Morphine suppression of neurotransmitter release evoked by sensory stimulation in vivo

Aquaporin-4 deficiency attenuates opioid dependence through suppressing glutamate transporter-1 down-regulation and maintaining glutamate homeostasis

BACKGROUND

Glutamate homeostasis plays a critical role in mediating the addiction-related behaviors. Therefore, preventing the disruption or reestablishing of it is a novel strategy for the treatment of addiction. Glutamate transporters are responsible for clearing extracellular glutamate and maintaining glutamate homeostasis. Our previous work demonstrated that aquaporin-4 (AQP4) deficiency attenuated morphine dependence, but the mechanisms are unclear. According to the recent evidence that AQP4 might form a functional complex with glutamate transporter-1 (GLT-1), this study focused on whether AQP4 participates in the modulation of GLT-1 and glutamate homeostasis in morphine-dependent mice.

RESULTS

We found that AQP4 knockout prevented the down-regulations of GLT-1 expression and glutamate clearance when mice were repeatedly treated with morphine. Further study revealed that inhibition of GLT-1 by dihydrokainic acid (DHK) initiated morphine dependence in AQP4 knockout mice. In addition, AQP4 knockout abolished both decreases and increases in the extracellular glutamate levels in the prefrontal cortex during repeated morphine treatment and naloxone-precipitated withdrawal.

CONCLUSION

AQP4 deficiency suppresses the down-regulation of GLT-1, and the disruption of glutamate homeostasis caused by repeated exposure to morphine, pointing to a strategy for maintaining glutamate homeostasis and thereby treating addiction through the modulation of AQP4 function and expression.

Differential regulation of morphine antinociceptive effects by endogenous enkephalinergic system in the forebrain of mice

BACKGROUND

Mice lacking the preproenkephalin (ppENK) gene are hyperalgesic and show more anxiety and aggression than wild-type (WT) mice. The marked behavioral changes in ppENK knock-out (KO) mice appeared to occur in supraspinal response to painful stimuli. However the functional role of enkephalins in the supraspinal nociceptive processing and their underlying mechanism is not clear. The aim of present study was to compare supraspinal nociceptive and morphine antinociceptive responses between WT and ppENK KO mice.

RESULTS

The genotypes of bred KO mice were confirmed by PCR. Met-enkephalin immunoreactive neurons were labeled in the caudate-putamen, intermediated part of lateral septum, lateral globus pallidus, intermediated part of lateral septum, hypothalamus, and amygdala of WT mice. Met-enkephalin immunoreactive neurons were not found in the same brain areas in KO mice. Tail withdrawal and von Frey test results did not differ between WT and KO mice. KO mice had shorter latency to start paw licking than WT mice in the hot plate test. The maximal percent effect of morphine treatments (5 mg/kg and 10 mg/kg, i.p.) differed between WT and KO mice in hot plate test. The current source density (CSD) profiles evoked by peripheral noxious stimuli in the primary somatosenstory cortex (S1) and anterior cingulate cortex (ACC) were similar in WT and KO mice. After morphine injection, the amplitude of the laser-evoked sink currents was decreased in S1 while the amplitude of electrical-evoked sink currents was increased in the ACC. These differential morphine effects in S1 and ACC were enhanced in KO mice. Facilitation of synaptic currents in the ACC is mediated by GABA inhibitory interneurons in the local circuitry. Percent increases in opioid receptor binding in S1 and ACC were 5.1% and 5.8%, respectively.

CONCLUSION

The present results indicate that the endogenous enkephalin system is not involved in acute nociceptive transmission in the spinal cord, S1, and ACC. However, morphine preferentially suppressed supraspinal related nociceptive behavior in KO mice. This effect was reflected in the potentiated differential effects of morphine in the S1 and ACC in KO mice. This potentiation may be due to an up-regulation of opioid receptors. Thus these findings strongly suggest an antagonistic interaction between the endogenous enkephalinergic system and exogenous opioid analgesic actions in the supraspinal brain structures.

Glutamatergic substrates of drug addiction and alcoholism

The past two decades have witnessed a dramatic accumulation of evidence indicating that the excitatory amino acid glutamate plays an important role in drug addiction and alcoholism. The purpose of this review is to summarize findings on glutamatergic substrates of addiction, surveying data from both human and animal studies. The effects of various drugs of abuse on glutamatergic neurotransmission are discussed, as are the effects of pharmacological or genetic manipulation of various components of glutamate transmission on drug reinforcement, conditioned reward, extinction, and relapse-like behavior. In addition, glutamatergic agents that are currently in use or are undergoing testing in clinical trials for the treatment of addiction are discussed, including acamprosate, N-acetylcysteine, modafinil, topiramate, lamotrigine, gabapentin and memantine. All drugs of abuse appear to modulate glutamatergic transmission, albeit by different mechanisms, and this modulation of glutamate transmission is believed to result in long-lasting neuroplastic changes in the brain that may contribute to the perseveration of drug-seeking behavior and drug-associated memories. In general, attenuation of glutamatergic transmission reduces drug reward, reinforcement, and relapse-like behavior. On the other hand, potentiation of glutamatergic transmission appears to facilitate the extinction of drug-seeking behavior. However, attempts at identifying genetic polymorphisms in components of glutamate transmission in humans have yielded only a limited number of candidate genes that may serve as risk factors for the development of addiction. Nonetheless, manipulation of glutamatergic neurotransmission appears to be a promising avenue of research in developing improved therapeutic agents for the treatment of drug addiction and alcoholism.

Morphine decreases extracellular levels of glutamate in the anterior cingulate cortex: an in vivo microdialysis study in freely moving rats

In the present study, we investigated the effect of morphine on the extracellular levels of glutamate in the anterior cingulate cortex (ACC) in freely moving rats using in vivo microdialysis coupled to high performance liquid chromatography and electrochemical detection. The results showed that either acute or chronic morphine treatment decreased the extracellular levels of glutamate in the ACC. Naloxone could reverse the decrease induced by chronic morphine treatment. The present study provided the first neurochemical evidence that morphine decreased extracellular levels of glutamate in the ACC, suggesting that glutamate in ACC is involved in the central actions of morphine.

Morphine inhibits glutamate exocytosis from rat cerebral cortex nerve terminals (synaptosomes) by reducing Ca2+ influx

Morphine, a mu-opioid agonist, suppressed the Ca(2+)-dependent release of glutamate that was evoked by exposing cerebrocortical synaptosomes to the potassium channel blocker 4-aminopyridine. The presynaptic inhibition produced by morphine was concentration-dependent and blocked by the nonselective opioid receptor antagonist naloxone. As determined by examining the mechanism of mu-opioid receptor-mediated inhibition of glutamate release, morphine caused a significant reduction in 4-aminopyridine-evoked increase in the cytoplasmic free Ca(2+) concentration ([Ca(2+)](c)), but failed to alter both 4-aminopyridine-evoked depolarization of the synaptosomal plasma membrane potential and Ca(2+) ionophore (ionomycin)-induced glutamate release. In addition, morphine was not capable of producing further inhibition on 4AP-evoked glutamate release in synaptosomes pretreated with the cannabinoid CB(1) receptor agonist WIN 55212-2, which has been shown to depress glutamate release through a suppression of presynaptic voltage-dependent Ca(2+) channel activity. These data suggest that morphine exerts its inhibitory effect presynaptically, likely through the reduction of Ca(2+) influx into nerve terminals, and thereby inhibits the release of glutamate in the cerebral cortex. This may therefore indicate that mu-opioid receptor agonists have neuroprotective properties, especially in the excessive glutamate release that occurs under certain pathological conditions.

Adaptive changes in M1 muscarinic receptors localized to specific rostral brain regions during and after morphine withdrawal

Morphine-dependent rats were allowed to undergo withdrawal by abrupt discontinuation of the drug. The regional expression of brain M1 muscarinic receptors was measured directly by autoradiographic determination with [(3)H] pirenzepine, and indirectly by quantifying the relative levels of M1 mRNA encoding the receptor protein. Patterns of receptor changes after morphine treatment were in general agreement using the two methods. Frontal cortical samples derived from morphine-dependent rats exhibited a 28% increase in M1 receptor mRNA measured at the end of the infusion. At the peak of the withdrawal, M1 mRNA levels for dependent rats were much lower (33.4%) than those for control rats. Hippocampal samples derived from morphine-dependent rats exhibited no changes in M1 mRNA levels after the morphine infusion. During the peak of withdrawal, however, hippocampal M1 mRNA levels were reduced (57%) compared with levels for controls. The M1 mRNA levels remained at this reduced degree of expression even after withdrawal symptoms had subsided. Addition of diisopropylflurophophate (DFP) to the morphine infusion schedule inhibited the adaptive changes in M1 mRNA levels induced by morphine. During the peak period of withdrawal, M1 mRNA levels in the hippocampus declined by only 18% as compared with 57% for the morphine control group. The adaptive decrease in hippocampal M1 receptors after withdrawal subsided may reflect prolonged heightened cholinergic activity in an area where such cholinergic innervation plays an important role in memory.