a, Lentiviruses of CMV-promoter-driven GFP were injected into the hypothalamic third ventricle (3V) of C57BL/6 mice via a pre-implanted cannula. One week after injection, brain sections were made and examined for GFP immunostaining. Scale bars, 50 μm. Images represent four independent experiments. b, c, AgRP-Cre mice and POMC-Cre mice received an injection in the hypothalamic third ventricle of Hsv-TK1 lentivirus followed by GCV treatment and were examined three months later for the number of AgRP and POMC neurons in the ARC through Cre immunostaining. d, AgRP-Cre mice and POMC-Cre mice were injected with rAAV2-FLEX-rev-ChR2:tdTomato virus or vehicle into the ARC, followed by injection of Sox2-promoter-driven Hsv-TK1 lentivirus (TK) or control lentivirus (Con) into the hypothalamic third ventricle. GCV was administrated into the third ventricle twice per week for three weeks. Subsequently these mice were subjected to an optogenetic stimulation-induced feeding response as described in the Methods. Food intake before and after optogenetic stimulation were also measured. e, MWM training information for Fig. 1f. **P < 0.01, ***P < 0.001; two-tailed Student’s t-test (b, c), one-way ANOVA with Tukey’s post hoc test (d, e); n = 4 mice per group (b, c), n = 5 mice per group (d) and n = 8 mice per group (e). Data are mean ± s.e.m. Source data