Peripheral projections of rat primary sensory neurons immunoreactive for neurotrophin 3

Sensory neurons can be classified into subpopulations based on a variety of characteristics, including their morphology and physiological modalities. Whether any of these classifications correlates with neurotrophic sensitivities has not been determined, We have recently reported that a subpopulation of large diameter sensory neurons of the rat contain neurotrophin 3‐like immunoreactivity (NT3‐ir). In this study, we have further characterised NT3‐ir sensory neurons by their size, segmental localization, and peripheral projections by combined techniques of retrograde tracing and immunohistochemistry. The size distribution showed that NT3‐ir was localised to a subpopulation of large‐diameter neurons ranging from 560 to 3,120 m Greater numbers of NT3‐ir neurons reside in trigeminal (43% of total), cervical (36%), and lumbar (39%) than in thoracic spinal ganglia (13–17%). In combination with Fluoro‐Gold retrograde tracing, it was found that about 30% of sensory neurons projecting to the tibial muscle were NT3‐ir, compared with 39% for tendon, 50% for whisker hair follicles, 17% for subdermis or epidermis, and only 1% for kidney or adrenal gland. These studies indicate that NT3‐ir sensory neurons mainly project to skin and muscles but not viscera. Thus, the characterization of NT3‐ir spinal sensory neurons suggests that large sensory neurons subserving proprioception and mechanoception require NT3 for the maintenance of normal function. © 1995 Wiley‐Liss, Inc.