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First published online November 2, 2007
Journal of Experimental Biology 210, 3970-3978 (2007)
Published by The Company of Biologists 2007
doi: 10.1242/jeb.010645

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Area postrema, a brain circumventricular organ, is the site of antidipsogenic action of circulating atrial natriuretic peptide in eels

Takehiro Tsukada^*, Shigenori Nobata, Susumu Hyodo and Yoshio Takei

Laboratory of Physiology, Ocean Research Institute, The University of Tokyo, Tokyo 164-8639, Japan

^* Author for correspondence at present address: Department of Cell Biology and Anatomy, The University of Arizona, 1656 E. Mabel-MRB330, Tucson, AZ 85724, USA (e-mail: tsukada{at}email.arizona.edu)

Accepted 3 September 2007

Accumulating evidence indicates that circulating atrial natriuretic peptide (ANP) potently reduces excess drinking to ameliorate hypernatremia in seawater (SW) eels. However, the cerebral mechanism underlying the antidipsogenic effect is largely unknown. To localize the ANP target site in the brain, we examined the distribution of ANP receptors (NPR-A) in eel brain immunohistochemically using an antiserum specific for eel NPR-A. The immunoreactive NPR-A was localized in the capillaries of various brain regions. In addition, immunoreactive neurons were observed mostly in the medulla oblongata, including the reticular formation, glossopharyngeal-vagal motor complex, commissural nucleus of Cajal, and area postrema (AP). Trypan Blue, which binds serum albumin and does not cross the blood–brain barrier, was injected peripherally and stained the neurons in the AP but not other NPR-A immunopositive neurons. These histological data indicate that circulating ANP acts on the AP, which was further confirmed by physiological experiments. To this end, the AP in SW eels was topically destroyed by electric cauterization or were by chemical lesion of its neurons by kainic acid, and ANP (100 pmol kg^–1) was then injected into the circulation. Both heat-coagulative and chemical lesions to the AP greatly reduced an antidipsogenic effect of ANP, but the ANP effect was retained in sham-operated eels and in those with lesions outside the AP. These results strongly suggest that the AP, a circumventricular organ without a blood–brain barrier, serves as a functional window of access for the circulating ANP to inhibit drinking in eels.

Key words: blood–brain barrier, ANP receptor (NPR-A), fish osmoregulation, drinking behaviour, circumvetricular organ