There are numerous studies on ∆9-THC and its role in cognitive functions, including learning and memory [3]. The role of synaptic plasticity in cognitive functions such as learning and memory formation has been extensively studied [36–38]. Synaptic plasticity is related to the up-regulation of AMPA and NMDA receptors in the membrane [39, 40] two critical events in which microtubules involve. Microtubules motor proteins (dynein) have a role in trafficking NMDA and AMPA receptor subunits to the membrane [41–43].
Moreover, microtubules are the main bricks of synaptic architecture. Synaptic architecture and function are sensitive to changes in microtubule dynamics [44]. Therefore, microtubule dysfunction may affect the synaptic plasticity and, consequently, learning and memory [45]. The major studies on cannabis focused on the effects of ∆9-THC through the cannabinoid receptor (CB1). ∆9-THC may exert its effect on cognitive functions by direct interaction with microtubule proteins.
In this study, we used the acute administration of ∆9-THC. Our behavioral data shows that ∆9-THC has a significant effect on rats’ behavior in both doses in comparison to the control group. THC-treated animals behaved poorly in the T-maze test and took longer to find the reward compared to the control group, which is notably similar to other behavioral studies [46, 47].
Turbidity measurement of BET following the administration of these doses showed ∆9-THC has a reliable effect on microtubule polymerization and kinetic parameters through decreasing the maximum absorbance (Amax) in the presence of both concentrations of ∆9-THC, compared to the control group.
On the contrary, t1/10 increased in THC-treated animals. Increasing tenth times demonstrates a longer lag phase in drug-treated animals compared to the control group. This suggests that the necessary time for tubulin dimers assembly in nucleation increased by injecting ∆9-THC to subjects. Observed data from purified tubulin studies were also consistent. In addition, by considering the other kinetically parameters, p parameter, in BET and purified tubulin, we realized that the nucleus size increased by increasing the ∆9-THC concentration implying that ∆9-THC has a negative effect on lag phase and nucleation.
The kobs value in BET and purified tubulin samples decreased strongly in a dose-dependent manner, show increasing ∆9-THC concentration has a critical role in reducing the elongation process. Therefore, ∆9-THC suppresses elongation. It seems that ∆9-THC decelerates the nucleation and elongation process and reduces the final amount of the protein polymer. Polymerization change could be a consequence of the ∆9-THC administration on tubulin dipole-dipole interactions by structural changes in tubulin.
∆9-THC is a hydrophobic structure that contains three cyclic structures: phenol ring, pyran ring, and a cyclohexane ring, cause rigidity in tubulin’s structure, and alkyl moiety [48]. The addition of ∆9-THC to the tubulin solution might induce exposure of hydrophobic amino acids normally buried inside α- helix secondary structures.
Our CD spectroscopy results elucidated that proportion of a-helix structures was reduced in ∆9-THC treated compared to the control. Additionally, the proportion of β-sheets and other structures increased. The addition of THC to the protein solution induces the secondary structures of the protein to transit to β- sheet and random coil structures aiming to expose more hydrophobic amino acids. Thus, we predict that ∆9-THC can bind to tubulin dimers through hydrophobic interactions. Our computer-aided molecular modeling supports this notion. Totally three major classes of tubulin-binding drugs have been identified[49]: I; Colchicine binding ligands [50, 51], ii; The Vinca domain drugs[52], and iii; Texans and Epothilone family. Vinca domain inhibitors, next to the nucleotide-binding region, cause inhibition of polymerization by preventing nucleic acid exchange [53, 54]. According to our computer-based study, tubulin has one binding site for ∆9-THC around the Vinca region and might inhibit the GDP-GTP exchange; therefore, prevents microtubule polymerization.