Dopamine induces intracellular Ca2+ signals mediated by alpha1B-adrenoceptors in rat pineal cells.

We have studied the functional interaction of dopamine with alpha1-adrenoceptor subtypes by measuring intracellular Ca2+ levels in pineal cells, a cell type where adrenoceptors are well characterized. We show that dopamine induces transient intracellular Ca2+ signals in only 70% of cells responding to phenylephrine. Dopamine-induced Ca2+ signals desensitise faster than Ca2+ transients elicited with phenylephrine and are selectively blocked by desipramine, imipramine, and alpha1B-adrenoceptor antagonists. These results suggest that dopamine induced Ca2+ signals are mainly due to the activation of one subtype of alpha-adrenoceptor, the alpha1B.

Estrogen modulates alpha(1)/beta-adrenoceptor- induced signaling and melatonin production in female rat pinealocytes.

Nocturnal rise in pineal melatonin output is due to the night-induced acceleration of noradrenergic transmission and alpha(1)- and beta-adrenoceptor activation. In addition, in female animals, cyclic oscillations in circulating levels of sex steroid hormones are accompanied by changes in the rate of pineal melatonin secretion. To investigate whether estrogen directly affects pineal adrenoceptor responsiveness, pinealocytes from 21-day-old ovariectomized rats were exposed to physiological concentrations of 17beta-estradiol (17beta-E(2)) and treated with noradrenergic agonists. Direct exposure to 17beta-E(2) reduced alpha(1)/beta-adrenoceptor-induced stimulation of melatonin synthesis and release. This effect was mediated by an estrogen-dependent inhibition of both beta-adrenoceptor-induced accumulation of cAMP and alpha(1)-adrenoceptor-induced phosphoinositide hydrolysis. Furthermore, estrogen reduced transient Ca(2+) signals elicited in single pinealocytes by alpha(1)-adrenoceptor activation or by potassium-induced depolarization. In the case of beta-adrenoceptor responsiveness, neither forskolin- nor cholera toxin-induced accumulation of cAMP were affected by previous exposure to 17beta-E(2). This indicates that estrogen effects must be exerted upstream from adenylylcyclase activation, and independent of modifications in G protein expression, therefore suggesting changes in either adrenoceptor expression or receptor-effector coupling mechanisms. Since estrogen effects upon adrenoceptor responsiveness in pineal cells was not mimicked by 17beta-E(2) coupled to bovine serum albumin and showed a latency of 48 h, this effect could be compatible with a genomic action mechanism. This is also consistent with the presence of two estrogen receptor proteins, alpha- and beta-subtypes, in female rat pinealocytes under the present experimental conditions.

Gonadal steroid modulation of neuroendocrine transduction: a transynaptic view.

1. Steroid hormones act on neuronal communication through different mechanisms, ranging from transynaptic modulation of neurotransmitter synthesis and release to development and remodeling of synaptic circuitry. Due the wide distribution of putative brain targets for steroid hormones, acute or sustained elevations of their circulating levels may affect, simultaneously, a variety of neuronal elements. In an elementary mode of interaction, steroids are able to modulate both the synthesis and release of a neurotransmitter at a particular synapse, and the response of its target postsynaptic cells. Using two neuroendocrine transducing systems-the rat pineal gland and the GT1-7 cell line-we have examined these interactions and the following findings are discussed in this article. 2. In the rat, pineal melatonin production is partially controlled by gonadal hormones. In females, melatonin synthesis and secretion is reduced during the night of proestrus, apparently as a consequence of elevated estradiol and progesterone levels. In males, circulating testosterone seems to be necessary to maintain the amplitude of the nocturnal melatonin peak. 3. Some gonadal effects on pineal activity are exerted on its noradrenergic input, since changes in circulating steroid hormone levels are able to induce acute modifications of tyrosine hydroxylase activity in pineal sympathetic nerve terminals. 4. Gonadal steroids are also able to regulate the response of pineal cells to adrenergic stimulation, since in vivo treatment of both male and female rats with steroid hormone blockers induces profound modifications in adrenergically-induced accumulation of cyclic AMP (cAMP) in dispersed pinealocytes. 5. Direct exposure of pineal cells from gonadectomized female and male rats to estradiol (E2) or testosterone (T), respectively, potentiates pinealocyte response to adrenergic activation. In addition, short-term (15 min) exposure to either progesterone (Pg) or progesterone coupled to bovine serum albumin (P-3-BSA) suppresses the E2-dependent potentiation of adrenergic response in female rat pinealocytes. 6. Exposure of GT1-7 cells to E2 completely blocked the norepinephrine (NE)-induced elevation of cAMP content. In E2-treated GT1-7 cells, additional exposure (15 min) to either Pg or P-3-BSA abolished E2-dependent inhibition of NE responsiveness. In addition, P-3-BSA alone increased basal cAMP levels in GT1-7 cells, regardless previous exposure to E2. 7. In conclusion, there are evidences, both from the current literature and from the present results, supporting the view that in some neuroendocrine systems gonadal hormones modulate neurotransmission by acting, simultaneously, at pre- and postsynaptic sites. The models presented here constitute appropriate examples of this transynaptic mode of steroid and, therefore, may offer a useful approach to investigate steroid hormone actions on the brain.

Ovarian hormones regulate alpha 1- and beta-adrenoceptor interactions in female rat pinealocytes.

We have examined the effects of alpha 1- and beta-receptor activation on cyclic AMP (cAMP) accumulation in cultured pinealocytes from female ovariectomized rats, and from intact rats at proestrus treated with the antiestrogen Tamoxifen, the antiprogestagen RU486, or with both. Isoproterenol (a beta-agonist) significantly increased cAMP levels in pinealocytes from intact and ovariectomized rats. This response was considerably enhanced in pinealocytes from rats treated with RU486 or Tamoxifen alone, but not with both. Moreover, incubation of pinealocytes with isoproterenol together with phenylephrine (an alpha 1-agonist) produced a synergistic effect in animals that had been ovariectomized, or that had received RU486, either alone or in combination with Tamoxifen. These results suggest that, in female rats, ovarian steroids may regulate the interactions between alpha 1- and beta-adrenergic receptors, and thus modulate pineal melatonin synthesis during the oestrous cycle through these mechanisms.