Effects of low doses of catecholamine receptor agonists on exploration in mice

Female mice were given 0.025--0.8 mg/kg of the dopamine and noradrenaline receptor agonists apomorphine and clonidine, respectively, and the effect on explorative behaviour of these drugs was tested in a Y-shaped runway for three minutes. Saline-treated controls displayed a stable exploratory behaviour, declining during the time period measured. The decline probably reflects adaptation to the new environment. Apomorphine depressed the activity, but the configuration of the time curve was the same as in controls. Clonidine also depressed the exploratory activity and further appeared to break the pattern of adaptation, the activity in clonidine-treated mice being low from the beginning. The higher doses of the drugs, alone or in combination, caused a pronounced suppression of the exploration. These mice were observed for ten minutes, under which period their exploration of the runway was not increased, as compared with the first three minutes. The smaller doses used are not known to cause functionally measureable activation of postsynaptic catecholamine receptors. On the other hand these doses are known as cause inhibition of firing in the respective presynaptic neurons and also inhibition of synthesis of the respective transmitter. Thus the present findings are interpreted to be due to presynaptic inhibition of the respective neurons. Clonidine caused a more pronounced inhibition of the exploration than apomorphine, suggesting nordrenaline to be of particular importance for exploratory behaviour.