Brain-Derived Neurotrofic Factor and its Receptor TrkB are Present, but Segregated, Within Mature Cutaneous Pacinian Corpuscles of Macaca fascicularis

Some mechanoreceptors in mammals depend totally or in part on the neurotrophins brain-derived neurotrophic factor (BDNF) and neurotrophin-4 (NT-4), and their receptor TrkB, for development and maintenance. These actions are presumably exerced regulating the survival of discrete sensory neurons in the dorsal root ganglia which form mechanoreceptors at the periphery. In addition, the cells forming the mechanoreceptors also express both neurotrophins and their receptors although large differences have been described among species. Pacinian corpuscles are rapidly adapting low-threshold mechanoreceptors whose dependence from neurotrophins is not known. In the present study, we analyzed expression of TrkB and their ligands BDNF and NT-4 in the cutaneous Pacinian corpuscles of Macaca fascicularis using immunohistochemistry and fluorescent microscopy. TrkB immunoreactivity was found in Pacinian corpuscles where it co-localized with neuron-specific enolase, and occasionally with S100 protein, thus suggesting that TrkB expression is primarily into axons but also in the lamellar cells and even in the outer core. On the other hand, BDNF immunoreactivity was found the inner core cells where it co-localized with S100 protein but also in the innermost layers of the outer core; NT-4 immunostaining was not detected. These results describe for the first time the expression and distribution of a full neurotrophin system in the axon-inner core complex of mature Pacinian corpuscles. The data support previous findings demonstrating large differences in the expression of BDNF-TrkB in mammalian mechanoreceptors, and also suggest the existence of a retrograde trophic signaling mechanism to maintain morphological and functional integrity of sensory neurons supplying Pacinian corpuscles.