Corticothalamic inhibition in the thalamic reticular nucleus

Mutual inhibition between the GABAergic cells of the thalamic reticular nucleus (RTN) is important in regulating oscillations in the thalamocortical network, promoting those in the spindle range of frequencies over those at lower frequencies. Excitatory inputs to the RTN from the cerebral cortex are numerically large and particularly powerful in inducing spindles. However, the extent to which corticothalamic influences can engage the inhibitory network of the RTN has not been fully explored. Focal electrical stimulation of layer VI in the barrel cortex of the mouse thalamocortical slice in vitro resulted in prominent di- or polysynaptic inhibitory postsynaptic currents (IPSCs) in RTN cells under the experimental conditions used. The majority of cortically induced responses consisted of mixed PSCs in which the inhibitory component predominated or of large IPSCs alone, implying inhibition of neighboring cells by other, cortically excited RTN cells. Within the mixed PSCs, fixed and variable latency components could commonly be identified. IPSCs could be blocked by application of ionotropic glutamate receptor antagonists or of GABA(A) receptor antagonists, also indicating their dependence on corticothalamic excitation triggering disynaptic or polysynaptic inhibition. Spontaneous GABA(A) receptor-dependent IPSCs were routinely observed in the RTN and, taken together with the results of cortical stimulation, indicate the existence of a substantial network of intrareticular inhibitory connections that can be effectively recruited by the corticothalamic system. These results suggest activation of cortical excitatory inputs triggers the propagation of inhibitory currents within the RTN and support the view that activation of the RTN from the somatosensory cortex, although focused by the topography of the corticothalamic projection, is capable of disynaptically engaging the whole inhibitory network of the RTN, by local and probably by reentrant GABA(A) receptor-based synapses, thus spreading the corticothalamic influence throughout the RTN.