Parvalbumin Basket Cells Preferentially Select Hippocampal CA1 Pyramidal Cells

The hippocampus is a brain structure known to play key roles in learning, memory, and spatial navigation, and is also involved with various neurological and psychiatric disorders. The CA1 region is a major output station of the hippocampus. Over the years, communication from local hippocampal interneurons to CA1 pyramidal cells has been studied on topics varying from neuroplasticity to cognition, but the diversity of their communication has yet to be further analyzed. Although the CA1 pyramidal cell layer has been reckoned a homogenous population of cells, studies have attempted to classify pyramidal cells using morphophysiological approaches. So far, pyramidal cells have been grouped based on their position from the border of the stratum radiatum and the stratum pyramidale: the superficial pyramidal cells (SPC) are found within the first 50 µm from this border while the deep pyramidal cells (DPC) are found 50 µm to 150 μm beyond this border. Also found in the stratum pyramidale are GABAergic interneurons. Of these interneurons, the parvalbumin basket cell (PVBC) makes synapses on the soma and proximal dendrites of CA1 pyramidal cells while sharing the same somatic location of pyramidal cells. Differential layer-based inhibition from PVBCs to the two aforementioned types of CA1 pyramidal cells has not been studied. A preferential selection of PVBCs to CA1 pyramidal cells via GABAergic neurotransmission could indicate that information is not completely homogenous across all sublayers of the CA1 pyramidal layer. This suggests that heterogeneity of these neurotransmissions is critical for the modulation of pyramidal cells. We hypothesize that differential GABAergic inhibitory regulation of CA1 pyramidal cells could be layer specific. In this study we focus our attention on the number of synaptic contacts from PVBCs to CA1 principal cells across the stratum pyramidale sublayers. We tested this hypothesis by visually counting the putative synaptic contacts between the axon of the PVBCs and the perisomatic area of the DPCs and SPCs. In order to identify the number of synaptic contacts, the anatomy of connected biocytin-filled neurons is examined under a 60X objective. After counting the contacts from the axon of identified PVBCs to 6 SPCs and to 5 DPCs, we found that PVBCs average 4.8 axonal contacts with SPCs and 9.35 axonal contacts with DPCs. Additional morphological analysis was conducted from 3-dimensional reconstructions of the cells. Our results indicate that PVBCs preferentially target DPCs, suggesting that PVBCs differentially innervate DPCs as compared with SPCs. This could play a crucial role in the regulation of CA1 pyramidal cell output, possibly suggesting different inhibition of PVBCs to SPCs, and to DPCs. This research was supported by the following NIH Grants: NIH-MBRS-IMSD Grant GM-55246, NS74432 to IS.