DESCENDING INHIBITION IN NEONATAL RAT SPINAL-CORD - ACTIONS OF PENTOBARBITAL AND MORPHINE

Descending inhibition plays an important role in modulating spinal noc iceptive neurotransmission. Barbiturates have been suggested to be poo r analgesics or anti-analgesic because they block descending inhibitio n from supraspinal centers to the spinal cord. Opiate analgesics, on t he other hand, are postulated to increase descending inhibition. We te sted this hypothesis in an isolated brain stem-spinal cord preparation from neonatal rats, using as the test response a nociceptive-related slow ventral root potential (sVRP) recorded in the lumbar region. Brai n stem and spinal cord were separately perfused. Transecting the spina l cord, applying the local anesthetic lidocaine to the brain stem, or cooling the brain stem increased the area of the sVRP, thus demonstrat ing that tonic descending inhibition is present in this preparation. P entobarbital (Pb) (1-10 mu M) applied to the spinal cord depressed the sVRP in a dose-dependent fashion. Spinal cord transection did not sig nificantly change Pb potency. Pb (5-10 mu M) applied to the brain stem alone did not significantly increase sVRP amplitude. Morphine (15-35 nM) applied to the spinal cord also depressed the sVRP but had no effe ct when applied to the brain stem. The results show that there are fun ctional synaptic connections mediating tonic descending inhibition in the neonatal rat. They do not support interaction with tonic descendin g inhibition as an explanation for morphine analgesia or as a reason f or lack of analgesic properties in the barbiturates.