Involvement of cholinergic mechanisms in the central control of respiration in the cane toad, Bufo marinus

Chemical substrates, central sites and central mechanisms underlying the re gulation of breathing in lower vertebrates have not been well characterized . The present study was undertaken to determine the effect of pH changes an d cholinergic agents on the central control of respiration in the cane toad , Bufo marinus. Adult toads were anesthetized, catheterized and unidirectio nally ventilated before exposing the brainstem. An airtight buccal cannula was also inserted through the tympanum to record buccal pressure. The anima l was decerebrated, anesthetic removed and the responses to pH changes of s olutions bathing the ventral surface of the medulla (VSM) were tested by su perfusing the VSM with mock cerebrospinal fluid (mCSF) of pH 7.8-normal, 7. 2-acidic and 8.4-basic. The acidic solution increased respiratory activity, the basic solution decreased activity and the normal solution had no effec t. In addition, cholinergeric agents (acetylcholine-ACh, physostigmine-Phy, nicotine-Nic, and atropine-Atr) dissolved in mCSF were applied bilaterally onto the VSM using filter paper pledgets. ACh, Phy and Nic increased episo dic breathing frequency by 14.3 +/- 9.7, 9.4 +/- 5.4 and 29.1 +/- 11.8%, re spectively, whereas, Atr caused a decrease (- 26.6 +/- 16.6%). These agents had no effect on blood pressure. It is therefore, concluded that the VSM i s pH sensitive and a cholinergic mechanism is involved in the central modul ation of respiration in Bufo. (C) 2001 Elsevier Science Inc. All rights res erved.