Labyrinthine influences on locus coeruleus neurons.

The locus coeruleus (LC) complex, located in the dorsolateral pontine tegmentum, is composed principally of noradrenergic neurons, which project to broad regions of the CNS, including the spinal cord. Experiments were performed in precollicular decerebrate cats to ascertain whether units histologically identified within the LC complex, and having the physiological characteristics of noradrenergic neurons, would respond to sinusoidal stimulation of labyrinth receptors. Among 141 LC complex neurons, 16 of which could be activated antidromically by stimulation of the spinal cord at T12-L1, 80 (i.e. 56.7%) responded to roll tilt of the animal at 0.15 Hz, +/- 10 degrees. The responses were particularly related to the extreme animal displacements, thus being attributed to stimulation of macular utricular receptors. The proportion of responsive units, and also the average gain of the responses, were higher in the LCd and the subcoerular (subLC) area than in the LCa. Moreover in the same structures the majority of units showed a beta-pattern of response (excitation during side-up tilt), which contrasted with the predominant alpha-pattern (excitation during side-down tilt) displayed by the previously recorded vestibulospinal neurons projecting to the same segments of the spinal cord. The role that the noradrenergic coeruleospinal neurons exert in the dynamic control of posture during the vestibulospinal reflexes is discussed.