Introduction

There may be more than just cultural value to the old German saying "grain tastes bitter for a satiated mouse" Sala 2013. Indeed, it is the state of an organism which determines what, if any, effect external sensory stimuli will have on the nervous system. Whether this is the satiation state of the mouse influencing taste receptors, or the feeding state of the leech which gates mechanosensory stimuli Gaudry 2010Gaudry 2009, or the locomotor state of flies which adjusts the gain in visual interneurons Chiappe 2010Maimon 2010Longden 2009Breugel 2014Tuthill 2014Suver 2012, sensory stimuli are rarely, if ever, directly transformed into motor outputs. Instead, all nervous systems seem to constantly balance external and internal demands before they arrive at any given action Brembs 2013Brembs 2017Heisenberg 2009Pezzulo 2016. Biogenic amines and neuropeptides have been shown to be crucially involved in orchestrating the processes needed to find this balance.