P.1.29 The decrease of amygdala hyperactivation in depression is associated with paroxetine serotonin transporter occupancy

Molecular neuropsychopharmacology P.1.28 Local administration of citalopram decreases extracellular levels of GABA in the rat hippocampus S.C.S. Rowley1 ° , A.C.E. Linthorst1 . 1 University of Bristol, HW-LINE, Bristol, United Kingdom Aberrant GABA transmission is becoming increasingly implicated in the pathophysiology of many debilitating mood disorders such as anxiety and depression. In addition to its synaptic action GABA can also elicit a tonic inhibition of activity via its action on extrasynaptic GABA receptors (Farrant & Nusser 2005). In vivo microdialysis has successfully been utilised to demonstrate stressorspecific effects on extracellular GABA levels in the brain (de Groote & Linthorst 2007). Hippocampal serotonin (5HT) is integral to the neuroendocrine and behavioural responses to stress and has been implicated in stress coping yet the interactions between 5-HT and GABA are poorly understood (Linthorst & Reul 2008). Therefore, the selective 5-HT re-uptake inhibitor citalopram was employed to acutely increase hippocampal 5-HT levels and hippocampal microdialysis was used to study the subsequent levels of extracellular GABA. Male Wistar rats were equipped with a guide cannula for microdialysis in the hippocampus (under isoflurane anaesthesia). After 7 days of recovery a microdialysis probe (4 mm length) was inserted into the hippocampal CA3-DG region and the animals were connected to a swivel and counterbalance arm system. Two days later 15-min sampling (flow rate of 2 ml/min) was started at 09:00 h. Samples were divided into two aliquots for the later electrochemical detection of 5-HT and GABA levels via HPLC. In drug-perfused animals after two hours of baseline recording citalopram at concentrations of 30 and 100 mM were subsequently applied locally via retrodialysis, for one hour per dose, followed by a final two hour washout. Resting baseline levels of GABA were found to be 24.18±2.07 (mean± SEM, fmol/ml sample, n = 6 animals). This is consistent with our previous data (de Groote & Linthorst 2007). Both doses of citalopram elicited a decrease in extracellular levels of GABA to around 83% and 75% of baseline values, respectively, followed by a recovery during the washout period (ANOVA with repeated measures F(15,75) = 2.53, P < 0.01). Preliminary data show that the concentrations of 5-HT indeed increased during the citalopram treatment and that this increase did not return to baseline values during the 2 hour washout period. 5-HIAA levels seem to remain unchanged by the citalopram treatment. This study demonstrates that local administration of citalopram to the hippocampus decreases extracellular

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concentrations of GABA. The effect of citalopram upon tonic inhibition could provide a novel mechanism of therapeutic effect with important implications for the pathophysiology of mood disorders such as anxiety. It will be important to elucidate whether the effect of citalopram on GABA concentrations is a consequence of increased serotonergic transmission, a direct effect of the drug itself or another unknown indirect effect. The hippocampus strongly expresses both 5-HT1A and 5-HT3 receptor subtypes and further research is underway using pharmacological agents for these receptors to more accurately establish the interplay between 5-HT and GABA in the hippocampus. This work is supported by the BBSRC and the Neuroendocrinology Charitable Trust. Reference(s) [1] de Groote, L., Linthorst, A.C.E., 2007, Exposure to novelty and forced swimming evoke stressordependent changes in extracellular GABA in the rat hippocampus. Neuroscience 148(3): 794–805. [2] Farrant, M., Nusser, Z., 2005, Variations on an inhibitory theme: phasic and tonic activation of GABA(A) receptors, Nat Rev Neurosci 6(3): 215– 229. [3] Linthorst, A.C.E., Reul, J.M.H.M., 2008, Stress and the brain: solving the puzzle using microdialysis. Pharmacol Biochem Behav 90(2): 163–173. P.1.29 The decrease of amygdala hyperactivation in depression is associated with paroxetine serotonin transporter occupancy H.G. Ruhe1 ° , M. Koster1 , J. Booij2 , D.J. Veltman3 , A.H. Schene1 . 1 Academic Medical Center, Department of Psychiatry, Amsterdam, The Netherlands; 2 Academic Medical Center, Department of Nuclear Medicine, Amsterdam, The Netherlands; 3 Academic Medical Center, Department of Addiction Research, Amsterdam, The Netherlands Background: The amygdala is a prominent limbic structure which plays a central role in emotional processing. In major depressive disorder (MDD) structural and functional changes of the amygdala are reported. Few studies investigated specific effects in the amygdala after antidepressant treatment with selective serotonin reuptake inhibitors (SSRIs). Functional Magnetic Resonance Imaging (fMRI) studies found a hyper-activation of the amygdala in MDD, which diminished after successful treatment with SSRIs [1,2]. To date, it is unclear whether this is a direct result of SSRIs.

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Molecular neuropsychopharmacology

Aim: To investigate the association between decreases in amygdala hyperactivation and the paroxetine occupancy of serotonin transporters (SERT) in the amygdala and the midbrain. Methods: Twenty drug-free MDD-patients (SCIDpositive, M+F, 25−55 years, Hamilton depression rating scale (HDRS17 )>18) underwent fMRI (angry + fearful [negative] Ekman faces versus blurred faces paradigm) (2) and [123 I]b-CIT single photon emission computed tomography (SPECT) scanning before the initiation of pharmacotherapy with paroxetine (20mg/day). Fourteen non-responders after 6 weeks of treatment (T0) received a randomized dose-escalation (30–50mg/day or placebo dose-escalation) [3]. Structural MRI scans were used to co register SPECT-scans and determine mean midbrain and amygdala SERT-availability relative to cerebellum. For 18 and 13 patients, both scans were repeated after 6 (T0 ) and 12 weeks (T1 ) of treatment, respectively. Repeated scans were used to calculate changes in amygdala hyperactivation, and paroxetine SERT-occupancies in the midbrain and amygdala, both relative to the pre-treatment scan. As a reference, we obtained fMRI and SERT scans of 19 healthy controls once. Results: At 6 and 12 weeks 4/18 (22%) and 11/18 (61%) patients responded. Repeated fMRI-scans showed decreases in amygdala hyperactivation, which were correlated with proportional decreases in HDRS17-scores. Midbrain and mean amygdala SERT-occupancy at T0 was 53.94 ±21.5 (SD) and 58.66 ±31.5 (SD), respectively, and did not increase significantly after dose-escalation (T1 ). We found an inverse association between increased paroxetine SERT-occupancy and decrease in left fMRI amygdala activation when viewing negative faces. This was found for mean amygdala (F1,23 = 13.112; p = 0.002) and midbrain (F1,23 = 21.927; p = 0.0001) SERT-occupancies. Right amygdala activation was not associated with amygdala, or midbrain SERT-occupancy. Midbrain SERToccupancy explained slightly more variance (r2 = 0.499) of fMRI amygdala activation than mean amygdala SERToccupancy, which was not significantly increased by entering mean amygdala occupancy (F1,21forchange = 0.655; p = 0.427). Conclusion: This is the first multimodality neuroimaging study which combined repeated fMRI imaging of amygdala-function and repeated SPECT-scans providing SERT-occupancies in the amygdala and midbrain during pharmacotherapy of MDD. Our findings point towards an increased serotonergic constraint over the amygdala after paroxetine treatment, and further corroborate amygdala function as a biomarker for successful pharmacological treatment. The stronger association between midbrain SERT-occupancy and decreased amygdala activation compared to mean amygdala SERT-occupancy might

be explained by dense serotonergic projections to the amygdala originating in the raphe nuclei. However, more indirect constraint of other regulatory parts of the brain (e.g. the prefrontal cortex and the cingulate cortex) might also constrain amygdala activation. Future research should clarify by which serotonergic pathway(s) amygdala activation is precisely regulated during treatment with SSRIs. Reference(s) [1] Sheline, Y.I., Barch, D.M., Donnelly, J.M., Ollinger, J.M., Snyder, A.Z., Mintun, M.A., 2001, Increased amygdala response to masked emotional faces in depressed subjects resolves with antidepressant treatment: an fMRI study. Biol Psychiatry 50: 651– 658. [2] Ruhe, H.G., Booij, J., Veltman, D.J., Reitsma, J., Michel, M.C., Schene, A.H., 2008, The effect of paroxetine on amygdala reactivity after emotional faces measured with fMRI. Eur Neuropsychopharmacol 18(Suppl 4): S332. [3] Ruhe, H.G., Booij, J., van Weert, H.C., Reitsma, J.B., Franssen, E.J.F., Michel, M.C., Schene, A.H., 2008, Evidence why dose-escalation of paroxetine in major depressive disorder is not effective: A 6-week, randomized-controlled trial with assessment of serotonin transporter occupancy. Neuropsychopharmacology. [E-pub. October 1st 2008; doi:10.1038/npp.2008.148]. P.1.30 Modulation by spermine of GABAA receptor endogenous phosphorylation and function M. Sid Ahmed1 ° , I. Kurcewicz1 , R. Pumain1 , J. Laschet1 . 1 Centre de Psychiatrie et de Neurosciences Broca-SainteAnne, Laboratoire de Neurobiologie & Pharmacologie Moleculaire, Paris, France In this study a potential new role for spermine in the regulation of the GABAA receptor (GABAA R) endogenous phosphorylation is described. The kinase of endogenous phosphorylation is glyceraldehyde-3-phosphate dehydrogenase (GAPDH), associated to the GABAA R at the membrane [1]. Recent publication [2] showed that this phosphorylation mechanism and GABAA R function are deficient in human epileptogenic tissue. It is also known that epileptogenic zone of human temporal lobe shows an increase of the content of spermine [3]. The first objective of this study was to investigate the effect of spermine on the endogenous phosphorylation of GABAA R. Secondly, we examined its role on the