[Show abstract][Hide abstract] ABSTRACT: Dopamine neurons in the ventral tegmental area (VTA) govern reward and motivation and dysregulated dopaminergic transmission may account for anhedonia and other symptoms of depression. Cyclin-dependent kinase 5 (Cdk5) is a proline-directed serine/threonine kinase that regulates a broad range of brain functions through phosphorylation of a myriad of substrates, including tyrosine hydroxylase (TH), the rate-limiting enzyme for dopamine synthesis. We investigated whether and how Cdk5 activity in VTA dopamine neurons regulated depression-related behaviors in mice. Using the Cre/LoxP system to selectively delete Cdk5 in the VTA or in midbrain dopamine neurons in Cdk5(loxP/loxP) mice, we showed that Cdk5 loss of function in the VTA induced anxiety- and depressive-like behaviors that were associated with decreases in TH phosphorylation at Ser31 and Ser40 in the VTA and dopamine release in its target region, the nucleus accumbens. The decreased phosphorylation of TH at Ser31 was a direct effect of Cdk5 deletion, whereas decreased phosphorylation of TH at Ser40 was likely caused by impaired cAMP/protein kinase A (PKA) signaling, because Cdk5 deletion decreased cAMP and phosphorylated cAMP response element-binding protein (p-CREB) levels in the VTA. Using Designer Receptors Exclusively Activated by Designer Drugs (DREADD) technology, we showed that selectively increasing cAMP levels in VTA dopamine neurons increased phosphorylation of TH at Ser40 and CREB at Ser133 and reversed behavioral deficits induced by Cdk5 deletion. The results suggest that Cdk5 in the VTA regulates cAMP/PKA signaling, dopaminergic neurotransmission, and depression-related behaviors.
Journal of Neuroscience 04/2014; 34(18):6352-6366. · 6.91 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The endocannabinoid (eCB) system regulates mood, emotion and stress coping, and dysregulation of the eCB system is critically involved in pathophysiology of depression. The eCB ligand 2-arachidonoylglycerol (2-AG) is inactivated by monoacylglycerol lipase (MAGL). Using chronic unpredictable mild stress (CUS) as a mouse model of depression, we examined how 2-AG signaling in the hippocampus was altered in depressive-like states and how this alteration contributed to depressive-like behavior. We report that CUS led to impairment of depolarization-induced suppression of inhibition (DSI) in mouse hippocampal CA1 pyramidal neurons, and this deficiency in 2-AG mediated retrograde synaptic depression was rescued by MAGL inhibitor JZL184. CUS induced depressive-like behaviors and decreased mammalian target of rapamycin (mTOR) activation in the hippocampus, and these biochemical and behavioral abnormalities were ameliorated by chronic JZL184 treatments. The effects of JZL184 were mediated by cannabinoid CB1 receptors. Genetic deletion of mTOR with adeno-associated viral (AAV) vector-carrying the Cre recombinase gene in the hippocampus of mTORf/f mice recapitulated depressive-like behaviors induced by CUS and abrogated the antidepressant-like effects of chronic JZL184 treatments. Our results suggest that CUS decreases eCB-mTOR signaling in the hippocampus, leading to depressive-like behaviors, whereas MAGL inhibitor JZL184 produces antidepressant-like effects through enhancement of eCB-mTOR signaling.Neuropsychopharmacology accepted article preview online, 30 January 2014. doi:10.1038/npp.2014.24.
Neuropsychopharmacology: official publication of the American College of Neuropsychopharmacology 01/2014; · 8.68 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Addictive drugs such as cocaine induce synaptic plasticity in discrete regions of the reward circuit. The goal of the present study is to investigate whether cocaine-evoked synaptic plasticity in the ventral tegmental area (VTA) and nucleus accumbens (NAc) is causally linked. Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) is a central regulator of long-term synaptic plasticity, learning and drug addiction. We examined whether blocking CaMKII activity in the VTA affected cocaine conditioned place preference (CPP) and cocaine-evoked synaptic plasticity in its target brain region, the NAc. TatCN21 is a CaMKII inhibitory peptide that blocks both stimulated and autonomous CaMKII activity with high selectivity. We report that intra-VTA microinjections of tatCN21 prior to cocaine conditioning blocked the acquisition of cocaine CPP, whereas intra-VTA microinjections of tatCN21 prior to saline conditioning did not significantly affect cocaine CPP, suggesting that the CaMKII inhibitor blocks cocaine CPP through selective disruption of cocaine-cue associated learning. Intra-VTA tatCN21 prior to cocaine conditioning blocked cocaine-evoked depression of excitatory synaptic transmission in the shell of the NAc slices ex vivo. In contrast, intra-VTA microinjection of tatCN21 just prior to the CPP test did not affect the expression of cocaine CPP and cocaine-induced synaptic plasticity in the NAc shell. These results suggest that CaMKII activity in the VTA governs cocaine-evoked synaptic plasticity in the NAc during the time window of cocaine conditioning.Neuropsychopharmacology accepted article preview online, 24 October 2013; doi:10.1038/npp.2013.299.
Neuropsychopharmacology: official publication of the American College of Neuropsychopharmacology 10/2013; · 8.68 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Advances in cell reprogramming technologies to generate patient-specific cells of a desired type will revolutionize the field of regenerative medicine. While several cell reprogramming methods have been developed over the last decades, the majority of these technologies require the exposure of cell nuclei to reprogramming large molecules via transfection, transduction, cell fusion or nuclear transfer. This raises several technical, safety and ethical issues. Chemical genetics is an alternative approach for cell reprogramming that uses small, cell membrane penetrable substances to regulate multiple cellular processes including cell plasticity. Recently, using the combination of small molecules that are involved in the regulation chromatin structure and function and agents that favor neural differentiation we have been able to generate neural-like cells from human mesenchymal stem cells. In this study, to improve the efficiency of neuronal differentiation and maturation, two specific inhibitors of SMAD signaling (SMAD1/3 and SMAD3/5/8) that play an important role in neuronal differentiation of embryonic stem cells, were added to our previous neural induction recipe. Results demonstrated that human mesenchymal stem cells grown in this culture conditions exhibited higher expression of several mature neuronal genes, formed synapse-like structures and exerted electrophysiological properties of differentiating neural stem cells. Thus, an efficient method for production of mature neuronal-like cells from human adult bone marrow derived mesenchymal stem cells has been developed. We concluded that specific combinations of small molecules that target specific cell signaling pathways and chromatin modifying enzymes could be a promising approach for manipulation of adult stem cell plasticity.
The international journal of biochemistry & cell biology 05/2013; · 4.89 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Antagonism of group I metabotropic glutamate receptors (mGluR 1 and mGluR5) reduces behavioral effects of drugs of abuse, including cocaine. However, the underlying mechanisms remain poorly understood. Activation of mGluR5 increases protein synthesis at synapses, and mGluR5-induced excessive protein synthesis has been implicated in the pathology of fragile X syndrome. It remains unknown whether group I mGluR-mediated protein synthesis is involved in any behavioral effects of drugs of abuse. We report that group I mGluR agonist DHPG induced more pronounced initial depression of inhibitory postsynaptic currents (IPSCs) followed by modest long-term depression (I-LTD) in dopamine neurons of rat ventral tegmental area (VTA) through the activation of mGluR1. The early component of DHPG-induced depression of IPSCs was mediated by the cannabinoid CB(1) receptors, while DHPG-induced I-LTD was dependent on protein synthesis. Western blotting analysis indicates that mGluR1 was coupled to extracellular signal-regulated kinase (ERK) and mammalian target of rapamycin (mTOR) signaling pathways to increase translation. We also show that cocaine conditioning activated translation machinery in the VTA via an mGluR1-dependent mechanism. Furthermore, intra-VTA microinjections of mGluR1 antagonist JNJ16259685 and protein synthesis inhibitor cycloheximide significantly attenuated or blocked the acquisition of cocaine-induced conditioned place preference (CPP) and activation of translation elongation factors. Taken together, these results suggest that mGluR1 antagonism inhibits de novo protein synthesis; this effect may block the formation of cocaine-cue associations and thus provide a mechanism for the reduction in CPP to cocaine.Neuropsychopharmacology accepted article preview online, 24 January 2013; doi:10.1038/npp.2013.29.
Neuropsychopharmacology: official publication of the American College of Neuropsychopharmacology 01/2013; · 8.68 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Endocannabinoid-mediated long-term depression of inhibitory synaptic transmission (I-LTD) in the ventral tegmental area (VTA) is implicated in cocaine-induced inhibitory synaptic plasticity and behavioral effects. It remains poorly understood, however, how this I-LTD is regulated and whether this regulation affects cocaine-seeking behavior. I-LTD requires cyclic adenosine 3', 5'-monophosphate (cAMP)-dependent protein kinase A (PKA) signaling, raising the possibility that modulators of cAMP/PKA signaling may regulate I-LTD and the reinforcement behavior. Phosphodiesterase (PDE) 4 hydrolyses cAMP and terminates cAMP/PKA signaling. Here, we report that selective PDE4 inhibitors rolipram and Ro 20-1724 blocked I-LTD and acute depression of inhibitory postsynaptic currents (IPSCs) induced by D(2) dopamine receptor and cannabinoid CB(1) receptor agonists in VTA dopamine neurons. We also show that intra-VTA microinjections of PDE4 inhibitor rolipram impaired the acquisition, but not the expression, of conditioned place preference (CPP) to cocaine. Systemic administration of rolipram also increased cAMP response element-binding protein (CREB) phosphorylation and activation in the VTA. Together, our results suggest that blockade of cocaine-induced inhibitory synaptic plasticity (I-LTD) and enhancement of CREB activation are two putative cellular mechanisms by which PDE4 inhibition impairs the acquisition of cocaine CPP.
Neuropsychopharmacology: official publication of the American College of Neuropsychopharmacology 06/2012; 37(11):2377-87. · 8.68 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Endocannabinoid (eCB) signaling is tightly regulated by eCB biosynthetic and degradative enzymes. The eCB 2-arachidonoylglycerol (2-AG) is hydrolyzed primarily by monoacylglycerol lipase (MAGL). Here, we investigated whether eCB signaling, synaptic function, and learning behavior were altered in MAGL knock-out mice. We report that MAGL⁻/⁻ mice exhibited prolonged depolarization-induced suppression of inhibition (DSI) in hippocampal CA1 pyramidal neurons, providing genetic evidence that the inactivation of 2-AG by MAGL determines the time course of the eCB-mediated retrograde synaptic depression. CB₁ receptor antagonists enhanced basal IPSCs in CA1 pyramidal neurons in MAGL⁻/⁻ mice, while the magnitude of DSI or CB₁ receptor agonist-induced depression of IPSCs was decreased in MAGL⁻/⁻ mice. These results suggest that 2-AG elevations in MAGL⁻/⁻ mice cause tonic activation and partial desensitization of CB₁ receptors. Genetic deletion of MAGL selectively enhanced theta burst stimulation (TBS)-induced long-term potentiation (LTP) in the CA1 region of hippocampal slices but had no significant effect on LTP induced by high-frequency stimulation or long-term depression induced by low-frequency stimulation. The enhancement of TBS-LTP in MAGL⁻/⁻ mice appears to be mediated by 2-AG-induced suppression of GABA(A) receptor-mediated inhibition. MAGL⁻/⁻ mice exhibited enhanced learning as shown by improved performance in novel object recognition and Morris water maze. These results indicate that genetic deletion of MAGL causes profound changes in eCB signaling, long-term synaptic plasticity, and learning behavior.
Journal of Neuroscience 09/2011; 31(38):13420-30. · 6.91 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The endocannabinoid (eCB) 2-arachidonoylglycerol (2-AG) is hydrolysed primarily by monoacylglycerol lipase (MAGL). Here, we investigated whether eCB-mediated retrograde synaptic depression in cerebellar slices was altered in MAGL knockout (MAGL(-/-)) mice. Depolarization-induced suppression of excitation (DSE) and metabotropic glutamate receptor (mGluR1)-mediated synaptic depression are mediated by 2-AG-induced activation of CB(1) receptors. We show that genetic deletion of MAGL prolonged DSE at parallel fibre (PF) or climbing fibre (CF) to Purkinje cell (PC) synapses. Likewise, mGluR1-mediated synaptic depression, induced either by high-frequency stimulation of PF or mGluR1 agonist DHPG, was prolonged in MAGL(-/-) mice. About 15% of 2-AG in the brain is hydrolysed by serine hydrolase α-β-hydrolase domain 6 and 12 (ABHD6 and ABHD12). However, the selective ABHD6 inhibitor WWL123 had no significant effect on cerebellar DSE in MAGL(+/+) and (-/-) mice. The CB(1) receptor antagonist SR141716 significantly increased the amplitude of basal excitatory postsynaptic currents (EPSCs) in MAGL(-/-) mice but not in MAGL(+/+) mice. Conversely, the CB(1) agonist WIN55212 induced less depression of basal EPSCs in MAGL(-/-) mice than in MAGL(+/+) mice. These results provide genetic evidence that inactivation of 2-AG by MAGL determines the time course of eCB-mediated retrograde synaptic depression and that genetic deletion of MAGL causes tonic activation and consequential desensitization of CB(1) receptors.
The Journal of Physiology 09/2011; 589(Pt 20):4847-55. · 4.38 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Drugs of abuse such as cocaine induce long-term synaptic plasticity in the reward circuitry, which underlies the formation of drug-associated memories and addictive behavior. We reported previously that repeated cocaine exposure in vivo facilitates long-term potentiation (LTP) in dopamine neurons of the ventral tegmental area (VTA) by reducing the strength of GABAergic inhibition and that endocannabinoid-dependent long-term depression at inhibitory synapses (I-LTD) constitutes a mechanism for cocaine-induced reduction of GABAergic inhibition. The present study investigated the downstream signaling mechanisms and functional consequences of I-LTD in the VTA in the rat. Extracellular signal-regulated kinase (ERK) signaling has been implicated in long-term synaptic plasticity, associative learning, and drug addiction. We tested the hypothesis that VTA ERK activity is required for I-LTD and cocaine-induced long-term synaptic plasticity and behavioral effects. We show that the activation of receptors required for I-LTD increased ERK1/2 phosphorylation and inhibitors of ERK activation blocked I-LTD. We further demonstrate that ERK mediates cocaine-induced reduction of GABAergic inhibition and facilitation of LTP induction. Finally, we show that cocaine conditioned place preference (CPP) training (15 mg/kg; four pairings) increased ERK1/2 phosphorylation in the VTA, while bilateral intra-VTA injections of a CB(1) antagonist or an inhibitor of ERK activation attenuated ERK1/2 phosphorylation and the acquisition, but not the expression, of CPP to cocaine. Our study has identified the CB(1) and ERK signaling cascade as a key mediator of several forms of cocaine-induced synaptic plasticity and provided evidence linking long-term synaptic plasticity in the VTA to rewarding effects of cocaine.
Journal of Neuroscience 08/2011; 31(31):11244-55. · 6.91 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The mechanisms subserving the ability of glucocorticoid signaling within the medial prefrontal cortex (mPFC) to terminate stress-induced activation of the hypothalamic-pituitary-adrenal (HPA) axis are not well understood. We report that antagonism of the cannabinoid CB(1) receptor locally within the mPFC prolonged corticosterone secretion following cessation of stress in rats. Mice lacking the CB(1) receptor exhibited a similar prolonged response to stress. Exposure of rats to stress produced an elevation in the endocannabinoid 2-arachidonoylglycerol within the mPFC that was reversed by pretreatment with the glucocorticoid receptor antagonist RU-486 (20 mg/kg). Electron microscopic and electrophysiological data demonstrated the presence of CB(1) receptors in inhibitory-type terminals impinging upon principal neurons within layer V of the prelimbic region of the mPFC. Bath application of corticosterone (100 nm) to prefrontal cortical slices suppressed GABA release onto principal neurons in layer V of the prelimbic region, when examined 1 h later, which was prevented by application of a CB(1) receptor antagonist. Collectively, these data demonstrate that the ability of stress-induced glucocorticoid signaling within mPFC to terminate HPA axis activity is mediated by a local recruitment of endocannabinoid signaling. Endocannabinoid activation of CB(1) receptors decreases GABA release within the mPFC, likely increasing the outflow of the principal neurons of the prelimbic region to contribute to termination of the stress response. These data support a model in which endocannabinoid signaling links glucocorticoid receptor engagement to activation of corticolimbic relays that inhibit corticosterone secretion.
Journal of Neuroscience 07/2011; 31(29):10506-15. · 6.91 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Hypoglossal motoneurons (HMs) innervate tongue muscles and are critical in maintaining patency of the upper airway during respiration. Abnormalities in HMs have been implicated in sudden infant death syndrome (SIDS) and obstructive sleep apnoea. Previously, we found a critical period in respiratory network development in rats around postnatal day (P) 12-13, when abrupt neurochemical, metabolic and physiological changes occurred. To test our hypothesis that an imbalance between inhibitory and excitatory synaptic transmission exists during the critical period, whole-cell patch-clamp recordings of HMs were done in brainstem slices of rats daily from P0 to P16. The results indicated that: (1) the amplitude and charge transfer of miniature excitatory postsynaptic currents (mEPSCs) were significantly reduced at P12-13; (2) the amplitude, mean frequency and charge transfer of miniature inhibitory postsynaptic currents (mIPSCs) were significantly increased at P12-13; (3) the kinetics (rise time and decay time) of both mEPSCs and mIPSCs accelerated with age; (4) the amplitude and frequency of spontaneous EPSCs were significantly reduced at P12-13, whereas those of spontaneous IPSCs were significantly increased at P12-13; and (5) both glycine and GABA contributed to mIPSCs. However, GABAergic currents fluctuated within a narrow range during the first three postnatal weeks, whereas glycinergic ones exhibited age-dependent changes comparable to those of total mIPSCs, indicating a reversal in dominance from GABA to glycine with development. Thus, our results provide strong electrophysiological evidence for an excitatory-inhibitory imbalance in HMs during the critical period of postnatal development in rats that may have significant implications for SIDS.
The Journal of Physiology 04/2011; 589(Pt 8):1991-2006. · 4.38 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The nucleus accumbens (NAc) is a critical component of the reward circuitry, and dysfunction of the NAc may account for anhedonia and other symptoms of depression. Here, we investigated whether alterations in endocannabinoid (eCB) signaling in the NAc contribute to depression-like behaviors induced by chronic unpredictable stress (CUS) in mice. We compared three types of eCB/CB1 receptor-mediated synaptic plasticity in slices prepared from the NAc core of control and stress-exposed mice: depolarization-induced suppression of excitation, long-term depression, and the depression of field excitatory postsynaptic potentials (fEPSPs) induced by group I metabotropic glutamate receptor agonist DHPG. CUS (5-6-week exposure to stressors), but not sub-CUS (1 week exposure to stressors), induces depression-like behaviors and impairs these forms of eCB/CB1 receptor-mediated plasticity examined in the NAc core. Neither sub-CUS nor CUS altered the tissue contents of the eCBs, anandamide and 2-arachidonoylglycerol in the striatum. However, exposure to CUS, but not to sub-CUS, attenuated the depression of fEPSPs induced by the CB1 receptor agonist WIN 55 212-2. CUS exposure reduced the maximal effect without affecting the EC(50) of WIN 55 212-2 to induce fEPSP depression. Thus, impaired CB1 receptor function could account for CUS-induced deficiency in eCB signaling in the NAc. Both CUS-induced deficiency in eCB signaling and depression-like behaviors were reversed by in vivo administration of antidepressant fluoxetine. These results suggest that downregulation of eCB signaling in the NAc occurs after CUS and contributes to the pathophysiology of depression.
Neuropsychopharmacology: official publication of the American College of Neuropsychopharmacology 10/2010; 35(11):2249-61. · 8.68 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Prolonged exposure to drugs of abuse, such as cannabinoids and opioids, leads to pharmacological tolerance and receptor desensitization in the nervous system. We found that a similar form of functional antagonism was produced by sustained inactivation of monoacylglycerol lipase (MAGL), the principal degradative enzyme for the endocannabinoid 2-arachidonoylglycerol. After repeated administration, the MAGL inhibitor JZL184 lost its analgesic activity and produced cross-tolerance to cannabinoid receptor (CB1) agonists in mice, effects that were phenocopied by genetic disruption of Mgll (encoding MAGL). Chronic MAGL blockade also caused physical dependence, impaired endocannabinoid-dependent synaptic plasticity and desensitized brain CB1 receptors. These data contrast with blockade of fatty acid amide hydrolase, an enzyme that degrades the other major endocannabinoid anandamide, which produced sustained analgesia without impairing CB1 receptors. Thus, individual endocannabinoids generate distinct analgesic profiles that are either sustained or transitory and associated with agonism and functional antagonism of the brain cannabinoid system, respectively.
[Show abstract][Hide abstract] ABSTRACT: Endocannabinoid (eCB) signaling mediates depolarization-induced suppression of excitation (DSE) and inhibition (DSI), two prominent forms of retrograde synaptic depression. N-Arachidonoylethanolamine (AEA) and 2-arachidonoylglycerol (2-AG), two known eCBs, are degraded by fatty acid amide hydrolase (FAAH) and monoacylglycerol lipase (MAGL), respectively. Selective blockade of FAAH and MAGL is critical for determining the roles of the eCBs in DSE/DSI and understanding how their action is regulated. 4-Nitrophenyl 4-(dibenzo[d][1,3]dioxol-5-yl(hydroxy)methyl)piperidine-1-carboxylate (JZL184) is a recently developed, highly selective, and potent MAGL inhibitor that increases 2-AG but not AEA concentrations in mouse brain. Here, we report that JZL184 prolongs DSE in Purkinje neurons in cerebellar slices and DSI in CA1 pyramidal neurons in hippocampal slices. The effect of JZL184 on DSE/DSI is mimicked by the nonselective MAGL inhibitor methyl arachidonyl fluorophosphonate. In contrast, neither the selective FAAH inhibitor cyclohexylcarbamic acid 3'-carbomoylbiphenyl-3-yl ester (URB597) nor FAAH knockout has a significant effect on DSE/DSI. JZL184 produces greater enhancement of DSE/DSI in mouse neurons than that in rat neurons. The latter finding is consistent with biochemical studies showing that JZL184 is more potent in inhibiting mouse MAGL than rat MAGL. These results indicate that the degradation of 2-AG by MAGL is the rate-limiting step that determines the time course of DSE/DSI and that JZL184 is a useful tool for the study of 2-AG-mediated signaling.
Journal of Pharmacology and Experimental Therapeutics 09/2009; 331(2):591-7. · 3.89 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Endocannabinoid (eCB) signaling mediates short-term and long-term synaptic depression (LTD) in many brain areas. In the ventral tegmental area (VTA) and striatum, D(2) dopamine receptors cooperate with group I metabotropic glutamate receptors (mGluRs) to induce eCB-mediated LTD of glutamatergic excitatory and GABAergic inhibitory (I-LTD) synaptic transmission. Because D(2) receptors and group I mGluR agonists are capable of inducing the release of eCBs, the predominant hypothesis is that the cooperation between these receptors to induce eCB-mediated synaptic depression results from the combined activation of type I cannabinoid (CB(1)) receptors by the eCBs. By determining the downstream effectors for D(2) receptor and group I mGluR activation in VTA dopamine neurons, we show that group I mGluR activation contributes to I-LTD induction by enhancing eCB release and CB(1) receptor activation. However, D(2) receptor activation does not enhance CB(1) receptor activation, but facilitates I-LTD induction via direct inhibition of cAMP-dependent protein kinase A (PKA) signaling. We further demonstrate that cAMP/PKA signaling pathway is the downstream effector for CB(1) receptors and is required for eCB-mediated I-LTD induction. Our results suggest that D(2) receptors and CB(1) receptors target the same downstream effector cAMP/PKA signaling pathway to induce I-LTD and D(2) receptor activation facilitates eCB-mediated I-LTD in dopamine neurons not by enhancing CB(1) receptor activation, but by enhancing its downstream effects.
Journal of Neuroscience 01/2009; 28(52):14018-30. · 6.91 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Drugs that increase GABA levels in the brain reduce cocaine seeking in rodents and humans, suggesting that GABAergic inhibition regulates cocaine-seeking behavior. We previously reported that repeated cocaine exposure in vivo facilitates long-term potentiation by reducing the strength of GABAergic inhibition in dopamine neurons of the ventral tegmental area (VTA). Selective blockade of cocaine-induced reduction of GABAergic inhibition in the VTA might diminish cocaine-induced aberrant synaptic plasticity and addictive behavior. Here, we investigated the mechanism for cocaine-induced reduction of GABAergic inhibition. We show that a pathophysiologically relevant concentration of cocaine enables a normally ineffective stimulus to induce long-term depression (LTD) of IPSCs (I-LTD) in VTA dopamine neurons of midbrain slices. Activation of D2 dopamine receptors and group I metabotropic glutamate receptors and subsequent recruitment of endocannabinoid signaling are required for I-LTD induction. We further demonstrate that in vivo pretreatment with antagonists to these receptors blocks cocaine-induced reduction of GABAergic inhibition and that repeated cocaine exposure in vivo occludes the subsequent induction of I-LTD ex vivo. Together, these results suggest that repeated cocaine exposure reduces the strength of GABAergic inhibition in dopamine neurons by inducing I-LTD-like modification in vivo.
Journal of Neuroscience 03/2008; 28(6):1385-97. · 6.91 Impact Factor