The GluN2 subunits that compose N-methyl-d-aspartate receptors (NMDARs) are attractive drug targets for therapeutic intervention in several diseases, in particular Parkinson's disease (PD). The precise roles and possible dysfunctions of NMDARs attributed to specific GluN2 subunits are however unresolved. Through the use of CIQ, a novel positive allosteric modulator of GluN2C/GluN2D-containing NMDARs, we have examined the functions and dysfunctions of NMDARs made of GluN2D in the striatum of control mice and of the 6-hydroxydopamine (6-OHDA)-lesioned mouse model of PD. We found that CIQ (20?M), applied to corticostriatal brain slices, increased the firing rate of spontaneously active cholinergic interneurons in the striatum of control mice and in the intact striatum of 6-OHDA-lesioned mice. CIQ also presynaptically depressed GABAergic neurotransmission through a cholinergic mechanism, but had no effect on glutamatergic neurotransmission, in medium spiny projection neurons (MSNs) of control and intact striatum. In the dopamine-depleted striatum, the effect of CIQ on the firing of cholinergic interneurons and GABAergic neurotransmission was lost. However, CIQ increased glutamatergic neurotransmission in MSNs. We also found that the protein levels of GluN2D were increased in the dopamine-depleted striatum as compared to the intact striatum. However, the contribution of GluN2D-containing NMDARs to whole-cell NMDA currents was reduced in cholinergic interneurons and increased in MSNs. These results demonstrate an impaired modulatory role of GluN2D-containing NMDARs on the activity of cholinergic interneurons and inhibitory transmission in the dopamine-depleted striatum. However, potentiation of excitatory neurotransmission occurs upon activation of these receptors. Thus, altered functions of GluN2D-containing NMDARs might contribute to adaptive changes in experimental Parkinsonism.
The GluN2 subunits that compose NMDA receptors (NMDARs) determine functional and pharmacological properties of the receptor. In the striatum, functions and potential dysfunctions of NMDARs attributed to specific GluN2 subunits have not been clearly elucidated, although NMDARs play critical roles in the interactions between glutamate and dopamine. Through the use of amperometry and field potential recordings in mouse brain slices, we found that NMDARs that contain the GluN2D subunit contribute to NMDA-induced inhibition of evoked dopamine release and of glutamatergic neurotransmission in the striatum of control mice. Inhibition is likely mediated through increased firing in cholinergic interneurons, which were shown to express GluN2D. Indeed, NMDA-induced inhibition of both dopamine release and glutamatergic neurotransmission is reduced in the presence of muscarinic receptor antagonists and is mimicked by a muscarinic receptor agonist. We have also examined whether this function of GluN2D-containing NMDARs is altered in a mouse model of Parkinson's disease. We found that the inhibitory role of GluN2D-containing NMDARs on glutamatergic neurotransmission is impaired in the 6-hydroxydopamine lesioned striatum. These results identify a role for GluN2D-containing NMDARs and adaptive changes in experimental Parkinsonism. GluN2D might constitute an attractive target for the development of novel pharmacological tools for therapeutic intervention in Parkinson's disease.
Age-related differences in various acute physiological and behavioral effects of alcohol have been demonstrated in humans and in other species. Adolescents are more sensitive to positive reinforcing properties of alcohol than adults, but the cellular mechanisms that underlie such a difference are not clearly established. We, therefore, assessed age differences in the ability of ethanol to modulate glutamatergic synaptic transmission in the mouse nucleus accumbens (NAc), a brain region importantly involved in reward mechanisms. We measured field excitatory postsynaptic potentials/population spikes (fEPSP/PS) in NAc slices from adolescent (22-30 days old) and adult (5-8 months old) male mice. We found that 50mM ethanol applied in the perfusion solution inhibits glutamatergic neurotransmission in the NAc of adolescent, but not adult, mice. This effect is blocked by the gamma-aminobutyric acid (GABA)A receptor antagonist bicuculline and by the GABAB receptor antagonist CGP 55845. Furthermore, bicuculline applied alone produces a stronger increase in the fEPSP/PS amplitude in adult mice than in adolescent mice. Activation of GABAA receptors with muscimol produces a stronger and longer lasting depression of neurotransmission in adolescent mice as compared with adult mice. Activation of GABAB receptors with SKF 97541 also depresses neurotransmission more strongly in adolescent than in adult mice. These results demonstrate that an increased GABA receptor function associated with a reduced inhibitory tone underlies the depressant action of ethanol on glutamatergic neurotransmission in the NAc of adolescent mice.
Natural rewards and addictive drugs are believed to exert their reinforcing actions by influencing synaptic plasticity in reward-related brain regions such as the nucleus accumbens (NAc). Long-lasting changes in the efficacy of excitatory synaptic transmission in the NAc are critically dependent on efficient interactions between the dopaminergic and the glutamatergic neurotransmitter systems. Potential targets to the actions of dopamine and of addictive drugs include the GluN2 subunits that compose the N-Methyl-D-Aspartate (NMDA) type of glutamate receptors. However, the ability of dopamine to induce synaptic plasticity by modulating specific subunits of the NMDA receptor has not been examined. The present study shows that in the mouse NAc, dopamine produces a long-lasting depression of NMDA responses which occludes long-term depression (LTD) induced by high frequency stimulation (HFS) of glutamatergic fibers. LTD induced by dopamine or by HFS does not involve a change in the subunit composition of NMDA receptors. Although GluN2B contributes to synaptic responses in the NAc and is affected by dopamine, this subunit might not be a direct target to the actions of dopamine. The results, however, identify a critical role for GluN2A in dopamine-induced and HFS-induced synaptic plasticity. This study suggests a possible mechanism of action for dopamine in the regulation of reward-related behaviors.
A major challenge in neuroscience is to resolve the connection between gene functionality, neuronal circuits, and behavior. Most, if not all, neuronal circuits of the adult brain contain a glutamatergic component, the nature of which has been difficult to assess because of the vast cellular abundance of glutamate. In this study, we wanted to determine the role of a restricted subpopulation of glutamatergic neurons within the forebrain, the Vglut2-expressing neurons, in neuronal circuitry of higher brain function. Vglut2 expression was selectively deleted in the cortex, hippocampus, and amygdala of preadolescent mice, which resulted in increased locomotor activity, altered social dominance and risk assessment, decreased sensorimotor gating, and impaired long-term spatial memory. Presynaptic VGLUT2-positive terminals were lost in the cortex, striatum, nucleus accumbens, and hippocampus, and a downstream effect on dopamine binding site availability in the striatum was evident. A connection between the induced late-onset, chronic reduction of glutamatergic neurotransmission and dopamine signaling within the circuitry was further substantiated by a partial attenuation of the deficits in sensorimotor gating by the dopamine-stabilizing antipsychotic drug aripiprazole and an increased sensitivity to amphetamine. Somewhat surprisingly, given the restricted expression of Vglut2 in regions responsible for higher brain function, our analyses show that VGLUT2-mediated neurotransmission is required for certain aspects of cognitive, emotional, and social behavior. The present study provides support for the existence of a neurocircuitry that connects changes in VGLUT2-mediated neurotransmission to alterations in the dopaminergic system with schizophrenia-like behavioral deficits as a major outcome.
Long-term changes in the efficacy of glutamatergic synaptic transmission in reward-related brain regions such as the nucleus accumbens (NAc) are proposed to contribute to neuroadaptations that lead to drug addiction. Although alcohol is a widely used addictive substance, the cellular mechanisms by which it influences synaptic plasticity in the NAc are not elucidated. We therefore examined whether acute ethanol (EtOH) alters long-term potentiation (LTP) in the core region of the NAc and investigated the possible underlying mechanisms.
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