THE HYPOTHALAMIC PARAVENTRICULAR NUCLEUS IN 2 TYPES OF WISTAR RATS WITH DIFFERENT STRESS RESPONSES .1. MORPHOMETRIC COMPARISON

The present study evaluates the role of the hypothalamic paraventricul ar nucleus (PVH) in stress 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 l ines of rats, indicated as APO-SUS (apomorphine-susceptible) and APO-U NSUS (apomorphine unsusceptible) rats on the basis of their reactivity to a subcutaneous injection of apomorphine, display a variety of phar macological 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 organizati on of the PVH in both lines, but distinct synaptic differences. The PV H (0.12 mm(3) volume with about 15,000 neurons on one side) has an ove rall vascular density of 5.6%, with significant differences between su bdivisions (parvocellular central part: 8.3%, parvocellular dorsal/ven tral/posterior part: 4.6-5.3%), which means that vascularity is a usef ul tool to delineate subdivisions in the parvocellular PVH. The neuron al density of 132 X 10(3)/mm(3) as found in the present study is two t imes higher than reported in a previous study (Kiss et al., J. Comp. N eurol., 313 (1991) 563-573). Possible reasons for this discrepancy are extensively discussed. The most significant finding of the present st udy is the observation that APO-SUS rats have a significantly higher s ynaptic density (158 X 10(6)/mm(3)) in the PVH than APO-UNSUS rats (10 8 X 10(6)/mm(3)). It is discussed in which way this synaptic differenc e may be correlated with the different activity of the hypothaiamo-pit uitary-adrenal axis in both lines of Wistar rats.