GABA spillover from single inhibitory axons suppresses low-frequency excitatory transmission at the cerebellar glomerulus.

GABA type B receptors (GABA(B)-Rs) are present on excitatory terminals throughout the CNS, but surprisingly little is known about their role in modulating neurotransmission under physiological conditions. We have investigated activation of GABA(B)-Rs on excitatory terminals within the cerebellar glomerulus, a structure where glutamatergic excitatory and GABAergic inhibitory terminals are in close apposition and make axodendritic synapses onto granule cells. Application of the GABA(B)-R agonist baclofen depressed evoked mossy fiber EPSCs by 54% at 1 Hz. The amplitude of miniature EPSCs recorded in tetrodotoxin was unchanged in the presence of baclofen, but the frequency was significantly reduced, indicating a purely presynaptic action of baclofen under our recording conditions. At physiological temperature (37 degrees C) presynaptic GABA(B)-Rs were not tonically activated by spontaneous GABA release from Golgi cells, which fire at approximately 8 Hz in slices at this temperature. However, tonic activation could be induced by blocking GABA uptake or by lowering temperature. GABA(B)-Rs were activated at physiological temperature when Golgi cell firing was increased above the basal level by stimulating a single inhibitory Golgi cell input at 50 Hz, suppressing the mossy fiber-evoked EPSC by 24% at 1 Hz. Furthermore, glutamate release was selectively inhibited at low-frequency mossy fiber inputs (<10 Hz) during Golgi cell stimulation. Our findings suggest that GABA spillover in the glomerulus modulates sensory input to the cerebellar cortex.