The nyctohemeral rhythm of plasma prolactin: effects of ganglionectomy, pinealectomy, constant light, constant darkness or 6-OH-dopamine administration.

In male rats maintained on a 12 h light-dark schedule (6 AM-6 PM), there is a nyctohemeral cycle of plasma prolactin which consists of a nadir at 11:30 AM and an apogee at approximately 11:30 PM. In rats exposed to constant darkness, this rhythm persists for 7 days. Seven days of constant light, however, reverses this diurnal variation such that plasma prolactin levels peak at 11:30 AM and reach a nadir at approximately 11:30 PM. In animals maintained on a 12 h light-dark cycle, ganglionectomy and lateral ventricular injections of 6-OH-dopamine (250 mug) also appear to reverse the diurnal variation of plasma prolactin, whereas a single injection of 6-OH-dopamine (250 mug) into the third ventricle decreases plasma prolactin values at all times intervals but does not alter the diurnal rhythm. Both sites of 6-OH-dopamine administration markedly deplete hypothalamic dopamine and norepinephrine, but injection of 6-OH-dopamine into the lateral ventricle destroys the catecholaminergic terminals in the pineal, whereas injection of 6-OH-dopamine into the third ventricle does not. Pinealectomy slightly increases the early morning values of plasma prolactin, but otherwise has no effect on the diurnal variation of prolactin. Five conclusions appear to be justified: 1) there is a nyctohemeral rhythm of plasma prolactin, which is reversed by constant light; 2) the pineal gland probably plays no role in the diurnal regulation of plasma prolactin secretion; 3) the diurnal rhythm of plasma prolactin is controlled by sympathetic input into the brain via the superior cervical ganglion; 4) a rhythm of plasma prolactin develops in constant light which is the exact opposite of the normal diurnal variation; 5) there appears to be a noradrenergic pathway in the hypothalamus or brainstem which stimulates release of prolactin.