Activation of Opioid μ-Receptors in the Commissural Subdivision of the Nucleus Tractus Solitarius Abolishes the Ventilatory Response to Hypoxia in Anesthetized Rats

Abstract

Background : The commissural subnucleus of the nucleus tractus solitarius (comNTS) is a key region in the brainstem responsible for the hypoxic ventilatory response (HVR) because it contains the input terminals of the carotid chemoreceptor. Because opioids inhibit the HVR via activating central μ-receptors that are expressed abundantly in the comNTS, the authors of the current study asked whether activating local μ-receptors attenuated the carotid body-mediated HVR. Methods : To primarily stimulate the carotid body, brief hypoxia (100% N2) and hypercapnia (15% CO2) for 10 s and/or intracarotid injection of NaCN (10 μg/100 μl) were performed in anesthetized and spontaneously breathing rats. These stimulations were repeated after: (1) microinjecting three doses of μ-receptor agonist [d-Ala2, N-Me-Phe4, Gly-ol]-Enkephalin (DAMGO) (approximately 3.5 nl) into the comNTS; (2) carotid body denervation; and (3) systemic administration of DAMGO (300 μg/kg) without and with previous intracomNTS injection of d-Phe-Cys-Tyr-d-Trp-Arg-Thr-Pen-Thr-NH2, a μ-receptor antagonist. Results : Study results showed that DAMGO at 0.25 and 2.5, but not 0.025 mM, caused a similar decrease in baseline ventilation (approximately 12%). DAMGO at 0.25 mM largely reduced (64%) the HVR, whereas DAMGO at 2.5 mM abolished the HVR (and the VE response to NaCN) and moderately attenuated (31%) the hypercapnic ventilatory response. Interestingly, similar HVR abolition and depression of the hypercapnic ventilatory response were observed after carotid body denervation. Blocking comNTS μ-receptors by d-Phe-Cys-Tyr-d-Trp-Arg-Thr-Pen-Thr-NH2 significantly attenuated the HVR depression by systemic DAMGO with little change in the DAMGO modulatory effects on baseline ventilation and the hypercapnic ventilatory response. Conclusion : The data suggest that opioids within the comNTS, via acting on μ-receptors, are able to abolish the HVR by affecting the afferent pathway of the carotid chemoreceptor.