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 (<10%). All of the CTB-labelled Ia boutons recovered in the sections incubated for glutamate (n=323) were enriched with glutamate immunoreactivity. One hundred and fifty of these disclosed synaptic contact in at least two ultrathin sections. In this sample, 50% (33-59%) appeared immunoreactive to receptor sub-units GluR1-4 in at least two ultrathin sections, whereas 35% were labelled in one section only. Distribution of gold particles relative to presynaptic and postsynaptic membrane profiles (n=23) revealed a close correlation between AMPA immunoreactivity and the postsynaptic membrane of the synapse. Finally, immunogold particles signalling GluR1 were observed much less frequently than particles signalling GluR2/3 or GluR4. Our results provide additional strong evidence that chemical transmission at Ia synapses is mediated by glutamate and identify GluR2/3 and GluR4 as important postsynaptic receptors.