Effects of (-)-Δ9-trans-Tetrahydrocannabinol on cell number and adult neurogenesis in the hippocampus

On hand thesis titled „Effects of (-)-Δ9-trans-Tetrahydrocannabinol on Hippocampal Cell Number and Adult Neurogenesis“ treats eventual alterations in neuron number, generation rate of neurons and also neuroinflammatory processes within healthy wildtype mice. Object of my present work was to evaluate potential adverse effects of chronic cannabis use on neuron number, neurogenesis and neuroinflammation in a mouse model. For that purpose, a total of 51 adult wildtype mice recieved a six weeks lasting daily intraperitoneal treatment with either THC or its vehicle solution. After daily treatment with either 20 mg per kilogram body weight THC or vehicle for 42 days in a row, the brains of half of the animals were extracted. The remaining mice were kept abstinent for another two months before they got killed as well. The brains were extracted and provided for the stereological and immunhistochemical examination performed within on hand project. At time of analysis the mice were all six and a half months old. Using design based stereolgy, total neuron number of the hippocampal Cornu ammonis 1-region as well as its volume was estimated. Between the overall four differently treated groups of animals there was no significant difference, neither in neuron numbers nor in hippocampal volume detected. Functioning as marker for adult neurogenesis, Doublecortin was immunhistochemically stained in a free-floating staining protocol. Doublecortin-positive neurons in the hippocampal Gyrus dentatus were counted. The results showed no significant effect of the preceding chronic THC treatment on the amount of adult neurogenesis in the examined brain area. In order to highlight eventual neuroinflammatoric effects of the treatment with THC, GFAP was stained as a marker for astrocytes in brains of female mice of all four differently treated groups. In the resulting microscopic pictures there was a significant downregulation of GFAP visible after treatment with THC in contrast to treatment with vehicle only, which happened to persist after the following time period spent in abstinence. Marking activated microglia, the staining against IBA1 showed significant higher levels after THC-treatment and following abstinence. The most remarkable change was percieved in pictures of the stainings against Cannabinoidreceptor CB1: Immediately after THC-treatment CB1 was strongly downregulated, whereas after the two months in abstinence CB1-expression increased highly above the levels of vehicle-treated control groups. On the whole there was no proof for direct neurotoxicity leading to hippocampal neuron loss or impaired neurogenesis found as part of this work, even though the THC-treatment was rather intense regarding dosis and duration of treatment. Nevertheless, the changes in expression of CB1 as well as the used markers for neuroinflammation, GFAP and IBA1, indicate functional alterations oft he mural brains following THC-exposition, which on one hand may work neuroprotective in inflammatoric situations, but seem on the other hand to impair homeostasis in neuroglial networks in a long term and not entirely reversible way.2021-03-1