GLUTAMATE-LIKE IMMUNOREACTIVITY IN SYNAPTIC TERMINALS OF THE POSTERIOR CINGULOPONTINE PATHWAY - A LIGHT AND ELECTRON-MICROSCOPIC STUDY IN THE RABBIT

A postembedding immunoperoxidase method for light microscopy was used to localize glutamate-like immunoreactivity in the rabbit basilar pont ine nuclei. Labelled fibre bundles, neuronal cell bodies and numerous puncta of diverse size were heavily glutamate immunoreactive throughou t all subdivisions of the pontine nuclei. To determine whether some of the glutamate-immunoreactive puncta were synaptic terminals of poster ior cingulate cortical neurons, a double-labelling technique involving an anterograde tract-tracing method and a postembedding immunogold pr ocedure for electron microscopy was used. A quantitative evaluation of gold particle densities revealed that anterogradely labelled cingulop ontine synaptic terminals were about twice as immunoreactive as their postsynaptic dendrites, perikaryal and glial profiles and about three times more than symmetric synaptic terminals. The present results indi cate that the posterior cingulopontine projection contains high levels of glutamate at its synaptic terminals. This observation provides fur ther support to the role for glutamate as a neurotransmitter in the co rticopontine pathway.