The effects of castration andtestosterone treatment on pineal and retinal hydroxyindole-O-methyl transferases (HIOMT) were investigated in male rats. Pineal and retinal HIOMT activities decreased 24–72 h after cas-tration. Testosterone propionate (TP) (0.1–1 mg/day) stimulated pineal HIOMT which higher doses inhibited significantly; TP (0.1–5 mg/day) increased retinal HIOMT activity. The simultaneous administration of norepinephrine hydrochloride (NE) (250–400 µ g) partially reversed the stimulation of pineal HIOMT caused by 0.5 mg/day of TP, and potentiated the inhibition induced by 5 mg/day of TP. NE reversed the TP-induced stimulation of retinal HIOMT. Testosterone was readily taken up and retained by the pineal gland; the radioactivity recovered from the pineal 60 min after a single injection of 3H-testosterone did not differ significantly from that present in the prostate, a classical androgen target organ. In contrast, the retina exhibited the lowest uptake of radioactivity among several organs. The testosterone-induced changes in the pineal HIOMT activity plus the significant amounts of radioactivity found in the pineal after 3H-testosterone administration, suggest that the pineal may be a target organ for androgens.

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