Descending inhibition in neonatal rat spinal cord: actions of pentobarbital and morphine.

ABSTRACT

Descending inhibition plays an important role in modulating spinal nociceptive neurotransmission. Barbiturates have been suggested to be poor analgesics or anti-analgesic because they block descending inhibition from supraspinal centers to the spinal cord. Opiate analgesics, on the other hand, are postulated to increase descending inhibition. We tested 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. Brain stem and spinal cord were separately perfused. Transecting the spinal cord, applying the local anesthetic lidocaine to the brain stem, or cooling the brain stem increased the area of the sVRP, thus demonstrating that tonic descending inhibition is present in this preparation. Pentobarbital (Pb) (1-10 microM) applied to the spinal cord depressed the sVRP in a dose-dependent fashion. Spinal cord transection did not significantly change Pb potency. Pb (5-10 microM) 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 effect when applied to the brain stem. The results show that there are functional synaptic connections mediating tonic descending inhibition in the neonatal rat. They do not support interaction with tonic descending inhibition as an explanation for morphine analgesia or as a reason for lack of analgesic properties in the barbiturates.