Abstract

Electrical stimulation of the medial mesencephalic central gray produces a strong rewarding effect, and laboratory animals can easily be trained to produce an operant response to obtain such a stimulation. Previous studies have shown that the rewarding effect of mesencephalic electrical stimulation is potentiated by intracerebroventricular and by ventral mesencephalic microinjections of neurotensin (NT); these findings suggest that this neuropeptide may be an important component of the mesencephalic reward-relevant pathway. The present work was aimed at testing this hypothesis by studying the effects of systemic administration of two selective NT antagonists, SR-48692 and SR-142948a, on responding for medial mesencephalic electrical stimulation. The curve-shift method adapted to operant responding for brain stimulation reward was used to assess reward and performance changes following intra-peritoneal injections of four doses (40, 80, 160, and 3000 og/kg) of SR-48692, three doses (40, 160, 640 og/kg) of SR-142948a, and the vehicle. Results show that SR-48692 and SR-142948a did not alter rewarding effectiveness of the stimulation nor the maximal rates of operant responding at any of the doses tested. These results raised the following hypotheses: (1) NT is not a component of the reward-relevant pathway activated by mesencephalic electrical stimulation; (2) reward-induced by mesencephalic electrical stimulation is not under tonic control of endogenous NT; and (3) NT is a component of the reward-relevant pathway but it acts through subtype(s) of NT receptor(s) that is/are not antagonized by SR-48692 and SR-142948a.