Number, origins, and chemical types of rat pallidostriatal projection neurons

Nigrostriatal lesion induces D2-modulated phase-locked activity in the basal ganglia of rats

There is a debate as to what modifications of neuronal activity underlie the clinical manifestations of Parkinson's disease and the efficacy of antiparkinsonian pharmacotherapy. Previous studies suggest that release of GABAergic striatopallidal neurons from D2 receptor-mediated inhibition allows spreading of cortical rhythms to the globus pallidus (GP) in rats with 6-hydroxydopamine-induced nigrostriatal lesions. Here this abnormal spreading was thoroughly investigated. In control urethane-anaesthetized rats most GP neurons were excited during the active part of cortical slow waves ('direct-phase' neurons). Two neuronal populations having opposite phase relationships with cortical and striatal activity coexisted in the GP of 6-hydroxydopamine-lesioned rats. 'Inverse-phase' GP units exhibited reduced firing coupled to striatal activation during slow waves, suggesting that this GP oscillation was driven by striatopallidal hyperactivity. Half of the pallidonigral neurons identified by antidromic stimulation exhibited inverse-phase activity. Therefore, spreading of inverse-phase oscillations through pallidonigral axons might contribute to the abnormal direct-phase cortical entrainment of basal ganglia output described previously. Systemic administration of the D2 agonist quinpirole to 6-hydroxydopamine-lesioned rats reduced GP inverse-phase coupling with slow waves, and this effect was reversed by the D2 antagonist eticlopride. Because striatopallidal hyperactivity was only slightly reduced by quinpirole, other mechanisms might have contributed to the effect of quinpirole on GP oscillations. These results suggest that antiparkinsonian efficacy may rely on other actions of D2 agonists on basal ganglia activity. However, abnormal slow rhythms may promote enduring changes in functional connectivity along the striatopallidal axis, contributing to D2 agonist-resistant clinical signs of parkinsonism.

Morphological and physiological properties of parvalbumin- and calretinin-containing gamma-aminobutyric acidergic neurons in the substantia nigra

Evidence for the existence of different populations of gamma-aminobutyric acid (GABA)-ergic neurons in the substantia nigra comes partially from anatomical studies, which have shown there to be little if any overlap between the calcium-binding proteins parvalbumin and calretinin in individual neurons, suggesting that these may represent neuronal subtypes with distinct electrophysiological and/or anatomical properties. We obtained whole-cell recordings from neurons in the substantia nigra pars reticulata in rat brain slices and labeled them with biocytin, followed by immunocytochemical staining for parvalbumin and calretinin. In other cases, neurons were retrogradely labeled from the thalamus or tectum and immunocytochemically identified to determine their projection sites. Intracellularly stained neurons were found to have a variety of somatic sizes and shapes. Reconstructions revealed that all parvalbumin- and calretinin-positive neurons issued at least one axon collateral, which ramified within the substantia nigra pars reticulata and/or pars compacta. Local collaterals were of medium caliber and branched modestly, expressing many long, smooth segments that then issued numerous en passant or terminal boutons, consistent with previous in vivo studies. There were no clear differences in the electrophysiological or morphological properties of neurons expressing parvalbumin or calretinin. Retrograde tracing experiments revealed that both parvalbumin- and calretinin-containing neurons project nonpreferentially to the thalamus or tectum. In sum, the parvalbumin- and calretinin-containing GABAergic neurons of the substantia nigra pars reticulata cannot be differentiated on the basis of their electrophysiological properties, morphological properties, or target nuclei, and both parvalbumin- and calretinin-containing projection neurons issue local axon collaterals that arborize within the substantia nigra.

Globus pallidus external segment

The external segment of the pallidum (GP(e)) is a relatively large nucleus located caudomedial to the neostriatum (Str). The GP(e) receives major inputs from two major basal ganglia input nuclei, the Str and the subthalamic nucleus (STN), and sends its output to many basal ganglia nuclei including the STN, the Str, the internal pallidal segment (GP(i)), and the substantia nigra (SN). Thus, the GPe can be placed at the center of the basal ganglia connection diagram (Fig. 1(A)). From the viewpoint that emphasizes the direct and indirect pathways of the basal ganglia, the GP(e) is a component of the indirect pathway that relays Str inputs to the STN. The indirect pathway can be traced in Fig. 1(A), although it comprises only a part of multiple indirect pathways. This chapter begins with a brief description of the anatomical organization of the GP(e) followed by physiological and pharmacological characterizations of GABAergic responses in the GP(e).

Repetitive Activation of Glutamatergic Inputs Evokes a Long-Lasting Excitation in Rat Globus Pallidus Neurons In Vitro

External globus pallidus (GPe) neurons express abundant metabotropic glutamate receptor 1 (mGluR1) in their somata and dendrites and receive glutamatergic inputs mainly from the subthalamic nucleus. We investigated whether synaptically released glutamate could activate mGluR1s using whole cell and cell-attached recordings in rat brain slice preparations. Repetitive internal capsule stimulation evoked EPSPs followed by a slow depolarizing response (sDEPO) lasting 10–20 s. Bath application of both GABAA and GABAB receptor antagonists increased the amplitude of sDEPOs. A mixture of AMPA/kainate and N-methyl-d-aspartate receptor antagonists did not alter sDEPOs. The induction of sDEPOs was only partially mediated by mGluR1 because mGluR1 antagonists reduced but failed to completely block the responses. Voltage-clamp recordings revealed that slow inward currents sensitive to mGluR1 antagonist were larger at −60 than at −100 mV, whereas the currents insensitive to mGluR1 antagonist were larger at −100 than at −60 mV. In cell-attached recordings, repetitive internal capsule stimulation evoked long-lasting excitations in GPe neurons, which were also partially suppressed by mGluR1 antagonists. Application of a glutamate uptake inhibitor or an mGluR1 agonist significantly increased the spontaneous firing rate but decreased the excitations to repetitive stimulation. These results suggest that synaptically released glutamate can activate mGluR1, contributing to the induction of long-lasting excitation in GPe neurons and that background mGluR1 activation suppresses the slow mGluR1 responses. Thus mGluR1 may play important roles in the control of GPe neuronal activity.

Control of the Subthalamic Innervation of the Rat Globus Pallidus by D2/3 and D4 Dopamine Receptors

The effects of activating dopaminergic D2/3 and D4 receptors during activation of the subthalamic projection to the globus pallidus (GP) were explored in rat brain slices using the whole cell patch-clamp technique. Byocitin labeling and both orthodromic and antidromic activation demonstrated the integrity of some subthalamopallidal connections in in vitro parasagittal brain slices. Excitatory postsynaptic currents (EPSCs) that could be blocked by CNQX and AP5 were evoked onto pallidal neurons by local field stimulation of the subthalamopallidal pathway in the presence of bicuculline. Bath application of dopamine and quinpirole, a dopaminergic D2-class receptor agonist, reduced evoked EPSCs by about 35%. This effect was only partially blocked by sulpiride, a D2/3 receptor antagonist. The sulpiride-sensitive reduction of the subthalamopallidal EPSC was associated with an increase in the paired-pulse ratio (PPR) and a reduction in the frequency but not the mean amplitude of spontaneous EPSCs (sEPSCs), indicative of a presynaptic site of action, which was confirmed by variance–mean analysis. The sulpiride-resistant EPSC reduction was mimicked by PD 168,077 and blocked by L-745,870, selective D4 receptor agonist and antagonist, respectively, suggesting the involvement of D4 receptors. The reduction of EPSCs produced by PD 168,077 was not accompanied by changes in PPR or the frequency of sEPSCs; however, it was accompanied by a reduction in mean sEPSC amplitude, indicative of a postsynaptic site of action. These results show that dopamine modulates subthalamopallidal excitation by presynaptic D2/3 and postsynaptic D4 receptors. The importance of this modulation is discussed.

Depolarization-induced suppression of inhibition mediated by endocannabinoids at synapses from fast-spiking interneurons to medium spiny neurons in the striatum

Endogenous cannabinoids (endocannabinoids) act as retrograde inhibitory messengers in various regions of the brain. We have recently reported that endocannabinoids mediate short-term retrograde suppression of excitatory synaptic transmission from the neocortex to medium spiny (MS) neurons, the major projection neurons from the striatum. However, it remains unclear whether endocannabinoids modulate inhibitory transmission in the striatum. Here we show that depolarization of MS neurons induces transient suppression of inhibition that is mediated by retrograde endocannabinoid signalling. By paired recording from a fast-spiking (FS) interneuron and an MS neuron, we demonstrated that FS-MS inhibitory synapses undergo endocannabinoid-mediated retrograde suppression. We verified that GABAergic inhibitory terminals immunopositive for parvalbumin (PV), a marker for FS interneurons, expressed CB1 receptors. These PV-CB1 double-positive terminals surrounded dopamine D1 receptor-positive and D2 receptor-positive MS neurons; these constitute direct and indirect pathways, respectively. These results suggest that endocannabinoid-mediated retrograde suppression of inhibition influences information flow along both direct and indirect pathways, depending on the activity of MS neurons.

Neurotensin effects on N-type calcium currents among rat pallidal neurons: an electrophysiological and immunohistochemical study

The tridecapeptide neurotensin (NT) is involved in the modulation of dopamine (DA)-mediated functions in the nigrostriatal and mesocorticolimbic pathways. Its relevance in mammalian globus pallidus (GP) is questioned. A recent electrophysiological study on GP slices described NT-mediated robust membrane depolarization, depending upon the suppression of potassium conductance and/or the activation of cation current. Here, we have studied whether NT also affected high-voltage-activated calcium (Ca(2+)) currents, by means of whole-cell recordings on isolated GP neurons. In our hands, the full peptide and the segment NT8-13 reversibly inhibited N-like Ca(2+) current in about 60% of the recorded dissociated neurons, irrespective of their capacitance. The NT-mediated modulation showed no desensitization and was antagonized by the NT1 antagonists SR48692 and SR142948. These results imply an abundant expression of NTS(1) on GP cell somata. Then, we performed a light and immunofluorescence-confocal microscopy study of NTS(1) localization among GP neurons. We found that NTS(1) is localized in about 56% of GP neurons in both subpopulations of neurons, namely parvalbumin positive and negative. We conclude that NT, likely released from the striatal terminals in GP, acts through the postsynaptic NTS(1) preferentially localized in the lateral aspects of the GP. These data suggest a new implication (neither merely presynaptic nor simply "excitatory") for NT in the modulation of GP firing pattern. In addition, NT might have a role in affecting the interplay among the endogenous release of GABA/glutamate and DA. This hypothesis might have implications on both sensori-motor and associative functions of the GP and should be tested in DA-denervated disease models.

Expression mapping of tetracycline-responsive prion protein promoter: digital atlasing for generating cell-specific disease models

We present a digital atlas system that allows mapping of molecular expression patterns at cellular resolution through large series of histological sections. Using this system, we have mapped the distribution of a distinct marker, encoded by the LacZ reporter gene driven by the tetracycline-responsive prion protein promoter in double transgenic mice. The purpose is to evaluate the suitability of this promoter mouse line for targeting genes of interest to specific brain regions, essential for construction of inducible transgenic disease models. Following processing to visualize the promoter expression, sections were counterstained to simultaneously display cytoarchitectonics. High-resolution mosaic images covering entire coronal sections were collected through the mouse brain at intervals of 200 microm. A web-based application provides access to a customized virtual microscopy tool for viewing and navigation within and across the section images. For each section image, the nearest section in a standard atlas is defined, and annotations of key structures and regions inserted. Putative categorization of labeled cells was performed with use of distribution patterns, followed by cell size and shape, as parameters that were compared to legacy data. Among the ensuing results were expression of this promoter in putative glial cells in the cerebellum (and not in Purkinje cells), in putative glial cells in the substantia nigra, in pallidal glial cells or interneurons, and in distinct cell layers and regions of the hippocampus. The study serves as a precursor for a database resource allowing evaluation of the suitability of different promoter mouse lines for generating disease models.

Striatal information signaling and integration in globus pallidus: timing matters

Advances in research on globus pallidus (GP) suggest that this 'long thought to be' relay in the 'indirect pathway' plays a unique and critical role in basal ganglia function. The traditional idea of parallel processing within the basal ganglia is also challenged by recent findings. It is now clear that axons of GP neurons form large, perisomatic baskets around target neurons in all major basal ganglia nuclei, thereby exerting a profound influence on the output of the entire basal ganglia. GP neurons are autonomously active both in vivo and in vitro. It is believed that temporal information carried along the corticostriatopallidal pathway is critical for proper motor execution. The importance of appropriately controlled discharge of GP neurons is highlighted by psychomotor disorders such as Parkinson's disease, in which alterations in the pattern and synchrony of discharge in GP neurons are thought to contribute to motor symptoms. Several lines of evidence suggest that the aberrant activity of GP neurons following dopamine depletion is caused by alteration in the synaptic input from both striatum and subthalamic nucleus. In normal subjects, the capability of striatal input in translating cortical input into precisely timed responses in GP neurons is mediated by (1) the expression of postsynaptic GABA(A) receptor composed of subunits with fast kinetic properties; (2) an effective GABA reuptake system in terminating the action of synaptically released GABA, and (3) the existence of dendritic HCN channels that actively abbreviate the time course of the inhibitory postsynaptic potentials and reset rhythmic discharge. Despite the rapid pace in uncovering the elements that shape the activity along the striatopallidosubthalamic pathway, the origin of rhythmic, synchronized bursting of GP neurons seen in parkinsonism has not been fully established experimentally. Further elucidation of the factors that control the information transfer in the striatopallidal synapses is thus critical to our understanding of basal ganglia function and establishing treatment for Parkinson's disease and other basal ganglia disorders.

Neural responses in multiple basal ganglia regions following unilateral dopamine depletion in behaving rats performing a treadmill locomotion task

To investigate basal ganglia (BG) neural responses to dopamine (DA) depletion, multiple channel, single unit recording was carried out in freely moving rats performing a treadmill locomotion task. Single unit activity from 64 microelectrodes in the striatum (STR), globus pallidus (GP), subthalamic nucleus (STN) and substantia nigra pars reticulata (SNr) was recorded simultaneously before and after a unilateral DA lesion induced by microinjection of 6-hydroxydopamine (6-OHDA) into the medial forebrain bundle. The DA lesion resulted in an impairment of treadmill walking manifested by a significant decrease in swing time of both forelimbs. The stance time, however, increased significantly only in ipsilateral (good) forelimbs, reflecting compensatory changes in the good limb for motor deficits. Neural activity in the STR and GP ipsilateral to the lesion decreased during the 7-day period following the DA lesion. Conversely, an increase in spike discharges appeared in the ipsilateral SNr and STN several days after the DA lesion. Changes in the type of neural response associated with treadmill locomotion were also found in some neurons after DA depletion. Such changes were most prominent in the STR. Limb movement-related neural activity increased significantly mainly in the SNr. Additionally, neural responses to the tone cue associated with the onset of the treadmill diminished greatly in the lesioned side of the BG. Increased activity in SNr neurons is consistent with the concept that inhibition of thalamus contributes to hypokinesis in the absence of DA. Substantial decrease in striatal activity supports a concept that DA loss leads to a global suppression of recurrent cortical striatal thalamic activity that degrades normal information flow in Parkinson's diseases.

Synaptically released GABA activates both pre- and postsynaptic GABA(B) receptors in the rat globus pallidus

The globus pallidus (GP) contains abundant GABAergic synapses and GABA(B) receptors. To investigate whether synaptically released GABA can activate pre- and postsynaptic GABA(B) receptors in the GP, physiological recordings were performed using rat brain slice preparations. Cell-attached recordings from GABA(A) antagonist-treated preparations revealed that repetitive local stimulation induced a GABA(B) antagonist-sensitive pause in spontaneous firings of GP neurons. Whole cell recordings revealed that the repetitive stimulation evoked fast excitatory postsynaptic potentials followed by a slow inhibitory postsynaptic potential (IPSP) in GP neurons. The slow IPSP was insensitive to a GABA(A) receptor antagonist, increased in amplitude with the application of ionotropic glutamate receptor antagonists, and was suppressed by the GABA(B) antagonist CGP55845. The reversal potential of the slow IPSP was close to the potassium equilibrium potential. These results suggest that synaptically released GABA activated postsynaptic GABA(B) receptors and induced the pause and the slow IPSP. On the other hand, in the neurons that were treated to block postsynaptic GABA(B) responses, CGP55845 increased the amplitudes of repetitive local stimulation-induced GABA(A)-mediated inhibitory postsynaptic currents (IPSCs) but not the ionotropic glutamate-mediated excitatory postsynaptic currents. Moreover, the GABA(B) receptor specific agonist baclofen reduced the frequency of miniature IPSCs without altering their amplitude distributions. These results suggest that synaptically released GABA also activated presynaptic GABA(B) autoreceptors, resulting in decreased GABA release in the GP. Together, we infer that both pre- and postsynaptic GABA(B) receptors may play crucial roles in the control of GP neuronal activity.

HCN2 and HCN1 channels govern the regularity of autonomous pacemaking and synaptic resetting in globus pallidus neurons

The globus pallidus (GP) is a critical component of the basal ganglia circuitry controlling motor behavior. Dysregulation of GP activity has been implicated in a number of psychomotor disorders, including Parkinson's disease (PD), in which a cardinal feature of the pathophysiology is an alteration in the pattern and synchrony of discharge in GP neurons. Yet the determinants of this activity in GP neurons are poorly understood. To help fill this gap, electrophysiological, molecular, and computational approaches were used to identify and characterize GABAergic GP neurons in tissue slices from rodents. In vitro, GABAergic GP neurons generate a regular, autonomous, single-spike pacemaker activity. Hyperpolarization-activated, cyclic nucleotide-gated cation (HCN) channels make an important contribution to this process: their blockade with ZD7288 significantly slowed discharge rate and decreased its regularity. HCN currents evoked by somatic voltage clamp had fast and slow components. Single-cell RT-PCR and immunohistochemical approaches revealed robust expression of HCN2 subunits as well as significant levels of HCN1 subunits in GABAergic GP neurons. Transient activation of striatal GABAergic input to GP neurons led to a resetting of rhythmic discharge that was dependent on HCN currents. Simulations suggested that the ability of transient striatal GABAergic input to reset pacemaking was dependent on dendritic HCN2/HCN1 channels. Together, these studies show that HCN channels in GABAergic GP neurons are key determinants of the regularity and rate of pacemaking as well as striatal resetting of this activity, implicating HCN channels in the emergence of synchrony in PD.

Directional analysis of coherent oscillatory field potentials in the cerebral cortex and basal ganglia of the rat

Population activity in cortico-basal ganglia circuits is synchronized at different frequencies according to brain state. However, the structures that are likely to drive the synchronization of activity in these circuits remain unclear. Furthermore, it is not known whether the direction of transmission of activity is fixed or dependent on brain state. We have used the directed transfer function (DTF) to investigate the direction in which coherent activity is effectively driven in cortico-basal ganglia circuits. Local field potentials (LFPs) were simultaneously recorded in the subthalamic nucleus (STN), globus pallidus (GP) and substantia nigra pars reticulata (SNr), together with the ipsilateral frontal electrocorticogram (ECoG) of anaesthetized rats. Directional analysis was performed on recordings made during robust cortical slow-wave activity (SWA) and "global activation". During SWA, there was coherence at approximately 1 Hz between ECoG and basal ganglia LFPs, with much of the coherent activity directed from cortex to basal ganglia. There were similar coherent activities at approximately 1 Hz within the basal ganglia, with more activity directed from SNr to GP and STN, and from STN to GP rather than vice versa. During global activation, peaks in coherent activity were seen at higher frequencies (15-60 Hz), with most coherence also directed from cortex to basal ganglia. Within the basal ganglia, however, coherence was predominantly directed from GP to STN and SNr. Together, these results highlight a lead role for the cortex in activity relationships with the basal ganglia, and further suggest that the effective direction of coupling between basal ganglia nuclei is dynamically organized according to brain state, with activity relationships involving the GP displaying the greatest capacity to change.

Role of Ionotropic Glutamatergic and GABAergic Inputs on the Firing Activity of Neurons in the External Pallidum in Awake Monkeys

The neurons in the external segment of the pallidum (GPe) in awake animals maintain a high level of firing activity. The level and pattern of the activity change with the development of basal ganglia disorders including parkinsonism and hemiballism. The GPe projects to most of the nuclei in the basal ganglia. Thus exploring the mechanisms controlling the firing activity is essential for understanding basal ganglia function in normal and pathological conditions. To explore the role of ionotropic glutamatergic and GABAergic inputs to the GPe, unit recordings combined with local injections of receptor antagonists were performed in awake monkeys. Observations on the effects of local application of the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)/kainate antagonist 1,2,3,4-tetrahydro-6-nitro-2, 3-dioxo-benzo[f]quinoxaline-7-sulfonamide, the N-methyl-d-aspartic acid (NMDA) antagonist 3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid, and the GABAA antagonist gabazine as well as the effects of muscimol blockade of the subthalamic nucleus on the spontaneous firing rate, firing patterns, and cortical stimulation induced responses in the GPe suggested the following: sustained glutamatergic and GABAergic inputs control the level of the spontaneous firing of GPe neurons; both AMPA/kainate and NMDA receptors are activated by glutamatergic inputs; some GPe neurons receive glutamatergic inputs originating from areas other than the subthalamic nucleus; no GPe neurons became silent after a combined application of glutamate and GABA antagonists, suggesting that GPe neurons have intrinsic properties or nonionotropic glutamatergic tonic inputs that sustain a fast oscillatory firing or a combination of a fast and a slow oscillatory firing in GPe neurons.

Different inhibitory inputs onto neostriatal projection neurons as revealed by field stimulation

This work investigated if diverse properties could be ascribed to evoked inhibitory postsynaptic currents (IPSCs) recorded on rat neostriatal neurons when field stimulation was delivered at two different locations: the globus pallidus (GP) and the neostriatum (NS). Previous work stated that stimulation in the GP could antidromically excite projection axons from medium spiny neurons. This maneuver would predominantly activate the inhibitory synapses that interconnect spiny cells. In contrast, intrastriatal stimulation would preferentially activate inhibitory synapses provided by interneurons. This study shows that, in fact, intensity-amplitude experiments are able to reveal different properties for IPSCs evoked from these two locations (GP and NS). In addition, while all IPSCs evoked from the GP were always sensitive to omega-conotoxin GVIA (Ca(V2.2)2.2 or N-channel blocker), one-half of the inhibition evoked from the NS exhibited little sensitivity to omega-conotoxin GVIA. Characteristically, all omega-conotoxin GVIA-insensitive IPSCs exhibited strong paired pulse depression, whereas omega-conotoxin GVIA-sensitive IPSCs evoked from either the GP or the NS could exhibit short-time depression or facilitation. omega-Agatoxin TK (Ca(V2.1)2.1+ or P/Q-channel blocker) blocked IPSCs evoked from both locations. Therefore 1) distinct inhibitory inputs onto projection neostriatal cells can be differentially stimulated with field electrodes; 2) N-type Ca2+ channels are not equally expressed in inhibitory terminals activated in the NS; and 3) synapses that interconnect spiny neurons use both N- and P/Q-type Ca2+ channels.

Opioid-mediated modulation of calcium currents in striatal and pallidal neurons following reserpine treatment: focus on kappa response

Previous work has shown that enkephalins target N-type calcium (Ca2+) channels in striatal and globus pallidus (GP) neurons, principally through activation of mu-like receptors. Here, we examined the effects of selective mu, delta, and kappa agonists on Ca2+ currents in striatal and GP neurons isolated from either control or reserpine-treated rats. In cells from control rats DAMGO and dynorphin (DYN) inhibited high-voltage-activated (HVA) Ca2+ currents preferentially in "medium-to-small" GP cells (likely to correspond to parvalbumin-negative cells). The kappa response was elicited by several agonists (DYN 17, DYN 13, BRL, U50-488-H), U50-488-H being the most effective (>30% maximal inhibition). U50-488-H affected both omega-CgTxGVIA-sensitive and nimodipine-sensitive Ca2+ conductances. The kappa-mediated effect (but not the mu response) was slow and blocked by chelerythrine, supporting the involvement of protein kinase C. In neurons from reserpinized rats we observed modest changes in the mu-inhibited fraction in small GP cells and a dramatic reduction of the kappa-sensitive fraction in principal striatal cells. These data imply that aminergic depletion alters opiate transmission differentially in the indirect and direct pathways. The suppression of the kappa response only in striatum reinforces the notion of an imbalance of endogenous opiates as relevant in extrapyramidal motor dysfunctions.

Activation of group III metabotropic glutamate receptors presynaptically reduces both GABAergic and glutamatergic transmission in the rat globus pallidus

To investigate the role of group III metabotropic glutamate receptors (mGluRs) in the globus pallidus (GP), whole-cell recordings were performed using rat brain slice preparations. Application of the group III mGluRs specific agonist L(+)-2-amino-4-phosphonobutyric acid (L-AP4) suppressed the amplitude of striatal stimulation-induced IPSCs and internal capsule stimulation-induced EPSCs in most of the GP neurons that were capable of generating repetitive firing without spike accommodation. The suppression of IPSCs and EPSCs was accompanied by an increase in the paired-pulse ratio. The L-AP4 effects were antagonized by (R,S)-alpha-cyclopropyl-4-phosphophenylglycine, a blocker for group II/III mGluRs. L-AP4 reduced the frequency of mIPSCs and mEPSCs without changing their amplitude distribution. L-AP4 failed to change iontophoretic glutamate induced responses. These results suggest that the subthalamo-pallidal glutamatergic input might homo- and hetero-synaptically control GABAergic and glutamatergic transmission in the GP.

Dopamine D4 receptor-induced postsynaptic inhibition of GABAergic currents in mouse globus pallidus neurons

Dopamine D4 receptors (D4R) are localized in the globus pallidus (GP), but their function remains unknown. In contrast, dopamine D2 receptor activation hyperpolarizes medium spiny neurons projecting from the striatum to the GP and inhibits GABA release. However, using slice preparations from D2R-deficient [D2 knock-out (D2KO)] mice, we found that dopamine inhibited GABA(A)-receptor-mediated currents in GP neurons. The paired-pulse ratio was statistically unchanged after dopamine application but was significantly elevated in D2KO wild-type littermates (WT). Furthermore, in D2KO mice, outward currents elicited by iontophoretically applied GABA were suppressed by dopamine. Dopamine (30 microm) decreased the amplitude of miniature IPSCs in both WT and D2KO mice, but the decrease in the frequency was observed only in the former but not significantly in the latter. Dopamine-induced suppression of IPSCs was blocked by selective D4R antagonists (clozapine or 3-[4-(4-iodophenyl)piperazin-1-yl]methyl-1H-pyrrolo[2,3-b]pyridine trihydrochloride), and a D4R-selective agonist N-[[4-(2-cyanophenyl)-1-piperazinyl]methyl]-3-methyl-benzamide reversibly and dose-dependently suppressed IPSCs, whereas agonists [SKF38,393 ((+/-)-1-phenyl-2,3,4,5-tetrahydro-(1H)-3-benzazepine-7,8-diol hydrochloride) or (+)-(4aR,10bR)-3,4,4a,10b-tetrahydro-4-propyl-2H,5H-[1]benzopyrano[4,3-b]-1,4-oxazin-9-ol] or antagonists [SCH23,390 (R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrochloride) or sulpiride] of other receptor subtypes had little effect. In GP neurons from D4R-deficient mice, dopamine-induced inhibition of GABAergic outward currents was undetectable. D4R activation suppressed the activity of protein kinase A in GP neurons, resulting in a decrease in the amplitude of GABAergic IPSCs. These findings showed that postsynaptic activation of D4R on the GP neurons reduces GABAergic currents through the suppression of PKA activity.

Is the songbird Area X striatal, pallidal, or both? an anatomical study

Anatomical and neurophysiological studies have established that Area X, a songbird nucleus essential for vocal learning, is a basal ganglia structure, with mammalian striatal properties. However, Area X also sends a gamma-aminobutyric acid (GABA)ergic projection to the medial portion of the dorsolateral thalamus (DLM), a projection characteristic of the pallidum. These findings suggested that Area X contains both striatal and pallidal neurons. To test this hypothesis further, we investigated the neurochemistry and connectivity of Area X and its projections by using neurotransmitter antibodies, in combination with tracing studies. Like the mammalian striatum, Area X contains small enkephalin- and substance P-immunopositive neurons. Choline acetyltransferase-positive cells of Area X do not retrogradely label from DLM and are probably cholinergic interneurons similar to those in mammals. Like pallidal cells, large GABAergic cells project from Area X to the thalamus, but they also contain enkephalin, a characteristic of striatal neurons projecting to indirect pathway pallidal neurons. Moreover, many Area X cells are labeled with the pallidal marker Nkx2.1, but these do not include any thalamus-projecting neurons, suggesting that the projection cells are not of pallidal embryonic origin. Thus, although Area X combines both striatal and pallidal features, it is not a simple recapitulation of the mammalian circuit or of the avian lateral striatopallidal pathway: some individual Area X neurons may function as pallidal-like projection neurons but have striatal characteristics as well. Such heterogeneity of basal ganglia circuitry, both within and across species, may be facilitated by the developmental history of basal ganglia, which involves extensive migration and cellular intermixing.

Rat intralaminar thalamic nuclei projections to the globus pallidus: A biotinylated dextran amine anterograde tracing study

The topographical organization and ultrastructural features of the intralaminar thalamic nuclei (ITN) projections to the globus pallidus (GP) were studied using the biotinylated dextran amine (BDA) anterograde tracing method in the rat. To assess the functional association of BDA injection sites in the ITN, the known topographical organization of the ITN-neostriatal (Str) projections and calcium binding protein (CaBP) immunostaining patterns of the Str and GP were used. BDA injection in the lateral part of the lateral parafascicular nucleus and the caudal part of the central lateral nucleus labeled fibers and boutons mainly in the dorsolateral sensorimotor territory of the Str and the middle territories of the GP. BDA injection in the medial part of the lateral parafascicular nucleus and the central lateral nucleus labeled mainly the middle association territory of the Str and the border and the caudomedial territories of the GP. BDA injection in the medial parafascicular nucleus and the central medial nucleus labeled mainly the medial limbic territory of the Str. The medial parafascicular nucleus projected to the medial-most region of the GP, while the central medial nucleus projection to the GP was very sparse. Electron microscopic observations indicated that BDA-labeled boutons form asymmetric synapses mainly on 0.5-2.0 microm diameter dendritic shafts in the GP. The boutons were small but had a relatively long active zone. The present observations together with the known topographical organization of striatopallidal projections indicated that the ITN-GP projections were topographically organized in parallel to the ITN-Str projections. Thus, each part of the ITN projecting to the sensorimotor, the association, and the limbic territories of the Str also projects to the corresponding functional territories of the GP.

Dopaminergic modulation of axon collaterals interconnecting spiny neurons of the rat striatum

Dopamine is a critical modulator of striatal function; its absence produces Parkinson's disease. Most cellular actions of dopamine are still unknown. This work describes the presynaptic actions of dopaminergic receptor agonists on GABAergic transmission between neostriatal projection neurons. Axon collaterals interconnect projection neurons, the main axons of which project to other basal ganglia nuclei. Most if not all of these projecting axons pass through the globus pallidus. Thus, we lesioned the intrinsic neurons of the globus pallidus and stimulated neostriatal efferent axons antidromically with a bipolar electrode located in this nucleus. This maneuver revealed a bicuculline-sensitive synaptic current while recording in spiny cells. D1 receptor agonists facilitated whereas D2 receptor agonists depressed this synaptic current. In contrast, a bicuculline-sensitive synaptic current evoked by field stimulation inside the neostriatum was not consistently modulated, in agreement with previous studies. The data are discussed in light of the most recent experimental and modeling results. The conclusion was that inhibition of spiny cells by axon collaterals of other spiny cells is quantitatively important; however, to be functionally important, this inhibition might be conditioned to the synchronized firing of spiny neurons. Finally, dopamine exerts a potentially important role regulating the extent of lateral inhibition.