What is the central question of this study? What are the contributions of allopregnanolone, the neuroactive metabolite of progesterone, and nuclear (nPR) and membrane (mPR) progesterone receptors to the respiratory effect of progesterone in newborn rats? What is the main finding and its importance? Acute progesterone injection increases the apnoea frequency, whereas finasteride (which blocks the conversion of progesterone to allopregnanolone) reduces apnoea frequency. An nPR agonist decreases apnoea frequency in males and an mPR agonist decreases apnoea frequency in males and females. Chronic injection of progesterone decreases the frequency of apnoea more efficiently in males than in females. We tested the hypothesis that the effects of progesterone on apnoea frequency in newborn rats are the result of a balance between its neuroactive metabolite, allopregnanolone (GABA receptor modulator), and progesterone receptors. We used male and female rats between 10 and 12 days of age and recorded respiratory and metabolic parameters (whole-body plethysmography), and assessed the frequency and duration of apnoeas in normoxia. We tested the effects of a single injection of progesterone (4 mg kg , i.p.), finasteride (10 mg kg , i.p.; a 5α-reductase antagonist, which blocks the conversion of progesterone to allopregnanolone), finasteride plus progesterone, or agonists of the nuclear or membrane progesterone receptors (R5020 or Org-od-02-0, 4 mg kg ). To test the hypothesis that chronic exposure to progesterone reduces the frequency of apnoeas, we used male and female rats treated daily with progesterone between postnatal days 3 and 12. The acute injection of progesterone reduced minute ventilation and metabolic rate and increased the frequency of apnoeas. Finasteride decreased the frequency of apnoeas, and finasteride plus progesterone did not increase apnoea frequency but decreased minute ventilation in female rats. Although R5020 decreased apnoea frequency only in males, Org-od-02-0 decreased apnoea frequency in males and females and decreased respiratory frequency in females. Chronic progesterone treatment reduced apnoea frequency more efficiently in males than in females, but in females (not in males) an acute injection of caffeine (the gold standard for the treatment of apnoea in preterm neonates) further reduced apnoea frequency. Apnoea frequency in newborn rats is, in part, determined by a sex-specific balance between allopregnanolone, GABA receptors and progesterone receptors.

Key pointsr In premature newborns, recurrent apnoea is systematically treated with caffeine to prevent long-term neurocognitive disorders, but a substantial percentage of apnoea persists particularly in neonates born before 28 weeks of gestation.r Progesterone has been proposed as a respiratory stimulant potentially suitable for the treatment of newborn apnoea persistent to caffeine. Accordingly we asked whether acute progesterone administration reduces apnoea frequency in newborn rats treated with caffeine.r Surprisingly our results show that in newborn rats treated with caffeine, administration of progesterone inhibits breathing and increases apnoea frequency.r Additional experiments showed an enhanced GABAergic inhibitory drive on breathing after caffeine treatment, and that progesterone is converted to allopregnanolone (an allosteric modulator of GABA A receptors) to inhibit breathing.r We conclude that combining progesterone and chronic caffeine is not an option in preterm neonates, unless the effects of allopregnanolone can be counteracted.Abstract Caffeine is the main treatment for apnoea in preterm neonates, but its interactions with other respiratory stimulants like progesterone are unknown. We tested the hypothesis that the addition of progesterone to caffeine treatments further stimulates ventilation. Newborn rats were treated with water (control) or caffeine (15 mg kg −1 ) by daily gavage between postnatal day (P)3 and P12. At P4 and P12, we measured apnoea frequency, ventilatory responses and metabolic parameters under both normoxia and hypoxia (12% O 2 , 20 min) following an acute administration of either saline or progesterone (4 mg kg −1 ; I.P.). Progesterone injection increased the serum levels of both progesterone and its neuroactive metabolite allopregnanolone. Progesterone had no effect on ventilation in control rats under normoxia. Progesterone depressed ventilation in P12 caffeine-treated rats under normoxia and hypoxia and increased apnoea frequency in both P4 and P12 rats. Because allopregnanolone is an allosteric modulator of GABA A receptors and caffeine may enhance GABAergic inhibition in newborns, we studied the effects of the GABA A receptor antagonist bicuculline at 0, 1, 2 and 3 mg kg −1 doses and allopregnanolone (10 mg kg −1 dose) in P12 rats. In caffeine-treated rats, bicuculline enhanced ventilation, while allopregnanolone decreased ventilation and increased total apnoea time. Progesterone had no effect on ventilation and apnoea frequency in caffeine-treated rats injected with finasteride, which blocks the conversion of progesterone to allopregnanolone. We conclude that combining progesterone and chronic caffeine therapy is not an option for the treatment of persistent apnoea in preterm neonates, unless the effects of allopregnanolone can be counteracted.