The long-term goal is to understand the cellular and molecular mechanisms linking sensory experience during early life to maturation of excitatory synapses in the somatosensory cortex. The maturation of cortical synapses can be detected biophysically by a switch from EPSP depression following repetitive presynaptic action potentials (immature) to EPSP facilitation (more mature). The general working hypothesis is that the molecular composition of postsynaptic structures correlates with and confers the degree of maturity upon the presynaptic axons. We will test this hypothesis by combining patch-recording of multiple, synaptically connected neurons within rat somatosensory cortical slices with electron microscopic (EM)- immuno-cytochemical (ICC) analysis of the recorded neurons to determine whether: (1) the more mature, facilitating synapses exhibit the NMDA receptor (NMDAR) subunits-NR1, NR2A, as well as the AMPA receptors and neuronal nitric oxide synthase (nNOS) at postsynaptic densities; (2) the immature, depressing synapses are characterized by 'pioneer' NMDARs that arrive to the plasma membrane first, along with neuronal nNOS via PSD-95; (3) activation of these pioneer NMDARs regulate recruitment of cytoplasmic NMDAR and AMPA receptor subunits to nascent postsynaptic sites; (4) pharmacological blockage of NMDAR will prevent the insertion of NR1/NR2A heteromers of NMDAR and AMPA receptors and also delay or abolish the switch at synapses from the depressing to the facilitating phenotype. The works of Aoki and Reyes indicate that synapse maturity can vary widely within single layers and even within single neurons. Thus, the combined EM, ICC and biophysical analysis of single synapses and single postsynaptic densities should be particularly helpful in elucidating functional links between ultrastructure, molecular composition, and physiological properties of excitatory synapses that form during early postnatal life in the somatosensory cortex and dictate life-long capacities of cortical neural function. The knowledge gained from such a study is required in designing molecular remedies for deficits caused by sensory deprivation during early life.
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