Ascorbic Acid Release In The Nucleus Accumbens Induced By Morphine

Administrated with morphine can produce many complex neurobiological effects in the central nervous system (CNS). In recent years, increasing evidence shows that ascorbic acid (AA) is a potent neuromodulator in the central nervous system to modulate some nervous activities. Drugs, such as morphine, induce AA release in the nucleus accumbens (NAc) after acute administration, suggesting that AA release in NAc might be a common feature in response to addictive drugs. Previous studies have shown the different effects of morphine on AA level following local infusion. The phenomenon indicates the existence of different regulating mechanisms of morphine in this nuclear. To further understand the neuromechanisms underlying morphine, in the present study we investigated the pharmacological, neuroanatomical, neurobiochemical and molecular alterations in the NAc following morphine administration.Firstly, the roles of the NAc glutamatergic pathways in local infusion of morphine-induced AA, glutamate (Glu) andγ-aminobutyric acid (GABA) release were studied by using microdialysis coupled to high performance liquid chromatography (HPLC) with electrochemical detection and HPLC with fluorescent detection. The results show that morphine (100μM,1 mM) dose-dependently increases the levels of AA and GABA, and decreases the level of Glu in the NAc. However, the effects of morphine on AA and GABA release could be reversed by naloxone. The results indicate that opioid receptor was involved in morphine-induced AA release in the NAc.To further investigate the effects of morphine, the method of NAc lesioned by kainic acid (KA) was used in this study. KA lesions eliminated the effects of morphine on the increasing level of AA and GABA in the NAc, but not the Glu. KA is thought to destroy the GABAergic neuronal cells. The muscimol (100 ng) was administrated to the NAc to prove the hypothesis. It was shown that muscimol could reverse the effect of KA lesion on morphine-induced AA and GABA release, indicating that GABAergic system in the NAc is involved in the morphine-induced AA release.In the present study, we also investigated the effects of KA lesion on morphine induced behavioral sensitivity. In the test of locomotor activity, lesions of NAc affected the locomotor activity of horizontal movement induced by morphine. Meanwhile, NAc lesions enhanced the pain threshold induced by morphine (5 mg/kg). In the behavioral sensitization tests, lesions of NAc blocked the induction of behavioral sensitization by repeated morphine, but not the hyperactivity by acute morphine and the induction of behavioral sensitization.With the consideration of the neuron projection in the limbic midbrain, we also examine the effects of medial prefrontal cortex (mPFC) and ventral tegmental area (VTA)-projection to the NAc on the AA release induced by morphine. Undercutting the mPFC could cut down the glutamatergic cortico-accumbens pathways. While the results showed it did not affect morphine-induced AA release in the NAc. Bicuculline (150 ng), the GABAA receptor antagonist, could inactivate GABAA receptors located on GABA interneurons, inhibit their activity and then, in turn, disinhibit the activity of DA cells, and increase the dopamine (DA) release in the NAc, which finally was related to the effects of blocking the AA release induced by morphine in the NAc. It was indicated that the effect of morphine on AA release in the NAc is partically regulated by the GABAA receptor-mediated action of DA afferents from the VTA.AA is transported into the brain and neurons via the sodium-dependent vitamin C transporter 2 (SVCT2), which causes accumulation of AA within cells against the concentration gradient. In this study, we also investigate the effects of morphine on the mRNA and protein expression of SVCT2. The results show that morphine (i.p.) significantly increase the expression of SVCT2 in the striatum, but not the NAc, while, morphine (1mM) significantly increased the expression of SVCT2 in the NAc. Naloxone (0.4 mM) and bicuculine (150 ng) could block the effect of morphine on the SVCT2, indicating an important role of SVTC2 in the AA release.In conclusion, the NAc is a key nuclear involved in the morphine-induced AA release, as well as the behavioral activity induced by morphine. The mediation may be related to the GABAergic system in the NAc, which may be contributed to the morphine-induced release of AA and GABA in the NAc. Moreover, this action of morphine in the NAc is regulated by the GABAA receptor mediated action of DA afferents from the VTA. In addition, the level of SVCT2 was correspondly changed during the procedure of morphine-induced AA release. Accordingly, several mechanisms are involved in the morphine-induced AA release in the NAc, which might be a potential target for treatment of the neurological disorders induced by morphine-like compounds.