Glutamate and AMPA receptor immunoreactivity in Ia synapses with motoneurons and neurons of the central cervical nucleus

Axonal tracing and high resolution immunocytochemistry were used to identify transmitter content and postsynaptic receptors in synapses between Ia primary afferents and motoneurons and in neurons of the central cervical nucleus (CCN), respectively, in the rat. The terminals, as well as the target neurons, were identified by postembedding immunogold detection of transganglionically or retrogradely, respectively, transported cholera toxin B subunit (CTB), and in adjacent sections postembedding immunogold was employed to demonstrate glutamate and AMPA receptors in the same synapses. A total of 390 CTB-labelled Ia boutons in apposition to CTB-labelled motoneurons, CCN neurons or unlabelled dendrites in the surrounding neuropil were traced in section series from two animals. A third animal was used as a control. In the motor nucleus, a majority of the synapses were with medium-sized dendrites, whereas in the CCN the distribution was skewed towards fine-calibre dendrites. In both nuclei, somatic and juxtasomatic synapses were quite infrequent (