Cell Type-Specific Differences in Chloride-Regulatory Mechanisms and GABA_AReceptor-Mediated Inhibition in Rat Substantia Nigra

Abstract

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 GABAA-mediated inhibition in nigral dopaminergic neurons compared with nigral GABAergic neurons may be related to their lack of KCC2.

Behavioral Electrophysiology of Psychostimulants

“…In fact, activity remains relatively stable even during DA receptor blockade in awake animals (Degos et al, 2005). Adjustments in the SNr autoactive rate depend instead on fluctuations in the level of inhibitory GABA input, which may arise from several sources including axon collaterals of SNr projection neurons (Fallon and Loughlin, 1995;Gulacsi et al, 2003;Rick and Lacey, 1994). It is difficult to determine, therefore, if a particular pattern of striatal activity is faithfully represented in SNr.…”

Pallidal control of substantia nigra dopaminergic neuron firing pattern and its relation to extracellular neostriatal dopamine levels

“…This is accompanied by a slowing or complete inhibition of spontaneous activity and a marked reduction in burst firing in vivo (Engberg et al, 1993). The GABA A inhibition is caused by an increase in conductance to chloride that leads to a hyperpolarization (Kaila, 1994;Gulacsi et al, 2003) whereas the GABA B inhibition is due to an increase in conductance to potassium (Lacey et al, 1988).…”

“…The most likely candidate mechanism that can generate the appropriate driving force is the sodium-dependent anion exchanger that exchanges Cl À for bicarbonate (NDAE; Payne et al, 2003;see Farrant and Kaila, this volume). When the IPSP reversal potential measurements were repeated in bicarbonate-free conditions that block the exchanger, the GABA A reversal potential in dopaminergic neurons was depolarized to around the resting membrane potential (À49 mV) whereas there was no significant change in the reversal potential in the GABAergic neurons (around À71 mV; Gulacsi et al, 2003) (Fig. 7).…”

“…In most mature CNS neurons, a neuron-specific K + Cl À cotransporter (KCC2) is responsible for extruding intracellular chloride, thereby generating an inwardly directed chloride gradient which makes increases in chloride conductance hyperpolarizing (Payne et al, 1996(Payne et al, , 2003DeFazio et al, 2000). Light and electron microscopic immunocytochemical labeling revealed the expression of KCC2 by GABAergic pars reticulata neurons (Gulacsi et al, 2003) (Fig. 6).…”

“…This neuron type-specific expression of Cl À regulatory mechanisms is reflected in differences in the reversal potential for GABA A IPSPs. GABA A IPSPs evoked by local stimulation and measured with perforated patch recordings in vitro exhibit a reversal potential that is significantly more hyperpolarized than those in the dopaminergic neurons, thus accounting in part for the increased sensitivity to GABA of the nigral GABAergic neurons compared to dopaminergic neurons (Gulacsi et al, 2003) (Fig. 7).…”