Effects of brainstem lesions on tonic immobility in the rabbit (Oryctolagus cuniculus)

Lesions were placed in discrete brainstem areas implicated in the generation of both tonic immobility (T1) and paradoxical sleep (PS) to examine postulated state and event correspondences between these states in the rabbit. Lesions were concentrated in the region of the nucleus locus coeruleus (LC)-an area implicated in the mediation of muscular atonia during PS-but also included other reticular (pontine gigantocellular tegmental field: FTG), and nonreticular (vestibular, cerebellar, central grey, collicular) areas. Polygraphic recordings of EEG, EMG and EOG activities were taken during sleep-waking states and measures of the TI response were obtained before (1 day prior) and after (5 and 14 days) lesions were made. None of the lesions was followed by a sustained, significant variation in either frequency of induction or duration of TI. Following LC lesions, and to a lesser extent after FTG lesions, sleep patterns were fragmented, with a reduction or absence of PS and the occurrence of phasic motor activation at times when PS periods might be expected to occur. The absence of PS and persistence of TI following specific brainstem lesions indicate a fundamental difference in mechanisms underlying these states. It is suggested that a major determinant of these results is the activation of phasic activity during PS but not TI, and that the possibility remains that both states may share a common mechanism of tonic motor control.