Interstimulus interval (ISI) discrimination of the conditioned eyeblink response in a rodent model of autism

Cerebellar associative sensory learning defects in five mouse autism models

Sensory integration difficulties have been reported in autism, but their underlying brain-circuit mechanisms are underexplored. Using five autism-related mouse models, Shank3+/ΔC, Mecp2(R308/Y), Cntnap2-/-, L7-Tsc1 (L7/Pcp2(Cre)::Tsc1(flox/+)), and patDp(15q11-13)/+, we report specific perturbations in delay eyeblink conditioning, a form of associative sensory learning requiring cerebellar plasticity. By distinguishing perturbations in the probability and characteristics of learned responses, we found that probability was reduced in Cntnap2-/-, patDp(15q11-13)/+, and L7/Pcp2(Cre)::Tsc1(flox/+), which are associated with Purkinje-cell/deep-nuclear gene expression, along with Shank3+/ΔC. Amplitudes were smaller in L7/Pcp2(Cre)::Tsc1(flox/+) as well as Shank3+/ΔC and Mecp2(R308/Y), which are associated with granule cell pathway expression. Shank3+/ΔC and Mecp2(R308/Y) also showed aberrant response timing and reduced Purkinje-cell dendritic spine density. Overall, our observations are potentially accounted for by defects in instructed learning in the olivocerebellar loop and response representation in the granule cell pathway. Our findings indicate that defects in associative temporal binding of sensory events are widespread in autism mouse models.

Modeling possible effects of atypical cerebellar processing on eyeblink conditioning in autism

Autism is unique among other disorders in that acquisition of conditioned eyeblink responses is enhanced in children, occurring in a fraction of the trials required for control participants. The timing of learned responses is, however, atypical. Two animal models of autism display a similar phenotype. Researchers have hypothesized that these differences in conditioning reflect cerebellar abnormalities. The present study used computer simulations of the cerebellar cortex, including inhibition by the molecular layer interneurons, to more closely examine whether atypical cerebellar processing can account for faster conditioning in individuals with autism. In particular, the effects of inhibitory levels on delay eyeblink conditioning were simulated, as were the effects of learning-related synaptic changes at either parallel fibers or ascending branch synapses from granule cells to Purkinje cells. Results from these simulations predict that whether molecular layer inhibition results in an enhancement or an impairment of acquisition, or changes in timing, may depend on (1) the sources of inhibition, (2) the levels of inhibition, and (3) the locations of learning-related changes (parallel vs. ascending branch synapses). Overall, the simulations predict that a disruption in the balance or an overall increase of inhibition within the cerebellar cortex may contribute to atypical eyeblink conditioning in children with autism and in animal models of autism.

Embryological exposure to valproic acid disrupts morphology of the deep cerebellar nuclei in a sexually dimorphic way

Autism spectrum disorders (ASD) is diagnosed in males at a much higher rate than females. For this reason, the majority of autism research has used male subjects exclusively. However; more recent studies using genetic sex as a factor find that the development of the male and female brain is differentially affected by ASD. That is, the natural sex-specific differences that exist between male and female brains lead to sexually dimorphic expressions of autism. Here we investigate the putative sexual dimorphism that exists in the deep cerebellar nuclei of male and female rats exposed to valproic acid (VPA) on embryological day 12.5. We find natural sex-specific differences in adult nucleus area, length, and estimated cell populations. Therefore VPA exposure during embryology creates some sex-specific deficits such as higher cell counts in the VPA males and lower cell counts in the VPA females. At the same time, some effects of VPA exposure occur regardless of sex. That is, smaller nucleus area and length lead to truncated nuclei in both VPA males and females. These deficits are more pronounced in the VPA males suggesting that genetic sex could play a role in teratogenic susceptibility to VPA. Taken together our results suggests that VPA exposure induces sexually dimorphic aberrations in morphological development along a mediolateral gradient at a discrete region of the hindbrain approximate to rhombomere (R) 1 and 2. Sex-specific disruption of the local and long-range projections emanating from this locus of susceptibility could offer a parsimonious explanation for the brain-wide neuroanatomical variance reported in males and females with ASD.

Exercise and environment as an intervention for neonatal alcohol effects on hippocampal adult neurogenesis and learning

Neonatal alcohol exposure impairs cognition and learning in adulthood and permanently damages the hippocampus. Wheel running (WR) improves hippocampus-associated learning and memory and increases the genesis and survival of newly generated neurons in the hippocampal dentate gyrus. WR significantly increases proliferation of newly generated dentate granule cells in alcohol-exposed (AE) and control rats on Postnatal Day (PD) 42 but only control rats show an increased number of surviving cells thirty days after WR (Helfer et al., 2009b). The present studies examined whether proliferation-promoting WR followed by survival-enhancing environmental complexity (EC) during adolescence could increase survival of new neurons in AE rats. On PD 4-9, pups were intubated with alcohol in a binge-like manner (5.25g/kg/day, AE), were sham-intubated (SI), or were reared normally (suckle control, SC). On PD 30 animals were assigned to WR (PD 30-42) followed by EC (PD 42-72; WR/EC) or were socially housed (SH/SH) for the duration of the experiment. All animals were injected with 200mg/kg bromodeoxyuridine (BrdU) on PD 41. In Experiment 1, survival of newly generated cells was significantly enhanced in the AE-WR/EC group in comparison with AE-SH/SH group. Experiment 2A examined trace eyeblink conditioning. In the SH/SH condition, AE impaired trace eyeblink conditioning relative to SI and SC controls. In the WR/EC condition, AE rats performed as well as controls. In Experiment 2B, the same intervention was examined using the context preexposure facilitation effect (CPFE); a hippocampus-dependent variant of contextual fear conditioning. Again, the WR/EC intervention reversed the deficit in conditioned fear to the context that was evident in the SH/SH condition. Post-weaning environmental manipulations promote cell survival and reverse learning deficits in rats that were exposed to alcohol during development. These manipulations may provide a basis for developing interventions that ameliorate learning impairments associated with human fetal alcohol spectrum disorders.

Children with autism spectrum disorders show abnormal conditioned response timing on delay, but not trace, eyeblink conditioning

Children with autism spectrum disorder (ASD) and age-matched typically-developing (TD) peers were tested on two forms of eyeblink conditioning (EBC), a Pavlovian associative learning paradigm where subjects learn to execute an appropriately-timed eyeblink in response to a previously neutral conditioning stimulus (CS). One version of the task, trace EBC, interposes a stimulus-free interval between the presentation of the CS and the unconditioned stimulus (US), a puff of air to the eye which causes the subjects to blink. In delay EBC, the CS overlaps in time with the delivery of the US, usually with both stimuli terminating simultaneously. ASD children performed normally during trace EBC, exhibiting no differences from TD subjects with regard to the learning rate or the timing of the conditioned response. However, when subsequently tested on delay EBC, subjects with ASD displayed abnormally-timed conditioned eye blinks that began earlier and peaked sooner than those of TD subjects, consistent with previous findings. The results suggest an impaired ability of children with ASD to properly time conditioned eye blinks which appears to be specific to delay EBC. We suggest that this deficit may reflect a dysfunction of the cerebellar cortex in which increases in the intensity or duration of sensory input can temporarily disrupt the accuracy of motor timing over short temporal intervals.

Improving basic and translational science by accounting for litter-to-litter variation in animal models

Background

Animals from the same litter are often more alike compared with animals from different litters. This litter-to-litter variation, or “litter effects”, can influence the results in addition to the experimental factors of interest. Furthermore, sometimes an experimental treatment can only be applied to whole litters rather than to individual offspring. An example is the valproic acid (VPA) model of autism, where VPA is administered to pregnant females thereby inducing the disease phenotype in the offspring. With this type of experiment the sample size is the number of litters and not the total number of offspring. If such experiments are not appropriately designed and analysed, the results can be severely biased as well as extremely underpowered.

Results

A review of the VPA literature showed that only 9% (3/34) of studies correctly determined that the experimental unit (n) was the litter and therefore made valid statistical inferences. In addition, litter effects accounted for up to 61% (p <0.001) of the variation in behavioural outcomes, which was larger than the treatment effects. In addition, few studies reported using randomisation (12%) or blinding (18%), and none indicated that a sample size calculation or power analysis had been conducted.

Conclusions

Litter effects are common, large, and ignoring them can make replication of findings difficult and can contribute to the low rate of translating preclinical in vivo studies into successful therapies. Only a minority of studies reported using rigorous experimental methods, which is consistent with much of the preclinical in vivo literature.

Eyeblink conditioning in the developing rabbit

Eyeblink classical conditioning in pre-weanling rabbits was examined in the present study. Using a custom lightweight headpiece and restrainer, New Zealand white littermates were trained once daily in 400 ms delay eyeblink classical conditioning from postnatal days (PD) 17-21 or PD 24-28. These ages were chosen because eyeblink conditioning emerges gradually over PD 17-24 in rats [Stanton et al., (1992) Behavioral Neuroscience, 106(4):657-665], another altricial species with neurodevelopmental features similar to those of rabbits. Consistent with well-established findings in rats, rabbits trained from PD 24-28 showed greater conditioning relative to littermates trained from PD 17-21. Both age groups displayed poor retention of eyeblink conditioning at retraining 1 month after acquisition. These findings are the first to demonstrate eyeblink conditioning in the developing rabbit. With further characterization of optimal conditioning parameters, this preparation may have applications to neurodevelopmental disease models as well as research exploring the ontogeny of memory.

Delay eyeblink classical conditioning is impaired in Fragile X syndrome

The authors examined 400 ms delay eyeblink classical conditioning in 20 participants with Fragile X syndrome ages 17 to 77 years, and 20 age-matched, healthy control participants. The participants in the Fragile X group demonstrated impaired learning and abnormal conditioned response timing. Adults with Fragile X (n=16) were also tested at two successive 12-month follow-up sessions to examine reacquisition and long-term retention. Participants in groups who were older and younger than 45 years demonstrated significant learning during each reacquisition session. Younger participants demonstrated greater retention of the conditioned stimulus/unconditioned stimulus association at each follow-up session than older participants. Fragile X impairs the acquisition and timing of conditioned eyeblink responses, but with repeated training adults with Fragile X syndrome show significant plasticity.