Dopamine gating of forebrain neural ensembles.

Dopamine may exert different actions depending on a number of factors. A common view is that D1 receptors may be responsible for excitatory actions whereas D2 receptors are involved in inhibitory actions. However, this position cannot be reconciled with several findings indicating otherwise. The role of dopamine on forebrain neural ensembles may be better understood in the light of functional states of the system. Pyramidal cortical neurons and striatal medium spiny neurons alternate between two membrane potential states ('up' and 'down') that could shape dopamine actions. It is proposed that D1 receptors can act as state-stabilizers by sustaining up states and thereby facilitating plasticity mechanisms by providing postsynaptic depolarization and increasing NMDA function. In this way, dopamine can sustain activity in depolarized units. This action is accompanied by a decrease in cell firing (perhaps mediated by D2 receptors), which renders the cells responsive only to strong stimuli. The result would be a net increase in signal-to-noise ratio in a selected assembly of neurons.