SEROTONERGIC DORSAL RAPHE NUCLEUS PROJECTIONS TO THE CHOLINERGIC AND NONCHOLINERGIC NEURONS OF THE PEDUNCULOPONTINE TEGMENTAL REGION - A LIGHT AND ELECTRON-MICROSCOPIC ANTEROGRADE TRACING AND IMMUNOHISTOCHEMICAL STUDY

The serotonergic dorsal raphe nucleus is considered an important modul ator of state-dependent neural activity via projections to cholinergic neurons of the pedunculopontine tegmental nucleus (PPT). Light and el ectron microscopic analysis of anterogradely transported biotinylated dextran, combined with choline acetyltransferase (ChAT) immunohistoche mistry, were employed to describe the synaptic organization of mesopon tine projections from the dorsal raphe to the PPT In a separate set of experiments, we utilized immunohistochemistry for the serotonin trans porter (SERT), combined with ChAT immunohistochemistry at the light an d electron microscopic levels, to determine whether PPT neurons receiv e serotonergic innervation. The results of these studies indicate that : (1) anterogradely labeled and SERT-immunoreactive axons and presumpt ive boutons invest the PPT at the light microscopic level; (2) at the ultrastructural level, dorsal raphe terminals in the PPT pars compacta synapse mainly with dendrites and axosomatic contacts were not observ ed; (3) approximately 12% of dorsal raphe terminals synapse with ChAT- immunoreactive dendrites; and (4) at least 2-4% of the total synaptic input to ChAT-immunoreactive dendrites is of dorsal raphe and/or serot onergic origin. This serotonergic dorsal raphe innervation may modulat e cholinergic PPT neurons during alterations in behavioral state. The role of these projections in the initiation of rapid eye movement (REM ) sleep and the ponto-geniculo-occipital waves that precede and accomp any REM sleep is discussed. J. Comp. Neurol. 382:302-322, 1997. (C) 19 97 Wiley-Liss, Inc.