Injection of glutamate into the pedunculopontine tegmental nuclei of anesthetized rat causes respiratory dysrhythmia and alters EEG and EMG power

Abstract

The pedunculopontine tegmental nucleus (PPT) has been shown to have important functions relevant to the regulation of behavioral states and various motor control systems, including breathing control. Our previous work has shown that the activation of neurons within the PPT, a structure that is typically active during rapid eye movement (REM) sleep, can produce respiratory disturbances in freely moving and anesthetized rats. The aim of this study was to test the hypothesis that respiratory modulation by the PPT in anesthetized rats can be evoked in the absence of other signs of an REM-sleep-like state. We characterized electroencephalogram (EEG) and electromyogram (EMG) changes during respiratory disturbances induced by glutamatergic stimulation of the PPT in spontaneously breathing, adult male Sprague-Dawley rats anesthetized with a ketamine/xylazine combination or with nembutal. Respiratory movements were monitored by a piezoelectric strain gauge. Two-barrel glass pipettes were used to pressure inject glutamate, to probe for respiratory effective sites within the PPT, and to inject oil red dye at the end of the experiments for histological verification of the injection sites. The EEGs were recorded from the sensorimotor cortex, hippocampus, and from the pons contralateral from the injection site. The EMGs were recorded from the genioglossus muscle. The initial response to glutamate injection into the respiratory modulating region of the PPT was always a respiratory pattern disturbance. Subsequent activation of EMG and EEG often occurred in ketamine/xylazine-anesthetized rats, but REM-sleep-like patterns were not observed. Respiratory pattern and EMG power changes in nembutal-anesthetized rats were similar, but EEG activation was never observed. Thus, we conclude that respiratory suppression produced by the local activation of PPT neurons may not necessarily be accompanied by an REM-sleep-like cortical state in this anesthetized model.

Modulation of respiratory pattern and upper airway muscle activity by the pedunculopontine tegmentum: role of NMDA receptors

“…Our previous work supports the possibility of direct respiratory pattern modulation by PPT neurons [14,35,36] and suggests that the relevant neuronal subpopulation may be differentially modulated by serotonin and noradrenaline-two key neuromodulators which are released maximally during wakefulness and minimally during REM sleep [40]. Together, these studies suggest that withdrawal of serotonergic damping of a critical neuronal pool within PPT during REM sleep may predispose to respiratory disturbance [36].…”

“…As we have previously observed in nembutal-anesthetized animals [35], the present study demonstrated no evidence of significant change in total EEG power during the first 60 s following injection (p>0.05 for all; data not shown).…”

“…There was no statistically significant difference between baseline EMG power and any preinjection EMG power (X 2 =0.2; p=0.98; Friedman ANOVA single factor). We have shown previously [35] that the onset of the tonic genioglossal activity increase is delayed 10 s in respect to the onset of respiratory pattern dysrhythmia induced by glutamate injection into PPT respiratory-modulating neuronal subpopulation. Ketamine injection at the same respiratory-modulating sites had no direct effect on the genioglossal tone (KET; p>0.05; Fisher's PLSD) and blocked any subsequent glutamateinduced increase in genioglossal activity (GLU2; p>0.05; Fisher's PLSD; Fig.…”

“…These breathing disturbances were observed to occur independently of any other REM sleep signs [35], suggesting that respiratory modulation by the PPT does not depend on brain phasic events. Nonetheless, the PPT respiratorymodulating region appears to be differentially regulated by two important REM sleep-related neurotransmitters (noradrenaline and serotonin) [36].…”

“…Although not fully quantitative, we chose the strain gauge method to avoid bypassing the upper airway, as required for intubation, or loading the airways by face mask because these methods may have significantly altered respiratory pattern responses to PPT stimulation. The details of this method are more fully explained elsewhere [14,35]. After conventional amplification and filtering (0.1-50 Hz bandpass; Grass Model 12, West Warwick), the analog data were digitized (sampling frequency, 100/s) and recorded using Brain Wave for Windows software (Datawave Systems, Longmont, CO, USA).…”