Excitotoxic lesions of the pedunculopontine tegmental nucleus of the rat. I. Comparison of the effects of various excitotoxins, with particular reference to the loss of immunohistochemically identified cholinergic neurons.

The pedunculopontine tegmental nucleus (PPTg) has been shown to have cholinergic connections with the thalamus and basal ganglia. The ability of various doses of the excitotoxins (alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) (AMPA), folate, ibotenate, kainate, N-methyl-D-aspartate (NMDA), quinolinate and quisqualate to make lesions in the PPTg was examined, with particular reference to their ability to destroy cholinergic neurons identified using choline acetyltransferase (ChAT) immunohistochemistry. All of the toxins induced convulsive activity on recovery from surgical anesthesia and all except folate made lesions in the PPTg and surrounding structures. The size of the lesions was computed following examination of Cresyl violet stained sections. The largest lesions were made by kainate = AMPA greater than NMDA = ibotenate greater than quisqualate = quinolinate. All of the toxins destroyed cholinergic neurons, higher doses producing greater loss than lower. The ratio of cholinergic cell loss to general neuronal loss (assessed by Cresyl violet staining) was also computed, revealing marked differences between the toxins. Statistical analysis showed that there were significant differences between excitotoxins in terms of this ratio, but these were accounted for by the low dose of quinolinate (24 nmol) producing a significantly greater ratio of damage (12.18:1) than every other toxin. (Next highest ratio: quisqualate 60 nmol, 6.22:1.) Between the other toxins (kainate, AMPA, ibotenate, quisqualate, NMDA and the high dose of quinolinate) there were no statistically significant differences. Intense calcium deposits (stained by Alizarin red) were found frequently and often defined the borders of the lesion. Tyrosine hydroxylase immunohistochemistry revealed axons running below and into the area of lesioned tissue suggesting strongly that fibers were undamaged by the lesions. We conclude that in the PPTg, different excitotoxins make discriminably different lesions, both quantitatively and qualitatively. Unlike excitotoxic lesions in the basal forebrain quinolinate, not quisqualate, made the most selective lesions of cholinergic neurons and, unlike excitotoxic lesions in the septal nuclei, non-myelinated fibers were spared by ibotenate. The implications of these data for research into brainstem mechanisms of Parkinson's disease are discussed.