The BTBR mouse model of autism spectrum disorders has learning and attentional impairments and alterations in acetylcholine and kynurenic acid in prefrontal cortex

Abstract

Autism is a complex spectrum of disorders characterized by core behavioral deficits in social interaction, communication, repetitive stereotyped behaviors and restricted interests. Autism frequently presents with additional cognitive symptoms, including attentional deficits and intellectual disability. Preclinical models are important tools for studying the behavioral domains and biological underpinnings of autism, and potential treatment targets. The inbred BTBR T+tf/J (BTBR) mouse strain has been used as an animal model of core behavioral deficits in autism. BTBR mice exhibit repetitive behaviors and deficits in sociability and communication, but other aspects of their cognitive phenotype, including attentional performance, are not well characterized. We examined the attentional abilities of BTBR mice in the 5-choice serial reaction time task (5-CSRTT) using an automated touchscreen testing apparatus. The 5-CSRTT is an analogue of the human continuous performance task of attention, and so both the task and apparatus have translational relevance to human touchscreen cognitive testing. We also measured basal extracellular levels of a panel of neurotransmitters within the medial prefrontal cortex, a brain region critically important for performing the 5-CSRTT. We found that BTBR mice have increased impulsivity, defined as an inability to withhold responding, and decreased motivation, as compared to C57Bl/6J mice. Both of these features characterize attentional deficit disorders in humans. BTBR mice also display decreased accuracy in detecting short stimuli, lower basal levels of extracellular acetylcholine and higher levels of kynurenic acid within the prefrontal cortex. Intact cholinergic transmission in prefrontal cortex is required for accurate performance of the 5-CSRTT, consequently this cholinergic deficit may underlie less accurate performance in BTBR mice. Based on our findings that BTBR mice have attentional impairments and alterations in a key neural substrate of attention, we propose that they may be valuable for studying mechanisms for treatment of cognitive dysfunction in individuals with attention deficits and autism.

Figure 1. The touchscreen apparatus.

The touchscreen apparatus as used in this experiment consisted of a touch sensitive screen and operant chamber mounted within a sound attenuating box and equipped with food reward delivery, a house light, a magazine light and tone generator (Campden Instruments, Ltd., Leicester, UK).

Figure 2. BTBR mice show increased grooming in a cage, but not in the touchscreen.

BTBR mice (n = 11) spend significantly more time grooming in the home cage test, but grooming is greatly reduced and not significantly different from C57 mice (n = 10) when both strains are assessed in the touchscreen apparatus. Grooming is shown as the total number of seconds spent grooming in a ten minute test period.

Figure 3. BTBR mice show slower habituation.

(A) BTBR (n = 12) mice consume significantly fewer rewards than C57 mice (n = 12) after three days of habituation to the touchscreen. Data are shown for day 3 of habituation. (B) After additional days of habituation, both BTBR and C57 mice are consuming the same number of rewards on the last day of habituation. Data are shown for the last day of total habituation (day 3 for C57 mice, and day 3–8 for BTBR mice depending on individual performance).

Figure 4. BTBR mice show slower initial learning.

BTBR mice (n = 12) took a significantly greater number of days to learn the initial screen-touch, as compared to C57 mice (n = 12; A). There was no significant difference in the number of days taken to learn the “punished” stage of the initial training (B) however the variability exhibited by the BTBR mice is very large compared to C57 mice. Finally, after pre-training was complete, both BTBR and C57 mice showed comparable accuracy performance in the last trial (C) indicating that their performance levels were equal before commencing training on the 5-CSRTT.

Figure 5. BTBR mice acquire the 5 choice serial reaction time task (5-CSRTT) more slowly than C57 mice.

BTBR mice (n = 9) took a significantly greater number of days than C57 mice (n = 12) to reach criterion of 80% accuracy with <20% omissions on the 5-CSRTT at a stimulus duration of 4 seconds.

Figure 6. BTBR mice show increased impulsivity.

On a long ITI probe session (10 second ITI; 8 second stimulus duration), BTBR mice (n = 12) showed a greater increase in the number of premature responses than C57 mice (n = 12; A). However this manipulation had no effect on accuracy (B) or omissions, although BTBR mice made more omissions in both ITI lengths (C).

Figure 7. BTBR mice show impaired accuracy, omissions and impulsivity.

Performance of BTBR (n = 12) and C57 mice (n = 12) on an accuracy probe session. In these sessions, ITI was held constant at 5 seconds, and mice were given one session at each of 4, 2, 0.8 and 0.4 second stimulus duration. For both strains, accuracy declines as stimulus duration is reduced (A) but BTBR mice are less accurate overall. BTBR mice consistently make more premature responses (B) but there is no effect of stimulus duration. Omissions increase for both strains as stimulus duration is decreased (C), BTBR mice consistently omit more trials than C57 mice. BTBR mice also show consistently longer magazine latencies (D) than C57 mice, again there is no effect of stimulus duration.

Figure 8. BTBR mice have lower acetylcholine, and higher kynurenic acid levels in mPFC.

Basal levels of acetylcholine (A; ACh) and kynurenic acid (B; KYNA) as measured by in vivo microdialysis in mPFC. In BTBR mice (n = 7) levels of acetylcholine were lower and kynurenic acid higher than C57 mice (n = 5) under baseline conditions.