Fig. 3.  H1 receptor, but not H2 receptor, blockade decreased the requirement of isoflurane to induce the loss of righting reflex and delayed the emergence time from isoflurane anesthesia. (A ) The dose–response curve for rats with intracerebroventricular injection of triprolidine (25 μg/1 μl) (blue lines)  and saline (red lines)  shows the percentage of animals that lost their righting reflex with stepwise increases in isoflurane concentration. The EC⁵⁰is smaller for triprolidine-treated rats (crosses)  than for saline-treated rats (open circles)  (n = 7 for both groups, P = 0.0011) (A ). Intracerebroventricular administration of triprolidine (5–25 μg/1 μl) failed to change time of onset of righting loss but prolonged emergence time in a dose-dependent manner after a 30-min exposure to isoflurane, 1.375% (n = 7 for both groups, *P < 0.05, compared with saline injection) (B ). Intracerebroventricular administration of the H2 receptor antagonist cimetidine (5–25 μg/1 μl) had no effect on the isoflurane dose–response curve (C ), onset time of righting loss, and emergence time (D ) (n = 6 for both groups, P > 0.05). Atm = atmosphere.