Ventral tegmental analgesia in two strains of rats: effects of amphetamine, naloxone and parachlorophenylalanine

Extracellular single unit responses were recorded from 381 neurons in the neostriatum of urethane-anesthetized rats. Single pulse stimulation of the sensory motor cortex produced strong excitatory responses in neostriatal neurons with a mean onset latency of 7.9 ms. A train of 10 pulses (0.15 ms, 300-600 microA) at 10 Hz delivered to the substantia nigra 100 ms before cortical stimulation attenuated the original excitatory response of 22 of 53 striatal neurons tested and enhanced the excitatory response of 12 of these neurons. These attenuating and enhancing effects were reduced or abolished by the systemic administration of haloperidol, a dopamine antagonist. Single pulse nigral stimulation 100 ms before the cortical stimulation had little or no effect on the excitatory response of neostriatal neurons. The iontophoretic application of dopamine also had attenuation and enhancement effects on the excitatory response of striatal neurons to cortical stimulation. The results suggest that the attenuation and enhancement of the excitatory responses of striatal neurons to cortical stimulation which resulted from nigral conditioning stimulation is mediated by nigrostriatal dopaminergic neurons.