Rostral ventrolateral medullary neurons projecting to locus coeruleus have cardiorespiratory inputs.

This study was designed to characterize some of the properties of the rostral ventrolateral medullary (RVLM) cells with axonal projection to the locus coeruleus (LC) in urethane anesthetized, vagotomized, paralyzed and artificially respirated rats. The vast majority of RVLM units antidromically (AD) activated from LC (RVLM-LC units) were silent and unresponsive to peripheral chemoreceptor stimulation or nociceptive stimulation. Twenty seven spontaneously active RVLM-LC neurons, AD activated from LC with currents below 30 microA (17 +/- 2 microA) were analyzed. AD mapping (n = 18) indicated that the lowest threshold for AD activation occurred within the LC itself. Axonal branching within or close to LC was suggested by the presence of sudden jumps in AD latency. Maximal AD latencies ranged from 7 to 37 ms. Most spontaneously active RVLM-LC neurons displayed marked central respiratory modulation characterized by either a post-inspiratory or an inspiratory pattern. The majority of the tested neurons were affected (excited or inhibited) by brief peripheral chemoreceptor stimulation (N2 inhalation). Most cells were inhibited by raising arterial pressure but none exhibited any detectable pulse synchrony. Reticulospinal sympathetic premotor neurons of RVLM were not found to project to LC (sample of 9) and very few RVLM cells with on-off respiratory discharges appeared to project to LC (2 out of 110). This study suggests that much of the information conveyed by the RVLM to LC could be of a mixed cardiorespiratory nature.