Previously we have detected the expression
Previously, we have detected the expression of APJ receptor on stomach projecting Phosphocreatine disodium salt australia in DMV, in addition, icv administration of apelin-13 induced a remarkable c-Fos activation in those cells which were previously marked by a retrograde neuronal tracer fast blue (Birsen et al., 2016). Moreover, we have demonstrated that the inhibitory action of intra-DMV injected apelin-13 on gastric tone and motility was abolished completely by fourth ventricle application of selective APJ receptor antagonist F13A suggesting that central apelin diminishes the vagal parasympathetic outflow through APJ receptor (Bulbul et al., 2018). The adrenergic sympathetic fibers innervate principally enteric neurons that reside in two major plexuses in the alimentary canal. It was demonstrated that the sympathetic fibers can act pre- and postsynaptically to modulate the activity of enteric neurons either via direct actions on the enteric neuronal membrane or indirectly via actions to modulate neurotransmitter release (Browning and Travagli, 2014; Lundgren, 2000). The sympathetic nerves may exert their action at a synapse in at least two ways: (i) they may inhibit the release of acetylcholine, for example, from an intramural synapse, and/or (ii) releasing noradrenaline released from sympathetic nerves may hyperpolarize the membrane potential of the postsynaptic neuron (Lundgren, 2000). Despite the dense sympathetic innervation to the GI tract, there appears to be little tonic sympathetic activity so that splanchnic nerve section does result in moderate increase in peristaltic activity (Browning and Travagli, 2014). On the other hand, there are evidences indicate that the sympathetic signaling pathway on GI motor functions appears to be activated predominantly under stressful conditions (Nakade et al., 2005; Nakade et al., 2006). In response to stressful stimuli, the expression of CRF is up-regulated in hypothalamic PVN, then it acts on the dorsal vagal complex neurocircuitry in the caudal brainstem to inhibit GE via its specific receptor (type-2) (Bhatia and Tandon, 2005; Nakade et al., 2005; Tache and Bonaz, 2007). On the other hand, Takahashi's group demonstrated previously that both centrally-injected and stress-induced upregulated CRF inhibit GE by disturbing the antro-pyloric coordination in conscious rats. Furthermore, these effects were antagonized by pretreatment of central astressin-2B, peripheral guanethidine, propranolol and CGX suggesting that CRF-induced gastroinhibitory action is mediated through central CRF receptors (type-2) and peripheral sympathetic neuronal pathways (Nakade et al., 2005; Nakade et al., 2006). We have shown in rats that along with the CRF-producing cells, the apelin-produding cells in hypothalamic PVN are activated by restraint stress. Our latter findings indicated that the inhibitory effect on gastric postprandial motility that elicited by apelin-13 was abolished by central pretreatment of CRF antagonist α-helical CRF9,41 (Bulbul et al., 2016). Interestingly, the increased c-Fos activity was demonstrated in hypothalamic PVN following central (icv) administration of apelin in rats (Kagiyama et al., 2005). Moreover, Lv and colleagues have shown in mice that central exogenous apelin-13 inhibited food intake, however, this anorexigenic effect was reversed by α-helical CRF9–41 (Lv et al., 2012). In line with the latter findings, central apelin-induced increases in plasma adrenocorticotropin releasing hormone and corticosterone were reduced significantly by pretreatment with α-helical CRF9–41 indicating the involvement of CRF in actions of centrally-administered apelin (Jaszberenyi et al., 2004; Newson et al., 2009). In keeping with our previous consideration, therefore, it appears to be convincible that apelin-induced effects involved with activation of sympathetic pathway is mediated partly by central CRF. In this study, we investigated the HRV as an indicator of the ANS activity. The efferent vagal parasympathetic activity is considered as the major contributor to the HF component. In contrast, both sympathetic and parasympathetic branches contribute to the LF component; therefore the ratio of LF:HF is commonly utilized as a measure of sympatho-vagal balance (Akita et al., 2002; Kuwahara et al., 1994). Another important result of our investigation which has been demonstrated for the first time that central exogenous apelin alters the LF:HF ratio through its modulatory action on both components. Our present findings have revealed that in rats underwent both CGX and VGX (but not CGX or VGX alone), the apelin-induced delayed GE was restored completely to the basal levels measured in the sham-operated rats. Likewise, a similar restoration was observed in rats pretreated with the combination of sympatholytic agent guanethidine and muscarinic receptor agonist bethanechol compared to the vehicle-injected rats. These results suggest the contribution of both sympathetic and parasympathetic branches and their relevant post-ganglionic neural pathways which are mediated through muscarinic receptors and adrenoceptors. In our study, pretreatment of L-NAME did not alter the actions of apelin indicating that the vagally-mediated NANC pathway appears to be irrelevant in apelin-induced gastroinhibition, unlike the vagal cholinergic pathway.