Noradrenergic modulation of respiratory motor output during tadpole development: role of α-adrenoceptors

Abstract

Noradrenaline (NA) is an important modulator of respiratory activity. Results from in vitro studies using immature rodents suggest that the effects exerted by NA change during development, but these investigations have been limited to neonatal stages. To address this issue, we used in vitro brainstem preparations of an ectotherm, Rana catesbeiana, at three developmental stages: premetamorphic tadpoles, metamorphic tadpoles and fully mature adult bullfrogs. We first compared the effects of NA bath application (0.02-10· mol·l -1 ) on brainstem preparations from both pre-metamorphic (Taylor-Köllros stages VII-XI) and metamorphic tadpoles (TK stages XVIII-XXIII) and adult frogs. The fictive lung ventilation frequency response to NA application was both dose-and stage-dependent. Although no net change was observed in the pre-metamorphic group, NA application decreased fictive lung burst frequency in preparations from more mature animals. These effects were attenuated by application of ␣ ␣-adrenoceptor antagonists. Conversely, NA application elicited dose-and stage-dependent increases in fictive buccal ventilation frequency. We then assessed the contribution of ␣ ␣-adrenoceptors towards these responses by applying specific agonists (␣ ␣ 1 : phenylephrine; ␣ ␣ 2 : clonidine; concentration range from 10 to 200· mol·l -1 for both). Of the two agonists used, only phenylephrine application consistently mimicked the lung burst frequency response observed during NA application in each stage group. However, both agonists decreased buccal burst frequency, thus suggesting that other (␤ ␤) adrenoceptor types mediate this response. We conclude that modulation of both buccal and lung-related motor outputs change during development. NA modulation affects both types of respiratory activities in a distinct fashion, owing to the different adrenoceptor type involved.

Developmental changes in central O2 chemoreflex in Rana catesbeiana: the role of noradrenergic modulation

“…At this point, little is known about the neural mechanisms underlying the central O 2 chemoreflex in amphibians. While we have recently shown that, under standard (hyperoxic) conditions, NA bath application onto bullfrog brainstem preparations elicits fictive lung ventilation responses that are similar to those observed during hypoxia (Fournier and Kinkead, 2006), a direct involvement of noradrenergic modulation in the O 2 chemoreflex remains to be demonstrated in this species. With that in mind, the main objective of the present study was to better understand the mechanisms underlying the central O 2 chemoreflex by testing the hypothesis that noradrenergic receptor activation is necessary to observe a fictive lung ventilation response to hypoxia in the brainstem preparation from Rana catesbeiana.…”

“…The brain was irrigated with ice-cold (0-5°C) artificial cerebrospinal fluid (aCSF) to reduce axonal conductance throughout the dissection procedure. The composition of the aCSF was identical to the one used in our previous studies (Kinkead et al, 1994;Kinkead et al, 2002;Fournier and Kinkead, 2006). For tadpoles, the aCSF consisted of (in mmol·l -1 ): 104 NaCl; 4 KCl; 1.4 MgCl 2 ; 10 D-glucose; 25 NaHCO 3 ; 2.4 CaCl 2 , and for adult bullfrogs: 75 NaCl; 4.5 KCl; 1 MgCl 2 ; 7.5 D-glucose; 40 NaHCO 3 ; 2.5 CaCl 2 ; 1 NaH 2 PO 4 .…”

“…Because we were concerned about potential carry-over effects, each preparation was exposed to one antagonist only. The choice of these pharmacological agents was based on our previous work showing that these receptors play a key role in the modulation of fictive lung ventilation (Fournier and Kinkead, 2006); doses were selected from other studies (e.g. Errchidi et al, 1991) (Fournier and Kinkead, 2006).…”

“…The choice of these pharmacological agents was based on our previous work showing that these receptors play a key role in the modulation of fictive lung ventilation (Fournier and Kinkead, 2006); doses were selected from other studies (e.g. Errchidi et al, 1991) (Fournier and Kinkead, 2006). The protocol began by recording baseline (drug-free) respiratory-related motor output for 10·min before the preparation was superfused with aCSF from a second reservoir containing 5·mol·l -1 NA for 10·min (pre-metamorphic, N=6; adult, N=6).…”

“…In those experiments, preparations were superfused with aCSF containing the bicuculline/strychnine mixture before the ␣ 1 receptor agonist phenylephrine (Phe; 25·mol·l -1 ), the ␣ 2 receptor agonist clonidine (Clo; 25·mol·l -1 ) or hypoxia was applied to the brainstem (pre-metamorphic, N=6; adult, N=6, for all), according to the protocol described previously. The selection of the agonist concentration was based on dose-response curves performed previously (Fournier and Kinkead, 2006). Although GABA A and glycine receptors are highly selective Cl -channels, HCO 3 -current (via GABA A receptors) affects GABA responses, especially in mature neurons (Yamada et al, 2004).…”