mTORC1-dependent protein synthesis underlying rapid antidepressant effect requires GABA B R signaling E.R. Workman, Farr Niere, Kimberly F. Raab-Graham * Center for Learning and Memory, Section of Neurobiology, University of Texas at Austin, Austin, TX 78712, USA article info Article history: Received 4 December 2012 Received in revised form 16 May 2013 Accepted 27 May 2013 Keywords: GABA B receptors NMDA receptors Protein synthesis Rapid antidepressant Hippocampal neurons Prefrontal cortex mTORC1 L-type calcium channels Calcium imaging abstract Administration of N-methyl-D-aspartate receptors (NMDAR) antagonists initiates a rapid anti-depressant response requiring mammalian Target of Rapamycin Complex 1 (mTORC1) kinase; however the mo- lecular mechanism is unknown. We have determined that upon NMDAR blockade, dendritic g-amino- butyric acid B receptors (GABA B R) facilitate dendritic calcium entry. The GABA B R-mediated increase in calcium signal requires the availability of dendritic L-type calcium channels. Moreover, GABA B R can activate mTOR and increase mTOR dependent expression of BDNF under the same NMDAR blocked conditions. In vivo, blocking GABA B R prevents the fast-acting, anti-depressant effect of the NR2B antagonist, Ro-25-6891, decreases active mTORC1 kinase, and reduces expression of BDNF and the AMPA receptor subunit GluA1. These findings propose a novel role for GABA B Rs in the antidepressant action of NR2B antagonists and as an initiator/regulator of mTORC1-mediated translation. Published by Elsevier Ltd. 1. Introduction Major depressive disorder (MDD) is a chronic disease with low remission rates (w33%) and relies on lengthy use of antidepres- sants for therapy (Kessler et al., 2003; Murrough, 2012; Rush et al., 2011, 2006). Several well-characterized N-methyl-D-aspartate re- ceptor (NMDAR) antagonists (AP-7, Ro-25-6981, MK-801, and ke- tamine) act as rapid onset antidepressants; however the mechanism by which this occurs is largely unknown (Autry et al., 2011; Trullas and Skolnick, 1990; Zarate et al., 2006). A recent finding demonstrates that rapid antidepressants can increase the activity of the mammalian target of rapamycin (mTOR), a serine/ threonine kinase critical for translation (Li et al., 2010). mTOR consists of two complexes mTORC1 and C2, with C1 mediating protein synthesis (Hay and Sonenberg, 2004; Hoeffer and Klann, 2010). Because activation of mTOR kinase is generally regarded as a consequence of NMDAR stimulation, it is unknown how NMDAR antagonists increase active mTOR (Gong et al., 2006; Raab-Graham et al., 2006). In this study we address the fundamental question of how mTOR kinase is activated during NMDAR blockade. Many molecular changes occur when NMDAR activity is altered. For example, NMDAR activation negatively regulates the surface expression of g-amino butyric acid type B receptors (GABA B Rs) (Chalifoux and Carter, 2011a; Guetg et al., 2010; Padgett and Slesinger, 2010). GABA B Rs inhibit neuronal activity by reducing neurotransmitter release, via the inhibition of presynaptic calcium channels, and by mediating slow inhibitory postsynaptic potentials via the activation of postsynaptic potassium channels. Whether changes in GABA B R signaling play a role in the molecular basis of rapid antidepressants is unknown. Herein, we demonstrate that a functional shift of postsynaptic GABA B Rs underlies the enhanced mTOR activity induced by NMDAR antagonists. GABA B Rs shift from reducing to increasing resting dendritic calcium signal and require L-type calcium channels to do so. Consistent with the requirement for new protein synthesis of plasticity related proteins, blocking GABA B R signaling in mice treated with the rapid antidepressant Ro-25- 6891 reduces the protein levels of the brain-derived neurotropic factor (BDNF) and the synaptic protein GluR1/GluA1 in the pre- frontal cortex. We extend these findings by demonstrating that block of GABA B Rs in vivo prevents both the up regulation of mTOR and the expected behavioral phenotype arising from blocking NMDARs. * Corresponding author. Tel.: þ1 5122320892. E-mail address: Kimberly@mail.clm.utexas.edu (K.F. Raab-Graham). Contents lists available at SciVerse ScienceDirect Neuropharmacology journal homepage: www.elsevier.com/locate/neuropharm 0028-3908/$ e see front matter Published by Elsevier Ltd. http://dx.doi.org/10.1016/j.neuropharm.2013.05.037 Neuropharmacology 73 (2013) 192e203