Optical intracranial self-stimulation (ICSS)

Three weeks following virus infection, mice labeled with Mor-Ens or Sal-Ens were given six daily intracranial self-stimulation (ICSS) training sessions. Behavioral training and testing were performed in a mouse-operant apparatus (Med Associates) interfaced with optogenetic stimulation equipment (Newdoon Inc and Inper Tech, Hangzhou, Zhejiang, China). Each operant behavior chamber was equipped with a dim-light source and a nose-poke hole equipped with infrared photobeams connected to a computer. A 1-h fixed-ratio one (FR1) schedule was performed every training day. A nose poke in the target hole produced 2 s of 473-nm light (5 ms, 20 Hz, 10 mW). For drug-infusion studies, mice received injections of either flupenthixol (0.5 mg/kg, Tocris, Bristol, UK) or saline vehicle intraperitoneally 30 min prior to ICSS sessions. For negative-reinforcement procedures [68], mice were placed into the chamber and delivered continuous 473-nm (5 ms, 20 Hz, 10 mW) optical stimulation with an interstimulus interval of 1 s. The mice were trained on a FR1 training schedule, in which each active nose poke produced a 20-s laser shut-off and the terminals were not optogenetically activated.