Role of glucocorticoid receptor in induction of neuronal apoptosis in spinal dorsal horn in chronic morphine tolerant rats

Abstract

Objective To investigate the role of glucocorticoid receptor (GR) in the induction of neuronal apoptosis in spinal dorsal horn in chronic morphine tolerant rats. Methods Twenty healthy male SD rats weighing 300-350 g in which intrathecal (IT) catheter was successfully implanted without complication were randomized into 4 groups ( n = 5 each): group Ⅰ control ( group C);group Ⅱ morphine ( group M );group Ⅲ morphine +RU38486 (GR antagonist) (group MR) and group Ⅳ morphine + dexamethasone (GR agonist) (group MD).Normal saline 10 μl, morphine 10 μg, morphine 10 μg + RU38486 2 μg and morphine 10μg + dexamethasone 4 μg were injected IT twice a day at 8:00 and 20:00 for6 consecutive days in group C, M, MR and MD respectively. Tail flick latency (TFL) to a thermal nociceptive stimulus was measured every day at 30 min after IT administration in the morning (8:00). Hyperalgesia was considered to be a sign of morphine tolerance. The animals were killed at 7 days after IT drug administration. The L3-5 segment of the spinal cord was isolated for determination of neuronal apoptosis in spinal dorsal horn by TUNEL staining. Apoptotic index was calculated ( the number of apoptotic neurons/the total number of neurons × 100% ). Results TFL was significantly prolonged at day 1 and 3 of IT morphine 10 μg twice a day and returned to baseline at day 5 and 7 indicating morphine tolerance. RU38486 inhibited while dexamethasone enhanced morphine tolerance. IT morphine significantly increased the number of apoptotic neurons in spinal dorsal horn. Morphine-induced neuronal apoptosis was decreased by IT RU38486 and increased by IT dexamethasone. Conclusion Glucocorticoid receptors may be involved in morphine tolerance by inducing neuronal apoptosis in spinal dorsal horn. Key words: Glucocorticoids; Morphine; Drug tolerance; Spinal cord; Apoptosis