Distribution of D1 and D2 dopamine receptors in the basal ganglia of the cat determined by quantitative autoradiography

Ventral pallidum DRD3 potentiates a pallido-habenular circuit driving accumbal dopamine release and cocaine seeking

Drugs of abuse induce persistent remodeling of reward circuit function, a process thought to underlie the emergence of drug craving and relapse to drug use. However, how circuit-specific, drug-induced molecular and cellular plasticity can have distributed effects on the mesolimbic dopamine reward system to facilitate relapse to drug use is not fully elucidated. Here, we demonstrate that dopamine receptor D3 (DRD3)-dependent plasticity in the ventral pallidum (VP) drives potentiation of dopamine release in the nucleus accumbens during relapse to cocaine seeking after abstinence. We show that two distinct VP DRD3+ neuronal populations projecting to either the lateral habenula (LHb) or the ventral tegmental area (VTA) display different patterns of activity during drug seeking following abstinence from cocaine self-administration and that selective suppression of elevated activity or DRD3 signaling in the LHb-projecting population reduces drug seeking. Together, our results uncover how circuit-specific DRD3-mediated plasticity contributes to the process of drug relapse.

Defining the Homo- and Heterodimerization Propensities of Metabotropic Glutamate Receptors

The eight metabotropic glutamate receptors (mGluRs) serve critical modulatory roles throughout the nervous system. The molecular diversity of mGluRs is thought to be further expanded by the formation of heterodimers, but the co-expression of mGluR subtypes at the cellular level and the relative propensities of heterodimer formation are not well known. Here, we analyze single-cell RNA sequencing data and find that cortical pyramidal cells express multiple mGluR subtypes with distinct profiles for different receptor combinations. We then develop quantitative, fluorescence-based assays to define the relative homo- and heterodimer propensities across group-I, -II, and -III mGluRs. We find a strong preference for heterodimerization in a number of cases, including mGluR2 with mGluR3, which we confirm in frontal cortex using in situ RNA hybridization and co-immunoprecipitation. Together, our findings support the biological relevance of mGluR heterodimerization and highlight the complex landscape of mGluR populations in the brain.

The role of dopamine pharmacotherapy and addiction-like behaviors in Parkinson's disease

Addictions involve a spectrum of behaviors that encompass features of impulsivity and compulsivity, herein referred to as impulsive-compulsive spectrum disorders (ICSDs). The etiology of ICSDs likely involves a complex interplay among neurobiological, psychological and social risk factors. Neurobiological risk factors include the status of the neuroanatomical circuits that govern ICSDs. These circuits can be altered by disease, as well as exogenous influences such as centrally-acting pharmacologics. The 'poster child' for this scenario is Parkinson's disease (PD) medically managed by pharmacological treatments. PD is a progressive neurodegenerative disease that involves a gradual loss of dopaminergic neurons largely within nigrostriatal projections. Replacement therapy includes dopamine receptor agonists that directly activate postsynaptic dopamine receptors (bypassing the requirement for functioning presynaptic terminals). Some clinically useful dopamine agonists, e.g., pramipexole and ropinirole, exhibit high affinity for the D2/D3 receptor subtypes. These agonists provide excellent relief from PD motor symptoms, but some patients exhibit debilitating ICSD. Teasing out the neuropsychiatric contribution of PD-associated pathology from the drugs used to treat PD motor symptoms is challenging. In this review, we posit that modern clinical and preclinical research converge on the conclusion that dopamine replacement therapy can mediate addictions in PD and other neurological disorders. We provide five categories of evidences that align with this position: (i) ICSD prevalence is greater with D2/D3 receptor agonist therapy vs PD alone. (ii) Capacity of dopamine replacement therapy to produce addiction-like behaviors is independent of disease for which the therapy is being provided. (iii) ICSD-like behaviors are recapitulated in laboratory rats with and without PD-like pathology. (iv) Behavioral pathology co-varies with drug exposure. (v) ICSD Features of ICSDs are consistent with agonist pharmacology and neuroanatomical substrates of addictions. Considering the underpinnings of ICSDs in PD should not only help therapeutic decision-making in neurological disorders, but also apprise ICSDs in general.

Cocaine Inhibition of Synaptic Transmission in the Ventral Pallidum Is Pathway-Specific and Mediated by Serotonin

The ventral pallidum (VP) is part of the basal ganglia circuitry and a target of both direct and indirect pathway projections from the nucleus accumbens. VP is important in cocaine reinforcement, and the firing of VP neurons is modulated in vivo during cocaine self-administration. This modulation of firing is thought to be indirect via cocaine actions on dopamine in the accumbens. Here, we show that cocaine directly inhibits synaptic transmission evoked by selective stimulation of indirect pathway projections to VP neurons. The inhibition is independent of dopamine receptor activation, absent in 5-HT1B knockout mice, and mimicked by a serotonin transporter (SERT) blocker. SERT-expressing neurons in dorsal raphe project to the VP. Optogenetic stimulation of these projections evokes serotonin transients and effectively inhibits GABAergic transmission to VP neurons. This study shows that cocaine increases endogenous serotonin in the VP to suppress synaptic transmission selectively from indirect pathway projections to VP neurons.

Dopaminergic Modulation of Synaptic Integration and Firing Patterns in the Rat Entopeduncular Nucleus

Dopamine is known to differentially modulate the impact of cortical input to the striatum between the direct and indirect pathways of the basal ganglia (BG). However, the role of extrastriatal dopamine receptors (DRs) in BG information processing is less clear. To investigate the role of extrastriatal DRs, we studied their distribution and function in one of the output nuclei of the BG of the rodent, the entopeduncular nucleus (EP). qRT-PCR indicated that all DR subtypes were expressed by EP neurons, suggesting that both D₁-like receptors (D1LRs) and D₂-like receptors (D2LRs) were likely to affect information processing in the EP. Whole-cell recordings revealed that striatal inputs to the EP were potentiated by D1LRs whereas pallidal inputs to the EP were depressed by D2LRs. Changes to the paired-pulse ratio of inputs to the EP suggested that dopaminergic modulation of striatal inputs is mediated by postsynaptic receptors, and that of globus pallidus-evoked inputs is mediated by presynaptic receptors. We show that these changes in synaptic efficacy changed the information content of EP neuron firing. Overall, the findings suggest that the dopaminergic system affects the passage of feedforward information through the BG by modulating input divergence in the striatum and output convergence in the EP.SIGNIFICANCE STATEMENT The entopeduncular nucleus (EP), one of the basal ganglia (BG) output nuclei, is an important station in information processing in BG. However, it remains unclear how EP neurons encode information and how dopamine affects this process. This contrasts with the well established role of dopamine in the striatum, which is known to redistribute cortical input between the direct and indirect pathways. Here we show that, in symmetry with the striatum, dopamine controls the rebalancing of information flow between the two pathways in the EP. Specifically, we demonstrate that dopamine regulates EP activity by differentially modulating striatal and pallidal GABAergic inputs. These results call for a reassessment of current perspectives on BG information processing by highlighting the functional role of extrastriatal dopamine receptors.

The external globus pallidus: progress and perspectives

The external globus pallidus (GPe) of the basal ganglia is in a unique and powerful position to influence processing of motor information by virtue of its widespread projections to all basal ganglia nuclei. Despite the clinical importance of the GPe in common motor disorders such as Parkinson's disease, there is only limited information about its cellular composition and organizational principles. In this review, recent advances in the understanding of the diversity in the molecular profile, anatomy, physiology and corresponding behaviour during movement of GPe neurons are described. Importantly, this study attempts to build consensus and highlight commonalities of the cellular classification based on existing but contentious literature. Additionally, an analysis of the literature concerning the intricate reciprocal loops formed between the GPe and major synaptic partners, including both the striatum and the subthalamic nucleus, is provided. In conclusion, the GPe has emerged as a crucial node in the basal ganglia macrocircuit. While subtleties in the cellular makeup and synaptic connection of the GPe create new challenges, modern research tools have shown promise in untangling such complexity, and will provide better understanding of the roles of the GPe in encoding movements and their associated pathologies.

The ventral pallidum: Subregion-specific functional anatomy and roles in motivated behaviors

The ventral pallidum (VP) plays a critical role in the processing and execution of motivated behaviors. Yet this brain region is often overlooked in published discussions of the neurobiology of mental health (e.g., addiction, depression). This contributes to a gap in understanding the neurobiological mechanisms of psychiatric disorders. This review is presented to help bridge the gap by providing a resource for current knowledge of VP anatomy, projection patterns and subregional circuits, and how this organization relates to the function of VP neurons and ultimately behavior. For example, ventromedial (VPvm) and dorsolateral (VPdl) VP subregions receive projections from nucleus accumbens shell and core, respectively. Inhibitory GABAergic neurons of the VPvm project to mediodorsal thalamus, lateral hypothalamus, and ventral tegmental area, and this VP subregion helps discriminate the appropriate conditions to acquire natural rewards or drugs of abuse, consume preferred foods, and perform working memory tasks. GABAergic neurons of the VPdl project to subthalamic nucleus and substantia nigra pars reticulata, and this VP subregion is modulated by, and is necessary for, drug-seeking behavior. Additional circuits arise from nonGABAergic neuronal phenotypes that are likely to excite rather than inhibit their targets. These subregional and neuronal phenotypic circuits place the VP in a unique position to process motivationally relevant stimuli and coherent adaptive behaviors.

Keep focussing: striatal dopamine multiple functions resolved in a single mechanism tested in a simulated humanoid robot

The effects of striatal dopamine (DA) on behavior have been widely investigated over the past decades, with “phasic” burst firings considered as the key expression of a reward prediction error responsible for reinforcement learning. Less well studied is “tonic” DA, where putative functions include the idea that it is a regulator of vigor, incentive salience, disposition to exert an effort and a modulator of approach strategies. We present a model combining tonic and phasic DA to show how different outflows triggered by either intrinsically or extrinsically motivating stimuli dynamically affect the basal ganglia by impacting on a selection process this system performs on its cortical input. The model, which has been tested on the simulated humanoid robot iCub interacting with a mechatronic board, shows the putative functions ascribed to DA emerging from the combination of a standard computational mechanism coupled to a differential sensitivity to the presence of DA across the striatum.

Reduced threshold for induction of LTP by activation of dopamine D1/D5 receptors at hippocampal CA1-subiculum synapses

The phasic release of dopamine in the hippocampal formation has been shown to facilitate the encoding of novel information. There is evidence that the subiculum operates as a detector and distributor of sensory information, which incorporates the novelty and relevance of signals received from CA1. The subiculum acts as the final hippocampal relay station for outgoing information. Subicular pyramidal cells have been classified as regular- and burst-spiking neurons. The goal of the present study was to study the effect of dopamine D1/D5 receptor activation on synaptic transmission and plasticity in the subicular regular-spiking neurons of 4–6 week old Wistar rats. We demonstrate that prior activation of D1/D5 receptors reduces the threshold for the induction of long-term potentiation (LTP) in subicular regular-spiking neurons. Our results indicate that D1/D5 receptor activation facilitates a postsynaptic form of LTP in subicular regular-spiking cells that is NMDA receptor-dependent, relies on postsynaptic Ca²⁺ signaling, and requires the activation of protein kinase A. The enhanced propensity of subicular regular-spiking cells to express postsynaptic LTP after activation of D1/D5 receptors provides an intriguing mechanism for the encoding of hippocampal output information.

Activation of dopamine D1/D5 receptors facilitates the induction of presynaptic long-term potentiation at hippocampal output synapses

Encoding of novel information has been proposed to rely on the time-locked release of dopamine in the hippocampal formation during novelty detection. However, the site of novelty detection in the hippocampus remains a matter of debate. According to current models, the CA1 and the subiculum act as detectors and distributors of novel sensory information. Although most CA1 pyramidal neurons exhibit regular-spiking behavior, the majority of subicular pyramidal neurons fire high-frequency bursts of action potentials. The present study investigates the efficacy of dopamine D1/D5 receptor activation to facilitate the induction of activity-dependent long-term potentiation (LTP) in rat CA1 regular-spiking and subicular burst-spiking pyramidal cells. Using a weak stimulation protocol, set at a level subthreshold for the induction of LTP, we show that activation of D1/D5 receptors for 5-10 min facilitates LTP in subicular burst-spiking neurons but not in CA1 neurons. The results demonstrate that D1/D5 receptor-facilitated LTP is NMDA receptor-dependent, and requires the activation of protein kinase A. In addition, the D1/D5 receptor-facilitated LTP is shown to be presynaptically expressed and relies on presynaptic Ca(2+) signaling. The phenomenon of dopamine-induced facilitation of presynaptic NMDA receptor-dependent LTP in subicular burst-spiking pyramidal cells is in accordance with observations of the time-locked release of dopamine during novelty detection in this brain region, and reveals an intriguing mechanism for the encoding of hippocampal output information.

A population level computational model of the basal ganglia that generates parkinsonian Local Field Potential activity

Recordings from the basal ganglia's subthalamic nucleus are acquired via microelectrodes immediately prior to the application of Deep Brain Stimulation (DBS) treatment for Parkinson's Disease (PD) to assist in the selection of the final point for the implantation of the DBS electrode. The acquired recordings reveal a persistent characteristic beta band peak in the power spectral density function of the Local Field Potential (LFP) signals. This peak is considered to lie at the core of the causality-effect relationships of the parkinsonian pathophysiology. Based on LFPs acquired from human subjects during DBS for PD, we constructed a computational model of the basal ganglia on the population level that generates LFPs to identify the critical pathophysiological alterations that lead to the expression of the beta band peak. To this end, we used experimental data reporting that the strengths of the synaptic connections are modified under dopamine depletion. The hypothesis that the altered dopaminergic modulation may affect both the amplitude and the time course of the postsynaptic potentials is validated by the model. The results suggest a pivotal role of both of these parameters to the pathophysiology of PD.

Dopamine D1/D5 receptors gate the acquisition of novel information through hippocampal long-term potentiation and long-term depression

Hebbian learning models require that neurons are able to both strengthen and weaken their synaptic connections. Hippocampal synaptic plasticity, in the form of long-term potentiation (LTP) and long-term depression (LTD), has been implicated in both spatial memory formation as well as novelty acquisition. In addition, the ventral tegmental area-hippocampal loop has been proposed to control the entry of information into long-term memory, whereas the dopaminergic system is believed to play an important role in information acquisition and synaptic plasticity. D1/D5 dopamine receptors are positively coupled to adenylyl cyclase and have been to modulate certain forms of synaptic plasticity, particularly in vitro. We investigated how D1/D5 dopamine receptors modify long-lasting synaptic plasticity at CA1 synapses of adult freely moving rats and found that receptor activation lowered the threshold for the induction of both LTP and LTD. Specific types of learning are associated with specific types of hippocampal synaptic plasticity. We found that object-configuration learning, facilitation of late-phase LTD by object exploration, and late-phase LTP by exploration of empty space were all prevented by D1/D5 receptor antagonism. Furthermore, receptor antagonism prevented electrically induced late-LTP, whereas receptor activation facilitated induction of both LTP and LTD by patterned electrical stimulation. These findings suggest that the dopaminergic system, acting via D1/D5 receptors, gates long-term changes in synaptic strength and that these changes are a critical factor in the acquisition of novel information.

Rats with unilateral median forebrain bundle, but not striatal or nigral, lesions by the neurotoxins MPP+ or rotenone display differential sensitivity to amphetamine and apomorphine

Rotenone and 1-methyl-4-phenyl pyridinium (MPP+) are two mitochondrial neurotoxins known to produce Parkinson's disease (PD) in experimental animals. In the present study, we compared drug-induced rotational asymmetry in rats lesioned using these neurotoxins at three distinct basal ganglia sites, the striatum, substantia nigra pars compacta (SNpc) and median forebrain bundle (MFB). The levels of dopamine (DA) in the ipsilateral striata of these hemiparkinsonian animals were assayed employing an HPLC-electrochemical procedure 2 days after the final rotational study. Rats infused with rotenone or MPP+ into the SNpc, but not into the striatum or MFB, exhibited contralateral rotations immediately after recovery from anesthesia. Irrespective of the lesion site or the toxin used, all the animals exhibited ipsilateral rotations when challenged with D-amphetamine. Apomorphine administration caused contralateral circling behavior in MFB-lesioned animals, but ipsilateral rotations in rats that received rotenone or MPP+ in the striatum or SNpc. Stereotaxic administration of rotenone into the MFB, SNpc or striatum caused a significant loss of DA in the ipsilateral striatum to varying degrees (96%, 62% and 30%, respectively, as compared to the contralateral side). However, unilateral MPP+ administration into the MFB, SNpc or striatum caused respectively about 98%, 74% and 59% loss of striatal DA. Behavioural observations and the neurochemical results indicate that, among the three anatomically distinct loci-lesioned, MFB-lesioned animals mimicked behavioral aberrations similar to nigral lesions caused by 6-hydroxydopamine, a classical parkinsonian neurotoxin. Moreover, the results point out that while both d-amphetamine and apomorphine-induced rotations could be considered as valuable behavioral indices to test novel drugs against PD, yet apomorphine-induced contralateral bias proves to be a more reliable indicator of specific destruction in the nigrostriatal pathway and development of post-synaptic DA receptor supersensitivity.

Control of the subthalamic innervation of substantia nigra pars reticulata by D1 and D2 dopamine receptors

The effects of activating dopaminergic D1 and D2 class receptors of the subthalamic projections that innervate the pars reticulata of the subtantia nigra (SNr) were explored in slices of the rat brain using the whole cell patch-clamp technique. Excitatory postsynaptic currents (EPSCs) that could be blocked by 6-cyano-7-nitroquinoxalene-2,3-dione and D-(-)-2-amino-5-phosphonopentanoic acid were evoked onto reticulata GABAergic projection neurons by local field stimulation inside the subthalamic nucleus in the presence of bicuculline. Bath application of (RS)-2,3,4,5-tetrahydro-7,8-dihydroxy-1-phenyl-1H-3-benzazepine hydrochloride (SKF-38393), a dopaminergic D1-class receptor agonist, increased evoked EPSCs by approximately 30% whereas the D2-class receptor agonist, trans-(-)-4aR-4,4a,5,6,7,8,8a,9-octahydro-5-propyl-1H-pyrazolo(3,4-g)quinoline (quinpirole), reduced EPSCs by approximately 25%. These apparently opposing actions were blocked by the specific D1- and D2-class receptor antagonists: R-(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetra-hydro-1H-3-benzazepinehydrochloride (SCH 23390) and S-(-)-5-amino-sulfonyl-N-[(1-ethyl-2-pyrrolidinyl)-methyl]-2-methoxybenzamide (sulpiride), respectively. Both effects were accompanied by changes in the paired-pulse ratio, indicative of a presynaptic site of action. The presynaptic location of dopamine receptors at the subthalamonigral projections was confirmed by mean-variance analysis. The effects of both SKF-38393 and quinpirole could be observed on terminals contacting the same postsynaptic neuron. Sulpiride and SCH 23390 enhanced and reduced the evoked EPSC, respectively, suggesting a constitutive receptor activation probably arising from endogenous dopamine. These data suggest that dopamine presynaptically modulates the subthalamic projection that targets GABAergic neurons of the SNr. Implications of this modulation for basal ganglia function are discussed.

Effect of intrastriatal 6-OHDA lesion on dopaminergic innervation of the rat cortex and globus pallidus

The present study examined in the rat the effect of a partial lesion of the nigrostriatal dopaminergic pathway induced by intrastriatal injection of 6-hydroxydopamine (6-OHDA), on the dopaminergic innervation of the cortex and the globus pallidus as revealed using tyrosine hydroxylase (TH) immunoreactivity. Twenty-eight days after unilateral injection of 6-OHDA into the dorsal part of the striatum, TH-positive fiber density was reduced by 41% in the dorsal and central part of the structure, and was accompanied by a retrograde loss of 33% of TH-positive neurons in the substantia nigra (SN), while the ventral tegmental area was completely spared. In the SN, TH-positive cell loss was most severe in the ventral part of the structure (-55%). In the same animals, a substantial loss of TH-positive fibers was evident in the dorsal part of the globus pallidus, and involved both thick fibers of passage and thin varicose terminal axonal branches. In the cortex, a loss of TH-positive fibers was prominent in the cingulate area, moderate in the motor area and less affected in the insular area, while the noradrenergic innervation revealed using dopamine-beta-hydroxylase immunoreactivity was preserved in all of these cortical subregions. These results demonstrate that the intrastriatal 6-OHDA lesion model in rats produces a significant loss of dopaminergic axons in extrastriatal structures including the pallidum and cortex, which may contribute to functional sequelae in this animal model of Parkinson's disease.

Behavioral differences in a rotenone-induced hemiparkinsonian rat model developed following intranigral or median forebrain bundle infusion

A mitochondrial complex-I inhibitor, rotenone was unilaterally infused into the substantia nigra pars compacta (SNpc) or median forebrain bundle (MFB) to create hemiparkinsonian animal models and investigated spontaneous and drug-induced stereotypic rotations, as well as certain postural behaviors in Sprague-Dawley rats. Animals infused intranigrally, but not intra-MFB, with rotenone exhibited spontaneous contralateral rotations immediately after recovery from anesthesia. Head position bias and elevated body swing test showed insignificant contralateral bias in animals with nigral damage but a significant ipsilateral bias in MFB-lesioned rats. General motor activity of the animals was reduced in both the groups as indicated by reduced performance on a Plus-Maze. Intranigrally, rotenone-infused animals exhibited progressive ipsilateral rotations when challenged with d-amphetamine on the 7th, 14th, 21st, and 28th days or with apomorphine on 9th, 16th, 23rd, and 30th days. However, animals that received rotenone in MFB exhibited ipsilateral or contralateral rotations when challenged respectively with d-amphetamine or apomorphine only in the 5th week (28th and 30th days). Stereotaxic administration of rotenone into SNpc or MFB caused a significant loss of dopamine in the ipsilateral striatum (>80% in SNpc; >95% in MFB), when assayed employing an HPLC equipped with electrochemical detector on the 32nd day. Neuronal loss in SNpc was confirmed in coronal sections stained with cresyl violet and revealed extension of lesion towards SN pars reticulata, in SNpc-infused animals. Our results demonstrate that rotenone-induced neurodegeneration is a slow, yet progressive process similar to that in idiopathic Parkinson's disease and unlike that observed in other classical neurotoxin-mediated lesions which are abrupt and developed in few hours to days. Thus, intranigral or intra-MFB infusion of rotenone could be used for producing hemiparkinsonian animal models in rats. These findings further suggest that, while both d-amphetamine and apomorphine-induced stereotypic rotations could be used as a valuable behavioral assay procedure to test novel drugs against Parkinson's disease, yet apomorpine-induced contralateral bias in turning is a reliable indicator of specific destruction in nigrostriatal pathway and development of postsynaptic dopamine receptor supersensitivity.

Intrapallidal D2 dopamine receptors control globus pallidus neuron activity in the rat

Because activation of D2 dopamine receptors inhibits gamma-aminobutyric acid (GABA) release from intrapallidal nerve terminals, we measured the effects of modifiers of dopamine D2 receptors on the firing rate of single neurons in the globus pallidus (GP) of the anesthetized rat. The predominant effect of intrapallidal administration of the selective D2 agonist quinpirole was an increase in the rate of spontaneous firing while the D2 blocker sulpiride caused a decrease. The spontaneous firing of GP neurons is inhibited by stimulation of the GABAergic striatopallidal projection. We therefore measured the effects of modifiers of D2 receptors on striatal inhibition of GP neurons and found that intrapallidal quinpirole blocked the inhibitory effects of striatal stimulation while sulpiride enhanced them. These experiments show that both the spontaneous rate of firing of pallidal neurons and its modification by striatopallidal inputs is controlled by intrapallidal dopamine D2 receptors. In addition, taken together with other findings in the literature, our results suggest that activation of dopamine D2 receptors within the globus pallidus leads to inhibition of GABA release from presynaptic terminals.

Changes in striatal dopamine D(2) receptors in relation to expression of and recovery from experimental parkinsonism

Changes in dopamine D(2) receptor number in 1-methyl-4-phenyl-1,2,3, 6-tetrahydropyridine (MPTP)-treated cats during various stages of experimental parkinsonism were examined. In the normal adult cat, D(2) receptors were expressed throughout the striatum. In symptomatic cats (assessed 7 days after the last MPTP administration), there was a slight elevation of D(2) receptors in all striatal regions. At 2 weeks after MPTP (animals still grossly symptomatic), D(2) receptor number was increased 60-75% above normal. At 3 weeks after MPTP (partial functional recovery), D(2) receptor number remained elevated at a level slightly less than that observed at 2 weeks. At 6 weeks after MPTP (full functional recovery), D(2) receptor levels were back to normal. Changes in D(2) receptor mRNA expression in the striatum essentially mirrored the changes in receptor number. Increases in D(2) receptor number and mRNA expression did not coincide with the onset of parkinsonian signs and peaked after the parkinsonism was established. Permanent reduction of parkinsonian signs corresponded to normalization of D(2) receptor number.

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.

Modulation of GABA release by dopamine in the substantia nigra

The role of specific dopamine receptor subtypes in the regulation of GABA release in the substantia nigra was investigated using microdialysis in the awake rat. Both basal and potassium-stimulated changes in the extracellular concentrations of GABA were examined in response to the local perfusion of tetrodotoxin (TTX), the D1 agonist SKF 38393, or the D2 agonist LY 171555 through the microdialysis probe in the substantia nigra. Although TTX (1 microM) did not alter the basal extracellular concentrations of GABA in the substantia nigra, it attenuated the potassium-stimulated (80 mM K+) release of GABA. SKF 38393 had no effect on basal extracellular concentrations of GABA, but did potentiate K+ -stimulated release of GABA in a concentration-dependent manner. The potentiated response at the highest concentration of SKF 38393 (100 microM) was blocked by the D1 antagonist SCH 23390. In contrast to the effect of the D1 agonist, the D2 agonist LY 171555 attenuated the stimulated release of GABA. These data indicate that although basal extracellular concentrations of GABA in the substantia nigra may not be derived from neuronal pools, K+ -stimulated release of GABA is impulse-mediated and is modulated by the D1 and the D2 receptors. Local interactions between dopamine and GABA in the substantia nigra may have important implications for the direct regulation of basal ganglia efferent activity and motor behavior.

Neurons exhibiting dopamine D2 receptor immunoreactivity in the substantia nigra of the mutant weaver mouse

Neurons exhibiting D2 receptor-like immunoreactivity were investigated in the substantia nigra pars compacta of weaver mice at the light and electron microscope levels using immunocytochemical techniques. At the light microscope level, there was significant loss of D2-like immunoreactive cells in weaver mice and the remaining labeled cells exhibited less intense immunoreactivity. At the ultrastructural level, there was a decrease in the number of immunoreactive profiles and fewer synapses were observed abutting labeled dendritic profiles. In addition, degenerative changes were noted in some of the D2 receptor-like immunoreactive profiles. Double labeling with D2 and tyrosine hydroxylase indicated that the majority of the labeled profiles were double labeled. Eight-week-old homozygous weavers were paired with wild-type littermates as controls and perfused with a buffered solution of acrolein/paraformadehyde. Midbrain sections were reacted immunocytochemically either with an antiserum to D2 or with antisera to D2 and tyrosine hydroxylase, using a double-labeling technique. Sections were processed for light and electron microscopy by standard methods. The results of this study confirm the autoreceptor-like activity of D2 receptors on nigral dopamine neurons. The cell degeneration, down-regulation of D2 receptors, and decreased dendritic and synaptic components in the neuropil suggest that the synaptic integrity of the substantia nigra has been compromised, which in turn would affect the functional efficacy of the basal ganglia circuitry. This altered circuity is expressed in the Parkinson-like symptoms displayed by this mutant mouse.

Action sequencing is impaired in D1A-deficient mutant mice

The role of dopamine in the production of behaviour is multifarious in that it can influence different aspects of movement (e.g. movement initiation, sensorimotor integration, and movement sequencing). A characteristic of the dopamine system which seems to be critical for the expression of this diverse influence is its varied receptor population. Previous studies have shown that specific receptor subtype activation leads to specific behavioural responses or alterations of selective aspects of movement. It is known that one of the important influences of dopamine includes sequential co-ordination of 'syntactic' patterns of grooming movements because moderate loss of the dopaminergic nigrostriatal projections specifically disrupts these patterns without affecting grooming actions in a general fashion (Berridge, K.C. Psychobiology, 15, 336, 1989). The specific receptors of the dopamine family which play a key part in this co-ordination of movement sequences is not known. In the present study, we examined the serial order of particular syntactic sequences or chains of grooming actions in mice lacking D1A receptors to explore the relationship between this receptor subtype and movement sequencing. Mutant mice had shorter grooming bouts and a disruption of the organization of sequential patterns compared with wild-type littermate controls. Sequential disruption was reflected in the failure of D1A mutants to follow the syntactic pattern of grooming to completion. This sequential disruption deficit appeared to be specific, as mutant mice initiated more syntactic chains than wild-type controls even though they were less likely to complete them. These results support the hypothesis that D1A receptor activation plays a part in the sequencing of natural action. This conclusion has important implications for the understanding of the functional heterogeneity of dopamine receptor subtypes and of the aetiology of symptoms observed in patients with basal ganglia disease.

Differences in D2 dopamine receptor binding in the neostriatum between cats hemidecorticated neonatally or in adulthood

In order to study differences in response to neocortical injury sustained at different ages at the neurotransmitter level, we examined the density in D2 dopamine receptors in the neostriatum of cats hemidecorticated neonatally (N = 4) or in adulthood (N = 4), as well as in intact brains (N = 6). Receptor densities were measured using quantitative autoradiography and [3H]-spiperone binding in 12 regions of the neostriatum and nucleus accumbens septi. We found that the anterior lateral caudate nucleus on both sides of the brain contained a higher D2 receptor density in neonatal-lesioned as compared to adult-lesioned brains. Ipsilateral to the lesion, the increase was 101% (P < 0.05) and contralaterally it amounted to 77% (P < 0.05). Moreover, this region of the ipsilateral caudate nucleus of neonatal-lesioned cats tended to be more densely labeled than that of intact brain by 58% (P < 0.1). D2 receptor densities in adult-lesioned cats did not differ from that of intact controls. Comparison of these data with those of a former morphological study using the same animals suggested that this bilateral elevation of D2 receptor density in neonatally lesioned brains represents a higher mean density of binding sites per neuron. The elevation in the neonatal-lesioned cats might be a response of the striatum to neuroplastic changes in the striatal neuropil, including the corticostriatal afferents, since such changes are different in neonatal- as compared to adult-lesioned cats.

Opposite rotation induced by dopamine agonists in rats with unilateral lesions of the globus pallidus or substantia nigra

Normal rats with a unilateral ibotenic acid lesion of substantia nigra pars reticulata (SNR, n = 12) or globus pallidus (GP, n = 12) were challenged systemically with the mixed dopaminergic agonist apomorphine (0.5 and 1.5 mg/kg) and the indirect acting d-amphetamine (1.5 mg/kg). The low dose of apomorphine produced a weak contralateral rotation only in the SNR-lesioned group, which showed an intense ipsilateral rotation following the administration of the higher dose. GP-lesioned rats also showed ipsilateral rotation after the high dose of apomorphine. d-Amphetamine produced ipsilateral rotation in GP-lesioned rats, contrasting with a vigorous contralateral rotation in SNR-lesioned rats. The unexpected opposite rotation after apomorphine and d-amphetamine, observed only in SNR-lesioned animals, indicates that the role of SNR in basal ganglia functions is less clear and more complex than what is expected from our current model of basal ganglia circuitry and functions. On the other hand, the GP lesion resulted in a consistent and predictable ipsilateral rotation after both apomorphine and d-amphetamine, indicating a more determinant effect on the output of the basal ganglia than heretofore believed. Our results may contribute to the recently expressed views challenging the established model of basal ganglia organisation.

Contribution of the nucleus accumbens to cocaine-induced responses of ventral pallidal neurons

The present study characterized the responses of ventral pallidal (VP) neurons to intravenously (iv) administered cocaine (0.003, 0.01, 0.03, 0.1, 0.3, and 1.0 mg/kg) in chloral hydrate-anesthetized rats. Eighty-four percent (16/19) of the tested neurons displayed rate changes following cocaine administration. Fifty-three percent responded by increasing firing rate, with an EMAX of 217 +/- 26% of basal activity and an ED50 of 0.07 +/- 0.03 mg/kg. Neurons that responded with a rate decrease (26%) had an EMAX of 14.3 +/- 9.0% of basal control and an ED50 of 0.04 +/- 0.02 mg/kg. One neuron (5%) displayed a biphasic response pattern. Haloperidol (0.2 mg/kg) attenuated cocaine-induced effects in 90% of the tested neurons. Given the responsiveness of VP neurons to cocaine, the extensive innervation of the VP by the nucleus accumbens (NAC), and the importance of the NAC in regulating cocaine-induced effects, it is likely that NAC activity may affect VP responses to cocaine. To test this possibility, the influence of NAC on cocaine-induced VP activity was evaluated. Unilateral inactivation of the NAC with microinjections of procaine (40 mu g/2 mu l/2 min) did not alter the proportion of VP neurons responsive to subsequent systemic administration of cocaine (0.1, 1.0 mg/kg iv) or the EMAX for those neurons showing a rate decrease. However, for the population of neurons showing a cocaine-induced rate increase, intra-NAC procaine significantly enhanced EMAX to 392 +/- 74% of control. These data suggest that the ability of VP neurons to respond to iv cocaine is independent of the NAC. However, the magnitude of the cocaine-induced effect appears to be dependent on NAC influences.

D-1 receptor mediated modulation of the release of gamma-aminobutyric acid by endogenous dopamine in the basal ganglia of the rat

1. Presynaptic D1 receptors are present on GABAergic terminals of neostriatal projections. 2. By activating these receptors, exogenous dopamine enhances the release of GABA. 3. Here the authors have explored whether endogenous dopamine was also able to activate the receptors, thus enhancing GABA release. 4. The effect of methamphetamine, a dopamine releaser, on the release of tritiated GABA was studied in slices of substantia nigra pars reticulata, entopeduncular nucleus and caudate-putamen, targets of the striatal projections. 5. Methamphetamine enhanced the release of the label. However the enhancement required an intact dopaminergic innervation, since it was lost in slices isolated from rats with 6-hydroxydopamine-induced lesions of the dopaminergic nigrostriatal system. 6. The activation of the receptors by endogenous dopamine was also judged by the effect of the selective D1 antagonist SCH 23390 in potassium depolarized slices. By preventing activation of the receptors by dopamine released as result of depolarization, the antagonist reduced GABA release. In 6-OHDA lesioned slices, no reduction was observed, even though the slices were also depolarized. 7. The results indicate that endogenous dopamine enhances GABA release from striatal terminals in the pars reticulata of the substantia nigra, entopeduncular nucleus and caudate-putamen. This would facilitate GABAergic neurotransmission. 8. The study suggests that the function of DA in the basal ganglia is widespread, modulating not only the firing of the striatal efferent neurons but also the transmission of the fired impulses across synapses in the target nuclei of these neurons.

Immunocytochemical localization of D1 and D2 dopamine receptors in the basal ganglia of the rat: light and electron microscopy

The modulatory actions of dopamine on the flow of cortical information through the basal ganglia are mediated mainly through two subtypes of receptors, the D1 and D2 receptors. In order to examine the precise cellular and subcellular location of these receptors, immunocytochemistry using subtype specific antibodies was performed on sections of rat basal ganglia at both the light and electron microscopic levels. Both peroxidase and pre-embedding immunogold methods were utilized. Immunoreactivity for both D1 and D2 receptors was most abundant in the neostriatum where it was mainly contained within spiny dendrites and in perikarya. Although some of the immunoreactive perikarya had characteristics of interneurons, most were identified as medium-sized spiny neurons. Immunoreactivity for D1 receptor but not D2 receptor was associated with the axons of the striatonigral pathway and axons and terminals in the substantia nigra pars reticulata and the entopeduncular nucleus. In contrast, D2 immunoreactivity but not D1 immunoreactivity was present in the dopaminergic neurons in the substantia nigra pars compacta and ventral pars reticulata. In the globus pallidus, little immunoreactivity for either D1 or D2 receptor was detected. At the subcellular level, D1 and D2 receptor immunoreactivity was found to be mainly associated with the internal surface of cell membranes. In dendrites and spines immunoreactivity was seen in contact with the membranes postsynaptic to terminals forming symmetrical synapses and less commonly, asymmetrical synapses. The morphological features and membrane specializations of the terminals forming symmetrical synapses are similar to those of dopaminergic terminals previously identified by immunocytochemistry for tyrosine hydroxylase. In addition to immunoreactivity associated with synapses, a high proportion of the immunoreactivity was also on membranes at non-synaptic sites. It is concluded that dopamine receptor immunoreactivity is mainly associated with spiny output neurons of the neostriatum and that there is a selective association of D1 receptors with the so-called direct pathway of information flow through the basal ganglia, i.e. the striatoentopeduncular and striatonigral pathways. Although there is an association of receptor immunoreactivity with afferent synaptic inputs a high proportion is located at extrasynaptic sites.

Alterations in dopamine uptake sites and D1 and D2 receptors in cats symptomatic for and recovered from experimental parkinsonism

The administration of the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) to adult cats severely disrupts the dopaminergic innervation of the striatum. Animals display a parkinson-like syndrome, consisting of akinesia, bradykinesia, postural instability, and rigidity, which spontaneously recovers by 4-6 weeks after the last administration of MPTP. In this study we used quantitative receptor autoradiography to examine changes in DA uptake sites and DA receptors in the basal ganglia of normal, and symptomatic and recovered MPTP-treated cats. Consistent with the destruction of the nigrostriatal DA pathway, there was a severe loss of DA uptake sites, labeled with [3H]-mazindol, in the caudate nucleus (64-82%), nucleus accumbens (44%), putamen (63%), and substantia nigra pars compacta (SNc, 53%) of symptomatic cats. Following behavioral recovery, there were no significant changes in DA uptake site density. Significant increases of [3H]-SCH 23390 binding to D1 DA receptors were observed in the dorsal caudate (> 24%; P < 0.05) of symptomatic cats and in all regions of the caudate-putamen (> 30%; P < 0.05) of recovered animals. [3H]-SCH 23390 binding in the substantia nigra pars reticulata was half of that in the striatum and showed no changes in symptomatic or recovered animals. No alterations in the binding of [125I]-epidepride to D2 receptors was observed in any region of the striatum in either symptomatic or recovered animals. [125I]-Epidepride binding in the SNc was decreased by > 36% (P < 0.05) following MPTP treatment. These data show that cats made parkinsonian by MPTP exposure have a significant decrease in the number of DA reuptake sites throughout the striatum and that recovery of sensorimotor function in these animals is not correlated with an increase in the number of striatal reuptake sites. Behavioral recovery, however, does seem to be correlated with a general elevation of D1 receptors throughout the striatal complex. The present data also show that direct correlations between changes in DA receptor regulation after a large DA depleting lesion and behavioral deficits or recovery from those deficits are difficult and that the relationships between DA receptors/transporters and behavior require further study.

Quantitative autoradiography of dopamine-D1 receptors, D2 receptors, and dopamine uptake sites in postmortem striatal specimens from schizophrenic patients

A number of previously published homogenate receptor binding studies have postulated that dopaminergic dysfunction in schizophrenia may be related to abnormalities in dopamine receptors. In this study, postmortem striatal specimens from patients with schizophrenia, normal controls, and psychiatric controls that had received neuroleptics were studied with quantitative autoradiography for dopamine receptors. Autoradiography with single concentrations of [3H]-SCH 23390 for D1 receptors, [3H]-raclopride for D2 receptors, and [3H]-CFT for dopamine uptake sites failed to define significant differences between the study groups. [3H]-CFT bound in a patchy distribution in the striatum that is believed to correspond to striosomal and matrix striatal compartments. There were no differences between groups when [3H]-CFT binding density was examined in the striosomal and matrix compartments. There were also no differences between groups in the percentage of striatal area occupied by striosomal or matrix compartments as defined by [3H]-CFT binding. We conclude that abnormalities of these dopamine receptor subtypes are probably not primary features of the schizophrenic syndrome in the brain collection examined. Previous reports of elevated D2 receptor binding in schizophrenia may have been related to drug treatment effects. Alternatively, the relatively high affinity of ligands used in previous studies for D4 receptors may explain the discrepancy in our findings. Unchanged [3H]-CFT binding in the schizophrenic group also suggests that the density of mesostriatal neuronal terminals is not altered in schizophrenia.

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.

Electrophysiological verification of the presence of D1 and D2 dopamine receptors within the ventral pallidum

The ventral pallidum is a basal forebrain region recently shown to receive dopaminergic projections from the midbrain. Binding sites for the D1 and D2 dopamine receptor families have been identified within the ventral pallidum, yet the consequences of activating these receptors have not been studied. Thus, to characterize the physiological pharmacology of D1 and D2 receptor subtypes for the ventral pallidum, extracellular single-neuron recording and microiontophoretic techniques were used in chloral hydrate-anesthetized rats. Half of the 93 ventral pallidal neurons tested were sensitive to iontophoresis of dopamine (DA), and both rate increases and decreases were observed. Co-iontophoresis of either the D1 antagonist SCH23390, or the D2 antagonist sulpiride, generally attenuated the DA-induced rate changes. Like DA, about half of the ventral pallidal neurons tested were sensitive to the D1 agonist, SKF38393. Yet in contrast to DA, rate suppression was observed almost exclusively, and the magnitude of this decrease was greater than that produced by DA. SKF38393-induced suppressions were antagonized by SCH23390, but not by sulpiride, demonstrating the specificity of the D1 agonist. Most of the neurons tested were not affected by quinpirole, but when responsive to the D2 agonist, rate increases were observed most often. The increases were antagonized by the D2 antagonist sulpiride, but not SCH23390, demonstrating that this response resulted from an activation of D2 receptors. These results support binding studies demonstrating that both D1 and D2 receptors are present in the ventral pallidum, and reveal that the independent activation of each of these is sufficient to alter neuronal activity.

Dopaminergic terminals form synaptic contacts with enkephalinergic striatal neurons in pigeons: an electron microscopic study

Medium spiny projection neurons of the striatum consist of two major neuropeptide-specific types, one type containing substance P and another type containing enkephalin. Both of these types have been shown to receive dopaminergic input onto their perikarya and proximal dendrites. However, whether each of these types receives direct dopaminergic input onto distal dendritic shafts and onto dendritic spines has not been explored in depth. In the present study, we used electron microscopic immunohistochemical double-label techniques to examine the synaptic organization of dopaminergic input onto enkephalin-positive (ENK+) striatal neurons in pigeons, in whom ENK+ striatal perikarya, dendritic shafts and spines can be readily labeled. Antibodies against tyrosine hydroxylase were used to label dopaminergic terminals using a silver-intensified immunogold method. ENK+ neurons were labeled using diaminobenzidine. We found that dopaminergic terminals make appositions and form symmetric synapses with the perikarya, dendritic shafts, and dendritic spine necks of ENK+ striatal neurons. Thus, nigral dopaminergic neurons provide a monosynaptic input onto ENK+ striatal neurons in a manner similar to that described previously by us for substance P-positive striatal medium spiny neurons.

N-methyl-D-aspartic acid-induced lesions of the nucleus accumbens and/or ventral pallidum fail to attenuate lateral hypothalamic self-stimulation reward

The role of ventral striatum in the maintenance and transmission of a hypothalamic intracranial self-stimulation (ICSS) reward signal was investigated using the rate-frequency multiple-curve shift paradigm. The excitotoxin N-methyl-D-aspartic acid (NMDA) was bilaterally administered into the nucleus accumbens (15 micrograms per side), the ventral pallidum (15 micrograms per side) or the juncture between the two structures (20 micrograms per side) creating three lesion groups. Both the nucleus accumbens (NAC) lesion group and the ventral pallidum (VP) lesion group displayed substantial NMDA-induced damage which was generally restricted to the intended limbic structure. The NMDA lesions in the third group displayed extensive damage to both the NAC and VP, as intended, but also typically diffused into adjacent medial structures. NMDA-induced lesions in all groups caused a suppression in motor/performance activity at all currents tested. Contrary to motor effects, reward efficacy was relatively unaffected for the NAC and VP groups. The lack of reward effects may be due to plasticity of neuronal systems and redundancy of circuit connections. However, this explanation is questionable given the fact that NMDA lesions which encompassed both the NAC and VP had little effect on reward efficacy. The above data suggests that the nucleus accumbens and the ventral pallidum are not critical for ICSS rewards stimulation and that hypothalamic ICSS reward signals are processed downstream from these limbic structures.

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.

Cholinergic neurons are distributed preferentially in areas rich in substance P-like immunoreactivity in the caudate nucleus of the adult cat

The distribution of cells stained immunocytochemically for the cholinergic marker choline acetyltransferase was compared to the pattern of substance P immunoreactivity in the caudate nucleus of adult cats using a double-label immunocytochemical protocol and three-dimensional reconstructions of adjacent sections single-labeled for either substance P or choline acetyltransferase. Substance P immunoreactivity was distributed in a highly complex mosaic within the caudate nucleus of the cat. In the dorsal caudate nucleus, substance P-rich zones consisting of either clusters of substance P-positive cell bodies or fibers were seen against a lighter staining background. The density of cholinergic neurons was found to be significantly greater within these substance P-rich patches in comparison to surrounding regions. The pattern of substance P immunoreactivity within the ventral caudate nucleus differed from that in more dorsal regions. Clear substance P-rich patches were not seen in this region, but a large substance P-rich area consisting of a dense plexus of substance P-containing fibers was visible. Embedded within this substance P-rich area were fairly discrete patches of light substance P staining. As in the dorsal caudate nucleus, increased numbers of cholinergic neurons and processes were associated with substance P-rich regions in the ventral caudate nucleus. Choline acetyltransferase-positive perikarya also appeared to be concentrated in substance P-rich areas in the nucleus accumbens and olfactory tubercle. The results of this study suggest that a close relationship exists between the distribution of substance P fibers and cholinergic perikarya in the striatum of the cat.

Heterogeneous distribution of D1, D2 and D5 receptor mRNAs in monkey striatum

The primate striatum has a compartmental organization reflected both in the topography of its afferent projections and in the segregation of its morphologically similar but neurochemically distinct efferent neurons. Discretely projecting mesostriatal neurons release dopamine (DA) which modulates the responses of striatal neurons to other afferent inputs. Multiple DA receptor (DAR) subtypes have been cloned and characterized and mapping their cellular expression is crucial for understanding the influence of DA on striatal function. We report the distribution of mRNAs for D1, D2 and D5 DAR subtypes (D2R, D2R and D5R) in the striatum of cynomolgus monkeys (Macaca fascicularis) studied by in situ hybridization histochemistry (ISH) using monkey-specific cRNA probes. Adjacent sections were stained for calbindin immunoreactivity to distinguish striosomal and matrix compartments for comparison with the patterns obtained with ISH. In the caudate nucleus, D1R mRNA was concentrated in calbindin-poor striosomes where dense grain clusters were seen overlying the majority of medium-sized neurons (diameter approximately 15 microns). D1R mRNA localization was relatively homogeneous in the putamen. By contrast, the distributions of D2R and D5R mRNAs showed no clear preference for the striosomal or matrix compartments of either caudate nucleus or putamen. In the ventral striatum (nucleus accumbens, olfactory tubercle and ventral portions of caudate nucleus and putamen), expression of D1R and D2R mRNA was sparse relative to dorsal striatum, while D5R mRNA expression was roughly equal in ventral and dorsal striatum. Circumscribed zones of hybridization associated with islands of tightly packed small cells occurred with all three DAR mRNA subtypes in the ventral striatum.(ABSTRACT TRUNCATED AT 250 WORDS)

Topography and functional role of dopaminergic projections from the ventral mesencephalic tegmentum to the ventral pallidum

A dopaminergic projection from the ventral tegmental area to the ventral pallidum was identified in the rat using anterograde tract tracing and combined retrograde tracing-immunocytochemistry. The projection was found to be topographically organized such that fibers innervating the ventromedial ventral pallidum arose from neurons located along the midline nuclei of the ventral mesencephalon, including the nucleus interfascicularis and nucleus linearis caudalis. Ventral tegmental neurons situated more laterally, in the nucleus parabrachialis pigmentosus and nucleus paranigralis, projected to the ventromedial and dorsolateral ventral pallidum. The substantia nigra did not supply a major contribution to this projection. The proportion of ventral tegmental area dopaminergic neurons projecting to the ventral pallidum ranged from approximately 30% to 60%. The functional significance of the projection is indicated since intra-ventral pallidum microinjections of dopamine elicited a dose-dependent increase in locomotor activity. Furthermore, whereas pretreatment of the ventral pallidum with the GABAA agonist muscimol has been shown to attenuate opioid-induced locomotor activity elicited from the ventral pallidum, it did not attenuate the dopamine-induced motor response. Thus, while mu-opioids in the ventral pallidum may presynaptically regulate GABAergic efferents from the nucleus accumbens, it appears that the dopaminergic input directly influences the ventral pallidal output neuron which is involved in locomotion.

Ultrastructural double-labeling demonstrates synaptic contacts between dopaminergic terminals and substance P-containing striatal neurons in pigeons

Immunohistochemical studies in rats have demonstrated dopaminergic input onto medium spiny neurons of the striatum. Medium spiny neurons, however, are known to consist of two major neuropeptide-specific types, those containing substance P (SP) and those containing enkephalin. Although both of these types have been shown to receive dopaminergic input onto their perikarya and proximal dendrites, the extent to which both types also receive direct dopaminergic input onto distal dendritic shafts or onto dendritic spines is uncertain. In the present study, we used EM immunohistochemical double-label techniques to examine the synaptic organization of dopaminergic input onto SP+ striatal neurons. We examined the striatum of pigeons, in whom SP+ striatal neurons, including their dendritic shafts and spines, can be readily labeled. Antibodies against tyrosine hydroxylase (TH) were used to identify dopaminergic terminals, which were labeled using silver-intensified immunogold. The SP+ neurons were labeled immunohistochemically using diaminobenzidine. We found that dopaminergic terminals make appositions and form symmetric synapses with the perikarya, dendritic shafts and dendritic spines of SP+ neurons. Thus, nigral dopaminergic neurons provide a monosynaptic input onto SP+ striatal neurons in a manner similar to that described for dopaminergic input onto striatal medium spiny neurons in general.

Organization and physiology of the substantia nigra

A comprehensive review of the literature on the anatomy, electrophysiology and pharmacology of the substantia nigra is presented. A diagram is developed taking into account the interneuronal interactions of neurotransmitters and receptors that control firing rates and neurotransmitter releases. The central features of the diagram are a positive dopaminergic feedforward process and a positive feedback mechanism mediated by extrasynaptic substance P diffusing from striatal terminals to dopaminergic dendrites of the zona compacta neurons. This loop can enhance the transmission of information from the striatum through the pars reticulata output neurons. The loop is controlled at the level of zona compacta neurons by a negative feedback supported by the dendritic release of dopamine and boosted by pedunculopontine activation mediated by muscarinic receptors. The output of the loop is controlled by two negative feedforward processes, both involving GABAergic striatonigral afferents. Application of the model to pharmacological studies of diverse behaviors including seizures, turning, and conditioned behaviors reveals unforseen relationships and may offer insights into, and directions for, further analysis of the mechanisms and functions involved.

Regional differences in the regulation of acetylcholine release upon D2 dopamine and N-methyl-D-aspartate receptor activation in rat nucleus accumbens and neostriatum

The effect of D2 dopamine receptor activation on either the electrically, or N-methyl-D-aspartate induced release of radiolabeled acetylcholine (ACh) was investigated in different areas of the nucleus accumbens and the neostriatum of rats, by using a superfusion technique. Sequential slices of 100 microns were chopped along either a rostrocaudal, mediolateral or dorsoventral axis. In every slice the effect of a supramaximal concentration of the selective D2 receptor agonist quinpirole on the release of ACh was measured. In the entire neostriatum the release of ACh was reduced by approximately 70% in the presence of quinpirole. By contrast, in the nucleus accumbens, a gradual decrease in the inhibitory effect of quinpirole on the release of ACh was observed along both the rostral-to-caudal and the lateral-to-medial axes. Whereas in the rostrolateral part a 50% inhibition could be observed, in the caudomedial part no significant inhibition could be detected. Also the N-methyl-D-aspartate induced release of ACh was smaller in the caudomedial part as compared to the rostrolateral part of the nucleus accumbens. It is concluded that the nucleus accumbens is a very heterogeneous structure with respect to the regulation of the release of ACh by D2 dopamine and N-methyl-D-aspartate receptor activation.

Effect of unilateral perinatal hypoxic-ischemic brain injury in the rat on dopamine D1 and D2 receptors and uptake sites: a quantitative autoradiographic study

The effect of a unilateral perinatal hypoxic-ischemic brain injury on dopamine D1 and D2 receptors and uptake sites was investigated in rats by using in vitro quantitative binding autoradiography, 2-3 weeks after the insult. We observed significant decreases in the Bmax and KD for [3H]SCH 23390-labeled D1 and in the Bmax for [3H]spiperone-labeled D2 receptors in the lesioned caudate-putamen in rats with moderate brain injury (visible loss in hemispheric volume ipsilateral to the injury) compared with the nonlesioned contralateral caudate-putamen or with control rats. Changes in [3H]SCH 23390 and [3H]spiperone binding predominated in the dorsolateral part of the lesioned caudate-putamen. Pronounced reduction in [3H]SCH 23390 binding was also observed in the substantia nigra pars reticulata on the side of the lesion. In contrast, we did not observe any significant change in Bmax or KD for [3H]mazindol-labeled dopamine uptake sites. Similarly, no significant changes in the levels of dopamine or its metabolites were found on the side of the lesion. The observed reductions in striatal dopamine D1 and D2 receptors are a reflection of striatal cell loss induced by the hypoxic-ischemic injury. The absence of changes in [3H]mazindol binding or dopamine levels in the lesioned caudate-putamen indicates that the dopaminergic presynaptic structures are preserved.

Dopamine D1 receptors in the sub-commissural part of the globus pallidus and their role in oro-facial dyskinesia in cats

The possible role of dopamine D1 receptors in the sub-commissural part of the globus pallidus in the induction of oro-facial dyskinesia was studied in cats. The present study reveals two findings. Firstly, bilateral injections of the D1 agonist (+/-)-SK& F38393 into the ventral pallidal area elicited oro-facial dyskinesia, which was quantified in terms of numbers of tongue protrusions. The results show that the dose-effect curve was bell-shaped (1.0, 1.75, 2.5, 5.0 micrograms/0.5 microliters (+/-)-SK&F38393). The oro-facial dyskinesia elicited by (+/-)-SK&F38393 was highly comparable to the oro-facial dyskinesia elicited by injections of the GABA antagonist picrotoxin or the acetylcholine agonist carbachol into the sub-commissural part of the globus pallidus. Secondly, the inactive enantiomer of SK&F38393, i.e. S(-)-SK&F38393, was found to be ineffective in eliciting oro-facial dyskinesia when injected in a dose equivalent to 50% of the most effective dose of the racemic mixture of (+/-)-SK&F38393. Furthermore, the effect elicited by 2.5 micrograms/0.5 microliters (+/-)-SK&F38393 was significantly attenuated by local injection of the D1 antagonist R(+)-SCH23390 in a dose which had no effect itself (1.0 micrograms/0.5 microliters). These findings indicate that the effects elicited by (+/-)-SK&F38393 are D1-specific. The present results thus clearly indicate that dopamine D1 receptors within the sub-commissural part of the globus pallidus are involved in mediating oro-facial dyskinesia in cats.

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.

Ontogeny of dopamine D1 and D2 receptor subtypes in rat basal ganglia: a quantitative autoradiographic study

The ontogeny of D1 and D2 dopamine (DA) receptors in rat basal ganglia was examined by quantitative autoradiography using the iodinated ligands [125I]SCH 23982 and [125I]iodobenzamide [( 125I]IBZM), respectively. Temporal and spatial differences in the development of the receptor subtypes were observed. Scatchard transformation of saturation isotherms conducted at postnatal day 10 (P10) and P60, showed that there was no age-related change in the affinity of [125I]SCH 23982 binding to D1 receptors (Kd = 2.6 nM) but there was a significant increase in the Bmax (771 compared to 2032 fmol/mg protein, P = 0.002). A statistically significant difference in Kd was noted between ages P10 and P60 for [125I]IBZM labelling of D2 receptors (0.62 vs 1.00 nM, respectively, P less than 0.01). A significant increase in the Bmax (211 and 721 fmol/mg protein, P less than 0.01) was also observed. D1 receptors were visible as distinct patches at P1. The highest density was found in the ventrolateral caudate-putamen (CPu). By P5 the patches were found in all subregions of the CPu and nucleus accumbens. Between P7 and P10 the binding became distinctly less patchy due to a marked increase in the density of D1 receptors in non-patch (matrix) regions. Adult levels of receptor were seen by P30. The concentration of DA (measured by HPLC) and binding of [3H]mazindol to DA uptake sites in whole striatum showed similar and nonlinear increases with age. The age-related change in the topography of binding sites for [3H]mazindol was similar to that of D1 receptors at the same ages. Both D2 receptors and [3H]hemicholinium-3 (HC-3) binding to high affinity transport sites for choline developed initially in the dorso-lateral CPu. Their topography was largely overlapping but distinct from that of the D1 receptor. D2 receptors were not consistently observed until P3 in the CPu, and zones of enriched binding were aligned with zones of low density for D1 receptors. The density of D2 receptors reached adult levels by P30. The differential development of the DA receptors was also evident in the substantia nigra (SN) and globus pallidus (GP). D1 receptors were found in SN prior to the appearance of D2 receptors and throughout development the density was greater in pars reticulata than in pars compacta, whereas the density of D2 receptors was higher in the pars compacta. At all ages the density of D1 receptors was greater than the density of D2 receptors in the GP and reached adult levels before reaching it in the CPu or SN.(ABSTRACT TRUNCATED AT 400 WORDS)

Dopamine D1 receptor behavioral responsitivity following selective lesions of the striatal patch compartment during development

The behavioral effects of selective destruction of the dopamine (DA) input to the patch compartment of rat striatum early in development was investigated. Rat pups were given bilateral intrastriatal (i.s.) injections of the neurotoxin 6-hydroxydopamine (6-OHDA) on day of birth (P0) or postnatal day 1 (P1), which resulted in selective behavioral alterations following DA agonist treatment in adulthood. Neonatally-lesioned rats exhibited self-biting behavior following treatment with the DA precursor L-dihydroxyphenylalanine (L-DOPA). In response to treatment with the selective D1 agonist SKF38393, there was an increased incidence of abnormal perioral movements. The cataleptogenic effects of the D1 antagonist SCH23390 and the D2 antagonist haloperidol were also studied. Neonatally-lesioned rats were significantly less cataleptic compared to control rats following D1 antagonist treatment, but not following D2 antagonist treatment. Autoradiographs of [3H]mazindol binding to DA uptake sites (a measure of DA terminal density) showed a 'patchy' loss of approx. 40-50% in striatal tissue sections derived from the i.s. lesioned rats. These data suggest that injections of 6-OHDA into the striatum during this early postnatal period cause a DA lesion that results in long-term effects on a D1 receptor system.

Quantitative image analysis with densitometry for immunohistochemistry and autoradiography of receptor binding sites--methodological considerations

Major technical progress in the development of computer-based image analysis has made possible the entry of autoradiography and immunohistochemistry into a new era where quantification by densitometry has become easily accessible. Autoradiography could become quantitative and displayed adequate reproducibility with the help of emulsion-coated films and the use of scales of standards of known radioactivity exposed and analyzed in parallel to the tissue sections. Immunohistochemistry after revelation by a color-based enzymatic technique can also become quantitative, providing that standardization of the crucial steps of the procedure and calibration through a parallel treatment of a scale of antigen standards can be ensured. Such an approach is described here in the rat with reference to tyrosine hydroxylase (TH), the main synthesizing enzyme for catecholamines, and with dopamine (DA) itself, a catecholaminergic neurotransmitter. The different parts of the procedure, which can influence the results, such as the fixation of the animals by perfusion and the evaluation of the fluctuations via the calibration curve, are discussed in detail. Biological validation of the proposed procedure is described by reference to experiments already well documented biochemically, such as the induction effect of reserpine on TH in the rat locus coeruleus and the depleting effect of alpha-methyltyrosine (AMPT), a well-known blocker of TH activity, on rat striatal DA content. Finally the importance of restricting the measurements to the (pseudo)linear portion of the calibration curve is illustrated by the autoradiographic identification of the differential intrastriatal repartition of the dopaminergic D1 and D2 receptor sites, particularly the dual patch-matrix compartments.