Inhibitory connections among rostral medullary expiratory neurones detected with cross-correlation in the decerebrate rat

Expiratory neurones, with a decrementing firing pattern during the first phase of expiration (E-DEC) and located in the rostral ventrolateral medulla, are thought to be involved in the network generating respiratory rhythm, which also includes expiratory neurones with augmenting firing patterns (E-AUG). We used cross-correlation to detect their synaptic interconnections and connections to phrenic motoneurones in 32 vagotomised, decerebrate, paralysed and ventilated male rats. Pairs of neurones were recorded extracellularly with glass-insulated tungsten microelectrodes and the whole phrenic nerve with bipolar silver wire electrodes. Of the 79 cross-correlograms computed between pairs of E-DEC neurones, 8 (similar to10%) showed evidence for inhibitory connections. Of the 67 cross-correlograms computed between E-DEC and E-AUG neurones, 5 (7.5%) showed evidence for a monosynaptic inhibition of the E-AUG neurone by the E-DEC neurone, while 3 (4.5%) showed evidence for a monosynaptic inhibition of the E-DEC neurone by the E-AUG neurone. An inhibitory connection from E-DEC neurones to phrenic motoneurones was detected in 5 (similar to2%) of the cross-correlograms, and from E-AUG neurones to phrenic motoneurones in 4 (similar to3.7%). These results are the first demonstration that network models of rhythm generation in the rat involving reciprocal inhibition between E-DEC and E-AUG neurones could have a neurophysiological basis, and the first to demonstrate that phrenic motoneurones are inhibited during the early phase of expiration by E-DEC neurones.