The mode of axonal regeneration of locus coeruleus neurons in the lateral geniculate nucleus following neonatal 6-hydroxydopamine treatment.

The effects of neonatal 6-hydroxydopamine (6-OHDA) treatment on the projection of locus coeruleus (LC) neurons to the dorsal part of the lateral geniculate nucleus (dLGN) were examined using histofluorescence and electrophysiological methods. Histofluorescence studies confirmed the previous results of Gustafson and Moore (1987, Dev. Brain Res. 37: 143-155) that catecholamine fibers in the cerebral cortex almost completely disappeared whereas the density of catecholamine fibers in the dLGN increased in the 6-OHDA-treated rats. To quantify the density of the LC projections, we assessed the percentage of LC neurons activated antidromically (projection index, P-index) from the cortex and dLGN. In the 6-OHDA-treated rats, the P-indices for the frontal and visual cortex were greatly reduced, whereas those for the dLGN were increased to twice the control values. In the control rats, about 50% of the LC neurons exhibited antidromic latencies from the dLGN of more than 50 ms, whereas all latencies obtained in the 6-OHDA-treated rats were shorter than 50 ms. These results suggest two types of axonal regeneration of LC neurons: (i) LC neurons projecting to the cerebral cortex lose their distal axonal arborization in the cortex and project alternatively to more proximal sites in the dLGN with regenerating axons; (ii) LC axons projecting to the dLGN with a long trajectory are lost, and regeneration occurs with axons of a shorter trajectory.