Reduced Excitability of Intrinsic Neuronal Properties during Development of the Medial Prefrontal Cortex in a Rodent Autism Model

Background:

One of the few known risk factors for autism is maternal exposure to valproic acid (VPA) during the first trimester of pregnancy. VPA is an anticonvulsant drug that is often prescribed for migraine and bipolar disorder in addition to epilepsy. An established rodent model in which pregnant rats are injected with a single dose of VPA at the time of neural tube closure results in offspring that exhibit both neuroanatomical and behavioral abnormalities similar to those observed in autistic humans. Therefore this model provides a useful tool for understanding the developmental biology of autism, and has not yet been fully explored. One remaining unexplored area of interest is the development of functional properties in the medial prefrontal cortex, an area that has been implicated in autistic dysfunction.  

Objectives:

In this project we have used the VPA rat model to study how fetal VPA exposure affects the development of intrinsic neuronal properties in the medial prefrontal cortex with the goal of understanding what abnormalities may exist, and whether any observed abnormalities are present close to birth or as a result of developmental programming gone awry. Upon discovering differences in the model animals the goal is now to identify the underlying mechanisms.  

Methods:

We performed whole cell patch clamp recordings from layer 2/3 pyramidal neurons in slices of postnatal rat brains (P4-P38) to characterize how passive and active electrical properties evolve beginning shortly after birth and continuing on into adulthood. Our two groups of animals consist of the experimental VPA pups and age matched saline-exposed control pups.  

Results:

Our data indicate that passive properties, including resting potential, resting input resistance, and membrane time constant, developed normally in the VPA-exposed animals. Active properties, however, were impaired compared to the neurons from control animals.  Neurons from the youngest VPA animals (<2 weeks) were markedly less excitable than the saline controls. Specifically, two properties that affect action potential firing, namely threshold potentials and rheobase currents, were higher while the number of action potentials generated by current steps was lower. These differences decreased over development such that responses to current steps were indistinguishable between VPA and control recordings in older animals (> 1 month).

Conclusions:

We conclude that VPA either induced a developmental delay in intrinsic properties of medial prefrontal layer 2/3 pyramidal neurons or caused an alteration in an ion channel current mediating action potential firing and homeostatic mechanisms were triggered so that these cells appeared "normal" by one month. One limitation of our techniques thus far is that simple DC current steps may not fully reveal the functional properties of these neurons. We are currently using other methods (such as dynamic clamp simulations) to probe the older neurons with more complex stimuli to determine what, if any, differences persist.  In addition, we are pursuing the cause of the observed differences by using Western blot analysis of ion channel proteins and electrophysiological isolation of specific ion channel currents.