PROJECT SUMMARY/ABSTRACT: Perseverative behaviors are prominent, disabling, and often treatment- resistant symptoms of several neuropsychiatric disorders including Obsessive Compulsive Disorder (OCD). Perseverative behavior refers to the repetition or continuation of a response that no longer results in a reward or expected outcome. Human neuroimaging data has associated abnormal activity within orbitofronto-striatal circuits with perseverative behavior in OCD patients. While early human neuroimaging studies identified overactivation of the entire orbitofrontal cortex (OFC), more recent studies have linked overactivation of the lateral OFC (lOFC) and hypoactivity of the medial OFC (mOFC) with OCD symptoms. Furthermore, preclinical work in rodents suggests that activating mOFC-striatal projection neurons drives perseverative behavior, while activating lOFC-striatal projection neurons suppresses perseverative behavior. Yet, a substantial gap in knowledge remains regarding the mechanisms underlying the differential regulation of perseverative behavior by the mOFC versus lOFC. This project will use chemogenetic and fiber photometry technologies in freely moving mice to determine how neuronal activity within the mOFC versus lOFC regulate perseverative behaviors. We will induce perseverative behavior in mice using acute challenge with a serotonin 1B receptor (5-HT1BR) agonist. Pharmacological challenge with agonists for the 5-HT1BR, previously termed 5-HT1Dβ in humans, exacerbates symptoms in OCD patients. Similarly, we found that treating mice acutely with a 5-HT1BR agonist induces perseverative behaviors including a highly repetitive form of hyperlocomotion termed “route stereotypy”, and perseverative responding in a delayed alternation task. These 5-HT1BR agonist-mediated effects can be prevented by the only effective monotherapy for OCD, 4 weeks of chronic treatment with SRIs. Thus, acute 5- HT1BR agonist treatment induces perseverative behaviors in mice with relevance to OCD. We will combine fiber photometry and chemogenetics with our well-validated behavioral methods to identify the mechanisms underlying the differential control over perseverative behavior by the mOFC versus lOFC. Previous work reported that optogenetic stimulation of mOFC-ventromedial striatal projections induces perseverative grooming in mice, while other work showed that optogenetic stimulation of lOFC-centromedial striatal projections suppresses perseverative grooming. Here, we will assess the effects of chemogenetic inhibition of neuronal projections from the mOFC versus lOFC to a large centromedial striatal region on 5-HT1BR agonist-induced perseverative behavior. We will also determine whether perseverative behavior is associated with differential changes in serotonin release into the mOFC versus lOFC using GRAB5-HT, and/or differential changes in the activity of GABAergic interneurons or orbitostriatal projection neurons within the mOFC versus lOFC. Since dysfunction with...