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J C Buckingham

Publications and source records attributed to J C Buckingham.

At least 55 records · Page 3Linked to original sources
PubMed1995

Immunoneutralization of lipocortin 1 reverses the acute inhibitory effects of dexamethasone on the hypothalamo-pituitary-adrenocortical responses to cytokines in the rat in vitro and in vivo.

Our recent studies suggest that lipocortin 1 (LC1), a potential mediator of the anti-inflammatory, antiproliferative and anti-fever actions of glucocorticoids in peripheral tissues, may also contribute to the powerful negative feedback actions of the steroids on the hypothalamo-pituitary-adrenal (HPA) axis. In the present study we have used (1) an in vitro model to examine the influence of a specific neutralizing monoclonal anti-LC1 antibody (anti-LC1 mAb) on the capacity of dexamethasone to suppress the cytokine-induced release of the 41-amino acid corticotropin-releasing factor (CRF-41) and arginine vasopressin (AVP) from the rat hypothalamus and (2) a passive immunization protocol to assess the contribution of LC1 to the inhibitory actions of dexamethasone on the HPA responses to immunological (i.p. injection of interleukin 1 beta, IL-1 beta) and surgical (laparotomy under ether anaesthesia) stress. In vitro, Il-1 alpha (0.2 ng/ml), IL-1 beta (0.5 ng/ml), IL-6 (10 ng/ml) and IL-8 (1 ng/ml) each caused significant increases in the release of immunoreactive (ir)-CRF-41 and ir-AVP from hypothalami removed from rats adrenalectomized 10-12 days before autopsy; these responses were readily inhibited by preincubation of the tissue with dexamethasone (10(-7) M). The inhibitory actions of the steroid were attenuated and, in many instances, abolished by inclusion in the medium of a monoclonal anti-LC1 antibody (LC1 mAb, diluted 1:15,000); an isotype-matched control antibody (antispectrin alpha+beta, diluted 1:15,000) was ineffective in this regard. IL-1 alpha (0.2 ng/ml), IL-1 beta (0.5 ng/ml) and IL-6 (10 ng/ml) also initiated similar increases in the release of CRF-41 and AVP from hypothalami from intact rats which were effectively blocked by dexamethasone (10(-7) M). However, although the inhibitory actions of the steroid on the pharmacologically evoked release of CRF-41 were specifically overcome by anti-LC1 mAb (diluted 1:15,000), the steroid blockade of AVP release was not. In vivo, rats pretreated with either a polyclonal anti-LC1 antibody (anti-LC1 pAb, 1 ml/day s.c. for 2 days) or a corresponding volume of a nonimmune sheep serum (NSS) responded to immunological (IL-1 beta, 3 micrograms/kg i.p.) or surgical (laparotomy under ether anaesthesia) trauma with significant increases in the serum ACTH and corticosterone concentrations. In the NSS-treated groups, dexamethasone (100 micrograms/kg), which had no effect on the prestress concentrations of ACTH and corticosterone in the blood, completely prevented the HPA responses to both IL-1 beta and laparotomy.(ABSTRACT TRUNCATED AT 400 WORDS)

Adrenalectomy↗
PubMed1995

Dexamethasone suppresses the release of prolactin from the rat anterior pituitary gland by lipocortin 1 dependent and independent mechanisms.

Glucocorticoids have been shown repeatedly to inhibit the release of prolactin (PRL) in the rat but their site and mode of action is unknown. In the present study, we used an in vitro model to examine the requirement for protein synthesis for dexamethasone to suppress the release of immunoreactive (ir)-PRL release from the rat pituitary gland. In addition we have performed a series of in vitro and in vivo experiments to investigate the potential role in this regard of lipocortin 1 (LC1), a protein shown previously not only to mediate aspects of the anti-inflammatory and anti-proliferative actions of the glucocorticoids but also to contribute to the regulatory actions of the steroids in the brain-neuroendocrine system. In vitro, the release of ir-PRL from rat anterior pituitary tissue initiated by submaximal concentrations of VIP (10 nM). TRH (10 nM) or the adenyl cyclase activator forskolin (100 microM) was reduced significantly (p < 0.01) by preincubation (2 h) of the tissue with dexamethasone (0.1 microM). By contrast, ir-PRL release evoked by a submaximal concentration of the L-Ca2+ channel opener BAY K8644 (10 microM) was unaffected by the steroid although readily antagonised (p < 0.01) by nifedipine (1-100 microM). Exposure of the pituitary tissue to dexamethasone (0.1 microM) also caused a pronounced and highly significant increase in de novo protein synthesis, as assessed by the incorporation of 14C-lysine into the tissue (p < 0.001). This response was reduced markedly by the inclusion of the RNA and protein synthesis inhibitors, actinomycin-D (0.5 micrograms/ml) or cycloheximide (1.0 micrograms/ml), in the incubation medium (p < 0.001), both of which also effectively abrogated (p < 0.01) the dexamethasone-induced inhibition of the release of ir-PRL evoked by TRH. VIP and forskolin. Lipocortin I was readily detectable by Western blotting in protein extracts of freshly excised anterior pituitary tissue: a small proportion of the protein was found to be attached to the outer surface of the cells where it was retained by a Ca(2+)-dependent mechanism. Exposure of the tissue in vitro to dexamethasone (0.1 microM) or corticosterone (0.1 microM) but not 17 beta-oestradiol (0.1 microM) caused a pronounced increase in the amount of LC1 attached to the outer surface of the cells and concomitant decrease in the LC1 content of the intracellular LC1 pool. Addition of an N-terminal LC1 fragment. LC11-188 (10 pg-10 ng/ml), to the incubation medium reduced significantly (p < 0.01) the increases in ir-PRL release induced in vitro by VIP (10 nM) and forskolin (100 microM). By contrast, at all concentrations tested. LC11-188 (10 pg-10 ng/ml) failed to influence (p < 0.05) the highly significant (p < 0.01) ir-PRL response to TRH (10 nM). Similarly, the inhibitory actions of dexamethasone (0.1 microM) on the release of ir-PRL induced by VIP (10 nM) or forskolin (100 microM) but not by TRH (10 nM) were substantially reversed (p < 0.01) by a specific monoclonal anti-LC1 antibody while an isotype-matched control antibody was without effect. In vivo, rats pretreated with either a polyclonal anti LC1 antiserum (anti-LC1 pAb, 1 ml/day s.c. for 2 days) or a corresponding volume of non-immune sheep serum (NSS) responded to stress (laparotomy under ether anaesthesia) with significant (p < 0.05) increases in the serum ir-PRL concentration. In the NSS-treated group, the ir-PRL response to stress was effectively inhibited by dexamethasone (100 micrograms/kg i.p.) which had no effect on the pre-stress serum ir-PRL concentration. By contrast, in rats pretreated with anti-LC1 pAb dexamethasone failed to block the stress-induced release of ir-PRL. The results show clearly that the inhibitory actions of dexamethasone on PRL release are dependent on de novo protein synthesis and provide novel evidence for the involvement of both LC1-dependent and LC1-independent mechanisms.

Animals↗
PubMed1995

Dexamethasone inhibits the release of TSH from the rat anterior pituitary gland in vitro by mechanisms dependent on de novo protein synthesis and lipocortin 1.

Glucocorticoids have been shown repeatedly to inhibit the secretion of TSH in experimental animals and in man but their site and mode of action are unknown. In the present study, we have used an in vitro model to examine the effects of dexamethasone on the resting and pharmacologically evoked secretion of TSH by the rat anterior pituitary gland, and to show how they are influenced by inhibitors of RNA/protein synthesis. In addition, we have investigated the potential role of lipocortin 1 (LC1), a protein shown previously to contribute to glucocorticoid action in several systems, as a mediator of the glucocorticoid-induced suppression of TSH release in our in vitro preparation. The significant (P < 0.01) increases in the release of immunoreactive (ir)TSH from rat anterior pituitary tissue initiated by submaximal concentrations of TRH (10 nmol/l), vasoactive intestinal polypeptide (VIP, 10 nmol/l) or the adenyl cyclase activator, forskolin (100 mumol/l) were reduced significantly (P < 0.05) by preincubation of the tissue with dexamethasone (0.1 mumol/l). In contrast, irTSH secretion evoked by a submaximal concentration of the L-Ca2+ channel opener BAY K8644 (10 mumol/l) was unaffected by the steroid, although readily antagonised (P < 0.01) by nifedipine (1-100 mumol/l). Inclusion of actinomycin D (1.78 mumol/l) or cycloheximide (0.8 mumol/l), inhibitors of RNA and protein synthesis respectively, in the medium effectively abrogated the inhibitory effects of dexamethasone (0.1 mumol/l) on the secretory responses to TRH (10 nmol/l), VIP (10 nmol/l) and forskolin (100 mumol/l). LC1 was readily detectable by Western blotting in protein extracts of freshly excised anterior pituitary tissue. A small proportion of the protein was found to be attached to the outer surface of the cells where it was retained by a Ca(2+)-dependent mechanism. Exposure of the tissue to dexamethasone (0.1 mumol/l) caused a pronounced increase in the amount of cellular LC1 attached to the outer surface of the cells and a concomitant decrease in the intracellular LC1 pool. Progesterone (0.1 mumol/l) and aldosterone (0.1 mumol/l) were also weakly active in this regard, but thyroxine and tri-iodothyronine (0.1 mumol/l) were not. Addition of an N-terminal LC1 fragment, LC1(1-188) (0.05-0.53 pmol/l) to the incubation medium reduced significantly (P < 0.01) the increases in irTSH release induced by TRH (10 nmol/l), VIP (10 nmol/l) and forskolin (100 mumol/l), but failed to influence (P < 0.05) those initiated by BAY K8644 (10 mumol/l). Furthermore, the inhibitory actions of dexamethasone (0.1 mumol/l) on the release of irTSH provoked by TRH (10 nmol/l), VIP (10 nmol/l) and forskolin (100 mumol/l) were substantially reversed (P < 0.01) by a specific monoclonal anti-LC1 antibody, while an isotype-matched control antibody was without effect. The results show clearly that dexamethasone, a semi-synthetic glucocorticoid, acts at the pituitary level to inhibit the neurochemically evoked release of irTSH. They also provide novel evidence that the inhibitory actions of the steroid are dependent upon de novo RNA/protein synthesis and that they involve an LC1 dependent mechanism.

3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethy↗
PubMed1994

Cytokines, glucocorticoids and neuroendocrine function.

Activation of the immune system is normally associated with widespread alterations in neuroendocrine activity, the profile of which depends upon the species and on the severity and duration of the stimulus. Particularly important in this regard is the activation of the hypothalamo-pituitary-adrenocortical (HPA) axis for the consequent rise in circulating glucocorticoids serves to contain the ensuing pathophysiological responses and thus to restore homeostasis. In the present study, in vivo and in vitro techniques have been used to examine the influence of various immunokines on the HPA axis and to determine whether their actions are modulated by glucocorticoids and lipocortin 1 (LC1). In vivo interleukin-1 beta (IL-1 beta), given orally or peripherally, produced increases (P < 0.01) the serum corticosterone concentration which were reversed by pretreatment with dexamethasone. IL-1 beta also produced glucocorticoid reversible increases in the release of the two corticotrophin releasing factors, CRF-41 and AVP, from the hypothalamus in vitro (P < 0.01) as also did IL-1 alpha, IL-6 and IL-8. By contrast, none of these cytokines influenced directly the release of ACTH from pituitary tissue in vitro. The inhibitory actions of the glucocorticoids on the HPA responses to cytokines observed in vivo and in vitro were mimicked by LC1 and reversed by neutralizing anti-LC1 antisera. Our results demonstrate a role of cytokines, glucocorticoids and LC1 in effecting the interplay between the brain-neuroendocrine and immune system which may be critical to host defence in conditions of both health and disease.

Animals↗
PubMed1993

Effects of lipocortin 1 and dexamethasone on the secretion of corticotrophin-releasing factors in the rat: in vitro and in vivo studies.

Lipocortin 1 (LC1: also called annexin 1) was first described as a putative second messenger protein for the anti-inflammatory steroids in peripheral tissues. In the present study, in vitro and in vivo methods were used to examine its potential role within the hypothalamus as a mediator of the regulatory actions of the glucocorticoids on the hypothalamo-pituitary-adrenocortical axis of the rat. In the in vitro studies, the effects of human recombinant LC1 (hu-r-LC1) on the concomitant release of the two major corticotrophin-releasing factors (CRF-41 and arginine vasopressin, AVP) from isolated hypothalami removed from chronically adrenalectomized rats were compared with those of dexamethasone in the presence and absence of appropriate secretagogues, namely phospholipase A2 (PLA2), interleukin-6 (IL-6) and a non-specific depolarizing agent, K+ (56 mM). The spontaneous release of CRF-41 in vitro was unaffected by either hu-r-LC1 (5 to 100 ng/ml) or dexamethasone (1 microM). Both compounds however reduced the release of the neuropeptide evoked by IL-6 (5 ng/ml) but failed to modify the secretory responses to PLA2 (25 U/ml) or K+ (56 mM). Dexamethasone (1 microM) had no effect on the basal release of AVP but effectively blocked the secretion of the peptide induced by either IL-6 (10 ng/ml) or PLA2 (25 U/ml). In complete contrast, hu-r-LC1 (5 to 100 ng/ml) stimulated the release of AVP and potentiated the secretory responses to IL-6 (10 ng/ml) and PLA2 (25 U/ml) but not to K+ (56 mM). The hypothalamic responses to PLA2 stimulation (25 U/ml) were associated with significant (P < 0.01) increases in prostaglandin E2 release which, in some instances, were potentiated by hu-r-LC1 (5 to 20 ng/ml). In vivo, administration of histamine (0.6 mg/100 g body wt, ip) produced significant (P < 0.01) increases in the serum corticosterone concentration and in the hypothalamic LC1 content. Neither hu-r-LC1 (0.6 to 1.2 micrograms) nor a polyclonal anti-LC1 antibody (3 microliters, diluted 1:200), injected intracerebroventricularly (icv), influenced either the resting serum corticosterone concentration or the hypersecretion of the steroid evoked by histamine stress. A lower dose of the recombinant protein (0.3 micrograms icv) also failed to alter basal corticosterone release but, in contrast to the higher doses, potentiated the pituitary-adrenocortical responses to histamine. The results suggest that LC1 may contribute to some aspects of peptide release in the hypothalamus but that its actions are not necessarily related to those of the glucocorticoids.

Adrenalectomy↗
PubMed1993

Modulation of the hypothalamo-pituitary-adrenocortical responses to cytokines in the rat by lipocortin 1 and glucocorticoids: a role for lipocortin 1 in the feedback inhibition of CRF-41 release?

Our recent studies indicate that lipocortin 1 (LC1), a putative second messenger protein for the anti-inflammatory steroids in peripheral tissues, may also contribute to the regulatory actions of the glucocorticoids on the hypothalamo-pituitary-adrenal axis. In the present study we have used in vitro and in vivo models to compare the effects of adrenalectomy, LC1 and dexamethasone on the cytokine-induced secretion of the 41-amino acid corticotrophin releasing factor (CRF-41) and arginine vasopressin (AVP) by the rat hypothalamus. In addition, western blot analysis was used to examine the influence of dexamethasone on the expression of LC1 in the hypothalamus. In vitro, interleukins- (IL-) 1 alpha (100 and 200 pg/ml), 1 beta (0.5 and 1.0 ng/ml) and 8 (0.25-1.0 ng/ml) readily initiated the release of CRF-41 and AVP from hypothalami removed from intact rats. IL-6 (10 and 20 ng/ml) was also an effective CRF-41 secretagogue but, unlike the other interleukins tested, it was ineffective with regard to AVP. Adrenalectomy 7-14 days prior to autopsy increased significantly (p < 0.01) the magnitude of the CRF-41 responses to IL-1 alpha, IL-1 beta and IL-6 but not to IL-8. In contrast however, while hypothalamic tissue from adrenalectomized rats, unlike that from intact animals, responded to IL-6 (5-20 ng/ml) with a pronounced hypersecretion of AVP, the AVP responses to IL-1 alpha and IL-1 beta were largely unaffected by adrenalectomy as too were those to IL-8. The marked increases in CRF-41 and AVP release from hypothalami from adrenalectomized rats initiated in vitro by IL1 alpha, IL-1 beta, IL-6 and IL-8 were readily overcome by preincubation of the tissue with dexamethasone (10(-7) M). In addition, the steroid caused 'externalization' of two species of immunoreactive (ir-) LC1 (37 and 58 kDa) by the hypothalamic cells but failed to influence the total LC1 content of the tissue. The inhibitory effects of dexamethasone on the cytokine-induced release of CRF-41 in vitro were mimicked by LC1 (10 ng/ml) which alone had no effect on the basal release of the peptide. However, unlike dexamethasone, LC1 failed to influence the concomitant release of AVP from the hypothalamic tissue elicited by IL1 alpha, IL-1 beta or IL-8 and potentiated that evoked by IL-6.(ABSTRACT TRUNCATED AT 400 WORDS)

Adrenal Cortex↗
PubMed1993

Lipocortin 1 mediates an early inhibitory action of glucocorticoids on the secretion of ACTH by the rat anterior pituitary gland in vitro.

Lipocortin 1 (LC1), a mediator of anti-inflammatory steroid action in some peripheral tissues, may contribute to the acute inhibitory effects of these steroids on hypothalamo-pituitary-adrenal (HPA) function. Accordingly, in the present study we have used an in vitro model to examine the potential role of this protein in the regulation of the release of corticotrophin (ACTH) from the anterior pituitary gland. Hypothalamic extracts (0.05-0.4 HE/ml), the 41 amino acid corticotrophin-releasing factor (CRF-41, 1-100 nM), the adenyl cyclase stimulator, forskolin (0.1 microM-10 mM), and the L-Ca2+ channel opener, BAY K8644 (0.01-10 mM), all caused concentration-dependent increases in the release in vitro of immunoreactive (ir)-ACTH from segments of rat anterior pituitary tissue. The secretory responses to submaximal concentrations of these secretagogues were overcome by preincubation of the tissue with dexamethasone (0.1 and 1 microM). LC1 was readily detectable by Western blotting in protein extracts of freshly excised pituitary tissue; a small proportion of the protein was found to be attached to the outer surface of the cell membranes where it was retained by a Ca(2+)-dependent mechanism. Exposure to dexamethasone (0.1 microM) in vitro did not affect the total LC1 content of the pituitary tissue but, over a 2-hour period, it caused a progressive two- to fivefold increase in the amount of LC1 attached to the outer surface of the cell; this response developed in parallel with the inhibitory effects of the steroid on ir-ACTH release. Both the dexamethasone-induced 'externalization' of LC1 by the pituitary tissue and the concomitant steroid-induced inhibition of peptide release were blocked by cycloheximide (1.0 microgram/ml) but not by actinomycin D (0.5 microgram/ml). A stable N-terminal lipocortin 1 fragment, LC1(1-188) (10 pg-10 ng/ml), attenuated (p < 0.01) the release of ir-ACTH evoked by HE (0.1 HE/ml), CRF-41 (1 nM), forskolin (1 mM) and BAY K8644 (1 nM). Conversely, inclusion of the anti-LC1 antibody in the medium substantially overcame the inhibitory effects of dexamethasone (0.1 microM) on the release of ir-ACTH evoked by the secretagogues whilst a control isotype matched antibody was without effect. The results suggest that LC1 plays a key role in effecting the acute inhibitory actions of glucocorticoids on the secretion of ir-ACTH by the rat anterior pituitary gland.

3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethy↗
PubMed1991

In vivo and in vitro studies on the opioidergic control of the secretion of gonadotrophin-releasing hormone and luteinizing hormone in sexually immature and adult male rats.

In vivo and in vitro methods have been used to compare the effects of opioid receptor blockade on the functional activity of the hypothalamo-pituitary-gonadal axis in adult (200 g) and sexually immature (50 g) male rats. In the adult, a single injection of the mu-receptor antagonist, naloxone (500 micrograms/100 g body weight, s.c.), produced hypersecretions of luteinizing hormone (LH) and testosterone. Maximal serum concentrations of the two hormones were attained within 20 and 60 min respectively. In contrast, neither ICI 174864 (100 micrograms/100 g body weight, s.c.) nor MR2266 (150 micrograms/100 g body weight, s.c.), which block delta- and kappa-receptors respectively, stimulated pituitary-gonadal activity; indeed, like the saline vehicle, both tended to depress the serum LH concentration. The injection procedure was sufficient to activate the hypothalamo-pituitary-adrenal axis and, thus, the vehicle-treated controls exhibited significant increases in the plasma adrenocorticotrophin and serum corticosterone concentrations. These effects were enhanced by naloxone (500 micrograms/100 g body weight, s.c.) and by the kappa-opioid receptor (MR2266, 150 micrograms/100 g body weight, s.c.) but not by the delta-opioid receptor antagonist (ICI 174864, 30-100 micrograms/100 g body weight, s.c.). The increases in serum corticosterone and LH concentration induced by naloxone in adult rats were not apparent in the sexually immature (50 g) animals. To the contrary, in the young rats naloxone (250 and 500 micrograms/100 micrograms body weight, s.c.) attenuated, in a dose-dependent manner, the pronounced hypersecretion of corticosterone induced by the vehicle injection. The higher dose of the antagonist (500 micrograms/100 g body weight, s.c.) also overcame the significant reductions in serum LH evident 20 (p less than 0.05) and 40 (p less than 0.01) min after the saline injection but the lower dose (250 micrograms/100 g body weight, s.c.) was ineffective in this respect. In vitro, hypothalami from both adult and sexually immature rats responded to the addition of naloxone (10(-8)-10(-6) M) to the incubation medium with significant (p less than 0.01) concentration-dependent increases in the release of gonadotrophin-releasing hormone (GnRH). In contrast, ICI 174864 (10(-7)-10(-6) M) and MR2266 (10(-7)-10(-6) M) had little effect on the secretion of the releasing hormone by hypothalami from rats of either group although, at the highest concentration tested, MR2266 (10(-6) M) precipitated a small increase in GnRH release from hypothalami from adult rats.(ABSTRACT TRUNCATED AT 400 WORDS)

Adrenal Glands↗
PubMed1991

Studies on the roles of phospholipase A2 and eicosanoids in the regulation of corticotrophin secretion by rat pituitary cells in vitro.

Dispersed anterior pituitary cells were used to investigate the possible roles of phospholipid metabolites released by phospholipase A2 (PLA2) in the control of immunoreactive ACTH (ir-ACTH) secretion in vitro. PLA2 (15,600-62,500 U/l), the PLA2 activator melittin (0.5-20 mg/l) and arachidonic acid (1 mmol/l) all produced increases in ir-ACTH release from the cells, whilst platelet-activating factor (PAF), prostaglandin F2 alpha (PGF2 alpha), the prostacyclin analogues iloprost and BW245C, the thromboxane A2 (TXA2) analogue U46619, and the leukotrienes LTB4 and LTC4 were ineffective in this respect. PGF2 alpha (100 nmol/l and 1 mumol/l), iloprost (1 mumol/l) and BW245C (100 nmol/l and 1 mumol/l) depressed corticotrophin-releasing factor-41-induced ir-ACTH secretion, while the PAF antagonist BN52021 (10 and 100 mumol/l) and LTC4 (100 nmol/l and 1 mumol/l) had no discernable effects. The secretory responses of the cells to hypothalamic extracts (0.2 hypothalami/ml) and arachidonic acid (1 mmol/l) were generally unaffected by the cyclooxygenase inhibitors ibuprofen (10 and 100 mumol/l) and indomethacin (10 mumol/l), the TXA2 synthetase inhibitor imidazole (10 mumol/l-1 mmol/l), the lipoxygenase inhibitor nordihydroguaiaretic acid (10 and 100 mumol/l) and the dual cyclo-oxygenase/lipoxygenase inhibitors phenidone (1-100 mumol/l) and BW755C (10 and 100 mumol/l). They were, however, inhibited by the dual cyclo-oxygenase/lipoxygenase inhibitor eicosatetraynoic acid (10 and 100 mumol/l), which also blocks epoxygenase and PLA2 activity and by the cytochrome P450 inhibitor SKF-525A (1 mmol/l). The results suggest that the stimulatory effects of PLA2 and arachidonic acid on ir-ACTH secretion are not effected by products generated by the cyclo-oxygenase or lipoxygenase pathways but may be mediated by metabolites generated by the cytochrome P450 pathway.

Adrenocorticotropic Hormone↗
PubMed1990

Peripubertal changes in the nature of the GnRH response to alpha-adrenoceptor stimulation in vitro and their modulation by testosterone.

Adrenergic mechanisms have been widely implicated in the regulation of GnRH secretion in adult rats but their role in young animals, in which the activity of the GnRH neurones is minimal, is unclear. These experiments were done to examine the effects of alpha-adrenoceptor stimulation on the secretion in vitro of GnRH by hypothalami from immature and adult male rats. The alpha 1-adrenoceptor agonist, phenylephrine (10(-9) - 10(-7) M), stimulated release of GnRH from hypothalami from adult (200 g) and peripubertal (150 g) rats but inhibited markedly the secretion of the releasing factor from the limited stores available in hypothalami from immature (50 or 100 g) rats. The stimulatory and inhibitory responses to phenylephrine, evident in adult and younger rats respectively, were concentration-dependent and antagonized readily by the selective alpha 1-adrenoceptor antagonist, alfuzosin (10(-6) M), but not by the beta-adrenoceptor antagonist, propranolol (10(-6) M). Hypothalami from 14-day castrated adult rats, in which the serum LH was elevated and hypothalamic GnRH content reduced, responded to alpha 1-adrenoceptor stimulation in vitro, like those from immature rats, with a marked reduction in GnRH release. In contrast, hypothalami from corresponding castrates bearing testosterone implants, which maintained the hypothalamic GnRH content and serum LH and testosterone concentrations at levels similar to those of intact controls, exhibited the normal 'adult' response to phenylephrine. Studies utilizing 3H-prazosin indicated that the number (Bmax) of hypothalamic alpha 1-adrenoceptor binding sites increases at puberty but that receptor affinity (KD) is unchanged.(ABSTRACT TRUNCATED AT 250 WORDS)

Adrenergic alpha-Antagonists↗