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

Abstract

Plasma levels of growth hormone in male rats exhibit a n ultradian rhythm having a periodicity of 3 to 4 h , bursts of growth hormone being separated by troughs when growth hormone secretion ceases. To determine if neurons in the vicinity of somatostatin neurons in the preopticlanterior hypothalamic area participate in the generation of t h e rhythm, we temporarily inhibited this region with injections of the y-arninobutyric acid agonist, muscimol, in unanaesthetized rats previously prepared with a venous catheter and stereotaxically implanted intracerebral guide tubes. The injection resulted in an immediate burst of growth hormone release, followed by a trough period and another growth hormone burst, 3.4k0.1 h later. The induction of a trough and synchronization of growth hormone bursts was not reproduced by a burst of intravenously injected exogenous growth hormone a t least as large as the first burst initiated by muscimol. These findings indicate that, in the short term, the ultradian rhythm is independent of growth hormone feedback and provide the first evidence that structures in the anterior hypothalamus receiving a y-aminobutyric acid input a r e an important component of the neural generator of the ultradian rhythm of growth hormone.

Influence of Fetal Alcohol Exposure on the GABAergic Regulation of Growth Hormone Release in Postnatal Rats

“…This also corresponds to a period of elevated endogenous G H surges. The decline in GABA sensitivity that occurs after puberty may be correlated with the random pattern of G H pulses in females (Gabriel et al, 1992), whereas the persistence of sensitivity in males may be associated with their precise pattern of G H pulses (Willoughby and Kapoor, 1990).…”