ELECTRON-MICROSCOPIC LOCALIZATION OF NERVE GROWTH-FACTOR RECEPTOR (P75)-IMMUNOREACTIVITY IN PARS CAUDALIS MEDULLARY DORSAL HORN OF THE CAT

Previous studies have demonstrated the presence of nerve growth factor receptor [NGFr(p75)]-immunoreactivity (IR) in the spinal trigeminal n ucleus of both 8-10 week-old kittens and mature cats. Most of the NGFr (p75)-IR is lost following retrogasserian rhizotomy, indicating that t he majority of the NGFr(p75)-IR within the spinal trigeminal nucleus i s of trigeminal primary afferent origin. Here, we examined the ultrast ructural localization of NGFr(p75)-IR within lamina II outer of pars c audalis/medullary dorsal horn in the mature cat. Lamina II outer repre sents a location where dense NGFr(p75)-IR is seen with the light micro scope. The NGFr(p75)-IR identified with the electron microscope was lo cated within small thinly myelinated and unmyelinated axons and within axon terminals. The terminals with NGFr(p75)-IR typically formed asym metric synaptic specializations onto dendritic profiles and at times w ere postsynaptic-to other axon terminals at symmetric synaptic special izations. The terminals with NGFr(p75)-IR were either simple (associat ed with a single profile) or more complex, such as those that typicall y formed the central element in synaptic glomeruli. The NGFr(p75)-IR i n terminals was especially prominent on microtubules and the plasmalem ma and these findings are consistent with proposed roles for NGFr(p75) in axoplasmic/neuronal transport and as a membrane protein, respectiv ely. The profiles with NGFr(p75)-IR seen with the electron microscope indicate a primary afferent origin and show some similarities when com pared to other markers of primary afferent fibers such as calcitonin g ene-related peptide. In addition, a possible role for NGFr(p75) in the transmission of nociceptive stimuli is also discussed.