Cholinergic synaptic activation due to HCO 3 − in the superior cervical ganglion of the rat

Influence of acid-base change on synaptic transmission was studied in the isolated superior cervical ganglion of the rat. Effects of changes inP_CO2 [HCO₃^–], or pH of the superfusing solution were studied, using as an index of synaptic excitation the amplitude of the initial negative deflection of surface potential induced by preganglionic stimulation. An increase or decrease in the extracellular fluid (ECF) pH by changing [HCO₃^–] at a normalP_CO2 elicited respectively augmentation or suppression of the negative deflection. Similar shifts in the ECF pH with varyingP_CO2 at a normal [HCO₃^–] had small or almost negligible effects on the negative deflection. Simultaneous increase in both theP_CO2 and [HCO₃^–], which compensated for the pH change in the ECF, induced a consistent increase in the amplitude of the negative deflection. The amplitude of negative deflection in various acid-base conditions was positively correlated with the ECF [HCO₃^–] but not with ECF pH orP_CO2. These results suggest that an increase in the ECF [HCO₃^–] activates cholinergic (nicotinic) synaptic transmission in the ganglion.