Developmental discord among markers for cholinergic differentiation: In vitro time courses for early expression and responses to skeletal muscle extract

The effects of skeletal muscle extract on the development of CAT, ACh synthesis, high affinity choline uptake, and AChE activities were studied in dissociated ventral spinal cord cultures prepared from 14-day gestational rat embryos. In the absence of muscle extract, the development of CAT and AChE follow biphasic time courses in which they show initial declines followed by periods of steadily increasing activity. In contrast, ACh synthesis and high affinity choline uptake both gradually increase throughout the entire culture period. The presence of muscle extract both prevents the initial decline of CAT and AChE as well as stimulates the rates of development of all four cholinergic markers; however, the degrees and time courses of stimulation differ markedly. The effects of muscle extract on the kinetic and pharmacological properties of ACh synthesis and choline uptake in rat ventral cord cultures were also investigated. Cells treated with muscle extract for 2 days express both high affinity (K_m = 1.6 μM) and low affinity (K_m = 22 μM) choline uptake mechanisms. Control cells, on the other hand, express only low affinity uptake at this stage but develop a high affinity uptake mechanism by Day 7. During this time both ACh synthesis and high affinity choline uptake become increasingly sensitive to inhibition by hemicholinium-3. These results demonstrate that skeletal muscle factors enhance the development of cholinergic properties in embryonic spinal cord cultures. However, differences in sensitivity to muscle extract concentration, time courses of development, and degrees of stimulation suggest that these changes may involve distinct cellular mechanisms which are differentially affected by skeletal muscle factors.