Restoration of Cyclo-Gly-Pro-induced salivary hyposecretion and submandibular composition by naloxone in mice

Igor Santana De Melo ,Návylla Candeia-Medeiros,Polliane Maria Cavalcante-Araújo,Ana Carolina Gomes Jardim,Robinson Sabino-Silva,Jandir M Hickmann,Renata Pereira Alves-Balvedi,Angelo Ricardo Fávaro Pipi,Olagide Wagner De Castro ,Cássio Eráclito Alves Dos Santos ,Emiliano Barreto ,Luíz Ricardo Goulart ,Emília Maria Gomes Aguiar ,Igor Andrade Santos ,Douglas Alexsander Alves ,Jaqueline Dos Santos Ferro ,Walter L Siqueira ,Tales Lyra De Oliveira ,Stephanie Wutke Oliveira ,Léia Cardoso-Sousa ,Luciano Pereira Rodrigues

doi:10.1371/journal.pone.0229761

Abstract

Cyclo-Gly-Pro (CGP) attenuates nociception, however its effects on salivary glands remain unclear. In this study, we investigated the acute effects of CGP on salivary flow and composition, and on the submandibular gland composition, compared with morphine. Besides, we characterized the effects of naloxone (a non-selective opioid receptor antagonist) on CGP- and morphine-induced salivary and glandular alterations in mice. After that, in silico analyses were performed to predict the interaction between CGP and opioid receptors. Morphine and CGP significantly reduced salivary flow and total protein concentration of saliva and naloxone restored them to the physiological levels. Morphine and CGP also reduced several infrared vibrational modes (Amide I, 1687-1594cm-1; Amide II, 1594-1494cm-1; CH2/CH3, 1488-1433cm-1; C = O, 1432-1365cm-1; PO2 asymmetric, 1290-1185cm-1; PO2 symmetric, 1135-999cm-1) and naloxone reverted these alterations. The in silico docking analysis demonstrated the interaction of polar contacts between the CGP and opioid receptor Cys219 residue. Altogether, we showed that salivary hypofunction and glandular changes elicited by CGP may occur through opioid receptor suggesting that the blockage of opioid receptors in superior cervical and submandibular ganglions may be a possible strategy to restore salivary secretion while maintaining antinociceptive action due its effects on the central nervous system.

Highlights

Saliva exerts multiple functions in the oral cavity such as protection against microorganisms, contribution to the taste and digestion and maintenance of oral health [1,2,3]
Experimental procedures were approved by the Ethical Committee of the Federal University of Alagoas (UFAL) (License 065/2011), according to Ethical Principles adopted by the Brazilian College of Animal Experimentation (COBEA)
Naloxone significantly increased the salivary secretion in mice treated with CGP and morphine (~20% and 30%, respectively; p

Summary

Introduction

Saliva exerts multiple functions in the oral cavity such as protection against microorganisms, contribution to the taste and digestion and maintenance of oral health [1,2,3]. Salivary function is controlled by sympathetic and parasympathetic nervous system, which innervate acinar, ductal, myoepithelial and vascular cells in salivary glands [4,5]. The activation of muscarinic receptors in the acinar cells is the most important control of salivary flow rates [6]. Electrical stimulation of sympathetic efferent branch to the salivary glands results in a low flow of saliva which is rich in proteins [7]. Sympathectomy generates decrease in salivary flow [8]. These findings demonstrate the complexity of the sympathetic regulation on salivary flow and salivary composition [9]. The activation of central pathways develops a great part in salivatory effects of intraperitoneal pilocarpine in rats [10]

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