Table 1.

Theoretical concentrations of microdialyzable ISF Aβ in vivo at zero-flow rate

	Aβ38 (pg/ml)		Aβ40 (pg/ml)		Aβ42 (pg/ml)		Aβ42:Aβ40
	3 mo	24 mo	3 mo	24 mo	3 mo	24 mo	3 mo	24 mo
[ISF Aβ] of J20 tg mice	930	658	2975	1231	880	166	0.30	0.13

Concentrations of endogenous hippocampal ISF Aβ₃₈, Aβ₄₀, and Aβ₄₂ were calculated by extrapolating the curves generated from the interpolated zero-flow method to zero PR (Fig. 5). Ratios of Aβ₄₂/Aβ₄₀ in the ISF of 3- and 24-month-old tg mice were calculated to be ∼0.30 and 0.13, respectively, reflecting a higher drop of Aβ₄₂ versus Aβ₄₀ in 24-month-old tg mice.