Fos expression in the rat brain and spinal cord evoked by noxious stimulation to low back muscle and skin

Study Design. Acute noxious stimulation delivered to lumbar muscles and ski n 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 lu mbar muscles and skin in rats. Summary of Background Data. Patients with low back pain sometimes show psyc hological symptoms, such as quiescence, loss of interest, decreased activit ies, appetite loss, and restlessness. The pathway of deep somatic pain to t he brain has been reported to be different from that of cutaneous pain. How ever, Fos expression has not been studied in the central nervous systems af ter stimulation of tow back muscles. Methods, Rats were injected with 100 L of 5% formalin into the multifidus m uscle (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 spi nal cord in the cutaneous pain group, but they were not seen in lamina It i n the deep pain group. In the brain, Fos-immunoreactive neurons were signif icantly more numerous in the deep pain group than in the cutaneous pain gro up in the piriform cortex, the accumbens nucleus core, the basolateral nucl eus of amygdala, the paraventricular hypothalamic nucleus, the ventral tegm ental area, and the ventrolateral periaqueductal gray. Conclusion. The finding that Fos-immunoreactive neurons were absent from la mina 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 ven trolateral periaqueductal gray in the deep pain group may represent a react ion of quiescence and a loss of interest, activities, or appetite. Furtherm ore, the detection of large numbers of Fos-immunoreactive neurons in the co re of accumbens nucleus, basolateral nucleus of amygdala, paraventricular h ypothalamic nucleus, and ventral tegmental area in the deep pain group may suggest a dominant reaction of dopaminergic neurons to stress, and a differ ent information processing pathway than from that of cutaneous pain.