In vivo and in vitro studies of the serotonin 1B receptor in relation to major depressive disorder and treatment with ketamine

Abstract

Despite the large implications of Major Depressive Disorder (MDD) on disease burden worldwide, current treatment options are suboptimal and a third of patients suffering from this disease do not respond to treatment. Therefore, an unmet need exists for the development of new treatment options and methods to aid appropriate treatment selection in individual patients. Selection of suitable biomarkers and reliable quantification methods are essential steps in this process. In recent research on MDD, more interest has arisen for the serotonin 1B (5-HT1B) receptor and for ketamine as a new antidepressant treatment option. This thesis focuses on the involvement of the 5-HT1B receptor and the related protein p11 in the pathophysiology of MDD and the antidepressant mechanism of action of ketamine. For quantification of 5-HT1B receptor densities, the nuclear imaging techniques Autoradiography (ARG) and Positron Emission Tomography (PET) were used. This work includes the development and application of an improved method for quantification of 5-HT1B receptor binding using PET. Quantification of p11 levels was performed in specific cell populations using Flow Cytometry.

In study I, 5-HT1B receptor binding densities and cortical distribution were examined using ARG in anterior cingulate cortex tissue of subjects with MDD, schizophrenia, bipolar disorder and healthy controls. Binding of the radioligand [3H]AZ10419369 in tissue of in total 52 subjects showed no significant differences between the subject groups. A distribution pattern with higher 5-HT1B receptor binding in supragranular layer compared to the infragranular layer was found, which correlated with glutamatergic N-methyl-D-aspartate receptor distribution. Female subjects had lower 5-HT1B receptor densities than male subjects, which was mostly profound in the MDD group.

In study II, an improved method was developed for delineation of Volumes of Interest (VOIs) for PET data with the radioligand [11C]AZ10419369. Based on a 3D [3H]AZ10419369 ARG model in post mortem brainstem tissue and literature findings, appropriate VOIs for quantification in PET were selected. Two previously developed semi-automatic VOI delineation methods, based on template or individual data, were evaluated on test-retest data of 8 healthy subjects and showed improved reliability compared to a conventional manual VOI. The VOIs created with PET template data of 52 healthy subjects can be automatically applied to future PET studies measuring 5-HT1B receptor binding in the brainstem.

Furthermore, in a randomized placebo-controlled study the effect of ketamine on cerebral [11C]AZ10419369 PET binding (study III) and peripheral p11 protein levels measured with Flow Cytometry (study IV) were examined in patients with Selective Serotonin Reuptake Inhibitor (SSRI) resistant MDD. An increase in 5-HT1B binding in the hippocampus and a decrease in p11 levels in both cytotoxic T cells and T-helper cells populations were seen in the ketamine group (n=20), although both did not differ from changes seen in the placebo group (n=10). Changes in Montgomery-Åsberg Depression Rating Scale (MADRS) score after ketamine treatment correlated significantly with baseline 5-HT1B receptor binding in the ventral striatum and baseline p11 levels in cytotoxic T cells. Future studies should be conducted on the role of 5-HT1B receptors and p11 in the antidepressant mechanism of action of ketamine and should clarify if these proteins could be used as biomarkers to predict ketamine treatment response in subjects with SSRI-resistant MDD.