Reward versus performance in self-stimulation: electrode-specific effects of alpha-methyl-p-tyrosine on reward in the rat.

The hypothesis that alpha-methyl-p-tyrosine (AMPT), an inhibitor of catecholamine synthesis, reduces brain stimulation reward was tested, using a measure of reward previously shown to be relatively unaffected by variables that alter performance but not reward. The rewarding effectiveness of stimulation was determined by the location of the sharp rise in the function relating running speed in an alley to the number of pulses received as a reward. For some electrodes, AMPT depressed self-stimulation performance (speed of running) without producing any sizable effect on the measure of reward (location of rise). For other electrodes, the rewarding effectiveness of the stimulation was greatly reduced by AMPT and restored by L-dopa. These opposing results could be repeatedly demonstrated on different electrodes in the same rat. The electrode-specific differential sensitivity to AMPT suggests neurochemically disparate substrates for reward.