The striatum, also known as the neostriatum or striate nucleus, is a subcortical part of the forebrain and a critical component of the reward system. It receives glutamatergic and dopaminergic inputs from different sources and serves as the primary input to the rest of the basal ganglia system. In all primates, the dorsal striatum is divided by a white matter tract called the internal capsule into two sectors called the caudate nucleus and the putamen.
The ventral striatum is composed of the nucleus accumbens and olfactory tubercle in primates. Functionally, the striatum coordinates multiple aspects of cognition, including motor and action planning, decision-making, motivation, reinforcement, and reward perception.
The corpus striatum, a macrostructure which contains the striatum, is composed of the entire striatum and the globus pallidus. The lenticular nucleus refers to the putamen together with the globus pallidus.
Spiny projection neurons, commonly referred to as “medium spiny neurons”, are the principal neurons of the striatum. 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.
Medium spiny neurons have two primary phenotypes (i.e., characteristic types): D1-type MSNs of the “direct pathway” and D2-type MSNs of the “indirect pathway”. A subpopulation of MSNs contain both D1-type and D2-type receptors, with approximately 40% of striatal MSNs expressing both DRD1 and DRD2 mRNA.
Cholinergic interneurons release acetylcholine, which has a variety of important effects in the striatum. In humans, non-human 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. The large aspiny cholinergic interneurons themselves are affected by dopamine through dopamine receptors D5.
There are many types of GABAergic interneurons. The best known are parvalbumin expressing interneurons, also known as fast-spiking interneurons, which participate in powerful feed-forward inhibition of principal neurons.
Also, there are GABAergic interneurons that express tyrosine hydroxylase, somatostatin, nitric oxide synthase and neuropeptide-Y. Recently, two types of neuropeptide-y expressing GABAergic interneurons have been described in detail, one of which translates synchronous activity of cholinergic interneurons into inhibition of principal neurons.
Adult humans continuously produce new neurons in the striatum, and these neurons could play a possible role in new treatments for neurodegenerative disorders.
The striatum is divided into ventral and dorsal subregions, based upon function and connectivity. The ventral striatum is composed of the nucleus accumbens and olfactory tubercle, whereas the dorsal striatum is composed of the caudate nucleus and putamen.
The dorsal striatum can be differentiated based on immunochemical characteristics—in particular with regard to acetylcholinesterase and calbindin — into “compartments”, consisting of “striosomes” and the surrounding “matrix”.
The ventral striatum, and the nucleus accumbens in particular, primarily mediates reward cognition, reinforcement, and motivational salience, whereas the dorsal striatum primarily mediates cognition involving motor function, certain executive functions, and stimulus-response learning.
There is a small degree of overlap, as the dorsal striatum is also a component of the reward system that, along with the nucleus accumbens core, mediates the encoding of new motor programs associated with future reward acquisition (e.g., the conditioned motor response to a reward cue).
Metabotropic dopamine 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.
In humans, the striatum is activated by stimuli associated with reward, but also by aversive, novel, unexpected, or intense stimuli, and cues associated with such events. fMRI evidence suggests that the common property linking these stimuli, to which the striatum is reacting, is salience under the conditions of presentation.
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.