Evidence for a Metabotropic Glycine Receptor: A Strychnine-Insensitive Inhibitory Effect of Glycine at the Bipolar to Ganglion Cell Synapse

Abstract

Purpose:: Glycine is an inhibitory ionotropic neurotransmitter in retina that acts by opening a chloride channel (iGlyR).To explore the possible presence in retina of a metabotropic glycine receptor (mGlyR).

Methods:: We examined the effects of glycine when ionotropic receptors were blocked by 10 µM strychnine, a potent competitive antagonist of iGlyRs.

Results:: Glycine in the presence of strychnine suppressed voltage-gated calcium channel current. Lack of a chloride current indicated iGlyRs were blocked. This effect was observed in both bipolar and ganglion cells in salamander retinal slice and also in isolated retinal neuron preparations. Strychnine appeared to completely block glycine-induced chloride current, indicating that the inhibition was not related to the traditional glycine-gated chloride current. Cell dialysis of 1mM GDP_ßS occluded this strychnine-insensitive effect of glycine, suggesting glycine activated a G-protein coupled receptor. A few antagonists of second messenger kinase pathways were tested. Forskolin, an adenylyl cyclase activator, suppressed glycine’s inhibition of calcium current. To determine if synaptic transmission was affected by this novel inhibitory effect of glycine on calcium channel current, we recorded from ganglion cells in the dark-adapted retinal slice preparation. 10µM glycine suppressed light-evoked EPSCs of ganglion cells. Forskolin prevented this effect of glycine on synaptic transmission. In addition, in a paired stimulus response (PSR) of retinal ganglion cells, the P2/P1 ratio was compared before and after adding glycine in the presence of strychnine. Glycine reduced the first response but increased the P2/P1 ratio. This PSR result suggests the strychnine insensitive effect of glycine was due to a presynaptic site of an action. The effect of glycine on the PSR was eliminated by forskolin.

Conclusions:: These results indicate that glycine inhibits neurotransmitter release from bipolar to ganglion cells by activating presynaptic strychnine-insensitive glycine receptors and suggest the presence of metabotropic glycine receptor, which is coupled to G-protein and intracellular PKA pathway.

Keywords: neurotransmitters/neurotransmitter systems • inhibitory receptors • synapse