Modulation of striatal neuronal activity by glutamate and GABA: iontophoresis in awake, unrestrained rats

To examine the effects of glutamate (GLU) and gamma-aminobutyric acid (GABA) and their interactions in the striatum under behaviorally relevant conditions, single-unit recording was combined with microiontophoresis in awake, unrestrained rats. Iontophoretically applied GLU (0-40 nA, 20 s) excited all spontaneously active neurons in dorsal (caudate-putamen) and ventral (accumbens, core) striatum; phasic GLU-induced excitations (mean threshold 19.7 nA) were dose-dependent, inversely correlated with rate of basal activity (excitation limit approximately 65 imp/s), and highly stable during repeated GLU applications. GLU also excited silent and sporadically active units, which greatly outnumbered spontaneously active cells, and enhanced neuronal excitations associated with movement. Both spontaneously active and GLU-stimulated striatal neurons were highly sensitive to GABA (0-40 nA, 20 s); most showed short-latency inhibitions during GABA diffusion from the pipette (0 nA) and the response quickly progressed to complete silence with a small increase in current. The GABA-induced inhibition was current-dependent, equally strong on spontaneously active and GLU-stimulated units, and independent of neuronal discharge rate, but less stable than the GLU-induced excitation during repeated drug applications. Prolonged GABA application (0-20 nA, 2-4 min) reduced basal impulse activity, but was less effective in attenuating the neuronal excitations induced by GLU or associated with movement. Our data support the role of GLU afferents in the phasic activation of striatal neurons and suggest that the effects of GLU strongly depend on the level of ongoing neuronal activity. The ability of GABA to modulate both basal and GLU-evoked activity suggests that GABA, released from efferent collaterals and interneurons, plays a critical role in regulating neuronal activity and responsiveness to phasic changes in excitatory input.