The striatum (pl.: striata) or corpus striatum[5] is a cluster of interconnected nuclei that make up the largest structure of the subcortical basal ganglia.[6] The striatum is a critical component of the motor and reward systems; receives glutamatergic and dopaminergic inputs from different sources; and serves as the primary input to the rest of the basal ganglia.
The striatum is the largest structure of the basal ganglia. The striatum is divided into two subdivisions, a ventral striatum and a dorsal striatum, based upon function and connections. It is also divisible into a matrix and embedded striosomes.
The dorsal striatum is composed of the caudate nucleus and the putamen.
Primarily it mediates cognition and involves motor and executive function. The dorsal striatum can be further subdivided into the dorsomedial striatum, and the dorsolateral striatum. Both of these areas have different roles in the acquisition of learnt behaviour and skill formation.[16] The dorsomedial region receives projections from the frontal and the parietal cortices. The dorsolateral region receives projections from the sensorimotor cortex.[17]
Matrix and striosomes
Neurochemistry studies have used staining techniques on the striatum that have identified two distinct striatal compartments, the matrix, and the striosome (or patch). The matrix is seen to be rich in acetylcholinesterase, while the embedded striosomes are acetylcholinesterase-poor.[18] The matrix forms the bulk of the striatum, and receives input from most areas of the cerebral cortex.[19] Clusters of neurons in the matrix, called matrisomes receive a similar input. Their output goes to both regions of the globus pallidus and to the substantia nigra pars reticulata.[19]
The striosomes receive input from the prefrontal cortex and give outputs to the substantia nigra pars compacta.[19] There are more striosomes present in the dorsal striatum making up 10-15% of the striatal volume, than in the ventral striatum.[18]
Cell types
Types of cells in the striatum include:
Medium spiny neurons (MSNs), which are the principal neurons of the striatum.[2] They are GABAergic and, thus, are classified as inhibitory neurons. Medium spiny projection neurons comprise 95% of the total neuronal population of the human striatum.[2] Medium spiny neurons have two characteristic types: D1-type MSNs and D2-type MSNs.[2][4][20] A subpopulation of MSNs contain both D1-type and D2-type receptors, with approximately 40% of striatal MSNs expressing both DRD1 and DRD2mRNA.[2][4][20]
Cholinergicinterneurons release acetylcholine, which has a variety of important effects in the striatum. In humans, other primates, and rodents, these interneurons respond to salient environmental stimuli with stereotyped responses that are temporally aligned with the responses of dopaminergic neurons of the substantia nigra.[21][22] The large aspiny cholinergic interneurons themselves are affected by dopamine through D5 dopamine receptors.[23] Dopamine also directly controls communication between cholinergic interneurons.[24][25]
There are many types of GABAergic interneurons.[26] The best known are parvalbumin expressing interneurons, also known as fast-spiking interneurons, which participate in powerful feedforward inhibition of principal neurons.[27] Also, there are GABAergic interneurons that express tyrosine hydroxylase,[28]somatostatin, nitric oxide synthase and neuropeptide-y. Recently, two types of neuropeptide-y expressing GABAergic interneurons have been described in detail,[29] one of which translates synchronous activity of cholinergic interneurons into inhibition of principal neurons.[30] These neurons of the striatum are not distributed evenly.[26]
There are two regions of neurogenesis in the brain – the subventricular zone (SVZ) in the lateral ventricles, and the dentate gyrus in the hippocampal formation. Neuroblasts that form in the lateral ventricle adjacent to the striatum, integrate in the striatum.[31][32] This has been noted in the human striatum following an ischemic stroke. Injury caused to the striatum stimulates the migration of neuroblasts from the SVZ, to the striatum, where they differentiate into adult neurons.[33] The normal passage of SVZ neuroblasts is to the olfactory bulb but this traffic is diverted to the striatum after an ischemic stroke. However, few of the new developed neurons survive.[34]
Inputs
The largest connection is from the cortex, in terms of cell axons. Many parts of the neocortexinnervate the dorsal striatum. The cortical pyramidal neurons projecting to the striatum are located in layers II-VI, with the most dense projections come from layer V.[35] They end mainly on the dendritic spines of the spiny neurons. They are glutamatergic, exciting striatal neurons.
Another well-known afferent is the nigrostriatal connection arising from the neurons of the substantia nigra pars compacta. While cortical axons synapse mainly on spine heads of spiny neurons, nigral axons synapse mainly on spine shafts.
In primates, the thalamostriatal afferent comes from the central median-parafascicular complex of the thalamus (see primate basal ganglia system). This afferent is glutamatergic. The participation of truly intralaminar neurons is much more limited.
The striatum also receives afferents from other elements of the basal ganglia such as the subthalamic nucleus (glutamatergic) or the external globus pallidus (GABAergic).
The main nucleus of the basal ganglia is the striatum which projects directly to the globus pallidus via a pathway of striatopallidal fibers.[40] The striato-pallidal pathway has a whitish appearance due to the myelinated fibers. This projection comprises successively the external globus pallidus (GPe), the internal globus pallidus (GPi), the pars compacta of the substantia nigra (SNc), and the pars reticulata of substantia nigra (SNr). The neurons of this projection are inhibited by GABAergic synapses from the dorsal striatum. Among these targets, the GPe does not send axons outside the system. Others send axons to the superior colliculus. Two others comprise the output to the thalamus, forming two separate channels: one through the internal segment of the globus pallidus to the ventral oralis nuclei of the thalamus and from there to the cortical supplementary motor area and another through the substantia nigra to the ventral anterior nuclei of the thalamus and from there to the frontal cortex and the occulomotor cortex.
The striatum is also thought to play a role in an at least partially dissociable executive control network for language, applied to both verbal working memory and verbal attention. These models take the form of a frontal-striatal network for language processing.[44] While the striatum is often not included in models of language processing, as most models only include cortical regions, integrative models are becoming more popular in light of imaging studies, lesion studies on aphasic patients, and studies of language disorders concomitant with diseases known to affect the striatum like Parkinson's and Huntington's disease.[45]
Metabotropicdopamine receptors are present both on spiny neurons and on cortical axon terminals. Second messenger cascades triggered by activation of these dopamine receptors can modulate pre- and postsynaptic function, both in the short term and in the long term.[46][47] In humans, the striatum is activated by stimuli associated with reward, but also by aversive, novel,[48]unexpected, or intense stimuli, and cues associated with such events.[49]fMRI evidence suggests that the common property linking these stimuli, to which the striatum is reacting, is salience under the conditions of presentation.[50][51] A number of other brain areas and circuits are also related to reward, such as frontal areas. Functional maps of the striatum reveal interactions with widely distributed regions of the cerebral cortex important to a diverse range of functions.[52]
The interplay between the striatum and the prefrontal cortex is relevant for behavior, particularly adolescent development as proposed by the dual systems model.[53]
Autism spectrum disorder (ASD) is characterized by cognitive inflexibility and poor understanding of social systems. This inflexible behavior originates in defects in the pre-frontal cortex as well as the striatal circuits.[59] The defects in the striatum seem to specifically contribute to the motor, social and communication impairments seen in ASD patients. In mice which have an ASD-like phenotype induced via the overexpression of the eukaryotic initiation of translation factor 4E, it has been shown that these defects seem to stem from the reduced ability to store and process information in the striatum, which leads to the difficulty seen in forming new motor patterns, as well as disengaging from existing ones.[60]
Dysfunction
Dysfunction in the ventral striatum can lead to a variety of disorders, most notably depression and obsessive-compulsive disorder. Because of its involvement in reward pathways, the ventral striatum has also been implicated in playing a critical role in addiction. It has been well established that the ventral striatum is strongly involved in mediating the reinforcing effects of drugs, especially stimulants, through dopaminergic stimulation.[61]
Language disorders
Lesions to the striatum have been associated with deficits in speech production and comprehension. While striatal damage can impact all levels of language, damage can broadly be characterized as affecting the ability to manipulate linguistic units and rules, resulting in the promotion of default linguistic forms in conflicting situations in which selection, inhibition, and monitoring load is increased.[62] Two subregions of the striatum have been shown to be particularly important in language: the caudate nucleus and left putamen. Lesions localized to the caudate nucleus, as well as direct electrical stimulation, can result in lexical paraphasias and perservations (continuations of an utterance after the stimulus has ceased), which is associated with inhibited executive control, in the sense that executive control allows for the selection of the best choice among competing alternatives).[63] Stimulation of the putamen results in the inhibition of articulatory sequences and the inability to initiate motor speech commands.[64][65]
History
In the seventeenth and eighteenth centuries, the term corpus striatum was used to designate many distinct, deep, infracortical elements of the[which?] hemisphere.[66] Etymologically, it is derived from (Latin) striatus[67] = "grooved, striated" and the English striated = having parallel lines or grooves on the surface.[68] In 1876 David Ferrier contributed decades of research to the subject; concluding that the corpus striatum was vital in the "organization and generation of voluntary movement".[69][70][71][72][73] In 1941, Cécile and Oskar Vogt simplified the nomenclature by proposing the term striatum for all elements in the basal ganglia built with striatal elements: the caudate nucleus, the putamen, and the fundus striati,[74] which is the ventral part linking the two preceding together ventrally to the inferior part of the internal capsule.
The term neostriatum was coined by comparative anatomists comparing the subcortical structures between vertebrates, because it was thought to be a phylogenetically newer section of the corpus striatum. The term is still used by some sources, including Medical Subject Headings.[75]
Other animals
In birds the term used was the paleostriatum augmentatum, and in the new avian terminology listing (as of 2002) for neostriatum this has been changed to the nidopallium.[76]
In non-primate species, the islands of Calleja are included in the ventral striatum.[13]
^ abcdefghijYager LM, Garcia AF, Wunsch AM, Ferguson SM (August 2015). "The ins and outs of the striatum: Role in drug addiction". Neuroscience. 301: 529–541. doi:10.1016/j.neuroscience.2015.06.033. PMC4523218. PMID26116518. [The striatum] receives dopaminergic inputs from the ventral tegmental area (VTA) and the substantia nigra (SNr) and glutamatergic inputs from several areas, including the cortex, hippocampus, amygdala, and thalamus (Swanson, 1982; Phillipson and Griffiths, 1985; Finch, 1996; Groenewegen et al., 1999; Britt et al., 2012). These glutamatergic inputs make contact on the heads of dendritic spines of the striatal GABAergic medium spiny projection neurons (MSNs) whereas dopaminergic inputs synapse onto the spine neck, allowing for an important and complex interaction between these two inputs in modulation of MSN activity ... It should also be noted that there is a small population of neurons in the NAc that coexpress both D1 and D2 receptors, though this is largely restricted to the NAc shell (Bertran- Gonzalez et al., 2008). ... Neurons in the NAc core and NAc shell subdivisions also differ functionally. The NAc core is involved in the processing of conditioned stimuli whereas the NAc shell is more important in the processing of unconditioned stimuli; Classically, these two striatal MSN populations are thought to have opposing effects on basal ganglia output. Activation of the dMSNs causes a net excitation of the thalamus resulting in a positive cortical feedback loop; thereby acting as a 'go' signal to initiate behavior. Activation of the iMSNs, however, causes a net inhibition of thalamic activity resulting in a negative cortical feedback loop and therefore serves as a 'brake' to inhibit behavior ... there is also mounting evidence that iMSNs play a role in motivation and addiction (Lobo and Nestler, 2011; Grueter et al., 2013). ... Together these data suggest that iMSNs normally act to restrain drug-taking behavior and recruitment of these neurons may in fact be protective against the development of compulsive drug use.
^ abcdefTaylor SB, Lewis CR, Olive MF (February 2013). "The neurocircuitry of illicit psychostimulant addiction: acute and chronic effects in humans". Subst. Abuse Rehabil. 4: 29–43. doi:10.2147/SAR.S39684. PMC3931688. PMID24648786. The DS (also referred to as the caudate-putamen in primates) is associated with transitions from goal-directed to habitual drug use, due in part to its role in stimulus–response learning.28,46 As described above, the initial rewarding and reinforcing effects of drugs of abuse are mediated by increases in extracellular DA in the NAc shell, and after continued drug use in the NAc core.47,48 After prolonged drug use, drug-associated cues produce increases in extracellular DA levels in the DS and not in the NAc.49 This lends to the notion that a shift in the relative engagement from the ventral to the dorsal striatum underlies the progression from initial, voluntary drug use to habitual and compulsive drug use.28 In addition to DA, recent evidence indicates that glutamatergic transmission in the DS is important for drug-induced adaptations and plasticity within the DS.50
^ abcdefghiFerré S, Lluís C, Justinova Z, Quiroz C, Orru M, Navarro G, Canela EI, Franco R, Goldberg SR (June 2010). "Adenosine-cannabinoid receptor interactions. Implications for striatal function". Br. J. Pharmacol. 160 (3): 443–453. doi:10.1111/j.1476-5381.2010.00723.x. PMC2931547. PMID20590556. Two classes of MSNs, which are homogeneously distributed in the striatum, can be differentiated by their output connectivity and their expression of dopamine and adenosine receptors and neuropeptides. In the dorsal striatum (mostly represented by the nucleus caudate-putamen), enkephalinergic MSNs connect the striatum with the external globus pallidus and express the peptide enkephalin and a high density of dopamine D2 and adenosine A2A receptors (they also express adenosine A1 receptors), while dynorphinergic MSNs connect the striatum with the substantia nigra (pars compacta and reticulata) and the entopeduncular nucleus (internal globus pallidus) and express the peptides dynorphin and substance P and dopamine D1 and adenosine A1 but not A2A receptors ... These two different phenotypes of MSN are also present in the ventral striatum (mostly represented by the nucleus accumbens and the olfactory tubercle). However, although they are phenotypically equal to their dorsal counterparts, they have some differences in terms of connectivity. First, not only enkephalinergic but also dynorphinergic MSNs project to the ventral counterpart of the external globus pallidus, the ventral pallidum, which, in fact, has characteristics of both the external and internal globus pallidus in its afferent and efferent connectivity. In addition to the ventral pallidum, the internal globus pallidus and the substantia nigra-VTA, the ventral striatum sends projections to the extended amygdala, the lateral hypothalamus and the pedunculopontine tegmental nucleus. ... It is also important to mention that a small percentage of MSNs have a mixed phenotype and express both D1 and D2 receptors (Surmeier et al., 1996).
^Macpherson, Tom; Hikida, Takatoshi (June 2019). "Role of basal ganglia neurocircuitry in the pathology of psychiatric disorders". Psychiatry and Clinical Neurosciences. 73 (6): 289–301. doi:10.1111/pcn.12830.
^Raz, Aeyal (1996). "Neuronal synchronization of tonically active neurons in the striatum of normal and parkinsonian primates". Journal of Neurophysiology. 76 (3): 2083–2088. doi:10.1152/jn.1996.76.3.2083. PMID8890317.
^Koós, Tibor; Tepper, James M. (May 1999). "Inhibitory control of neostriatal projection neurons by GABAergic interneurons". Nature Neuroscience. 2 (5): 467–472. doi:10.1038/8138. PMID10321252. S2CID16088859.
^Rosell, Antonio; Giménez-Amaya, José Manuel (September 1999). "Anatomical re-evaluation of the corticostriatal projections to the caudate nucleus: a retrograde labeling study in the cat". Neuroscience Research. 34 (4): 257–269. doi:10.1016/S0168-0102(99)00060-7. PMID10576548. S2CID31392396.
^Robbins, Trevor W.; Everitt, Barry J. (April 1992). "Functions of dopamine in the dorsal and ventral striatum". Seminars in Neuroscience. 4 (2): 119–127. doi:10.1016/1044-5765(92)90010-Y.
^ abMalenka RC, Nestler EJ, Hyman SE (2009). Sydor A, Brown RY (eds.). Molecular Neuropharmacology: A Foundation for Clinical Neuroscience (2nd ed.). New York: McGraw-Hill Medical. pp. 147–148, 321, 367, 376. ISBN978-0-07-148127-4. VTA DA neurons play a critical role in motivation, reward-related behavior (Chapter 15), attention, and multiple forms of memory. This organization of the DA system, wide projection from a limited number of cell bodies, permits coordinated responses to potent new rewards. Thus, acting in diverse terminal fields, dopamine confers motivational salience ("wanting") on the reward itself or associated cues (nucleus accumbens shell region), updates the value placed on different goals in light of this new experience (orbital prefrontal cortex), helps consolidate multiple forms of memory (amygdala and hippocampus), and encodes new motor programs that will facilitate obtaining this reward in the future (nucleus accumbens core region and dorsal striatum). In this example, dopamine modulates the processing of sensorimotor information in diverse neural circuits to maximize the ability of the organism to obtain future rewards. ... Functional neuroimaging in humans demonstrates activation of the prefrontal cortex and caudate nucleus (part of the striatum) in tasks that demand inhibitory control of behavior. ... The brain reward circuitry that is targeted by addictive drugs normally mediates the pleasure and strengthening of behaviors associated with natural reinforcers, such as food, water, and sexual contact. Dopamine neurons in the VTA are activated by food and water, and dopamine release in the NAc is stimulated by the presence of natural reinforcers, such as food, water, or a sexual partner. ... The NAc and VTA are central components of the circuitry underlying reward and memory of reward. As previously mentioned, the activity of dopaminergic neurons in the VTA appears to be linked to reward prediction. The NAc is involved in learning associated with reinforcement and the modulation of motoric responses to stimuli that satisfy internal homeostatic needs. The shell of the NAc appears to be particularly important to initial drug actions within reward circuitry; addictive drugs appear to have a greater effect on dopamine release in the shell than in the core of the NAc.