Ventral Tegmentum - Reward System

Reward System

The dopamine reward circuitry in the human brain involves two projection systems from the ventral midbrain to the nucleus accumbens-olfactory tubercle complex. First, the posteromedial VTA and central linear raphe cells selectively project to the ventromedial striatum, which includes the medial olfactory tubercle and the medial NAC shell. Secondly, the lateral VTA largely projects to the ventrolateral striatum, which includes the NAC core, the medial NAC shell, and the lateral olfactory tubercle. These pathways are respectively called the meso-ventromedial and the meso-ventrolateral striatal dopamine systems. The medial projection system is important in the regulation of arousal characterized by affect and drive and plays a different role in goal-directed behavior than the lateral projection system. Unlike the lateral part, the medial one is activated not by rewarding but by noxious stimuli. Therefore, the NAC shell and the posterior VTA are the primary areas involved in the reward system.

Normally, the dopaminergic neurons are only phasically active. When they are excited they fire a barrage of action potentials and dopamine is released in the NAC. The medium spiny neurons of the NAC are much more responsive to this increase in dopamine if there is coincident excitatory input form the telencephalic structures such as the amygdala and orbital-medial prefrontal cortex. The activated striatal neurons (NAC neurons) then project to the ventral pallidum where they inhibit the inhibitory GABA neurons. This inhibition in the pallidum disinhibits the thalamic target of the limbic loop, which is the mediodorsal nucleus. The thalamus then innervates the cortical division of the limbic forebrain. This final connection is reinforced by activity in direct cortical projections from the dopaminergic neurons of the VTA.