| AMPA-type glutamate receptors (alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate receptors) mediate the majority of fast excitatory neurotransmission in the central nervous system (CNS). Whole-cell electrophysiological techniques were used in order to examine the functional development of AMPA receptor (AMPAR) properties at individual glutamatergic synapses on principal neurons in the CA1 region of the rat hippocampus. Following pharmacological isolation of AMPAR-mediated glutamatergic synaptic currents (evoked by stimulation of the Schaffer Collateral pathway, sEPSCs), we substituted extracellular strontium for calcium to take advantage of the properties of asynchronous EPSCs (aEPSCs) in order to analyze populations of individual synapses. During early postnatal development (P5-P22) at CA1 synapses, we found that for AMPAR-mediated aEPSCs, quantal amplitudes increased while single-channel conductances were relatively consistent across development. aEPSC amplitudes were directly modulated by the number of activated receptors and showed the highest coefficient of variation (CV) at P5-7. AMPAR kinetics (aEPSC taudecay) were variable from P5-P7 but became slower from P8 until P18. As with quantal amplitude, aEPSC tau decay exhibited the greatest CV at P5-7. Further analysis demonstrated that in many neurons at P5-7, two functionally distinct populations of synapses were activated. One population was characterized by fast kinetics, while the other was characterized by slower kinetics similar to those found at P8-18. AMPAR-mediated sEPSCs had greater inward-rectification at ages P5-P7 compared to older ages, suggesting an age-dependent mechanism of calcium regulation with implications for synaptic plasticity. Synapses that utilize faster kinetics may selectively cluster inward-rectifying and likely calcium- permeable AMPARs. These findings support a mechanism of tightly controlled developmental regulation of AMPAR expression: at P5-7 two different types of AMPAR synapses are present within an individual neuron, suggesting functional synaptic receptor heterogeneity, whereas at P8-22 synapses are more uniform. |
