NEUROANATOMICAL DIFFERENTIATION IN THE BRAIN OF THE SPONTANEOUSLY HYPERTENSIVE RAT (SHR) .1. VOLUMETRIC COMPARISONS WITH WKY CONTROL

A series of measurements was made to assess the morphology of the brai n of the spontaneously hypertensive rat (SHR). The SHR brain was small er than that of age-matched normotensive Wistar-Kyoto (WKY) controls i n a majority of measures of external surface landmarks. This reduction in size was evident in the youngest age group examined (94 days) and persisted in older groups (170, 240 and 350 days). The brain of the SH R was also smaller in terms of brain weight and brain weight:body weig ht ratios. Section-by-section digitized analyses of coronal histologic sections from 94-day-old rats revealed significant reductions in mean cross-sectional area and volume of midbrain/pons (10%) and hindbrain (11%) regions, but not of forebrain, in the SHR. Alterations in the me diolateral dimension, particularly within the pontomedullary brainstem , accounted for more of these volumetric changes than those in the dor soventral dimension. Using the same coronal sections, it was found tha t surface areas and volumes of five individual nuclei/fiber tracts, se lected for their involvement in central cardiovascular regulation, wer e significantly decreased in the SHR. The largest reduction in volume (30%) was found in the nucleus tractus solitarius, the primary site of termination of afferent baroreceptor fibers. No differences in surfac e area or volume were found in that portion of the cerebroventricular system (aqueduct of Sylvius) associated with the periventricular grey region, or in the inferior colliculus, which is not thought to be invo lved in cardiovascular control. These observations not only have pract ical implications, but suggest that the pathophysiological condition e xpressed as spontaneous hypertension in this widely-used model may be related to morphological alterations in the central nervous system.