Facilitatory and inhibitory processes in the thalamic ventrobasal nucleus of the rat.

Experiments were carried out on anesthetized rats to clarify the actions of the corticothalamic input on the thalamic ventrobasal neuron (VB). The field potentials and the unit activities were evoked in the VB by the electrical stimulation of the contralateral (cSCx) and ipsilateral somatosensory cortex (SCx), the thalamic radiation (TR), and the medial lemniscus (ML). The corticothalamic axons could be activated from cSCx via the ipsilateral SCx, with the result of a clear excitation followed by the inhibition of VB neurons. These VB responses were also evoked from SCx, TR, or even ML with other responses. Paired stimulation of these sites revealed that the corticothalamic input exerted facilitation on the succeeding corticothalamic input for 690 +/- 74.2 ms (n = 5). It was confirmed that these corticothalamic actions were abolished by the lesion of SCx. In contrast to the corticothalamic-induced VB responses, the VB responses to ML input received powerful inhibitory effects from ML or SCx for 1,150 +/- 35.4 ms (n = 5). It is concluded that the corticothalamic input can excite the VB neuron in a mechanism that differs from that of ML input.