Developmental changes in transmission of respiratory rhythm in the rat.

We used cross-correlation to examine the short time-scale synchronisation of left and right phrenic nerve discharges in in-situ preparations of rats over a range of ages, to investigate the development of respiratory rhythm transmission to phrenic motoneurones. We found central peaks in the cross-correlograms, indicative of short time-scale synchronisation, at all ages (2-41 days), whose half-amplitude widths varied inversely with age (40-1.8 ms). In 10 preparations < or =5-days-old the central peaks were unaffected by a mid-sagittal section from C3 to C6. Carbenoxalone (CBX), a gap junction blocker, and its inactive analogue glycyrrhzic acid (GZA), eliminated central peaks in preparations younger than 12 days but not in older preparations. We concluded that in rats older than approximately 12 days short time-scale synchronisation is produced by bilaterally-projecting axons of medullary pre-motor neurones, whereas in younger rats it is due to pre-synaptic synchronisation of left and right medullary pre-motor neurones. While the latter mechanism may be gap junction connections, these experiments cannot unequivocally demonstrate it.