Investigation of the hippocampal information processing in freely moving rats
Extracellular electrophysiological recordings of the hippocampus were carried out in freely moving Long-Evans rats. Large-scale and high-density electrodes were used to target all subregions simultaneously. We were able to record both local field potentials (LFPs) and single unit activity, which all...
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| Dokumentumtípus: | Disszertáció |
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2016-11-08
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| Tárgyszavak: | |
| doi: | 10.14232/phd.3145 |
| mtmt: | 3195443 |
| Online Access: | http://doktori.ek.szte.hu/3145 |
| Tartalmi kivonat: | Extracellular electrophysiological recordings of the hippocampus were carried out in freely moving Long-Evans rats. Large-scale and high-density electrodes were used to target all subregions simultaneously. We were able to record both local field potentials (LFPs) and single unit activity, which allowed for the analysis of population activity and individual cells, respectively. We recorded the animals while they performed spatial navigational tasks and during sleep in their home cages. We focused our investigations on the information processing of the hippocampus during its two main general functions: spatial navigation and memory consolidation. In particular, two main questions were addressed in this work:How the different subregions of the hippocampus contribute to spatial coding within the hippocampus. How the distinct subregions of the hippocampus coordinate to generate sharp-wave ripples complexes, known to be essential in memory consolidation To investigate the contribution of the different subregions of the hippocampus to spatial coding, we recorded simultaneously place cells (neurons which selectively respond to crossing locations in the environment) from CA1, CA2 and CA3 during spatial navigation.We analized the fine function of the distinct anatomical portions of these areas separating CA1 into proximal, intermediate and distal and CA3 into CA3c, CA3b and CA3a subregions.We could also record neurons located at several depths of the soma layers so we treated the deep (toward stratum oriens) and the superficially (toward stratum radiatum) located cells inthe CA1, CA2 and CA3 regions separately. We analyzed the distinct properties of the place cells located in the different subregions and found significant differences which characterize the spatial coding in these parts and correlate with the anatomy. The CA1 and CA2 regions in general had a higher number of place cells than the CA3region. Furthermore, more fields per cell were found in CA1 neurons and CA2 compared to their CA3 peers. Firing rates inside the fields and peak firing rates were also higher for CA1 and CA2. The more spatially informative place cells were the ones located in CA3, and the less informative the CA2 ones. Inside the different subregions, CA1 proximal cells appeared 43 to be more spatially informative, showed higher firing rates in-field and a tended to have one single place field whereas toward CA1 distal they fired less, had more place fields and contained less spatial information. For the CA3 region, more spatially informative and selective place cells were located in CA3c, with preferentially single fields but lower firing rates than in their CA3a peers. No significant differences were found within the cells located at different depths in the CA3 area, whereas in CA1 and CA2 a tendency characterized the deep cells with more number of place cells, higher in-field firing rates but less selective and spatially informative than their superficial peers. These findings are in correlation with the axonal distribution of the different afferents to the hippocampus, mainly from the entorhinal cortex. The medial part of the entorhinal cortex (where the highly spatially selective grid cells are located) is preferentially connected with the proximal CA1 (more spatial selectivity than the distal part) and the lateral portion of the entorhinal cortex (where mainly object and environmental cues coding cells are located) mainly target the distal part of CA1 (more place fields per cell and less spatially informative). Unlike the CA1, cells in the CA3 region have strong recurrent collaterals, which can account for the lower number and highly spatially selective place cells in this region. The least informative cells during spatial navigation were the ones located in CA2, which is in line with the discovery of a specific network which code for space during immobility inside this region. The phase precession was weaker in place cells from the CA2 region, which can be also explained by the distinct combination of inputs arriving to this part of the hippocampus. The theta phase locking of the different subregions also correlates with anatomical inputs. While a relatively preserved phase preference was found in CA1 cells from all subregions during RUN (ascending phase of the cycle), a gradual shift was notable from the CA2-CA3a border (ascending phase) toward the CA3c (descending phase). During REM, an important percentage of cells shift their firing with 180º in the CA1 region, but this is not the case for the cells located in CA2 or CA3 regions. Therefore, this is assumed to be related to the specific entorhinal layer III input to the CA1 region... |
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