L-arginine potentiates GABA-mediated synaptic transmission by a nitric oxide-independent mechanism in rat dopamine neurons

Effect of L-arginine in the nervous system are often attributed to nitric oxide. Using whole-cell patch pipettes to record membrane currents in voltage-clamp from dopamine neurons in the rat midbrain slice, the present studies found that L-arginine potentiates GABA-dependent membrane currents via a nitric oxide-independent mechanism. L-Arginine (0.3 10 mM) increased the peak amplitude, half-width duration and time constant of decay of GABA(B) receptor-mediated inhibitory postsynaptic currents in a concentration- dependent manner. In the presence of CGP 35348 (300 μM), a GABA(B) receptor antagonist, L-arginine also prolonged the duration of inhibitory postsynaptic currents mediated by GABA(A) receptors, but their amplitudes were reduced. L- Arginine (10mM) also evoked 17±3 pA of outward current (at -60 mV) which was significantly increased in the presence of exogenous GABA (100 μM). Pressure-ejection of GABA from micropipette produced outward currents mediated by GABA(B) receptors (recorded in bicuculline) or GABA(A) receptors (recorded in CGP 35348): both types of receptor-mediated currents were increased by L-arginine (10mM). In contrast, outward currents evoked by baclofen, a GABA(B) receptor agonist, were not potentiated by L-arginine. The GABA transport inhibitors NO 711 (1 μM) and nipecotic acid (1 mM) significantly increased the half-width duration and time-constant of decay of GABA(B)-mediated inhibitory postsynaptic currents, thus mimicking effects of L-arginine. However, nitric oxide donors failed to mimic effects of L- arginine on GABA(B) inhibitory postsynaptic currents, and inhibitors of nitric oxide synthesis failed to selectively block the action of L-arginine. These findings suggest that L-arginine potentiates GABA synaptic transmission by a nitric oxide-independent mechanism. Similarities between effects of L- arginine, NO 711 and nipecotic acid suggest that L-arginine inhibits a GABA transporter.