Electron microscopic evidence for different myelination of rat septohippocampal fibres

Rat septohippocampal fibres are known to originate from GABAergic parvalbum in-containing, fast-firing, fast-conducting neurons and from cholinergic sl ow-firing, slow-conducting neurons. In the present electron microscopic stu dy, based on immunocytochemical demonstration of parvalbumin and choline ac etyltransferase in transverse and horizontal septal sections, it was shown that parvalbumin-immunoreactive fibres are myelinated, but the vast majorit y of cholinergic fibres are not. As revealed, especially in horizontal sect ions, the cholinergic axons show considerably finer calibres than parvalbum in-containing ones. These results confirm and extend our previous light mic roscopic findings. It can be concluded that differences in conduction veloc ities, presence or absence of myelin sheaths and differences in axonal diam eters are correlated in the septohippocampal pathway. NeuroReport 12:17-20 (C) 2001 Lippincott Williams & Wilkins.