ANTI-NOCICEPTIVE EFFECT OF MORPHINE, OPIOID ANALGESICS AND HALOPERIDOL INJECTED INTO THE CAUDATE-NUCLEUS OF THE RAT

The effect of intracaudate (i.c.) microinjections of morphine, opioid analgesics and haloperidol was determined on the tail-flick response evoked by radiant heat in rats. Bilateral injections (0.2 μl on each side) into the caudate nuclei of morphine 5 μg, pethidine 50 μg, levorphanol 4 μg, dextrorphan 10 μg and haloperidol 5 μg significantly increased the reaction time of the tail-flick response. The antinociceptive effect of an i.c. injection of morphine or levorphanol was abolished by an intraperitoneal (i.p.) injection of naloxone 0.2 mg/kg or apomorphine 2 mg/kg. The anti-nociceptive effect of pethidine, dextrorphan and haloperidol was reduced but not abolished by an i.p. injection of naloxone 0.2 mg/kg. An i.p. injection of apomorphine 2 mg/kg abolished the effect of an i.c. injection of haloperidol. A bilateral i.c. injection of naloxone 5 μg or apomorphine 10 μg reduced the anti-nociceptive effect of an i.p. injection of morphine 2 mg/kg or haloperidol 2 mg/kg, but did not abolish it. It is concluded that (1) an anti-nociceptive effect can be achieved by an action on the caudate nucleus of the drugs tested; (2) the anti-nociceptive effect exerted by morphine and levorphanol in the caudate nucleus is due to a specific action mediated by opiate receptors, whilst that produced by pethidine and dextrorphan is due to a specific and/or unspecific action; (3) the anti-nociceptive effect of haloperidol in the caudate nucleus is due to an impairment of dopaminergic impulse transmission, which is also involved in the effect of morphine and levorphanol. © 1979 Springer-Verlag.