The presence and distribution of cannabinoid type 1 and 2 receptors in the mandibular gland: The influence of different physical forms of diets on their expression in piglets

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There is growing evidence that, in laboratory animals, the physical forms of a diet can affect the functional structure of the mandibular gland (Johnson, 1981; Johnson & Cardenas, 1993; Kurahashi, 2002). In rats, the weight of the mandibular gland increased when the diet had a lower water content and thus required more intense masticatory activity. On the contrary, its weight decreased when rats were fed liquid diets that require almost no masticatory activities. In addition, the weight of the mandibular gland of the animals was related to qualitative changes of saliva secretion, characterized by a net decrease in salivary protein content (Johnson, 1982).
Recently, Cappai et al. (2016) reported morpho‐functional changes in the mandibular gland of growing pigs fed with different physical forms of the same diet. They showed that a coarser diet increased the expression of leptin and its receptor in the epithelial cells of striated ducts (Cappai et al., 2016). These findings suggested a link between the type of diet and the functional role of molecules, such as leptin, which are likely involved in the functional regulation of the mandibular gland in human as well in other animal species (Bohlender, Rauh, Zenk, & Gröschl, 2003; Dall'Aglio, Maranesi, Pascucci, Mercati, & Ceccarelli, 2012; Leone et al., 2012). Interestingly, however, the type of the diet fed to pigs differentially affected the parotid and mandibular glands. In fact, while tannins or other toxic substances promptly affected the parotid but not the mandibular gland, the physical form of the diet had an opposite effect (Aboling, Drotleff, Cappai, & Kamphues, 2016; Cappai, Wolf, Dimauro, Pinna, & Kamphues, 2014; Cappai et al., 2010).
Among the various regulatory molecules identified to date in the mandibular gland, the endocannabinoids, such as anandamide, as ligands for specific cognate receptors composing the so‐called cannabinoid system, might also have a role in the control of salivary secretion. Several studies on laboratory animals have shown the presence of cannabinoid receptors in the secretory portion, of the major salivary glands, which controlled the quality and quantity of saliva produced (Hipkaeo, Watanabe, & Kondo, 2015; Kopach et al., 2012). Similar observations were found in humans where the prolonged use of marijuana decreased saliva secretion. In another study, anandamide administration to rats decreased the production of saliva through the activation of cannabinoid receptors identified in the major salivary glands (Prestifilippo et al., 2006).
From a physiological point of view, the cannabinoid system is involved in appetite regulation, memory and pain sensation. The cannabinoid receptors belong to the family of the G protein‐coupled receptor and are membrane receptors. There are at least two types of cannabinoid receptors: CB1 and CB2. The CB1 receptor is expressed mainly in the brain (Jin et al., 2014), as well as in several peripheral organs such as lungs, liver, kidneys, placenta and pituitary (Abán et al., 2013; Dall'Aglio et al., 2013). The CB2 receptor is involved in the control of immune processes being primarily expressed in the immune system and hematopoietic cells (Galiègue et al., 1995). Recently, the CB2 receptor has been found in other peripheral organs as well (Campora et al., 2012; Peralta et al., 2011).
Currently, data regarding the presence and localization of cannabinoid receptors in the major salivary glands of domestic animals are limited to the dog (Dall'Aglio et al., 2010). Therefore, the aim of this study was to evaluate (i) whether CB1 and CB2 are expressed in the mandibular gland of pigs and (ii) whether different physical types of the same diet modulate their expression in different ways.
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