Effect of growth hormone-releasing stimuli in streptozotocin diabetic rats

Enhanced Pulsatile Growth Hormone Secretion and Altered Metabolic Hormones by in Vivo Hexarelin Treatment in Streptozotocin-Induced Diabetic Rats

Significant growth hormone (GH) reductions have been reported in diabetic animal models with disturbed metabolic balance coinciding with GH deficiency. Therefore, enhanced GH secretion may have beneficial effects in controlling diabetes. Thus, we aim to investigate the effect of hexarelin, a synthetic GH secretagogue (GHS), on GH secretion in streptozotocin (STZ, 65 mg/kg)-induced diabetic rats. Daily hexarelin (100 μg/kg) treatment was performed for two weeks in four-week-long STZ-diabetic and vehicle control rats. Pulsatile GH secretion in STZ-rats was significantly reduced in total, pulsatile, basal, and mass of GH secretion per burst. In addition, impaired GH secretion was followed by an increase in fasting-level free fatty acids (FFAs) and a decrease in insulin-like growth factor 1 (IGF-1) compared to control rats. After hexarelin treatment, pulsatile GH secretion in STZ-rats was significantly increased in total, pulsatile, and basal, but not in the mass GH secretion per burst, compared to STZ-rats without hexarelin treatment. However, there was no significant elevation in GH secretion in the hexarelin-treated control group. In addition, hexarelin-treated STZ-rats showed a significant decrease in fasting level FFAs, whereas suppression of fasting level for IGF-1 was maintained. These results suggest that STZ-induced diabetic rats have impaired pulsatile GH secretion, causing increased FFAs and decreased IGF-1 levels in circulation. Hexarelin injections for two weeks is able to normalize impaired pulsatile GH secretion with normal fasting levels of FFAs, but fails to recover IGF-1 levels.

GH responses to GHRH and GHRP-6 in Streptozotocin (STZ)-diabetic rats

GH responses to GHRH, the physiologic hypothalamic stimulus, and GHRP-6, a synthetic hexapeptide that binds the Ghrelin receptor, were studied in rats treated with streptozotocin (STZ), an experimental model of diabetes. Sprague-Dawley male rats received a single injection either of STZ (70 mg/Kg in 0.01 M SSC, i.p.) or of the vehicle (0.01 M SSC). GH responses were challenged with two different doses of GHRH (1 and 10 microg/kg) or GHRP-6 (3 and 30 microg/kg) and with a combination of both at low (1 + 3 microg/kg) or high (10 + 30 microg/kg) doses, respectively. We observed a dose-dependent effect for GH responses to GHRH both in STZ-treated rats and in controls. However, we could not find significant differences between STZ-rats and controls. GH responses to GHRP-6 occurred in a dose-dependent manner in STZ-rats, but not in controls. GH responses to GHRP-6 in both groups were clearly lower than those elicited by GHRH. GH responses to 30 microg/Kg of GHRP-6 were significantly greater in STZ-rats than in controls (AUC: 3549.9 +/- 1001.4 vs. 2046.4 +/- 711.7; p<0.05). The combined administration of GHRH plus GHRP-6 was the most potent stimuli for GH in both groups. The administration of doses in the lower range (1 + 3 microg/Kg, GHRH + GHRP-6 respectively) induced a great peak of GH in STZ-rats and in control rats, revealing a synergistic effect of GHRH and GHRP-6 in both groups. When the higher doses were administered (10 + 30 microg/kg), GH levels in time 5, and AUC were significantly higher in control rats. In addition, a negative correlation between WT (weight tendency) values and GH responses, represented as AUC, could be established in STZ-rats (r2=-0.566, p=0.004 for GHRH; r2=-0.412, p=0.028 for GHRP-6). Thus, the more negative the values of WT were, the more severe the metabolic alteration and, therefore, the higher the GH response to GHRH and GHRHP-6. In conclusion, our results do not support the existence of a functional hypothalamic hypertone of SS in diabetic rats, as GH responses were not usually reduced in STZ-rats, except when both secretagogues were administered together at the higher doses. Besides, GH responses to GHRH and GHRP-6 were inversely correlated with the severity of the metabolic alteration in STZ-rats, meaning that worse glycaemic control promoted higher GH secretion. These results resemble those found in humans, where GH responses to secretagogues are increased in type-1 diabetes and depend on hyperglycaemia, and are representative of not well-controlled insulin-dependent diabetic status.

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.

Altered thermoregulatory responses to clonidine in streptozotocin-diabetic rats

1. The effects of streptozotocin (STZ) treatment on alpha 2-adrenoceptor regulation of body temperature were studied by monitoring the response of colonic temperature to administration of clonidine. 2. A dose-dependent fall in colonic temperature occurred in control rats given clonidine challenge (0.05-2.0 mg kg-1, s.c.); this response was inhibited by prior administration of either yohimbine or idazoxan (2 mg kg-1, s.c.) but not by the peripherally-acting alpha 2-adrenoceptor antagonist L-659,066 (10 mg kg-1, s.c.). 3. In rats treated with STZ (65 mg kg-1, i.v.) administration of clonidine elicited a dose-independent hyperthermia (circa 1 degree C.); this effect was unaltered by prior administration of yohimbine or idazoxan. 4. Naloxone (5 mg kg-1, s.c.) elicited a small fall in temperature (< 1 degree C.) in both control and STZ-treated rats; naloxone pretreatment did not alter the temperature response to clonidine in either group. 5. Nicotinic acid (10 mg kg-1, s.c.) caused a similar small elevation in temperature in both groups. 6. Administration of replacement insulin to STZ-treated rats maintained weight gain and low blood glucose while the thermoregulatory response to clonidine slowly reverted to normal. 7. These results show that altered central temperature control is an element of the generalised abnormality of alpha 2-receptor function induced by STZ.

Effects of streptozotocin-induced diabetes on lymphocyte POMC and growth hormone gene expression in the rat

Diabetes in the rat is associated with a change in the profiles of several neuroendocrine hormones resulting in poor growth and decreased immune function. Since lymphocytes can also serve as a source of neuroendocrine hormones, we have examined whether the change in hormone profiles are accompanied by an impairment of lymphocyte GH and POMC gene expression in the immune system. Diabetes was induced by the administration of streptozotocin (STZ; 10 mg/100 g body weight) and 3 days later GH and ACTH protein and mRNA were determined. The results show a modest diminution of GH RNA in the spleen of diabetic animals whereas the expression of POMC mRNA and ACTH by the thymus was enhanced. The expression of POMC in the spleen appeared unaltered while the increase of POMC RNA in the thymus was evident after the first day of STZ treatment. STZ had no direct effect on GH or POMC expression in the spleen or thymus cells in vitro. Insulin does not appear to be involved in the expression of lymphocyte GH or POMC. The administration of insulin to the diabetic animals had no significant effect on the expression of GH or POMC by the immune cells. In addition, lymphocytes do not appear to serve as a source of insulin or are the expression of genes for lymphocyte GH or ACTH altered by insulin in vitro. Taken together, the findings are the first to report on the expression of neuroendocrine genes in lymphocytes during diabetes. The mechanism for the inhibition of GH and stimulation of POMC expression by lymphocytes in diabetic animals is unknown, but it is tempting to speculate an important role in the development of the autoimmunity that characterizes this complex disease.

The changes in the hypothalamo-pituitary-gonadal axis of streptozotocin-treated male rats depend from age at diabetes onset

The influence of age at diabetes onset and of capillary microangiopathy on the severity and evolution of hypothalamo-pituitary-gonadal changes was studied morphologically and morphometrically in male rats 4 and 8 months after streptozotocin injection. At each time period we studied 2 groups of rats, one made diabetic before (age 1 month), the other after puberty (age 3 months), and compared them with corresponding controls. The size of hypothalamic axons, numerical density and size of pituitary gonadotrophs, size of testicular tubules, and basement membrane thickness of retinal capillaries were measured. Major differences were found at 8 months. Changes of pituitary glands (i.e. small and numerous gonadotrophs) and testes (i.e. small tubular size) were more important in pre- than in postpubertal diabetic rats. This was a consequence of the aggravating prepubertal diabetes between 4 and 8 months. On the contrary, these changes partially regressed in postpubertal diabetic animals. Pituitary and testicular changes were correlated. Other lesions, such as swollen axonal processes in the hypothalamus, increased thickness of seminiferous epithelium and of capillary basement membranes, though very evident in diabetics, were independent from age at induction. Neither microangiopathy nor glycemia were correlated with any other change which confirmed their secondary role in diabetic neuroendocrine disorders. Thus, two types of diabetic disorders of the hypothalamo-pituitary-gonadal axis could be distinguished: 1) those with irreversible effects on immature yet partially reversible effects on mature structures; and 2) those independent from age at induction.

Diabetes-induced changes in monoamine concentrations of rat hypothalamic nuclei

Streptozotocin-induced diabetes produced marked alterations of monoamine concentrations in several hypothalamic nuclei of male and female rats. Norepinephrine (NE) concentrations were significantly elevated in the median eminence (ME), supraoptic nucleus (SON) and periventricular nucleus (PEVN) in both sexes of diabetic rats. NE concentrations in the suprachiasmatic nucleus (SCN) and ventromedial nucleus (VMN) of male and female diabetic animals remained unaltered. Serotonin (5-HT) concentrations were increased in PEVN of male and female diabetic rats. No significant changes in hypothalamic dopamine (DA) concentrations were observed. Insulin treatment reversed the diabetes-related changes in monoamine concentrations in most of the nuclei. The significance of these biochemical changes relative to the endocrine and behavioral abnormalities in diabetes is discussed.