Differences in glial and neuronal labeling following [ 3H]proline or [ 3H]leucine injections into the dorsal column nuclei of cats

The differential incorporation of the amino acids proline and leucine by cells in the cat cuneate nucleus was investigated with electron microscopic autoradiography. Following [ 3H]leucine injections into the cuneate nucleus, all neurons located in either its clusters or non-clusters portion were densely labeled. In contrast, following [ 3H]proline injections, no neurons, regardless of their size and shape, were densely labeled in the clusters region. In the non-clusters region, some smaller neurons were labeled, but only at a moderate density. At all [ 3H]proline injection sites outside the area damaged by the pipette, the only densely-labeled cells were macroglial cells, both astrocytes and oligodendrocytes. Some densely labeled macroglial cells were also found at [ 3H]leucine injection sites, but fewer than at [ 3H]proline injection sites. Microglial cells were at most only sparsely labeled at both sites. These results suggest that most of the ‘small cells’ which were densely labeled by [ 3H]proline in earlier light-microscopic experiments ( Künzle & Cuénod, 1973; Felix & Künzle, 1974; Berkley, 1975; Groenewegen & Voogd, 1977) are macroglial cells. The preferential incorporation of [ 3H]proline by macroglial cells in the cuneate nucleus could be taken to indicate that proline serves as a neurotransmitter in the cuneate nucleus, either directed towards or produced by its neurons. Although the results of other experiments so far do not support this suggestion, they are insufficient to eliminate it. It is also possible that the unusual [ 3H]proline uptake pattern reflects regional variations in neuronal or glial metabolic needs. These and a number of other possible explanations for proline's incorporation pattern are discussed but none of them is as yet more appropriate than the others. Whatever the explanations prove to be, the results have implications for the use of proline in auto-radiographic tracing studies of neuronal projections. As shown earlier, unlike [ 3H]leucine, [ 3H]proline alone cannot always be relied upon to demonstrate all of the projections of a group of neurons. In addition, although neurons in the clusters region of the cuneate nucleus fail to be densely labeled by [ 3H]proline, dense labeling still can be observed in one of the terminal targets of the cuneate nucleus, the inferior olive ( Berkley, 1975). This result suggests that, along with neurons, glial cells may also be involved in the transfer of certain incorporated amino acids