An Ultra-Low Dose of ∆9-Tetrahydrocannabinol Alleviates Alzheimer's Disease-Related Cognitive Impairments and Modulates TrkB Receptor Expression in a 5XFAD Mouse Model

Abstract

Alzheimer's disease (AD) is the most common form of dementia, but there is still no available treatment. Δ9-tetrahydrocannabinol (THC) is emerging as a promising therapeutic agent. Using THC in conventional high doses may have deleterious effects. Therefore, we propose to use an ultra-low dose of THC (ULD-THC). We previously published that a single injection of ULD-THC ameliorated cognitive functioning in several models of brain injuries as well as in naturally aging mice. Here, 5xFAD AD model mice received a single treatment of ULD-THC (0.002 mg/kg) after disease onset and were examined in two separate experiments for cognitive functions, neurotropic, and inflammatory factors in the hippocampus. We show that a single injection of ULD-THC alleviated cognitive impairments in 6- and 12-month-old 5xFAD mice. On the biochemical level, our results indicate an imbalance between the truncated TrkB receptor isoform and the full receptor, with AD mice showing a greater tendency to express the truncated receptor, and ULD-THC improved this imbalance. We also investigated the expression of three AD-related inflammatory markers and found an ameliorating effect of ULD-THC. The current research demonstrates for the first time the beneficial effects of a single ultra-low dose of THC in a mouse model of AD after disease onset.

Keywords: 5xFAD mice model; Alzheimer’s disease; cognition; full TrkB receptor; truncated TrkB receptor; ultra-low dose of THC.

Figure 1

Cognitive improvement after ULD-THC. The learning of AD mice was impaired in the Morris water maze (indicative of learning ability and spatial memory) compared to WT mice, and the ULD-THC treatment alleviated this impairment at 6 months (A) and 12 months (B). On the probe phase on day 4 (C), 6-month-old WT mice spent significantly more time at the platform zone than AD mice, indicating the impaired long-term memory acquisition of AD mice. Importantly, ULD-THC-treated AD mice also spend more time than non-treated AD mice. In the novel location recognition test at six months (D), there were significant differences between the groups in long-term associative-spatial memory. Performance was impaired in AD mice compared to WT mice. Importantly, AD-treated mice exhibited a better performance compared to non-treated AD mice and spent significantly more time sniffing the object in the new location. According to the Y-maze, short-term spatial memory (E) was impaired in 12-month-old AD mice compared to WT mice. Importantly, treated AD mice exhibited better performance than vehicle-treated AD mice and spent significantly more time in the new arm. * indicates p > 0.05. ^ indicates the difference between WT groups and TG groups. %—Difference between WT control and TG groups. $—Difference between TG treated and TG control groups. Data in graphs (C–E) are presented as % from WT. n = 7–10. Mean ± SEM.

Figure 2

TrkB Neurotropic receptor imbalance. Six-month-old AD mice displayed higher TrkB.T1 expression in RT-PCR compared to WT mice, while no such upregulation was observed between the AD mice (A). There was no difference in the gene expression levels of TrkB-full receptor between AD mice and WT mice, although we did see an upregulation in the ULD-THC-treated WT mice compared to the non-treated WT mice (B). Non-treated AD mice had a higher ratio of expressed TrkB.T1: TrkB-full compared to WT mice, while no such tendency was observed in treated AD mice (C). 12-month-old AD mice displayed higher TrkB.T1 expression compared to WT mice, while no such upregulation was observed between the treated AD mice (D). There was no difference in the gene expression levels of TrkB-full receptor between AD mice and WT mice, although we did see an upregulation in the ULD-THC-treated WT mice compared to the non-treated WT mice (E). Non-treated AD mice had a higher ratio of expressed TrkB.T1: TrkB-full compared to WT mice, while no such tendency was observed in treated AD mice (F). * indicates p > 0.05. # indicates p = 0.08 Mean ± SEM.

Figure 3

Inflammatory and glia markers of AD at six months. Relative expression in RT-PCR of Tissue Inhibitor of Metalloproteinase 3′ (TIMP-3) was elevated in non-treated AD mice compared to WT mice. Importantly, AD-treated mice had lower expression compared to non-treated mice (A). Relative expression of the Nfkbia gene was elevated in non-treated AD mice compared to WT mice. Importantly, AD-treated mice had lower expression compared to non-treated mice (B). Relative expression of Glucocorticoid activated kinase (Sgk) was higher in non-treated AD mice and in AD-treated mice compared to WT mice (C). The relative expression of both aif1 gene (D), related to microglia and GFAP gene (E); related astrocytes were upregulated in non-treated AD mice compared to WT mice. Notably, in the aif1 gene, AD-treated mice showed a trend toward less gliosis compared to non-treated mice. n = 7–10. * indicates p > 0.05, # indicates p = 0.06, Mean ± SEM.

Figure 4

Study design. The 5xFAD tg mice model at different ages and their WT littermates were treated with a single i.p. injection of either ULD-THC or a vehicle and tested for short- and long-term spatial memory three weeks later.