Response of thalamocortical neurons to hypoxia: A whole-cell patch- clamp study

Abstract

The effect of hypoxia (3-4 min of 95% N2, 5% CO2) on thalamocortical (TC) neurons was investigated using the whole-cell patch-clamp technique in rat dorsal lateral geniculate nucleus slices kept submerged at 32°C. The predominant feature of the response of TC neurons to hypoxia was an increase in input conductance (ΔG(N) = 117 ± 15%, n = 33) that was accompanied by an inward shift in baseline holding current (I(BH)) at -65 and -57 mV (ΔI(BH) = -45 ± 6 pA, n = 18, and -25 ± 8 pA, n = 33, respectively) but not at -40 mV. The hypoxia-induced increase in G(N) (as well as the shift in I(BH)) was abolished by procedures that are known to block I(h), i.e., bath application of 4-(N-ethyl-N-phenylamino)-1,2-dimethyl-6-(methylamino)- pyrimidinium chloride (100-300 μM) (ΔG(N) = 5 ± 13%, n = 11) and CsCl (2- 3 mM) (ΔG(N) = 16 ± 16%, n = 5), or low [Na+](o) (ΔG(N) = 10 ± 10%, n = 5), whereas bath application of BaCl2 (0.1-2.0 mM) had no significant effect (ΔG(N) = 128 ± 14%, n = 8). The hypoxic response was also abolished in low [Ca+2](o) (ΔG(N) = 25 ± 16%, ΔI(BH) = -6 ± 8 pa, n = 13), but was unaffected by recording with electrodes containing EGTA (10 mM), BAPTA (10-30 mM), Cs+, or Cl-, as well as in the presence of external tetraethylammonium and 4-aminopyridine. Furthermore, preincubation of the slices with botulinum toxin A (100 nM), which is known to reduce Ca2+- dependent transmitter release, blocked the hypoxic response (ΔG(N) = -3 ± 15%, ΔI(BH) = 10 ± 5 pA, n = 4). We suggest that a positive shift in the voltage-dependence of I(h) and a change in its activation kinetics, which transforms it into a fast activating current, may be responsible for the hypoxia-induced changes in G(N) and I(BH), probably via an increase in Ca+2-dependent transmitter release.