Distribution of D1 and D2 receptor- immunoreactive neurons in the paraventricular nucleus of the hypothalamus in the rat.

The paraventricular nucleus (PVN) of the hypothalamus is an important center in regulating the functions of internal organs, and it is the key nucleus for coordinating endocrine, behavior and autonomic stress responses. Dopamine receptors are widely expressed in the hypothalamus and participate in the regulation of many vital activities, such as blood pressure and glycolipid metabolism, but the details of the cellular localization of dopamine receptors in the PVN are not clear. The present study investigated the distribution of D1 and D2 receptor-immunoreactive (IR) neurons in the subnuclei of the PVN and their co-labeling with vasopressin (VP) and oxytocin (OT) in rats by means of Nissl staining and immunofluorescence. The PVN is subdivided into magnocellular and parvicellular subnuclei. The results showed that the D1 receptor-IR neurons were primarily located in the magnocellular subnuclei and distributed throughout several regions of the PVN, including the paraventricular hypothalamic nucleus, lateral magnocellular part (PaLM), medial magnocellular part (PaMM) and posterior part (PaPO). Additionally, the neurons co-labeled with OT and VP. D2 receptor-IR neurons had a distribution pattern similar to that of D1 receptors. The D1- and D2-IR neurons in the subnuclei of the PVN were in relatively close proximity to the fornix. Several subnuclei gradually shifted from the ventral medial aspect of the fornix to the dorsal side when examined along the rostrocaudal axis. In summary, D1 and D2 receptors are widely distributed in the subnuclei of PVN in a similar distribution pattern with a slightly less number of D2 receptor immunoreactive neurons than that of D1. Moreover, D1 and D2 receptors are coexisted with VP and OT. The present study provides important morphological evidence for the function of dopamine receptors in the PVN.