Peripheral projections of a subpopulation of dorsal root ganglion neurons defined by ovalbumin immunoreactivity

Previous studies from this laboratory have used antisera to aldehyde-conjugated ovalbumin to localize ovalbumin-like immunoreactivity within a subpopulation of sensory neurons. We have now combined retrograde tracing and immunohistochemical procedures to identify the tissues innervated by sensory neurons which are either immunoreactive or non-immunoreactive for ovalbumin. The fluorescent tracer Di-I was administered to feather follicles, flexor ulnar muscle, subdermis, expansor secundariorum, heart and liver and identified seven days later within corresponding dorsal root ganglia. Most neurons innervating the follicles had large cell somata, and fewer than 3% were immunoreactive for ovalbumin. In contrast, most sensory neurons projecting to subdermis, muscle and expansor secundariorum muscle were of a medium diameter. Approximately 25% of those neurons projecting to the expansor secundariorum, and 60% projecting to the subdermis and muscle, were immunoreactive for ovalbumin. Sensory neurons innervating heart and liver were the smallest, and only 8% were immunoreactive for ovalbumin. The study indicates that sensory neurons innervating different organs have somata with significantly different sizes, suggesting a functional specificity. Moreover, neurons demonstrating either the ovalbumin-IR positive or negative phenotypes show distinct peripheral projections, suggesting that this phenotype may be at least partially controlled by retrograde signals derived from the cells they innervate.