Here, we use an example from the development of skill sets for complex social-emotional regulation. Fundamental abilities are assembled in a hierarchical fashion throughout, which is comprised of basic functions building upon each other to develop the complex skill set. The underlying neurobiological circuits underlying these individual skill sets are shown, also developing in hierarchical fashion, with the more complex, top-down regulatory circuits forming late in development. Rather than measuring end-state, complex functions, dissecting components during assembly in development will provide key insights regarding pathogenesis. For example, genetic and environmental challenges to the organism can lead to widespread disruptions (A), intermediate disruptions (B), or disruptions confined to particular, late developing skill sets (C), depending on the maturational stage of the neural circuitry when the perturbation occurs. (D) For adult-onset disorders with a neurodevelopmental basis, challenges acutely may not impact function overtly, but over time, for example through allostatic overload (see Figure 2), perturbations may emerge. In each of these examples, intermediate developmental pathophysiological states can be examined in animal models in far more detail, yielding information useful for intervention or prevention designs.

Here, we illustrate homeostasis and neural adaptation in response to challenging stimuli (adapted from (Koob and Le Moal, 2001)). An organism must continuously respond to both external and internal stimuli, maintaining an equilibrated balance (solid red line in A), in which there are peak and trough responses to stimuli (dotted red lines in A). In neurotypical individuals these responses average out into a constant homeostatic set point. If the system is challenged by either a single, potent stimuli, or through repeated, subthreshold stimuli, the system may not have the time or capacity to equilibrate to homeostasis (allostatic load). Instead, a weakened peak response to stimuli is generated, which is matched in intensity by the same opponent response (dotted green line in B). As the system attempts to create homeostasis between these two responses, a new homeostatic set-point is achieved (solid green line in B). Extreme challenges to homeostasis (perhaps a combination of genetic load interacting with potent environmental stimulus) can produce allostatic overload, resulting in allostasis existing outside the typical boundaries for homeostasis (purple lines in C). This overload may have a profound impact on developmental trajectories and expression of pathophysiologic functions.

Some elements, like ethologically relevant behaviors, species-specific timing of histogenic events and species-unique environmental impact are noteworthy for their influence on model design. Others are typically not considered, like differences in regulation and patterns of gene expression, and neural and non-neural system interactions in a species-relevant manner. These components provide opportunities to examine disorder-relevant processes from the perspective of neurobiological and behavioral hierarchies.