Kappa opioid receptor is expressed in the rat cerebellar cortex

Opioid receptors modulate parallel fiber-Purkinje cell synaptic transmission in mouse cerebellum

Opioid receptors play important roles in, among others, learning and memory, emotional responses, addiction, and pain. In recent years, the cerebellum has received increasing attention for its role in non-motor functions. The Purkinje cell (PC) is the only efferent neuron in the cerebellar cortex, and receives glutamatergic synaptic inputs from the parallel fibers (PF) formed by the axons of granule cells. Studies have shown that opioid receptors are expressed during the development of cerebellar cells. However, the distribution of opioid receptors, their subtypes in cerebellar PF-PC synapses, and their effects on synaptic transmission remain unclear. To examine these questions, we used whole-cell patch clamp recordings and pharmacological methods to determine the effects of activating three different opioid receptor subtypes on synaptic transmission at PF-PC synapses. In the presence of picrotoxin, mouse cerebellar slices were perfused with agonists or blockers of different opioid receptor subtypes, and the changes in excitatory postsynaptic currents (EPSCs) were examined. Both agonists of µ-opioid receptors (MOR) and δ-opioid receptors (DOR) significantly reduced the amplitude and area under the curve of PF-PC EPSCs in a concentration-dependent manner, accompanied by an increase in the paired-pulsed ratio (PPR). These effects could be blocked by respective receptor antagonists. In contrast, no significant changes were found after the application of κ-opioid receptor (KOR) agonists. In conclusion, MOR and DOR are present at the axon terminals of PF in the mouse cerebellar cortex, whereas no or negligible amounts of KOR are found. Activation of MOR and DOR regulates PF-PC synaptic transmission via inhibition of glutamate (Glu) release in cerebellar cortex in mice. We also found that endogenous opioid peptides are present in PF-PC synapses of mouse cerebellum, which also can inhibit the release of Glu.

Addiction and the cerebellum with a focus on actions of opioid receptors

Increasing evidence suggests that the cerebellum could play a role in the higher cognitive processes involved in addiction as the cerebellum contains anatomical and functional pathways to circuitry controlling motivation and saliency. In addition, the cerebellum exhibits a widespread presence of receptors, including opioid receptors which are known to play a prominent role in synaptic and circuit mechanisms of plasticity associated with drug use and development of addiction to opioids and other drugs of abuse. Further, the presence of perineural nets (PNNs) in the cerebellum which contain proteins known to alter synaptic plasticity could contribute to addiction. The role the cerebellum plays in processes of addiction is likely complex, and could depend on the particular drug of abuse, the pattern of use, and the stage of the user within the addiction cycle. In this review, we discuss functional and structural modifications shown to be produced in the cerebellum by opioids that exhibit dependency-inducing properties which provide support for the conclusion that the cerebellum plays a role in addiction.

Proteomic Analysis Unveils Expressional Changes in Cytoskeleton- and Synaptic Plasticity-Associated Proteins in Rat Brain Six Months after Withdrawal from Morphine

Drug withdrawal is associated with abstinence symptoms including deficits in cognitive functions that may persist even after prolonged discontinuation of drug intake. Cognitive deficits are, at least partially, caused by alterations in synaptic plasticity but the precise molecular mechanisms have not yet been fully identified. In the present study, changes in proteomic and phosphoproteomic profiles of selected brain regions (cortex, hippocampus, striatum, and cerebellum) from rats abstaining for six months after cessation of chronic treatment with morphine were determined by label-free quantitative (LFQ) proteomic analysis. Interestingly, prolonged morphine withdrawal was found to be associated especially with alterations in protein phosphorylation and to a lesser extent in protein expression. Gene ontology (GO) term analysis revealed enrichment in biological processes related to synaptic plasticity, cytoskeleton organization, and GTPase activity. More specifically, significant changes were observed in proteins localized in synaptic vesicles (e.g., synapsin-1, SV2a, Rab3a), in the active zone of the presynaptic nerve terminal (e.g., Bassoon, Piccolo, Rims1), and in the postsynaptic density (e.g., cadherin 13, catenins, Arhgap35, Shank3, Arhgef7). Other differentially phosphorylated proteins were associated with microtubule dynamics (microtubule-associated proteins, Tppp, collapsin response mediator proteins) and the actin–spectrin network (e.g., spectrins, adducins, band 4.1-like protein 1). Taken together, a six-month morphine withdrawal was manifested by significant alterations in the phosphorylation of synaptic proteins. The altered phosphorylation patterns modulating the function of synaptic proteins may contribute to long-term neuroadaptations induced by drug use and withdrawal.

Brief prenatal ethanol exposure alters behavioral sensitivity to the kappa opioid receptor agonist (U62,066E) and antagonist (Nor-BNI) and reduces kappa opioid receptor expression

BACKGROUND

Approximately 10 to 15% of women consume alcohol (ethanol [EtOH]) during pregnancy in the United States. Even low amounts of EtOH consumption during pregnancy can elicit long-term consequences. Prenatal experience with as few as 3 drinks has been associated with increase problem drinking in adulthood. Such effects are corroborated in rodents; however, the underlying neural adaptations contributing to this effect are not clear. In the current set of experiments, we investigated whether changes in EtOH responding following prenatal EtOH exposure involved kappa opioid receptor activation and expression.

METHODS

Sprague-Dawley rats were prenatally exposed to low levels of alcohol (1.0 g/kg) during late gestation (gestational days 17 to 20 [GD17-20]) via intragastric intubation of pregnant dams. Following birth, EtOH intake, kappa- and mu-opioid-induced place conditioning, and kappa opioid receptor expression in mesolimbic brain regions were assessed in infant rats (postnatal days 14 to 15 [PD14-15]) that were offspring of dams given EtOH, vehicle, or untreated, during pregnancy.

RESULTS

Animals exposed to prenatal alcohol drank more alcohol later in life and exhibited significant changes in the kappa opioid system. While control subjects found kappa opioid activation aversive, animals exposed to EtOH prenatally exhibited either no aversion or appetitive responding. Further analysis revealed that synaptosomal kappa opioid receptor expression was significantly decreased in brain areas implicated in responding to EtOH.

CONCLUSIONS

Overall, these data suggest that prenatal EtOH affects kappa opioid function and expression and that these changes may be involved in increased drinking later in life.

Reward processing by the opioid system in the brain

The opioid system consists of three receptors, mu, delta, and kappa, which are activated by endogenous opioid peptides processed from three protein precursors, proopiomelanocortin, proenkephalin, and prodynorphin. Opioid receptors are recruited in response to natural rewarding stimuli and drugs of abuse, and both endogenous opioids and their receptors are modified as addiction develops. Mechanisms whereby aberrant activation and modifications of the opioid system contribute to drug craving and relapse remain to be clarified. This review summarizes our present knowledge on brain sites where the endogenous opioid system controls hedonic responses and is modified in response to drugs of abuse in the rodent brain. We review 1) the latest data on the anatomy of the opioid system, 2) the consequences of local intracerebral pharmacological manipulation of the opioid system on reinforced behaviors, 3) the consequences of gene knockout on reinforced behaviors and drug dependence, and 4) the consequences of chronic exposure to drugs of abuse on expression levels of opioid system genes. Future studies will establish key molecular actors of the system and neural sites where opioid peptides and receptors contribute to the onset of addictive disorders. Combined with data from human and nonhuman primate (not reviewed here), research in this extremely active field has implications both for our understanding of the biology of addiction and for therapeutic interventions to treat the disorder.

Effects of neonatal stress and morphine on kappa opioid receptor signaling

BACKGROUND

Critically ill neonates experience multiple stressors during hospitalization. Opioids are commonly prescribed to ameliorate their pain and stress. However, the enduring effects of stress and opioids are not understood. The kappa opioid system is important in the mediation of stress in adults, but little is known about its function in neonates.

OBJECTIVES

To characterize kappa opioid receptor (KOR) distribution in the neonatal mouse brain and test whether neonatal exposure to morphine, stress, or both, change KOR signaling.

METHODS

Five groups of wild-type C57BL/6 or prodynorphin (Pdyn) knockout mice were tested: (1) untreated control (dam-reared, no handling), (2) saline-injected control, (3) morphine-injected control, (4) stressed with saline injections and (5) stressed with morphine injections. Mice were treated from postnatal day 5 to postnatal day 9, after which their brains were immunolabeled with a phospho-specific KOR antibody (KOR-P), glial fibrillary acidic protein or glutamic acid decarboxylase.

RESULTS

There were no effects of saline or morphine injection on KOR-P immunoreactivity. Neonatal stress increased KOR-P labeling in wild-type brains (p < 0.05), but not in Pdyn(-/-) animals. Mice exposed to stress and morphine showed region-specific increases in KOR-P immunoreactivity from 38 to 500% (p < 0.05 to p < 0.001), with marked gliosis. In stressed morphine-treated Pdyn(-/-) animals, KOR-P immunoreactivity was absent, but gliosis increased compared to wild-type animals.

CONCLUSIONS

Neonatal stress increases KOR activation via the dynorphin system. Neonatal stress plus morphine treatment further increased this response and also resulted in hippocampal gliosis. Enhanced gliosis noted in Pdyn(-/-) animals suggests that the endogenous dynorphin may play a role in downregulating this inflammatory response.

Altered gene expression and functional activity of opioid receptors in the cerebellum of CB1 cannabinoid receptor knockout mice after acute treatments with cannabinoids

Numerous studies have shown functional links between the cannabinoid and opioid systems. The goal of this study was to evaluate whether acute treatments by endogenous cannabinoid agonist, selective CB1 or CB2 receptor antagonists modulate the expression of mu- (MOR) and delta- (DOR) opioid receptor mRNA levels and functional activity in the cerebellum of transgenic mice deficient in the CB1 type of cannabis receptors. We examined the effect of noladin ether (endogenous cannabinoid agonist) pretreatment on MOR and DOR mRNA expression by using reverse transcription and real-time polimerase chain reaction (PCR) and the ability of subsequent application of the opioid agonists to activate G-proteins, as measured by [35S]GTPgammaS binding, in wild-type (CB1+/+) and CB1 cannabinoid receptor deficient (CB1-/-, 'knockout', K.O.) mice. The acute administration of noladin ether markedly reduced MOR-mediated G-protein activation and caused a significant increase in the level of MOR mRNAs in the cerebella of wildtype, but not in the CB1-/- mice. No significant differences were observed in DOR functional activity and mRNA expression in wild-type animals. In CB1-/- mice the expression of DOR mRNA increased after noladin ether treatment, but no changes were found in DOR functional activity. In addition, Rimonabant (selective central cannabinoid CB1 receptor antagonist) and SR144528 (selective peripheral cannabinoid CB2 receptor antagonist) caused significant potentiation in MOR functional activity in the wild-type animals, whereas DOR mediated G-protein activation was increased in the CB1-/- mice. In contrast, Rimonabant and SR144528 decreased the MOR and DOR mRNA expressions in both CB1+/+ and CB1-/- mice. Taken together, these results indicate that acute treatment with cannabinoids causes alterations in MOR and DOR mRNA expression and functional activity in the cerebella of wild-type and CB1 knockout mice indicating indirect interactions between these two signaling systems.

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).

Recent advances in elucidating pain mechanisms

PURPOSE OF REVIEW

The purpose of this review is to highlight the important recent advances in this fast developing field of pain mechanisms. It is now recognized that acute tissue and neural injuries can result in nociceptor sensitization (primary hyperalgesia) and spinal cord hyperexcitability or central sensitization that results in secondary hyperalgesia and allodynia.

RECENT FINDINGS

New findings regarding peripheral and central sensitization are presented in this review. Newer ideas on pain modulation, pain states and pain syndromes, mechanisms of chronic pain, newer opioid and non opioid medications including newer N-methyl-D-asparate antagonists, cyclooxygenase-2 inhibitors and membrane stabilizing analgesics as well as pain control at the genetic level are discussed.

SUMMARY

The research of the last decade has focused on the biochemical and structural plasticity of the nervous system following tissue and nerve injury. The mechanisms involved in the transition from acute to chronic pain are complex with the involvement of interacting receptor systems and intracellular ion flux, second messenger systems, new synaptic connections and apoptosis.