Changes in VOR adaptation after local injection of beta-noradrenergic agents in the flocculus of rabbits

Abstract

Noradrenaline (NA) has been implicated as a neuromodulator in plasticity, presumably facilitating adaptive processes. Since the flocculus receives noradrenergic afferents, and ablation of the flocculus interferes with the normal adaptive changes in the VOR gain, experiments were performed to find out whether bilateral injection of monoaminergic substances into the flocculus of rabbits could modify the adaptive changes of the VOR. The visual world surrounding the rabbit was oscillated in opposite direction to the platform on which the rabbit was mounted, which resulted in an adaptive increase in the VOR gain; this adaptation was measured either in light or in darkness. Floccular injection of the beta-agonist isoproterenol did not greatly affect the adaptation of the VOR measured in light. In darkness, however, the increase in gain after injection of isoproterenol was larger than during normal adaptation. The beta-antagonist sotalol reduced the adaptation of the VOR gain significantly in light as well as in darkness. In a control condition without pressure for adaptation (only intermittent testing of the VOR gain over a period of 2.5 h), the gain of the VOR was not significantly affected by similar injections of beta-adrenergic agents. We conclude that the noradrenergic system facilitates the adaptation of the VOR gain to retinal slip in rabbits without affecting the VOR gain directly. At least part of this influence is exerted through beta-receptors located in the cerebellar flocculus.