Stress decreases serotonin tone in the nucleus accumbens in male mice to promote aversion and potentiate cocaine preference via decreased stimulation of 5-HT receptors.

Stress-induced release of dynorphins (Dyn) activates kappa opioid receptors (KOR) in serotonergic neurons to produce dysphoria and potentiate drug reward; however, the circuit mechanisms responsible for this effect are not known. In male mice, we found that conditional deletion of KOR from Slc6a4 (SERT)-expressing neurons blocked stress-induced potentiation of cocaine conditioned place preference (CPP). Within the dorsal raphe nucleus (DRN), two overlapping populations of KOR-expressing neurons: Slc17a8 (VGluT3) and SERT, were distinguished functionally and anatomically. Optogenetic inhibition of these SERT neurons potentiated subsequent cocaine CPP, whereas optical inhibition of the VGluT3 neurons blocked subsequent cocaine CPP. SERT/VGluT3 expressing neurons were concentrated in the lateral aspect of the DRN. SERT projections from the DRN were observed in the medial nucleus accumbens (mNAc), but VGluT3 projections were not. Optical inhibition of SERT neurons produced place aversion, whereas optical stimulation of SERT terminals in the mNAc attenuated stress-induced increases in forced swim immobility and subsequent cocaine CPP. KOR neurons projecting to mNAc were confined to the lateral aspect of the DRN, and the principal source of dynorphinergic (Pdyn) afferents in the mNAc was from local neurons. Excision of Pdyn from the mNAc blocked stress-potentiation of cocaine CPP. Prior studies suggested that stress-induced dynorphin release within the mNAc activates KOR to potentiate cocaine preference by a reduction in 5-HT tone. Consistent with this hypothesis, a transient pharmacological blockade of mNAc 5-HT receptors potentiated subsequent cocaine CPP. 5-HT is known to be expressed on 5-HT terminals in NAc, and 5-HT transcript was also detected in Pdyn, Adora2a and ChAT (markers for direct pathway, indirect pathway, and cholinergic interneurons, respectively). Following stress exposure, 5-HT transcript was selectively elevated in Pdyn cells of the mNAc. These findings suggest that Dyn/KOR regulates serotonin activation of 5HT receptors within the mNAc and dynamically controls stress response, affect, and drug reward.