Catecholamine neurons in fetal brain: effects on breathing movements and electrocorticogram.

We investigated the effect of increasing the synaptic concentration of catecholamines released from central pathways on breathing movements and electrocortical (ECoG) activity in fetal sheep in utero. In 11 trials (9 fetuses) intravenous infusion of the noradrenergic uptake inhibitor desipramine (DMI) resulted in an initial increase in the incidence of breathing movements from 47.8 +/- 2.2%/h to a maximum of 77.9 +/- 5.5%/h (P less than 0.05) followed by a subsequent decrease to 18.8 +/- 2.7%/h (P less than 0.05), which was associated with a decrease in the incidence of low-voltage ECoG activity. Mean breath amplitude also increased after DMI infusion from 5.8 +/- 0.2 mmHg to a peak of 8.9 +/- 0.9 mmHg (P less than 0.05). In five fetuses, intravenous injection of the alpha 1-receptor antagonist prazosin 30 min after the start of the DMI infusion blocked the increase of breath amplitude observed when DMI was given alone. Intracisternal infusion of 6-hydroxydopamine (6-OHDA, which causes an initial displacement of transmitter stores followed by a neurotoxic action) in six fetuses caused an increase in mean breath amplitude from 6.6 +/- 0.3 mmHg (control) to 22.3 +/- 5.5 mmHg (P less than 0.05) and a prolonged episode of breathing movements (168.5 +/- 47.2 min). The incidence of low-voltage ECoG activity was also increased significantly. A second infusion of 6-OHDA, 2 days after the first, had no effect on breathing movements or ECoG activities. The two treatments with 6-OHDA also resulted in a significant reduction in the response to intravenous infusion of DMI, indicating that endogenous release of norepinephrine had been reduced by the neurotoxin. These results suggest that 1) there is normally some tonic release of norepinephrine from central pathways, but this release is insufficient to stimulate sustained fetal breathing, and 2) accumulation in the synaptic cleft of catecholamines from central neurons is able to stimulate fetal breathing and promote low-voltage ECoG activity. Possible mechanisms that normally limit the activity of central catecholamine pathways before birth are discussed.