POSTNATAL CHANGES IN THE MAMMALIAN RESPIRATORY NETWORK AS REVEALED BYTHE TRANSVERSE BRAIN-STEM SLICE OF MICE

1. Spontaneous rhythmic activity in hypoglossal (XII) rootlets is gene rated at all postnatal stages from postnatal day (P) 0 to P22 in the t ransverse brainstem slice of mice containing the pre-Botzinger complex (PBC). The PBC is known to be a region essential for respiratory rhyt hm generation. It contains neurones generating periodic bursts that oc cur in synchrony with rhythmic XII activity. This synchrony indicates that the rhythmic PBC activity generated by the transverse slice is th e central respiratory rhythm. 2. The strength of coupling between XII bursts and PBC bursts decreased during early postnatal development. In younger mice (P0-4) each burst in XII rootlets corresponded to one bu rst in the PBC. In older mice (P5-18) one burst in XII rootlets occurr ed only every third to fourth burst in PBC neurones. 3. Cycle length a nd burst duration of rhythmic XII activity did not change significantl y during the first three postnatal creeks. However, the pattern of XII bursts changed from decrementing (P0-7) to bell shaped (P8-18) while the rate of rise of XII bursts decreased significantly. 4. The rate of rise of rhythmic depolarizations in neurones of the PBC discharging i n phase with XII bursts ('inspiratory neurones') decreased with postna tal development. During interburst intervals, membrane potentials of n eurones of older mice (P6-18) were characterized by waves of synaptic input that were not observed in neonatal animals (PO-5). 5. Blockade o f glycine receptors by strychnine increased the frequency of rhythmic XII activity in neonatal. and older mice (P0-22). Although in expirato ry PBC neurones glycinergic transmission was blocked at 10 mu M strych nine, in inspiratory PBC neurones and XII rootlets even higher concent rations of up to 50 mu M strychnine failed to abolish rhythmic activit y.