ALTERATIONS IN MYSTACIAL PAD INNERVATION IN THE AGED RAT

It is well established that sensory perception becomes impaired with a dvancing age and that, in parallel, dystrophy and degeneration of axon s occur in sensory pathways. In this study, the impact of aging was ex amined in the mystacial pad, which receives a large variety of sensory nerve endings organized in a highly predictable pattern. Mystacial pa d specimens from aged (30 months old) and young adult (2-3 months old) female Sprague-Dawley rats were processed, in parallel. for immunohis tochemical analyses with antibodies against human neuronal cytoplasmic protein (protein gene product 9.5), transmitter enzymes, and several neuropeptides. Several changes in cutaneous innervation including both degenerative and regenerative processes were evident in the aged rat: (1) the Merkel endings and lanceolate endings that emanate from large -caliber afferents in the whisker follicles were reduced and shouted s ig ns of degeneration. Furthermore, a reduction of piloneural complexe s at the intervibrissal hairs were evident, but only in aged rats that showed more severe behavioral sensorimotor disturbances. In contrast, Ruffini endings as well as mechanoreceptors emanating: from medium-ca liber axons, i.e., transverse lanceolate and reticular endings, appear ed normal. (2) A reduction was evident among two sets of unmyelinated epidermal endings; however, the epidermal innervation affiliated with the intervibrissal hairs appeared normal in the aged rat. (3) A loss o f sympathetic neuropeptide tyrosine (NPY) or tyrosine hydroxylase-immu noreactive (IR) and somatosensory Calcitonin gene-related peptide (CGR P)-IR perivascular axons was paralleled by an increase in presumed par asympathetic NPY/CGRP-IR axons. (4) Two ''novel'' networks of fine-cal iber axons were observed in the outer and inner root sheaths of the wh isker follicles in the aged rat. (5) NPY was present in a population o f small-caliber, somatosensory CGRP-IR axons in the aged rat. This may represent a de novo synthesis, since, normally, NPY-like immunoreacti vity is not observed in this set of axons. Our results suggest that th e sensory impairments occurring with advancing age are part of a perip heral process instigated by changes in nerve-target interactions and/o r incapacitation of the neuronal machinery to sustain the axonal integ rity.