Marabelli_2013_J.Physiol_591_3289

Glycine receptors mediate fast synaptic inhibition in spinal cord and brainstem. Two alpha subunits are present in adult neurones, alpha1, which forms most of the synaptic glycine receptors, and alpha3. The physiological role of alpha3 is not known, despite the fact that alpha3 expression is concentrated in areas involved in nociceptive processing, such as the superficial dorsal horn. In the present study, we characterized the kinetic properties of rat homomeric alpha3 glycine receptors heterologously expressed in HEK293 cells. We analysed steady state single channel activity at a range of different glycine concentrations by fitting kinetic schemes and found that alpha3 channels resemble alpha1 receptors in their high maximum open probability (99.1% cf. 98% for alpha1), but differ in that maximum open probability is reached when all five binding sites are occupied by glycine (cf. three out of five sites for alpha1). alpha3 activation was best described by kinetic schemes that allow the channel to open also when partially liganded and that contain more than the minimum number of shut states, either as desensitized distal states (Jones and Westbrook scheme) or as pre-open gating intermediates (flip scheme). We recorded also synaptic-like alpha3 currents elicited by the rapid application of 1 ms pulses of high concentration glycine to outside-out patches. These currents had fast deactivation, with a time constant of decay of 9 ms. Thus, if native synaptic currents can be mediated by alpha3 glycine receptors, they are likely to be very close in their kinetics to alpha1-mediated synaptic events.