Action Potentials Are Required for the Lateral Transmission of Glycinergic Transient Inhibition in the Amphibian Retina

Abstract

Transient lateral inhibition (TLI), the suppression of responses of a ganglion cell to light stimuli in the receptive field center by changes in illumination in the receptive field surround, was studied in light-adapted mud puppy and tiger salamander retinas using both eyecup and retinal slice preparations. In the eyecup, TLI was measured in on-off ganglion cells as the ability of rotating, concentric windmill patterns of 500-1200 micron inner diameter to suppress the response to a small spot stimulus in the receptive field center. Both the suppression of the spot response and the hyperpolarization produced in ganglion cells by rotation of the windmill were blocked in the presence of 2 microM strychnine or 500 nM tetrodotoxin (TTX), but not by 150 microM picrotoxin. In the slice preparation in which GABA-mediated currents were blocked with picrotoxin, IPSCs elicited by diffuse illumination were blocked by strychnine and strongly reduced by TTX. The TTX-resistant component was probably attributable to illumination of the receptive field center. TTX had a much greater effect in reducing the glycinergic inhibition elicited by laterally displaced stimulation versus nearby focal electrical stimulation. Strychnine enhanced light-evoked excitatory currents in ganglion cells, but this was not mimicked by TTX. The results suggest that local glycinergic transient inhibition does not require action potentials and is mediated by synapses onto both ganglion cell dendrites and bipolar cell terminals. In contrast, the lateral spread of this inhibition (at least over distances >250 micron) requires action potentials and is mainly onto ganglion cell dendrites.

Spike-Dependent GABA Inputs to Bipolar Cell Axon Terminals Contribute to Lateral Inhibition of Retinal Ganglion Cells

“…Our results are also consistent with previous studies, which showed that TTX did not affect the magnitude of light-evoked EPSCs recorded in salamander ganglion cells (Cook et al 1998). Taken together, these findings suggest that regenerative sodium currents do not play a major role in controlling glutamate release from salamander bipolar cells.…”

“…The IPSCs were GABAergic because they were blocked by a combination of picrotoxin and bicuculline (data not shown) in agreement with previous studies (Lukasiewicz and Shields 1998;Roska et al 1998). Because glycinergic inhibition depends on action potentials (Cook et al 1998), GABAergic inhibition was isolated by blocking glycine receptors with strychnine (3 M). IPSCs were recorded when cells were voltage clamped to 0 mV, the reversal potential for nonspecific cation conductances (E cation ).…”

“…Our recordings from bipolar cells showed that TTX did not affect their light-evoked EPSCs, indicating that photoreceptor to bipolar cell transmission did not depend on regenerative sodium currents. Consistent with these findings, Cook and McReynolds (1998) showed that TTX did not affect voltage responses to light in second-order retinal neurons in salamander.…”

“…These reports demonstrated that lateral inhibition in the IPL contributed a significant fraction of surround inhibition of third-order neurons in amphibian and mammalian retinas (Bloomfield and Xin 2000;Cook and McReynolds 1998a;Cook et al 1998;Taylor 1999). This surround had a postsynaptic component arising from TTXsensitive, inhibitory inputs mediated by ganglion cell GABA A receptors (Bieda and Copenhagen 1999;Cook and McReynolds 1998a;Flores-Herr et al 2001;Lukasiewicz and Shields 1998).…”

“…Several types of amacrine cell in salamander retina use sodium action potentials to propagate lateral inhibitory signals in the IPL (Cook and McReynolds 1998a;Cook and Werblin 1994;Cook et al 1998). Inhibitory signaling to bipolar cell terminals in the inner retina is mediated by GABAergic amacrine cells (Lukasiewicz and Shields 1998;.…”