GABA(C) receptors control adaptive changes in a glycinergic inhibitory pathway in salamander retina

We studied the role of GABA in adaptive changes in a lateral inhibitory sys tem in the tiger salamander retina. In dark-adapted retinal slice preparati ons picrotoxin caused a slow enhancement of glycine-mediated IPSCs in gangl ion cells. The enhancement of glycinergic IPSCs developed slowly over the c ourse of 5-20 min, even though picrotoxin blocked both GABA(A) and GABA(C) receptors within a few seconds. The slow enhancement of glycinergic IPSCs b y picrotoxin was much weaker in light-adapted preparations. The slow enhanc ement of glycinergic inhibitory inputs was not produced by bicuculline, ind icating that it involved GABA(C) receptors. The responses of ganglion cells to direct application of glycine were not enhanced by picrotoxin, indicati ng that the enhancement was not caused by an action on glycine receptors. I n dark-adapted eyecup preparations picrotoxin caused a slow enhancement of glycinergic IPSPs and transient lateral inhibition produced by a rotating w indmill pattern, similar to the effect of light adaptation. The results sug gest that the glycinergic inhibitory inputs are modulated by an unknown sub stance whose synthesis and/or release is inhibited in dark-adapted retinas by GABA acting at GABA(C) receptors.