Differential uptake of HRP by intact axon terminals versus transected axons: a study on bulbospinal fibers in the dorsolateral funiculus.

A comparison was made of the amount of HRP uptake from transected versus intact axonal endings of rostral ventromedial medulla (RVMM) and locus coeruleus cells projecting via the dorsolateral funiculus in the rat. We found that retrograde labelling in the RVMM was reliably different between treatments, while locus coeruleus retrograde labelling was highly variable and not reliably different. HRP applied to intact endings retrogradely labelled approximately twice as many RVMM cells, including 25 times as many 5-hydroxytryptamine (B3) cells, as HRP applied to transected axons. In this first quantitative assessment of HRP uptake from transected versus intact endings, reliable differences have been found. These results have implications for the neural circuitry involved in pain modulatory systems.