Influence of dorsal periaqueductal gray activation on respiratory occlusion reflexes in rats

The dorsal periaqueductal gray (dPAG) is an essential neural component for central integration of defense behavior and associated autonomic regulation. Electrical and chemical stimulation of this region results in a significant decrease of inspiratory (Ti) and expiratory (Te) breathing durations. In the present study it was hypothesized that breath timing changes elicited by dPAG activation would modulate respiratory load compensation volume-timing reflexes. The effect of dPAG activation of breathing pattern was investigated using chemical stimulation with microinjection of the excitatory amino acid D,L-homocysteic acid (DLH) or disinhibition with GABA(A) (gamma-aminobutyric acid) receptor antagonist bicuculline (BIC) in urethane-anesthetized, spontaneously breathing rats. Volume-timing reflexes were studied using inspiratory and expiratory occlusion before and after DLH or BIC dPAG activation. Both BIC and DLH activation of the dPAG significantly increased breathing frequency. Inspiratory occlusion significantly increased Ti before dPAG activation and DLH and BIC potentiated the relative increase in Ti during occlusion. BIC elicited a greater increase in inspiratory occlusion Ti than DLH. Inspiratory occlusion alone also significantly increased diaphragm EMG activity and this response was potentiated with dPAG activation. Expiratory occlusion significantly increased Te and DLH and BIC potentiated the relative increase in Te during expiratory occlusion. These results suggest that dPAG activation modulates the respiratory neural network, resulting in an increased respiratory drive and a decrease in volume-timing reflex sensitivity.