Subtle alterations in breathing and heart rate control in the 5-HT1A receptor knockout mouse in early postnatal development

Abstract

We hypothesized that absence of the 5-HT(1A) receptor would negatively affect the development of cardiorespiratory control. In conscious wild type (WT) and 5-HT(1A) receptor knockout (KO) mice, we measured resting ventilation (Ve), oxygen consumption (Vo(2)), heart rate (HR), breathing and HR variability, and the hypercapnic ventilatory response (HCVR) at postnatal day 5 (P5), day 15 (P15), and day 25 (P25). In KO mice compared with WT, we found a 17% decrease in body weight at only P5 (P < 0.01) and no effect on Vo(2). Ve was significantly (P < 0.001) lower at P5 and P25, but there was no effect on the HCVR. Breathing variability (interbreath interval), measured by standard deviation, the root mean square of the standard deviation (RMSSD), and the product of the major (L) and minor axes (T) of the Poincaré first return plot, was 57% to 187% higher only at P5 (P < 0.001). HR was 6-10% slower at P5 (P < 0.001) but 7-9% faster at P25 (P < 0.001). This correlated with changes in the spectral analysis of HR variability; the low frequency to high frequency ratio was 47% lower at P5 but 68% greater at P25. The RMSSD and (L × T) of HR variability were ~2-fold greater at P5 only (P < 0.001; P < 0.05). We conclude that 5-HT(1A) KO mice have a critical period of potential vulnerability at P5 when pups hypoventilate and have a slower respiratory frequency and HR with enhanced variability of both, suggesting abnormal maturation of cardiorespiratory control.

Fig. 1.

Schematic of the experimental protocol employed to assess breathing and heart rate (HR) responses to hypercapnia in 5-HT_1A knockout (KO) and wild type (WT) pups at postnatal day 5 (P5), postnatal day 15 (P15), and postnatal day 25 (P25). In P5 pups (top), the flow through the plethysmograph was stopped for 1 min at a time after the 30 min and the 5 min HR recordings in room air (RA) and 5% CO₂, respectively, to amplify the signal associated with inspiration and expiration, whereas breathing and HR were recorded continuously in the older pups (bottom).

Fig. 2.

Breathing in 5-HT_1A KO and WT pups in normoxia (RA) and during hypercapnia (5% CO₂) at P5, P15, and P25. A: KO pups had a significantly lower minute ventilation (V̇e) during normoxia and hypercapnia at P5 (*P < 0.01; interaction between gas and genotype) and P25 (*P < 0.05; interaction between gas and genotype). Oxygen consumption (V̇o₂) was not different between KO and WT pups at any age. V̇e/V̇o₂ was significantly lower in KO compared with WT pups at P5 (*P < 0.05; interaction between gas and genotype) and at P25 (*P < 0.01; effect of genotype). One-way repeated measures analysis of variance (ANOVA) at each age, with genotype as the between subjects factor and gas as the repeated factor; Bonferroni post hoc comparisons. B: The hypercapnic ventilatory response, expressed as the percent change in V̇e and V̇e/V̇o₂, was not different between KO and WT pups at any age (P > 0.05). One-way ANOVA at each age, with genotype as the between subjects factor. Data shown as means ± standard deviation (SD).

Fig. 3.

Interbreath interval variability in 5-HT_1A KO and WT pups in normoxia (RA) and during hypercapnia (5% CO₂) at P5, P15, and P25. Breathing variability, expressed as SD, root of the mean square of successive differences (RMSSD), and an estimate of the Poincaré distribution (L × T), was greater in KO compared with WT pups at P5 (*P < 0.05; interaction between gas and genotype), but was lower than WT pups at P25 (*P < 0.05; interaction between gas and genotype). One-way repeated measures ANOVA at each age, with genotype as the between subjects factor and gas as the repeated factor; Bonferroni post hoc comparisons. Data shown as means ± SD.

Fig. 4.

HR and the low frequency to high frequency ratio (LF/HF) in 5-HT_1A KO and WT pups in normoxia (RA) and during hypercapnia (5% CO₂) at P5, P15, and P25.Top: HR was slower in KO compared with WT pups at P5 (***P < 0.0001; effect of genotype), but faster than WT pups at P25 (*P < 0.01; interaction between gas, gender, and genotype). Two-way repeated measures ANOVA at each age, with gender and genotype as the between subjects factors and gas as the repeated measure; Bonferroni post hoc comparisons. Bottom: LF/HF ratio was lower in KO compared with WT pups at P5 (*P < 0.05; effect of genotype) but higher than WT pups at P25 (*P < 0.05; effect of genotype). One-way repeated measures ANOVA at each age, with genotype as the between subjects factor and gas as the repeated factor. Data shown as means ± SD.

Fig. 5.

Heart rate variability (HRV) in 5-HT_1A KO and WT pups in normoxia (RA) and during hypercapnia (5% CO₂) at P5, P15, and P25. HRV, expressed as RMSSD and L × T, was greater in KO compared with WT pups at P5 (*P < 0.05, **P < 0.001; effect of genotype), but was not different from WT pups at P15 and P25 (P > 0.05). One-way repeated measures ANOVA at each age, with genotype as the between subjects factors and gas as the repeated measure. Data shown as means ± SD.

Fig. 6.

Bradycardia frequency in 5-HT_1A KO and WT pups in normoxia (RA) and during hypercapnia (5% CO₂) at P5, P15, and P25. At P25, KO pups had more spontaneous bradycardias during hypercapnia compared with WT pups (***P < 0.0001; interaction between gas and genotype) and compared with normoxia (††P < 0.001; interaction between gas and genotype). One-way repeated measures ANOVA at each age, with genotype as the between subjects factor and gas as the repeated measure; Bonferroni post hoc comparisons. Data shown as means ± SD.