Preventing regeneration of infraorbital axons does not alter the ganglionic or transganglionic consequences of neonatal transection of this trigeminal branch

Abstract

Retrograde and transganglionic tracing with a combination of horseradish peroxidase (HRP) and wheatgerm agglutinin (WGA) — conjugated HRP (WGA-HRP) was employed to determine whether transection of the infraorbital (IO) nerve on the day of birth and prevention of regeneration by retransecting it at weekly intervals until the time of a terminal anatomical experiment had effects upon ganglion cell survival and innervation of the brainstem by this trigeminal (V) branch that differed from those which followed a single transection of the same nerve on the day of birth without any attempt to prevent peripheral regeneration of the cut axons. Counts of labelled ganglion cells and examination of the brainstem labelling produced by application of HRP and WGA-HRP to the IO nerve proximal to the point of transection(s) at 6 weeks of age demonstrated no differential effects of preventing regeneration of the cut nerve. In animals subjected to a single transection of the nerve ( n = 9), we counted an average of 5001.2 (S.D. = 1286.9) labelled ganglion cells and these had an average diameter of 22.7 μm (S.D. = 6.3). In the rats ( n = 9) that sustained multiple nerve cuts, the average number of labelled ganglion cells was 4447.8 (S.D. = 1060.9). The mean diameter for these primary afferent neurons was 21.5 μm (S.D. = 6.6). Neither of these values were significantly different from those from the rats subjected to a single nerve cut. The cell counts from both of these groups were significantly lower than those obtained after application of HRP and WGA-HRP to the IO nerve in normal rats ( n = 3, ¯X = 12,553.3, S.D. = 1454.8), but the average cell diameter in the normals (X = 23.2, S.D. = 6.6) was not significantly greater than that in the nerve-damaged animals. The pattern of brainstem labelling observed in the rats subjected to multiple nerve cuts was the same as that in the rats which sustained a single transection of the IO nerve on the day of birth 16. Very little terminal labelling was observed in nucleus principalis, subnucleus oralis, subnucleus interpolaris or the magnocellular portion of caudalis. There was, however, very heavy labelling in laminae I and II of the latter nucleus.