Interaction of norepinephrine with cerebellar activity evoked by mossy and climbing fibers

Abstract

Previous studies of the inhibitory actions of norepinephrine (NE) and γ-aminobutyric acid (GABA) on Purkinje neurons were extended to determine the effect on the responses to afferent input from climbing fibers, mossy fibers, and cerebellar interneurons. Complex spike excitation, simple spike excitation and inhibition of Purkinje cell discharge were elicited by electrical stimulation of the appropriate afferent pathways and quantitated by computing poststimulus-time histograms of the neuronal response, recorded extracellularly in anesthetized rats. Histograms were compared before, during, and after microiontophoresis of NE or GABA. The effect of both these putative neurotransmitters was to reduce spontaneous activity more profoundly than either simple or complex spike excitation. In addition, an apparent increase in counts in histograms detecting complex spike excitations was often recorded, particularly with NE, due to an increase in the number of full-size action potentials in the complex spike. The inhibitory response of the Purkinje cell to activation of basket and stellate cell afferents was also potentiated by NE. This effect was often seen at NE dosages causing only a minimal change in spontaneous activity. Thus, NE enhances excitatory or inhibitory evoked responses of Purkinje cells relative to spontaneous activity. The data imply that the action of NE is more complex than a simple depression of activity. In the context of current concepts of cerebellar function, NE input may be able to modulate many dimensions of information processing in the cerebellum.