NORADRENERGIC AGENTS IN THE CEREBELLAR VERMIS AFFECT ADAPTATION OF THE VESTIBULOSPINAL REFLEX GAIN

In precollicular decerebrate cats, the vestibulospinal reflex (VSR) wa s intermittently recorded from the triceps brachii during sinusoidal r oll tilt of the whole animal (at 0.15 Hz, +/-10 degrees), leading to s elective stimulation of labyrinth receptors. This reflex, tested durin g and after a 3-h period of sustained animal tilt at the same paramete rs indicated above, showed an adaptive increase in gain in some experi ments but not in others. In a second group of experiments, however, ro tation of the head (at 0.15 Hz, +/-10 degrees) was associated with a s ynchronous body rotation (at 0.15 Hz, +/-12.5 degrees) which led to an additional neck input, due to 2.5 degrees of out-phase body-to-head d isplacement. In these experiments, the VSR, tested every 10-15 min, co nsistently showed an adaptive increase in gain during and after a 3-h period of sustained vestibular and neck stimulation. Microinjection in to the cerebellar anterior vermis of beta-adrenergic agents (0.25 mu l at 8 mu g/mu l saline) produced slight and short-lasting changes in t he basic amplitude of the VSR, due to the neuromodulatory influence of these agents on the Purkinje cells activity. In addition, the beta-ad renergic agonist isoproterenol brought to the light an adaptive proces s in those experiments in which no adaptation occurred during a sustai ned roll tilt of the whole animal. On the other hand, the beta-adrener gic antagonists propranolol or sotalol either suppressed the increase in gain of the VSR which occurred in other experiments during sustaine d animal rotation, or prevented the occurrence of an adaptive increase in gain during a continuous out-phase head and body rotation. We conc lude that the adaptive changes in gain of the VSR are facilitated by t he noradrenergic system acting within the cerebellar cortex through be ta-adrenoceptors.