The striatum is a subcortical structure in the brain that is involved in movement, habit, and reward. Striatal medium spiny neurons receive cortical glutamatergic and mesolimbic dopaminergic afferents. Calcineurin and its relationship to glutamate and dopamine signaling were examined within striatum. Two members of the calcineurin signaling pathway were studied: Regulator of Calmodulin Signaling (RCS) and Nuclear Factor of Activated T-cells (NFAT), which are upstream and downstream of calcineurin, respectively.
RCS is a striatally-abundant protein that is phosphorylated by Protein Kinase A (PKA) at Ser55. Phosphorylation at Ser55 enables RCS to bind to and sequester calmodulin and thereby inhibit calcineurin. The role of RCS in striatum was assessed using wild-type and knockout mice in a paradigm measuring instrumental responding for food reinforcement. While RCS knockout mice showed normal acquisition of food-motivated behavior, they exhibited a lower breakpoint on a progressive ratio schedule. In addition, behaviors reliant on the amygdala were examined. RCS knockout mice displayed decreased exploration in the open arms of the plus maze and in the center of an open field, suggestive of enhanced anxiety-like behavior. Biochemical studies revealed a reduction in the levels of Dopamine and cAMP-Regulated Phosphoprotein (DARPP-32) and of the GluR1 glutamate receptor in RCS knockout mice striata. As DARPP-32 and GluR1 are important in reward-mediated behavior and GluR1 is a key regulator of synaptic plasticity, this supports behavioral data indicating impaired striatal function.
Nuclear Factor of Activated T-cells (NFAT) proteins are calcium-dependent transcription factors that are activated when dephosphorylated by calcineurin. In primary striatal cultures, glutamate was found to act on AMPA/kainate receptors via calcineurin, while dopamine was found to act on D1 receptors independently of calcineurin. In addition, it was determined that cAMP-dependent signaling induced NFAT nuclear translocation. This likely involved a novel signaling pathway involving Glycogen Synthase Kinase 3 inactivation. In addition, a synergy in NFAT activation was observed when glutamate and dopamine were co-applied. Regulation of NFAT by glutamate and dopamine may play a role in striatal plasticity. Together these two studies provide a basis for the complex interplay between calcineurin, glutamate, and dopamine signaling within the striatum.
|Advisor:||Nairn, Angus C.|
|School Location:||United States -- Connecticut|
|Source:||DAI-B 70/06, Dissertation Abstracts International|
|Keywords:||Calcineurin, Calmodulin, Medium spiny neurons, NFAT, Striatum|
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