The paraventricular nucleus (PVN) of the hypothalamus has a key role in the regulation of sympathetic outflow, neuroendocrine secretion and behavioral homeostatic responses. One factor important in the regulation of PVN neuronal discharge and sympathetic outflow is the peptide angiotensin II (Ang II). The Ang II type 1 (AT1R) that are present in the PVN contribute to the pressor action of centrally injected Ang II, and it is also clear that over activity of brain AT1R, including those in the PVN, contributes to neurogenic hypertension. Thus it is important to understand the central mechanisms that modulate the activity of PVN neurons. To this end we have demonstrated that the cytokine macrophage migration inhibitory factor (MIF) exerts a negative regulatory influence over the AT1R-mediated increases in blood pressure produced by centrally injected Ang II in normotensive rats. MIF is known as a as a pro-inflammatory cytokine in the peripheral immune system, acting largely through its plasma membrane CD-74 receptor. However, MIF is also internalized and exerts intracellular actions via a CXXC motif that is part of its intrinsic structure. As such it has been proposed to be a member of the thioredoxin (Trx) family of proteins (1). Similar to Trx, this CXXC motif allows MIF to exert thiol-protein oxidoreductase (TPOR) activity, which confers the ability to scavenge reactive oxygen species (ROS) and reduce proteins. Our collective in vitro and in vivo experiments have indicated that that the negative regulatory action of MIF at the PVN over AT1R-mediated responses is due to an intracellular action via its TPOR activity, and scavenging of ROS (2). Since MIF appears to be an important regulator of Ang II actions at the PVN, and increased levels and activity of AT1R in the PVN contribute to hypertension, it is necessary to understand whether this MIF regulatory mechanism is at fault in hypertensive animals. Thus, we assessed the relative levels of MIF expression in the brains of both pre-hypertensive SHR (spontaneously hypertensive rats) and SHR with established hypertension. SHR were chosen as a model system as they exhibit increased levels and activity of CNS AT1R, including those in the PVN. The data indicate that newborn (pre-hypertensive) SHR express significantly lower (P< 0.05; n=6 rats/strain) levels of both MIF mRNA and protein within the hypothalamus compared with control Wistar Kyoto (WKY) rats. No such differences in MIF expression were observed in the medulla of newborn SHR and WKY rats. Immunostaining revealed the presence of immunoreactive MIF within neurons and astroglia in the PVN of newborn WKY rats. However, a similar approach indicated that MIF is absent from PVN neurons of newborn SHR, but is retained in astroglia. In comparison, immunoreactive Trx was present within PVN neurons of both newborn SHR and WKY rats, suggesting a specific deficit of MIF rather than other Trx-related proteins. Analyses of PVN tissue from 2 and 6 week old SHR revealed similar significant reductions in MIF mRNA (P< 0.05; n=4 rats/strain) compared with age-matched WKY and Sprague Dawley (SD) rats. In contrast, the levels of MIF mRNA and protein in the PVN of 20 week old SHR were similar to those found in WKY and SD rats of similar age. Interestingly, PVN neurons of 20 week old SHR were still devoid of immunoreactive MIF, similar to the situation in newborn SHR. Thus, we developed the idea that a lack of MIF within PVN neurons of SHR might contribute to the development of high blood pressure in these animals. This was tested by chronic restoration of MIF into PVN neurons of 8 week old SHR via bilateral microinjection of AAV2-CBA-MIF. Treatment of SHR (n=7) in this manner alleviated the development of high blood pressure and cardiac hypertrophy compared with SHR (n=7) that had received control injections of AAV2-CBA-GFP. All vector injections were performed with the rat situated in a stereotaxic apparatus. Anaesthesia was induced using a mixture of oxygen (1L/min) and isoflurane (4%), and maintained during the injections by delivery of oxygen and isoflurane (1.5-2%) via a specialised nose cone. The negative regulatory actions of MIF within the PVN over blood pressure appear to be important in multiple situations. For example, normotensive rats that are subjected to acute restraint stress respond with increases in blood pressure and heart rate, effects mediated via AT1R. In a preliminary study we have demonstrated that increased expression of MIF within the PVN of adult WKY rats via AAV2-CBA-MIF elicits a significant decrease in the pressor response to restraint stress. Collectively these data suggest that MIF plays an important role in regulating the output of the PVN neurons that are involved in controlling blood pressure, and in particular in regulating AT1R-mediated responses via the PVN. Further, the data suggest that a lack of MIF in PVN neurons contributes to hypertension in the SHR model of this disease.