INNERVATION OF PUTATIVE RAPIDLY ADAPTING MECHANORECEPTORS BY CALBINDIN-IMMUNOREACTIVE AND CALRETININ-IMMUNOREACTIVE PRIMARY SENSORY NEURONSIN THE RAT

Calbindin and calretinin are two homologous calcium-binding proteins t hat are expressed by subpopulations of primary sensory neurons. In the present work, we have studied the distribution of the neurons express ing calbindin and calretinin in dorsal root ganglia of the rat and the ir peripheral projections. Calbindin and calretinin immunoreactivities were expressed by subpopulations of large- and small-sized primary se nsory neurons and colocalized in a majority of large-sized ones. The a xons emerging from calbindin- or calretinin-immunoreactive neurons inn ervated muscle spindles, Pacini corpuscles and subepidermal lamellar c orpuscles in the glabrous skin, formed palisades of lanceolate endings around hairs and vibrissae, and gave rise to intraepidermal nerve end ings in the digital skin. Since most of these afferents are considered as rapidly adapting mechanoreceptors, it is concluded that calbindin- or calretinin-expressing neurons innervate particular mechanoreceptor s that display physiological characteristics of rapid adaptation to st imuli.