Mechanisms by which Dopamine Receptors May Influence Synaptic Plasticity

While dopamine (DA) receptors mediate acute effects of amphetamine and cocaine, chronic drug administration produces many glutamate-dependent adaptations, including LTP in reward-related neuronal circuits. An important question presents itself: How do DA receptors influence glutamate-dependent synaptic plasticity? Alterations in AMPA receptor phosphorylation and trafficking are critical for LTP. We hypothesize that D1 DA receptors modulate these processes, that chronic drug-induced adaptations in D1 receptor signaling, therefore, trigger compensatory changes in AMPA receptor function, and that this ultimately contributes to inappropriate plasticity in addiction-related neuronal circuits. Postnatal rat nucleus accumbens (NAc) cultures were used to study D1 receptor regulation of the AMPA receptor subunit GluR1. We found that D1 receptor stimulation enhances phosphorylation of GluR1 at the protein kinase A (PKA) site. Furthermore, D1 receptor stimulation increases GluR1 surface expression by increasing the rate of GluR1 externalization. The latter effect is prevented by the PKA inhibitors KT5720 and RpcAMPS, whereas the PKA activator SpcAMPS increases the rate of GluR1 externalization. These findings indicate that PKA phosphorylation is important in determining AMPA receptor surface expression and suggest a mechanism by which DA-releasing drugs of abuse may directly tap into fundamental mechanisms that enable synaptic plasticity. A limitation of our current model is that there are no intrinsic glutamate neurons in the NAc and thus no glutamate synapses in NAc cultures. To address this problem, we have restored excitatory synaptic inputs to NAc neurons by co-culturing them with prefrontal cortex (PFC) neurons. We are also studying GluR1 trafficking in PFC cultures. In both systems, synaptic AMPA receptors can be defined based on colocalization of GluR1 and the synaptic marker synaptobrevin. Preliminary results suggest that D1 receptor stimulation or PKA activation leads to increased surface GluR1 expression in PFC neurons but not to insertion into synaptic sites.