GHRP‐6‐Induced Changes in Electrical Activity of Single Cells in the Arcuate, Ventromedial and periventricular nucleusriventricular Nuclei of a Hypothalamic Slice Preparation in Vitro

Abstract

Previously, we demonstrated that systemic injection of the growth hormone secretagogue, growth hormone-releasing peptide (GHRP)-6, selectively activated cells in the hypothalamic arcuate nucleus, as reflected by increased electrical activity and induction of the immediate early gene c-fos. The growth hormone secretagogue receptor distribution is not confined to the arcuate nucleus, suggesting that additional sites of action may exist. In the present study we characterized the electrophysiological responses of cells in the arcuate nucleus, ventromedial nucleus and periventricular nucleus in an in-vitro hypothalamic slice preparation, following bath application of GHRP-6. Additionally, since central somatostatin administration has been shown to attenuate the induction of the c-fos gene by GHRP-6, we sought to determine whether the arcuate cells activated by GHRP-6 are also somatostatin-sensitive. Male Wistar rats (100-150 g body weight (BW)) were anaesthetized (urethane; 1.2 g/kg BW) and the brains removed. Coronal sections (400 microm thickness) were cut through a block of hypothalamus and were transferred to a slice chamber perfused with artificial cerebrospinal fluid. Forty-one arcuate nucleus cells were tested with bath application of 15 microm GHRP-6 for 10 min, 16 of which were tested subsequently (>30 min later) with application of 10 microM somatostatin. Following GHRP-6 administration, 19 cells (46. 3%) showed a significant increase in firing rate during the 15-min period after GHRP-6 application (P<0.001), 17 cells (41.5%) did not respond and the remaining five cells (12.2%) were significantly inhibited. Six of the eight arcuate nucleus cells that were excited by GHRP-6 were significantly inhibited by somatostatin. By contrast, five of the six arcuate nucleus cells that were unresponsive to GHRP-6 were also unresponsive to somatostatin. In the ventromedial nucleus, of 19 cells tested, eight cells (42.1%) were excited by GHRP-6, eight cells (42.1%) were unresponsive and the remaining three cells (15.8%) were significantly inhibited. Of 19 cells recorded in the periventricular nucleus, 13 (68.4%) were unresponsive to GHRP-6 and six (31.6%) were significantly inhibited. Thus, electrophysiological studies in vitro suggest that: (1) neurones in the hypothalamic arcuate nucleus, ventromedial nucleus and periventricular nucleus show changes in electrical activity in response to GHRP-6; and (2) the arcuate nucleus cells excited by GHRP-6 are also subject to inhibitory control by somatostatin.

Agouti-Related Peptide, Neuropeptide Y, and Somatostatin-Producing Neurons Are Targets for Ghrelin Actions in the Rat Hypothalamus

“…On the other hand, the lack of expression of the GHS-R in the lateral hypothalamus (LHA) (17), where a large number of orexigenic neurons are located (22,23), suggested that this hypothalamic nucleus was unlikely to be a target of the effects of ghrelin on food intake. However, some of the biological effects of ghrelin might be mediated by as yet unknown receptors, different from the only one cloned until now (24,25). In keeping with this possibility, it was found that ghrelin administration leads to increased c-Fos expression in neurons of the LHA (26).…”

Peptide YY Directly Inhibits Ghrelin-Activated Neurons of the Arcuate Nucleus and Reverses Fasting-Induced c-Fos Expression

“…Ghrelin stimulates food intake after both central and peripheral administration [25, 26]. As demonstrated by recent studies, ghrelin [27, 28]and artificial growth hormone secretagogues [29, 30], which also bind to the GHS-R, directly activate leptin-responsive cells of the ArcM, where NPY-expressing neurons are regionally clustered [20]. As an indirect effect, ghrelin presynaptically inhibits neurons in the lateral Arc (ArcL) [28], where POMC-expressing neurons predominate [20].…”

Role of endogenous ghrelin in growth hormone secretion, appetite regulation and metabolism

“…Many ARC GHRH neurons have GHS-R [31] and GHS stimulation increased both ARC c-fos expression [32,33] and ARC electrical activity in vivo [34] and in vitro [35]. Pretreatment of rats with GHRH antiserum [36] or administration of a GHRH-R antagonist to humans [37] blocked most of the GH response to GHRP-6.…”