Castration reduces the nocturnal rise of pineal melatonin levels in the male rat by impairing its noradrenergic input.

Abstract The effects of castration and testosterone treatment on pineal day-night rhythms were studied in male rats. Bilateral gonadectomy was performed at 21 days of age. Testosterone propionate was given subcutaneously to castrated animals in a dose of 10 mug/100 g body weight during two consecutive days before sacrifice. Animals were killed 40 days after gonadectomy at four different times of a 12:12 h light-dark cycle (1600, 2400, 0400 and 0800h). Tyrosine hydroxylase activity was measured in individual pineals by means of high-performance liquid chromatography determination of L-DOPA formed. Pineal levels of norepinephrine, dopamine, 5-hydroxytryptamine and 5-hydroxyindole acetic acid were determined by high-performance liquid chromatography with amperometric detection, while pineal melatonin content was measured by radioimmunoassay. Castration abolished the day-night rhythms of pineal tyrosine hydroxylase activity and norepinephrine content, both by elevating their daytime levels and by blocking their nocturnal rise. In addition, gonadectomy drastically modified pineal indoleamine metabolism by increasing daytime levels of both 5-hydroxytryptamine and 5-hydroxyindole acetic acid, and by reducing the nocturnal elevation of pineal melatonin content. Testosterone treatment was unable to prevent the effect of orchidectomy on pineal rhythms of tyrosine hydroxylase activity, 5-hydroxytryptamine or 5-hydroxyindole acetic acid content, however it partially restored the day-night pineal rhythms of both norepinephrine and melatonin content. These results are indicative of a possible participation of reproductive hormones in the control of pineal rhythmic activity in the male rat. Apparently, since gonadectomy abolished the nocturnal rise of both pineal tyrosine hydroxylase activity and norepinephrine content, the primary site of action of reproductive hormones could be at the level of the superior cervical ganglion.