Effect of morphine on hypothalamic tyrosine hydroxylase mRNA levels in dopaminergic neurons and on preoptic DOPAC levels measured by microdialysis.
Morphine not only suppresses norepinephrine-induced increases in LHRH mRNA levels but, in these same animals, it simultaneously amplifies norepinephrine (NE)-induced LH release. These observations suggest that NE may activate parallel mechanisms which independently increase LHRH mRNA levels and LHRH release and suggest that some of these effects could be mediated indirectly via morphine's action on different components of the hypothalamic dopamine (DA) system. Accordingly, in the present studies we examined the effects of morphine on various components of this dopamine system using as our index of altered DA neuronal activity, the changes which occur in tyrosine hydroxylase (TH) mRNA levels following morphine. As an ancillary index of changes which occur in dopamine neuronal activity, we measured, by microdialysis, the changes which occur in preoptic dihydroxyphenylacetic acid (DOPAC) levels after either subcutaneous injections or following microinfusions of morphine into the zona incerta (ZI). In a final study, we evaluated whether DA when given alone (icv infusion) or prior to icv NE would altered LH release. Single cell levels of TH mRNA in preoptic A15 and periventricular anterior hypothalamic A14 DA neurons were not affected by morphine 1, 5 and 24 h later. In contrast, within 1 h after morphine, TH mRNA levels in ZI A13 neurons were significantly elevated and they remained high at 5 nd 24 h compared to controls. Morphine also resulted in a significant rise in TH mRNA levels in tuberoinfundibular DA neurons (TIDA) (A12) within 1 h and these levels remained high to 5 h. Thereafter, by 24 h, message levels had returned to control values. Morphine also resulted in a rapid rise in plasma prolactin (Prl) with peak values occurring at 20 min and then returning to baseline by 90 min. When morphine was given sc it resulted, within 15 min, in a rapid rise in preoptic DOPAC levels and these levels continued to rise such that they were 217% higher than pretreatment values by 105 min. Preoptic 5-hydroxyindoleacetic acid (5-HIAA) levels also increased by 25-75% after sc morphine. The microinfusion of morphine into ZI also resulted in elevated preoptic DOPAC but not 5-HIAA levels within 15 min. The icv infusion of DA alone had no effect on plasma LH whereas, NE (icv) produced a modest but significant increase in plasma LH. When DA was given 15 min prior to the infusion of NE, neither amplification nor inhibition of NE-induced LH release occurred. From these and other studies we conclude that the morphine-induced suppression of TIDA neuronal activity may allow NE to release greater amounts of LHRH from axon terminals in the median eminence.(ABSTRACT TRUNCATED AT 400 WORDS)