Expression of the mu-opioid receptor is induced in dentate gyrus granule cells after focal cerebrocortical ischaemia and stimulation of entorhinal afferents

Differential regulation of CXCL12 and PACAP mRNA expression after focal and global ischemia

Pituitary adenylate cyclase activating peptide (PACAP) and the chemokine stromal cell-derived factor (SDF-1) have been implicated in neuroprotection, neurogenesis, and regeneration. Focal ischemia is associated with rapid upregulation of PACAP in perifocal neurons and delayed induction of SDF-1 in hypoxic/ischemic tissues, the latter process being involved in the recruitment of stem cells and inflammatory cells. Here, we studied mRNA patterns of PACAP, SDF-1 and the cognate receptors PAC1 and CXCR4 by in situ hybridization in the rat hippocampus after transient global ischemia, a rat model for programmed death of CA1 pyramidal neurons. Cell death in CA1 was not associated with local induction of PACAP and SDF-1 expression or recruitment of CXCR4-expressing infiltrates. However, there was a transient, almost complete loss of SDF-1 expression in microvessels in all hippocampal regions. Granule cells transiently showed a decrease of SDF-1 and an increase of PACAP expression. While PAC1 mRNA was moderately decreased throughout the hippocampus, CXCR4 expression was selectively increased in the subgranular layer. We propose that altered PACAP and SDF-1 gene expression in granule cells plays a role in regulated neurogenesis after global ischemia. The finding that programmed neuronal death after global ischemia was not associated with SDF-1 upregulation or recruitment of CXCR4-expressing cells is in sharp contrast to SDF-1/CXCR4-mediated infiltration of infarct tissue after focal ischemia. Hence, the different modes of neuronal death after focal and global ischemia are associated with distinct SDF-1 and PACAP gene regulation patterns and distinct reorganization mechanisms.

Age matters

The age of an experimental animal can be a critical variable, yet age matters are often overlooked within neuroscience. Many studies make use of young animals, without considering possible differences between immature and mature subjects. This is especially problematic when attempting to model traits or diseases that do not emerge until adulthood. In this commentary we discuss the reasons for this apparent bias in age of experimental animals, and illustrate the problem with a systematic review of published articles on long-term potentiation. Additionally, we review the developmental stages of a rat and discuss the difficulty of using the weight of an animal as a predictor of its age. Finally, we provide original data from our laboratory and review published data to emphasize that development is an ongoing process that does not end with puberty. Developmental changes can be quantitative in nature, involving gradual changes, rapid switches, or inverted U-shaped curves. Changes can also be qualitative. Thus, phenomena that appear to be unitary may be governed by different mechanisms at different ages. We conclude that selection of the age of the animals may be critically important in the design and interpretation of neurobiological studies.

Long-term alterations in mu, delta and kappa opioidergic receptors following middle cerebral artery occlusion in mice

Alterations in the opioidergic system may play a role in the molecular mechanisms underlying neurochemical responses to cerebral ischaemia. The present study aimed to determine the delayed expression of mu, delta and kappa opioid receptors, following 1, 2, 7, and 30 days of middle cerebral artery occlusion (MCAO) in mice. Using quantitative autoradiography, we highlighted significant decreases in mu, delta and kappa opioid receptor expression in ipsilateral cortices from day 1 post-MCAO. Moreover, in contralateral nucleus lateralis thalami pars posterior, ipsi- and contralateral nucleus medialis dorsalis thalami, and ipsilateral substantia nigra, pars reticulata (SNr), kappa receptors were increased; mu receptor densities were decreased in nucleus ventralis thalami, pars posterior (VThP), and SNr. delta-Binding sites were increased in the striatum on day 30 post-MCAO. The alterations in opioid receptors in cortical infarcts were correlated with strong histological damage. Further reductions in opioid receptor densities in cortical infarcts were observed at later time points. In subcortical brain regions, opioid receptor densities were also altered but no histological damage was seen, except in the VThP, in which cell density was increased on day 30. Delayed reductions in opioid receptor densities in the infarct appeared as the continuation of the early processes previously demonstrated. However, changes in subcortical opioid receptor expression may correlate with neuronal alterations in remote brain regions. Changes in opioidergic receptor expression in these regions may be involved in the long-term consequences of stroke and could be used as biomarker of neuronal alteration through the use of imaging techniques in the clinic.

Pituitary adenylate cyclase-activating polypeptide is up-regulated in cortical pyramidal cells after focal ischemia and protects neurons from mild hypoxic/ischemic damage

The protective effect of pituitary adenylate cyclase-activating polypeptide (PACAP) in stroke models is poorly understood. We studied patterns of PACAP, vasoactive intestinal peptide, and the PACAP-selective receptor PAC1 after middle cerebral artery occlusion and neuroprotection by PACAP in cortical cultures exposed to oxygen/glucose deprivation (OGD). Within hours, focal ischemia caused a massive, NMDA receptor (NMDAR)-dependent up-regulation of PACAP in cortical pyramidal cells. PACAP expression dropped below the control level after 2 days and was normalized after 4 days. Vasoactive intestinal peptide expression was regulated oppositely to that of PACAP. PAC1 mRNA showed ubiquitous expression in neurons and astrocytes with minor changes after ischemia. In cultured cortical neurons PACAP27 strongly activated Erk1/2 at low and p38 MAP kinase at higher nanomolar concentrations via PAC1. In astrocyte cultures, effects of PACAP27 on Erk1/2 and p38 were weak. During OGD, neurons showed severely reduced Erk1/2 activity and dephosphorylation of Erk1/2-regulated Ser112 of pro-apoptotic Bad. PACAP27 stimulation counteracted Erk1/2 inactivation and Bad dephosphorylation during short-term OGD but was ineffective after expanded OGD. Consistently, PACAP27 caused MEK-dependent neuroprotection during mild but not severe hypoxic/ischemic stress. While PACAP27 protected neurons at 1-5 nmol/L, full PAC1 activation by 100 nmol/L PACAP exaggerated hypoxic/ischemic damage. PACAP27 stimulation of astrocytes increased the production of Akt-activating factors and conferred ischemic tolerance to neurons. Thus, ischemia-induced PACAP may act via neuronal and astroglial PAC1. PACAP confers protection to ischemic neurons by maintaining Erk1/2 signaling via neuronal PAC1 and by increasing neuroprotective factor production via astroglial PAC1.

Distinct effects of individual opioids on the morphology of spines depend upon the internalization of mu opioid receptors

This study has examined the relationship between the effects of opioids on the internalization of mu opioid receptors (MORs) and the morphology of dendritic spines. Several opioids (morphine, etorphine, DAMGO or methadone) were applied to cultured hippocampal neurons. Live imaging and biochemical techniques were used to examine the dynamic changes in MOR internalization and spine morphology. This study reveals that MOR internalization can regulate opioid-induced morphological changes in dendritic spines: (1) Chronic treatment with morphine, which induced minimal receptor internalization, caused collapse of dendritic spines. In contrast, "internalizing" opioids such as DAMGO and etorphine induced the emergence of new spines. It reveals that opioid-induced changes in spines vary greatly depending on how the applied opioid agonist affects MOR internalization. (2) The blockade of receptor internalization by dominant negative mutant of dynamin, K44E, reversed the effects of DAMGO and etorphine. It indicates that receptor internalization is necessary for the distinct effects of DAMGO and etorphine on spines. (3) In neurons that were cultured from MOR knock-out mice and had been co-transfected with DsRed and MOR-GFP, morphine caused collapse of spines whereas DAMGO induced emergence of new spines, indicating that opioids can alter the structure of spines via postsynaptic MORs. (4) Methadone at a low concentration induced minimal internalization and had effects that were similar to morphine. At a high concentration, methadone induced robust internalization and had effects that are opposite to morphine. The concentration-dependent opioid-induced changes in dendritic spines might also contribute to the variation in the effects of individual opioids.

Endogenous opiates and behavior: 2005

This paper is the 28th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over a quarter-century of research. It summarizes papers published during 2005 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, neurophysiology and transmitter release (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); immunological responses (Section 17).