Ventral tegmental self-stimulation selectively induces opioid peptide release in rat CNS.

Intracranial self-stimulation (ICS) is thought to activate neuronal systems involved in processing natural reinforcing agents. Metabolic mapping studies have previously demonstrated a subset of CNS structures specifically engaged by ICS in animals receiving stimulation actively vs. passively. Since opiates are known to enhance ICS behavior and presumably its reinforcing properties, the current study addressed the question of the role of opioid peptides as mediators of ICS. Rats were trained on a fixed ration (FR) 20 schedule of responding maintained by ICS. Following response stabilization, rats were assigned either to an active or a corresponding yoked stimulation group at 1 of 2 schedules of reinforcement (i.e., FR1-YFR1, FR20-YFR20, or sedentary control), and opioid peptide release was inferred from in vivo receptor occupancy. Autoradiographic analyses identified 3 groups of structures. Treatment-induced alterations in occupancy were seen in the medial dorsal nucleus of the thalamus, basolateral amygdala, ventral pallidum, medial habenula, dorsal raphe, posterior hypothalamus, substantia nigra pars compacta, agranular preinsular cortex, and zona incerta. Depending upon the structure, peptide release was dependent upon stimulus contingency (active vs. yoked) and/or schedule (FR1 vs. FR20). Evidence for ICS-induced inhibition of peptide release was found in the habenula and preinsular cortex. Nine additional structures, all components of, or receiving projections from, the limbic system, revealed complex interactions between ICS treatment and the electrode side. Finally, a widespread ipsilateral increase in receptor binding was seen rostrally from the cingulate, olfactory tubercle, and nucleus accumbens, along the lateral hypothalamus and hippocampus, and extending caudally to the substantia nigra and ventral tegmentum. These later effects appear to be related to stimulation-induced changes in blood flow and subsequent ligant presentation increases. Collectively, these data point towards the ability of rewarding brain stimulation to activate discrete neuronal opioid systems contingent upon specific behavioral as well as stimulus conditions.