RESUMEN
Methyl-CpG-binding protein 2 (MeCP2) encoded by the MECP2 gene is a transcriptional regulator whose mutations cause Rett syndrome (RTT). Mecp2-deficient mice show fear regulation impairment; however, the cellular and molecular mechanisms underlying this abnormal behavior are largely uncharacterized. Here, we showed that Mecp2 gene deficiency in cholinergic interneurons of the nucleus accumbens (NAc) dramatically impaired fear learning. We further found that spontaneous activity of cholinergic interneurons in Mecp2-deficient mice decreased, mediated by enhanced inhibitory transmission via α2-containing GABAA receptors. With MeCP2 restoration, opto- and chemo-genetic activation, and RNA interference in ChAT-expressing interneurons of the NAc, impaired fear retrieval was rescued. Taken together, these results reveal a previously unknown role of MeCP2 in NAc cholinergic interneurons in fear regulation, suggesting that modulation of neurons in the NAc may ameliorate fear-related disorders.
Asunto(s)
Neuronas Colinérgicas/metabolismo , Miedo/fisiología , Interneuronas/metabolismo , Proteína 2 de Unión a Metil-CpG/metabolismo , Receptores de GABA-A/metabolismo , Animales , Modelos Animales de Enfermedad , Aprendizaje/fisiología , Proteína 2 de Unión a Metil-CpG/genética , Ratones , Ratones Endogámicos C57BL , Ratones Noqueados , Núcleo Accumbens/metabolismo , Interferencia de ARNRESUMEN
Substance P (SP) and its receptors are involved in anxiety-related behaviours and regulate the intake of drugs of abuse and alcohol. Within the midbrain ventral tegmental area (VTA), a region that is clearly involved in the control of these behaviours, SP is released by stress and has been shown to trigger relapse. SP activates neurokinin (NK) receptors, which excites midbrain dopamine (DA) neurons and leads to increased DA in target regions. In this study, we have investigated the mechanisms underlying SP actions in the VTA, specifically investigating interactions between SP and GABA(B) receptors. We show that in VTA neurons, NK receptor activation closes an inwardly rectifying potassium channel, and moreover inhibits GABA(B) receptor-mediated transmission through an interaction that depends upon phospholipase C (PLC), intracellular calcium and protein kinase C (PKC).
Asunto(s)
Antagonistas del GABA , Antagonistas de Receptores de GABA-B , Receptores de Neuroquinina-2/efectos de los fármacos , Transducción de Señal/efectos de los fármacos , Sustancia P/farmacología , Área Tegmental Ventral/efectos de los fármacos , Animales , Baclofeno/farmacología , Quelantes/farmacología , Dopamina/fisiología , Ácido Egtácico/análogos & derivados , Ácido Egtácico/farmacología , Electrofisiología , Canales de Potasio Rectificados Internamente Asociados a la Proteína G/efectos de los fármacos , Agonistas del GABA/farmacología , Inmunohistoquímica , Técnicas In Vitro , Masculino , Potenciales de la Membrana/efectos de los fármacos , Técnicas de Placa-Clamp , Proteína Quinasa C/fisiología , Ratas , Ratas Sprague-Dawley , Receptores de Neuroquinina-1/efectos de los fármacos , Tirosina 3-Monooxigenasa/metabolismoRESUMEN
In the present study, we investigated the effects of psychostimulant exposure on kappa-opioid peptide (KOP) receptor signaling in the rat mesolimbic system. A single subcutaneous (s.c.) injection of amphetamine (2.5 mg/kg) reduced the KOP receptor-mediated inhibition of glutamate release in the nucleus accumbens shell, as a consequence of KOP receptor desensitization. This effect was blocked by dopamine (DA) receptor antagonists or the nonselective opioid antagonist, naltrexone (1 mg/kg, s.c.), and mimicked by the KOP receptor agonists U69593 (0.32 mg/kg, s.c.) and dynorphin (1 microM), indicating that an amphetamine-induced release of dynorphin is producing a long-lasting desensitization of the KOP receptor. Despite the fact that amphetamine also increases dynorphin release in the ventral tegmental area (VTA), KOP receptor function in this region was not affected by amphetamine; there was no difference in the KOP receptor-mediated change in firing rate or resting membrane potential measured in VTA neurons from saline- or amphetamine-treated animals. This study demonstrates that amphetamine can produce regionally selective adaptations in KOP receptor signaling, which may, in turn, alter the effects of subsequent drug exposure.
Asunto(s)
Anfetamina/farmacología , Estimulantes del Sistema Nervioso Central/farmacología , Núcleo Accumbens/efectos de los fármacos , Receptores Opioides kappa/fisiología , Analgésicos Opioides/farmacología , Análisis de Varianza , Animales , Bencenoacetamidas/farmacología , Dopaminérgicos/farmacología , Relación Dosis-Respuesta a Droga , Interacciones Farmacológicas , Dinorfinas/metabolismo , Dinorfinas/farmacología , Estimulación Eléctrica , Encefalina Ala(2)-MeFe(4)-Gli(5)/farmacología , Potenciales Postsinápticos Excitadores/efectos de los fármacos , Regulación de la Expresión Génica/efectos de los fármacos , Masculino , Antagonistas de Narcóticos/farmacología , Técnicas de Placa-Clamp , Pirrolidinas/farmacología , Ratas , Ratas Sprague-Dawley , Receptores Opioides kappa/agonistasRESUMEN
Intense motor neuron activity induces a long-term facilitation (LTF) of synaptic transmission at crayfish neuromuscular junctions (NMJs) that is accompanied by an increase in the accumulation of presynaptic Ca2+ ions during a test train of action potentials. It is natural to assume that the increased Ca2+ influx during action potentials is directly responsible for the increased transmitter release in LTF, especially as the magnitudes of LTF and increased Ca2+ influx are positively correlated. However, our results indicate that the elevated Ca2+ entry occurs through the reverse mode operation of presynaptic Na+/Ca2+ exchangers that are activated by an LTF-inducing tetanus. Inhibition of Na+/Ca2+ exchange blocks this additional Ca2+ influx without affecting LTF, showing that LTF is not a consequence of the regulation of these transporters and is not directly related to the increase in [Ca2+]i reached during a train of action potentials. Their correlation is probably due to both being induced independently by the strong [Ca2+]i elevation accompanying LTF-inducing stimuli. Our results reveal a new form of regulation of neuronal Na+/Ca2+ exchange that does not directly alter the strength of synaptic transmission.
Asunto(s)
Calcio/metabolismo , Unión Neuromuscular/fisiología , Plasticidad Neuronal/fisiología , Intercambiador de Sodio-Calcio/fisiología , Transmisión Sináptica/fisiología , Potenciales de Acción/fisiología , Animales , Astacoidea , Estimulación Eléctrica , Neuronas Motoras/fisiología , Músculo Esquelético/inervación , Músculo Esquelético/fisiología , Sodio/metabolismo , ATPasa Intercambiadora de Sodio-Potasio/fisiología , Regulación hacia Arriba/fisiologíaRESUMEN
Metastin is an antimetastatic peptide encoded by the KiSS-1 gene in cancer cells. Recent studies found that metastin is a ligand for the orphan G-protein-coupled receptor GPR54, which is highly expressed in specific brain regions such as the hypothalamus and parts of the hippocampus. This study shows that activation of GPR54 by submicromolar concentrations of metastin reversibly enhances excitatory synaptic transmission in hippocampal dentate granule cells in a mitogen-activated protein (MAP) kinase-dependent manner. Synaptic enhancement by metastin was suppressed by intracellular application of the G-protein inhibitor GDP-beta-S and the calcium chelator BAPTA. Analysis of miniature excitatory postsynaptic currents (mEPSCs) revealed an increase in the mean amplitude but no change in event frequency. This indicates that GPR54 and the mechanism responsible for the increase in EPSCs are postsynaptic. Metastin-induced synaptic potentiation was abolished by 50 microM PD98059 and 20 microM U0126, two inhibitors of the MAP kinases ERK1 and ERK2. The effect was also blocked by inhibitors of calcium/calmodulin-dependent kinases and tyrosine kinases. RT-PCR experiments showed that both KiSS-1 and GPR54 are expressed in the hippocampal dentate gyrus. Metastin is thus a novel endogenous factor that modulates synaptic excitability in the dentate gyrus through mechanisms involving MAP kinases, which in turn may be controlled upstream by calcium-activated kinases and tyrosine kinases.
Asunto(s)
Potenciales de Acción/fisiología , Potenciales Postsinápticos Excitadores/fisiología , Hipocampo/fisiología , Sistema de Señalización de MAP Quinasas/fisiología , Proteínas Quinasas Activadas por Mitógenos/metabolismo , Proteínas/farmacología , Transmisión Sináptica/fisiología , Potenciales de Acción/efectos de los fármacos , Animales , Células Cultivadas , Potenciales Postsinápticos Excitadores/efectos de los fármacos , Hipocampo/efectos de los fármacos , Kisspeptinas , Ratas , Ratas Sprague-Dawley , Transmisión Sináptica/efectos de los fármacos , Proteínas Supresoras de TumorRESUMEN
Earlier studies showed that positive modulators of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors enhance synaptic responses and facilitate synaptic plasticity. Those studies focused mainly on hippocampal functions. However, AMPA receptors have regionally distinct subunit compositions and thus potencies and efficacies of modulators may vary across the brain. The present study compared the effects of CX546 [1-(1,4-benzodioxan-6-ylcarbonyl) piperidine], a benzamide-type modulator, on synaptic transmission in neurons of the reticular thalamic nucleus (RTN), which regulates the firing mode of relay cells in other thalamic nuclei, and on hippocampal CA1 pyramidal cells. CX546 greatly prolonged synaptic responses in CA1 pyramidal cells, but at the same concentration it had only weak modulatory effects in RTN neurons. Effects on miniature excitatory postsynaptic currents (EPSCs) were similar to those on EPSCs in both regions, suggesting that variations in neuronal morphology and transmitter release kinetics do not account for the differences. Relay cells in the ventrobasal thalamus also exhibited weak modulatory effects that were comparable with those in RTN neurons. Regionally different effects on response duration were also observed with CX516 [BDP-12, 1-(quinoxalin-6-ylcarbonyl)piperidine], a second benzamide drug. In contrast, 100 microM cyclothiazide produced comparable synaptic enhancements in hippocampus and RTN. The regional selectivity of benzamide drugs (ampakines) may be explained, at least in part, by a lower potency at thalamic AMPA receptors, perhaps due to the prevalence of the subunits GluR3 and 4. Although regional preferences of the ampakines were modest in their extent, they may be sufficient to be of relevance when considering future therapeutic applications of such compounds.
Asunto(s)
Hipocampo/efectos de los fármacos , Receptores AMPA/agonistas , Transmisión Sináptica/efectos de los fármacos , Tálamo/efectos de los fármacos , Animales , Benzotiadiazinas/farmacología , Dioxoles/farmacología , Potenciales Postsinápticos Excitadores/efectos de los fármacos , Técnicas In Vitro , Masculino , Potenciales de la Membrana/efectos de los fármacos , Técnicas de Placa-Clamp , Piperidinas/farmacología , Ratas , Ratas Sprague-DawleyRESUMEN
CX516 (BDP-12) and CX546, two first-generation benzamide-type AMPA receptor modulators, were compared with regard to their influence on AMPA receptor-mediated currents, autaptic responses in cultured hippocampal neurons, hippocampal excitatory postsynaptic currents, synaptic field potentials, and agonist binding. The two drugs exhibited comparable potencies in most tests but differed in their efficacy and in their relative impact on various response parameters. CX546 greatly prolonged the duration of synaptic responses, and it slowed 10-fold the deactivation of excised-patch currents following 1-ms pulses of glutamate. The effects of CX516 on those measures were, by comparison, small; however, the drug was equally or more efficacious than CX546 in increasing the amplitude of synaptic responses. This double dissociation suggests that amplitude and duration of synaptic responses are governed by different aspects of receptor kinetics, which are differentially modified by the two drugs. These effects can be reproduced in receptor simulations if one assumes that CX516 preferentially accelerates channel opening while CX546 slows channel closing. In binding tests, CX546 caused an approximately 2-fold increase in the affinity for radiolabeled agonists, whereas CX516 was ineffective. More importantly, even millimolar concentrations of CX516 did not influence the dose-response relation for CX546, suggesting the possibility that they bind to different sites. Taken together, the evidence suggests that benzamide modulators from the Ampakine family form two subgroups with different modes and sites of action. Of these, CX516-type drugs may have the greater therapeutic utility because of their limited efficacy in prolonging synaptic responses and in attenuating receptor desensitization.
Asunto(s)
Benzamidas/farmacología , Dioxoles/farmacología , Piperidinas/farmacología , Receptores AMPA/metabolismo , Transmisión Sináptica/efectos de los fármacos , Regulación Alostérica/efectos de los fármacos , Animales , Benzamidas/química , Benzamidas/metabolismo , Sitios de Unión/efectos de los fármacos , Sitios de Unión/fisiología , Dioxoles/química , Dioxoles/metabolismo , Relación Dosis-Respuesta a Droga , Hipocampo/efectos de los fármacos , Hipocampo/metabolismo , Técnicas In Vitro , Masculino , Neuronas/efectos de los fármacos , Neuronas/metabolismo , Técnicas de Placa-Clamp , Piperidinas/química , Piperidinas/metabolismo , Ratas , Ratas Sprague-Dawley , Receptores AMPA/agonistas , Transmisión Sináptica/fisiologíaRESUMEN
Alkyl-substituted benzothiadiazides (BTDs) were tested for their effects on (R,S)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-type glutamate receptors. In excised patches, the 5'-ethyl derivative "D1" blocked the desensitization of AMPA receptor currents during prolonged application of glutamate (EC(50), 36 microM), and it slowed deactivation of responses elicited by 1-ms glutamate pulses greater than 10-fold. [(3)H]Fluorowillardiine binding to rat synaptic membranes was increased by D1 by a factor of 3.6 (EC(50), 17 microM) with a Hill coefficient near 2. In hippocampal slices, the compound reversibly increased excitatory postsynaptic currents and field excitatory postsynaptic potentials (EPSPs) with thresholds around 10 microM. The size of the alkyl substituent influenced both the potency and nature of the drug effect on synaptic currents: 5'-methyl compounds had a 2-fold greater effect on response amplitude than on response duration, whereas 5'-ethyl compounds like D1 caused greater increases in duration than amplitude. In tests with recombinantly expressed AMPA receptor subunits, D1 preferred the glutamate receptor (GluR) subunit GluR4 flip (0.64 microM) over GluR4 flop (5.3 microM); similar affinities but with smaller flip-flop differences were obtained for GluR1 through 3. These results show that D1 and congeners are significantly more potent than the parent compound IDRA-21 and that they differ in two fundamental aspects from cyclothiazide, the most widely studied BTD: 1) D1 markedly increases the agonist affinity of AMPA receptors and 2) it has immediate and large effects on field EPSPs. The large gain in potency conferred by alkyl substitution suggests that the 5' substituent is in intimate contact with the receptor, with the size of the substituent determining the way in which receptor kinetics is changed.