Prolonged morphine application modulates Bax and Hsp70 levels in primary rat neurons

Morphine-induced MOR-1X and ASF/SF2 Expressions Are Independent of Transcriptional Regulation: Implications for MOR-1X Signaling

Recently, multiple μ-opioid receptor (MOR) isoforms have been identified that originate from a single gene, OPRM1; however, both their regulation and their functional significance are poorly characterized. The objectives of this study were to decipher, first, the regulation of alternatively spliced μ-opioid receptor isoforms and the spliceosome components that determine splicing specificity and, second, the signaling pathways utilized by particular isoforms both constitutively and following agonist binding. Our studies demonstrated that the expression of a particular splice variant, MOR-1X, was up-regulated by morphine, and this coincided with an increase in the essential splicing factor ASF/SF2. Structural comparison of this isoform to the prototypical variant MOR-1 revealed that the unique distal portion of the C-terminal domain contains additional phosphorylation sites, whereas functional comparison found distinct signaling differences, particularly in the ERK and p90 RSK pathways. Additionally, MOR-1X expression significantly reduced Bax expression and mitochondrial dehydrogenase activity, suggesting a unique functional consequence for MOR-1X specific signaling. Collectively, these findings suggest that alternative splicing of the MOR is altered by exogenous opioids, such as morphine, and that individual isoforms, such as MOR-1X, mediate unique signal transduction with distinct functional consequence. Furthermore, we have identified for the first time a potential mechanism that involves the essential splicing factor ASF/SF2 through which morphine regulates splicing specificity of the MOR encoding gene, OPRM1.

Differential modulatory effects of morphine on acute and chronic stress induced neurobehavioral and cellular markers in rats

The present study evaluated the effects of morphine treatments on elevated plus maze test parameters, oxidative stress markers and Hsp70 expression in normal and stressed rats. Acute and chronic stress caused neurobehavioral suppression, altered prooxidant-antioxidant balance and increased Hsp70 expression in brain homogenates in a differential manner. Morphine (1 and 5mg/kg) attenuated RS induced anxiogenesis, changes in MDA and GSH but further enhanced Hsp70 expression. Similar anxiolytic and Hsp70 enhancing effects were seen after morphine in normal rats (no RS). Exposure to chronic RS did not elicit any appreciable neurobehavioral response in EPM but enhanced MDA, lowered GSH and exaggerated the Hsp70 expression. Pretreatment with morphine did not affect the neurobehavioral response to chronic RS, but reverted the GSH and Hsp70 expression. The results suggest that morphine differentially influences acute and chronic stress induced changes in anxiety behavior and complex interactions between oxidative stress markers and Hsp70 expression which may contribute to these effects.

Molecular chaperone heat shock protein 70 participates in the labile phase of the development of behavioural sensitization induced by a single morphine exposure in mice

De-novo protein synthesis is required in the development of behavioural sensitization. A prior screening test from our laboratory has implicated heat shock protein 70 (Hsp70) as one of the proteins required in this behavioural plasticity. Thus, this study was designed to extend our understanding of the role of Hsp70 in the development of behavioural sensitization induced by a single morphine exposure in mice. First, by employing transcription inhibitor actinomycin D (AD) and protein synthesis inhibitor cycloheximide (CHX), we identified a protein synthesis-dependent labile phase (within 4 h after the first morphine injection) in the development of behavioural sensitization to a single morphine exposure. Second, Hsp70 protein expression in the nucleus accumbens correlated positively with locomotor responses of sensitized mice and, more importantly, the expression of Hsp70 increased within 1 h after the first morphine injection. Third, AD and CHX both prevented expression of Hsp70 and disrupted the development of the single morphine induced behavioural sensitization, which further implied Hsp70 was highly associated with behavioural sensitization. Finally, the selective Hsp70 inhibitor pifithrin-μ (PES) i.c.v. injected in mice prevented the development of behavioural sensitization and, critically, this inhibitory effect occurred only when PES was given within 1 h after the first morphine injection, which was within the labile phase of the development period. Taken together, we draw the conclusion that Hsp70 is crucially involved in the labile phase of the development of behavioural sensitization induced by a single morphine exposure, probably functioning as a molecular chaperone.

Influences of calcitonin gene-related peptide on mu opioid receptors in nucleus accumbens neurons of rats

The Mu opioid receptor (MOR) has been shown to participate in the analgesic effect of the calcitonin gene-related peptide (CGRP) in the nucleus accumbens (NAc) of adult rats. However, it is not clear whether and how CGRP regulates the MOR at the molecular levels. In the present study, it is found that the level of MORs on the cell membrane of NAc neurons was increased twice more than the control level following CGRP treatment (1μM, 30min), which is a phenomenon that was blocked by the peptidergic antagonist CGRP8-37. No direct physical interaction was observed between MORs and CGRP receptors, and neither brefeldin A nor dynosore preincubation affected such effects of CGRP. However, addition of 20μM monensin 1h before CGRP treatment significantly blocked the action of CGRP on surface MORs. In living animals, microinjection of CGRP (1nmol in 1μl) into the NAc partially restored morphine antinociception in morphine-tolerant rats, and the effect of CGRP on surface MORs extended beyond normal NAc neurons to chronic morphine-treated NAc neurons. To conclude, these results demonstrate that CGRP can act on MORs and increase the number of surface MORs in NAc neurons, partially explaining the involvement of opioid receptors in CGRP-induced antinociception in the rat NAc.

Transcription and protein synthesis inhibitors reduce the induction of behavioural sensitization to a single morphine exposure and regulate Hsp70 expression in the mouse nucleus accumbens

New protein synthesis has been implicated as necessary for long-lasting changes in neuronal function. Behavioural sensitization to a single exposure to addictive drugs is a form of neuroplasticity, but little is known about the importance of new protein synthesis in the underlying mechanism. This study was designed to investigate the effects of the transcription inhibitor actinomycin D (AD) and the protein synthesis inhibitor cycloheximide (CHX) on induction of behavioural sensitization to a single morphine exposure in mice. In combination with behavioural experiments, changes in gene and protein expression in the mouse nucleus accumbens (NAc) were analysed by RT-PCR array and Western blot respectively. Behavioural sensitization was evident in mice pretreated only once with morphine at the doses of 20 and 40 mg/kg, but not 5 and 10 mg/kg. Mice pretreated with morphine (20 mg/kg) and challenged with a lower dose (5 mg/kg) after a period of 4-21 d washout showed sensitized locomotion. At the doses that did not affect locomotion in mice, AD or CHX significantly suppressed hyperactivity induced by acute treatment, but not challenge with morphine, and blocked induction of behavioural sensitization to a single morphine exposure in a dose-related manner. The results from RT-PCR array and Western blot indicated that the changes of Hsp70 expression in the NAc of mice were associated with behavioural sensitization induced by a single morphine exposure. Together, these findings suggest that induction of behavioural sensitization to a single morphine exposure requires new protein synthesis, potentially involving Hsp70 expression in the NAc of mice.

Endogenous opiates and behavior: 2008

This paper is the 31st consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2008 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

Intracellular amyloid induces impairments on electrophysiological properties of cultured human neurons

The role of intracellular amyloid beta (iAbeta) in Alzheimer's disease (AD) initiation and progression attracts more and more attention in recent years. To address whether iAbeta induces early alterations of electrophysiological properties in cultured human primary neurons, we delivered iAbeta with adeno-virus and measured the electrophysiological properties of infected neurons with whole-cell recordings. Our results show that iAbeta induces an increase in neuronal resting membrane potentials, a decrease in K(+) currents and a hyperpolarizing shift in voltage-dependent activation of K(+) currents. These results suggest the electrophysiological impairments induced by iAbeta may be responsible for its neuronal toxicity.