Angiotensin AT₁-receptors depolarize neonatal spinal motoneurons and other ventral horn neurons via two different conductances

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 (V_H -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.