Effects of chronic lithium exposure in a modified rodent ketamine-induced hyperactivity model of mania.

Bipolar illness is characterized by periods of "mania" - high energy, irritability, and increased psychomotor activation. While the neurobiological investigation of mania has been limited by the lack of reliable animal models, researchers have recently reported that daily subanesthetic doses of ketamine produce a lithium-reversible increase in rodent locomotor activity. Such studies have typically employed short-term (2 week) exposure to daily intraperitoneal-injected lithium and extremely brief (i.e., 5-min) open-field tests of hyperactivity. To increase the translational utility of the model, the effects of 70-days of orally administered lithium were examined on ketamine-induced hyperlocomotion during 30-min test sessions. Rats consumed 2.0 mEq/kg lithium chloride (LiCl) presented daily in a high incentive food (10 g of peanut butter). Control animals ingested peanut butter infused with an equimolar concentration of sodium chloride (NaCl). After 60 days of treatment, a 30-min baseline revealed no differences in the locomotor activity of LiCl and NaCl animals. During the next 10 days, animals received single daily supplemental injections of 25 mg/kg IP ketamine. A subset of animals was injected daily with saline and served as non-ketamine controls. Behavioral testing on the final two days of treatment confirmed that ketamine administration produced a profound increase in locomotor activity that was significantly attenuated in the LiCl group. Additionally, blood plasma levels of lithium were found to be comparable to low-moderate human therapeutic levels. These data confirm the viability and utility of ketamine-induced hyperlocomotion as a rodent model of mania.