Central CO₂chemoreception in developing bullfrogs: anomalous response to acetazolamide

Abstract

Central CO(2) chemoreception and the role of carbonic anhydrase were assessed in brain stems from Rana catesbeiana tadpoles and frogs. Buccal and lung rhythms were recorded from cranial nerve VII and spinal nerve II during normocapnia and hypercapnia before and after treatment with 25 microM acetazolamide. The lung response to acetazolamide mimicked the hypercapnic response in early-stage and midstage metamorphic tadpoles and frogs. In late-stage tadpoles, acetazolamide actually inhibited hypercapnic responses. Acetazolamide and hypercapnia decreased the buccal frequency but had no effect on the buccal duty cycle. Carbonic anhydrase activity was present in the brain stem in every developmental stage. Thus more frequent lung ventilation and concomitantly less frequent buccal ventilation comprised the hypercapnic response, but the response to acetazolamide was not consistent during metamorphosis. Therefore, acetazolamide is not a useful tool for central CO(2) chemoreceptor studies in this species. The reversal of the effect of acetazolamide in late-stage metamorphosis may reflect reorganization of central chemosensory processes during the final transition from aquatic to aerial respiration.

Effects of maturation and acidosis on the chaos-like complexity of the neural respiratory output in the isolated brainstem of the tadpole,Rana esculenta

“…Third, the very small size of R. (Pelophylax) esculenta prevented us from safely applying suction electrodes to SN II. Consequently, we could not record a neural output purely reflecting the activity of the lung oscillator (22,23,55,60). This could have led to an underestimation of the number of lung bursts, or to some degree of ambiguity in their identification.…”

Locus coeruleus is a central chemoreceptive site in toads

“…According to Milsom (33), once central chemoreceptors appear, they take on the predominant role of providing chemosensory drive under steady-state conditions. In amphibians, this dominant role of central chemoreceptors appears at metamorphosis, when these receptors assume the generation of the respiratory drive (48,51). The present study adds information specifically related to the LC as a chemosensitive site in the central nervous system (CNS) of adult toads to this scenario.…”

Afferent input modulates the chronic hypercapnia-induced increase in respiratory-related central pH/CO2 chemosensitivity in the cane toad (Bufo marinus)

“…The cranial case was removed with Rongeurs and bone shears and placed into a Sylgard-coated dissecting dish. The brain was exposed and superfused with ice-cold oxygenated artificial cerebral spinal fluid (aCSF) (in mmol·l -1 ; NaCl, 103; KCl, 4.05; MgCl 2 , 1.38; glucose, 10; NaHCO 3 , 29.2; CaCl 2 , 2.45; pH·7.8; Sigma) (Taylor et al, 2003a;Taylor et al, 2003b;Gheshmy et al, 2006). The rostral forebrain was removed and the remaining brain tissue was continually superfused with the aCSF.…”