Electron Microscopic Localization of M2-muscarinic receptors in Cholinergic and Non-Cholinergic Neurons of the Laterodorsal Tegmental and Pedunculopontine Nuclei of the Rat Mesopontine Tegmentum

• Published in: Journal of comparative neurology (1911) 2016-04, Vol.524 (15), p.3084-3103
• Main Authors: Garzón, Miguel, Pickel, Virginia M.
• Format: Article
• Language: English
• ISSN: 0021-9967; 1096-9861
• DOI: 10.1002/cne.24010

Muscarinic m2 receptors (M2Rs) are implicated in autoregulatory control of cholinergic output neurons located within the pedunculopontine (PPT) and laterodorsal tegmental (LTD) nuclei of the mesopontine tegmentum (MPT). However, these nuclei contain many non-cholinergic neurons in which activation of M2R heteroceptors may contribute significantly to the decisive role of the LTD and PPT in sleep-wakefulness. We examined the electron microscopic dual immunolabeling of M2Rs and the vesicular acetylcholine transporter (VAchT) in the MPT of rat brain to identify the potential sites for M2R activation. M2R immunogold labeling was predominately seen in somatodendritic profiles throughout the PPT/LTD complex. In somata, M2R immunogold particles were often associated with Golgi lamellae and cytoplasmic endomembrannes, but were rarely in contact with the plasma membrane as was commonly seen in dendrites. Approximately 36% of the M2R-labeled somata and 16% of the more numerous M2R-labeled dendrites co-expressed VAchT. M2R and M2R/VAchT-labeled dendritic profiles received synapses from inhibitory- and excitatory-type axon terminals, over 88% of which were unlabeled and others contained exclusively M2R or VAchT immunoreactivity. In axonal profiles M2R immunogold was localized to plasmalemmal and cytoplasmic regions and showed a similar distribution in many VAchT-negative glial profiles. These results provide ultrastructural evidence suggestive of somatic endomembrane trafficking of M2Rs, whose activation serves to regulate the postsynaptic excitatory and inhibitory responses in dendrites of cholinergic and non-cholinergic neurons in the MPT. They also suggest the possibity that M2Rs in this brain region mediate the effects of acetylcholine on the release of other neurotransmitters and on glial signaling. Muscarinic M2 receptors are identified mainly on plasma membranes and endomembranes of cholinergic and non-cholinergic neurons and rarely on axonal and glial processes in the mesopontine tegmentum. This has important implications for intracellular M2R trafficking and MR2-mediated control of sleep-wake cycle through autoreceptors and heteroreceptors in this brain region.