GABA-EVOKED AND GLUTAMATE-EVOKED RESPONSES IN THE RAT VENTRAL PALLIDUM ARE MODULATED BY DOPAMINE

Microiontophoresis was used to investigate the influence of dopamine o n GABA- and glutamate-induced responses from ventral pallidal neurons recorded extracellularly in chloral hydrate-anaesthetized rats. Modula tion was determined by comparing dopamine-induced alterations in amino acid-induced activity ('signal') with dopamine-induced effects on spo ntaneous firing ('noise'). A dopamine ejection current-response curve was generated to determine the current levels that did not alter spont aneous firing ('subthreshold') and those that produced similar to 50% of the maximal dopamine-induced response (ECur(50)) Co-iontophoresis o f dopamine with GABA generally diminished the inhibitory influence of GABA on pallidal neuron firing; 70% of neurons tested with ECur(50) do pamine demonstrated a decrease in the signal-to-noise ratio whereas 10 % displayed an increase. At subthreshold dopamine ejection currents, 5 9% of neurons responded with a decrease and 18% responded with an incr ease in the GABA signal-to-noise ratio. When ECur(50) dopamine was co- iontophoresed with glutamate, 84% of the neurons displayed a decrease in the signal-to-noise ratio for glutamate-evoked excitations whereas 11% demonstrated an increase. Subthreshold dopamine ejection currents decreased the signal-to-noise ratio in 62% of the ventral pallidal neu rons excited by glutamate and increased the ratio in 23%. These data i llustrate that dopamine substantially alters GABA- and glutamate-evoke d responses even at ejection currents that are below those necessary t o change spontaneous firing. Thus, it appears that neuromodulation is an important means by which dopamine influences Ventral pallidal neuro nal activity.