The present study was conducted to establish a simple method for measuring
muscular rigidity in rats, which could be used for screening and is able to discriminate between rigidity and akinesia/
catalepsy. Therefore, we treated rats with
morphine (30 mg/kg i.p.), since large doses of
morphine lead to
muscular rigidity and akinesia. We measured
muscular rigidity with a new method by determining the resistance of the hindlimb to passive flexion in the 'balance test' and also checked
haloperidol (3 mg/kg i.p.) treated rats for
muscular rigidity. Furthermore,
catalepsy was also tested after administration of each of these drugs. Then, the influence of D(1)-like and D(2)-like
dopamine receptor stimulation on
muscular rigidity and
catalepsy was studied. Therefore, the partial D(1) agonist
SKF 38393 (3 and 8 mg/kg s.c.), the D(2)/D(1) agonist
pergolide (0.25 and 0.5 mg/kg i.p.) and the
dopamine precursor
L-DOPA (50 and 100 mg/kg i.p.) were administered up to 30 min before
muscular rigidity was measured in
morphine-treated rats. The results showed that
morphine, but not
haloperidol led to
muscular rigidity, whereas both drugs led to positive scores in the
catalepsy test. The
dopaminergic drugs partly antagonized the
morphine-induced
muscular rigidity in the doses applied, but not the
catalepsy. Apparently, rigidity, akinesia/
catalepsy produced by
morphine can be discriminated from that produced by
haloperidol in simple and quick tests.