Involvement of the catecholaminergic system in the inhibitory effect of brain histamine on growth hormone secretion

The effects of intracerebroventricular (icv) injection of histamine (HA, 0.4 μmol/rat) or of the inhibitor of HA synthesis α-fluoromethylhistidine (αFMH, 100 mg/kg, intracarotid (ia)) on growth hormone (GH) secretion induced by GH-releasing hormone (hGHRH1-40, 2 μg/kg, ia) in freely moving rats were examined. HA significantly inhibited GH release induced by GHRH whereas pretreatment (3 h before) with αFMH enhanced the GH response to GHRH. The inhibitory effect of HA on GHRH-induced GH release is still evident in somatostatin-depleted rats with cysteamine (300 mg/kg, sc, 4 h before) but is completely overcome by the inhibitor of catecholamine synthesis (α-methyl-p-tyrosine, 200 mg/kg, ip, 2 h before) suggesting that HA influences GH secretion by modulation of catecholaminergic neurons. This suggestion is supported by the biochemical data showing an increase in noradrenaline and dopamine contents in the hypothalamus and striatum of rats pretreated with αFMH.