Spiny projection neuron striatum

The NAc is the major component of the ventral striatum, and of the so-called limbic striatum. This striatal region corresponds to the entire anterior and ventromedial sector of the striatum and continues anteriorly into the NAc and the olfactory tubercle (Heimer and Wilson, 1975 Nauta, 1989). In the NAc, the principal neurons, which are the medium spiny projection neurons, make up approximately 90% of the total neuronal population and are generally similar to those of the striatal (i.e., neostriatal) counterpart. The local circuit neurons represent approximately 10% of the NAc neurons and vary greatly in size,... 

Fig. 3. View of a single striatal spiny projection neuron, intracellularly filled with biocytin, in a sagittal section of the striatum (A) and at higher magnification (B). Corticofugal fiber fascicles are clearly evident coursing through the striatum. Spiny projection neurons are labeled within the striatum with calbindin iramunoreactiv-...

Spiny projection neurons receive inputs from the cortex, thalamus and amygdala, which make asymmetric synapses on dendritic spines, and to a lesser degree, dendritic shafts. These inputs provide the major excitatory input to these neurons. In addition, a number of inputs from outside the striatum, and from within the striatum provide inputs that function to modify the responsiveness of spiny neurons to the excitatory input. These include inputs from dopamine afferents from the substantia nigra, inhibitory GABA inputs from the axon collaterals of other spiny neurons, inhibitory inputs from GABA (and peptide containing) striatal interneurons, and inputs from cholinergic striatal interneurons. 

Fig. 13. A) Diagram showing an example of inputs to the globus pallidus (GP) from striatal spiny projection neurons. Typically there are two major sites of axonal arborization, one in the region immediately adjacent to the striatum and a second in the central region of the GP. B) Stylized drawing of two pallidal neurons showing how the dendrites of neurons are confined within the two regions of the GP that conform to the pattern of striatal inputs. C) The axonal projection of a globus pallidus neuron of the type with discoid dendrites, which provides collaterals to the striatum (CP), to the entopeduncular nucleus (EP), subthalamic nucleus (stn) and substantia nigra (SN). Adapted from Kita and Kitai 1994.

Fig. 24. Patch and matrix striatal compartments are labeled with neurochemical markers. A) The patch compartment is labeled with 3H-naloxone binding to mu opiate receptors (white in the darkfield photomicrograph). B) The matrix compartment is labeled with calbindin-immunoreactivity, which labels spiny projection neurons that provide inputs to the substantia nigra pars reticulata. The correspondence between calbindin-poor zones (black arrows) and mu opiate binding sites (white arrows) is seen to occur in all regions of the striatum. Calbindin-immunoreactivity is relatively weak in the dorso-lateral striatum, which nonetheless contains opiate receptor patches.

Fig. 29. Coronal sections through the striatum showing mu-opiate receptor with 3H-naloxone binding of patches (A and B) and in adjacent sections spiny projection neurons labeled by in situ hybridization histochemistry with probes directed against substance P mRNA (A ) and enkephalin mRNA (B ). Substance P and enkephalin are expressed by different populations of spiny projection neurons, each comprising about half of the population and each evenly distributed in both patch and matrix compartments (arrows show patches in the corresponding sections). From Gerfen and Young (1987).

The ultrastructural organization of dopaminergic boutons in the striatum has been extensively investigated (see, inter alia, the reviews of Smith and Bolam, 1990 Sesack, 2003). Extrastriatal inputs, including the dopaminergic one, terminate mainly on the more distal part of the dendritic tree of medium spiny neurons, while intrinsic inputs terminate mainly on the proximal parts of the dendritic shaft and on the cell body. A small proportion of dopaminergic axons also contact the cell body of striatal projection neurons (Fig. 18B). 

Dopamine acts on G-protein-coupled receptors belonging to the D1 -family of receptors (so-called D1-like receptors , or DlLRs, comprised of Dl- and D5-receptors), and the D2-family of receptors ( D2-like receptors , or D2LRs comprised of D2-, D3- and D4-receptors). Dl LRs stimulate adenylate cyclase activity and, possibly, also phosphoinosit-ide hydrolysis, while D2LRs reduce adenylate cyclase activity. In the striatum, DlLRs are predominately associated with medium spiny neurons of the direct pathway, while D2LRs have been found as autoreceptors on dopaminergic terminals, as heteroreceptors on cholinergic interneurons, and on indirect pathway neurons. In the SNr, DlLRs are located on terminals of the direct pathway projection, while D2LRs appear to function as autoreceptors. 

Other cholinergic pathways in the brain include a network of intrinsic neurons in the striatum, and also various nuclei in the lower brain stem which project to the cerebellum are the origins of the cranial nerves. Striatal cholinergic neurons project mainly to spiny neurons which are the principal locus for the relay of cortical information flowthrough the basal ganglia (Calabresi et al.. 

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