Enhanced Sensitivity to Hyperpolarizing Inhibition in Mesoaccumbal Relative to Nigrostriatal Dopamine Neuron Subpopulations

Muscarinic Receptor Activation Preferentially Inhibits Rebound in Vulnerable Dopaminergic Neurons

Dopaminergic subpopulations of the substantia nigra pars compacta (SNc) differentially degenerate in Parkinson's disease and are characterized by unique electrophysiological properties. The vulnerable population expresses a T-type calcium channel-mediated afterdepolarization (ADP) and shows rebound activity upon release from inhibition, whereas the resilient population does not have an ADP and is slower to fire after hyperpolarization. This rebound activity can trigger dopamine release in the striatum, an important component of basal ganglia function. Using whole-cell patch-clamp electrophysiology on ex vivo slices from adult mice of both sexes, we find that muscarinic activation with the nonselective muscarinic agonist oxotremorine inhibits rebound activity more strongly in vulnerable versus resilient SNc neurons. Here, we show that this effect depends on the direct activation of muscarinic receptors on the SNc dopaminergic neurons. Through a series of pharmacological and transgenic knock-out experiments, we tested whether the muscarinic inhibition of rebound was mediated through the canonical rebound-related ion channels: T-type calcium channels, hyperpolarization-activated cation channels (HCN), and A-type potassium channels. We find that muscarinic receptor activation inhibits HCN-mediated current (I h) in vulnerable SNc neurons but that I h activity is not necessary for the muscarinic inhibition of rebound activity. Similarly, we find that oxotremorine inhibits rebound activity independently of T-type calcium channels and A-type potassium channels. Together these findings reveal new principles governing acetylcholine and dopamine interactions, showing that muscarinic receptors directly affect SNc rebound activity in the midbrain at the somatodendritic level and differentially modify information processing in distinct SNc subpopulations.

The cation channel mechanisms of subthreshold inward depolarizing currents in the mice VTA dopaminergic neurons and their roles in the chronic-stress-induced depression-like behavior

The slow-intrinsic-pacemaker dopaminergic (DA) neurons originating in the ventral tegmental area (VTA) are implicated in various mood- and emotion-related disorders, such as anxiety, fear, stress and depression. Abnormal activity of projection-specific VTA DA neurons is the key factor in the development of these disorders. Here, we describe the crucial role of the NALCN and TRPC6, non-selective cation channels in mediating the subthreshold inward depolarizing current and driving the firing of action potentials of VTA DA neurons in physiological conditions. Furthermore, we demonstrate that down-regulation of TRPC6 protein expression in the VTA DA neurons likely contributes to the reduced activity of projection-specific VTA DA neurons in chronic mild unpredictable stress (CMUS) depressive mice. In consistent with these, selective knockdown of TRPC6 channels in the VTA DA neurons conferred mice with depression-like behavior. This current study suggests down-regulation of TRPC6 expression/function is involved in reduced VTA DA neuron firing and chronic stress-induced depression-like behavior of mice.

VTA dopamine neurons are hyperexcitable in 3xTg-AD mice due to casein kinase 2-dependent SK channel dysfunction

Alzheimer's disease (AD) patients exhibit neuropsychiatric symptoms that extend beyond classical cognitive deficits, suggesting involvement of subcortical areas. Here, we investigated the role of midbrain dopamine (DA) neurons in AD using the amyloid + tau-driven 3xTg-AD mouse model. We found deficits in reward-based operant learning in AD mice, suggesting possible VTA DA neuron dysregulation. Physiological assessment revealed hyperexcitability and disrupted firing in DA neurons caused by reduced activity of small-conductance calcium-activated potassium (SK) channels. RNA sequencing from contents of single patch-clamped DA neurons (Patch-seq) identified up-regulation of the SK channel modulator casein kinase 2 (CK2), which we corroborated by immunohistochemical protein analysis. Pharmacological inhibition of CK2 restored SK channel activity and normal firing patterns in 3xTg-AD mice. These findings identify a mechanism of ion channel dysregulation in VTA DA neurons that could contribute to behavioral abnormalities in AD, paving the way for novel treatment strategies.

Corticonigral projections recruit substantia nigra pars lateralis dopaminergic neurons for auditory threat memories

Dopaminergic neurons (DANs) in the lateral substantia nigra project to the tail of striatum (TS), which is involved in threat conditioning. Auditory cortex also contributes to threatening behaviors, but whether it directly interacts with midbrain DANs and how these interactions might influence threat conditioning remain unclear. Here, functional mapping revealed robust excitatory input from auditory and temporal association cortexes to substantia nigra pars lateralis (SNL) DANs, but not to pars compacta (SNc) DANs. SNL DANs exhibited unique firing patterns, with irregular pacemaking and higher maximal firing, reflecting different channel complements than SNc DANs. Behaviorally, inhibiting cortex to SNL projections impaired memory retrieval during auditory threat conditioning. Thus, we demonstrate robust corticonigral projections to SNL DANs, contrasting with previous observations of sparse cortical input to substantia nigra DANs. These findings distinguish SNL DANs from other nigral populations, highlighting their role in threatening behaviors and expanding knowledge of cortex to midbrain interactions.

Muscarinic receptor activation preferentially inhibits rebound in vulnerable dopaminergic neurons

Dopaminergic subpopulations of the substantia nigra pars compacta (SNc) differentially degenerate in Parkinson's disease and are characterized by unique electrophysiological properties. The vulnerable population expresses a T-type calcium channel-mediated afterdepolarization (ADP) and shows rebound activity upon release from inhibition, whereas the resilient population does not have an ADP and is slower to fire after hyperpolarization. This rebound activity can trigger dopamine release in the striatum, an important component of basal ganglia function. Using whole-cell patch clamp electrophysiology on ex vivo slices from adult mice of both sexes, we find that muscarinic activation with the non-selective muscarinic agonist Oxotremorine inhibits rebound activity more strongly in vulnerable vs resilient SNc neurons. Here, we show that this effect depends on the direct activation of muscarinic receptors on the SNc dopaminergic neurons. Through a series of pharmacological and transgenic knock-out experiments, we tested whether the muscarinic inhibition of rebound was mediated through the canonical rebound-related ion channels: T-type calcium channels, hyperpolarization-activated cation channels (HCN), and A-type potassium channels. We find that muscarinic receptor activation inhibits HCN-mediated current (Ih) in vulnerable SNc neurons, but that Ih activity is not necessary for the muscarinic inhibition of rebound activity. Similarly, we find that Oxotremorine inhibits rebound activity independently of T-type calcium channels and A-type potassium channels. Together these findings reveal new principles governing acetylcholine and dopamine interactions, showing that muscarinic receptors directly affect SNc rebound activity in the midbrain at the somatodendritic level and differentially modify information processing in distinct SNc subpopulations.

Impact of Unitary Synaptic Inhibition on Spike Timing in Ventral Tegmental Area Dopamine Neurons

Midbrain dopamine neurons receive convergent synaptic input from multiple brain areas, which perturbs rhythmic pacemaking to produce the complex firing patterns observed in vivo. This study investigated the impact of single and multiple inhibitory inputs on ventral tegmental area (VTA) dopamine neuron firing in mice of both sexes using novel experimental measurements and modeling. We first measured unitary inhibitory postsynaptic currents produced by single axons using both minimal electrical stimulation and minimal optical stimulation of rostromedial tegmental nucleus and ventral pallidum afferents. We next determined the phase resetting curve, the reversal potential for GABAA receptor-mediated inhibitory postsynaptic currents (IPSCs), and the average interspike membrane potential trajectory during pacemaking. We combined these data in a phase oscillator model of a VTA dopamine neuron, simulating the effects of unitary inhibitory postsynaptic conductances (uIPSGs) on spike timing and rate. The effect of a uIPSG on spike timing was predicted to vary according to its timing within the interspike interval or phase. Simulations were performed to predict the pause duration resulting from the synchronous arrival of multiple uIPSGs and the changes in firing rate and regularity produced by asynchronous uIPSGs. The model data suggest that asynchronous inhibition is more effective than synchronous inhibition, because it tends to hold the neuron at membrane potentials well positive to the IPSC reversal potential. Our results indicate that small fluctuations in the inhibitory synaptic input arriving from the many afferents to each dopamine neuron are sufficient to produce highly variable firing patterns, including pauses that have been implicated in reinforcement.

VTA dopamine neurons are hyperexcitable in 3xTg-AD mice due to casein kinase 2-dependent SK channel dysfunction

Alzheimer's disease (AD) patients exhibit neuropsychiatric symptoms that extend beyond classical cognitive deficits, suggesting involvement of subcortical areas. Here, we investigated the role of midbrain dopamine (DA) neurons in AD using the amyloid + tau-driven 3xTg-AD mouse model. We found deficits in reward-based operant learning in AD mice, suggesting possible VTA DA neuron dysregulation. Physiological assessment revealed hyperexcitability and disrupted firing in DA neurons caused by reduced activity of small-conductance calcium-activated potassium (SK) channels. RNA sequencing from contents of single patch-clamped DA neurons (Patch-seq) identified up-regulation of the SK channel modulator casein kinase 2 (CK2). Pharmacological inhibition of CK2 restored SK channel activity and normal firing patterns in 3xTg-AD mice. These findings shed light on a complex interplay between neuropsychiatric symptoms and subcortical circuits in AD, paving the way for novel treatment strategies.

Neuronal implementation of the temporal difference learning algorithm in the midbrain dopaminergic system

The temporal difference learning (TDL) algorithm has been essential to conceptualizing the role of dopamine in reinforcement learning (RL). Despite its theoretical importance, it remains unknown whether a neuronal implementation of this algorithm exists in the brain. Here, we provide an interpretation of the recently described signaling properties of ventral tegmental area (VTA) GABAergic neurons and show that a circuitry of these neurons implements the TDL algorithm. Specifically, we identified the neuronal mechanism of three key components of the TDL model: a sustained state value signal encoded by an afferent input to the VTA, a temporal differentiation circuit formed by two types of VTA GABAergic neurons the combined output of which computes momentary reward prediction (RP) as the derivative of the state value, and the computation of reward prediction errors (RPEs) in dopamine neurons utilizing the output of the differentiation circuit. Using computational methods, we also show that this mechanism is optimally adapted to the biophysics of RPE signaling in dopamine neurons, mechanistically links the emergence of conditioned reinforcement to RP, and can naturally account for the temporal discounting of reinforcement. Elucidating the implementation of the TDL algorithm may further the investigation of RL in biological and artificial systems.

Estrous Cycle Mediates Midbrain Neuron Excitability Altering Social Behavior upon Stress

The estrous cycle is a potent modulator of neuron physiology. In rodents, in vivo ventral tegmental area (VTA) dopamine (DA) activity has been shown to fluctuate across the estrous cycle. Although the behavioral effect of fluctuating sex steroids on the reward circuit is well studied in response to drugs of abuse, few studies have focused on the molecular adaptations in the context of stress and motivated social behaviors. We hypothesized that estradiol fluctuations across the estrous cycle acts on the dopaminergic activity of the VTA to alter excitability and stress response. We used whole-cell slice electrophysiology of VTA DA neurons in naturally cycling, adult female C57BL/6J mice to characterize the effects of the estrous cycle and the role of 17β-estradiol on neuronal activity. We show that the estrous phase alters the effect of 17β-estradiol on excitability in the VTA. Behaviorally, the estrous phase during a series of acute variable social stressors modulates subsequent reward-related behaviors. Pharmacological inhibition of estrogen receptors in the VTA before stress during diestrus mimics the stress susceptibility found during estrus, whereas increased potassium channel activity in the VTA before stress reverses stress susceptibility found during estrus as assessed by social interaction behavior. This study identifies one possible potassium channel mechanism underlying the increased DA activity during estrus and reveals estrogen-dependent changes in neuronal function. Our findings demonstrate that the estrous cycle and estrogen signaling changes the physiology of DA neurons resulting in behavioral differences when the reward circuit is challenged with stress.SIGNIFICANCE STATEMENT The activity of the ventral tegmental area encodes signals of stress and reward. Dopaminergic activity has been found to be regulated by both local synaptic inputs as well as inputs from other brain regions. Here, we provide evidence that cycling sex steroids also play a role in modulating stress sensitivity of dopaminergic reward behavior. Specifically, we reveal a correlation of ionic activity with estrous phase, which influences the behavioral response to stress. These findings shed new light on how estrous cycle may influence dopaminergic activity primarily during times of stress perturbation.

Extracellular S100B inhibits A-type voltage-gated potassium currents and increases L-type voltage-gated calcium channel activity in dopaminergic neurons

Parkinson's disease (PD) is associated with an increase in secreted S100B within the midbrain and cerebrospinal fluid. In addition, S100B overexpression in mice accelerates the loss of substantia nigra pars compacta dopaminergic (DA) neurons, suggesting a role for this protein in PD pathogenesis. We found that in the mouse SNc, S100B labeled astrocytic processes completely envelop the somata of tyrosine hydroxylase (TH) expressing DA neurons only in male mice. These data suggest that an increase in S100B secretion by astrocytes within the midbrain could play a role in DA dysfunction during early PD. We therefore asked if acute exposure to extracellular S100B alters the activity of identified TH expressing DA neurons in primary mouse midbrain cultures. Acute exposure to 50 pM S100B specifically inhibited A-type voltage-gated potassium currents in TH+ , but not TH- neurons. This was accompanied by ~2-fold increases in the frequency of both intrinsic firing, as well as L-type voltage-gated calcium channel (VGCC)-mediated calcium fluxes only in TH+ neurons. Further, exposure to 100 μM 4-aminopyridine (4-AP), an A-type voltage-gated potassium channel inhibitor, mimicked the S100B mediated increase in intrinsic firing and L-type VGCC-mediated calcium fluxes in TH+ neurons. Taken together, our finding that extracellular S100B alters the activity of native DA neurons via an inhibition of A-type voltage-gated potassium channels has important implications for understanding the pathophysiology of early PD.

Striatonigrostriatal circuit architecture for disinhibition of dopamine signaling

The basal ganglia operate largely in closed parallel loops, including an associative circuit for goal-directed behavior originating from the dorsomedial striatum (DMS) and a somatosensory circuit important for habit formation originating from the dorsolateral striatum (DLS). An exception to this parallel circuit organization has been proposed to explain how information might be transferred between striatal subregions, for example, from the DMS to the DLS during habit formation. The "ascending spiral hypothesis" proposes that the DMS disinhibits dopamine signaling in the DLS through a tri-synaptic, open-loop striatonigrostriatal circuit. Here, we use transsynaptic and intersectional genetic tools to investigate both closed- and open-loop striatonigrostriatal circuits. We find strong evidence for closed loops, which would allow striatal subregions to regulate their own dopamine release. We also find evidence for functional synapses in open loops. However, these synapses are unable to modulate tonic dopamine neuron firing, questioning the prominence of their role in mediating crosstalk between striatal subregions.

Regulation of Rostral Nucleus of the Solitary Tract Responses to Afferent Input by A-type K+ Current

Responses in the rostral (gustatory) nucleus of the solitary tract (rNST) are modified by synaptic interactions within the nucleus and the constitutive membrane properties of the neurons themselves. The potassium current IA is one potential source of modulation. In the caudal NST, projection neurons with IA show lower fidelity to afferent stimulation compared to cells without. We explored the role of an A-type K+ current (IA) in modulating the response to afferent stimulation and GABA-mediated inhibition in the rNST using whole cell patch clamp recording in transgenic mice that expressed channelrhodopsin (ChR2 H134R) in GABAergic neurons. The presence of IA was determined in current clamp and the response to electrical stimulation of afferent fibers in the solitary tract was assessed before and after treatment with the specific Kv4 channel blocker AmmTX3. Blocking IA significantly increased the response to afferent stimulation by 53%. Using dynamic clamp to create a synthetic IA conductance, we demonstrated a significant 14% decrease in responsiveness to afferent stimulation in cells lacking IA. Because IA reduced excitability and is hyperpolarization-sensitive, we examined whether IA contributed to the inhibition resulting from optogenetic release of GABA. Although blocking IA decreased the percent suppression induced by GABA, this effect was attributable to the increased responsiveness resulting from AmmTX3, not to a change in the absolute magnitude of suppression. We conclude that rNST responses to afferent input are regulated independently by IA and GABA.

Daily electrical activity in the master circadian clock of a diurnal mammal

Circadian rhythms in mammals are orchestrated by a central clock within the suprachiasmatic nuclei (SCN). Our understanding of the electrophysiological basis of SCN activity comes overwhelmingly from a small number of nocturnal rodent species, and the extent to which these are retained in day-active animals remains unclear. Here, we recorded the spontaneous and evoked electrical activity of single SCN neurons in the diurnal rodent Rhabdomys pumilio, and developed cutting-edge data assimilation and mathematical modeling approaches to uncover the underlying ionic mechanisms. As in nocturnal rodents, R. pumilio SCN neurons were more excited during daytime hours. By contrast, the evoked activity of R. pumilio neurons included a prominent suppressive response that is not present in the SCN of nocturnal rodents. Our modeling revealed and subsequent experiments confirmed transient subthreshold A-type potassium channels as the primary determinant of this response, and suggest a key role for this ionic mechanism in optimizing SCN function to accommodate R. pumilio's diurnal niche.

How neurons adjust to diurnality

Being active during the day requires a slow-closing ion channel that dampens the activity of neurons in a specific area of the brain.

Inactivation mode of sodium channels defines the different maximal firing rates of conventional versus atypical midbrain dopamine neurons

Two subpopulations of midbrain dopamine (DA) neurons are known to have different dynamic firing ranges in vitro that correspond to distinct projection targets: the originally identified conventional DA neurons project to the dorsal striatum and the lateral shell of the nucleus accumbens, whereas an atypical DA population with higher maximum firing frequencies projects to prefrontal regions and other limbic regions including the medial shell of nucleus accumbens. Using a computational model, we show that previously identified differences in biophysical properties do not fully account for the larger dynamic range of the atypical population and predict that the major difference is that originally identified conventional cells have larger occupancy of voltage-gated sodium channels in a long-term inactivated state that recovers slowly; stronger sodium and potassium conductances during action potential firing are also predicted for the conventional compared to the atypical DA population. These differences in sodium channel gating imply that longer intervals between spikes are required in the conventional population for full recovery from long-term inactivation induced by the preceding spike, hence the lower maximum frequency. These same differences can also change the bifurcation structure to account for distinct modes of entry into depolarization block: abrupt versus gradual. The model predicted that in cells that have entered depolarization block, it is much more likely that an additional depolarization can evoke an action potential in conventional DA population. New experiments comparing lateral to medial shell projecting neurons confirmed this model prediction, with implications for differential synaptic integration in the two populations.

We developed a theoretical and mathematical framework that could explain the major electrophysiological differences between the conventional midbrain dopamine (DA) neurons with a low maximum firing rate, and the more recently identified atypical DA neurons. Testable predictions from this framework were then verified with in vitro patch-clamp recordings from DA neurons with identified phenotypes and projection targets. Since different subpopulations of DA neurons participate in different circuits, and these circuits are likely differentially dysregulated in diseases such as addiction, Parkinson disease, and schizophrenia, it is important to identify the differences of their intrinsic electrophysiological properties as a prelude to developing more precisely targeted therapies.

Refining the Identity and Role of Kv4 Channels in Mouse Substantia Nigra Dopaminergic Neurons

Substantia nigra pars compacta (SNc) dopaminergic (DA) neurons display a peculiar electrical phenotype characterized in vitro by a spontaneous tonic regular activity (pacemaking activity), a broad action potential (AP) and a biphasic postinhibitory response. The transient A-type current (I_A) is known to play a crucial role in this electrical phenotype, and so far, this current was considered to be carried exclusively by Kv4.3 potassium channels. Using Kv4.3−/− transgenic mice, we demonstrate that the constitutive loss of this channel is associated with increased exploratory behavior and impaired motor learning at the behavioral level. Consistently, it is also associated with a lack of compensatory changes in other ion currents at the cellular level. Using antigen retrieval (AR) immunohistochemistry, we then demonstrate that Kv4.2 potassium channels are also expressed in SNc DA neurons, although their contribution to I_A appears significant only in a minority of neurons (∼5–10%). Using correlative analysis on recorded electrophysiological parameters and multicompartment modeling, we then demonstrate that, rather than its conductance level, I_A gating kinetics (inactivation time constant) appear as the main biophysical property defining postinhibitory rebound delay and pacemaking frequency. Moreover, we show that the hyperpolarization-activated current (I_H) has an opposing and complementary influence on the same firing features.

The gating pore blocker 1-(2,4-xylyl)guanidinium selectively inhibits pacemaking of midbrain dopaminergic neurons

Although several ionic mechanisms are known to control rate and regularity of the slow pacemaker in dopamine (DA) neurons, the core mechanism of pacing is controversial. Here we tested the hypothesis that pacemaking of SNc DA neurons is enabled by an unconventional conductance. We found that 1-(2,4-xylyl)guanidinium (XG), an established blocker of gating pore currents, selectively inhibits pacemaking of DA neurons. The compound inhibited all slow pacemaking DA neurons that were tested, both in the substantia nigra pars compacta, and in the ventral tegmental area. Interestingly, bursting behavior was not affected by XG. Furthermore, the drug did not affect fast pacemaking of GABAergic neurons from substantia nigra pars reticulata neurons or slow pacemaking of noradrenergic neurons. In DA neurons, current-clamp analysis revealed that XG did not appear to affect ion channels involved in the action potential. Its inhibitory effect persisted during blockade of all ion channels previously suggested to contribute to pacemaking. RNA sequencing and voltage-clamp recordings yielded no evidence for a gating pore current to underlie the conductance. However, we could isolate a small subthreshold XG-sensitive current, which was carried by both Na⁺ and Cl^- ions. Although the molecular target of XG remains to be defined, these observations represent a step towards understanding pacemaking in DA neurons.

Dopamine Transporter Is a Master Regulator of Dopaminergic Neural Network Connectivity

Dopaminergic neurons of the substantia nigra pars compacta (SNC) and ventral tegmental area (VTA) exhibit spontaneous firing activity. The dopaminergic neurons in these regions have been shown to exhibit differential sensitivity to neuronal loss and psychostimulants targeting dopamine transporter. However, it remains unclear whether these regional differences scale beyond individual neuronal activity to regional neuronal networks. Here, we used live-cell calcium imaging to show that network connectivity greatly differs between SNC and VTA regions with higher incidence of hub-like neurons in the VTA. Specifically, the frequency of hub-like neurons was significantly lower in SNC than in the adjacent VTA, consistent with the interpretation of a lower network resilience to SNC neuronal loss. We tested this hypothesis, in DAT-cre/loxP-GCaMP6f mice of either sex, when activity of an individual dopaminergic neuron is suppressed, through whole-cell patch clamp electrophysiology, in either SNC or VTA networks. Neuronal loss in the SNC increased network clustering, whereas the larger number of hub-neurons in the VTA overcompensated by decreasing network clustering in the VTA. We further show that network properties are regulatable via a dopamine transporter but not a D2 receptor dependent mechanism. Our results demonstrate novel regulatory mechanisms of functional network topology in dopaminergic brain regions.SIGNIFICANCE STATEMENT In this work, we begin to untangle the differences in complex network properties between the substantia nigra pars compacta (SNC) and VTA, that may underlie differential sensitivity between regions. The methods and analysis employed provide a springboard for investigations of network topology in multiple deep brain structures and disorders.

In vivo patch-clamp recordings reveal distinct subthreshold signatures and threshold dynamics of midbrain dopamine neurons

The in vivo firing patterns of ventral midbrain dopamine neurons are controlled by afferent and intrinsic activity to generate sensory cue and prediction error signals that are essential for reward-based learning. Given the absence of in vivo intracellular recordings during the last three decades, the subthreshold membrane potential events that cause changes in dopamine neuron firing patterns remain unknown. To address this, we established in vivo whole-cell recordings and obtained over 100 spontaneously active, immunocytochemically-defined midbrain dopamine neurons in isoflurane-anaesthetized adult mice. We identified a repertoire of subthreshold membrane potential signatures associated with distinct in vivo firing patterns. Dopamine neuron activity in vivo deviated from single-spike pacemaking by phasic increases in firing rate via two qualitatively distinct biophysical mechanisms: 1) a prolonged hyperpolarization preceding rebound bursts, accompanied by a hyperpolarizing shift in action potential threshold; and 2) a transient depolarization leading to high-frequency plateau bursts, associated with a depolarizing shift in action potential threshold. Our findings define a mechanistic framework for the biophysical implementation of dopamine neuron firing patterns in the intact brain.

Kv4 channel expression and kinetics in GABAergic and non-GABAergic rNST neurons

The rostral nucleus of the solitary tract (rNST) serves as the first central relay in the gustatory system. In addition to synaptic interactions, central processing is also influenced by the ion channel composition of individual neurons. For example, voltage-gated K⁺ channels such as outward K⁺ current (I_A) can modify the integrative properties of neurons. I_A currents are prevalent in rNST projection cells but are also found to a lesser extent in GABAergic interneurons. However, characterization of the kinetic properties of I_A, the molecular basis of these currents, as well as the consequences of I_A on spiking properties of identified rNST cells is lacking. Here, we show that I_A in rNST GABAergic (G+) and non-GABAergic (G-) neurons share a common molecular basis. In both cell types, there was a reduction in I_A following treatment with the specific Kv4 channel blocker AmmTx3. However, the kinetics of activation and inactivation of I_A in the two cell types were different with G- neurons having significantly more negative half-maximal activation and inactivation values. Likewise, under current clamp, G- cells had significantly longer delays to spike initiation in response to a depolarizing stimulus preceded by a hyperpolarizing prepulse. Computational modeling and dynamic clamp suggest that differences in the activation half-maximum may account for the differences in delay. We further observed evidence for a window current under both voltage clamp and current clamp protocols. We speculate that the location of Kv4.3 channels on dendrites, together with a window current for I_A at rest, serves to regulate excitatory afferent inputs.NEW & NOTEWORTHY Here, we demonstrate that the transient outward K⁺ current I_A occurs in both GABAergic and non-GABAergic neurons via Kv4.3 channels in the rostral (gustatory) solitary nucleus. Although found in both cell types, I_A is more prevalent in non-GABAergic cells; a larger conductance at more negative potentials leads to a greater impact on spike initiation compared with GABAergic neurons. An I_A window current further suggests that I_A can regulate excitatory afferent input to the nucleus.

Functional Dissection of Basal Ganglia Inhibitory Inputs onto Substantia Nigra Dopaminergic Neurons

Substantia nigra (SNc) dopaminergic neurons respond to aversive stimuli with inhibitory pauses in firing followed by transient rebound activation. We tested integration of inhibitory synaptic inputs onto SNc neurons from genetically defined populations in dorsal striatum (striosome and matrix) and external globus pallidus (GPe; parvalbumin- and Lhx6-positive), and examined their contribution to pause-rebound firing. Activation of striosome projections, which target "dendron bouquets" in the pars reticulata (SNr), consistently quiets firing and relief from striosome inhibition triggers rebound activity. Striosomal inhibitory postsynaptic currents (IPSCs) display a prominent GABA-B receptor-mediated component that strengthens the impact of SNr dendrite synapses on somatic excitability and enables rebounding. By contrast, GPe projections activate GABA-A receptors on the soma and proximal dendrites but do not result in rebounding. Lastly, optical mapping shows that dorsal striatum selectively inhibits the ventral population of SNc neurons, which are intrinsically capable of rebounding. Therefore, we define a distinct striatonigral circuit for generating dopamine rebound.

Heterogeneity of dopamine release sites in health and degeneration

Despite comprising only ~ 0.001% of all neurons in the human brain, ventral midbrain dopamine neurons exert a profound influence on human behavior and cognition. As a neuromodulator, dopamine selectively inhibits or enhances synaptic signaling to coordinate neural output for action, attention, and affect. Humans invariably lose brain dopamine during aging, and this can be exacerbated in disease states such as Parkinson's Disease. Further, it is well established in multiple disease states that cell loss is selective for a subset of highly sensitive neurons within the nigrostriatal dopamine tract. Regional differences in dopamine tone are regulated pre-synaptically, with subcircuits of projecting dopamine neurons exhibiting distinct molecular and physiological signatures. Specifically, proteins at dopamine release sites that synthesize and package cytosolic dopamine, modulate its release and reuptake, and alter neuronal excitability show regional differences that provide linkages to the observed sensitivity to neurodegeneration. The aim of this review is to outline the major components of dopamine homeostasis at neurotransmitter release sites and describe the regional differences most relevant to understanding why some, but not all, dopamine neurons exhibit heightened vulnerability to neurodegeneration.

In vivo functional diversity of midbrain dopamine neurons within identified axonal projections

Functional diversity of midbrain dopamine (DA) neurons ranges across multiple scales, from differences in intrinsic properties and connectivity to selective task engagement in behaving animals. Distinct in vitro biophysical features of DA neurons have been associated with different axonal projection targets. However, it is unknown how this translates to different firing patterns of projection-defined DA subpopulations in the intact brain. We combined retrograde tracing with single-unit recording and labelling in mouse brain to create an in vivo functional topography of the midbrain DA system. We identified differences in burst firing among DA neurons projecting to dorsolateral striatum. Bursting also differentiated DA neurons in the medial substantia nigra (SN) projecting either to dorsal or ventral striatum. We found differences in mean firing rates and pause durations among ventral tegmental area (VTA) DA neurons projecting to lateral or medial shell of nucleus accumbens. Our data establishes a high-resolution functional in vivo landscape of midbrain DA neurons.

Dimensions of control for subthreshold oscillations and spontaneous firing in dopamine neurons

Dopaminergic neurons (DAs) of the rodent substantia nigra pars compacta (SNc) display varied electrophysiological properties in vitro. Despite this, projection patterns and functional inputs from DAs to other structures are conserved, so in vivo delivery of consistent, well-timed dopamine modulation to downstream circuits must be coordinated. Here we show robust coordination by linear parameter controllers, discovered through powerful mathematical analyses of data and models, and from which consistent control of DA subthreshold oscillations (STOs) and spontaneous firing emerges. These units of control represent coordinated intracellular variables, sufficient to regulate complex cellular properties with radical simplicity. Using an evolutionary algorithm and dimensionality reduction, we discovered metaparameters, which when regressed against STO features, revealed a 2-dimensional control plane for the neuron’s 22-dimensional parameter space that fully maps the natural range of DA subthreshold electrophysiology. This plane provided a basis for spiking currents to reproduce a large range of the naturally occurring spontaneous firing characteristics of SNc DAs. From it we easily produced a unique population of models, derived using unbiased parameter search, that show good generalization to channel blockade and compensatory intracellular mechanisms. From this population of models, we then discovered low-dimensional controllers for regulating spontaneous firing properties, and gain insight into how currents active in different voltage regimes interact to produce the emergent activity of SNc DAs. Our methods therefore reveal simple regulators of neuronal function lurking in the complexity of combined ion channel dynamics.

Electrophysiological activity of the neuronal membrane and concomitant ion channel properties are highly variable within groups of neurons of the same type from the same brain region. Reconciliation of the mechanisms generating neuronal activity is challenging due to the complexity of the interactions between the channel currents involved. Here we present a set of mathematical analyses that uncover the low-dimensional intracellular parameter combinations capable of regulating features of subthreshold oscillations and spontaneous firing in empirically constrained models of nigral dopaminergic neurons. This method generates, from a naive starting point, linear combinations of ion channel properties that are surprisingly capable of reliably controlling a wide variety of emergent electrophysiological activity, thereby predicting drug effects and shedding light on unsuspected compensatory mechanisms that contribute to neuronal function.

Inhibitory Plasticity of Mesocorticolimbic Circuits in Addiction and Mental Illness

Behavioral adaptations occur through remodeling of brain circuits, as arising, for instance, from experience-dependent synaptic plasticity. Drugs of abuse and aversive stimuli, such as stress, act on the mesocorticolimbic system, dysregulating adaptive mechanisms and leading to a variety of aberrant behaviors associated with neuropsychiatric disorders. Until recently, research in the field has commonly focused on experience-dependent synaptic plasticity at excitatory synapses. However, there is growing evidence that synaptic plasticity within inhibitory circuits is an important contributor to maladaptive behaviors. We speculate that restoring normal inhibitory synaptic transmission is a promising therapeutic target for correcting some of the circuit abnormalities underlying neuropsychiatric disorders.

The Evolving Understanding of Dopamine Neurons in the Substantia Nigra and Ventral Tegmental Area

In recent years, the population of neurons in the ventral tegmental area (VTA) and substantia nigra (SN) has been examined at multiple levels. The results indicate that the projections, neurochemistry, and receptor and ion channel expression in this cell population vary widely. This review centers on the intrinsic properties and synaptic regulation that control the activity of dopamine neurons. Although all dopamine neurons fire action potentials in a pacemaker pattern in the absence of synaptic input, the intrinsic properties that underlie this activity differ considerably. Likewise, the transition into a burst/pause pattern results from combinations of intrinsic ion conductances, inhibitory and excitatory synaptic inputs that differ among this cell population. Finally, synaptic plasticity is a key regulator of the rate and pattern of activity in different groups of dopamine neurons. Through these fundamental properties, the activity of dopamine neurons is regulated and underlies the wide-ranging functions that have been attributed to dopamine.