This review examines dopamine (DA) and glutamate receptor interactions in the neostriatum (NS) primarily from a neurophysiological perspective. Historically, a clear understanding of the function of DA in the NS has been difficult because it was considered a classical neurotransmitter with either excitatory or inhibitory actions and because many of the data were obtained by use of varying methodologies. When DA is considered a neuromodulator whose role is to alter how NS cells respond to glutamatergic inputs, many of its actions can be accounted for and predicted with great accuracy within a model of receptor subtype. In this model, DA via activation of D1 receptors potentiates responses mediated by activation of N-methyl-D-aspartate (NMDA) receptors. DA via activation of D2 receptors attenuates responses mediated by activation of non-NMDA receptors. Outcomes of combinations of NMDA and D2 and non-NMDA and D1 receptors are not as predictable. The mechanisms underlying the D1-NMDA receptor interactions appear to involve alterations in cell excitability mediated by activation of Ca²⁺ conductances and/or phosphorylation of NMDA receptors. Less is known about mechanisms underlying the D2-non-NMDA receptor interaction. The functional implications of this model in setting membrane potentials, signal-to-noise ratio, plasticity and excitotoxicity are discussed.

Dopamine (DA), via activation of D1 receptors, enhances N-methyl-d-aspartate (NMDA)-evoked responses in striatal neurons. The present investigation examined further the properties of this enhancement and the potential mechanisms by which this enhancement might be effected. Dissociated medium-sized striatal neurons were obtained from intact rats and mice or mutant mice lacking the DA and cyclic adenosine 3′,5′ monophosphate (cAMP)-regulated phosphoprotein of M _R 32,000 (DARPP-32). NMDA (10–1,000 μM) induced inward currents in all neurons. In acutely dissociated neurons from intact rats or mice, activation of D1 receptors with the selective agonist, SKF 81297, produced a dose-dependent enhancement of NMDA currents. This enhancement was reduced by the selective D1 receptor antagonist SKF 83566. Quinpirole, a D2 receptor agonist alone, produced small reductions of NMDA currents. However, it consistently and significantly reduced the enhancement of NMDA currents by D1 agonists. In dissociated striatal neurons, in conditions that minimized the contributions of voltage-gated Ca²⁺ conductances, the D1-induced potentiation was not altered by blockade of L-type voltage-gated Ca²⁺ conductances in contrast to results in slices. The DARPP-32 signaling pathway has an important role in D1 modulation of NMDA currents. In mice lacking DARPP-32, the enhancement was significantly reduced. Furthermore, okadaic acid, a protein phosphatase 1 (PP-1) inhibitor, increased D1-induced potentiation, suggesting that constitutively active PP-1 attenuates D1-induced potentiation. Finally, activation of D1 receptors produced differential effects on NMDA and gamma aminobutyric acid (GABA)-induced currents in the same cells, enhancing NMDA currents and inhibiting GABA currents. Thus simultaneous activation of D1, NMDA, and GABA receptors could predispose medium-sized spiny neurons toward excitation. Taken together, the present findings indicate that the unique potentiation of NMDA receptor function by activation of the D1 receptor signaling cascade can be controlled by multiple mechanisms and has major influences on neuronal function.