 These changes in synaptic plasticity and the accompanying learning is dependent upon activation of striatal D1 and NMDA receptors.The intracellular cascade activated by D1 receptors involves the recruitment of protein kinase A, and through resulting phosphorylation of DARPP-32, the inhibition of phosphatases that deactivate ERK.NMDA receptors activate ERK through a different but interrelated Ras-Raf-MEK-ERK pathway.