EFFECT OF MODIFIED BRAIN HISTAMINE CONTENTS ON PROLACTIN AND THYROTROPIN SECRETION IN MALE-RATS

Effects of modified brain histamine contents on thyrotropin and prolac tin secretion were studied in male rats. Under basal conditions the hi stamine content in the hypothalamus was approximately 8-10-fold higher than that in the striatum and the rest of the brain. L-Histidine (100 0 mg/kg, ip), a histamine precursor, and metoprine (20 mg/kg, ip), an inhibitor of histamine methyltransferase, elevated histamine content i n the brain by 65% and 167%, respectively. When the treatments were gi ven together an additive effect (119-250% increase) on brain histamine was observed. Metoprine significantly decreased serum prolactin level s, while L-histidine had no effect. This effect of metoprine was not m odified by treatment with L-histidine. Thus, metoprine has an inhibito ry effect on prolactin secretion that is not related to elevated brain histamine contents. The increased brain histamine content after L-his tidine treatment had no effect on prolactin secretion. Basal levels of serum thyrotropin were decreased by both L-histidine and metoprine, L -histidine being more potent, In rats treated with alpha-fluoromethlhi stidine, an inhibitor of L-histidine decarboxylase, the cold-induced ( rats kept for 60 min at 14 degrees C) thyrotropin secretion was increa sed while the stress-induced prolactin secretion was decreased. In the se rats, metoprine did not affect thyrotropin release but blunted the prolactin response. In conclusion, endogenous histamine inhibits thyro tropin secretion but does not affect prolactin release. Owing to its o ther effects, metoprine is not suitable as a tool to elevate endogenou s histamine contents in the brain, at least when the regulation of ant erior pituitary hormone release is being studied.