HOW QUICKLY CAN GABA(A) RECEPTORS OPEN

We have examined GABA(A) receptor activation by making rapid applicati ons of GABA to outside-out patches excised from cultured postnatal rat cerebellar neurons. The rate of development of current increases with increasing GABA concentration from a low to a high concentration asym ptote. The low concentration asymptote is about 10 s-1 for patches tak en from granule cells and 4 s-1 for patches from Purkinje cells. The h igh concentration asymptote is about 6000 s-1 for patches taken from e ither granule cells or Purkinje cells. The high concentration asymptot e gives an estimate of the fastest rate at which these channels can op en and indicates that agonist binding steps are not rate limiting. The concentration dependence of the development of current indicates that more than one GABA molecule is bound to most receptors with open chan nels and that the final binding step is of low affinity (about 500 muM ). A comparison with GABA-mediated postsynaptic currents suggests that the properties of the GABA(A) receptor play a major role in determini ng the shape of inhibitory synaptic responses and that the cleft conce ntration of GABA reaches at least 500 muM.