Caloric intake stimulates growth hormone secretion in food-deprived rats with anterolateral deafferentation of the medial basal hypothalamus or administered antiserum to somatostatin

Masculinizing effect of dihydrotestosterone on growth hormone secretion is inhibited in ovariectomized rats with anterolateral deafferentation of the medial basal hypothalamus or in intact female rats

There is a striking sex difference in the pattern of growth hormone (GH) secretion in rats. Our previous studies showed that short-term administration of pharmacological doses of testosterone or dihydrotestosterone (DHT) masculinized the GH secretory pattern in ovariectomized (OVX) rats. The locus where testosterone or DHT interacts with the somatotropic axis is believed to be the hypothalamus. To obtain insights into this phenomenon, we administered a single dose of DHT s.c. to adult OVX rats at 0.01, 0. 1 or 1 mg/rat. Blood GH concentrations were measured in unanaesthetized rats. Six to 12 h after the s.c. administration of all three doses of DHT, the GH secretory pattern revealed a male-like secretory pattern as shown by episodic bursts occurring at 2-3-h intervals with low or undetectable trough levels. When anterolateral deafferentation of the medial basal hypothalamus (ALC) was performed, the blood concentrations revealed irregularly occurring small fluctuations, instead of the usual high bursts, but the basal GH concentration was significantly higher than that of OVX-sham-operated rats. DHT treatment did not elicit pulsatile GH secretion or alter GH concentrations in OVX rats with ALC. When intact adult female rats received DHT at a dose of 1 mg/rat, the male-like GH secretory pattern was not induced. These results suggest that neural inputs from the anterolateral direction to the medial basal hypothalamus are necessary for the masculinizing effect of DHT on the GH secretory pattern in OVX rats, and that oestrogen in intact female rats prevents the masculinizing effect of DHT.

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.

Inhibitory effect of neuropeptide Y on growth hormone secretion in rats is mediated by both Y1- and Y2-receptor subtypes and abolished after anterolateral deafferentation of the medial basal hypothalamus

Neuropeptide Y (NPY) may play a physiological role in the regulation of growth hormone (GH) secretion by acting via somatostatin (SS) in the periventricular nucleus (PeV), as well as via the GH-releasing factor in the arcuate nucleus (ARC) of the medial basal hypothalamus (MBH). The objectives of the present study were to determine the neuron structures and receptor subtypes necessary for mediating the inhibitory effect of NPY on GH secretion in unanesthetized male rats. To eliminate the influence of hypophyseotropic SS, anterolateral deafferentation (ALC) of the hypothalamus was performed. Intracerebroventricular (i.c.v.) administration of 1.17 nmol of NPY decreased the blood level of GH for 3-4 h in sham-operated rats, while the procedure was without effect in ALC rats. The i.c.v. administration of 1.17 nmol of a Y1-receptor agonist ([Leu31, Pro34]-NPY) or a Y2-receptor agonist (NPY 13-36 and NPY 3-36) similarly suppressed the blood GH level. The data support the hypothesis that neuron structures anterolateral to the MBH are required for NPY-induced inhibition of GH secretion that is mediated via Y1- and Y2-receptor subtypes. Combined with data of other investigators, SS is likely the neurohumoral mediator of the effect of NPY on GH secretion.

Effects of hyper- and hypoglycemia on blood growth hormone level in free-feeding rats with anterolateral deafferentation of the medial basal hypothalamus

In rats with anterolateral deafferentation of the medial basal hypothalamus, the growth hormone (GH) level in the blood showed irregular and small fluctuations instead of the usual high bursts and low trough level, and the baseline GH level was higher than that in sham-operated rats. Continuous infusion of a glucose solution to operated rats increased the baseline level, GH pulse and pulse amplitude. I.v. bolus injection of the glucose solution resulted in a significant but transient increase in GH level. Insulin-induced hypoglycemia decreased the blood GH level in operated rats more effectively than in sham-operated ones and that was prevented by simultaneous infusion of glucose. Since SS influence on GH secretion had been largely eliminated in rats with anterolateral deafferentation of the medial basal hypothalamus, it is highly unlikely that the effects of hyperglycemia or hypoglycemia on GH secretion were the consequence of altered SS secretion.