Sex-specific respiratory effects of acute and chronic caffeine administration in newborn rats

Caffeine is widely used for the treatment of apnea of prematurity (AoP) but whether this effect varies with sex is unknown. To shed some light on this question, we present a summary of data obtained on the effects of caffeine on the respiratory chemoreflexes and apnea frequency in 1- and 12-days old male and female rats. Caffeine was either administered as a single acute injection (10 mg/kg, i.p.) or for 10 consecutive days (7.5 mg/kg/day between 3 and 12 days of life by gavage, simulating its clinical use). Acute caffeine had little effects on breathing in 1-day old male and female rats. In 12-days old female rats caffeine reduced the response to hypercapnia (not hypoxia) compared to males. During the steady state of hypoxia females had a lower frequency of apneas than males, and acute injection of caffeine decreased the frequency of apnea, suppressing the differences between males and females. In 12-days old rats chronic administration of caffeine stimulated basal breathing and decreased the frequency of apnea similarly in males and females. In response to hypoxia, chronic caffeine administration also masked the difference in respiratory frequency between males and females observed in control rats. Female rats had lower frequency of apnea than males with or without caffeine treatment. These observations indicate that sex influences the respiratory responses to caffeine and this effect seems to depend on the modality of administration (acute vs chronic) and environmental oxygen (normoxia vs hypoxia).