Differential contribution of mesoaccumbens and mesohabenular dopamine to intracranial self-stimulation.

The contribution of mesoaccumbens dopamine transmission to intracranial self-stimulation is well-established. However, although the nucleus accumbens comprises two main subregions, the shell and the core, little is known of the contribution of each to this behaviour. Our first aim was to study the effects of d-amphetamine infusions into the shell and core in order to understand their relative importance to reward and operant responding. Our second aim was to examine the contribution of a lesser studied group of dopamine neurons, those within the mesohabenular pathway, to intracranial self-stimulation. Male Sprague-Dawley rats were implanted with bilateral cannulae in the nucleus accumbens shell, core or in the lateral habenula and a monopolar stimulation electrode in the posterior mesencephalon, a brain site that is sensitive to changes in dopamine transmission. Using curve-shift scaling, we measured the reward- and performance-enhancing effects of intra-accumbens (1-20 μg) and intra-habenular (10-40 μg) infusions of d-amphetamine or vehicle. Within the nucleus accumbens, the use of multiple doses and long test sessions allowed us to observe an interaction between drug effect and infusion site. We show, for the first time, differences in the minimal doses necessary to enhance rewarding effectiveness and operant responding, in the magnitude of these enhancements as well as in their duration. Conversely, regardless of dose, intra-habenular D-amphetamine did not alter rewarding effectiveness or operant rate, highlighting the differential contribution of mesoaccumbens and mesohabenular dopamine pathways to intracranial self-stimulation.