Participação dos receptores NK-1 no locus coeruleus na resposta cardiorrespiratória e termorreguladora à hipercapnia

Abstract

The locus coeruleus (LC) has been suggested as a CO2 chemoreceptor site in mammals. Substance P (SP) has been used as a marker of respiratory neurons and it plays an important role in compensatory responses to hypercapnia in several sites of the central nervous system. Neurokinin-1 (NK-1) receptor immunoreactive (NK1Rir) neurons and processes are widely distributed within the LC. Thus, the present study assessed the role of NK-1 receptors in the LC in the cardiorespiratory and thermal responses to hypercapnia. To this end, substance P-saporin conjugate (SPSAP; 2μM) was injected in the LC to kill NK1R-ir neurons, or IgG-SAP as a control in male Wistar rats. The animals that the drug reached the fourth ventricle (4ºV) were considered as a 4ºV group. Pulmonary ventilation (VE, body plethysmograph), mean arterial pressure (MAP), heart rate (HR) and body core temperature were measured followed by 60 min of hypercapnic exposure (7% CO2). To verify the correct placement and effectiveness of the chemical lesions, immunohistochemistry for NK1R was performed. In addition, tyrosine hydroxylase (TH) immunoreactivity was performed to verify if noradrenergic neurons were eliminated. Fluoro-Jade technique was used to evaluate neuronal degeneration. A reduced NK1R (72% of reduction) and TH immunoreactivity (66% of reduction) was observed seven days after the injections of SP-SAP in the LC and an intense Fluoro-Jade staining, showing the effectiveness of the lesion. Focal lesions of NK1R-ir did not affect basal ventilation in the SP-SAP in LC and SP-SAP in 4ºV groups. Hypercapnia caused an increase in pulmonary ventilation in all groups, which was a result of increases in respiratory frequency (fR) and tidal volume (VT), SP-SAP treatment in the LC and in the fourth ventricle attenuated the hypercapnic ventilatory response (30% and 20%, respectivally), due to a reduction in the VT. SP-SAP in the LC and SP SAP in the 4ºV 11 lesion did not affect MAP, but caused an increase in HR in both groups. The results suggest that NK1R-ir neurons in the LC modulate hypercapnic ventilatory response but play no role in breathing control under resting conditions. Additionally, NK1R-ir neurons seem to play no role in body temperature and MAP regulation in resting conditions and during hypercapnia, but modulate HR during CO2 exposure. This modulation may be due to a change in the noradrenaline release.