Kainate-activated cobalt uptake in the primary gustatory nucleus in goldfish: Visualization of the morphology and distribution of cells expressing AMPA/kainate receptors in the vagal lobe

Gustatory afferent fibers of the vagus nerve that innervate taste buds of t he oropharynx of the goldfish, Carassius auratus, project to the vagal lobe , which is a laminated gustatory nucleus in the dorsal medulla. As in the m ammalian gustatory system, responses by second-order cells in the goldfish medulla are mediated by N-methyl-D-aspartate (NMDA) and non-NMDA ionotropic glutamate receptors. We utilized a cobalt uptake technique to label vagal lobe neurons that possess cobalt-permeable ionotropic glutamate receptors. Vagal lobe slices were bathed in kainate (40 muM) or glutamate (0.5 or 1 mM ) in the presence of CoCl2, which can pass into cells through the ligand-ga ted cation channels of non-NMDA receptors made up of certain subunit combin ations. Cobalt-filled cells and dendrites were observed in slices that were activated by kainate or glutamate, but not in control slices that were bat hed in CoCl2 alone, nor in slices that were bathed with the non-NMDA recept or antagonist 6,7-dinitroquinoxaline-2,3-dione (10 muM) in addition to an a gonist. Likewise, simple depolarization of the cells with KCl failed to ind uce cobalt loading. Cobalt-filled round unipolar cells, elongate or globula r bipolar cells, and multipolar cells with elongate or polygonal perikarya were distributed throughout the cell layers in the sensory zone of the vaga l lobe. Numerous labeled neurons had dendrites spanning layers IV and VI, t he two principal layers of primary afferent input. Apical and basal dendrit es often extended radially through neigh boring laminae, but many cells als o extended dendrites tangential to the lamination of the sensory zone. In t he motor layer, cell bodies and proximal dendrites of small, multipolar neu rons, and large motoneurons were regularly loaded with cobalt. J. Comp. Neu rol. 431: 59-74, 2001. (C) 2001 Wiley-Liss, Inc.