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Mesostriatal and mesolimbic projections of midbrain neurons immunoreactive for estrogen receptor beta or androgen receptors in rats

The dopamine (DA) inputs to the caudate putamen, the nucleus accumbens, and the amygdala in rats are sensitive to circulating estrogens and androgens. One mechanism for the hormone modulation of these systems may be via actions at cognate intracellular estrogen and androgen receptors. However, although it is known that specific subsets of midbrain DA neurons are immunopositive for estrogen receptor β (ERβ) or androgen receptors (ARs), it is not known where these receptor‐bearing cells project. To address this issue, we combined double‐label immunocytochemistry with retrograde tract tracing to identify the forebrain projections of ERβ‐ or AR‐immunoreactive (IR) midbrain neurons. Specifically, Fluoro‐Gold and/or cholera toxin were injected into discrete subregions of the caudate‐putamen, the nucleus accumbens, or the amygdala. Evaluations of the resultant midbrain labeling revealed that ERβ‐IR neurons sent collateral projections mainly to both the ventral caudate‐putamen and the amygdala, but not to the dorsal caudate or nucleus accumbens. In contrast, AR‐IR neurons projected either to the amygdala or the nucleus accumbens but not to the caudate‐putamen. The organization of these forebrain projections concurs with some of the known hormone sensitivities of mesostriatal and mesolimbic DA systems in rats and provides an anatomical model that predicts separate influences for androgens and estrogens over mesostriatal and mesolimbic DA systems. J. Comp. Neurol. 476:348–362, 2004. © 2004 Wiley‐Liss, Inc.