Intermittent hypercapnia enhances CO₂ responsiveness and overcomes serotonergic dysfunction

Abstract

Serotonergic dysfunction compromises ventilatory chemosensitivity and may enhance vulnerability to pathologies such as the Sudden Infant Death Syndrome (SIDS). We have shown raphé contributions to central chemosensitivity involving serotonin (5-HT)-and γ-aminobutyric acid (GABA)-mediated mechanisms. We tested the hypothesis that mild intermittent hypercapnia (IHc) induces respiratory plasticity, due in part to strengthening of GABA mechanisms. Rat pups were IHc-pretreated (eight consecutive cycles; 5 min 5% CO2 - air, 10 min air) or constant normocapnia-pretreated as a control, each day for 5 consecutive days beginning at P12. We subsequently assessed CO2 responsiveness using the in situ perfused brainstem preparation. Hypercapnic responses were determined with and without pharmacological manipulation. Results show IHc-pretreatment induces plasticity sufficient for responsiveness despite removal of otherwise critical ketanserin-sensitive mechanisms. Responsiveness following IHc-pretreatment was absent if ketanserin was combined with GABAergic antagonism, indicating that plasticity depends on GABAergic mechanisms. We propose that IHc-induced plasticity could reduce the severity of reflex dysfunctions underlying pathologies such as SIDS.

Keywords: Chemosensitivity; GABA; Respiration; SIDS; Serotonin.

Fig. 1

Shown are typical eupneic raw (above) and integrated (below) phrenic neurogram recording with ventilatory bursts representing the neural correlate of ventilation (Eldridge, 1971; St.-John and Paton, 2000). Single raw neurogram bursts are also shown with an expanded timescale. When the in situ preparation was exposed to perfusate containing 9 % CO₂ (4 % hypercapnic challenge; bottom panel) there was an increase in both frequency and NVE (frequency*amplitude) of ventilatory events compared to 5 % CO₂ exposure (normocapnia; top panel). Both panels are representative records of the last minute of the indicated CO₂ treatment. Records from a preparation derived from a pup that received intermittent air exposure (sham treatment) and no pharmacological manipulations are shown.

Fig. 2

Shown are integrated phrenic neurogram recordings illustrating hypercapnic responses in a normocapnic pretreated preparation without pharmacological maniputaion (A) and the absence of a response with ketanserin treatment (B). Similar recordings illustrate the enhanced hypercapnic response in an IHC pretreated preparation without pharmacological treatment (C) and preserved responses in an IHC preparation treated with ketanserin (D). Shown also are recordings from an IHC preparation lacking a hypercapnic response under combined ketanserin, bicuculline and saclofen treatment (E). There was an apparent increase in burst duration, not present with ketanserin treatment but observed when ketanserin was combined with bicuculline and saclofen. This observation may suggest a role of GABA mediated inhibition in burst generation, although such influences were not characterized in the current study.

Fig. 3

Hypercapnia increased ventilatory frequency, normalized to normocapnia, in preparations derived from pups that received IHc air-control exposures without drugs (no fill bar, n = 20), those that received IHc and no drugs (horizontal stripe bar, n = 16) and those that received IHc and ketanserin (vertical stripe bar, n = 27). Significant increase from normocapnia to hypercapnia (one-way repeated-measures ANOVA): ***P < 0.001. Between groups comparison of difference in hypercapnic sensitivities determined using one-way ANOVA: †P < 0.05, †††P < 0.001.

Fig. 4

Hypercapnia increased ventilatory NVE, normalized to normocapnia, in preparations derived from pups that received IHc air-control exposures without drugs (no fill bar), those that received IHc and no drugs (horizontal stripe bar) and those that received IHc and ketanserin (vertical stripe bar). Significant increase from normocapnia to hypercapnia (1-way repeated-measures ANOVA): *P < 0.05, ***P < 0.001. Between groups comparison of difference in hypercapnic sensitivities determined using 1-way ANOVA: †P ≤ 0.05, ††P < 0.01.