Sympathetic activation is a characteristic feature of many cardiovascular diseases. Sympathetic activity is the output from a range of central brain nuclei, one of which is the paraventricular nucleus of the hypothalamus (PVN), a key regulator of mean sympathetic nerve activity1. Changes within the endogenous neurotransmitters within this nucleus controlling sympathetic outflow have been shown to contribute to the sympathoexcitation evident in a number of cardiovascular pathologies1. In order to elucidate the molecular basis for these changes, we have begun to characterise the subunit expression of the inhibitory GABAA receptor within the PVN in different models of cardiovascular homeostasis and determine their relationship with spinally projecting neurones emanating from this nucleus. All experiments were performed in accordance with the Animals (Scientific Procedures) Act, 1986. To determine GABAA receptor subunit distribution in the PVN, rats (normotensive, n=3, spontaneously hypertensive rat (SHR), n=2, pregnancy, n=2) were humanely killed with sodium pentobarbital (60mg/kg) and perfuse-fixed with 4% paraformaldehyde. Brains were removed and cryoprotected in 30% sucrose. Forty micron sections were incubated in 1.5µg/ml primary antibody appropriate to the subunit of interest (24hrs) followed by 7.5µg/ml biotinylated secondary antibody appropriate to the species in which the primary was raised (2hr). This was followed with avidin-biotin complex (1hr) and finally 0.4mM 3,3’-diaminobenzidine (DAB, 5mins). To determine the relationship between GABAA receptors and spinally projecting neurones, male Wistar rats (n=2) were anaesthetised with an intraperitoneal injection of medetomidine (0.6mg/kg) and ketamine (30mg/kg) and 1µl Flurogold injected into the lateral horn of the spinal cord. Following a 7-day recovery period, the animals underwent the same fixation procedure as described above. The sections were incubated as before, substituting streptavidin Alexa Fluor 594 (2µg/ml) as the chromagen. The tissue was examined using bright field and epifluorescent microscopy. The subunits α1, α2, α3, α5, β2, β3, γ2 and δ were distributed throughout the parvocellular subdivisions of the PVN. For SHR and pregnant animals, α1, α2, β2 and γ2 subunits were found to be present in those same regions in the PVN. In addition, the α1 subunit was shown to be associated with the majority of spinally projecting neurones. This is the first study to reveal the subunit expression of the GABAA receptor within the major subdivisions of the PVN and show their relationship with sympathetic spinally projecting neurones. These preliminary findings will provide insights into how alterations in subunit composition may contribute to the sympathoexcitation in cardiovascular disease.