Neurotensin Release from Dopamine Neurons Drives Long-Term Depression of Substantia Nigra Dopamine Signaling.

Midbrain dopamine neurons play central physiological roles in voluntary movement, reward learning, and motivated behavior. Inhibitory signaling at somatodendritic dopamine D2 receptor (D2R) synapses modulates excitability of dopamine neurons. The neuropeptide neurotensin is expressed by many inputs to the midbrain and induces LTD of D2R synaptic currents (LTDDA); however, the source of neurotensin that is responsible for LTDDA is not known. Here we show, in brain slices from male and female mice, that LTDDA is driven by neurotensin released by dopamine neurons themselves. Optogenetic stimulation of dopamine neurons was sufficient to induce LTDDA in the substantia nigra, but not the VTA, and was dependent on neurotensin receptor signaling, postsynaptic calcium, and vacuolar-type H+-ATPase activity in the postsynaptic cell. These findings reveal a novel form of signaling between dopamine neurons involving release of the peptide neurotensin, which may act as a feedforward mechanism to increase dopamine neuron excitability.SIGNIFICANCE STATEMENT Dopamine neurons in the midbrain play a critical role in reward learning and the initiation of movement. Aberrant dopamine neuron function is implicated in a range of diseases and disorders, including Parkinson's disease, schizophrenia, obesity, and substance use disorders. D2 receptor-mediated PSCs are produced by a rare form of dendrodendritic synaptic transmission between dopamine neurons. These D2 receptor-mediated PSCs undergo LTD following application of the neuropeptide neurotensin. Here we show that release of neurotensin by dopamine neurons themselves is sufficient to induce LTD of dopamine transmission in the substantia nigra. Neurotensin signaling therefore mediates a second form of interdopamine neuron communication and may provide a mechanism by which dopamine neurons maintain excitability when nigral dopamine is elevated.

Autism-associated mutations in KV7 channels induce gating pore current.

Autism spectrum disorder (ASD) adversely impacts >1% of children in the United States, causing social interaction deficits, repetitive behaviors, and communication disorders. Genetic analysis of ASD has advanced dramatically through genome sequencing, which has identified >500 genes with mutations in ASD. Mutations that alter arginine gating charges in the voltage sensor of the voltage-gated potassium (KV) channel KV7 (KCNQ) are among those frequently associated with ASD. We hypothesized that these gating charge mutations would induce gating pore current (also termed ω-current) by causing an ionic leak through the mutant voltage sensor. Unexpectedly, we found that wild-type KV7 conducts outward gating pore current through its native voltage sensor at positive membrane potentials, owing to a glutamine in the third gating charge position. In bacterial and human KV7 channels, gating charge mutations at the R1 and R2 positions cause inward gating pore current through the resting voltage sensor at negative membrane potentials, whereas mutation at R4 causes outward gating pore current through the activated voltage sensor at positive potentials. Remarkably, expression of the KV7.3/R2C ASD-associated mutation in vivo in midbrain dopamine neurons of mice disrupts action potential generation and repetitive firing. Overall, our results reveal native and mutant gating pore current in KV7 channels and implicate altered control of action potential generation by gating pore current through mutant KV7 channels as a potential pathogenic mechanism in autism.

A history of ethanol drinking increases locomotor stimulation and blunts enhancement of dendritic dopamine transmission by methamphetamine.

Ethanol and psychostimulant use disorders exhibit comorbidity in humans and cross-sensitization in animal models, but the neurobiological underpinnings of this are not well understood. Ethanol acutely increases dopamine neuron excitability, and psychostimulants such as cocaine or methamphetamine increase extracellular dopamine through inhibition of uptake through the dopamine transporter (DAT) and/or vesicular monoamine transporter 2 (VMAT2). Psychostimulants also depress dopamine neuron activity by enhancing dendritic dopamine neurotransmission. Here, we show that mice with a previous history of ethanol drinking are more sensitive to the locomotor-stimulating effects of a high dose (5 mg/kg), but not lower doses (1 and 3 mg/kg) of methamphetamine or any tested dose of cocaine (3, 10, and 18 mg/kg), compared with water-drinking controls. We next investigated the impact of a history of ethanol drinking, in a separate group of mice, on methamphetamine- or cocaine-induced enhancement of dendritic dopamine transmission using whole-cell voltage clamp electrophysiology in mouse brain slices. Methamphetamine, applied at a concentration (10 µM) that affects both DAT and VMAT2, enhanced D2 receptor-mediated inhibitory postsynaptic currents (D2-IPSCs) in both groups, but this effect was blunted in mice with a history of ethanol drinking. As methamphetamine action at VMAT2 disrupts dopamine neurotransmission, these results may suggest enhanced action of methamphetamine at VMAT2. Furthermore, there were no differences in low-dose methamphetamine or cocaine-induced enhancement of D2-IPSCs, suggesting intact DAT function. Disruption of methamphetamine-induced enhancement of dendritic dopamine transmission would result in decreased inhibition of dopamine neurons, ultimately increasing downstream release and the behavioral effects of methamphetamine.

Translating the atypical dopamine uptake inhibitor hypothesis toward therapeutics for treatment of psychostimulant use disorders.

Medication-assisted treatments are unavailable to patients with cocaine use disorders. Efforts to develop potential pharmacotherapies have led to the identification of a promising lead molecule, JJC8-091, that demonstrates a novel binding mode at the dopamine transporter (DAT). Here, JJC8-091 and a structural analogue, JJC8-088, were extensively and comparatively assessed to elucidate neurochemical correlates to their divergent behavioral profiles. Despite sharing significant structural similarity, JJC8-088 was more cocaine-like, increasing extracellular DA concentrations in the nucleus accumbens shell (NAS) efficaciously and more potently than JJC8-091. In contrast, JJC8-091 was not self-administered and was effective in blocking cocaine-induced reinstatement to drug seeking. Electrophysiology experiments confirmed that JJC8-091 was more effective than JJC8-088 at inhibiting cocaine-mediated enhancement of DA neurotransmission. Further, when VTA DA neurons in DAT-cre mice were optically stimulated, JJC8-088 produced a significant leftward shift in the stimulation-response curve, similar to cocaine, while JJC8-091 shifted the curve downward, suggesting attenuation of DA-mediated brain reward. Computational models predicted that JJC8-088 binds in an outward facing conformation of DAT, similar to cocaine. Conversely, JJC8-091 steers DAT towards a more occluded conformation. Collectively, these data reveal the underlying molecular mechanism at DAT that may be leveraged to rationally optimize leads for the treatment of cocaine use disorders, with JJC8-091 representing a compelling candidate for development.

Acute and subchronic PCP attenuate D2 autoreceptor signaling in substantia nigra dopamine neurons.

Phencyclidine (PCP) administration is commonly used to model schizophrenia in laboratory animals. While PCP is well-characterized as an antagonist of glutamate-sensitive N-methyl-D-aspartate (NMDA) receptors, its effects on dopamine signaling are not well understood. Here we used whole-cell and cell-attached patch-clamp electrophysiology of substantia nigra dopamine neurons to determine the effects of acute and subchronic PCP exposure on both dopamine D2 autoreceptor-mediated currents and burst firing evoked by glutamate receptor activation. Acute PCP affected D2 autoreceptor-mediated currents through two apparently distinct mechanisms: a low-concentration dopamine transporter (DAT) inhibition and a high-concentration potassium (GIRK) channel inhibition. Subchronic administration of PCP (5 mg/kg, i.p., every 12 h for 7 days) decreased sensitivity to low dopamine concentrations, and also enhanced evoked burst firing of dopamine neurons. These findings suggest the effects of PCP on dopaminergic signaling in the midbrain could enhance burst firing and contribute to the development of schizophreniform behavior.

Diverse actions of the modulatory peptide neurotensin on central synaptic transmission.

Neurotensin (NT) is a 13 amino acid neuropeptide that is expressed throughout the central nervous system and is implicated in the etiology of multiple diseases and disorders. Many primary investigations of NT-induced modulation of neuronal excitability at the level of the synapse have been conducted, but they have not been summarized in review form in nearly 30 years. Therefore, the goal of this review is to discuss the many actions of NT on neuronal excitability across brain regions as well as NT circuit architecture. In the basal ganglia as well as other brain nuclei, NT can act through diverse intracellular signaling cascades to enhance or depress neuronal activity by modulating activity of ion channels, ionotropic and metabotropic neurotransmitter receptors, and presynaptic release of neurotransmitters. Further, NT can produce indirect effects by evoking endocannabinoid release, and recently has itself been identified as a putative retrograde messenger. In the basal ganglia, the diverse actions and circuit architecture of NT signaling allow for input-specific control of reward-related behaviors.

Progressively disrupted somatodendritic morphology in dopamine neurons in a mouse Parkinson's model.

BACKGROUND: Parkinson's disease is characterized by the progressive loss of dopamine neurons in the substantia nigra, leading to severe motor deficits. Although the disease likely begins to develop years before observable motor symptoms, the specific morphological and functional alterations involved are poorly understood. OBJECTIVES: MitoPark mice lack the gene coding for mitochondrial transcription factor A specifically in dopamine neurons, which over time produces a progressive decline of neuronal function and related behavior that phenotypically mirrors human parkinsonism. Our previous work identified a progressive decrease in cell capacitance in dopamine neurons from MitoPark mice, possibly suggesting reduced membrane surface area. We therefore sought to identify and quantify somatodendritic parameters in this model across age. METHODS: We used whole-cell patch clamp and fluorescent labeling to quantify somatodendritic morphology of single, neurobiotin-filled dopamine neurons in acutely isolated brain slices from MitoPark mice. RESULTS: We found that MitoPark mice exhibit an adult-onset, age-dependent reduction of neuritic branching and soma size in dopamine neurons. This decline proceeds similarly in MitoPark mice of both sexes, but does not begin until after the age that early decrements in ion channel physiology and behavior have previously been observed. CONCLUSIONS: A progressive and severe decline in somatodendritic morphology occurs prior to cell death, but is not responsible for the subtle decrements observable in the earliest stages of neurodegeneration. This work could help identify the ideal time window for specific treatments to halt disease progression and avert debilitating motor deficits in Parkinson's patients. © 2018 International Parkinson and Movement Disorder Society.

Neurotensin speeds inhibition of dopamine neurons through temporal modulation of GABAA and GABAB receptor-mediated synaptic input.

Midbrain dopamine neurons play physiological roles in many processes including reward learning and motivated behavior, and are tonically inhibited by γ-aminobutyric acid (GABA)ergic input from multiple brain regions. Neurotensin (NT) is a neuropeptide which acutely modulates midbrain dopamine neuron excitability through multiple mechanisms, one of which is a decrease of GABA-mediated inhibition. However, the mechanisms through which NT depresses GABA signaling are not known. Here we used whole cell patch-clamp electrophysiology of dopamine neurons in mouse brain slices to show that NT acts both presynaptically to increase GABAA and postsynaptically to decrease GABAB receptor-mediated currents in the substantia nigra. The active peptide fragment NT8-13 enhanced GABAA signaling presynaptically by causing an increase in the size of the readily releasable pool of GABA via activation of the NT type-1 receptor and protein kinase A. Conversely, NT8-13 depressed GABAB signaling postsynaptically via the NT type-2 receptor in a process that was modulated by protein kinase C. Both forms of plasticity could be observed simultaneously in single dopamine neurons. Thus, as the kinetics of GABAA signaling are significantly faster than those of GABAB signaling, NT functionally speeds GABAergic input to midbrain dopamine neurons. This finding contributes to our understanding of how neuropeptide-induced plasticity can simultaneously differentiate and integrate signaling by a single neurotransmitter in a single cell and provides a basis for understanding how neuropeptides use temporal shifts in synaptic strength to encode information.