Intrathecal administration of 5,6-DHT or 5,7-DHT reduces morphine and substance P-antinociceptive activity in the rat.

The administration of SP (15 and 50 ug) and morphine sulphate (10 ug) either into the IV ventricle or intrathecally produces an analgesic effect. This effect was attenuated when the rats received previously an intrathecal dose of 5,6-DHT or 5,7-DHT. The consequences of neurotoxin administration upon monoamine descending systems were evaluated by measuring 14C-5HT and 3H-NA synaptosomal uptake in different structures of the CNS. SP levels were also determined in the animals injected with the neurotoxins. Our results confirm the existence of a relationship injected with the neurotoxins. Our results confirm the existence of a relationship between the 5HT and SP descending systems, which control the nociceptive information at the level of the dorsal horn of the spinal cord.