Abstract
The CA2 region of the mammalian hippocampus is a unique region with its own distinctive properties, inputs and pathologies. Disruption of inhibitory circuits in this region appears to be linked with the pathology of specific psychiatric disorders, promoting interest in its local circuitry, its role in hippocampal function and its dysfunction in disease. In previous studies, CA2 interneurons, including a novel subclass of CA2 dendrite-preferring interneurons that has not been identified in other CA regions, have been shown to display physiological, synaptic and morphological properties unique to this sub-field and may therefore play a crucial role in the hippocampal circuitry. The distributions of immuno-labeled interneurons in dorsal CA2 were studied and compared with those of interneurons in CA1 and CA3. Like those in CA1 and CA3, the somata of CA2 parvalbumin-immunoperoxidase-labeled interneurons were located primarily in Stratum Pyramidale (SP) and Stratum Oriens (SO), with very few cells in Stratum Radiatum (SR) and none in Stratum Lacunosum Moleculare (SLM). There was, however, a greater proportion of GAD-positive cells were immunopositive for PV in SP in CA2 than in CA1 or CA3. CA2 SP also contained a larger density of somatostatin-, calbindin-, and VIP-immunopositive somata than CA1 and/or CA3. Like those in CA1 and CA3, CCK-immunopositive somata in CA2 were mostly located in SR. Reelin- and NPY- immunolabeled cell bodies were located in all layers of the three CA regions. However, a higher density of Reelin-positive somata was found in SP and SR of CA2 than in CA1 or CA3.
Highlights
The intrinsic hippocampal circuitry was for many years considered to involve three main functional subregions: the dentate gyrus, the CA3 and the CA1 regions
PV-immunopositive somata represented a larger proportion of GAD-immunopositive cells in stratum pyramidale (SP) in CA2 than in CA1 or CA3 (MW U, P < 0.05, UCA1 = 0, UCA3 = 2) (Figure 4C), with a density of 24.1 103 ± 1.6 103 PV-positive cells/mm3 in CA2 SP compared with 13.3 103 ± 1.5 103 cells/mm3 in CA1 SP and 9 103 ± 0.3 103 cells/mm3 in CA3 SP (Figure 3)
The present study describes in detail the distributions of interneurons expressing widely used markers in the CA2 region for the first time and compare these distributions with those in the www.frontiersin.org
Summary
The intrinsic hippocampal circuitry was for many years considered to involve three main functional subregions: the dentate gyrus, the CA3 and the CA1 regions. More than 20 classes of interneurons have been reported in CA1 and CA3 (Somogyi and Klausberger, 2005; Ascoli et al, 2008; Fuentealba et al, 2008; Klausberger, 2009) They are generally identified on the basis of their dendritic and axonal arborisations, and the post-synaptic domain of the pyramidal cell that they target (Han et al, 1993; Sik et al, 1995; Buhl et al, 1996). Some CA2 PV-immunopositive basket and bistratified cells had partly spiny horizontal dendrites that extended horizontally along stratum oriens (SO) into both CA1 and CA3 These neurons displayed spike frequency adaptation and a “sag,” in response to hyperpolarizing current injection. CA2 basket cell axons arborized in stratum pyramidale (SP) of all three CA regions, while the axons of CA2 bistratified cells were very strongly polarized, ramifying in SO and SR of the CA2 and CA1 regions but stopping abruptly at the CA2/CA3
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