Angiotensin receptors are highly expressed in neonatal spinal cord. To identify their influence on neuronal excitability, we used patch-clamp recordings in spinal cord slices to assess responses of neonatal rat (5-12 days) ventral horn neurons to bath-applied angiotensin II (ANG II; 1 μM). In 14/34 identified motoneurons tested under current clamp, ANG II induced a slowly rising and prolonged membrane depolarization, blockable with Losartan (n = 5) and (Sar1, Val5, Ala8)-ANG II (Saralasin, n = 4) but not PD123319 (1 μM each; n = 4). Under voltage clamp (VH -65 mV), 7/22 motoneurons displayed an ANG-II-induced tetrodotoxin-resistant inward current (-128 ± 31 pA) with a similar time course, an associated reduction in membrane conductance and net current reversal at -98.8 ± 3.9 mV. Losartan-sensitive ANG II responses were also evoked in 27/78 tested ventral horn "interneurons." By contrast with motoneurons, their ANG-II-induced inward current was smaller (-39.9 ± 5.2 pA) and analysis of their I-V plots revealed three patterns. In eight cells, membrane conductance decreased with net inward current reversing at -103.8 ± 4.1 mV. In seven cells, membrane conductance increased with net current reversing at -37.9 ± 3.6 mV. In 12 cells, I-V lines remained parallel with no reversal within the current range tested. Intracellular dialysis with GTP-γ-S significantly prolonged the ANG II effect in seven responsive interneurons and GDP-β-S significantly reduced the ANG II response in four other cells. Peak inward currents were significantly reduced in all 13 responding neurons recorded in slices incubated in pertussis toxin (5 μg/ml) for 12-18 h or in 12 neurons perfused with N-ethylmaleimide. Of 29 interneurons sensitive to percussis toxin or N-ethylmaleimide treatment, 9 cells displayed a decrease in membrane conductance that reversed at -101.3 ± 3.8 mV. In eight cells, membrane conductance increased and reversed at -38.7 ± 3.4 mV. In 12 cells, the I-V lines remained parallel with no reversal within the current range tested, suggesting that both conductances are modulated by pertussis toxin-sensitive G proteins. These observations reveal a direct, G-protein-mediated depolarizing action of ANG II on neonatal rat ventral horn neurons. They also imply involvement of two distinct conductances that are differentially distributed among different cell types.
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