Spontaneously Hypertensive Rat substrains show differences in model traits for addiction risk and cocaine self-administration: Implications for a novel rat reduced complexity cross

Forward genetic mapping of F2 crosses between closely related substrains of inbred rodents - referred to as a reduced complexity cross (RCC) - is a relatively new strategy for accelerating the pace of gene discovery for complex traits, such as drug addiction. RCCs to date were generated in mice, but rats are thought to be optimal for addiction genetic studies. Based on past literature, one inbred Spontaneously Hypertensive Rat substrain, SHR/NCrl, is predicted to exhibit a distinct behavioral profile as it relates to cocaine self-administration traits relative to another substrain, SHR/NHsd. Direct substrain comparisons are a necessary first step before implementing an RCC. We evaluated model traits for cocaine addiction risk and cocaine self-administration behaviors using a longitudinal within-subjects design. Impulsive-like and compulsive-like traits were greater in SHR/NCrl than SHR/NHsd, as were reactivity to sucrose reward, sensitivity to acute psychostimulant effects of cocaine, and cocaine use studied under fixed-ratio and tandem schedules of cocaine self-administration. Compulsive-like behavior correlated with the acute psychostimulant effects of cocaine, which in turn correlated with cocaine taking under the tandem schedule. Compulsive-like behavior also was the best predictor of cocaine seeking responses. Heritability estimates indicated that 22 %-40 % of the variances for the above phenotypes can be explained by additive genetic factors, providing sufficient genetic variance to conduct genetic mapping in F2 crosses of SHR/NCrl and SHR/NHsd. These results provide compelling support for using an RCC approach in SHR substrains to uncover candidate genes and variants that are of relevance to cocaine use disorders.

Differences in home cage behavior and endocrine parametres in rats of four strains

OBJECTIVE

The majority of studies that involve behavioral testing in different rat strains is based on measuring behavioral responses to stressful situations in a novel environment. Very little is known on the spontaneous behavior in rat strains. The aim of the present study was to compare home cage behavior and basal hormone levels in two outbred rat strains, Sprague-Dawley (SD) and Wistar as well as two inbred strains, Lewis and Spontaneously Hypertensive Rats (SHR).

METHODS

Twenty-eight male rats from four strains of rats (n=7/strain) were used in this study. Behavior of each rat in his home cage was recorded by a video camera for 45 minutes during the dark phase of the day. The parameters considered included rearing, jumps, ambulation, grooming, feeding/drinking and no movements. Blood plasma was analyzed for aldosterone, plasma renin activity (PRA), adrenocorticotropin hormone (ACTH) and corticosterone by specific radioimmunoassays.

RESULTS

One-way ANOVA revealed significant inter-strain differences in counts of jumps and rearing. In comparison with Sprague-Dawley and Wistar rats, Lewis rats displayed significantly more jumps and rearing. Statistical analysis showed significant inter-strain differences in the levels of aldosterone and of plasma renin activity. The highest levels of aldosterone were found in Lewis rats. Plasma renin activity was significantly lower in SHR than in Sprague-Dawley rats. Correlation analysis failed to reveal any relationships between behavioral and endocrine parameters. Significant strain differences were observed also in relative weights of the spleen, adrenals and thymus.

CONCLUSION

Plasma renin activity and basal mineralocorticoid secretion did not show parallel pattern if compared among different rat strains. Locomotor activity in the home cage, which could represent general activity to be considered in evaluating emotional responses, was highest in the Lewis rats.

Aging effects on elevated plus maze behavior in spontaneously hypertensive, Wistar–Kyoto and Sprague–Dawley male and female rats

Male and female spontaneously hypertensive (SHR), Wistar-Kyoto (WKY), and Sprague-Dawley (SD) rats were assessed at one of two ages (postnatal day 74 or 346) for open field locomotor activity and anxiety-related behavior in the elevated plus maze (EPM). In general, the SHR displayed the least anxiety-related behavior, an effect that was magnified with age. At 11 months of age, the SHR more frequently entered and remained longer in the open arms than either the SD or the WKY strains. EPM behavior of the WKY strain was much less affected by age than that of the SD strain which displayed increased anxiety-related behavior with age. At the younger age, the typical sex effects were apparent; specifically, females exhibited a shorter duration in the closed arms. While the SHR were the most active strain in the EPM at both ages, they were more active in the open field only at the older age. In general, age-related changes in open field activity mirrored those of the EPM. These results provide a more comprehensive illustration of aging-related behavioral changes in male and female SHR, WKY and SD rats.

Does hypertension alone lead to cognitive decline in spontaneously hypertensive rats?

Spontaneously hypertensive rats (SHR) represent an animal model of cognitive decline associated with hypertension. Few studies have systematically investigated this decline in aging. We assessed spatial learning performances of SHR, the genetically similar Wistar-Kyoto rats (WKY), and Sprague-Dawley rats (SD), in a water maze (WM) task. The following age ranges were studied: 6, 12, and 20 months. The results demonstrated that all three strains show the same level of impairment at the age of 20 months. The rate of cognitive decline, however, is different: both SHR and WKY show moderate degrees of impairment at all age ranges, while SD display good cognitive abilities at 6 months, declining at 12, and reaching the performance level of the other two groups at 20 months. Besides, the nine groups showed substantial differences in swim velocity and WKY exhibited a peculiar motor behavior. These results suggest the following: (a) the decline in cognitive level exhibits different trends in the three strains; (b) learning impairment of aged SHR might not be entirely explained by hypertension; (c) WKY should be used cautiously as normotensive control for SHR, due to their unusual behavior and low learning abilities; (d) analysis of escape distances is mandatory for the comparison of different strains in the WM test.

Behavioral validation of the spontaneously hypertensive rat (SHR) as an animal model of attention-deficit/hyperactivity disorder (AD/HD)

A good model of a disorder is one that: (a) mimics, although in a simpler form than the full-blown clinical case, the fundamentals of the behavioral characteristics, in this case of people with Attention-Deficit/Hyperactivity Disorder (AD/HD;face validity); (b) conforms with a theoretical rationale for the disorder (construct validity); and (c) is able to predict aspects of behavior, genetics and neurobiology previously uncharted in the clinics (predictive validity). This article discusses the Spontaneously Hypertensive Rat (SHR) and some other putative animal models of AD/HD. It is argued that although other strains and species may be hyperactive and/or show attention deficits following genetic, environmental or pharmacological interventions, the SHR is presently the only strain shown to have the major behavioral symptoms of AD/HD. This does not mean that investigating other models cannot give valuable information.

Concurrent reductions in blood pressure and metabolic rate during fasting in the unrestrained SHR

Fasting produces multiple cardiovascular, metabolic, and behavioral responses. To examine the interrelationship between these responses, male spontaneously hypertensive rats (SHR; n = 8) implanted with cardiovascular telemetry devices were housed in metabolic chambers at 23 degrees C for 22-h daily measurements of physiological variables. The experimental apparatus was designed so that ingestive behavior was detected by photobeams and locomotion was detected by a load sensor. Cardiovascular and metabolic status were determined as both a function of the circadian cycle (12-h dark and 10-h light), as well as during periods of inactivity (no ingestion and minimal locomotion) within the dark and light phases. Data were obtained during baseline, 48-h of caloric deprivation, and 6 days of refeeding. Fasting produced significant reductions in mean arterial pressure (dark: -9.2+/-1.3 from 143.7+/-3.7 mm Hg; light: -8.6+/-1.8 from 140.1+/-3.7 mm Hg), heart rate (dark: -43.4+/-5.2 from 330.0+/-5.2 beats/min; light: -27.4+/-5.2 from 294.0+/-5.2 beats/min), and oxygen consumption (dark: -5.0+/-0.6 from 20.6+/-0.3 ml x min(-1) x kg (0.75); light: -2.7+/-0.2 from 14.9 +/-0.2 ml x min(-1) x kg(0.75)). Analysis of inactive periods during both light and dark phases revealed that these reductions were not dependent on behavioral effects. We conclude that fasting produces concurrent and interrelated reductions in cardiovascular and metabolic function in the SHR. The merging of cardiovascular telemetry, indirect calorimetry, and behavioral monitoring provides a powerful approach for investigation of the integrative physiological responses to energetic challenges.

The spontaneously hypertensive rat: insight into the pathogenesis of irritative symptoms in benign prostatic hyperplasia and young anxious males

Recent epidemiological studies have shown that hypertensive men are more likely to undergo surgical intervention for irritative voiding symptoms from BPH than age-matched controls. Indeed, noradrenergic nerves which regulate vascular tone also participate in the functional component of bladder outlet obstruction due to BPH. Newer, less invasive therapies for BPH such as thermal therapy can relieve symptoms yet do not eliminate obstruction based on urodynamic studies. Coincidentally, drugs such as alpha-adrenoceptor antagonists, which have been thought to relieve obstruction due to a peripheral effect, can be given intrathecally in animals to relieve urinary frequency due to obstruction. Taken together these observations implicate both peripheral and central sympathetic pathways in the motor control of the urinary bladder especially with disease states. We have used the hypertensive and behaviourally hyperactive spontaneously hypertensive rat (SHR), to investigate the roles sympathetic pathways or micturition. Elevated nerve growth factor (NGF) derived from vascular and bladder smooth muscle cells of the SHR appears to direct morphological, biochemical, and functional changes. The increase in NGF can apparently be explained by stabilization of its mRNA leading to increased synthesis in NGF. Bladders from SHRs develop a profuse noradrenergic hyperinnervation compared with the control WKY strain. Since afferents supplying the SHR bladder are hypertrophied, changes in afferent pathways are also likely. These differences in innervation and NGF in the SHR may explain changes in function. SHRs void 3 times as frequently as their genetic controls. Urinary frequency can be reduced by alpha-adrenoceptor antagonists. Cystometrograms performed in SHRs reveal lower bladder capacities and micturition volumes and the presence of unstable contractions compared with the WKY rat. Intrathecal, rather than intra-arterial administration of the alpha-adrenoceptor antagonist doxazosin reduces unstable contractions in the SHR. In vitro muscle bath studies have shown enhanced responses of SHR bladder smooth muscle to alpha-adrenoceptor agonists. It is likely that upregulation of NGF production causes sensory and possibly noradrenergic pathways to elicit hyperactive voiding. Increase in NGF in the adult bladder due to pathological conditions yields similar, yet distinct, consequences for voiding behaviour and innervation. Likewise, increased NGF in adult bladders following obstruction or inflammation triggers neuronal hypertrophy, enhanced reflex activity and urinary frequency. In contrast to the SHR, hyper-innervation is not observed. Moreover, peripheral or spinal alpha-adrenoceptor blockade eliminates urinary frequency following obstruction. These observations support the role for sympathetic pathways in the motor function of the bladder, especially in congenital or adult disease states. A similar process may underlie the neuroplasticity involved in alterations after obstruction or inflammation of the lower urinary tract in humans. The SHR strain raises the possibility that a common genetic defect exists capable of predisposing to both hypertension and overactivity of the urinary bladder. Whether a genetic predisposition to sustained bladder overactivity in response to inflammatory stimuli in obstruction exists in humans is an intriguing prospect.

Strain difference in behavioral response to a new environment in rats

Locomotor activity in rats throughout a 24-hour period in a new environment was examined for strain differences and for the capacity for adaptation to that environment. Fischer 344 rats (F344), spontaneously hypertensive rats (SHR) and Wistar normotensive Kyoto rats (WKY) were used. The horizontal locomotor activity of individual rats was measured by photocell-utilizing activity-recording devices. The locomotor activity counts on the second day, after 1 day of adaptation, were compared with those after 5 days of adaptation (on the sixth day). In WKY, there was no difference in activity at any period of the day between the second and sixth days. In SHR, the locomotor activity on the second day between 6:00 h and 9:00 h (in the light phase) and between 24:00 h and 3:00 h (in the dark phase) was higher than on the sixth day. In F344, the locomotor activity on the second day between 18:00 h and 2:00 h was higher than on the sixth day. The capacity for adaptation in SHR and F344 was thus poorer than in WKY. The poor adaptation in SHR and F344 was similar to that in depressive patients induced by moving house. These findings suggested that SHR or F344 were suitable for depression research.

Comparison of anxiety measured in the elevated plus-maze, open-field and social interaction tests between spontaneously hypertensive rats and Wistar EPM-1 rats

On the basis of open-field and plus-maze results it has been proposed that spontaneously hypertensive (SHR) rats are less emotionally reactive than their normotensive controls, Wistar-Kyoto (WKY). However, the proposed "anxiolytic characteristics" of SHR rats may be questioned in view of the significant hypoactivity presented by WKY rats. In the present study, the behavioral response of spontaneously hypertensive (SHR) and equally active normotensive Wistar EPM-1 (EPM-1) rats (4-month old males, 10-13 animals per group) were evaluated in the open-field, social interaction and elevated plus-maze tests. In the open-field study, no differences were observed for total locomotion frequency and immobility duration, but SHR rats presented a higher central square locomotion frequency (23.8 +/- 2.1 vs 10.3 +/- 1.6) as compared to EPM-1. SHR rats also exhibited a greater duration of social interaction when compared to EPM-1 rats (mean +/- SEM values were 113.9 +/- 8.7 s for SHR vs 72.7 +/- 8.6 s for EPM-1 rats after 8-min observation). In the elevated plus-maze test, SHR rats presented an increased percent of entries (52.8 +/- 3.3 vs 28.3 +/- 4.5) and time in the open arms (65.6 +/- 6.0 vs 11.1 +/- 1.9) as compared to EPM-1 rats, although the total number of arm entries (9.2 +/- 0.9 vs 9.7 +/- 1.0) was unchanged. These results suggest that the "anxiolytic behavior" of SHR rats in relation to normotensive controls is not related to differences in motility levels.

The spontaneously hypertensive rat (SHR) as an animal model of childhood hyperactivity (ADHD): changed reactivity to reinforcers and to psychomotor stimulants

Childhood hyperactivity (attention-deficit hyperactivity disorder, ADHD) is a behavior disorder affecting 2-6% of grade-school children. The main symptoms are attention problems and hyperkinesis. The disorder is commonly treated with psychomotor stimulants, usually methylphenidate hydrochloride (ritalin) or d-amphetamine. Neither the cause of the disorder nor the basis of the effectiveness of the drug treatment is well understood. Differences in reinforcement processes have been implicated as part of the underlying problem. The main purpose of the present research was to investigate reinforcement processes and motor characteristics with and without stimulant medication in SHR, as an animal model of ADHD, and WKY controls, its normoactive progenitor strain. SHR behavior turned out to be more sensitive to immediate reinforcement and proportionately less sensitive to delayed reinforcement when compared to the behavior of WKY, as demonstrated by systematic changes in rates of responding throughout fixed-interval schedules of reinforcement of bar-presses by water. The psychomotor stimulants weakened the control by immediate reinforcers and strengthened the control by delayed reinforcers, with the effect of the drugs being more pronounced in WKY than in SHR. The results are consistent with clinical observations that ADHD children are less willing than others to accept "delayed gratification" and that methylphenidate increases the control of delayed reward over their behavior.