Heart rate variability during carbachol-induced REM sleep and cataplexy

The Transition Between Slow-Wave Sleep and REM Sleep Constitutes an Independent Sleep Stage Organized by Cholinergic Mechanisms in the Rostrodorsal Pontine Tegmentum

There is little information on either the transition state occurring between slow-wave sleep (SWS) and rapid eye movement (REM) sleep, as well as about its neurobiological bases. This transition state, which is known as the intermediate state (IS), is well-defined in rats but poorly characterized in cats. Previous studies in our laboratory demonstrated that cholinergic stimulation of the perilocus coeruleus α nucleus (PLCα) in the pontine tegmentum of cats induced two states: wakefulness with muscle atonia and a state of dissociated sleep we have called the SPGO state. The SPGO state has characteristics in common with the IS, such including the presence of ponto-geniculo-occipital waves (PGO) and EEG synchronization with δ wave reduction. Therefore, the aims of the present study were (1) to characterize the IS in the cat and, (2), to study the analogy between the SPGO and the different sleep stages showing PGO activity, including the IS. Polygraphic recordings of 10 cats were used. In seven cats carbachol microinjections (20-30 nL, 0.01-0.1 M) were delivered in the PLCα. In the different states, PGO waves were analyzed and power spectra obtained for the δ, θ, α, and β bands of the EEG from the frontal and occipital cortices, and for the θ hippocampal band. Statistical comparisons were made between the values obtained from the different states. The results indicate that the IS constitutes a state with characteristics that are distinct from both the preceding SWS and the following REM sleep, and that SPGO presents a high analogy with the IS. Therefore, the SPGO state induced by administering carbachol in the PLCα nucleus seems to be an expression of the physiological IS of the cat. Consequently, we propose that the PLCα region, besides being involved in the mechanisms of muscle atonia, may also be responsible for organizing the transition from SWS to REM sleep.

EEG dissociation induced by muscarinic receptor antagonists: Coherent 40 Hz oscillations in a background of slow waves and spindles

Mesopontine and basal forebrain cholinergic neurons are involved in the control of behavioral states and cognitive functions. Animals treated with cholinergic muscarinic receptor antagonists display a dissociated state characterized by behavioral wakefulness (W) associated with high amplitude slow oscillations and spindles in the electroencephalogram (EEG), similar to those that occur during non-REM (NREM) sleep. Oscillations in the gamma frequency band (≈ 40 Hz) of the EEG also play a critical role during W and cognition. Hence, the present study was conducted to determine the effect of muscarinic antagonists on the EEG gamma band power and coherence. Five cats were implanted with electrodes in different cortices to monitor the EEG. The effects of atropine and scopolamine on power and coherence within the low gamma frequency band (30-45 Hz) from pairs of EEG recordings were analyzed and compared to gamma activity during sleep and W. Muscarinic antagonists induced a NREM sleep-like EEG profile that was accompanied by a large increase in gamma power and coherence. The values of gamma coherence were similar to that occurring during alert W (AW), and greater than in quiet W, NREM and REM sleep. We conclude that under atropine or scopolamine, functional interactions between cortical areas in the gamma frequency band remain high, as they are during AW. This significant functional connectivity at high frequency may explain why the animals remain awake in spite of the presence of slow waves and spindles.

Neocortical 40 Hz oscillations during carbachol-induced rapid eye movement sleep and cataplexy

Higher cognitive functions require the integration and coordination of large populations of neurons in cortical and subcortical regions. Oscillations in the gamma band (30-45 Hz) of the electroencephalogram (EEG) have been involved in these cognitive functions. In previous studies, we analysed the extent of functional connectivity between cortical areas employing the 'mean squared coherence' analysis of the EEG gamma band. We demonstrated that gamma coherence is maximal during alert wakefulness and is almost absent during rapid eye movement (REM) sleep. The nucleus pontis oralis (NPO) is critical for REM sleep generation. The NPO is considered to exert executive control over the initiation and maintenance of REM sleep. In the cat, depending on the previous state of the animal, a single microinjection of carbachol (a cholinergic agonist) into the NPO can produce either REM sleep [REM sleep induced by carbachol (REMc)] or a waking state with muscle atonia, i.e. cataplexy [cataplexy induced by carbachol (CA)]. In the present study, in cats that were implanted with electrodes in different cortical areas to record polysomnographic activity, we compared the degree of gamma (30-45 Hz) coherence during REMc, CA and naturally-occurring behavioural states. Gamma coherence was maximal during CA and alert wakefulness. In contrast, gamma coherence was almost absent during REMc as in naturally-occurring REM sleep. We conclude that, in spite of the presence of somatic muscle paralysis, there are remarkable differences in cortical activity between REMc and CA, which confirm that EEG gamma (≈40 Hz) coherence is a trait that differentiates wakefulness from REM sleep.