Mechanism-based pharmacokinetics-pharmacodynamics studies of harmine and harmaline on neurotransmitters regulatory effects in healthy rats: Challenge on monoamine oxidase and acetylcholinesterase inhibition

β-Carboline alkaloid harmine (HAR) and harmaline (HAL) are monoamine oxidase (MAO) and acetylcholinesterase (AChE) inhibitors. However, whether HAR and HAL inhibit MAO or AChE selectively and competitively is unclear. The purpose of this study was to investigate the potential competition inhibition of HAR and HAL on MAO and AChE in brain endothelial cells (RBE4) and in healthy rats to provide a basis for the application of the inhibitors in the treatment of patients with depression and with Parkinson's disease or Alzheimer's disease. The transport properties of HAR and HAL by using blood-brain barrier models constructed with RBE4 were systematically investigated. Then, the modulation effects of HAR and HAL on CNS neurotransmitters (NTs) in healthy rat brains were determined by a microdialysis method coupled with LC-MS/MS. The competition inhibition of HAR and HAL on MAO and AChE was evaluated through real time-PCR, Western blot analysis, and molecular docking experiments. Results showed that HAL and HAR can be detected in the blood and striatum 300 min after intravenous injection (1 mg/kg). Choline (Ch), gamma-aminobutyric acid (GABA), glutamate (Glu), and phenylalanine (Phe) levels in the striatum decreased in a time-dependent manner after the HAL treatment, with average velocities of 1.41, 0.73, 3.86, and 1.10 (ng/ml)/min, respectively. The Ch and GABA levels in the striatum decreased after the HAR treatment, with average velocities of 1.16 and 0.22 ng/ml/min, respectively. The results of the cocktail experiment using the human liver enzyme indicated that the IC50 value of HAL on MAO-A was 0.10 ± 0.08 µm and that of HAR was 0.38 ± 0.21 µm. Their IC50 values on AChE were not obtained. These findings indicated that HAL and HAR selectively acted on MAO in vitro. However, RT-PCR and Western blot analysis results showed that the AChE mRNA and protein expression decreased in a time-dependent manner in RBE4 cells after the HAR and HAL treatments. NT analysis results showed that HAL and HAR selectively affect AChE in vivo. HAL and HAR may be highly and suitably developed for the treatment of Alzheimer's disease. [Display omitted]