Differences in GluN2B-containing NMDA receptors result in opposite long-term plasticity and dopaminergic modulation at ipsilateral vs. contralateral cortico-striatal synapses

Abstract

Excitatory neurons in the primary motor cortex project bilaterally to the striatum. However, whether synaptic structure and function in ipsilateral and contralateral cortico-striatal pathways is identical or different remains largely unknown. Here, we describe that excitatory synapses in the contralateral pathway have higher levels of NMDA-type of glutamate receptors (NMDARs) than those in the ipsilateral pathway, although both synapses utilize the same presynaptic vesicular glutamate transporter. We also show that NMDARs containing the GluN2B subunit, but not GluN2A, contribute to this difference. The altered NMDAR subunit composition in these two pathways results in opposite synaptic plasticity: long-term depression in the ipsilateral pathway and long-term potentiation in the contralateral pathway. Furthermore, we demonstrate that activation of D1 and D2 dopamine (DA) receptors by either selective pharmacological agonists or light-induced release of endogenous DA have no effect on NMDAR-mediated neurotransmission in either pathway. However, blocking basal DAergic tone with either D1 or D2 with selective antagonists revealed that GluN2B-containing NMDARs are modulated by D1 receptors in the contralateral pathway and by D2 receptors in the ipsilateral pathway. Such distinct modulatory actions seem to be permissive rather than sufficient for the induction of long-term synaptic plasticity. Altogether, our results provide novel and unexpected evidence for the lack of bilaterality of NMDAR-mediated synaptic transmission at cortico-striatal pathways due to differences in the expression of GluN2B subunits, which results in differences in bidirectional synaptic plasticity and modulation by dopaminergic inputs.