Behavioral changes induced by Toxoplasma infection of rodents are highly specific to aversion of cat odors

Abstract

The protozoan parasite Toxoplasma gondii blocks the innate aversion of rats for cat urine, instead producing an attraction to the pheromone; this may increase the likelihood of a cat predating a rat. This is thought to reflect adaptive, behavioral manipulation by Toxoplasma in that the parasite, although capable of infecting rats, reproduces sexually only in the gut of the cat. The "behavioral manipulation" hypothesis postulates that a parasite will specifically manipulate host behaviors essential for enhancing its own transmission. However, the neural circuits implicated in innate fear, anxiety, and learned fear all overlap considerably, raising the possibility that Toxoplasma may disrupt all of these nonspecifically. We investigated these conflicting predictions. In mice and rats, latent Toxoplasma infection converted the aversion to feline odors into attraction. Such loss of fear is remarkably specific, because infection did not diminish learned fear, anxiety-like behavior, olfaction, or nonaversive learning. These effects are associated with a tendency for parasite cysts to be more abundant in amygdalar structures than those found in other regions of the brain. By closely examining other types of behavioral patterns that were predicted to be altered we show that the behavioral effect of chronic Toxoplasma infection is highly specific. Overall, this study provides a strong argument in support of the behavioral manipulation hypothesis. Proximate mechanisms of such behavioral manipulations remain unknown, although a subtle tropism on part of the parasite remains a potent possibility.

Fig. 1.

Infection caused a transient increase in luminescent signals emanating from parasites. The series of images reflects the spread of luminescent signal at successive days after infection. Photon flux is coded by a range of pseudocolors (lowest, blue; highest, red).

Fig. 2.

Infection abolished aversion to bobcat urine in rats, instead producing an attraction. (A) In a circular arena, infection increased the ratio of occupancy in the quadrant laced with bobcat urine versus that with rabbit urine. The ordinate depicts relative occupancy in the bobcat quadrant relative to total occupancy in the bobcat plus rabbit quadrant. The dotted line depicts the chance level. Control, n = 10; infected, n = 10. ∗, P < 0.01 relative to control (Student's t test). (B) Infected rats spent more of their time nearer to bobcat urine compared with control animals, as illustrated in the leftward shift in the cumulative frequency plot of distance from bobcat urine during the trial. The 50% mark (dotted line) for the total n represents the median values. ∗, P < 0.001 (Student's t test). (C and D) Representative scatter plots depicting the location of a control rat (C) and an infected rat (D) during trial, with respect to bobcat and rabbit urine.

Fig. 3.

Infection abolished aversion to bobcat urine and cat collar in mice. (A) Infection increased the ratio of occupancy in the bobcat quadrant versus the rabbit quadrant. The dotted line depicts the chance level. Control, n = 8; infected, n = 13. ∗, P < 0.05 relative to control (Student's t test). (B) Infection enhanced the occupancy of mice in a bisect of a rectangular arena containing a collar worn by a cat relative to the other half containing a sham collar. Control, n = 5; infected, n = 5.

Fig. 4.

Infection did not affect anxiety-like behavior of rats in the open-field arena. Control, n = 15; infected, n = 15. Ordinates depict the total distance traveled during a 30-min trial (A), time spent in the center (inner two-thirds of the arena; the dotted line represents the chance level) relative to the total duration of the trial (B), and the number of progression segments (C).

Fig. 5.

Infection did not affect freezing during various phases of fear conditioning in rats. The ordinate depicts the percentage of time spent in freezing. Control, n = 18; infected, n = 15. P < 0.2 (Student's t test). Panels (from left to right) represent freezing during training, strength of conditioning to context, strength of cued conditioning, and extinction of cued conditioning.

Fig. 6.

Localization of tissue cysts in the brain. (A) Detection of parasites in acute brain slices using bioluminescence imaging revealed the presence of parasites along a broad range of coronal levels. The ordinate depicts the total photon flux (number of photons per second; section thickness = 1,000 μm). Animals infected with Pru strain parasites lacking the luciferase gene served as controls in this experiment. Negative control, n = 3; infected, n = 7. (Inset) Representative luminescence of eight successive sections (Bregma = 1.70 mm to Bregma = −6.30 mm) derived from an infected animal. A threshold was previously determined by using brain slices of control animals. All pixels registering photon flux more than the threshold are depicted. (B) Tissue cysts were observed in a variety of brain regions examined. (C) Determination of tissue cyst density using the physical dissector method showed that, among the brain regions analyzed, amygdalar areas (medial and basolateral amygdala) had higher cyst density compared with nonamygdalar areas either responsive to or nonresponsive to cat odor (n = 6 animals).