The role of the locus coeruleus in the gain regulation of vestibulospinal reflexes.

In precollicular decerebrate cats, activation of limb extensors during side-down roll tilt of the animal depends on both an increased discharge of excitatory vestibulospinal neurons and a reduced discharge of inhibitory medullary reticulospinal (mRS) neurons. These inhibitory neurons are tonically excited by a cholinergic pontine reticular formation (pRF) system, which is in turn inhibited by norepinephrine (NE)-containing locus coeruleus (LC) neurons. Functional inactivation of LC neurons, produced by local injection of clonidine, which acts on the somatodendritic alpha 2-adrenoceptors by enhancing recurrent and/or lateral inhibition of the NE neurons, enhanced the response gain of the triceps brachii to animal tilt. We postulate that inactivation of LC neurons releases the cholinergic pRF system and the related mRS system from the suppressive influence exerted by the noradrenergic terminals. The increased discharge of these neuronal systems in the animal at rest would lead to a greater disinhibition which affects the limb extensor motoneurons during side-down tilt, thus increasing the response gain of the corresponding muscle to the same amount of labyrinth stimulation.