Effects of 8-OH-DPAT, lisuride and some ergot-related compounds on the acoustic startle response in the rat.

Five ergot-related compounds were examined for their effects on the acoustic startle response in the rat. The startle amplitude and the startle latency were registered. 8-Hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT; 0.5-8 mg/kg) and lisuride (0.05-0.8 mg/kg) were found to enhance the startle amplitude, while the mainly DA receptor active ergot derivatives pergolide (0.2-0.8 mg/kg), bromocriptine (5-20 mg/kg) and LY 141865 (5-20 mg/kg) had no, or even the reverse, effect. All five compounds caused a prolongation of the startle latency. The increased startle amplitude caused by 8-OH-DPAT (2 mg/kg) and lisuride (0.2 mg/kg) was successfully antagonized by the 5-HT receptor antagonist methiothepin (0.1 mg/kg) but not by metergoline (1 mg/kg). 5-Hydroxy-L-tryptophan (L-5-HTP; 12.5-50 mg/kg), administered to pargyline- and benserazide-pretreated animals, was included for comparison. The serotonin precursor caused a marked increase in the startle amplitude and a shortening of the startle latency.