Catecholamine receptor agonists

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Abstract
Motor activity during the first 5 min in a motility meter was measured in mice given 0.025–3.2 mg/kg of the dopamine and noradrenaline receptor agonists apomorphine and clonidine, respectively. The accumulation of Dopa, as induced by the inhibitor of aromatic amino acid decarboxylase, NSD 1015, was measured in parallel in two dopamine-rich regions, i.e. the limbic system and the corpus striatum, and in two noradrenaline-rich regions, i.e. the neocortex and the lower brain stem. Low doses (0.025–0.2 mg/kg) of apomorphine reduced locomotion in a dose-dependent manner, while the reduction after higher doses was less pronounced, indicating a biphasic dose-response relationship. Clonidine caused a dose-dependent locomotor depression. When low doses of the two drugs were combined, the inhibitory effect observed was at least additive. When clonidine was combined with a high dose of apomorphine (0.8 mg/kg), it caused a significant inhibition of locomotion in a dose of 0.1–0.2 mg/kg, but not after 0.8 mg/kg, indicating a biphasic dose-response relationship. Either drug given alone reduced Dopa accumulation after inhibition of its decarboxylation, in all regions, but smaller doses of apomorphine had a clearcut effect only in the dopamine-rich regions, whereas the lowest dose of clonidine investigated (0.05 mg/kg) had an inhibitory effect on Dopa formation only in the neocortex. The relationship between the dose of apomorphine and Dopa formation in the neocortex appeared biphasic, the highest dose (3.2 mg/kg) having no significant effect. Further, apomorphine in this dose accelerated the disappearance of noradrenaline after inhibition of synthesis by α-methyltyrosine. Reversal of reserpine-induced suppression of motor activity was taken to indicate postsynaptic receptor activation. The threshold dose of apomorphine causing reversal was 0.2 mg/kg. The inhibitory effect of e.g. 0.05 mg/kg on locomotion and on Dopa formation suggests a preferential activation of inhibitory autoregulatory dopamine receptors by low doses of this drug. A similar trend was observed for clonidine. The basal importance of dopamine neurones for the locomotor function studied in the present paper is illustrated by the marked inhibition by low doses of apomorphine. On the other hand, the observations with clonidine suggest a somewhat less striking and perhaps less direct influence of noradrenaline neurones on motor activity. Mice with a low motor activity, as induced e.g. by reserpine or, in another experiment, mice adapted to the motility meter, displayed an increased motor activity after higher doses of apomorphine (from 0.2 and 2 mg/kg, respectively), whereas all doses depressed the initial high motor activity. Probably, high motor activity requires active dopamine neurones, making this behaviour more susceptible to interference with autoregulatory mechanisms, whereas a low basal activity may be more affected by activation of postsynaptic dopamine receptors.