Vulnerability of cerebellar development in malnutrition—II. Intrinsic determination of total synaptic area on Purkinje cell spines

Abstract

In a previous investigation, we demonstrated a greater reduction in the ratio of granule cells to Purkinje cells in malnourished female rats than male rats that were reared under the same conditions ( Hillman & Chen, 1981 ). In this study, the synaptic relationships between these two types of cells were analyzed to determine how this difference in ratio was accommodated in cerebellar circuitry. Control diets (25% casein) and experimental diets (8% casein), having the same calories, were administered pregestationally and through lactation with continuation of the respective diets to offspring until death at 60 days. Molecular layer volume and synaptic density were determined and the relative number of synapses on each Purkinje cell was estimated from the number of synapses in the molecular layer and total number of Purkinje cells. The average length of synaptic profiles on Purkinje cell spines was measured and the average synaptic contact area on each Purkinje cell was calculated. We found that the average number of synapses on individual Purkinje cells decreased in the deficient female group but not in the deficient male group and there was an increase in the average length of synaptic profiles in the malnourished female group but not in the deficient male group. In some cerebella of females, elongated synaptic profiles on giant spines were seen on Purkinje cells. Comparison of the average number of synapses on each Purkinje cell with average length of synaptic profiles revealed an inverse relationship between size and number of synapses for two control and two experimental groups and a subgroup of females with giant spines. After conversion of average profile length to average contact area and taking into account the number of synapses on each Purkinje cell, we found that Purkinje cells for all five groups had the same total synaptic area. This inverse relationship was believed to reflect a constant total contact area for parallel fiber synapses on Purkinje cells. It is suggested that a constancy in total synaptic area means that the amounts of macromolecules that are incorporated into postsynaptic membrane specializations are determined intrinsically by the Purkinje cell. However, the distribution of these molecules to synaptic sites on dendritic trees can be shifted by interaction of parallel fiber synapses with the Purkinje cell.