Central and peripheral mechanisms underlying respiratory deficits in a mouse model of accelerated senescence.

Aging invariably decreases sensory and motor stimuli and affects several neuronal systems and their connectivity to key brain regions, including those involved in breathing. Nevertheless, further investigation is needed to fully comprehend the link between senescence and respiratory function. Here, we investigate whether a mouse model of accelerated senescence could develop central and peripheral respiratory abnormalities. Adult male Senescence Accelerated Mouse Prone 8 (SAMP8) and the control SAMR1 mice (10 months old) were used. Ventilatory parameters were assessed by whole-body plethysmography, and measurements of respiratory input impedance were performed. SAMP8 mice exhibited a reduction in the density of neurokinin-1 receptor immunoreactivity in the entire ventral respiratory column. Physiological experiments showed that SAMP8 mice exhibited a decreased tachypneic response to hypoxia (FiO2 = 0.08; 10 min) or hypercapnia (FiCO2 = 0.07; 10 min). Additionally, the ventilatory response to hypercapnia increased further due to higher tidal volume. Measurements of respiratory mechanics in SAMP8 mice showed decreased static compliance (Cstat), inspiratory capacity (IC), resistance (Rn), and elastance (H) at different ages (3, 6, and 10 months old). SAMP8 mice also have a decrease in contractile response to methacholine compared to SAMR1. In conclusion, our findings indicate that SAMP8 mice display a loss of the NK1-expressing neurons in the respiratory brainstem centers, along with impairments in both central and peripheral respiratory mechanisms. These observations suggest a potential impact on breathing in a senescence animal model.

The pontine Kölliker-Fuse nucleus is important for reduced postinspiratory airflow elicited by stimulation of the ventral respiratory parafacial region.

Considering that the retrotrapezoid nucleus/respiratory parafacial region (RTN/pFRG) would be an important center in the central nervous system involved in the maintenance and modulation of respiratory activity, we hypothesized that neurons in this nucleus would also be involved in the postinspiratory (post-I) phase of the respiratory cycle through a connection with the pontine Kölliker-Fuse (KF) region. Here, we performed pharmacogenetic manipulation (AAV-hM3D(Gq)-mCherry or AAV-hM4D(Gi)-mCherry) in VGlut2-cre, Ai6 conscious mice to evaluate breathing parameters through whole body plethysmography under baseline conditions (normoxia: [Formula: see text] = 0.21) or under hypercapnia or hypoxia challenges ([Formula: see text] = 0.07 or [Formula: see text] = 0.08). Under normoxia, selective stimulation of RTN/pFRG resulted in a smaller increase in V̇e (1,272 ± 102.5, vs. RTN/pFRG stimulation: 1,878 ± 122.1 mL/kg/min), due to a smaller increase in VT (5.4 ± 0.35, vs. RTN/pFRG stimulation: 7.77 ± 0.21 mL/kg) without changing fR in a condition of KF inhibition. However, inhibition of the VGlut2 neurons in the KF did affect the TE1 produced by selective activation of RTN/pFRG (119.9 ± 2.53, vs. RTN/pFRG stimulation: 104 ± 2.46 ms). Both the hypercapnia and hypoxia ventilatory response were reduced after inhibition of VGlut2-expressing KF neurons. Therefore, consistent with anatomical projections RTN/pFRG neurons regulate lung ventilation by controlling all aspects of breathing, i.e., breathing frequency, inspiration, postinspiration, and active expiration. All the modulation seems to be dependent on the integrity of the glutamatergic neurons in the KF region.NEW & NOTEWORTHY Our research reveals specific roles and interactions between the retrotrapezoid nucleus/respiratory parafacial region (RTN/pFRG) and the pontine Kölliker-Fuse (KF) region in controlling respiratory phases. RTN/pFRG neurons are key in regulating all aspects of breathing, including frequency, inspiration, postinspiration, and active expiration. This regulation depends on the functional integrity of glutamatergic neurons in the KF region, aligning with anatomical projections.

The latency to awake from induced-obstructive sleep apnea is reduced in rats with chronic epilepsy.

OSA is known to increase the risk for SUDEP in persons with epilepsy, but the relationship between these two factors is not clear. Also, there is no study showing the acute responses to obstructive apnea in a chronic epilepsy model. Therefore, this study aimed to characterize cardiorespiratory responses to obstructive apnea and chemoreceptor stimulation in rats. In addition, we analyzed respiratory centers in the brain stem by immunohistochemistry. Epilepsy was induced with pilocarpine. About 30-60 days after the first spontaneous seizure, tracheal and thoracic balloons, and electrodes for recording the electroencephalogram, electromyogram, and electrocardiogram were implanted. Intermittent apneas were made by inflation of the tracheal balloon during wakefulness, NREM sleep, and REM sleep. During apnea, respiratory effort increased, and heart rate fell, especially with apneas made during wakefulness, both in control rats and rats with epilepsy. Latency to awake from apnea was longer with apneas made during REM than NREM, but rats with epilepsy awoke more rapidly than controls with apneas made during REM sleep. Rats with epilepsy also had less REM sleep. Cardiorespiratory responses to stimulation of carotid chemoreceptors with cyanide were similar in rats with epilepsy and controls. Immunohistochemical analysis of Phox2b, tryptophan hydroxylase, and NK1 in brain stem nuclei involved in breathing and sleep (retrotrapezoid nucleus, pre-Bötzinger complex, Bötzinger complex, and caudal raphe nuclei) revealed no differences between control rats and rats with epilepsy. In conclusion, our study showed that rats with epilepsy had a decrease in the latency to awaken from apneas during REM sleep, which may be related to neuroplasticity in some other brain regions related to respiratory control, awakening mechanisms, and autonomic modulation.

Pilocarpine-induced status epilepticus reduces chemosensory control of breathing.

One of the possible causes of death in epilepsy is breathing disorders, especially apneas, which lead to an increase in CO2 levels (hypercapnia) and/or a decrease in O2 levels in arterial blood (hypoxemia). The respiratory neurons located in the ventral brainstem respiratory column are the main groups responsible for controlling breathing. Recent data from our group demonstrated respiratory changes in two experimental models of epilepsy, i.e. audiogenic epilepsy, and amygdala rapid kindling. Here, we aimed to evaluate respiratory changes in the classic model of temporal lobe epilepsy induced by intra-hippocampal injection of pilocarpine. Adult Wistar rats with stainless-steel cannulas implanted in the hippocampus region were used. The animals were submitted to pilocarpine injection (2.4 mg/µL, N = 12-15) or saline (N = 9) into the hippocampus. The respiratory parameters analyzed by whole-body plethysmography were respiratory rate (fR), tidal volume (VT) and ventilation (VE). Respiratory mechanics such as Newtonian airway resistance (Rn), viscance of the pulmonary parenchyma (G) and the elastance of the pulmonary parenchyma (H) were also investigated. No changes in baseline breathing were detected 15 or 30 days after pilocarpine-induced status epilepticus (SE). However, 30 days after pilocarpine-induced SE, a significant reduction in VE was observed during hypercapnic (7% CO2) stimulation, without affecting the hypoxia (8% O2) ventilatory response. We also did not observe changes in respiratory mechanics. The present results suggest that the impairment of the hypercapnia ventilatory response in pilocarpine-induced SE could be related to a presumable degeneration of brainstem respiratory neurons but not to peripheral mechanisms.