Fig. 7. Group data showing the effects of microinjection of DAMGO into the caudal medullary raphe region on the minute ventilation (V^E;  A ), respiratory frequency (f;  B ), and tidal volume (V^T;  C ) responsiveness to progressive hypercapnia and the locations of the microinjections (  D ). The linear regression equations before and 5, 30, 60, and 90 min after microinjection of DAMGO were (1) for V^E, y = 19.9x − 433.2, y = 14.1x − 272.2, y = 13.3x − 350.5, y = 17.1x − 371.5, and y = 18.8x − 406.8, respectively; (2) for f, y = 0.78x + 73.66, y = 0.38x + 94.21, y = 0.30x + 101.01, y = 0.54x + 82.80, and y = 0.61x + 90.16, respectively; and (3) for V^T, y = 0.15x − 2.89, y = 0.11x − 2.01, y = 0.12x − 3.41, y = 0.14x − 3.07, and y = 0.14x − 2.52, respectively. The range of  R ²in all equations was 0.94–0.99. “0” on the x-axis indicates the onset of microinjection of DAMGO. Data are presented as mean ± SE; n = 7. *  P < 0.05, **  P < 0.01 compared with the control. The coronal slice (  D ) shows locations of the microinjections 12.0 mm caudal to the bregma and either 8.3 mm (  squares ) or 9.3 mm (  triangles ) ventral to the cerebellar surface. There were six microinjections outside the caudal medullary raphe region that are represented by an  open circle in three rats. Amb = ambiguus nucleus; NTS = nucleus of solitary tract; ROb = raphe obscurus nucleus; RPa = raphe pallidus nucleus; RVL = rostroventrolateral reticular nucleus; Sp5I = spinal 5 nucleus, interpolar part.