M2 muscarinic receptor mediates arginine-vasopressin synthesis possibly through decreasing presynaptic GABA release in the supraoptic nuclei

[Speaker] Hiroshi Nagano:1
[Co-author] Jeffrey G Tasker:2, Hayato Matsuyama:3, Yuki Sobue:3, Shoichiro Saito:4, Hiroki Sakai:5,6, Toshiaki Ishii:7, Yasuyuki Tanahashi:8, Toshihiro Unno:3
1:Department of Pathogenetic Veterinary Science, United Graduate School of Veterinary Science, Gifu University, Japan, 2:Department of Cell and Molecular Biology, Tulane University, USA, 3:Laboratory of Veterinary Pharmacology, Faculty of Applied Biological Science, Gifu University, Japan, 4:Laboratory of Veterinary Anatomy, Faculty of Applied Biological Science, Gifu University, Japan, 5:Laboratory of Veterinary Pathology, Faculty of Applied Biological Science, Gifu University, Japan, 6:Center for Highly Advanced Integration of Nano and Life Sciences, Gifu University, Japan, 7:Department of Basic Veterinary Medicine, Obihiro University of Agriculture and Veterinary Medicine, Japan, 8:Department of Animal Medical Sciences, Faculty of Life Sciences, Kyoto Sangyo University, Japan

Background: Arginine-vasopressin (AVP) is synthesized in the paraventricular (PVN) and supraoptic nuclei (SON) of the hypothalamus to cause the reabsorption of water in the kidney. Muscarinic acetylcholine receptors have been suggested to promote AVP secretion although its subtype is unclear. The present study was carried out to examine whether M2 muscarinic receptors in the PVN and SON regulate AVP secretion.

Methods: Female M2 receptor knockout (M2KO) mice, wild-type (WT) mice (3-4 months old) and AVP-enhanced green fluorescent protein (eGFP) transgenic rats (1.5-2.5 months old) were used in the following experiments. The AVP neurons were identified by immunohistochemistry and the number was counted. Plasma AVP level was measured by radioimmunoassay. Water intake, urinary volume, and urinary frequency were measured using metabolic cages. The function of renal AVP receptors was assessed by a decrease of urinary volume after administration of a V2 receptor agonist, desmopressin. Miniature inhibitory postsynaptic currents (mIPSCs), reflecting properties of GABAergic synapses, were recorded from SON AVP neurons in rat hypothalamic slices in the presence of tetrodotoxin using whole-cell voltage clamp recording. The effect of muscarinic receptor activation on the amplitude and frequency of mIPSCs was determined by administration of a non-selective muscarinic agonist, muscarine, and an M2 receptor antagonist, AFDX-116.

Results: In M2KO mice, the number of AVP neurons was significantly decreased in the SON, but not in the PVN, compared with WT mice. Plasma AVP levels were lower in M2KO than in WT mice. Urinary volume and frequency and water intake were significantly increased in M2KO mice. The increased urinary volume of M2KO mice was decreased to almost the same level as WT by administration of desmopressin. In rat AVP neurons, the frequency, but not amplitude, of mIPSCs was reduced by muscarine, and the muscarinic suppression of mIPSCs was blocked by AFDX-116.

Conclusions: These results suggest that M2 receptors stimulate AVP release from SON magnocellular neurons by suppressing synaptic GABA release, which is crucial for the regulation of body fluid.
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