Regulation of unit activity in nucleus reticularis thalami by the mesencephalic reticular formation and the frontal granular cortex.

Recruiting responses and related synchronous activities appear to be mediated by thalamic inhibition originating in nucleus reticularis thalami, a structure jointly regulated by an ascending projection from mesencephalic reticular formation and a descending influence from the frontal cortex. Extracellular unit activity was recorded in the anterior nucleus reticularis thalami (RVA) during recruiting responses, augmenting responses, stimulation of the mesencephalic reticular formation (MRF), and cryogenic blockade of the inferior thalamic peduncle (ITP). During recruiting responses, RVA units responded to medial thalamic (MT) stimulation with prolonged high frequency bursts. Analysis of the post-stimulus time histograms of these responses showed tham to have the same latency, duration, incrementing character, and envelope shape as the phasic thalamic inhibitory postsynaptic potentials (IPSPs) which appear to mediate recruiting responses. Brief stimulation of the MRF, which abolishes recruiting responses and thalamic IPSPs, prevented the response of RVA units to MT stimuli, and inhibited the spontaneous discharge of these units for 20 sec or more. We propose that the desynchronizing effect of MRF activation results from the abolition of thalamic inhibition originating in RVA. Cryogenic blockade of the ITP, which abolishes recruiting responses in the thalamus and cortex, also prevented R units from responding to MT stimuli. This result suggests that the MT activates R units via a thalamo-frontocortico-R pathway and explains the long latency of R bursts and thalamic IPSPs following MT stimulation. R cells that fired prolonged bursts during recruiting responses did not respond during augmenting responses. This result suggests that separate thalamic inhibitory mechanisms are involved in these two types of synchronization.