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The hypothalamic paraventricular nucleus in two types of Wistar rats with different stress responses

The present study evaluates the role of the hypothalamic paraventricular nucleus (PVH) in stresvs regulation by a morphometric comparison of the vascular, neuronal and synaptic properties of this nucleus in two lines of Wistar rats, It has been previously reported that these two lines of rats, indicated as APO-SUS (apomorphine-susceptible) and APO-UNSUS (apomorphine-unsusceptible) rats on the basis of their reactivity to a subcutaneous injection of apomorphine, display a variety of pharmacological and behavioral differences, including differences in their stress-coping mechanisms (Cools et al., Neuropsychobiology, 28 (1993) 100-105). The results show a similar vascular and neuronal organization of the PVH in both lines, but distinct synaptic differences. The PVH (0.12 mm3 volume with about 15,000 neurons on one side) has an overall vascular density of 5.6%, with significant differences between subdivisions (parvocellular central part: 8.3%, parvocellular dorsal/ventral/posterior part: 4.6-5.3%), which means that vascularity is a useful tool to delineate subdivisions in the parvocellular PVH. The neuronal density of 132 X 103/m m 3 as found in the present study is two times higher than reported in a previous study (Kiss et al., J. Comp. Neurol., 313 (1991) 563-573). Possible reasons for this discrepancy are extensively discussed. The most significant finding of the present study is the observation that APO-SUS rats have a significantly higher synaptic density (158 X 106/m m 3) in the PVH than APO-UNSUS rats (108 X lOVmum3). It is discussed in which way this synaptic difference may be correlated with the different activity of the hypothalamo-pituitary-adrenal axis in both lines of Wistar rats.