SEROTONERGIC SYNAPTIC INPUT TO CHOLINERGIC NEURONS IN THE RAT MESOPONTINE TEGMENTUM

Serotonergic synaptic inputs to cholinergic neurons in the laterodorsa l and pedunculopontine tegmental nuclei were examined with pre-embeddi ng dual-label immunoelectron microscopy. Numerous serotonin-immunoreac tive axon terminals visualized with a silver-enhanced immunogold metho d were present in both of these tegmental nuclei. Serotonergic termina ls occasionally made synaptic contacts with the soma and proximal dend rites of cholinergic tegmental neurons labelled with a choline acetylt ransferase-immunoreactive peroxidase-anti-peroxidase diaminobenzidine reaction product. In the rostralmost region of the laterodorsal tegmen tal nucleus, a few serotonergic neurons of the dorsal raphe nucleus we re interspersed among cholinergic neurons. Some dendrites of these ser otonergic neurons appeared to contain synaptic vesicles. Both myelinat ed and unmyelinated serotonergic axons were present in the mesopontine tegmentum. The presence of serotonergic synapses onto tegmental choli nergic neurons is consistent with previous behavioral and electrophysi ological findings suggesting an inhibitory role of serotonin in the in duction of rapid eye movement sleep and its phenomenology through an a ction on cholinergic neurons in the mesopontine tegmentum.