GABA-evoked chloride currents do not differ between dendrites and somata of rat neocortical neurons

We performed patch-clamp recordings on acutely isolated somata and dendritic segments of rat neocortical neurons, in order to compare the reversal potential (EGABA) and relative density of GABAA receptor-mediated Cl− currents in these two cellular compartments. Currents were recorded with the Cl−-impermeable pore former gramicidin (25–75 μg ml−1) in HCO3−-free bath solution. Voltage ramps (−110 to −30 mV) from a holding potential (Vh) of −60 mV in the absence and presence of 2 μM GABA were used to construct instantaneous current-voltage relationships. Currents were abolished by co-application of GABA with the GABAA receptor antagonist bicuculline (40 μM). GABA conductance, normalized to membrane surface area, was not different in somata and dendrites. In addition, EGABA was not different in the two compartments. Replacement of intracellular K+ with Cs+ resulted in a significantly more depolarized EGABA in both somata and dendrites. These results suggest that the resting intracellular Cl− concentration ([Cl−]i) is similar in somata and dendrites and that an outward Cl− transporter system maintains low [Cl−]i. In mature neocortex, GABAA receptor-mediated inhibitory postsynaptic potentials (IPSPs) have reversal potentials more negative than the resting membrane potential (Connors et al. 1988; van Brederode & Spain, 1995). The reversal potential for GABAA receptor-mediated neurotransmission (EGABA) depends on the transmembrane gradients for Cl− and HCO3− (Thompson et al. 1988; Kaila et al. 1993). Thus, the principal mechanism of synaptic inhibition is membrane hyperpolarization due to a net inward anion flux. Immature cortical neurons are strongly depolarized by GABAA receptor activation, however (Owens et al. 1996), and under these conditions GABAA receptor activation might paradoxically result in excitation. In addition, intense or prolonged GABA application in mature neocortex can also give rise to depolarizing responses (Staley et al. 1995; Cerne & Spain, 1997). Indirect evidence, obtained with intracellular recording electrodes, indicates that local application of GABA to dendrites elicits predominantly depolarizing responses (Barker & Ransom, 1978; Alger & Nicoll, 1982; Scharfmann & Sarvey, 1987). Since the extracellular concentrations of Cl− and HCO3− are relatively constant, intracellular anion concentration and/or GABAA receptors with different permeability ratios for Cl− and HCO3− are most likely to underlie the observed variability of EGABA (Hara et al. 1992; Staley et al. 1995; Staley & Proctor, 1999; Perkins, 1999). Little direct evidence is available on the intracellular anion concentration in dendrites. In order to compare the mechanisms that control the direction and magnitude of dendritic and somatic GABAergic synaptic currents we examined the density of GABA currents and the regulation of intracellular Cl− homeostasis in these two compartments. For this purpose we used recordings from acutely dissociated dendritic segments and somata (Takigawa & Alzheimer, 1999). This allowed us to compare dendritic and somatic GABAA receptor-mediated Cl− currents under nominally HCO3−-free conditions, such that EGABA equals the Cl− reversal potential (ECl). In addition, by using the gramicidin-perforated patch technique (Ebihara et al. 1995), we were able to avoid internal dialysis of the cell and the resulting changes in [Cl−]i.