Toxicol Sci. 2002 Sep;69(1):191-201.  

2,3,7,8-tetrachlorodibenzo-p-dioxin toxicity in the zebrafish embryo: local
circulation failure in the dorsal midbrain is associated with increased
apoptosis.

Dong W, Teraoka H, Yamazaki K, Tsukiyama S, Imani S, Imagawa T, Stegeman JJ,
Peterson RE, Hiraga T.

Department of Toxicology, School of Veterinary Medicine, Rakuno Gakuen
University, Ebetsu 069-8501, Japan.

Effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on local circulation and
apoptosis in the midbrain were investigated in zebrafish (Danio rerio) embryos
during early development. Embryos were exposed to TCDD from 24 h post
fertilization (hpf) until observation, in water maintained at 28.5 degrees C.
TCDD decreased blood flow in the mesencephalic vein, the only vessel perfusing
the dorsal midbrain of the embryo. At 50 hpf, blood flow was maximally reduced
in this vessel and gradually returned to the control level at 60 hpf. In
contrast, blood flows in the trunk and in other vessels of the head of the
embryo did not significantly change until 72 hpf. Furthermore, TCDD exposure
caused apoptosis in the midbrain at 60 hpf, and the TCDD dose response
relationship for this effect was similar to that for reduced blood flow in the
mesencephalic vein at 50 hpf. The effects of TCDD on apoptosis in the midbrain,
but not on blood flow, were abolished by Z-VAD-FMK, a general caspase inhibitor.
TCDD effects on both endpoints were mimicked by beta-naphthoflavone (BNF), an
aryl hydrocarbon receptor (AHR) agonist, and almost abolished by concomitant
exposure to TCDD and alpha-naphthoflavone (ANF), an AHR antagonist. Concomitant
exposure to TCDD and either an inhibitor of cytochrome P450 (CYP) (SKF525A or
miconazole) or an antioxidant (N-acetylcysteine or ascorbic acid) inhibited
these effects of TCDD. The incidence of apoptosis in the midbrain was inversely
related to blood flow in this brain region following these various treatments
and graded TCDD exposure concentrations (r = -0.91). The same range of TCDD
exposure concentrations that reduced blood flow and increased apoptosis in the
midbrain greatly enhanced CYP1A mRNA expression and immunoreactivity at 50 hpf
in endothelial cells of blood vessels including the mesencephalic vein and the
heart, but not the brain parenchyma. Taken together, these results suggest that
TCDD induces apoptosis in the midbrain of the zebrafish embryo secondary to
local circulation failure, which could be related to AHR activation, induction
of