Noradrenergic Modulation of Spontaneous Inhibitory Postsynaptic Currents in the Hypothalamic Paraventricular Nucleus

Noradrenergic Modulation of Spontaneous Inhibitory Postsynaptic Currents in the Hypothalamic Paraventricular Nucleus

<P> Previous studies have suggested that brain stem noradrenergic inputs differentially modulate neurons in the paraventricular nucleus (PVN). Here, we compared the effects of norepinephrine (NE) on spontaneous GABAergic inhibitory postsynaptic currents (sIPSCs) in identified PVN neurons using slice patch technique. In 17 of 18 type I neurons, NE (30∼100μM) reversibly decreased sIPSC frequency to 41&#8273;7% of the baseline value (4.4&#8273;0.8 Hz, p＜0.001). This effect was blocked by yohimbine (2∼20μM), an α<SUB>2</SUB>-adrenoceptor antagonist and mimicked by clonidine (50μM), an α<SUB>2</SUB>-adrenoceptor agonist. In contrast, NE increased sIPSC frequency to 248&#8273;32% of the control (3.06&#8273;0.37 Hz, p＜0.001) in 31 of 54 type II neurons, but decreased the frequency to 41&#8273;7% of the control (5.5&#8273;1.3 Hz) in the rest of type II neurons (p＜0.001). In both types of PVN neurons, NE did not affect the mean amplitude and decay time constant of sIPSCs. In addition, membrane input resistance and amplitude of sIPSC of type I neurons were larger than those of type II neurons tested (1209 vs. 736 MΩ, p＜0.001; 110 vs. 81 pS, p＜0.001). The results suggest that noradrenergic modulation of inhibitory synaptic transmission in the PVN decreases the neuronal excitability in most type I neurons via α<SUB>2</SUB>-adrenoceptor, however, either increases in about 60% or decreases in 40% of type II neurons.