SEASONAL EFFECTS OF ANTIHISTAMINE ON MEAN SERUM CONCENTRATIONS OF LUTEINIZING-HORMONE, GROWTH-HORMONE AND PROLACTIN IN OVARIECTOMIZED EWES

Ovariectomized ewes (n = 5) were treated with a histamine H-1-receptor antagonist, diphenhydramine, once each month to determine if: (1) his tamine affects serum concentrations of luteinizing hormone (LH), growt h hormone (GH) and prolactin (PRL) during the breeding and anestrous s easons and (2) if histamine regulates the release of pituitary hormone s through common or discrete mechanisms. Effects of diphenhydramine (2 00 mg) on serum concentrations of LH, GH and PRL were analyzed during the breeding season, anestrous season and periods of transition into, or out of, the breeding season. Treatment suppressed (P < 0.05) serum concentrations of LH (by 0.81 +/- 0.25 ng ml-1) during the breeding se ason. Consistent effects were not noted during the anestrous season or periods of transition. Diphenhydramine suppressed (P < 0.05) serum co ncentrations of GH (by 1.11 +/- 0.45 ng ml-1) during the anestrous sea son, but had no effect during the breeding season. Serum concentration s of PRL were not affected (P > 0.05) by diphenhydramine. The possibil ity that histamine may function as a general modulator of overall hypo thalamic function is unlikely because diphenhydramine differentially a ffected release of LH, GH and PRL. The absence of consistent effects o f diphenhydramine on secretion of LH throughout the year do not suppor t the premise that histamine is a direct modulator of release of gonad otropin-releasing hormone.