Subthalamic Neurons Research Articles

Tonic activation of presynaptic GABAB receptors on rat pallidosubthalamic terminals

The subthalamic nucleus plays a critical role in the regulation of movement, and abnormal activity of its neurons is associated with some basal ganglia motor symptoms. We examined the presence of functional presynaptic GABA(B) receptors on pallidosubthalamic terminals and tested whether they were tonically active in the in vitro subthalamic slices. Whole-cell patch-clamp recordings were applied to acutely prepared rat subthalamic nucleus slices. The effects of specific GABA(B) agonist and antagonist on action potential-independent inhibitory postsynaptic currents (IPSCs), as well as holding current, were examined. Superfusion of baclofen, a GABA(B) receptor agonist, significantly reduced the frequency of GABA(A) receptor-mediated miniature IPSCs (mIPSCs), in a Cd2+-sensitive manner, with no effect on the amplitude, indicating presynaptic inhibition on GABA release. In addition, baclofen induced a weak outward current only in a minority of subthalamic neurons. Both the pre- and post-synaptic effects of baclofen were prevented by the specific GABA(B) receptor antagonist, CGP55845. Furthermore, CGP55845 alone increased the frequency of mIPSCs, but had no effect on the holding current. These findings suggest the functional dominance of presynaptic GABA(B) receptors on the pallidosubthalamic terminals over the postsynaptic GABA(B) receptors on subthalamic neurons. Furthermore, the presynaptic, but not the postsynaptic, GABA(B) receptors are tonically active, suggesting that the presynaptic GABA(B) receptors in the subthalamic nucleus are potential therapeutic target for the treatment of Parkinson disease.

Electrophysiological alterations in subthalamic neurons after unilateral dopamine depletion in the rat

The present study examined the effects of unilateral 6-hydroxydopamine (6-OHDA) lesions of the substantia nigra pars compacta (SNc) on electrophysiological properties of subthalamic neurons (STN) in adult rats. Most neurons displayed regular spontaneous tonic firing patterns in both control and lesioned animals; however, the percentage of neurons with spontaneous burst firing at hyperpolarized membrane potentials was increased significantly in lesioned animals compared with controls (45% vs. 14% respectively). In the presence of bicuculline, a gamma-aminobutyric acid type A (GABAA) receptor antagonist, electrical stimulation of the internal capsule produced monosynaptic excitatory postsynaptic potentials (EPSPs) in almost all recorded neurons. DA (50 microM) increased the amplitude and/or duration of the EPSPs in neurons from both groups, whereas the DA D1 receptor agonist SKF 81297 (10 microM) produced a significant increase in amplitude and/or duration of EPSPs in neurons from the lesioned group only. This latter increase was blocked by pretreatment with the DA D1 antagonist SCH 23390 (10 microM). These data suggest that unilateral degeneration of DA neurons in the SNc changes firing properties and enhances electrophysiological responsiveness of STN neurons to activation of DA D1 receptors.

The anatomy of the porcine subthalamic nucleus evaluated with immunohistochemistry and design-based stereology

This study provides a light-microscopic description of the organization, morphology and number of neurons in the subthalamic nucleus (STN) of the Göttingen minipig. It is based on histological material stained with Nissl, Golgi and autometallographic techniques, and employs design-based stereological estimation of the total neuron number. The organization of several neurotransmitters in the STN has been evaluated in histological preparations stained for acetylcholinesterase (AChE) and immunostained for choline acetyltransferase (ChAT), tyrosine hydroxylase (TH), glutamic acid decarboxylase (GAD) and glutamate. In all of the stained preparations the STN appeared as a distinct lens-shaped structure located in the caudal diencephalon, medial to the internal capsule and ventrolateral to the zona incerta. Rostrally, the STN approached the globus pallidus pars interna, whereas caudally the ventromedial part of the STN was adjacent to the rostral part of the substantia nigra pars compacta (SNc), where some of the neurons of the two nuclei merged. The neurons in the STN had medium-sized (25-40 microm) ovoid or fusiform cell bodies, from which three to six large dendrites emanated in a direction predominantly parallel to the long axis of the STN. Immunohistochemistry revealed that most of the subthalamic neurons were glutamatergic and differed significantly in appearance from the large stellate TH-positive cells of the adjacent SNc. Numerous TH-positive bouton-rich fibers traversed the STN. The GAD-staining revealed a large number of terminals within the boundaries of the STN. The STN was highly AChE-positive, reflecting a prominent innervation by ChAT-positive terminals. The total number of subthalamic neurons in one hemisphere was estimated to be approximately 56,000. We conclude that the neuroarchitecture of the porcine STN is similar to primates, including humans, and appears well-suited for further studies examining the role of the STN in movement disorders.

Thalamic innervation of striatal and subthalamic neurons projecting to the rat entopeduncular nucleus.

The present study analyses the anatomical arrangement of the projections linking the Wistar rat parafascicular thalamic nucleus (PF) and basal ganglia structures, such as the striatum and the subthalamic nucleus (STN), by using neuroanatomical tract-tracing techniques. Both the thalamostriatal and the striato-entopeduncular projections were topographically organized, and several areas of overlap between identified circuits were noticed, sustaining the existence of up to three separated channels within the Nauta-Mehler loop. Thalamic afferents arising from dorsolateral PF territories are in register with striatofugal neurons located in dorsolateral striatal areas, which in turn project to dorsolateral regions of the entopeduncular nucleus (ENT). Medial ENT regions are innervated by striatal neurons located within medial striatal territories, these neurons being the target for thalamic afferents coming from medial PF areas. Finally, afferents from neurons located in ventrolateral PF areas approached striatal neurons in ventral and lateral striatal territories, which in turn project towards ventral and lateral ENT regions. Efferent STN neurons projecting to ENT were found to be the apparent postsynaptic target for thalamo-subthalamic axons. The thalamo-subthalamic projection was also topographically organized. Medial, central and lateral STN territories are innervated by thalamic neurons located within medial, ventrolateral and dorsolateral PF areas, respectively. Thus, each individual PF subregion projects in a segregated fashion to specific parts of the striato-entopeduncular and subthalamo-entopeduncular systems. These circuits enabled the caudal intralaminar nuclei to modulate basal ganglia output.

Calcium-dependent subthreshold oscillations determine bursting activity induced by N-methyl-D-aspartate in rat subthalamic neurons in vitro.

We used whole-cell patch recordings in current clamp to investigate the ionic dependence of burst firing induced by N-methyl-d-aspartate (NMDA) in neurons of the subthalamic nucleus (STN) in slices of rat brain. NMDA (20 microm) converted single-spike firing to burst firing in 87% of STN neurons tested. NMDA-induced bursting was blocked by AP5 (50 microm), and was not mimicked by the non-NMDA receptor agonist AMPA (0.6 microm). Tetrodotoxin (1 microm) converted bursts to oscillations of membrane potential, which were most robust when oscillations ranged between -50 and -70 mV. The NMDA bursts were blocked by an elevated extracellular concentration of Mg(2+), but superfusate containing no added Mg(2+) either reduced or increased burst firing, depending upon the amount of intracellular current injection. Block of K(+) conductances by apamin and tetraethylammonium prolonged burst duration, but iberiotoxin had no effect. NMDA-induced burst firing and membrane oscillations were completely blocked by superfusate containing no added Ca(2+), and they were significantly reduced when patch pipettes contained BAPTA. Selective antagonists for T-type (mibefradil, 10 microm), L-type (nifedipine, 3 microm), and N-type (omega-conotoxin GVIA, 1 micro m) Ca(2+) channels had no effect on NMDA burst firing. Superfusate containing a low concentration of Na(+) (20 mm) completely abolished NMDA-induced burst firing. Flufenamic acid (10 microm), which blocks current mediated by Ca(2+)-activated nonselective cation channels (I(CAN)), reversibly abolished NMDA-depended bursting. These results are consistent with the hypothesis that NMDA-induced burst firing in STN neurons requires activation of either an I(CAN) or a Na(+)-Ca(2+) exchanger.

Effect of High-Frequency Stimulation of the Subthalamic Nucleus on Subthalamic Neurons: An Intracellular Study

Background/Aims: The precise mechanism of action of deep brain stimulation in the subthalamic nucleus (STN) for the treatment of Parkinson’s disease and epilepsy is unknown. In the present study, the intracellular effects on STN neurons following high-frequency stimulation (HFS) of STN were examined to test the hypothesis that HFS results in either an increase or a decrease in neuronal action potential generation. Methods: Intracellular electrophysiological recordings were made in the rat STN neurons in in vitro slice preparations. A concentric bipolar stimulating electrode was placed in the STN, and electrical stimulation (duration, 100–2000 ms; amplitude, 10–500 µA, and frequency, 10–200 Hz) was delivered while simultaneously recording intracellularly from a STN neuron using a sharp electrode. Results: HFS of STN resulted in the generation of excitatory postsynaptic potentials and an increase in action potential firing during the stimulation period followed by a period of poststimulation inhibition of firing in STN neurons. The degree of increase in action potentials from HFS was critically dependent on the frequency of electrical stimulation, i.e. at approximately 100–140 Hz, maximal increase was obtained, but at 200 Hz, the activity was blocked. Interestingly, the duration of poststimulation inhibition of firing was dependent on the duration of stimulation, i.e. the longer the HFS, the longer the inhibition. Conclusions: These results suggest that the mechanism of action of deep brain stimulation involves initial excitation followed by later inhibition of STN neurons at a cellular level rather than primary inhibition, as previously hypothesized.

Dynamics of beta-oscillations in human subthalamic neurons: a single-case study

Dynamics of beta-oscillations in human subthalamic neurons: a single-case study

A Model of Reverse Spike Frequency Adaptation and Repetitive Firing of Subthalamic Nucleus Neurons

Subthalamic nucleus neurons exhibit reverse spike-frequency adaptation. This occurs only at firing rates of 20-50 spikes/s and higher. Over this same frequency range, there is an increase in the steady-state frequency-intensity (F-I) curve's slope (the secondary range). Specific blockade of high-voltage activated calcium currents reduced the F-I curve slope and reverse adaptation. Blockade of calcium-dependent potassium current enhanced secondary range firing. A simple model that exhibited these properties used spike-triggered conductances similar to those in subthalamic neurons. It showed: 1) Nonaccumulating spike afterhyperpolarizations produce positively accelerating F-I curves and spike-frequency adaptation that is complete after the second spike. 2) Combinations of accumulating aftercurrents result in a linear F-I curve, whose slope depends on the relative contributions of inward and outward currents. Spike-frequency adaptation can be gradual. 3) With both accumulating and nonaccumulating aftercurrents, primary and secondary ranges will be present in the F-I curve. The slope of the primary range is determined by the nonaccumulating conductance; the accumulating conductances govern the secondary range. The transition is determined by the relative strengths of accumulating and nonaccumulating currents. 4) Spike-threshold accommodation contributes to the secondary range, reducing its slope at high firing rates. Threshold accommodation can stabilize firing when inward aftercurrents exceed outward ones. 5) Steady-state reverse adaptation results when accumulated inward aftercurrents exceed outward ones. This requires spike-threshold accommodation. Transient speedup arises when inward currents are smaller than outward ones at steady state, but accumulate more rapidly. 6) The same mechanisms alter firing in response to irregular patterns of synaptic conductances, as cell excitability fluctuates with changes in firing rate.

MGluRs: a target for pharmacotherapy in Parkinson disease

Metabotropic glutamate receptors (mGluRs) are G-protein-coupled excitatory amino acid (glutamate) receptors and are abundantly expressed in basal ganglia nuclei. We used behavioral, regional glucose uptake metabolic mapping, and FOS protein expression to examine the effects of stimulating striatal and subthalamic mGluRs in rats. Stimulation of striatal Group I mGluRs produced behavioral effects mediated by polysynaptic activation of subthalamic neurons. Stimulation of subthalamic Group II mGluRs produced similar effects. Excessive activity of subthalamic neurons is a key feature of parkinsonism. mGluR Group I or Group II antagonists may prove to be useful for symptomatic treatment of parkinsonism. Stimulation of Group III mGluRs produced behavioral effects in only 6-hydroxydopamine-lesioned animals. Regional glucose uptake metabolic mapping and FOS expression studies suggested that striatal dopamine denervation produced increased sensitivity of Group III mGluRs. Agents active at Group III mGluRs may also be useful for treatment of parkinsonism.

Dual Effect of High-Frequency Stimulation on Subthalamic Neuron Activity

Although it is well known that high-frequency stimulation (HFS) of the subthalamic nucleus (STN) alleviates the cardinal symptoms of Parkinson's disease, the underlying mechanisms are not fully understood. We investigated the effect of stimulation from low to high frequencies on rat STN neurons in naive and dopamine-depleted slices using whole-cell, current-clamp techniques and on-line artifact suppression. Stimulation at 10 Hz evoked 10 Hz single spikes but did not significantly modify ongoing STN activity. In contrast, at therapeutically relevant frequencies (80-185 Hz), stimulation had a dual effect: it fully suppressed STN spontaneous activity and generated a robust pattern of recurrent bursts of spikes, with each spike being time-locked to a stimulus pulse. Neither the suppression of spontaneous activity nor the generation of spikes was prevented by the antagonists of the metabotropic and ionotropic receptors of glutamate and gamma-aminobutyric acid. Tetrodotoxin, the Na+ channel blocker, suppressed all HFS-evoked spikes, whereas nifedipin, an L-type Ca2+-channel blocker, abolished the membrane oscillations underlying bursts. Therefore, we conclude that HFS drives the STN neuronal activity by directly activating the neuronal membrane. We suggest that this pattern may remove the deleterious activity of the basal ganglia network in the parkinsonian state and drive target neurons to a high-frequency state of activity, dependent on the characteristics of STN efferent synapses and resonant properties of target membranes.

Noradrenaline increases the firing rate of a subpopulation of rat subthalamic neurones through the activation of α 1-adrenoceptors

In the rat subthalamic nucleus, which plays a critical role in the control of motor behaviour, specific binding of [ 3H]-prazosin was detected by radioligand binding to homogenates and by autoradiography in slices. [ 3H]-Prazosin binding to homogenates (B max 71±5 fmol/mg protein; K d 0.27±0.05 nM) was competed for by α 1-antagonists. In subthalamic nucleus slices and in the presence of 10 mM LiCl, noradrenaline (100 μM) produced a modest, but consistent, stimulation of [ 3H]-inositol phosphate accumulation (146±6% of basal), reversed by the α 1-antagonist prazosin (1 μM). Extracellular single-unit recordings in slices showed that in a subpopulation (61 out of 94 cells) of rat subthalamic neurones with regular, single-spike firing pattern, noradrenaline induced a concentration-dependent increase in the firing rate (EC 50 2.5±0.2 μM, maximum effect 272±33% of basal). The action of noradrenaline was mimicked by the selective α 1-agonist phenylephrine but not by selective α 2- or β-agonists, and was blocked by the α 1-antagonist prazosin but not by α 2- or β-antagonists. The excitatory effect of noradrenaline was not prevented by perfusion with low Ca 2+/high Mg 2+ solution. In four out of 11 neurones perfusion with 3 μM noradrenaline resulted in a shift from bursting to regular firing. Taken together, our results indicate that rat subthalamic neurones express α 1-adrenoceptors responsible for noradrenaline-induced stimulation of the firing rate of a subpopulation of neurones. By modulating the spontaneous activity of STN neurones, noradrenergic pathways might have a significant role in regulating basal ganglia function and thus motor activity.

Subthreshold Sodium Currents and Pacemaking of Subthalamic Neurons: Modulation by Slow Inactivation

Neurons of the subthalamic nucleus (STN) are spontaneously active. By voltage clamping dissociated rat STN neurons with their own firing patterns, we found that pacemaking is driven by two kinds of subthreshold sodium current: a steady-state “persistent” sodium current and a dynamic “resurgent” sodium current, which promotes rapid firing by flowing immediately after a spike. These currents are strongly regulated by a process of slow inactivation that is active at physiological firing frequencies. Slow inactivation of the pacemaking sodium currents promotes a constant frequency of tonic firing in the face of small, steady changes in input and constitutes a form of adaptation at the single-cell level. Driving cells at a high rate (75 Hz) produced pronounced slow inactivation (60%–70%) of resurgent, persistent, and transient components of sodium current. This inactivation is likely to contribute to effects of clinical deep-brain stimulation on STN excitability.

Behavioral/neurophysiological investigation of effects of combining a quinolinic acid entopeduncular lesion with a fetal mesencephalic tissue transplant in striatum of the 6-OHDA hemilesioned rat.

Behavioral and electrophysiological methods were used to investigate the effects of combining a unilateral quinolinic acid lesion of the entopeduncular nucleus (QA/EP) with a striatal transplant of fetal ventral mesencephalic tissue in the 6-hydroxydopamine (6-OHDA) hemilesioned rat model for Parkinson's disease. The subjects were 6-OHDA-lesioned rats selected because their response to amphetamine treatment was a strongly biased rotation toward the side of the 6-OHDA lesion in the substantia nigra, at the expense of other evoked behaviors associated with amphetamine. Two experiments were performed. In the first, the motor effects of the QA/EP lesion alone and of the combination of QA/EP lesion with striatal transplant were determined by measuring six aspects of the motor response. In the second the electrophysiological effects of the two interventions on the responses of neurons in the subthalamic nucleus to amphetamine and apomorphine were determined in the 6-OHDA-lesioned rats. The results of the first experiment show that the QA/EP lesion by itself produced an attenuation of the rotation response and, simultaneously, an increase of oral stereotypy. They also show that the combination of QA/EP lesion with striatal transplant was more effective than the single intervention, inducing more attenuation of rotation and more oral stereotypy. The results of the second experiment show that the responses of subthalamic neurons to amphetamine in the behaving hemiparkinsonian rat with combined QA/EP lesion and transplant were larger than the responses in the hemiparkinsonian rat with the QA/EP lesion alone. However, even these larger responses in the rats with combined intervention were not as large as those recorded at the same time in the subthalamus in the opposite, intact, hemisphere. The results of the two experiments, both of which show enhanced motor and neuronal sensitivity to amphetamine after the combined intervention, suggest that such a multiple approach might prove more beneficial than a one-site intervention targeting either the EP or the striatum.

D5 (Not D1) Dopamine Receptors Potentiate Burst-Firing in Neurons of the Subthalamic Nucleus by Modulating an L-Type Calcium Conductance

Dopamine is a crucial factor in basal ganglia functioning. In current models of basal ganglia, dopamine is postulated to act on striatal neurons. However, it may also act on the subthalamic nucleus (STN), a key nucleus in the basal ganglia circuit. The data presented here were obtained in brain slices using whole-cell patch clamp. They reveal that D5 dopamine receptors strengthen electrical activity in the subset of subthalamic neurons endowed with burst-firing capacity, resulting in longer discharges of spontaneous or evoked bursts. To distinguish between D1 and D5 subtypes, the action of agonists in the D1/D5 receptor family was first investigated on rat subthalamic neurons. Single-cell reverse transcription-PCR profiling showed that burst-competent neurons only expressed D5 receptors. Accordingly, receptors localized in postsynaptic membranes within the STN were labeled by a D5-specific antibody. Second, agonists in the D1/D5 family were tested in mouse brain slices. It was found that these agonists were active in D1 receptor knock-out mice in a similar way to wild-type mice or rats. This proved that D5 rather than D1 receptors were involved. Pharmacological tools (dihydropyridines, omega-conotoxins, and calciseptine) were used to identify the target of D5 receptors as an L-type channel. This was reached via G-protein and protein kinase A. The action of dopamine on D5 receptors therefore shapes neuronal activity. It contributes to normal information processing in basal ganglia outside striatum. This finding may be useful in drug therapy for various disorders involving changes in STN activity, such as Parkinson's disease and related disorders.

Excitation by dopamine of rat subthalamic nucleus neurones in vitro—a direct action with unconventional pharmacology

Recent anatomical and physiological studies have pointed to a functional innervation of the subthalamic nucleus by dopamine. This nucleus has a pivotal role in basal ganglia function and voluntary movement control and the possibility that dopamine, and dopaminergic medication used in Parkinson’s disease, might directly influence its activity is of considerable interest. We have evaluated electrophysiologically the action and pharmacology of dopamine on single subthalamic neurones in rat brain slices. Dopamine increased firing rate to up to a mean of 60% in 98% of the 261 neurones tested when examined using extracellular single-unit recording. This excitation was unaffected by the GABA antagonist picrotoxin, and the glutamate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione, and persisted in a low Ca 2+/raised Mg 2+ solution, indicative of a direct action, independent of synaptic transmission. Of the 33 cells examined using whole patch-clamp recording, only 13 showed measurable increases in firing rate and/or depolarisations in response to dopamine. Dopamine-responsive cells displayed significantly greater access resistance, suggesting that an unidentified cytoplamic constituent, removed by whole-cell dialysis, was required for the response. Using extracellular recording, the D2-like dopamine receptor agonists quinpirole and bromocryptine, but not the D1-like receptor agonist 1-phenyl-2,3,4,5-tetrahydro-(1H)-3-benzazepine-7,8-diol, also consistently caused an excitation. This was mimicked by the catecholamine releaser amphetamine in 60% of cells tested. However, the dopamine excitation was not significantly reduced either by the D1-like receptor antagonist 7-chloro8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine or the D2-like receptor antagonists (−)-sulpiride, eticlopride and (+)-butaclamol, and the quinpirole excitation was also unaffected by (−)-sulpiride. In contrast, (−)-sulpiride, eticlopride and (+)-butaclamol all abolished the D2-like receptor-mediated inhibition by dopamine of substantia nigra pars compacta neurones. The α-adrenoceptor antagonist phentolamine was a weak antagonist of dopamine excitations, but not of those caused by quinpirole. Dopamine excitations also showed weak sensitivity to the 5-HT 2 antagonist ritanserin, but were unaffected by the α 1-adrenoceptor antagonist prazocin and the β-adrenoceptor antagonist propranolol. The pharmacology of this dopamine excitation is inconsistent with an action on any known catecholamine receptor. However, the effect of amphetamine indicates that an unidentified monamine—possibly dopamine—can be released within the subthalamic nucleus to cause an excitation. The anomalies of its pharmacological characterisation do not strongly support a physiologically relevant direct action of dopamine in the rat subthalamic nucleus.

High-frequency stimulation of the subthalamic nucleus silences subthalamic neurons: a possible cellular mechanism in Parkinson’s disease

The subthalamic nucleus participates in the control of movement and is considered a surgical target in the treatment of parkinsonian symptoms. Using the rat brain in vitro slice technique we show that sustained high-frequency (>100 Hz) electrical stimulation (i.e., ‘tetanic stimulation’) of the nucleus, as used in humans to treat Parkinson’s disease, silenced subthalamic neurons. Two main cell types were identified. ‘Tonic cells’ (68%) showed delayed inward rectification, fired continuously, switched to bursting and stopped firing when strongly depolarized with injected current. Tetanic stimulation of the nucleus induced a steady depolarization (≈18 mV) that triggered action potentials at a high rate followed by bursts and finally (≈25 s) totally silenced tonic cells. The control tonic activity was recovered rapidly (<10 s) after ending stimulation. ‘Phasic cells’ (25%) discharged a single initial brief burst of action potentials both when depolarized by prolonged current injection and tetanic stimulation and did not show inward rectification. An infrequent cell type called ‘phasic–tonic’ (7%) showed a mixed discharge. We suggest that the silencing effect of tetanic stimulation is not a frequency-dependent presynaptic depression and could result from the gradual inactivation of Na +-mediated action potentials. These findings suggest that the remission of parkinsonian symptoms by treatment with high-frequency electrical stimulation of the subthalamic nucleus in humans may primarily reside on its capacity to suppress the action potential activity of subthalamic neurons.

The Switch of Subthalamic Neurons From an Irregular to a Bursting Pattern Does Not Solely Depend on Their GABAergic Inputs in the Anesthetic-Free Rat

The subthalamic nucleus (STN) powerfully controls basal ganglia outputs and has been implicated in movement disorders observed in Parkinson's disease because of its pathological mixed burst firing mode and hyperactivity. A recent study suggested that reciprocally connected glutamatergic STN and GABAergic globus pallidus (GP) neurons act in vitro as a generator of bursting activity in basal ganglia. In vivo, we reported that GP neurons increased their firing rate in wakefulness (W) compared with slow-wave sleep (SWS) without any change in their random pattern. In contrast, STN neurons exhibited similar firing rates in W and SWS, with an irregular pattern in W and a bursty one in SWS. Thus, the pallidal GABAergic tone might control the STN pattern. This hypothesis was tested by mimicking such variations with microiontophoresis of GABA receptor ligands. GABA agonists specifically decreased the STN firing rate but did not affect its firing pattern. GABA(A) (but not GABA(B)) antagonists strongly enhanced the STN mean discharge rate during all vigilance states up to three to five times its basal activity. However, such applications did not change the typical W random pattern. When applied during SWS, GABA(A) antagonists strongly reinforced the spontaneous bursty pattern into a particularly marked one with instantaneous frequencies reaching 500-600 Hz. SWS-W transitions occurring during ongoing antagonist iontophoresis invariably disrupted the bursty pattern into a random one. Thus GABA(A) receptors play a critical, but not exclusive, role in regulating the excitatory STN influence on basal ganglia outputs.

Deep brain stimulation of subthalamic neurons increases striatal dopamine metabolism and induces contralateral circling in freely moving 6-hydroxydopamine-lesioned rats

Deep brain stimulation (DBS) of the subthalamic nucleus (STN) alleviates Parkinson's disease (PD) symptoms. Although widely used, the mechanisms of action are still unknown. In an attempt to elucidate those mechanisms, we have previously demonstrated that STN-DBS increases striatal extracellular dopamine (DA) metabolites in anaesthetized rats. PD being a movement disorder, it remains to be determined whether these findings are related to any relevant motor or behavioural changes. Thus, this study investigates concomitant behavioural changes during STN-DBS and extracellular striatal DA metabolites measured using microdialysis in freely moving 6-hydroxydopamine-lesioned rats. STN-DBS induced an increase of striatal DA metabolites in awake, freely moving animals. Furthermore, we observed concomitant contralateral circling behaviour. Taken together, these results suggest that STN-DBS could disinhibit (consequently activate) substantia nigra compacta neurons via inhibition of gamma-aminobutyric acid-ergic substantia nigra reticulata neurons.

Pharmacological identification of inward current evoked by dopamine in rat subthalamic neurons in vitro

Dopaminergic mechanisms in the subthalamic nucleus (STN) are implicated in the pathophysiology of Parkinson’s disease. Here, electrophysiological responses of STN neurons to dopamine (DA) were investigated by using whole-cell patch-clamp recordings in the rat brain slice preparation. Under current-clamp, DA depolarized membrane potential and increased the frequency of spontaneous action potentials of STN neurons. Under voltage-clamp, DA (3–300 μM) produced a reversible concentration-dependent inward current ( I DA; 6–40 pA) with an EC 50 of 13 μM. This DA-induced current had a negative slope conductance which reversed at −102 mV. It was partially reduced by barium and by superfusion with an elevated concentration of extracellular K +. Moreover, TTX and glutamate receptor antagonists (CNQX and AP5) did not significantly affect the DA responses, indicating that I DA is not dependent upon afferent synaptic activity in the STN. Quinpirole, a D 2 receptor agonist, mimicked the DA action more effectively than did the D 1 agonist SKF-38393. The D 2 antagonist sulpiride, but not the D 1 antagonist SCH-23390, blocked responses induced by DA. Intracellular application of G-protein inhibitor GDP-β-S also suppressed I DA. GTP-γ-S, added to the pipette solution, evoked a sustained inward shift in the absence of DA. These results suggest that DA increases the activity of STN neurons via activation of G-protein-coupled D 2-like receptors which reduce a K + conductance.

The parafascicular thalamic complex and basal ganglia circuitry: further complexity to the basal ganglia model

This work is focused on the study of neuronal circuits arising from the rodent caudal intralaminar nuclei and their presumed role on basal ganglia function. Emphasis was placed on the analysis of the architecture of thalamostriatal and thalamo-subthalamic projections in albino rats. Our major interest was to elucidate whether thalamic inputs were related to projection neurons or local circuit neurons within targeted structures (striatum and subthalamic nucleus). Projections coming from the parafascicular nucleus (PF) to the striatum displayed a patchy organization throughout the matrix compartment. These patches are composed by dense terminal axonal arborizations, often containing striatal neurons projecting to the entopeduncular nucleus (ENT) (medial globus pallidus in primates) and neurons projecting to the substantia nigra reticulata (SNR). The thalamostriatal projections under scrutiny were also seen to be in register with all the major classes of striatal interneurons (nitrergic neurons, neurons containing the calcium binding protein parvalbumin (PV) and cholinergic interneurons). Subthalamic neurons projecting to the ENT are the presumed postsynaptic target for fibers coming from the sensorimotor part (dorsolateral) of the PF. In summary, glutamatergic axons arising from the PF might exert a dual control of the striatal output, either by directly exciting striatal projection neurons or indirectly by means of a previous synaptic contact onto an striatal interneuron which in turn modulates the activity of projection neurons. Furthermore, thalamic inputs can also gain access to basal ganglia output nuclei via subthalamo-pallidal projecting neurons, neurons receiving glutamatergic thalamo-subthalamic projections. Thus, activation of either circuit has an opposite physiological effect on the basal ganglia output nucleus. Taken together, these data suggest that the PF may influence neuronal activity in the direct and indirect circuits and could be considered as an additional component of the basal ganglia motor loops.