Noradrenergic innervation of the cerebellar cortex in normal and in Purkinje cell degeneration mutant mice: Evidence for long term survival following loss of the two major cerebellar cortical neuronal populations

Abstract

Purkinje cell degeneration mutant mice were examined during the course of Purkinje cell death (26 and 35 days old) and at 3, 5, 9 and 12 months of age. Glyoxylic acid fluorescence histochemistry for catecholamines was used to investigate possible alterations or reorganization of the noradrenergic fibers from the coeruleo-cerebellar system in response to the degeneration of two major cell types in the cerebellar cortex, of which one, the Purkinje cell, is reported to be the major target neuron. In control mice, noradrenergic fibers traveled in linear and tortuous profiles through the granule cell layer, formed pericellular arrays alongside Purkinje cell somata, and branched profusely into both radially oriented and longitudinally oriented chains. The density of noradrenergic varicosities diminished in the molecular layer with age. In the mutants, concomitant with the progressive shrinkage of the molecular layer, there was a progressive increase in the density of noradrenergic varicosities. This was most conspicuous at 9 and 12 months of age, at which time the molecular layer has been depleted not only of Purkinje cell dendrites, but also of parallel fibers. Noradrenergic fibers in these zones formed dense parallel bundles of varicose profiles whose density reached 621.3 ± 122.8% (mean ± SD, n = 4) at 9–12 months of age, compared with age-matched controls. Neurochemical measurement of norepinephrine content in whole cerebellum of the Purkinje cell degeneration mutants revealed no change compared with age-matched controls. We conclude that noradrenergic innervation persists in the cerebellar cortex despite the death of Purkinje cells and most of the granule cells. Although we found an increased density of varicosities in the molecular layer of mutant mice, progressing with age, we believe that this can be explained on the basis of the resultant geometry of the altered cerebellar cortex. It appears that the health of the environment surrounding the noradrenergic fibers in cerebellar cortex has little influence on their anatomical integrity.