Three divisions of the mouse caudal striatum differ in the proportions of dopamine D1 and D2 receptor-expressing cells, distribution of dopaminergic axons, and composition of cholinergic and GABAergic interneurons.

The greater part of the striatum is composed of striosomes and matrix compartments, but we recently demonstrated the presence of a region that has a distinct structural organization in the ventral half of the mouse caudal striatum (Miyamoto et al. in Brain Struct Funct 223:4275-4291, 2018). This region, termed the tri-laminar part based upon its differential immunoreactivities for substance P and enkephalin, consists of medial, intermediate, and lateral divisions. In this study, we quantitatively analyzed the distributions of both projection neurons and interneurons in each division using immunohistochemistry. Two types of projection neurons expressing either the dopamine D1 receptor (D1R) or D2 receptor (D2R) showed complementary distributions throughout the tri-laminar part, but the proportions significantly differed among the three divisions. The proportion of D1R-expressing neurons in the medial, intermediate, and lateral divisions was 88.6 ± 8.2% (651 cells from 3 mice), 14.7 ± 3.8% (1025 cells), and 49.3 ± 4.5% (873 cells), respectively. The intermediate division was further characterized by poor innervation of tyrosine hydroxylase immunoreactive axons. The numerical density of choline acetyltransferase immunoreactive neurons differed among the three divisions following the order from the medial to lateral divisions. In contrast, PV-positive somata were distributed throughout all three divisions at a constant density. Two types of GABAergic interneurons labeled for nitric oxide synthase and calretinin showed the highest cell density in the medial division. The present results characterize the three divisions of the mouse caudal striatum as distinct structures, which will facilitate studies of novel functional loops in the basal ganglia.

Structural organization of the dendritic reticulum linked by gap junctions in layer 4 of the visual cortex.

Neuronal gap junctions are ubiquitous in the brain, but their precise positions in actual neuronal circuits have been uncertain, and their functional roles remain unclear. In this study, I visualized connexin36-immunoreactive gap junctions and examined the structural features of the interconnected dendrites arising from parvalbumin (PV)-positive interneurons in layer 4 of the feline visual cortex. I observed evidence for net-like dense linkages of dendrites among virtually all PV neurons (56/58 cells, 96.6%) in the tissue. This dendritic reticulum established connections among clustered cells and further among remote cells. The latter connectivity exhibited a preference for vertical direction, including translaminar linkages, but was also characterized by lateral continuity. Measurement of the distances from each dendritic gap junction back to the two connected somata revealed that at least one of two somata was within 50µm from the junction in 77.5% of the cases and within 75µm in 91.2% of the cases. Thus, distal gap junctions mediated morphologically asymmetrical connection where one soma was close to, but the other soma was far from the connecting junction. This connectivity was typically observed between neurons located apart in the same columnar space, where a long vertical dendrite bridged two neurons through an asymmetrically positioned gap junction. In contrast, gap junctions formed between nearby cells were close to both somata. Thalamocortical afferents established synapses densely on somata of layer 4 PV neurons, indicating the possible involvement of proximal gap junctions in visual stimulus-driven feedforward regulation. These findings provide a new structural basis for cortical investigations.

Selective Thalamic Innervation of Rat Frontal Cortical Neurons.

Most glutamatergic inputs in the neocortex originate from the thalamus or neocortical pyramidal cells. To test whether thalamocortical afferents selectively innervate specific cortical cell subtypes and surface domains, we investigated the distribution patterns of thalamocortical and corticocortical excitatory synaptic inputs in identified postsynaptic cortical cell subtypes using intracellular and immunohistochemical staining combined with confocal laser scanning and electron microscopic observations in 2 thalamorecipient sublayers, lower layer 2/3 (L2/3b) and lower layer 5 (L5b) of rat frontal cortex. The dendrites of GABAergic parvalbumin (PV) cells preferentially received corticocortical inputs in both sublayers. The somata of L2/3b PV cells received thalamic inputs in similar proportions to the basal dendritic spines of L2/3b pyramidal cells, whereas L5b PV somata were mostly innervated by cortical inputs. The basal dendrites of L2/3b pyramidal and L5b corticopontine pyramidal cells received cortical and thalamic glutamatergic inputs in proportion to their local abundance, whereas crossed-corticostriatal pyramidal cells in L5b exhibited a preference for thalamic inputs, particularly in their distal dendrites. Our data demonstrate an exquisite selectivity among thalamocortical afferents in which synaptic connectivity is dependent on the postsynaptic neuron subtype, cortical sublayer, and cell surface domain.

Immunohistochemical study on the neuronal diversity and three-dimensional organization of the mouse entopeduncular nucleus.

The entopeduncular nucleus (EPN) is one of the major output nuclei of the basal ganglia in rodents. Previous studies have divided it into rostral and caudal halves, with the former containing somatostatin (SOM)-immunoreactive neurons and the latter dominated by parvalbumin (PV)-containing neurons, respectively. However, it is unclear whether this simple rostrocaudal segmentation is appropriate, and the possibility of the existence of other neuronal populations remains to be investigated. In this study the cytoarchitecture of the mouse EPN was analyzed immunohistochemically. Substance P (SP)-immunoreactivity determined the extent of the EPN, which was 800 µm-long along the rostrocaudal axis. PV-positive neurons were concentrated in the caudal two-thirds of this range. PV-negative neurons were abundant in the rostral half but were further located caudally around the PV neuron-rich core. PV(+)/SOM(-) and PV(-)/SOM(+) neurons constituted 28.6% and 45.7% of EPN neurons, respectively, whereas the remaining population (25.7%) exhibited neither immunoreactivity. Eleven percent of EPN neurons lacked immunoreactivity for glutamic acid decarboxylase, indicating their non-GABAergic nature. Three-dimensional reconstruction revealed that PV-rich/SP-poor core was surrounded by PV-poor/SP-rich shell region. Therefore, presumptive thalamus-targeting PV neurons are outnumbered by other populations, and the regional heterogeneity shown here might be related to functionally distinct pathways through the basal ganglia.