Phytocannabinoids as potential tools for ameliorating Rett Syndrome-like phenotype in Mecp2-null mice.

Rett syndrome (RTT) is an X-linked neurodevelopmental disorder with a prevalence rate of 1 in 10,000 females. RTT patients have apparently normal perinatal development until about 6-18 months of age, after which they undergo a period of rapid regression, characterized by the appearance of autistic features, stereotypic hand movements and loss of language. RTT girls also have seizures during childhood, breathing arrhythmias, develop scoliosis and lose mobility between ages 1 and 4 years. Recently, there has been growing interest in the therapeutic potential of phytocannabinoids in the context of neurological diseases. Interestingly, several preclinical and clinical data support the ability of some phytocannabinoids to modulate cognitive and motor functions, mood and neuronal excitability, all of which are altered in RTT. Despite this evidence, so far no studies have addressed the potential therapeutic application of phytocannabinoids in RTT. Based on these premises, the aim of this project was to evaluate the ability of chronic administration of the phytocannabinoids, cannabidivarin (CBDV) and cannabidiolic acid (CBDA), to affect neurological and motor defects as well as cognitive deficits in a mouse model of RTT, namely Mecp2 knockout (KO) mice. To this aim, Mepc2 KO mice were treated daily with CBDV (or relative vehicle) at the doses of 0.2, 2, 20 or 200 mg/kg or CBDA (or relative vehicle) at the dose of 2 and 20 mg/kg from postnatal day (PND) 28 to 66. During the whole treatment schedule, motor and neurological signs were scored while short- and long-term memory deficits were evaluated at PND 41, 56 and 66. 24 hours after the last injection, brain tissues were collected to investigate the presence of alterations on neurotrophic factors (BDNF and IGF-1), inflammatory markers (CD11b, GFAP and TNFα) as well as components of the endocannabinoid system. The present findings provide for the first time direct evidence that CBDV and CBDA improve motor and neurological signs as well as cognitive deficits in Mecp2 KO mice. In particular, CBDV administration delays the appearance neurological and motor signs in Mecp2 KO mice in a time window between 5 and 7 weeks of age. Conversely, CBDA administration ameliorates motor signs only at later stages of the disease progression, i.e. 8 and 9 weeks of age. Remarkably, both phytocannabinoids exert a complete and enduring beneficial effect towards short- and long-term memory deficits in Mecp2 KO animals. At the biochemical level, chronic treatments with CBDV and CBDA enhance the expression of both BDNF and IGF-1 and reduce microglia activation in the brain of Mecp2 KO mice. Moreover, Mecp2 deletion results in alterations in the endocannabinoid system that could likely sustain RTT-like phenotype, and chronic CBDV treatment further modulates them. Although further studies are needed to directly assess the mechanism(s) through which CBDV and CBDA can improve RTT-like phenotype in Mecp2 KO mice, overall these findings suggest for the first time a potential therapeutic application of the phytocannabinoids CBDV and CBDA in the context of RTT.