In experiments on rats it has been observed that the intensity of
amphetamine-induced (6 mg/kg s.c.) stereotypy is reduced by the blockers of the
serotonin (5-HT),
GABA- and
opiate receptors, respectively danitracene (3 mg/kg i.p.),
picrotoxin (2 mg/kg i.p.) and
naloxone (10 mg/kg s.c.). Applied 24 hours after the
serotonin depletor
para-chlorophenylalanine (PCPA--300 mg/kg i.p.),
amphetamine induced more pronounced but earlier disappearing stereotypy. Also more intensive but longer was the stereotypy induced by
apomorphine (2.5 mg/kg i.p.) applied after PCPA. The stimulator of the
serotonin receptors quipazine (10 mg/kg i.p.) potentiated
amphetamine stereotypy and eliminated
haloperidol catalepsy (1 mg/kg i.p.). The blocker of the
cholinergic receptors atropine (30 mg/kg i.p.) increased and prolonged
amphetamine stereotypy. In the interpretation of the results obtained an attempt is made to substantiate the thesis that
dopamine (DA) released under the effect of
amphetamine by the nigrostriatal terminals, acts at least partially not as inhibitory, as it is usually accepted, but as excitatory transmitter. It is considered that the results obtained show that DA released from the nigrostriatal pathway exercises its typical inhibitory influence on the striatum, by stimulating
5-HT,
GABA and
opiate neurons situated in nucleus caudatus. The ostensibly contradictory results from the experiments with PCPA to the proposed hypothesis are explained with the development of sensitization of the
5-HT receptors.