Alteration in serotonin 5HT_2A receptors and dopamine D₂-like receptors in human Alzheimer’s disease brains

Objectives Accumulation of senile plaques (SP) and neurofibrillary tangles (NFT) affects neurotransmitter-receptor functions in the Alzheimer’s disease (AD) brain. Previous studies have shown some loss of serotonin and dopamine receptors in AD. We report our results on the evaluation of serotonin 5HT_2A and dopamine D₂-like receptors in frontal cortex (FC) in post-mortem human AD and control brain in relation to Aβ -amyloid.

Methods Human post-mortem brain samples (AD, n=6, age 75-92, SP Stage C and controls, n=6; age 73-90, SP Stage 0-A) were obtained from UCI ADRC and Banner Health. Brain slices (10 μm thick) of FC and cerebellum (CB) of the frozen tissue were obtained on a Leica 1850 cryotome. Adjacent brain slices were incubated with ³H-spiperone (0.4 μCi/cc), in the presence of sulpiride (100 μM), or ketanserin (10 μM) in 50 mM Tris/pH 7.4 buffer 24 °C for 1 hr. Adjacent sections were tested for Aβ using ³H-PIB (0.03 μCi/cc; 80 Ci/mmol). Using the Optiquant program, regions of interest were drawn to quantify the percentage change in binding of the radiotracers.

Results Presence of Aβ -amyloid plaques in the FC of AD subjects was confirmed using 4G8 immunostains and ³H-PIB. Binding of ³H-spiperone was evident in FC of all subjects while CB exhibited little binding. Gray matter (GM) to white matter (WM) ratios computed for all subjects showed control=3.37 and AD=2.55, showing a 24% decrease in AD. Using ratio of total binding in the absence/presence of ketanserin, control=2.96 and AD=3.49, an 18% increase was measured, while using WM ratios a 26% decrease was observed. In the presence of sulpiride, control=1.61 and AD=1.38, a 15% decrease was seen and using WM a 19% decrease was measured.

Conclusions Our preliminary results are consistent with previous findings which indicate that both serotonin 5HT_2Aand dopamine D₂-like receptors are affected in AD FC regions that exhibit high Aβ -amyloid as evidenced by ³H-PIB binding. Further analysis and studies on additional subjects are underway in order to evaluate reliability of WM and CB as a reference region.

Research Support NIH/NIA R33AG030524, R01AG029479.