Phosphorylation-dependent modulation of hypothalamic magnocellular neurons by neurosteroids
Description
The magnocellular neurons of the supraoptic nucleus (SON) of the hypothalamus secrete vasopressin and oxytocin into the bloodstream to regulate cardiovascular and reproductive functions of the body. The secretion of these hormones depends on the electrical activity of the magnocellular neurons, which is governed by the excitatory and inhibitory synaptic inputs to these cells. In the SON gamma-amino-butyric-acid (GABA) has been found to be the main inhibitory neurotransmitter (Randle et al., 1986; Randle and Renaud, 1987) regulating the magnocellular neurons, with its inhibitory actions mediated primarily by GABAA receptors. GABAA receptors are under post-translational regulatory control by several protein kinases and phosphatases, and GABAA receptor function is modulated by neurosteroids synthesized and stored in the central nervous system Using whole-cell patch-clamp recordings, we tested whether continuous phosphorylation of the GABAA receptor complex is necessary for synaptic GABAA receptor function in magnocellular neurons of the SON. We established that spontaneous GABA-mediated inhibitory postsynaptic currents (IPSCs) in magnocellular neurons do not run down when phosphate donor levels are depleted by intracellular dialysis with a patch solution lacking ATP and GTP. Blocking protein kinase function with specific kinase blockers did not affect the IPSCs. These results suggest that synaptic GABAA receptor function does not require constitutive phosphorylation of the receptor complex in supraoptic neurons GABA mediated inhibitory synaptic inputs are modulated by various neuromodulators. Steroids, for a long time, were not considered as traditional neuromodulators, their action taking place primarily at the level of transcriptional regulation. Recently, a group of steroids has been shown to have fast modulatory effects on neuronal activity through direct actions on neurotransmitter receptors. These steroids, which mainly affect the action of the inhibitory transmitter GABA through allosteric modulation of the GABAA receptor, are named neurosteroids. The modulatory effects of allopregnanolone, the most potent of the neurosteroids, on synaptic GABA currents were investigated in our study. Allopregnanolone stereospecifically increased the time of decay of spontaneous GABA mediated inhibitory postsynaptic currents recorded in magnocellular neurons of the SON. This effect of allopregnanolone was G-protein/protein kinase dependent, since blocking G-protein and protein kinase C function completely abolished the effect. G-protein and protein kinase C activation in the absence of the neurosteroid had no effect on spontaneous IPSCs, suggesting that the neurosteroid does not act at a G-protein-coupled receptor separate from the GABAA receptor complex The results of our study indicate that the spontaneous synaptic function of the GABAA receptor in magnocellular neurons does not require phosphorylation, but that constitutive G-protein/protein kinase function and phosphorylation of the GABAA receptor is necessary for neurosteroid modulation