Changes in the responsiveness of perifused rat adenohypophysial cells to repeated stimulation with luteinizing hormone releasing hormone

The ability of luteinizing hormone releasing hormone (LRH) to stimulate the release of luteinizing hormone (LH) from columns of enzymatically dispersed perfused adenohypophysial cells is being used to study the mechanisms controlling the secretion of LH. LRH stimulated the release in vitro of LH from columns of rat pituitary cells. However, when exposed repeatedly (1 pulse every 12 min) to the same submaximal dose (8 nmol/l) of LRH the cells always exhibited a marked progressive increase and subsequent decrease in their responsiveness. Similar effects occurred when the interval between pulses was extended to 20, 30 or 45 min. The enhanced responsiveness of the cells was prevented by the inclusion of protein synthesis inhibitors, cycloheximide or puromycin, in the perifusion fluid. Cells removed from rats ovariectomized 14 days previously also failed to exhibit increased responsiveness when stimulated repeatedly with LRH. LH secretion was also elicited by K+ (50 nmol/l), 8-bromoadenosine 3'-5'-cyclic monophosphate (8-Br-cAMP, 6 nmol/l), 8-bromoguanosine 3'-5'-cyclic monophosphate (8-Br-cGMP, 6 nmol/l) and a calcium ionophore (A23187, 40 mumol/l) but the responses to these secretagogues differed markedly from those to LRH for the tachyphylaxis which resulted from repeated exposure was not preceded by an increase in responsiveness. The decreased responsiveness to K+ developed in parallel with that to LRH. Diminished responses to the cyclic nucleotides and the Ca++ ionophore developed more rapidly, but the refractory cells responded readily to stimulation with LRH or K+.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of chronic administration of melanin-concentrating hormone on corticotrophin, melanotrophin, and pigmentation in the trout

This paper reports the effects of salmonid melanin-concentrating hormone (MCH), administered via an Alzet minipump, on pigmentation and secretion of pituitary melanotrophin (MSH) and corticotrophin (ACTH) by black-adapted, adult rainbow trout. The drug induced melanin concentration in the skin melanophores and prevented melanogenesis. Both the cytological appearance of the pituitary pars intermedia and determinations of plasma alpha-MSH suggest that MCH prevented the increase in secretory activity of the melanotrophic cells seen normally in black-adapted trout. Resting plasma cortisol titres were similar in all groups of fish but anterior pituitary glands taken from stressed, MCH-treated fish released less ACTH in vitro than those from corresponding saline-treated fish.

Stimulation and inhibition of corticotrophin releasing factor secretion by beta endorphin

The influence of beta-endorphin on the secretion of corticotrophin releasing factor (CRF) by isolated rat hypothalami in vitro was studied. beta-Endorphin (10(-11)-10(-10) M) caused dose-related increases in the CRF contents of the hypothalami and of the medium in which they were incubated. Its effects were antagonized by naloxone (10(-8)-10(-7) M). In contrast, in higher concentrations (10(-7) - 10(-5) M), it reduced, in a dose-dependent manner, both the spontaneous release of CRF from the hypothalami and the release which normally occurred in response to acetylcholine, 5-hydroxytryptamine, morphine, met-enkephalin and leu-enkephalin. The inhibition of CRF release was associated with a rise in the tissue content of the hormone and was not blocked readily by naloxone. The results support the concept that opioid substances may be involved in the control of hypothalamo-pituitary-adrenocortical function.

Effects of naloxone on hypothalamo-pituitary-adrenocortical activity in the rat

The influences of morphine and naloxone on hypothalamo-pituitary-adrenocortical (HPA) function were studied in the rat to investigate further the role of opioidergic mechanisms in the control of the secretion of corticotrophin and its hypothalamic releasing factor (CRF). Morphine not only caused rises in hypothalamic CRF content and plasma ACTH concentration but also potentiated the HPA response to stress. Its effects were antagonized by naloxone which, when given alone, did not influence basal plasma concentrations of ACTH and corticosterone but which inhibited, in a dose-dependent manner, the release of both of these hormones which normally occurs in response to stress. Naloxone also attenuated the exaggeration in stress-induced HPA activity but did not affect the increases in plasma ACTH concentration which followed adrenalectomy. The findings suggest that opioidergic mechanisms may be involved in the regulation of the HPA response to stress.

Pharmacological characterization of opioid receptors influencing the secretion of corticotrophin releasing factor in the rat

The effects of selective agonists and antagonists of opioid receptors on the secretion of corticotrophin releasing factor (CRF) by isolated rat hypothalami in vitro were studied. Morphine (10(-8)-10(-6) M) and the mu-opioid receptor agonists, FK33-824CH (10(-8)-10(-6) M) and Tyr-D-Ala-Gly-MePhe-NH(CH2)2OH (10(-8)-10(-6) M), caused dose-related increases in the release of CRF from isolated hypothalami. The kappa-opioid receptor agonist, U50,488 (10(-8)-10(-6) M), was also weakly active in this respect but the delta-opioid receptor agonist, (D-Pen2,D-Pen5)-enkephalin (2 X 10(-10)-2 X 10(-7) M), was not. The stimulatory actions of morphine and Tyr-D-Ala-Gly-MePhe-NH(CH2)2OH on CRF release were antagonized by naloxone (10(-8) M) and by the mu/delta-opioid receptor antagonist, beta-funaltrexamine (beta-FNA, 10(-9) M), but not by the delta-opioid receptor antagonist, ICI-154129 (5 X 10(-6) M). The effects of U50,488 on CRF release were unaffected by either beta-FNA or ICI-154129 but were antagonized by high doses of naloxone (10(-6) M). The results suggest that both mu- and kappa-opioid receptors are involved in the stimulation of CRF secretion but that delta-opioid receptors are not important in this respect.

Salmonid melanin-concentrating hormone inhibits corticotrophin release

Melanin-concentrating hormone (MCH) is a neural peptide associated with colour change in fishes. We show here that it also inhibits corticotrophin (ACTH) secretion. Synthetic salmonid MCH at a concentration of 100pmol/l reduced the in-vitro release of ACTH by pars distales (pDs) taken from stressed trout. At lower concentrations (10pmol/l) the peptide inhibited CRF-41-induced secretion of ACTH by pDs removed from unstressed trout, while at higher concentrations (10nmol/l) it reduced the corticotrophic response of rat pituitary tissue to CRF-41.

Corticotrophin-like bioactivity in the pituitary gland and brain of the pacific hagfish, Eptatretus stouti

The cytochemical bioassay for corticotrophin (ACTH) was used in an attempt to detect ACTH-like activity in the Pacific hagfish, Eptatretus stouti. Extracts of the pituitary gland and brain were active in this assay system but those of liver and skeletal muscles were not. The slopes of the dose-response lines of the pituitary extracts were less than those of the mammalian corticotrophin standard preparation but greater than those of the brain extracts. The results suggest that the corticotrophic factor in the hagfish differs from mammalian ACTH.

Peripubertal changes in the nature of LH

Administration of pregnant mare serum gonadotrophin (PMSG) to peripubertal rats, aged 27 days, induces ovulation provided the animals weigh more than 60 g at the time of the injection. In an attempt to determine whether the apparent immaturity of the ovaries in smaller rats is associated with an inability of the pituitary gland to secrete LH, the biological and immunological properties of LH in peripubertal PMSG-treated rats were examined. A single injection of PMSG caused a marked hypersecretion of LH in rats aged 27 days. The LH in the plasma of rats weighing more than 60 g was active in both the radioimmunoassay and the cytochemical bioassay but that in smaller rats was active only in the former. Plasma from both groups of rats stimulated the release of testosterone from dispersed Leydig cells. Luteinizing hormone-releasing hormone stimulated the secretion, in vitro, of immunoreactive, cytochemically active LH by pituitary tissue from rats weighing over 60 g. The LH released in vitro from tissue from the smaller animals, like that in their plasma, was active in the radioimmunoassay but not in the cytochemical system. The results suggest that an abrupt change in the nature of LH occurs at puberty and that ovulatory cycles commence only when the pituitary gland secretes the adult form of LH with a full spectrum of biological activity.

Influence of growth hormone, corticosterone, corticotrophin and changes in the environmental temperature on pituitary-ovarian function in the immature rat

The influence of various endocrine and environmental factors on pituitary-ovarian function was studied in peripubertal rats treated with pregnant mare serum gonadotrophin (PMSG). Pregnant mare serum gonadotrophin induced ovulation in rats aged 27 days provided they weighed over 60 g. The response was preceded by a marked hypersecretion of LH which was detectable by radioimmunological and biological assay methods. In contrast, smaller rats of the same age did not ovulate in response to PMSG apparently because of the secretion of a pleiomorphic form of LH which, although immunoreactive, appeared to be biologically inactive. Ovarian function, assessed by response to exogenous gonadotrophins and by measurement of 125I-labelled human chorionic gonadotrophin binding, was normal despite the presence of the biologically inactive pleiomorph. Exposure of the small PMSG-treated rats to a high environmental temperature (39 degrees C) or treatment with corticosterone or GH altered the nature of the LH in the blood so that it was active in both assay systems and facilitated ovulation as also did ACTH. The results suggest that the abrupt change in the nature of the LH released by the pituitary gland essential for the initiation of ovulation may be affected by GH, corticosterone or a raised environmental temperature.

Differences in hypothalamo-pituitary-adrenocortical activity in the rat after acute and prolonged treatment with morphine

The influence of opioid substances on the secretion in vivo and in vitro of corticosterone, corticotrophin (ACTH) and corticotrophin releasing factor (CRF) in the rat was studied. Rats given a single injection of morphine exhibited a marked hypersecretion of ACTH and an exaggeration of the hypothalamo-pituitary-adrenocortical (HPA) response to stress. In contrast, animals rendered tolerant to morphine failed to release ACTH or corticosterone in response either to a subsequent injection of the opiate or to stress. The development of the inhibitory effect paralleled the development of tolerance to the analgesic actions of the drug. The production of ACTH by pituitary segments removed from control animals was not affected by the addition of opioid substances to the incubation medium. However, morphine, met-enkephalin and leu-enkephalin stimulated the secretion of CRF by hypothalami and their effects were competitively antagonized by naloxone. The secretory activity of hypothalami removed from rats treated acutely with morphine was enhanced. In contrast hypothalami from morphine-tolerant rats failed to secrete CRF in response to morphine, met-enkephalin, acetylcholine or 5-hydroxytryptamine. Neither the density nor the affinity of 3H-naloxone binding sites in the hypothalamus was influenced by the morphine treatment. The results suggest the opioid peptides and their receptors play a major role in the regulation of HPA function.

Inhibition of corticotrophin releasing factor secretion in the pentobarbitone-morphine-treated rat

The concentrations of ACTH in the plasma and adenohypophysis and corticotrophin releasing factor (CRF) in the hypothalamus were determined and the functional capacities of the adenohypophysis and hypothalamus assessed in male rats treated with pentobarbitone and morphine. A single injection of morphine increased markedly the concentrations of ACTH in both the plasma and the adenohypophysis and the CRF content in the hypothalamus. When the opiate was given to rats injected 10 min previously with sodium pentobarbitone its effects were substantially reduced and the hypothalamo-pituitary-adrenocorticotrophic response to stress was abolished. The functional activity in vitro of pituitary tissue removed from control rats was not affected by the addition to the incubation medium of pentobarbitone and/or morphine. Pituitary glands of drug-treated rats also responded normally both in vivo and in vitro, with respect to ACTH secretion, to hypothalamic extracts prepared from control animals, CRF-41 and arginine vasopressin (AVP). On the other hand, hypothalami removed from pentobarbitone/morphine treated rats showed a marked reduction in their ability to secrete CRF in response to acetylcholine or 5-hydroxytryptamine as also did hypothalami from control animals after preincubation with pentobarbitone and/or morphine. The results suggest that the hypothalamus is an important site of action of pentobarbitone and morphine in modulating the functional activity of the hypothalamo-pituitary-adrenocortical system and are in accord with the view that the pentobarbitone/morphine-treated rat is suitable for the detection and quantification of potential corticotrophin releasing factors.

A study of corticotrophic and melanotrophic activities in the pituitary and brain of the lamprey Lampetra fluviatilis

Biological and radioimmunological assay methods were used in an attempt to detect and determine corticotrophic and melanotrophic activity in the pituitary gland and the central nervous system of the lamprey, Lampetra fluviatilis. The activities of various corticotrophin (1-39 ACTH)-related peptides in the cytochemical assay for ACTH could be readily distinguished on the basis of their dose-response lines. Extracts of the pituitary gland and brain but not of spinal cord were also active in this test. The dose-response lines of the pars distalis extracts were parallel with those of 1-39 ACTH; those of the neurointermediate lobe and the brain resembled des-Ac-alpha-MSH and alpha-MSH, respectively, although subsequent tests showed that the active molecules in the extracts differed from these standard peptides in other respects. Pituitary extracts were active in the Anolis bioassay for melanotrophin but not in a radioimmunoassay for alpha-MSH. Moreover, the electrophoretic Rf value of lamprey melanotrophic activity was distinct from both alpha-MSH and des-Ac-alpha-MSH. Brain extracts showed only low melanotrophic bioactivity and again no immunoreactivity. The results suggest that small amounts of corticotrophin occur in the pars distalis of the lamprey and that the melanotrophic factors in the neurointermediate lobe and brain are not identical to mammalian alpha-MSH and des-Ac-alpha-MSH.

Biological assay of luteinizing hormone-releasing hormone (gonadorelin)

The abilities of luteinizing hormone releasing hormone (LHRH) to stimulate the release of luteinizing hormone (LH) in vitro from dispersed pituitary cells or from segments of adenohypophysial tissue have both been exploited in an attempt to develop biological assays for its determination. Columns of dispersed adenohypophysial cells were highly sensitive to LHRH. The responses appeared to be specific, but the marked continual variation in their magnitude rendered the system nonprecise. Pituitary segments were less sensitive to LHRH than the dispersed cells. However, their responsiveness was increased by pretreatment ('priming') with the releasing hormone. Over a wide range of doses, there was a linear relationship between the LH released by 'primed' segments and the logarithm of the concentration of LHRH. The dose-response lines of hypothalamic extracts were parallel with those of the synthetic peptide. The method using segments is accurate, specific, and precise and fulfils the essential requirements of a biological assay.

Effects of adrenocortical and gonadal steroids on the secretion in vitro of corticotrophin and its hypothalamic releasing factor

The effects of adrenocortical and gonadal steroids on the secretion in vitro of ACTH by adenohypophysial segments and corticotrophin releasing factor (CRF) by isolated hypothalami were studied in the rat. Corticosterone (1.25 X 10(-6) mol/l), betamethasone (2.5 X 10(-8) mol/l) and progesterone (2.5 X 10(-7) mol/l) reduced the hypothalamic extract-induced secretion of ACTH by pituitary tissue in vitro but aldosterone (2 X 10(-7) mol/l), testosterone, androsterone, androstenedione (2 x 10(-7) mol/l), oestradiol, oestriol, and oestrone (10(-6) mol/l) did not. Corticosterone (2.5 +/- 10(-9) mol/l), aldosterone (2 X 10(-8) mol/l) and betamethasone (2 X 10(-10) mol/l) inhibited and oestradiol, oestriol and oestrone (10(-8) - 10(-6) mol/l) potentiated the production of CRF by isolated hypothalami which occurred when acetylcholine or 5-hydroxytryptamine were added to the incubation medium but progesterone (2.5 X 10(-7) mol/l), testosterone, androsterone and androstenedione (2 X 10(-7) mol/l) had no effects. The results indicate that hypothalamo-pituitary-adrenocorticotrophic activity may be modified not only by glucocorticoids but also by other steroids.

Secretion of corticotrophin and its hypothalamic releasing factor in response to morphine and opioid peptides

The influence of morphine and enkephalins on the functional activity of the hypothalamo-pituitary-adrenocortical system in the rat was studied by investigating their effects on the secretion in vivo and in vitro of corticotrophin (ACTH) by the pituitary gland and corticotrophin-releasing factor (CRF) by the hypothalamus. A single injection of morphine caused a rise followed by a fall in hypothalamic CRF content and increases in the concentrations of ACTH in the plasma and adenohypophysis. In addition, the stress-induced increments in hypothalamic CRF and pituitary and plasma ACTH were exaggerated in morphine-treated rats. The production of ACTH by pituitary segments in vitro was not affected by the addition to the incubation medium of morphine, met-enkephalin, leu-enkephalin or naloxone. However, morphine and the enkephalins stimulated the secretion of CRF by isolated hypothalami and their effects were antagonized by naloxone. The results indicate that morphine and the enkephalins evoke hypothalamo-pituitary-adrenocortical activity by stimulating specific receptors in the hypothalamus and raise the possibility that opioid peptides and their receptors are physiologically important in the control of the secretion of CRF.

A cytochemical bioassay method for the determination of luteinizing hormone in biological fluids and tissues

1 The ability of luteinizing hormone (LH) to alter the reducing activity of corpora lutea in rat ovarian sections has been exploited to develop a new cytochemical bioassay for the hormone. 2 Sections of ovaries, removed from mature rats during the second day of dioestrus, were incubated with either standard LH or samples diluted for assay, stained immediately for reducing potential and the intensity of the stain measured by scanning and integrating microdensitometry. 3 An inverse linear relationship existed between the density of the stain and the logarithm of the concentration of standard LH (68/40) and the dose-response lines of serial dilutions of rat or human plasma were parallel with those of the standard. 4 The method was found to be accurate, specific, sensitive, precise and suitable for the determination of LH in the rat and in man.

The influence of vasopressin on hypothalamic corticotrophin releasing activity in rats with inherited diabetes insipidus

1. The corticotrophin releasing activity of hypothalamic extracts from rats congenitally lacking vasopressin (Brattleboro strain) has been studied in the presence and absence of arginine vasopressin and its antiserum. 2. Hypothalamic extracts from Brattleboro rats stimulated the production of corticotrophin by pituitary segments in vitro but both their potency and the slopes of their dose-response lines were significantly less than those of controls. Arginine vasopressin also stimulated pituitary-adrenocorticotrophic activity in vitro but only in concentrations considerably greater than those present in hypothalamic extracts from normal rats. 3. In low, physiological concentrations arginine vasopressin did not affect the corticotrophin releasing activity of hypothalamic extracts from controls but potentiated markedly the activity of extracts from Brattleboro rats and rendered the slopes of their dose-response lines parallel with those of the controls. 4. The antiserum to arginine vasopressin did not affect the corticotrophin releasing activity of Brattleboro extracts but reduced the activity of control extracts and rendered the slopes of their dose-response lines parallel with those of Brattleboro extracts. 5. The results suggest that vasopressin acts synergistically with the corticotrophin releasing factor and is essential for the full expression of pituitary-adrenocorticotrophic activity.

Hypothalamo-pituitary-adrenocortical function in rats with inherited diabetes insipidus

1. Hypothalamo-pituitary-adrenocortical activity in male homozygous and heterozygous Brattleboro rats was compared with that in normal (Long Evans) controls in an attempt to elucidate the role of vasopressin the control of the secretion of corticotrophin. 2. The concentrations of corticosterone and corticotrophin in the plasma, corticotrophin in the adenohypophyses and corticotrophin releasing factor in the hypothalami were lower in the heterozygotes than in the controls and lower still in the homozygotes. 3. The capacities of adenohypophyses and hypothalami to secrete in vitro corticotrophin and corticotrophin releasing factor respectively in response to trophic stimuli were also reduced in the heterozygotes and, to an even greater extent, in the homozygotes. 4. The results suggest that vasopressin is not the corticotrophin releasing factor but they do not exclude the possibility that it may be involved in the sequence of events which leads to the secretion of corticotrophin in the rat.