Cholinergic neurons of the rat and primate striatum are morphologically different

Targeted Transgene Expression in Cholinergic Interneurons in the Monkey Striatum Using Canine Adenovirus Serotype 2 Vectors

The striatum, the main input structure of the basal ganglia, is critical for action selection and adaptive motor control. To understand the neuronal mechanisms underlying these functions, an analysis of microcircuits that compose the striatum is necessary. Among these, cholinergic interneurons (ChIs) provide intrinsic striatal innervation whose dysfunction is implicated in neuropsychiatric diseases, such as Parkinson’s disease and Tourette syndrome. The ability to experimentally manipulate the activity of ChIs is critical to gain insights into their contribution to the normal function of the striatum and the emergence of behavioral abnormalities in pathological states. In this study, we generated and tested CAV-pChAT-GFP, a replication-defective canine adenovirus type 2 (CAV-2) vector carrying the green fluorescent protein (GFP) sequence under the control of the human choline acetyltransferase (ChAT) promoter. We first tested the potential specificity of CAV-pChAT-GFP to label striatal ChIs in a rat before performing experiments on two macaque monkeys. In the vector-injected rat and monkey striatum, we found that GFP expression preferentially colocalized with ChAT-immunoreactivity throughout the striatum, including those from local circuit interneurons. CAV-2 vectors containing transgene driven by the ChAT promoter provide a powerful tool for investigating ChI contributions to circuit function and behavior in nonhuman primates.

Thalamic degeneration in MPTP-treated Parkinsonian monkeys: impact upon glutamatergic innervation of striatal cholinergic interneurons

In both Parkinson's disease (PD) patients and MPTP-treated non-human primates, there is a profound neuronal degeneration of the intralaminar centromedian/parafascicular (CM/Pf) thalamic complex. Although this thalamic pathology has long been established in PD (and other neurodegenerative disorders), the impact of CM/Pf cell loss on the integrity of the thalamo-striatal glutamatergic system and its regulatory functions upon striatal neurons remain unknown. In the striatum, cholinergic interneurons (ChIs) are important constituents of the striatal microcircuitry and represent one of the main targets of CM/Pf-striatal projections. Using light and electron microscopy approaches, we have analyzed the potential impact of CM/Pf neuronal loss on the anatomy of the synaptic connections between thalamic terminals (vGluT2-positive) and ChIs neurons in the striatum of parkinsonian monkeys treated chronically with MPTP. The following conclusions can be drawn from our observations: (1) as reported in PD patients, and in our previous monkey study, CM/Pf neurons undergo profound degeneration in monkeys chronically treated with low doses of MPTP. (2) In the caudate (head and body) nucleus of parkinsonian monkeys, there is an increased density of ChIs. (3) Despite the robust loss of CM/Pf neurons, no significant change was found in the density of thalamostriatal (vGluT2-positive) terminals, and in the prevalence of vGluT2-positive terminals in contact with ChIs in parkinsonian monkeys. These findings provide new information about the state of thalamic innervation of the striatum in parkinsonian monkeys with CM/Pf degeneration, and bring up an additional level of intricacy to the consequences of thalamic pathology upon the functional microcircuitry of the thalamostriatal system in parkinsonism. Future studies are needed to assess the importance of CM/Pf neuronal loss, and its potential consequences on the neuroplastic changes induced in the synaptic organization of the thalamostriatal system, in the development of early cognitive impairments in PD.

The role of the intrinsic cholinergic system of the striatum: What have we learned from TAN recordings in behaving animals?

Cholinergic interneurons provide rich local innervation of the striatum and play an important role in controlling behavior, as evidenced by the variety of movement and psychiatric disorders linked to disrupted striatal cholinergic transmission. Much progress has been made in recent years regarding our understanding of how these interneurons contribute to the processing of information in the striatum. In particular, investigation of the activity of presumed striatal cholinergic interneurons, identified as tonically active neurons or TANs in behaving animals, has pointed to their role in the signaling and learning of the motivational relevance of environmental stimuli. Although the bulk of this work has been conducted in monkeys, several studies have also been carried out in behaving rats, but information remains rather disparate across studies and it is still questionable whether rodent TANs correspond to TANs described in monkeys. Consequently, our current understanding of the function of cholinergic transmission in the striatum is challenged by the rapidly growing, but often confusing literature on the relationship between TAN activity and specific behaviors. As regards the precise nature of the information conveyed by the cholinergic TANs, a recent influential view emphasized that these local circuit neurons may play a special role in the processing of contextual information that is important for reinforcement learning and selection of appropriate actions. This review provides a summary of recent progress in TAN physiology from which it is proposed that striatal cholinergic interneurons are crucial elements for flexible switching of behaviors under changing environmental conditions.

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.

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.

Functional properties of striatal fast-spiking interneurons

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

Uncoordinated firing rate changes of striatal fast-spiking interneurons during behavioral task performance

Basal ganglia circuits make key contributions to decision making. Distributed, synchronous feedforward inhibition of striatal medium spiny neurons by fast-spiking GABAergic interneurons (FSIs) has been argued to be important for the suppression of unwanted actions, and a deficit in FSIs has been found in human patients with Tourette syndrome. However, no studies have yet examined how striatal FSIs change their activity during behavioral tasks. Here I describe 36 presumed striatal FSIs recorded in rats during well practiced performance of a radial maze win-stay task. Although most FSIs showed robust task-related activity, the temporal patterns of firing rate change were highly idiosyncratic. In contrast to other classes of striatal neurons, FSIs showed little or no coordinated population response to major task events such as instruction cues or rewards. Even when multiple FSIs were recorded simultaneously from the same local region of striatum, firing rate changes were dissimilar, and no clear evidence for synchronous firing was found using cross-correlograms (18 FSI pairs examined). These results suggest that FSIs play a more complex role in the information processing achieved by striatal microcircuits than supposed by current theoretical models.

Effects of haloperidol and clozapine on Fos expression in the primate striatum

In cynomolgus monkeys, the typical neuroleptic haloperidol induced strong expression of the immediate early gene product Fos in both the nucleus accumbens shell and the dorsal striatum. In the caudate nucleus, haloperidol induced staining was more marked in the striosomes than the matrix. The atypical neuroleptic clozapine also induced Fos expression in the nucleus accumbens, but, in contrast to haloperidol, had only a small effect in the dorsal striatum. Additionally, clozapine was more potent than haloperidol at inducing Fos-like immunoreactivity in the islands of Calleja. These results are similar to those typically obtained in rodents, and suggest that the basic mechanisms underlying the regional specificity of the effects of atypical neuroleptics are likely be conserved between these two mammalian orders.

Tonically active neurons in the primate striatum and their role in the processing of information about motivationally relevant events

Analysis of recordings of single neuronal activity in the striatum of monkeys engaged in behavioural tasks has shown that tonically active neurons (TANs) can be distinguished by their distinct spontaneous firing and functional properties. As TANs are assumed to be cholinergic interneurons, the study of their physiological characteristics allows us to gain an insight into the role of a particular type of local-circuit neuron in the processing of information at the striatal level. In monkeys performing various behavioural tasks, the change in the activity of TANs, unlike the diversity of task-related activations exhibited by the phasically active population of striatal neurons, involves a transient depression of the tonic firing related to environmental events of motivational significance. Such events include primary rewards and stimuli that have acquired a reward value during associative learning. These neurons also respond to an aversive air puff, indicating that their responsiveness is not restricted to appetitive conditions. Another striking feature of the TANs is that their responses can be modulated by predictions about stimulus timing. Temporal variations in event occurrence have been found to favour the responses of TANs, whereas the responses are diminished or abolished in the presence of external cues that predict the time at which events will occur. These data suggest that the TANs respond as do detectors of motivationally relevant events, but they also demonstrate that these neurons are influenced by predictive information based on past experience with a given temporal context. TANs represent a unique subset of striatal neurons that might serve a modulatory function, monitoring for temporal relationships between environmental events.

Improvement of levodopa induced dyskinesias by thalamic deep brain stimulation is related to slight variation in electrode placement: possible involvement of the centre median and parafascicularis complex

OBJECTIVE

To define the reason why two teams using the same procedure and the same target for deep brain stimulation (DBS) obtained different results on levodopa induced dyskinesias, whereas in both, parkinsonian tremor was improved or totally suppressed.

METHODS

Deep brain stimulation can replace lesions in the surgical treatment of abnormal movements. After 10 years of experience with DBS in Parkinson's disease, a comparison of results between the teams of Lille (A) and Grenoble (B) was carried out, for as long as they used intraoperative ventriculography. Both teams aimed at the same target, the ventralis intermedius nucleus of the thalamus (VIM), but team A found a clear improvement of choreic peak dose dyskinesias, whereas team B did not consistently. Therefore all teleradioanatomical data of both teams were re-examined and compared with the therapeutic effects. Location of 99 monopolar electrodes of thalamic stimulation applied to treat parkinsonian tremor has been retrospectively measured (team A included 21 patients, 22 electrodes; team B included 52 patients, 74 electrodes). Peak dose levodopa dyskinesias were suppressed by DBS in all nine patients of team A, four of which were severely disabling. Only eight out of 32 patients from team B experienced a moderate (four) or clear (four) improvement of dyskinesias, whereas in the remaining 24 patients, dyskinesias were unchanged with stimulation.

RESULTS

The mean centre of team A's electrodes was on average 2.9 mm deeper, more posterior and medial than team B's (t=8.05; p<0.0001). This does not correspond to the coordinates of the VIM, but seems to be closer to those of the centre median and parafascicularis complex (CM-Pf), according to stereotaxic atlases. Considering only the dyskinetic patients, significant differences were found in the electrode position according to the therapeutic effects on levodopa dyskinesias, but they were not related to the team membership. Improvement in levodopa dyskinesias was significantly associated with deeper and more medial placement of electrodes.

CONCLUSION

The retrospective analysis of patients treated with DBS using comparable methodologies provides important information concerning electrode position and therapeutic outcome. The position of the electrode is related to the therapeutic effects of DBS. The results support the hypothesis that patients experiencing an improvement of dyskinesias under DBS are actually stimulated in a structure which is more posterior, more internal, and deeper than the VIM, very close to the CM-Pf. These results are consistent with neuroanatomical and neurophysiological data showing that the CM-Pf is included in the motor circuits of the basal ganglia system and receives an important input from the internal pallidum. This suggests that the CM-Pf could be involved specifically in the pathophysiology of levodopa peak dose dyskinesias.

Prenatal and perinatal striatal injury: a hypothetical cause of attention-deficit-hyperactivity disorder?

Experimental data indicate a particular vulnerability of striatal neurons in the developing brain, and together with the idea that the striatum is important for context recognition and behavior, these data have led the author to search for subtle striatal lesions, in the form of biochemical changes, in children who have suffered perinatal adverse events. Evidence is presented to demonstrate that the composition of metabolites in the striatum is altered, primarily in the form of an elevated level of lactate, in human neonates who have suffered various perinatal disorders, such as germinal matrix hemorrhage, intrauterine growth retardation, and asphyxia. An elevated level of lactate suggests tissue hypoxia, which may interfere with the formation of frontostriatal circuits and may play a role in the pathogenesis of the behavioral disturbances observed in a proportion of children with a history of perinatal adverse events.

Haloperidol induces Fos expression in the globus pallidus and substantia nigra of cynomolgus monkeys

Systemic injections of the dopamine antagonist haloperidol (0.1-2.5 mg/kg) induced a dose dependent increase in Fos-like immunoreactivity (FLI) in the internal segment of the globus pallidus (GPi) and in the substantia nigra (SN) of cynomolgus monkeys. These findings are consistent with models of basal ganglia organization which predict that blockade of dopamine receptors should result in a disinhibition of cells in these structures. In the GPi, labeling was most pronounced along the ventral, lateral and medial borders of the nucleus and none of the pallidal cells expressing FLI were immunopositive for choline acetyltransferase. In the SN, immunoreactive nuclei were concentrated in the pars reticulata and the majority of labeled nigral neurons did not display tyrosine hydroxylase-like immunoreactivity. A small number of cells displaying FLI were also observed in the external pallidal segment, but no labeling was seen in the subthalamic nucleus. These findings indicate that blockade of dopamine receptors induces a characteristic pattern of Fos expression in the primate brain which strongly resembles that previously reported in rodents.

Large neostriatal neurons in humans and their possible role in neuronal networks

The Golgi method was used to study the structure of large neostriatal neurons in adult humans. Four types of large interneurons were found (spider cells, hairy cells, asymmetric fan cells, and giant stretched cells), along with two types of large projection cells (large reticular cells with spines and giant reticular cells with smooth dendrites). The structural features and possible mediators of these cells are discussed, along with their roles in neostriatal neuronal networks and in the development of pathological symptoms in chorea and progressive supranuclear paralysis.

Dopamine receptor-mediated mechanisms involved in the expression of learned activity of primate striatal neurons

To understand the mechanisms by which basal ganglia neurons express acquired activities during and after behavioral learning, selective dopamine (DA) receptor antagonists were applied while recording the activity of striatal neurons in monkeys performing behavioral tasks. In experiment 1, a monkey was trained to associate a click sound with a drop of reward water. DA receptor antagonists were administered by micropressure using a stainless steel injection cannula (300 microm ID) through which a Teflon-coated tungsten wire for recording neuronal activity had been threaded. Responses to sound by tonically active neurons (TANs), a class of neurons in the primate striatum, were recorded through a tungsten wire electrode during the application of either D1- or D2-class DA receptor antagonists (total volume <1 microl, at a rate of 1 microl/5-10 min). Application of the D2-class antagonist, (-)-sulpiride (20 micrograms/microl, 58 mM, pH 6.8), abolished the responses of four of five TANs examined. In another five TANs, neither the D2-class antagonist nor the D1-class antagonists, SCH23390 (10 micrograms/microl, 31 mM, pH 5.7) or cis-flupenthixol (30 micrograms/microl, 59 mM, pH 6.6) significantly suppressed responses. In experiment 2, four- or five-barreled glass microelectrodes were inserted into the striatum. The central barrel was used for extracellular recording of activity of TANs. Each DA receptor antagonist was iontophoretically applied through one of the surrounding barrels. SCH23390 (10 mM, pH 4.5) and (-)-sulpiride (10 mM, pH 4.5) were used. The effects of iontophoresis of both D1- and D2-class antagonists were examined in 40 TANs. Of 40 TANs from which recordings were made, responses were suppressed exclusively by the D2-class antagonist in 19 TANs, exclusively by the D1-class antagonist in 3 TANs, and by both D1- and D2-class antagonists in 7 TANs. When 0.9% NaCl, saline, was applied by pressure (<1 microl) or by iontophoresis (<30 nA) as a control, neither the background discharge rates nor the responses of TANs were significantly influenced. Background discharge rate of TANs was also not affected by D1- or D2-class antagonists applied by either micropressure injection or iontophoresis. It was concluded that the nigrostriatal DA system enables TANs to express learned activity primarily through D2-class and partly through D1-class receptor-mediated mechanisms in the striatum.