Heroin sensitization induces circumventricular organ activation in the rat brain.

Using c-Fos protein immunohistochemistry we previously demonstrated various sites of activation in the rat forebrain according to the animal's drug history. This study originates from a more detailed evaluation ex-post of the same specimens. A discrete number of c-Fos protein immunoreactive nuclei could be observed in some circumventricular organs, including the vascular organ of terminal lamina (OVLT) and subfornical organ (SFO) and in the nucleus of solitary tract near the area postrema, but only in specimens from sensitized rats. We therefore suggest that repeated drug injections activate the normally low drug metabolizing enzyme activity in the circumventricular organs thus implicating these organs in the complex mechanisms underlying behavioral sensitization.