D2 antagonist-induced c-fos in an identified subpopulation of globus pallidus neurons by a direct intrapallidal action

Spatiomolecular Characterization of Dopamine D2 Receptors Cells in the Mouse External Globus Pallidus

The external globus pallidus (GPe) is part of the basal ganglia circuit and plays a key role in controlling the actions. Although, many evidence indicate that dopamine through its activation of dopamine D2 receptors (D2Rs) modulates the GPe neuronal activity, the precise spatiomolecular characterization of cell populations expressing D2Rs in the mouse GPe is still lacking. By combining single molecule in situ hybridization, cell type-specific imaging analyses, and electrophysiology slice recordings, we found that GPe D2R cells are neurons preferentially localized in the caudal portion of GPe. These neurons comprising pallido-striatal, pallido-nigral, and pallido-cortical neurons segregate into two distinct populations displaying molecular and electrophysiological features of GPe GABAergic PV/NKX2.1 and cholinergic neurons respectively. By clarifying the spatial molecular identity of GPe D2R neurons in the mouse, this work provides the basis for future studies aiming at disentangling the action of dopamine within the GPe.

Dopaminergic control of the globus pallidus through activation of D2 receptors and its impact on the electrical activity of subthalamic nucleus and substantia nigra reticulata neurons

The globus pallidus (GP) receives dopaminergic afferents from the pars compacta of substantia nigra and several studies suggested that dopamine exerts its action in the GP through presynaptic D2 receptors (D2Rs). However, the impact of dopamine in GP on the pallido-subthalamic and pallido-nigral neurotransmission is not known. Here, we investigated the role of dopamine, through activation of D2Rs, in the modulation of GP neuronal activity and its impact on the electrical activity of subthalamic nucleus (STN) and substantia nigra reticulata (SNr) neurons. Extracellular recordings combined with local intracerebral microinjection of drugs were done in male Sprague-Dawley rats under urethane anesthesia. We showed that dopamine, when injected locally, increased the firing rate of the majority of neurons in the GP. This increase of the firing rate was mimicked by quinpirole, a D2R agonist, and prevented by sulpiride, a D2R antagonist. In parallel, the injection of dopamine, as well as quinpirole, in the GP reduced the firing rate of majority of STN and SNr neurons. However, neither dopamine nor quinpirole changed the tonic discharge pattern of GP, STN and SNr neurons. Our results are the first to demonstrate that dopamine through activation of D2Rs located in the GP plays an important role in the modulation of GP-STN and GP-SNr neurotransmission and consequently controls STN and SNr neuronal firing. Moreover, we provide evidence that dopamine modulate the firing rate but not the pattern of GP neurons, which in turn control the firing rate, but not the pattern of STN and SNr neurons.

Nurr1 blocks the mitogenic effect of FGF-2 and EGF, inducing olfactory bulb neural stem cells to adopt dopaminergic and dopaminergic-GABAergic neuronal phenotypes

The transcription factor Nurr1 is expressed in the mouse olfactory bulb (OB), although it remains unknown whether it influences the generation of dopaminergic neurons (DA) (DA neurons) in cells isolated from this brain region. We found that expressing Nurr1 in proliferating olfactory bulb stem cells (OBSCs) produces a marked inhibition of cell proliferation and the generation of immature neurons immunoreactive for tyrosine hydroxylase (TH) concomitant with marked upregulations of Th, Dat, Gad, and Fgfr2 transcripts. In long-term cultures, these cells develop neurochemical and synaptic markers of mature-like mesencephalic DA neurons, expressing GIRK2, VMAT2, DAT, calretinin, calbindin, synapsin-I, and SV2. Concurring with the increase in both Th and Gad expression, a subpopulation of induced cells was both TH- and GAD-immunoreactive indicating that they are dopaminergic-GABAergic neurons. Indeed, these cells could mature to express VGAT, suggesting they can uptake and store GABA in vesicles. Remarkably, the dopamine D1 receptor agonist SKF-38393 induced c-Fos in TH(+) cells and dopamine release was detected in these cultures under basal and KCl-evoked conditions. By contrast, cotransducing the Neurogenin2 and Nurr1 transcription factors produced a significant decrease in the number of TH-positive neurons. Our results indicate that Nurr1 overexpression in OBSCs induces the formation of two populations of mature dopaminergic neurons with features of the ventral mesencephalon or of the OB, capable of responding to functional dopaminergic stimuli and of releasing dopamine. They also suggest that the accumulation of Fgfr2 by Nurr1 in OBSCs may be involved in the generation of DA neurons.

Reduced striatal dopamine DA D2 receptor function in dominant-negative GSK-3 transgenic mice

Glycogen synthase kinase-3 (GSK-3) is a serine/threonine kinase with constitutive activity involved in cellular architecture, gene expression, cell proliferation, fate decision and apoptosis, among others. GSK-3 expression is particularly high in brain where it may be involved in neurological and psychiatric disorders such as Alzheimer׳s disease, bipolar disorder and major depression. A link with schizophrenia is suggested by the antipsychotic drug-induced GSK-3 regulation and by the involvement of the Akt/GSK-3 pathway in dopaminergic neurotransmission. Taking advantage of the previous development of dominant negative GSK-3 transgenic mice (Tg) showing a selective reduction of GSK-3 activity in forebrain neurons but not in dopaminergic neurons, we explored the relationship between GSK-3 and dopaminergic neurotransmission in vivo. In microdialysis experiments, local quinpirole (DA D2-R agonist) in dorsal striatum reduced dopamine (DA) release significantly less in Tg mice than in wild-type (WT) mice. However, local SKF-81297 (selective DA D1-R agonist) in dorsal striatum reduced DA release equally in both control and Tg mice indicating a comparable function of DA D1-R in the direct striato-nigral pathway. Likewise, systemic quinpirole administration - acting preferentially on presynaptic DA D2- autoreceptors to modulate DA release-reduced striatal DA release similarly in both control and Tg mice. Quinpirole reduced locomotor activity and induced c-fos expression in globus pallidus (both striatal DA D2-R-mediated effects) significantly more in WT than in Tg mice. Taking together, the present results show that dominant negative GSK-3 transgenic mice show reduced DA D2-R-mediated function in striatum and further support a link between dopaminergic neurotransmission and GSK-3 activity.

Differential regulation of MeCP2 phosphorylation in the CNS by dopamine and serotonin

Systemic administration of amphetamine (AMPH) induces phosphorylation of MeCP2 at Ser421 (pMeCP2) in select populations of neurons in the mesolimbocortical brain regions. Because AMPH simultaneously activates multiple monoamine neurotransmitter systems, here we examined the ability of dopamine (DA), serotonin (5-HT), and norepinephrine (NE) to induce pMeCP2. Selective blockade of the DA transporter (DAT) or the 5-HT transporter (SERT), but not the NE transporter (NET), was sufficient to induce pMeCP2 in the CNS. DAT blockade induced pMeCP2 in the prelimbic cortex (PLC) and nucleus accumbens (NAc), whereas SERT blockade induced pMeCP2 only in the NAc. Administration of selective DA and 5-HT receptor agonists was also sufficient to induce pMeCP2; however, the specific combination of DA and 5-HT receptors activated determined the regional- and cell-type specificity of pMeCP2 induction. The D(1)-class DA receptor agonist SKF81297 induced pMeCP2 widely; however, coadministration of the D(2)-class agonist quinpirole restricted the induction of pMeCP2 to GABAergic interneurons of the NAc. Intra-striatal injection of the adenylate cyclase activator forskolin was sufficient to induce pMeCP2 in medium-spiny neurons, suggesting that the combinatorial regulation of cAMP by different classes of DA and 5-HT receptors may contribute to the cell-type specificity of pMeCP2 induction. Consistent with the regulation of pMeCP2 by multiple monoamine neurotransmitters, genetic disruption of any single monoamine transporter in DAT-, SERT-, and NET-knockout mice failed to eliminate AMPH-induced pMeCP2 in the NAc. Together, these studies indicate that combinatorial signaling through DA and 5-HT receptors can regulate the brain region- and cell-type specific pMeCP2 in the CNS.

Functional properties of striatal fast-spiking interneurons

Striatal fast-spiking interneurons (FSIs) have a major influence over behavioral output, and a deficit in these cells has been observed in dystonia and Tourette syndrome. FSIs receive cortical input, are coupled together by gap junctions, and make perisomatic GABAergic synapses onto many nearby projection neurons. Despite being critical components of striatal microcircuits, until recently little was known about FSI activity in behaving animals. Striatal FSIs are near-continuously active in awake rodents, but even neighboring FSIs show uncorrelated activity most of the time. A coordinated "pulse" of increased FSI firing occurs throughout striatum when rats initiate one chosen action while suppressing a highly trained alternative. This pulse coincides with a drop in globus pallidus population activity, suggesting that pallidostriatal disinhibition may have a important role in timing or coordinating action execution. In addition to changes in firing rate, FSIs show behavior-linked modulation of spike timing. The variability of inter-spike intervals decreases markedly following instruction cues, and FSIs also participate in fast striatal oscillations that are linked to rewarding events and dopaminergic drugs. These studies have revealed novel and unexpected properties of FSIs, that should help inform new models of striatal information processing in both normal and aberrant conditions.

Extrastriatal dopaminergic circuits of the Basal Ganglia

The basal ganglia are comprised of the striatum, the external and internal segment of the globus pallidus (GPe and GPi, respectively), the subthalamic nucleus (STN), and the substantia nigra pars compacta and reticulata (SNc and SNr, respectively). Dopamine has long been identified as an important modulator of basal ganglia function in the striatum, and disturbances of striatal dopaminergic transmission have been implicated in diseases such as Parkinson's disease (PD), addiction and attention deficit hyperactivity disorder. However, recent evidence suggests that dopamine may also modulate basal ganglia function at sites outside of the striatum, and that changes in dopaminergic transmission at these sites may contribute to the symptoms of PD and other neuropsychiatric disorders. This review summarizes the current knowledge of the anatomy, functional effects and behavioral consequences of the dopaminergic innervation to the GPe, GPi, STN, and SNr. Further insights into the dopaminergic modulation of basal ganglia function at extrastriatal sites may provide us with opportunities to develop new and more specific strategies for treating disorders of basal ganglia dysfunction.

Selective activation of striatal fast-spiking interneurons during choice execution

Basal ganglia circuits are essential for the organization and execution of voluntary actions. Within the striatum, fast-spiking interneurons (FSIs) are thought to tightly regulate the activity of medium-spiny projection neurons (MSNs) through feed-forward inhibition, yet few studies have investigated the functional contributions of FSIs in behaving animals. We recorded presumed MSNs and FSIs together with motor cortex and globus pallidus (GP) neurons, in rats performing a simple choice task. MSN activity was widely distributed across the task sequence, especially near reward receipt. By contrast, FSIs showed a coordinated pulse of increased activity as chosen actions were initiated, in conjunction with a sharp decrease in GP activity. Both MSNs and FSIs were direction selective, but neighboring MSNs and FSIs showed opposite selectivity. Our findings suggest that individual FSIs participate in local striatal information processing, but more global disinhibition of FSIs by GP is important for initiating chosen actions while suppressing unwanted alternatives.

Opposite effects of stimulant and antipsychotic drugs on striatal fast-spiking interneurons

Psychomotor stimulants and typical antipsychotic drugs have powerful but opposite effects on mood and behavior, largely through alterations in striatal dopamine signaling. Exactly how these drug actions lead to behavioral change is not well understood, as previous electrophysiological studies have found highly heterogeneous changes in striatal neuron firing. In this study, we examined whether part of this heterogeneity reflects the mixture of distinct cell types present in the striatum, by distinguishing between medium spiny projection neurons (MSNs) and presumed fast-spiking interneurons (FSIs), in freely moving rats. The response of MSNs to both the stimulant amphetamine (0.5 or 2.5 mg/kg) and the antipsychotic eticlopride (0.2 or 1.0 mg/kg) remained highly heterogeneous, with each drug causing both increases and decreases in the firing rate of many MSNs. By contrast, FSIs showed a far more uniform, dose-dependent response to both drugs. All FSIs had decreased firing rate after high eticlopride. After high amphetamine most FSIs increased firing rate, and none decreased. In addition, the activity of the FSI population was positively correlated with locomotor activity, whereas the MSN population showed no consistent response. Our results show a direct relationship between the psychomotor effects of dopaminergic drugs and the firing rate of a specific striatal cell population. Striatal FSIs may have an important role in the behavioral effects of these drugs, and thus may be a valuable target in the development of novel therapies.

Involvement of Basal Ganglia network in motor disabilities induced by typical antipsychotics

Background

Clinical treatments with typical antipsychotic drugs (APDs) are accompanied by extrapyramidal motor side-effects (EPS) such as hypokinesia and catalepsy. As little is known about electrophysiological substrates of such motor disturbances, we investigated the effects of a typical APD, α-flupentixol, on the motor behavior and the neuronal activity of the whole basal ganglia nuclei in the rat.

Methods and Findings

The motor behavior was examined by the open field actimeter and the neuronal activity of basal ganglia nuclei was investigated using extracellular single unit recordings on urethane anesthetized rats. We show that α-flupentixol induced EPS paralleled by a decrease in the firing rate and a disorganization of the firing pattern in both substantia nigra pars reticulata (SNr) and subthalamic nucleus (STN). Furthermore, α-flupentixol induced an increase in the firing rate of globus pallidus (GP) neurons. In the striatum, we recorded two populations of medium spiny neurons (MSNs) after their antidromic identification. At basal level, both striato-pallidal and striato-nigral MSNs were found to be unaffected by α-flupentixol. However, during electrical cortico-striatal activation only striato-pallidal, but not striato-nigral, MSNs were found to be inhibited by α-flupentixol. Together, our results suggest that the changes in STN and SNr neuronal activity are a consequence of increased neuronal activity of globus pallidus (GP). Indeed, after selective GP lesion, α-flupentixol failed to induce EPS and to alter STN neuronal activity.

Conclusion

Our study reports strong evidence to show that hypokinesia and catalepsy induced by α-flupentixol are triggered by dramatic changes occurring in basal ganglia network. We provide new insight into the key role of GP in the pathophysiology of APD-induced EPS suggesting that the GP can be considered as a potential target for the treatment of EPS.

Striatal and extrastriatal dopamine in the basal ganglia: an overview of its anatomical organization in normal and Parkinsonian brains

Degeneration of the nigrostriatal dopaminergic system is the characteristic neuropathological feature of Parkinson's disease and therapy is primarily based on a dopamine replacement strategy. Dopamine has long been recognized to be a key neuromodulator of basal ganglia function, essential for normal motor activity. The recent years have witnessed significant advances in our knowledge of dopamine function in the basal ganglia. Although the striatum remains the main functional target of dopamine, it is now appreciated that there is dopaminergic innervation of the pallidum, subthalamic nucleus, and substantia nigra. A new dopaminergic- thalamic system has also been uncovered, setting the stage for a direct dopamine action on thalamocortical activity. The differential distribution of D1 and D2 receptors on neurons in the direct and indirect striato-pallidal pathways has been re-emphasized, and cholinergic interneurons are recognized as an intermediary mediator of dopamine-mediated communication between the two pathways. The importance and specificity of dopamine in regulating morphological changes in striatal projection neurons provides further evidence for the complex and multifarious mechanisms through which dopamine mediates its functional effects in the basal ganglia. In this review, the role of basal ganglia dopamine and its functional relevance in normal and pathological conditions will be discussed.

Dopamine manipulation alters immediate-early gene response of striatal parvalbumin interneurons to cortical stimulation

Cortical projections provide the major excitatory inputs to the striatum. In addition to innervating medium spiny cells, these axons contact striatal interneurons that are parvalbumin-immunoreactive (PV-ir). PV-ir interneurons make synaptic connections with many medium spiny cells, and thus can modulate striatal output. The striatum also receives dopaminergic projections from the substantia nigra, but it has been challenging to study the impact of dopamine (DA) cell injury on corticostriatal activity in vivo due to limitations in the methods used to induce cortical activity. Using epidural application of the GABA(A) antagonist picrotoxin, which produces a topographically restricted region of striatal immediate-early gene expression, we have investigated the effect of DA cell injury or DA receptor antagonism on immediate-early gene (IEG) expression in striatal medium spiny cells and PV-ir interneurons. Epidural application of picrotoxin to the rat's M1 motor cortex induced Fos in ipsilateral dorsolateral striatum. Animals previously given 6-hydroxydopamine (6-OHDA) injections into the ascending DA pathways had greater total numbers of cortical stimulation-induced striatal Fos-ir cells but fewer Fos-ir/PV-ir cells, compared to sham-operates. In a separate experiment, rats given cortical stimulation and treated with the DA D2-class antagonist eticlopride (0.10 mg/kg) exhibited fewer Fos-ir/PV-ir cells than did vehicle-treated rats. Taken together, these results indicate that DA may importantly control striatal output via influences on PV-ir interneurons. Possible mechanisms for these influences are discussed.

Glutamic acid decarboxylase 67 mRNA regulation in two globus pallidus neuron populations by dopamine and the subthalamic nucleus

The globus pallidus (GP) consists of two neuron populations, distinguished according to their immunoreactivity for parvalbumin (PV). The PV-immunoreactive (PV+) neurons project preferentially to "downstream" targets such as the subthalamic and entopeduncular nuclei, whereas neurons lacking PV (PV- neurons) project preferentially to the striatum, suggesting a role for PV- cells in feedback to striatal neurons. Although dopamine D2 antagonist administration induces immediate early gene expression preferentially in PV- GP neurons, little is known about long-term regulation of PV- versus PV+ GP neurons. Nigral 6-hydroxydopamine (6-OHDA) lesions or repeated D2-class antagonist injections have been shown to increase pallidal expression of glutamate decarboxylase (GAD(67) isoform) mRNA. This increase in GAD(67) is believed to be secondary to activation of excitatory subthalamopallidal projections. The current study examined the effects of subthalamic nucleus (STN) lesion on 6-OHDA- or repeated D2 antagonist-induced changes in GP GAD(67) mRNA expression in PV+ and PV- neurons. Five or 21 d after nigral 6-OHDA injections or after 3, 7, or 21 d of D2 antagonist administration, GAD(67) mRNA increased in both the PV- and PV+ GP neurons, but the magnitude of the increase was significantly greater in PV- neurons. By contrast, STN lesion resulted in declines in GAD(67) mRNA in both cell populations, with the decreases in PV+ neurons exceeding those in PV- neurons. Furthermore, STN lesion completely blocked 6-OHDA- or D2 antagonist-induced GAD(67) mRNA increases in PV+ cells but only partly offset the GAD(67) mRNA increase in PV- pallidal neurons. Thus, the PV+ and PV- neurons are influenced in qualitatively similar ways by dopamine and the STN, but these cell types exhibit contrasting degrees of regulation by the dopaminergic and STN perturbations. This pattern of results has implications for pallidal control of striatal versus downstream basal ganglia nuclei.

Molecular, chemical, and anatomical characterization of globus pallidus dopamine D2 receptor mRNA-containing neurons

Essential for normal movement, the globus pallidus (GP) is a prominent nucleus whose neurons project to all other basal ganglia nuclei. The GP is composed of at least two distinct neuron populations. GP neurons of the rodent contain either the calcium-binding protein parvalbumin (PV) or preproenkephalin (PPE) mRNA, differentially innervate several basal ganglia structures, and have distinct immediate early gene responses to dopamine agonists or antagonists. Recent research has revealed that dopamine directly influences GP neurons, with D2 receptors contributing to both pre- and postsynaptic effects of dopaminergic agents. The existence of D2 mRNA-expressing (D2+) GP neurons has been established, but little is known concerning their numbers, regional distribution, or relationship to pallidal subpopulations identified on the basis of PV immunocytochemistry, PPE mRNA, or axonal targets. Detection of pallidal D2 mRNA with a 35S-cRNA probe revealed that D2+ neurons are found throughout the GP, comprising approximately one-half of pallidal neurons, but they are most dense within a dorsoventral band in lateral GP. While a substantial proportion (42-51%) of all chemically and anatomically labeled pallidal neuron subpopulations expressed D2 transcript, the D2+ neurons exhibited both population-based and regional heterogeneities. Overall, the pallidostriatal cells had a greater density of D2 mRNA than did pallidosubthalamic cells. Also, compared to other pallidal regions, the ventromedial GP contained fewer D2+ cells, and the PPE mRNA-expressing cells in this region had lower densities of D2 mRNA per neuron. These results reveal heterogeneous chemical and anatomical properties of the extensive population of D2+ GP neurons, a potential cellular substrate for dopamine's effects in pallidum.