Tryptamine: Bimodal actions on the EEG and behavior of rabbits

EEG tracings from conscious restrained rabbits were analyzed by inspection and amplitude integration (electrogenesis). Intravenously, tryptamine in doses of 0.1 and 0.5 mg/kg increased electrogenesis, whereas 1 mg/kg initially decreased and subsequently increased electrogenesis. Higher doses (3, 6 and 12 mg/kg) always caused an immediate and dose dependent decrease in electrogenesis. Neither atropine nor reserpine pretreatment prevented this decrease, whereas chlorpromazine and methysergide abolished it. Intravenous infusions of tryptamine at 1 mg/kg/min caused a nonsignificant increase in electrogenesis but 2 and 4 mg/kg/min infusions caused a sustained decrease. These results show that tryptamine has a biphasic effect on the cortical EEG: low doses enhance synchrony while high doses alert the EEG. Moreover, analysis of the behavioral, EEG and peripheral effects of tryptamine indicate that different sites of action account for the multiple responses observed. Chlorpromazine and methysergide selectively inhibit the excitatory responses.