Mechanisms Underlying the Chronotropic Effect of Angiotensin II on Cultured Neurons From Rat Hypothalamus and Brain Stem

Wang, Desuo, Craig H. Gelband, Colin Sumners, and Philip Posner. Mechanisms underlying the chronotropic effect of angiotensin II on cultured neurons from rat hypothalamus and brain stem. J. Neurophysiol. 78: 1013–1020, 1997. The chronotropic effect of angiotensin II (Ang II) was studied in cultured neurons from rat hypothalamus and brain stem with the use of the patch-clamp technique. Ang II (100 nM) increased the neuronal spontaneous firing rate from 0.8 ± 0.3 (SE) Hz in control to 1.3 ± 0.4 Hz ( n = 7, P < 0.05). The amplitude of threshold stimulation was decreased by Ang II (100 nM) from 82 ± 4 pA to 62 ± 5 pA ( n = 4, P < 0.05). These actions of Ang II were reversed by the angiotensin type 1 (AT1) receptor antagonist losartan (1 μM). In the presence of tetrodotoxin, Ang II (100 nM) significantly increased the frequency and the amplitude of the Cd2+-sensitive subthreshold activity of the cultured neurons. Ang II also stimulated the subthreshold early afterdepolarizations (EADs) to become fully developed action potentials. Similar to the action of Ang II, the protein kinase C (PKC) activator phorbol 12-myristate 13-acetate (PMA, 100 nM) increased the firing rate from 0.76 ± 0.3 Hz to 2.3 ± 0.5 Hz ( n = 6, P < 0.05) and increased the neuronal subthreshold activity. After neurons were intracellularly dialyzed with PKC inhibitory peptide (PKCIP, 5 μM), PMA alone, Ang II alone, or PMA plus Ang II no longer increased the action potential firing initiated from the resting membrane potential level. However, superfusion of PMA plus Ang II or Ang II alone increased the number of EADs that reached threshold and produced action potentials even in the presence of PKCIP (5 μM, n = 4). The actions of Ang II could also be mimicked by depolarizing pulse and K+ channel blockers (tetraethylammonium chloride or 4-aminopyridine). These results indicate that Ang II by activation of AT1 receptors increases neuronal excitability and firing frequency, and that this may involve both PKC dependent and-independent mechanisms.