GLUTAMATE-IMMUNOREACTIVE CLIMBING FIBERS IN THE CEREBELLAR CORTEX OF THE RAT

The climbing fibre system, one of the two main excitatory inputs to th e cerebellar cortex, is anatomically and physiologically well characte rized, while the nature of its neurotransmitter is still a matter of d ebate. We wished to determine whether glutamate-immunoreactive profile s with the morphological characteristics of climbing fibres could be f ound in the rat cerebellar cortex. For this purpose, a monoclonal 'ant i-glutamate' antibody has been used in combination with a sensitive po stembedding immunoperoxidase method on semithin sections or in combina tion with a postembedding immunogold method on ultrathin sections. At the light microscopic level, climbing fibers appeared as strongly stai ned fibrous profiles, chains of interconnected varicosities or heavily labelled dots of various sizes, often in close apposition to principa l Purkinje cell dendrites. At the electron microscopic level, certain labelled varicosities or more elongated profiles resembling climbing f ibre terminals were in synaptic contact with dendritic spines of Purki nje cells. Quantitative analysis of gold particle densities showed tha t such elements were about three to four times more heavily labelled t han their postsynaptic partners. The results obtained in this study de monstrate that at least a subset of climbing fibres and their terminal s contain relatively high levels of glutamate-like immunoreactivity an d provide additional evidence for a role of glutamate as transmitter i n these cerebellar afferents.