Evaluation of hippocampal arylalkylamine N-acetyltransferase activity in amyloid-β neurotoxicity

Arylalkylamine N-acetyltransferase (AANAT), a rate-limiting enzyme in melatonin synthesis, is present in extra-pineal tissues such as the hippocampus. The hippocampal AANAT activity in amyloid β (Aβ) neurotoxicity has not been exactly defined. Adult male rats received bilateral intra-CA1 Aβ administration. The hippocampus tissue sampling was performed 2, 12, and 24 h after Aβ injection in the morning and night. The inflammation was monitored using tumor necrosis factor-alpha (TNF-α) immunohistochemistry. The AANAT enzyme activity and melatonin levels were measured using western blotting and high-performance liquid chromatography. The sampling in the morning vs night showed no significant differences in the AANAT activity. The Aβ increased the area of TNF-α positive staining 24 h after injection, which indicated the induction of an inflammatory context. It was accompanied by a significant reduction in AANAT activity and hippocampal melatonin. A reverse correlation was also detected between TNF-α and AANAT activity in the 24-h group. The TNF-α positive area was significantly increased in the 24-h group as compared to the 12-h group. Data showed that inflammatory processes began 12 h after the Aβ injection and augmented 24 h later. In the second experiment, the impact of Aβ injection on hippocampus AANAT activity was examined in the pinealectomized (PIN×) animals. The PIN× per se did not affect the hippocampal AANAT and melatonin levels. However, there was a significant decrease in hippocampal melatonin in the PIN×+Aβ group. The findings suggest the accompanying hippocampal inflammatory context and AANAT enzyme activity reduction in early stages after Aβ administration. Understanding the underlying mechanism of the decreased AANAT activity may suggest new treatment strategies.