Differences between cation-chloride co-transporter functions in the visual cortex of pigmented and albino rats

Albinism in mammals is accompanied by specific morphological and functional alterations of the visual system. To understand their cellular basis we studied the physiological characteristics and transmembrane currents of pyramidal neurons in 350-microm-thick slices of visual cortex from pigmented and albino rats using whole-cell and gramicidin perforated patch-clamp recordings. The resting membrane potential was significantly more positive and the rheobase was significantly lower in neurons of layers II/III and V in albinos as compared with pigmented rats. No significant differences were found in the input resistance, time constant and chronaxy. Whereas the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor-mediated currents were not significantly different, the maximum gamma-aminobutyric acid (GABA)(A) receptor (GABA(A)R)-mediated currents and miniature inhibitory postsynaptic currents showed significantly lower amplitudes in neurons of layer V in visual cortex of albinos as compared with pigmented rats. The reversal potential of the GABA(A)R-mediated currents (E(GABA)) was significantly shifted to more positive values in albinos. Pharmacological experiments showed that this shift could be caused by an increased action of the inward chloride co-transporter NKCC1 and reduced action of the outward chloride co-transporter KCC2 in albino rats. This difference seems to be restricted to the visual cortex because in pyramidal neurons from frontal cortex E(GABA) was not significantly different in albinos as compared with pigmented rats. These results are discussed in relation to functional alterations in the albino visual system.