Like the H3 receptor, the H4 receptor belongs to the family of G;/o coupled receptors. Studies with transfected cells show that the H4 receptor inhibits forskolin-induced cAMP accumulation (Liu et al. 2001a; Nakamura et al. 2000; Oda et al. 2000), stimulates [35S]GTPyS binding (Morse etal. 2001), and activates MAP kinase (Morse etal. 2001) in a PTX-sensitive manner. Co-expression studies using either Ga15 or Ga16 (Morse etal. 2001; Oda etal. 2000) have shown that the H4 receptor can also activate these G proteins, leading to a mobilization of intracellular Ca2+]. As the receptor is strongly expressed in leucocytes, coupling of the receptor to G15/16 seems to represent a natural second messenger pathway of the H4 receptor (Oda et al. 2000). Indeed, Raible et al. (1994) reported on a new HA receptor on human eosinophils linked to mobilization of intracellular [Ca2+]; this response had a pharmacological profile resembling that of the H4 receptor. At this moment, no data on the regulation of H4-receptor responses are known.
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