Dopamine receptors and ergot drugs. Evidence that an ergolene derivative is a differential agonist at subcortical limbic dopamine receptors.

The actions of a new type of ergolene derivative, MPME ((5R, 8R)-8-(4-p-methoxyphenyl-1-piperazinyl methyl)-6-methylergolene), have been evaluated on central catecholamine (CA) neurons in the rat by means of a combined biochemical, histochemical and behavioural analysis. The evidence suggests that this ergolene derivative is a preferential agonist at subcortical limbic dopamine (DA) receptors and at DA receptors belonging to the neostriatal DA islands. (1) MPME does not change the DA and noradrenaline (NA) levels 4 h after the injection in doses ranging from 0.1 to 5 mg/kg. MPME significantly reduced DA turnover in doses from 0.5 to 5 mg/kg, whereas the NA turnover was increased in the same dose range. (2) Histochemically, using quantitative microfluorometry, DA levels were unchanged, except in the islands of the nucleus caudatus following administration of MPME. The drug selectively reduced DA turnover in the subcortical limbic regions (tuberculum olfactorium and nucleus accumbens) and in the DA terminal islands of the nucleus caudatus in doses of 1-5 mg/kg, whereas the large diffuse DA terminal systems of the nucleus caudatus were unaffected. Using this ergolene derivative the islandic small neostriatal DA system can be excellently demonstrated also in the adult rat. The effects of MPME on DA turnover, are blocked by haloperiodol but not by methergoline (which blocks 5-HT receptors). (3) Studies on uptake of tritiated DA in the nucleus caudatus and tuberculum olfactorium reveal a weak inhibition of DA uptake and retention only in high concentrations (10(-5)-10(-6) M). Such actions therefore can probably not explain the changes in DA turnover observed. (4) Behavioural effects of MPME were evaluated in the rotometer model of Ungerstedt25. This model will reveal actions on supersensitive striatal DA receptors. MPME was found to mimic the action of apomorphine and cause a prolonged rotational behaviour towards the nonoperated side in doses of 0.25-0.5 mg/kg. A marked potentiation of the action of MPME was obtained by means of pretreatment with phosphodiesterase inhibitors suggesting that the effect of MPME might be mediated by cyclic AMP. Studies with the DA receptor blocking agent pimozide indicated a high affinity of MPME for the supersensitive striatal DA receptors, since only very high doses of pimozide (15 mg/kg) were capable of blocking the actions of MPME. (5) Studies on the effect of MPME on DA sensitive adenylate cyclase in the nucleus caudatus and the subcortical limbic system (mainly tuberculum olfactorium and nucleus accumbens) suggested that MPME is a partial DA receptor agonist with different intrinsic activity on the DA receptors of the subcortical limbic system and of the nucleus caudatus, the effects in the subcortical limbic system being considerably larger than in the nucleus caudatus. Thus, the present paper gives evidence that the various DA receptor populations in the brain are sufficiently different to allow their preferential activation by drugs.