Acetylcholine/dopamine interaction in planaria

Planaria represents the most primitive example of centralization and cephal ization of nervous system. Previous reports indicate that planaria shows sp ecific behavioral patterns, analogous to mammalian stereotypes, in response to drugs acting on acetylcholine or dopamine transmission. Here we further characterized these responses, and investigated the interactions between c holinergic and dopaminergic systems by means of behavioral methods. Exposur e to cholinergic agonists physostigmine or nicotine produced hypokinesia wi th 'bridge-like' and 'walnut' positions, respectively. Blockade of muscarin ic receptors by atropine produced 'screw-like' hyperkinesia. Exposure to do pamine agonists (nomifensine, apomorphine) produced marked hyperkinesia wit h 'screw-like' movements. Finally, exposure to dopamine antagonists produce d immobility or 'bridge-like' position. Pre-exposure to physostigmine block ed the behavioral effects of nomifensine and reduced and markedly delayed t he behavioral effects of apomorphine. Pre-exposure to apomorphine slightly reduced and delayed the behavioral changes by physostigmine. Finally, plana ria exposed to atropine after either SCH23388 or sulpiride showed 'C-like' or 'screw-like' hyperkinesia, respectively. Thus, reduction of cholinergic transmission seems to play a pivotal role in determining hyperkinesia in pl anaria. Under these conditions, different patterns of hyperkinetic activiti es occur, according to the subpopulation of dopamine receptors stimulated b y drugs. These findings suggest that interactions between cholinergic and d opaminergic systems occur very early in animal phylogeny. (C) 2000 Elsevier Science Inc. All rights reserved.