Innervation of putative rapidly adapting mechanoreceptors by calbindin- and calretinin-immunoreactive primary sensory neurons in the rat.

Calbindin and calretinin are two homologous calcium-binding proteins that are expressed by subpopulations of primary sensory neurons. In the present work, we have studied the distribution of the neurons expressing calbindin and calretinin in dorsal root ganglia of the rat and their peripheral projections. Calbindin and calretinin immunoreactivities were expressed by subpopulations of large- and small-sized primary sensory neurons and colocalized in a majority of large-sized ones. The axons emerging from calbindin- or calretinin-immunoreactive neurons innervated muscle spindles, Pacini corpuscles and subepidermal lamellar corpuscles in the glabrous skin, formed palisades of lanceolate endings around hairs and vibrissae, and gave rise to intraepidermal nerve endings 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 mechanoreceptors that display physiological characteristics of rapid adaptation to stimuli.