Ultrastructural localization of brain-derived neurotrophic factor in rat primary sensory neurons

In a previous study we have shown that brain-derived neurotrophic factor (BDNF) is present in a subpopulation of small- to medium-sized sensory neurons in the dorsal root ganglia (DRG) and is anterogradely transported in both the peripheral and central processes. Within the spinal cord, BDNF is localized to varicosities of sensory nerve terminals in laminae I and II of the dorsal horn. This study raised the question of whether BDNF is localized in synaptic vesicles of the afferent nerve terminals. Using immunohistochemical and immunocytochemical techniques we have now investigated the ultrastructural localization of BDNF in the spinal cord of the rat. In addition, its colocalization with the low affinity neurotrophin receptor, p75, and calcitonin gene related peptide (CGRP) was also investigated. In lamina II of the spinal cord, BDNF immunoreactivity was restricted to nerve terminals. The reaction product appeared associated with dense-cored and clear vesicles of terminals superficial laminae. Double labelling experiments at the light microscopic level showed that 55% of BDNF immunoreactive neurons in DRG are colocalized with CGRP and many nerve terminals in laminae I and II of the spinal cord contained both BDNF and CGRP immunoreactivities. The results of double labelling at the ultrastructural level showed that most BDNF-ir (immunoreactive) nerve terminals contained CGRP or the low affinity neurotrophin receptor, p75, but not vice versa. These results point to the conclusion that BDNF may be released in parallel with neurotransmitters from nerve terminals in the spinal cord from a subpopulation of nociceptive primary afferents.