Original Article Homozygote inositol transporter knockout mice show a lithium-like phenotype There are three major sources of intracellular myo- inositol: (i) recycling in the phosphatidylinositol cycle; (ii) de novo synthesis from glucose-6-phos- phate by myo-inositol-1-phosphate synthase and inositol monophosphatase; and (iii) uptake from extracellular fluid (1–3). Dietary myo-inositol is an important source of intracellular inositol. Extracel- lular inositol is taken up by the cells mainly through membrane-associated transporters. The major inositol transporter is believed to be the sodium- dependent myo-inositol cotransporter-1 (SMIT1), which utilizes the electrochemical gradient of Na + across the plasma membrane to import inositol (4, 5). SMIT1 is also designated SLC5A3, a member of the sodium / glucose co-transporter family SLC5 (6). Inositol depletion by exogenously administered lithium has been proposed as a mechanism of action of lithium [i.e., the ‘inositol depletion hypothesis’ (7)]. Hallcher and Sherman (8) first reported that lithium inhibits the enzyme inositol mono-phosphatase. Replenishment of inositol in animal models of lithium action can reverse some lithium effects, in particular, lithium enhancement Bersudsky Y, Shaldubina A, Agam G, Berry GT, Belmaker RH. Homozygote inositol transporter knockout mice show a lithium-like phenotype. Bipolar Disord 2008: 10: 453–459. ª Blackwell Munksgaard, 2008 Objective: Lithium inhibits inositol monophosphatase and also reduces inositol transporter function. To determine if one or more of these mechanisms might underlie the behavioral effects of lithium, we studied inositol transporter knockout mice. We previously reported that heterozygous knockout mice with reduction of 15–37% in brain inositol had no abnormalities of pilocarpine sensitivity or antidepressant-like behavior in the Porsolt forced swim test. We now report on studies of homozygous inositol transporter knockout mice. Methods: Homozygote knockout mice were rescued by 2% inositol supplementation to the drinking water of the dam mice through pregnancy and lactation. Genotyping was carried out by polymerase chain reaction followed by agarose electrophoresis. Brain free myo- inositol levels were determined gas-chromatographically. Motor activity and coordination were assessed by the rotarod test. Behavior of the mice was studied in lithium–pilocarpine seizure models for lithium action and in the Porsolt forced swim test model for depression. Results: In homozygote knockout mice, free inositol levels were reduced by 55% in the frontal cortex and by 60% in the hippocampus. There were no differences in weight or motor coordination by the rotarod test. They behaved similarly to lithium-treated animals in the model of pilocarpine seizures and in the Porsolt forced swimming test model of depression. Conclusions: Reduction of brain inositol more than 15–37% may be required to elicit lithium-like neurobehavioral effects. Yuly Bersudsky a , Alona Shaldubina a , Galila Agam a , Gerard T Berry b and Robert H Belmaker a a Stanley Research Center, Faculty of Health Sciences, Ben Gurion University of the Negev, Beersheva, Israel, b Department of Pediatrics, Thomas Jefferson University, Philadelphia, PA, USA Key words: animal models of lithium action – behavioral phenotyping – depression – knockout mice – myo-inositol Received 22 February 2007, revised and accepted for publication 15 August 2007 Corresponding author: Robert H Belmaker, MD, Ben Gurion University of the Negev, PO Box 4600, Beersheva, Israel. Fax: +972 8 640 1621; e-mail: belmaker@bgu.ac.il The authors of this paper do not have any commercial associations that might pose a conflict of interest in connection with this manu- script. Bipolar Disorders 2008: 10: 453–459 Copyright ª Blackwell Munksgaard 2008 BIPOLAR DISORDERS 453