Comparison of synaptic transmission and plasticity between sensory and cortical synapses on relay neurons in the ventrobasal nucleus of the rat thalamus

Relay neurons in the ventrobasal nucleus of the thalamus transmit somatosensory information to the cerebral cortex and receive sensory and cortical (feedback) synaptic inputs via, respectively, medial lemniscal (ML) and corticothalamic (CT) fibres. Here, we report that calcium-permeable AMPA receptors are expressed at CT synapses, but not ML synapses, and that the NMDA receptor (NMDAR)-mediated/non-NMDAR-mediated synaptic current ratio is significantly larger at CT synapses than at ML synapses. Moreover, NMDAR-dependent LTP and L-type voltage-gated calcium channel-dependent LTD are readily induced at CT synapses, but not ML synapses. In particular, LTD of CT synaptic transmission is induced by spiking of postsynaptic relay neurons in continuous mode, but not burst mode, in current-clamp recordings. These results show that the strength of the cortical input to thalamic relay neurons is selectively subjected to use-dependent modification, which could be a mechanism for regulation of thalamocortical-corticothalamic interactions and the underlying sensory processing. The ventrobasal nucleus (VBN) of the thalamus serves as a major gateway for relaying somatosensory information to the cerebral cortex. In addition to synaptic inputs from sensory afferents, VBN neurons also receive strong cortical feedback. We report differences in the ionotropic glutamate receptor content of synapses of sensory and cortical inputs on VBN neurons, which may account for the observation that the function of cortical synapses is more plastic than that of sensory synapses. These results provide a better understanding of the regulation of the cortex-thalamus interaction and the underlying sensory process.