Bidirectional Dysregulation of AMPA Receptor-Mediated Synaptic Transmission and Plasticity in Brain Disorders

AMPA receptors (AMPARs) are glutamate-gated ion channels that mediate the majority of fast excitatory synaptic transmission throughout the brain. Changes in the properties and postsynaptic abundance of AMPARs are pivotal mechanisms in synaptic plasticity, such as long-term potentiation (LTP) and long-term depression (LTD) of synaptic transmission. A wide range of neurodegenerative, neurodevelopmental and neuropsychiatric disorders, despite their extremely diverse etiology, pathogenesis and symptoms, exhibit brain region-specific and AMPAR subunit-specific aberrations in synaptic transmission or plasticity. These include abnormally enhanced or reduced AMPAR-mediated synaptic transmission or plasticity. Bidirectional reversal of these changes by targeting AMPAR subunits or trafficking ameliorates drug-seeking behavior, chronic pain, epileptic seizures, or cognitive deficits. This indicates that bidirectional dysregulation of AMPAR-mediated synaptic transmission or plasticity may contribute to the expression of many brain disorders and therefore serve as a therapeutic target. Here, we provide a synopsis of bidirectional AMPAR dysregulation in animal models of brain disorders and review the preclinical evidence on the therapeutic targeting of AMPARs.publishedVersio

The Nociceptin/Orphanin FQ System and the Regulation of Memory

International audienceNociceptin/orphanin FQ (N/OFQ) is an endogenous neuropeptide of 17 amino acids, related to opioid peptides but with its own receptor, distinct from conventional opioid receptors, the ORL1 or NOP receptor. The NOP receptor is a G protein-coupled receptor which activates Gi/o proteins and thus induces an inhibition of neuronal activity. The peptide and its receptor are widely expressed in the central nervous system with a high density of receptors in regions involved in learning and memory. This review describes the consequences of the pharmacological manipulation of the N/OFQ system by NOP receptor ligands on learning processes and on the consolidation of various types of long-term memory. We also discuss the role of endogenous N/OFQ release in the modulation of learning and memory. Finally we propose several putative neuronal mechanisms taking place at the level of the hippocampus and amygdala and possibly underlying the behavioral amnestic or promnesic effects of NOP ligands

To Eat or Not To Eat: Contributions of Dorsal Hippocampal Neurons and Memory to Meal Onset

There is extensive research regarding the neural mechanisms that control satiety and meal termination; in contrast, there is very limited understanding of how the central nervous system regulates meal onset and thus the duration of the postprandial intermeal interval (ppIMI) and meal frequency. Based on emerging evidence, we hypothesize that dorsal hippocampal neurons, which are critical for episodic memory, form a memory of a meal and inhibit meal onset during the ppIMI. To test whether hippocampal neurons form a memory of a meal, we first determined that ingesting sucrose or isopreferred concentrations of the non-caloric sweetener saccharin increased the expression of the plasticity-related immediate early gene activity-regulated cytoskeleton-associated protein (Arc) in dorsal CA1 hippocampal (dCA1) neurons in Sprague-Dawley rats. Furthermore, repeated exposure to the sucrose meal attenuated the ability of the sucrose to induce Arc expression. Together, these data indicate that orosensory stimulation produced by a sweet taste is sufficient to induce synaptic plasticity in dCA1 neurons in an experience-dependent manner. Second, we showed that reversibly inactivating dorsal hippocampal neurons with infusions of the GABAA agonist muscimol after the end of a sucrose meal accelerated the onset of the next meal, indicating that dorsal hippocampal neurons inhibit meal onset. Lastly, using a clinically-relevant animal model of early life inflammatory injury, we found that neonatal injury (1) impairs hippocampal-dependent memory, (2) decreases the ppIMI and increases sucrose intake, (3) increases body mass, (4) attenuates sucrose-induced Arc expression in dCA1 neurons, and that (5) blocking inflammatory pain with morphine at the time of injury reverses the effects of injury on memory, energy intake and Arc expression. Collectively, the findings of this dissertation support the overarching hypothesis that dorsal hippocampal neurons inhibit meal onset during the ppIMI and suggest that dorsal hippocampal dysfunction may contribute to the development and/or maintenance of diet-induced obesity

The effect of repeated early injury on reward-related processing in the adult rat

Pain during early life can affect the developing central nervous system, leading to altered neural function in the adult organism. In this thesis, I investigate the long-term effects of repeated early pain on reward-related processing in the adult rat. I hypothesised that the reward system was likely to be sensitive to early activation of pain pathways, as the brain systems involved in both pain and reward overlap extensively, and virtually all centrally acting analgesic drugs are also drugs of reward. To begin, I investigate the extent to which the developing reward system is activated by a classic analgesic and drug of abuse, morphine. Comparing neonatal and adult activation of the dopaminergic system, results show that a single morphine challenge activates neonatal reward pathways, but that there are qualitative differences in the neonatal response to repeated morphine. Next, I show how reward-related behaviours of adult animals repeatedly injured as neonates differ from those of uninjured littermates, and finally propose the lateral hypothalamic orexin system as a biomarker reflecting this behaviour. The results provide evidence that neonatal injury interferes with the normal development of reward systems during a critical period of development, resulting in characteristic changes in reward behaviour and cell signalling in the adult animal

Drug-induced synaptic plasticity in addiction : the mesolimbic dopamine pathway and benzodiazepines

Drug addiction is defined as a chronically relapsing disorder of the brain. The characteristic compulsive drug use behaviour despite the negative consequences has been explained by a maladaptive learning phenomenon. Synaptic plasticity, i.e. the ability of neuronal connections to change on demand, forms the basis for learning and memory storage in the brain. Interestingly, many drugs of abuse have been shown to converge in their actions on the mesolimbic dopamine (DA) pathway as they induce synaptic plasticity in the DA neurons of the ventral tegmental area (VTA) that also mediate the recognition of natural rewards and reward-driven learning. This drug-induced plasticity is thought to facilitate future use of the drug i.e. reinforce drug-seeking behavior. Benzodiazepines are anxiolytic and sedative drugs that unfortunately can be addictive in a subset of users. The role of the DA system in benzodiazepine addiction is still controversial and thus this thesis was aimed at studying possible synaptic alterations in the VTA DA neurons induced by acute administration of benzodiazepine ligands. Diazepam and zolpidem were shown to induce persistent plasticity at the glutamatergic synapses in VTA DA neurons; a phenomenon claimed to be a common feature of different kinds of drugs of abuse. The possible effects of this synaptic plasticity at the behavioral level were examined by challenging the locomotor activity of mice with morphine and amphetamine, as the DA system plays a central role in their actions. We found that indeed the locomotor reactions of mice were altered: morphine-induced hyperlocomotion and amphetamine-induced sensitization were attenuated in mice while there was diazepam-induced plasticity at VTA DA neurons (24-72 h after diazepam administration). These findings indicated that the effects of morphine and amphetamine on DA neuron activity might be blunted during diazepam-induced synaptic plasticity. Orexins are neuropeptides synthesized in the hypothalamus; they posses important roles in several behaviors such as the regulation of sleep-wake cycle, arousal, energy balance, stress, motivation and reward. In particular, orexinergic signaling in VTA has been postulated to be important for drug-induced plasticity. The effects of diazepam on the activity of this neuronal population were studied by c-Fos immunohistochemistry as the levels of c-Fos are believed to mirror the activity of neurons. An anxiolytic dose of diazepam prevented the stress-induced increment in the orexinergic activity whereas a clearly sedative dose reduced the activity of these neurons even from the basal levels. These findings led to the formation of the hypothesis that orexinergic activity in the brain could have a role in the anxiolytic, sedative and hypnotic actions of benzodiazepines. In conclusion, this project revealed new aspects about the brain areas and the mechanisms mediating both the therapeutic properties as well as the addictive features of benzodiazepines. New evidence was found for a role of the mesolimbic DA system in mediating the addictive potential of benzodiazepines and a novel hypothesis was devised about how the orexinegic system may play a role in the therapeutic actions of benzodiazepines.Huumeriippuvuus on sairaus, jonka perustana ovat huumeen aikaansaamat muutokset aivojen nk. palkkioradalla, jonka keskeinen osa on mesolimbinen dopamiinirata. Palkkiorata huolehtii, että ympäristön mielihyvää antavat, selviämisen ja suvunjatkamisen kannalta tärkeät asiat (ruoka, juoma, seksi, suoja) havaitaan ja niitä lähestytään ja käytetään. Huumeet aktivoivat rataa hyvin voimakkaasti aikaansaaden intensiivistä mielihyvää. Valitettavasti palkkiorata näin ollen myös arvioi huumeen erittäin hyödylliseksi, ja ehdollistaa yksilöä käyttäytymään tavalla, joka johtaa huumeen etsimiseen ja käyttämiseen yhä uudestaan. Huumeet aikaansaavat dopamiini hermosoluissa synaptista plastisuutta. Tämä on nimitys tapahtumille, jotka johtavat hermosolujen välisten yhteyksien, synapsien, vahvistumiseen tai heikkenemiseen, siis hermosoluyhteyksien muovautuvuuteen, mikä on perusta mm. muistamiselle ja oppimiselle. Huumeriippuvuuden kehittymistä voisi kuvata epätarkoituksenmukaiseksi voimakkaaksi oppimisprosessiksi. Bentsodiatsepiinit (BZt) ovat tehokkaita lääkkeitä ahdistuneisuuden ja univaikeuksien hoidossa. Valitettavasti osalle potilaista kehittyy vaikea riippuvuus. Aivojen dopamiiniradan roolista BZ-riippuvuudessa ei ole päästy yksimielisyyteen, vaan paradoksaalisti BZt aktivoivat dopamiinisoluja, mutta vähentävät dopamiinin määrää synapseissa. Väitöskirjatutkimuksessani selvitettiin sähköfysiologisin menetelmin, aiheuttavatko BZt dopamiini soluissa synaptista plastisuutta, kuten muut riippuvuutta aiheuttavat aineet. BZn havaittiin aikaansaavan tämän synaptisen ilmiön, mikä kesti useita päiviä yhden lääkeaineannoksen jälkeen ennen palaamista normaalitilaan. Seuraavaksi tarkasteltiin, vaikuttaako dopamiinisolujen muuttunut tilanne näiden solujen välittämään käyttäytymiseen; morfiini ja amfetamiini lisäävät eläinten liikkumista aktivoidessaan dopamiini järjestelmää, ja kun BZn aiheuttama plastisuus oli käynnissä, morfiinin ja amfetamiinin vaikutukset olivat muuttuneet. Kolmannessa osatyössä tarkasteltiin aivoleikevärjäyksin, miten BZ vaikuttaa eri aivoalueiden aktiivisuuteen. Tulokset tukevat dopamiiniradan merkitystä BZ-riippuvuuden kehittymisessä. Lisäksi, BZn aikaansaama plastisuus aivoissa vaikuttaa dopamiinisolujen välittämään käyttäytymiseen useita päiviä, vielä kun itse lääkeaine on jo poistunut. Tutkimus toi uutta tietoa myös BZn rauhoittavista ja nukuttavista mekanismeista, kun havaittiin, että ne hiljentävät aivoissa tietyn mm. valvetta ja vireyttä ylläpitävän oreksiini hermosolujoukon. Riippuvuus on erittäin vaikea BZn sivuvaikutus, mutta parempia rauhoittavia lääkkeitä ilman riippuvuuspotentiaalia ei ole pystytty kehittämään. Väitöskirjani tulokset antavat uusia ideoita siitä, millaisiin reseptoripopulaatioihin ja mihin hermosolujoukkoihin uusien lääkeainemolekyylien tulisi vaikuttaa, jotta mahdollisesti saavutettaisiin rauhoittava vaikutus ilman riippuvuuden riskiä

The sigma-1 receptor as key common factor in cocaine and food seeking behaviors

Addiction and eating disorders involve brain reward circuits. A previous history of binge eating predisposes to addictive behavior, while the cessation of exposure to drugs of abuse leads to reward activities, including intake of tasty foods. Cocaine use is associated with a decrease in food intake, with reversal after the drug use is stopped. Exciting new findings show that receptors for the 'hunger' hormone, ghrelin, directly interact with the sigma-1 receptors (1R), which is a target of cocaine. 1R are key players in regulating dopaminergic neurotransmission and ghrelin-mediated actions. This review focuses on the 1 receptor as general neuroendocrine regulator by directly interacting with neuronal G-protein-coupled receptors. This review also covers the early mechanisms by which cocaine binding to 1 blocks the food-seeking behavior triggered by ghrelin. Such new findings appear as fundamental to understand common mechanisms in drug addiction and eating disorders

Lentiviral Vector-Mediated Gene Transfer and RNA Silencing Technology in Neuronal Dysfunctions

Lentiviral-mediated gene transfer in vivo or in cultured mammalian neurons can be used to address a wide variety of biological questions, to design animals models for specific neurodegenerative pathologies, or to test potential therapeutic approaches in a variety of brain disorders. Lentiviruses can infect non-dividing cells, thereby allowing stable gene transfer in post-mitotic cells such as mature neurons. An important contribution has been the use of inducible vectors: the same animal can thus be used repeatedly in the doxycycline-on or -off state, providing a powerful mean for assessing the function of a gene candidate in a disorder within a specific neuronal circuit. Furthermore, lentivirus vectors provide a unique tool to integrate siRNA expression constructs with the aim to locally knockdown expression of a specific gene, enabling to assess the function of a gene in a very specific neuronal pathway. Lentiviral vector-mediated delivery of short hairpin RNA results in persistent knockdown of gene expression in the brain. Therefore, the use of lentiviruses for stable expression of siRNA in brain is a powerful aid to probe gene functions in vivo and for gene therapy of diseases of the central nervous system. In this chapter I review the applications of lentivirus-mediated gene transfer in the investigation of specific gene candidates involved in major brain disorders and neurodegenerative processes. Major applications have been in polyglutamine disorders, such as synucleinopathies and Parkinson's disease, or in investigating gene function in Huntington's disease, dystonia, or muscular dystrophy. Recently, lentivirus gene transfer has been an invaluable tool for evaluation of gene function in behavioral disorders such as drug addiction and attention-deficit hyperactivity disorder or in learning and cognitio

Investigations into the role of the metabotropic glutamate receptor, mGluR5, in incentive learning and some behavioural and neurobiological effects of cocaine

The metabotropic glutamate receptor, mGluR5, is densely expressed in brain regions involved in incentive learning processes. There is considerable evidence to suggest that following exposure to addictive drugs such as cocaine, adaptations in these brain areas may underlie the development and maintenance of behavioural responses related to addictive processes. The present thesis examines the role of mGluR5 in both incentive learning processes and some behavioural and neurobiological effects of cocaine. First, using a novel mutant mouse line in which mGluR5 is selectively knocked down in cells that express dopamine D1 receptors (D1R), I argue that this mGluR5 population is critically important for specific incentive learning processes. By blocking mGluR5 in wild-type mice with a selective antagonist, I then propose mGluR5 as necessary for the acquisition, but not the expression of an incentive association. Next, I present data showing that mGluR5 on dopaminoceptive neurons are not necessary for the „conditioned rewarding‟ properties of cocaine, measured in the conditioned place preference model, but do contribute to the psychomotor activating effects of cocaine. Finally, I present an immunohistochemistry study that examines cocaine-induced activation of the extracellular-signal related kinase (ERK) pathway. In the mGluR5 knock-down mice, activation of the ERK pathway in the striatum is disrupted following an acute injection of cocaine. Given the importance of the ERK pathway in establishing and maintaining long term memories, I propose that disruption of this pathway could contribute, in part, to some findings reported in the present thesis. Taken together, this thesis will argue that signalling through mGluR5 on D1R expressing neurons is important for the formation of incentive associations, and may contribute to neural adaptations necessary for the development and maintenance of behavioural responses related to addictive processes

Naloxone's Pentapeptide Binding Site on Filamin A Blocks Mu Opioid Receptor–Gs Coupling and CREB Activation of Acute Morphine

Chronic morphine causes the mu opioid receptor (MOR) to switch its coupling from Gi/o to Gs, resulting in excitatory signaling via both Gαs and its Gβγ dimer. Ultra-low-dose naloxone (NLX) prevents this switch and attenuates opioid tolerance and dependence. This protective effect is mediated via a high-affinity interaction of NLX to a pentapeptide region in c-terminal filamin A (FLNA), a scaffolding protein interacting with MOR. In organotypic striatal slice cultures, we now show that acute morphine induces a dose-dependent Go-to-Gs coupling switch at 5 and 15 min that resolves by 1 hr. The acute Gs coupling induced by 100 µM morphine was completely prevented by co-treatment with 100 pM NLX, (+)NLX, or naltrexone (NTX), or their pentapeptide binding site (FLNA2561–2565), which we show can act as a decoy for MOR or bind to FLNA itself. All of these co-treatments presumably prevent the MOR–FLNA interaction. Since ultra-low-dose NTX also attenuates the addictive properties of opioids, we assessed striatal cAMP production and CREB phosphorylation at S133. Correlating with the Gs coupling, acute morphine induced elevated cAMP levels and a several-fold increase in pS133CREB that were also completely blocked by NLX, NTX or the FLNA pentapeptide. We propose that acute, robust stimulation of MOR causes an interaction with FLNA that allows an initially transient MOR–Gs coupling, which recovers with receptor recycling but persists when MOR stimulation is repeated or prolonged. The complete prevention of this acute, morphine-induced MOR–Gs coupling by 100 pM NLX/NTX or 10 µM pentapeptide segment of FLNA further elucidates both MOR signaling and the mechanism of action of ultra-low-dose NLX or NTX in attenuating opioid tolerance, dependence and addictive potential

Recent advances in understanding the roles of hypocretin/orexin in arousal, affect, and motivation [version 1; referees: 3 approved]

The hypocretins (Hcrts) are two alternatively spliced neuropeptides (Hcrt1/Ox-A and Hcrt2/Ox-B) that are synthesized exclusively in the hypothalamus. Data collected in the 20 years since their discovery have supported the view that the Hcrts play a broad role in the control of arousal with a particularly important role in the maintenance of wakefulness and sleep-to-wake transitions. While this latter point has received an overwhelming amount of research attention, a growing literature has begun to broaden our understanding of the many diverse roles that the Hcrts play in physiology and behavior. Here, we review recent advances in the neurobiology of Hcrt in three sections. We begin by surveying findings on Hcrt function within normal sleep/wake states as well as situations of aberrant sleep (that is, narcolepsy). In the second section, we discuss research establishing a role for Hcrt in mood and affect (that is, anxiety, stress, and motivation). Finally, in the third section, we briefly discuss future directions for the field and place an emphasis on analytical modeling of Hcrt neural activity. We hope that the data discussed here provide a broad overview of recent progress in the field and make clear the diversity of roles played by these neuromodulators