Carbon dioxide regulates the tonic activity of locus coeruleus neurons by modulating a proton- and polyamine-sensitive inward rectifier potassium current

Abstract

The electrophysiological effects of CO 2 on locus coeruleus noradrenergic neurons were investigated in rat brain slices. Under control conditions, when slices were perfused with artificial cerebrospinal fluid containing 24 mM NaHCO 3/5% CO 2 (pH ∼7.34, 33°C) and exposed to 5% CO 2/95% O 2 arriving through an interface chamber, locus coeruleus neurons discharged spontaneously at ∼1 Hz. Extracellular recordings showed that lowering CO 2 that arrived through the chamber below 5% resulted in reductions in firing rate, often with a complete cessation of activity when exogenous CO 2 was removed completely. Intracellular recordings revealed that lowering CO 2 produced an outward current with an increase in slope conductance and a reversal potential near the potassium equilibrium potential; doubling the concentration of external potassium shifted the reversal potential of the current activated by CO 2 removal by ∼+20 mV. Raising CO 2 above 5% induced an increase in firing rate, an inward current, a decreased slope conductance at potentials near resting membrane voltage, and an increased slope conductance at more negative potentials. These effects of CO 2 were mimicked by other manipulations that are known to affect intracellular pH. For example, NH 4Cl, which acutely induces intracellular alkalinization, caused a marked reduction in firing rate, an outward current and an increased slope conductance that reversed near the potassium equilibrium potential. Bath-applied barium blocked the effects induced by removal of CO 2 or addition of NH 4Cl. The polyamine spermine (tetrahydrochloride) applied via intracellular micropipettes blocked the outward current induced by removal of CO 2 or addition of NH 4Cl. Spermine (free base) or an equivalent concentration of putrescine failed to alter the CO 2 (0%)- or NH 4Cl-induced effects. We conclude that CO 2 maintains the tonic activity of locus coeruleus neurons by decreasing intracellular pH which, in turn, closes inward rectifier potassium channels, an effect that may be mediated by a protonated polyamine. According to this model, when there is alkalinization of locus coeruleus cells through removal of CO 2 or addition of NH 4Cl, endogenous spermine or a similar polyamine becomes partially deprotonated, releasing the channel block and allowing the cell to hyperpolarize. The possible implications of these results for the physiological effects of CO 2 in the locus coeruleus are discussed.