SPONTANEOUS ACTIVITY IN THE THALAMIC RETICULAR NUCLEUS DURING THE SLEEP-WAKE CYCLE OF THE FREELY-MOVING RAT

Neurons of the somatosensory thalamic reticular nucleus (TRN) were studied by extracellular recordings through the sleep/wake cycle in the unanesthetized, freely-moving rat. All electrophysiologically-identified TRN neurons expressed rhythmic patterns of discharge that altered with shifts in sleep/wake state. During slow wave (SW) sleep, neurons displayed spike-burst discharges in long trains followed by pauses. High-frequency oscillations in auto-correlograms in the spindle-frequency range (approximately 10 Hz) reflected the rhythm of interburst intervals within the trains whereas low-frequency oscillations (0.3-0.2 Hz) displayed the rhythm of intertrain intervals. During rapid eye movement (REM) sleep, a more continuous pattern of spike-burst discharges was prominent, resulting in absence of a detectable, low-frequency rhythm but persistence of spindle-frequency firing. At the transitions between SW and REM sleep, cell discharge was more tonic than during either sleep state and lacked a dominant rhythm. During the wake (AW) state, neurons fired in a single-spike mode that also lacked rhythmicity. Unlike their pattern of discharge, TRN neurons' mean rate of discharge did not distinguish sleep/wake state. The mean discharge rates were: SW, 18.4+/-1.3; REM, 17.4+/-1.2; AW, 22.3+/-2.1 (Hz +/-S.E.M.). Mean discharge rate during transitions from SW to REM sleep (28.6+/-2.1) was significantly higher, however, than during any sleep/wake state. Compelling evidence was lacking for segregation of TRN neurons into discrete populations according to absolute discharge rate. Neurons recorded simultaneously from the same electrode discharged synchronous trains of spike-bursts and pauses during SW sleep. This phenomenon may be related to generation of EEG slow waves. The individual, high-frequency bursts recorded from neuron-pairs in SW and REM sleep were not correlated in time. These results are discussed with respect to the similarities and differences in spontaneous activity of TRN neurons in rat versus cat.