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 t he adult mammalian spinal cord. Following dorsal root injury (rhizotomy), D RG 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 t o compare how neurites growing from embryonic (E13) and postnatal (P0 and a dult) 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 rhizot omized DREZ, However, embryonic DRG neurons more readily grow neurites acro ss cryosections of the uninjured and 7-day post-lesion (dpl) DREZ and are d ramatically better able to cross the 21 dpl DREZ. This enhanced growth was abolished by co-incubation with a function-blocking antiserum to beta 1-int egrin receptors, whilst immunoreactivity for some beta 1-integrin ligands ( tenascin-C and fibronectin) increased at the DREZ by 21 dpl, suggesting tha t beta 1-integrin ligands may stimulate the growth of embryonic neurites ac ross the 2f dpl DREZ. Fluorescence time-lapse videomicroscopy was used to r ecord the behaviour of dye-labelled postnatal DRG neurites as they encounte r the uninjured adult DREZ in vitro. Neurites rarely turned around at the D REZ, but instead became paralysed. Of a variety of chemical modifications t o uninjured DREZ cryosections, only treatment with methanol, chloroform, or the protease inhibitor D-phe-pro-arg chloromethylketone hydrochloride (PPA CK, 100 mu M) caused any increase in the proportion of postnatal neurites w hich crossed the DREZ. GLIA 26:309-323, 1999. ()C1999 Wiley-Liss, Inc.