Cell type-specific differences in chloride-regulatory mechanisms and GABA_A receptor-mediated inhibition in rat substantia nigra

The regulation of intracellular chloride has important roles in neuronal function, especially by setting the magnitude and direction of the Cl ^- flux gated by GABA_A receptors. Previous studies have shown that GABA_A-mediated inhibition is less effective in dopaminergic than in GABAergic neurons in substantia nigra. We studied whether this phenomenon may be related to a difference in Cl-regulatory mechanisms. Light-microscopic immunocytochemistry revealed that the potassium-chloride cotransporter 2 (KCC2) was localized only in the dendrites of nondopaminergic (primarily GABAergic) neurons in the substantia nigra, whereas the voltage-sensitive chloride channel 2 (ClC-2) was observed only in the dopaminergic neurons of the pars compacta. Electron-microscopic immunogold labeling confirmed that KCC2 is localized in the dendritic plasma membrane of GABAergic neurons close to inhibitory synapses. Confocal microscopy showed that ClC-2 was selectively expressed in the somatic and dendritic cell membranes of the dopaminergic neurons. Gramicidin-perforated-patch recordings revealed that the GABA_A IPSP reversal potential was significantly less negative and had a much smaller hyperpolarizing driving force in dopaminergic than in GABAergic neurons. The GABA_A reversal potential was significantly less negative in bicarbonate-free buffer in dopaminergic but not in GABAergic neurons. The present study suggests that KCC2 is responsible for maintaining the low intracellular Cl^- concentration in nigral GABAergic neurons, whereas a sodium-dependent anion (Cl^--HCO₃^-) exchanger and ClC-2 are likely to serve this role in dopaminergic neurons. The relatively low efficacy of GABA_A-mediated inhibition in nigral dopaminergic neurons compared with nigral GABAergic neurons may be related to their lack of KCC2.