Activation of the midbrain periaqueductal gray induces airway smooth muscle relaxation

In this study, we examined effects of chemical stimulation of the ventrolateral region of the midbrain periaqueductal gray (vl PAG) on airway smooth muscle tone. We observed that in anesthetized, paralyzed, and artificially ventilated ferrets, vl PAG stimulation elicited airway smooth muscle relaxation. To clarify the mechanisms underlying this observation, we examined the GABA-GABA_A receptor signalling pathway by 1) examining the expression of GABA_A receptors on airway-related vagal preganglionic neurons (AVPNs) located in the rostral nucleus ambiguus region (rNA), by use of receptor immunochemistry and confocal microscopy; 2) measuring GABA release and within the rNA by using microdialysis; and 3) performing physiological experiments to determine the effects of selective blockade of GABA_A receptors expressed by AVPNs in the rNA region on vl PAG-induced airway relaxation, thereby defining the role of the GABA_A receptors subtype in this process. We observed that AVPNs located in the rNA region do express the GABA_A receptor β-subtype. In addition, we demonstrated that activation of vl PAG induced GABA release with the rNA region, and this release associated with airway smooth muscle relaxation. Blockade of the GABA_A receptor subtype expressed by AVPNs in the rNA by bicuculline diminished the inhibitory effects of vl PAG stimulation on the airway smooth muscle tone. These data indicate, for the first time, that activation vl PAG dilates the airways by a release of GABA and activation of GABA_A receptors expressed by AVPNs.