Depletion of TrkB Receptors From Adult Serotonergic Neurons Increases Brain Serotonin Levels, Enhances Energy Metabolism and Impairs Learning and Memory.

Neurotrophin brain-derived neurotrophic factor (BDNF) and neurotransmitter serotonin (5-HT) regulate each other and have been implicated in several neuronal mechanisms, including neuroplasticity. We have investigated the effects of BDNF on serotonergic neurons by deleting BDNF receptor TrkB from serotonergic neurons in the adult brain. The transgenic mice show increased 5-HT and Tph2 levels with abnormal behavioral phenotype. In spite of increased food intake, the transgenic mice are significantly leaner than their wildtype littermates, which may be due to increased metabolic activity. Consistent with increased 5-HT, the proliferation of hippocampal progenitors is significantly increased, however, long-term survival of newborn cells is unchanged. Our data indicates that BDNF-TrkB signaling regulates the functional phenotype of 5-HT neurons with long-term behavioral consequences.

Antidepressant drugs act by directly binding to TRKB neurotrophin receptors.

It is unclear how binding of antidepressant drugs to their targets gives rise to the clinical antidepressant effect. We discovered that the transmembrane domain of tyrosine kinase receptor 2 (TRKB), the brain-derived neurotrophic factor (BDNF) receptor that promotes neuronal plasticity and antidepressant responses, has a cholesterol-sensing function that mediates synaptic effects of cholesterol. We then found that both typical and fast-acting antidepressants directly bind to TRKB, thereby facilitating synaptic localization of TRKB and its activation by BDNF. Extensive computational approaches including atomistic molecular dynamics simulations revealed a binding site at the transmembrane region of TRKB dimers. Mutation of the TRKB antidepressant-binding motif impaired cellular, behavioral, and plasticity-promoting responses to antidepressants in vitro and in vivo. We suggest that binding to TRKB and allosteric facilitation of BDNF signaling is the common mechanism for antidepressant action, which may explain why typical antidepressants act slowly and how molecular effects of antidepressants are translated into clinical mood recovery.

Dicer and microRNAs protect adult dopamine neurons.

MicroRNAs (miRs) are important post-transcriptional regulators of gene expression implicated in neuronal development, differentiation, aging and neurodegenerative diseases, including Parkinson's disease (PD). Several miRs have been linked to PD-associated genes, apoptosis and stress response pathways, suggesting that deregulation of miRs may contribute to the development of the neurodegenerative phenotype. Here, we investigate the cell-autonomous role of miR processing RNAse Dicer in the functional maintenance of adult dopamine (DA) neurons. We demonstrate a reduction of Dicer in the ventral midbrain and altered miR expression profiles in laser-microdissected DA neurons of aged mice. Using a mouse line expressing tamoxifen-inducible CreERT2 recombinase under control of the DA transporter promoter, we show that a tissue-specific conditional ablation of Dicer in DA neurons of adult mice led to decreased levels of striatal DA and its metabolites without a reduction in neuronal body numbers in hemizygous mice (DicerHET) and to progressive loss of DA neurons with severe locomotor deficits in nullizygous mice (DicerCKO). Moreover, we show that pharmacological stimulation of miR biosynthesis promoted survival of cultured DA neurons and reduced their vulnerability to thapsigargin-induced endoplasmic reticulum stress. Our data demonstrate that Dicer is crucial for maintenance of adult DA neurons, whereas a stimulation of miR production can promote neuronal survival, which may have direct implications for PD treatment.

Chronic nicotine modifies the effects of morphine on extracellular striatal dopamine and ventral tegmental GABA.

Previously, we have shown that 7-week oral nicotine treatment enhances morphine-induced behaviors and dopaminergic activity in the mouse brain. In this study, we further characterized the nicotine-morphine interaction in the mesolimbic and nigrostriatal dopaminergic systems, as well as in the GABAergic control of these systems. In nicotine-pretreated mice, morphine-induced dopamine release in the caudate putamen and nucleus accumbens was significantly augmented, as measured by microdialysis. Chronic nicotine treatment did not change basal extracellular concentrations of dopamine and its metabolites in the caudate putamen and nucleus accumbens, nor did it affect the rate of dopamine synthesis, as assessed by 3-hydroxybenzylhydrazine dihydrochloride-induced DOPA accumulation. GABAergic control of dopaminergic activity was studied by measuring extracellular GABA in the presence of nipecotic acid, an inhibitor of GABA uptake. Acute (0.3 mg/kg or 0.5 mg/kg i.p.) and chronic nicotine, as well as morphine (15 mg/kg s.c.) in control mice decreased nipecotic acid-induced increase in extracellular GABA in the ventral tegmental area/substantia nigra (VTA/SN). In contrast, in nicotine-treated mice, morphine increased GABA levels in the presence of nipecotic acid. We did not find any alterations in GABA(B)-receptor function after chronic nicotine treatment. Thus, our data show that chronic nicotine treatment sensitizes dopaminergic systems to morphine and affects GABAergic systems in the VTA/SN.

In heterozygous GDNF knockout mice the response of striatal dopaminergic system to acute morphine is altered.

Glial cell line-derived neurotrophic factor (GDNF) regulates striatal dopaminergic neurons. To study whether reduced endogenous GDNF affect morphine's effects on striatal dopamine transmission, we estimated extracellular concentrations of dopamine and its metabolites by microdialysis in vivo and tissue concentrations post mortem in mice lacking one GDNF allele (GDNF+/- mice). In the wild-type mice, acute morphine (5 and 10 mg/kg s.c.) increased accumbal dopamine output dose-dependently (maximally by 30 and 80%, respectively). In the GDNF+/- mice, 5 mg/kg of morphine enhanced the accumbal dopamine output maximally by 110%, and significantly more than morphine 10 mg/kg (maximally by 60%). Also, the response of extracellular accumbal DOPAC to acute morphine was significantly altered in the GDNF+/- mice. In mice of both genotypes, the responses to morphine in the caudate putamen were similar to but much less intense than those in the nucleus accumbens. Morphine at the doses 5, 10, and 30 mg/kg dose-dependently elevated the striatal tissue concentrations of DOPAC and HVA, but the effect of 30 mg/kg was significantly smaller in the GDNF+/- mice than in their wild-type littermates. The binding of [(3)H]DAMGO to striatal membrane homogenates was similar between the genotypes. However, morphine induced antinociception in the GDNF+/- mice at a smaller dose than in the controls. The finding that reduced GDNF level alters the effects of morphine on striatal dopamine and our previous findings of elevated extracellular striatal dopamine concentrations and FosB/DeltaFosB expression in the GDNF+/- mice show the importance of GDNF in the regulation of striatal dopaminergic system.

Increased extracellular dopamine concentrations and FosB/DeltaFosB expression in striatal brain areas of heterozygous GDNF knockout mice.

Glial cell line-derived neurotrophic factor (GDNF) has been shown to be involved in the maintenance of striatal dopaminergic neurons. To study whether reduced levels of endogenous GDNF affect the striatal dopaminergic transmission we estimated the basal extracellular levels of dopamine in vivo, the basal expression of FosB-related proteins in striatal brain areas as well as the effects of acute and repeated cocaine on locomotor activity and dopamine output in mice lacking one GDNF allele (heterozygous GDNF+/- mice). As expected the striatal GDNF protein content was found to be smaller in the GDNF+/- mice than in their wild-type littermates. Unexpectedly the extracellular dopamine concentration in the GDNF+/- mice in the dorsal striatum (CPu) was 2.0-fold, and in the nucleus accumbens (NAc) 1.6-fold the concentration found in the wild-type littermates. Also FosB/DeltaFosB-like immunoreactivity was found to be elevated in the CPu as well as in the core and in the shell of NAc of the GDNF+/- mice as compared with the wild-type mice. This suggests chronic postsynaptic activation of these brain areas and is in line with elevated extracellular dopamine concentrations. Cocaine's effects acutely and after repeated treatment on locomotor activity were similar in the GDNF+/- and the wild-type mice. Neither did cocaine's acute effects on dopamine output differ between the mice of the two strains. Our findings demonstrate that reduced levels of endogenous GDNF induce alterations in dorsal striatal and accumbal dopaminergic transmission, and stress the importance of endogenous GDNF in the regulation of the dopaminergic neurons.