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Posttraumatic Stress Disorder Symptoms are Associated with Impaired Mnemonic Pattern Separation but Paradoxically Improved Pattern Separation with Comorbid Heavy Cannabis Use

Jacob Ross
University of California San Diego

Co-Authors: Bruna Cuccurazzu1, Cindy Napan1, Dean T Acheson1,2, Dewleen G Baker1,2, Victoria B Risbrough1,2, Daniel M Stout1,2
1University of California San Diego, 2VA San Diego Healthcare System

Background: Posttraumatic stress disorder (PTSD) is associated with impaired ability to use contextual cues to regulate emotional responses. Contextual learning is a hippocampus-dependent process that underlies the ability to differentiate novel cues from previously learned but similar stimuli. Heavy cannabis use is highly prevalent in individuals with PTSD. With the advent of recreational and medicinal cannabis availability, the impact of heavy cannabis use in individuals with PTSD on contextual learning mechanisms is poorly understood.

Objective: To test the hypothesis that heavy cannabis use would be associated with significant disruptions in hippocampal-dependent memory in individuals with PTSD symptoms using the Mnemonic Similarity Task (MST).

Methods: To investigate the relationship between heavy cannabis use and PTSD on pattern separation ability, data from two independent samples with the same inclusion criteria were combined. Adult participants (n = 123; 31 female) with and without heavy cannabis use and with a range of PTSD symptoms completed the mnemonic similarity task to assay mnemonic pattern separation ability, and self-report measures of PTSD severity (PCL-5), depression symptoms (BDI-II), and cannabis use (CDDR) were collected.

Results: In the no cannabis use group, increasing PTSD symptom severity was associated with lower mnemonic pattern separation performance (p=.005). In contrast, in heavy cannabis users, increasing levels of PTSD symptom severity was associated with greater pattern separation performance (p=.032), but after including depression symptoms (BDI-II) as a covariate, this relationship was no longer significant (p=.96). Secondary analyses indicated that at low levels of PTSD symptom severity, heavy cannabis use is associated with lower pattern separation ability compared to those without heavy cannabis use; we also show preliminary evidence that this pattern is stronger in women than in men.

Conclusions: These results provide evidence that heavy cannabis use may have a detrimental impact on contextual learning ability but may spare this ability in individuals with elevated PTSD symptoms. Future work should include collection of hormonal assays and measures of both endogenous and exogenous cannabinoid levels to examine their interacting contributions on hippocampal-dependent mnemonic processes.