Effect of fetal infraorbital nerve transection upon trigeminal primary afferent projections in the rat

Abstract

Transganglionic tracing with a combination of horseradish peroxidase (HRP) and wheat germ agglutinin-conjugated HRP (WGA-HRP) was employed to compare the trigeminal (V) innervation of the brainstem in adult rats that sustained transection of the infraorbital nerve (ION) on either the day of birth or just prior to the beginning of the 17th embryonic day (E-17). The same methods were also employed to assess the effects of such lesions upon the innervation of the brainstem by the lingual, inferior alveolar, mylohyoid, and auriculotemporal V branches. Previous experiments (Chiaia et al.: Dev. Brain Res. 36:75-88, '87) showed that application of HRP and WGA-HRP to the ION in normal adult rats (N = 3) labelled 12,553 +/- 1,455 (mean +/- s.d.) V ganglion cells while application of these tracers to the regenerated ION after neonatal transection (N = 9) labelled 5,001 +/- 1,287 ganglion cells. Application of HRP and WGA-HRP to the regenerated ION in adulthood (N = 6) after fetal transection labelled 5,476 +/- 3,056 ganglion cells. Thus, the numbers of ganglion cells giving rise to the regenerated ION after fetal and neonatal transection were equivalent (P greater than .05). The central projections of the ION after fetal transection were qualitatively different from those observed after neonatal injury. After neonatal transection, the central terminal field of regenerated ION fibers in adulthood is almost completely restricted to layers I and II of subnucleus caudalis (SpC; Jacquin and Rhoades: Brain Res. 269:137-144, '83; Chiaia et al.: Dev. Brain Res. 36:75-88, '87). After fetal transection, regenerated ION axons terminate heavily in all portions of the V brainstem complex. After neonatal ION transection, we (Jacquin and Rhoades: J. Comp. Neurol. 235:129-143, '85) have been unable to detect central sprouting of undamaged V mandibular axons by means of transganglionic tracing with HRP and WGA-HRP. Such sprouting was evident in both V subnucleus interpolaris (SpI) and SpC after fetal ION transection. We carried out one additional experiment to determine whether ION ganglion cells that survived fetal axotomy were more resistant to axonal damage than the population of neurons that normally contribute to this nerve on the day of birth. Rats (N = 5) sustained transection of the ION on E-17 and again on the day of birth. The regenerated ION was then labelled with HRP and WGA-HRP when the animals reached adulthood.(ABSTRACT TRUNCATED AT 400 WORDS)