The electrophysiological effects of CO2 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 NaHCO3/5% CO2 (pH approximately 7.34, 33 degrees C) and exposed to 5% CO2/95% O2 arriving through an interface chamber, locus coeruleus neurons discharged spontaneously at approximately 1 Hz. Extracellular recordings showed that lowering CO2 that arrived through the chamber below 5% resulted in reductions in firing rate, often with a complete cessation of activity when exogenous CO2 was removed completely. Intracellular recordings revealed that lowering CO2 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 CO2 removal by approximately +20 mV. Raising CO2 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 CO2 were mimicked by other manipulations that are known to affect intracellular pH. For example, NH4Cl, 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 CO2 or addition of NH4Cl. The polyamine spermine (tetrahydrochloride) applied via intracellular micropipettes blocked the outward current induced by removal of CO2 or addition of NH4Cl. Spermine (free base) or an equivalent concentration of putrescine failed to alter the CO2 (0%)- or NH4Cl-induced effects. We conclude that CO2 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 CO2 or addition of NH4Cl, 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 CO2 in the locus coeruleus are discussed.