Effects of intracisternal monoamines on breathing movements in fetal sheep

The effects of intracisternal administration of tyramine on fetal breathing movements (FBM), electrocortical (ECoG) and nuchal muscle activities, and cerebrospinal fluid (CSF) monoamine concentrations have been studied in unanesthetized fetal sheep (124-140 days gestation) in utero. In 18 trials (8 fetuses) infusion of 50 or 100 micrograms/kg tyramine increased the incidence of FBM from 32.2 +/- 2.0 to 79.4 +/- 4.7%/h (P < 0.05) and increased mean breath amplitude from 6.4 +/- 0.4 to 11.8 +/- 1.6 mmHg (P < 0.05). FBM incidence during high-voltage ECoG activity increased from 3.3 +/- 0.6 to 22.5 +/- 3.6%/h (P < 0.05). Tyramine infusion (100 micrograms/kg) significantly increased (P < 0.05) the CSF concentrations of dopamine from 129.5 +/- 26.2 to 10,222.4 +/- 1,103.6 pg/ml, of norepinephrine from 74.7 +/- 11.0 to 2,238.6 +/- 143.5 pg/ml, and of serotonin from 1,824.5 +/- 340.7 to 3,888.7 +/- 1,335.2 pg/ml. Intracisternal injection of dopamine or norepinephrine (10-20 micrograms) caused the rapid onset of large-amplitude FBM, which often continued throughout high-voltage ECoG activity. In contrast, serotonin (20-40 micrograms) caused cessation of FBM and change of the ECoG from low- to high-voltage activity. These results indicate that neuronal release of catecholamines in the CNS has excitatory effects on FBM.