Dopamine neuron responses depend exponentially on pacemaker interval

Dopamine transporter blockade during adolescence increases adult dopamine function, impulsivity, and aggression

Sensitive developmental periods shape neural circuits and enable adaptation. However, they also engender vulnerability to factors that can perturb developmental trajectories. An understanding of sensitive period phenomena and mechanisms separate from sensory system development is still lacking, yet critical to understanding disease etiology and risk. The dopamine system is pivotal in controlling and shaping adolescent behaviors, and it undergoes heightened plasticity during that time, such that interference with dopamine signaling can have long-lasting behavioral consequences. Here we sought to gain mechanistic insight into this dopamine-sensitive period and its impact on behavior. In mice, dopamine transporter (DAT) blockade from postnatal (P) day 22 to 41 increases aggression and sensitivity to amphetamine (AMPH) behavioral stimulation in adulthood. Here, we refined this sensitive window to P32-41 and identified increased firing of dopaminergic neurons in vitro and in vivo as a neural correlate to altered adult behavior. Aggression can result from enhanced impulsivity and cognitive dysfunction, and dopamine regulates working memory and motivated behavior. Hence, we assessed these behavioral domains and found that P32-41 DAT blockade increases impulsivity but has no effect on cognition, working memory, or motivation in adulthood. Lastly, using optogenetics to drive dopamine neurons, we find that increased VTA but not SNc dopaminergic activity mimics the increase in impulsive behavior in the Go/NoGo task observed after adolescent DAT blockade. Together our data provide insight into the developmental origins of aggression and impulsivity that may ultimately improve diagnosis, prevention, and treatment strategies for related neuropsychiatric disorders.

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.

Functioning of K channels during sleep

The functioning of voltage-dependent K channels (Kv) may correlate with the physiological state of brain in organisms, including the sleep in Drosophila. Apparently, all major types of K currents are expressed in CNS of this model organism. These are the Shab-Kv2, Shaker-Kv1, Shal-Kv4, and Shaw-Kv3 α subunits and can be deciphered by patch-clamp technique. Although it is plausible that some of these channels may play a prevailing role in sleep or wakefulness, several of recent data are not conclusive. It needs to be defined that indeed the frequency of action potentials in large ventral lateral pacemaker neurons is either higher or lower during the morning or night because of an increased Kv3 and Kv4 currents, respectively. The outcomes of dynamic-clamp approach in combination with electrophysiology in insects are unreliable in contrast to those in mammalian neurons. Since the addition of virtual Kv conductance during any Zeitgeber time should not significantly alter the resting membrane potential. This review explains the Drosophila sleep behavior based on neural activity with respect to K current-driven action potential rate.

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.

Interactions with PDZ proteins diversify voltage-gated calcium channel signaling

Voltage-gated Ca²⁺ (Ca_V ) channels are crucial for neuronal excitability and synaptic transmission upon depolarization. Their properties in vivo are modulated by their interaction with a variety of scaffolding proteins. Such interactions can influence the function and localization of Ca_V channels, as well as their coupling to intracellular second messengers and regulatory pathways, thus amplifying their signaling potential. Among these scaffolding proteins, a subset of PDZ (postsynaptic density-95, Drosophila discs-large, and zona occludens)-domain containing proteins play diverse roles in modulating Ca_V channel properties. At the presynaptic terminal, PDZ proteins enrich Ca_V channels in the active zone, enabling neurotransmitter release by maintaining a tight and vital link between channels and vesicles. In the postsynaptic density, these interactions are essential in regulating dendritic spine morphology and postsynaptic signaling cascades. In this review, we highlight the studies that demonstrate dynamic regulations of neuronal Ca_V channels by PDZ proteins. We discuss the role of PDZ proteins in controlling channel activity, regulating channel cell surface density, and influencing channel-mediated downstream signaling events. We highlight the importance of PDZ protein regulations of Ca_V channels and evaluate the link between this regulatory effect and human disease.

Robustness to Axon Initial Segment Variation Is Explained by Somatodendritic Excitability in Rat Substantia Nigra Dopaminergic Neurons

In many neuronal types, axon initial segment (AIS) geometry critically influences neuronal excitability. Interestingly, the axon of rat SNc dopaminergic (DA) neurons displays a highly variable location and most often arises from an axon-bearing dendrite (ABD). We combined current-clamp somatic and dendritic recordings, outside-out recordings of dendritic sodium and potassium currents, morphological reconstructions and multicompartment modeling on male and female rat SNc DA neurons to determine cell-to-cell variations in AIS and ABD geometry, and their influence on neuronal output (spontaneous pacemaking frequency, action potential [AP] shape). Both AIS and ABD geometries were found to be highly variable from neuron to neuron. Surprisingly, we found that AP shape and pacemaking frequency were independent of AIS geometry. Modeling realistic morphological and biophysical variations helped us clarify this result: in SNc DA neurons, the complexity of the ABD combined with its excitability predominantly define pacemaking frequency and AP shape, such that large variations in AIS geometry negligibly affect neuronal output and are tolerated.SIGNIFICANCE STATEMENT In many neuronal types, axon initial segment (AIS) geometry critically influences neuronal excitability. In the current study, we describe large cell-to-cell variations in AIS length or distance from the soma in rat substantia nigra pars compacta dopaminergic neurons. Using neuronal reconstruction and electrophysiological recordings, we show that this morphological variability does not seem to affect their electrophysiological output, as neither action potential properties nor pacemaking frequency is correlated with AIS morphology. Realistic multicompartment modeling suggests that this robustness to AIS variation is mainly explained by the complexity and excitability of the somatodendritic compartment.

Cell-Autonomous Excitation of Midbrain Dopamine Neurons by Endocannabinoid-Dependent Lipid Signaling

The major endocannabinoid in the mammalian brain is the bioactive lipid 2-arachidonoylglycerol (2-AG). The best-known effects of 2-AG are mediated by G-protein-coupled cannabinoid receptors. In principle, 2-AG could modify neuronal excitability by acting directly on ion channels, but such mechanisms are poorly understood. Using a preparation of dissociated mouse midbrain dopamine neurons to isolate effects on intrinsic excitability, we found that 100 nM 2-AG accelerated pacemaking and steepened the frequency-current relationship for burst-like firing. In voltage-clamp experiments, 2-AG reduced A-type potassium current (I_A) through a cannabinoid receptor-independent mechanism mimicked by arachidonic acid, which has no activity on cannabinoid receptors. Activation of orexin, neurotensin, and metabotropic glutamate G_q/11-linked receptors mimicked the effects of exogenous 2-AG and their actions were prevented by inhibiting the 2-AG-synthesizing enzyme diacylglycerol lipase α. The results show that 2-AG and related lipid signaling molecules can directly tune neuronal excitability in a cell-autonomous manner by modulating I_A.

Non-linear developmental trajectory of electrical phenotype in rat substantia nigra pars compacta dopaminergic neurons

Neurons have complex electrophysiological properties, however, it is often difficult to determine which properties are the most relevant to neuronal function. By combining current-clamp measurements of electrophysiological properties with multi-variate analysis (hierarchical clustering, principal component analysis), we were able to characterize the postnatal development of substantia nigra dopaminergic neurons' electrical phenotype in an unbiased manner, such that subtle changes in phenotype could be analyzed. We show that the intrinsic electrical phenotype of these neurons follows a non-linear trajectory reaching maturity by postnatal day 14, with two developmental transitions occurring between postnatal days 3-5 and 9-11. This approach also predicted which parameters play a critical role in phenotypic variation, enabling us to determine (using pharmacology, dynamic-clamp) that changes in the leak, sodium and calcium-activated potassium currents are central to these two developmental transitions. This analysis enables an unbiased definition of neuronal type/phenotype that is applicable to a range of research questions.

Somatodendritic ion channel expression in substantia nigra pars compacta dopaminergic neurons across postnatal development

Dopaminergic neurons of the substantia nigra pars compacta (SNc) are involved in the control of movement, sleep, reward, learning, and nervous system disorders and disease. To date, a thorough characterization of the ion channel phenotype of this important neuronal population is lacking. Using immunohistochemistry, we analyzed the somatodendritic expression of voltage-gated ion channel subunits that are involved in pacemaking activity in SNc dopaminergic neurons in 6-, 21-, and 40-day-old rats. Our results demonstrate that the same complement of somatodendritic ion channels is present in SNc dopaminergic neurons from P6 to P40. The major developmental changes were an increase in the dendritic range of the immunolabeling for the HCN, T-type calcium, Kv4.3, delayed rectifier, and SK channels. Our study sheds light on the ion channel subunits that contribute to the somatodendritic delayed rectifier (Kv1.3, Kv2.1, Kv3.2, Kv3.3), A-type (Kv4.3) and calcium-activated SK (SK1, SK2, SK3) potassium currents, IH (mainly HCN2, HCN4), and the L- (Cav1.2, Cav1.3) and T-type (mainly Cav3.1, Cav3.3) calcium currents in SNc dopaminergic neurons. Finally, no robust differences in voltage-gated ion channel immunolabeling were observed across the population of SNc dopaminergic neurons for each age examined, suggesting that differing levels of individual ion channels are unlikely to distinguish between specific subpopulations of SNc dopaminergic neurons. This is significant in light of previous studies suggesting that age- or region-associated variations in the expression profile of voltage-gated ion channels in SNc dopaminergic neurons may underlie their vulnerability to dysfunction and disease.

Pacemaker rate and depolarization block in nigral dopamine neurons: a somatic sodium channel balancing act

Midbrain dopamine (DA) neurons are slow intrinsic pacemakers that undergo depolarization (DP) block upon moderate stimulation. Understanding DP block is important because it has been correlated with the clinical efficacy of chronic antipsychotic drug treatment. Here we describe how voltage-gated sodium (Na(V)) channels regulate DP block and pacemaker activity in DA neurons of the substantia nigra using rat brain slices. The distribution, density, and gating of Na(V) currents were manipulated by blocking native channels with tetrodotoxin and by creating virtual channels and anti-channels with dynamic clamp. Although action potentials initiate in the axon initial segment and Na(V) channels are distributed in multiple dendrites, selective reduction of Na(V) channel activity in the soma was sufficient to decrease pacemaker frequency and increase susceptibility to DP block. Conversely, increasing somatic Na(V) current density raised pacemaker frequency and lowered susceptibility to DP block. Finally, when Na(V) currents were restricted to the soma, pacemaker activity occurred at abnormally high rates due to excessive local subthreshold Na(V) current. Together with computational simulations, these data show that both the slow pacemaker rate and the sensitivity to DP block that characterizes DA neurons result from the low density of somatic Na(V) channels. More generally, we conclude that the somatodendritic distribution of Na(V) channels is a major determinant of repetitive spiking frequency.

Functional characterization of ether-à-go-go-related gene potassium channels in midbrain dopamine neurons - implications for a role in depolarization block

Bursting activity by midbrain dopamine neurons reflects the complex interplay between their intrinsic pacemaker activity and synaptic inputs. Although the precise mechanism responsible for the generation and modulation of bursting in vivo has yet to be established, several ion channels have been implicated in the process. Previous studies with nonselective blockers suggested that ether-à-go-go-related gene (ERG) K(+) channels are functionally significant. Here, electrophysiology with selective chemical and peptide ERG channel blockers (E-4031 and rBeKm-1) and computational methods were used to define the contribution made by ERG channels to the firing properties of midbrain dopamine neurons in vivo and in vitro. Selective ERG channel blockade increased the frequency of spontaneous activity as well as the response to depolarizing current pulses without altering spike frequency adaptation. ERG channel block also accelerated entry into depolarization inactivation during bursts elicited by virtual NMDA receptors generated with the dynamic clamp, and significantly prolonged the duration of the sustained depolarization inactivation that followed pharmacologically evoked bursts. In vivo, somatic ERG blockade was associated with an increase in bursting activity attributed to a reduction in doublet firing. Taken together, these results show that dopamine neuron ERG K(+) channels play a prominent role in limiting excitability and in minimizing depolarization inactivation. As the therapeutic actions of antipsychotic drugs are associated with depolarization inactivation of dopamine neurons and blockade of cardiac ERG channels is a prominent side effect of these drugs, ERG channels in the central nervous system may represent a novel target for antipsychotic drug development.

Ca²⁺/cAMP-sensitive covariation of I(A) and I(H) voltage dependences tunes rebound firing in dopaminergic neurons

The level of expression of ion channels has been demonstrated to vary over a threefold to fourfold range from neuron to neuron, although the expression of distinct channels may be strongly correlated in the same neurons. We demonstrate that variability and covariation also apply to the biophysical properties of ion channels. We show that, in rat substantia nigra pars compacta dopaminergic neurons, the voltage dependences of the A-type (I(A)) and H-type (I(H)) currents exhibit a high degree of cell-to-cell variability, although they are strongly correlated in these cells. Our data also demonstrate that this cell-to-cell covariability of voltage dependences is sensitive to cytosolic cAMP and calcium levels. Finally, using dynamic clamp, we demonstrate that covarying I(A) and I(H) voltage dependences increases the dynamic range of rebound firing while covarying their amplitudes has a homeostatic effect on rebound firing. We propose that the covariation of voltage dependences of ion channels represents a flexible and energy-efficient way of tuning firing in neurons.

In vivo ethanol experience increases D(2) autoinhibition in the ventral tegmental area

Alcoholism is characterized by compulsive alcohol intake after a history of chronic consumption. A reduction in mesolimbic dopaminergic transmission observed during abstinence may contribute to the negative affective state that drives compulsive intake. Although previous in vivo recording studies in rodents have demonstrated profound decreases in the firing activity of ventral tegmental area (VTA) dopamine neurons after withdrawal from long-term ethanol exposure, the cellular mechanisms underlying this reduced activity are not well understood. Somatodendritic dopamine release within the VTA exerts powerful feedback inhibition of dopamine neuron activity via stimulation of D(2) autoreceptors and subsequent activation of G protein-gated inwardly rectifying K(+) (GIRK) channels. Here, by performing patch-clamp recordings from putative dopamine neurons in the VTA of mouse brain slices, we show that D(2) receptor/GIRK-mediated inhibition becomes more potent and exhibits less desensitization after withdrawal from repeated in vivo ethanol exposure (2 g/kg, i.p., three times daily for 7 days). In contrast, GABA(B) receptor/GIRK-mediated inhibition and its desensitization are not affected. Chelating cytosolic Ca(2+) with BAPTA augments D(2) inhibition and suppresses its desensitization in control mice, while these effects of BAPTA are occluded in ethanol-treated mice. Furthermore, inositol 1,4,5-trisphosphate (IP(3))-induced intracellular Ca(2+) release and Ca(2+)/calmodulin-dependent protein kinase II are selectively involved in the desensitization of D(2), but not GABA(B), receptor signaling. Consistent with this, activation of metabotropic glutamate receptors that are coupled to IP(3) generation leads to cross-desensitization of D(2)/GIRK-mediated responses. We propose that enhancement of D(2) receptor-mediated autoinhibition via attenuation of a Ca(2+)-dependent desensitization mechanism may contribute to the hypodopaminergic state during ethanol withdrawal.

Chronic activation of the D2 autoreceptor inhibits both glutamate and dopamine synapse formation and alters the intrinsic properties of mesencephalic dopamine neurons in vitro

Dysfunctional dopamine (DA)-mediated signaling is implicated in several diseases including Parkinson's disease, schizophrenia and attention deficit and hyperactivity disorder. Chronic treatment with DA receptor agonists or antagonists is often used in pharmacotherapy, but the consequences of these treatments on DA neuron function are unclear. It was recently demonstrated that chronic D2 autoreceptor (D2R) activation in DA neurons decreases DA release and inhibits synapse formation. Given that DA neurons can establish synapses that release glutamate in addition to DA, we evaluated the synapse specificity of the functional and structural plasticity induced by chronic D2R activation. We show that chronic activation of the D2R with quinpirole in vitro caused a parallel decrease in the number of dopaminergic and glutamatergic axon terminals. The capacity of DA neurons to synthesize DA was not altered, as indicated by the lack of change in protein kinase A-mediated Ser(40) phosphorylation of tyrosine hydroxylase. However, the spontaneous firing rate of DA neurons was decreased and was associated with altered intrinsic properties as revealed by a prolonged latency to first spike after release from hyperpolarization. Moreover, D2R function was decreased after its chronic activation. Our results demonstrate that chronic activation of the D2R induces a complex neuronal reorganization involving the inhibition of both DA and glutamate synapse formation and an alteration in electrical activity, but not in DA synthesis. A better understanding of D2R-induced morphological and functional long-term plasticity may lead to improved pharmacotherapy of DA-related neurological and psychiatric disorders.

Firing properties and functional connectivity of substantia nigra pars compacta neurones recorded with a multi-electrode array in vitro

Dopamine (DA) neurones of the substantia nigra pars compacta (SNc) are involved in a wide variety of functions, including motor control and reward-based learning. In order to gain new insights into the firing properties of neuronal ensembles in the SNc, we recorded extracellular single units from spontaneously active neurones, using a multi-electrode array (MEA) device in midbrain slices. The majority of neurones (50.21%) had a low firing frequency (1-3 Hz) and a stable pacemaker-like pattern, while others (44.84%) were irregular, but still firing at a low rate. The remaining population (4.95%) comprised neurones with a regular higher firing rate (5-10 Hz). High rate neurones, on the whole, were insensitive to DA (30 mum), while low rate neurones were mostly inhibited by DA, although responding either with a prominent or a weak inhibition. However, we recorded low rate regular neurones that were insensitive to DA, or irregular low rate neurones excited by DA. Interestingly, we found pairs of active neurones (12.10 +/- 3.14%) with a significant proportion of spikes occurring synchronously. Moreover, the crosscorrelation probability in each pair tended to increase in response to DA. In conclusion, MEA recordings in midbrain slices reveal a much more complex picture than previously reported with regard to the firing pattern and DA sensitivity of spontaneously active SNc neurones. Moreover, the study opens new prospectives for the in vitro investigation of functional connectivity in the midbrain dopaminergic system, thus proposing new targets for the pharmacological treatment of DA-dependent neurological disorders.

Regulation of firing frequency in a computational model of a midbrain dopaminergic neuron

Dopaminergic (DA) neurons of the mammalian midbrain exhibit unusually low firing frequencies in vitro. Furthermore, injection of depolarizing current induces depolarization block before high frequencies are achieved. The maximum steady and transient rates are about 10 and 20 Hz, respectively, despite the ability of these neurons to generate bursts at higher frequencies in vivo. We use a three-compartment model calibrated to reproduce DA neuron responses to several pharmacological manipulations to uncover mechanisms of frequency limitation. The model exhibits a slow oscillatory potential (SOP) dependent on the interplay between the L-type Ca(2+) current and the small conductance K(+) (SK) current that is unmasked by fast Na(+) current block. Contrary to previous theoretical work, the SOP does not pace the steady spiking frequency in our model. The main currents that determine the spontaneous firing frequency are the subthreshold L-type Ca(2+) and the A-type K(+) currents. The model identifies the channel densities for the fast Na(+) and the delayed rectifier K(+) currents as critical parameters limiting the maximal steady frequency evoked by a depolarizing pulse. We hypothesize that the low maximal steady frequencies result from a low safety factor for action potential generation. In the model, the rate of Ca(2+) accumulation in the distal dendrites controls the transient initial frequency in response to a depolarizing pulse. Similar results are obtained when the same model parameters are used in a multi-compartmental model with a realistic reconstructed morphology, indicating that the salient contributions of the dendritic architecture have been captured by the simpler model.

An intrinsic neuronal oscillator underlies dopaminergic neuron bursting

Dopaminergic neurons of the ventral midbrain fire high-frequency bursts when animals are presented with unexpected rewards, or stimuli that predict reward. To identify the afferents that can initiate bursting and establish therapeutic strategies for diseases affected by altered bursting, a mechanistic understanding of bursting is essential. Our results show that bursting is initiated by a specific interaction between the voltage sensitivity of NMDA receptors and voltage-gated ion channels that results in the activation of an intrinsic, action potential-independent, high-frequency membrane potential oscillation. We further show that the NMDA receptor is uniquely suited for this because of the rapid kinetics and voltage dependence imparted to it by Mg(2+) ion block and unblock. This mechanism explains the discrete nature of bursting in dopaminergic cells and demonstrates how synaptic signals may be reshaped by local intrinsic properties of a neuron before influencing action potential generation.

Cav1.3 channel voltage dependence, not Ca2+ selectivity, drives pacemaker activity and amplifies bursts in nigral dopamine neurons

Ca(v)1.3 (alpha 1D) L-type Ca(2+) channels have been implicated in substantia nigra (SN) dopamine (DA) neuron pacemaking and vulnerability to Parkinson's disease. These effects may arise from the depolarizing current and cytoplasmic Ca(2+) elevation produced by Ca(v)1.3 channels at subthreshold membrane potentials. However, the assumption that the Ca(2+) selectivity of Ca(v)1.3 channels is essential has not been tested. In this study the properties of SN DA neuron L-type Ca(2+) channels responsible for driving pacemaker activity in juvenile rat brain slices were probed by replacing native channels blocked with the dihydropyridine nimodipine with virtual channels generated by dynamic clamp. Surprisingly, virtual L-type channels that mimic native and recombinant Ca(v)1.3 channels supported pacemaker activity even though dynamic clamp currents are not carried by Ca(2+). This effect is specific because pacemaker activity could not be restored by tonic current injection, virtual nonselective leak channels or virtual NMDA receptors, which share with L-type channels a negative slope conductance region in their current-voltage (I-V) curve. Altering virtual channels showed that the production of pacemaker activity depended on the characteristic voltage dependence of DA neuron L-type channels, while activation kinetics and reversal potential were not critical parameters. Virtual L-type channels also supported slow oscillatory potentials and enhanced firing rate during evoked bursts. Thus, Ca(v)1.3 channel voltage dependence, rather than Ca(2+) selectivity, drives pacemaker activity and amplifies bursts in SN DA neurons.