An adrenal-mediated, naloxone-reversible increase in reaction time in the tail-flick test following intrathecal administration of substance P at the lower thoracic spinal level in the rat.

We have shown previously that intrathecal administration of substance P to the lower thoracic vertebral level increases sympathetic output and increases the adrenal output of catecholamines. As opioid peptides are co-released with catecholamines from the adrenal medullae, experiments were done to determine whether the intrathecal administration of substance P to the eighth thoracic vertebral level would alter reaction time in the tail-flick test. Intrathecal administration of substance P (6.5 nmoles in artificial cerebrospinal fluid) in the awake restrained rat increased the reaction time at 1 and 6 min after injection to about 130-140% of the preadministration values; reaction time returned to preadministration values by 11 min. Similar administration of cerebrospinal fluid was without effect. Administration of 6.5 nmoles of thyrotropin-releasing hormone or oxytocin, peptides which also increase sympathetic output, failed to mimic the effects of substance P. The substance P-induced increase in reaction time was absent in rats which had been medullectomized and in rats pretreated with naloxone (10 mg/kg). Pretreatment with 10 mg/kg of either phentolamine or the quaternary opiate antagonists, nalorphine methochloride and naloxone methobromide, had no effect on the substance P-induced increase in reaction time. These results suggest that substance P given intrathecally to the eighth thoracic vertebral level may activate spinal sympathetic neurons to the adrenal medullae to cause the release of an opioid into the circulation. This circulating opioid may in turn play a role in the regulation of the tail-flick reflex by a centrally-mediated depression.