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
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