Fig. 2.
Experimentally observed dose–response and dose–intake functions for intravenous self-administration of cocaine represented as the following: means ± SE for the entire group of animals (A); individual values (B); separate means ± SE for animals with a high and low intake of cocaine at the highest dose (1 mg/kg per infusion) (C, top, middle); and correlation between the number of responses (nose-pokes) in the active device for doses of the ascending (0.06 mg/kg per infusion) and descending (0.25 mg/kg per infusion) limbs of the dose–response function (C, bottom). There were large individual differences in the number of responses (nose-pokes) in the active device (the ones delivering a drug infusion) (B,top) and cocaine intake (B,bottom). These differences were derived by vertical shifts in individual dose–response functions. Thus, HI animals compared with LI animals showed upward shifted dose–response (C, top) and dose– intake (C, bottom) functions that did not differ for the ED50 (as calculated on the dose–intake function). Furthermore, the number of responses in the ascending and descending limbs of the dose–response function were positively correlated (C, bottom). The individual values used for computing the graphs were the mean of the last 2 d of testing at each dose.