Effects of X-radiation on the subcellular distribution of cholinergic enzymes in the developing rat cerebellum

Abstract

The developmental pattern of ChAc and AChE activity studied in cerebellar homogenates and subcellular fractions of control and neonatally X-radiated rats. The activity of AChE in cerebellar homogenate increases with age, while that of ChAc decreases; yet the developmental pattern of total enzymatic activity in the cerebellar structure as a whole shows a progressive increase with age, common to both enzymes. The divergence in the developmental pattern of the two enzymes with respect to activity in the homogenate was also observed in all fractions except in the nuclear fraction, which contains the mossy fiber endings, where a progressive increase was observed in both enzymes. When results were expressed in terms of specific activity, we noted that ChAc progressively decreased with age in homogenates as well as in all subfractions, again with the exception of the nuclear fraction, in which it did not change significantly. The specific activity of AChE, however, showed no significant changes in the homogenate and soluble fraction, an increase in the nuclear fraction and progressive decrease, similar to that observed in ChAc, in the microsomal and mitochondrial compartments as well as in mitochondrial subcomponents (myelin, synaptosomes and free mitochondrial). We have concluded that despite the increase in total cholinergic activity with age, the increase in cerebellar mass is proportionately greater and, therefore, the specific activity of ChAc and AChE shows a decrease or no change in cerebellar homogenates and a decrease in most fractions and, particularly, the synaptosomal fraction. The divergent pattern of AChE and ChAc activity in cerebellar homogenates as well as the much higher activity of AChE relative to ChAc is thought to represent a more active synthesis of AChE, associated with a high breakdown of ACh at this time. Neonatal X-radiation induces an increase in the activity and specific activity of both enzymes, and this increase is manifested in all subfractions but most significantly in the microsomal and synaptosomal fractions. In spite of the increase in activity and specific activity observed in both enzymes, total cerebellar activity was depressed, indicating that although the number and/or activity of cholinergic neurons is depressed by X-radiation, the depression of noncholinergic neurons and overall cerebellar mass is greater, so that the net effect on enzymatic activity is an increase.