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Biomedical subjects

P G Knight

Publications and source records attributed to P G Knight.

At least 91 records · Page 5Linked to original sources
PubMed1990

Isolation of bovine ovarian inhibin, its immunoneutralization in vitro and immunolocalization in bovine ovary.

A purification scheme involving gel permeation chromatography, anion exchange chromatography and reversed-phase high performance liquid chromatography (RP-HPLC) was used to isolate from bovine follicular fluid (FF) biologically-active inhibin of molecular weight 32 kDa. Chromatographic fractions were monitored for inhibin-like biological activity (ILA) using a simplified bioassay procedure in which a suppression of total basal FSH production by rat pituitary cells in monolayer culture indicates the presence of ILA. Approximately 3 mg protein having an ILA potency (ED50 value in in vitro bioassay) of 1.7 ng/ml was obtained from 4 1 crude bovine FF (260 g protein; ILA potency 3750 ng/ml) reflecting an approximate 2200-fold purification factor with an overall recovery of about 3%. The isolated material appeared as a single major UV absorbance peak on RP-HPLC and as a single band (32 kDa) when subjected to SDS-PAGE (15% gel) under non-reducing conditions. Under reducing conditions the molecule dissociated into 2 subunits of apparent molecular weight 22 and 14 kDa confirming that it is probably identical to the 31/32 kDa form of bovine ovarian inhibin previously reported by two other independent research groups. An antiserum raised in a chicken against the isolated material completely neutralized the suppressive effects of both 32 kDa inhibin and bovine FF on basal production of FSH by rat pituitary cells in vitro but only partially reversed the suppressive effects of both porcine and human FF. Immunohistochemical staining of sections of bovine ovary and of isolated preparations of bovine granulosa cells using this antiserum confirmed that granulosa cells are a major source of inhibin. The observation that specific immunostaining was not confined to these cells, however, suggests that they may not be the exclusive source of immunoreactive inhibin in the bovine ovary.

Animals↗
PubMed1990

Demonstration of a nonsteroidal factor in human follicular fluid that attenuates the self-priming action of gonadotropin-releasing hormone on pituitary gonadotropes.

The self-priming effect of gonadotropin-releasing hormone (GnRH) on luteinizing hormone (LH) release can be demonstrated in vitro by perfusing pituitary tissue with a continuous GnRH stimulus. A characteristic biphasic response is produced. We have used this system to investigate whether or not human follicular fluid (hFF) contains a nonsteroidal substance that can attenuate the GnRH-induced LH secretion in perfused rat pituitary glands. Steroid-extracted hFF, added to the perfusing medium, attenuated the self-priming action of GnRH in a dose-dependent manner. This was not abolished by selectively depleting the inhibin content of hFF by 97% on an immunoaffinity column. Furthermore, the biological activity of the substance was resistant to heating and was removed by dialysis. It is concluded that hFF contains a nonsteroidal factor, distinct from inhibin, that can attenuate the self-priming action of GnRH on pituitary gonadotropes.

Animals↗
PubMed1990

Inhibin immunoneutralization by antibodies raised against synthetic peptide sequences of inhibin alpha subunit: effects on gonadotrophin concentrations and ovulation rate in sheep.

Two experiments were conducted to explore the effectiveness of synthetic peptide-based vaccines for active and passive autoimmunization of sheep against inhibin. In the first experiment, adult Romney ewes (n = 20) were actively immunized against a synthetically produced peptide that corresponded to the N-terminus of the alpha-subunit of bovine inhibin (bI alpha(1-29)-Tyr30). This peptide was conjugated to tuberculin purified protein derivative (PPD) to increase its antigenic properties. Control groups comprised non-immunized (n = 10) and PPD-immunized (n = 10) ewes. Primary immunization (400 micrograms conjugate/ewe) was followed by two booster immunizations (200 micrograms conjugate/ewe), given 5 and 8 weeks later. Following synchronization of oestrus using progestagen sponges, ovulation rates were assessed by laparoscopy. Weekly blood samples were taken throughout the experiment. All inhibin-immunized ewes produced antibodies which bound 125I-labelled bovine inhibin (Mr 32,000), and ovulation rate in inhibin-immunized ewes (2.15 +/- 0.22; mean +/- S.E.M.) was significantly (P less than 0.01) greater than in both non-immunized (0.90 +/- 0.23) and PPD-immunized (1.20 +/- 0.13) control groups. Immunization against the peptide, but not against PPD alone, resulted in a modest rise in plasma FSH, with mean levels after the second boost being significantly (P less than 0.025) higher (22%) than those before immunization. Moreover, when blood samples were taken (2-h intervals) from randomly selected groups of control (n = 7) and inhibin-immunized (n = 7) ewes for an 84-h period following withdrawal of progestagen sponges, the mean plasma concentration of FSH during the 48 h immediately before the preovulatory LH surge was 37% greater (P less than 0.025) in immunized than in control animals. However, more frequent blood sampling (every 15 min for 12 h) during follicular and mid-luteal phases of the oestrous cycle revealed no significant differences between treatment groups in mean plasma concentrations of FSH. In addition, neither mean concentrations of LH nor the frequency and amplitude of LH episodes differed between immunized and control ewes. However, the mean response of LH to a 2 micrograms bolus of gonadotrophin-releasing hormone, given during the luteal phase, was significantly (P less than 0.05) less in immunized than in control ewes. These findings indicate that active immunization of Romney ewes against a synthetic fragment of inhibin can promote a controlled increase in ovulation rate, but this response cannot be unequivocally related to an increase in plasma levels of FSH.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1990

Comparison of the effects of crude and highly purified bovine inhibin (Mr 32,000) on plasma concentrations of FSH and LH in chronically ovariectomized prepubertal heifers.

Chronically ovariectomized prepubertal heifers were used for a comparison of the effects of highly purified bovine inhibin (Mr 32,000) and steroid-free bovine follicular fluid (bFF) on the secretion of FSH and LH. In view of the limited availability of highly purified inhibin, an initial study was undertaken to establish the optimal method for administration of bFF inhibin activity. In comparison with the FSH response to a single large i.v. bolus injection of bFF (50 ml; 3250 mg protein), a far more effective suppression of plasma FSH concentrations was achieved when considerably less bFF (6.3 ml; 410 mg protein) was administered gradually over an extended time-period (2 days) either as a continuous i.v. infusion or as a series of 2-hourly i.v. injections. Following a single i.v. bolus injection of bFF, immunoreactive inhibin was cleared rapidly from the circulation (half-life 51 +/- 8 (S.E.M.) min, n = 5), presumably accounting for its limited ability to suppress FSH secretion when administered in this manner. In a second experiment, treatment of ovariectomized heifers (three per group) with highly purified Mr 32,000 bovine inhibin at a dose rate of 15 micrograms/2 h for 2 days significantly (P less than 0.05) suppressed plasma FSH concentrations, which reached their minimum values (40% suppression) during day 2 of treatment. At a lower dose rate (5 micrograms/2 h), inhibin did not significantly affect plasma FSH levels. Administration of bFF was also associated with a dose-dependent suppression of FSH secretion. For each of three dose rates tested (three heifers per group), plasma FSH concentrations were maximally suppressed during day 2 of treatment (65 mg/2 h, 86% suppression, P less than 0.001; 21.7 mg/2 h, 66% suppression, P less than 0.001; 7.2 mg/2 h, 15% suppression, P greater than 0.05). Neither highly purified inhibin nor bFF significantly affected mean plasma LH concentrations, LH pulse frequency or LH pulse amplitude. Thus we have shown for the first time that highly purified Mr 32,000 bovine inhibin does possess in-vivo biological activity in cattle, promoting a selective suppression of plasma FSH concentrations qualitatively similar to that evoked by steroid-free bFF.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1990

Effects of crude and highly purified bovine inhibin (Mr 32,000 form) on gonadotrophin production by ovine pituitary cells in vitro: inhibin enhances gonadotrophin-releasing hormone-induced release of LH.

Primary cultures of ovine pituitary cells (from adult ewes) were used to investigate the actions of steroid-free bovine follicular fluid (bFF) and highly-purified Mr 32,000 bovine inhibin on basal and gonadotrophin-releasing hormone (GnRH)-induced release of FSH and LH. Residual cellular contents of each hormone were also determined allowing total gonadotrophin content/well to be calculated. As in rats, both crude and highly purified inhibin preparations promoted a dose (P less than 0.001)- and time (P less than 0.001)-dependent suppression of basal and GnRH-induced release of FSH as well as an inhibition of FSH synthesis, reflected by a fall in total FSH content/well. However, while neither inhibin preparation affected basal release of LH or total LH content/well, GnRH-induced LH release was significantly (P less than 0.001) increased by the presence of either bFF (+75%) or highly-purified inhibin (+64%) in a dose- and time-dependent manner. This unexpected action of bFF on GnRH-induced LH release was abolished in the presence of 5 microliters specific anti-inhibin serum, confirming that the response was indeed mediated by inhibin. Furthermore, neither oestradiol-17 beta (1 pmol/1-10 nmol/l) nor monomeric alpha-subunit of bovine inhibin (2.5-40 ng/ml) significantly affected basal or GnRH-induced release of LH. These in-vitro findings for the ewe lend support to a number of recent in-vivo observations and indicate that, in addition to its well-documented suppressive effect on the synthesis and secretion of FSH, inhibin may actually facilitate LH release in this species, in marked contrast to its action in the rat.

Animals↗
PubMed1989

Evidence that the bovine ovary secretes large amounts of monomeric inhibin alpha subunit and its isolation from bovine follicular fluid.

Analysis of bovine follicular fluid (FF) using sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) coupled with a sensitive immunoblotting procedure resolved several components that were immunoreactive with an antiserum directed against the n-terminus of the alpha subunit of human inhibin (hI alpha(1-32]. Under non-reducing conditions, three intensely stained bands having apparent Mr values of 116,000, 44,000 and 25,000 were present, whilst under reducing conditions only two intensely stained bands (Mr 43,000 and 21,000) were detected. The Mr 44,000 and 25,000 immunoreactive forms (non-reducing conditions) were also demonstrated in bovine utero-ovarian vein and peripheral venous plasma after subjecting samples (40 ml) to immunoaffinity concentration using Sepharose beads coupled to anti-hI alpha(1-32), SDS-PAGE and immunoblotting. The same approach revealed the presence of the smaller (Mr 25,000) form in bovine granulosa cell-conditioned culture medium (GCCM). Gel-permeation chromatography (Sephacryl S-200), immunoaffinity chromatography (Sepharose-anti-hI alpha(1-32] and reversed-phase high-performance liquid chromatography (RP-HPLC; C18 and C8 columns) were employed to isolate from bFF (30 ml, 19.5 g protein) 750 micrograms protein which appeared essentially homogeneous by RP-HPLC and SDS-PAGE and had an Mr of 25,000 (non-reducing conditions)/21,000 (reducing conditions), identical to that of the immunoreactive component of lowest Mr found in bovine FF, utero-ovarian vein plasma, peripheral plasma and GCCM. The isolated material was highly immunoreactive with antisera against both hI alpha(1-32) and purified Mr 32,000 bovine inhibin but was devoid of biological activity when tested in a rat pituitary cell inhibin bioassay. Amino-terminal analysis revealed an amino acid sequence (residues 1-14) identical to that reported elsewhere for the alpha subunit (Mr 20,000/21,000) of bovine inhibin. In conclusion, the present study has revealed that the bovine ovary secretes considerable quantities of monomeric inhibin alpha subunit. The unexpected presence of this material in peripheral blood is likely to hinder attempts to obtain physiologically relevant data on circulating levels of inhibin in cattle using conventional radioimmunoassays.

Animals↗
PubMed1989

Treatment of ovariectomized heifers with bovine follicular fluid specifically suppresses pituitary levels of FSH-beta mRNA.

To investigate the inhibin-induced suppression of FSH secretion by the anterior pituitary, chronically ovariectomized heifers (three per group) were treated for 56-58 h with either steroid-free bovine follicular fluid (bFF; 8 ml i.v. every 8 h) or 0.9% (w/v) NaCl (8 ml i.v. every 8 h). Blood was withdrawn at 8-h intervals for analysis of plasma concentrations of FSH and LH by radioimmunoassay. At the end of the treatment period, heifers were slaughtered and pituitary glands recovered for determination of gonadotrophin contents and levels of mRNA encoding FSH-beta, LH-beta, TSH-beta and common alpha glycoprotein hormone subunits using [32P]cDNA probes in total RNA dot and Northern blot assays. Treatment with bFF markedly suppressed plasma FSH by 85% (P less than 0.001 compared with pretreatment period), but did not affect plasma LH concentrations. Plasma FSH and LH concentrations did not vary significantly in the saline-injected control heifers. The level of FSH-beta subunit mRNA was reduced by 60% (P less than 0.001) in heifers treated with bFF, whereas no significant differences between control and bFF-treated heifers were observed in the levels of mRNA encoding LH-beta, TSH-beta or common alpha subunits. Treatments with bFF, however, did not affect pituitary content of either FSH or LH. These results support the conclusion that inhibin exerts its selective suppressive effect on the secretion of FSH by the bovine pituitary, at least in part, by directly inhibiting expression of the gene encoding the FSH-beta subunit.

Animals↗
PubMed1988

Endogenous opioid peptide modulation of LH secretion in the ewe lamb: possible involvement of 5-hydroxytryptamine.

Evidence from several species suggest that the endogenous opioid peptides participate in the regulation of gonadotrophin and prolactin secretion. The aim of the present study involving intact and ovariectomized prepubertal ewe lambs was to compare the effects in vivo of an opioid peptide agonist [D-Ala2,N-Phe4,Met(0)ol5]-enkephalin (FK 33-824) and antagonist, naloxone, on concentrations of LH and prolactin in plasma, and levels of neurotransmitter metabolites in cerebrospinal fluid (CSF), with their effects in vitro on the release of gonadotrophin-releasing hormone (GnRH) and neurotransmitters from isolated median eminences. Infusion of FK 33-824 (0.5 mg/30 min) in vivo depressed plasma LH levels in both intact and ovariectomized lambs; this effect could be reversed by naloxone. In ovariectomized lambs, the inhibitory action of FK 33-824 on plasma LH levels was associated with a 13% rise in the concentration of the metabolite of 5-hydroxytryptamine, 5-hydroxyindolacetic acid (5-HIAA). Concurrent administration of naloxone resulted in an abrupt 33% fall in CSF levels of 5-HIAA. No significant changes in plasma concentrations of prolactin or CSF concentrations of the metabolites of dopamine were observed in response to the administration of FK 33-824 or FK 33-824 plus naloxone. That FK 33-824 inhibited LH release through a central mechanism was confirmed using superfused median eminences in vitro. Thus FK 33-824 (1 mumol/l) greatly diminished the release of GnRH induced by the introduction of a depolarizing stimulus (36 mmol K+/l) in tissue obtained from both intact and ovariectomized ewe lambs.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1988

Effects of bovine follicular fluid on gonadotrophin secretion in intact and chronically ovariectomized ewes before and after desensitization of pituitary gonadotrophs to gonadotrophin-releasing hormone.

Intact and chronically ovariectomized ewes were treated for 4 days with charcoal-treated bovine follicular fluid (FF) or charcoal-treated bovine serum during the late-anoestrous period, and the effects on basal and gonadotrophin-releasing hormone (GnRH)-induced secretion of LH and FSH observed. Subsequently, ewes received s.c. implants containing a sustained-release formulation of a potent GnRH agonist D-Ser(But)6-Azgly10-LHRH (ICI 118630) to desensitize pituitary gonadotrophs to hypothalamic stimulation, and the effects of bovine FF and bovine serum were re-assessed 2 weeks later. Chronic exposure (for 2-3 weeks) to ICI 118630 significantly reduced basal levels of LH and FSH in both intact and ovariectomized ewes and completely abolished both spontaneous LH pulses as well as exogenous GnRH-induced acute increases in plasma LH and FSH levels. Treatment with bovine FF significantly reduced plasma FSH levels, but not LH levels, in both intact and ovariectomized ewes before and after chronic exposure to ICI 118630. In intact ewes before exposure to ICI 118630, treatment with bovine FF actually enhanced pulsatile LH secretion and raised mean plasma LH levels by 240% (P less than 0.05). No such stimulatory effect of bovine FF on LH secretion was observed in intact ewes exposed to ICI 118630 or in ovariectomized ewes before or after exposure to ICI 118630, suggesting that the effect probably involved an alteration in ovarian steroid feedback affecting hypothalamic GnRH output. Treatment with bovine FF did not significantly affect the magnitude of GnRH-induced surges of LH or of FSH observed in either intact or ovariectomized ewes before exposure to ICI 118630.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1987

Involvement of endogenous opioid peptides in the neuroendocrine response of ewe lambs to the introduction of a ram.

Prepubertal ewes can, under certain circumstances, be stimulated to ovulate by the novel introduction of a ram. The endocrine response to the presence of the ram is characterized by a rapid increase in the frequency of episodic release of LH. The purpose of this study was to investigate the effect of the presence of a ram on LH pulse frequency in vivo, gonadotrophin-releasing hormone (GnRH) and beta-endorphin concentrations in the median eminence, and on the influence of the endogenous opioid peptide agonist [D-Ala2,N-Phe4,Met(0)ol5]-enkephalin (FK 33-824) on basal and depolarization-induced release of GnRH from median eminence tissue superfused in vitro. The study was performed at two prepubertal ages in August and September. In September, the introduction of a ram resulted in an increase in pulsatile release of LH, which was associated with an increase in the rate of basal release of GnRH from median eminence tissue superfused in vitro, and the development of a marked ability of FK 33-824 to suppress depolarization-induced release of GnRH. The concentration of beta-endorphin in the median eminence was reduced in animals exposed to the ram at this time. In contrast, the introduction of a ram in August failed to stimulate an increase in LH pulse frequency, basal release of GnRH in vitro was not altered and FK 33-824 was ineffective in reducing depolarization-induced release of GnRH. These results suggest that the premature onset of reproductive activity induced by exposure to the ram may involve the participation of the endogenous opioid peptide system.

Aging↗
PubMed1986

Evidence that opioid peptides and dopamine participate in the suckling-induced release of prolactin in the ewe.

A pharmacological approach was used to study the involvement of opioid peptides and dopamine in mediating the suckling-induced release of prolactin in the lactating ewe (10-20 days post partum). To promote reliable and predictable suckling activity lambs were fitted with elasticated masks to prevent sucking for 4.5 h. After a 1-hour control period of frequent blood sampling, ewes were treated (i.v. injections every 5 min) for a further 75 min with either saline vehicle, an opioid antagonist (naloxone; 4.17 mg/5 min), a dopamine antagonist (metoclopramide; 1.25 mg/5 min), a mixture of naloxone + metoclopramide or a dopamine agonist (apomorphine; 6.6 mg/5 min). Blood was withdrawn at 5-min intervals for determination of plasma prolactin and luteinizing hormone (LH) by radioimmunoassay. Plasma LH concentrations (less than or equal to 1 microgram/l) were not significantly affected by any of the drug treatments and there was no evidence for an acute fall in LH associated with suckling- or TRH-induced increases in prolactin secretion. Naloxone significantly (p less than 0.05) reduced the mean incremental change in prolactin concentration (delta PRL) in response to suckling (+7 +/- 18 micrograms/ml) compared with saline-infused controls (+79 +/- 26 micrograms/ml), an effect which was completely reversed by combined treatment with naloxone and metoclopramide (+146 +/- 56 micrograms/ml). Metoclopramide alone raised basal prolactin levels by 46% (p less than 0.01) but did not affect delta PRL in response to suckling (+115 +/- 52 micrograms/ml). Neither naloxone, metoclopramide nor a combination of the two drugs affected the subsequent prolactin to TRH (10 micrograms). Apomorphine, however, completely abolished both the suckling- and TRH-induced release of prolactin.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1986

Effects of passive immunization against oestradiol-17 beta on some endocrine values of the male lamb.

Passive immunization of male lambs against oestradiol-17 beta from 2 to 16 weeks of age significantly elevated androgen concentrations in plasma and depressed the median eminence content of dopamine. Removal of endogenous oestrogens had no significant effects on plasma FSH, LH or prolactin concentrations or on testicular growth and hypothalamic content of GnRH. These results suggest that endogenous oestrogens may indirectly suppress testicular androgen secretion by exerting a stimulatory influence on hypothalamic dopaminergic neurones, which in turn may inhibit GnRH secretion by the median eminence.

Androgens↗
PubMed1985

Release of LHRH in vitro and anterior pituitary responsiveness to LHRH in vivo during sexual maturation in pullets (Gallus domesticus).

An in-vitro superfusion technique was used to study basal and depolarization-induced (32 mmol K+/l) release of LHRH from the mediobasal hypothalamus (MBH) of pullets at 8-25 weeks of age. Plasma LH concentrations and the incremental change (delta LH) after an i.v. injection of 1 or 15 micrograms synthetic ovine LHRH/kg body weight were also determined. Between 8 and 25 weeks of age, significant (P less than 0.01) increases in basal and depolarization-induced release of LHRH (93 and 330%, respectively) were accompanied by a significant (P less than 0.01) rise in the residual LHRH content of MBH tissue (152%), observations which suggest that the ability of the hypothalamus to synthesize and secrete LHRH increases as sexual maturation proceeds. However, plasma LH, which reached a maximum concentration of 2.05 +/- 0.43 micrograms/l at 15 weeks, fell significantly (P less than 0.05) to 1.14 +/- 0.05 micrograms/l at 25 weeks. Since delta LH in response to exogenous LHRH showed a marked and progressive decline between 12 and 20 weeks of age, the low plasma concentration of LH typical of the mature hen is probably attributable to a direct negative-feedback action of ovarian steroids on the anterior pituitary gland rather than to an impaired secretion of LHRH from the median eminence. It is suggested that a dramatic increase in the responsiveness of LHRH nerve terminals in the MBH to depolarization by 32 mmol K+/l between 20 and 25 weeks of age (mean age at onset of lay 21.9 weeks; range 19-25 weeks) may reflect the development of hypothalamic responsiveness to the positive feedback action of progesterone.

Animals↗
PubMed1984

Hypothalamic contents of LHRH and catecholamines during the ovulatory cycle of the hen (Gallus domesticus).

Concentrations of LHRH, dopamine, noradrenaline and adrenaline in the anterior hypothalamus-preoptic region (AH-POR) and posterior hypothalamus-median eminence (PH-me) were determined in hens killed at different times in relation to the first ovulation of a sequence. The occurrence of a preovulatory rise in plasma LH concentration 4-6 h before the expected time of ovulation was confirmed. This rise in plasma LH was accompanied by a significant (P less than 0.01) 50% reduction in the LHRH content of the AH-POR and PH-me while the subsequent fall in plasma LH was accompanied by a restoration of the LHRH content of both regions to their former levels. Although no significant fluctuations in the hypothalamic content of either dopamine, noradrenaline or adrenaline were detected during the ovulatory cycle, significant correlations between LHRH content and catecholamine content were observed in the AH-POR (P less than 0.05) and PH-me (P less than 0.01). Thus mean levels of each amine followed the same temporal pattern as LHRH content with minimum values being observed shortly before the peak of the preovulatory surge of LH. These findings support the conclusion that an enhanced secretion of LHRH from the median eminence, possibly associated with an increased activity of catecholaminergic neurones, is a prerequisite for the preovulatory release of LH in the hen.

Animals↗
PubMed1983

Concentrations of immunoreactive luteinizing hormone releasing hormone in discrete brain regions of the cockerel: effects of castration and testosterone replacement therapy.

The validity of using a radioimmunoassay employing an antiserum raised against synthetic luteinizing hormone releasing hormone (LH-RH) for the quantification of luteinizing hormone releasing factor (LH-RF) in birds was investigated. Extracts of avian hypothalamus yielded displacement curves which were parallel to that of the synthetic LH-RH standard and the immunoreactive potencies of a number of extracts assayed concurrently using two different anti-LH-RH sera were found to be similar. Moreover, after chromatography of cockerel hypothalamic extract on carboxymethyl-cellulose, immunoreactive and biologically active LH-RF were found in the same eluate fractions. Immunoreactive LH-RH was shown to be widely distributed in cockerel hypothalamus with the highest concentrations present in the mediobasal hypothalamus (MBH; 6.55 +/- 1.86 pg/microgram protein, n = 6) and medial preoptic region (POR; 0.95 +/- 0.07 pg/microgram protein, n = 6). The postcastration rise in plasma LH in the cockerel was accompanied by significant (P less than 0.05) increases in the concentration of LH-RH in five hypothalamic areas including the POR; testosterone replacement therapy completely reversed these effects. Although castration raised the mean concentration of LH-RH in four other hypothalamic areas including the MBH, these differences were not significant. However, testosterone replacement therapy depressed LH-RH in all four regions to levels significantly (P less than 0.05) less than those in castrated cockerels. These findings constitute the first direct evidence that the negative feedback action of testosterone on LH secretion in the cockerel is mediated, at least in part, by an action on hypothalamic LH-RF-producing neurones.

Animals↗
PubMed1983

Evidence for the involvement of central conversion of testosterone to oestradiol-17 beta in the regulation of luteinizing hormone secretion in the cockerel.

Treatment of intact cockerels with the synthetic antioestrogen tamoxifen caused a significant increase in the plasma concentration of LH. In contrast, passive immunization with an antiserum raised against oestradiol-17 beta did not lead to an increase in plasma LH. A pronounced depressive effect of injections of 0.1 mg testosterone propionate (TP) or 0.1 mg oestradiol benzoate (OB) on plasma concentrations of LH was prevented by tamoxifen. Furthermore, a pronounced rise in the concentration of LH releasing hormone in the posterior hypothalamus after the injection of cockerels with OB was completely inhibited by tamoxifen. Neither 0.1 nor 0.5 mg androstenedione modified the concentration of LH in plasma. A dose of 0.05 mg TP, which failed to depress the concentration of LH in plasma of intact cockerels, caused a marked fall in plasma LH in castrated cockerels. Tamoxifen itself exhibited weak oestrogen agonist activity in castrated cockerels by causing a reduction in the concentration of LH in plasma. However, tamoxifen prevented any further depressive effect on LH resulting from the injection of TP. These findings suggest that testosterone exerts an inhibitory influence on LH secretion at the central neural level, partially at least, by means of the product of its aromatization, oestradiol-17 beta.

Animals↗
PubMed1983

Variations in hypothalamic luteinizing hormone releasing hormone content and release in vitro and plasma concentrations of luteinizing hormone and testosterone in developing cockerels.

An in-vitro superfusion system was used to study age-dependent changes in the functional activity of LH releasing hormone (LHRH) neurones terminating in the mediobasal hypothalamus (MBH) of the cockerel. Fragments of MBH tissue were obtained from cockerels killed at 8, 12, 16 and 20 weeks of age and both the rate of release of LHRH in vitro and the residual content of LHRH were determined by radioimmunoassay. Blood was collected from a similar group of cockerels of the same age for determination of plasma LH and testosterone concentrations. Superfused cockerel MBH showed both basal and depolarization-induced release of LHRH and the calcium-dependency of the release process was demonstrated. The viability of MBH in vitro was indicated by the observation that the rate of CO2 production by the tissue remained constant during a 3-h period of superfusion. Both the basal rate of release of LHRH (7.24 +/- 0.63 pg/2MBH per h; mean +/- S.E.M., n = 4) and the residual content of LHRH (1.10 +/- 0.24 ng/2MBH) were lowest in the 8-week-old cockerel and increased progressively to reach levels three (P less than 0.001) and 13 (P less than 0.001) times greater, respectively, by 20 weeks of age. Concentrations of LH and testosterone in plasma did not increase significantly until 16 weeks of age when the respective values were 5.86 +/- 0.37 micrograms/l and 1.88 +/- 0.31 nmol/l (n = 11). Whereas plasma testosterone increased further to 5.76 +/- 0.42 nmol/l (n equal 11) at 20 weeks of age, plasma LH fell significantly (P less than 0.02) to a concentration of 4.08 +/- 0.41 micrograms/l (n = 11).(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗