Glutamate-immunoreactive climbing fibres in the cerebellar cortex of the rat

The climbing fibre system, one of the two main excitatory inputs to the cerebellar cortex, is anatomically and physiologically well characterized, while the nature of its neurotransmitter is still a matter of debate. We wished to determine whether glutamate-immunoreactive profiles with the morphological characteristics of climbing fibres could be found in the rat cerebellar cortex. For this purpose, a monoclonal 'anti-glutamate' antibody has been used in combination with a sensitive postembedding immunoperoxidase method on semi-thin sections or in combination with a postembedding immunogold method on ultrathin sections. At the light microscopic level, climbing fibres appeared as strongly stained fibrous profiles, chains of interconnected varicosities or heavily labelled dots of various sizes, often in close apposition to principal Purkinje cell dendrites. At the electron microscopic level, certain labelled varicosities or more elongated profiles resembling climbing fibre terminals were in synaptic contact with dendritic spines of Purkinje cells. Quantitative analysis of gold particle densities showed that such elements were about three to four times more heavily labelled than their postsynaptic partners. The results obtained in this study demonstrate that at least a subset of climbing fibres and their terminals contain relatively high levels of glutamate-like immunoreactivity and provide additional evidence for a role of glutamate as transmitter in these cerebellar afferents.