RELATIVE CONTRIBUTION OF SYMPATHETIC AND SENSORY NERVES TO THERMAL NOCICEPTION AND TISSUE TROPHISM IN RATS

Neonatal injection of 6-hydroxydopamine (420 mg/kg s.c.) lowered therm al nociceptive threshold (hot plate and tail immersion tests) and incr eased levels of substance P-like immunoreactivity in the skin (paws, t ail, area of vibrissae) of Wistar rats. Chemical ablation of primary a fferents, induced in either neonatal or adult rats by systemic adminis tration of capsaicin, increased thermal nociceptive threshold (hot pla te), irrespective of 6-hydroxydopamine pretreatment, and reduced subst ance P-like immunoreactivity in the hind-paw skin of either control or sympathectomized rats. Capsaicin pretreatment of neonatal but not adu lt rats produced antinociceptive effect in the tail-immersion test and completely reversed the hyperalgesic effect of sympathectomy, without affecting levels of substance P-like immunoreactivity in the tail ski n. These findings indicate that sympathetic nerves and different subse ts of capsaicin-sensitive primary afferents are involved in the proces sing of thermal nociceptive input. Corneal and cutaneous lesions were induced by neonatal sensory denervation with capsaicin. Sympathectomy afforded protection against the development of corneal pathology, whil e it did not affect the occurrence of cutaneous lesions. It appears th at a balance in the neuronal activity between sympathetic neurons and trigeminal sensory neurons is critical for maintaining the normal trop hism of the cornea, and that sensory neuropeptides play a key role in the maintenance of normal trophism of the skin.