Zoopharmacognosy in diseased laboratory mice: conflicting evidence

Abstract

Zoopharmacognosy denotes a constellation of learned ingestive responses that promote healing and survival of infected or poisoned animals. A similar self-medication phenomenon was reported in diseased laboratory rodents. In particular, a series of studies revealed that autoimmune MRL/lpr mice readily consume solutions paired or laced with cyclophosphamide (CY), an immunosuppressive drug that prevents inflammatory damage to internal organs. However, due to design limitations, it could not be elucidated whether such a response reflects the learned therapeutic effect of CY, or a deficit in sensory input. We presently assess the behavioural effects of prolonged consumption of CY-laced, 16% sucrose solution in a continuous choice paradigm, with tap water available ad lib. Contrary to overall expectation, MRL/lpr mice did not increase their intake of CY with disease progression. Moreover, they ingested lower doses of CY and preferred less CY-laced sucrose solution than age-matched controls. The results obtained could not confirm zoopharmacognosy in diseased MRL/lpr mice, likely due to impaired responsiveness to palatable stimulation, or attenuated survival mechanisms after prolonged inbreeding in captivity. However, by revealing the effectiveness of unrestricted drinking of drug-laced sucrose solution on behavior and immunity, the current study supports broader use of such an administration route in behavioural studies sensitive to external stressors.

Figure 1. Mean body weight from 11–22 weeks of age.

Autoimmune MRL/lpr mice were significantly lighter when compared to congenic controls. However, prolonged immunosuppression further reduced their body weight, but not in MRL +/+ mice.

Figure 2. Daily fluid consumption from 11–22 weeks of age.

(A) Despite fluctuations in daily water intake over the course of the study, Veh MRL +/+ mice consumed the most water in the 2-bottle test. They also drank significantly more than their CY +/+ counterparts, a trend that was not noted in the MRL/lpr substrain. (B) Lacing sweetened solution with CY significantly reduced daily consumption. However, autoimmune MRL/lpr mice consumed significantly less CY solution in comparison to MRL +/+ mice, particularly after the first week of treatment.

Figure 3. Weekly CY dose ingested.

Although both substrains received similar CY dose at the starting of the treatment period, voluntary drinking in subsequent weeks was higher in less symptomatic MRL +/+ than in MRL/lpr mice.

Figure 4. Weekly preference for sweet solutions.

Initially, CY lpr mice displayed a higher preference for CY/sucrose in comparison to control CY +/+ group at 11 weeks of age. However, this difference was transient and became the lowest of all groups by the end of the study.

Figure 5. Behavioural effects of chronic exposure to CY/sucrose solution.

(A) Chronic immunosuppression reduced step-down latency in diseased MRL/lpr mice, but had the opposite effect on congenic controls. (B) In addition to the previously-documented substrain differences in sucrose consumption , CY had a significant detrimental effect on the performance in both groups. (C) Although a trend was noted, CY could not abolish previously-reported differences in overall floating in the forced swim test . (D) Prolonged immunosuppression improved spontaneous ambulatory activity in MRL/lpr mice, but had a detrimental effect on MRL +/+ controls. (E) Conversely, CY exposure significantly improved (but did not normalize) running wheel activity in MRL/lpr mice.

Figure 6. Immunosuppressive effects of continuous exposure to CY/sucrose solution.

Generalized immunosuppression in CY-exposed MRL/lpr mice evidenced by normalized spleen weight (A), as well as significant reductions in serum levels of autoantibodies directed towards dsDNA (B), cardiolipin (C), and PR3 (D) antigens.