Dorsal raphe serotonin neurons inhibit operant responding for reward via inputs to the ventral tegmental area but not the nucleus accumbens: evidence from studies combining optogenetic stimulation and serotonin reuptake inhibition

Optogenetic stimulation causes dorsal raphe nucleus (DRN) 5-HT neuron activation and enhances downstream 5-HT release, but does not alter operant responding for reward. a Expression of ChR2-EYFP specifically in 5-HT neurons was achieved by crossing Ai32+/+ mice and ePet-cre+/？ mice. b Representative coronal section showing ChR2-EYFP expression in the DRN: left, 5-HT neurons immunostained for tryptophan hydroxylase-2, a marker of 5-HT neurons; middle, cells expressing ChR2-EYFP; right, merge of tryptophan hydroxylase-2 and ChR2-EYFP channels. Scale bar represents 100？μm. c Schematic for ex vivo patch-clamp recordings of ChR2-EYFP expressing DRN neurons. d Characteristic electrophysiological response of an EYFP-positive serotonergic DRN neuron to current steps and 2？Hz photostimulation trains. e Top: average (±SEM) firing frequencies of DRN 5-HT neurons (n？=？13) elicited by 30？s trains of optogenetic stimulation (2–10？Hz). Bottom: Averaged trace illustrating that extended photostimulation (2？Hz, 200？s) can maintain a significantly elevated firing frequency compared to baseline (paired-samples t-test: t(7)？=？5.52, p？<？0.001), when tested in a subset of DRN 5-HT neurons. f Schematic for in vivo microdialysis with optogenetic stimulation of ChR2-EYFP expressing DRN 5-HT neurons. g Average (±SEM) extracellular 5-HT concentration within the NAc from anesthetized ChR2+ mice (n？=？5) during periods of photostimulation (blue shaded areas) or no photostimulation (unshaded areas). *p？<？0.05 vs. previous baseline sample from post hoc analysis. h Schematic for optogenetic stimulation (473？nm light, 10？mW, 10？ms pulses) of DRN 5-HT neurons throughout tests of operant responding for saccharin. i Optogenetic stimulation of DRN 5-HT neurons does not alter operant responding for saccharin: mean lever presses (±SEM) for saccharin was not different between ChR2？ (n？=？10; white bars) and ChR2+ mice (n？=？11; green bars) at any photostimulation frequency tested (Group？×？Frequency: F(3,57)？=？0.06, ns; Group: F(3,57)？=？2.94, ns; independent-samples t-tests: all t(19)？<？1.57, all p？>？0.066