GLUTAMATE, ASPARTATE AND COLOCALIZATION WITH CALBINDIN IN THE MEDIAL THALAMUS - AN IMMUNOHISTOCHEMICAL STUDY IN THE RAT

Topographical and quantitative features of medial thalamic neurons in which aspartate (ASP) or glutamate (GLU) might act as neurotransmitter s were investigated in the rat. The calcium-binding protein calbindin D-28k (CB) was exploited as a marker of neuronal subsets, thus allowin g us to study also the relationships between the CB-containing neurons and those immunoreactive to excitatory amino acids. Double immunocyto chemistry of ASP and CB or GLU and CB was performed in 40-mu m-thick s ections. The three markers were distributed in the thalamic midline, m ediodorsal, anterior intralaminar and ventromedial nuclei, with region al variations. ASP-immunoreactive neurons appeared more numerous than the GLU-immunoreactive ones throughout these structures; ASP-CB or GLU -CB double-immunostained neurons were evident. ASP-, CLU- and CB-immun oreactive cells were then quantitatively evaluated in 5-mu m-thick con secutive sections. Interindividual variations and different anti-ASP a nd anti-GLU antibodies did not result in significant differences. ASP and GLU were not co-localized. Single ASP- or GLU-immunoreactive neuro ns accounted for 60% of the total number of immunostained cells, and s ingle ASP-immunopositive cells represented more than half of these neu rons. Among the CB-immunoreactive cells (40% of the total), half were double immunostained; the proportion of double CB-ASP-immunopositive n eurons was sevenfold higher than that of the CB-GLU-immunoreactive one s. These results indicate that ASP may act as excitatory neurotransmit ter in a relatively high proportion of medial thalamic neurons, in whi ch ASP frequently coexists with CB. Approximately 50% of the CB-immuno reactive cells did not contain either ASP or GLU, suggesting that some medial thalamic neurons may utilize a different neurotransmitter.