Respiratory responses elicited by rostral versus caudal dorsal periaqueductal gray stimulation in rats

The periaqueductal gray (PAG) is a central neural region essential for defense behavior and coordination of accompanying autonomic responses. Activation of rostral versus caudal dorsal (dPAG) regions mediates different cardiovascular response patterns. Stimulation of the dPAG also elicits increased respiratory activity, however, it is unknown if there is a regional difference in dPAG modulation of respiratory pattern. The present study was undertaken to identify whether activation of rostral vs caudal dPAG modulates respiration differently. In anesthetized, spontaneously breathing rats, chemical and electrical stimulation in rostral and caudal dPAG evoked an increased respiratory frequency (fR) with significant shortening of both inspiratory (Ti) and expiratory time (Te). Stimulation in the dPAG also evoked significant increases in electromyography activity of the diaphragm (dEMG), arterial pressure, and heart rate. Caudal dPAG stimulation evoked a greater increase in fR due to a significantly greater decrease in Ti and Te than the rostral dPAG. Caudal dPAG stimulation also evoked a greater increase in baseline dEMG activity and elicited a significantly greater increase in dEMG amplitude above baseline than rostral dPAG. There was a rostro-caudal difference in the post-stimulus respiratory recovery response, with the caudal dPAG eliciting a longer sustained effect. No regional differences were identified in the arterial blood pressure and heart rate during dPAG stimulation. The results demonstrate that the magnitude of the respiratory response during and immediately after activation of the caudal dPAG is greater than during rostral dPAG stimulation.