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.