Presynaptic alpha 2 -adrenergic stimulation leads to growth hormone release in the dog

Neuroendocrine and other studies of the mechanism of antidepressant action of desipramine

It is not known whether in depressed patients antidepressant treatment increases or reduces monoaminergic neurotransmission. Clinical studies are therefore reviewed that investigate adaptive changes at adrenoceptors in depressed patients treated with desipramine, and the net effect of these changes upon neurotransmission. Although in animals chronic desipramine treatment enhances the responsiveness of alpha 1-adrenoceptors to phenylephrine, no such effect could be demonstrated in patients upon the responsiveness of pupil diameter to phenylephrine. However, in keeping with animal studies, clinical evidence of altered responsiveness of alpha 2-adrenoceptors could be demonstrated after chronic desipramine treatment. The alpha 2-mediated growth hormone response to clonidine was increased after one week's treatment with desipramine and then reduced during the second and third weeks of treatment. No clinical measure of the responsiveness of central beta-adrenoceptors is available. However, the secretion of melatonin is a measure of neurotransmission at noradrenergic terminals in the pineal with alpha 1-, alpha 2- and beta 1-adrenoceptors. In normal volunteers the secretion of melatonin was increased by the noradrenaline uptake inhibitors desipramine and (+)-oxaprotiline; (-)-oxaprotiline had no effect. In depressed patients melatonin secretion was increased after three weeks' treatment with desipramine. These and other clinical studies suggest that antidepressant treatments increase noradrenergic neurotransmission in depressed patients.

Neuroendocrine control of growth hormone secretion

The secretion of growth hormone (GH) is regulated through a complex neuroendocrine control system, especially by the functional interplay of two hypothalamic hypophysiotropic hormones, GH-releasing hormone (GHRH) and somatostatin (SS), exerting stimulatory and inhibitory influences, respectively, on the somatotrope. The two hypothalamic neurohormones are subject to modulation by a host of neurotransmitters, especially the noradrenergic and cholinergic ones and other hypothalamic neuropeptides, and are the final mediators of metabolic, endocrine, neural, and immune influences for the secretion of GH. Since the identification of the GHRH peptide, recombinant DNA procedures have been used to characterize the corresponding cDNA and to clone GHRH receptor isoforms in rodent and human pituitaries. Parallel to research into the effects of SS and its analogs on endocrine and exocrine secretions, investigations into their mechanism of action have led to the discovery of five separate SS receptor genes encoding a family of G protein-coupled SS receptors, which are widely expressed in the pituitary, brain, and the periphery, and to the synthesis of analogs with subtype specificity. Better understanding of the function of GHRH, SS, and their receptors and, hence, of neural regulation of GH secretion in health and disease has been achieved with the discovery of a new class of fairly specific, orally active, small peptides and their congeners, the GH-releasing peptides, acting on specific, ubiquitous seven-transmembrane domain receptors, whose natural ligands are not yet known.

Psychoneuroendocrinology of depression. Growth hormone

The release of growth hormone (GH) from the anterior pituitary is regulated by hypothalamic peptides especially GH-releasing hormone (GHRH) and somatostatin, which in turn are controlled by classic neurotransmitters such as noradrenaline, dopamine, and acetylcholine, as well as negative feedback from GH and insulin-like growth factor-1. There has been extensive investigation of this axis in patients with depression. The most consistently reported abnormality is in noradrenergic-mediated GH release, which probably occurs via GHRH containing neurones. ACh-induced GH release through the somatostatin system, GABA, and also GHRH-stimulated release are reported as abnormal by some researchers.

An improved radioimmunoassay for the determination of canine growth hormone based on commercially available reagents

Determination of canine growth hormone (cGH) is of importance when evaluating the growth hormone status in dogs, for instance when diagnosing pituitary dwarfism and acromegaly. At present, analysis for the cGH content in canine blood has mainly been restricted to specialized laboratories which have had the capacity to develop suitable methods. We therefore formulated the objective of the present study to be the development of a generally accessible method utilizing commercially available reagents. We describe a radioimmunoassay (RIA) for the determination of cGH which was based on reagents developed for the determination of porcine growth hormone (pGH), thereby drawing benefit from the similarity between growth hormones from the two species. The RIA method was characterized by satisfying precision and accuracy parameters: intra- and inter-assay coefficients of variation (CV%) were 4-27% and 4-20%, respectively, depending on the cGH concentration of the samples. Recovery of added cGH and parallelism between standard curve and sample dilutions were acceptable, thus indicating identity between the calibrator system and the material under investigation (i.e. plasma samples). When measuring individual plasma samples or samples taken during clonidin response tests from normal dogs, dogs suffering from pituitary dwarfism and from dogs suffering from acromegaly, the results were comparable with previous findings.

Growth hormone responses to growth hormone-releasing hormone and clonidine in dogs with Cushing's syndrome

The effects of clonidine, a growth hormone (GH) secretagogue, acting at the hypothalamic level and synthetic GH-releasing hormone (GHRH), a physiological stimulus of somatotrophs, on GH secretion were measured in 11 dogs with Cushing's syndrome. Eight healthy dogs served as controls. After the administration of GHRH the dogs with hyperadrenocorticism had a mean (SEM) GH peak of 11.2 (2.5) ng ml-1 which was significantly lower than the peak of 48.6 (13.4) ng ml-1 observed in the healthy dogs. Similarly, the GH response to clonidine was inhibited in the dogs with hyperadrenocorticism, the mean GH peak being 9.3 (3.3) ng ml-1 compared with 135.6 (43.3) ng ml-1 in the control dogs. No significant difference between the GH responses to GHRH and clonidine was observed in the dogs with Cushing's syndrome, the areas under the response curves being 567.9 (78.2) and 478.0 (102.6) ng.min ml-1, respectively. These results demonstrate that the function of somatotrophs is abnormal in dogs with Cushing's syndrome. There is evidence that the likely action of clonidine in dogs is to inhibit the release of somatostatin and the results therefore suggest that the effect of an excess of glucocorticoid in the dog is probably not mediated through an increase in somatostatin tone.

Growth hormone response to clonidine in the nervous pointer dog model of anxiety

Blunted growth hormone responses to clonidine have been reported in most studies of humans with panic disorder but have been an inconsistent finding in the study of other anxiety syndromes. The growth hormone response to oral clonidine (100 micrograms/kg) was investigated in the adult nervous pointer dog, a genetic animal model of anxiety. Compared with placebo, clonidine produced significant increases in plasma levels of growth hormone; however, there were no differences in the growth hormone (GH) responses to clonidine in the nervous compared with the normal pointer dogs. Findings in this animal model are discussed within the context of noradrenergic-hypothalamic-GH dysfunction reported in some but not all types of anxiety disorders in humans.

Endogenous catecholamines modulate growth hormone release in the conscious rat during hypoglycaemia but not in the basal state

To investigate the role of endogenous catecholamines and 5-hydroxytryptamine in the control of growth hormone (GH) secretion, secretory profiles of GH and prolactin were measured in conscious, male rats following intravenous administration of either 1) alpha 2 antagonist idazoxan 2 mg/kg, a dose that blocked alpha 2 agonist induced GH rise, 2) alpha 1 antagonist prazosin 1 mg/kg, 3) non-specific beta-blocker propranolol 1.5 mg/kg, a dose that prevented beta 2 agonist (salbutamol) induced inhibition, 4) serotonin antagonist cyproheptadine 0.5 mg/kg, a dose that inhibited serotonin agonist quipazine induced GH rise, or 5) control. No drug altered mean GH or prolactin levels and pulsatile GH release persisted. Unilateral injections of prazosin, propranolol and idazoxan were made into the medial basal hypothalamus and preoptic-anterior hypothalamic area and of cyproheptadine into the medial basal hypothalamus, all with no effect on short-term GH release. GH and prolactin secretory profiles were measured after giving rats 6 units/kg intravenous insulin. Blood glucose levels fell to less than 50% basal. Hypoglycaemia caused a non-significant 30% fall in mean 2 h GH. Intravenous idazoxan, prazosin, propranolol and cyproheptadine (doses as in first study) did not modify the blood glucose fall, but idazoxan produced a significant reduction of mean GH compared to insulin alone (4 +/- 1.1 ng/ml SEM, idazoxan/insulin versus 16 +/- 5.6 ng/ml, saline/insulin). The lack of an effect of alpha- and beta-blockers on normal, pulsatile GH release is against a role for endogenous catecholamines in controlling this release.(ABSTRACT TRUNCATED AT 250 WORDS)

Platelet α2-adrenoreceptors in depression: a critical examination

During the past decade, results from radioligand studies comparing platelet α(2)-adrenoreceptors in depressed patients and healthy volunteers have been inconsistent, especially when related to the known functional characteristics of these receptors. Despite the availability of radioligands for α(2)-adrenoreceptors, inherent methodological problems exist which make data from these studies difficult to interpret. The authors review the overall data from radioligand studies using [(3)H] clonidine and [(3)H] yohimbine of platelet α(2)-adrenoreceptors in depressed patients and healthy volunteers. Theoretical and methodological issues are critically examined in the light of recent findings. Finally, alternative strategies for studying α( 2)-adrenoreceptors in clinical populations are considered.

Beta-adrenoceptor activity change after prolonged treatment with growth hormone and somatostatin

1. The effect of 10 days treatment with growth hormone (GH) (1 mg/kg body wt/day) and somatostatin (SRIF) (0.25 mg/kg body wt/day) subcutaneously on the activity of beta-adrenoceptors in rat hypothalamic, pituitary and cerebral cortical membrane fractions was studied using [3H]dihydroalprenolol ([3H]DHA) as radioligand. 2. The administration of GH significantly increased the beta-adrenoceptor binding affinity and the administration of SRIF decreased the beta-adrenoceptor binding capacity in the hypothalamus. 3. In the pituitary the beta-adrenoceptor binding affinity was significantly decreased after both hormonal applications. 4. In the cerebral cortex the beta-adrenoceptor binding affinity was significantly decreased after the GH treatment and increased after the SRIF treatment. 5. The present study provides direct evidence for GH and SRIF effects on the activity of rat beta-adrenoceptors and supports the view about the involvement of beta-adrenergic mechanisms in the neurotransmitter regulation of GH secretion in the rat.

The possible role of androgenic hormones in maturation of alpha-adrenergic mechanisms in the rat

Abstract In order to get better characterization of androgenic hormones on the functionality of the alpha(2)-adrenergic system and widen our previous studies, we investigated the effect of clonidine on the growth hormone (GH) secretion in male Wistar rats of various ages: 7-, 14- and 30-day old and adult, adult male castrated rats with and without testosterone treatment. Two different patterns of the GH response to clonidine have been observed in the control and testosterone-treated young animals: clonidine at the dose 15 mug/kg intraperitoneally had no effect on the GH secretion in 7- to 14-day old rat pups. In contrast, its effect appeared following the increase in the plasma testosterone concentration induced by pretreatment with testosterone (5 mg/kg subcutaneously for 4 days) in these animals. In 30-day old rats clonidine affected GH secretion and this influence was more pronounced in the testosterone-treated animals than in the controls. The decrease in the circulating testosterone levels caused by castration in adult male rats caused a decreased GH response to clonidine. Moreover, there was a tendency for the GH response to return in 4-week old animals. The effect of clonidine has been restored by testosterone replacement of castrates. Testosterone administration decreased basal plasma GH levels in the pups. However, it triggered the ultradian surges of GH secretion which were absent in the young animals. Clonidine had no effect on the corticosterone secretion in 7-day old animals. Testosterone treatment induced a response in the 7-day old rat and markedly potentiated its effect on the secretion of this hormone in 14- and 30-day old animals, respectively. Neither progesterone nor hydrocortisone influenced the GH-releasing effect of clonidine. Hydrocortisone markedly inhibited the basal- and clonidine-induced corticosterone secretion. The results of the present study indicate an important role of androgenic hormones in inducing and/or maintaining the effectiveness/sensitivity of the alpha(2)-adrenergic receptor system and suggest a possible role for the androgenic hormones in the maturation of alpha-adrenergic mechanisms in the rat.

The neuroendocrine approach to psychiatric disorders: a critical appraisal

Tremendous efforts have been made to exploit the strategy of measuring the secretion of hormones into the plasma by the pituitary as a "window to the brain" and therefore as an insight into potential neurotransmitter receptor lesions in patients with psychiatric disorders. This contribution focuses upon the advantages and drawbacks of the neuroendocrine approach, caution and objectiveness necessary for the critical evaluation and interpretations of the data. Factors related to the neurobiology of the medial basal hypothalamus and its peculiar features, the information that can be derived from the administration of a specific neuroregulatory hormone or a neuroactive compound and evaluation of the evoked hormone release, the multiple constraints related to the drug itself and/or the physiology or coexisting pathology of the psychiatric patient under examination, are thoroughly discussed.

Growth hormone response induced by synthetic human growth hormone-releasing factor (1-44) in healthy dogs

The growth hormone (GH) secretory response to varying doses (1, 5 and 25 micrograms/kg) of intravenous (i.v.) human GH releasing factor (GRF) (1-44) was measured in four, 16-month-old female dogs and compared to the GH response to placebo (vehicle solubilizing GRF) and standard clonidine test (10 micrograms/kg i.v.). The three doses of GRF had no apparent clinical side effects and were capable of eliciting a rapid increase in the mean plasma GH level. The rise in plasma GH occurred within 5 minutes with a peak response at 15-30 minutes. However inter-subject variability of the GH response was large in terms of magnitude and time course. By analysis of the areas under the response curves it was shown that GRF 1-44 was as effective in stimulating GH release as clonidine and that a dose of 1 microgram/kg GRF 1-44 could be a good choice for use as a standard test dose in the dog.

Growth hormone response to clonidine after recovery in patients with endogenous depression

The growth hormone response to clonidine was measured in ten drug-free recovered patients, seven of whom had previously been tested when endogenously depressed, and compared with the response in ten individually matched controls. In eight of the patients there was an impairment of the growth hormone response, despite clinical recovery, although the hypotensive effect of clonidine in these patients was normal. This is suggestive of a persisting abnormal alpha2-adrenoceptor function in forebrain regions after recovery from an episode of endogenous depression, and may represent a trait marker for this condition.

Effects of parachlorophenylalanine, quipazine and cyproheptadine on growth hormone and adrenocorticotropin secretion in steers

Adrenergic and perhaps dopaminergic neurons provide inhibitory regulation of growth hormone (GH) secretion in ruminants. This suggests that either serotonergic or other neurons regulate the stimulatory release of GH. The nature of neurotransmitter control of adrenocorticotropin (ACTH) secretion in ruminants has not been determined. Parachlorophenylalanine (PCPA; serotonin synthesis inhibitor), quipazine (serotonin receptor agonist) and cyproheptadine (serotonin receptor antagonist) were utilized in Holstein steers to determine whether serotonin receptors mediate stimulatory actions on GH and ACTH secretion. PCPA (100 mg/kg BW) administered each day at 1900 hr for three successive days did not alter mean GH concentrations, amplitude of GH peaks, nor the number of GH peaks. Likewise, PCPA altered none of these parameters for ACTH. Quipazine injected iv at .1 or .5 mg/kg BW increased plasma GH (P less than .05) and ACTH (P less than .001) concentrations. There was a dose effect of quipazine on both GH (P less than .05) and ACTH (P less than .0001) secretion. Pretreatment of steers with cyproheptadine (.06 and .6 mg/kg BW) reduced the stimulation of GH by quipazine (P less than .0001) and decreased basal GH concentrations (P less than .0004). Cyproheptadine at .06 mg/kg BW did not alter quipazine effects on ACTH, however, the higher dose decreased the peak ACTH response (P less than .02) to quipazine. Studies with quipazine and cyproheptadine indicated that serotonergic mechanisms are likely involved in the regulation of GH and ACTH secretion in steers.