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) histamine affects serum concentrations of luteinizing hormone (LH), growth hormone (GH) and prolactin (PRL) during the breeding and anestrous seasons and (2) if histamine regulates the release of pituitary hormones through common or discrete mechanisms. Effects of diphenhydramine (200 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 season. Consistent effects were not noted during the anestrous season or periods of transition. Diphenhydramine suppressed (P < 0.05) serum concentrations of GH (by 1.11 +/- 0.45 ng ml-1) during the anestrous season, but had no effect during the breeding season. Serum concentrations of PRL were not affected (P > 0.05) by diphenhydramine. The possibility that histamine may function as a general modulator of overall hypothalamic function is unlikely because diphenhydramine differentially affected release of LH, GH and PRL. The absence of consistent effects of diphenhydramine on secretion of LH throughout the year do not support the premise that histamine is a direct modulator of release of gonadotropin-releasing hormone.