5-HT2A Receptor Binding in the Frontal Cortex of Parkinson's Disease Patients and Alpha-Synuclein Overexpressing Mice: A Postmortem Study

Abstract

The 5-HT2A receptor is highly involved in aspects of cognition and executive function and seen to be affected in neurodegenerative diseases like Alzheimer's disease and related to the disease pathology. Even though Parkinson's disease (PD) is primarily a motor disorder, reports of impaired executive function are also steadily being associated with this disease. Not much is known about the pathophysiology behind this. The aim of this study was thereby twofold: (1) to investigate 5-HT2A receptor binding levels in Parkinson's brains and (2) to investigate whether PD associated pathology, alpha-synuclein (AS) overexpression, could be associated with 5-HT2A alterations. Binding density for the 5-HT2A-specific radioligand [(3)H]-MDL 100.907 was measured in membrane suspensions of frontal cortex tissue from PD patients. Protein levels of AS were further measured using western blotting. Results showed higher AS levels accompanied by increased 5-HT2A receptor binding in PD brains. In a separate study, we looked for changes in 5-HT2A receptors in the prefrontal cortex in 52-week-old transgenic mice overexpressing human AS. We performed region-specific 5-HT2A receptor binding measurements followed by gene expression analysis. The transgenic mice showed lower 5-HT2A binding in the frontal association cortex that was not accompanied by changes in gene expression levels. This study is one of the first to look at differences in serotonin receptor levels in PD and in relation to AS overexpression.

Figure 1

Receptor binding analysis and alpha-synuclein levels in postmortem prefrontal cortex of Parkinson's disease (PD) patients. (a) Binding assays with [³H] MDL100.907 comparing membrane suspensions from postmortem prefrontal cortex of Parkinson's disease (PD) patients and controls show higher maximum binding levels (B _max) of the 5-HT_2A receptor in the PD group. (b) Western blot showing higher levels of alpha-synuclein proteins in the PD brains included in the binding assay. Control = normal controls; PD = Parkinson's disease patients. Significant ^∗ p < 0.05. Student's t-test with mean ± SEM.

Figure 2

5-HT_2A receptor mouse prefrontal cortex autoradiography binding analysis and gene expression levels. (a) Results from [³H] MDL 100.907 (5-HT_2A) binding. FrA [³H] MDL 100.907 binding is significantly lower in alpha-synuclein overexpressing (SNCA) mice compared to wt mice. Bars: grey: wild-type mice (n = 9); black: SNCA mice (n = 10). ^∗ p < 0.05. Multiple t-test. Data illustrated with mean ± SEM. (b) Results from RT-qPCR show CT ratios (calculated using CT-ratio (receptor/reference) = 2^{(CT  (reference)  −  CT  (receptor))}) comparing alpha-synuclein overexpressing (SNCA) and wt mice. Reference genes are Rpl13a and GAPDH. No difference in receptor gene expression was found. Bars show mean ± SEM.