Differential modulation of active expiration during hypercapnia by the medullary raphe in unanesthetized rats

Respiratory activity during seizures induced by pentylenetetrazole

This study investigated the respiratory activity in adult Wistar rats across different behavioral seizure severity induced by pentylenetetrazole (PTZ). Animals underwent surgery for electrodes implantation, allowing simultaneous EEG and diaphragm EMG (DIAEMG) recordings and the respiratory frequency and DIAEMG amplitude were measured. Seizures were acutely induced through PTZ injection and classified based on a pre-established score, with absence-like seizures (spike wave discharge (SWD) events on EEG) representing the lowest score. The respiratory activity was grouped into the different seizure severities. During absence-like and myoclonic jerk seizures, the breathing frequency decreased significantly (∼50% decrease) compared to pre- and post-ictal periods. Pronounced changes occurred with more severe seizures (clonic and tonic) with periods of apnea, especially during tonic seizures. Apnea duration was significantly higher in tonic compared to clonic seizures. Notably, during PTZ-induced tonic seizures the apnea events were marked by tonic DIAEMG contraction (tonic-phase apnea). In the majority of animals (5 out of 7) this was a fatal event in which the seizure-induced respiratory arrest preceded the asystole. In conclusion, we provide an assessment of the respiratory activity in the PTZ-induced acute seizures and showed that breathing dysfunction is more pronounced in seizures with higher severity.

Role of hydrogen sulfide in ventilatory responses to hypercapnia in the medullary raphe of adult rats

NEW FINDINGS

What is the central question of this study? There is evidence that H₂ S plays a role in the control of breathing: what are its actions on the ventilatory and thermoregulatory responses to hypercapnia via effects in the medullary raphe, a brainstem region that participates in the ventilatory adjustments to hypercapnia? What is the main finding and its importance? Hypercapnia increased the endogenous production of H₂ S in the medullary raphe. Inhibition of endogenous H₂ S attenuated the ventilatory response to hypercapnia in unanaesthetized rats, suggesting its excitatory action via the cystathionine β-synthase-H₂ S pathway in the medullary raphe.

ABSTRACT

Hydrogen sulfide (H₂ S) has been recently recognized as a gasotransmitter alongside carbon monoxide (CO) and nitric oxide (NO). H₂ S seems to modulate the ventilatory and thermoregulatory responses to hypoxia and hypercapnia. However, the action of the H₂ S in the medullary raphe (MR) on the ventilatory responses to hypercapnia remains to be elucidated. The present study aimed to assess the role of H₂ S in the MR (a brainstem region that contains CO₂ -sensitive cells and participates in the ventilatory adjustments to hypercapnia) in the ventilatory responses to hypercapnia in adult unanaesthetized Wistar rats. To do so, aminooxyacetic acid (AOA; a cystathionine β-synthase (CBS) enzyme inhibitor), propargylglycine (PAG; a cystathionine γ-lyase enzyme inhibitor) and sodium sulfide (Na₂ S; an H₂ S donor) were microinjected into the MR. Respiratory frequency (f_R ), tidal volume (V_T ), ventilation ( V ̇ E ), oxygen consumption ( V ̇ O 2 ) and body temperature (T_b ) were measured under normocapnic (room air) and hypercapnic (7% CO₂ ) conditions. H₂ S concentration within the MR was determined. Microinjection of the drugs did not affect f_R , V_T and V ̇ E during normocapnia when compared to the control group. However, the microinjection of AOA, but not PAG, attenuated f_R and V ̇ E during hypercapnia in comparison to the vehicle group, but had no effects on T_b . In addition, we observed an increase in the endogenous production of H₂ S in the MR during hypercapnia. Our findings indicate that endogenously produced H₂ S in the MR plays an excitatory role in the ventilatory response to hypercapnia, acting through the CBS-H₂ S pathway.

Excitatory and inhibitory modulation of parafacial respiratory neurons in the control of active expiration

In order to increase ventilation, the respiratory system engages active expiration through recruitment of abdominal muscles. Here, we reviewed the new advances in the modulation of parafacial respiratory (pF) region to trigger active expiration. In addition, we also made a comprehensive discussion of experiments indicating that the lateral aspect of the pF (pFL) is anatomically and functionally distinct from the adjacent and partially overlapping chemosensitive neurons of the ventral aspect of the pF (pFV) also named the retrotrapezoid nucleus. Recent evidence suggest a complex network responsible for the generation of active expiration and neuromodulatory systems that influence its activity. The activity of the pFL is tonically inhibited by inhibitory inputs and also receives excitatory inputs from chemoreceptors (central x peripheral) as well as from catecholaminergic C1 neurons. Therefore, the modulatory inputs and the physiological conditions under which these mechanisms are used to recruit active expiration and increase ventilation need further investigation.

Lesion of Serotonergic Afferents to the Retrotrapezoid Nucleus Impairs the Tachypneic Response to Hypercapnia in Unanesthetized Animals

Hypercapnia promotes an increase in pulmonary ventilation due to the stimulation of brainstem chemosensory cells that are connected to the respiratory network. Among these cells are the raphe serotonergic neurons which widely send projections to distinct central respiratory compartments. Nevertheless, the physiological role of specific raphe serotonergic projections to other chemosensitive sites on the emergence of hypercapnia ventilatory response in vivo still remains to be elucidated. Here we investigated whether the ventilatory response to hypercapnia requires serotonergic inputs to the chemosensitive cells of the retrotrapezoid nucleus (RTN) in the ventrolateral medulla. To test this, pulmonary ventilation was evaluated under baseline conditions and during hypercapnia (7% CO₂) in unanesthetized juvenile Holtzman rats (60-90 g) that received bilateral microinjections of either vehicle (control) or anti-SERT-SAP (0.1 mM, 10 pmol/100 nl) toxin in the RTN to retrogradely destroy serotonergic afferents to this region. Fifteen days after microinjections, baseline ventilation was not different between anti-SERT-SAP (n = 8) and control animals (n = 9). In contrast, the ablation of RTN-projecting serotonergic neurons markedly attenuated the hypercapnia-induced increase in respiratory frequency which was correlated with reduced numbers of serotonergic neurons in the raphe obscurus and magnus, but not in the raphe pallidus. The increase in tidal volume during hypercapnia was not significantly affected by anti-SERT-SAP microinjections in the RTN. Our data indicate that serotoninergic neurons that send projections to the RTN region are required for the processing of ventilatory reflex response during exposure to high CO₂ in unanesthetized conditions.