The effects of age on atrophy and recovery in denervated fiber types of the rat nasolabialis muscle

Abstract

This study investigates the effects of advancing age on responses of nasolabialis muscle fibers to denervation and reinnervation. The nasolabialis is innervated by the facial nerve and is responsible for the whisking movement of the animal's large vibrissae. In young adult (3-month) and middle-aged (15-month) rats the muscle on one side of the head was denervated by crushing the facial nerve. At specific days postcrush, animals were sacrificed and thick sections of muscle were incubated to demonstrate cytochrome oxidase activity, a mitochondrial enzyme, which differentiated between red, white, and intermediate fiber types. The rate and extent of atrophy and recovery were evaluated using light microscope morphometric methods for which transverse fiber areas were measured and compared to fibers on the contralateral control side. There was an age-related delay in the time of functional return since older animals resumed normal whisking behavior 6 days later than the younger animals. In both age groups, white and intermediate fibers atrophied to the greatest extent and red fibers showed least atrophy. Despite the different responses of the fiber types to denervation, there was no age difference in the maximum degree of fiber atrophy within each fiber type. Age differences did occur in the rate of the denervation response since the middle-aged fibers consistently showed a more rapid significant atrophy than the young adult fibers. During recovery, older fibers may be limited in their ability to attain the size of fibers on the control side. The results indicate that through middle age, the process of advancing age increases the susceptibility of the nasolabialis muscle to denervation but does not alter the maximum extent of atrophy. The ability to recovery to normal fiber size, at least 2 months after denervation, is also age-related.

Age effects on cytochrome oxidase activities during denervation and recovery of three muscle fiber types

“…In this study, the distance between site of injury and nasolabialis muscle was the same (40 mm) in both the young and middle-aged animals, although 15-month animals were heavier and had larger muscle fibers than 3-month animals (White and Vaughan, 1990). Upon the initial crush injury, animals were unable to move their whiskers voluntarily.…”

“…In this report, COX activity is quantified in the rat nasolabialis muscle, a muscle innervated by the facial nerve and responsible for moving the animal's large mystacial vibrissae (Dorfl, 1982;Watson et al, 1982). The nasolabialis is comprised of a mosaic of red, white, and intermediate muscle fiber types which can be recognized by the intensity of reaction 8 1991 WILEY-LISS, INC product within them following incubation in a variety of media, including that for COX (Hogenhuis and Engel, 1965;White and Vaughan, 1990). As described by several investigators (Stein and Padykula, 1962;Gauthier, 1969), red fibers are mitochondrial rich and react intensely upon incubation in reaction medium used to demonstrate mitochondrial enzymes, white fibers have sparse mitochondria and react minimally, and intermediate fibers have a moderate amount of mitochondria and their reaction is intermediate between the red and white fibers.…”

“…In the present study, tissue sections of rat nasolabialis are reacted to demonstrate COX activity, and the density of reaction product is quantified with microdensitometry in individual muscle fibers. Previous studies of the effects of advancing age on the responses of this rat facial motor system to axotomy include the analysis of the morphological responses of these fibers (White and Vaughan, 1990), and investigations of both the morphological (Vaughan, 1990a) and enzymatic (Vaughan, 1990b) changes occurring centrally in the facial nucleus.…”

Effects of peripheral axotomy on presynaptic axon terminals with GABA‐like immunoreactivity

“…Since the facial nucleus consists entirely of motor neurons (Courville, 1966;McCall and Aghajanian, 1979), all cell somata and dendrites postsynaptic to these axon terminals can be assumed to be those of facial motor neurons, all of which have experienced axotomy and are involved in regrowing an axon. Our own previous studies employing this animal model provide a background of temporal patterns of responses of not only the neuronal somata, but also, in the periphery, their regenerating axons and the denervated muscle fibers (e.g., Vaughan, 1990aVaughan, , 1992White and Vaughan, 1991). The observations in this study are confined to the region of the nucleus, the lateral part, which contains motor neurons whose axons project to the muscle fibers responsible for whisking the animal's vibrissae (Klein and Rhoades, 1985;Friauf, 1986).…”

“…Consequently, change in the neuropil can be correlated with the stage of axon reinnervation since the condition of target innervation can be correlated grossly with loss and recovery of the whisking behavior of the animal. The present study examines motor neurons whose axons have not yet reinnervated the denervated muscles but will, beginning 2 days later (e.g., White and Vaughan, 1991).…”