Fig. 5.

A model describing the neural circuitries by which the vSub may modulate DA neuron activity. Stimulation of the vSub by infusions of NMDA activates glutamatergic afferents to the NAc. This in turn is proposed to inhibit the activity of neurons in the VP. As a result, there is a removal of tonic inhibition of neurons in the VTA. In this model, we propose that the VP–VTA projection provides sufficient inhibitory influence to hold a subpopulation of DA neurons in an inactive state. Thus, inhibition of VP firing would lead to an increase in the number of VTA DA neurons firing spontaneously. The question mark beside the VP to VTA path indicates that, although this pathway is known to be present in the rat, its involvement in this response remains speculative at the present time.