The participation of histaminergic receptors of the rostral hypothalamus on the tonic release of luteinizing hormone (LH) in adult spayed rats under estrogen and progesterone treatment

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Abstract
The influence of histaminergic sites in the preoptic-anterior hypothalamic area (POA-AHA) on the basal release of luteinizing hormone (LH) under a continuous regimen of estradiol, progesterone, or both was studied in ovariectomized rats. Different groups of animals were subjected to the following experimental schedule: at day 1, rats received a s.c. silastic implant filled with oil, estradiol, progesterone, or estradiol plus progesterone. Seven days later (day 7), animals were implanted into the POA-AHA with microinjection cannulae. At day 8 and 9, the different groups of rats were microinjected with 1 Μl of saline solution containing 35 nMol of pyrilamine or metiamide, or 20 nMol of alpha-fluoro-methyl-histidine. At day 10, blood samples were taken through a permanent jugular cannulae implanted in situ the day before. LH concentrations were determined in plasma by RIA. Results showed that the increase of LH plasma levels induced by the ovariectomy was inhibited by the estrogen implant, as expected. Treatment of metiamide or alpha-fluoro-methyl-histidine did not affect the pattern of LH secretion. Nevertheless, treatment of metiamide induced a transient increase in the gonadotropin concentrations that extended for two hours (16:00 and 17:00 H). No change in LH plasma levels was observed in rats bearing the progesterone implant. Treatments (pyrilamine, metiamide, or alpha-fluoro-methyl-histidine into the POA-AHA) had no effect. The transient increase in the hormone levels observed in rats treated with pyrilamine in the estrogen-implanted rats was absent in rats bearing the estrogen-progesterone implant. Present data support the concept that histamine is involved in the POA-AHA to control the pituitary LH release and emphasize the role of plasma estrogen to facilitate the expression of HA receptors.