Motor neuron scaling and somatic growth in normal and transgenic rainbow trout, Oncorhynchus mykiss

Abstract

Teleost fishes demonstrate indeterminate growth and a dramatic increase in body mass over their life cycle. Myotonal muscle mass comprises >60% of the adult body mass in active species and effective growth and development of functional capability requires interactions between multuinucleated muscle fibers (myocytes) and motor neurons. Basically, as all organisms grow and develops, the nervous system must compensate for the change in size. While numerous studies have examined myogenesis, muscle fiber recruitment and hypertrophy, and phenotypic plasticity in fishes, our understanding of the myocyte/neuron linkage during development is extremely limited. The goal of this project is to determine how motor neurons in rainbow trout change through development and how this change can be viewed through principles of scaling. Sections of spinal cord were removed at the rostral, medial, and caudal positions from normal and genetically modified rainbow trout to overexpress follistatin, which antagonizes myostatin and increases myotonal muscle. Increased muscling in these transgenic trout is due primarily to myocyte hyperplasia. Spinal cords (n= 35) were sectioned on a microtome and stained with acridine orange. Sections were analyzed to 1) measure the cross sectional area of the spinal cord and ventral horn motor neurons and 2) determine how motor neurons develop to compensate for growth in total muscle mass. The average motor neuron area from normal rainbow trout revealed an unusual allometric relationship compared to endothermic animals. As the mass of the fish increased there was a positive (r2 = 0.961) correlation to the motor neuron area. Compared with normal rainbow trout, transgenic trout had an average neuron area similar to that of the normal trout, but relative to per gram of body mass motor neuron is significantly less, suggesting that the motor unit size – the number of skeletal muscle fibers per motor neuron ‐ increases with an increase in muscle mass. Previous studies suggest that neuron size in endothermic organisms follow a pattern of metabolic allometric scaling, yet motor neuron from ectotherms in this study had scaling coefficient of just 0.204. These findings suggest that as the axial muscle increases mass in normal and transgenic rainbow trout, the peripheral nervous system is not undergoing a similar growth in neuron size.Support or Funding InformationNIH Grant #P20 RR0116454