Glycine Receptors and Glycinergic Synaptic Input at the Axon Terminals of Mammalian Retinal Rod Bipolar Cells

Abstract

Abstract: : Purpose: Glycine is one of the major inhibitory neurotransmitters in the retina. In this study, we investigated the properties of glycine receptors and glycinergic synaptic inputs at the axon terminals of mammalian rod bipolar cells (RBCs). Methods:Whole-cell patch-clamp recordings were performed in isolated axon terminal and retinal slice preparations from the rat. Intracellular Ca²⁺ transients were monitored by Ca²⁺ imaging recordings. Results: Glycine current density at terminal endings was found to be more than one order of magnitude higher than that in the somatic/dendritic region. On the other hand, glycine currents in both the terminal and somatic/dendritic regions manifested similar sensitivity to picrotoxinin. Single channel activities recorded from terminals and somata displayed a similar main-state conductance (~46 pS). Application of glycine was found to effectively suppress K⁺-evoked intracellular Ca²⁺ increases at the terminals. Furthermore, in the presence of GABA_A and GABA_C antagonists, strychnine-sensitive chloride currents were evoked in RBCs by puffing kainate onto the inner plexiform layer of retinal slices. No such currents were observed in RBCs lacking axon terminals or if Ca²⁺ was replaced by Co²⁺ in the extracellular solution. Glycinergic synaptic currents also displayed discrete miniature events, which were partially blocked by TTX. Conclusions: Glycine receptors are highly concentrated at the axon terminals of RBCs. The pharmacological and physiological properties of glycine receptors located at axon terminals and at somatic/dendritic regions appear to be the same. This study provides evidence for the existence of functional glycinergic synaptic input at RBC axon terminals that may play a role in modulating their synaptic transmission.

Keywords: inhibitory receptors • retina: proximal(bipolar, amacrine, and gangli • bipolar cells