Role of cholinergic-nicotinic receptors on hypoxic chemoreflex during postnatal development in rats

We tested the hypothesis that the function of cholinergic-nicotinic receptors on respiration is age dependent. To this end, we used whole body plethysmography to measure breathing frequency (fR), tidal volume (V(T)) and minute ventilation (V (E)) under normoxia (21% O(2)) in rats at 1, 4, 7, 12 and 21 postnatal days before and after administration of epibatidine (nicotinic agonist 5 microg/kg, i.p.). In normoxia, epibatidine increased fR and V (E) in a proportionally age-dependent manner (p for age <0.001), without affecting V(T). We then focused on P4 and P12 rats, as representative of this developmental pattern, to measure ventilatory response to moderate hypoxia (12% O(2), 20 min) after i.p. injection of saline (control), epibatidine (5 microg/kg), or (nicotinic antagonist, 1mg/kg). At these doses, both drugs selectively target peripheral nicotinic receptors. Epibatidine enhanced the hypoxic ventilatory response while hexamethonium significantly reduced it. These effects were significantly greater in P12 than in P4 rats (p for age <0.001). In P12 rats, in vitro recordings of carotid sinus nerve activity showed that superfusion with nicotine enhanced chemosensory discharge rate in normoxia and hexamethonium reduced the discharge rate in hypoxia. We also identified the nicotinic receptor alpha7 subunit by immunohistochemistry in carotid bodies from P12 rat. These data show that the role of cholinergic-nicotinic receptor on hypoxic chemoreflex is age dependent and this effect likely implicates carotid body nicotinic receptor activation.