Fos expression in spinothalamic and postsynaptic dorsal column neurons following noxious visceral and cutaneous stimuli

The spinothalamic tract (STT) has been classically viewed as the major ascending pathway for pain transmission while the dorsal column (DC) was thought to be involved primarily in signaling innocuous stimuli. Recent clinical studies have shown that limited midline myelotomy, which transects fibers in the DC, offers good pain relief in patients with visceral cancer pain. Experimental studies provided evidence that a DC lesion decreases the activation of thalamic neurons by visceral stimuli and suggested that this effect is due to transection of the axons of postsynaptic dorsal column (PSDC) neurons. In our study, Fos protein expression in retrogradely labeled STT and PSDC neurons in the lumbosacral enlargement in rats was used as an anatomical marker of enhanced activation to compare the role of these neurons in cutaneous and visceral pain. The noxious stimuli used were intradermal injection of capsaicin and distention of the ureter. Retrogradely labeled PSDC neurons were found in laminae III–IV and in the vicinity of the central canal. STT neurons were located in laminae I, III–VII and X. Ureter distention evoked Fos expression in PSDC and STT neurons located in all laminae in which retrogradely labeled cells were found, with the maximum in the L 2 spinal segment. The Fos-positive PSDC neurons represented a significantly higher percentage of the retrogradely labeled PSDC neurons (19.3±2.3% SEM) than of the STT Fos-positive neurons (13.2±1.5% SEM). Intradermal capsaicin injection also evoked Fos expression in both PSDC and STT neurons, but with no significant difference between these two, when expressed as a percentage of the retrogradely labeled cells (11.6±2.9% SEM, 10.8±1.1% SEM). These results show that both PSDC and STT neurons are activated by cutaneous and visceral noxious stimuli. Their particular role in transmission and modulation of painful stimuli needs to be investigated further.