Neurotoxicity, blood-brain barrier breakdown, demyelination and remyelination associated with NMDA-induced lesions of the rat lateral hypothalamus.

Excitotoxins have been widely used to make lesions in the brains of experimental animals because they have the ability to destroy neurones while sparing fibres of passage. Because loss of fibres of passage can confound the interpretation of lesion effects, this property is of considerable value. Recently, however, there have been reports indicating that excitotoxins acting at different sites within the rat CNS not only destroy neurones but also strip myelin from fibres and compromise the integrity of the blood-brain barrier. However, some reports also indicate that the myelin content of the lesioned area recovers. Excitotoxic lesions of the lateral hypothalamus have been shown to produce local demyelination. The present studies sought to investigate this effect further by (1) defining the time course of demyelination and possible remyelination after excitotoxic lesions of the lateral hypothalamus made with N-methyl-D-aspartate (NMDA); (2) establishing the relationships between neuronal loss, de- and remyelination after various doses of NMDA; and (3) examining the integrity of the blood-brain barrier using an immunohistochemical probe. Our data show that after injection of NMDA into the lateral hypothalamus there was neuronal loss, blood-brain barrier disruption (followed by recovery over approximately 12 days), triggering of reactive gliosis, invasion of the lesioned area by cells from outwith the CNS, demyelination over an area coexistent with but not exceeding the area of neuronal loss, and remyelination. Remyelination occurred over a period of 3 months following the production of the lesion and was associated initially with blood vessels. It occurred across the whole of the lesioned area, not by encroachment from the borders. All doses of NMDA that produced neuronal death also produced demyelination. These data confirm that excitotoxic lesions of the lateral hypothalamus demyelinate fibres, but show for the first time that remyelination occurs here. They are consistent with reports concerning excitotoxin actions at other CNS sites and indicate that de- and remyelination after excitotoxic lesions is a ubiquitous process. Consideration should be given to this when using excitotoxins to make fibre-sparing lesions.