Brain Research, 370 (1986) 327-332 327 Elsevier

BRE 21469

Progressive postnatal dilation of brain ventricles in spontaneously hypertensive rats

SUE RITrER and THU T. DINH

Department of Veterinary and Comparative Anatomy, Pharmacology and Physiology, College of Veterinary Medicine, Washington State University, Pullman, WA 99164-6520 (U.S.A.)

(Accepted December 3rd, 1985)

Key words: spontaneously hypertensive rat - - hypertension - - brain ventricle - - hydrocephalus - - Wistar-Kyoto rat

Cross-sectional areas of the forebrain ventricles were measured from coronal sections in spontaneously hypertensive rats (SHRs) 4, 8, 12, 16, 21 and 56 weeks of age and in age-matched Wistar-Kyoto (WKY) and Sprague-Dawley (SD) normotensive rats. Progres- sive ventricular dilation and associated attrition of brain tissue was observed in SHRs of both sexes after 4 weeks of age, and was pres- ent in animals obtained from two different suppliers. In some SHRs, ventricle size was increased to 270% of control. Hence, it seems likely that some systemic and behavioral signs which are concomitant with hypertension in the SHR may be attributable to hydroce- phalus and its neuropathological correlates.

The spontaneously hypertensive rat (SHR) has

been extensively invest igated with regard to the pa-

thophysiology of hypertensionl4 and is currently re-

garded by many to be the best available animal mod-

el of human essential hypertension6,2k In the course

of our studies of adult SHRs, we observed that the ce-

rebral ventricles of these rats appeared to be di lated

in comparison to age-matched W i s t a r - K y o t o

(WKY) rats. Since circumventr icular structures play

an impor tant role in the control of b lood pressure and

body fluid homeostasisS, 17, ventr icular pa thology

could have impor tant implications for the pathophy-

siology of hypertension. In addit ion, hydrocephalus

or its neuropathological correlates could contr ibute

to the behavioral abnormali t ies , including hyper-

reactivity and hyperresponsiveness to stress 7,sA5,16,

which are well-recognized parts of the S H R syn-

drome. Therefore , we systematical ly analyzed ven-

tricular size in hypertensive rats of several ages and

in age-matched normotensive rats.

For our first exper iment , male O k a m o t o - A o k i

SHRs and W K Y normotensive rats were obta ined

from Taconic Farms, Inc. Since the O k a m o t o - A o k i

strain of SHRs was originally der ived by selective

inbreeding from rats of the W K Y strain, WKYs

are usually considered to be the most appropr ia te

normotensive control for the SHRs a2A3. In some of

our comparisons we also used normotensive controls

of the S p r a g u e - D a w l e y (SD) strain. These were ob-

tained from Harlan S p r a g u e - D a w l e y , Inc. Systolic

blood pressure was measured in the exper imental an-

imals at 4, 8, 12, 16, 21 and 56 weeks of age using an

inflatable tail cuff with a photoelectr ic sensor

( I . I .T.C. , Inc.) and they were then sacrificed by le-

thal overdose of sodium pentobarb i ta l and perfused

transcardially with 0.9% saline followed by a 4%

paraformaldehyde solution. Perfusates were deliv-

ered by gravity flow from elevated reservoirs of a

fixed height (1.3 m). Brains were fixed in situ over-

night, then removed from the skull, soaked in fixa-

tive for 3 days and cryoprotec ted by immersion in

30% sucrose for 3 -5 days. Coronal cryostat sections

(40 /~m in thickness) were mounted directly onto

slides from the knife blade and stained with cresyl vi-

olet for microscopic examinat ion. Cross-sectional

areas of the entire brain section and of the ventricular

lumen were measured from the same mounted sec-

tions by computer-assis ted image analysis at 3 rostro-

Correspondence: S. Ritter, Department of Veterinary and Comparative Anatomy, Pharmacology and Physiology, College of Veteri- nary Medicine, Washington State University, Pullman, WA 99164-6520, U.S.A.

0006-8993/86/$03.50 © 1986 Elsevier Science Publishers B.V. (Biomedical Division)

3 ~

caudal levels: at the decussation of the anterior com-

missure, at the level of the subfornical organ and at

the level of the posterior commissure. Equivalent

sections were carefully selected from animals of each

group for analysis. All ventricular space in a given

section was summed. The cerebral aqueduct and

fourth ventricle were examined in serial sections for

evidence of pathology or ventricular obstruction, but

the cross-sectional areas of these ventricular spaces

were not analyzed in this experiment.

The numbers of animals in the various age groups

were as follows: 4 weeks of age (SHR = 8, WKY = 8,

SD = 6); 8 weeks of age (SHR = 9, WKY = 9, SD =

6): 12 weeks of age (SHR = 7, WKY = 7, SD = 6); 16

weeks of age (SHR = 12, WKY = 12, SD = 6); 21

weeks of age (SHR = 2, WKY = 2)', 56 weeks of age

(SHR = 4, WKY = 4, SD = 4). In addition, 7 SHR

and 7 WKY females (Taconic Farms), approximately

52 weeks of age, were examined. Differences be-

tween groups were analyzed statistically at each age

using Student 's t-test for independent means.

We found that ventricular size was significantly

greater in SHRs than in WKYs or SDs in all but the

youngest animals and that the differences between

the hypertensive and normotensive rats increased

with age. Examinat ion of serial sections in each rat

revealed no evidence of ventricular obstruction at

any age in the SHRs. Fig. 1 shows the mean cross-

sectional area (+ S.E.M.) of the ventricular lumen at

the 3 levels analyzed for each group of rats. At 4

weeks of age ventricular size did not differ in SHRs

and WKYs. However, small but significant differ-

ences were present between SHRs and SDs: the ven-

tricles were significantly larger in the SHRs than in

SDs at the level of the anterior commissure, but sig-

nificantly smaller than in SDs at the level of the pos-

terior commissure. In SHRs 8 weeks of age or older,

however, the cross-sectional area of the ventricles

was significantly greater than in either WKYs or SDs

and every hypertensive animal examined was af-

fected. As revealed in Fig. 1, ventricular dilation was

apparent in SHRs at all 3 levels analyzed and ap-

peared to be progressive throughout adulthood. Al-

though we pooled the areas for lateral and third ven-

tricles, it was clear from our analysis that both ventri-

cles were enlarged in the SHRs.

In contrast to ventricular area, brain cross-section-

al area did not differ at the 3 levels analyzed between

E E

v </)

o

t--

>

<

c .o

¢)

I

o

1 5 A n t e r i o r C o m m i s s u r e . -

10 ~ j ~ 7

* ** ~ ~.. . ~ l l j

0 +I I I I i , i--.-.~

2O S u b f o r n i c a l O r g a n

0 i i i i i ,

// P o s t e r i o r C o m m i s s u r e

2 0

*

10

5

o ,~ ~ 1'2 ¢6 2'~ " s~

Age (weeks)

Fig. 1. Cross-sectional area of the cerebral ventricles measured at 3 anatomical levels (anterior commissure, subfornical organ and posterior commissure) in 4, 8, 12, 16, 21 and 56 week old male spontaneously hypertensive rats (SHRs) and normoten- sire Wistar-Kyoto (WKY) and Sprague-Dawley (SD) rats. *, P < 0.05, WKY vs SHR; *, P < 0.01 WKY vs SHR; +, P < 0.05 SD vs SHR; +, + P < 0.01 SD vs SHR.

SHRs and WKYs of any age group (Fig. 2). Similar-

ly, mature SDs did not differ from SHRs and WKYs

with respect to brain cross-sectional area although

minor differences were present in the 4- and 8-week-

old animals. Thus, the ventricular enlargement in

SHRs is not a function of a larger brain size and ob-

viously must occur at the expense of brain tissue.

Our measurements also showed that ventricular

cross-sectional area was increased and brain cross-

sectional area was decreased in the 56-week-old

WKY normotensive rats, compared to the younger

adult WKYs. Ventricular dilation during normal ag-

o-WKY

170 [ A'SHR

160 [

1sol ~, 14o[ E 13o[

,o lzo[

i ::: f Subfornical Organ 13o[

f O I I I t i ~ I

r "-- Posterior

°:::f 150[ f i i i I

4 a 1~ 1'6 ~1 a6 Age (Weeks)

Fig. 2. Cross-sectional areas of anatomically corresponding brain sections from 3 rostro-caudal levels in SHRs and WKYs of different ages. Measurements were made from the same brain sections used for measurements of ventricular area, shown in Fig. 1.

ing has been reported in humans, and is thought to be related to atrophy of underlying brain tissue 2. Thus,

the changes we observed in the 56-week-old normo-

tensive rats may represent the effects of normal ag-

ing.

Fig. 3 shows representative coronal sections from a 21-week-old male SHR and WKY at the 3 brain lev-

els analyzed. The enlarged ventricular lumen of the

SHR is readily apparent in this Figure and in addi- tion, attrition of brain tissue can be seen at some loci.

Loss of brain tissue is apparent in the septal region and caudate (A'), in the hippocampal commissure

and caudate (B') and in the hippocampus, amygdala,

and entorhinal and temporal cortices (C'). In the 56- week-old SHRs the loss of tissue in the latter areas was much more pronounced.

Blood pressures obtained from the experimental

animals (Table I) confirm the presence of hyperten- sion in the SHRs used in this study and indicate that the ventricular dilation in the SHRs occurs over a

329

time course which is roughly parallel to the devel-

opment of their hypertension.

At the one age examined, female SHRs also re- vealed significant ventricular dilation at all 3 rostro-

caudal levels analyzed, compared to age- and sex-

matched WKYs (P < 0.01 for comparisons at all 3

levels). Ventricular cross-sectional area for the

WKY females was 5.69 + 0.27, 7.33 + 2.80, 11.86 +

1.11 mm 2 at the level of the anterior commissure,

subfornical organ and posterior commissure, respec- tively. For SHR females ventricular area was 8.20 +

0.20, 11.03 + 1.41 and 20.80 + 2.72 mm 2, respective-

ly, at these levels. Blood pressures of the female rats

at the time of sacrifice were 121 + 1 mm Hg for the

WKYs and 199 + 6 mm Hg for the SHRs.

Taconic Farms SHRs have been bred in a closed

colony since 1972 (ref. 18). For this reason, it seemed possible to us that traits might be present in this col- ony that would not be present in animals of the same

strain from an independent colony. Therefore, we

conducted a second experiment to determine wheth- er ventricular enlargement is also present in SHRs

from the Charles River Breeding Laboratories, Inc.

Although Charles River Breeding Laboratories and

Taconic Farms maintain independent colonies,

SHRs from both of these suppliers are derived from

Okamoto-Aoki breeding stock originally provided

by the National Institutes of Health Animal Genetic

Resource. In this experiment, we measured ventricular area,

as described above, in 12-week-old male SHRs (n =

6) and WKYs (n = 5) from Charles River. Our re-

sults reveal that ventricular dilation is also present in

the Charles River SHRs. Ventricular area was signif-

icantly larger in SHRs than in WKYs (P < 0.01) at all

3 anatomical levels investigated. Cross-sectional

areas (mm 2) for SHRs and WKYs, respectively, were 7.0 + 0.5 vs 4.7 + 0.3 at the level of the anterior

commissure, 7.0 + 0.3 vs 5.3 + 0.7 at the level of the

subfornical organ and 9.8 + 1.3 vs 4.6 + 0.8 at the

level of the posterior commissure. In Charles River SHRs, the degree of ventricular enlargement was

somewhat less than observed at 12 weeks of age in the Taconic Farms animals. However, a comparison of the brain cross-sectional areas revealed that at this

age, the brains of the Charles River SHRs were ap- proximately 7% smaller than those of the age-match WKYs from the same supplier. Smaller brain size in

330

Fig. 3. Photomicrographs of coronal sections from 21-week-old male WKY and SHR rats at the level of the anterior commissure (A and A', respectively), the subfornical organ (B and B') and the posterior commissure (C and C') revealing ventricutar dilation and commensurate atrophy of brain tissue in SHRs.

SHRs, compared to age-matched WKYs, has been

reported previously II, but as noted above, was not

observed in the Taconic Farms animals.

Our results indicate that postnatal ventricular dila-

tion and associated loss of brain tissue are common

traits of O k a m o t o - A o k i SHRs. Unfortunately, these

data do not suggest the mechanism responsible for

ventricular enlargement. However, in speculating

about the etiology and pathogenesis of cerebroven-

tricular dilation in SHRs, several potential mecha-

331

TABLE I

Systolic blood pressure of spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto (WKY) and Sprague- Dawley (SD) rats of various ages

Age Blood pressure (mm Hg)

(weeks) SHR WKY SD

4 111 + 2 89 + 2* 88_+ 5* 8 146 + 6 111 + 3* 115 + 1'

12 152 + 9 112 + 5* 113 + 6* 12 (CR) 1 168 + 5 122 + 4** - 16 168 + 6 122 + 5* 132 + 5 21 204 + 5 126 + 6* -

* P < 0.01, vs age-matched SHRs. ** P < 0.01, vs CR SHRs. i 'CR' designates SHR and WKY rats obtained from Charles River Breeding Laboratories, Inc. Other SHRs and WKYs were obtained from Taconic Farms, Inc. and SDs were ob- tained from Harlan Sprague-Dawley, Inc. All SHRs were of the Okamoto-Aoki strain.

nisms may be suggested. It is conceivable that hydro-

cephalus could result from the hypertension itself.

For example, hypertension may increase the rate of

secretion or decrease the rate of clearance of cere- brospinal fluid, resulting in increased intraventricu-

lar pressure and hydrocephalus. Alternatively, the

multifold consequences of hypertension for cerebro- vascular function 3,19,20 might predispose SHRs to

progressive brain tissue atrophy. If so, ventricular di-

lation may be secondary to neuronal degeneration,

as described for several pathological states including

Alzheimer's disease 2 and Huntington's chorea 4. An-

other possibility is that the ventricular dilation may

result from one of the membrane abnormalities or

body fluid and electrolyte homeostatic mechanisms which are disturbed in SHRs 1,10,21-23. For example,

increased membrane Na ÷ permeability and de-

creased Na+-K ÷ cotransport have been described for

vascular smooth muscle cells and erythrocytes from SHRs. If ion permeability and transport are altered

in choroid plexus, the ionic composition of cerebros-

pinal fluid might be altered, leading perhaps to in-

creased brain extracellular fluid volume.

Other potential mechanisms also exist which could account for the ventricular dilation in SHRs, includ-

ing the possibility that ventricular dilation in SHRs may not be related to their hypertension at all, but

may be an independently inherited genetic trait. In discovering the actual mechanisms involved in the

etiology of ventricular dilation in SHRs, it will be im-

portant to examine cerebrospinal fluid dynamics in

these rats and to determine whether this disorder is

present in other forms of experimental hypertension

or in other spontaneously hypertensive rat strains.

The causal relationship between the ventricular dila-

tion and the brain tissue atrophy also requires further

investigation. In addition, a detailed investigation of

the specific morphological changes in brain tissue as-

sociated with the hydrocephalic process might yield

important information regarding ttie possible contri-

bution of these changes to the SHR syndrome.

The pathophysiological effects of the ventricular

dilation in the SHRs are also matters for speculation

at this point. However, it seems reasonable to as-

sume that the ventricular dilation may have impor-

tant physiological and behavioral consequences since

the dilation appears to be associated with loss of

brain tissue. For example, during the development of

ventricular dilation in SHRs, obvious brain tissue at-

rophy occurs in cortical, septal, hippocampal and

possibly medial hypothalamic sites. A large volume

of behavioral evidence 9 suggests that damage to

these particular structures and, if present, increased

intraventricular pressure, could contribute to the

syndrome of hyperexcitability and hyperresponsivity which are recognized attributes of SHR behav- ior 7,8,15,16. Therefore, the cerebral ventricular dila-

tion which we have shown to be a correlate of sponta-

neous hypertension in the O k a m o t o - A o k i SHR

should be taken into account as a pathophysiological factor in the study of the SHR syndrome.

1 Chrysant, S.G., Walsh, G.M. and Frohlich, D.C., Hemo- dynamic and metabolic evidence of salt sensitivity in spon- taneously hypertensive rats, Kidney Int., 15 (1979) 33-37.

2 De Leon, M.J. and George, A.E., Computed tomography in aging and senile dementia of the Alzheimer type, Adv. Neurol., 38 (1983) 103-122.

3 Dinsdale, H.B., Consequences of elevated arterial pres-

sure in brain. In G.P. Guthrie, Jr. and T.A. Kotchen (Eds.), Hypertension and the Brain, Futura Publishing Co., New York, 1984, pp. 203-220.

4 Dreese, M.J. and Netsky, M.G., Degenerative disorders of the basal ganglia. In J. Minckler (Ed.), Pathology of the Nervous System, Vol. 1, McGraw-Hill Book Co., New York, 1968, pp. 1186-1190.

332

5 Fitzsimons. J.T., Angiotensin stimulation of the central nervous system, Rev. Physiol. Bioehem, t'harmacol.. 87 (1980) 117-167.

6 Grollman, A., The spontaneous hypertensive rat: an exper- imental analogue of essential hypertension in the human being. In K. Okamoto (Ed.), Spontaneous Hypertension: Its Pathogenesis and Complications. Igaku Shoin, Tokyo, 1972, pp. 238-242.

7 McCarty, R., Chiueh, C.C. and Kopin, l.J., Differential behavioral responses of spontaneously hypertensive (SHR) and normotensive (WKY) rats to l>ampbetamine. Pharma- col. Biochem. Behav.. 12 (1980) 53-59,

8 McCarty, R. and Kopin, I.J., Patterns of behavioral devel- opment in spontaneously hypertensive rats and Wistar-- Kyoto normotensive controls, Dev. Psychobiol.. 12 (1979) 239-243.

9 Mogenson, G.J., Jones, D.L. and Yim, C.Y., From moti- vation to action: functional interface between the limbic system and the motor system, Prog. Neurobiol.. 14 (1980) 69-97.

10 Nagaoka, A., Kakihana, K., Fujiwara, K. and Shimakawa, K., Reduced ability to excrete sodium and water in young spontaneously hypertensive rats. In W. Rascher, D. Clugh and D. Ganten (Eds.), Hypertensive Mechanisms: The Spontaneously Hypertensive Rat as a Model to Study Hu- man Hypertension. Proceedings of the 4th International Symposium on Rats with Spontaneous Hypertension and Re- lated Studies. F.K. Schattauer Verlag, Heidelberg, 1982. pp. 249-251.

11 Nelson, D.O. and Boulant, J.A., Altered CNS neuroana- tomical organization of spontaneously hypertensive (SHR) rats, Brain Research, 226 (1981) 119-130,

12 Okamoto, K. and Aoki, K., Development of a strain of spontaneously hypertensive rats, Jpn. Circ. J., 27 (1963) 282-293.

13 Okamoto, K., Yamori, Y., Ooshima, A., Park, C., Haeba- ra, H., Matsumoto, M., Tanaka, T., Okuda, T., Hazama, F. and Kyogoku. M., Establishment of the inbred strain of the spontaneously hypertensive rat and genetic factors in- volved in hypertension. In K. Okamoto (Ed.), Spontaneous Hypertension: Its Pathogenesis and Complications, Igaku

Shoin. l()kyc~.1972, pp, ~7 4L), 14 Rascher, W., ('lugh, 1). and Ganlcn, 1). ( l ids.) tl.vpcrtcv~

sire Meeham's'nzs: ;t'tze 5pontatteous.l 3 tlyperten,~l~, Rat as a Model to Stltdv tluma~z ttyperlension. Proceedings ~I" lhc 4th International Svmposiam on Rats with Spontaneous Hy- pertension and Related Studies, F.K. Schattaucr Verlag. Heidelberg, 1982, pp. 2-19-251.

15 Schaefer, C.F., Brackelt, D.,I., (iunn, C.G. and Wilson, M.F.. Behavioral hypcrreaclivity in the spontaneously h~- pertensive rat compared to its normotensive progenitor, Pavlovian J. Biol. ,Sci. , 13 (1978) 211-216.

16 Schaefer. C.F., Brackett, D.J.. Wilson, M.F. and Gunn, C.G., Lifehmg hyperarousal in the spontaneously hyper- tensive rat indicated by operant behavior, Pal'lovian .1. Biol. Sci,. 13 (1978) 217-225.

17 Simpson, J.B., The circumvcntricular organs and the cen- tral actions of angiotensin. Neuroendocrinology, 32 (I981) 248-256.

18 Taconic Farms Technical Bulletin, 5 (1985). 19 Wiener, J. and Giacomelli, F., Hypertensive vascular dis-

ease. In J. Gencst. O. Kuchel, P, Hamet and M. Cantin (Eds.), Hypertensiol~. Physiology and Treatment, Mc- Graw-Hill Book Co., New York, 1983, pp. 498-524.

20 Winn, R.tI., Clarke, 11,. Jr. and Berne. R.M., Cerebral blood flow regulation in normotension and hypertension. In G.P. Guthrie, Jr. and T.A. Kotchen (Eds.), Hypertension and the Brain, Futura Publishing Co.. New York, 1984, pp, 157-189.

21 Yamori, Y., Physiopathology of the various strains of spon- taneously hypertensive rats. In J. Genest, O. Kuchel, P. Hamet and M. Cantin (Eds.), Hypertension: Physiopatho- logy and Treatment. McGraw-Hill Book Co., New York, 1983, pp. 556-581.

22 Yamori, Y., ttorie. R., Ohtake, M., Nara, Y, and Ooshi- ma, A., Electrolyte balance in stroke-prone and -resistant SHR, Jpn. HeartJ.. 2(t, Suppl. I (1979) 65-67.

23 Yamori, Y., Nara, Y., Horie, R. and Ooshima, A., Abnor- mal membrane characteristics of erythrocytes in rat models and men with predisposition to stroke, Clin, Exp. ttyper- tens.. 2 (1980) 1009-1021.