OBJECTIVE Quantitate interspecies toxicodynamic differences during neocortical development of the rat and primate brain. BACKGROUND  Ethanol is a particularly harmful developmental neurotoxicant and is the leading known cause of mental retardation in the Western world (1).  Extensive research on ethanol-induced developmental neurotoxicity provides a rich data set for application and subsequent assessment of our biologically based computational models for developmental toxicology.  Numerous human, non-human primate and rat behavioral, histological, and stereological studies suggest the neocortex may be particularly sensitive to prenatal ethanol exposure.  The present computational model is a application of the model developed by Leroux (1996) which uses a stochastic approach to describe the branching process of cell kinetics during development (Figure 1)(2). EVALUATION OF INTERSPECIES VARIABILITY DURING NEOCORTICAL NEUROGENESIS USING BIOLOGICALLY BASED COMPUTATIONAL MODELS J.M. Gohlke, W.C. Griffith, E.M. Faustman Institute for Risk Analysis and Risk Communication and Department of Environmental Health, University of Washington, Seattle, Washington, USA

Figure 1. The anatomy of neocortical neuronogenesis and how it relates to model building. a. A mammalian nervous system at the 5 vesicle stage in lateral view showing progenitor cells (orange)are generated in the pseudostratefied ventricular epithelium (PVE). During G1 newly generated cells either stay in the proliferative population (P fraction) or become postmitotic (Q fraction- blue cells) and begin migration through the intermediate zone (IZ) to the cortical plate (CP). In the mouse, the neuronogenesis period is six days long and traverses eleven cell cycles (CC1-CC11) whereas in the rhesus monkey it is 60 days long transversing at least 28 cell cycles b. Basic model framework from Leroux (1996) which was modified as a model for neocortical neuronogenesis where Type X cells represent neuronal progenitor cells in the PVE (orange circles) and Type Y cells represent postmitotic neurons leaving the PVE (blue circles).

I. KEY DIFFERENCES BETWEEN RODENT AND PRIMATE NEOCORTICAL NEUROGENESIS Figure 2. Cell cycle length changes over time in the mouse, rat and monkey. During rodent neurogenesis the cell cycle lengthens over time, whereas in the rhesus monkey the cell cycle length peaks at E60 and thereafter shortens. Data was derived from references Key Points:  The length of neocortical neurogenesis is increased in the primate (6 days in the mosue compared with 60 days in the rhesus monkey).  The cell cycle length is longer during primate neurogenesis (between 2 to 4 times on average).  The time dependent change in the cell cycle is different in rodents and the rhesus monkey (See Fig. 2).  The founder cell population is larger in the primate (approximately 4 times larger than the mouse founder cell population).

II. APPLICATION OF MODEL TO MOUSE, RAT AND PRIMATE CONTROL DATASETS Key Point:  Our model predictions closely match stereologically determined neuronal counts in the mouse, rat, and monkey. Figure 3. Comparison of mouse and rat neocortical neuronal output predictions. This figure shows a plot of our model predictions for the normal mouse (▲), rat (●), and rhesus monkey ( ) neuronal output (total Y cells). For comparison we plotted stereological estimates of total neurons in the neocortex of the adult mouse and rat adjusted for a 35-50% reduction in postnatal cells due to normal processes of cell death ( ׀ ). The length of the vertical line represents the variation due to differences in the mean estimates from different stereological studies (9-11); (12-16) and variation due to 35-50% reduction (17) of population in the cell death period.

Key Points:  At human blood ethanol concentrations (BEC) that occur after 3-5 drinks (~150 mg/dl), our model predicts a 30-35% neocortical cellular deficit by the end of neurogenesis in the rat model, which match independent stereological studies on ethanol-induced cellular loss (19).  At BECs ranging from 15 to 50 mg/dl (occuring after.5 to 1.5 drinks) the model predicts a gain in neurons over normal.  Application of rat toxicity data to the primate neurogenesis model indicates primates may be more sensitive to ethanol induced neocortical neuronal loss during neurogenesis at BECs above 75 mg/dl. III.APPLICATION OF RAT ETHANOL NEUROTOXICITY DATA TO PRIMATE MODEL  Ethanol exposure (maternal BEC ~150 mg/dl) during neocortical neurogenesis lengthens the cell cycle prematurely in the rat (4).  The time-dependent effect of dose on the neocortical progenitor division rate is based on an in vitro study of cell cycle inhibition (18) and is modeled using the equation: rate = untreated baseline * e (drp*dose) where drp is the time-dependent dose-response parameter Figure 4. Prediced ethanol dose-response curve for necortical cell loss in the rat and monkey model. Rat toxicity data showing a lengthening of the cell cycle what applied to our monkey model. The differences in the dose response curves highlight toxicodynamic differences across species may cause differences in susceptibility.

Acknowledgements This study was supported by the Center for Child Environmental health Risks research through EPA grant R and NIEHS 1PO1ES CONCLUSIONS  We have developed stochastic models for mouse, rat and primate neocortical neurogenesis allowing for cross species comparisons of toxicodynamic processes.  We have applied rodent toxicity data to the new primate model to compare the neurodevelopmental impacts of ethanol on neocortical development across species.  Our model predictions indicate dynamic differences in normal neocortical neurogenesis may enhance the suseptibility of the primate brain to neuron loss after exposure to ethanol during neocortical neurogenesis.