Effects of drugs influencing brain catecholamines on GH release in rats with hypothalamic surgery

The effect of pretreatment with alpha-methyl-p-tyrosine (alpha-MT) on the basal levels of plasma growth hormone (GH) and the responses to chlorpromazine (CPZ) were investigated in urethane-anesthetized rats with either complete hypothalamic deafferentation (c.d.), hypothalamic ablation (H.A.) or sham operation (Sham). Basal GH levels were high in C.D. rats, intermediate in H.A. rats, and low in Sham rats without any pretreatment. Pretreatment with alpha-MT caused a significant increase in basal GH levels in both C.D. and Sham rats, but not in H.A. rats. GH release following the intravenous injection of CPZ, which was observed in C.D. and Sham rats without alpha-MT pretreatment, was blunted by treatment with alpha-MT. In H.A. rats CPZ failed to stimulate the secretion of GH regardless of alpha-MT pretreatment. Neither the injection of L-DOPA nor DL-DOPS affected basal GH levels in non-alpha-MT pretreated C.D. rats. However, plasma GH levels significantly decreased following the injection of L-DOPA, but not DL-DOPS, in C.D. rats pretreated with alpha-MT. These findings suggest that the injection of CPZ causes an enhancement of GH release by inhibiting the catecholaminergic (dopaminergic) mechanism, which is active within the basal medial hypothalamus (BMH) and plays an inhibitory role in GH secretion. They also suggest that the extrahypothalamic inhibitory neural pathway, which is connected to the BMH and is interrupted by hypothalamic deafferentation, is not catecholaminergic.