Functional topography of the rat hypothalamic dopamine neuron systems: retrograde tracing and immunohistochemical study.

The origins of the dopamine (DA)-containing nerve terminals in the external layer of the median eminence and in the neurointermediate pituitary were determined in rats by a combination of retrograde labeling with wheat germ agglutinin (WGA) and immunohistochemistry for tyrosine hydroxylase (TH). Biotinylated WGA (b-WGA) was injected into the posterior pituitary (group 1) and into the median eminence (group 2). In group 1 animals, all the magnocellular neurons of the paraventricular, supraoptic, and accessory nuclei, and many parvicellular neurons in the rostral periventricular region (RPR) were labeled with WGA. In group 2 animals, many neurons were labeled in the arcuate nucleus and the RPR, and in a small population of the preoptic-septal region. In group 1 animals, about 39% of TH neurons in the RPR were labeled with WGA, whereas only a few TH neurons (1%) in the arcuate nucleus were labeled with WGA. In group 2 animals, on the contrary, almost all TH neurons (73%) in the arcuate nucleus carried WGA, whereas in the RPR, only some of the TH neurons (19%) were labeled with WGA. It is concluded that DA neurons involved in the hypothalamic-anterior pituitary axis are located in the arcuate nucleus; those involved in neuro-intermediate lobe function in the RPR.