Effect of ketamine on spinal astrocytes in mice tolerant to morphine

Objective Recent studies have shown that activation of spinal astrocytes (ASTs) may be involved in the development of morphine tolerance. The purpose of this study was to investigate the effect of ketamine (K) on spinal ASTs in mice tolerant to morphine (M) .Methods Thirty Kun-Ming mice of both sexes weighing 18-22 g were randomly divided into 5 groups of six animals each : (A) control group received only subcutaneous (s.c.) and intraperitoneal (i.p.) normal saline (NS); (B) chronic M-tolerance group received M 10 mg·kg-1 s.c. followed after 30 min by NS 10 ml·kg-1 i.p. twice a day (at 8:00 and 17:00) for 9 days;(C), (D), (E) K group received M 10 mg·kg-1 s.c. followed after 30 min by K 5 mg· kg-1(C), 10 mg·kg-1 (D) or 20 mg·kg-1 (E) i.p. twice a day for 9 days. Pain threshold was estimated by measuring paw withdrawal response to Von Frey filament stimulation every other day (1st, 3rd, 5th, 7th, 9th) In after second administration of drugs. The percentage of maximal possible effect (MPE% ) was calculated : MPE% = [ (test group PWTV - control group PWTV) / (15 - control group PWTV)] × 100% (PWTV = paw withdrawal threshold value). On the 9th day after pain threshold was measured the animals were sacrificed and lumbosacral segment of spinal cord was removed. The changes in spinal ASTs were detected by immunohistochemistry. The average areas of GFAP immuno-reactive cells in the dorsal horn were measured to show the degree of spinal AST activation. Results 1. MPE% was 0 at all time points in group A. In group B MPE% was 42.8% on the 1st and 3rd day and gradually decreasing on the 5th and 7th day and became 0 on the 9th day signifying full development of morphine tolerance. In group C the change in MPE% was almost the same as in group B. In group D and E MPE % tended to decrease but was still above 30% at all time points signifying that ketamine 10 and 20 mg·kg-1 could partly antagonize the development of morphine tolerance. 2. In group B the staining of GFAP immuno-reactive cells was heavier and the average areas were significantly larger than in group A (P 0.01) . In group D and E the average areas of GFAP immuno-reactive cells were significantly smaller than those in group B (P 0.01) .Conclusion The activation of ASTs may be involved in the development of morphine tolerance. Ketamine can partly block morphine tolerance by inhibiting the activation of ASTs.