Spontaneous activity and properties of two types of principal neurons from the ventral tegmental area of rat

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.

Contributions of the Sodium Leak Channel NALCN to Pacemaking of Medial Ventral Tegmental Area and Substantia Nigra Dopaminergic Neurons

We tested the role of the sodium leak channel, NALCN, in pacemaking of dopaminergic neuron (DAN) subpopulations from adult male and female mice. In situ hybridization revealed NALCN RNA in all DANs, with lower abundance in medial ventral tegmental area (VTA) relative to substantia nigra pars compacta (SNc). Despite lower relative abundance of NALCN, we found that acute pharmacological blockade of NALCN in medial VTA DANs slowed pacemaking by 49.08%. We also examined the electrophysiological properties of projection-defined VTA DAN subpopulations identified by retrograde labeling. Inhibition of NALCN reduced pacemaking in DANs projecting to medial nucleus accumbens (NAc) and others projecting to lateral NAc by 70.74% and 31.98%, respectively, suggesting that NALCN is a primary driver of pacemaking in VTA DANs. In SNc DANs, potentiating NALCN by lowering extracellular calcium concentration speeded pacemaking in wildtype but not NALCN conditional knockout mice, demonstrating functional presence of NALCN. In contrast to VTA DANs, however, pacemaking in SNc DANs was unaffected by inhibition of NALCN. Instead, we found that inhibition of NALCN increased the gain of frequency-current plots at firing frequencies slower than spontaneous firing. Similarly, inhibition of the hyperpolarization-activated cyclic nucleotide-gated (HCN) conductance increased gain but had little effect on pacemaking. Interestingly, simultaneous inhibition of NALCN and HCN resulted in significant reduction in pacemaker rate. Thus, we found NALCN makes substantial contributions to driving pacemaking in VTA DAN subpopulations. In SNc DANs, NALCN is not critical for pacemaking but inhibition of NALCN makes cells more sensitive to hyperpolarizing stimuli.SIGNIFICANCE STATEMENT Pacemaking in midbrain dopaminergic neurons (DAN) relies on multiple subthreshold conductances, including a sodium leak. Whether the sodium leak channel, NALCN, contributes to pacemaking in DANs located in the VTA and the SNc has not yet been determined. Using electrophysiology and pharmacology, we show that NALCN plays a prominent role in driving pacemaking in projection-defined VTA DAN subpopulations. By contrast, pacemaking in SNc neurons does not rely on NALCN. Instead, the presence of NALCN regulates the excitability of SNc DANs by reducing the gain of the neuron's response to inhibitory stimuli. Together, these findings will inform future efforts to obtain DAN subpopulation-specific treatments for use in neuropsychiatric disorders.

Ongoing habenular activity is driven by forebrain networks and modulated by olfactory stimuli

Ongoing neural activity, which represents internal brain states, is constantly modulated by the sensory information that is generated by the environment. In this study, we show that the habenular circuits act as a major brain hub integrating the structured ongoing activity of the limbic forebrain circuitry and the olfactory information. We demonstrate that ancestral homologs of amygdala and hippocampus in zebrafish forebrain are the major drivers of ongoing habenular activity. We also reveal that odor stimuli can modulate the activity of specific habenular neurons that are driven by this forebrain circuitry. Our results highlight a major role for the olfactory system in regulating the ongoing activity of the habenula and the forebrain, thereby altering brain's internal states.

Resonance in the Mouse Ventral Tegmental Area Dopaminergic Network Induced by Regular and Poisson Distributed Optogenetic Stimulation in-vitro

Neurons in many brain regions exhibit spontaneous, intrinsic rhythmic firing activity. This rhythmic firing activity may determine the way in which these neurons respond to extrinsic synaptic inputs. We hypothesized that neurons should be most responsive to inputs at the frequency of the intrinsic oscillation frequency. We addressed this question in the ventral tegmental area (VTA), a dopaminergic nucleus in the midbrain. VTA neurons have a unique propensity to exhibit spontaneous intrinsic rhythmic activity in the 1-5 Hz frequency range, which persists in the in-vitro brain slice, and form a network of weakly coupled oscillators. Here, we combine in-vitro simultaneous recording of action potentials from a 60 channel multi-electrode-array with cell-type-specific optogenetic stimulation of the VTA dopamine neurons. We investigated how VTA neurons respond to wide-band stochastic (Poisson input) as well as regular laser pulses. Strong synchrony was induced between the laser input and the spike timing of the neurons, both for regular pulse trains and Poisson pulse trains. For rhythmically pulsed input, the neurons demonstrated resonant behavior with the strongest phase locking at their intrinsic oscillation frequency, but also at half and double the intrinsic oscillation frequency. Stochastic Poisson pulse stimulation provided a more effective stimulation of the entire population, yet we observed resonance at lower frequencies (approximately half the oscillation frequency) than the neurons' intrinsic oscillation frequency. The non-linear filter characteristics of dopamine neurons could allow the VTA to predict precisely timed future rewards based on past sensory inputs, a crucial component of reward prediction error signaling. In addition, these filter characteristics could contribute to a pacemaker role for the VTA in synchronizing activity with other regions like the prefrontal cortex and the hippocampus.

Modulation of Functional Connectivity Between Dopamine Neurons of the Rat Ventral Tegmental Area in vitro

Micro Electrode Arrays were used to simultaneously record spontaneous extracellular action potentials from 10 to 30 dopamine neurons in acute brain slices from the lateral Ventral Tegmental Area (VTA) of the rat. The spike train of an individual neuron was used to characterize the firing pattern: firing rate, firing irregularity and oscillation frequency. Functional connectivity between a pair of neurons was quantified by the Paired Phase Consistency (PPC), taking the oscillation frequency as reference. Under baseline conditions the PPC was significantly different from zero and 42 of the 386 pairs of VTA neurons showed significant coupling. Fifty percent of the recorded dopamine neurons were part of the coupled VTA network. Raising extracellular potassium from 3.5 to 5 mM increased the mean firing rate of the dopamine neurons by 45%. The same increase could be induced by bath application of 300 μm glutamate. High potassium reduced the PPC, but it did not change during the glutamate application. Our findings imply that manipulating excitability has distinct and specific consequences for functional connectivity in the VTA network that cannot be directly predicted from the changes in neuronal firing rates. Functional connectivity reflects the spatial organization and synchronization of the VTA output and thus represents a unique element of the message that is sent to the mesolimbic projection area. It adds a dimension to pharmacological manipulation of the VTA micro circuit that might help to understand the pharmacological (side) effects of e.g., anti-psychotic drugs.

Ionic currents influencing spontaneous firing and pacemaker frequency in dopamine neurons of the ventrolateral periaqueductal gray and dorsal raphe nucleus (vlPAG/DRN): A voltage-clamp and computational modelling study

Dopamine (DA) neurons of the ventrolateral periaqueductal gray (vlPAG) and dorsal raphe nucleus (DRN) fire spontaneous action potentials (APs) at slow, regular patterns in vitro but a detailed account of their intrinsic membrane properties responsible for spontaneous firing is currently lacking. To resolve this, we performed a voltage-clamp electrophysiological study in brain slices to describe their major ionic currents and then constructed a computer model and used simulations to understand the mechanisms behind autorhythmicity in silico. We found that vlPAG/DRN DA neurons exhibit a number of voltage-dependent currents activating in the subthreshold range including, a hyperpolarization-activated cation current (I_H), a transient, A-type, potassium current (I_A), a background, ‘persistent’ (I_NaP) sodium current and a transient, low voltage activated (LVA) calcium current (I_CaLVA). Brain slice pharmacology, in good agreement with computer simulations, showed that spontaneous firing occurred independently of I_H, I_A or calcium currents. In contrast, when blocking sodium currents, spontaneous firing ceased and a stable, non-oscillating membrane potential below AP threshold was attained. Using the DA neuron model we further show that calcium currents exhibit little activation (compared to sodium) during the interspike interval (ISI) repolarization while, any individual potassium current alone, whose blockade positively modulated AP firing frequency, is not required for spontaneous firing. Instead, blockade of a number of potassium currents simultaneously is necessary to eliminate autorhythmicity. Repolarization during ISI is mediated initially via the deactivation of the delayed rectifier potassium current, while a sodium background ‘persistent’ current is essentially indispensable for autorhythmicity by driving repolarization towards AP threshold.

Ventral Tegmental Area Dopamine Neurons Firing Model Reveals Prenatal Nicotine Induced Alterations

The dopamine (DA) neurons found in the ventral tegmental area (VTA) are widely involved in the addiction and natural reward circuitry of the brain. Their firing patterns were shown to be important modulators of dopamine release and repetitive burst-like firing activity was highlighted as a major firing pattern of DA neurons in the VTA. In the present study we use a state space model to characterize the DA neurons firing patterns, and trace transitions of neural activity through bursting and non-bursting states. The hidden semi-Markov model (HSMM) framework, which we use, offers a statistically principled inference of bursting states and considers VTA DA firing patterns to be generated according to a Gamma process. Additionally, the explicit Gamma-based modeling of state durations allows efficient decoding of underlying neural information. Consequently, we decode and segment our single unit recordings from DA neurons in VTA according to the sequence of statistically discriminated HSMM states. The segmentation is used to study bursting state characteristics in data recorded from rats prenatally exposed to nicotine (6 mg/kg/day starting with gestational day 3) and rats from saline treated dams. Our results indicate that prenatal nicotine exposure significantly alters burst firing patterns of a subset of DA neurons in adolescent rats, suggesting nicotine exposure during gestation may induce severe effects on the neural networks involved in addiction and reward.

Activation of group I metabotropic glutamate receptors regulates the excitability of rat retinal ganglion cells by suppressing Kir and I h

Group I metabotropic glutamate receptor (mGluR I) activation exerts a slow postsynaptic excitatory effect in the CNS. Here, the issues of whether and how this receptor is involved in regulating retinal ganglion cell (RGC) excitability were investigated in retinal slices using patch-clamp techniques. Under physiological conditions, RGCs displayed spontaneous firing. Extracellular application of LY367385 (10 µM)/MPEP (10 µM), selective mGluR1 and mGluR5 antagonists, respectively, significantly reduced the firing frequency, suggesting that glutamate endogenously released from bipolar cells constantly modulates RGC firing. DHPG (10 µM), an mGluR I agonist, significantly increased the firing and caused depolarization of the cells, which were reversed by LY367385, but not by MPEP, suggesting the involvement of the mGluR1 subtype. Intracellular Ca²⁺-dependent PI-PLC/PKC and calcium/calmodulin-dependent protein kinase II (CaMKII) signaling pathways mediated the DHPG-induced effects. In the presence of cocktail synaptic blockers (CNQX, D-AP5, bicuculline, and strychnine), which terminated the spontaneous firing in both ON and OFF RGCs, DHPG still induced depolarization and triggered the cells to fire. The DHPG-induced depolarization could not be blocked by TTX. In contrast, Ba²⁺, an inwardly rectifying potassium channel (Kir) blocker, and Cs⁺ and ZD7288, hyperpolarization-activated cation channel (I _h) blockers, mimicked the effect of DHPG. Furthermore, in the presence of Ba²⁺/ZD7288, DHPG did not show further effects. Moreover, Kir and I _h currents could be recorded in RGCs, and extracellular application of DHPG indeed suppressed these currents. Our results suggest that activation of mGluR I regulates the excitability of rat RGCs by inhibiting Kir and I _h.

Ventral tegmental area dopamine and GABA neurons: Physiological properties and expression of mRNA for endocannabinoid biosynthetic elements

The ventral tegmental area (VTA) is involved in adaptive reward and motivation processing and is composed of dopamine (DA) and GABA neurons. Defining the elements regulating activity and synaptic plasticity of these cells is critical to understanding mechanisms of reward and addiction. While endocannabinoids (eCBs) that potentially contribute to addiction are known to be involved in synaptic plasticity mechanisms in the VTA, where they are produced is poorly understood. In this study, DA and GABAergic cells were identified using electrophysiology, cellular markers, and a transgenic mouse model that specifically labels GABA cells. Using single-cell RT-qPCR and immunohistochemistry, we investigated mRNA and proteins involved in eCB signaling such as diacylglycerol lipase α, N-acyl-phosphatidylethanolamine-specific phospholipase D, and 12-lipoxygenase, as well as type I metabotropic glutamate receptors (mGluRs). Our results demonstrate the first molecular evidence of colocalization of eCB biosynthetic enzyme and type I mGluR mRNA in VTA neurons. Further, these data reveal higher expression of mGluR1 in DA neurons, suggesting potential differences in eCB synthesis between DA and GABA neurons. These data collectively suggest that VTA GABAergic and DAergic cells have the potential to produce various eCBs implicated in altering neuronal activity or plasticity in adaptive motivational reward or addiction.

Effects of ST1936, a selective serotonin-6 agonist, on electrical activity of putative mesencephalic dopaminergic neurons in the rat brain

The serotonin-6 (5-HT6) receptor is the most recently discovered serotonin receptor, and it represents an increasingly promising target for improving cognition in both normal and disease states. Recently, a new selective 5-HT6 receptor agonist, 2-(5 chloro-2-methyl-1H-indol-3-yl)-N,N-dimethylethanamine (ST1936), with nanomolar affinity for 5-HT6 receptors was described. We performed in-vivo electrophysiological studies to investigate the physiological role of 5-HT6 receptors in the control of the function of the substantia nigra pars compacta (SNc) and ventral tegmental area (VTA). Extracellular single-unit recordings were performed from putative dopamine-containing neurons in the SNc and VTA of anesthetised rats. In the SNc, acute systemic administration of ST1936 had no effects on basal firing activity of these dopamine neurons; however, in the VTA, ST1936 induced either dose-related increases (45% of cells) or decreases in basal activity of these dopaminergic neurons. Local application of ST1936 into the VTA caused excitation in all of the dopamine neurons, but had no effects on non-dopamine VTA neurons. Both effects of systemic and microiontophoretic ST1936 were completely reversed by the potent and selective 5-HT6 receptor antagonist 5-chloro-N-(4-methoxy-3-piperazin-1-ylphenyl)-3-methyl-2- benzothiophene-sulfonamide (SB271046). Systemic application of another 5-HT6 agonist, 2-(1-{6-chloroimidazo[2,1-b] [1,3]thiazole-5-sulfonyl}-1H-indol-3-yl)ethan-1-amine (WAY-181187), induced dose-dependent inhibition of these VTA dopaminergic neurons. ST1936 and WAY-181187 appear to have different effects on these VTA dopaminergic neurons, potentially due to different mechanisms of action or to the complexity of 5-HT6 receptor functions. Our data demonstrate the need for further investigations into the use of 5-HT6 receptor agonists to control cognitive disfunction, such as in schizophrenia and depression.

Pacemaking in dopaminergic ventral tegmental area neurons: depolarizing drive from background and voltage-dependent sodium conductances

Dopaminergic neurons in the ventral tegmental area (VTA) fire spontaneously in a pacemaker-like manner. We analyzed the ionic currents that drive pacemaking in dopaminergic VTA neurons, studied in mouse brain slices. Pacemaking was not inhibited by blocking hyperpolarization-activated cation current (I(h)) or blocking all calcium current by Mg(2+) replacement of Ca(2+). Tetrodotoxin (TTX) stopped spontaneous activity and usually resulted in stable resting potentials near -60 mV to -55 mV, 10-15 mV below the action potential threshold. When external sodium was replaced by N-methyl-D-glucamine (NMDG) with TTX present, cells hyperpolarized by an average of -11 mV, suggesting a significant resting sodium conductance not sensitive to TTX. Voltage-clamp experiments using slow (10 mV/s) ramps showed a steady-state, steeply voltage-dependent current blocked by TTX that activates near -60 mV, as well as a sodium "background" current with little voltage sensitivity, revealed by NMDG replacement for sodium with TTX present. We quantified these two components of sodium current during the pacemaking trajectory using action potential clamp. The initial phase of depolarization, up to approximately -55 mV, is driven mainly by non-voltage-dependent sodium background current. Above -55 mV, TTX-sensitive voltage-dependent "persistent" Na current helps drive the final phase of depolarization to the spike threshold. Voltage-dependent calcium current is small at all subthreshold voltages. The pacemaking mechanism in VTA neurons differs from that in substantia nigra pars compacta (SNc) neurons, where subthreshold calcium current plays a dominant role. In addition, we found that non-voltage-dependent background sodium current is much smaller in SNc neurons than VTA neurons.

Electrophysiological characteristics of dopamine neurons: a 35-year update

This chapter consists of four sections. The first section provides a general description of the electrophysiological characteristics of dopamine (DA) neurons in both the substantia nigra and ventral tegmental area. Emphasis is placed on the differences between DA and neighboring non-DA neurons. The second section discusses the ionic mechanisms underlying the generation of action potential in DA cells. Evidence is provided to suggest that these mechanisms differ not only between DA and non-DA neurons but also between DA cells located in different areas, with different projection sites and at different developmental stages. Some of the differences may play a critical role in the vulnerability of a DA neuron to cell death. The third section describes the firing patterns of DA cells. Data are presented to show that the current "80/160 ms" criteria for burst identification need to be revised and that the burst firing, originally described by Bunney et al., can be described as slow oscillations in firing rate. In the ventral tegmental area, the slow oscillations are, at least partially, derived from the prefrontal cortex and part of prefrontal information is transferred to DA cells indirectly through inhibitory neurons. The final section focuses on the feedback regulation of DA cells. New evidence suggests that DA autoreceptors are coupled to multiple effectors, and both D1 and D2-like receptors are involved in long-loop feedback control of DA neurons. Because of the presence of multiple feedback and nonfeedback pathways, the effect of a drug on a DA neuron can be far more complex than an inhibition or excitation. A better understanding of the intrinsic properties of DA neurons and their regulation by afferent input will, in time, help to point to the way to more effective and safer treatments for disorders including schizophrenia, drug addiction, and Parkinson's disease.

Dynamic, nonlinear feedback regulation of slow pacemaking by A-type potassium current in ventral tegmental area neurons

We analyzed ionic currents that regulate pacemaking in dopaminergic neurons of the mouse ventral tegmental area by comparing voltage trajectories during spontaneous firing with ramp-evoked currents in voltage clamp. Most recordings were made in brain slice, with key experiments repeated using acutely dissociated neurons, which gave identical results. During spontaneous firing, net ionic current flowing between spikes was calculated from the time derivative of voltage multiplied by cell capacitance, signal-averaged over many firing cycles to enhance resolution. Net inward interspike current had a distinctive nonmonotonic shape, reaching a minimum (generally <1 pA) between -60 and -55 mV. Under voltage clamp, ramps over subthreshold voltages elicited a time- and voltage-dependent outward current that peaked near -55 mV. This current was undetectable with 5 mV/s ramps and increased steeply with depolarization rate over the range (10-50 mV/s) typical of natural pacemaking. Ramp-evoked subthreshold current was resistant to alpha-dendrotoxin, paxilline, apamin, and tetraethylammonium but sensitive to 4-aminopyridine and 0.5 mM Ba2+, consistent with A-type potassium current (I(A)). Same-cell comparison of currents elicited by various ramp speeds with natural spontaneous depolarization showed how the steep dependence of I(A) on depolarization rate results in small net inward currents during pacemaking. These results reveal a mechanism in which subthreshold I(A) is near zero at steady state, but is engaged at depolarization rates >10 mV/s to act as a powerful, supralinear feedback element. This feedback mechanism explains how net ionic current can be constrained to <1-2 pA but reliably inward, thus enabling slow, regular firing.

Intrinsic excitability of cholinergic neurons in the rat parabigeminal nucleus

Cholinergic neurons in the parabigeminal nucleus of the rat midbrain were studied in an acute slice preparation. Spontaneous, regular action potentials were observed both with cell-attached patch recordings as well as with whole cell current-clamp recordings. The spontaneous activity of parabigeminal nucleus (PBN) neurons was not due to synaptic input as it persisted in the presence of the pan-ionotropic excitatory neurotransmitter receptor blocker, kynurenic acid, and the cholinergic blockers dihydro-beta-erythroidine (DHbetaE) and atropine. This result suggests the existence of intrinsic currents that enable spontaneous activity. In voltage-clamp recordings, I(H) and I(A) currents were observed in most PBN neurons. I(A) had voltage-dependent features that would permit it to contribute to spontaneous firing. In contrast, I(H) was significantly activated at membrane potentials lower than the trough of the spike afterhyperpolarization, suggesting that I(H) does not contribute to spontaneous firing of PBN neurons. Consistent with this interpretation, application of 25 microM ZD-7288, which blocked I(H), did not affect the rate of spontaneous firing in PBN neurons. Counterparts to I(A) and I(H) were observed in current-clamp recordings: I(A) was reflected as a slow voltage ramp observed between action potentials and on release from hyperpolarization, and I(H) was reflected as a depolarizing sag often accompanied by rebound spikes in response to hyperpolarizing current injections. In response to depolarizing current injections, PBN neurons fired at high frequencies, with relatively little accommodation. Ultimately, the spontaneous activity in PBN neurons could be used to modulate cholinergic drive in the superior colliculus in either positive or negative directions.

Somatostatinergic modulation of firing pattern and calcium-activated potassium currents in medium spiny neostriatal neurons

Somatostatin is synthesized and released by aspiny GABAergic interneurons of the neostriatum, some of them identified as low threshold spike generating neurons (LTS-interneurons). These neurons make synaptic contacts with spiny neostriatal projection neurons. However, very few somatostatin actions on projection neurons have been described. The present work reports that somatostatin modulates the Ca(2+) activated K(+) currents (K(Ca) currents) expressed by projection cells. These actions contribute in designing the firing pattern of the spiny projection neuron; which is the output of the neostriatum. Small conductance (SK) and large conductance (BK) K(Ca) currents represent between 30% and 50% of the sustained outward current in spiny cells. Somatostatin reduces SK-type K(+) currents and at the same time enhances BK-type K(+) currents. This dual effect enhances the fast component of the after hyperpolarizing potential while reducing the slow component. Somatostatin then modifies the firing pattern of spiny neurons which changed from a tonic regular pattern to an interrupted "stuttering"-like pattern. Semi-quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) tissue expression analysis of dorsal striatal somatostatinergic receptors (SSTR) mRNA revealed that all five SSTR mRNAs are present. However, single cell RT-PCR profiling suggests that the most probable receptor in charge of this modulation is the SSTR2 receptor. Interestingly, aspiny interneurons may exhibit a "stuttering"-like firing pattern. Therefore, somatostatin actions appear to be the entrainment of projection neurons to the rhythms generated by some interneurons. Somatostatin is then capable of modifying the processing and output of the neostriatum.

The ventral tegmental area revisited: is there an electrophysiological marker for dopaminergic neurons?

The ventral tegmental area (VTA) and in particular VTA dopamine (DA) neurons are postulated to play a central role in reward, motivation and drug addiction. However, most evidence implicating VTA DA neurons in these functions is based on indirect electrophysiological characterization, rather than cytochemical identification. These physiological criteria were first established in the substantia nigra pars compacta (SNc), but their validity in the VTA is uncertain. In the current study we found that while 88 +/- 2% of SNc neurons labelled by the neuronal marker NeuN were co-labelled for the catecholamine enzyme tyrosine hydroxylase (TH), a much smaller percentage (55 +/- 2%) of VTA neurons co-expressed TH. In addition, using in vitro whole-cell recordings we found that widely accepted physiological criteria for VTA DA neurons, including the hyperpolarization-activated inwardly rectifying non-specific cation current (I(h)), spike duration, and inhibition by DA D2 receptor agonists, do not reliably predict the DA content of VTA neurons. We could not distinguish DA neurons from other VTA neurons by size, shape, input resistance, I(h) size, or spontaneous firing rate. Although the absence of an I(h) reliably predicted that a VTA neuron was non-dopaminergic, and I(h)(-) neurons differ from I(h)(+) neurons in firing rate, interspike interval (ISI) standard deviation, and ISI skew, no physiological property examined here is both sensitive and selective for DA neurons in the VTA. We conclude that reliable physiological criteria for VTA DA neuron identification have yet to be determined, and that the criteria currently being used are unreliable.

Ethanol inhibition of m-current and ethanol-induced direct excitation of ventral tegmental area dopamine neurons

Ethanol-induced excitation of ventral tegmental area dopamine (DA VTA) neurons is thought to be critical for the reinforcing effects of ethanol. Although ligand-gated ion channels are known to be the targets of ethanol, ethanol modulation of voltage-dependent ion channels of central neurons has not been well studied. We have demonstrated that ethanol excites DA VTA neurons by the reduction of sustained K(+) currents and recently reported that M-current (I(M)) regulates action potential generation through fast and slow afterhyperpolarization phases. In the present study we thus examined whether ethanol inhibition of I(M) contributes to the excitation of DA VTA neurons using nystatin-perforated patch current- and voltage-clamp recordings. Ethanol (20-120 mM) reduced I(M) in a concentration-dependent manner and increased the spontaneous firing frequency of DA VTA neurons. Ethanol-induced increase in spontaneous firing frequency correlated positively with ethanol inhibition of I(M) with a slope value of 1.3. Specific I(M) inhibition by XE991 (0.3-10 microM) increased spontaneous firing frequency which correlated positively with I(M) inhibition with a slope value of 0.5. In the presence of 10 muM XE991, a concentration that produced maximal inhibition of I(M), ethanol still increased the spontaneous firing frequency of DA VTA neurons in a concentration-dependent manner. Thus we conclude that, although ethanol causes inhibition of I(M) and this results in some increase in the firing frequency of DA VTA neurons, another effect of ethanol is primarily responsible for the ethanol-induced increase in firing rate in these neurons.

The effects of long chain-length n-alcohols on the firing frequency of dopaminergic neurons of the ventral tegmental area

The dopaminergic neurons of the ventral tegmental area (DA VTA neurons) have been implicated in the reinforcing properties of drugs of abuse, including ethanol (ethyl alcohol). Ethanol increases the spontaneous firing frequency of DA VTA neurons in vitro, in both brain slices and acutely dissociated neurons, and also in vivo. In many systems, longer n-alkyl alcohols have a more potent effect than ethanol, and the potency is a function of the number of carbons in the alkyl chain. We studied n-alcohols of chain length 1 (methanol) to 5 (pentanol) on the firing rate of DA VTA neurons in brain slice preparations. All of the alcohols studied produced increases in the spontaneous firing frequency in DA VTA neurons; as the chain length increased, lower concentrations of the alcohols were needed to produce the same percentage increase in firing. With very high concentrations of all the alcohols except methanol, we observed apparent depolarization block of firing. In addition, trichloroethanol (TCE), the active metabolite of chloral hydrate, increased the firing frequency of DA VTA neurons, and the EC(40) (concentration to produce a 40% increase in firing rate) of TCE was below that of ethanol. These studies indicate that excitation of VTA dopamine neurons by n-alcohols is related to the chain length of the carbons. This is likely to be a characteristic of the ethanol-sensitive element of DA VTA neurons and may be useful in identifying the element of the membrane that is responsible for ethanol-induced excitation.

A-type K+ current of dopamine and GABA neurons in the ventral tegmental area

A-type K(+) current (I(A)) is a rapidly inactivating voltage-dependent potassium current which can regulate the frequency of action potential (AP) generation. Increased firing frequency of ventral tegmental area (VTA) neurons is associated with the reinforcing effects of some drugs of abuse like nicotine and ethanol. In the present study, we classified dopamine (DA) and GABA VTA neurons, and investigated I(A) properties and the physiological role of I(A) in these neurons using conventional whole cell current- and voltage-clamp recording. DA VTA neurons had a mean firing frequency of 3.5 Hz with a long AP duration. GABA VTA neurons had a mean firing frequency of 16.7 Hz with a short AP duration. For I(A) properties, the voltage-dependence of steady-state I(A) activation and inactivation was similar in DA and GABA VTA neurons. I(A) inactivation was significantly faster and became faster at positive voltages in GABA neurons than DA neurons. Recovery from inactivation was significantly faster in DA neurons than GABA neurons. I(A) current density at full recovery was significantly larger in DA neurons than GABA neurons. In DA and GABA VTA neurons, latency to the first AP after the recovery from membrane hyperpolarization (repolarization latency) was measured. Longer repolarization latency was accompanied by larger I(A) current density in DA VTA neurons, compared with GABA VTA neurons. We suggest that I(A) contributes more to the regulation of AP generation in DA VTA neurons than in GABA VTA neurons.

Characterization of M-current in ventral tegmental area dopamine neurons

M-current (I(M)) is a voltage-gated potassium current (KCNQ type) that affects neuronal excitability and is modulated by some drugs of abuse. Ventral tegmental area (VTA) dopamine (DA) neurons are important for the reinforcing effects of drugs of abuse. Therefore we studied I(M) in acutely dissociated rat DA VTA neurons with nystatin-perforated patch recording. The standard deactivation protocol was used to measure I(M) during voltage-clamp recording with hyperpolarizing voltage steps to -65 mV (in 10-mV increments) from a holding potential of -25 mV. I(M) amplitude was voltage dependent and maximal current amplitude was detected at -45 mV. The deactivation time constant of I(M) was voltage dependent and became shorter at more negative voltages. The I(M)/KCNQ antagonist XE991 (0.3-30 microM) caused a concentration-dependent reduction in I(M) amplitude with an IC(50) of 0.71 microM. Tetraethylammonium (TEA, 0.3-10 mM) caused a concentration-dependent inhibition of I(M) with an IC(50) of 1.56 mM. In current-clamp recordings, all DA VTA neurons were spontaneously active. Analysis of evoked action potential shape indicated that XE991 (1-10 microM) reduced the fast and slow components of the spike afterhyperpolarization (AHP) without affecting the middle component of the AHP. Action potential amplitude, duration, and threshold were not affected by XE991. In addition, 10 microM XE991 significantly shortened the interspike intervals in evoked spike trains. In conclusion, I(M) is active near threshold in DA VTA neurons, is blocked by XE991 (10 microM) and TEA (10 mM), may contribute to the shape of the AHP, and may decrease excitability of these neurons.

Ca2+-activated K+-current density is correlated with soma size in rat vestibular-afferent neurons in culture

Vestibular-afferent neurons (VANs) transmit information about linear and angular accelerations during head movements from vestibular end organs to vestibular nuclei. In situ, these neurons show heterogeneous discharge patterns that may be produced by differences in their intrinsic properties. However, little is known about the ionic currents underlying their different firing patterns. Using the whole cell patch-clamp technique, we analyzed the expression of Ca(2+) and Ca(2+)-activated K(+) currents (I(KCa)) in primary cultured neurons isolated from young rats (p7-p10). We found two overlapping subpopulations of VANs classified according to low-threshold Ca(2+)-current [low-voltage-activated (LVA)] expression; LVA (-) neurons, formed by small cells, and LVA (+) neurons composed of medium to large cells. The I(KCa) in both cell-groups was carried through channels of high (BK), intermediate (IK), and low conductance (SK), besides a resistant channel to classical blockers (IR). BK was expressed preferentially in LVA (+) cells, whereas IR expression was preferentially in LVA (-) cells. No correlation between SK and IK expression with the soma size was found. Current-clamp experiments showed that BK participates in the adaptation of discharge and in the duration of the action potential, whereas SK and IK did not show a significant contribution to electrical discharge of cultured VANs. However, because of the low number of VANs in culture with repetitive firing it is difficult to interpret our results in terms of discharge patterns. Our results demonstrate that vestibular-afferent neurons possess different Ca(2+)-activated K(+) (K(Ca)) channels and that their expression, heterogeneous among the cells, would contribute to explain some of the differences in the electrical-firing properties of these neurons.