Sub-populations of pars compacta neurons in the substantia nigra: the significance of qualitatively and quantitatively distinct conductances

Current Review of the Function and Regulation of Tuberoinfundibular Dopamine Neurons

Tuberoinfundibular dopamine (TIDA) neurons have cell bodies located in the arcuate nucleus of the mediobasal hypothalamus. They project to the external zone of the median eminence, and the dopamine (DA) released there is carried by the hypophysial portal vasculature to the anterior pituitary. The DA then activates D2 receptors to inhibit prolactin (PRL) secretion from lactotrophs. The TIDA neuronal population is the principal regulatory factor controlling PRL secretion. The neuroendocrine role subserved by TIDA neurons sets them apart from other dopaminergic populations like the nigrostriatal and mesolimbic DA neurons. TIDA neurons exhibit intrinsic oscillatory fluctuations in their membrane potential that give rise to phasic firing and bursting activity. TIDA neuronal activity is sexually differentiated and modulated by gonadal hormones and PRL, as well as an array of small molecule and peptide neurotransmitters. This review covers these characteristics.

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.

TRPM4 Contributes to Subthreshold Membrane Potential Oscillations in Multiple Mouse Pacemaker Neurons

Select neuronal populations display steady rhythmic neuronal firing that provides tonic excitation to drive downstream networks and behaviors. In noradrenergic neurons of the locus coeruleus (LC), circadian neurons of the suprachiasmatic nucleus (SCN), and CO₂/H⁺-activated neurons of the brainstem retrotrapezoid nucleus (RTN), large subthreshold membrane potential oscillations contribute to the pacemaker-like action potential discharge. The oscillations and firing in LC and SCN involve contributions from leak sodium (NALCN) and L-type calcium channels while recent work from RTN suggested an additional pivotal role for a secondary calcium-activated and voltage-gated cationic current sensitive to TRPM4 channel blockers. Here, we tested whether TRPM4 contributes to subthreshold oscillations in mouse LC and SCN. By RNAscope in situ hybridization, Trpm4 transcripts were detected in both cell groups. In whole-cell recordings from acute slice preparations, prominent voltage-dependent membrane potential oscillations were revealed in LC and SCN after blocking action potentials. These oscillations were inhibited by two chemically-distinct blockers of TRPM4, 9-phenanthrol (9-pt) and 4-chloro-2-[[2-(2-chlorophenoxy)acetyl]amino]benzoic acid (CBA). Under whole-cell voltage clamp, inward currents evoked by oscillation voltage waveforms were inhibited in LC by blocking L-type calcium channels and TRPM4. These data implicate TRPM4 in the large subthreshold membrane potential oscillations that underlie tonic action potential discharge in LC and SCN, providing a voltage-dependent and calcium-dependent cationic current to augment the depolarizing inward Na⁺ and Ca²⁺ currents previously associated with this distinctive electroresponsive property.

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.

TRPM4 mediates a subthreshold membrane potential oscillation in respiratory chemoreceptor neurons that drives pacemaker firing and breathing

Brainstem networks that control regular tidal breathing depend on excitatory drive, including from tonically active, CO₂/H⁺-sensitive neurons of the retrotrapezoid nucleus (RTN). Here, we examine intrinsic ionic mechanisms underlying the metronomic firing activity characteristic of RTN neurons. In mouse brainstem slices, large-amplitude membrane potential oscillations are evident in synaptically isolated RTN neurons after blocking action potentials. The voltage-dependent oscillations are abolished by sodium replacement; blocking calcium channels (primarily L-type); chelating intracellular Ca²⁺; and inhibiting TRPM4, a Ca²⁺-dependent cationic channel. Likewise, oscillation voltage waveform currents are sensitive to calcium and TRPM4 channel blockers. Extracellular acidification and serotonin (5-HT) evoke membrane depolarization that augments TRPM4-dependent oscillatory activity and action potential discharge. Finally, inhibition of TRPM4 channels in the RTN of anesthetized mice reduces central respiratory output. These data implicate TRPM4 in a subthreshold oscillation that supports the pacemaker-like firing of RTN neurons required for basal, CO₂-stimulated, and state-dependent breathing.

In Vivo Protein Complementation Demonstrates Presynaptic α-Synuclein Oligomerization and Age-Dependent Accumulation of 8-16-mer Oligomer Species

Intracellular accumulation of α-synuclein (α-syn) and formation of Lewy bodies are neuropathological characteristics of Parkinson's disease (PD) and related α-synucleinopathies. Oligomerization and spreading of α-syn from neuron to neuron have been suggested as key events contributing to the progression of PD. To directly visualize and characterize α-syn oligomerization and spreading in vivo, we generated two independent conditional transgenic mouse models based on α-syn protein complementation assays using neuron-specifically expressed split Gaussia luciferase or split Venus yellow fluorescent protein (YFP). These transgenic mice allow direct assessment of the quantity and subcellular distribution of α-syn oligomers in vivo. Using these mouse models, we demonstrate an age-dependent accumulation of a specific subtype of α-syn oligomers. We provide in vivo evidence that, although α-syn is found throughout neurons, α-syn oligomerization takes place at the presynapse. Furthermore, our mouse models provide strong evidence for a transsynaptic cell-to-cell transfer of de novo generated α-syn oligomers in vivo.

CaV1.3 L-Type Calcium Channels Increase the Vulnerability of Substantia Nigra Dopaminergic Neurons in MPTP Mouse Model of Parkinson's Disease

Mechanisms underlying the selective vulnerability of dopaminergic (DA) neurons in the substantia nigra pars compacta (SNpc) over those in the ventral tegmental area (VTA) to degeneration in Parkinson’s disease (PD) remain poorly understood. DA neurons of SNpc and VTA are autonomous pacemakers but pacemaking in SNpc but not in VTA is accompanied by calcium influx through L-type calcium channel, Ca_V1.3 contributing to increased intracellular calcium and hence to cell death. Ca_V1.3_42A, an alternatively spliced short variant of Ca_V1.3 has increased calcium influx. We, therefore studied the role of Ca_V1.3₄₂ (full-length channel) and Ca_V1.3_42A in mouse SNpc in PD pathogenesis by quantifying mRNA levels of Ca_V1.3₄₂ and Ca_V1.3_42A in SNpc and followed the change in their levels in MPTP induced parkinsonism mouse model. Using in situ hybridization and immunohistochemistry we observed the localization of mRNA of Ca_V1.3₄₂ and Ca_V1.3_42A in tyrosine hydroxylase (TH) positive DA neurons. Further, mRNA levels of Ca_V1.3_42A were higher in SNpc as compared to the cortex. Upon MPTP treatment, mRNA levels of Ca_V1.3₄₂ and Ca_V1.3_42A maintained their levels in SNpc in spite of the loss of ~50% of the DA neurons. This indicates that the expression of Ca_V1.3₄₂ and Ca_V1.3_42A is maintained at a robust level during the degenerative process in the parkinsonism model.

Ineffectiveness of saxagliptin as a neuroprotective drug in 6-OHDA-lesioned rats

OBJECTIVES

To determine whether the drug saxagliptin, a dipeptidyl peptidase-4 (DPP-4) inhibitor which is utilized for the treatment of Diabetes Mellitus, has neuroprotective effects in the animal model of Parkinson's disease (PD) induced by 6-hydroxydopamine (6-OHDA) in rats.

METHODS

Male Wistar rats (weighing 280-300 g) received a bilateral infusion of 6-OHDA in the substantia nigra. Twenty-four hours later, they were treated with saxagliptin (1 mg/kg, p.o) once daily, for 21 days. The motor function was evaluated using the open field and rotarod (RT) tests. In addition, cognition was assessed with the novel object recognition test (ORT). After the evaluation of the behavioural tests, the animals were transcardially perfused to perform immunohistochemistry staining for tyrosine hydroxylase (TH) in the substantia nigra pars compacta (SNpc).

KEY FINDINGS

Saxagliptin impaired the memory of animals in the sham group.

CONCLUSIONS

Saxagliptin treatment did not exhibit neuroprotection and it did not improve the cognitive and motor deficits in the 6-OHDA model of PD. Interestingly, when saxagliptin was administered to the sham animals, a cognitive decline was observed. Therefore, this drug should be investigated as a possible treatment for PTSD.

Synergy of AMPA and NMDA Receptor Currents in Dopaminergic Neurons: A Modeling Study

Dopaminergic (DA) neurons display two modes of firing: low-frequency tonic and high-frequency bursts. The high frequency firing within the bursts is attributed to NMDA, but not AMPA receptor activation. In our models of the DA neuron, both biophysical and abstract, the NMDA receptor current can significantly increase their firing frequency, whereas the AMPA receptor current is not able to evoke high-frequency activity and usually suppresses firing. However, both currents are produced by glutamate receptors and, consequently, are often co-activated. Here we consider combined influence of AMPA and NMDA synaptic input in the models of the DA neuron. Different types of neuronal activity (resting state, low frequency, or high frequency firing) are observed depending on the conductance of the AMPAR and NMDAR currents. In two models, biophysical and reduced, we show that the firing frequency increases more effectively if both receptors are co-activated for certain parameter values. In particular, in the more quantitative biophysical model, the maximal frequency is 40% greater than that with NMDAR alone. The dynamical mechanism of such frequency growth is explained in the framework of phase space evolution using the reduced model. In short, both the AMPAR and NMDAR currents flatten the voltage nullcline, providing the frequency increase, whereas only NMDA prevents complete unfolding of the nullcline, providing robust firing. Thus, we confirm a major role of the NMDAR in generating high-frequency firing and conclude that AMPAR activation further significantly increases the frequency.

Balance between the proximal dendritic compartment and the soma determines spontaneous firing rate in midbrain dopamine neurons

Midbrain dopamine (DA) neurons are slow intrinsic pacemakers that require the elaborate composition of many ion channels in the somatodendritic compartments. Understanding the major determinants of the spontaneous firing rate (SFR) of midbrain DA neurons is important because they determine the basal DA levels in target areas, including the striatum. As spontaneous firing occurs synchronously at the soma and dendrites, the electrical coupling between the soma and dendritic compartments has been regarded as a key determinant for the SFR. However, it is not known whether this somatodendritic coupling is served by the whole dendritic compartments or only parts of them. In the rat substantia nigra pars compacta (SNc) DA neurons, we demonstrate that the balance between the proximal dendritic compartment and the soma determines the SFR. Isolated SNc DA neurons showed a wide range of soma size and a variable number of primary dendrites but preserved a quite consistent SFR. The SFR was not correlated with soma size or with the number of primary dendrites, but it was strongly correlated with the area ratios of the proximal dendritic compartments to the somatic compartment. Tetrodotoxin puff and local Ca(2+) perturbation experiments, computer simulation, and local glutamate uncaging experiments suggest the importance of the proximal dendritic compartments in pacemaker activity. These data indicate that the proximal dendritic compartments, not the whole dendritic compartments, play a key role in the somatodendritic balance that determines the SFR in DA neurons.

Interaction of NMDA receptor and pacemaking mechanisms in the midbrain dopaminergic neuron

Dopamine neurotransmission has been found to play a role in addictive behavior and is altered in psychiatric disorders. Dopaminergic (DA) neurons display two functionally distinct modes of electrophysiological activity: low- and high-frequency firing. A puzzling feature of the DA neuron is the following combination of its responses: N-methyl-D-aspartate receptor (NMDAR) activation evokes high-frequency firing, whereas other tonic excitatory stimuli (α-amino-3-hydroxyl-5-methyl-4-isoxazolepropionate receptor (AMPAR) activation or applied depolarization) block firing instead. We suggest a new computational model that reproduces this combination of responses and explains recent experimental data. Namely, somatic NMDAR stimulation evokes high-frequency firing and is more effective than distal dendritic stimulation. We further reduce the model to a single compartment and analyze the mechanism of the distinct high-frequency response to NMDAR activation vs. other stimuli. Standard nullcline analysis shows that the mechanism is based on a decrease in the amplitude of calcium oscillations. The analysis confirms that the nonlinear voltage dependence provided by the magnesium block of the NMDAR determine its capacity to elevate the firing frequency. We further predict that the moderate slope of the voltage dependence plays the central role in the frequency elevation. Additionally, we suggest a repolarizing current that sustains calcium-independent firing or firing in the absence of calcium-dependent repolarizing currents. We predict that the ether-a-go-go current (ERG), which has been observed in the DA neuron, is the best fit for this critical role. We show that a calcium-dependent and a calcium-independent oscillatory mechanisms form a structure of interlocked negative feedback loops in the DA neuron. The structure connects research of DA neuron firing with circadian biology and determines common minimal models for investigation of robustness of oscillations, which is critical for normal function of both systems.

The h-current in the substantia Nigra pars compacta neurons: a re-examination

The properties of the hyperpolarization-activated cation current (I_h) were investigated in rat substantia nigra - pars compacta (SNc) principal neurons using patch-clamp recordings in thin slices. A reliable identification of single dopaminergic neurons was made possible by the use of a transgenic line of mice expressing eGFP under the tyrosine hydroxylase promoter. The effects of temperature and different protocols on the I_h kinetics showed that, at 37°C and minimizing the disturbance of the intracellular milieu with perforated patch, this current actually activates at potentials more positive than what is generally indicated, with a half-activation potential of −77.05 mV and with a significant level of opening already at rest, thereby substantially contributing to the control of membrane potential, and ultimately playing a relevant function in the regulation of the cell excitability. The implications of the known influence of intracellular cAMP levels on I_h amplitude and kinetics were examined. The direct application of neurotransmitters (DA, 5-HT and noradrenaline) physiologically released onto SNc neurons and known to act on metabotropic receptors coupled to the cAMP pathway modify the I_h amplitude. Here, we show that direct activation of dopaminergic and of 5-HT receptors results in I_h inhibition of SNc DA cells, whereas noradrenaline has the opposite effect. Together, these data suggest that the modulation of I_h by endogenously released neurotransmitters acting on metabotropic receptors –mainly but not exclusively linked to the cAMP pathway- could contribute significantly to the control of SNc neuron excitability.

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.

Differences in Na+ conductance density and Na+ channel functional properties between dopamine and GABA neurons of the rat substantia nigra

Dopamine (DA) neurons and GABA neurons of the substantia nigra (SN) promote distinct functions in the control of movement and have different firing properties and action potential (AP) waveforms. APs recorded from DA and GABA neurons differed in amplitude, maximal rate of rise, and duration. In addition, the threshold potential for APs was higher in DA neurons than in GABA neurons. The activation of voltage-gated Na(+) channels accounts largely for these differences as the application of a low concentration of the voltage-gated Na(+) channel blocker TTX had an effect on all of these parameters. We have examined functional properties of somatic Na(+) channels in nucleated patches isolated from DA and GABA neurons. Peak amplitudes of macroscopic Na(+) currents were smaller in DA neurons in comparison to those in GABA neurons. The mean peak Na(+) conductance density was 24.5 pS microm(-2) in DA neurons and almost twice as large, 41.6 pS microm(-2), in GABA neurons. The voltage dependence of Na(+) channel activation was not different between the two types of SN neurons. Na(+) channels in DA and GABA neurons, however, differed in the voltage dependence of inactivation, the mean mid-point potential of steady-state inactivation curve being more positive in DA neurons than in GABA neurons. The results suggest that specific Na(+) channel gating properties and Na(+) conductance densities in the somatic membrane of SN neurons may have consequences on synaptic signal integration in the soma of both types of neurons and on somatodendritic release of dopamine by DA neurons.

Ca2+ influx through NMDA-gated channels activates ATP-sensitive K+ currents through a nitric oxide-cGMP pathway in subthalamic neurons

Excessive burst firing of action potentials in subthalamic nucleus (STN) neurons has been correlated with the bradykinesia and rigidity seen in Parkinson's disease. Consequently, there is much interest in characterizing mechanisms that promote burst firing, such as the regulation of NMDA receptor function. Using whole-cell recording techniques in rat brain slices, we report that inward currents evoked by NMDA are greatly potentiated by ATP-sensitive K(+) (K-ATP) channel blocking agents in STN neurons but not in dopamine neurons in the substantia nigra. Moreover, we found that the ability of NMDA to evoke K-ATP current was blocked by inhibitors of nitric oxide synthase, guanylyl cyclase, and calcium/calmodulin. By altering firing patterns of STN neurons, this NMDA/K-ATP interaction may exert an important influence on basal ganglia output and thereby affect the clinical expression of Parkinson's disease.

Endogenous calcium buffering capacity of substantia nigral dopamine neurons

Dopamine (DA)-containing cells from the substantia nigra pars compacta (SNc) play a major role in the initiation of movement. Loss of these cells results in Parkinson's disease (PD). Changes in intracellular calcium ion concentration ([Ca(2+)](i)) elicit several events in DA cells, including spike afterhyperpolarizations (AHPs) and subthreshold oscillations underlying autonomous firing. Continuous Ca(2+) load due to Ca(2+)-dependent rhythmicity has been proposed to cause the death of DA cells in PD and normal aging. Because of the physiological and pathophysiological importance of [Ca(2+)](i) in DA cells, we characterized their intrinsic Ca(2+)-buffering capacity (K(S)) in brain slices. We introduced a fluorescent Ca(2+)-sensitive exogenous buffer (200 microM fura-2) and cells were tracked from break-in until steady state by stimulating with a single action potential (AP) every 30 s and measuring the Ca(2+) transient from the proximal dendrite. DA neurons filled exponentially with a tau of about 5-6 min. [Ca(2+)](i) was assumed to equilibrate between the endogenous Ca(2+) buffer and the exogenous Ca(2+) indicator buffer. Intrinsic buffering was estimated by extrapolating from the linear relationships between the amplitude or time constant of the Ca(2+) transients versus [fura-2]. Extrapolated Ca(2+)-transients in the absence of fura-2 had mean peak amplitudes of 293.7 +/- 65.3 nM and tau = 124 +/- 13 ms (postnatal day 13 [P13] to P17 animals). Intrinsic buffering increased with age in DA neurons. For cells from animals P13-P17, K(S) was estimated to be about 110 (n = 20). In older animals (P25-P32), the estimate was about 179 (n = 10). These relatively low values may reflect the need for rapid Ca(2+) signaling, e.g., to allow activation of sK channels, which shape autonomous oscillations and burst firing. Low intrinsic buffering may also make DA cells vulnerable to Ca(2+)-dependent pathology.

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.

Roles of subthreshold calcium current and sodium current in spontaneous firing of mouse midbrain dopamine neurons

We used a preparation of acutely dissociated neurons to quantify the ionic currents driving the spontaneous firing of substantia nigra pars compacta neurons, isolated from transgenic mice in which the tyrosine hydroxylase promoter drives expression of human placental alkaline phosphatase (PLAP) on the outer surface of the cell membrane. Dissociated neurons identified by fluorescent antibodies to PLAP showed firing properties similar to those of dopaminergic neurons in brain slice, including rhythmic spontaneous firing of broad action potentials and, in some cells, rhythmic oscillatory activity in the presence of tetrodotoxin (TTX). Spontaneous activity in TTX had broader, smaller spikes than normal pacemaking and was stopped by removal of external calcium. Normal pacemaking was also consistently silenced by replacement of external calcium by cobalt and was slowed by more specific calcium channel blockers. Nimodipine produced a slowing of pacemaking frequency. Pacemaking was also slowed by the P/Q-channel blocker omega-Aga-IVA, but the N-type channel blocker omega-conotoxin GVIA had no effect. In voltage-clamp experiments, using records of pacemaking as command voltage, cobalt-sensitive current and TTX-sensitive current were both sizeable at subthreshold voltages between spikes. Cobalt-sensitive current was consistently larger than TTX-sensitive current at interspike voltages from -70 to -50 mV, with TTX-sensitive current larger at voltages positive to -45 mV. These results support previous evidence for a major role of voltage-dependent calcium channels in driving pacemaking of midbrain dopamine neurons and suggest that multiple calcium channel types contribute to this function. The results also show a significant contribution of subthreshold TTX-sensitive sodium current.

Transient high-frequency firing in a coupled-oscillator model of the mesencephalic dopaminergic neuron

Dopaminergic neurons of the midbrain fire spontaneously at rates <10/s and ordinarily will not exceed this range even when driven with somatic current injection. When driven at higher rates, these cells undergo spike failure through depolarization block. During spontaneous bursting of dopaminergic neurons in vivo, bursts related to reward expectation in behaving animals, and bursts generated by dendritic application of N-methyl-d-aspartate (NMDA) agonists, transient firing attains rates well above this range. We suggest a way such high-frequency firing may occur in response to dendritic NMDA receptor activation. We have extended the coupled oscillator model of the dopaminergic neuron, which represents the soma and dendrites as electrically coupled compartments with different natural spiking frequencies, by addition of dendritic AMPA (voltage-independent) or NMDA (voltage-dependent) synaptic conductance. Both soma and dendrites contain a simplified version of the calcium-potassium mechanism known to be the mechanism for slow spontaneous oscillation and background firing in dopaminergic cells. The compartments differ only in diameter, and this difference is responsible for the difference in natural frequencies. We show that because of its voltage dependence, NMDA receptor activation acts to amplify the effect on the soma of the high-frequency oscillation of the dendrites, which is normally too weak to exert a large influence on the overall oscillation frequency of the neuron. During the high-frequency oscillations that result, sodium inactivation in the soma is removed rapidly after each action potential by the hyperpolarizing influence of the dendritic calcium-dependent potassium current, preventing depolarization block of the spike mechanism, and allowing high-frequency spiking.

Nigral GABAergic inhibition upon mesencephalic dopaminergic cell groups in rats

Synaptic inhibition from the substantia nigra pars reticulata (SNr) to the mesencephalic dopaminergic neurons, which was mediated by gamma (gamma)-amino-butyric acid (GABA), was investigated in a midbrain slice preparation of Wistar rats. Whole-cell patch-clamp recordings were used to record synaptic potentials/currents from the dopaminergic neurons (n = 93) located in the retrorubral field (n = 22), the substantia nigra pars compacta (n = 47) and the ventral tegmental area (n = 24). In the presence of ionotropic glutamate receptor antagonists electrical stimulation of the SNr induced inhibitory postsynaptic potentials (IPSPs) and/or currents (IPSCs) in 83 neurons. The IPSPs/IPSCs were comprised early and late components. The early IPSPs/IPSCs were mediated by chloride currents through GABA(A) receptors. The late IPSPs/IPSCs were mediated by potassium currents through GABA(B) receptors. Both GABA(A)- and GABA(B)-IPSPs were amplified by repetitive stimuli with frequencies between 25 and 200 Hz. This frequency range covers the firing frequencies of SNr neurons in vivo. It was observed that an application of a GABA(B) receptor antagonist increased the amplitude of the GABA(A)-IPSPs. The amplification was followed by a rebound depolarization that induced transient firing of dopaminergic neurons. These properties of the IPSPs were common in all of the three dopaminergic nuclei. These results suggest that postsynaptic GABA(A)- and GABA(B)-inhibition contribute to transient and persistent alternations of the excitability of dopaminergic neurons, respectively. These postsynaptic mechanisms may be, in turn, regulated by presynaptic GABA(B)-inhibition. Nigral GABAergic input may provide the temporospatial regulation of the background excitability of mesencephalic dopaminergic systems.

Influence of R-type (Cav2.3) and t-type (Cav3.1-3.3) antagonists on nigral somatodendritic dopamine release measured by microdialysis

The release of dopamine from soma and dendrites of dopaminergic neurons in substantia nigra has been reported to be calcium-dependent, but it remains to be determined which calcium channels mediate this effect. We have used in vivo microdialysis in rat substantia nigra and striatum to investigate the effect of Ca(v)3.1-3.3 (T-type) and Ca(v)2.3 (R-type) calcium channel antagonists on somatodendritic and terminal dopamine release. Local reverse dialysis administration of 0.1-10 microM of the Ca(v)2.3 inhibitor SNX-482, or 100 microM of mibefradil, decreased the concentrations of dopamine and its metabolites in dialysate from substantia nigra, whereas 1 microM mibefradil or 40-80 microM nickel(II) induced an increase in nigral dialysate dopamine concentrations. Dopamine concentrations in striatal dialysates were decreased only by 10 microM of SNX-482 or 100 microM of mibefradil. Nickel(II) induced an increase in striatal dialysate dopamine concentration similar to that in substantia nigra. The results indicate a role for Ca(v)2.3 (R-type) voltage sensitive calcium channels in the calcium dependency of somatodendritic dopamine release, but argue against a calcium dependency mediated substantially by Ca(v)3.1-3.3 (T-type) channels.

Dendritic projections and dye-coupling in dopaminergic neurons of the substantia nigra examined in horizontal brain slices from young rats

We examined the rostro-caudal dendritic spread of striatally projecting dopaminergic neurons of the Substantia Nigra pars compacta (SNc) and investigated the presence of dye-coupling after labeling these cells with a mixture of lucifer yellow (LY) and neurobiotin (NB) or with LY alone. Whole cell recordings were made from horizontal brain slices (400 microm) obtained from P5-P20 rats. SNc neurons retrogradely labeled with Fluoro-Gold and located in the region containing tyrosine hydroxylase-immunoreactive cells displayed Ih current and other properties characteristic of SNc neurons. To prevent extracellular leakage, dyes were introduced into patch pipettes after the establishment of whole cell configuration, and cells were filled under visual control. In contrast to previous studies conducted in coronal sections that identified dendritic projections of SNc neurons mainly in the medio-lateral and ventral directions, almost all neurons labeled in our study (53/54) additionally displayed a large rostro-caudal dendritic span (649 +/- 219 microm). Dye-coupling between SNc neurons was not observed under basal conditions, in the presence of gap junction "openers" (forskolin, trimethylamine), or after neurons were filled with LY using sharp intracellular microelectrodes. As a "positive control," dye-coupling was demonstrated in four hippocampal dentate gyrus neurons that were filled using the same patch pipette technique. In addition, none of the tested SNc cells (n = 12) showed expression of connexin 36 (the "neuronal" connexin) when tested with single-cell RT-PCR. In conclusion, this study revealed extensive rostro-caudal dendritic projections of SNc neurons. Under our in vitro conditions, no evidence was found for dye-coupling among these neurons.

Actions of the sodium channel inhibitor 202W92 on rat midbrain dopaminergic neurons

Excessive glutamatergic activity is implicated in Parkinson's disease (PD) and sodium channel blockade, resulting in inhibition of glutamate release, is a potential therapeutic approach to PD therapy. Beneficial effects of riluzole and lamotrigine have been reported in animal models of PD, but these compounds have relatively low potency as sodium channel inhibitors and also inhibit N and P/Q-type calcium channels. 202W92, a structural analog of lamotrigine, is a potent sodium channel inhibitor, with no effect on N, P/Q-type channels. Here we present the effects of 202W92 on single patch-clamped dopaminergic neurons. 202W92 (> or =10 microM) inhibited spontaneous action potential firing and reduced amplitude and frequency of evoked action potentials. It also inhibited the frequency of 4-aminopyridine (4-AP)- and electrically evoked excitatory postsynaptic currents (EPSCs) and GABAergic inhibitory postsynaptic currents (IPSCs), with >80% inhibition at 10 microM (IC(50) 1.5 microM). EPSC and IPSC amplitudes were partially inhibited. 202W92 did not affect postsynaptic responses to locally applied glutamate and GABA, nor spontaneously occurring mini-IPSCs. These actions of 202W92 are compatible with sodium channel inhibition and depression of transmitter release.

Differential regulation of somatodendritic and nerve terminal dopamine release by serotonergic innervation of substantia nigra

Nigrostriatal dopaminergic neurons release dopamine from dendrites in substantia nigra and axon terminals in striatum. The cellular mechanisms for somatodendritic and axonal dopamine release are similar, but somatodendritic and nerve terminal dopamine release may not always occur in parallel. The current studies used in vivo microdialysis to simultaneously measure changes in dendritic and nerve terminal dopamine efflux in substantia nigra and ipsilateral striatum respectively, following intranigral application of various drugs by reverse dialysis through the nigral probe. The serotonin releasers (+/-)-fenfluramine (100 micro m) and (+)-fenfluramine (100 micro m) significantly increased dendritic dopamine efflux without affecting extracellular dopamine in striatum. The non-selective serotonin receptor agonist 1-(m-chlorophenyl)-piperazine (100 micro m) elicited a similar pattern of dopamine release in substantia nigra and striatum. NMDA (33 micro m) produced an increase in nigral dopamine of a similar magnitude to mCPP or either fenfluramine drug. However, NMDA also induced a concurrent increase in striatal dopamine. The D2 agonist quinpirole (100 micro m) had a parallel inhibitory effect on dopamine release from dendritic and terminal sites as well. Taken together, these data suggest that serotonergic afferents to substantia nigra may evoke dendritic dopamine release through a mechanism that is uncoupled from the impulse-dependent control of nerve terminal dopamine release.

Dopamine-mediated volume transmission in midbrain is regulated by distinct extracellular geometry and uptake

Somatodendritic release of dopamine (DA) in midbrain is, at least in part, nonsynaptic; moreover, midbrain DA receptors are predominantly extrasynaptic. Thus somatodendritic DA mediates volume transmission, with an efficacy regulated by the diffusion and uptake characteristics of the local extracellular microenvironment. Here, we quantitatively evaluated diffusion and uptake in substantia nigra pars compacta (SNc) and reticulata (SNr), ventral tegmental area (VTA), and cerebral cortex in guinea pig brain slices. The geometric parameters that govern diffusion, extracellular volume fraction (alpha) and tortuosity (lambda), together with linear uptake (k'), were determined for tetramethylammonium (TMA(+)), and for DA, using point-source diffusion combined with ion-selective and carbon-fiber microelectrodes. TMA(+)-diffusion measurements revealed a large alpha of 30% in SNc, SNr, and VTA, which was significantly higher than the 22% in cortex. Values for lambda and k' for TMA(+) were similar among regions. Point-source DA-diffusion curves fitted theory well with linear uptake, with significantly higher values of k' for DA in SNc and VTA (0.08--0.09 s(-1)) than in SNr (0.006 s(-1)), where DA processes are sparser. Inhibition of DA uptake by GBR-12909 caused a greater decrease in k' in SNc than in VTA. In addition, DA uptake was slightly decreased by the norepinephrine transport inhibitor, desipramine in both regions, although this was statistically significant only in VTA. We used these data to model the radius of influence of DA in midbrain. Simulated release from a 20-vesicle point source produced DA concentrations sufficient for receptor activation up to 20 microm away with a DA half-life at this distance of several hundred milliseconds. Most importantly, this model showed that diffusion rather than uptake was the most important determinant of DA time course in midbrain, which contrasts strikingly with the striatum where uptake dominates. The issues considered here, while specific for DA in midbrain, illustrate fundamental biophysical properties relevant for all extracellular communication.

Neurotrophin-4/5 promotes dendritic outgrowth and calcium currents in cultured mesencephalic dopamine neurons

Ca(2+) currents and their modulation by neurotrophin-4/5 were studied in cultured mesencephalic neurons. Tyrosine hydroxylase-positive neurons consistently had larger somas than tyrosine hydroxylase-negative neurons. Neurons with larger somas were therefore targeted for recording. In both control and neurotrophin-4/5-treated cultured neurons, isolation of Ca(2+) currents in cultured mesencephalic neurons revealed prominent low- and high-voltage-activated currents. These currents were separable based upon their voltage dependence of activation, the response to replacement of Ca(2+) with Ba(2+) and the response to Ca(2+) channel blockers. Replacement of Ca(2+) with Ba(2+) resulted in a slight reduction of low-voltage-activated currents and a significant enhancement of high-voltage-activated currents. Cd(2+) blocked a larger fraction of the high-voltage-activated current than Ni(2+). The synthetic conotoxins SNX-124 and SNX-230 selectively blocked high-voltage-activated currents. Morphological analysis of mesencephalic cultures pretreated with neurotrophin-4/5 revealed an increase in soma size and dendritic length in tyrosine hydroxylase-positive neurons. In agreement with the neurotrophin-4/5 induction of growth, neurotrophin-4/5 also increased cell capacitance in whole-cell recordings. Neurotrophin-4/5 significantly enhanced both low- and high-voltage-activated currents, but normalization for changes in capacitance revealed only a significant increase in high-voltage-activated current density. This study demonstrates the existence of low-voltage-activated and multiple classes of high-voltage-activated calcium currents in cultured mesencephalic neurons. Morphological and physiological data demonstrate that the increases in calcium currents due to neurotrophin-4/5 pretreatment are associated with somatodendritic growth, but an increase in high-voltage-activated Ca(2+) channel expression also occurred.

Coupled oscillator model of the dopaminergic neuron of the substantia nigra

Calcium imaging using fura-2 and whole cell recording revealed the effective location of the oscillator mechanism on dopaminergic neurons of the substantia nigra, pars compacta, in slices from rats aged 15-20 days. As previously reported, dopaminergic neurons fired in a slow rhythmic single spiking pattern. The underlying membrane potential oscillation survived blockade of sodium currents with TTX and was enhanced by blockade of voltage-sensitive potassium currents with TEA. Calcium levels increased during the subthreshold depolarizing phase of the membrane potential oscillation and peaked at the onset of the hyperpolarizing phase as expected if the pacemaker potential were due to a low-threshold calcium current and the hyperpolarizing phase to calcium-dependent potassium current. Calcium oscillations were synchronous in the dendrites and soma and were greater in the dendrites than in the soma. Average calcium levels in the dendrites overshot steady-state levels and decayed over the course of seconds after the oscillation was resumed after having been halted by hyperpolarizing currents. Average calcium levels in the soma increased slowly, taking many cycles to achieve steady state. Voltage clamp with calcium imaging revealed the voltage dependence of the somatic calcium current without the artifacts of incomplete spatial voltage control. This showed that the calcium current had little or no inactivation and was half-maximal at -40 to -30 mV. The time constant of calcium removal was measured by the return of calcium to resting levels and depended on diameter. The calcium sensitivity of the calcium-dependent potassium current was estimated by plotting the slow tail current against calcium concentration during the decay of calcium to resting levels at -60 mV. A single compartment model of the dopaminergic neuron consisting of a noninactivating low-threshold calcium current, a calcium-dependent potassium current, and a small leak current reproduced most features of the membrane potential oscillations. The same currents much more accurately reproduced the calcium transients when distributed uniformly along a tapering cable in a multicompartment model. This model represented the dopaminergic neuron as a set of electrically coupled oscillators with different natural frequencies. Each frequency was determined by the surface area to volume ratio of the compartment. This model could account for additional features of the dopaminergic neurons seen in slices, such as slow adaptation of oscillation frequency and may produce irregular firing under different coupling conditions.

Regulation of action potential size and excitability in substantia nigra compacta neurons: sensitivity to 4-aminopyridine

Slow, pacemaker-like firing is due to intrinsic membrane properties in substantia nigra compacta (SNc) neurons in vitro. How these properties interact with afferent synaptic inputs is not fully understood. In this study, intracellular recordings from SNc neurons in brain slices showed that spontaneous action potentials (APs) were attenuated when generated from lower than normal threshold. Such APs were blocked by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and could be related to non-N-methyl-D-aspartate (NMDA) receptor-mediated spontaneous excitatory postsynaptic potentials (EPSPs). The AP attenuation was reproduced by stimulus-evoked EPSPs and by current injections to the soma. APs evoked from holding potentials between -40 and -60 mV were reduced in width by Cd(2+) (0. 2 mM). Tetraethylammonium chloride (TEA, 10 mM) or 4-aminopyridine (4-AP, 5 mM) increased the AP width. However, at more negative holding potentials, Cd(2+) and TEA were inefficacious, whereas 4-AP enlarged the AP, partly via induction of a Cd(2+)-sensitive component. A monophasic afterhyperpolarization (AHP), following attenuated APs, was little affected by either Cd(2+) or TEA, but inhibited by 4-AP, which induced an additional, slow component, sensitive to Cd(2+) or apamin (100 nM). The AP delay showed a discontinuous relation to the amplitude or slope of the injected current (delay shift), which was sensitive to low doses of 4-AP (0. 05 mM). The initial time window before the delay shift was longer than the rise time of EPSPs. It is suggested that a 4-AP-sensitive current prevents or postpones discharge during slow depolarization's, but allows direct excitation by fast EPSPs. Fast excitation leads to AP attenuation, primarily due to strong activation of 4-AP-sensitive current. This seems to cause inhibition of the Ca(2+) current during the AP and reduction of Ca(2+)-dependent K(+) currents. Together, these properties are likely to influence the excitability and the local, somatodendritic effects of the AP, in a manner that discriminates between firing induced by the intrinsic pacemaker mechanism and fast synaptic potentials.

Afferent modulation of dopamine neuron firing patterns

In recent studies examining the modulation of dopamine (DA) cell firing patterns, particular emphasis has been placed on excitatory afferents from the prefrontal cortex and the subthalamic nucleus. A number of inconsistencies in recently published reports, however, do not support the contention that tonic activation of NMDA receptors is the sole determinate of DA neuronal firing patterns. The results of work on the basic mechanism of DA firing and the action of apamin suggest that excitatory projections to DA neurons from cholinergic and glutamatergic neurons in the tegmental pedunculopontine nucleus, and/or inhibitory GABAergic projections, are also involved in modulating DA neuron firing behavior.

Blockade of SK-type Ca2+-activated K+ channels uncovers a Ca2+-dependent slow afterdepolarization in nigral dopamine neurons

Sharp electrode current-clamp recording techniques were used to characterize the response of nigral dopamine (DA)-containing neurons in rat brain slices to injected current pulses applied in the presence of TTX (2 microM) and under conditions in which apamin-sensitive Ca2+-activated K+ channels were blocked. Addition of apamin (100-300 nM) to perfusion solutions containing TTX blocked the pacemaker oscillation in membrane voltage evoked by depolarizing current pulses and revealed an afterdepolarization (ADP) that appeared as a shoulder on the falling phase of the voltage response. ADP were preceded by a ramp-shaped slow depolarization and followed by an apamin-insensitive hyperpolarizing afterpotential (HAP). Although ADPs were observed in all apamin-treated cells, the duration of the response varied considerably between individual neurons and was strongly potentiated by the addition of TEA (2-3 mM). In the presence of TTX, TEA, and apamin, optimal stimulus parameters (0.1 nA, 200-ms duration at -55 to -68 mV) evoked ADP ranging from 80 to 1,020 ms in duration (355.3 +/- 56.5 ms, n = 16). Both the ramp-shaped slow depolarization and the ensuing ADP were markedly voltage dependent but appeared to be mediated by separate conductance mechanisms. Thus, although bath application of nifedipine (10-30 microM) or low Ca2+, high Mg2+ Ringer blocked the ADP without affecting the ramp potential, equimolar substitution of Co2+ for Ca2+ blocked both components of the voltage response. Nominal Ca2+ Ringer containing Co2+ also blocked the HAP evoked between -55 and -68 mV. We conclude that the ADP elicited in DA neurons after blockade of apamin-sensitive Ca2+-activated K+ channels is mediated by a voltage-dependent, L-type Ca2+ channel and represents a transient form of the regenerative plateau oscillation in membrane potential previously shown to underlie apamin-induced bursting activity. These data provide further support for the notion that modulation of apamin-sensitive Ca2+-activated K+ channels in DA neurons exerts a permissive effect on the conductances that are involved in the expression of phasic activity.

Electrophysiological characterization of GABAergic neurons in the ventral tegmental area

GABAergic neurons in the ventral tegmental area (VTA) play a primary role in local inhibition of mesocorticolimbic dopamine (DA) neurons but are not physiologically or anatomically well characterized. We used in vivo extracellular and intracellular recordings in the rat VTA to identify a homogeneous population of neurons that were distinguished from DA neurons by their rapid-firing, nonbursting activity (19.1 +/- 1.4 Hz), short-duration action potentials (310 +/- 10 microseconds), EPSP-dependent spontaneous spikes, and lack of spike accommodation to depolarizing current pulses. These non-DA neurons were activated both antidromically and orthodromically by stimulation of the internal capsule (IC; conduction velocity, 2.4 +/- 0.2 m/sec; refractory period, 0.6 +/- 0.1 msec) and were inhibited by stimulation of the nucleus accumbens septi (NAcc). Their firing rate was moderately reduced, and their IC-driven activity was suppressed by microelectrophoretic application or systemic administration of NMDA receptor antagonists. VTA non-DA neurons were recorded intracellularly and showed relatively depolarized resting membrane potentials (-61.9 +/- 1.8 mV) and small action potentials (68.3 +/- 2.1 mV). They were injected with neurobiotin and shown by light microscopic immunocytochemistry to be multipolar cells and by electron microscopy to contain GABA but not the catecholamine-synthesizing enzyme tyrosine hydroxylase (TH). Neurobiotin-filled dendrites containing GABA received asymmetric excitatory-type synapses from unlabeled terminals and symmetric synapses from terminals that also contained GABA. These findings indicate that VTA non-DA neurons are GABAergic, project to the cortex, and are controlled, in part, by a physiologically relevant NMDA receptor-mediated input from cortical structures and by GABAergic inhibition.

Electrophysiological evidence for putative subtypes of neurotensin receptors in guinea-pig mesencephalic dopaminergic neurons

Electrophysiologically identified mesencephalic dopaminergic neurons were examined by means of extra- and intracellular microelectrodes in coronal slices of guinea-pig brain. Neurotensin and its C-terminal fragment (8-13) were equipotent in the enhancement of spontaneous neuronal firing rate (EC50 values 81.9 and 72.6nM, respectively). The duration of response was significantly longer and more variable for neurotensin compared to neurotensin fragment (8-13) (mean half-time of recovery 423+/-44 and 100+/-14 s, respectively, for peptides applied at 300 nM). The initial fast phase of excitatory responses to neurotensin receptor agonists was associated with membrane depolarization (when assessed in current-clamp mode) or with inward currents (when assessed in voltage-clamp mode), whereas prolonged excitation was associated with a slowly occurring and long-lasting change in the late afterhyperpolarization. Two kinetically distinct components were revealed in responses to neurotensin and neurotensin fragment (8-13) by the use of SR48692 and SR142948, two selective non-peptide neurotensin receptor antagonists. SR142948 (100 nM) potently antagonized responses to both agonists [response was reduced by 661 5% and 74+/-9% for neurotensin and neurotensin fragment (8-13), respectively] and caused a rightward shift in the concentration-response curve for neurotensin. On the other hand, SR48692 (100 nM) selectively inhibited the slow (late afterhyperpolarization-dependent) component, without altering the response amplitude; the half-time of recovery was reduced by 71+/-6% and 65+/-5% of control values for responses induced by neurotensin (300 nM) and neurotensin fragment (8-13) (300 nM), respectively. In addition, neurotensin, but not neurotensin fragment (8-13), provoked SR48692-sensitive and long-lasting attenuation of dopamine-induced inhibitory responses. It is suggested that two subtypes of neurotensin receptors are present in dopaminergic neurons, based on the differences in agonist and antagonist sensitivity, kinetic properties and the membrane mechanisms involved.

Axonal expression sites of tyrosine hydroxylase, calretinin- and calbindin-immunoreactivity in striato-pallidal and septal nuclei of the rat brain: a double-immunolabelling study

Besides the dopaminergic afferent projection system, calbindin (CALB)- and calretinin (CR)-immunoreactive fibres of intrinsic and extrinsic origin represent the most abundant axonal categories in the rat striatal and lateral septal areas. The question arises whether or not they may represent separate populations, or whether they form subgroups which co-express more than one of these antigens. Therefore, the present study is focused on the distribution patterns of the axons single-immunolabelled by the catecholaminergic marker tyrosine hydroxylase (TH), and on TH-immunoreactive axons displaying also CR- and/or CALB-immunoreactivity in double-immunostained sections. Striking differences were found between the patch and matrix compartments of the caudate-putamen (CP). Whereas the vast majority of TH-immunoreactive fibres in the patches and a patch-associated subcallosal layer co-expressed CR but not CALB, fibres mono-labelled by the TH-immunoreactivity were predominant in the matrix. The matrix-like regions of the core of nucleus accumbens (CACC), fundus striati (FS), the striatal cell bridges (CB) and the striatal part of olfactory tubercle (OTU) coincided in this respect with the matrix in CP. The absence of CR-immunoreactivity was also characteristic of the TH-immunoreactive fibres in the patch-like areas of the accumbal core, although a high number of separate CR-immunoreactive axons were present. In the shell of nucleus accumbens (SACC) which receives a rich catecholaminergic innervation, fibres co-expressing either one of the calcium-binding proteins were absent. The islands of Calleja (CJI) displaying a strongly TH-immunoreactive centre and a periphery of lower staining intensity, showed only a low number of TH-immunoreactive fibres co-expressing CR or CALB. The broad shell-like band of TH-immunoreactive axons between medial and lateral part of the septum was single-stained with the TH-immunoreactivity. In contrast, the TH-positive fibres forming basket-like arrangements around some neurons in the dorsal lateral septal nucleus co-expressed also CR, but not CALB. The results are discussed in view of the recent concepts of basal forebrain organization and the cytochemical characteristics of mesencephalic dopaminergic nuclei giving rise to the vast majority of the striatal and septal TH-immunoreactive fibre supply, in order to correlate the known projection patterns with the content of calcium-binding proteins in TH-immunolabelled fibres and presumed cells of origin. The TH-immunoreactive fibres in the striatal patches displaying CR- but not CALB-immunoreactivity may originate mainly from neurons in the ventral tier of pars compacta (SNC) and from the pars reticulata of substantia nigra (SNR) which show identical cytochemical properties. Axons in the matrix of CP and the accumbal core as well as in the islands of Calleja single-labelled by the TH-immunoreactivity or additionally containing CALB and CR may originate from neurons in the dorsal tier of mesencephalic nuclei like SN, pars compacta and ventral tegmental area. CR-containing TH-immunoreactive basket-like axon terminations in the dorsal lateral septal nucleus are likely to originate either from mesencephalic nuclei or from the supramammillary region.

Association between the low threshold calcium spike and activation of NMDA receptors in guinea-pig substantia nigra pars compacta neurons

The aim of this study was to examine the interaction between N-methyl-D-aspartate (NMDA) receptor activation and the low threshold calcium spike (LTS) of phasically firing neurons in the rostral part of the substantia nigra pars compacta (SNpc) in mid-brain slices. Bath perfusion of 10 microM NMDA gradually increased the LTS area and the effect reached a maximum after 6 min of perfusion. This enhancement of the LTS by NMDA was blocked both by a competitive and non-competitive NMDA receptor antagonist, 50 microM D-AP5 and 10 microM MK801, respectively, demonstrating that this effect of NMDA was mediated through NMDA receptors. Prolonged exposure to increasing concentrations of NMDA (0.1-100 microM) progressively decreased the LTS area. The higher doses led to an irreversible marked depolarization and decrease of the membrane resistance. These results suggest that the LTS of SNpc neurons can trigger a NMDA receptor-dependent response which may have physiological and pathological roles.

Burst firing in midbrain dopaminergic neurons

Midbrain dopaminergic (DA) neurons fire bursts of activity in response to sensory stimuli, including those associated with primary reward. They are therefore conditional bursters - the bursts conveying, amongst other things, motivationally relevant information to the forebrain. In the forebrain, bursts give rise to a supra-additive release of dopamine, and possibly favour the release of co-localised neuropeptides. Evidence is presented that in rat DA neurons, bursts are engendered by the activity of cortically-regulated afferents. Certain factors are identified which, in combination, lead to burst production: (1) A burst of activity in EAAergic afferents to DA neurons arising from non-cortical sources, but controlled by the medial prefrontal cortex; (2) N-methyl-D-aspartate receptor activation, producing a slow depolarising wave in the recipient neuron; (3) activation of a high threshold, dendritically located calcium conductance which produces a 'plateau potential'; (4) activation of a calcium-activated potassium conductance, which terminates the burst. These factors are argued to operate in the context of an 'optimal' level of intracellular calcium buffering for bursting. Other factors which appear to be involved in bursting in other systems, in particular a low threshold calcium conductance, are rejected as being necessary for bursting in DA neurons. The factors which do play a crucial role in burst production in DA neurons are integrated into a theory from which arises a series of hypotheses amenable to empirical investigation. Additional factors are discussed which may modulate bursting. These may either act indirectly through changes in membrane potential (or intracellular calcium concentration), or they may act directly through an interaction with certain conductances, which appear to promote or inhibit burst firing in DA neurons.

Electrophysiological and immunocytochemical characterization of GABA and dopamine neurons in the substantia nigra of the rat

Neurons in the substantia nigra pars reticulata and pars compacta of the rat were studied using a combination of intracellular electrophysiological recording in in vitro and subsequent immunocytochemical double and triple labelling techniques. The neurons recorded in the pars reticulata were identified as either GABA or dopamine neurons: neurons were considered to be GABA neurons if they were immunopositive for glutamate decarboxylase, whereas those neurons which were immunopositive for tyrosine hydroxylase were considered to be dopaminergic. The GABA neurons had short duration action potentials (0.45+/-0.03 ms halfwidth), no apparent rectifying currents, no low threshold calcium spikes, were spontaneously active (7.4+/-3.7 Hz), and could maintain high firing rates. The dopamine neurons had long duration action potentials (1.49+/-0.10 ms), displayed both anomalous inward and transient outward rectifying currents, and more than half (12/17 neurons) displayed a low threshold calcium spike. Their spontaneous firing rate was lower than that of the GABA neurons (2.3+/-1.0 Hz), and they displayed strong frequency adaptation. Morphological reconstruction of neurobiotin-filled neurons revealed that the pars reticulata GABA neurons had more extensive local dendritic arborization than the dopamine neurons from either the pars reticulata or the pars compacta. All of the neurons recorded from the pars compacta were dopamine neurons; they were found not to be different either electrophysiologically or morphologically from pars reticulata dopamine neurons. The electrophysiology of the GABA neurons suggests that input activity is translated linearly to spike frequency. These GABA neurons probably represent the projection neurons of the pars reticulata, and it is thus likely that this basal ganglia output is frequency coded. The close similarity between the dopamine neurons in the pars compacta, which give rise to the nigrostriatal pathway, and those in the pars reticulata supports the notion that the dopamine neurons in these two regions are part of the same neuronal population.

Ionic mechanisms involved in the spontaneous firing of tegmental pedunculopontine nucleus neurons of the rat

We have previously defined three types of tegmental pedunculopontine nuclei neurons based on their electrophysiological characteristics: Type I neurons characterized by low-threshold Ca2+ spikes, Type II neurons which displayed a transient outward current (A-current), and Type III neurons having neither low-threshold spikes nor A-current [Kang Y. and Kitai S. T. (1990) Brain Res. 535, 79-95]. In this report, ionic mechanisms underlying repetitive firing of Type I (n=15) and Type II (n=69) neurons were studied in in vitro slice preparations. Type I neurons did not fire rhythmically but their spontaneous firing frequency ranged from 0 to 19.5 spikes/s (mean 9.7 spikes/s). The spontaneous firing of Type II neurons was rhythmic, with a mean frequency of 9.6 spikes/s (range 3.5-16.0 spikes/s). Choline acetyltransferase immunohistochemistry combined with biocytin labeling indicated that none of the Type I neurons were immunopositive to choline acetyltransferase, while 60% (42 of 69) of Type II neurons were immunopositive. There was no apparent difference in the electrophysiological membrane properties of immunopositive and immunonegative Type II neurons. At membrane potentials subthreshold for Na+ spikes (-50 mV), spontaneous membrane oscillations (11.6 Hz) were observed: these underlie the spontaneous repetitive firing of Type I neurons. The subthreshold membrane oscillation was tetrodotoxin sensitive but was not affected by Ca2+-free medium. A similar tetrodotoxin-sensitive subthreshold membrane oscillation (10.5 Hz) was also observed in Type II neurons. However, in Type II neurons a membrane oscillation was also observed at higher membrane potentials (-50 mV). This high-threshold oscillation was insensitive to tetrodotoxin and Na+-free medium, but was eliminated in Ca2+-free conditions. The amplitude and frequency of the high-threshold oscillation was increased upon membrane depolarization. At the most prominent oscillatory level (around -40 mV), the high-threshold oscillation had a mean frequency of 8.8 Hz. The high-threshold Ca2+ spike was triggered from the peak potential (-35 to -30mV) of the high-threshold oscillation. Application of tetraethylammonium chloride (< 5 mM) increased the amplitude of the high-threshold oscillation, while nifedipine greatly attenuated the high-threshold oscillation without changing the shape of the high-threshold Ca2+ spike. Application of Cd2+ eliminated both the high-threshold oscillation and the high-threshold Ca2+ spike, and omega-conotoxin reduced the size of the high-threshold Ca2+ spike without affecting the frequency of the high-threshold oscillation. Nickel did not have any effect on either the high-threshold oscillation or the high-threshold Ca2+ spike. These data suggest an involvement of N- and L-type Ca2+ channels in the generation of the high-threshold oscillation and the high-threshold Ca2+ spike, respectively. The results indicate that a persistent Na+ conductance plays a crucial role in the subthreshold membrane oscillation, which underlies spontaneous repetitive firing in Type I neurons. On the other hand, in addition to a persistent Na+ conductance for subthreshold membrane oscillation, a voltage-dependent Ca2+ conductance with Ca2+-dependent K+ conductance (for the high-threshold oscillation) may be responsible for rhythmic firing of Type II neurons.

Tachykinin neurokinin-1 and neurokinin-3 receptor-mediated responses in guinea-pig substantia nigra: an in vitro electrophysiological study

The effects of tachykinin receptor agonists and antagonists were investigated using intra- and extracellular recordings on spontaneously firing nigral neurons in guinea-pig brain slices. In 70 of 76 electrophysiologically identified dopaminergic neurons, a concentration-dependent increase in firing rate was induced by the selective neurokinin-3 tachykinin agonist senktide and by the natural tachykinin agonists neurokinin B and substance P, with EC50 values of 14.7, 31.2 and 12200 nM respectively. These responses were inhibited in a concentration- and time-dependent manner by the selective non-peptide neurokinin-3 receptor antagonist SR 142801 (1-100 nM; n=23), but neither by its S-enantiomer SR 142806 (100 nM; n=4) nor by selective antagonists of neurokinin-1 (SR 140333) or neurokinin-2 (SR 48968) receptors (both at 100 nM; n=3). The selective neurokinin-1 agonist [Sar9,Met(O2)11]substance P (30-100 nM; n=23) and the selective neurokinin-2 agonist [Nle10]neurokinin A(4-10)(30-100 nM; n=13) were without any effect on dopaminergic cells. In 13 of 21 electrophysiologically identified, presumably GABAergic neurons located in the pars compacta of the substantia nigra, excitatory responses were evoked concentration dependently by substance P and [Sar9,Met(O2)11]substance P, with EC50 values of 18.6 and 41.9 nM respectively. These responses were inhibited by SR 140333 (100 nM; n=3), but neither by its R-enantiomer SR 140603 nor by SR 142801 (both at 100 nM; n=3). Senktide and [Nle10]neurokinin A(4-10) (both at 30-100 nM; n=10) were without effect on these presumed GABAergic neurons. A small population (12%) of pars compacta neurons was insensitive to any of the three selective tachykinin agonists. In the nigral pars reticulata, 12 neurons were recorded which had an electrophysiological profile similar to that of presumed GABAergic neurons in the pars compacta. Of these 12 cells, seven did not respond to any of the selective tachykinin agonists tested, while five were excited by senktide in a concentration-dependent manner (EC50=98.5 nM). Although this value was significantly higher than that found for dopaminergic neurons in the pars compacta, senktide-evoked responses were inhibited by SR 142801 (100 nM; n=3). We conclude that, in the guinea-pig substantia nigra, tachykinins evoke excitatory responses in both dopaminergic and non-dopaminergic neurons; however, the sensitivity to tachykinin agonists (neurokinin-1 versus neurokinin-3) depends on both neuronal type and localization.

6-Hydroxy dopamine lesions block a tolbutamide-sensitive K+ conductance in the guinea-pig substantia nigra

A population of neurons in the anterior substantia nigra pars compacta of the guinea-pig display an outward K+ conductance, which can be activated by a brief period of hypoxia, and blocked in a dose-dependent manner by tolbutamide (100-500 microns): these features are those of an ATP-sensitive K+ channel (KATP). Typically hyperpolarisation of the cell is accompanied by a mean decrease in input resistance (IR) of 41.29 +/- 3.69 M omega (SEM), a mean decrease in resting membrane potential (RMP) of 8.58 +/- 2.41 mV and a cessation of action potential generation. These cells do not show positive immunoreactivity for tyrosine hydroxylase (TH), but they are present in slices that contain TH and release dopamine (DA). Given the intimacy of these cells with nigral DA systems, and given the fact that they have D2 receptors and respond to dopaminergic (DAergic) agents, it was of interest to see if lesions of DAergic neurons had any effect on the above response. The neurotoxin 6-hydroxy dopamine (6-OHDA) was injected into the median forebrain bundle to destroy the DAergic neurons of the nigrostriatal pathway. Unilateral lesion of this pathway produced a significant reduction in the above conductance, in the lesioned substantia nigra pars compacta (SNpc) (mean IR decrease of only 17.99 +/- 4.44 M omega (and a mean RMP decrease of 1.40 +/- 1.76 mV), whilst the hypoxia induced change in potential on the non-lesioned side was still marked and indistinguishable from that in control animals. The KATP channel would therefore appear to be under the influence of the DAergic neurons of the SNpc.

Characteristics of electrically evoked somatodendritic dopamine release in substantia nigra and ventral tegmental area in vitro

Somatodendritic dopamine (DA) release from neurons of the midbrain represents a nonclassical form of neuronal signaling. We assessed characteristics of DA release during electrical stimulation of the substantia nigra pars compacta (SNc) in guinea pig midbrain slices. With the use of parameters optimized for this region, we compared stimulus-induced increases in extracellular DA concentration ([DA]o) in medial and lateral SNc, ventral tegmental area (VTA), and dorsal striatum in vitro. DA release was monitored directly with carbon-fiber microelectrodes and fast-scan cyclic voltammetry. Detection of DA in SNc was confirmed by electrochemical, pharmacological, and anatomic criteria. Voltammograms of the released substance had the same peak potentials as those of DA obtained during in vitro calibration, but different from those of the indoleamine 5-hydroxytryptamine. Similar voltammograms were also obtained in the DA-rich striatum during local electrical stimulation. Contribution from the DA metabolite 3,4-dihydroxyphenylacetic acid to somatodendritic release was negligible, as indicated by the lack of effect of the monoamine oxidase inhibitor pargyline (20 microM) on the signal. Lastly, DA voltammograms could only be elicited in regions that were subsequently determined to be positive for tyrosine hydroxylase immunoreactivity (TH-ir). The frequency dependence of stimulated DA release in SNc was determined over a range of 1-50 Hz, with a constant duration of 10 s. Release was frequency dependent up to 10 Hz, with no further increase at higher frequencies. Stimulation at 10 Hz was used in all subsequent experiments. With this paradigm, DA release in SNc was tetrodotoxin insensitive, but strongly Ca2+ dependent. Stimulated [DA]o in the midbrain was also site specific. At the midcaudal level examined, DA efflux was significantly greater in VTA (1.04 +/- 0.05 microM, mean +/- SE) than in medial SNc (0.52 +/- 0.05 microM), which in turn was higher than in lateral SNc (0.35 +/- 0.03 microM). This pattern followed the apparent density of TH-ir, which was also VTA > medial SNc > lateral SNc. This report has introduced a new paradigm for the study of somatodendritic DA release. Voltammetric recording with electrodes of 2-4 microns tip diameter permitted highly localized, direct detection of endogenous DA. The Ca2+ dependence of stimulated release indicated that the process was physiologically relevant. Moreover, the findings that somatodendritic release was frequency dependent across a range characteristic of DA cell firing rates and that stimulated [DA]o varied markedly among DA cell body regions have important implications for how dendritically released DA may function in the physiology and pathophysiology of substantia nigra and VTA.

Heterogeneity of electrically evoked dopamine release and reuptake in substantia nigra, ventral tegmental area, and striatum

Somatodendritic dopamine (DA) released in substantia nigra pars compacta (SNc) and the ventral tegmental area (VTA) may mediate extrasynaptic neuronal signaling. The concentration of extracellular DA ([DA]o) attained during somatodendritic activation will be governed by the density of release sites and properties of DA uptake. We evaluated these factors in SNc, VTA, and dorsal striatum with carbon-fiber microelectrodes and fast-scan cyclic voltammetry to monitor [DA]o during local electrical stimulation (10 Hz, 5 s) in guinea pig brain slices. Stimulated DA efflux was site specific, with significantly higher [DA]o in caudal (0.48 +/- 0.03 microM, mean +/- SE) than rostral SNc (0.16 +/- 0.01 microM), averaged over their mediolateral extents, and higher [DA]o in VTA (0.74 +/- 0.07 microM) than in medial (0.43 +/- 0.04 microM) or lateral SNc (0.29 +/- 0.05 microM), averaged rostrocaudally. Throughout SNc, evoked [DA]o correlated positively (r = 0.91) with the density of tyrosine-hydroxylase-immunoreactive cells. Modulation of evoked [DA]o by uptake was also site specific. The selective DA uptake inhibitor GBR 12909 significantly increased evoked [DA]o in caudal SNc (to 185 +/- 27%) and striatum (408 +/- 24%), but had no effect in rostral SNc or VTA. Conversely, the norepinephrine (NE) uptake inhibitor desipramine did not alter stimulated [DA]o in caudal SNc or striatum, but caused significant enhancement in rostral SNc (196 +/- 17%) and VTA (126 +/- 12%). Paroxetine, a selective 5-hydroxytryptamine uptake inhibitor had little effect in any region tested. Site-specific sensitivity to desipramine mandated evaluation of dopamine-beta-hydroxylase immunoreactivity (D beta H-ir) in midbrain. The density of filaments positive for D beta H-ir was greater in rostral SNc and VTA than in caudal SNc, suggesting DA clearance via the NE transporter in these regions. Importantly, D beta H-ir was most dense in sections rostral to SNc where no catecholamine signal was detected and no enhancement was observed with desipramine, indicating a lack of NE contribution to evoked release in any region examined. Taken together, these data confirmed that evoked somatodendritic [DA]o depends on DA cell density and on local uptake properties. Uptake was less efficient in SNc and VTA than in striatum. Moreover, enhancement of stimulated [DA]o by GBR 12909 demonstrated that evoked release from dendrites is not via reversal of the DA transporter. Lastly, the heterogeneous patterns of DA uptake within SNc and VTA were consistent with the pattern of degeneration in Parkinson's disease; less vulnerable DA cells, e.g., those in VTA, have less DA uptake than the more vulnerable cells of caudal SNc.

The role of ventral tegmental dopamine neurons in locomotor sensitization following quinpirole or (+)-amphetamine: ex vivo voltammetric evidence

Behavioural sensitization to the locomotor stimulating effects of (+)-amphetamine or quinpirole was induced in rats by intermittent drug administration. Following expression of sensitization, locomotor activity scores on day 9 were: vehicle 87 +/- 9, (+)-amphetamine 1441 +/- 227 and quinpirole 2078 +/- 214. Electrically stimulated dopamine release was measured on day 12 in ventral tegmental slices using fast cyclic voltammetry. Dopamine release was significantly elevated in the (+)-amphetamine- and quinpirole-treated groups when compared to vehicle-treated controls over a wide range of stimulation frequencies (5-200 Hz) and pulses (1-200). Quinpirole (1 microM) in the perfusion fluid attenuated dopamine release following 40-pulse, 200-Hz electrical stimulation, by 31.6 +/- 2.8% in the ventral tegmental area of the vehicle-treated group, by 14.8 +/- 5.6% in the (+)-amphetamine-treated group and 8 +/- 7.3% in the quinpirole-treated group. This study shows that dopamine release is increased in the ventral tegmental area following sensitization with either a direct or indirectly acting dopamine agonist. The findings that dopamine release was elevated at all stimulation frequencies in sensitized animals, and that quinpirole only attenuated this release at the highest stimulation frequency, would suggest that in addition to D2 autoreceptor subsensitivity, other mechanisms contribute to the enhanced release of dopamine in these animals.

Comparison of the behavioural effects of infusion of carbachol and acetylcholinesterase into the rat substantia nigra

It has been postulated for many years that acetylcholinesterase (AChE) may play a nonclassical role in the substantia nigra, unrelated to its ability to hydrolyse acetylcholine. In this study the behavioural effects of unilateral infusion of AChE and a cholinergic agonist, carbachol, were compared. Carbachol induced ipsiversive circling over a very short time scale (minutes), whereas AChE induced contraversive circling, but over a longer time course-10 days. Both agents showed selectivity of response within the substantia nigra: acetylcholinesterase was only effective when infused into the most rostral region of the substantia nigra and its effects were limited to the pars compacta. In contrast, carbachol had effects in both the pars compacta and reticulata, with a graded sensitivity to carbachol in the rostral/caudal plane; infusions into rostral regions induced high rates of circling compared to more caudal areas, suggesting that the cholinergic input to the substantia nigra is not homogenous, but greater in rostral regions. This disparity between the effects of carbachol and AChE would, therefore, suggest that AChE is not exerting its long-term behavioural actions via a cholinergic mechanism, both in terms of time course of the response and the areas within the substantia nigra sensitive to these agents.

Characterization of a barium-sensitive outward current following glutamate application on rat midbrain dopaminergic cells

Using intracellular electrophysiological recordings in dopaminergic (principal) neurons of the rat mesencephalon maintained in vitro, we studied a postexcitatory amino acid response (PEAAR). Under current-clamp mode, bath application of glutamate produced a depolarization that was followed by a hyperpolarization when the perfusion of the excitatory amino acid was discontinued. Under single-microelectrode voltage-clamp mode, an outward current followed the glutamate-induced inward current. The PEAAR was associated with an increase in membrane conductance and reversed polarity at about-85 mV (2.5 mM extracellular K+). The null potential for the PEAAR was independent of the intracellular loading of chloride ions and was shifted towards less negative values (approximately 23 mV) by increasing extracellular K+ from 2.5 to 8.5 mM. The PEAAR was present in neurons treated with tetraethylammonium (5-10 mM), apamin (1 microM) or glibenclamide (1-300 microM). However, it was strongly depressed or blocked by extracellular barium (300 microM to 1 mM), by low-calcium (0.5 mM) plus cadmium (100 microM) or magnesium (10 mM), and by low-sodium solutions. An outward response was also generated after an inward current induced by the perfusion of the specific agonists for the ionotropic excitatory amino acid receptors NMDA, alpha-amino-3-hydroxy-5-methyl-4-isoxazole (AMPA) and kainate. The PEAAR was not affected by tetrodotoxin (1 microM), saclofen (100-300 microM), bicuculline (30 microM), sulpiride (1 microM) or strychnine (1 microM). In addition, the inhibition of the ATP-dependent Na(+)-K+ pump by ouabain and strophanthidin (1-10 microM) prolonged the glutamate-induced membrane depolarization/inward current while the subsequent PEAAR was reduced or not observed. Our data indicate that the PEAAR mainly results from the activation of a barium-sensitive potassium current. This response might limit the excitatory and eventually neurotoxic effects of glutamate.

Fast Na+ spike generation in dendrites of guinea-pig substantia nigra pars compacta neurons

Electric fields were applied to study the regenerative properties of substantia nigra pars compacta neurons in guinea-pig brain slices. Two types of spikes, of high or low amplitude, were generated in both the soma-hyperpolarizing and the soma-depolarizing directions of the field. The different sensitivity of the spikes to somatic polarization suggested that the high-amplitude spikes were generated near the cell body, whereas the low-amplitude spikes were generated at a distance from the soma. Application of tetrodotoxin or intracellular injection of QX 314 abolished both types of spike. The spikes were not inhibited in the presence of glutamate receptor antagonists or during Ca2+ channel blockade. Blockers of gap junctional conductance (sodium propionate, octanol and halothane) did not affect the field-induced spikes. The spike generation was highly sensitive to changes in membrane conductance induced by current injection in the soma or by external field application. The ability of a conditioning field stimulation to affect the spike generation in different neuronal compartments suggested that a transient outward current was generated in the dendrites. The field-induced spikes were facilitated by synaptic stimulation and, in some neurons, low-amplitude spikes were generated by synaptic potentials in the absence of field application. These results suggest that channels responsible for Na+ spike generation reside in the dendrites, and are influenced by spatially distributed voltage-dependent K+ currents and by synaptic input.

'Intact' dopaminergic midbrain neurons of the rat display unclamped dendritic Ca2+ currents

Calcium channels play an important role in generating the complex electrophysiology of dopaminergic (DAergic) neurons in the substantia nigra pars compacta (SNc). To directly study these calcium currents in dendritically intact neurons, two issues needed to be addressed: the identity of the neuron (DAergic versus non-DAergic) and control of putative dendritic calcium conductances. A measure of the anomalous rectifier (Ih) was used to identify DAergic neurons. Both groups of neurons produced large (approximately 1 nA), sustained inward currents that persisted as 'plateau currents' for hundreds of milliseconds after the termination of the depolarizing voltage clamp step. We conclude that intact DAergic neurons can be identified under conditions optimal for recording calcium currents (i.e. Cs+ internal solution), but powerful dendritic currents limit rigorous biophysical analysis.

Involvement of the NMDA receptor in a non-cholinergic action of acetylcholinesterase in guinea-pig substantia nigra pars compacta neurons

Evidence is accumulating that a soluble, secretory form of acetylcholinesterase may have novel, non-cholinergic functions in certain brain regions, such as the substantia nigra. In this study, application of human recombinant acetylcholinesterase (rhAChE) to pars compacta neurons in the rostral substantia nigra resulted in a sustained hyperpolarization that was not only mimicked by application of N-methyl-D-aspartate (NMDA) but also blocked by the NMDA receptor antagonists MK8O1 and 2-amino-5-phosphonopentanoic acid. Neither the rhAChE- nor the NMDA-induced hyperpolarization was seen when the calcium chelator BAPTA was injected into the neuron; hence the effect is mediated by accumulation of intracellular calcium. This intracellular calcium appears sufficient to compromise neuronal metabolism since the rhAChE-induced hyperpolarization was reversed by application of the K-ATP channel antagonist tolbutamide. Butyrylcholinesterase, a protein of similar molecular weight to acetylcholinesterase, which also hydrolyses acetylcholine, had no effect whatsoever. The results suggest that, independent of its normal catalytic function, acetylcholinesterase can act via the NMDA receptor complex to enhance calcium entry into nigral neurons and jeopardize cell metabolism. This non-classical action of acetylcholinesterase might thus be an important factor in the mechanisms underlying parkinsonian neurodegeneration.

Stimulation of the prefrontal cortex in the rat induces patterns of activity in midbrain dopaminergic neurons which resemble natural burst events

Evidence suggests that excitatory amino acid-containing afferents from the prefrontal cortex (PFC) play an important role in the induction of burst firing in midbrain dopaminergic (DA) neurons. In the present study, the extracellular activity of individual DA neurons (A10 and A9 cell groups) was recorded during single pulse electrical stimulation (0.25 and 1 mA) of the PFC. The majority of cells were responsive, and two main patterns of activity were elicited: responses characterised by an initial excitation (E responses; 41.8% of responses at 0.25 mA and 26.6% at 1 mA; cell groups combined) and responses characterised by excitation following an initial inhibition (IE responses; 43.3% of responses at 0.25 mA and 56.6% at 1 mA; cell groups combined). Burst analysis performed on the excitatory phase of E and IE responses revealed that the excitation contained events which fulfilled the criteria for natural bursts in DA neurons. A procedure was developed for assessing whether these bursts were time-locked to the stimulus. This showed that 27.9% of E responses and 33.3% of IE responses were accompanied by time-locked bursts (currents and cell groups combined). It is argued that time-locked bursts during IE responses were produced by rebound activation of a low threshold calcium conductance, whereas time-locked bursts during E responses were produced by excitatory afferents. Since natural bursts in DA neurons also seem to involve cortically induced excitation, the hypothesis that the PFC plays a role in the production of natural bursts in DA neurons is strengthened.

Comparison of somatodendritic and axon terminal dopamine release in the ventral tegmental area and the nucleus accumbens

Fast cyclic voltammetry at a carbon fibre microelectrode was used to measure dopamine release following electrical or chemical stimulation in rat brain slices incorporating either the ventral tegmental area or the core region of the nucleus accumbens. Electrical or chemical stimulation gave clear voltammetric signals which corresponded to dopamine; less dopamine was released in the ventral tegmental area than in the nucleus accumbens. In contrast to the nucleus accumbens, electrically stimulated dopamine release in the ventral tegmental area was not sensitive to tetrodotoxin, was not modified by the presence of dopamine uptake inhibitors, or agonist or blockers acting at dopamine D2 autoreceptors.