EVIDENCE FOR CENTRAL CHEMORECEPTION IN THE MIDLINE RAPHE

We injected acetazolamide (AZ; 5 x 10(-6) M; 1 nl; n = 14), its inacti ve analogue cetylamino-1,3,4-thiadiazole-5-sulfon-t-butylamide (5 x 10 (-5) M; n = 6), or mock cerebrospinal fluid (n = 5) into the caudal ra phe in the midline brain stem of anesthetized paralyzed ventilated rat s. These AZ injections have been shown to produce a focal region of ti ssue acidosis with a radius < 350 mu m and are used as a probe for sit es of central chemosensitivity. Compared with control injections, AZ i njection into the raphe, as demonstrated by anatomic analysis of injec tion location, significantly increased the amplitude of the integrated phrenic neurogram over 10-40 min. Not all raphe injections produced s uch a response. AZ injections identified as responders (n = 8 of 14) i ncreased integrated phrenic amplitude 43.3 +/- 10.7% (SE) of baseline 20 min after the injection. We conclude that the midline caudal raphe contains sites of ventilatory chemoreception.