Inhibition of striatal dopamine release by the L-type calcium channel inhibitor isradipine co-varies with risk factors for Parkinson's.

Ca2+ entry into nigrostriatal dopamine (DA) neurons and axons via L-type voltage-gated Ca2+ channels (LTCCs) contributes, respectively, to pacemaker activity and DA release and has long been thought to contribute to vulnerability to degeneration in Parkinson's disease. LTCC function is greater in DA axons and neurons from substantia nigra pars compacta than from ventral tegmental area, but this is not explained by channel expression level. We tested the hypothesis that LTCC control of DA release is governed rather by local mechanisms, focussing on candidate biological factors known to operate differently between types of DA neurons and/or be associated with their differing vulnerability to parkinsonism, including biological sex, α-synuclein, DA transporters (DATs) and calbindin-D28k (Calb1). We detected evoked DA release ex vivo in mouse striatal slices using fast-scan cyclic voltammetry and assessed LTCC support of DA release by detecting the inhibition of DA release by the LTCC inhibitors isradipine or CP8. Using genetic knockouts or pharmacological manipulations, we identified that striatal LTCC support of DA release depended on multiple intersecting factors, in a regionally and sexually divergent manner. LTCC function was promoted by factors associated with Parkinsonian risk, including male sex, α-synuclein, DAT and a dorsolateral co-ordinate, but limited by factors associated with protection, that is, female sex, glucocerebrosidase activity, Calb1 and ventromedial co-ordinate. Together, these data show that LTCC function in DA axons and isradipine effect are locally governed and suggest they vary in a manner that in turn might impact on, or reflect, the cellular stress that leads to parkinsonian degeneration.

Impaired dopamine release in Parkinson's disease.

Parkinson's disease is the second most common neurodegenerative disease and yet the early pathophysiological events of the condition and sequences of dysfunction remain unclear. The loss of dopaminergic neurons and reduced levels of striatal dopamine are descriptions used interchangeably as underlying the motor deficits in Parkinson's disease. However, decades of research suggest that dopamine release deficits in Parkinson's disease do not occur only after cell death, but that there is dysfunction or dysregulation of axonal dopamine release before cell loss. Here we review the evidence for dopamine release deficits prior to neurodegeneration in Parkinson's disease, drawn from a large and emerging range of Parkinson's disease models, and the mechanisms by which these release deficits occur. The evidence indicates that impaired dopamine release can result from disruption to a diverse range of Parkinson's disease-associated genetic and molecular disturbances, and can be considered as a potential pathophysiological hallmark of Parkinson's disease.

Dopamine Release in Nucleus Accumbens Is under Tonic Inhibition by Adenosine A1 Receptors Regulated by Astrocytic ENT1 and Dysregulated by Ethanol.

Striatal adenosine A1 receptor (A1R) activation can inhibit dopamine release. A1Rs on other striatal neurons are activated by an adenosine tone that is limited by equilibrative nucleoside transporter 1 (ENT1) that is enriched on astrocytes and is ethanol sensitive. We explored whether dopamine release in nucleus accumbens core is under tonic inhibition by A1Rs, and is regulated by astrocytic ENT1 and ethanol. In ex vivo striatal slices from male and female mice, A1R agonists inhibited dopamine release evoked electrically or optogenetically and detected using fast-scan cyclic voltammetry, most strongly for lower stimulation frequencies and pulse numbers, thereby enhancing the activity-dependent contrast of dopamine release. Conversely, A1R antagonists reduced activity-dependent contrast but enhanced evoked dopamine release levels, even for single optogenetic pulses indicating an underlying tonic inhibition. The ENT1 inhibitor nitrobenzylthioinosine reduced dopamine release and promoted A1R-mediated inhibition, and, conversely, virally mediated astrocytic overexpression of ENT1 enhanced dopamine release and relieved A1R-mediated inhibition. By imaging the genetically encoded fluorescent adenosine sensor [GPCR-activation based (GRAB)-Ado], we identified a striatal extracellular adenosine tone that was elevated by the ENT1 inhibitor and sensitive to gliotoxin fluorocitrate. Finally, we identified that ethanol (50 mm) promoted A1R-mediated inhibition of dopamine release, through diminishing adenosine uptake via ENT1. Together, these data reveal that dopamine output dynamics are gated by a striatal adenosine tone, limiting amplitude but promoting contrast, regulated by ENT1, and promoted by ethanol. These data add to the diverse mechanisms through which ethanol modulates striatal dopamine, and to emerging datasets supporting astrocytic transporters as important regulators of striatal function.SIGNIFICANCE STATEMENT Dopamine axons in the mammalian striatum are emerging as strategic sites where neuromodulators can powerfully influence dopamine output in health and disease. We found that ambient levels of the neuromodulator adenosine tonically inhibit dopamine release in nucleus accumbens core via adenosine A1 receptors (A1Rs), to a variable level that promotes the contrast in dopamine signals released by different frequencies of activity. We reveal that the equilibrative nucleoside transporter 1 (ENT1) on astrocytes limits this tonic inhibition, and that ethanol promotes it by diminishing adenosine uptake via ENT1. These findings support the hypotheses that A1Rs on dopamine axons inhibit dopamine release and, furthermore, that astrocytes perform important roles in setting the level of striatal dopamine output, in health and disease.

Inhibition of Nigrostriatal Dopamine Release by Striatal GABAA and GABAB Receptors.

Nigrostriatal dopamine (DA) is critical to action selection and learning. Axonal DA release is locally influenced by striatal neurotransmitters. Striatal neurons are principally GABAergic projection neurons and interneurons, and a small minority of other neurons are cholinergic interneurons (ChIs). ChIs strongly gate striatal DA release via nicotinic receptors (nAChRs) identified on DA axons. Striatal GABA is thought to modulate DA, but GABA receptors have not been documented conclusively on DA axons. However, ChIs express GABA receptors and are therefore candidates for potential mediators of GABA regulation of DA. We addressed whether striatal GABA and its receptors can modulate DA release directly, independently from ChI regulation, by detecting DA in striatal slices from male mice using fast-scan cyclic voltammetry in the absence of nAChR activation. DA release evoked by single electrical pulses in the presence of the nAChR antagonist dihydro-ß-erythroidine was reduced by GABA or agonists of GABAA or GABAB receptors, with effects prevented by selective GABA receptor antagonists. GABA agonists slightly modified the frequency sensitivity of DA release during short stimulus trains. GABA agonists also suppressed DA release evoked by optogenetic stimulation of DA axons. Furthermore, antagonists of GABAA and GABAB receptors together, or GABAB receptors alone, significantly enhanced DA release evoked by either optogenetic or electrical stimuli. These results indicate that striatal GABA can inhibit DA release through GABAA and GABAB receptors and that these actions are not mediated by cholinergic circuits. Furthermore, these data reveal that there is a tonic inhibition of DA release by striatal GABA operating through predominantly GABAB receptors.SIGNIFICANCE STATEMENT The principal inhibitory transmitter in the mammalian striatum, GABA, is thought to modulate striatal dopamine (DA) release, but definitive evidence for GABA receptors on DA axons is lacking. Striatal cholinergic interneurons regulate DA release via axonal nicotinic receptors (nAChRs) and also express GABA receptors, but they have not been eliminated as potentially critical mediators of DA regulation by GABA. Here, we found that GABAA and GABAB receptors inhibit DA release without requiring cholinergic interneurons. Furthermore, ambient levels of GABA inhibited DA release predominantly through GABAB receptors. These findings provide further support for direct inhibition of DA release by GABA receptors and reveal that striatal GABA operates a tonic inhibition on DA output that could critically influence striatal output.

Diabetes Causes Dysfunctional Dopamine Neurotransmission Favoring Nigrostriatal Degeneration in Mice.

BACKGROUND: Numerous studies indicate an association between neurodegenerative and metabolic diseases. Although still a matter of debate, growing evidence from epidemiological and animal studies indicate that preexisting diabetes increases the risk to develop Parkinson's disease. However, the mechanisms of such an association are unknown. OBJECTIVES: We investigated whether diabetes alters striatal dopamine neurotransmission and assessed the vulnerability of nigrostriatal neurons to neurodegeneration. METHODS: We used streptozotocin-treated and genetically diabetic db/db mice. Expression of oxidative stress and nigrostriatal neuronal markers and levels of dopamine and its metabolites were monitored. Dopamine release and uptake were assessed using fast-scan cyclic voltammetry. 6-Hydroxydopamine was unilaterally injected into the striatum using stereotaxic surgery. Motor performance was scored using specific tests. RESULTS: Diabetes resulted in oxidative stress and decreased levels of dopamine and its metabolites in the striatum. Levels of proteins regulating dopamine release and uptake, including the dopamine transporter, the Girk2 potassium channel, the vesicular monoamine transporter 2, and the presynaptic vesicle protein synaptobrevin-2, were decreased in diabetic mice. Electrically evoked levels of extracellular dopamine in the striatum were enhanced, and altered dopamine uptake was observed. Striatal microinjections of a subthreshold dose of the neurotoxin 6-hydroxydopamine in diabetic mice, insufficient to cause motor alterations in nondiabetic animals, resulted in motor impairment, higher loss of striatal dopaminergic axons, and decreased neuronal cell bodies in the substantia nigra. CONCLUSIONS: Our results indicate that diabetes promotes striatal oxidative stress, alters dopamine neurotransmission, and increases vulnerability to neurodegenerative damage leading to motor impairment. © 2020 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

Representation of spontaneous movement by dopaminergic neurons is cell-type selective and disrupted in parkinsonism.

Midbrain dopaminergic neurons are essential for appropriate voluntary movement, as epitomized by the cardinal motor impairments arising in Parkinson's disease. Understanding the basis of such motor control requires understanding how the firing of different types of dopaminergic neuron relates to movement and how this activity is deciphered in target structures such as the striatum. By recording and labeling individual neurons in behaving mice, we show that the representation of brief spontaneous movements in the firing of identified midbrain dopaminergic neurons is cell-type selective. Most dopaminergic neurons in the substantia nigra pars compacta (SNc), but not in ventral tegmental area or substantia nigra pars lateralis, consistently represented the onset of spontaneous movements with a pause in their firing. Computational modeling revealed that the movement-related firing of these dopaminergic neurons can manifest as rapid and robust fluctuations in striatal dopamine concentration and receptor activity. The exact nature of the movement-related signaling in the striatum depended on the type of dopaminergic neuron providing inputs, the striatal region innervated, and the type of dopamine receptor expressed by striatal neurons. Importantly, in aged mice harboring a genetic burden relevant for human Parkinson's disease, the precise movement-related firing of SNc dopaminergic neurons and the resultant striatal dopamine signaling were lost. These data show that distinct dopaminergic cell types differentially encode spontaneous movement and elucidate how dysregulation of their firing in early Parkinsonism can impair their effector circuits.

LRRK2 BAC transgenic rats develop progressive, L-DOPA-responsive motor impairment, and deficits in dopamine circuit function.

Mutations in leucine-rich repeat kinase 2 (LRRK2) lead to late-onset, autosomal dominant Parkinson's disease, characterized by the degeneration of dopamine neurons of the substantia nigra pars compacta, a deficit in dopamine neurotransmission and the development of motor and non-motor symptoms. The most prevalent Parkinson's disease LRRK2 mutations are located in the kinase (G2019S) and GTPase (R1441C) encoding domains of LRRK2. To better understand the sequence of events that lead to progressive neurophysiological deficits in vulnerable neurons and circuits in Parkinson's disease, we have generated LRRK2 bacterial artificial chromosome transgenic rats expressing either G2019S or R1441C mutant, or wild-type LRRK2, from the complete human LRRK2 genomic locus, including endogenous promoter and regulatory regions. Aged (18-21 months) G2019S and R1441C mutant transgenic rats exhibit L-DOPA-responsive motor dysfunction, impaired striatal dopamine release as determined by fast-scan cyclic voltammetry, and cognitive deficits. In addition, in vivo recordings of identified substantia nigra pars compacta dopamine neurons in R1441C LRRK2 transgenic rats reveal an age-dependent reduction in burst firing, which likely results in further reductions to striatal dopamine release. These alterations to dopamine circuit function occur in the absence of neurodegeneration or abnormal protein accumulation within the substantia nigra pars compacta, suggesting that nigrostriatal dopamine dysfunction precedes detectable protein aggregation and cell death in the development of Parkinson's disease. In conclusion, our longitudinal deep-phenotyping provides novel insights into how the genetic burden arising from human mutant LRRK2 manifests as early pathophysiological changes to dopamine circuit function and highlights a potential model for testing Parkinson's therapeutics.

Transcription factors FOXA1 and FOXA2 maintain dopaminergic neuronal properties and control feeding behavior in adult mice.

Midbrain dopaminergic (mDA) neurons are implicated in cognitive functions, neuropsychiatric disorders, and pathological conditions; hence understanding genes regulating their homeostasis has medical relevance. Transcription factors FOXA1 and FOXA2 (FOXA1/2) are key determinants of mDA neuronal identity during development, but their roles in adult mDA neurons are unknown. We used a conditional knockout strategy to specifically ablate FOXA1/2 in mDA neurons of adult mice. We show that deletion of Foxa1/2 results in down-regulation of tyrosine hydroxylase, the rate-limiting enzyme of dopamine (DA) biosynthesis, specifically in dopaminergic neurons of the substantia nigra pars compacta (SNc). In addition, DA synthesis and striatal DA transmission were reduced after Foxa1/2 deletion. Furthermore, the burst-firing activity characteristic of SNc mDA neurons was drastically reduced in the absence of FOXA1/2. These molecular and functional alterations lead to a severe feeding deficit in adult Foxa1/2 mutant mice, independently of motor control, which could be rescued by L-DOPA treatment. FOXA1/2 therefore control the maintenance of molecular and physiological properties of SNc mDA neurons and impact on feeding behavior in adult mice.

Cortical Control of Striatal Dopamine Transmission via Striatal Cholinergic Interneurons.

Corticostriatal regulation of striatal dopamine (DA) transmission has long been postulated, but ionotropic glutamate receptors have not been localized directly to DA axons. Striatal cholinergic interneurons (ChIs) are emerging as major players in striatal function, and can govern DA transmission by activating nicotinic receptors (nAChRs) on DA axons. Cortical inputs to ChIs have historically been perceived as sparse, but recent evidence indicates that they strongly activate ChIs. We explored whether activation of M1/M2 corticostriatal inputs can consequently gate DA transmission, via ChIs. We reveal that optogenetic activation of channelrhodopsin-expressing corticostriatal axons can drive striatal DA release detected with fast-scan cyclic voltammetry and requires activation of nAChRs on DA axons and AMPA receptors on ChIs that promote short-latency action potentials. By contrast, DA release driven by optogenetic activation of intralaminar thalamostriatal inputs involves additional activation of NMDA receptors on ChIs and action potential generation over longer timescales. Therefore, cortical and thalamic glutamate inputs can modulate DA transmission by regulating ChIs as gatekeepers, through ionotropic glutamate receptors. The different use of AMPA and NMDA receptors by cortical versus thalamic inputs might lead to distinct input integration strategies by ChIs and distinct modulation of the function of DA and striatum.

Substance P Weights Striatal Dopamine Transmission Differently within the Striosome-Matrix Axis.

The mammalian striatum has a topographical organization of input-output connectivity, but a complex internal, nonlaminar neuronal architecture comprising projection neurons of two types interspersed among multiple interneuron types and potential local neuromodulators. From this cellular melange arises a biochemical compartmentalization of areas termed striosomes and extrastriosomal matrix. The functions of these compartments are poorly understood but might confer distinct features to striatal signal processing and be discretely governed. Dopamine transmission occurs throughout striosomes and matrix, and is reported to be modulated by the striosomally enriched neuromodulator substance P. However, reported effects are conflicting, ranging from facilitation to inhibition. We addressed whether dopamine transmission is modulated differently in striosome-matrix compartments by substance P.We paired detection of evoked dopamine release at carbon-fiber microelectrodes in mouse striatal slices with subsequent identification of the location of recording sites with respect to µ-opioid receptor-rich striosomes. Substance P had bidirectional effects on dopamine release that varied between recording sites and were prevented by inhibition of neurokinin-1 receptors. The direction of modulation was determined by location within the striosomal-matrix axis: dopamine release was boosted in striosome centers, diminished in striosomal-matrix border regions, and unaffected in the matrix. In turn, this different weighting of dopamine transmission by substance P modified the apparent center-surround contrast of striosomal dopamine signals. These data reveal that dopamine transmission can be differentially modulated within the striosomal-matrix axis, and furthermore, indicate a functionally distinct zone at the striosome-matrix interface, which may have key impacts on striatal integration.

Serotonin spillover onto the axon initial segment of motoneurons induces central fatigue by inhibiting action potential initiation.

Motor fatigue induced by physical activity is an everyday experience characterized by a decreased capacity to generate motor force. Factors in both muscles and the central nervous system are involved. The central component of fatigue modulates the ability of motoneurons to activate muscle adequately independently of the muscle physiology. Indirect evidence indicates that central fatigue is caused by serotonin (5-HT), but the cellular mechanisms are unknown. In a slice preparation from the spinal cord of the adult turtle, we found that prolonged stimulation of the raphe-spinal pathway--as during motor exercise--activated 5-HT1A receptors that decreased motoneuronal excitability. Electrophysiological tests combined with pharmacology showed that focal activation of 5-HT1A receptors at the axon initial segment (AIS), but not on other motoneuronal compartments, inhibited the action potential initiation by modulating a Na(+) current. Immunohistochemical staining against 5-HT revealed a high-density innervation of 5-HT terminals on the somatodendritic membrane and a complete absence on the AIS. This observation raised the hypothesis that a 5-HT spillover activates receptors at this latter compartment. We tested it by measuring the level of extracellular 5-HT with cyclic voltammetry and found that prolonged stimulations of the raphe-spinal pathway increased the level of 5-HT to a concentration sufficient to activate 5-HT1A receptors. Together our results demonstrate that prolonged release of 5-HT during motor activity spills over from its release sites to the AIS of motoneurons. Here, activated 5-HT1A receptors inhibit firing and, thereby, muscle contraction. Hence, this is a cellular mechanism for central fatigue.

Deficits in dopaminergic transmission precede neuron loss and dysfunction in a new Parkinson model.

The pathological end-state of Parkinson disease is well described from postmortem tissue, but there remains a pressing need to define early functional changes to susceptible neurons and circuits. In particular, mechanisms underlying the vulnerability of the dopamine neurons of the substantia nigra pars compacta (SNc) and the importance of protein aggregation in driving the disease process remain to be determined. To better understand the sequence of events occurring in familial and sporadic Parkinson disease, we generated bacterial artificial chromosome transgenic mice (SNCA-OVX) that express wild-type α-synuclein from the complete human SNCA locus at disease-relevant levels and display a transgene expression profile that recapitulates that of endogenous α-synuclein. SNCA-OVX mice display age-dependent loss of nigrostriatal dopamine neurons and motor impairments characteristic of Parkinson disease. This phenotype is preceded by early deficits in dopamine release from terminals in the dorsal, but not ventral, striatum. Such neurotransmission deficits are not seen at either noradrenergic or serotoninergic terminals. Dopamine release deficits are associated with an altered distribution of vesicles in dopaminergic axons in the dorsal striatum. Aged SNCA-OVX mice exhibit reduced firing of SNc dopamine neurons in vivo measured by juxtacellular recording of neurochemically identified neurons. These progressive changes in vulnerable SNc neurons were observed independently of overt protein aggregation, suggesting neurophysiological changes precede, and are not driven by, aggregate formation. This longitudinal phenotyping strategy in SNCA-OVX mice thus provides insights into the region-specific neuronal disturbances preceding and accompanying Parkinson disease.

Gating of dopamine transmission by calcium and axonal N-, Q-, T- and L-type voltage-gated calcium channels differs between striatal domains.

KEY POINTS: The voltage-gated Ca(2+) channels (VGCCs) that catalyse striatal dopamine transmission are critical to dopamine function and might prime subpopulations of neurons for parkinsonian degeneration. However, the VGCCs that operate on mesostriatal axons are incompletely defined; previous studies encompassed channels on striatal cholinergic interneurons that strongly influence dopamine transmission. We define that multiple types of axonal VGCCs operate that extend beyond classic presynaptic N/P/Q channels to include T- and L-types. We reveal differences in VGCC function between mouse axon types that in humans are vulnerable versus resistant to Parkinson's disease. We show for the first time that this is underpinned by different sensitivity of dopamine transmission to extracellular Ca(2+) and by different spatiotemporal intracellular Ca(2+) microdomains. These data define key principles of how Ca(2+) and VGCCs govern dopamine transmission in the healthy brain and reveal differences between neuron types that might contribute to vulnerability in disease. ABSTRACT: The axonal voltage-gated Ca(2+) channels (VGCCs) that catalyse dopamine (DA) transmission are incompletely defined. Yet, they are critical to DA function and might prime subpopulations of DA neurons for parkinsonian degeneration. Previous studies of VGCCs will have encompassed those on striatal cholinergic interneurons, which strongly influence DA transmission. We identify which VGCCs on DA axons govern DA transmission, we determine their dynamic properties and reveal an underlying basis for differences between the caudate putamen (CPu) and nucleus accumbens (NAc). We detected DA release evoked electrically during nicotinic receptor blockade or optogenetically by light activation of channel rhodopsin-expressing DA axons in mouse striatal slices. Subtype-specific VGCC blockers indicated that N-, Q-, T- and L-VGCCs govern DA release in CPu, but in NAc, T and L-channels are relatively silent. The roles of the most dominant channels were inversely frequency-dependent, due to low-pass filtering of DA release by Ca(2+)-dependent relationships between initial release probability and short-term plasticity. Ca(2+) concentration-response curves revealed that differences between CPu and NAc were due to greater underlying Ca(2+) sensitivity of DA transmission from CPu axons. Functions for 'silent' L- and T-channels in NAc could be unmasked by elevating extracellular [Ca(2+)]. Furthermore, we identified a greater coupling between BAPTA-sensitive, fast Ca(2+) transients and DA transmission in CPu axons, and evidence for endogenous fast buffering of Ca(2+) in NAc. These data reveal that a range of VGCCs operate dynamically on DA axons, depending on local driving forces. Furthermore, they reveal dramatic differences in Ca(2+) handling between axonal subpopulations that show different vulnerability to parkinsonian degeneration.

The impact of a parkinsonian lesion on dynamic striatal dopamine transmission depends on nicotinic receptor activation.

Dopamine function is disturbed in Parkinson's disease (PD), but whether and how release of dopamine from surviving neurons is altered has long been debated. Nicotinic acetylcholine receptors (nAChRs) on dopamine axons powerfully govern dopamine release and could be critical contributing factors. We revisited whether fundamental properties of dopamine transmission are changed in a parkinsonian brain and tested the potentially profound masking effects of nAChRs. Using real-time detection of dopamine in mouse striatum after a partial 6-hydroxydopamine lesion and under nAChR inhibition, we reveal that dopamine signals show diminished sensitivity to presynaptic activity. This effect manifested as diminished contrast between DA release evoked by the lowest versus highest frequencies. This reduced activity-dependence was underpinned by loss of short-term facilitation of dopamine release, consistent with an increase in release probability (Pr). With nAChRs active, the reduced activity-dependence of dopamine release after a parkinsonian lesion was masked. Consequently, moment-by-moment variation in activity of nAChRs may lead to dynamic co-variation in dopamine signal impairments in PD.

Region-specific deficits in dopamine, but not norepinephrine, signaling in a novel A30P α-synuclein BAC transgenic mouse.

Parkinson's disease (PD) is a neurodegenerative disorder classically characterized by the death of dopamine (DA) neurons in the substantia nigra pars compacta and by intracellular Lewy bodies composed largely of α-synuclein. Approximately 5-10% of PD patients have a familial form of Parkinsonism, including mutations in α-synuclein. To better understand the cell-type specific role of α-synuclein on DA neurotransmission, and the effects of the disease-associated A30P mutation, we generated and studied a novel transgenic model of PD. We expressed the A30P mutant form of human α-synuclein in a spatially-relevant manner from the 111kb SNCA genomic DNA locus on a bacterial artificial chromosome (BAC) insert on a mouse null (Snca-/-) background. The BAC transgenic mice expressed α-synuclein in tyrosine hydroxylase-positive neurons and expression of either A30P α-synuclein or wildtype α-synuclein restored the sensitivity of DA neurons to MPTP in resistant Snca-/- animals. A30P α-synuclein mice showed no Lewy body-like aggregation, and did not lose catecholamine neurons in substantia nigra or locus coeruleus. However, using cyclic voltammetry at carbon-fiber microelectrodes we identified a deficit in evoked DA release in the caudate putamen, but not in the nucleus accumbens, of SNCA-A30P Snca-/- mice but no changes to release of another catecholamine, norepinephrine (NE), in the NE-rich ventral bed nucleus of stria terminalis. SNCA-A30P Snca-/- mice had no overt behavioral impairments but exhibited a mild increase in wheel-running. In summary, this refined PD mouse model shows that A30P α-synuclein preferentially perturbs the dopaminergic system in the dorsal striatum, reflecting the region-specific change seen in PD.

Distinct contributions of nicotinic acetylcholine receptor subunit alpha4 and subunit alpha6 to the reinforcing effects of nicotine.

Nicotine is the primary psychoactive component of tobacco. Its reinforcing and addictive properties depend on nicotinic acetylcholine receptors (nAChRs) located within the mesolimbic axis originating in the ventral tegmental area (VTA). The roles and oligomeric assembly of subunit α4- and subunit α6-containing nAChRs in dopaminergic (DAergic) neurons are much debated. Using subunit-specific knockout mice and targeted lentiviral re-expression, we have determined the subunit dependence of intracranial nicotine self-administration (ICSA) into the VTA and the effects of nicotine on dopamine (DA) neuron excitability in the VTA and on DA transmission in the nucleus accumbens (NAc). We show that the α4 subunit, but not the α6 subunit, is necessary for ICSA and nicotine-induced bursting of VTA DAergic neurons, whereas subunits α4 and α6 together regulate the activity dependence of DA transmission in the NAc. These data suggest that α4-dominated enhancement of burst firing in DA neurons, relayed by DA transmission in NAc that is gated by nAChRs containing α4 and α6 subunits, underlies nicotine self-administration and its long-term maintenance.

Psychosis as a disorder of muscarinic signalling: psychopathology and pharmacology.

Dopaminergic receptor antagonism is a crucial component of all licensed treatments for psychosis, and dopamine dysfunction has been central to pathophysiological models of psychotic symptoms. Some clinical trials, however, indicate that drugs that act through muscarinic receptor agonism can also be effective in treating psychosis, potentially implicating muscarinic abnormalities in the pathophysiology of psychosis. Here, we discuss understanding of the central muscarinic system, and we examine preclinical, behavioural, post-mortem, and neuroimaging evidence for its involvement in psychosis. We then consider how altered muscarinic signalling could contribute to the genesis and maintenance of psychotic symptoms, and we review the clinical evidence for muscarinic agents as treatments. Finally, we discuss future research that could clarify the relationship between the muscarinic system and psychotic symptoms.

Lights, fiber, action! A primer on in vivo fiber photometry.

Fiber photometry is a key technique for characterizing brain-behavior relationships in vivo. Initially, it was primarily used to report calcium dynamics as a proxy for neural activity via genetically encoded indicators. This generated new insights into brain functions including movement, memory, and motivation at the level of defined circuits and cell types. Recently, the opportunity for discovery with fiber photometry has exploded with the development of an extensive range of fluorescent sensors for biomolecules including neuromodulators and peptides that were previously inaccessible in vivo. This critical advance, combined with the new availability of affordable "plug-and-play" recording systems, has made monitoring molecules with high spatiotemporal precision during behavior highly accessible. However, while opening exciting new avenues for research, the rapid expansion in fiber photometry applications has occurred without coordination or consensus on best practices. Here, we provide a comprehensive guide to help end-users execute, analyze, and suitably interpret fiber photometry studies.

Striatal α5 nicotinic receptor subunit regulates dopamine transmission in dorsal striatum.

Polymorphisms in the gene for the α5 nicotinic acetylcholine receptor (nAChR) subunit are associated with vulnerability to nicotine addiction. However, the underlying normal functions of α5-containing nAChRs in the brain are poorly understood. Striatal dopamine (DA) transmission is critical to the acquisition and maintenance of drug addiction and is modulated strongly by nicotine acting at heteromeric ß2-containing (ß2*) nAChRs. We explored whether α5 subunits, as well as α4, α6, and ß3 subunits, participate in the powerful regulation of DA release probability by ß2* nAChRs in nucleus accumbens (NAc) core and in dorsal striatum [caudatoputamen (CPu)]. We detected evoked dopamine release using fast-scan cyclic voltammetry at carbon-fiber microelectrodes in striatal slices from mice with deletions of α4, α5, α6, or ß3 subunits. We show that the nAChR subtypes that dominantly regulate dopamine transmission depend critically upon α5 subunits in the dorsal CPu in α4α5(non-α6)ß2-nAChRs but not in NAc core, where α4α6ß2ß3-nAChRs are required. These data reveal the distinct populations of nAChRs that govern DA transmission in NAc core versus dorsal CPu. Furthermore, they indicate that α5 subunits are critical to the regulation of DA transmission by α4ß2* nAChRs in regions of striatum associated with habitual and instrumental responses (dorsal CPu) rather than pavlovian associations (NAc).

Striatal dopamine transmission is reduced after chronic nicotine with a decrease in α6-nicotinic receptor control in nucleus accumbens.

Nicotine directly regulates striatal dopamine (DA) neurotransmission via presynaptic nicotinic acetylcholine receptors (nAChRs) that are α6ß2 and/or α4ß2 subunit-containing, depending on region. Chronic nicotine exposure in smokers upregulates striatal nAChR density, with some reports suggesting differential impact on α6- or α4-containing nAChRs. Here, we explored whether chronic nicotine exposure modifies striatal DA transmission, whether the effects of acute nicotine on DA release probability persist and whether there are modifications to the regulation of DA release by α6-subunit-containing (*) relative to non-α6* nAChRs in nucleus accumbens (NAc) and in caudate-putamen (CPu). We detected electrically evoked DA release at carbon-fiber microelectrodes in striatal slices from mice exposed for 4-8 weeks to nicotine (200 µg/mL in saccharin-sweetened drinking water) or a control saccharin solution. Chronic nicotine exposure subtly reduced striatal DA release evoked by single electrical pulses, and in NAc enhanced the range of DA release evoked by different frequencies. Effects of acute nicotine (500 nm) on DA release probability and its sensitivity to activity were apparent. However, in NAc there was downregulation of the functional dominance of α6-nAChRs (α6α4ß2ß3), and an emergence in function of non-α6* nAChRs. In CPu, there was no change in the control of DA release by its α6 nAChRs (α6ß2ß3) relative to non-α6. These data suggest that chronic nicotine subtly modifies the regulation of DA transmission, which, in NAc, is through downregulation of function of a susceptible population of α6α4ß2ß3 nAChRs. This imbalance in function of α6:non-α6 nAChRs might contribute to DA dysregulation in nicotine addiction.