[Studies on reinforcing effects of methylephedrine, caffeine and their mixture with intravenous-self administration in rhesus monkeys].

: The reinforcing effects of methylephedrine hydrochloride (ME), anhydrous caffeine (CA) and their mixture (ME+CA) were studied by the intravenous cross self-administration experiment and by the progressive ratio experiment in four male rhesus monkeys each. In the intravenous cross self-administration experiment, ME, CA and ME+CA were found to have reinforcing effects. Self-administration rates above the level of cocaine, a typical reinforcing drug, were not observed in ME or CA alone, but observed in their mixture (120 + 126 micrograms/kg/inj.: M dose or 480 + 504 micrograms/kg/inj.: H dose). The minimum reinforcing doses were 120 micrograms/kg/inj. (M dose) for ME, 126 micrograms/kg/inj.: (M dose) for CA, and ME 30 micrograms+CA 32 micrograms/kg/inj. (L dose) for the mixture. Vomiting was observed during the session in monkeys which showed higher self-administration rates of ME and the mixture, and decreases in the rates were observed in these animals on the next day. One animal at H dose of the mixture, which showed a high self-administration rate on both the 1st and 2nd days, died after the end of the session on the 2nd day. In the progressive ratio experiment, the final ratios at 120 micrograms/kg/inj. of ME, 126 micrograms/kg/inj. of CA and the mixture of ME 120 micrograms/CA 126 micrograms/kg/inj. were almost equal to or less than that of saline (negative control). Thus, these drugs didn't show reinforcing effects at the above levels. However, a reinforcing effect was observed in three out of four monkeys administered 1920 micrograms/kg/inj. of ME, 2016 micrograms/kg/inj. of CA, and the mixture of ME 1920 and CA 2016 micrograms/kg/inj.(ABSTRACT TRUNCATED AT 250 WORDS)