On the selectivity of intravenous mu- and kappa-opioids between nociceptive and non-nociceptive reflexes in the spinalized rat.
1. In electrophysiological experiments in spinalized, alpha-chloralose anaesthetized rats, opioids and anaesthetics were tested intravenously (i.v.) on the responses of individual motoneurones to alternating noxious (heat or pinch) and non-noxious (tap or vibration) stimuli. 2. On cells that were sensitive to low doses of mu-opioids, both fentanyl (0.5-4 micrograms kg-1 i.v.) and morphine (0.5 mg kg-1 i.v.) selectivity reduced reflexes to noxious stimuli to a greater degree than the higher doses required to reduce nociceptive reflexes (fentanyl 8 micrograms kg-1 i.v.; morphine 1-8 mg kg-1 i.v.) depressed non-nociceptive reflexes to a similar degree. 3. A similar spectrum of selectivity was seen with U-50,488 (0.5-16 mg kg-1 i.v.) although statistically significant selective depression of reflexes was only evident at the lowest dose tested (0.5 mg kg-1 i.v.). All effects of U-50,488 were readily reversed by low doses of the opioid antagonist, naloxone (10-100 micrograms kg-1 i.v.). 4. The dissociative anaesthetic/PCP ligand ketamine (0.5-4 mg kg-1 i.v.) was similar in having selective actions at low doses on sensitive cells but non-selective actions when higher doses were required. In contrast, the general anaesthetics methohexitone (4 mg kg-1 i.v.) and alphadolone/alphaxalone (1 mg kg-1 i.v.) were consistently non-selective between reflexes to noxious and non-noxious stimuli. alpha-Chloralose (20-40 mg kg-1 i.v.) had very little effect on reflexes to any of the synaptic inputs tested.
Parsons
C G
CG
Department of Physiology, School of Medical Sciences, University of Bristol.
Headley
P M
PM
D019900:3,4-Dichloro-N-methyl-N-(2-(1-pyrrolidinyl)-cyclohexyl)-benzeneacetamide, (trans)-Isomer; D000777:Anesthetics / Q000494:pharmacology; D000818:Animals; D003655:Decerebrate State; D005283:Fentanyl / Q000494:pharmacology; D007267:Injections; D009020:Morphine / Q000494:pharmacology; D009046:Motor Neurons / Q000187:drug effects; D009294:Narcotics / Q000008:administration & dosage / Q000494:pharmacology*; D009619:Nociceptors / Q000187:drug effects*; D011759:Pyrrolidines / Q000494:pharmacology; D051381:Rats; D011957:Receptors, Opioid / Q000502:physiology*; D017473:Receptors, Opioid, kappa; D017450:Receptors, Opioid, mu; D012018:Reflex / Q000187:drug effects*
D000777:Anesthetics; D009294:Narcotics; D011759:Pyrrolidines; D011957:Receptors, Opioid; D017473:Receptors, Opioid, kappa; D017450:Receptors, Opioid, mu; D019900:3,4-Dichloro-N-methyl-N-(2-(1-pyrrolidinyl)-cyclohexyl)-benzeneacetamide, (trans)-Isomer; D009020:Morphine; D005283:Fentanyl