Objective To observe the activation effect of histamine on mast cells in the acupoint area and its analgesic effects similar to acupuncture stimulation,so as to reveal the underlying mechanism of acupoint injection therapy.Methods A total of 60 SD rats were divided into control,model,normal saline(NS),histamine-injection and acupuncture groups(12 rats/group).Acute adjuvant-induced-arthritis(AIA) model was duplicated by injection of complete Freunds' Adjuvant(0.05 mL) into the left ankle articular cavity.For rats of saline and histamine groups,normal saline(50 ?L) and histamine(50 ?L,100 ?g/mL) were injected into the left "Zusanli"(ST 36) acupoint area.For rats of acupuncture group,manual acupuncture(lifting-thrusting and twisting needle intermittently) was applied to ST 36 for 20 min.Paw withdrawal(pain threshold,PT) was detected by radiant heat irradiation of the hindpaw.Skin and muscle tissues of ST 36 area were sampled to be fixed in formalin,then,sliced(5 ?m),and stained with Toluidine Blue,separately,followed by counting the degranulation rate of mast cells under microscope.Results Compared with normal control group and pre-modeling,PT of model,NS,acupuncture and histamine groups decreased significantly(P