not present. These data indicate that an increase in intrasynaptosomal Ca2+ concentration is necessary for the enhancement of [3H]DA release by phorbol esters to occur. The combination of TPA and the Ca2+ ionophore A23187 does not show the marked synergism observed in some other systems, that is maximal release was not reinstated. This suggests that in dopaminergic nerve terminals, activation of PKC has a modulatory, rather than a mediating, effect on release. Recently, we have shown that hyperprolactinemia stimulated [3H]DA release from median eminence synaptosomes by an external Ca2+-independent mechanism which might involve the PKC pathway. However, in the present work we found that the TPA and PRL effects on evoked [3H]DA release were additive, suggesting that two independent mechanisms are involved. A marked difference in the sensitivity of median eminence and striatal synaptosomes to calcium ionophore was discovered. The concentration of A23187 required to support significant [3H]DA release from median eminence synaptosomes was 3-fold greater than that in striatal synaptosomes. This suggests that some difference in calcium homeostatic processes exists, such as a higher resting striatal Ca2+ concentration, in these two kinds of dopaminergic nerve terminals. These data support the hypothesis that PKC activation potentiates the intrasynaptosomal stimulus-secretion coupling mechanism(s) and that nigrostriatal and tuberoinfundibular dopaminergic nerve terminals are affected by phorbol esters in a similar manner.

Animals↗