DECREASE IN GLUTAMIC-ACID DECARBOXYLASE LEVEL IN THE HYPOTHALAMUS OF SPONTANEOUSLY HYPERTENSIVE RATS

Background A reduction in gamma-aminobutyric (GABA)mediated inhibition of presser sites in the caudal hypothalamus of spontaneously hyperten sive rats compared with that of normotensive Wistar-Kyoto rats has rec ently been demonstrated.Objective To determine whether the reduction i n GABA-mediated inhibition of the caudal hypothalamus of the spontaneo usly hypertensive rats results from reductions both in the number of G ABA-synthesizing neurons and in the amount of the GABA-synthesizing en zyme, glutamic acid decarboxylase messenger RNA (mRNA), Design and met hods A polyclonal antibody (Chemicon) for the 67 kDa isoform of glutam ic acid decarboxylase (GAD(67)) was used to immunocytochemically label GABAergic neurons in the caudal hypothalamus of spontaneously hyperte nsive and Wistar-Kyoto rats that had been treated beforehand with colc hicine. The labeled cells were counted for both strains by a blinded a nalysis and compared. Caudal hypothalamic tissues from spontaneously h ypertensive and Wistar-Kyoto rats were analysed for GAD(67) mRNA by No rthern blotting. The signal intensities of the radioactive probe speci fic for GAD(67) for the two strains were analyzed by using a phosphori mager and compared. Control areas for the immunocytochemical (zona inc erta) and Northern blotting (cortex, midbrain, cerebellum, and brain s tem) experiments were used to determine regional differences in expres sion of GAD(67). Results Both the hypothalamus and cerebellum of spont aneously hypertensive and Wistar-Kyoto rats contained GAD(67)-immunore active neurons; however, there were 42% fewer GAD(67) neurons in the c audal hypothalamus of spontaneously hypertensive rats than there were in that of Wistar-Kyoto rats. Furthermore, a 33% reduction in the amou nt of GAD(67) messenger RNA in the caudal hypothalamus of spontaneousl y hypertensive rats compared with that for Wistar-Kyoto rats was demon strated. Analysis of the expression of GAD(67) in the cortex, midbrain , cerebellum, brain stem, and total brain revealed no difference betwe en spontaneously hypertensive and Wistar-Kyoto rats. Conclusions Our f indings demonstrate that the spontaneously hypertensive rat has fewer neurons synthesizing GABA and less GAD(67) mRNA in the caudal hypothal amus than do Wistar-Kyoto rats. This deficit in the GABAergic system i n the caudal hypothalamus, a well-known cardiovascular regulatory site , could contribute to the essential hypertension in this animal model. (C) 1998 Lippincott-Raven Publishers.