Rationale <p>Our laboratory has demonstrated that stressors regulate the 5-HT system with consequences in behavioral models of psychiatric disorders including addiction. We have employed behavioral pharmacology methods to show that stressors engage corticotropin-releasing factor afferents to 5-HT dorsal raphe nucleus (DRN) neurons with effects on negative affective state and stress-induced opioid relapse models. Given the ongoing opioid crisis, there remains a need to elucidate the neural mechanisms underlying opioid use disorder with molecular, temporal and spatial precision using newer circuit manipulation strategies in animal models.</p> Objectives <p>We sought to characterize the behavioral effects of chemogenetic manipulations of 5-HT DRN neurons in male and female Tph2-iCre rats.</p> Methods <p>Subjects received intra-DRN viral infusions of Cre-dependent inhibitory or excitatory Designer Receptors Exclusively Activated by Designer Drugs (DREADDs). Three behavioral paradigms were used to examine the effect of clozapine-N-oxide activation or inhibition of 5-HT DRN neurons: elevated plus maze (EPM), forced swim test (FST), and stress-induced reinstatement of morphine conditioned place preference (CPP).</p> Results <p>Chemogenetic activation of 5-HT DRN neurons induces anxiety-like behavior in the EPM in a transgenic rat model.</p> Conclusions <p>This study confirms some of the findings of prior chemo- and optogenetic studies in mice showing a role of 5-HT DRN neurons in anxiety-like behaviors but did not support effects in the FST model or in reinstatement of morphine CPP. Given the growing availability of transgenic rat lines, future studies in transgenic rats are needed to help dissect a complex literature on the behavioral functions of the 5-HT system.</p>