STIMULATION OF H3-HISTAMINE RECEPTORS INCREASES THE RELEASE OF PROLACTIN IN MALE-RATS

Histamine (HA) stimulates prolactin secretion via H-1 and H-2 receptor s. In the present study, we examined the role of a third subtype of re ceptor recently described in brain, the H-3-HA receptor, on prolactin secretion in male rats. R(-)alpha-methyl-HA (alpha-MHA), a selective H -3 receptor agonist, was injected into the lateral ventricle of the br ain in freely moving rats. Alpha-MHA produced a dose-dependent (1-5 mu g) and long-lasting increase in plasma prolactin levels. This increase was observed from 15 to 60 min after injection of alpha-MHA. Its stim ulatory action was prevented by thioperamide (20 mug i.v.t.), a select ive H-3 antagonist. This compound, injected intraventricularly, lacked effect by itself on basal plasma prolactin levels. Neither pyrilamine (H-1 antagonist; 60 mug i.v.t.) nor ranitidine (H-2 antagonist; 60 mu g i.v.t.) affected alpha-MHA-induced prolactin release. The stimulator y effect was still present when brain HA was depleted by alpha-fluorom ethylhistidine (30 mg/kg i.p.). Our findings suggest that alpha-MHA ev okes prolactin release by activation of postsynaptic H-3 receptors.