Heterogeneity and diversity of striatal GABAergic interneurons

Acetylcholine-based entropy in response selection: a model of how striatal interneurons modulate exploration, exploitation, and response variability in decision-making

The basal ganglia play a fundamental role in decision-making. Their contribution is typically modeled within a reinforcement learning framework, with the basal ganglia learning to select the options associated with highest value and their dopamine inputs conveying performance feedback. This basic framework, however, does not account for the role of cholinergic interneurons in the striatum, and does not easily explain certain dynamic aspects of decision-making and skill acquisition like the generation of exploratory actions. This paper describes basal ganglia acetylcholine-based entropy (BABE), a model of the acetylcholine system in the striatum that provides a unified explanation for these phenomena. According to this model, cholinergic interneurons in the striatum control the level of variability in behavior by modulating the number of possible responses that are considered by the basal ganglia, as well as the level of competition between them. This mechanism provides a natural way to account for the role of basal ganglia in generating behavioral variability during the acquisition of certain cognitive skills, as well as for modulating exploration and exploitation in decision-making. Compared to a typical reinforcement learning model, BABE showed a greater modulation of response variability in the face of changes in the reward contingences, allowing for faster learning (and re-learning) of option values. Finally, the paper discusses the possible applications of the model to other domains.

A novel functionally distinct subtype of striatal neuropeptide Y interneuron

We investigated the properties of neostriatal neuropeptide Y (NPY)-expressing interneurons in transgenic GFP (green fluorescent protein)-NPY reporter mice. In vitro whole-cell recordings and biocytin staining demonstrated the existence of a novel class of neostriatal NPY-expressing GABAergic interneurons that exhibit electrophysiological, neurochemical, and morphological properties strikingly different from those of previously described NPY-containing, plateau-depolarization low-threshold spike (NPY-PLTS) interneurons. The novel NPY interneuron type (NPY-neurogliaform) differed from previously described NPY-PLTS interneurons by exhibiting a significantly lower input resistance and hyperpolarized membrane potential, regular, nonaccommodating spiking in response to depolarizing current injections, and an absence of plateau depolarizations or low-threshold spikes. NPY-neurogliaform interneurons were also easily distinguished morphologically by their dense, compact, and highly branched dendritic and local axonal arborizations that contrasted sharply with the sparse and extended axonal and dendritic arborizations of NPY-PLTS interneurons. Furthermore, NPY-neurogliaform interneurons did not express immunofluorescence for somatostatin or nitric oxide synthase that was ubiquitous in NPY-PLTS interneurons. IPSP/Cs could only rarely be elicited in spiny projection neurons (SPNs) in paired recordings with NPY-PLTS interneurons. In contrast, the probability of SPN innervation by NPY-neurogliaform interneurons was extremely high, the synapse very reliable (no failures were observed), and the resulting postsynaptic response was a slow, GABA(A) receptor-mediated IPSC that has not been previously described in striatum but that has been elicited from NPY-GABAergic neurogliaform interneurons in cortex and hippocampus. These properties suggest unique and distinctive roles for NPY-PLTS and NPY-neurogliaform interneurons in the integrative properties of the neostriatum.

Muscarinic modulation of striatal function and circuitry

Striatal cholinergic interneurons are pivotal modulators of the striatal circuitry involved in action selection and decision making. Although nicotinic receptors are important transducers of acetylcholine release in the striatum, muscarinic receptors are more pervasive and have been more thoroughly studied. In this review, the effects of muscarinic receptor signaling on the principal cell types in the striatum and its canonical circuits will be discussed, highlighting new insights into their role in synaptic integration and plasticity. These studies, and those that have identified new circuit elements driven by activation of nicotinic receptors, make it clear that temporally patterned activity in cholinergic interneurons must play an important role in determining the effects on striatal circuitry. These effects could be critical to the response to salient environmental stimuli that serve to direct behavior.

Integrating neurotransmission in striatal medium spiny neurons

The striatum is a major entry structure of the basal ganglia. Its role in information processing in close interaction with the cerebral cortex and thalamus has various behavioral consequences depending on the regions concerned, including control of body movements and motivation. A general feature of striatal information processing is the control by reward-related dopamine signals of glutamatergic striatal inputs and of their plasticity. This relies on specific sets of receptors and signaling proteins in medium-sized spiny neurons which belong to two groups, striatonigral and striatopallidal neurons. Some signaling pathways are activated only by dopamine or glutamate, but many provide multiple levels of interactions. For example, the cAMP pathway is mostly regulated by dopamine D1 receptors in striatonigral neurons, whereas the ERK pathway detects a combination of glutamate and dopamine signals and is essential for long-lasting modifications. These adaptations require changes in gene expression, and the signaling pathways linking synaptic activity to nuclear function and epigenetic changes are beginning to be deciphered. Their alteration underlies many aspects of striatal dysfunction in pathological conditions which include a decrease or an increase in dopamine transmission, as encountered in Parkinson's disease or exposure to addictive drugs, respectively.

Early minocycline treatment prevents a decrease in striatal dopamine in an SIV model of HIV-associated neurological disease

HIV-infected individuals, even with antiretroviral therapy, often display cognitive, behavioral and motor abnormalities and have decreased dopamine (DA) levels. Minocycline prevents encephalitis and neurodegeneration in SIV models, suggesting that it might also protect against nigrostriatal dopaminergic system dysfunction. Using an SIV/macaque model of HIV-associated CNS disease, we demonstrated that striatal levels of DA were significantly lower in macaques late in infection and that levels of the metabolite DOPAC also tended to be lower. DA levels declined more than its metabolites, indicating a dysregulation of DA production or catabolism. Minocycline treatment beginning at 12 but not 21 days postinoculation prevented striatal DA loss. DA decline was not due to direct loss of dopaminergic projections to the basal ganglia as there was no difference in tyrosine hydroxylase, dopamine transporter, vesicular monoamine transporter 2 or synaptophysin between minocycline-treated and untreated macaques. SIV-infected macaques had significantly higher monoamine oxidase (MAO) activity than uninfected macaques, although MAO activity was not affected by minocycline. Oxidative/nitrosative stress was examined by nitrotyrosine staining in the deep white matter and was lower in SIV-infected, minocycline-treated macaques compared with untreated macaques. These data suggest that minocycline, which has antioxidant activity, has a protective effect on DA homeostasis when administered at an appropriate time in SIV neuropathogenesis.

Opioidergic interactions between striatal projection neurons

Medium spiny striatal projection neurons (MSNs) release opioid neuropeptides, but the role of these neurotransmitters is still poorly understood. While presynaptic inhibition of corticostriatal axons by opioid receptors has been demonstrated using exogenous ligands, the action of synaptically released opioids in the striatum has not been investigated. We performed single and paired whole-cell recordings from rat MSNs while corticostriatal fibers were electrically activated. In single recording experiments, we also activated antidromically the axons of a population of MSNs. Corticostriatal fibers were stimulated once every 10 s and every other stimulation was preceded by 5 antidromic spikes (at 100 Hz). This burst of antidromic spikes produced robust inhibition of evoked corticostriatal responses. This inhibition was not affected by the δ-opioid receptor antagonist SDM25N, but was completely abolished by the μ-opioid receptor antagonist CTOP. Inhibitory effects were maximal (on average 29.6 ± 11.4%) when the burst preceded the corticostriatal stimulation by 500 ms and became undetectable for intervals >2 s. Paired recordings from MSNs located <100 μm apart revealed that, in 30 of 56 (54%) pairs, a burst of five action potentials in one of the MSNs caused significant inhibition (17.1 ± 5.7%) of evoked glutamatergic responses in the other MSN. In 5 of these pairs, reciprocal inhibition of corticostriatal inputs was present. These effects were maximal 500 ms after the burst and were completely blocked by CTOP. Thus, these results reveal a novel, strong opioid-mediated communication between MSNs and provide a new cellular substrate for competitive dynamics in the striatum.

Bidirectional plasticity in striatonigral synapses: a switch to balance direct and indirect basal ganglia pathways

There is no hypothesis to explain how direct and indirect basal ganglia (BG) pathways interact to reach a balance during the learning of motor procedures. Both pathways converge in the substantia nigra pars reticulata (SNr) carrying the result of striatal processing. Unfortunately, the mechanisms that regulate synaptic plasticity in striatonigral (direct pathway) synapses are not known. Here, we used electrophysiological techniques to describe dopamine D(1)-receptor-mediated facilitation in striatonigral synapses in the context of its interaction with glutamatergic inputs, probably coming from the subthalamic nucleus (STN) (indirect pathway) and describe a striatonigral cannabinoid-dependent long-term synaptic depression (LTD). It is shown that striatonigral afferents exhibit D(1)-receptor-mediated facilitation of synaptic transmission when NMDA receptors are inactive, a phenomenon that changes to cannabinoid-dependent LTD when NMDA receptors are active. This interaction makes SNr neurons become coincidence-detector switching ports: When inactive, NMDA receptors lead to a dopamine-dependent enhancement of direct pathway output, theoretically facilitating movement. When active, NMDA receptors result in LTD of the same synapses, thus decreasing movement. We propose that SNr neurons, working as logical gates, tune the motor system to establish a balance between both BG pathways, enabling the system to choose appropriate synergies for movement learning and postural support.

Modulation of striatal projection systems by dopamine

The basal ganglia are a chain of subcortical nuclei that facilitate action selection. Two striatal projection systems--so-called direct and indirect pathways--form the functional backbone of the basal ganglia circuit. Twenty years ago, investigators proposed that the striatum's ability to use dopamine (DA) rise and fall to control action selection was due to the segregation of D(1) and D(2) DA receptors in direct- and indirect-pathway spiny projection neurons. Although this hypothesis sparked a debate, the evidence that has accumulated since then clearly supports this model. Recent advances in the means of marking neural circuits with optical or molecular reporters have revealed a clear-cut dichotomy between these two cell types at the molecular, anatomical, and physiological levels. The contrast provided by these studies has provided new insights into how the striatum responds to fluctuations in DA signaling and how diseases that alter this signaling change striatal function.

Removal of GABA(A) receptor γ2 subunits from parvalbumin neurons causes wide-ranging behavioral alterations

We investigated the behavioral significance of fast synaptic inhibition by αβγ2-type GABA_A receptors on parvalbumin (Pv) cells. The GABA_A receptor γ2 subunit gene was selectively inactivated in Pv-positive neurons by Cre/loxP recombination. The resulting Pv-Δγ2 mice were relatively healthy in the first postnatal weeks; but then as Cre started to be expressed, the mice progressively developed wide-ranging phenotypic alterations including low body weight, motor deficits and tremor, decreased anxiety levels, decreased pain sensitivity and deficient prepulse inhibition of the acoustic startle reflex and impaired spatial learning. Nevertheless, the deletion was not lethal, and mice did not show increased mortality even after one year. Autoradiography with t-butylbicyclophosphoro[³⁵S]thionate suggested an increased amount of GABA_A receptors with only α and β subunits in central nervous system regions that contained high levels of parvalbumin neurons. Using BAC-transgenesis, we reduced some of the Pv-Δγ2 phenotype by selectively re-expressing the wild-type γ2 subunit back into some Pv cells (reticular thalamic neurons and cerebellar Pv-positive neurons). This produced less severe impairments of motor skills and spatial learning compared with Pv-Δγ2 mice, but all other deficits remained. Our results reveal the widespread significance of fast GABAergic inhibition onto Pv-positive neurons for diverse behavioral modalities, such as motor coordination, sensorimotor integration, emotional behavior and nociception.

The ionic mechanism of gamma resonance in rat striatal fast-spiking neurons

Striatal fast-spiking (FS) cells in slices fire in the gamma frequency range and in vivo are often phase-locked to gamma oscillations in the field potential. We studied the firing patterns of these cells in slices from rats ages 16-23 days to determine the mechanism of their gamma resonance. The resonance of striatal FS cells was manifested as a minimum frequency for repetitive firing. At rheobase, cells fired a doublet of action potentials or doublets separated by pauses, with an instantaneous firing rate averaging 44 spikes/s. The minimum rate for sustained firing was also responsible for the stuttering firing pattern. Firing rate adapted during each episode of firing, and bursts were terminated when firing was reduced to the minimum sustainable rate. Resonance and stuttering continued after blockade of Kv3 current using tetraethylammonium (0.1-1 mM). Both gamma resonance and stuttering were strongly dependent on Kv1 current. Blockade of Kv1 channels with dendrotoxin-I (100 nM) completely abolished the stuttering firing pattern, greatly lowered the minimum firing rate, abolished gamma-band subthreshold oscillations, and slowed spike frequency adaptation. The loss of resonance could be accounted for by a reduction in potassium current near spike threshold and the emergence of a fixed spike threshold. Inactivation of the Kv1 channel combined with the minimum firing rate could account for the stuttering firing pattern. The resonant properties conferred by this channel were shown to be adequate to account for their phase-locking to gamma-frequency inputs as seen in vivo.

Dopaminergic modulation of striatal neurons, circuits, and assemblies

In recent years, there has been a great deal of progress toward understanding the role of the striatum and dopamine in action selection. The advent of new animal models and the development of optical techniques for imaging and stimulating select neuronal populations have provided the means by which identified synapses, cells, and circuits can be reliably studied. This review attempts to summarize some of the key advances in this broad area, focusing on dopaminergic modulation of intrinsic excitability and synaptic plasticity in canonical microcircuits in the striatum as well as recent work suggesting that there are neuronal assemblies within the striatum devoted to particular types of computation and possibly action selection.

Neurochemical characterization of the tree shrew dorsal striatum

The striatum is a major component of the basal ganglia and is associated with motor and cognitive functions. Striatal pathologies have been linked to several disorders, including Huntington’s, Tourette’s syndrome, obsessive–compulsive disorders, and schizophrenia. For the study of these striatal pathologies different animal models have been used, including rodents and non-human primates. Rodents lack on morphological complexity (for example, the lack of well defined caudate and putamen nuclei), which makes it difficult to translate data to the human paradigm. Primates, and especially higher primates, are the closest model to humans, but there are ever-increasing restrictions to the use of these animals for research. In our search for a non-primate animal model with a striatum that anatomically (and perhaps functionally) can resemble that of humans, we turned our attention to the tree shrew. Evolutionary genetic studies have provided strong data supporting that the tree shrews (Scadentia) are one of the closest groups to primates, although their brain anatomy has only been studied in detail for specific brain areas. Morphologically, the tree shrew striatum resembles the primate striatum with the presence of an internal capsule separating the caudate and putamen, but little is known about its neurochemical composition. Here we analyzed the expression of calcium-binding proteins, the presence and distribution of the striosome and matrix compartments (by the use of calbindin, tyrosine hydroxylase, and acetylcholinesterase immunohistochemistry), and the GABAergic system by immunohistochemistry against glutamic acid decarboxylase and Golgi impregnation. In summary, our results show that when compared to primates, the tree shrew dorsal striatum presents striking similarities in the distribution of most of the markers studied, while presenting some marked divergences when compared to the rodent striatum.

Enhanced high-frequency membrane potential fluctuations control spike output in striatal fast-spiking interneurones in vivo

Fast-spiking interneurones (FSIs) constitute a prominent part of the inhibitory microcircuitry of the striatum; however, little is known about their recruitment by synaptic inputs in vivo. Here, we report that, in contrast to cholinergic interneurones (CINs), FSIs (n = 9) recorded in urethane-anaesthetized rats exhibit Down-to-Up state transitions very similar to spiny projection neurones (SPNs). Compared to SPNs, the FSI Up state membrane potential was noisier and power spectra exhibited significantly larger power at frequencies in the gamma range (55-95 Hz). The membrane potential exhibited short and steep trajectories preceding spontaneous spike discharge, suggesting that fast input components controlled spike output in FSIs. Spontaneous spike data contained a high proportion (43.6 ± 32.8%) of small inter-spike intervals (ISIs) of <30 ms, setting FSIs clearly apart from SPNs and CINs. Cortical-evoked inputs had slower dynamics in SPNs than FSIs, and repetitive stimulation entrained SPN spike output only if the stimulation was delivered at an intermediate frequency (20 Hz), but not at a high frequency (100 Hz). Pharmacological induction of an activated ECoG state, known to promote rapid FSI spiking, mildly increased the power (by 43 ± 55%, n = 13) at gamma frequencies in the membrane potential of SPNs, but resulted in few small ISIs (<30 ms; 4.3 ± 6.4%, n = 8). The gamma frequency content did not change in CINs (n = 8). These results indicate that FSIs are uniquely responsive to high-frequency input sequences. By controlling the spike output of SPNs, FSIs could serve gating of top-down signals and long-range synchronisation of gamma-oscillations during behaviour.

An Improved BAC Transgenic Fluorescent Reporter Line for Sensitive and Specific Identification of Striatonigral Medium Spiny Neurons

The development of BAC transgenic mice expressing promoter-specific fluorescent reporter proteins has been a great asset for neuroscience by enabling detection of neuronal subsets in live tissue. For the study of basal ganglia physiology, reporters driven by type 1 and 2 dopamine receptors have been particularly useful for distinguishing the two classes of striatal projection neurons – striatonigral and striatopallidal. However, emerging evidence suggests that some of the transgenic reporter lines may have suboptimal features. The ideal transgenic reporter line should (1) express a reporter with high sensitivity and specificity for detecting the cellular subset of interest and that does not otherwise alter the biology of the cells in which it is expressed, and (2) involve a genetic manipulation that does not cause any additional genetic effects other than expression of the reporter. Here we introduce a new BAC transgenic reporter line, Drd1a-tdTomato line 6, with features that approximate these ideals, offering substantial benefits over existing lines. In this study, we investigate the integrity of dopamine-sensitive behaviors and test the sensitivity and specificity of tdTomato fluorescence for identifying striatonigral projection neurons in mice. Behaviorally, hemizygous Drd1a-tdTomato line 6 mice are similar to littermate controls; while hemizygous Drd2-EGFP mice are not. In characterizing the sensitivity and specificity of line 6 mice, we find that both are high. The results of this characterization indicate that line 6 Drd1a-tdTomato+/− mice offer a useful alternative approach to identify both striatonigral and striatopallidal neurons in a single transgenic line with a high degree of accuracy.

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.