Neuropsychopharmacology (2013) 38, 2666–2674 & 2013 American College of Neuropsychopharmacology. All rights reserved 0893-133X/13 www.neuropsychopharmacology.org Subanesthetic Doses of Ketamine Transiently Decrease Serotonin Transporter Activity: A PET Study in Conscious Monkeys 1 1 1 1 1 Shigeyuki Yamamoto , Hiroyuki Ohba , Shingo Nishiyama , Norihiro Harada , Takeharu Kakiuchi , 1 ,2 Hideo Tsukada and Edward F Domino* 1 2 Central Research Laboratory, Hamamatsu Photonics KK, Hamakita, Japan; Department of Pharmacology, University of Michigan, Ann Arbor, MI, USA Subanesthetic doses of ketamine, an N-methyl-D-aspartic acid (NMDA) antagonist, have a rapid antidepressant effect which lasts for up to 2 weeks. However, the neurobiological mechanism regarding this effect remains unclear. In the present study, the effects of subanesthetic doses of ketamine on serotonergic systems in conscious monkey brain were investigated. Five young monkeys 11 underwent four positron emission tomography measurements with [ C]-3-amino-4-(2-dimethylaminomethyl-phenylsulfanyl)benzoni- 11 trile ([ C]DASB) for the serotonin transporter (SERT), during and after intravenous infusion of vehicle or ketamine hydrochloride in a 11 dose of 0.5 or 1.5 mg/kg for 40 min, and 24 h post infusion. Global reduction of [ C]DASB binding to SERT was observed during ketamine infusion in a dose-dependent manner, but not 24 h later. The effect of ketamine on the serotonin 1A receptor (5-HT1A-R) and dopamine transporter (DAT) was also investigated in the same subjects studied with [11C]DASB. No significant changes were observed in either 5-HT -R or DAT binding after ketamine infusion. Microdialysis analysis indicated that ketamine infusion transiently increased 1A serotonin levels in the extracellular fluid of the prefrontal cortex. The present study demonstrates that subanesthetic ketamine selectively enhanced serotonergic transmission by inhibition of SERT activity. This action coexists with the rapid antidepressant effect of subanesthetic doses of ketamine. Further studies are needed to investigate whether the transient combination of SERT and NMDA reception inhibition enhances each other’s antidepressant actions. Neuropsychopharmacology (2013) 38, 2666–2674; doi:10.1038/npp.2013.176; published online 21 August 2013 Keywords: subanesthetic ketamine; serotonin transporer; [11C]DASB; PET; monkey INTRODUCTION inhibitors (SSRIs) and selective serotonin/noradrenalin inhibitors are the most common treatments for patients Prevalence estimates for major depressive disorder in an with depression (Celada et al, 2004). Because the serotonin American population is ca. 16% (Kessler et al, 2003). transporter (SERT) located on the presynaptic nerve Pharmacological therapy of depression was revolutionized terminal has a key role in the regulation of the serotonin in the late 1950s. Two classes of drugs were serendipitously levels in the synaptic cleft, inhibition of SERT would be found to be effective antidepressants. The first antidepres- expected to result in enhanced serotonergic neurotransmis- sant was iproniazid, originally developed as an antituber- sion (Salomon et al, 1993). Especially, increased serotonin cular drug, and the second the tricyclic antidepressant level in prefrontal cortex is thought to be a key step in the imipramine arose from antihistamine research (for review, therapeutic mechanism of SSRIs (Bel and Artigas, 1992; see Nestler et al, 2002). The acute mechanism of action Invernizzi et al, 1992, 1996). Positron emission tomography of both antidepressant medications had been identified. 11 (PET) studies with [ C]-3-amino-4-(2-dimethylamino- Iproniazid is a monoamine oxidase inhibitor, and imipra- 11 methyl-phenylsulfanyl)benzonitrile ([ C]DASB; Wilson mine inhibits serotonin and/or norepinephrine reuptake et al, 2000) have shown that 80% occupancy of SERT by transporters (Frazer, 1997). Today, there is a wide range of SSRIs is needed for the improvement of depression available antidepressants. Selective serotonin reuptake symptom (Meyer et al, 2001, 2004; Suhara et al, 2003). However, only one-third of patients show significant mood *Correspondence: Dr Edward F Domino, Department of Pharmacology, improvement in response to an initial antidepressant University of Michigan, Ann Arbor, MI 48109-05632, USA, Tel: treatment (Trivedi et al, 2006). Moreover, there is a time +1 734 764 9115, Fax: +1 734 763 4450, E-mail: [email protected] lag of several weeks before a therapeutic effect is observed Received 15 April 2013; revised 18 June 2013; accepted 17 July 2013; (Krystal., 2010). This lengthy time to achieve remission is accepted article preview online 24 July 2013 thought to be caused by indirect activation of the serotonin Decreased serotonin transporter by low-dose ketamine S Yamamoto et al 2667 1A receptor (5-HT1A-R) (Chaput et al, 1986; Invernizzi et al, MATERIALS AND METHODS 1996). Elevated extracellular serotonin levels in response to Subjects and Drug acute blockade of SERT engaged inhibition of the 5-HT1A-R in presynaptic neurons (autoreceptor) of the dorsal raphe. Experiments were conducted in accordance with the This inhibits serotonergic neural activity, resulting in recommendations of the US National Institutes of Health reduced subsequent serotonin release in terminal brain and the guidelines of the Central Research Laboratory, areas such as a frontal cortex (Bel and Artigas, 1992; Hamamatsu Photonics. Eight male rhesus monkeys Gartside et al, 1995; Invernizzi et al, 1992, 1996). In (Macaca mulatta;7.8±0.8 years old, weighing 6.4±1.4 kg) contrast, after chronic administrations of SSRIs, these 5- were studied. Five monkeys participated in the PET experi- HT1A autoreceptors are desensitized, resulting in a pro- ments, and another three were used for the microdialysis nounced increase in serotonin levels in the prefrontal cortex experiment. The doses of ketamine hydrochloride were based (Invernizzi et al, 1994). on human clinical studies (Diazgranados et al, 2010). Saline We previously investigated the clinical pharmacological or each dose of ketamine was infused intravenously for effects of CI-581, now known as ketamine, a noncompetitive 40 min. PET scans were started after the end of ketamine N-methyl-D-aspartic acid (NMDA) glutamate receptor infusion. Vital signs including heart rate, respiration rate, antagonist (Domino et al, 1965). Ketamine has analgesic systolic and diastolic blood pressure, and body temperature and dissociative anesthetic properties (Reich and Silvay, were monitored throughout the ketamine infusion. 1989). The brain distribution and kinetics of ketamine have 11 already been studied with PET. The uptake of [ C]ketamine PET Experiments reflects the distribution of NMDA receptors in the brain with a rapid brain-plasma exchange rate (Hartvig et al, A high-resolution animal PET scanner (SHR-7700; Hama- 1994; Kumlien et al, 1999). Ketamine in anesthetic doses matsu Photonics, Hamamatsu, Japan) with a transaxial affects several monoaminergic neuronal systems. In con- resolution of 2.6 mm full-width half-maximum in the scious monkeys and animal, PET anesthetic doses of enhanced 2D mode and a center-to-center distance of ketamine significantly alter the synthesis rate of dopamine 3.6 mm (Watanabe et al, 1997) was used. PET images were (Tsukada et al, 2000). Although dopamine D2 receptor reconstructed by a filtered backprojection method with a binding was decreased (Ohba et al, 2009; Onoe et al, 1994; 4.5-mm Hanning filter, resulting in an in-plane recon- Tsukada et al, 2000), dopamine transporter (DAT) avail- structed resolution of 4.5 mm. PET scans with [11C]DASB ability is increased in living monkey brains (Harada et al, and [18F]MPPF were performed with arterial blood sam- 2004; Tsukada et al, 2001). Although an inhibitory effect of pling. To avoid excessive arterial blood sampling, PET scans ketamine on SERT was reported in vitro (Martin et al, 1990; with [11C]b-CFT were performed without sampling. A Nishimura et al, 1998; Zhao and Sun, 2008), no one has saphenous venous cannula in an inferior limb and another evaluated the effects of subanesthetic doses of ketamine on cannula in the femoral artery of the other leg were inserted. SERT in vivo. The trained animal’s head was rigidly fixed to the upper The rapid antidepressant effect of ketamine at subanes- frame of a monkey chair using an acrylic head-restraining thetic dose for treatment-resistant depressed patients device. The animal sitting in a restraining chair was placed suggests a possible new approach for its therapy, compared at a fixed position in the PET gantry with stereotactic with the standard medications required for several weeks coordinates aligned parallel to the orbitomeatal line. (Krystal, 2010). In addition to robust and rapid antide- Transmission data with a 68Ge–68Ga pin source were pressant effects after a single dose, ketamine has sustained obtained for an attenuation correction. After i.v. bolus antidepressant effects in depressed patients for 1–2 weeks injection of each radiotracer, PET scans were acquired for (Berman et al, 2000; Diazgranados et al, 2010; Price et al, 91 min. The injected dose of [11C]DASB, [18F]MPPF, and 2009; Zarate et al, 2006). This is surprising because of an [11C]b-CFT was 212.1±37.0, 98.3±13.2, and 199.4± approximate 3-h half-life of ketamine in plasma (Clements 65.3 MBq/kg (mean±SD, n ¼ 5), respectively. A summation et al, 1982) and its absence in brain in 24 h. These findings image from 28–40 min postinjection was obtained. The