Relief of synaptic depression produces long-term enhancement in thalamocortical networks.

Thalamocortical synapses may be able to undergo activity-dependent long-term changes in efficacy, such as long-term potentiation. Indeed, studies conducted in vivo have found that theta-burst stimulation (TBS) of the thalamus induces a long-term enhancement (LTE) of field potential responses evoked in the neocortex of adult rodents. Because the thalamus and neocortex form a complex interconnected network that is highly active in vivo, it is possible that a change in thalamic excitability would be reflected in the neocortex. To test this possibility, we recorded from barrel neocortex and applied TBS to the thalamic radiation while the somatosensory thalamus was inactivated with muscimol. Thalamocortical LTE was absent when the thalamus was inactivated, suggesting that changes in thalamic excitability are involved. Single-unit recordings from thalamocortical cells revealed that TBS causes a significant reduction in the spontaneous firing rate of thalamocortical cells. Reducing the spontaneous firing of thalamocortical cells directly enhances the efficacy of the thalamocortical pathway because it relieves the tonic depression of the thalamocortical connection caused by thalamocortical activity. Because these changes in thalamic excitability are triggered by corticothalamic activity, this may be a useful top-down mechanism to regulate afferent sensory input to the neocortex during behavior as a function of experience.