Axonal Transport Blockade in the Neonatal Rat Optic Nerve Induces Limited Retinal Ganglion Cell Death

Abstract

Optic nerve section in the newborn rat results in a rapid apoptotic degeneration of most axotomized retinal ganglion cells (RGCs). This massive process of neuronal death has been ascribed mainly to the interruption of a trophic factor supply from target structures rather than to the axonal damage per se. To distinguish between these two possibilities, we induced a reversible axonal transport blockade in the developing optic nerve by topical application of a local anesthetic (lidocaine). Light and electron microscopy showed no alterations in the fine structure of treated optic nerves. Retinae of treated and control rats were stained with cresyl violet and examined at different times after surgery. We found that axonal transport blockade induced only a limited number of pyknotic RGCs. Degeneration of these cells was completely prevented by inhibiting protein synthesis during lidocaine application. We conclude that the rapid degeneration of RGCs after axotomy can be ascribed only partly to the loss of retrogradely transported trophic factors.Key words: neuronal death; axonal transport; retinal ganglion cells; apoptosis; rat optic nerve; retrograde degenerationRetinal ganglion cells (RGC s), as well as other mammalian CNS neurons, eventually die as a result of axonal injury (Grafstein and Ingoglia, 1982;Bregman and Reier, 1986;Hefti, 1986; VillegasPerez et al., 1993). Transection of the optic nerve is known to induce massive RGC death by an apoptotic process in both developing (Rabacchi et al., 1994a) and adult rats (Berkelaar et al., 1994;Garcia-Valenzuela et al., 1994). The molecular nature of the signal triggering the loss of axotomized neurons is largely unknown. According to the trophic theory, the effects of axotomy can be ascribed to the disconnection of neurons from their targets, resulting in the sudden loss of retrogradely transported trophic molecules (Oppenheim, 1991;Snider et al., 1992;Johnson and Deckwerth, 1993). An alternative hypothesis is that axonal injury per se can induce cell death. In particular, acute alterations in cellular homeostasis and ionic imbalances subsequent to plasmalemmal leakage, such as C a 2ϩ deregulation, have been implicated (Choi, 1992;Snider et al., 1992).Especially in the neonate, a lack of target-derived trophic signals is proposed as the main cause of death for axotomized RGC s. It is known that trophic factors, such as NGF and BDNF, are synthesized in the major retinal projection fields, the optic tectum (Hofer et al., 1990;Maisonpierre et al., 1990;Friedman et al., 1991) and the dorsal lateral geniculate nucleus (dLGN) (Schoups et al., 1995). E xogenous administration of NGF and BDN F, as well as other growth factors, can partly prevent RGC death after optic nerve lesion (C armignoto et al., 1989;Mey and Thanos, 1993;Cohen et al., 1994;Mansour-Robaey et al., 1994;Rabacchi et al., 1994b). Moreover, the survival of highly purified RGCs in culture can be enhanced by both tectal cells (McCaffery et al., 1982;Armson and Bennett, 1983) and soluble tectal factors (Meyer-Fran...

Retrograde Signaling in the Optic Nerve Is Necessary for Electrical Responsiveness of Retinal Ganglion Cells

“…39 Here, we addressed this problem by recording the PERG-a signal that depends on the physiologic integrity of RGC-before and after manipulations at postretinal level that impair retrograde signaling in the retinocollicular pathway. Lidocaine is a well established method to block axon transport [40][41][42] without damaging optic nerve structures, 43 and acts at very low concentrations. 44 At sufficient concentrations, lidocaine also is known to alter signal conduction by blocking voltage-gated sodium channels in the neuronal cell membrane, 45 thereby suppressing postsynaptic activity.…”

“…Reversible blockade of axon transport was not expected to cause damage to RGC, 43,46 whereas for optic nerve crush RGC loss was expected to start approximately 5 days after surgery. 21,47 However, the effects of the optic nerve crush could include an acute physiologic effect on RGC signaling 48 that added to the effect mediated by impairment of retrograde signaling.…”

Free Radical Scavenging and Inhibition of Nitric Oxide Synthase Potentiates the Neurotrophic Effects of Brain-Derived Neurotrophic Factor on Axotomized Retinal Ganglion CellsIn Vivo

“…Because of good surgical accessibility, lesion of the retinocollicular projection in the rat serves as a convenient model to study secondary death of injured CNS neurons (Villegas-Pérez et al, 1988Eschweiler and Bähr 1993;Mey and Thanos, 1993;Mansour-Robaey et al, 1994). In comparison with lesion models in newborn rats, transection of the adult ON holds the advantage that injury-induced RGC loss is not confounded by additional RGC degeneration attributable to deprivation of target-derived trophic support during the period of naturally occurring cell death (C arpenter et al, 1986;Fagiolini et al, 1997). For this reason, axotomy of the ON in the adult rat, a model with well documented spatiotemporal kinetics of cell loss, better reflects the f undamental pathophysiological sequelae of brain injury seen in clinical neurotraumatology.…”

Pressure related apoptosis in neuronal cell lines

“…Theories of glaucoma pathogenesis include mechanodistortion of the optic nerve at its exit from the eyeball affecting RGC transport mechanisms, and compression of optic nerve head microvasculature compromising neuronal circulation (reviewed by Quigley, 1995). It has been reported that inhibition of retrograde axonal flow can only partly explain RGC loss post axotomy (Fagiolini et al, 1997). There is evidence from other studies that RGC loss may be mediated independently of these mechanisms (Radius 1981;Johansson 1983Johansson , 1988.…”