Psilocybin has a biphasic dose-response effect on the startle reflex in rats. Low doses (0.75-2.0 mg/kg) increased startle amplitude, while high doses (4.0-8.0 mg/kg) depressed it. Equimolar doses of psilocin produced comparable effects. This pattern is consistent with the hypothesis that startle increases when midbrain raphe neuron firing is selectively inhibited but decreases when neurons postsynaptic to raphe cells are also inhibited.
Rats were tested for shock-elicited fighting after receiving various doses of N,N-dimethyltryptamine (0.12 to 8.0 mg/kg) and 5-methoxy-N,N-dimethyltryptamine (0.06 to 2.0 mg/kg). Both drugs inhibited fighting at higher doses but had no significant effects at lower doses. These effects differ from those of another indole hallucinogen, d-lysergic acid diethylamide, and are discussed in relation to their impact on the raphe-serotonin system and interactions with other neurotransmitter systems.