Modulation of vestibulospinal reflexes through microinjection of GABAergic agents in the dorsal pontine tegmentum of decerebrate cats.

Abstract 1. The locus coeruleus (LC) neurons control posture as well as the gain of the vestibulospinal (VS) reflexes either through direct coeruleospinal (CS) projections or by inhibiting the dorsal pontine reticular formation (pRF) and the related medullary inhibitory reticulospinal (RS) systems. After the demonstration that the LC neurons are under the inhibitory control of GABAergic afferents, experiments were performed to investigate whether the same system could also suppress the activity of the dorsal pRF, thus intervening in the control of posture and VS reflexes. 2. In precollicular decerebrate cats, unilateral microinjection into the dorsal pRF of the GABA-A antagonist bicuculline or the GABA-B antagonist faclofen (0.13-0.25 microliter at 2.5 micrograms/microliter saline) decreased the tonic contraction of the ipsilateral triceps brachii, but greatly increased the amplitude of the EMG responses of the same muscle to roll tilt of the animal (at 0.15 Hz, +/- 10 degrees). This increase in gain of the VS reflexes was not associated with great changes in phase angle of the responses, which were always characterized by an increased EMG activity during ipsilateral tilt and a decreased activity during contralateral tilt. The same injection also produced in some instances a slight decrease of the extensor tonus of the contralateral forelimb, associated with a moderate increase in the amplitude of modulation and thus in the response gain of contralateral triceps brachii to labyrinth stimulation. However, when the ipsilateral effects were prominent, the response gain of the contralateral triceps muscle was slightly depressed, while the pattern of response reversed. 3. The changes in posture and reflexes described above appeared 5-10 min after unilateral injection of the GABA antagonists in the pRF, reached the highest values in about 30 min and fully recovered to the control levels within 2 h after the injection. These effects were not due to irritative phenomena following injection of the fluid, since neither changes in posture nor in the response gain of the triceps brachii to labyrinth stimulation were observed after injection of an equal volume of vehicle in the pRF of that side. On the other hand, an increase in postural activity associated with a decrease in the response gain of the ipsilateral and contralateral triceps brachii to animal tilt was observed after unilateral microinjection into the dorsal pRF of GABA agonists. 4. Histological controls indicated that the structure responsible for the postural and reflex changes was located in the dorsal pontine reticular formation, ventral to the LC.(ABSTRACT TRUNCATED AT 400 WORDS).