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.

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.

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.

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 GABA_A and postsynaptically to decrease GABA_B receptor-mediated currents in the substantia nigra. The active peptide fragment NT_8-13 enhanced GABA_A 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, NT_8-13 depressed GABA_B 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 GABA_A signaling are significantly faster than those of GABA_B 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.