Recurrent axon collaterals of corticothalamic projection neurons in rat primary somatosensory cortex contribute to excitatory and inhibitory feedback-loops
Abstract
Intrinsic circuitry within the primary somatosensory cortex of the rat was examined in a combined light and electron microscope study. Corticothalamic projection neurons were retrogradely labeled by applying Phaseolus vulgaris leucoagglutinin (PHA-L) into the ventro-posteromedial thalamic nucleus (VPM). Most labeled neurons were pyramidal cells of layer VI. Postsynaptic targets of recurrent axon collaterals originating from these neurons were assessed in layers IV and V. Single labeled cells, complete with recurrent collaterals, could be isolated in "barrels" in which no anterograde transport had taken place. These findings were confirmed by first eliminating thalamocortical projections from the VPM with kainic acid and then applying PHA-L into the same nucleus. This procedure led to selective retrograde accumulation of tracer in layer VI pyramidal cells. Reconstructed portions of labeled axonal trees reached layer IV, bringing numerous boutons to layers IV, V and VI. The boutons had characteristic drumstick-like shapes. In order to identify postsynaptic targets, 4 sections of axons stemming from 3 neurons were reembedded and serially sectioned for electron microscopy. The ultrastructure of 72 asymmetric synapses, all belonging...Continue Reading
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