本研究課題では, 背側海馬と腹側海馬の機能の違いが, 海馬から他の脳領域への活動伝播の違いに反映されているか検証した。海馬CA1錘体神経細胞に光感受性陽イオンチャネル(ChR2)が発現している遺伝子改変マウスを用いて, 全脳活動を機能的MRI(fMRI)装置で計測している最中に, 海馬の背側あるいは腹側へ光照射することで海馬神経細胞を活性化し, これに応答する脳領域を特定した。この結果, 海馬の背側と腹側とを活性化した時に応答する脳領域群が異なることを見出した。さらに, 海馬の遠心性軸索線維の多寡と, 海馬活動への応答の大きさとは必ずしも対応しないことを見つけた。
The dorsal and ventral hippocampal regions (dHP and vHP) are proposed to have distinct functions. Nevertheless, the extra-hippocampal influences of dHP and vHP activities remain unclear. In this study, we compared the spatial distribution of brain-wide responses upon dHP or vHP activation. To achieve this, we performed optogenetic fMRI using an anesthetized transgenic mouse, whose CA1 pyramidal neurons expressed a step-function opsin variant of channelrhodopsin-2 (ChR2). Activation at the dHP, but not the vHP, evoked BOLD responses at the retrosplenial cortex (RSP), which is in line with anatomical evidence. In contrast, BOLD responses at the lateral septum (LS) were induced only upon vHP activation, even though both dHP and vHP send axonal fibers to the LS. Our findings suggest that the primary targets of dHP and vHP activation are distinct, which concurs with attributed functions of the dHP and RSP in spatial memory, as well as of the vHP and LS in emotional responses.
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