Development of tolerance to the antinociceptive effect of metergoline

Metergoline given IP reduced the response to noxious stimulation in the mouse formalin test. Tolerance to this effect developed after a chronic treatment schedule consisting of ten daily injections of 5 mg/kg. Twenty four hours after the last injection a test dose of metergoline (2.5 mg/kg) reduced the licking time in the formalin test by 28% in the chronic metergoline group, compared to 68% reduction in the vehicle-treated animals. In addition, the antinociceptive effect of the 5-hydroxytryptamine releasing compoundp-chloroamphetamine (PCA) was reduced following chronic treatment with metergoline. The reduced effect of PCA may have been caused by down-regulation of 5-HT₂ receptors. However, this finding is also compatible with the contention that metergoline may act as an agonist at postsynaptic serotonergic receptors.