Postsynaptic Targets of Purkinje Cell Terminals in the Cerebellar and Vestibular Nuclei of the Rat

The cerebellar and vestibular nuclei consist of a heterogeneous group of inhibitory and excitatory neurons. A major proportion of the inhibitory neurons provides a GABAergic feedback to the inferior olive, while the excitatory neurons exert more direct effects on motor control via non‐olivary structures. At present it is not clear whether Purkinje cells innervate all types of neurons in the cerebellar and vestibular nuclei or whether an individual Purkinje cell axon can innervate different types of neurons. In the present study, we studied the postsynaptic targets of Purkinje cell axons in the rat using a combination of pre‐embedding immunolabelling of the Purkinje cell terminals by L7, a Purkinje cell‐specific marker, and postembedding GABA and glycine immunocytochemistry. In the cerebellar nuclei, vestibular nuclei and nucleus prepositus hypoglossi Purkinje cell terminals were found apposed to GABAergic and glycinergic neurons as well as to larger non‐GABAergic, non‐glycinergic neurons. In the cerebellar and vestibular nuclei individual Purkinje cell terminals innervated both the inhibitory and excitatory neurons. Both types of neurons were contacted not only by GABAergic Purkinje cell terminals but also by GABA‐containing terminals that were not labelled for L7 and by non‐GABAergic, non‐glycinergic terminals that formed excitatory synapses. Glycine‐containing terminals were relatively scarce (<2% of the GABA‐containing terminals) and frequently contacted the larger non‐GABAergic, non‐glycinergic neurons. To summarize, Purkinje cell axons evoke their effects through different types of neurons present in the cerebellar and vestibular nuclear complex. The observation that individual Purkinje cells can innervate both excitatory and inhibitory neurons suggests that the excitatory cerebellar output system and the inhibitory feedback to the inferior olive are controlled simultaneously.