Edith D. Hendley

Behavior of hypertensive and hyperactive rat strains: hyperactivity is not unitarily determined.

The spontaneously hypertensive rat (SHR) is behaviorally hyperactive relative to the Wistar-Kyoto rat (WKY). By breeding SHR with WKY, followed by inbreeding, two new strains have been developed in which hypertension seems to be separated from hyperactivity to novel stimuli: the WKHT and the WKHA strains. The main purpose of the present study was to determine which behavioral characteristics of SHR have been dissociated from the hypertensive trait in the WKHA strain. Male SHR, WKY, WKHT, and WKHA were subjected to three protocols: 1) Two forced-exploration tests, where the results showed that both the SHR and the WKHA rats were hyperactive. 2) A free-exploration open field, where the SHR was more active than the other strains, showing shorter latencies to leave the home cage, spending more time in the field, ambulating and rearing more. Furthermore, the WKHT behavior was more similar to the SHR behavior than the WKHA behavior. 3) A two-component schedule of reinforcement, where one component (fixed-interval 2 min) was signaled by houselight on and the other (extinction, EXT) by houselight off. In this test, the SHR behavior was markedly different from that of the three other strains: the fixed-interval scallop, the accelerated responding towards the end of the interval, was steeper in SHR than in the other groups. The SHR emitted more responses during the extinction component of the schedule. The SHR hyperactivity was dependent upon the reinforcement value of the water deliveries and was increased even further by sensory-reinforcing respones feedback lights. Thus, the hyperactivity of the WKHA strain seems to be less pervasive than that of the SHR.(ABSTRACT TRUNCATED AT 250 WORDS)

Attenuation of hyperactivity in the spontaneously hypertensive rat by amphetamine

Several experiments show that spontaneously hypertensive rats (SHR) are behaviorally hyperactive when compared with their normotensive Wistar Kyoto (WKY) progenitor rat strain. Behavioral hyperactivity was present in both 3-min open-field tests as well as in 1-hr tests using an automated activity recording chamber. In addition, under certain conditions, d-amphetamine (1.25–3.5 mg/kg) decreased activity in the SHR while inducing the expected increase in activity in the WKY. Further analysis of these data shows that the attenuation of SHR behavioral hyperactivity by amphetamine can be predicted based upon rate dependency of the actions of d-amphetamine. The SHR may provide a valuable animal model for studying spontaneous hyperactivity and for investigating the neurochemical basis of the so-called “paradoxical response” to amphetamine as seen in children.

Dissociation of genetic hyperactivity and hypertension in SHR.

The Wistar Kyoto strain of spontaneously hypertensive rat (SHR) has been characterized as behaviorally hyperactive as well as hypertensive. The relationship between these two inbred traits remains uncertain, and their coexistence in the SHR has complicated studies of central nervous system mechanisms underlying the hypertensive process. A breeding program was initiated to examine the possible genetic linkage of these two traits which, if separable, would allow us to develop substrains of SHR that are hypertensive without being hyperactive, or hyperactive without being hypertensive. We crossed SHR males with Wistar Kyoto, normotensive (WKY) female rats and produced F1 hybrids which were then randomly inbred to produce an F2 population. When tested at 12 weeks of age, F2 rats exhibited the expected wide range of mean systolic blood pressures (BP), from 111 to 174 mm Hg, as determined using indirect tail plethysmography. The BP in the parental rats at the time of breeding (16 weeks) was 187 +/- 4.5 mm Hg (SHR males, n = 7) and 111 +/- 2.4 (WKY females, n = 7). Locomotor activity was determined in an automated activity cage in F1 and F2 rats at 12 weeks of age. These strains exhibited a wide range of phenotypic distribution of locomotor activity scores, and the mean scores were intermediary between those of SHR rats and WKY rats of the same age. Among individual rats of both the F1 and F2 hybrid strains, there was no correlation between the activity score and the level of the BP at 12 weeks of age. These findings indicated that the genes responsible for the hypertensive trait and those responsible for the hyperactivity trait were not tightly linked in the hybrid populations, suggesting that different genetic factors were involved in the transmission of each of these traits. Accordingly, it should be possible to separate the two traits by further selective, recombinant inbreeding procedures.

Changes in catecholamine neuronal uptake and receptor binding in the brains of spontaneously hypertensive rats (SHR).

In these studies we have characterized differences between spontaneously hypertensive rats (SHR) and normotensive Wistar/Kyoto (WKY) rats with respect to rates of neuronal uptake of norepinephrine (NE) and dopamine (DA), and beta-adrenergic receptor (dihydroalprenolol; [3H]DHA) binding in the central nervous system. We find that SHR have greater rates of NE uptake in the frontal cortex, cerebellum, hypothalamus and pons-medulla during early development, and that these changes are accounted for, at least in the cerebral cortex, by an increased Vmax of the NE uptake mechanism. In addition, we find a decrease in the Bmax for [3H]DHA binding, suggestive of down-regulation of beta-adrenergic receptors of this region. In contrast to the results for NE uptake, we have measured significant decreases in DA uptake in the frontal cortex of the SHR at several postnatal ages. Decreases in DA uptake were also observed in the striatum of SHR although these changes were found only in animals approximately 6 weeks of age. From these results we have suggested that NE neurons projecting to a number of brain regions have elevated functional activity, while more regionally selective decreases in dopaminergic functional activity are characteristic of the SHR. We have further proposed that these changes in catecholamine neurons of the central nervous system may play an important role in the development of both the hypertension and behavioral hyperactivity exhibited by these animals.

Inbreeding of Wistar–Kyoto rat strain with hyperactivity but without hypertension

A genetic inbreeding program using Wistar-Kyoto rat strains as progenitors was used to combine the hyperactivity trait of the spontaneously hypertensive rat (SHR) with the normotensive trait of the WKY genetic control strain. From an SHR X WKY cross we produced a gene-assorting F2 population from which selected brother-sister matings were carried out through seven successive inbred populations. This program produced a new strain of hyperactive rats with normotensive mean systolic blood pressure levels, and we have designated the new strain as the Wistar-Kyoto hyperactive (WK/HA) rat. Another behavioral characteristic of the SHR rat, poor habituation in a nonreinforcing novel environment, did not appear as a characteristic trait of the new strain of WK/HA rats, suggesting a separate underlying genetic basis for the two traits that had been apparently fortuitously fixed in the SHR genotype as a result of intensive inbreeding of that strain. The new WK/HA strain, together with the WKY control strain, is considered as more suitable for subjects in studying hyperactivity in rats than the original SHR strain with its concomitant hypertension and poor habituation traits.

Behavioral and neuroendocrine reactivity to stress in the WKHA/WKY inbred rat strains: a multifactorial and genetic analysis

Genetic factors have been shown to influence the nature and the intensity of the stress responses. In order to understand better the genetic mechanisms involved, we have studied the behavioral and neuroendocrine responses to novel environments in the WKHA/WKY inbred strains and we have investigated the genetic relationships between these traits in a segregating F2 intercross. The animals were submitted to behavioral tests known to provide both indices of activity and fear (activity cages, open field and elevated plus-maze). The plasma levels of prolactin, ACTH, corticosterone, glucose and renin activity were determined after a 10-min exposure to novelty. Our results showed that WKHA rats, compared to WKYs, were more active in a familiar as well as in novel environments. They exhibited also less anxiety-related behaviors and lower neuroendocrine responses. A principal component analysis performed on the behavioral F2 results defined three independent factors: general activity, anxiety and defecation, none of them being correlated with the neuroendocrine measures. Thus this study suggests that these different responses to stress are independent components that may have distinct molecular bases.

Age, sex and strain differences in activity and habituation in SHR and WKY Rats

The highly inbred strain of Wistar-Kyoto spontaneously hypertensive rat (SHR) and its normotensive, genetic control (WKY) were examined with respect to strain differences in spontaneous activity scores in a novel environment (small activity cage) and in ability to habituate to that environment. These behaviors were examined in experimentally naive rats, 197 SHR and WKY, males and females, at varying ages from 4 to 56 weeks, in order to determine whether there are sex and age differences in addition to the well-known strain differences in these behaviors. Total activity scores, determined in a 15 min test in the activity cage, were higher in SHR than WKY rats; females were significantly more active than males in either strain, and activity scores varied significantly with age both within strains and between strains. Ability to habituate to the test cage was determined by repeating the 15 min activity test at hourly intervals for three additional trials on the same day. The results indicate that the SHR, males and females and at all ages tested, habituate poorly if at all to the test cage as compared with WKY rats. Moreover, despite the variability of baseline activity scores (first trial) observed across ages, sexes and strains, the habituation patterns of either strain remained relatively fixed throughout the first year of life.

Independence of blood pressure and locomotor hyperactivity in normotensive and genetically hypertensive rat

The spontaneously hypertensive rat (SHR) exhibits locomotor hyperactivity in comparison to its normotensive progenitor Wistar-Kyoto (WKY) strain. We asked whether the hyperactive behavior was a direct consequence of elevated blood pressure in the hypertensive rat. Three experimental protocols were used to chronically alter blood pressure. In the first protocol, a group of adult SHRs was given hydralazine (20 mg/kg/day) in their drinking water to lower blood pressure. These animals exhibited a significant decrease in blood pressure, but no change in locomotor activity. In the second protocol, young SHRs (4 weeks of age) were treated with the same dosage of hydralazine until 16 weeks of age. Blood pressure was significantly decreased in these animals with no change in locomotor activity. In the third protocol, normotensive WKY and Sprague-Dawley (SD) rats were made hypertensive with unilateral renal clips. The resulting increase in blood pressure in these animals did not alter locomotor activity. These results suggest that locomotor hyperactivity is an inherent property of the SHR and is independent of blood pressure.

Anti-CD 18 monoclonal antibody slows experimental aortic aneurysm expansion☆☆☆★★★

PURPOSE Inflammation has been implicated as a contributing factor in the expansion of abdominal aortic aneurysms (AAA). To test this hypothesis, we examined the effects of a monoclonal antibody (MAB) to the leukocyte CD18 adhesion molecule on the expansion of experimental AAA. METHODS Aneurysms were induced by perfusion of an isolated segment of the infrarenal aorta with elastase in 22 normotensive (WKY) and 17 genetically hypertensive (WKHT) rats. Animals of both strains were randomly allocated to control or MAB-treated groups (MAB, 5 microgram/100 gm body weight intraperitoneally, daily, beginning on the operative day for a total of four doses). The activity of the MAB against rat leukocytes had first been determined by in vitro immunofluorescence flow cytometry. Aortic size was directly measured initially and on day 14. At that time, a segment of aorta was stained with hematoxylin and eosin and mononuclear leukocytes and neutrophils were counted in each of 10 microscopic fields (400X). RESULTS The initial aortic size in all animals was 1.11+/-0.15 mm. All groups developed aneurysms significantly larger than the initial aortic size (p<0.01). However, the MAB-treated animals had significantly smaller aneurysms than the untreated controls (mm): WKY: 3.63+/-1.26, WKY-MAB: 2.08+/-0.30, WKHT: 4.54+/-1.86, WKHT-MAB: 2.37+/-0.40, p<0.0001. There also were significantly fewer monocytes in the MAB-treated normotensive rats: WKY:35.5+/-29.9, WKHT:40.6+/-28.8, WKY-MAB: 8.9+/-8.5, WKHT-MAB: 32.3+/-25.7, p=0.03. Neutrophil counts did not differ significantly between the groups. CONCLUSIONS Treatment with anti-CD18 monoclonal antibody slows the expansion of AAA in this experimental model. The associated inflammatory process at day 14, as indicated by monocyte infiltration, is reduced, but this effect may be opposed by the presence of hypertension. Further evaluation of the role of leukocytes and adhesion molecules in the expansion of AAA is warranted.

Expansion of aortic aneurysms is reduced by propranolol in a hypertensive rat model

PURPOSE It has been suggested that propranolol has unique effects that slow aneurysm expansion by remodeling the structural proteins of the aorta. These effects are believed to be independent of blood pressure reduction, a hypothesis we tested in this investigation with a rat model of abdominal aortic aneurysm (AAA). METHODS With an established model, AAA were induced in normotensive Wistar-Kyoto (WKY) rats and genetically hypertensive Wistar-Kyoto (WKHT) rats by perfusing an isolated segment of the infrarenal aorta with elastase. A propranolol dose-response was studied for each strain: (1) saline solution controls (n = 18); (2) propranolol, 10 mg/kg subcutaneously (n = 18); (3) propranolol, 30 mg/kg (n = 14). Systolic blood pressure was determined by tail plethysmography before operation and on day 14, as well as by direct recording at surgery and on day 14. Rats were killed at 14 days, and aneurysm diameter was measured. RESULTS The initial tail BP was 129 +/- 22 mm Hg in WKY animals and 158 +/- 21 mm Hg in WKHT animals (p < 0.0001). Tail BP and intraaortic systolic, diastolic, and mean blood pressure (BP) were not significantly decreased by propranolol treatment in either strain of rats. However, BP tended to rise in WKY rats, whereas it fell slightly in WKHT rats. Initial aortic size in all animals was 1.06 +/- 0.12. The final aortic size in untreated, hypertensive rats was more than twice that of untreated normotensive controls: 1: WKHT, 3.0 +/- 0.73 mm, 1: WKY, 6.9 +/- 3.5 mm (p < 0.01). After treatment with both doses of propranolol, hypertensive aneurysms were significantly smaller than the untreated WKHT group (p < 0.05) and not significantly different from aneurysms in all groups of normotensive animals: 2: WKY, 3.1 +/- 1.13 mm, 2: WKHT, 4.0 +/- 1.81 mm; 3: WKY, 4.1 +/- 0.41 mm, 3: WKHT, 2.9 +/- 1.24 mm. There was no significant difference in aortic size between the three normotensive WKY groups. CONCLUSIONS Hypertension increases the size of aortic aneurysms in this experimental model. Propranolol significantly reduces the size of experimental AAA in hypertensive animals independently of the dose and by a mechanism that may be unrelated to simple BP reduction.