Behaviour of DRG sensory neurites at the intact and injured adult rat dorsal root entry zone: postnatal neurites become paralysed, whilst injury improves the growth of embryonic neurites.

The dorsal root entry zone is a PNS-CNS junction between Schwann cells and astrocytes, defining the site where dorsal root ganglia (DRG) axons enter the adult mammalian spinal cord. Following dorsal root injury (rhizotomy), DRG axons regenerate within the PNS environment of the root but stop at the DREZ and fail to re-enter the spinal cord. We have used an in vitro model to compare how neurites growing from embryonic (E13) and postnatal (P0 and adult) DRG neurons behave at the uninjured and rhizotomized adult rat DREZ. We find that both freshly dissected and conditioned-lesioned postnatal DRG neurons seldom grow neurites across cryosections of the uninjured or rhizotomized DREZ. However, embryonic DRG neurons more readily grow neurites across cryosections of the uninjured and 7-day post-lesion (dpl) DREZ and are dramatically better able to cross the 21 dpl DREZ. This enhanced growth was abolished by co-incubation with a function-blocking antiserum to beta1-integrin receptors, whilst immunoreactivity for some beta1-integrin ligands (tenascin-C and fibronectin) increased at the DREZ by 21 dpl, suggesting that beta1-integrin ligands may stimulate the growth of embryonic neurites across the 21 dpl DREZ. Fluorescence time-lapse video-microscopy was used to record the behaviour of dye-labelled postnatal DRG neurites as they encounter the uninjured adult DREZ in vitro. Neurites rarely turned around at the DREZ, but instead became paralysed. Of a variety of chemical modifications to uninjured DREZ cryosections, only treatment with methanol, chloroform, or the protease inhibitor D-phe-pro-arg chloromethylketone hydrochloride (PPACK, 100 microM) caused any increase in the proportion of postnatal neurites which crossed the DREZ.