Effects of Ondansetron in Obsessive-compulsive and Tic Disorders

Many psychiatric disorders are associated with altered sensory experiences arising from within the body. Examples include increased experience of sensations or urges in muscles, skins, joints or visceral organs in Tic/Tourette's Disorders, OCD patients with symptoms of "not just right experiences" or disgust sensitivity, and other disorders such as trichotillomania or excoriation disorder. In OCD, these sensory phenomena occur in approximately half of patients, are associated with earlier age of onset, and may be harder to treat with classic cognitive-behavioral approaches to OCD. Of interest, sensory phenomena in OCD are associated with Tourette's syndrome and respond to pharmacological treatments primarily used for tics. As such, abnormal sensory processing may be a basic mechanism that links various psychiatric disorders. The process of attending to body sensations is referred to as interoception, abnormality of which may be related to sensory phenomena. Research has revealed a cortical interoceptive circuit involving insula, anterior cingulate cortex (ACC), and sensorimotor cortex. Ondansetron (OND) is a good candidate for the modulation of the above-described interoceptive circuit. It is a selective 5-HT3 (serotonin) receptor antagonist that acts on both peripheral and central receptors. OND has long been used to treat nausea and vomiting due to chemotherapy, radiation therapy, anesthesia, and opioid-induced emesis. It has also been used alone or as adjunctive therapy for the treatment of both OCD and Tourette's disorder, showing some efficacy in small clinical trials. The mechanisms by which ondansetron improves symptoms in OCD and tic disorders are unknown, although the investigator's earlier study found that single doses of ondansetron reduce activation of insula and somatosensory cortex in healthy controls. As a follow-up to this work, the current protocol will compare the effects of 24 mg/day of ondansetron vs. placebo for 4 weeks in patients with OCD or Tic Disorders on symptoms and brain functioning.