Evaluation of serotonin 5HT_1A receptors in postmortem Alzheimer’s disease brains

Objectives Serotonin 5HT_1A receptors may be affected in Alzheimer’s disease (AD) by Aβ-amyloid plaques (Aβ) and neurofibrillary tangles in the hippocampus (HP) and frontal cortex (FC). We report autoradiographic studies in postmortem AD brains of serotonin 5HT_1A receptors using antagonists ¹⁸F -Mefwayand ³H -WAY100635 (WAY) and agonist ³H -8-OH-DPAT (DPAT) which were correlated with ¹¹C /³H -PIB.

Methods Human post-mortem brain sections (AD, n=4, age 77-89, SP Stage C; controls (CN), n=4; age 81-90 SP Stage 0-A) from UCI ADRC were sliced into 7 μm thick HP and FC sections. Adjacent brain slices were incubated with either ¹⁸F -Mefway (5 μCi/cc) or WAY (0.03 μCi/cc) in 50 mM Tris/pH 7.4 buffer and DPAT (0.03 μCi/cc) in 50 mM Tris/pH 7.7 at 37 oC for 1 hr. The effect of Gpp(NH)p (30 μM) was evaluated for high-affinity state binding and 10 μM WAY was used for nonspecific binding. Adjacent sections were tested for Aβ using ¹¹C -PIB (5-25 μCi/cc) in 40% ethanol and ³H - PIB (0.03 μCi/cc) in 10% ethanol and 90% PBS at 37 oC for 0.5-1 hr. Regions of interest were used to quantify the radiotracer’s percent change of binding.

Results The 4G8 immunostains confirmed Aβ presence on AD HP and FC sections while CN sections showed minimal Aβ presence. ¹⁸F -Mefway binding was seen in both the CN and AD HP, with HP/white matter ratios comparable to rodents (>20). ¹¹C -PIB positive HP did not show a significant reduction of ¹⁸F -Mefway compared to CN. Binding of WAY in the FC of AD brains overlapped with ¹¹C /³H -PIB binding. No significant difference was found between AD and CN brain FC sections in the binding of WAY. High-affinity state binding of DPAT was reduced (>50%) in the presence of Gpp(NH)p in both AD and CN brains, suggesting 5HT_1Areceptors in AD retain their G-protein coupling.

Conclusions Preliminary results indicate that serotonin 5HT_1Areceptors of AD brains are not reduced significantly. However, more complete brain analyses must be conduct in order to ascertain this finding. Further experiments are examining the differences between DPAT, WAY and ¹⁸F-Mefway.

Research Support NIH/NIA R33 AG030524 (JM)