Kinetic properties of the α₂ homo-oligomeric glycine receptor impairs a proper synaptic functioning

Abstract

Ionotropic glycine receptors (GlyRs) are present in the central nervous system well before the establishment of synaptic contacts. Immature nerve cells are known, at least in the spinal cord, to express α₂ homomeric GlyRs, the properties of which are relatively unknown compared to those of the adult synaptic form of the GlyR (mainly α₁/β heteromeres). Here, the kinetics properties of GlyRs at the single-channel level have been recorded in real-time by means of the patch-clamp technique in the outside-out configuration coupled with an ultra-fast flow application system (< 100 µs). Recordings were performed on chinese hamster ovary (CHO) cells stably transfected with the α₂ GlyR subunit. We show that the onset, the relaxation and the desensitisation of α₂ homomeric GlyR-mediated currents are slower by one or two orders of magnitude compared to synaptic mature GlyRs and to other ligand-gated ionotropic channels involved in fast synaptic transmission. First latency analysis performed on single GlyR channels revealed that their slow activation time course was due to delayed openings. When synaptic release of glycine was mimicked (1 mm glycine; 1 ms pulse duration), the opening probability of α₂ homomeric GlyRs was low (P_o ≈ 0.1) when compared to mature synaptic GlyRs (P_o = 0.9). This low P_o is likely to be a direct consequence of the relatively slow activation kinetics of α₂ homomeric GlyRs when compared to the activation kinetics of mature α₁/β GlyRs. Such slow kinetics suggest that embryonic α₂ homomeric GlyRs cannot be activated by fast neurotransmitter release at mature synapses but rather could be suited for a non-synaptic paracrine-like release of agonist, which is known to occur in the embryo.