Carlos A. Bolaños

Dnmt3a regulates emotional behavior and spine plasticity in the nucleus accumbens

Despite abundant expression of DNA methyltransferases (Dnmts) in brain, the regulation and behavioral role of DNA methylation remain poorly understood. We found that Dnmt3a expression was regulated in mouse nucleus accumbens (NAc) by chronic cocaine use and chronic social defeat stress. Moreover, NAc-specific manipulations that block DNA methylation potentiated cocaine reward and exerted antidepressant-like effects, whereas NAc-specific Dnmt3a overexpression attenuated cocaine reward and was pro-depressant. On a cellular level, we found that chronic cocaine use selectively increased thin dendritic spines on NAc neurons and that DNA methylation was both necessary and sufficient to mediate these effects. These data establish the importance of Dnmt3a in the NAc in regulating cellular and behavioral plasticity to emotional stimuli.

Brain-derived neurotrophic factor in the ventral midbrain–nucleus accumbens pathway: a role in depression

BACKGROUND Previous work has shown that brain-derived neurotrophic factor (BDNF) and its receptor, tyrosine kinase receptor B (TrkB), are involved in appetitive behavior. Here we show that BDNF in the ventral tegmental area-nucleus accumbens (VTA-NAc) pathway is also involved in the development of a depression-like phenotype. METHODS Brain-derived neurotrophic factor signaling in the VTA-NAc pathway was altered in two complementary ways. One group of rats received intra-VTA infusion of vehicle or BDNF for 1 week. A second group of rats received intra-NAc injections of vehicle or adeno-associated viral vectors encoding full-length (TrkB.FL) or truncated (TrkB.T1) TrkB; the latter is kinase deficient and serves as a dominant-negative receptor. Rats were examined in the forced swim test and other behavioral tests. RESULTS Intra-VTA infusions of BDNF resulted in 57% shorter latency to immobility relative to control animals, a depression-like effect. Intra-NAc injections of TrkB.T1 resulted in and almost fivefold longer latency to immobility relative to TrkB.FL and control animals, an antidepressant-like effect. No effect on anxiety-like behaviors or locomotion was seen. CONCLUSIONS These data suggest that BDNF action in the VTA-NAc pathway might be related to development of a depression-like phenotype. This interpretation is intriguing in that it suggests a role for BDNF in the VTA-NAc that is opposite of the proposed role for BDNF in the hippocampus.

[Delta]FosB in brain reward circuits mediates resilience to stress and antidepressant responses

In contrast with the many studies of stress effects on the brain, relatively little is known about the molecular mechanisms of resilience, the ability of some individuals to escape the deleterious effects of stress. We found that the transcription factor ΔFosB mediates an essential mechanism of resilience in mice. Induction of ΔFosB in the nucleus accumbens, an important brain reward-associated region, in response to chronic social defeat stress was both necessary and sufficient for resilience. ΔFosB induction was also required for the standard antidepressant fluoxetine to reverse behavioral pathology induced by social defeat. ΔFosB produced these effects through induction of the GluR2 AMPA glutamate receptor subunit, which decreased the responsiveness of nucleus accumbens neurons to glutamate, and through other synaptic proteins. Together, these findings establish a previously unknown molecular pathway underlying both resilience and antidepressant action.

The hypothalamic neuropeptide melanin-concentrating hormone acts in the nucleus accumbens to modulate feeding behavior and forced-swim performance

Melanin-concentrating hormone (MCH) is a hypothalamic neuropeptide with a prominent role in feeding and energy homeostasis. The rodent MCH receptor (MCH1R) is highly expressed in the nucleus accumbens shell (AcSh), a region that is important in the regulation of appetitive behavior. Here we establish a role for MCH and MCH1R in mediating a hypothalamic-limbic circuit that regulates feeding and related behaviors. Direct delivery of an MCH1R receptor antagonist to the AcSh blocked feeding and produced an antidepressant-like effect in the forced swim test, whereas intra-AcSh injection of MCH had the opposite effect. Expression studies demonstrated that MCH1R is present in both the enkephalin- and dynorphin-positive medium spiny neurons of the AcSh. Biochemical analysis in AcSh explants showed that MCH signaling blocks dopamine-induced phosphorylation of the AMPA glutamate receptor subunit GluR1 at Ser845. Finally, food deprivation, but not other stressors, stimulated cAMP response element-binding protein-dependent pathways selectively in MCH neurons of the hypothalamus, suggesting that these neurons are responsive to a specific set of physiologically relevant conditions. This work identifies a novel hypothalamic-AcSh circuit that influences appetitive behavior and mediates the antidepressant activity of MCH1R antagonists.

Methylphenidate treatment during pre- and periadolescence alters behavioral responses to emotional stimuli at adulthood.

BACKGROUND Methylphenidate (MPH) is a psychomotor stimulant medication widely used for the treatment of attention-deficit/hyperactivity disorder (ADHD). Given the extent of prescribed use of MPH, and because MPH interacts with the same brain pathways activated by drugs of abuse, most research has focused on assessing MPHs potential to alter an individuals risk for adult drug addiction. Data examining other potential long-term behavioral consequences of early MPH administration are lacking, however. METHODS We investigated the long-term behavioral consequences of chronic administration of MPH (2.0 mg/kg) during pre- and periadolescent development in adult rats by assessing their behavioral reactivity to a variety of emotional stimuli. RESULTS The MPH-treated animals were significantly less responsive to natural rewards such as sucrose, novelty-induced activity, and sex compared with vehicle-treated control animals. In contrast, MPH-treated animals were significantly more sensitive to stressful situations, showed increased anxiety-like behaviors, and had enhanced plasma levels of corticosterone. CONCLUSIONS Chronic exposure to MPH during development leads to decreased sensitivity to rewarding stimuli and results in enhanced responsivity to aversive situations. These results highlight the need for further research to improve understanding of the effects of stimulants on the developing nervous system and the potential enduring effects resulting from early-life drug exposure.

CREB regulation of nucleus accumbens excitability mediates social isolation-induced behavioral deficits

Here, we characterized behavioral abnormalities induced by prolonged social isolation in adult rodents. Social isolation induced both anxiety- and anhedonia-like symptoms and decreased cAMP response element–binding protein (CREB) activity in the nucleus accumbens shell (NAcSh). All of these abnormalities were reversed by chronic, but not acute, antidepressant treatment. However, although the anxiety phenotype and its reversal by antidepressant treatment were CREB-dependent, the anhedonia-like symptoms were not mediated by CREB in NAcSh. We found that decreased CREB activity in NAcSh correlated with increased expression of certain K+ channels and reduced electrical excitability of NAcSh neurons, which was sufficient to induce anxiety-like behaviors and was reversed by chronic antidepressant treatment. Together, our results describe a model that distinguishes anxiety- and depression-like behavioral phenotypes, establish a selective role of decreased CREB activity in NAcSh in anxiety-like behavior, and provide a mechanism by which antidepressant treatment alleviates anxiety symptoms after social isolation.

An essential role for ΔFosB in the nucleus accumbens in morphine action

The transcription factor ΔFosB is induced in the nucleus accumbens (NAc) and dorsal striatum by the repeated administration of drugs of abuse. Here, we investigated the role of ΔFosB in the NAc in behavioral responses to opiates. We achieved overexpression of ΔFosB by using a bitransgenic mouse line that inducibly expresses the protein in the NAc and dorsal striatum and by using viral-mediated gene transfer to specifically express the protein in the NAc. ΔFosB overexpression in the NAc increased the sensitivity of the mice to the rewarding effects of morphine and led to exacerbated physical dependence, but also reduced their sensitivity to the analgesic effects of morphine and led to faster development of analgesic tolerance. The opioid peptide dynorphin seemed to be one target through which ΔFosB produced this behavioral phenotype. Together, these experiments demonstrated that ΔFosB in the NAc, partly through the repression of dynorphin expression, mediates several major features of opiate addiction.

Subsensitivity to dopaminergic drugs in periadolescent rats: a behavioral and neurochemical analysis

It has been reported that post-natal day (PD) 30-40 rats respond differently to the behavioral effects of dopaminergic drugs when compared to younger or older rats. In this study, the behavioral effects of amphetamine (AMPH) on motor behavior and the effects of dopaminergic drugs on striatal acetylcholine (ACh) release were evaluated in periadolescent (PD35) and adult rats. AMPH increased dopamine (DA)-mediated motor behaviors (locomotor activity and stereotypy) in periadolescent and adult rats; however, these responses were of a lesser magnitude in periadolescent rats. In adult rats, cocaine and nomifensine inhibited ACh overflow in a dose-dependent manner. In periadolescent rats, ACh overflow was maximally inhibited at a lower drug concentration (5 microM) than in adult rats (10 microM) signifying increased sensitivity in these rats. Apomorphine inhibited ACh overflow in a dose-dependent fashion in slices from adult rats. In contrast, apomorphine did not consistently inhibit ACh overflow in striatal slices prepared from periadolescent rats. Collectively, the results of this study demonstrate behavioral subsensitivity to AMPH in periadolescent rats. Examination of the effects of DA reuptake blockers on DA modulation of striatal cholinergic neurons failed to reveal a corresponding subsensitivity. In fact, ACh release was more sensitive to DA reuptake blockers in periadolescent rats. This latter finding suggests that undisclosed factors override dopaminergic modulation of striatal neurons in the mediation of behavior in periadolescent rats. We propose that during periadolescence, DA transmission is transiently elevated. This results in post-synaptic supersensitivity of cholinergic receptors and consequently induces behavioral subsensitivity when challenged with dopaminergic drugs. Increased cholinergic tone may mediate behavioral subsensitivity despite drug-induced elevations in DA.

Regulation of Drug Reward by cAMP Response Element-Binding Protein: Evidence for Two Functionally Distinct Subregions of the Ventral Tegmental Area

The transcription factor cAMP response element binding protein (CREB) is implicated in the actions of drugs of abuse in several brain areas, but little information is available about a role for CREB in the ventral tegmental area (VTA), one of the key reward regions of the brain. Here, we demonstrate that chronic exposure to drugs of abuse induces CREB activity throughout the VTA. Using viral-mediated gene transfer, we expressed green fluorescent protein (GFP)-tagged CREB or mCREB (a dominant-negative form of CREB) in the VTA and, using a conditioned place-preference paradigm, found that CREB activation within the rostral versus caudal subregions of the VTA produces opposite effects on drug reward. We identified VTA subregion-specific differences in the proportion of dopaminergic and GABAergic neurons and in the dopaminergic projections to the nucleus accumbens, another brain region implicated in drug reward, and suggest that this may contribute to behavioral differences in this study. We also measured expression levels of tyrosine hydroxylase and the AMPA glutamate receptor subunit GluR1, both of which are known to contribute to drug reward in the VTA, and found that both of these genes are upregulated following the expression of CREB-GFP and downregulated following expression of mCREB-GFP, raising the possibility that CREB may exert its effects on drug reward, in part, via regulation of these genes. These results suggest a novel role for CREB in mediating drug-induced plasticity in the VTA and establish two functionally distinct subregions of the VTA in which CREB differentially regulates drug reward.

Neurotrophic mechanisms in drug addiction.

The involvement of neurotrophic factors in neuronal survival and differentiation is well established. The more recent realization that these factors also play pivotal roles in the maintenance and activity-dependent remodeling of neuronal functioning in the adult brain has generated excitement in the neurosciences. Neurotrophic factors have been implicated in the modulation of synaptic transmission and in the mechanisms underlying learning and memory, mood disorders, and drug addiction. Here the evidence for the role of neurotrophins and other neurotrophic factors—and the signaling pathways they activate—in mediating long-term molecular, cellular, and behavioral adaptations associated with drug addiction is reviewed.