Changes in cat medullary neurone firing rates and synchrony following induction of respiratory long-term facilitation

1. Long-term facilitation is a respiratory memory expressed as an increase in motor output lasting more than an hour. This change is induced by repeat ed hypoxia, stimulation of carotid chemoreceptors, or electrical stimulatio n of the carotid sinus nerve or brainstem mid-line. The present work addres sed the hypothesis that persistent changes in medullary respiratory neural networks contribute to long-term facilitation. 2. Carotid chemoreceptors were stimulated by close arterial injection of CO 2-saturated saline solution. Phrenic nerve efferent activity and up to 30 s ingle medullary neurones were recorded simultaneously in nucleus tractus so litarii (NTS) including the dorsal respiratory group (DRG), Botzinger-ventr al respiratory group (Bot-VRG), and nucleus raphe obscurus of nine adult ca ts, anaesthetized, injected with a neuromuscular blocking agent, vagotomize d and artificially ventilated. 3. The firing rates of 87 of 105 neurones (83%) changed following induction of long-term facilitation. Nine of eleven DRG and Bot-VRG putative premoto r inspiratory neurones had increased firing rates with long-term facilitati on. Fourteen of twenty-one raphe obscurus neurones with control firing rate s less than 4 Hz had significant long-term increases in activity. 4. Cross-correlogram analysis suggested that there were changes in effectiv e connectivity of neuron pairs with long-term facilitation. Joint peristimu lus time histograms and pattern detection methods used with 'gravity' analy sis also detected changes in short time scale correlations associated with long-term facilitation. 5. The results suggest that changes in firing rates and synchrony of VRG an d DRG premotor neurones and altered effective connectivity among other func tionally antecedent elements of the medullary respiratory network contribut e to the expression of long-term facilitation.