The Effect of Growth Hormone Secretagogues and Neuropeptide Y on Hypothalamic Hormone Release from Acute Rat Hypothalamic Explants

Abstract

Growth hormone (GH) secretagogues (GH-releasing peptides and their non-peptide analogues) stimulate growth hormone release via specific G-protein coupled receptors both directly from the pituitary gland and through stimulation of the hypothalamus. The exact mechanism of action in the hypothalamus is not known. The presence of endogenous GH releasing hormone (GHRH) seems to be necessary for the in-vivo actions of growth hormone secretagogues (GHSs), but data suggest that further factors must be involved as well. The effect of GHSs is not entirely specific for the GH axis; they release prolactin and stimulate the hypothalamo-pituitary-adrenal axis causing elevations in circulating ACTH and cortisol levels in both animal and human studies. Recently, it has also been suggested that GHSs stimulate hypothalamic neuropeptide Y (NPY) neurones. In the present study, we have therefore investigated the direct effect of several GHSs (GHRP-6, hexarelin and the non-peptide analogues L-692, 429 and L-692, 585) on GHRH, somatostatin (SS), corticotrophin-releasing hormone (CRH) and arginine vasopressin (AVP) release in vitro in an acute rat hypothalamic incubation system. We also assessed the effect of NPY on GHRH, SS and AVP release. Freshly removed hypothalami were incubated in control media for 20 min and then in 1-4 consecutive 20-min periods in each of the test substances at different concentrations. There was no significant change in either the basal or potassium-stimulated release of GHRH or SS at low concentrations of any of the secretagogues; however, at millimolar doses a paradoxical inhibition of GHRH was observed with GHRP-6, hexarelin and L-692 585 (data are expressed as the ratio of treated to preceding basal release; at 20 min control group: 0.97+/-0.02, GHRP-6: 0.55+/-0.04, P<0.001 compared to control group; hexarelin: 0. 56+/-0.06, P<0.001, L-692,585: 0.70+/-0.03, P<0.001), while SS was stimulated after 60 or 80 min (at 80 min control: 0.80+/-0.03, hexarelin: 1.23+/-0.07, P<0.05 and L-692,585: 1.37+/-0.11, P<0.05). GHSs stimulated hypothalamic AVP release (at 20 min control: 0. 99+/-0.06 ratio to basal release, 10-4 M concentration of GHRP-6: 6. 31+/-1, P<0.001, hexarelin: 1.88+/-0.4, P<0.01, L-692,429: 1.90+/-0. 5, P<0.05 and L-692,585: 2.34+/-0.96, P<0.01), while no stimulatory effect was found on CRH release. NPY significantly stimulated SS and inhibited basal and potassium-stimulated GHRH release, while potentiating potassium-evoked AVP secretion. The Y1 receptor antagonist BIBP 3226 did not inhibit the effects of NPY on SS, GHRH or AVP release. We therefore conclude that, in this in-vitro rat hypothalamic incubation model, growth hormone secretagogues stimulate the release of AVP but have no effect on either GHRH, SS or CRH at low doses; at high doses paradoxically they inhibit the hypothalamic GH axis similar to in-vivo data in the rat. We speculate that these effects might be mediated by NPY.

Ghrelin in obesity and endocrine diseases

“…There are data to A c c e p t e d M a n u s c r i p t 19 suggest that ghrelin stimulates the HPA axis at a hypothalamic level, at least in part via arginine vasopressin (AVP) and CRH (Coiro et al, 2005;Korbonits et al, 1999a;Mozid et al, 2003). It has been shown that NPY can stimulate CRH and AVP release from the rat hypothalamus (Korbonits et al, 1999b), therefore the effect of ghrelin on CRH and AVP could occur through direct action on CRH and AVP neurons in the paraventricular nucleus, or may occur indirectly via NPY (Mozid et al, 2003). Ghrelin does not have direct stimulatory effects on adrenal hormone secretion (Andreis et al, 2003;Barreiro et al, 2002).…”

Abnormalities of the somatotrophic axis in the obese agouti mouse

“…In addition, NPY may suppress the somatotrophic axis via stimulation of hypothalamic somatostatin release. 44,45 Hence, the observed reduction in PeVN somatostatin mRNA expression in obese A y /a mice may be secondary to reduced Arc NPY mRNA expression. Since NPY inhibits the growth axis, 20,21 a reduction in Arc NPY would increase circulating GH and hence IGFI levels.…”

Fasting-induced changes in the hypothalamic-pituitary-GH axis in the absence of GH expression: lessons from the spontaneous dwarf rat

“…In contrast to the fasted human, the fasted male rat shows dramatically reduced pulsatile GH release (Tannenbaum et al 1979, Bruno et al 1990, Janowski et al 1993. The fasting-induced fall in GH is thought to be due to a decrease in hypothalamic GHRH expression (Bruno et al 1990, Ghigo et al 1997, Vuagnat et al 1998, Carro et al 1999) brought about by the inhibitory actions of neuropeptide Y (NPY) (Minami et al 1998, Korbonits et al 1999. However, consistent with the fasted human, food deprivation in the rat leads to peripheral GH resistance and reduced IGF-I (Bornfeldt et al 1989), and increased pituitary sensitivity to GHRH (Sugihara et al 1996).…”