ADRENOCEPTOR-MEDIATED ELEVATION OF AMBIENT GABA LEVELS ACTIVATES PRESYNAPTIC GABA(B) RECEPTORS IN RAT SENSORIMOTOR CORTEX

At inhibitory synapses in the mature neocortex and hippocampus in vitr o, spontaneous action-potential-dependent and independent release of g amma-aminobutyric acid (GABA) activates postsynaptic GABA(A) receptors but not pre- or postsynaptic GABA(B) receptors. Elevation of synaptic GABA levels with pharmacological agents or electrical stimulation can cause activation of GABA(B) receptors, but the physiological conditio ns under which such activation occurs need further elucidation. In rod ent sensorimotor cortex, epinephrine produced a depression in the ampl itude of evoked monosynaptic inhibitory postsynaptic currents (IPSCs) and a concomitant, adrenoceptor-mediated increase in the frequency of spontaneous IPSCs. Blockade of GABA(B) receptors prevented the depress ion of evoked IPSC amplitude by epinephrine but did not affect the inc rease in spontaneous IPSC frequency. These data show that adrenoceptor -mediated increases in spontaneous IPSCs can cause activation of presy naptic GABA(B) receptors and indirectly modulate impulse-related GABA release, presumably through elevation of synaptic GABA levels.