TGF-ß Signaling in Dopaminergic Neurons Regulates Dendritic Growth, Excitatory-Inhibitory Synaptic Balance, and Reversal Learning.

Neural circuits involving midbrain dopaminergic (DA) neurons regulate reward and goal-directed behaviors. Although local GABAergic input is known to modulate DA circuits, the mechanism that controls excitatory/inhibitory synaptic balance in DA neurons remains unclear. Here, we show that DA neurons use autocrine transforming growth factor ß (TGF-ß) signaling to promote the growth of axons and dendrites. Surprisingly, removing TGF-ß type II receptor in DA neurons also disrupts the balance in TGF-ß1 expression in DA neurons and neighboring GABAergic neurons, which increases inhibitory input, reduces excitatory synaptic input, and alters phasic firing patterns in DA neurons. Mice lacking TGF-ß signaling in DA neurons are hyperactive and exhibit inflexibility in relinquishing learned behaviors and re-establishing new stimulus-reward associations. These results support a role for TGF-ß in regulating the delicate balance of excitatory/inhibitory synaptic input in local microcircuits involving DA and GABAergic neurons and its potential contributions to neuropsychiatric disorders.

Dopaminergic Neurons and Brain Reward Pathways: From Neurogenesis to Circuit Assembly.

Midbrain dopaminergic (DA) neurons in the substantia nigra pars compacta and ventral tegmental area regulate extrapyramidal movement and important cognitive functions, including motivation, reward associations, and habit learning. Dysfunctions in DA neuron circuitry have been implicated in several neuropsychiatric disorders, including addiction and schizophrenia, whereas selective degeneration of DA neurons in substantia nigra pars compacta is a key neuropathological feature in Parkinson disease. Efforts to understand these disorders have focused on dissecting the underlying causes, as well as developing therapeutic strategies to replenish dopamine deficiency. In particular, the promise of cell replacement therapies for clinical intervention has led to extensive research in the identification of mechanisms involved in DA neuron development. It is hoped that a comprehensive understanding of these mechanisms will lead to therapeutic strategies that improve the efficiency of DA neuron production, engraftment, and function. This review provides a comprehensive discussion on how Wnt/ß-catenin and sonic hedgehog-Smoothened signaling mechanisms control the specification and expansion of DA progenitors and the differentiation of DA neurons. We also discuss how mechanisms involving transforming growth factor-ß and transcriptional cofactor homeodomain interacting protein kinase 2 regulate the survival and maturation of DA neurons in early postnatal life. These results not only reveal fundamental mechanisms regulating DA neuron development, but also provide important insights to their potential contributions to neuropsychiatric and neurodegenerative diseases.

Aldehyde dehydrogenase 1a1 mediates a GABA synthesis pathway in midbrain dopaminergic neurons.

Midbrain dopamine neurons are an essential component of the basal ganglia circuitry, playing key roles in the control of fine movement and reward. Recently, it has been demonstrated that γ-aminobutyric acid (GABA), the chief inhibitory neurotransmitter, is co-released by dopamine neurons. Here, we show that GABA co-release in dopamine neurons does not use the conventional GABA-synthesizing enzymes, glutamate decarboxylases GAD65 and GAD67. Our experiments reveal an evolutionarily conserved GABA synthesis pathway mediated by aldehyde dehydrogenase 1a1 (ALDH1a1). Moreover, GABA co-release is modulated by ethanol (EtOH) at concentrations seen in blood alcohol after binge drinking, and diminished ALDH1a1 leads to enhanced alcohol consumption and preference. These findings provide insights into the functional role of GABA co-release in midbrain dopamine neurons, which may be essential for reward-based behavior and addiction.

Temporal and spatial requirements of Smoothened in ventral midbrain neuronal development.

BACKGROUND: Several studies have indicated that Sonic hedgehog (Shh) regulates the expansion of dopaminergic (DA) progenitors and the subsequent generation of mature DA neurons. This prevailing view has been based primarily on in vitro culture results, and the exact in vivo function of Shh signaling in the patterning and neurogenesis of the ventral midbrain (vMB) remains unclear. METHODS: We characterized the transcriptional codes for the vMB progenitor domains, and correlated them with the expression patterns of Shh signaling effectors, including Shh, Smoothened, Patched, Gli1, Gli2 and Gli3. RESULTS: While Shh and its downstream effectors showed robust expression in the neurogenic niche for DA progenitors at embryonic day (E)8 to E8.5, their expression shifted to the lateral domains from E9.5 to E12.5. Consistent with this dynamic change, conditional mutants with region-specific removal of the Shh receptor Smoothened in the vMB progenitors (Shh-Cre;Smo(fl/fl)) showed a transient reduction in DA progenitors and DA neurons at E10.5, but had more profound defects in neurons derived from the more lateral domains, including those in the red nucleus, oculomotor nucleus, and raphe nuclei. Conversely, constitutive activation of Smoothened signaling in vMB (Shh-Cre;SmoM2) showed transient expansion of the same progenitor population. To further characterize the nature of Shh-Smoothened signaling in vMB, we examined the BAT-GAL reporter and the expression of Wnt1 in vMB, and found that the antagonistic effects of Shh and Wnt signaling critically regulate the development of DA progenitors and DA neurons. CONCLUSION: These results highlight previously unrecognized effects of Shh-Smoothened signaling in the region-specific neurogenesis within the vMB.

Interactions of Wnt/beta-catenin signaling and sonic hedgehog regulate the neurogenesis of ventral midbrain dopamine neurons.

Signaling mechanisms involving Wnt/beta-catenin and sonic hedgehog (Shh) are known to regulate the development of ventral midbrain (vMB) dopamine neurons. However, the interactions between these two mechanisms and how such interactions can be targeted to promote a maximal production of dopamine neurons are not fully understood. Here we show that conditional mouse mutants with region-specific activation of beta-catenin signaling in vMB using the Shh-Cre mice show a marked expansion of Sox2-, Ngn2-, and Otx2-positive progenitors but perturbs their cell cycle exit and reduces the generation of dopamine neurons. Furthermore, activation of beta-catenin in vMB also results in a progressive loss of Shh expression and Shh target genes. Such antagonistic effects between the activation of Wnt/beta-catenin and Shh can be recapitulated in vMB progenitors and in mouse embryonic stem cell cultures. Notwithstanding these antagonistic interactions, cell-type-specific activation of beta-catenin in the midline progenitors using the tyrosine hydroxylase-internal ribosomal entry site-Cre (Th-IRES-Cre) mice leads to increased dopaminergic neurogenesis. Together, these results indicate the presence of a delicate balance between Wnt/beta-catenin and Shh signaling mechanisms in the progression from progenitors to dopamine neurons. Persistent activation of beta-catenin in early progenitors perturbs their cell cycle progression and antagonizes Shh expression, whereas activation of beta-catenin in midline progenitors promotes the generation of dopamine neurons.