Localization of the M2 muscarinic cholinergic receptor in dendrites, cholinergic terminals, and noncholinergic terminals in the rat basolateral amygdala: An ultrastructural analysis

• Published in: Journal of comparative neurology (1911) Vol. 524; no. 12; pp. 2400 - 2417
• Main Authors: Muller, Jay F., Mascagni, Franco, Zaric, Violeta, Mott, David D., McDonald, Alexander J.
• Format: Journal Article
• Language: English
• Published: United States Blackwell Publishing Ltd 15-08-2016; Wiley Subscription Services, Inc
• Subjects: acetylcholine; Animals; autoreceptor; Basolateral Nuclear Complex - metabolism; Basolateral Nuclear Complex - ultrastructure; Cholinergic Neurons - metabolism; Cholinergic Neurons - ultrastructure; Dendrites - metabolism; Dendrites - ultrastructure; electron microscopy; immunocytochemistry; Male; Presynaptic Terminals - metabolism; Presynaptic Terminals - ultrastructure; Rats; Rats, Sprague-Dawley; Receptor, Muscarinic M2 - biosynthesis; Receptor, Muscarinic M2 - ultrastructure; RRID:AB_572269; RRID:AB_94952; vesicular acetylcholine transporter protein; RRID:AB_94952; electron microscopy; immunocytochemistry; acetylcholine; autoreceptor; vesicular acetylcholine transporter protein; RRID:AB_572269
• ISSN: 0021-9967; 1096-9861
• DOI: 10.1002/cne.23959

ABSTRACT
Activation of M2 muscarinic receptors (M2Rs) in the rat anterior basolateral nucleus (BLa) is critical for the consolidation of memories of emotionally arousing events. The present investigation used immunocytochemistry at the electron microscopic level to determine which structures in the BLa express M2Rs. In addition, dual localization of M2R and the vesicular acetylcholine transporter protein (VAChT), a marker for cholinergic axons, was performed to determine whether M2R is an autoreceptor in cholinergic axons innervating the BLa. M2R immunoreactivity (M2R‐ir) was absent from the perikarya of pyramidal neurons, with the exception of the Golgi complex, but was dense in the proximal dendrites and axon initial segments emanating from these neurons. Most perikarya of nonpyramidal neurons were also M2R–negative. About 95% of dendritic shafts and 60% of dendritic spines were M2 immunoreactive (M2R+). Some M2R+ dendrites had spines, suggesting that they belonged to pyramidal cells, whereas others had morphological features typical of nonpyramidal neurons. M2R‐ir was also seen in axon terminals, most of which formed asymmetrical synapses. The main targets of M2R+ terminals forming asymmetrical (putative excitatory) synapses were dendritic spines, most of which were M2R+. The main targets of M2R+ terminals forming symmetrical (putative inhibitory or neuromodulatory) synapses were unlabeled perikarya and M2R+ dendritic shafts. M2R‐ir was also seen in VAChT+ cholinergic terminals, indicating a possible autoreceptor role. These findings suggest that M2R‐mediated mechanisms in the BLa are very complex, involving postsynaptic effects in dendrites as well as regulating release of glutamate, γ‐aminobutyric acid, and acetylcholine from presynaptic axon terminals. J. Comp. Neurol. 524:2400–2417, 2016. © 2016 Wiley Periodicals, Inc.
Electron micrograph of an axon terminal (V/M2t) with immunoreactivity for both vesicular acetylcholine transporter (diffuse reaction product) and the M2 muscarinic receptor (granular reaction product, arrows) forming synapses (arrowheads) with a dendrite (M2d) and spine (M2sp) that exhibit granular M2 reaction product. These data suggest an autoreceptor role for M2.