Tetrodotoxin-resistant fibres and spinal Fos expression: differences between input from muscle and skin
STUDY DESIGN Acute noxious stimulation delivered to lumbar muscles and skin of rats was used to study Fos expression patterns in the brain and spinal cord. OBJECTIVES The present study was conducted to determine the differences in Fos expression in the brain and spinal cord as evoked by stimuli delivered to lumbar muscles and skin in rats. SUMMARY OF BACKGROUND DATA Patients with low back pain sometimes show psychological symptoms, such as quiescence, loss of interest, decreased activities, appetite loss, and restlessness. The pathway of deep somatic pain to the brain has been reported to be different from that of cutaneous pain. However, Fos expression has not been studied in the central nervous systems after stimulation of low back muscles. METHODS Rats were injected with 100 L of 5% formalin into the multifidus muscle (deep pain group; n = 10) and into the back skin of the L5 dermatome (cutaneous pain group; n = 10). Two hours after injection, the distribution of Fos-immunoreactive neurons was studied in the brain and spinal cord. RESULTS Fos-immunoreactive neurons were observed in laminae I-V in the spinal cord in the cutaneous pain group, but they were not seen in lamina II in the deep pain group. In the brain, Fos-immunoreactive neurons were significantly more numerous in the deep pain group than in the cutaneous pain group in the piriform cortex, the accumbens nucleus core, the basolateral nucleus of amygdala, the paraventricular hypothalamic nucleus, the ventral tegmental area, and the ventrolateral periaqueductal gray. CONCLUSION The finding that Fos-immunoreactive neurons were absent from lamina II of the spinal cord in the deep pain group is similar to that of the projection pattern of the visceral pain pathway. Fos expression in the ventrolateral periaqueductal gray in the deep pain group may represent a reaction of quiescence and a loss of interest, activities, or appetite. Furthermore, the detection of large numbers of Fos-immunoreactive neurons in the core of accumbens nucleus, basolateral nucleus of amygdala, paraventricular hypothalamic nucleus, and ventral tegmental area in the deep pain group may suggest a dominant reaction of dopaminergic neurons to stress, and a different information processing pathway than from that of cutaneous pain.