INTERAKTION VON S-(+)-KETAMIN MIT OPIATREZEPTOREN

To check for suspected opioid-receptor mediated hypnotic and antinociceptive effects of S(+)-ketamine, highly selective antagonists were used after the anaesthetic. Methods. To determine the hypnotic effects of increasing doses of S(+)ketamine (2-5-10-20 mg/kg given at 10-min intervals), EEG power spectra (δ, θ, α, β) were derived (Lifescan®), and antinociceptive potency was evaluated using the somatosensory evoked potential (SEP, Lifescan®) in awake, trained dogs (n = 10). To check for an opioid-receptor-related interaction, an antagonist of the methoxymorphinane series (HS-275, 80 μg/kg i.v.) with higher selectivity than naloxone for the μ-receptor was given at the end. After washout the same animals were exposed to S(+)-ketamine. This time, however, the highly selective δ-antagonist naltrindole (160 μg/kg i.v.) was given. To show up any respiratory depression arterial blood gases were taken after each dose. Results. S(+)-Ketamine induced a dose-related increase in power in the theta band (3-8 Hz), with a ceiling effect at 10 mg/kg. The changes were reversed by both antagonists. In the beta band (13-30 Hz) and in the delta domain, power decreased or increased, respectively, in a highly significant manner (P < 0.005) at 20 mg/kg. Both effects reversed after the antagonists with an overshoot in beta (+12% and +14%, respectively) and a decrease in delta (-45% and -62%, respectively) compared with control. S(+)-Ketamine induced a dose-dependent increase in peak latency and depression of the SEP amplitude by a maximum of over 50%. Latency changes were completely reversed only by HS-275. Amplitude height was only partly restored by both antagonists. A clinical relevant decrease in P(a)O₂ and increase in P(a)CO₂ increase were seen at 20 mg/kg. Hypoxia was reversed by both antagonists; hypercapnia was only partially reversed. Conclusion. The results confirm the suspicion that S(+)-ketamine induces an opioid θ- and δ-receptor-mediated deep hypnotic effect. Blockade of nociceptive impulses in afferent sensory nervous pathways suggests an efficient analgesic effect mediated partly by the opioid μ-receptor. Other mechanisms, such as an interaction with the NMDA receptor, have to be taken into consideration to account for the full antinociceptive effect. Respiratory depression may be of clinical importance when high dosages of S(+)ketamine are given.