Effects of Aging on Properties of the Local Circuit in Rat Primary Somatosensory Cortex (S1) In Vitro

During aging receptive field properties degrade, the ability of the circuit to process temporal information is impaired and behaviors mediated by the circuit can become impaired. These changes are mediated by changes in the properties of neural circuits, particularly the balance of excitation and inhibition, the intrinsic properties of neurons, and the anatomy of connections in the circuit. In this study, properties of thalamorecipient pyramidal neurons in layer 3 were examined in the hindpaw region of rat primary somatosensory cortex (S1) in vitro. Excitatory and inhibitory postsynaptic currents (IPSCs) resulting from trains of electrical stimulation of thalamocortical afferents were recorded. Excitatory postsynaptic currents were larger in old S1, but showed no difference in temporal dynamics; IPSCs showed significantly less suppression across the train in old S1, partly due to a decrease in GABA(B) signaling. Neurons in old S1 were more likely to exhibit burst firing, due to an increase in T-current. Significant differences in dendritic morphology were also observed in old S1, accompanied by a decrease in dendritic spine density. These data directly demonstrate changes in the properties of the thalamorecipient circuit in old S1 and help to explain the changes observed in responses during aging.