[Changes in cAMP and cGMP levels in neonatal rat histaminergic neurons of tuberomammillary nucleus following 48-hour morphine exposure and effects of sinomenine intervention].

OBJECTIVE: To explore the effects of long-term morphine exposure on cAMP and cGMP levels in primary cultured tuberomammillary nucleus (TM) neurons of neonatal rats and the effects of sinomenine on morphine-dependent TM cells. METHODS: TM neurons after a 7-day primary culture were further cultured in the medium containing 100 micromol/L morphine for 48 h to prepare the cell model of morphine dependence. Serial doses of histamine or sinomenine were administered 30 min naloxone treatment, the cAMP and cGMP levels of the TM cells were determined by enzyme immunoassay. cAMP and cGMP levels were also determined in normal TM cells treated by histamine or sinomenine. RESULTS: After treatment with 100 micromol/L morphine for 48 h, cAMP and cGMP levels in the TM neurons were increased markedly. Treatment with 100 micromol/L naloxone added in the culture media caused an overshoot of cellular cAMP and a marked declination of cGMP, resulting in significantly increased cAMP/cGMP ratio. Sinomenine at 30 and 100 micromol/L and histamine at 40 micromol/L failed to obviously affect cAMP and cGMP levels in normal TM neurons, but sinomenine at 300 micromol/L and histamine at 80 micromol/L significantly increased the intracellular cAMP level. After pre-treatment with sinomenine at the above 3 doses or histamine at 40 micromol/L, the TM neurons with morphine dependence exhibited significant reduction in intracellular cAMP level but increment in cGMP level after naloxone treatment, with significantly reduced cAMP/cGMP ratio. CONCLUSION: Long-term morphine (100 micromol/L) exposure for 48 h can induce marked changes of cAMP and cGMP levels in the TM neurons. The central histaminergic nervous system may be responsible for the development of morphine dependence and withdrawal. Sinomenine can significantly reduce the cAMP level and enhance cGMP level of morphine-dependent TM neurons precipitated by naloxone, which results in a near-normal ratio of cAMP and cGMP.