Correlates of motivation and reward

Ikemoto, Satoshi

Abstract

Scientific questions on brain reward systems were prompted by the discovery that animals learn to lever-press for electrical stimulation of certain brain areas, a phenomenon known as intracranial self-stimulation. Since then, numerous studies using anatomy, pharmacology and electrophysiology methods have established dopamine neurons in the ventral tegmental area (VTA) projecting to the nucleus accumbens (NAc) as a key reward substrate. Recent studies using optogenetics confirmed that animals learn to self-stimulate VTA dopamine neurons, suggesting that excitation of dopamine neurons is sufficient in inducing reward. The question remains is how excitation of VTA dopamine neurons influences downstream brain areas. We sought to identify firing patterns that encode dopamine neuron-mediated reward in the NAc. We injected the AAV-ChR2 viruses and implanted optical fibers in the VTA area of TH::Cre mice, a procedure that allowed us to specifically activate dopamine neurons through optical stimulation; a bundle of 8 tetrodes (32 wires) was implanted in the NAc shell for neural activity recording. We found that VTA optical stimulation in freely-behaving mice evoked fast excitatory local field potential (LFP) responses in the NAc, and the amplitudes of this LFP correlated well with the animals self-stimulation rates. Consistent with the LFP activity, 35% of the recorded NAc neurons showed fast phasic excitations, suggesting an excitatory input to the NAc from VTA dopamine neurons. We also recorded neurons that showed phasic inhibitions (17%). To determine whether these firing pattern changes were mediated by dopamine, mice were systemically injected with the dopamine D1 receptor antagonist SCH 23390. Although the antagonist decreased majority basal firing in the NAc, it did not abolish optical stimulation-evoked neural responses, suggesting that transmitters other than dopamine were released by VTA dopamine neurons. In light of recent in vitro studies showing that dopamine neurons can also release glutamate and GABA to depolarize or hyperpolarize post-synaptic neurons, our above results could be explained by the co-release of glutamate or GABA from dopamine neurons. To determine whether fast excitation of NAc neurons evoked by VTA photostimulation is mediated by glutamate release from dopamine neurons and whether co-release of glutamate is important in SS responding, we used the mouse line with a conditional deletion of type 2 vascular glutamate transporter (Vglut2) in dopamine neurons (Vglut2f/f;DAT-Cre) (Birgner et al., 2010). The use of Vglut2f/f;DAT-Cre combined with optogenetics allowed us to selectively stimulate dopamine neurons without glutamate co-release. We injected the Cre-dependent AAV-ChR2 into the VTA of Vglut2f/f;DAT-Cre and DAT-Cre mice. VTA photostimulation significantly increased leverpresses in both Vglut2f/f;DAT-Cre and DAT-Cre mice, and we found no notable difference in the acquisition of SS. The above mentioned Vglut2f/f;DAT-Cre and DAT-Cre mice received implantation of electrodes, and their nucleus accumbens neurons were recorded as a function of VTA photostimulation. As predicted, Vglut2f/f;DAT-Cre neurons displayed little fast excitatory responses compared to those of DAT-Cre mice. These results suggest that the photostimulation-evoked fast response in the accumbens is likely mediated by the co-release of glutamate from the dopamine neuron. Together these results suggest that the fast NAc responses evoked by the VTA photostimulation were not mediated by the D1 or D2 dopamine receptors. In particular, fast excitatory responses appear to be mediated by glutamate release from dopamine neurons. While the functional role of glutamate co-release is unclear, it may not be involved in reinforcement.