Intracranial facial nerve lesion : experimental study on neural degeneration and its treatment

Abstract

The aim of this thesis was to develop an experimental model that would generate extensive motor neuronal degeneration in adult rats, and furthermore to study mechanisms causing nerve cell death and effects of putative neuroprotective compounds. The facial nerve was chosen because it is well characterized and allows evaluation of functional recovery by analyzing vibrissae movements. Using a microsurgical technique, the facial nerve was exposed in the petrosal bone and along its intracranial course in the subarachnoid space where it was cut or compressed near the brainstem. This proximal injury may reflect the situation for some patients suffering from long-lasting facial palsies due to skull-base fractures or tumor growth in the pontine angle.

We found that the intracranial portion of the facial nerve was more sensitive than the distal part of the nerve, in terms of neuronal degeneration. A massive motor neuronal death was found after intracranial facial nerve transection (73% cell death after one month). Intracranial facial nerve crush also generated nerve cell death (13% after one month). These data suggest that the intracranial portion of the facial nerve is vulnerable in a unique fashion, in comparison to other peripheral nerves. Furthermore, adult facial motor neurons demonstrated apoptotic death after intracranial transection, in contrast to a distal injury, which is yet another unique response to transection of adult peripheral motor neurons. Moreover, following intracranial transection, reactive microglia transformed into ED I-expressing macrophages, in the ipsilateral facial nucleus, which was not the case after the distal injury. This reaction pattern, which distinguishes intracranial facial nerve transection from the distal lesion as well as from other peripheral nerve injuries, may partly explain the poor outcome that occur in some patients, when the intracranial portion of the facial nerve is lesioned in patients.

Several mechanisms have been suggested to contribute to neuronal death following axotomy, such as loss of trophic support from the periphery, accumulation of neurotoxic compounds in the vicinity of neuronal somata, like reactive oxygen species and excitotoxic amino acids. Some of these neurotoxic compounds are released from reactive microglial cells. Here we have studied yet another potential cell death promoting mechanism, the activation of the complement cascade. Using immunocytochemistry and in situ hybridization, we provide evidence for activation of the complement cascade locally in the ipsilateral facial nucleus, in the immediate vicinity to the axotomized neurons. Facial motor neurons also responded with an upregulation of the complement regulators cluster in and CD59. Reactive oxygen species as well as the other neurotoxic components mentioned above, including the complement cascade, may cause an intracellular overload of calcium, however, by different mechanisms. We have therefore within the frame of the present thesis also evaluated potential neuroprotective effects of two antioxidants, betamethasone and the pyrrolopyrimidine PNU-101033-E, as well as the calcium flow antagonist nimodipine. No neuroprotective effects of high-dose betamethasone could be found, however, a dramatic neuroprotective effect of pyrrolopyrimidines as well as nimodipine could be demonstrated. In addition, nimodipine promoted axonal regeneration. We found increased numbers of myelinated axons as well as increased axon and myelin dimensions ipsilateral to intracranial facial nerve crush, and furthermore, an improved facial nerve function, studied by analysis of vibrissae movements.

In conclusion, proximal, intracranial lesion of the adult facial nerve cause severe consequences in the facial nucleus in terms of neuronal degeneration, some of which might be due to substances released by reactive microglia, like complement. This intense degeneration can be reversed by administration of pyrrolopyrimidines or nimodipine, both of which may become new strategies for treatment of patients suffering from cranial or other peripheral nerve injuries. We suggest that these drugs should be further evaluated for clinical use.