Glial cells: Possible determinants of neuronal uptake of tritiated proline

Abstract

When tritiated proline is injected into various sensory and integrative areas of the brain, it fails to be incorporated into the proteins of neuronal soma located within the injection site. In contrast, such incorporation does occur when [ 3H]proline is injected into dorsal root ganglia. The basis for this difference is unclear because brain and dorsal root ganglion tissue differ in configurational factors (e.g. synapses, dendrites) as well as in the embryological origin of their respective neuronal and non-neuronal cell populations. To determine if configurational factors might account for [ 3H]proline's incorporation into somal neuronal proteins in dorsal root ganglia, [ 3H]proline was injected into autonomie (pelvic and superior cervical) and sensory (dorsal root and nodose) ganglia in the rat. These ganglia differ in synaptic and cellular configurations, but have the same neural origin (neural crest). Virtually all neuronal soma were labeled in autoradiograms of all of these injection sites, suggesting that configurational factors do not account for the labeling of dorsal root ganglion neurons by [ 3H]proline. To address the issue of embryological origin, cellular labeling patterns after [ 3H]proline injection into the hypoglossal nucleus, dorsal motor nucleus of the vagus and the ventral horn of spinal cord were compared with those after [ 3H]proline injections into the adjacent solitary nucleus, gracile nucleus and central cervical nucleus of the spinal cord. The neurons in the former three nuclei (i.e. motoneurons) originate from the neural tube, but their axons are associated primarily with Schwann cells which originate from the neural crest. Although neurons in the latter three regions also originate from the neural tube, their axons are myelinated entirely by neural tube-derived glia (i.e. oligodendrocytes). Whereas motoneuronal soma in the former three areas were labeled with [ 3H]proline (72, 47 and 93%, respectively), few neuronal soma in the other three areas were labeled (14, 9 and 13%, respectively). These results indicate that the location of neurons or the axonal projections of neurons in the periphery is correlated with the uptake of proline, while containment of the entire neuron and its processes in the CNS is correlated with the lack of proline uptake. The results therefore suggest that the incorporation of proline by neuronal soma depends upon the presence of Schwann cells and/or the absence of oligodendrocytes on some portion of the neuron's axon and implicate neuronal incorporation of proline in regenerative processes of the nervous system.