Acute sodium depletion modifies septo-preoptic neuron sensitivities to neurohormones

Intrinsic properties of the sodium sensor neurons in the rat median preoptic nucleus

The essential role of the median preoptic nucleus (MnPO) in the integration of chemosensory information associated with the hydromineral state of the rat relies on the presence of a unique population of sodium (Na+) sensor neurons. Little is known about the intrinsic properties of these neurons; therefore, we used whole cell recordings in acute brain slices to determine the electrical fingerprints of this specific neural population of rat MnPO. The data collected from a large sample of neurons (115) indicated that the Na+ sensor neurons represent a majority of the MnPO neurons in situ (83%). These neurons displayed great diversity in both firing patterns induced by transient depolarizing current steps and rectifying properties activated by hyperpolarizing current steps. This diversity of electrical properties was also present in non-Na+ sensor neurons. Subpopulations of Na+ sensor neurons could be distinguished, however, from the non-Na+ sensor neurons. The firing frequency was higher in Na+ sensor neurons, showing irregular spike discharges, and the amplitude of the time-dependent rectification was weaker in the Na+ sensor neurons than in non-Na+ sensor neurons. The diversity among the electrical properties of the MnPO neurons contrasts with the relative function homogeneity (Na+ sensing). However, this diversity might be correlated with a variety of direct synaptic connections linking the MnPO to different brain areas involved in various aspects of the restoration and conservation of the body fluid homeostasis.

Endogenous angiotensin II facilitates GABAergic neurotransmission afferent to the Na+-responsive neurons of the rat median preoptic nucleus

The median preoptic nucleus (MnPO) is densely innervated by efferent projections from the subfornical organ (SFO) and, therefore, is an important relay for the peripheral chemosensory and humoral information (osmolality and serum levels ANG II). In this context, controlling the excitability of MnPO neuronal populations is a major determinant of body fluid homeostasis and cardiovascular regulation. Using a brain slice preparation and patch-clamp recordings, our study sought to determine whether endogenous ANG II modulates the strength of the SFO-derived GABAergic inputs to the MnPO. Our results showed that the amplitude of the inhibitory postsynaptic currents (IPSCs) were progressively reduced by 44 ± 2.3% by (Sar1, Ile8)-ANG II, a competitive ANG type 1 receptor (AT1R) antagonist. Similarly, losartan, a nonpeptidergic AT1R antagonist decreased the IPSC amplitude by 40.4 ± 5.6%. The facilitating effect of endogenous ANG II on the GABAergic input to the MnPO was not attributed to a change in GABA release probability and was mimicked by exogenous ANG II, which potentiated the amplitude of the muscimol-activated GABAA/Cl− current by 53.1 ± 11.4%. These results demonstrate a postsynaptic locus of action of ANG II. Further analysis reveals that ANG II did not affect the reversal potential of the synaptic inhibitory response, thus privileging a cross talk between postsynaptic AT1 and GABAA receptors. Interestingly, facilitation of GABAergic neurotransmission by endogenous ANG II was specific to neurons responding to changes in the ambient Na+ level. This finding, combined with the ANG II-mediated depolarization of non-Na+-responsive neurons reveals the dual actions of ANG II to modulate the excitability of MnPO neurons.