Ultrastructural features of the choline acetyltransferase-containing neurons and relationships with nigral dopaminergic and cortical afferent pathways in the rat striatum

Absolute Number of Thalamic Parafascicular and Striatal Cholinergic Neurons, and the Three-Dimensional Spatial Array of Striatal Cholinergic Neurons, in the Sprague-Dawley Rat

The absolute number of neurons and their spatial distribution yields important information about brain function and species comparisons. We studied thalamic parafascicular neurons and striatal cholinergic interneurons (CINs) because the parafascicular neurons are the main excitatory input to the striatal CINs. This circuit is of increasing interest due to research showing its involvement in specific types of learning and behavioral flexibility. In the Sprague-Dawley rat, the absolute number of thalamic parafascicular neurons and striatal CINs is unknown. They were estimated in this study using modern stereological counting methods. From each of six young adult rats, complete sets of serial 40 µm glycol methacrylate sections were used to quantify neuronal numbers in the right parafascicular nucleus (PFN). From each of five young adult rats, complete sets of serial 20 µm frozen sections were immunostained and used to quantify cholinergic neuronal numbers in the right striatum. The spatial distribution, in three dimensions, of striatal CINs was also determined from exhaustive measurement of the x, y, z coordinates of each large interneuron in 40 µm glycol methacrylate sections in sampled sets of five consecutive serial sections from each of two rats. Statistical analysis of spatial distribution was conducted by comparing observed three-dimensional data with computer models of 10,000 pseudorandom distributions, using measures of nearest neighbor distance and Ripley's K-function for inhomogeneous samples. We found that the right PFN consisted, on average, of 30,073 neurons (with a coefficient of variation of 0.11). The right striatum consisted, on average, of 10,778 CINs (0.14). The statistical analysis of spatial distribution showed no evidence of clustering of striatal CINs in three dimensions in the rat striatum, consistent with previous findings in the mouse striatum. The results provide important data for the transfer of information through the PFN and striatum, species comparisons, and computer modeling.

Extrinsic and intrinsic control of striatal cholinergic interneuron activity

The striatum is an integrated component of the basal ganglia responsible for associative learning and response. Besides the presence of the most abundant γ-aminobutyric acid (GABA-ergic) medium spiny neurons (MSNs), the striatum also contains distributed populations of cholinergic interneurons (ChIs), which bidirectionally communicate with many of these neuronal subtypes. Despite their sparse distribution, ChIs provide the largest source of acetylcholine (ACh) to striatal cells, have a prominent level of arborization and activity, and are potent modulators of striatal output and play prominent roles in plasticity underlying associative learning and reinforcement. Deviations from this tonic activity, including phasic bursts or pauses caused by region-selective excitatory input, neuromodulator, or neuropeptide release can exert strong influences on intrinsic activity and synaptic plasticity via diverse receptor signaling. Recent studies and new tools have allowed improved identification of factors driving or suppressing cholinergic activity, including peptides. This review aims to outline our current understanding of factors that control tonic and phasic ChI activity, specifically focusing on how neuromodulators and neuropeptides interact to facilitate or suppress phasic ChI responses underlying learning and plasticity.

Dopaminergic Perturbation in the Aetiology of Neurodevelopmental Disorders

Brain development may be influenced by both genetic and environmental factors, with potential consequences that may last through the lifespan. Alterations during neurogenesis are linked to neurodevelopmental cognitive disorders. Many neurotransmitters and their systems play a vital role in brain development, as most are present prior to synaptogenesis, and they are involved in the aetiology of many neurodevelopmental disorders. For instance, dopamine (DA) receptor expression begins at the early stages of development and matures at adolescence. The long maturation period suggests how important it is for the stabilisation and integration of neural circuits. DA and dopaminergic (DAergic) system perturbations have been implicated in the pathogenesis of several neurological and neuropsychiatric disorders. The DAergic system controls key cognitive and behavioural skills including emotional and motivated behaviour through DA as a neurotransmitter and through the DA neuron projections to major parts of the brain. In this review, we summarise the current understanding of the DAergic system's influence on neurodevelopment and its involvement in the aetiology and progression of major disorders of the developing brain including autism, schizophrenia, attention deficit hyperactivity disorder, down syndrome, and fragile X syndrome.

Enhancement of Haloperidol-Induced Catalepsy by GPR143, an L-Dopa Receptor, in Striatal Cholinergic Interneurons

Dopamine neurons play crucial roles in pleasure, reward, memory, learning, and fine motor skills and their dysfunction is associated with various neuropsychiatric diseases. Dopamine receptors are the main target of treatment for neurologic and psychiatric disorders. Antipsychotics that antagonize the dopamine D2 receptor (DRD2) are used to alleviate the symptoms of these disorders but may also sometimes cause disabling side effects such as parkinsonism (catalepsy in rodents). Here we show that GPR143, a G-protein-coupled receptor for L-3,4-dihydroxyphenylalanine (L-DOPA), expressed in striatal cholinergic interneurons enhances the DRD2-mediated side effects of haloperidol, an antipsychotic agent. Haloperidol-induced catalepsy was attenuated in male Gpr143 gene-deficient (Gpr143-/y ) mice compared with wild-type (Wt) mice. Reducing the endogenous release of L-DOPA and preventing interactions between GPR143 and DRD2 suppressed the haloperidol-induced catalepsy in Wt mice but not Gpr143-/y mice. The phenotypic defect in Gpr143-/y mice was mimicked in cholinergic interneuron-specific Gpr143-/y (Chat-cre;Gpr143flox/y ) mice. Administration of haloperidol increased the phosphorylation of ribosomal protein S6 at Ser240/244 in the dorsolateral striatum of Wt mice but not Chat-cre;Gpr143flox/y mice. In Chinese hamster ovary cells stably expressing DRD2, co-expression of GPR143 increased cell surface expression level of DRD2, and L-DOPA application further enhanced the DRD2 surface expression. Shorter pauses in cholinergic interneuron firing activity were observed after intrastriatal stimulation in striatal slice preparations from Chat-cre;Gpr143flox/y mice compared with those from Wt mice. Together, these findings provide evidence that GPR143 regulates DRD2 function in cholinergic interneurons and may be involved in parkinsonism induced by antipsychotic drugs.

Striatal Cholinergic Signaling in Time and Space

The cholinergic interneurons of the striatum account for a small fraction of all striatal cell types but due to their extensive axonal arborization give the striatum the highest content of acetylcholine of almost any nucleus in the brain. The prevailing theory of striatal cholinergic interneuron signaling is that the numerous varicosities on the axon produce an extrasynaptic, volume-transmitted signal rather than mediating rapid point-to-point synaptic transmission. We review the evidence for this theory and use a mathematical model to integrate the measurements reported in the literature, from which we estimate the temporospatial distribution of acetylcholine after release from a synaptic vesicle and from multiple vesicles during tonic firing and pauses. Our calculations, together with recent data from genetically encoded sensors, indicate that the temporospatial distribution of acetylcholine is both short-range and short-lived, and dominated by diffusion. These considerations suggest that acetylcholine signaling by cholinergic interneurons is consistent with point-to-point transmission within a steep concentration gradient, marked by transient peaks of acetylcholine concentration adjacent to release sites, with potential for faithful transmission of spike timing, both bursts and pauses, to the postsynaptic cell. Release from multiple sites at greater distance contributes to the ambient concentration without interference with the short-range signaling. We indicate several missing pieces of evidence that are needed for a better understanding of the nature of synaptic transmission by the cholinergic interneurons of the striatum.

Morphological Study of the Cortical and Thalamic Glutamatergic Synaptic Inputs of Striatal Parvalbumin Interneurons in Rats

Parvalbumin-immunoreactive (Parv+) interneurons is an important component of striatal GABAergic microcircuits, which receive excitatory inputs from the cortex and thalamus, and then target striatal projection neurons. The present study aimed to examine ultrastructural synaptic connection features of Parv+ neruons with cortical and thalamic input, and striatal projection neurons by using immuno-electron microscopy (immuno-EM) and immunofluorescence techniques. Our results showed that both Parv+ somas and dendrites received numerous asymmetric synaptic inputs, and Parv+ terminals formed symmetric synapses with Parv- somas, dendrites and spine bases. Most interestingly, spine bases targeted by Parv+ terminals simultaneously received excitatory inputs at their heads. Electrical stimulation of the motor cortex (M1) induced higher proportion of striatal Parv+ neurons express c-Jun than stimulation of the parafascicular nucleus (PFN), and indicated that cortical- and thalamic-inputs differentially modulate Parv+ neurons. Consistent with that, both Parv + soma and dendrites received more VGlut1+ than VGlut2+ terminals. However, the proportion of VGlut1+ terminal targeting onto Parv+ proximal and distal dendrites was not different, but VGlut2+ terminals tended to target Parv+ somas and proximal dendrites than distal dendrites. These functional and morphological results suggested excitatory cortical and thalamic glutamatergic inputs differently modulate Parv+ interneurons, which provided inhibition inputs onto striatal projection neurons. To maintain the balance between the cortex and thalamus onto Parv+ interneurons may be an important therapeutic target for neurological disorders.

Pauses in Striatal Cholinergic Interneurons: What is Revealed by Their Common Themes and Variations?

Striatal cholinergic interneurons, the so-called tonically active neurons (TANs), pause their firing in response to sensory cues and rewards during classical conditioning and instrumental tasks. The respective pause responses observed can demonstrate many commonalities, such as constant latency and duration, synchronous occurrence in a population of cells, and coincidence with phasic activities of midbrain dopamine neurons (DANs) that signal reward predictions and errors. Pauses can however also show divergent properties. Pause latencies and durations can differ in a given TAN between appetitive vs. aversive outcomes in classical conditioning, initial excitation can be present or absent, and a second pause can variably follow a rebound. Despite more than 20 years of study, the functions of these pause responses are still elusive. Our understanding of pause function is hindered by an incomplete understanding of how pauses are generated. In this mini-review article, we compare pause types, as well as current key hypotheses for inputs underlying pauses that include dopamine-induced inhibition through D₂-receptors, a GABA input from ventral tegmental area, and a prolonged afterhyperpolarization induced by excitatory input from the cortex or from the thalamus. We review how each of these mechanisms alone explains some but not all aspects of pause responses. These mechanisms might need to operate in specific but variable sets of sequences to generate a full range of pause responses. Alternatively, these mechanisms might operate in conjunction with an underlying control mechanism within cholinergic interneurons which could potentially provide a framework to generate the common themes and variations seen amongst pause responses.

Striatal cholinergic interneurons and cortico-striatal synaptic plasticity in health and disease

Basal ganglia disorders such as Parkinson's disease, dystonia, and Huntington's disease are characterized by a dysregulation of the basal ganglia neuromodulators (dopamine, acetylcholine, and others), which impacts cortico-striatal transmission. Basal ganglia disorders are often associated with an imbalance between the midbrain dopaminergic and striatal cholinergic systems. In contrast to the extensive research and literature on the consequences of a malfunction of midbrain dopaminergic signaling on the plasticity of the cortico-striatal synapse, very little is known about the role of striatal cholinergic interneurons in normal and pathological control of cortico-striatal transmission. In this review, we address the functional role of striatal cholinergic interneurons, also known as tonically active neurons and attempt to understand how the alteration of their functional properties in basal ganglia disorders leads to abnormal cortico-striatal synaptic plasticity. Specifically, we suggest that striatal cholinergic interneurons provide a permissive signal, which enables long-term changes in the efficacy of the cortico-striatal synapse. We further discuss how modifications in the striatal cholinergic activity pattern alter or prohibit plastic changes of the cortico-striatal synapse. Long-term cortico-striatal synaptic plasticity is the cellular substrate of procedural learning and adaptive control behavior. Hence, abnormal changes in this plasticity may underlie the inability of patients with basal ganglia disorders to adjust their behavior to situational demands. Normalization of the cholinergic modulation of cortico-striatal synaptic plasticity may be considered as a critical feature in future treatments of basal ganglia disorders.

Cholinergic interneurons in the dorsal and ventral striatum: anatomical and functional considerations in normal and diseased conditions

Striatal cholinergic interneurons (ChIs) are central for the processing and reinforcement of reward-related behaviors that are negatively affected in states of altered dopamine transmission, such as in Parkinson's disease or drug addiction. Nevertheless, the development of therapeutic interventions directed at ChIs has been hampered by our limited knowledge of the diverse anatomical and functional characteristics of these neurons in the dorsal and ventral striatum, combined with the lack of pharmacological tools to modulate specific cholinergic receptor subtypes. This review highlights some of the key morphological, synaptic, and functional differences between ChIs of different striatal regions and across species. It also provides an overview of our current knowledge of the cellular localization and function of cholinergic receptor subtypes. The future use of high-resolution anatomical and functional tools to study the synaptic microcircuitry of brain networks, along with the development of specific cholinergic receptor drugs, should help further elucidate the role of striatal ChIs and permit efficient targeting of cholinergic systems in various brain disorders, including Parkinson's disease and addiction.

Differential organization of cortical inputs to striatal projection neurons of the matrix compartment in rats

In prior studies, we described the differential organization of corticostriatal and thalamostriatal inputs to the spines of direct pathway (dSPNs) and indirect pathway striatal projection neurons (iSPNs) of the matrix compartment. In the present electron microscopic (EM) analysis, we have refined understanding of the relative amounts of cortical axospinous vs. axodendritic input to the two types of SPNs. Of note, we found that individual dSPNs receive about twice as many axospinous synaptic terminals from IT-type (intratelencephalically projecting) cortical neurons as they do from PT-type (pyramidal tract projecting) cortical neurons. We also found that PT-type axospinous synaptic terminals were about 1.5 times as common on individual iSPNs as IT-type axospinous synaptic terminals. Overall, a higher percentage of IT-type terminals contacted dSPN than iSPN spines, while a higher percentage of PT-type terminals contacted iSPN than dSPN spines. Notably, IT-type axospinous synaptic terminals were significantly larger on iSPN spines than on dSPN spines. By contrast to axospinous input, the axodendritic PT-type input to dSPNs was more substantial than that to iSPNs, and the axodendritic IT-type input appeared to be meager and comparable for both SPN types. The prominent axodendritic PT-type input to dSPNs may accentuate their PT-type responsiveness, and the large size of axospinous IT-type terminals on iSPNs may accentuate their IT-type responsiveness. Using transneuronal labeling with rabies virus to selectively label the cortical neurons with direct input to the dSPNs projecting to the substantia nigra pars reticulata, we found that the input predominantly arose from neurons in the upper layers of motor cortices, in which IT-type perikarya predominate. The differential cortical input to SPNs is likely to play key roles in motor control and motor learning.

Potentiation of NMDA receptor-mediated transmission in striatal cholinergic interneurons

Pauses in the tonic firing of striatal cholinergic interneurons (CINs) emerge during reward-related learning in response to conditioning of a neutral cue. We have previously reported that augmenting the postsynaptic response to cortical afferents in CINs is coupled to the emergence of a cell-intrinsic afterhyperpolarization (AHP) underlying pauses in tonic activity. Here we investigated in a bihemispheric rat-brain slice preparation the mechanisms of synaptic plasticity of excitatory afferents to CINs and the association with changes in the AHP. We found that high frequency stimulation (HFS) of commissural corticostriatal afferents from the contralateral hemisphere induced a robust long-term depression (LTD) of postsynaptic potentials (PSP) in CINs. Depression of the PSP of smaller magnitude and duration was observed in response to HFS of the ipsilateral white matter or cerebral cortex. In Mg²⁺-free solution HFS induced NMDA receptor-dependent potentiation of the PSP, evident in both the maximal slope and amplitude of the PSP. The increase in maximal slope corroborates previous findings, and was blocked by antagonism of either D1-like dopamine receptors with SCH23390 or D2-like dopamine receptors with sulpiride during HFS in Mg²⁺-free solution. Potentiation of the slower PSP amplitude component was due to augmentation of the NMDA receptor-mediated potential as this was completely reversed on subsequent application of the NMDA receptor antagonist AP5. HFS similarly potentiated NMDA receptor currents isolated by blockade of AMPA/kainate receptors with CNQX. The plasticity-induced increase in the slow PSP component was directly associated with an increase in the subsequent AHP. Thus plasticity of cortical afferent synapses is ideally suited to influence the cue-induced firing dynamics of CINs, particularly through potentiation of NMDA receptor-mediated synaptic transmission.

Striatal cholinergic dysfunction as a unifying theme in the pathophysiology of dystonia

Dystonia is a movement disorder of both genetic and non-genetic causes, which typically results in twisted posturing due to abnormal muscle contraction. Evidence from dystonia patients and animal models of dystonia indicate a crucial role for the striatal cholinergic system in the pathophysiology of dystonia. In this review, we focus on striatal circuitry and the centrality of the acetylcholine system in the function of the basal ganglia in the control of voluntary movement and ultimately clinical manifestation of movement disorders. We consider the impact of cholinergic interneurons (ChIs) on dopamine-acetylcholine interactions and examine new evidence for impairment of ChIs in dysfunction of the motor systems producing dystonic movements, particularly in animal models. We have observed paradoxical excitation of ChIs in the presence of dopamine D2 receptor agonists and impairment of striatal synaptic plasticity in a mouse model of DYT1 dystonia, which are improved by administration of recently developed M1 receptor antagonists. These findings have been confirmed across multiple animal models of DYT1 dystonia and may represent a common endophenotype by which to investigate dystonia induced by other types of genetic and non-genetic causes and to investigate the potential effectiveness of pharmacotherapeutics and other strategies to improve dystonia.

Striatal cholinergic interneuron regulation and circuit effects

The striatum plays a central role in motor control and motor learning. Appropriate responses to environmental stimuli, including pursuit of reward or avoidance of aversive experience all require functional striatal circuits. These pathways integrate synaptic inputs from limbic and cortical regions including sensory, motor and motivational information to ultimately connect intention to action. Although many neurotransmitters participate in striatal circuitry, one critically important player is acetylcholine (ACh). Relative to other brain areas, the striatum contains exceptionally high levels of ACh, the enzymes that catalyze its synthesis and breakdown, as well as both nicotinic and muscarinic receptor types that mediate its postsynaptic effects. The principal source of striatal ACh is the cholinergic interneuron (ChI), which comprises only about 1-2% of all striatal cells yet sends dense arbors of projections throughout the striatum. This review summarizes recent advances in our understanding of the factors affecting the excitability of these neurons through acute effects and long term changes in their synaptic inputs. In addition, we discuss the physiological effects of ACh in the striatum, and how changes in ACh levels may contribute to disease states during striatal dysfunction.

Phasic dopaminergic activity exerts fast control of cholinergic interneuron firing via sequential NMDA, D2, and D1 receptor activation

Phasic increases in dopamine (DA) are involved in the detection and selection of relevant sensory stimuli. The DAergic and cholinergic system dynamically interact to gate and potentiate sensory inputs to striatum. Striatal cholinergic interneurons (CINs) respond to relevant sensory stimuli with an initial burst, a firing pause, or a late burst, or a combination of these three components. CIN responses coincide with phasic firing of DAergic neurons in vivo. In particular, the late burst of CINs codes for the anticipated reward. To examine whether DAergic midbrain afferents can evoke the different CIN responses, we recorded from adult olfactory tubercle slices in the mouse ventral striatum. Olfactory inputs to striatal projection neurons were gated by the cholinergic tone. Phasic optogenetic activation of DAergic terminals evoked combinations of initial bursts, pauses, and late bursts in subsets of CINs by distinct receptor pathways. Glutamate release from midbrain afferents evoked an NMDAR-dependent initial burst followed by an afterhyperpolarization-induced pause. Phasic release of DA itself evoked acute changes in CIN firing. In particular, in CINs without an initial burst, phasic DA release evoked a pause through D2-type DA receptor activation. Independently, phasic DA activated a slow depolarizing conductance and the late burst through a D1-type DA receptor pathway. In summary, DAergic neurons elicit transient subsecond firing responses in CINs by sequential activation of NMDA, D2-type, and D1-type receptors. This fast control of striatal cholinergic tone by phasic DA provides a novel dynamic link of two transmitter systems central to the detection and selection of relevant stimuli.

Multiphasic modulation of cholinergic interneurons by nigrostriatal afferents

The motor and learning functions of the striatum are critically dependent on synaptic transmission from midbrain dopamine neurons and striatal cholinergic interneurons (CINs). Both neural populations alter their discharge in vivo in response to salient sensory stimuli, albeit in opposite directions. Whereas midbrain dopamine neurons respond to salient stimuli with a brief burst of activity, CINs exhibit a distinct pause in firing that is often followed by a period of increased excitability. Although this "pause-rebound" sensory response requires dopaminergic signaling, the precise mechanisms underlying the modulation of CIN firing by dopaminergic afferents remain unclear. Here, we show that phasic activation of nigrostriatal afferents in a mouse striatal slice preparation is sufficient to evoke a pause-rebound response in CINs. Using a combination of optogenetic, electrophysiological, and pharmacological approaches, we demonstrate that synaptically released dopamine inhibits CINs through type 2 dopamine receptors, while another unidentified transmitter mediates the delayed excitation. These findings imply that, in addition to their direct effects on striatal projection neurons, midbrain dopamine neurons indirectly modulate striatal output by dynamically controlling cholinergic tone. In addition, our data suggest that phasic dopaminergic activity may directly participate in the characteristic pause-rebound sensory response that CINs exhibit in vivo in response to salient and conditioned stimuli.

Melatonin inhibits manganese-induced motor dysfunction and neuronal loss in mice: involvement of oxidative stress and dopaminergic neurodegeneration

Excessive manganese (Mn) induces oxidative stress and dopaminergic neurodegeneration. However, the relationship between them during Mn neurotoxicity has not been clarified. The purpose of this study was to investigate the probable role of melatonin (MLT) against Mn-induced motor dysfunction and neuronal loss as a result of antagonizing oxidative stress and dopaminergic neurodegeneration. Mice were randomly divided into five groups as follows: control, MnCl2, low MLT + MnCl2, median MLT + MnCl2, and high MLT + MnCl2. Administration of MnCl2 (50 mg/kg) for 2 weeks significantly induced hypokinesis, dopaminergic neurons degeneration and loss, neuronal ultrastructural damage, and apoptosis in the substantia nigra and the striatum. These conditions were caused in part by the overproduction of reactive oxygen species, malondialdehyde accumulation, and dysfunction of the nonenzymatic (GSH) and enzymatic (GSH-Px, superoxide dismutase, quinone oxidoreductase 1, glutathione S-transferase, and glutathione reductase) antioxidative defense systems. Mn-induced neuron degeneration, astrocytes, and microglia activation contribute to the changes of oxidative stress markers. Dopamine (DA) depletion and downregulation of DA transporter and receptors were also found after Mn administration, this might also trigger motor dysfunction and neurons loss. Pretreatment with MLT prevented Mn-induced oxidative stress and dopaminergic neurodegeneration and inhibited the interaction between them. As a result, pretreatment with MLT significantly alleviated Mn-induced motor dysfunction and neuronal loss. In conclusion, Mn treatment resulted in motor dysfunction and neuronal loss, possibly involving an interaction between oxidative stress and dopaminergic neurodegeneration in the substantia nigra and the striatum. Pretreatment with MLT attenuated Mn-induced neurotoxicity by means of its antioxidant properties and promotion of the DA system.

Dopamine neurons control striatal cholinergic neurons via regionally heterogeneous dopamine and glutamate signaling

Midbrain dopamine neurons fire in bursts conveying salient information. Bursts are associated with pauses in tonic firing of striatal cholinergic interneurons. Although the reciprocal balance of dopamine and acetylcholine in the striatum is well known, how dopamine neurons control cholinergic neurons has not been elucidated. Here, we show that dopamine neurons make direct fast dopaminergic and glutamatergic connections with cholinergic interneurons, with regional heterogeneity. Dopamine neurons drive a burst-pause firing sequence in cholinergic interneurons in the medial shell of the nucleus accumbens, mixed actions in the accumbens core, and a pause in the dorsal striatum. This heterogeneity is due mainly to regional variation in dopamine-neuron glutamate cotransmission. A single dose of amphetamine attenuates dopamine neuron connections to cholinergic interneurons with dose-dependent regional specificity. Overall, the present data indicate that dopamine neurons control striatal circuit function via discrete, plastic connections with cholinergic interneurons.

Cortical and thalamic excitation mediate the multiphasic responses of striatal cholinergic interneurons to motivationally salient stimuli

Cholinergic interneurons are key components of striatal microcircuits. In primates, tonically active neurons (putative cholinergic interneurons) exhibit multiphasic responses to motivationally salient stimuli that mirror those of midbrain dopamine neurons and together these two systems mediate reward-related learning in basal ganglia circuits. Here, we addressed the potential contribution of cortical and thalamic excitatory inputs to the characteristic multiphasic responses of cholinergic interneurons in vivo. We first recorded and labeled individual cholinergic interneurons in anesthetized rats. Electron microscopic analyses of these labeled neurons demonstrated that an individual interneuron could form synapses with cortical and, more commonly, thalamic afferents. Single-pulse electrical stimulation of ipsilateral frontal cortex led to robust short-latency (<20 ms) interneuron spiking, indicating monosynaptic connectivity, but firing probability progressively decreased during high-frequency pulse trains. In contrast, single-pulse thalamic stimulation led to weak short-latency spiking, but firing probability increased during pulse trains. After initial excitation from cortex or thalamus, interneurons displayed a "pause" in firing, followed by a "rebound" increase in firing rate. Across all stimulation protocols, the number of spikes in the initial excitation correlated positively with pause duration and negatively with rebound magnitude. The magnitude of the initial excitation, therefore, partly determined the profile of later components of multiphasic responses. Upon examining the responses of tonically active neurons in behaving primates, we found that these correlations held true for unit responses to a reward-predicting stimulus, but not to the reward alone, delivered outside of any task. We conclude that excitatory inputs determine, at least in part, the multiphasic responses of cholinergic interneurons under specific behavioral conditions.

GABAergic inputs from direct and indirect striatal projection neurons onto cholinergic interneurons in the primate putamen

Striatal cholinergic interneurons (ChIs) are involved in reward-dependent learning and the regulation of attention. The activity of these neurons is modulated by intrinsic and extrinsic γ-aminobutyric acid (GABA)ergic and glutamatergic afferents, but the source and relative prevalence of these diverse regulatory inputs remain to be characterized. To address this issue, we performed a quantitative ultrastructural analysis of the GABAergic and glutamatergic innervation of ChIs in the postcommissural putamen of rhesus monkeys. Postembedding immunogold localization of GABA combined with peroxidase immunostaining for choline acetyltransferase showed that 60% of all synaptic inputs to ChIs originate from GABAergic terminals, whereas 21% are from putatively glutamatergic terminals that establish asymmetric synapses, and 19% from other (non-GABAergic) sources of symmetric synapses. Double pre-embedding immunoelectron microscopy using substance P and Met-/Leu-enkephalin antibodies to label GABAergic terminals from collaterals of "direct" and "indirect" striatal projection neurons, respectively, revealed that 47% of the indirect pathway terminals and 36% of the direct pathway terminals target ChIs. Together, substance P- and enkephalin-positive terminals represent 24% of all synapses onto ChIs in the monkey putamen. These findings show that ChIs receive prominent GABAergic inputs from multiple origins, including a significant contingent from axon collaterals of direct and indirect pathway projection neurons.

Pause and rebound: sensory control of cholinergic signaling in the striatum

Cholinergic interneurons have emerged as one of the key players controlling network functions in the striatum. Extracellularly recorded cholinergic interneurons acquire characteristic responses to sensory stimuli during reward-related learning, including a pause and subsequent rebound in spiking. However, the precise underlying cellular mechanisms have remained elusive. Here, we review recent advances in our understanding of the regulation of cholinergic interneuron activity. We discuss evidence of mechanisms that have been proposed to underlie sensory responses, including antagonistic actions by dopamine, recurrent inhibition via local interneurons, and an intrinsically generated membrane hyperpolarization in response to excitatory inputs. The review highlights outstanding questions and concludes with a model of the sensory responses and their downstream effects through dynamic acetylcholine receptor activation.

Centrality of striatal cholinergic transmission in Basal Ganglia function

Work over the past two decades revealed a previously unexpected role for striatal cholinergic interneurons in the context of basal ganglia function. The recognition that these interneurons are essential in synaptic plasticity and motor learning represents a significant step ahead in deciphering how the striatum processes cortical inputs, and why pathological circumstances cause motor dysfunction. Loss of the reciprocal modulation between dopaminergic inputs and the intrinsic cholinergic innervation within the striatum appears to be the trigger for pathophysiological changes occurring in basal ganglia disorders. Accordingly, there is now compelling evidence showing profound changes in cholinergic markers in these disorders, in particular Parkinson's disease and dystonia. Based on converging experimental and clinical evidence, we provide an overview of the role of striatal cholinergic transmission in physiological and pathological conditions, in the context of the pathogenesis of movement disorders.

Number and type of synapses on the distal dendrite of a rat striatal cholinergic interneuron: a quantitative, ultrastructural study

Knowledge of the innervation of interneurons within the striatum is critical to determining their role in the functioning of the striatal network. To this end, the synaptic innervation of a distal dendrite of a rat striatal cholinergic interneuron was quantified for the first time. These synaptic data were compared to three other dendrites from rat striatal interneurons and to published data from dendrites in the mammalian cerebral cortex. To label the cholinergic interneurons and their distal dendrites, a male Wistar rat was perfused and the striatum was double-immunolabelled with an antibody to choline acetyltransferase (ChAT) and an antibody to m2 muscarinic receptor. After processing for transmission electron microscopy, a cholinergic interneuron was located and an m2-labelled distal dendrite identified by tracing it through serial ultrathin sections to this double-immunolabelled soma. Two interneuronal distal dendrites in the same tissue, and another from a second rat, were used for comparison. The widths and lengths of the four distal dendrites, the total number and type of synapses, and the number of synapses per mum for each distal dendrite were measured. Symmetric synapses were the most common type on all four dendrites. There were 0.73 synapses per mum on the distal dendrite of the identified striatal cholinergic interneuron. Two other interneuronal dendrites that were positive for the m2 muscarinic receptor antibody showed similar synaptic densities of 0.62 and 0.83 synapses per microm of distal dendrite, respectively. On a third unlabelled interneuronal distal dendrite located in the lateral striatum, there were 2.17 synapses per microm. This interneuron was thought to be a parvalbumin interneuron rather than a calretinin interneuron, which would more likely be medially located. These data suggest that the number of synapses per microm on the distal dendrite of the cholinergic interneuron, and possibly two other cholinergic interneurons, is three times lower than that of a likely parvalbumin interneuron in the rat striatum. The number of synapses per microm of distal dendrite for a striatal cholinergic interneuron is also lower than the published 1.22-3.3 synapses per microm of dendrite for neurons in the mammalian cerebral cortex. Such anatomical data are important for the construction of new generation computer models that are better able to emulate the operation of striatal cholinergic interneurons.

IH current generates the afterhyperpolarisation following activation of subthreshold cortical synaptic inputs to striatal cholinergic interneurons

Pauses in the tonic firing of striatal cholinergic interneurons emerge during reward-related learning and are triggered by neutral cues which develop behavioural significance. In a previous in vivo study we have proposed that these pauses in firing may be due to intrinsically generated afterhyperpolarisations (AHPs) evoked by excitatory synaptic inputs, including those below the threshold for action potential firing. The aim of this study was to investigate the mechanism of the AHPs using a brain slice preparation which preserved both cerebral hemispheres. Augmenting cortically evoked postsynaptic potentials (PSPs) by repetitive stimulation of cortical afferents evoked AHPs that were unaffected by blocking either GABA(A) receptors with bicuculline, or GABA(B) receptors with saclofen or CGP55845. Apamin (a blocker of small conductance Ca(2+)-activated K(+) channels) had minimal effects, while chelation of intracellular Ca(2+) with BAPTA reduced the AHP by about 30%. In contrast, blocking hyperpolarisation and cyclic nucleotide activated (HCN) cation current (I(H)) with ZD7288 or Cs(+) diminished the size of the AHPs by 60% and reduced the proportion of episodes that contained this hyperpolarisation. The reversal potential (20 mV) and voltage dependence of the AHPs were consistent with the hypothesis that a transient deactivation of I(H) caused most of the AHP at hyperpolarised potentials, while the slow AHP-type Ca(2+)-activated K(+) channels increasingly contributed at more depolarised membrane potentials. Subthreshold somatic current injections yielded similar AHPs with a median duration of approximately 700 ms that were not affected by firing of a single action potential. These results indicate that transient deactivation of HCN channels evokes pauses in tonic firing of cholinergic interneurons, an event likely to be elicited by augmentation of afferent synaptic inputs during learning.

Oxidative damage and neurodegeneration in manganese-induced neurotoxicity

Exposure to excessive manganese (Mn) levels results in neurotoxicity to the extrapyramidal system and the development of Parkinson's disease (PD)-like movement disorder, referred to as manganism. Although the mechanisms by which Mn induces neuronal damage are not well defined, its neurotoxicity appears to be regulated by a number of factors, including oxidative injury, mitochondrial dysfunction and neuroinflammation. To investigate the mechanisms underlying Mn neurotoxicity, we studied the effects of Mn on reactive oxygen species (ROS) formation, changes in high-energy phosphates (HEP), neuroinflammation mediators and associated neuronal dysfunctions both in vitro and in vivo. Primary cortical neuronal cultures showed concentration-dependent alterations in biomarkers of oxidative damage, F2-isoprostanes (F2-IsoPs) and mitochondrial dysfunction (ATP), as early as 2 h following Mn exposure. Treatment of neurons with 500 microM Mn also resulted in time-dependent increases in the levels of the inflammatory biomarker, prostaglandin E2 (PGE2). In vivo analyses corroborated these findings, establishing that either a single or three (100 mg/kg, s.c.) Mn injections (days 1, 4 and 7) induced significant increases in F2-IsoPs and PGE2 in adult mouse brain 24 h following the last injection. Quantitative morphometric analyses of Golgi-impregnated striatal sections from mice exposed to single or three Mn injections revealed progressive spine degeneration and dendritic damage of medium spiny neurons (MSNs). These findings suggest that oxidative stress, mitochondrial dysfunction and neuroinflammation are underlying mechanisms in Mn-induced neurodegeneration.

Dopaminergic synapses in the caudate of subjects with schizophrenia: relationship to treatment response

The typical symptoms of schizophrenia (SZ) are psychotic symptoms (hallucinations, delusions, disorders of thought or speech, grossly disorganized behavior) as well as cognitive impairments and negative symptoms. Not all patients respond to treatment and in those who do, only psychotic symptoms are usually improved. Imaging studies have shown that SZ subjects with high striatal dopamine release are far more responsive to antipsychotic drugs than those patients who have dopamine levels lower than or comparable to that of normal controls. In the present study we hypothesized that there was a link between psychosis and the number of dopaminergic synapses in the caudate nucleus in SZ. We examined dopaminergic synapses at the electron microscopic level in postmortem caudate from cases obtained from the Maryland Brain Collection. SZs were subdivided based on treatment response or resistance. The tissue was processed for the immunocytochemical localization of tyrosine hydroxylase (TH), the synthesizing enzyme for dopamine, and prepared for electron microscopy. The density of all TH labeled synapses was 43% greater in treatment responders than in controls and 62% greater in than in treatment resistant SZ. Axodendritic, but not axospinous, TH-labeled synapses showed this increase. TH-labeled axodendritic synapses in treatment responders were elevated in density (1.95 +/- 0.093/10 microm(3)) compared to treatment resistant SZ (0.04 +/- 0.017/10 microm(3)) and controls (0.11 +/- 0.044/10 microm(3)). The results of the present study suggest that one anatomical underpinning of good treatment response may be a higher density of dopaminergic synapses and support a biological basis to treatment response and resistance. Moreover, these data have important implications for linking specific neuropathology with particular symptoms.

Aging of the striatum: mechanisms and interventions

Motor function declines with increasing adult age. Proper regulation of the balance between dopamine (DA) and acetylcholine (ACh) in the striatum has been shown to be fundamentally important for motor control. Although other factors can also contribute to this age-associated decline, a decrease in the concentration and binding potential of the DA D(2) receptor subtype in the striatum, especially in the cholinergic interneurons, are involved in the mechanism. Our studies have shown that gene transfer of the DA D(2) receptor subtype with adenoviral vectors is effective in ameliorating age-associated functional decline of the striatal cholinergic interneurons. These achievements confirm that an age-associated decrease of D(2)R contributes functional alteration of the interaction of DA and ACh in the striatum and demonstrate that these age-associated changes indeed are modifiable.

High-resolution neuroanatomical tract-tracing for the analysis of striatal microcircuits

Although currently available retrograde tracers are useful tools for identifying striatal projection neurons, transported tracers often remained restricted within the neuronal somata and the thickest, main dendrites. Indeed, thin dendrites located far away from the cell soma as well as post-synaptic elements such as dendritic spines cannot be labeled unless performing intracellular injections. In this regard, the subsequent use of anterograde tracers for the labeling of striatal afferents often failed to unequivocally elucidate whether a given afferent makes true contacts with striatal projections neurons. Here we show that such a technical constraint can now be circumvented by retrograde tracing using rabies virus (RV). Immunofluorescence detection with a monoclonal antibody directed against the viral phosphoprotein resulted in a consistent Golgi-like labeling of striatal projection neurons, allowing clear visualization of small-size elements such as thin dendrites as well as dendritic spines. The combination of this retrograde tracing together with dual anterograde tracing of cortical and thalamic afferents has proven to be a useful tool for ascertaining striatal microcircuits. Indeed, by taking advantage of the trans-synaptic spread of RV, different subpopulations of local-circuit neurons modulating striatal efferent neurons can also be identified. At the striatal level, structures displaying labeling were visualized under the confocal laser-scanning microscope at high resolution. Once acquired, confocal stacks of images were firstly deconvoluted and then processed through 3D-volume rendering in order to unequivocally identify true contacts between pre-synaptic elements (axon terminals from cortical or thalamic sources) and post-synaptic elements (projection neurons and/or interneurons labeled with RV).

Striatal dopamine transporter levels correlate with apathy in neurodegenerative diseases

OBJECTIVES

The aim of the present study was to stress the relationship between neuropsychiatric symptoms and most particularly apathy and striatal dopamine uptake in patients with Alzheimer's disease (AD) or dementia with Lewy body (DLB).

PATIENTS AND METHODS

Twenty-two patients (AD n=14; DLB n=8) were included. All patients had neuropsychological and behavioral examination including Mini Mental Test Examination (MMSE), Neuropsychiatric Inventory (NPI), and UPDRS for the motor activity assessment. Apathy dimensions, emotional blunting, lack of initiative and lack of interest were assessed using the Apathy Inventory (AI). Dopamine transporter (DAT) striatal uptake was assessed using (123)I-FP-CIT (DaTSCAN) SPECT. Quantitative measurements were obtained in 3D using a method which compensates for physical detection biases including partial volume effect.

RESULTS

We observed a correlation between DAT uptake and NPI's domains only for apathy. More specifically using the AI, lack of initiative significantly correlated with bilateral putamen DAT uptake. Using partial correlation coefficients controlling for the UPDRS score, the correlation remained significant between lack of initiative and right and left putamen DAT uptake.

CONCLUSION

These results demonstrate a relationship between apathy and DAT levels independent from motor activity. They suggest that the patients with neurodegenerative diseases presenting with apathy are characterized by some degree of dopaminergic neuronal loss.

Manganese-Induced Neurotoxicity: The Role of Astroglial-Derived Nitric Oxide in Striatal Interneuron Degeneration

Chronic exposure to excessive manganese (Mn) is the cause of a neurodegenerative movement disorder, termed manganism, resulting from degeneration of neurons within the basal ganglia. Pathogenic mechanisms underlying this disorder are not fully understood but involve inflammatory activation of glial cells within the basal ganglia. It was postulated in the present studies that reactive astrocytes are involved in neuronal injury from exposure to Mn through increased release of nitric oxide. C57Bl/6 mice subchronically exposed to Mn by intragastric gavage had increased levels of Mn in the striatum and displayed diminutions in both locomotor activity and striatal DA content. Mn exposure resulted in neuronal injury in the striatum and globus pallidus, particularly in regions proximal to the microvasculature, indicated by histochemical staining with fluorojade and cresyl fast violet. Neuropathological assessment revealed marked perivascular edema, with hypertrophic endothelial cells and diffusion of serum albumin into the perivascular space. Immunofluorescence studies employing terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate (DUTP)-biotin nick-end labeling revealed the presence of apoptotic neurons expressing neuronal nitric oxide synthase (NOS), choline acetyltransferase, and enkephalin in both the striatum and globus pallidus. In contrast, soma and terminals of dopaminergic neurons were morphologically unaltered in either the substantia nigra or striatum, as indicated by immunohistochemical staining for tyrosine hydroxylase. Regions with evident neuronal injury also displayed increased numbers of reactive astrocytes that coexpressed inducible NOS2 and localized with areas of increased neuronal staining for 3-nitrotyrosine protein adducts, a marker of NO formation. These data suggest a role for astrocyte-derived NO in injury to striatal-pallidal interneurons from Mn intoxication.

Functional significance of HCN2/3-mediated I(h) in striatal cells at early developmental stages

Hyperpolarization-activated cAMP-gated cation currents (I(h)) were recently linked to pre- and postnatal developmental processes in several brain regions, including the ventral telencephalon. To evaluate the role of I(h) in striatal development, we used short-term cultured cells from the lateral ganglionic eminence at embryonic day 14 (E14) and postnatal days 1-3 (P1-3) as well as the embryonic striatal progenitor cell line ST14A. Western blot analysis of the I(h) underlying subunit proteins HCN1-4 revealed strong HCN2 expression in proliferating ST14A cells and weak expression in postmitotic ST14A cells and in cells from the developing brain. We also found HCN3 expression only in ST14A cells at both proliferative and nonproliferative stages but not in short-term cultured striatal cells. In all cases, HCN1 and HCN4 transcripts were below the detection level. Despite the selective protein expression, RT-PCR analysis showed stable expression of HCN2-4 but not HCN1 mRNA in all short-term-cultured striatal cells and in the ST14A cell line. Consistent with the strong protein expression, an I(h) was recorded with features of an HCN2-mediated current in ST14A cells at the proliferative stage and in short-term-cultured E14 cells. Of particular importance is that we detected no currents upon hyperpolarization in the ST14A cells at the nonproliferative stage when only HCN3 protein was present. These results suggest the potential importance of ST14A cells in defining the molecular mechanisms regulating I(h) expression and function.

Dopamine D5 receptor localization on cholinergic neurons of the rat forebrain and diencephalon: a potential neuroanatomical substrate involved in mediating dopaminergic influences on acetylcholine release

The study of dopaminergic influences on acetylcholine release is especially useful for the understanding of a wide range of brain functions and neurological disorders, including schizophrenia, Parkinson's disease, Alzheimer's disease, and drug addiction. These disorders are characterized by a neurochemical imbalance of a variety of neurotransmitter systems, including the dopamine and acetylcholine systems. Dopamine modulates acetylcholine levels in the brain by binding to dopamine receptors located directly on cholinergic cells. The dopamine D5 receptor, a D1-class receptor subtype, potentiates acetylcholine release and has been investigated as a possible substrate underlying a variety of brain functions and clinical disorders. This receptor subtype, therefore, may prove to be a putative target for pharmacotherapeutic strategies and cognitive-behavioral treatments aimed at treating a variety of neurological disorders. The present study investigated whether cholinergic cells in the dopamine targeted areas of the cerebral cortex, striatum, basal forebrain, and diencephalon express the dopamine D5 receptor. These receptors were localized on cholinergic neurons with dual labeling immunoperoxidase or immunofluorescence procedures using antibodies directed against choline acetyltransferase (ChAT) and the dopamine D5 receptor. Results from this study support previous findings indicating that striatal cholinergic interneurons express the dopamine D5 receptor. In addition, cholinergic neurons in other critical brain areas also show dopamine D5 receptor expression. Dopamine D5 receptors were localized on the somata, dendrites, and axons of cholinergic cells in each of the brain areas examined. These findings support the functional importance of the dopamine D5 receptor in the modulation of acetylcholine release throughout the brain.

Nucleus accumbens nitric oxide immunoreactive interneurons receive nitric oxide and ventral subicular afferents in rats

The nitric oxide generating neurons of the nucleus accumbens exert a powerful influence over striatal function, in addition, these nitrergic inputs are in a position to regulate the dopaminergic and glutamatergic inputs on striatal projection neurons. It was the aim of this study to establish the source of the glutamatergic drive to nitric oxide synthase interneurons of the nucleus accumbens. The nucleus accumbens nitric oxide-generating neurons receive asymmetrical, excitatory, presumably glutamatergic inputs. Possible sources of these inputs could be the limbic and cortical regions known to project to this area. To identify sources of the excitatory inputs to the nitric oxide synthase-containing interneurons of the nucleus accumbens in the rat we first examined the ultrastructural morphology of asymmetrical synaptic specializations contacting nitric oxide synthase-immunohistochemically labeled interneurons in the nucleus accumbens. Neurons were selected from different regions of the nucleus accumbens, drawn using camera lucida, processed for electron microscopic analysis, and the boutons contacting nitric oxide synthase-labeled dendrites were photographed and correlated to the drawings. Using vesicle size as the criterion the source was predicted to be either the prefrontal cortex or the ventral subiculum of the hippocampus. To examine this prediction, a further study used anterograde tracing from both the prefrontal cortex and the ventral subiculum, and nitric oxide synthase immunohistochemistry with correlated light and electron microscopy. Based on appositions by anterogradely labeled fibers, selected nitric oxide synthase-labeled neurons within the nucleus accumbens, were examined with electron microscopic analysis. With this technique we confirmed the prediction that subicular afferent boutons make synaptic contact with nitric oxide synthase interneurons, and demonstrated anatomically that nitric oxide synthase boutons make synaptic contact with the dendritic arbors of nitric oxide synthase interneurons. We suggest that the subicular input may excite the nitric oxide synthase neurons synaptically, while the nitric oxide synthase-nitric oxide synthase interactions underlie a nitric oxide signaling network which propagates hippocampal information, and expands the hippocampus's influence on 'gating' information flow across the nucleus accumbens.

Thalamic regulation of striatal acetylcholine efflux is both direct and indirect and qualitatively altered in the dopamine-depleted striatum

Striatal cholinergic interneurons play a pivotal role in the integrative sensorimotor functions of the basal ganglia. The major excitatory input to these interneurons arises from glutamatergic neurons of the parafascicular nucleus of the thalamus (Pf). Thalamic regulation of cholinergic interneurons, however, may also include an indirect inhibitory component mediated by the axon collaterals of GABAergic medium spiny neurons that are also innervated by Pf. The present study examined thalamic regulation of striatal cholinergic interneurons by employing dual probe in vivo microdialysis in freely moving animals to determine the effect of pharmacological manipulation of Pf on acetylcholine (ACh) efflux in intact and dopamine-lesioned striata. In intact animals, reverse dialysis application of the GABA(A) antagonist bicuculline (50 microM) into Pf, likely disinhibiting Pf neurons, significantly decreased striatal ACh efflux. When striatal GABA(A) receptors were blocked by simultaneous reverse dialysis application of bicuculline (10 microM), however, the same manipulation significantly increased ACh efflux. Qualitatively similar results were obtained in experiments employing a higher concentration of bicuculline (200 microM). Application of the GABA agonist muscimol (500 microM) into Pf, likely inhibiting Pf neurons, decreased ACh efflux only when the experiment was conducted under blockade of striatal GABA(A) receptors. These data are consistent with the existence of an indirect, inhibitory, GABA(A) receptor-mediated component of ACh regulation that is most clearly manifested when Pf is disinhibited and with the existence of a direct excitatory component of ACh regulation, evident when Pf is inhibited. Manipulation of Pf using very high concentrations of drug (500 microM bicuculline, 2 mM muscimol), however, yielded data consistent only with direct excitatory thalamic regulation. In contrast to results obtained in intact animals, in animals with prior (3 weeks) unilateral lesion of the dopaminergic nigrostriatal pathway, bicuculline application (50 muM) in Pf significantly increased striatal ACh efflux, irrespective of simultaneous blockade of striatal GABA(A) receptors. The results of experiments in which muscimol (500 microM) was applied in Pf were similar to those obtained in intact animals, however. Baseline ACh efflux was not significantly elevated in dopamine-lesioned animals. These results indicate a qualitative alteration in the effectiveness of an inhibitory component of the thalamic regulation of ACh efflux in the dopamine depleted striatum, evident during increased thalamostriatal input. Such altered regulation of striatal ACh output is likely to have profound consequences for integrative function in the parkinsonian basal ganglia.

The corticostriatal input to giant aspiny interneurons in the rat: a candidate pathway for synchronising the response to reward-related cues

Tonically active neurons (TANs) in the mammalian striatum show a pause in their ongoing firing activity in response to an auditory cue that is paired with a reward. This response to reward-related cues develops through learning and becomes expressed synchronously by TANs located throughout the striatum. The pause response is abolished by inactivating the thalamic inputs to the striatum but a short-latency excitatory response to reward-related cues remains, which may originate in the cortex. We investigated the cortical inputs to striatal neurons to determine the electrophysiological properties of their cortical projections. We made in vivo intracellular recordings from 14 giant aspiny interneurons (which correspond to the TANs) and from a control group of spiny projection neurons (n=18) in urethane-anaesthetised rats. All giant aspiny interneurons were tonically active (firing rate: 3.0+/-1.5 Hz) and displayed small-amplitude subthreshold fluctuations in membrane potential. These fluctuations in membrane potential were correlated with the cortical electroencephalogram (EEG). Test stimulation of the contralateral cortex induced postsynaptic potentials (PSPs) in giant aspiny interneurons. These PSPs were significantly shorter in latency (5.1+/-1.6 ms) than those measured in spiny projection neurons (9.3+/-2.8 ms; p<0.01), whereas the latencies of ipsilaterally evoked PSPs did not differ. Taken together, these observations suggest that giant aspiny interneurons are under the significant influence of spontaneous excitatory inputs and receive specialised input from either faster conducting or less branching cortical fibres than spiny projection neurons. These inputs may be involved in the synchronised convergence of reward-related cues from spatially distinct cortical areas onto giant aspiny interneurons.

Hippocampal long-term potentiation attenuated by lesions in the marginal division of neostriatum

The marginal division (MrD) is a spindled-neurons consisted zone at the caudal border of the neostriatum in the mammalian brain and has been verified as contributing to associative learning and declarative memory in the rat and human with behavior and functional magnetic resonance imaging methods. It was proved to have functional connections with the limbic system. Whether the MrD has influence on the hippocampal long-term potentiation (LTP) was investigated in this study. LTP was induced from the dentate gyrus (DG) in the hippocampus by high-frequency stimulation (HFS) to the perforant path (PP). The amplitude of the population spike (PS) and the slope of the excitatory postsynaptic potential (EPSP) increased significantly to form LTP in the DG of the hippocampus after HFS of PP in normal and saline-injected control groups of rats. Lesions introduced in the MrD reduced significantly both the amplitude of PS and the slope of the EPSP following HFS of the PP. The results indicated that lesions in the MrD could attenuate LTP formation in the hippocampus. Our data suggest that the MrD might very possibly have excitatory functional influence on the hippocampus and therefore might influence the function of the hippocampus.

Muscarinic, adenosine A(2) and histamine H(3) receptor modulation of haloperidol-induced c-fos expression in the striatum and nucleus accumbens

It is generally believed that haloperidol exerts its motor side effects and therapeutic effects mainly by antagonizing dopamine D(2) receptors in the striatum and the nucleus accumbens, respectively. Several neurotransmitters/modulators, including glutamate, acetylcholine, adenosine and histamine, affect dopaminergic activity in these centers. We have recently shown that N-methyl-D-aspartate receptor-mediated modulation of haloperidol-induced c-fos expression differs in functionally specific regions of the striatum and the nucleus accumbens. In the present study, the entire striatum and the nucleus accumbens were comprehensively examined for the pattern of modulation of haloperidol-induced c-fos expression by adenosine A(2), histamine H(3) and muscarinic receptor antagonists. Blockade of muscarinic and H(3) receptors resulted in a profound suppression of haloperidol-induced c-fos expression in the dorsolateral part of the striatum. In addition, the H(3) receptor antagonist suppressed the effects of haloperidol in the ventrolateral aspect of the striatum and the rostral parts of the medial striatum. Muscarinic receptor antagonists suppressed haloperidol-induced c-fos expression throughout the shell and in the mid-level of the core of the nucleus accumbens while A(2) and H(3) receptor antagonists did not.We found that the muscarinic and H(3) receptor antagonists suppress the induction of c-fos by haloperidol in the dorsolateral aspect of the striatum, an area implicated in the development of extrapyramidal motor symptoms following chronic haloperidol treatment. By contrast, haloperidol-induced c-fos expression in the nucleus accumbens, an area implicated in the therapeutic effects of haloperidol, was suppressed by the muscarinic receptor antagonist, but not by the H(3) receptor antagonist. Therefore we conclude that H(3) receptor modulation may provide a useful therapeutic target in future efforts to minimize neuroleptic-induced motor side effects.

A sequential protocol combining dual neuroanatomical tract-tracing with the visualization of local circuit neurons within the striatum

We describe here an experimental approach designed to aid in the identification of complex brain circuits within the rat corpus striatum. Our aim was to characterize in a single section (i) striatal thalamic afferents, (ii) striatopallidal projection neurons and (iii) striatal local circuit interneurons. To this end, we have combined anterograde tracing using biotinylated dextran amine and retrograde neuroanatomical tracing with Fluoro-Gold. This dual tracing protocol was further implemented with the visualization of different subpopulations of striatal interneurons. The subsequent use of three different peroxidase substrates enabled us to unequivocally detect structures that were labeled within a three-color paradigm.

Decreased choline acetyltransferase immunoreactivity in discrete striatal subregions following chronic haloperidol in rats

Neuronal loss within the basal ganglia has been hypothesized to play a role in movement disorders (e.g., tardive dyskinesia) that often occur following chronic neuroleptic treatment. Previous studies in animal models have provided some support to this possibility, but have not assessed regionally specific changes after chronic neuroleptic administration. The present study examined whether counts of neurons containing acetylcholine, described as large aspiny type II neurons, were altered in subregions of the corpus striatum and nucleus accumbens following chronic haloperidol administration in rats. Rats were administered haloperidol decanoate (21 mg/kg, i.m.) or vehicle every third week for 24 weeks. Following 4 weeks of withdrawal from the drug, predefined regions were examined for choline acetyltransferase (ChAT) immunoreactive (ir) cells. Compared to the vehicle group, the haloperidol group showed significant reductions in ChAT-ir cell counts in the ventrolateral striatum, nucleus accumbens core, and nucleus accumbens lateral shell. No significant differences were found in the other regions examined: dorsolateral striatum, dorsomedial striatum, ventromedial striatum, nucleus accumbens medial shell, and horizontal limb of the diagonal band. These findings indicate that there may be regionally specific alterations in ChAT-ir cells following chronic haloperidol treatment, supporting previous hypotheses of striatal cholinergic cell loss resulting from chronic neuroleptic treatment. More importantly, the regions affected (ventrolateral striatum and nucleus accumbens) are critical in the regulation of oral movements, thus suggesting that alterations in cholinergic cell activity, and perhaps actual loss of cholinergic cells in these regions, may be important in the manifestation of late-onset oral dyskinesia.

Chemical anatomy of striatal interneurons in normal individuals and in patients with Huntington's disease

This paper reviews the major anatomical and chemical features of the various types of interneurons in the human striatum, as detected by immunostaining procedures applied to postmortem tissue from normal individuals and patients with Huntington's disease (HD). The human striatum harbors a highly pleomorphic population of aspiny interneurons that stain for either a calcium-binding protein (calretinin, parvalbumin or calbindin D-28k), choline acetyltransferase (ChAT) or NADPH-diaphorase, or various combinations thereof. Neurons that express calretinin (CR), including multitudinous medium and a smaller number of large neurons, are by far the most abundant interneurons in the human striatum. The medium CR+ neurons do not colocalize with any of the known chemical markers of striatal neurons, except perhaps GABA, and are selectively spared in HD. Most large CR+ interneurons display ChAT immunoreactivity and also express substance P receptors. The medium and large CR+ neurons are enriched with glutamate receptor subunit GluR2 and GluR4, respectively. This difference in AMPA GluR subunit expression may account for the relative resistance of medium CR+ neurons to glutamate-mediated excitotoxicity that may be involved in HD. The various striatal chemical markers display a highly heterogeneous distribution pattern in human. In addition to the classic striosomes/matrix compartmentalization, the striosomal compartment itself is composed of a core and a peripheral region, each subdivided by distinct subsets of striatal interneurons. A proper knowledge of all these features that appear unique to humans should greatly help our understanding of the organization of the human striatum in both health and disease states.

Cortical inputs to m2-immunoreactive striatal interneurons in rat and monkey

Previous anatomical studies have been unsuccessful in demonstrating significant cortical inputs to cholinergic and somatostatinergic striatal interneurons in rats. On the other hand, electrophysiological studies have shown that cortical stimulation induces monosynaptic EPSPs in cholinergic interneurons. It has been proposed that the negative anatomical findings might have been the result of incomplete labeling of distal dendrites. In the present study, we reinvestigated this issue using m2 muscarinic receptor antibodies as a selective marker for cholinergic and somatostatinergic interneurons in the striatum. This was combined with injections of either the anterograde tracer biotinylated dextran amine (BDA) in the monkey prefrontal cortex or aspiration lesion of the sensorimotor cortex in rats. The results showed that, in both species, a small percentage (1-2%) of cortical terminals make asymmetric synaptic contacts with m2-immunoreactive interneurons in the striatum. Interestingly, the majority of these synapses are onto small dendritic spines or spine-like appendages, as opposed to dendritic shafts and/or cell bodies. Thus, m2-containing striatal interneurons do receive direct cortical inputs and can, therefore, integrate and modulate cortical information flow through the striatum. Although the density of cortical terminals in contact with individual striatal interneurons is likely to be relatively low compared to the massive cortical input to projection neurons, both cholinergic and somatostatinergic interneurons display intrinsic properties that allow even small and distal inputs to influence their overall state of neuronal activity.

The expression of the calcium binding protein calretinin in the rat striatum: effects of dopamine depletion and L-DOPA treatment

The activity of the striatum is regulated by glutamate and dopamine neurotransmission. Consequent to striatal dopamine depletion the corticostriatal excitatory input is increased, which in turn can raise intracellular calcium levels. We investigated changes in the neuronal expression of the calcium binding protein calretinin related to dopamine depletion and l-DOPA administration. Immunohistochemical methods were used to assess calretinin in the striatum of rats with unilateral lesions of the nigrostriatal system. In these animals we observed a loss of the patchy distribution of calretinin fibers. Moreover, after dopaminergic depletion we detected two new, not previously described, calretinin cell types, the presence of which could be related to morphological changes induced by loss of a dopaminergic input. We also found an increase in the number of calretinin-labeled cells in the striatum ipsilateral to the lesion compared to the contralateral striatum or to the striatum of normal rats. This increase was mostly evident at 3 weeks postlesion and tended to decrease toward normal levels at 6, 10, and 18 weeks postlesion. In unlesioned animals, l-DOPA administration did not induce changes in the expression of calretinin. In unilaterally lesioned animals, l-DOPA reversed the increase in the number of calretinin-positive cells induced by the lesion. However, chronic l-DOPA administration was less effective than acute l-DOPA in reversing the effect of the lesion. The present data suggests that striatal calretinin neurons are sensitive to dopamine depletion. Increased expression of calretinin in striatal cells may be consequent to enhanced striatal excitatory input.

Control by GABA and tachykinins of the evoked release of acetylcholine in striatal compartments under different modalities of NMDA receptor stimulation

The contribution of endogenously released dopamine, GABA and its co-transmitters, substance P (SP) and neurokinin A (NKA), to the control of the evoked release of acetylcholine was investigated in vitro in the striosomes and the matrix of the rat striatum under various modalities of NMDA receptor stimulation (NMDA 50 microM or 1 mM without or with 10 microM D-serine). Sulpiride, bicuculline, SR140333 and SR48968, the antagonists of D(2), GABA A, NK(1) and NK(2) tachykinin receptors, respectively, were used for this purpose. (1) In both striatal compartments, the dopamine-mediated inhibitory regulation of the evoked release of acetylcholine only occurred when D-serine was co-applied with 50 microM or 1 mM NMDA. (2) In striosomes, the dopamine-dependent inhibitory effects of SP and NKA on the evoked release of acetylcholine only occurred when D-serine was co-applied with 50 microM or 1 mM NMDA. (3) A similar inhibitory regulation by NKA, but not SP, was found in the matrix when 1 mM NMDA was co-applied with D-serine. (4) In contrast, the dopamine-dependent facilitatory effect of GABA on the evoked release of acetylcholine did not require added D-serine and was more important with 1 mM than 50 microM NMDA. In the presence of D-serine, and depending on the NMDA concentration, the facilitatory regulation of GABA was reduced (matrix) or suppressed (striosomes). This latter effect was partially restored in the presence of SR48968. Therefore, the dopamine-dependent inhibitory effects of tachykinins on the evoked release of acetylcholine only occurred when NMDA receptors were stimulated in the presence of saturating concentrations of D-serine.

The synaptic framework for chemical signaling in nucleus accumbens

Our knowledge of the organization of the nucleus accumbens has been greatly advanced in the last two decades, but only now are we beginning to understand the complex neural circuitry that underlies the mix of behaviors attributed to this nucleus. Superimposed on the neurochemically defined territories of the shell and core are four or more conduits for information flow. Each of these behaviorally relevant pathways can be characterized by the spatial distribution of inputs to its central unit: the GABAergic projection neuron, a spiny cell that also contains the opioid peptides, enkephalin or dynorphin. In this review, current models of accumbal circuits will be examined and, with the aid of recent anatomical findings, further extended to shed light on how functionally diverse information is processed in this nucleus. However complex, accumbal wiring is not fixed, and, as we will show, psychostimulants, dopamine-deleting lesions, and chronic blockade of dopaminergic receptors can alter the anatomical substrate, synaptology, and neurotrophic factors that govern circuits through the shell and core.

Thalamic input to parvalbumin-immunoreactive GABAergic interneurons: organization in normal striatum and effect of neonatal decortication

The neocortex and thalamus send dense glutaminergic projections to the neostriatum. The neocortex makes synaptic contact with spines of striatal projection neurons, and also targets a distinct class of GABAergic interneurons immunoreactive for the calcium-binding protein parvalbumin. We determined whether the parafascicular thalamic nucleus also targets striatal parvalbumin-immunoreactive interneurons. The anterograde tracer biotinylated dextranamine was injected into the parafascicular nucleus of adult rats. Double-labeled histochemistry/immunohistochemistry revealed overlapping thalamic fibers and parvalbumin-immunoreactive neurons in the neostriatum. Areas of overlap within the sensorimotor striatum were analysed by electron microscopy. Of 311 synaptic boutons originating from the parafascicular nucleus, 75.9% synapsed with unlabeled dendrites, 22.5% with unlabeled spines, and 1.3% had parvalbumin-immunoreactive dendrites as a postsynaptic target. Only 4% of all asymmetric synapses on parvalbumin-immunoreactive dendrites were derived from the parafascicular nucleus. A separate group of animals underwent bilateral neocortical deafferentation on the third postnatal day, prior to injection of anterograde tracer into the parafascicular nucleus of adult animals. These experiments were performed with the dual purpose of (i) reducing the possibility that thalamic inputs to parvalbumin-immunoreactive neurons are the result of transsynaptic uptake of tracer by a thalamo-cortico-striatal route, and (ii) determining whether competitive interactions between developing corticostriatal and thalamostriatal fibers may account for the relatively sparse thalamic input onto parvalbumin-immunoreactive interneurons. In decorticates, 219 striatal synaptic contacts derived from the parafascicular nucleus, out of which 77.2% were on unlabeled dendrites, 20.9% were upon unlabeled spines, and 0.9% targeted parvalbumin-immunoreactive dendrites. We conclude that the thalamic parafascicular nucleus indeed sends synaptic input to parvalbumin-immunoreactive striatal neurons. Parafascicular nucleus inputs to striatal parvalbumin-immunoreactive interneurons are sparse in comparison to other asymmetric inputs, most of which are likely to be of cortical origin. The synaptic profile of thalamostriatal inputs to parvalbumin-immunoreactive neurons and unlabeled elements is unchanged following neonatal decortication. This suggests that competitive interaction between developing thalamostriatal and corticostriatal projections is not a major mechanism determining synaptic input to striatal subpopulations.

Diverse pre- and post-synaptic expression of m1-m4 muscarinic receptor proteins in neurons and afferents in the rat neostriatum

We have utilized subtype specific antibodies to determine the cellular and subcellular distributions of the muscarinic acetylcholine receptor subtypes that are highly expressed in the rat striatum (m1-m4). Each receptor is expressed in distinct populations of striatal neurons in the relative proportions predicted by their mRNAs. They concentrate at post-synaptic sites and each of the four subtypes are also transported to pre-synaptic sites. m2 appears to be the only presynaptic autoreceptor in the striatum, but it is also localized in non-cholinergic terminals. These distinct pre- and post-synaptic localizations suggest that muscarinic receptor subtype diversity evolved to enable increasingly complex responses to acetylcholine release.

Role of arachidonic acid in the regulation of the NMDA-evoked release of acetylcholine in striatal compartments

The role of endogenously released arachidonic acid in the control of the NMDA (50 microM)-evoked release of [3H]-acetylcholine previously formed from [3H]-choline was investigated in striosome-enriched areas and in the matrix of the rat striatum using a microsuperfusion procedure in vitro. Experiments were performed with either mepacrine (0.2 microM) or bovine serum albumin (BSA, 0.02%) which inhibits phospholipase A2 activity or binds endogenously released arachidonic acid, respectively. Both treatments similarly reduce the NMDA-evoked release of [3H]-acetylcholine, this effect being more pronounced in striosomes than in the matrix. These reductions result from a facilitation of dopamine release, since they were not observed in the presence of (-)sulpiride, the D2 dopamine receptor antagonist. Moreover, the superfusion with BSA was shown to enhance the release of [3H]-dopamine (formed from [3H]-tyrosine), this effect being of larger amplitude in striosomes than in the matrix. In control conditions, due to the blockade of the presynaptic inhibitory effect of GABA on dopamine release, bicuculline (GABA(A) receptor antagonist) reduces the NMDA-evoked release of [3H]-acetylcholine in both striatal compartments. Bicuculline was no longer effective following superfusions with either mepacrine or BSA, suggesting that these treatments eliminate the GABAergic presynaptic inhibitory control on dopamine transmission and thus lead to the dopamine-mediated inhibition of [3H]-acetylcholine release. These results indicate that arachidonic acid endogenously formed under weak stimulation of NMDA receptors contributes to the regulation of the evoked release of [3H]-acetylcholine by facilitating GABAergic transmission and that this process is more important in striosomes than in the matrix.