In mice that were scored for the length of time they remained immobile in awkward postures (cataleptic) on an inclined wire grid, a large IP dose of pilocarpine (80 mg/kg) caused a clear catalepsy, which was prevented by both dopamine agonists that were tested, apomorphine (4 or 8 mg/kg, IP) and bromocriptine (8 mg/kg, IP). In other experiments, haloperidol (2.5 mg/kg) caused mild catalepsy. As expected, neither 4 nor 8 mg/kg apomorphine caused much effect when given alone, but both doses produced profound and long-lasting catalepsy in the haloperidol-treated animals. Bromocriptine also had little effect when given alone, and neither 4 nor 8 mg/kg enhanced the haloperidol catalepsy. Apomorphine alone produced catalepsy at low doses. Repeated testing after a low (0.3 mg/kg) dose of apomorphine showed that catalepsy was most profound at 5 min postinjection, with progressive decline thereafter. Apomorphine, but not bromocriptine, thus can produce catalepsy under certain conditions of DA receptor blockade or in low dose. Catalepsy, and perhaps other forms of hypomotility, appear to be differentially mediated by a subclass of dopaminergic receptors.
- Dose-Response Relationship, Drug
- Time Factors
- Motor Activity
- Receptors, Dopamine