Interaction of estradiol and 2-hydroxy-estradiol with histamine receptors at hypothalamic level

Sexual dimorphism in [3H]histamine binding sites of rat cerebral cortex

[3H]Histamine binding sites, identified on rat brain homogenate membranes, displayed sexual dimorphism with higher density and lower affinity in preparations from adult female compared to the males. The ontogenetic development of [3H]histamine binding sites was studied in male and female brain cortex neural membranes. Histamine binding sites were detectable in newborn-10 day old rats. Density increased with age, reaching adult levels at 37-45 days. No significant differences between sexes were observed in the binding constants until 37 days after birth, when the [3H]histamine binding characteristics began to differentiate according to sex. Sexual dimorphism in brain histamine binding site development appeared to depend on ovarian steroids. Binding patterns in immature female rats which were ovariectomized at 21 days and killed at 37+ were similar to those found in males. On the other hand, when oestradiol replacement was administered for 10 days to ovariectomized rats a total recovery of [3H]histamine binding pattern was obtained, which was comparable to that observed in intact female rats of the same age.

Effect of in vivo antiestrogen pretreatment on rabbit atrial chronotropic response to histamine

The chronotropic response (delta rate) to histamine (1.4 to 18 X 10(-6) M) of isolated atria from antiestrogen (tamoxifen)-pretreated immature female rabbit was investigated. Tamoxifen treatment (1.0 and 10.0 mg/kg/day for 14 days) had no significant effect on the delta rate. The Rmax and D1/2max were not significantly different in the two tamoxifen-treated groups compared to the oil-treated (1.0 ml/kg/day for 14 days) control group. Cimetidine (2.8 X 10(-7) M) inhibited the delta rate to histamine in all groups: control, 27%; tamoxifen (1.0 mg/kg), 38%; and tamoxifen (10.0 mg/kg), 28%. Only the low dose of tamoxifen was found to be estrogenic (uterotropic). We conclude that tamoxifen pretreatment, both at estrogen-agonist and estrogen-antagonist doses, is without effect on atrial chronotropic response to histamine.

Modulation by estradiol of rabbit atrial chronotropic response to histamine

The chronotropic response (delta rate) to histamine of atria from estradiol 17-beta (E2; 0.1 mg/kg)-treated (14 days) rabbit was significantly greater compared to those from control rabbits. However, the maximum response of atria from rabbits treated with a higher dose of E2 (1.0 mg/kg) was not significantly different from control. Cimetidine (2.8 X 10(-7) M) inhibited the theoretical maximum income in rate to histamine in all 3 groups, control 25%, E2 (0.1 mg) 42% and E2 (1.0 mg) 35%.

Histamine and the hypothalamus

The chemical tools that could be used to examine the function of histamine in the brain are considered together with the evidence linking histamine specifically with the hypothalamus. The distribution of histamine and the enzymes responsible for its synthesis and metabolism is consistent with there being both mast cells and histaminergic nerve terminals within the hypothalamus. Iontophoresis, mepyramine binding and histamine-stimulated adenylate cyclase studies suggest that both histamine H1- and H2- receptors are present in the hypothalamus. In addition, intracerebroventricularly injected histamine receptor agonists and antagonists affect many functions associated with the hypothalamus such as cardiovascular control, food intake, body temperature control, and pituitary hormones whose release is mediated via the hypothalamus, such as corticotropin, growth hormone, thyroid stimulating hormone, prolactin, gonadotropins and vasopressin. However, only in the case of thyroliberin release, prolactin release, body fluid control and blood pressure control is there evidence yet that such effects are mediated via histamine receptors actually in the hypothalamus. The effects of enzyme inhibitors suggest endogenous histamine may be involved in the physiological control of thyroid stimulating hormone, growth hormone and blood pressure, and the effects of receptor antagonists support a role for endogenous histamine in prolactin control. Otherwise, there is little evidence for a physiological role for endogenous, as against exogenous, histamine whether it be from histaminergic terminals or mast cells. In addition, few studies have tried to distinguish possible effects on presynaptic receptors, postsynaptic receptors, hypothalamic blood vessels or the hypophyseal portal blood vessels. It is concluded that although there is good evidence now linking histamine and the hypothalamus more specific studies are required, for instance using microinjection or in vitro techniques and the more specific chemical tools now available, to enable a clearer understanding of the physiological role of histamine in the hypothalamus.

Histamine-activated adenylate cyclase in brain homogenates of several species

Histamine has been shown to activate cyclic AMP synthesis in brain slices and homogenates of certain species, although less is known about species differences in brain homogenates. Dutch Belted and New Zealand White rabbit brain homogenates contained a histamine-responsive adenylate cyclase similar to that of the guinea pig. In contrast, adenylate cyclase of gerbil and hamster brain exhibited little or no stimulation by histamine. Male rat hypothalamic homogenates contained adenylate cyclase, but also exhibited minimal stimulation by histamine, in disagreement with some recent reports. Detailed studies of the conditions of assay failed to resolve this discrepancy.

Neural gonadal steroid actions

Neurons sensitive to gonadal steroids are located strategically within neural circuits that mediate behaviors broadly related to the reproductive process. Some neuronal events and properties are regulated by these hormones. Variability in the occurrence and distribution of particular neural hormonal sensitivities across species may be related to variations in the hormonal requirements for sexual differentiation and for activation of reproductive behaviors.