in the acquisition training of mice may have prevented a floor effect in response rate data in Experiment 2. Enhanced sensitivity of the mice in Experiment 1 appeared to be selective for endocannabinoids rather than for all cannabinoid agonists, as the mice did not exhibit a similar decrease in response rate when lower doses of THC were administered. While the reason for the potentiated potency of anandamide in decreasing responding in these mice remains unclear, it could be related to procedural variables such as route of administration or to the absence of a response rate criterion during acquisition. Whatever the cause, this anomaly may have interfered with adequate assessment of anandamide’s THC-like effects in this group of mice. Further, it may account for the different effects of the two FAAH inhibitors, URB597 and PF3845. PF3845 increased substitution of anandamide in THC-trained mice in Experiment 2, but only at doses of anandamide that could not be tested in combination with URB597 due to suppression of overall response rate.
