On the selectivity of intravenous μ‐ and κ‐opioids between nociceptive and non‐nociceptive reflexes in the spinalized rat

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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.