Long-term supraspinal analgesic tolerance produced by agonists of brain CB1Rs. Upper panel: Mice were injected icv with methanandamide, ACEA or WIN55,212-2 at the doses indicated, and analgesia was evaluated at various intervals post-injection using the "tail-flick" test with a thermal stimulus of a 52°C water bath. Middle panel: A priming dose of methanandamide (50 nmol), ACEA (40 nmol), WIN55,212-2 (20 nmol), or saline was administered icv to mice (n = 10 per group). After 24 h, single-dose tolerance was examined by administering a test dose of each agonist (same dose as the priming dose) to mice that had received either saline (open bars) or the priming doses (grey bars). In another set of assays, mice that had received the priming doses of the corresponding agonists were injected icv 24 h later with the PKC kinase inhibitor Gö7874 (1 nmol), and 30 min later with test doses of the cannabinoid agonists (dark grey bars) (n = 10 per group). The "tail-flick" test was conducted 10 min after agonist administration. Bars represent mean ± SEM. *Significantly different from the saline control group (P < 0.05). Lower panel: Recovery from the acute analgesic tolerance produced by the CB1R agonists. The "tail-flick" test was conducted 10 min after injection of the test dose. Data are expressed as mean ± SEM from groups of 8 mice.