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Fentanyl, a μ-opioid receptor agonist, has been studied for its neuro/psycho-pharmacological effects since its first clinical use; however, its effect on the release rate of the Central Nervous System (CNS) neurotransmitters has not been yet elucidated. In the present study the influence of fentanyl on the release rates of glutamate and GABA is investigated. Specifically, we examined the effects of intravenous (10μg/kg) as well as intrahypothalamic (0.1nmol/min) fentanyl administration on the release rates of GABA and glutamate in the superfusate of anterior hypothalamus, under tail pinch manipulation. The release rate of the neurotransmitters was monitored by the push–pull superfusion technique. To investigate the role of fentanyl the opioid antagonist, naloxone 0.1mg/kg was administered intravenously, or 50nmol/min intrahypothalamicaly. The amino acids were determined by High Performance Liquid Chromatography (HPLC) and fluorimetric detection after NBD-Cl derivatisation. After intravenous fentanyl administration a significant decrease of glutamate and increase of GABA release rates were observed. However during the pain manipulations, the release rate of glutamate was increased. Intravenous naloxone did not affect significantly the release rates of both amino acids, while intrahypothalamic antagonist administration reversed the alterations in both neurotransmitters release rates. Our results demonstrate that there is an opioid-glutamatergic transmission pathway, located in hypothalamus and that opioids can activate NMDA receptors, thus reducing the nociceptive threshold and the opioid analgesic effect.