Glutamate-immunoreactivity in identified vagal afferent terminals of the cat: A study combining horseradish peroxidase tracing and postembedding electron microscopic immunogold staining

Using electron microscopic immunohistochemistry we have shown that strong glutamate-immunoreactivity (glutamate-ir) is present in neuronal cell bodies of the nodose ganglion, axons in the tractus solitarius and afferent terminals in the nucleus tractus solitarii. Vagal afferent fibres were specifically labelled by transganglionic retrograde transport of horseradish peroxidase (HRP). Fifty-seven per cent of the HRP-labelled terminals in the dorsomedial medulla were found to contain a high level of glutamate-ir, suggesting that a population of vagal afferent fibres uses glutamate as a neurotransmitter substance. There were no apparent ultrastructural differences between glutamate-ir and non-glutamate-ir vagal afferent terminals, both classes mainly containing rounded vesicles and forming asymmetric synapses. However, some difference in their preference for postsynaptic target was noted. The great majority (83%) of non-glutamate-ir vagal afferent terminals made axodendritic synapses, but only just over half (57%) of the glutamate-ir vagal terminals made synaptic contact with dendrites. Approximately 13% of the HRP-labelled terminals were found to make synaptic contact with HRP-labelled dendrites or soma of motoneurones of the dorsal vagal motor nucleus, confirming the existence of monosynaptic connections between vagal afferent fibres and vagal motoneurones.