Morphine modulation of GABA- and glutamate-induced changes of ventral pallidal neuronal activity

Microiontophoresis was used to investigate the influence of morphine on the GABA- and glutamate-evoked responses of ventral pallidal neurons recorded extracellularly from chloral hydrate-anesthetized rats. Of the GABA-sensitive neurons (50 of 69 tested) in the ventral pallidum, all displayed a decreased firing rate when GABA was applied, whereas all of the glutamate-sensitive neurons (29 of 40 tested) increased neuronal activity in the presence of glutamate. The majority of ventral pallidal cells tested (65 of 83) were sensitive to iontophoretically applied morphine, and both increases and decreases in neuronal activity were observed. The ability of morphine to alter the ratio between amino acid-evoked activity ("signal") and spontaneous firing ("noise") was used as an indicator of morphine modulation. A morphine subthreshold ejection current, i.e. one that did not change spontaneous firing rate, and a morphine ejection current that produced approximately 50% of the maximum opioid-induced neuronal response were chosen for this evaluation. When morphine was co-iontophoresed with GABA or glutamate, attenuation of the amino acid signal-to-noise ratio was generally seen, though some potentiations were observed. These changes were independent of the direction of morphine-induced changes in spontaneous firing rate. Both sub- and suprathreshold ejection currents were capable of affecting GABA- and glutamate-evoked responses. These data suggest that morphine is a robust ventral pallidal neuromodulator. As ventral pallidal amino acid activity is important in the integration of sensorimotor information, opioid modulation of amino acid transmission in the ventral pallidum may have a profound effect on this integration.