Electrical Coupling: Novel Mechanism for Sleep-Wake Control

THE PEDUNCULOPONTINE NUCLEUS (PPN), THE CHOLINERGIC ARM OF THE RETICULAR ACTIVATING SYSTEM (RAS), CONTAINS MEDIUM AND LARGE CHOLINERGIC neurons as well as small noncholinergic cells1 and has both ascending and descending projections. Early work established that electrical stimulation of the region of the PPN induced desynchronization of the electroencephalogram, similar to that seen during waking and rapid eye movement (REM) sleep.2 PPN neurons show increased rates of firing during waking and REM sleep, but fire less during slow wave sleep.3 Lesions of the PPN4,5 or pharmacologic blockade of PPN efferents6 decrease or eliminate REM sleep and diminish waking. Fast cortical oscillations in waking and REM sleep are triggered by PPN depolarization (push) of thalamocortical relay neurons via muscarinic inhibition of a K+ conductance, whereas PPN hyperpolarization (pull) of reticular thalamic neurons blocks spindles.7 In addition, the majority of PPN neurons project to the “nonspecific” intralaminar thalamus,8 especially the parafascicular nucleus in primates.9 The “nonspecific” thalamic system, including the parafascicular nucleus, is thought to allow sensory input to access the machinery that presumably generates conscious experience, the thalamocortical 40-Hz rhythm.10 The PPN also sends descending projections throughout the pontomedullary reticular formation, including the anterior pontine region.11,12 Injections of cholinergic agonists into a region called the pontine inhibitory area induce signs of a REM sleep-like state (atonia and pontogeniculooccipital waves, depending on species and preparation).13,14 Lesioning of this pontine region, termed the subcoeruleus, can produce REM sleep without atonia or decreases in REM sleep signs.15–17 Recent studies on subcoeruleus cells reported neurons excited by the cholinergic agonist carbachol (presumed to be REM-on cells) with low threshold spikes (LTS) and cells inhibited by carbachol, some with LTS, some with Ia current.18 We observed the presence of spikelets and dye coupling in neurons in the PPN and subcoeruleus and recently published indirect evidence for electrical coupling in the subcoeruleus.19 The present study was conducted to provide direct evidence of electrical coupling in the PPN and subcoeruleus by recording from pairs of coupled neurons and to study correlated activity between these cells, especially that induced by carbachol. Electrical coupling in the mammalian brain was first described in the 1970s, with connexin 36 (Cx36) being the only gap junction protein between neurons.20 Spikelets are stereotypical, usually rhythmic, subthreshold depolarizing potentials thought to reflect synchronous firing in the coupled neurons, as opposed to noncoupled cells.21 Electrical synapses appear mainly between GABAergic neurons in the thalamic reticular nucleus and in the cortex, where they may enhance the synchrony of gamma oscillations.22,23 A recent report suggests that the stimulant modafinil acts by increasing electrical coupling in cortical, reticular thalamic, and inferior olive neurons.24 Our results demonstrate the presence of coupled neurons in RAS neurons of the PPN and subcoeruleus, and their modulation by carbachol and modafinil to induce oscillations, emphasizing the role of electrical coupling as a novel mechanism for sleep-wake control. Preliminary findings have been reported in abstract form.25