Fig. 4. Analgesic response to a combination of systemic dexmedetomidine (Dex) and nitrous oxide (N²O). Tail-flick latency (mean ± SEM; n = 5) was assessed 30 min after coadministration of nitrous oxide and dexmedetomidine. (A ) Effect of nitrous oxide alone or combined with 12.5 μg/kg intraperitoneal dexmedetomidine (white square ) on the analgesic response. (B ) Effect of dexmedetomidine alone or combined with 45% N²O (white square ) on the analgesic response (mean ± SEM; n = 4). (C ) Isobologram of the analgesic interaction between intraperitoneal dexmedetomidine and nitrous oxide. The solid diagonal line  is the line of additivity, constructed by joining the ED⁵⁰± SEM for each agent (black square ). Black triangle = theoretical ED⁵⁰s ± SEMs if their combined effect is additive; gray circle = ED⁵⁰for dexmedetomidine when combined with 45% N²O; gray square = ED⁵⁰for nitrous oxide when combined with 12.5 μg/kg intraperitoneal dexmedetomidine. MPE = maximal possible effect.