Kainate receptors in primary afferents to the rat gracile nucleus.

In a previous work, we demonstrated that under weak paraformaldehyde fixation, kainate receptors (KR) (GluR5/6/7) are expressed in primary afferent terminals in superficial dorsal horn. We extended our study to primary afferents to the gracile nucleus; immunostaining for GluR5/6/7 in weakly fixed sections was in puncta of variable size. In double-stained sections, the majority of immunostained puncta colocalized with synaptophysin. Because of their large size and relations with smaller puncta, single-stained for synaptophysin, these terminals were presumed to be of dorsal column primary afferents. This was confirmed by anterograde labeling with cholera toxin B and with electron microscopy, which showed that GluR5/6/7 was present in terminals with morphology of primary afferents. These observations demonstrate that expression of presynaptic KR is a general feature of primary afferents with different functional properties.