THE BIDIRECTIONAL INTERACTION BETWEEN VENTRAL TEGMENTAL REWARDING ANDHINDBRAIN AVERSIVE-STIMULATION EFFECTS IN THE RAT

We used the curve-shift procedure in self-stimulating rats to examine the interaction of aversive and rewarding electrical stimuli in terms of duration and direction. The subjects were implanted with two moveab le electrodes, one in a region supporting self-stimulation (the ventra l tegmental area, VTA) and another in a region supporting escape (the nucleus reticularis gigantocellularis, Gi). The function relating self -stimulation rate to pulse frequency (RF function) was first obtained and then replicated in a condition where each VTA pulse was followed 0 .2 or 2.0 ms later by a Gi pulse. The intensity of Gi pulses was set a t a value previously found to elicit escape within less than 5 sec. Th e following observations were made: (1) rats self-stimulated consisten tly, despite the presence of Ci pulses, (2) the presence of Gi pulses shifted the RF function rightward (decreased the rewarding efficacy of VTA stimulation), with little effect on the maximum rate, (3) after 2 to 5 VTA-Gi self-stimulation sessions, the Gi pulses progressively lo st their ability to shift the RF function, and (4) at the end of testi ng, escape was no longer detectable using Gi pulses alone. It was conc luded that (1) the interaction between rewarding VTA and aversive Gi s timulation effects is bidirectional, thus suggesting the presence of a lgebraic summation; (2) the effect of Gi on VTA reward is transient wh ereas that of VTA on Gi aversion cumulates and eventually results in t otal abolition of Gi aversion. The present study represents the first account of cumulative and long-lasting suppression of aversion followi ng brain stimulation in the rat.