Synthesis, In Silico, and Biological Evaluation of a Borinic Tryptophan-Derivative That Induces Melatonin-like Amelioration of Cognitive Deficit in Male Rat

Abstract

Preclinical and clinical evidence supports melatonin and its analogues as potential treatment for diseases involving cognitive deficit such as Alzheimer's disease. In this work, we evaluated by in silico studies a set of boron-containing melatonin analogues on MT1 and MT2 receptors. Then, we synthesized a compound (borolatonin) identified as potent agonist. After chemical characterization, its evaluation in a rat model with cognitive deficit showed that it induced ameliorative effects such as those induced by equimolar administration of melatonin in behavioral tests and in neuronal immunohistochemistry assays. Our results suggest the observed effects are by means of action on the melatonin system. Further studies are required to clarify the mechanism(s) of action, as the beneficial effects on disturbed memory by gonadectomy in male rats are attractive.

Keywords: Alzheimer; amyloid; androgen depletion; boron; boron-containing compounds; boroxazolidone; cognitive deficit; male rat; melatonin; melatonin receptors.

Figure 1

The chemical structures of tryptophan, melatonin, and borolatonin (adduct of tryptophan and 2-APB).

Figure 2

Synthesis procedure of the borolatonin compound.

Figure 3

The 3-D models of human (cyan) or rat (gray) of MT1 (A) and MT2 (B) melatonin receptors. High similarity in the general disposition, conformations, and binding sites of the four receptors is noteworthy.

Figure 4

Overlay of docked ligand structures (in stick and balls representation) on human MT1 ((A), with agomelatine) and MT2 ((B), with ramelteon) melatonin receptors as they are found in the crystal complexes. The sidechain of some residues considered key in the binding pocket are depicted as reference. High similarity in the binding sites of both receptors is notable. Oxygen atoms are in red color, nitrogen atoms are in blue color.

Figure 5

Docking of borolatonin (in sticks and balls representation) on melatonin receptors. Binding site on the crystalized human MT1 (A), MT2 (B), and the built models of rat MT1 (C) and MT2 (D). All sidechains of residues are labeled in licorice representation. Underlined residues marked in the lower panels are different in the binding site of the homologous human receptor. Asterisk is in the boron atom of each borolatonin molecule. In the D panel, Thr 191 and Asn 175 were not included for clarity. Oxygen atoms are in red color, nitrogen atoms are in blue color.

Figure 6

Relationship between estimated (theoretical) and reported (experimental) affinity of tested ligands on human MT1 (A) or MT2 (B) melatonin receptors. Linear regression equation and coefficients of determination are shown in each plot.

Figure 7

Affinity of tested ligands on human melatonin receptors. Arrows are for labeling the endogenous agonist melatonin, the tested compound borolatonin, and a region for predicted affinity values for the well-known melatonin-receptor ligands (*WKML).

Figure 8

Post-treatment short-term (A) and long-term (B) memory performance. Results are expressed as mean of time in the safe compartment ± Standard Error of the Mean (SEM, n = 8). * p < 0.05, compared to orchiectomized control group (GDX); MEL, melatonin; BORO, Borolatonin.

Figure 9

Motor performance in the open-field test at the post-treatment evaluation. No difference was found among groups. Results are expressed as mean ± SEM (n = 8). GDX, gonadectomized; MEL, melatonin; BORO, Borolatonin.

Figure 10

Neuronal survival. Above are photomicrographs of NeuN positive labeling observed in the hippocampus. Magnification 40×. Scale bar represents 100 μm. In the plot the columns represent the number of NeuN-positive neurons, bars represent SEM. Significant differences in groups compared with the control group (n = 4, * p < 0.01); GDX, gonadectomized; MEL, melatonin; BORO, Borolatonin.

Figure 11

Amyloid beta accumulation. Above are photomicrographs displaying Aβ-positive labeling (arrows) seen in the rat hippocampus. Magnification 40×. Scale bar represents 100 µm. In the plot columns represent the number of positive Aβ-regions related to plaques in the hippocampus ± SEM (as bars). Significant differences in groups compared with to orchiectomized control group (GDX) group (n = 4, * p < 0.01); MEL, melatonin; BORO, borolatonin.