Intrahypothalamic Actions of Somatostatin and Growth Hormone Releasing Factor on Growth Hormone Secretion

Abstract

Possible effects of GRF on somatostatin neurons and of somatostatin on GRF neurons were examined by measuring the effects of localised intracerebral injections of these peptides on growth hormone (GH) secretion. Serial GH concentrations were measured in plasma in unanaesthetized male rats chronically prepared with venous and intracerebral cannulae, before and after treatment with bilateral intracerebral injections of somatostatin or GRF in the preoptic anterior hypothalamic area (PO/AHA) and medial basal hypothalamus. Injections of 0.1 and lnmol of GRF in medial basal hypothalamus or 10 nmol somatostatin in the PO/AHA, respectively, had stimulatory or inhibitory effects on GH, which were assumed to be due to diffusion of the peptide from the injection site to the median eminence and pituitary gland. Injection of lower doses of somatostatin around GRF neurons in the medial basal hypothalamus were without significant effect on secretion of GH, but 0.1 nmol somatostatin in the PO/AHA resulted in an increase in GH concentrations from 128 ± 61 to 524 ± 103 ng/ml, p < 0.02. Injections of GRF in lower doses amongst somatostatin neurons in the PO/AH or amongst GRF neurons in the medial basal hypothalamus were both without effect on GH secretion. We conclude that somatostatin may stimulate GH secretion by an effect on or close to somatostatin neurons in the PO/AHA. Somatostatin, though present in terminals on GRF neurons, is without effect at this site in our model. Furthermore, we have been unable to demonstrate any sigificant intrahypothalamic effect of GRF on GH regulation.

Electrical Stimulation of Hypothalamic Periventricular Nucleus Is Followed by a Large Rebound Secretion of Growth Hormone in Unanesthetized Rats

“…Immunocytochemical studies show that GRF containing perikarya in hypothalamic arcuate nucleus are surrounded by SS-containing nerve fibers [6], It is expected that the inhibitory influence on GRF neurons is conveyed via SS-containing fibers originating from PVN. However, recent studies by Willoughby et al [25,26] suggest that SS and GRF neurons do not inter communicate directly at the level of their cell bodies. They as sume that SS exerts an inhibitory influence on GRF release act ing at terminals of G RF within the median eminence.…”

¹²⁵l-Somatostatin-Labeled Cells in the Anterior Arcuate Nucleus Mediate Somatostatin Effects on Growth Hormone but Not Prolactin Secretion

“…(2) Auto crine, short negative feedback of SRIH on periventricular SRIH-producing neurons. Intrahypothalamic SRIH stim ulation of GH serum levels is located near the periventric ular but not in the arcuate nucleus [25], and inhibition by SRIH analogues of endogenous SRIH release in vitro has been reported [26,27]; it should lead to a decreased fiux of the peptide from the median eminence to the pituitary and could thus be involved in the paradoxical GH stimu lation observed after intraventricular infusion of SRIH. (3) Intervention of a small population of SRIH interneu rons located in the arcuate itself.…”

Activation of Hypothalamic Gamma‐Aminobutyric Acid Receptors Resets the Pendulum of the Growth Hormone Clock

“…Synchronization was achieved by a neural mechanism independent of immediate feedback effects of These results provide the first direct evidence localizing the generator of the 3-hourly G H rhythm t o the anterior hypothalamus. O u r hypothesis is that the clock imposes a 3-hourly rhythm o n GH release by effects of somatostatin a t the somatotroph and, as well, a t GRF nerve terminals in the median eminence (22). …”