Mirela Barros Dias

Focal CO2 dialysis in raphe obscurus does not stimulate ventilation but enhances the response to focal CO2 dialysis in the retrotrapezoid nucleus

Simultaneous inhibition of the retrotrapezoid nucleus (RTN) and raphe obscurus (ROb) decreased the systemic CO(2) response by 51%, an effect greater than inhibition of RTN (-24%) or ROb (0%) alone, suggesting that ROb modulates chemoreception by interaction with the RTN (19). We investigated this interaction further by simultaneous dialysis of artificial cerebrospinal fluid equilibrated with 25% CO(2) in two probes located in or adjacent to the RTN and ROb in conscious adult male rats. Ventilation was measured in a whole body plethysmograph at 30 degrees C. There were four groups (n = 5): 1) probes correctly placed in both RTN and ROb (RTN-ROb); 2) one probe correctly placed in RTN and one incorrectly placed in areas adjacent to ROb (RTN-peri-ROb); 3) one probe correctly placed in ROb and one probe incorrectly placed in areas adjacent to RTN (peri-RTN-ROb); and 4) neither probe correctly placed (peri-RTN-peri-ROb). Focal simultaneous acidification of RTN-ROb significantly increased ventilation (Ve) up to 22% compared with baseline, with significant increases in both breathing frequency and tidal volume. Focal acidification of RTN-peri-ROb increased Ve significantly by up to 15% compared with baseline. Focal acidification of ROb and peri-RTN had no significant effect. The simultaneous acidification of regions just outside the RTN and ROb actually decreased Ve by up to 11%. These results support a modulatory role for the ROb with respect to central chemoreception at the RTN.

Serotonergic neurons in the nucleus raphe obscurus contribute to interaction between central and peripheral ventilatory responses to hypercapnia

Serotonergic (5-HT) neurons in the nucleus raphe obscurus (ROb) are involved in the respiratory control network. However, it is not known whether ROb 5-HT neurons play a role in the functional interdependence between central and peripheral chemoreceptors. Therefore, we investigated the role of ROb 5-HT neurons in the ventilatory responses to CO2 and their putative involvement in the central–peripheral CO2 chemoreceptor interaction in unanaesthetised rats. We used a chemical lesion specific for 5-HT neurons (anti-SERT-SAP) of the ROb in animals with the carotid body (CB) intact or removed (CBR). Pulmonary ventilation (VE), body temperature and the arterial blood gases were measured before, during and after a hypercapnic challenge (7% CO2). The lesion of ROb 5-HT neurons alone (CB intact) or the lesion of 5-HT neurons of ROb+CBR did not affect baseline VE during normocapnic condition. Killing ROb 5-HT neurons (CB intact) significantly decreased the ventilatory response to hypercapnia (p < 0.05). The reduction in CO2 sensitivity was approximately 15%. When ROb 5-HT neurons lesion was combined with CBR (anti-SERT-SAP+CBR), the VE response to hypercapnia was further decreased (−31.2%) compared to the control group. The attenuation of CO2 sensitivity was approximately 30%, and it was more pronounced than the sum of the individual effects of central (ROb lesion; −12.3%) or peripheral (CBR; −5.5%) treatments. Our data indicate that ROb 5-HT neurons play an important role in the CO2 drive to breathing and may act as an important element in the central–peripheral chemoreception interaction to CO2 responsiveness.

Purinergic transmission in the rostral but not caudal medullary raphe contributes to the hypercapnia-induced ventilatory response in unanesthetized rats

The medullary raphe (MR) is a putative central chemoreceptor site, contributing to hypercapnic respiratory responses elicited by changes in brain PCO2/pH. Purinergic mechanisms in the central nervous system appear to contribute to central chemosensitivity. To further explore the role of P2 receptors within the rostral and caudal MR in relation to respiratory control in room air and hypercapnic conditions, we performed microinjections of PPADS, a non-selective P2X antagonist, in conscious rats. Microinjections of PPADS into the rostral or caudal MR produced no changes in the respiratory frequency, tidal volume and ventilation in room air condition. The ventilatory response to hypercapnia was attenuated after microinjection of PPADS into the rostral but not in the caudal MR when compared to the control group (vehicle microinjection). These data suggest that P2X receptors in the rostral MR contribute to the ventilatory response to CO2, but do not participate in the tonic maintenance of ventilation under room air condition in conscious rats.

Ionotropic glutamatergic receptors in the rostral medullary raphe modulate hypoxia and hypercapnia-induced hyperpnea

It has been suggested that the medullary raphe (MR) plays a key role in the physiological responses to hypoxia and hypercapnia. We assessed the role of ionotropic glutamate receptors in the rostral MR (rMR) in the respiratory responses to hypoxia and hypercapnia by measuring pulmonary ventilation (V.(E)) and body temperature (Tb) of male Wistar rats before and after microinjecting Kynurenic acid (KY, an ionotropic glutamate receptors antagonist, 0.1mM) into the rMR followed by 60 min of hypoxia (7% O₂) or hypercapnia exposure (7% CO₂). Compared to the control group, the ventilatory response to hypoxia was attenuated in animals treated with KY intra-rMR, however the ventilatory response to hypercapnia increased significantly. No differences in Tb among groups were observed during hypoxia or hypercapnia. These data suggest that the glutamate acting on ionotropic receptors in the rMR exerts an excitatory modulation on hyperventilation induced by hypoxia but an inhibitory modulation on the hypercapnia-induced hyperpnea.

Central heme oxygenase-carbon monoxide pathway participates in the lipopolysaccharide-induced tolerance in rats.

Recently, heme oxygenase-carbon monoxide (HO-CO) pathway has been reported to be involved in the development of lipopolysaccharide (LPS) fever. However, no information exists about its participation in LPS tolerance, which is defined by an attenuation of the febrile response to repeated administrations of LPS. Thus, we tested the hypothesis that HO-CO pathway plays a role in endotoxin tolerance, which was induced by means of three consecutive LPS intraperitoneal injections (i.p.) at 24-h intervals. Body temperature (Tb) was measured by biotelemetry. Induction of the HO pathway using intracerebroventricular (i.c.v.) heme lysinate reversed tolerance, and this effect could be prevented by pretreatment with ODQ [a soluble guanylate cyclase (sGC) inhibitor; i.c.v.]. These results indicate that HO-CO pathway seems to be down-regulated during LPS tolerance, and that CO is the HO product that can prevent LPS tolerance, acting via cGMP. In further support, either biliverdine or iron (the others HO products; i.c.v.) had no effect in LPS-induced tolerance.