Dopamine attenuates the effects of GABA on single unit activity in the globus pallidus

Breaking Away: The Role of Homeostatic Drive in Perpetuating Depression

We propose that the complexity of regulatory interactions modulating brain neurochemistry and behavior is such that multiple stable responses may be supported, and that some of these alternate regulatory programs may play a role in perpetuating persistent psychological dysfunction. To explore this, we constructed a model network representing major neurotransmission and behavioral mechanisms reported in literature as discrete logic circuits. Connectivity and information flow through this biobehavioral circuitry supported two distinct and stable regulatory programs. One such program perpetuated a depressive state with a characteristic neurochemical signature including low serotonin. Further analysis suggested that small irregularities in glutamate levels may render this pathology more directly accessible. Computer simulations mimicking selective serotonin reuptake inhibitor (SSRI) therapy in the presence of everyday stressors predicted recidivism rates similar to those reported clinically and highlighted the potentially significant benefit of concurrent behavioral stress management therapy.

Increasing Resilience to Traumatic Stress: Understanding the Protective Role of Well-Being

The brain maintains homeostasis in part through a network of feedback and feed-forward mechanisms, where neurochemicals and immune markers act as mediators. Using a previously constructed model of biobehavioral feedback, we found that in addition to healthy equilibrium another stable regulatory program supported chronic depression and anxiety. Exploring mechanisms that might underlie the contributions of subjective well-being to improved therapeutic outcomes in depression, we iteratively screened 288 candidate feedback patterns linking well-being to molecular signaling networks for those that maintained the original homeostatic regimes. Simulating stressful trigger events on each candidate network while maintaining high levels of subjective well-being isolated a specific feedback network where well-being was promoted by dopamine and acetylcholine, and itself promoted norepinephrine while inhibiting cortisol expression. This biobehavioral feedback mechanism was especially effective in reproducing well-being's clinically documented ability to promote resilience and protect against onset of depression and anxiety.

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.

Functional connectivity and integrative properties of globus pallidus neurons

The globus pallidus consists of the external (GPe) and the internal (GPi) segments. The GPe and GPi have different functional roles. The GPe is located centrally within multiple basal ganglia feedforward and feedback connections. The GPi is an output nucleus of the basal ganglia. A complex interplay between intrinsic pacemaking conductances and the balance of glutamatergic and GABAergic input largely determines the rate and pattern of firing of pallidal neurons. The initial part of this article introduces recent findings made in vivo that are related to the roles of glutamatergic and GABAergic inputs in the control of pallidal activity. The latter part describes the roles of intrinsic mechanisms of GPe neurons in the integration of the synaptic inputs. The presence of dendritic voltage-gated sodium channels may allow the initiation of dendritic spikes, giving distal inputs on the long and thin GPe dendrites an opportunity to strongly shape spiking activity. Basal ganglia disorders including Parkinson's disease, hemiballismus, and dystonias are accompanied by increased irregularity and synchronized bursts of pallidal activity. These changes may be in part due to changes in the GABA release in the GPe and GPi, but also involve intrinsic cellular changes in pallidal neurons.

Pre-synaptic histamine H₃ receptors modulate glutamatergic transmission in rat globus pallidus

The globus pallidus, a neuronal nucleus involved in the control of motor behavior, expresses high levels of histamine H(3) receptors (H(3)Rs) most likely located on the synaptic afferents to the nucleus. In this work we studied the effect of the activation of rat pallidal H(3)Rs on depolarization-evoked neurotransmitter release from slices, neuronal firing rate in vivo and turning behavior. Perfusion of globus pallidus slices with the selective H(3)R agonist immepip had no effect on the release of [(3)H]-GABA ([(3)H]-γ-aminobutyric acid) or [(3)H]-dopamine evoked by depolarization with high (20 mM) K(+), but significantly reduced [(3)H]-d-aspartate release (-44.8 ± 2.6% and -63.7 ± 6.2% at 30 and 100 nM, respectively). The effect of 30 nM immepip was blocked by 10 μM of the selective H(3)R antagonist A-331440 (4'-[3-[(3(R)-dimethylamino-1-pyrrolidinyl]propoxy]-[1,1-biphenyl]-4'-carbonitrile). Intra-pallidal injection of immepip (0.1 μl, 100 μM) decreased spontaneous neuronal firing rate in anaesthetized rats (peak inhibition 68.8±10.3%), and this effect was reversed in a partial and transitory manner by A-331440 (0.1 μl, 1 mM). In free-moving rats the infusion of immepip (0.5 μl; 10, 50 and 100 μM) into the globus pallidus induced dose-related ipsilateral turning following systemic apomorphine (0.5 mg/kg, s.c.). Turning behavior induced by immepip (0.5 μl, 50 μM) and apomorphine was partially prevented by the local injection of A-331440 (0.5 μl, 1 mM) and was not additive to the turning evoked by the intra-pallidal injection of antagonists at ionotropic glutamate receptors (0.5 μl, 1 mM each of AP-5, dl-2-amino-5-phosphonovaleric acid, and CNQX, 6-nitro-7-sulphamoylbenzo[f]quinoxaline-2,3-dione). These results indicate that pre-synaptic H(3)Rs modulate glutamatergic transmission in rat globus pallidus and thus participate in the control of movement by basal ganglia.

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.

Presynaptic actions of D2-like receptors in the rat cortico-striato-globus pallidus disynaptic connection in vitro

The cerebral cortex, the neostriatum (Str), and the external segment of the globus pallidus (GPe) form a cortico-Str-GPe disynaptic connection, which is one of the major connections in the basal ganglia circuitries and a target of dopamine modulation. The aim of this study was to examine the actions of D2-like dopamine receptors (D2LRs) in this connection using rat brain slice preparations. Electrical stimulation of the frontal cortex evoked disynaptic inhibitory postsynaptic currents (IPSCs) in cesium-filled GPe neurons voltage-clamped at 0 mV. The IPSCs evoked by threshold stimulation were small, <10 pA. Bath or local applications of the D2LR agonist quinpirole to Str decreased the amplitude of the cortical stimulation-induced IPSCs. Electrical stimulation of Str evoked monosynaptic IPSCs in GPe neurons. Local application of quinpirole to GPe decreased the Str stimulation-induced IPSCs. Bath application of quinpirole decreased the frequency of large miniature IPSCs (mIPSCs) that were considered to be evoked by local collateral axons of GPe neurons. These results suggested that activation of D2LRs decrease the gain of the cortico-Str-GPe disynaptic connection, with the decrease attributed to activation of D2LRs in Str and GPe, and that both Str-GPe and GPe-GPe GABAergic inhibitions are under the control of presynaptic D2LRs.

Effects of physiological and pharmacological stimuli on dopamine release in the rat globus pallidus

A major aspect of understanding functions of the globus pallidus (GP) within the basal ganglia is the significance of its dopamine innervation. Here, we used in vivo-microdialysis in rats to characterize pallidal dopamine release in response to a number of physiological and pharmacological stimuli known to activate dopamine neurons. Results reveal that an aversive stimulus, i.e. handling for 20 min, significantly increased dialysate dopamine in the globus pallidus to about 130% of baseline levels. Likewise, a novel and appetitive stimulus, i.e. presentation of unfamiliar, palatable food, significantly elevated pallidal dopamine to about 150% of baseline levels both in rats which did and did not consume the food reward. These findings provide evidence that increases of dopamine (DA) efflux may largely reflect stimulus saliency implicating an involvement of pallidal dopamine signalling in control of behaviour governed by salient stimuli. Results further showed that reverse microdialysis of D-amphetamine and cocaine in augmenting concentrations of 0.1-100 microM elevated dialysate dopamine in a concentration-dependent manner suggesting a role of pallidal dopamine in mediating behavioural effects of psychostimulant drugs.

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.

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.

Neurotensin depolarizes globus pallidus neurons in rats via neurotensin type-1 receptor

The globus pallidus is a major component in the indirect pathway of the basal ganglia. There is evidence that neurotensin receptors exist in this nucleus. To determine the electrophysiological effects of neurotensin on pallidal neurons, whole-cell patch-clamp recordings were performed in the acutely prepared brain slices. Under current-clamp recordings, neurotensin at 1 microM depolarized pallidal neurons. Voltage-clamp recordings also showed an inward current induced by neurotensin. The depolarizing effect of neurotensin could be mimicked by the C-terminal fragment, neurotensin (8-13), but not by the N-terminal fragment, neurotensin (1-8). Both SR 142948A, a non-selective neurotensin receptor type-1 and type-2 antagonist, and SR 48692, a selective type-1 receptor antagonist, blocked the depolarizing effect of neurotensin, and which themselves had no effect on membrane potential. Thus, neurotensin type-1 receptors appear to mediate the effect of neurotensin. The depolarization evoked by neurotensin persisted in the presence of tetrodotoxin, ionotropic and metabotropic glutamate and GABA receptor antagonists, indicating that neurotensin excited the pallidal neurons by activating the receptor expressed on the neurons recorded. Current-voltage relationship revealed that both the suppression of a potassium conductance and the activation of a cationic conductance are involved in the neurotensin-induced depolarization. Based on the action of neurotensin in the globus pallidus we hypothesize that alterations of the striatopallidal neurotensin system contribute to symptoms of basal ganglia motor disorders.

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

Much research now supports the view that the dopaminergic innervation of the globus pallidus external segment (GP) influences basal ganglia information processing via pallidal dopamine (DA) D2, D3, and possibly D1 receptors. Systemic DA agonists, or systemic or intrapallidal dopamine D2-class antagonist administration, can induce immediate early gene expression (IEG) in the rat GP. In view of the distinct chemical phenotypes and axonal projections of the GP neurons, it is important to characterize the population(s) of pallidal neurons responding to local DA manipulations. Parvalbumin (PV) immunostaining was used to identify one of the two principal GP neuron populations. Awake, behaving rats received intrapallidal infusions of the dopamine D2 antagonist sulpiride (50 or 100 ng), the D1-class antagonist SCH-23390 (100 ng), the D2-class agonist quinpirole (500 ng), the GABA(A) antagonist picrotoxin (0.25, 0.5 or 1 microg) or bicuculline (20 ng), the GABA(A) agonist muscimol (15 ng) or vehicle. Intrapallidal GABA manipulations were used to assess the likelihood that the effects of the DAergic drugs on Fos induction occurred secondarily to altering intrapallidal GABA release. Using Fos and PV double immunolabeling procedures, we found that several treatments induced GP Fos, but that intrapallidal sulpiride induced Fos almost exclusively in PV-lacking pallidal neurons. No other intrapallidal drug-induced Fos showed similar population specificity. These results support evidence suggesting that GP DA can play a unique and critical role in modulating pallidal neuron function, and that the cessation of pallidal dopamine transmission can activate gene expression within the pallidal neuron subpopulation that maintains extensive axonal projections to caudate-putamen.

Further characterization of preproenkephalin mRNA-containing cells in the rodent globus pallidus

The globus pallidus (external pallidum of primates) is an essential nucleus within basal ganglia circuitry, in part because it receives at least one-half of striatal efferent projections. Neurons of the globus pallidus can be divided into subpopulations based on anatomical, physiological, and chemical features. Globus pallidus neurons project to several structures (the striatum, subthalamic nucleus, entopeduncular nucleus, and substantia nigra pars reticulata), have one of two alternative waveforms (positive/negative versus negative/positive), contain either the calcium binding protein parvalbumin or the neuropeptide precursor preproenkephalin mRNA and show differential immediate early gene responses to dopamine receptor agonists and antagonists. The objective of the present study was to characterize in greater detail the preproenkephalin mRNA-containing pallidal neurons using Sprague-Dawley rats. In situ hybridization for preproenkephalin mRNA was combined with immunocytochemical detection of: (i) the neuron-specific nuclear protein, NeuN, (ii) FluoroGold-labeled pallidostriatal and pallidosubthalamic cells, or (iii) Fos induced by either systemic combined D1-class/D2-class dopamine receptor agonists or a D2-class receptor antagonist. These experiments demonstrated that a substantial population (42%) of globus pallidus neurons contains preproenkephalin mRNA, and that globus pallidus neurons retrogradely labeled after FluoroGold injections into the striatum are more frequently preproenkephalinergic, compared to the population of pallidosubthalamic neurons. Furthermore, systemic administration of a D2 receptor antagonist, eticlopride, induced Fos immunoreactivity predominantly in globus pallidus neurons expressing preproenkephalin mRNA, while combined administration of D1 and D2 receptor agonists induced Fos predominantly in pallidal neurons lacking preproenkephalin mRNA.These results support the conclusion that preproenkephalin mRNA identifies one of the two major subpopulations of pallidal neurons. This preproenkephalin mRNA-expressing pallidal subpopulation preferentially targets the striatum and is more readily activated in its immediate early gene expression by D2 receptor antagonists than by dopamine receptor agonists. This projection provides a pallidal substrate for the dopaminergic regulation of striatal information processing.

Dopamine D2 receptor mediated presynaptic inhibition of striatopallidal GABA(A) IPSCs in vitro

The modulation of GABA release within the globus pallidus (GP) by dopamine was studied using whole-cell patch clamp recordings from visually identified neurones. In sagittal slices, single shock electrical stimulation in the striatum evoked GABA(A) inhibitory postsynaptic currents (IPSCs), which were inhibited by dopamine in a dose-dependent manner (0.3-30 microM) with an IC(50) value of 0.7 microM. The inhibition was accompanied by an increase in paired pulse facilitation, indicative of a presynaptic effect. In coronal slices, stimulation within the GP adjacent to the recording site evoked GABA(A) IPSCs which were relatively unaffected by dopamine indicating the lack of modulation of GABA release from terminals of local GP axon collaterals. No consistent changes in holding current, membrane potential, firing rate or the frequency of spontaneous IPSCs was observed.Tetrodotoxin-resistant miniature (m)IPSCs were recorded in chloride-loaded cells. Dopamine (3-30 microM) reduced the frequency of mIPSCs, but was without effect on mIPSC amplitude, confirming a presynaptic effect. The addition of the "D2 like" agonist quinpirole (3 microM), but not the "D1 like" agonist SKF 38393 (10 microM), mimicked these effects. The "D2 like" antagonist sulpiride (10 microM), while having no effect alone, blocked the action of dopamine. In contrast the dopamine D4 selective antagonist L745, 870 (1 microM) or D1 antagonist SCH 23390 (10 microM) were without effect. These results indicate that dopamine acts on presynaptic D2 receptors on striatopallidal terminals to reduce the release of GABA in the GP. Attenuation of this mechanism following the depletion of dopamine may contribute to the changes in GP neuronal activity observed in animal models of Parkinson's disease.

Dopamine-opiate interaction in the regulation of neostriatal and pallidal neuronal activity as assessed by opioid precursor peptides and glutamate decarboxylase messenger RNA expression

Neostriatal GABAergic neurons projecting to the globus pallidus synthesize the opioid peptide enkephalin, while those innervating the substantia nigra pars reticulata and the entopeduncular nucleus synthesize dynorphin. The differential control exerted by dopamine on the activity of these two efferent projections concerns also the biosynthesis of these opioid peptides. Using in situ hybridization histochemistry, we investigated the role of opioid co-transmission in the regulation of neostriatal and pallidal activity. The expression of the messenger RNAs encoding glutamate decarboxylase-the biosynthetic enzyme of GABA-and the precursor peptides of enkephalin (preproenkephalin) and dynorphin (preprodynorphin) were measured in rats after a sustained blockade of opioid receptors by naloxone (s.c. implanted osmotic minipump, eight days, 3 mg/kg per h), and/or a subchronic blockade of D2 dopamine receptors by haloperidol (one week, 1.25 mg/kg s.c. twice a day). The density of mu opioid receptors in the neostriatum and globus pallidus was determined by autoradiography. Naloxone treatment resulted in a strong up-regulation of neostriatal and pallidal mu opioid receptors that was not affected by the concurrent administration of haloperidol. Haloperidol alone produced a moderate down-regulation of neostriatal and pallidal micro opioid receptors. Haloperidol strongly stimulated the expression of neostriatal preproenkephalin and preprodynorphin messenger RNAs. This effect was partially attenuated by naloxone, which alone produced moderate increases in preproenkephalin and preprodynorphin messenger RNA levels. In the neostriatum, naloxone did not affect either basal or haloperidol-stimulated glutamate decarboxylase messenger RNA expression. A strong reduction of glutamate decarboxylase messenger RNA expression was detected over pallidal neurons following either naloxone or haloperidol treatment, but concurrent administration of the two antagonists did not result in a further decrease. The amplitude of the variations of mu opioid receptor density and of preproenkephalin and preprodynorphin messenger RNA levels suggests that the regulation of neostriatal and pallidal micro opioid receptors is more susceptible to a direct opioid antagonism, while the biosynthesis of opioid peptides in the neostriatum is more dependent on the dopaminergic transmission. The down-regulation of mu opioid receptors following haloperidol represents probably an adaptive change to increased enkephalin biosynthesis and release. The haloperidol-induced increase in neostriatal preprodynorphin messenger RNA expression might result from an indirect, intermittent stimulation of neostriatal D1 receptors. The haloperidol-induced decrease of pallidal glutamate decarboxylase messenger RNA expression suggests, in keeping with the current functional model of the basal ganglia, that the activation of the striatopallidal projection produced by the interruption of neostriatal dopaminergic transmission reduces the GABAergic output of the globus pallidus. The reduction of pallidal glutamate decarboxylase messenger RNA expression following opioid receptor blockade indicates an indirect, excitatory influence of enkephalin upon globus pallidus neurons and, consequently, a functional antagonism between the two neuroactive substances (GABA and enkephalin) of the striatopallidal projection in the control of globus pallidus output. Through this antagonism enkephalin could partly attenuate the GABA-mediated effects of a dopaminergic denervation on pallidal neuronal activity.

Tyrosine hydroxylase-immunoreactive elements in the human globus pallidus and subthalamic nucleus

In contrast to the well-established dopaminergic innervation of the neostriatum, the existence of dopaminergic innervation of the subthalamic nucleus and globus pallidus is controversial. In the present study, tyrosine hydroxylase (TH)-immunoreactive elements were observed by light microscopy after antigen retrieval in the subthalamic nucleus and in the internal and external segments of the globus pallidus in postmortem human brain. Small islands of apparent neostriatal tissue with abundant arborization of fine, TH-immunoreactive axons in the vicinity of calbindin-positive small neurons resembling neostriatal medium spiny neurons were present in the external segment of the globus pallidus. Large numbers of medium-large, TH-immunoreactive axons were observed passing above and through the subthalamic nucleus and through both pallidal segments; these are presumed to be axons of passage on their way to the neostriatum. In addition, fine, TH-immunoreactive axons with meandering courses, occasional branches, and irregular outlines, morphologically suggestive of terminal axon arborizations with varicosities, were seen in both pallidal segments, including the ventral pallidum, and the subthalamic nucleus, consistent with a catecholaminergic (probably dopaminergic) innervation of these nuclei. This finding suggests that, in Parkinson's disease and in animal models of this disorder, loss of dopaminergic innervation might contribute to abnormal neuronal activation in these three nuclei.

Dopaminergic modulation of pallidal preproenkephalin mRNA

The present study examines dopaminergic regulation of neuropeptide gene expression within a relatively poorly characterized population of cells, the preproenkephalin (PPE) mRNA containing neurons of the globus pallidus (GP). Rats that received 6-hydroxydopamine (6-OHDA) lesions or repeated D1 or D2 antagonist administration were compared to control animals. One month after 6-OHDA lesions, PPE mRNA was elevated in the GP ipsilateral to the lesion, with a smaller elevation also being observed in the contralateral GP. Repeated administration of eticlopride, but not SCH 23390, also resulted in elevated PPE mRNA expression in the GP. These data reveal a novel effect of decreased dopamine transmission on the GP, and draw attention to this subpopulation of pallidal neurons.

A neurocomputational theory of the dopaminergic modulation of working memory functions

The dopaminergic modulation of neural activity in the prefrontal cortex (PFC) is essential for working memory. Delay-activity in the PFC in working memory tasks persists even if interfering stimuli intervene between the presentation of the sample and the target stimulus. Here, the hypothesis is put forward that the functional role of dopamine in working memory processing is to stabilize active neural representations in the PFC network and thereby to protect goal-related delay-activity against interfering stimuli. To test this hypothesis, we examined the reported dopamine-induced changes in several biophysical properties of PFC neurons to determine whether they could fulfill this function. An attractor network model consisting of model neurons was devised in which the empirically observed effects of dopamine on synaptic and voltage-gated membrane conductances could be represented in a biophysically realistic manner. In the model, the dopamine-induced enhancement of the persistent Na+ and reduction of the slowly inactivating K+ current increased firing of the delay-active neurons, thereby increasing inhibitory feedback and thus reducing activity of the "background" neurons. Furthermore, the dopamine-induced reduction of EPSP sizes and a dendritic Ca2+ current diminished the impact of intervening stimuli on current network activity. In this manner, dopaminergic effects indeed acted to stabilize current delay-activity. Working memory deficits observed after supranormal D1-receptor stimulation could also be explained within this framework. Thus, the model offers a mechanistic explanation for the behavioral deficits observed after blockade or after supranormal stimulation of dopamine receptors in the PFC and, in addition, makes some specific empirical predictions.

Globus pallidus lesions depress the excitatory responses to apomorphine but not amphetamine in the subthalamic nucleus of the behaving rat with a 6-OHDA nigra lesion

The role of the dopaminergic innervation of the basal ganglia on the activity in the subthalamic nucleus (STN) evoked by amphetamine and apomorphine in the behaving rat was examined. The aim was to determine the relationship between that neural activity and the movements evoked by the drugs. Bilateral electrolytic lesions of the globus pallidus (GP), superimposed on the earlier unilateral lesion in substantia nigra (SN) with 6-hydroxydopamine (6-OHDA) affected differently the excitatory responses in the STN evoked by amphetamine and apomorphine and the motor responses to the drugs recorded concurrently. Before the GP lesions, the administration of amphetamine, 5 mg/kg, to the unilaterally deafferented rat induced increased activity in the STN and simultaneously increased movement in the animal. After the GP lesions, the excitatory response to amphetamine in the STN was not different from that seen before the GP lesions. The motor response was also unchanged. In contrast, the GP lesions altered the excitatory response to apomorphine, 3 mg/kg. Before these lesions, the administration of apomorphine to the 6-OHDA lesioned animal evoked a robust and long-lasting excitatory response in the STN and, concurrently, a long-lasting motor response. After the GP lesions, both responses to apomorphine were attenuated. These differential effects of the GP lesions on the unit and motor responses to the two drugs are viewed as representing the effects of the damage in the GP on the dopaminergic innervation contributing to the regulation of activity in the STN. In the 6-OHDA animal, the dopamine afferents innervating the basal ganglia had already been dramatically reduced by 6-OHDA. The GP lesions did not significantly add to the number of these afferents previously eliminated; therefore, the excitatory and motor responses to amphetamine were not changed by the GP lesions. But the GP damage served to eliminate the dopamine receptor in the GP and thus reduced the density of the dopamine receptor in the basal ganglia available for binding to apomorphine. Therefore, the excitatory and motor responses to apomorphine were attenuated after the GP lesions compared to the responses before these lesions.

Substance P attenuates and DAMGO potentiates amygdala glutamatergic neurotransmission within the ventral pallidum

The amygdala (AMG), nucleus accumbens (NA) and ventral pallidum (VP) influence goal-oriented behaviors. However, the nature of the interactions among these regions has not been well characterized. Anatomical studies indicate that excitatory amino acids are contained in VP inputs from the AMG, and the NA is a primary source of VP substance P (SP) and opioids. The present study was designed to functionally characterize the NA and AMG projections to the VP, and to assess if opioids and SP can modulate AMG-mediated excitatory neurotransmission within the VP. To do so, extracellularly recorded electrophysiological responses of single VP neurons to electrical activation of VP afferents were monitored during microiontophoretic application of treatment ligands in chloral hydrate-anesthetized rats. The anatomically described glutamatergic inputs from the AMG, and SP inputs from the NA, were pharmacologically verified. It also was determined that even though iontophoretically applied SP increased the spontaneous activity of VP neurons, at ejection current levels that were below those necessary to produce this effect (termed sub-threshold), the tachykinin attenuated AMG stimulation-evoked glutamatergic neurotransmission. SP failed to modulate the excitations induced by iontophoretically applied glutamate suggesting that SP modulation of AMG-evoked excitations were mediated via a decrease in the pre-synaptic release of glutamate. Like SP, the effects of sub-threshold ejection currents of micro opioid agonist DAMGO on AMG-evoked responses were not predicted by the opioid's effects on spontaneous VP neuronal activity; DAMGO inhibited spontaneous firing but potentiated AMG-evoked glutamatergic neurotransmission. The opioid also potentiated effects of exogenous glutamate implying an interaction at a post-synaptic site. These results indicate that tachykinin and opioid neuropeptides contained in NA projection neurons can differentially modulate AMG glutamatergic inputs to the VP.

GABA- and glutamate-evoked responses in the rat ventral pallidum are modulated by dopamine

Microiontophoresis was used to investigate the influence of dopamine on GABA- and glutamate-induced responses from ventral pallidal neurons recorded extracellularly in chloral hydrate-anaesthetized rats. Modulation was determined by comparing dopamine-induced alterations in amino acid-induced activity ('signal') with dopamine-induced effects on spontaneous firing ('noise'). A dopamine ejection current-response curve was generated to determine the current levels that did not alter spontaneous firing ('subthreshold') and those that produced approximately 50% of the maximal dopamine-induced response (ECur50). Co-iontophoresis of dopamine with GABA generally diminished the inhibitory influence of GABA on pallidal neuron firing; 70% of neurons tested with ECur50 dopamine demonstrated a decrease in the signal-to-noise ratio whereas 10% displayed an increase. At subthreshold dopamine ejection currents, 59% of neurons responded with a decrease and 18% responded with an increase in the GABA signal-to-noise ratio. When ECur50 dopamine was co-iontophoresed with glutamate, 84% of the neurons displayed a decrease in the signal-to-noise ratio for glutamate-evoked excitations whereas 11% demonstrated an increase. Subthreshold dopamine ejection currents decreased the signal-to-noise ratio in 62% of the ventral pallidal neurons excited by glutamate and increased the ratio in 23%. These data illustrate that dopamine substantially alters GABA- and glutamate-evoked responses even at ejection currents that are below those necessary to change spontaneous firing. Thus, it appears that neuromodulation is an important means by which dopamine influences ventral pallidal neuronal activity.

Morphine modulation of GABA- and glutamate-induced changes of ventral pallidal neuronal activity

Microiontophoresis was used to investigate the influence of morphine on the GABA- and glutamate-evoked responses of ventral pallidal neurons recorded extracellularly from chloral hydrate-anesthetized rats. Of the GABA-sensitive neurons (50 of 69 tested) in the ventral pallidum, all displayed a decreased firing rate when GABA was applied, whereas all of the glutamate-sensitive neurons (29 of 40 tested) increased neuronal activity in the presence of glutamate. The majority of ventral pallidal cells tested (65 of 83) were sensitive to iontophoretically applied morphine, and both increases and decreases in neuronal activity were observed. The ability of morphine to alter the ratio between amino acid-evoked activity ("signal") and spontaneous firing ("noise") was used as an indicator of morphine modulation. A morphine subthreshold ejection current, i.e. one that did not change spontaneous firing rate, and a morphine ejection current that produced approximately 50% of the maximum opioid-induced neuronal response were chosen for this evaluation. When morphine was co-iontophoresed with GABA or glutamate, attenuation of the amino acid signal-to-noise ratio was generally seen, though some potentiations were observed. These changes were independent of the direction of morphine-induced changes in spontaneous firing rate. Both sub- and suprathreshold ejection currents were capable of affecting GABA- and glutamate-evoked responses. These data suggest that morphine is a robust ventral pallidal neuromodulator. As ventral pallidal amino acid activity is important in the integration of sensorimotor information, opioid modulation of amino acid transmission in the ventral pallidum may have a profound effect on this integration.

Modulation of inhibitory transmission by dopamine in rat basal forebrain nuclei: activation of presynaptic D1-like dopaminergic receptors

The effects of dopamine (DA) on inhibitory transmission onto identified magnocellular neurons were examined in rat basal forebrain slices using whole-cell recording. IPSCs evoked by focal stimulation within basal forebrain nuclei were reversibly blocked by 10 microM bicuculline and had a decay time constant of 20.1 +/- 0.77 msec in the presence of 6-cyano-7-nitroquinoxalline-2,3-dione (5 mM). Bath application of DA reduced the amplitude of IPSCs up to 71.1 +/- 1.49% in a concentration-dependent manner between 0.003 and 1 mM (the IC50 value being 6.6 microM), without any effect on the holding current at -70 mV. DA (10 microM) reduced the frequency of miniature IPSCs (mIPSCs) recorded in the presence of TTX (0.5 microM), without affecting their mean amplitude, rise time, and decay time constant. Furthermore, the DA-induced effect on mIPSCs remained unaffected by 100 microM cadmium, suggesting a presynaptic mechanism independent of calcium influx. SKF 81297, a D1-like agonist, mimicked DA-induced effect on evoked IPSCs (IC50, 10.9 microM), whereas R(-)-TNPA or (-)-quinpirole, D2-like agonists (30 microM), had little or no effect on the amplitude of evoked IPSCs. R(+)-SCH 23390, a D1-like antagonist, antagonized the DA-induced effect on IPSCs (K(B) 0.82 microM), whereas S(-)-eticlopride, a D2-like antagonist, showed slight antagonism (K(B) 7.8 microM). Forskolin (10 microM) reduced the amplitude of evoked IPSCs to approximately 58% of the control and occluded the inhibitory effect of DA. These findings indicate that DA reduces inhibitory transmission onto magnocellular basal forebrain neurons by activating presynaptic D1-like receptors.

Functional significance of mesolimbic dopamine

Although a large body of neuropharmacological evidence suggests that the mesolimbic dopamine system (ML DA) is critical for goal-directed behaviors, exactly which aspects of behavior are mediated or modulated by this system remains a matter of conjecture. By measuring changes in DA cell firing patterns and extracellular DA concentrations in target areas of ML DA cells during the development and performance of goal-directed behavior, it is possible to directly examine the relationship between ML DA transmission and various stages and components of behavior. This permits tests of hypotheses concerned with the functional significance of ML DA. This review will discuss recent electrophysiological, microdialysis and electrochemical data on behavior-associated changes in firing activity of ML DA cells and fluctuations in DA concentrations in target areas of these cells. Although application of an electrochemical technique to study behavior-associated changes in DA transmission is an area of hot debates, a close correlation between DA-dependent electrochemical signal changes and separate behavioral components, with a generally similar pattern of rapid signal fluctuations found in trained animals during operant lever-pressing behavior maintained by palatable food, cocaine or heroin, suggests that extrasynaptic DA may have some important functions in regulating behavior. This review will discuss possible mechanisms underlying phasic and tonic changes in ML DA transmission accompanying development and performance of positively-reinforced behavior, the contribution of learning, behavioral and pharmacological variables in the mediation of these changes, and their relevance for the organization and regulation of goal-directed behavior.

Electrophysiology of dopamine D-1 receptors in the basal ganglia: old facts and new perspectives

1. The dopamine (DA) D1-receptor family is highly represented in the mammalian brain and particularly in the nigrostriatal system, whose integrity is crucial for the execution of motor performances. 2. In the last decade, our understanding of the electrophysiology of D1 receptors on caudate-putamen neurons has greatly improved. The effects of the activation of striatal D1 receptors were studied by extracellular single unit recordings in the intact animal as well as by intracellular recordings in rat brain slice preparation. More recently, whole-cell recordings on isolated striatal neurons have further addressed this issue and confirmed the inhibitory modulatory role of D1 receptor on the electrical activity of striatal neurons. 3. Several important questions, however, concerning the functional effects of D1 receptor activation in the basal ganglia are still debated: the cellular segregation of the distribution of D1-D2-like receptors, their synergistic or opposite functional roles at the second messenger level, the effects of D1 receptor activation on the transmitter release and the modifications of D1 receptor pharmacology in dopamine-denervated striata. 4. A different perspective will also be discussed: the involvement of D1 receptors in long-term changes of synaptic efficacy in the striatum as a possible correlate of motor learning.

Dopaminergic innervation of the telencephalon of the pigeon (Columba livia): a study with antibodies against tyrosine hydroxylase and dopamine

The dopaminergic structures in the telencephalon of the pigeon were investigated with antisera against glutaraldehyde-conjugated dopamine (DA) and tyrosine hydroxylase (TH). Our goal was to describe the morphological patterns of the labelled axons and to provide a detailed map of the density and regional distribution of the dopaminergic innervation in relation to cytoarchitectonic areas. DA- and TH-like fibers reached their highest density in the paleostriatum augmentatum and the lobus parolfactorius of the basal ganglia. The paleostriatum primitivum was characterized by a dichotomous DA-positive innervation with a diffuse fiber network contacting enpassant granular cells and a more specific input that completely wrapped up large cells, which probably represent relay neurons. Two distinct DA-positive pathways could be followed back from the forebrain leading to the dopaminergic cell groups of the nucleus tegmenti pedunuculopontinus pars lateralis and the area ventralis tegmentalis. The primary sensory areas of the visual, auditory, somatosensory, and trigeminal systems within the forebrain of the pigeon were virtually devoid of DA-like fibers and demonstrated only TH-positive axons, probably of a noradrenergic nature. Among the limbic structures, the neostriatum caudolaterale (a possible equivalent of the mammalian prefrontal cortex), the septum, the nucleus accumbens, and parts of the archistriatum were heavily labelled by DA-like axons. A highly characteristic morphological feature of the catecholaminergic innervation was the presence of "baskets," which are constituted by TH- and DA-positive fibers coiled up around large perikarya, so that the surrounded somata were virtually visible by the presence of labelled axons. The density of basket and nonbasket type innervations seemed to be independently regulated, so that each forebrain structure could be characterized by a mixture consisting of the individual degrees of these two features. Our results demonstrate that the dopaminergic innervation of the forebrain of the pigeon is widespread but shows important regional variations. Similar to mammals, associative and motor structures are heavily innervated by dopaminergic fibers, whereas sensory areas are dominated by their noradrenergic input. The basket and nonbasket type innervations observed in virtually all of these subdivisions of the telencephalon may indicate the presence of two main classes of catecholaminergic afferents with different mechanisms of modulation of forebrain activity patterns.

Activation of dopaminergic neurons modulates ventral pallidal responses evoked by amygdala stimulation

The ventral pallidum is a basal forebrain region that is thought to integrate cognitive processes with motoric behaviors. These functions are influenced by ventral pallidal inputs, which include projections from the amygdala and the ventral tegmental area/substantia nigra zona compacta. By examining the consequences of this convergence at the neuronal level, the present study indicates that electrical activation of ventral tegmental regions releases dopamine in the ventral pallidum which subsequently modulates pallidal electrophysiological responses evoked by stimulating the amygdala. Stimulation-evoked responses were characterized for extracellular single unit recordings of spontaneously active ventral pallidal neurons from chloral hydrate anesthetized rats. Stimulation of the amygdala evoked short latency (< or = 12 ms; possibly monosynaptic) and/or long latency (> 12 ms; polysynaptic) responses in all ventral pallidal neurons tested. Fifty-nine per cent of the tested neurons responded to ventral tegmental stimulation with short latency inhibition, and these neurons were often sensitive to microiontophoretically applied dopamine. Iontophoresis of dopamine antagonists SCH23390 (a D1 antagonist) or sulpiride (a D2 antagonist) attenuated the ventral tegmental-induced inhibitions. These observations indicate that the evoked responding was the result of endogenously released dopamine, and that D1 and D2 receptors were involved in this effect. Ninety-two per cent of the ventral pallidal neurons that demonstrated short latency responses to amygdala stimulation also exhibited short latency responses to activation of the ventral tegmentum. This suggests that these inputs often converge onto the same pallidal neurons. Amygdala-evoked responses were consistently attenuated by prior stimulation of the ventral tegmentum. Similarly, microiontophoretic ejection of dopamine attenuated amygdala-evoked effects. These results indicate that dopamine modulates amygdala-evoked pallidal responses. Such modulation may contribute to the integrative functions of the ventral pallidum.

Dopamine mechanisms of cocaine addiction

The ability of cocaine to induce a compulsive addictive behavior is the most astonishing feature of this drug. Attempting to understand the mechanisms underlying cocaine's addictive properties, two major questions should be considered: a) why and how organism's interaction with cocaine results in the development of new, drug-seeking and drug-taking behavior and b) why and how cocaine maintains this behavior when the drug is available. Since a large body of neuropharmacological evidence suggest that the mesocorticolimbic dopamine (DA) system has exclusive importance for the development and maintenance of cocaine addictive behavior, and cocaine is known to interfere in activity of this brain system, examination of mesocorticolimbic DA activity during cocaine self-administration behavior may provide some clues for understanding the drug's additive properties and regulation of this maladaptive goal-directed behavior. The aim of this paper is to discuss the literature and own experimental data on cocaine's action on the mesocorticolimbic DA system that may be involved in mediating its addictive properties. Based on these data, it is suggested that an inhibiting action of cocaine on reuptake of released DA, although essential, but not sufficient mechanism for the development and maintenance of addictive behavior. It is hypothesized, that coexistence of functionally antagonistic, inhibiting actions of cocaine on the mesolimbic DA release and reuptake of released DA may be responsible for biphasic fluctuations in DA transmission that appear to be a critical component of central oscillatory mechanism which drives and regulates cyclic drug-taking behavior.

Brainstem dopaminergic, cholinergic and serotoninergic afferents to the pallidum in the squirrel monkey

The retrograde tracer cholera toxin B subunit (CTb) was used in combination with immunohistochemistry for tyrosine hydroxylase (TH), calbindin D-28k (CaBP), choline acetyltransferase (ChAT) and 5-hydroxytryptamine (5-HT) to determine the distribution and relative proportion of brainstem chemospecific neurons that project to the pallidum in the squirrel monkey (Saimiri sciureus). Large injections of CTb involving both pallidal segments produce numerous retrogradely labeled neurons in the substantia nigra (SN), the pedunculopontine tegmental nucleus (PPN) and the dorsal raphe nucleus (DR). Labeled neurons are distributed uniformly in SN with a slight numerical increase at the junction between the pars compacta (SNc) and the ventral tegmental area (VTA). Retrogradely labeled neurons abound also in PPN, principally in its pars dissipata, whereas other CTb-labeled cells are scattered throughout the rostrocaudal extent of DR. After CTb injection involving specifically the internal pallidal segment (GPi), the same pattern of cell distribution is found in SN, PPN and DR, except that the number of retrogradely labeled cells is lower than after large pallidal complex injections. Approximately 70% of all CTb-labeled neurons in SNc-VTA complex display TH immunoreactivity, whereas 20% are immunoreactive for CaBP. About 39% of all retrogradely labeled neurons in PPN are immunoreactive for ChAT, whereas approximately 38% of the labeled neurons in DR display 5-HT immunoreactivity. Following CTb injection in the external pallidal segment (GPe), the number of labeled cells is much smaller than after GPi injection. The majority of CTb-labeled cells in SNc-VTA complex are located in the lateral half of SNc and approximately 93% of these neurons display TH immunoreactivity compared to 10% that are immunoreactive for CaBP; very few CTb-labeled cells occur in PPN. Retrogradely labeled cells in DR are located more laterally than those that projects to the GPi and about 25% of them are immunoreactive for 5-HT. These results suggest that, in addition to their action at striatal and/or nigral levels, the brainstem dopaminergic, cholinergic and serotoninergic neurons influence the output of the primate basal ganglia by acting directly upon GPi neurons.

Contribution of the amygdala and nucleus accumbens to ventral pallidal responses to dopamine agonists

Neurons recorded from ventral pallidum/substantia innominata (VP) of the basal forebrain respond to dopaminergic agonists that activate either the D1 or D2 the receptor subtype. Major afferent systems to the VP originate within amygdaloid nuclei (AMN) and the nucleus accumbens (NA). Since both the AMN and the NA are dopaminoceptive, the present study sought to analyze the contribution of these afferent systems to VP responses to dopaminergic agonists. Single VP neurons were electrophysiologically recorded in vivo from chloral hydrate-anesthetized rats, and the following determinations were made. 1) Effects of pharmacologic inactivation of an afferent system were assessed by monitoring VP neurons during intracerebral microinjections of the local anesthetic procaine, administered directly into either the AMN or the NA. 2) With procaine-induced VP rate changes used to indicate an afferent influence on the recorded neuron, VP responses to apomorphine (an agonist that acts at D1 and D2 receptor subtypes), SKF38393 (a D1 agonist), or quinpirole (a D2 agonist) were determined and compared with responses in rats not receiving the procaine pretreatment. Following pharmacologic inactivation of either the AMN or the NA, approximately 80% of the VP neurons monitored demonstrated rate changes, illustrating that spontaneous neuronal firing in the Vp is dependent on tonically input systems. Following afferent cessation, responses to apomorphine and quinpirole remained intact, suggesting that the AMN or NA is not necessary for VP responding to the systemic administration of dopaminergic agonists that act at D2 receptors. In contrast, the number of neurons that responded to SKF38393 was diminished follow intra-AMN (but not intra-NA) procaine. This suggests that D1-induced VP responses are mediated, at least in part, via the AMN.

Dopamine modulates the inhibition induced by GABA in rat cerebral cortex: an iontophoretic study

Effects of iontophoresed gamma-aminobutyric acid (GABA) and two GABA agonists, 4,5,6,7-tetrahydroisooxazolo-[5,4-c]pyridine-3-ol (THIP) and baclofen were quantitatively compared in the anterior cingulate, frontal, and parietal cortex of urethane-anesthetized intact rats after catecholamine (CA) depletion with alpha-methyl-p-tyrosine (alpha-MPT) or selective dopamine (DA) denervation with 6-hydroxydopamine (6-OHDA). As assessed with to the IT50 index, the postsynaptic sensitivity to GABA was significantly higher in anterior cingulate than in frontal and parietal cortex. The responsiveness to GABA was also greater in frontal than in parietal cortex. Sensitivity to GABA was significantly reduced in both anterior cingulate and frontal cortex after CA depletion, and similarly, after DA denervation with 6-OHDA. The difference in the sensitivity to GABA between the three cortical regions in intact rats as well as after CA depletion did not seem to be correlated with either GABAA or GABAB receptors since the responsiveness to both GABA agonists in every region examined was comparable in intact rats, and remained unchanged after alpha-MPT treatment. This finding raises the possibility that some GABA receptors in the cerebral cortex may be pharmacologically distinct from the two main subtypes of GABA receptors, GABAA and GABAB. When GABA was administered by iontophoresis in the anterior cingulate cortex during continuous applications of subthreshold currents of DA, the inhibition induced by GABA was either increased or decreased. As DA innervation density is nearly two-fold greater in anterior cingulate than in frontal cortex, and 30-fold greater in anterior cingulate than in parietal cortex, these results suggest that responsiveness to GABA may be correlated with the regional density of DA innervation and that elevated levels of DA may enhance the sensitivity to GABA.

Relative sparing of the dopaminergic innervation of the globus pallidus in monkeys made hemi-parkinsonian by intracarotid MPTP infusion

Tyrosine hydroxylase immunohistochemical analysis was performed on tissue sections through the pallidal complex from Nemistrina monkeys which had been made hemi-parkinsonian by intracarotid MPTP infusion 8-12 months earlier. The side contralateral to the MPTP infusion showed a dense dopaminergic innervation of the pallidum (both internal and external segments), but particularly the internal pallidum. The side of the brain ipsilateral to the MPTP infusion showed a remarkable sparing of the pallidal dopaminergic innervation, despite almost total loss of the dopaminergic innervation of the caudate and putamen. These results support the view that in the primate, the nigropallidal projection is mostly distinct from the nigrostriatal projection. It is also suggested that perhaps the sparing of pallidal dopamine at least in part may contribute to some of the recovery of function observed in some monkeys following exposure to MPTP.

Acute monoaminergic depletion in the rat potentiates the excitatory effect of the subthalamic nucleus in the substantia nigra pars reticulata but not in the pallidal complex

Recent neurochemical evidence suggests that chemical or electrical stimulation of the subthalamic nucleus (STH) increases dopamine release in the substantia nigra (SN) with a subsequent decrease in the striatum. In a previous paper, we reported that bicuculline-induced activation of the STH increases neuronal activity in the substantia nigra pars reticulata (SNpr) and in the pallidal complex. In order to investigate the role played by the dopaminergic system in the observed activation, the neuronal responses of subthalamic nucleus target structures were studied in amine depleted rats following subthalamic stimulation. Amine depletion was accomplished by pretreating the rats with reserpine (2 mg/kg; S.C.) and with alpha-methyl-para-tyrosine (alpha-mpt; 50 mg/kg; I.P.). Following this treatment, dopamine levels were reduced by 94% in the striatum as measured by HPLC. Amine depletion significantly increased the spontaneous activity of subthalamic cells by 53%. In the SNpr, no significant changes in the spontaneous neuronal activity were observed, but the excitatory responses to bicuculline-induced stimulation of the STH were potentiated as compared to non-treated animals. In the pallidial complex (GP-EP), no potentiation was found. The data suggest that the spontaneous pattern of discharge of the STH is probably under monoaminergic control. They also suggest a reciprocal interaction between dopamine and glutamatergic afferent terminals from the STH within the SNpr, but not in the pallidal complex.

The role of striatopallidal neurones utilizing gamma-aminobutyric acid in the pathophysiology of MPTP-induced parkinsonism in the primate: evidence from [3H]flunitrazepam autoradiography

The GABA/benzodiazepine receptor complex in the basal ganglia of primates treated with the neurotoxin n-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) has been studied by semi-quantitative autoradiography with [3H]flunitrazepam ([3H]FNZ). Systemic treatment with MPTP produced a stable and lasting parkinsonian condition, with pronounced bradykinesia, akinesia and tremor. In the lateral segment of the globus pallidus (GPL) there was a significant reduction of [3H]FNZ binding compared with non-treated animals. There were no significant changes in the [3H]FNZ binding in the caudate nucleus, putamen and medial globus pallidus (GPM). This suggests that MPTP-treatment increases GABA release within the GPL exclusively. In view of the available evidence suggesting increased striatal output, and reduced unit activity within the GPL of the MPTP-treated primate, it seems likely that the striatal GABAergic output to the GPL is overactive in this model of Parkinson's disease. Furthermore, as there is no evidence for a change in GABA function within the GPM using this measure, the striatal neurones which innervate the GPM may be differentially affected by loss of dopamine innervation. In line with structural evidence and extrastriatal dopamine receptor distribution this suggests that the two striatopallidal systems are functionally heterogeneous. A hemi-parkinsonian primate model has also been used in this study. This model was produced by injection of MPTP directly into one carotid artery. The substantia nigra pars compacta (SNc) was destroyed on the injected side alone, and consequently the appearance of parkinsonian symptoms was confined to the contralateral side. [3H]FNZ binding in the GPL appears to be bilaterally reduced in this model, suggesting an interaction between the treated and non-treated side of the brain. In addition there is increased binding in the putamen and GPM with respect to the non-treated side of the brain. The increased [3H]FNZ binding in the GPM of the unilateral model may be due to the greater disruption of the nigropallidal and/or nigrostiatal dopamine neurones relative to the systemic model. The former would have the effect of uncoupling D1 dopamine receptors located on the terminals of striatal efferents from nigropallidal dopamine input, and as D1 dopamine receptors are implicated in the presynaptic control of GABA release from the terminals of striatal efferents, this would consequently reduce the level of GABA release in the GPM. The latter possibility would suggest that striatopallidal neurones projecting to GPM are more resistant to the effects of dopaminergic denervation than those projecting to GPL.

Distribution and synaptic organization of dopaminergic axons in the lateral geniculate nucleus of the rat

In the present study, immunocytochemistry with an antiserum against dopamine (DA) revealed hitherto unknown terminal fields of DA axons in the lateral geniculate nucleus (LGN) of the rat. The innervation of all subdivisions of the LGN is achieved by a common set of afferent fibers that branch to form terminal fields of uneven density. The ventral lateral geniculate nucleus (LGv) receives slightly more DA axons than the dorsal lateral geniculate nucleus (LGd), whereas within the latter, DA afferents innervate the lateral part of the nucleus slightly more densely. Labeled axon terminals and varicosities, examined in single and serial ultrathin sections, were found in the extraglomerular neuropil in the LGd and in the neuropil of the LGv characterized by relatively simple synaptic relationships. They formed predominantly asymmetrical synaptic contacts with dendritic profiles. Occasionally, the postsynaptic elements were found to be presynaptic dendrites of presumptive interneurons. Some of the possible roles of this newly demonstrated DA afferent system in the physiology of the LGN and in the pathophysiology of diseases associated with impairment of dopaminergic activity are discussed.

Differential effects of D1 and D2 dopamine receptor agonists on substantia nigra pars reticulata neurons

Dopamine was shown in previous studies to exert a dual effect on non-dopaminergic neurons of the substantia nigra pars reticulata: it increases the firing rates of about 50% of cells, and consistently lessens the ability of iontophoretically applied or endogenously released GABA to inhibit their firing. These studies were undertaken to determine (1) whether the two effects could occur independently and, (2) whether different dopamine receptor subtypes might mediate the two responses. Extracellular, single unit activities of pars reticulata neurons were monitored in male rats anesthetized with chloral hydrate. Repeated 30-s iontophoretic pulses of GABA were delivered at an ejection current sufficient to inhibit cell firing by at least 50%, but not totally. After establishing a consistent response to GABA, co-iontophoresis of a test compound was initiated to determine its effects on basal firing rates and responsiveness to GABA. When acetylcholine and glutamate were evaluated in the test paradigm using ejection currents which excited cells by 54.0 +/- 4.9%, neither compound consistently altered the inhibition elicited by GABA. This confirmed that increases in cell firing could occur without concurrent GABA-attenuating effects, and supported the contention that the dual effects of dopamine could be dissociated and perhaps independently mediated. To examine whether the effects of dopamine involve actions at different dopamine receptor subtypes within the nigra, the D1 agonist SKF 38393 and the D2 agonist LY 171555 were substituted in the procedure. Applications of R,S(+/-)-SKF 38393 caused current-dependent increases in firing with a maximal increase at 8 nA of 55 +/- 18% above baseline (n = 14). The excitatory effect appeared to be D1-mediated since R(+)-SKF 38393, but not the inactive S(+)-enantiomer, could elicit the response. Conversely, graded applications of LY 171555 caused only occasional and more modest increases in basal activities, but consistently and markedly attenuated responses to GABA, decreasing GABA's inhibitory potency by 60.9 +/- 4.3% at 10 nA (n = 17). These results provide support for discrete roles of D1 and D2 receptors in substantia nigra pars reticulata, and suggest mechanistically distinct ways by which dendritically released dopamine could act to modify basal ganglia output from this region.

The effects of striatal lesion on turning behavior and globus pallidus single unit response to dopamine agonist administration

In normal rats, globus pallidus neurons are excited by the systemic administration of postsynaptically active doses of apomorphine. The role of the striatum in mediating this phenomenon was examined by investigating the effects of apomorphine on neuronal activity in the globus pallidus and on turning behavior in rats with unilateral quinolinic acid lesions of the striatum. The lesion markedly reduced striatal choline acetyltransferase activity and GABA content and significantly attenuated apomorphine's effect on the activity of pallidal neurons. Both the extent of attenuation of the electrophysiological response of pallidal neurons in lesioned animals and the neurotoxin-induced decreases in choline acetyltransferase activity and GABA content in the caudal striatum were correlated with the degree of apomorphine-induced turning. The data indicate that striatopallidal neurons contribute to apomorphine's excitatory effect on the activity of pallidal neurons in normal animals.

Dopaminergic innervation of the basal ganglia in the squirrel monkey as revealed by tyrosine hydroxylase immunohistochemistry

The organization of the dopaminergic mesostriatal fibers and their patterns of innervation of the basal ganglia in the squirrel monkey (Saimiri sciureus) were studied immunohistochemically with an antiserum raised against tyrosine hydroxylase (TH). Numerous fibers arose from midbrain TH-positive cell bodies of the substantia nigra pars compacta (group A9), the retrorubral area (group A8), and the lateral portion of the ventral tegmental area (group A10). These fibers accumulated dorsomedially to the rostral pole of the substantia nigra where they formed a massive bundle that coursed through the prerubral field and ascended along the laterodorsal aspect of the medial fore-brain bundle in the lateral hypothalamus. Some ventrally located fibers ran throughout the rostrocaudal extent of the lateral preopticohypothalamic area and could be followed up to the olfactory tubercle, whereas other fibers turned laterodorsally to invade the head of the caudate nucleus. At more dorsal levels in the lateral hypothalamus, many fiber fascicles detached themselves from the main bundle and swept laterally to reach the globus pallidus, the putamen, and the amygdala. Several TH-positive fibers coursed along the dorsal surface of the subthalamic nucleus, and some invaded the dorsomedial third of this structure. The remaining portion of the subthalamic nucleus contained relatively few TH-positive elements. In contrast, the globus pallidus received a dense dopaminergic innervation deriving mostly from two fascicles that coursed backward along the two major output pathways of the pallidum: the lenticular fasciculus caudodorsally and the ansa lenticularis rostroventrally. At the pallidal level, the labeled fibres merged within the medullary laminae and arborized profusely in the internal pallidal segment and less abundantly in the external pallidal segment. However, the caudoventral portion of the external pallidum displayed a dense field of TH-positive axonal varicosities. Other fibers ran through the dorsal two-thirds of the external pallidum en route to the putamen. The striatum contained a multitude of thin axonal varicosities among which a few long and varicosed fibers were scattered. These immunoreactive neuronal profiles were rather uniformly distributed along the rostrocaudal extent of the striatum but appeared slightly more numerous in the ventral striatum than in the dorsal striatum. The pattern of distribution of the TH-positive axonal varicosities in the dorsal striatum was markedly heterogeneous: it consisted of typical zones of poor TH immunoreactivity lying within a matrix of dense terminal labeling.(ABSTRACT TRUNCATED AT 400 WORDS)

Responses of pallidal neurons to striatal stimulation in monkeys with MPTP-induced parkinsonism

Extracellular single unit activity was recorded from neurons of the internal (GPi) and external (GPe) pallidal segments, and from 'border cells' (Bor) which are part of the nucleus basalis, in 2 cynomolgus monkeys rendered parkinsonian by MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine). Cell counts showed that at least 90% of the nigral neurons of the compacta-type were degenerated. Electrical stimulation was applied to 3 sites bilaterally in the striatum: one in the caudate nucleus and 2 in the putamen. The results were compared to those obtained in intact monkeys. In the parkinsonians, more neurons of the 3 types responded to ipsilateral stimulation. The difference was even greater for contralateral responses, except in the case of Bor neurons. Greater proportions of the 3 types of neurons also responded to 2 and 3 sites and showed convergent responses to both the caudate nucleus and the putamen. The magnitude of the responses was larger. These results are in accordance with the excessive and unselective responses of the same neurons to passive limb movement, obtained in the same animals and described previously. The electrical stimulation allowed more detailed analyses of the responses. The major change in the responses of GPi and Bor neurons was the more frequent and larger late inhibitions, whereas the excitations were larger in GPe neurons. Long lasting oscillatory responses occurred frequently in the parkinsonians, mainly in GPi, and at frequencies close to the tremor displayed by the animals. Responses beginning with early inhibition were displayed by neurons located in the center of the pallidal zone of influence of each striatal stimulation site, as in intact animals, but in the GPi of the parkinsonians they were less frequently curtailed by excitation. Moreover, in the parkinsonians, the zones of influence were larger in both GPi and GPe, mainly because of the expansion of their periphery, where responses began with excitation and had lower thresholds than in intact animals. The dopamine agonist apomorphine normalized the responses in the parkinsonians. Thus, both the temporal and spatial magnitudes of inhibitions and excitations are abnormal at the output of the basal ganglia in parkinsonism.

Chronic haloperidol administration increases GABA binding and enhances neuronal responsiveness to iontophoresed GABA in rat globus pallidus

Experiments were conducted to assess the effects of chronic haloperidol (CHAL) administration on gamma-aminobutyric acid (GABA) receptor binding within the rat globus pallidus (GP) and on the responsiveness of individual pallidal neurons to microiontophoretically applied GABA and glycine. Rats were administered haloperidol in their food for 30 days in increasing concentrations and the experiments were conducted 2 days after termination of the haloperidol treatment. GABA receptor binding and neuronal responsiveness to GABA were significantly increased within the GP following CHAL treatment. The mean EC50 value for GABA was significantly decreased in the CHAL-treated rats, but there was no change in the EC50 for glycine. Scatchard analysis of [3H]muscimol binding demonstrated a single high-affinity binding site (Kd = 5 nM) within both control and CHAL-treated rats. The binding capacity (Bmax) of this high-affinity site was significantly increased in CHAL-treated rats without any change in the dissociation constant (Kd) for this site. These results suggest that CHAL administration may lead to a decrease in GABA release by striatopallidal efferents. The results of this study, combined with those of our previous study on SNR neurons, have demonstrated that blockade of striatally mediated dopamine (DA) neurotransmission leads to similar changes in GABAergic mechanisms at the level of the GP and SNR and suggest that DA regulation of the striatopallidal and striatonigral GABAergic pathways need not be differentially organized as has previously been postulated.

Evidence for a distinct nigropallidal dopaminergic projection in the squirrel monkey

Injections of the retrograde fluorescent tracer fast blue in the striatum (STR) and nuclear yellow in the internal segment of the globus pallidus (GPi) in the squirrel monkey (Saimiri sciureus) revealed a nigropallidal projection whose cellular origin was largely distinct from that of the nigrostriatal pathway. Neurons containing the tracer injected in GPi were scattered throughout the substantia nigra-ventral tegmental area complex where they formed approximately 20-25% of the total number of retrogradely labeled cells. Only about 5-10% of all positive neurons were double-labeled after STR-GPi injections. In experiments combining the use of the fluorescent tracer propidium iodide with immunofluorescence, the majority of neurons projecting to GPi displayed tyrosine hydroxylase immunoreactivity. Hence, in addition to their important role at striatal level, midbrain dopaminergic neurons may influence directly the output neurons of the basal ganglia at pallidal level in primates.

Low doses of accumbens dopamine modulate amygdala suppression of spontaneous exploratory activity in rats

The effect of pharmacological stimulation of the amygdala on spontaneous locomotor activity in the rat and its modulation by accumbens dopamine were investigated. Bilateral injection of N-methyl-D-aspartic acid into the basolateral nucleus of the amygdala produced a dose-dependent suppression of spontaneous locomotor activity in the rat. The suppression of locomotor activity was reversed completely by injection of L-glutamic acid diethyl ester, a putative glutamatergic antagonist, into the nucleus accumbens but partially enhanced by injection of nipecotic acid, a GABA uptake inhibitor, into the ventral pallidum. Furthermore, low doses of dopamine injected into the accumbens, which by itself did not elicit hyperactivity in the animals, completely reversed the suppression of locomotor activity following amygdala stimulation. These results show that the projection from the amygdala to nucleus accumbens has an inhibitory effect on spontaneous locomotor activity in rats and that dopamine in the accumbens attenuated this suppression effect possibly due to its neuromodulatory action as demonstrated in previous electrophysiological experiments.

Unilateral lesion of the nigrostriatal pathway decreases the firing rate and alters the firing pattern of globus pallidus neurons in the rat

Activities of spontaneously firing neurons in the globus pallidus of intact rats and rats that survived unilateral lesions of the nigrostriatal pathway for 3 days, 1 week, or 6-11 weeks were compared. No significant differences in neuronal firing rate, firing pattern, and number of cells per pass were observed between chloral hydrate-anesthetized control and lesioned animals. However, in locally anesthetized animals, pallidal cells fired significantly faster than in chloral hydrate-anesthetized animals, and the lesion caused a decrease in the firing rates of pallidal cells 1 week and 6-9 weeks postlesion. In addition, significant differences in the firing pattern of pallidal cells, as determined by the ratio of the mean to median interspike intervals, were seen between locally anesthetized controls and animals surviving 3 days, 1 week, and 6-9 weeks postlesion. This altered firing pattern tended to return to normal with time. The number of cells per pass was not significantly altered by the lesion. Data from this study suggest that, in locally anesthetized animals, the removal of the tonic dopaminergic input to the basal ganglia causes pallidal cells to decrease their firing rates in a time-dependent fashion and causes reversable firing pattern changes. This suggests that tonically active dopamine neurons, probably acting through the striatopallidal pathway, regulate the firing rate and mechanisms controlling the temporal ordering of spontaneous discharges of globus pallidus neurons.

Differential dopaminergic innervation of the two pallidal segments in the squirrel monkey (Saimiri sciureus)

Immunohistochemical studies with an antiserum raised against tyrosine hydroxylase have allowed us to demonstrate a dense dopaminergic innervation of the globus pallidus in the squirrel monkey. This innervation derived mostly from two fascicles that detached themselves from the major ascending dopaminergic bundle arising from midbrain dopamine cell bodies and running in the lateral hypothalamus. Dopaminergic fibers reached the globus pallidus by coursing along its two major output pathways: the lenticular fasciculus dorsally and the ansa lenticularis ventrally. At pallidal levels, dopaminergic fibers abounded in medullary laminae and arborized profusely within the internal pallidal segment, whereas the external pallidum displayed only few short fibers that prevailed in its dorsal portion. These findings provide the first evidence that the primate globus pallidus receives a massive and differentially distributed dopaminergic input.

Biochemical and morphological studies on GABA neurons in reaggregate culture

Dissociated cells from the 14-day fetal mouse corpus striatum (CS), rostral mesencephalic tegmentum (RMT) and tectum were reaggregated in rotation-mediated cultures in the following combinations: CS alone, RMT-CS, RMT-tectum, and tectum alone. Reaggregates were cultured for 1-4 weeks. An even distribution of gamma-aminobutyric acid (GABA)-positive cells was observed in all the reaggregate combinations that were cultured for 1 and 2 weeks. With increasing time in culture, cellular staining was decreased while positive fiber staining increased. CS and RMT-CS co-cultures increased their capacity to take up [3H]GABA with time in culture. All reaggregates reached a maximum uptake/accumulation capacity of 30-40 pmol/mg protein/30 min, by 4 weeks in vitro. There were no significant differences between the various co-aggregate combinations in the accumulation capacity. [3H]GABA accumulation in the reaggregates was largely blocked by the putative neuronal GABA uptake inhibitor, diaminobutyric acid, and was inhibited to a much lesser degree by the putative glial uptake inhibitor beta-alanine. All reaggregates released [3H]GABA to 70 mM potassium depolarization, in a calcium-dependent manner. One-three week CS reaggregates released more [3H]GABA in response to the potassium-induced depolarization than RMT-CS co-cultures. Since nigral dopamine neurons within the RMT proliferate processes and actively release dopamine only when co-cultured with CS target cells, it is suggested that these dopamine neurons might chronically inhibit striatal GABA neurons in the RMT-CS co-cultures, thereby depressing stimulated release of [3H]GABA.