Voluntary exercise is motivated by ghrelin, possibly related to the central reward circuit. Issue 3 (29th April 2021) - Mifune, Hiroharu; Tajiri, Yuji; Sakai, Yusuke; Kawahara, Yukie; Hara, Kento; Sato, Takahiro; Nishi, Yoshihiro; Nishi, Akinori; Mitsuzono, Ryouichi; Kakuma, Tatsuyuki; Kojima, Masayasu