Deletion of the glutamate receptor 5 subunit of kainate receptors affects the development of morphine tolerance

Interneuronal GluK1 kainate receptors control maturation of GABAergic transmission and network synchrony in the hippocampus

Kainate type glutamate receptors (KARs) are strongly expressed in GABAergic interneurons and have the capability of modulating their functions via ionotropic and G-protein coupled mechanisms. GABAergic interneurons are critical for generation of coordinated network activity in both neonatal and adult brain, yet the role of interneuronal KARs in network synchronization remains unclear. Here, we show that GABAergic neurotransmission and spontaneous network activity is perturbed in the hippocampus of neonatal mice lacking GluK1 KARs selectively in GABAergic neurons. Endogenous activity of interneuronal GluK1 KARs maintains the frequency and duration of spontaneous neonatal network bursts and restrains their propagation through the hippocampal network. In adult male mice, the absence of GluK1 in GABAergic neurons led to stronger hippocampal gamma oscillations and enhanced theta-gamma cross frequency coupling, coinciding with faster spatial relearning in the Barnes maze. In females, loss of interneuronal GluK1 resulted in shorter sharp wave ripple oscillations and slightly impaired abilities in flexible sequencing task. In addition, ablation of interneuronal GluK1 resulted in lower general activity and novel object avoidance, while causing only minor anxiety phenotype. These data indicate a critical role for GluK1 containing KARs in GABAergic interneurons in regulation of physiological network dynamics in the hippocampus at different stages of development.

Different Roles of Beclin1 in the Interaction Between Glia and Neurons after Exposure to Morphine and the HIV- Trans-Activator of Transcription (Tat) Protein

Previously we showed that Beclin1 has a regulatory role in the secretion of inflammatory molecules in glia after exposure to morphine and Tat (an HIV protein). Here we show increased secretion of neuronal growth factors and increased neuronal survival in Beclin1-deficient glia. However, without glia co-culture, neurons deficient in Beclin1 showed greater death and enhanced dendritic beading when compared to wild-type neurons, suggesting that glial-secreted growth factors compensate for the damage reduced autophagy causes neurons. To assess if our ex vivo results correlated with in vivo studies, we used a wild-type (Becn1+/+) and Beclin1-deficient (Becn1+/+) mouse model and intracranially infused the mice with Tat and subcutaneously administered morphine pellets. After morphine implantation, significantly impaired locomotor activities were detected in both Becn1+/+ and Becn1+/- mice, irrespective of Tat infusion. After induction of pain, morphine-induced antinociception was detected. Interestingly, co-exposure to morphine and Tat increased sensitivity to pain in Becn1+/+ mice, but not in similarly treated Becn1+/- mice. Brain homogenates from Becn1+/+ mice exposed to Tat, alone and in combination with morphine, showed increased secretion of pro-inflammatory cytokines and reduced expression of growth factors when compared to similarly treated Becn1+/- mice. Likewise, increased neuronal loss was detected when both Tat and morphine were administered to Becn1+/+ mice, but not in similarly treated Becn1+/- mice. Overall, our findings show that there is a Beclin1-driven interaction between Tat and morphine in glia and neurons. Moreover, reduced glial-Beclin1 may provide a layer of protection to neurons under stressful conditions, such as when exposed to morphine and Tat.

The prolactin receptor long isoform regulates nociceptor sensitization and opioid-induced hyperalgesia selectively in females

Pain is more prevalent in women for reasons that remain unclear. We have identified a mechanism of injury-free nociceptor sensitization and opioid-induced hyperalgesia (OIH) promoted by prolactin (PRL) in females. PRL signals through mutually inhibitory long (PRLR-L) and short (PRLR-S) receptor isoforms, and PRLR-S activation induces neuronal excitability. PRL and PRLR expression were higher in females. CRISPR-mediated editing of PRLR-L promoted nociceptor sensitization and allodynia in naïve, uninjured female mice that depended on circulating PRL. Opioids, but not trauma-induced nerve injury, decreased PRLR-L promoting OIH through activation of PRLR-S in female mice. Deletion of both PRLR-L and PRLR-S (total PRLR) prevented, whereas PRLR-L overexpression rescued established OIH selectively in females. Inhibition of circulating PRL with cabergoline, a dopamine D2 agonist, up-regulated PRLR-L and prevented OIH only in females. The PRLR-L isoform therefore confers protection against PRL-promoted pain in females. Limiting PRL/PRLR-S signaling pharmacologically or with gene therapies targeting the PRLR may be effective for reducing pain in a female-selective manner.

HIV-1 Tat and Morphine Differentially Disrupt Pyramidal Cell Structure and Function and Spatial Learning in Hippocampal Area CA1: Continuous versus Interrupted Morphine Exposure

About half the people infected with human immunodeficiency virus (HIV) have neurocognitive deficits that often include memory impairment and hippocampal deficits, which can be exacerbated by opioid abuse. To explore the effects of opioids and HIV on hippocampal CA1 pyramidal neuron structure and function, we induced HIV-1 transactivator of transcription (Tat) expression in transgenic mice for 14 d and co-administered time-release morphine or vehicle subcutaneous implants during the final 5 d (days 9-14) to establish steady-state morphine levels. Morphine was withheld from some ex vivo slices during recordings to begin to assess the initial pharmacokinetic consequences of opioid withdrawal. Tat expression reduced hippocampal CA1 pyramidal neuronal excitability at lower stimulating currents. Pyramidal cell firing rates were unaffected by continuous morphine exposure. Behaviorally, exposure to Tat or high dosages of morphine impaired spatial memory Exposure to Tat and steady-state levels of morphine appeared to have largely independent effects on pyramidal neuron structure and function, a response that is distinct from other vulnerable brain regions such as the striatum. By contrast, acutely withholding morphine (from morphine-tolerant ex vivo slices) revealed unique and selective neuroadaptive shifts in CA1 pyramidal neuronal excitability and dendritic plasticity, including some interactions with Tat. Collectively, the results show that opioid-HIV interactions in hippocampal area CA1 are more nuanced than previously assumed, and appear to vary depending on the outcome assessed and on the pharmacokinetics of morphine exposure.

HIV-1 Tat and opioids act independently to limit antiretroviral brain concentrations and reduce blood-brain barrier integrity

Poor antiretroviral penetration may contribute to human immunodeficiency virus (HIV) persistence within the brain and to neurocognitive deficits in opiate abusers. To investigate this problem, HIV-1 Tat protein and morphine effects on blood-brain barrier (BBB) permeability and drug brain penetration were explored using a conditional HIV-1 Tat transgenic mouse model. Tat and morphine effects on the leakage of fluorescently labeled dextrans (10-, 40-, and 70-kDa) into the brain were assessed. To evaluate effects on antiretroviral brain penetration, Tat+ and Tat- mice received three antiretroviral drugs (dolutegravir, abacavir, and lamivudine) with or without concurrent morphine exposure. Antiretroviral and morphine brain and plasma concentrations were determined by LC-MS/MS. Morphine exposure, and, to a lesser extent, Tat, significantly increased tracer leakage from the vasculature into the brain. Despite enhanced BBB breakdown evidenced by increased tracer leakiness, morphine exposure led to significantly lower abacavir concentrations within the striatum and significantly less dolutegravir within the hippocampus and striatum (normalized to plasma). P-glycoprotein, an efflux transporter for which these drugs are substrates, expression and function were significantly increased in the brains of morphine-exposed mice compared to mice not exposed to morphine. These findings were consistent with lower antiretroviral concentrations in brain tissues examined. Lamivudine concentrations were unaffected by Tat or morphine exposure. Collectively, our investigations indicate that Tat and morphine differentially alter BBB integrity. Morphine decreased brain concentrations of specific antiretroviral drugs, perhaps via increased expression of the drug efflux transporter, P-glycoprotein.

Adolescent oxycodone self administration alters subsequent oxycodone-induced conditioned place preference and anti-nociceptive effect in C57BL/6J mice in adulthood

Adolescent and young adult abuse of short-acting MOP-r agonists such as oxycodone is a pressing public health issue. Few preclinical studies have examined how adolescent exposure to oxycodone impacts its effects in the transition to adulthood.

OBJECTIVE

To determine in mice how chronic adolescent oxycodone self-administration (SA) affects subsequent oxycodone-induced conditioned place preference (CPP), locomotor activity, and anti-nociception once mice reach early adulthood.

METHODS

Adolescent C57BL/6J male mice (4 weeks old, n = 6-11) and adult mice (10 weeks old, n = 6-10) were surgically implanted with indwelling jugular catheters. Mice then acquired oxycodone self-administration (14 consecutive 2-hr daily sessions; 0.25 mg/kg/infusion) followed by a 14-day drug-free (withdrawal) period in home cage. After the 14-day drug-free period, mice underwent a 10-day oxycodone CPP procedure (0, 1, 3, 10 mg/kg i.p.) or were tested for acute oxycodone-induced antinociception in the hot plate assay (3.35, 5, 7.5 mg/kg i.p.).

RESULTS

Mice that self-administered oxycodone during adolescence exhibited greater oxycodone-induced CPP (at the 3 mg/kg dose) than their yoked saline controls and mice that self-administered oxycodone during adulthood. Oxycodone dose-dependently increased locomotor activity, but sensitization developed only to the 3 mg/kg in the mice that underwent oxycodone self-administration as adolescents. Mice that self-administered oxycodone as adolescents decreased in the anti-nociceptive effects of oxycodone in one dose (5 mg/kg), whereas animals that self-administered oxycodone as adults did not show this effect.

CONCLUSION

Chronic adolescent oxycodone self-administration led to increased oxycodone-induced CPP (primarily 1 and 3 mg/kg, i.p.) and reduced antinociceptive effect of oxycodone (5 mg/kg, i.p.) in adulthood.

Activation of P2X7 receptors in the midbrain periaqueductal gray of rats facilitates morphine tolerance

Opiates such as morphine exhibit analgesic effect in various pain models, but repeated and chronic morphine administration may develop resistance to antinociception. The purinergic signaling system is involved in the mechanisms of pain modulation and morphine tolerance. This study aimed to determine whether the P2X7 receptor in the ventrolateral midbrain periaqueductal gray (vlPAG) is involved in morphine tolerance. Development of tolerance to the antinociceptive effect of morphine was induced in normal adult male Sprague-Dawley (SD) rats through subcutaneous injection of morphine (10mg/kg). The analgesic effect of morphine (5mg/kg, i.p.) was assessed by measuring mechanical withdrawal thresholds (MWTs) in rats with an electronic von Frey anesthesiometer. The expression levels and distribution of the P2X7 receptor in the vlPAG was evaluated through Western blot analysis and immunohistochemistry. The acute effects of intra-vlPAG injection of the selective P2X7 receptor agonist Bz-ATP, the selective P2X7 receptor antagonist A-740003, or antisense oligodeoxynucleotide (AS ODN) targeting the P2X7 receptor on morphine-treated rats were also observed. Results demonstrated that repeated morphine administration decreased the mechanical pain thresholds. By contrast, the expression of the P2X7 receptor protein was up-regulated in the vlPAG in morphine tolerant rats. The percent changes in MWT were markedly but only transiently attenuated by intra-vlPAG injection of Bz-ATP (9nmol/0.3μL) but elevated by A-740003 at doses of 10 and 100nmol/0.3μL. AS ODN (15nmol/0.3μL) against the P2X7 receptor reduced the development of chronic morphine tolerance in rats. These results suggest that the development of antinociceptive tolerance to morphine is partially mediated by activating the vlPAG P2X7 receptors. The present data also suggest that the P2X7 receptors may be a therapeutic target for improving the analgesic effect of morphine in treatments of pain when morphine tolerance occurs.

Persistent peripheral inflammation attenuates morphine-induced periaqueductal gray glial cell activation and analgesic tolerance in the male rat

Morphine is among the most prevalent analgesics prescribed for chronic pain. However, prolonged morphine treatment results in the development of analgesic tolerance. An abundance of evidence has accumulated indicating that central nervous system glial cell activity facilitates pain transmission and opposes morphine analgesia. While the midbrain ventrolateral periaqueductal gray (vlPAG) is an important neural substrate mediating pain modulation and the development of morphine tolerance, no studies have directly assessed the role of PAG glia. Here we test the hypothesis that morphine-induced increases in vlPAG glial cell activity contribute to the development of morphine tolerance. As morphine is primarily consumed for the alleviation of severe pain, the influence of persistent inflammatory pain was also assessed. Administration of morphine, in the absence of persistent inflammatory pain, resulted in the rapid development of morphine tolerance and was accompanied by a significant increase in vlPAG glial activation. In contrast, persistent inflammatory hyperalgesia, induced by intraplantar administration of complete Freund's adjuvant (CFA), significantly attenuated the development of morphine tolerance. No significant differences were noted in vlPAG glial cell activation for CFA-treated animals versus controls. These results indicate that vlPAG glia are modulated by a persistent pain state, and implicate vlPAG glial cells as possible regulators of morphine tolerance.

PERSPECTIVE

The development of morphine tolerance represents a significant impediment to its use in the management of chronic pain. We report that morphine tolerance is accompanied by increased glial cell activation within the vlPAG, and that the presence of a persistent pain state prevented vlPAG glial activation and attenuated morphine tolerance.

Therapeutic potential of kainate receptors

Glutamate receptors are key mediators of brain communication. Among ionotropic glutamate receptors, kainate receptors (KARs) have been least explored and their relevance to pathophysiology is relatively obscure. This is in part due to the relatively low abundance of KARs, the regulatory function in network activity they play, the lack of specific agonists and antagonists for this receptor subtype, as well as to the absence of striking phenotypes in mice deficient in KAR subunits. Nonetheless, it is now well established that KARs are located presynaptically whereby they regulate glutamate and GABA release, and thus, excitability and participate in short‐term plasticity. In turn, KARs are also located postsynaptically and their activation contributes to synaptic integration. The development of specific novel ligands is helping to further investigate the contribution of KARs to health and disease. In this review, I summarize current knowledge about KAR physiology and pharmacology, and discuss their involvement in cell death and disease. In addition, I recapitulate the available data about the use of KAR antagonists and receptor subunit deficient mice in experimental paradigms of brain diseases, as well as the main findings about KAR roles in human CNS disorders. In sum, subunit specific antagonists have therapeutic potential in neurodegenerative and psychiatric diseases as well as in epilepsy and pain. Knowledge about the genetics of KARs will also help to understand the pathophysiology of those and other illnesses.

Endogenous opiates and behavior: 2009

This paper is the 32nd consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2009 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).

Spinal mediators that may contribute selectively to antinociceptive tolerance but not other effects of morphine as revealed by deletion of GluR5

Several groups maintain that morphine tolerance and dependence correlate with increased activity of protein kinases ERK1/2 and P38 MAPK and PKC as well as elevated levels of the neuropeptides dynorphin (DYN), substance P (sP), and calcitonin gene-related peptide (CGRP) in spinal cord dorsal horn (SCDH). They demonstrate that tolerance and dependence can be prevented, and sometimes reversed, by constitutive genetic deletion or pharmacological inhibition of these factors. Recently, we showed that mice with a constitutive deletion of the GluR5 subunit of kainate receptors (GluR5 KO) are not different from wild type (WT) littermates with respect to baseline nociceptive thresholds as well as acute morphine antinociception, morphine physical dependence and conditioned place preference. However, unlike WT, GluR5 KO mice do not develop antinociceptive tolerance following systemic morphine administration. In this report, we examined levels of these mediators in SCDH of WT and GluR5 KO mice following subcutaneous implantation of placebo or morphine pellets. Surprisingly, spinal DYN and CGRP, along with phosphorylated ERK2 (pERK2), P38 (pP38) and PKCgamma (pPKCgamma) are elevated by deletion of GluR5. Additionally, chronic systemic morphine administration increased spinal pERK2, pP38 and pPKCgamma levels in both tolerant WT and non-tolerant GluR5 KO mice. In contrast, while morphine increased spinal DYN and CGRP in WT mice, DYN remained unchanged and CGRP was reduced in GluR5 KO mice. These observations suggest that spinal ERK2, P38 and PKCgamma are likely involved in multiple adaptive responses following systemic morphine administration, whereas DYN and CGRP may contribute selectively to the development of antinociceptive tolerance.

Deletion of the GluR5 subunit of kainate receptors affects cocaine sensitivity and preference

We have demonstrated previously that mice expressing a constitutive deletion of the kainate receptor subunit GluR5 (GluR5 KO) do not differ from wildtype (WT) littermates of a congenic C57BL/6 background with regard to both the development of morphine physical dependence as measured by naloxone-precipitated withdrawal signs and to morphine reward as revealed by the expression of conditioned place preference (CPP). However, unlike WT, GluR5 KO mice fail to develop antinociceptive tolerance following repeated systemic morphine administration. In this report, we examined the impact of GluR5 deletion on cocaine-mediated CPP and locomotor sensitization. Expression of CPP was evident in WT mice following repeated daily administration of 20mg/kg (but not 10mg/kg) i.p. cocaine. Interestingly, GluR5 KO mice exhibited enhanced cocaine preference as compared with WT mice at both 10 and 20mg/kg doses. In addition, while GluR5 KO mice did not differ from WT with respect to baseline locomotor activity, mutant mice demonstrated increased locomotor hyperactivity versus WT mice after repeated injection of 15mg/kg i.p. cocaine. Collectively, these data indicate that GluR5 appears to negatively modulate some psychostimulant and rewarding properties of cocaine, as demonstrated by heightened sensitization and salience in mutant mice.

Effect of KEPI (Ppp1r14c) deletion on morphine analgesia and tolerance in mice of different genetic backgrounds: when a knockout is near a relevant quantitative trait locus

We previously identified KEPI as a morphine-regulated gene using subtractive hybridization and differential display PCR. Upon phosphorylation by protein kinase C, KEPI becomes a powerful inhibitor of protein phosphatase 1. To gain insights into KEPI functions, we created KEPI knockout (KO) mice on mixed 129S6xC57BL/6 genetic backgrounds. KEPI maps onto mouse chromosome 10 close to the locus that contains the mu-opioid receptor (Oprm1) and provides a major quantitative trait locus for morphine effects. Analysis of single nucleotide polymorphisms in and near the Oprm1 locus identified a doubly-recombinant mouse with C57BL/6 markers within 1 Mb on either side of the KEPI deletion. This strategy minimized the amount of 129S6 DNA surrounding the transgene and documented the C57BL/6 origin of the Oprm1 gene in this founder and its offspring. Recombinant KEPIKO mice displayed (a) normal analgesic responses and normal locomotion after initial morphine treatments, (b) accelerated development of tolerance to analgesic effects of morphine, (c) elevated activity of protein phosphatase 1 in thalamus, (d) attenuated morphine reward as assessed by conditioned place preference. These data support roles for KEPI action in adaptive responses to repeated administration of morphine that include analgesic tolerance and drug reward.