Chronic hypercapnia modulates respiratory-related central pH/CO2 chemoreception in an amphibian,Bufo marinus

Abstract

SUMMARY Anuran amphibians have multiple populations of pH/CO2-sensitive respiratory-related chemoreceptors. This study examined in cane toads(Bufo marinus) whether chronic hypercapnia (CHC) altered the pH/CO2 sensitivity of central respiratory-related chemoreceptors in vitro and whether CHC altered the acute hypercapnic ventilatory response (HCVR; 5% CO2) in vivo. Toads were exposed to CHC(3.5% CO2) for 9 days. In vitro brainstem–spinal cord preparations were used to examine central respiratory-related pH/CO2 chemosensitivity. CHC augmented in vitro fictive breathing as the pH of the superfusate was lowered from 8.2 to 7.4. Midbrain transection in vitro (at a level known to reduce the clustering of breaths) did not alter this augmentation. In vivo, CHC did not alter the acute HCVR but midbrain transection changed the breathing pattern and increased the overall level of ventilation. CHC did not alter the effect of olfactory CO2 chemoreceptor denervation on the acute HCVR in vivo but did alter the response when returned to normal air. The results indicate that CHC increases the response of central pH/CO2chemoreceptors to changes in cerebrospinal fluid pH in vitro yet this increase is not manifest as an increase in the HCVR in vivo.

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

“…This pH range approximates that used in previous studies on amphibian brainstem-spinal cord preparations (Kinkead et al, 1994;McLean et al, 1995a;McLean et al, 1995b;Gheshmy et al, 2006). All experiments were performed at room temperature (approximately 22°C).…”

“…Toads were anaesthetised by emersion in a solution of 3-aminobenzoic acid ethyl ester (MS222, 1.0·g·l -1 ; Sigma) buffered to pH·7.0 with sodium bicarbonate (Reid and Milsom, 1998;Reid et al, 2000a;Reid et al, 2000b;Gheshmy et al, 2006). Animals were kept in the anaesthetic until the eye-blink and toe-pinch reflexes were eliminated.…”

“…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.…”

“…The preparation was then pinned, ventral-side up, onto a fine stainless-steel mesh within a superfused recording chamber. The mesh divided the chamber into upper and lower compartments, which ensured simultaneous superfusion of both surfaces of the preparation (Kinkead et al, 1994;McLean et al, 1995a;McLean et al, 1995b;Gheshmy et al, 2006). The preparation was continuously superfused with oxygenated aCSF, at a rate of 10·ml·min -1 , using peristaltic pumps that delivered and removed the aCSF from the chamber.…”

“…Fictive breaths in a given episode were defined as occurring within 2·s of each other according to general practices in the literature (Kinkead et al, 1994;Kinkead et al, 1997;Reid et al, 2000a;Gheshmy et al, 2006).…”