Microinjections of GABAergic agents in the locus coeruleus modify the gain of vestibulospinal reflexes in decerebrate cats.

1. Precollicular decerebrate cats show a good postural activity in the four limbs and a small-amplitude vestibulospinal (VS) reflex, as revealed by recording the EMG response of the triceps brachii to sinusoidal roll tilt of the animal at 0.15 Hz, +/- 10 degrees. This preparation was used to study whether inactivation of the locus coeruleus (LC) elicited by local administration either of the GABA-A agonist muscimol or of the GABA-B baclofen produced changes in posture as well as in gain of the VS reflexes. 2. Microinjection into the LC of one side of 0.13-0.25 microliter of a muscimol or baclofen solution (at 2.5 or 1 microgram/microliter saline, respectively) stained with 5% pontamine, usually decreased the extensor tonus in the ipsilateral limbs and to a lesser extent also in the contralateral limbs. However, the amplitude of modulation and thus the gain of the first harmonic component of the multiunit EMG responses of the ipsilateral and to a lesser extent also of the contralateral triceps brachii to the same parameters of labyrinth stimulation increased. The phase angle of the responses remained nearly unchanged by this treatment. The effects appeared 5-10 min after the injection of muscimol, fully developed in 30 min and progressively declined within the following 2-2.5 hours. 3. The increased gain of the VS reflexes acting on the triceps brachii did not depend on the decreased postural activity following the injection of the GABA-A or GABA-B agonist, since it was still observed if the reduced EMG activity following the injection was compensated by an increased static stretch of the muscle. Moreover, the postural and reflex changes described above were not due to irritative events following the injection, since they were not observed in control experiments following injection of 0.25 microliter saline within the LC-complex. On the other hand, postural and reflex changes opposite in sign to those elicited by muscimol or baclofen were obtained by local injection into the LC-complex of a GABA-A or GABA-B antagonist (0.25 microliter of bicuculline or faclofen at 8 micrograms/microliters saline). 4. In some experiments, the decrease in postural activity of the forelimbs after injection of muscimol into the LC complex was followed by transient episodes of postural atonia, which affected not only the ipsilateral but also the contralateral triceps muscles.(ABSTRACT TRUNCATED AT 400 WORDS)