Estrogen receptor-alpha immunoreactive neurons in the ventrolateral periaqueductal gray receive monosynaptic input from the lumbosacral cord in the rhesus monkey.

Estrogen affects female sexual behavior, analgesia, and micturition in mammals. One of the possible sites at which estrogen might exert its effect on these functions is the periaqueductal gray (PAG). The PAG is involved in each of these functions, it receives sensory input relevant to these functions from the lumbosacral cord, and contains estrogen receptor-alpha immunoreactive (ER-alpha IR) neurons. The present light (LM) and electron microscopic (EM) study seeks to determine whether there are monosynaptic projections from the lumbosacral cord to ER-alpha IR neurons in the PAG of the female rhesus monkey. Tracer was injected into the lumbosacral cord to visualize the lumbosacral-PAG projection, and the distribution of ER-alpha IR neurons in the PAG was studied immunohistochemically. The medial part of the ventrolateral caudal PAG received the densest projection from the lumbosacral cord. Another prominent projection was found in the lateral PAG at the intercollicular level. Although ER-alpha IR neurons were widely distributed throughout the PAG, approximately 40% of ER-alpha IR PAG neurons were located as a distinct cluster in the medial portion of the ventrolateral, caudal PAG. Double labeling experiments showed that the location of this cluster precisely overlapped with the densest lumbosacral-PAG projection. EM revealed that axons from the lumbosacral cord made asymmetrical synaptic contacts with unlabeled dendrites and ER-alpha IR neuronal somata in the ventrolateral PAG. It is concluded that there exists a specific, monosynaptic pathway from lumbosacral neurons to ER-alpha expressing PAG neurons in the rhesus monkey. This pathway might be involved in the mechanisms of analgesia, blood pressure, mating behavior, and micturition.