Interactions of morphine with apomorphine: behavioural and biochemical studies.

The interactions of morphine with the agonist at dopamine receptors apomorphine were studied on the behavioural and biochemical level. Apomorphine (0.5 mg/kg s.c.) produced stereotyped sniffing and some licking behaviour. Pretreatment with morphine enhanced licking behaviour and, in addition, produced some gnawing behaviour, a sign which is seen after a larger dose of apomorphine alone as well. This enhancement by morphine was maximal after 3.3 mg/kg i.p. and less pronounced after smaller or larger doses of morphine; naloxone (1 mg/kg i.p.) antagonized the enhancement. Morphine did not affect the decrease in the concentration of 3,4-dihydroxyphenylacetic acid (DOPAC) produced by apomorphine in striatum and nucleus accumbens. In contrast, morphine increased the concentration of 3-methoxytyramine (3-MT) in both areas after pretreatment with pargyline (75 mg/kg i.p.), suggesting that it increased the release of dopamine, which might explain the enhancement of apomorphine-induced stereotyped behaviour. The enhancement by morphine of stereotyped behaviour produced by lisuride (2 or 4 mg/kg i.p.), another agonist at dopamine receptors, was much less pronounced than on apomorphine-induced stereotypies.