Regulation of gene expression is known to contribute to the long-term adaptations taking place in response to drugs of abuse. Recent studies highlighted the regulation of gene transcription in neurons by chromatin remodeling, a process in which posttranslational modifications of histones play a major role. To test the involvement of epigenetic regulation on drug-reinforcing properties, we submitted rats to the cocaine operant self-administration paradigm. Using the fixed ratio 1 schedule, we found that the histone deacetylase (HDAC) inhibitors trichostatin A and phenylbutyrate dose-dependently reduced cocaine self-administration. Under the progressive ratio schedule, both trichostatin A and depudecin significantly reduced the breaking point, indicating that HDAC inhibition attenuated the motivation of rats for cocaine. Conversely, HDAC inhibition did not decrease self-administration for the natural reinforcer sucrose. This observation was correlated with measurements of HDAC activity in the frontal cortex, which was inhibited in response to cocaine, but not to sucrose self-administration. Control experiments showed that the decrease in the motivation for the drug was not attributable to a general motivational dysfunction because trichostatin A had no adverse effect on locomotion during the habituation session or on cocaine-induced hyperlocomotion. It was not attributable to anhedonia because the inhibitor had no effect on the sucrose preference test. In contrast, trichostatin A completely blocked the cocaine-induced behavioral sensitization. Together, the data show that epigenetic regulation of gene transcription in adult brain is able to influence a motivated behavior and suggest that HDAC inhibition may counteract the neural sensitization leading to drug dependence.

Gamma-hydroxybutyrate (GHB) is an endogenous neuromodulator with therapeutical applications in anesthesia, sleep disorders, and drug addiction. We report the cloning of a GHB receptor from a rat hippocampal cDNA library. This receptor has a molecular mass of 56 kDa and belongs to the seven-transmembrane receptor family. The peptidic sequence has no significant homology with any known receptor, including GABA(B) receptors. Its mRNA is restricted to the brain and is particularly abundant in the hippocampus, cortex, striatum, thalamus, olfactory bulbs, and cerebellum, matching the distribution of GHB binding sites in rat brain. Southern blot revealed the presence of homologous sequences in several species including the human. Binding assays on transfected CHO cells showed a dissociation constant (Kd) of 426 nM for GHB and no affinity for GABA, baclofen, or glutamate. In patch-clamp experiments, transfected CHO cells revealed a functional G-protein-coupled receptor as demonstrated by GTP-gamma-S-induced irreversible activation. Application of 0.1-15 microM GHB specifically induced an inward current at negative membrane potentials that was not reproduced by application of baclofen (10 microM). CGP-55845, a GABA(B) receptor antagonist, did not inhibit the GHB-induced response nor did the GHB receptor antagonist NCS-382, suggesting that the GHB receptor system includes several subtypes.

Abnormal signaling by retinoids or n-3 polyunsaturated fatty acids has been implicated in clinical depression. The converging point in activities of these two classes of molecules is transcriptional activation of retinoid X receptors (Rxr). We show here that ablation of Rxrgamma in mice leads to depressive-like behaviors including increased despair and anhedonia, which were accompanied by reduced expression of dopamine D2 receptor in the shell of nucleus accumbens (NAc) and altered serotonin signaling. While abnormal serotonin signaling is not sufficient to generate the depressive behaviors, increasing D2r expression by chronic fluoxetine (Prozac) treatment or adenoassociated virus type2 (AAV2) mediated expression of Rxrgamma or D2r in the NAc of Rxrgamma(-/-) mice normalizes depressive-like behaviors in Rxrgamma(-/-) animals. Conversely, NAc infusion of raclopride, a D2r antagonist prevents AAV2-Rxrgamma-mediated rescue of despair behaviors in Rxrgamma(-/-) mice. Combined, our data argue that control of NAc D2r expression is critical for Rxrgamma-mediated modulation of affective behaviors.

Tryptophan metabolism through the kynurenine pathway leads to several neuroactive compounds, including kynurenic and picolinic acids. Xanthurenic acid (Xa) has been generally considered as a substance with no physiological role but possessing toxic and apoptotic properties. In the present work, we present several findings which support a physiological role for endogenous Xa in synaptic signalling in brain. This substance is present in micromolar amounts in most regions of the rat brain with a heterogeneous distribution. An active vesicular synaptic process inhibited by bafilomycin and nigericin accumulates xanthurenate into pre‐synaptic terminals. A neuronal transport, partially dependant on adenosine 5′‐triphosphate (ATP), sodium and chloride ions exists in NCB‐20 neurons which could participate in the clearance of extracellular xanthurenate. Both transports (neuronal and vesicular) are greatly enhanced by the presence of micromolar amounts of zinc ions. Finally, electrical in vivo stimulation of A10‐induced Xa release in the extracellular spaces of the rat prefrontal cortex. This phenomenon is reproduced by veratrine, K+ ions and blocked by EGTA and tetrodotoxin. These results strongly argue for a role for Xa in neurotransmission/neuromodulation in the rat brain, thus providing the existence of specific Xa receptors.