The other possibility for the lack of

The other possibility for the lack of effects of H1 receptor agonists, e.g. histamine, on the NMDAR-mediated responses is that other neurotransmitters may cause more profound effects than histamine on the gliotransmitter release from astrocytes. For example, recent studies showed that noradrenaline, endocannabinoids and Cy3 TSA Fluorescence System Kit regulate astrocytic Ca2+ signaling, thereby modulating the release of D-serine (Rasooli-Nejad et al., 2014; Lalo et al., 2018; Papouin et al., 2017b). It is therefore possible that tonic release of such neurotransmitters may efface the H1 receptor agonist actions because of its relatively weak actions on astrocytic Ca2+ signaling and D-serine release. Recent findings of constitutive signaling associated with GPCR-mediated pathways have fundamentally changed our insight into receptor pharmacology: GPCRs can activate their associated signal transduction pathways both upon agonist activation and with constitutive receptor activity in the absence of agonists (Seifert and Wenzel-Seifert, 2002). Histamine H1 receptors linking to PLC cascade and nuclear factor-kappa B (NFκB) can display constitutive activities, and most H1 receptor antagonists, including pyrilamine and cetirizine, exhibit the property of inverse agonist to inhibit the constitutive H1 receptor activity (Bakker et al., 2001; Fitzsimons et al., 2004). Thus, the constitutive activity of H1 receptors should not be ignored in the pyrilamine- and cetirizine-induced attenuation of hippocampal NMDAR-mediated synaptic responses, namely EPSCs and LTP, observed in this study. Histamine facilitates glutamate release from the presynaptic terminal via activation of H1 and H2 receptors in hippocampal synaptosome preparations (Rodriguez et al., 1997). However, our data are inconsistent with the possibility that blockade of presynaptic H1 receptors regulating synaptic release of glutamate in the CA1 may underlie the pyrilamine- and cetirizine-induced attenuation of hippocampal NMDAR-mediated EPSCs, because the H1 receptor antagonist/inverse agonists did not alter PPRs of AMPAR-mediated EPSCs, a parameter that reflects changes in the amount of neurotransmitter release (see Fig. S2). The fact that the pretreatment of slices with D-serine or FAC occluded the inhibitory effects of H1 receptor antagonist/inverse agonists on NMDAR-mediated EPSCs and LTP suggested that pyrilamine and cetirizine attenuate the release of D-serine from hippocampal astrocytes. It has been demonstrated that D-serine plays a pivotal role as an endogenous agonist for the glycine binding-site of NMDARs and Ca2+-dependent release of D-serine from astrocytes can control NMDAR-mediated synaptic responses and plasticity at various brain excitatory synapses (Henneberger et al., 2010). However, the detailed mechanism of how intracellular calcium elevation may lead to the release of gliotransmitters remains unclear (Wenker, 2010). More recently, at least two classes of α7-nicotinic acetylcholine and cannabinoid CB1 receptors have been shown to regulate the release of synaptic D-serine following elevation of intracellular free Ca2+ concentration ([Ca2+]i) in astrocytes, thereby resulting in enhancement of NMDAR-mediated synaptic responses in the hippocampal CA1 region (Papouin et al., 2017b; Robin et al., 2018). Three types of histamine receptors (H1, H2, and H3) are shown to occur in astrocytes where they can participate in neurogenesis, neuroprotection and release of gliotransmitters, such as ATP and glutamate (Ambrée et al., 2014; Kárpáti et al., 2018; Xu et al., 2018). In particular, stimulation of astrocytic H1 receptors evokes the elevation of [Ca2+]i in type 2 astrocytes that have a characteristic morphology of small cell bodies with long radial processes (Inagaki et al., 1991). Therefore, it is likely that astrocytic H1 receptors activated by tonically released histamine could induce the liberation of gliotransmitters, including D-serine from hippocampal astrocytes in the CA1 region. This possibility is supported by behavioral studies showing that intra-hippocampal injection of D-cycloserine, a glycine binding-site partial agonist of NMDAR, improved an H1 receptor antagonist/inverse agonist-induced working memory deficit (Nakazato et al., 2000; Masuoka et al., 2008), which is consistent with the finding in this study that the pretreatment of hippocampal slices with D-serine markedly attenuated the H1 receptor antagonist/inverse agonist pyrilamine-induced suppression of LTP at excitatory synapses between Schaffer collaterals and CA1 pyramidal neurons (Fig. 6). Agulhon et al. (2010), however, reported that Ca2+ signaling following the activation of Gq-coupled GPCR in astrocytes did not affect the spontaneous and action potential-driven excitatory synaptic transmission or short- and long-term plasticity at Shaffer colateral-CA1 pyramidal neuron synapses. A possible explanation of these inconsistent observations is that activation of individual GPCRs couples to multiple signaling pathway modes (Milligan, 1993): e.g., H1 receptors activate not only the Gq protein-mediated PLC cascade but also NFκB and adenylyl cyclase-mediated cascades (Maruko et al., 2005).