Development of thyrotropin-releasing hormone and norepinephrine potentiation of inspiratory-related hypoglossal motoneuron discharge in neonatal and juvenile mice in vitro.

1. The ontogeny of thyrotropin-releasing hormone (TRH) and norepinephrine (NE) potentiation of inspiratory-related hypoglossal (XII) motor nerve discharge was studied in medullary slices from P0-3, P7, and P11-14 mice that retain functional networks for respiration. 2. TRH, applied locally to the XII motor nucleus, had no effect on XII inspiratory burst amplitude in slices from P0-3 mice. By P7 there was significant potentiation of burst amplitude that increased more than three-fold by P11-14. NE applied to the XII nucleus produced significant potentiation in all age groups. Potentiation increased developmentally with the largest increase occurring between P0-3 and P7. 3. Thus catecholamine and TRH modulation of inspiratory-related XII nerve activity increases during the first two weeks of life; the potentiating effects of NE appear prior to those of TRH. 4. With rhythmically active, transverse medullary slices from mice up to 2 wk of age, it is now possible to study the development of XII motoneuron modulation as it relates to respiratory control of the upper airway.