ELECTRON-MICROSCOPIC EVIDENCE FOR A CHOLINERGIC INNERVATION OF GABAERGIC PARVALBUMIN-IMMUNOREACTIVE NEURONS IN THE RAT MEDIAL SEPTUM

The presence of interconnections between cholinergic and parvalbumin ( PARV)-containing gamma aminobutyric acid (GABA)ergic septohippocampal projection neurons is still a matter of debate. To search for contacts of cholinergic collateral axon terminals in the septal-diagonal band region the immunotoxin 192IgG-saporin was applied, which was proved to selectively destroy cholinergic basal forebrain neurons, Seven and 10 days after administration of the inununotoxin, choline acetyltransfer ase immunoreactivity had disappeared, and numerous neuronal somata and dendrites as well as axonal terminals revealed characteristics of ele ctron-lucent degeneration, Electron-dense degeneration was never obser ved in dendrites and synaptic boutons, Degenerating terminals were fou nd in contact with PARV-immunopositive and PARV-negative neurons. Beca use only cholinergic cells were degenerating, the terminals should be collaterals from cholinergic neurons. In addition to such contacts, PA RV-immunoreactive boutons were seen in contact with PARV-positive and PARV-negative cells, but were not identified at degenerating postsynap tic profiles. As suggested in other studies, cholinergic boutons conta cting GABAergic PARV-containing septal projection cells may influence hippocampal theta activity. Furthermore, multiple synaptic connections of both neuronal populations forming the septohippocampal pathway may contribute to their high rate of survival after fimbria-fornix transe ction. J. Neurosci. Res. 54:243-253, 1998. (C) 1998 Wiley-Liss, Inc.