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ABSTRACT
Circulating GH concentrations were suppressed 24 h after intracerebroventricular (i.c.v.) injection of native or recombinant DNA-derived chicken GH (rcGH) (10 μg in 4 μl vehicle) into the right lateral ventricles of conscious 6-week-old cockerels. Plasma concentrations of LH were suppressed within 1 h of i.c.v. administration of GH at doses of 1 or 10 μg, and remained suppressed for 24 h in birds injected with the highest concentration of GH. In contrast, concentrations of plasma prolactin were increased 24 h after i.c.v. administration of GH (10 μg). The i.v. administration of rcGH (at concentrations of 10 or 100 μg/kg) did not mimic the effects of i.c.v. injection of GH.
These results demonstrate central effects of GH on pituitary function in the domestic fowl.
Journal of Endocrinology (1990) 125, 409–415
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ABSTRACT
Thyrotrophin-releasing hormone (TRH) stimulates GH secretion in domestic fowl by actions at pituitary and central nervous system sites. The possibility that this central action might be mediated by hypothalamic catecholamines or indoleamines was therefore investigated. When TRH was administered into the lateral ventricles of anaesthetized fowl the concentration of 3,4-dihydroxyphenylacetic acid (DOPAC, a metabolite of dopamine (DA)) in the medial basal hypothalamus (MBH) was increased within 20 min. The concentrations of MBH noradrenaline (NA), DA, serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) were, however, unaffected by the intracerebroventricular (i.c.v.) administration of TRH, although the MBH concentrations of somatostatin and TRH were concomitantly reduced. A rapid increase in DA release into MBH extracellular fluid and its metabolism to DOPAC was also observed after i.c.v. or i.v. administration of TRH, in birds in which the MBH was perfused in vivo with Ringer's solution. Microdialysate concentrations of NA, 5-HT and 5-HIAA were not, however, affected by central or peripheral injections of TRH. Diminished GH responses to i.v. TRH challenge occurred in birds pretreated with reserpine (a catecholamine depletor), α-methyl-paratyrosine (a DA synthesis inhibitor) and pimozide (a DA receptor antagonist). These results therefore provide evidence for the involvement of a hypothalamic dopaminergic pathway in the induction of GH release following the central or peripheral administration of TRH. In contrast with its inhibitory actions at peripheral sites, DA would appear to have a central stimulatory role in regulating GH release in birds.
Journal of Endocrinology (1993) 138, 225–232
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ABSTRACT
GH administered centrally or peripherally inhibits basal or secretagogue-induced GH secretion in domestic fowl. Since the release of pituitary GH is neurally regulated by the hypothalamus, GH autoregulation may be mediated by changes in the content or metabolism of hypothalamic monoamines. When chicken GH (500 μg/kg body weight) was injected i.v. into laying hens, tissue catecholamine (adrenaline, noradrenaline and dopamine) concentrations in the preoptic area (POA) and medial basal hypothalamus (MBH) were depleted for 2–24 h, as were concentrations of dihydroxyphenylacetic acid, a dopamine metabolite. The serotonin (5-HT) content of the POA and MBH was unaffected by i.v. GH administration, although a reduction in MBH 5-hydroxyindoleacetic acid suggested a tissue-specific inhibition of 5-HT turnover. Qualitatively similar results were observed in laying hens 24 h after the intracerebroventricular injection of chicken GH (10 μg/bird). These results therefore demonstrate aminergic actions of GH within the chicken hypothalamus which may mediate GH autoregulation. However, as amine metabolism is not only suppressed when endogenous GH secretion is reduced, but also at times when normal GH secretion is restored, these aminergic effects may also reflect other actions of GH on central function.
Journal of Endocrinology (1993) 136, 245–251
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ABSTRACT
Peripheral plasma concentrations of GH in adult chickens were increased, in a dose-related manner, between 5 and 30 min after the intracerebroventricular (i.c.v.) injection of 0·1 or 10 μg TRH. In contrast, i.v. administration of comparable doses of TRH had no significant effect on circulating GH concentrations. [3H]3-methyl-histidine2-TRH ([3H]Me-TRH) was located in the pituitary gland and peripheral plasma within 5 min of its i.c.v. administration, although in amounts that were unlikely to affect directly pituitary function. [3H]Me-TRH rapidly accumulated in the hypothalamus following its i.c.v. administration (but not after i.v. injection), and the central effect of TRH on GH secretion in birds is therefore likely to be induced by effects at hypothalamic sites.
Journal of Endocrinology (1990) 126, 83–88
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ABSTRACT
Plasma prolactin began to increase significantly about 5 days after the onset of incubation in both sexes of the dove to reach a peak at the time of hatch. At this time, the concentration of prolactin in the plasma of the female was significantly higher than in the male. In the middle of the incubation period prolactin levels measured over a 24-h period remained constant in both sexes, although the male sits during the middle of the day and the female for the rest of the time. Nest deprivation resulted in a sharp, significant decline in the concentration of prolactin in both sexes. Newly hatched squabs stimulated the release of prolactin only in those doves which had been incubating eggs for several days.
A distinct sex difference was observed in the expression of nest defence behaviour of the ring dove during the breeding cycle. At the time of laying, the female was significantly more aggressive than the male and her aggression increased only slightly up to the time of hatching. In contrast, male aggression increased gradually from a low level at laying to reach a peak at the time of hatching. The levels of plasma progesterone in the female showed a significant increase around the time of lay. No significant changes occurred in the plasma concentration of either progesterone or 17α-hydroxyprogesterone in the male.
Administration of prolactin increased the length of time of incubation of infertile eggs. Nest manipulations which had the effect of inducing the doves to begin incubation 4 days before laying showed that (1) the length of time of incubation of infertile eggs is fixed and independent of events which occur at courtship or oviposition, (2) the initiation of the increase in plasma prolactin concentration during incubation is independent of events which occur at courtship and oviposition and (3) the termination of incubation is always preceded by a fall in the concentration of plasma prolactin.
It is concluded that the length of time of incubation is dependent upon sustained raised levels of plasma prolactin. The concentration of plasma prolactin increases several days after the onset of incubation in response to the tactile stimulation of sitting. High levels, if maintained by visual stimulation from the nest, maintain incubation for a fixed period. After this, if the eggs fail to hatch, prolactin levels fall and the doves cease incubation and begin a new cycle.
J. Endocr. (1986) 110, 447–458
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ABSTRACT
Testicular weights and concentrations of LH in plasma were measured in individually housed adult male ring doves given five daily intracerebroventricular injections of saline–NaHCO3 vehicle (2 μl), ovine prolactin (0·1, 0·5, 1·0 or 2·0 μg/day), turkey prolactin (1·0 μg/day), turkey GH (1·0 μg/day) or ovine GH (1·0 μg/day). Administration of ovine prolactin resulted in a dose-dependent suppression of plasma LH concentration, with values in the two highest dose groups averaging three- to fivefold less than those of vehicle-injected controls. Reductions of similar magnitude were obtained following intracranial administration of turkey, ovine or human GH. Whilst effective in reducing plasma LH, turkey prolactin was less effective than an equivalent dose of ovine prolactin. Testicular regression was observed in all treatment groups which showed a significant decrease in plasma LH concentrations. Because crop sacs remained undeveloped in all treatment groups, it was concluded that these centrally administered hormones acted primarily at the level of the brain or pituitary to exert their suppressive effects. The possibility that prolactin and GH interact with different binding sites to inhibit LH secretion is discussed, together with evidence for a possible role of prolactin and GH in gonadotrophin regulation under normal physiological conditions.
J. Endocr. (1988) 118, 33–40
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ABSTRACT
Intracerebroventricular (i.c.v.) administration of GH-releasing factor (GRF) (at 1 or 10 μg) to anaesthetized immature (6- to 8-weeks-old) or adult (> 24-weeks-old) domestic fowl had no effect on basal GH concentrations in peripheral plasma, but suppressed (after 20 min) the acute GH response to exogenous (i.v.) thyrotrophin-releasing hormone (TRH) (1 μg/kg). The i.c.v. injection of GRF also reduced the content of somatostatin (SRIF) and dopamine (DA) in the hypothalamus, while increasing the concentration of the DA metabolite 3,4-dihydroxyphenyl acetic acid (DOPAC) and the DOPAC/DA ratio. The release of SRIF from hypothalamic tissue was stimulated in vitro by 100 nmol GRF/l. The inhibitory effect of i.c.v. GRF on TRH-induced GH secretion was blocked when it was simultaneously injected i.c.v. with SRIF antiserum.
These results demonstrate central effects of GRF on avian hypothalamic function and suggest an inhibitory role for this peptide in GH regulation, possibly mediated through increased SRIF secretion.
Journal of Endocrinology (1991) 128, 13–19
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ABSTRACT
The effects of pinealectomy on the daily rhythms of concentrations of tri-iodothyronine (T3) and thyroxine (T4) were investigated in sexually immature female chickens exposed to 21-, 24- and 27-h cycles of light and darkness, or to extended periods of light or darkness for more than 24 h. In pinealectomized and control birds, rhythms in levels of plasma T3 and T4 were entrained by all lighting cycles and decreased in amplitude or disappeared in continuous light or darkness. In pinealectomized and control birds held on 21-h (11 h light: 10 h darkness; 11L: 10D) and 24-h (14L: 10D) lighting cycles, the peak of the T4 rhythm coincided with, or lagged, the trough in the rhythm of T3 while in birds held on a 27-h (14L: 13D) lighting cycle, the peak of the T4 rhythm preceded the trough in the rhythm of T3.
Pinealectomy resulted in significant effects on the phases or amplitudes of rhythms of T3 or T4 in all lighting schedules except 4L: 20D. However, these effects were not consistent in direction between experimental groups and were, therefore, of doubtful physiological significance. Pinealectomy increased the mean level of plasma T4 in birds exposed to continuous light or darkness or to 4L: 20D. A corresponding reduction in mean levels of plasma T3 was seen in birds exposed to continuous light or darkness.
It is concluded that under the lighting conditions investigated pinealectomy had no clear effect on the phases or amplitude of daily rhythms of levels of T4 or T3. However, after the effects of the feeding pattern on thyroid hormone rhythms imposed by the lighting cycle were removed by placing birds in constant lighting conditions, pinealectomy appeared to exert an inhibitory action on thyroid function.
J. Endocr. (1984) 103, 337–345
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ABSTRACT
The interactions between broody behaviour and changes in concentrations of plasma prolactin and LH were investigated in bantam hens. Adoption of newly hatched chicks caused incubating hens to leave their nests and prevented plasma prolactin decreasing as rapidly as in hens deprived of their nests and not given chicks. Further, the hens allowed to rear chicks came back into lay later (P< 0·001) than the hens not allowed chicks. Plasma prolactin decreased and plasma LH increased in hens deprived of their nests: these changes were reversed when the hens re-nested. The changes in plasma LH and prolactin in nest-deprived and re-nesting birds were not always synchronous; this was particularly clear immediately after nest deprivation when the increase in plasma LH preceded the decrease in the plasma prolactin. Readiness to incubate disappeared between 48 and 72 h after nest deprivation and corresponded with the time when plasma prolactin decreased to baseline values. Administration of ovine prolactin depressed (P<0·01) the initial increase in plasma LH after nest deprivation, but repeated administration of prolactin for up to 72 h failed to suppress plasma LH to the values found in incubating hens. Repeated administration of ovine prolactin at 5- to 8-h intervals for 72 h maintained readiness to incubate in nest-deprived hens. It is concluded that the secretion of prolactin in broody hens is facilitated by the presence of chicks and that increased concentrations of plasma prolactin maintain incubation behaviour. In incubating hens the secretion of LH and prolactin may be partly regulated independently. In addition, LH secretion may also be inhibited by increased plasma prolactin.
J. Endocr. (1988) 118, 279–286
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ABSTRACT
The i.c.v. administration of 0·1 or 10 μg ovine (o)GH to 12- to 16-week-old hypothyroid chickens of a sex-linked dwarf (SLD) strain suppressed the basal plasma GH concentrations, measured 24 h afterwards. The GH response of the oGH-injected SLDs to TRH was suppressed, in a dose-related way, in comparison with that induced by TRH in birds given control injections (10 μg) of bovine serum albumin (BSA). Basal circulating concentrations of GH in euthyroid K strain birds of the same age were even lower than in the SLDs following injection of 10 μg oGH, and were not further reduced by oGH administration. The GH response to TRH in the K strain birds injected i.c.v. with 0·1 or 10 μg oGH was, nevertheless, suppressed in comparison with the BSA-injected K strain controls. The i.c.v. administration of oGH also suppressed circulating concentrations of LH and the LH response to TRH in the K strain birds.
Twenty-four hours after i.c.v. administration of oGH (10 μg), the somatostatin (SRIF) content in the medial basal hypothalamus of 8-week-old euthyroid cockerels was greater than that in BSA (10 μg)-injected controls. At the same time, the binding of [3H]3-methyl-histidine2-TRH to the pituitary caudal and cephalic lobes of GH-injected birds was less than that in the controls.
These results suggest that GH regulation in avian species is partly mediated by an inhibitory short-loop mechanism (mediated by hypothalamic SRIF and a down-regulation of pituitary TRH-binding sites) that suppresses basal and secretagogue-induced GH release.
Journal of Endocrinology (1990) 126, 237–244
