The Central Mechanism Of Orexin On Respiratory Regulation In Rat

Purpose Medulla plays an important role in the generation of respiratory rhythm. Orexins are important hypothalamic neuropeptides,and were thought to regulate feeding,energy metabolism,endocrine,cadiovescular system,respiration, sleeping-arousal,arousal maintain and other physiological activities.The present study was to investigate the expression of orexinA(OXA) and orexin type 1 receptor(OX₁R) in the medulla of neonatal and adult Sprague-Dawley(SD) rats and to observe whether the effects of OXA on respiratory activity via OX₁R and P2X receptor in central nervous system of adult rats.Methods 1.Experiments were performed on neonatal(1～5d) and adult(6～8w) SD rats.Expression and distribution of OXA-like-immunoreactivity(OXA-like-ir) and OX₁R-like-immunoreactivity(OX₁R-like-ir) were observed and compared in the medulla by using immunohistochemistry and software of ImageMeasure.2.Rats were anesthetized with urethane(700mg/kg) andα-chloralose(35mg/kg),and the trachea,femoral artery and extemal jugular vein were cannulated respectively. Unilateral phrenic nerve was dissociated.The rats were placed on a stereotaxic frame in a prostrate position.Exposure of the injection sites on the brain surface.Finally, the phrenic nerve discharge(PND),respiratory rate(RR),artery blood pressure(BP) and heart rate(HR) were recorded.OXA,ATP,SB408124(OX₁R antagonist) and NF279(P2X receptor antagonist) were microinjected into lateral ventricle(LV) or pre-B(?)tzinger complex(PBC) respectively.Results(1) Expression of OXA-like- and OX₁R-like-ir was found throughout the medulla of neonatal and adult rats.OXA-like-ir fibers were obvious in the nucleus tractus solitarius(NTS),hypoglossal nucleus(â…«) and ventrolateral medulla(VLM). OX₁R-like-ir positive cells were heavily stained in the hypoglossal nucleus(â…«) and ventrolateral medulla(VLM).In theâ…«and VLM,relative optical density of OX₁R-like-ir positive cells of neonatal rats was more less than that of adult rats (P＜0.001).(2)After microinjecting OXA(100μg/ml,5μl) into LV,PND increased, BP rose and HR accelerated.Increase of PND was dose-dependent at 10,100, 1000μg/ml concentrations(P＜0.05,P＜0.01,P＜0.001 vs N.S.).The increase of RR at three concentrations was not significant.The acceleration of HR was also dose-dependent at 10,100,1000μg/ml concentrations(P＞0.05,P＜0.01,P＜0.01 vs N.S.).The increase of mean arterial pressure(MAP) was significant at 100 and 1000μg/ml concentrations(P＜0.05,P＜0.01 vs N.S.respectively).(3)After microinjecting ATP(10mM,5μl) into LV,PND increased,BP rose and HR accelerated.Although the PND increased with the concentration of ATP,it was only statistical significance at 100mM(P＜0.05 vs N.S.).There was no significant change in RR.MAP at three different concentrations rose,but that was not significant.The acceleration of HR at 1Mm,10mM,100Mm concentrations was significant(P＜0.05 vs N.S.).(4)After microinjecting OXA(100μg/ml,0.1μl),SB408124(10μg/ml, 0.1μl),SB408124(10μg/ml,0.1μl) + OXA(100μg/ml,0.1μl),NF279(10mM,0.1μl) +OXA(100μg/ml,0.1μl),ATP(10mM,0.1μl),NF279(10mM,0.1μl) + ATP (10mM,0.1μl) into the pre-B(?)tzinger complex,MAP,HR and RR have no significant changes.While PND significantly increased(P＜0.01 vs N.S.) after microinjection of OXA(100μg/ml,0.1μl),PND significantly decreased(P＜0.05 vs N.S.) after microinjection of SB408124(10μg/ml,0.1μl).PND significantly decreased(P＜0.05 vs N.S.) after microinjection of ATP(10mM,0.1μl).There was not significant change in PND after microinjection of SB408124(10μg/ml,0.1μl) + OXA(100μg/ml,0.1μl) and NF279(10mM,0.1μl) + OXA(100μg/ml,0.1μl),but there was significant change in PND(P＜0.001,P＜0.001 vs OXA respectively).Conclusion(1) Expression of OXA -like- and OX₁R-like-ir was found throughout the medulla of neonatal and adult rats.OXA-like-ir fibers were obvious in the nucleus tractus solitarius(NTS),hypoglossal nucleus(â…«) and ventrolateral medulla(VLM). OX₁R -like-ir positive cells were heavily stained in the hypoglossal nucleus(â…«) and ventrolateral medulla(VLM).The expression of OX₁R-like-ir increased with maturation.(2) In the brain,OXA produced a dose-dependent increase of respiratory activity. Although ATP also excited respiratory activity,dose-response was not observed.(3) OXA enhanced respiratory activity in PBC,and OX₁R mediated the effect of the endogenous or exogenous OXA.However,ATP attenuated respiratory activity in PBC.P2X receptor was involved in the effect of OXA on respiratory activity.