Effect of melatonin-free pineal extract on prolactin in castrated and thyrotropin releasing hormone-stimulated rats.

Administration of a melatonin-free pineal extract to the rat induced the following effects on prolactin: a) under basal conditions, after 4 hrs from a single injection or after 3 days of treatment, there was a decrease of 34% and 28% respectively, in serum prolactin, with no change in the pituitary content of this hormone; b) when prolactin was stimulated by castration the decrease of the serum level of this hormone (-37%) was accompanied by a decrease in its pituitary content (-33%) and concentration (-35%); c) under conditions of exogenous TRH stimulation the decrease in serum prolactin (-26%) was accompanied by a decrease in pituitary prolactin content (-27%) and concentration (-21%), the latter two without statistical significance. The role of the pineal gland on prolactin is discussed.