Discrimination of morphine- and haloperidol-induced muscular rigidity and akinesia/catalepsy in simple tests in rats

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 D₁-like and D₂-like dopamine receptor stimulation on muscular rigidity and catalepsy was studied. Therefore, the partial D₁ agonist SKF 38393 (3 and 8 mg/kg s.c.), the D₂/D₁ 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.