The Role Of Striatal NR2B Subunits In Motor Learning And Memory

The striatum, also known as the neostriatum, is a subcortical part of the forebrain. It is the major input station of the basal ganglia system. Striatum is closely related to the physiological function of the motor learning, decision-making, goal-directed learning, reward and stimulation, etc. The key role of the striatum in many neurons disorders can be clearly reflected, for example, Huntington’s syndrome, Parkinson’s disease, obsessive-compulsive disorder, dystonia or drug addiction, etc.The striatum is heterogeneous in terms of neurons. It is mainly composed of Medium spiny neurons (MSNs), making up 96% of the striatum. A huge amount of NMDA receptors express on the MSNs cells. The NMDA receptors are heteromeric complexes consisting of subunit (NR1), and NR2 subunits (A and B). The formation of functional NMDA receptors requires a combination of NR1 subunits and at least one of NR2 subunits, and the type of the NR2 subunit critically determines additional biophysical and pharmacological properties of NMDARs. In the striatum, most of NMDA receptors are NR2B-containing NMDA receptors. Recent studies showed that NMDA receptors in striatum contribute to motor learning. However, the role of NR2B subunits for the striatum plasticity and related to motor learning function is not well characterized. By employing morphological, behavioral, electrophysiological, RNAi and pharmacological techniques, we measured the influences of up-and down-regulation of striatal NR2B subunits on functions of NMDA receptors, cortico-dorsal striatal LTP, and motor learning and memory in NR2B overexpressing transgenic mice and NR2B siRNA mice.The main results are as follows:1. Up-regulation of NR2B subunit expression promoted the motor learning abilities and long term potentiation (LTP) in NR2B Tg mice.(1) Study on NR2B expression in the striatum of NR2B transgenic mice. The data from Real-time PCR and western blot experiment revealed that the expression of NR2B subunits increased in the striatum of the transgenic mice. NR2B transgenic mice did not exhibit any gross anatomical alternations in the cortex and striatum, as determined by Nissl staining. Golgi staining showed that the shapes and architecture of dendritic spines in the striatum are also normal in transgenic mice.(2) Overexpressing of NR2B significantly promoted the motor learning abilities in accelerated rotarod test in NR2B Tg mice. Furthermore, no significant difference was found in locomotor activities, motor coordination abilities or muscle strength in NR2B Tg mice compared to Wt mice.(3) No significant differences were measured in the input-output relationship of synapse responses and paired-pulse response in the path of cortico-dorsal striatum of NR2B transgenic mice, indicating the release of glutamate from presynapse and postsynaptic AMPA receptors are normal.(4) Long-term potentiation was enhanced in dorsomedial striatum of NR2B transgenic mice.(5) Long-term depression was unchanged in dorsolateral striatum of NR2B transgenic mice.(6) The NMDA receptor-dependent EPSP was enhanced in dorsal lateral striatum of NR2B transgenic mice.(7) NR2B transgenic mice exhibited higher amplitude NMDAR-mediated responses recorded under voltage clamp of evoked excitatory postsynaptic currents (eEPSCs) of MSNs cells in dorsomedial striatum.(8) The concentration of dopamine, but not glutamate, was significantly enhanced in striatum of transgenic mice.2. Blocking or down-regulation of NR2B subunit expression of dorsal striatum impaired the motor learning abilities and striatal LTP.(1) Motor learning ability was significantly impaired by intraperitoneal injection or striatum local injection of NR2B antagonist, RO256981 in mice. While no significant difference was found on locomotor activities, motor coordination abilities or muscle strength.(2) The data from Real-time PCR, immunohistochemistry experiment revealed that NR2B siRNA mice were well created.(3) Motor learning ability was impaired in accelerated rotarod test while no significant difference was found on locomotor activities, motor coordination abilities or muscle strength in NR2B siRNA mice.(4) No significant differences were measured in the input-output relationship of synapse responses and paired-pulse depression in dorsomedial striatum of NR2B siRNA, indicating the presynaptic release of glutamate and postsynaptic AMPA receptors are normal.(5) Long-term potentiation was impaired in dorsomedial striatum in NR2B siRNA mice.(6) The NMDA receptor-dependent EPSP was impaired in dorsomedial striatum of NR2B siRNA mice.(7) NR2B siRNA mice showed lower amplitude NMDAR-mediated responses recorded under voltage clamp of evoked excitatory postsynaptic currents (eEPSCs) of MSNs cells in dorsomedial striatum.(8) The concentration of dopamine, was decreased in striatum of NR2B siRNA mice.In summary, up- or down-regulation of NR2B expression in the striatum could enhance or impair motor learning ability in mice, respectively. Our research also proved that the effects were not only through improving or decreasing the activities of NMDA receptor, but also through changing the concentration of dopamine in striatum. The present findings will help us to understand how NR2B subunits play the role in motor learning, and also provide a new clue for curing the disorder of motor learning.