EFFECTS OF SEASON AND ESTRADIOL ON CONCENTRATIONS OF HISTAMINE WITHIN DISCRETE BRAIN-REGIONS OF OVARIECTOMIZED EWES

Histamine has been implicated as a neuromodulator of secretion of gonadotropins in several species. Concentrations of histamine were analyzed within discrete brain regions and endocrine tissues to help determine whether this amine has the potential to exert a similar function in ewes expected to have dramatically different serum concentrations of LH. Following collection of blood samples at 12-min intervals for 4 hr, ovariectomized (OVX) and OVX-estradiol treated (OVX-E) ewes were slaughtered during the breeding and anestrous seasons (five animals/group). Concentrations of LH were depressed by treatment with estradiol (E; P<.01), but to a greater extent (P<.05) during the anestrous season compared to the breeding season. Concentrations of histamine in tissues (ng/mg) differed (P<.01) between the breeding and anestrous seasons, in the medial thalamus (39.2 +/- 14.1 vs 109.9 +/- 13.0), posterior pituitary gland (247.6 +/- 50.7 vs 23.0 +/- 9.1) and midbrain tegmentum (10.4 +/- 5.6 vs 50.7 +/- 3.9). Estradiol containing implants decreased (P<.05) concentrations of histamine in the midbrain tegmentum (20.3 +/- 7.1 vs 37.7 +/- 7.8) and posterior pituitary gland (87.3 +/- 24.0 vs 258.2 +/- 67.5) compared to non-estradiol treated controls. Histamine concentrations in the pineal and anterior pituitary glands and brain regions: stalk-median eminence, medial basal hypothalamus, preoptic area, cerebellum, parietal neocortex, were not (P>.05) affected solely by either season or E. An interaction between effects of season and estradiol on concentrations of histamine occurred (P<.05) in the posterior pituitary gland and the preoptic area. Mean concentrations of histamine (ng/mg) in the posterior pituitary gland of OVX ewes during the anestrous and breeding seasons were 23.3 +/- 10.3 and 376.1 +/- 77.7, respectively, and were 23.7 +/- 15 and 119.1 +/- 30.7 in OVX-E ewes during the same periods. Mean concentrations of histamine in the preoptic area of OVX ewes were 63.3 +/- 10.5 and 12.5 +/- 7.3 during the anestrous season and breeding seasons and 29.8 +/- 8.5 and 42.9 +/- 12.8, respectively, in OVX-E ewes. In a second experiment, OVX (n=5) and OVX-E (n=5) ewes were slaughtered during the breeding season and their brains dissected into micro-punches to determine effects of E on concentration of histamine in more discrete loci. Differences (P<.05) in concentrations of histamine were noted only in the posterior thalamus (144.1 +/- 7.8 vs 79.0 +/- 10.0) in OVX and OVX-E ewes, respectively. In conclusion histamine may function as a intermediary neuromodulator in the neural circuitry which regulates secretion of gonadotropins in ewes. Neither season or estradiol alone, however, significantly influenced concentrations of histamine in brain regions which closely regulate release of GnRH.