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Hormonal-Neural Integration in the Female Rat Ventromedial Hypothalamus: Triple Labeling for Estrogen Receptor-, Retrograde Tract Tracing from the Periaqueductal Gray, and Mating-Induced Fos Expression

Institute of Neurological Sciences (L.H.C., L.M.F.-C.), and Department of Psychology (L.M.F.-C.), University of Pennsylvania, Philadelphia, Pennsylvania 19104

Address all correspondence and requests for reprints to: Dr. Loretta M. Flanagan-Cato, Department of Psychology, University of Pennsylvania, 3815 Walnut Street, Philadelphia, Pennsylvania 19104. E-mail: flanagan{at}psych.upenn.edu.

The lordosis reflex, a stereotypic posture adopted by female rats during sexual behavior, requires the convergence of a hormonal signal, estrogen, with a descending neural pathway from the ventromedial hypothalamic nucleus (VMH). The VMH contains at least three lordosis-relevant neural populations: estrogen receptor- immunoreactive (ER-IR) neurons, VMH neurons that project to the periaqueductal gray (PAG), and neurons that are ER-IR and project to the PAG. Expression of Fos, a marker for neuronal activation, is increased in the VMH after mating. However, it is unknown which, if any, of these lordosis-relevant populations is activated. The majority of ER-IR and projection neurons were not colocalized. Of the Fos-positive neurons, 41% neither contained ER nor projected to the PAG, and 35% contained ER but did not project to the PAG. Only 25% of Fos-positive neurons projected to the PAG, including projection neurons that expressed ER. Our results suggest that mating activates several distinct VMH neuron types. However, ER-IR neurons are activated to a greater extent compared with the PAG-projecting neurons.