Can Oral THC Produce DNA-Damage Signals in Rat Brain Cells Without Changing Cholinesterase? A 30-Rat Study

Answer: In a controlled experiment involving 30 male Wistar rats, oral delta-9-tetrahydrocannabinol (THC) at 7 mg/kg per day was associated with increased DNA-migration signals in several alkaline-comet measurements of brain and white blood cells. After one day, three biochemical measures in brain tissue also differed significantly from sesame-oil controls: thiobarbituric-acid-reactive substances and glutathione were higher, and superoxide dismutase activity was lower. But the researchers did not find significant changes in measured plasma oxidative-stress markers or plasma/brain cholinesterase activity at that one-day time point. These are controlled rat-cell and tissue findings, not proof of a human DNA mutation, traumatic brain injury, loss of memory, impairment at a specific blood THC concentration, or a cause of any individual event.

The original primary report is Kopjar N, Fuchs N, Žunec S, et al., DNA Damaging Effects, Oxidative Stress Responses and Cholinesterase Activity in Blood and Brain of Wistar Rats Exposed to Δ9-Tetrahydrocannabinol, Molecules (2019), 24(8):1560, doi:10.3390/molecules24081560. The conclusions below are limited to the methods, Figure 1, Tables 1–2, results and discussion of that complete original experiment.

What question did this study actually test?

The investigators asked whether a fixed, deliberately high oral THC dose would change three families of laboratory endpoints in rats: DNA migration under alkaline comet-assay conditions, selected biochemical responses associated with oxidation, and cholinesterase enzyme activity. That is materially different from asking whether a blood specimen contains a known amount of THC, whether a driver was impaired, or whether cannabis caused illness in a named person. A tissue-damage marker is not itself a drug-concentration result or a clinical outcome.

The study is particularly useful for teaching why apparently discordant toxicology results can coexist. An exposure can produce detectable cellular endpoints in one biological compartment while different biochemical tests in another compartment show no statistically significant change. Different assays answer different questions, respond over different timescales, and have different susceptibility to biological and methodological variation. The study does not establish which test is a forensic diagnostic of intoxication.

Who received THC, and what was the actual exposure?

Researchers randomly assigned 30 male Wistar rats to six treatment-duration-by-condition groups, with five rats in each group. Each THC group was given 7 mg THC per kilogram of rat body weight by mouth in sesame oil once daily for one, three or seven days. Each duration had a corresponding group receiving the same volume of sesame-oil vehicle without THC. Rats were assessed 24 hours after their final administration. Consequently, the study includes three distinct terminal measurement times rather than repeated biological sampling of the same five animals over all three durations.

Seven milligrams per kilogram is an administered oral mass normalized to animal body weight. It is not an observed concentration in human forensic blood, a blood-alcohol equivalent, a verified systemic THC dose or an experimentally established threshold for adverse effects in people. The investigators did not provide a dose-response series in this experiment, did not include females, and did not evaluate vaping, smoked cannabis, driving, witnessed behavior, memory performance or legal fitness.

For each time group, whole blood was collected and partly used immediately to prepare cell slides; a separate portion was centrifuged to obtain plasma for biochemical assays. Brain tissue was dissected and separately prepared for cellular and enzyme measurements. These distinctions matter because a white-cell comet result from whole blood and an antioxidant reading from plasma do not represent the same matrix or analyte.

What does the alkaline comet assay measure—and what does it not?

The alkaline comet assay embeds individual nucleated cells in agarose, subjects them to lysis and alkaline electrophoresis, and images how DNA migrates from each cell’s nucleus. Investigators assessed tail intensity (the proportion of fluorescent DNA in the tail), tail length and total comet area. These are operational endpoints of DNA migration, associated with strand breaks and alkali-labile lesions under the assay’s conditions. They do not identify a particular DNA sequence mutation, distinguish all possible lesion mechanisms, measure brain function, or establish that the sampled cell will undergo apoptosis or become malignant.

The study scored 200 comets per rat on duplicate slides, or approximately 1,000 scored cell images per experimental group. These 1,000 images are not 1,000 independent rats; the independent biological group size remained five. When evaluating statistical precision and broader reproducibility, the hierarchy of repeated cell measurements nested within animals should not be confused with independent exposure subjects. Figure 1 reports group mean ± standard deviation and marks several comparisons significant using Mann–Whitney U testing at p < 0.05. The plotted image, not a fabricated reconstruction, is the source of those significance markers.

Likewise, a slide prepared from brain tissue contains mixed cell populations. The experiment did not separately quantify neurons, astrocytes and other glia, so a result for a brain-cell suspension is not a direct count of DNA breaks occurring exclusively in neurons. White-blood-cell slides also contained mixed nucleated leukocyte types rather than a purified immune-cell subtype.

Which comet-assay findings were significant?

Figure 1 showed significant comet-parameter differences in exposed rats versus duration-matched controls. After one day, white blood cells showed greater DNA-in-tail intensity; the brain-cell signal was more prominent in the investigators’ interpretation. Other endpoints, including tail length and total comet area, also showed marked comparisons at different exposure durations, but their direction and magnitude were not uniformly parallel across matrices or across the one-, three- and seven-day terminal groups. It would misstate the study to turn this figure into a single monotonic percentage increase in “brain DNA damage” or to infer a human dose-response curve.

The table summarizes the actual comparison design and the qualitative results reported by the investigators. It deliberately does not invent exact bar heights from Figure 1, whose individual group estimates were presented graphically rather than as a numerical data table.

What happened to oxidative-stress markers after one day?

The study evaluated ferric reducing antioxidant power (FRAP), thiobarbituric-acid-reactive substances (TBARS), glutathione (GSH), catalase activity and superoxide dismutase (SOD) activity after the one-day treatment. These biochemical endpoints were not repeated as a 3- or 7-day longitudinal oxidative-stress series. In the dissected brain preparation, three markers differed significantly at the authors’ unadjusted threshold. Brain TBARS were higher (control 10.597 ± 4.637 versus THC 17.256 ± 1.353 μmol/L); brain GSH was higher (28.027 ± 8.302 versus 38.537 ± 2.293 μg/mL); and brain SOD activity was lower (3.533 ± 0.612 versus 1.409 ± 0.330 IU/g protein). The original Table 1 marks these three brain comparisons p < 0.05.

Brain FRAP and catalase were not marked statistically significant. The authors also reported no significant treatment-related difference in any of the five plasma oxidative-stress measures at that time point. A nonsignificant result is neither a demonstration that THC cannot affect plasma markers nor an assurance that all such markers are biologically equivalent between exposed and control animals; it reports what these particular assays and small groups detected.

TBARS is an indirect, non-specific assay of substances reacting with thiobarbituric acid, not a unique measurement of oxidative DNA adducts or a chemical identification of the source of free radicals. GSH is part of antioxidant defense and can rise as a compensatory response; it should not be classified mechanically as proof of antioxidant benefit. SOD is an enzyme activity, not a THC concentration. A pattern of higher brain TBARS and GSH alongside lower SOD supports the investigators’ discussion of altered oxidative balance under these conditions, without proving the mechanism of the observed comet-tail changes.

Did THC reduce brain or plasma cholinesterase activity?

Not significantly in the tested setting. Twenty-four hours after one oral dose, measured cholinesterase activities in plasma and brain did not differ significantly from controls. For total activity, the original Table 2 gives plasma median 0.107 IU/g protein for controls versus 0.104 after THC; brain median 21.8 versus 18.1 IU/g protein. These were descriptive median contrasts with overlapping reported ranges and no statistically significant treatment effect. The same conclusion applied to the separately reported acetylcholinesterase and butyrylcholinesterase fractions.

The investigators noted that plasma acetylcholinesterase accounted for an unexpectedly large share of the total assayed activity and suggested that free enzyme and some red-cell hemolysis during separation might have contributed. They did not establish the contribution of hemolysis quantitatively, so that explanation is a caution about matrix preparation rather than a proven correction factor. It would also be incorrect to infer the opposite proposition—that THC has no possible cholinesterase effects at any dose or duration—from this single-dose rat comparison.

Why do plasma and brain tissue give different answers?

The plasma and brain measurements were not assays of the same chemical at two concentrations. Plasma was obtained by centrifuging blood; brain was excised and homogenized. TBARS, glutathione and enzyme activities depend on tissue composition, cellular metabolism and sample processing. The absence of a significant plasma contrast alongside significant brain contrasts therefore is not a laboratory contradiction. It reflects different biological materials, analytical signals and uncertainty. Readers comparing biological matrices in forensic toxicology should retain the specimen label for each result.

Whole blood was used for the leukocyte comet assay, but that did not make the study a whole-blood THC quantification project. The source did not report paired gas-chromatographic or liquid-chromatographic determinations of parent THC, 11-hydroxy-THC or THC-COOH in rat whole blood, plasma or brain. Consequently, no concentration-effect slope, impairment cutoff, individualized back-extrapolation or numerical conversion to a human blood result can be supported from this paper.

Does a comet-assay difference show a permanent mutation or dead neurons?

No. The assay captures a momentary distribution of DNA migration and may reflect DNA strand discontinuities, alkali-labile lesions, processing responses and ongoing repair. The original investigators discussed apoptosis, repair saturation and differential susceptibility of brain cell types as biologically plausible possibilities, but did not directly demonstrate permanent mutations, neuronal loss, a specific DNA lesion identity, or a chronic neurodegenerative syndrome. The same caution applies to the repeated-duration groups, which were euthanized at separate terminal time points rather than followed for long-term neurological outcomes.

Moreover, biochemical TBARS elevation is not a direct sequencing result or specific chemical fingerprint of a DNA lesion. Showing concordant changes in two laboratory endpoint families can generate a mechanistic hypothesis, but it does not establish that the oxidative marker caused the measured DNA migration in each cell. A causal mediation claim would require additional controlled experiments, temporal measurements and appropriate molecular identification.

What were the most important design and statistical limits?

The independent biological sample is small: only five rats in each condition-duration group, all male. Multiple comet endpoints were evaluated in two broad cell preparations at three durations; five oxidative markers in two matrices and three cholinesterase measures in both brain and plasma were also considered. The source reports conventional p < 0.05 comparisons and does not provide a comprehensive, prospectively controlled adjustment for this family of multiple tests. Some apparent effects could be sensitive to sampling and multiplicity, so an individual asterisk should not be inflated into a precise human risk estimate.

The repeated cellular images and triplicate laboratory readings provide technical replication, not more animals. The experiment used a single THC dose and oral oil preparation, with no exposed females, dose-ranging control, parallel human concentration measurements, behavioral impairment tasks, histological counts of dead neurons, or long-term post-treatment recovery series. Assay-specific sources of uncertainty include TBARS non-specificity and possible hemolysis in plasma enzyme studies. Those qualifications do not erase the observed contrasts; they define their scientific reach.

How should the study be used in a forensic toxicology review?

First, identify the precise proposition. If the proposition is that high oral THC exposure can be associated with measurable cellular DNA-migration and brain oxidative-marker changes in a rat model, this study offers limited controlled experimental support. If the proposition concerns a person’s measured THC concentration, use history, reaction time, memory deficit, collision mechanism or irreversible brain damage, this experiment did not test it. Applying a rat cellular result directly to such a proposition would skip several necessary evidentiary links.

Second, request the actual case-specific evidence appropriate to the proposition: the collected specimen and its handling record, method validation, numerical analytical report, uncertainty information, timing, observed behavior and relevant medical findings. Confirmatory cannabinoid analysis determines a chemical in the validated specimen; it is not interchangeable with an alkaline comet measure of DNA migration or an indirect TBARS readout. A legal conclusion about impairment, exposure source or causation must be evaluated on its own factual record.

Third, maintain the boundary between evidence of biological response and evidence of harmful clinical effect. The researchers did not measure rat memory or motor performance and did not follow human patients. Results under a selected rat oral regimen do not identify a patient’s cause of illness or prove that a driver was impaired by THC. Both the significant and null observations must be retained in an accurate review.

Frequently asked questions

1. Did this study give THC to people?

No. All experimental subjects were male Wistar rats. The 30 rats were divided into six groups of five; the administered dose was 7 mg/kg orally on one, three or seven consecutive days.

2. Does the reported whole-blood result give a blood THC concentration?

No. Whole blood provided nucleated white cells for comet imaging. Plasma was separately used for biochemical assays. Neither is a parent-THC blood concentration in this paper.

3. Does an alkaline comet tail establish a permanent DNA mutation?

No. DNA migration under alkaline conditions can indicate primary lesions and repair intermediates. Permanent sequence changes, specific lesion identity and cell fate were not measured by the assay.

4. Did every oxidative-stress marker increase after THC?

No. Brain TBARS and GSH were higher and SOD activity lower after one day, while brain FRAP and catalase and all tested plasma oxidative-stress markers were not reported significantly different.

5. Was cholinesterase inhibited?

No statistically significant inhibition was detected in plasma or brain 24 hours after the single tested oral dose. This null finding cannot rule out effects at untested doses, time points or species.

6. Is 7 mg/kg in rats a human impairment threshold?

No. It describes mass administered per kilogram in an animal experiment. The authors did not establish corresponding human blood THC concentrations, cognitive impairment values, or legal cutoffs.

7. Does the paper prove THC caused a named person’s neurological injury?

No. Its endpoints were mixed rat cell preparations and selected biochemical assays. It has no case-specific medical record, individual human exposure measurements, neuropathological diagnosis or causal evaluation of a real injury.

Bottom line: keep each analytical endpoint in its evidentiary lane

The most defensible reading is narrow: oral THC at 7 mg/kg in small male-rat groups was associated with selected comet-assay DNA migration differences and significant one-day brain biochemical changes, while measured plasma markers and brain/plasma cholinesterase activities did not show significant one-day treatment effects. The paper does not validate a clinical genotoxicity diagnosis, individual human impairment opinion or permanent-neuron-injury attribution. Scientific accuracy depends on preserving the distinction between administered dose, specimen, measured assay endpoint, statistical contrast, clinical outcome and incident-specific causation.

Primary research: Kopjar N, Fuchs N, Žunec S, et al. Molecules. 2019;24(8):1560. https://doi.org/10.3390/molecules24081560.