Selective firing of rat pontine gigantocellular neurons during movement and REM sleep.

Neurons in the gigantocellular field of the pontine reticular formation (PGF) fire at very slow rates during waking (W) and slow wave sleep (SWS) and increase their discharge as much as 100 times during selective segments of the rapid eye movement (REM) sleep cycle8,1L Hobson has proposed that these neurons form a center for the control of the various phasic and/or tonic events that are specific to REM sleep 7. This hypothesis is supported by the observations that lesions within the pontine reticular formation eliminate elements of REM sleep~, 6 and that prolonged electrical stimulation in this region increases the total amount of REM sleep 5,15. The data of Hobson and co-workers s,12 were taken from recordings in restrained cats. In a preliminary report in the partially restrained cat, it was demonstrated that PGF neurons that fire at high rates during REM show an equally increased rate of discharge during waking with head movements a3. In the awake, freely moving rat it was incidentally observed that a very large percentage of PGF neurons show markedly increased activity during movement 2a. If these movement-correlated cells also show the pattern of selective REM-associated activity described by Hobson et al. s, the significance of increased PGF activity in REM would require reinterpretation. Our data were taken from 15 unrestrained male Sprague-Dawley rats (350-450 g) prepared with chronically implanted cervical electromyogram (EMG), bipolar dorsal hippocampal and cortical (skull screw) recording electrodes. A skull-mounted micro-drive advanced a metal microelectrode for recording from PGF neurons 22. After a 5-7-day recovery period, each rat was sleep-deprived overnight by the inverted flower pot method16; recording began on the following day. The microelectrode was advanced in increments less than 30/zm through successive cerebellar Purkinje cell layers which were identified by a high frequency, high amplitude multiunit response. Entry into the dorsal pons was marked by a significant reduction in spontaneous cell activity, and single, well isolated, large amplitude spikes were encountered approximately 1 mm ventral to that point. From that location ven-trally any neuron that had an extracellular action potential of 250/~V or greater was considered a PGF cell and recorded from. The only criteria, then, used in the selection of cells were that they be localized to the PGF region of the brain stem and that they