Spillover-mediated transmission at inhibitory synapses promoted by high affinity α₆ subunit GABA(A) receptors and glomerular geometry

Divergence and convergence of synaptic connections make a crucial contribution to the information processing capacity of the brain. Until recently, it was thought that transmitter released at a synapse affected only a specific postsynaptic cell. We show here that spillover of inhibitory transmitter at the Golgi to granule cell synapse produces significant cross- talk to non-postsynaptic cells, which is promoted both by the anatomical specialization of this glomerular synapse and by the presence of the high affinity α₆ subunit-containing GABA(A) receptor in granule cells. Cross- talk is manifested as a novel slow rising and decaying small amplitude inhibitory postsynaptic current (IPSC) that can also contribute a long- lasting component to more typical IPSCs, which is prolonged by inhibition of the neuronal GABA transporter GAT-1. Because of the long duration of IPSCs generated by spillover, the total charge carried is three times that of IPSCs generated by directly connected terminals. GABA spillover within the mossy fiber glomerulus may play an important role in regulating the number of granule cells active in the cerebellar cortex, a regulation that is suggested by theoretical models to optimize cerebellar information processing.