Antidepressant ketamine via oral gavage impairs fear memory, suppresses 22 kHz ultrasonic vocalizations, lowers GluN2A/B expression, and reduces medial habenula activity in rats

Ketamine, an N-methyl-D-aspartate receptor (NMDAR) antagonist known for its rapid-acting antidepressant properties, has been extensively studied through intravenous and intraperitoneal routes. However, despite its growing use in pharmacotherapy, research on oral administration of ketamine remains limited. This study aims to investigate the behavioral and neural effects of fixed-dose oral ketamine delivered via gavage. Adult male Wistar rats were assigned to four groups receiving either saline (vehicle) or ketamine at 15 mg/kg, 30 mg/kg, or 45 mg/kg doses. Animals were tested in the forced swim test (FST), open field test, cued fear conditioning, and elevated plus maze (EPM). Three doses of ketamine were administered via oral gavage every two days, each given 30 min before the FST, fear extinction, and EPM. ELISA was used to measure expression levels of the NMDAR subunit GRIN1 (GluN1), while immunohistochemistry was used for the GluN2A and GluN2B subunits as well as c-Fos. 45 mg/kg ketamine reduced immobility in the FST, transiently impaired fear memory retrieval and reduced ultrasonic vocalizations during Extinction 1. GRIN1 levels were reduced in the hypothalamus for all doses, but increased in the thalamus for higher doses. The antidepressant-like dose decreased the number of GluN2A and GluN2B expressing neurons in the paraventricular nucleus of the thalamus, basolateral amygdala, and habenula. High dose groups also showed diminished c-Fos + cells in the medial habenula following acute stress. These results suggest that 45 mg/kg ketamine via oral gavage produces antidepressant-like effects, through regulation of NMDAR subunits in several depression-related structures like the medial habenula.