GluR23Y, a biologically active peptide, was utilized in an ELISA-based cell-surface assay to assess insulin-stimulated endocytosis of native AMPA receptors in cultured hippocampal neurons. GluR23Y effectively inhibited the insulin-induced decrease in receptor presence. The inhibition of insulin effects was achieved by introducing GluR23Y into neurons through fusion with the membrane transduction domain of HIV-1 TAT.