Excessive thyrotropin concentrations in the circulation of the spontaneously hypertensive rat

Differential Hypothalamic-pituitary-adrenal Response to Stress among Rat Strains: Methodological Considerations and Relevance for Neuropsychiatric Research

The hormones of the hypothalamic-pituitary-adrenal (HPA) axis, particularly glucocorticoids (GCs), play a critical role in the behavioral and physiological consequences of exposure to stress. For this reason, numerous studies have described differences in HPA function between different rodent strains/lines obtained by genetic selection of certain characteristics not directly related to the HPA axis. These studies have demonstrated a complex and poorly understood relationship between HPA function and certain relevant behavioral characteristics. The present review first remarks important methodological considerations regarding the evaluation and interpretation of resting and stress levels of HPA hormones. Then, it presents works in which differences in HPA function between Lewis and Fischer rats were explored as a model for how to approach other strain comparisons. After that, differences in the HPA axis between classical strain pairs (e.g. High and Low anxiety rats, Roman high- and low-avoidance, Wistar Kyoto versus Spontaneously Hypertensive or other strains, Flinder Sensitive and Flinder Resistant lines) are described. Finally, after discussing the relationship between HPA differences and relevant behavioral traits (anxiety-like and depression-like behavior and coping style), an example for main methodological and interpretative concerns and how to test strain differences is offered.

Antisense inhibition of thyrotropin-releasing hormone reduces arterial blood pressure in spontaneously hypertensive rats

Thyrotropin-releasing hormone (TRH) plays an important role in central cardiovascular regulation. Recently, we described that the TRH precursor gene overexpression induces hypertension in the normal rat. In addition, we published that spontaneously hypertensive rats (SHR) have central extrahypothalamic TRH hyperactivity with increased TRH synthesis and release and an elevated TRH receptor number. In the present study, we report that intracerebroventricular antisense (AS) treatment with a phosphorothioate oligonucleotide against the TRH precursor gene significantly diminished up to 72 hours and in a dose-dependent manner the increased diencephalic TRH content, whereas normalized systolic blood pressure (SABP) was present in the SHR compared with Wistar-Kyoto (WKY) rats. Although basal thyrotropin was higher in SHR compared with WKY rats and this difference disappeared after antisense treatment, no differences were observed in plasma T4 or T3 between strains with or without AS treatment, indicating that the effect of the AS on SABP was independent of the thyroid status. Because the encephalic renin-angiotensin system seems to be crucial in the development and/or maintenance of hypertension in SHR, we investigated the effect of antisense inhibition of TRH on that system and found that TRH antisense treatment significantly diminished the elevated diencephalic angiotensin II (Ang II) content in the SHR without any effect in control animals, suggesting that the Ang II system is involved in the TRH cardiovascular effects. To summarize, the central TRH system seems to be involved in the etiopathogenesis of hypertension in this model of essential hypertension.

Skeletal muscle adaptation to physical training and beta-adrenergic blockade in spontaneously hypertensive rats

The effects of training alone or in combination with long-term, non-selective, beta-adrenergic blockade on histochemical and biochemical properties of fast-twitch [extensor digitorum longus muscle (EDL)] and slow-twitch [soleus muscle (Sol)] muscle were analyzed in spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto strain rats (WKY). Fiber type distribution of Sol was drastically modified in SHR with fewer type I fibers and more type IIA fibers. No such histochemical alterations were observed in EDL. While prolonged swimming training remained ineffective in inducing both histochemical and biochemical improvement in WKY, SHR displayed a significant enhancement of capillarization and oxidative capacity in both Sol and EDL. However, in long-term beta-blocks rats training failed to improve significantly the oxidative capacity of SHR muscles, suggesting that beta-adrenoreceptor stimulation is necessary for a fully efficient adaptation of muscular metabolism to physical training.

Quantitative autoradiography of TRH receptors in discrete brain regions of different mammalian species

The results clearly show marked heterogeneity and ubiquity of the CNS distribution of TRH receptors across several mammalian species including man. The use of high resolution autoradiography coupled with image analysis has permitted the visualization and quantification of TRH receptor density in even very small regions and nuclei of the CNS. This technique will undoubtedly help elucidate the other areas of TRH receptor localization that have thus far escaped detection in mammals and that are yet to be studied in lower vertebrates. Although an attempt has been made to correlate the presence of the peptide, its receptors, and its possible physiological functions, only further detailed physiological/behavioral investigations will ultimately unravel and support the diverse neurotransmitter and trophic roles of TRH in CNS and endocrine function.

Studies on the mechanism of the enhanced cold-induced TSH secretion in spontaneously hypertensive rats

Various noradrenergic and tryptaminergic antagonists as well as pinealectomy significantly inhibited cold-induced TSH secretion in SHR as in control rats.

Hypothyroidism as a cause of hypertension

To study whether there is an association between hypertension and hypothyroidism, measurements of blood pressure and thyroid function were determined in 477 female patients with chronic thyroiditis. Based on the blood levels of thyroxine (T4) and thyroid stimulating hormone (TSH), 308 patients were considered euthyroid and 169 were hypothyroid [T4 = 2.9 +/- 0.1 micrograms/dl and TSH = 105.8 +/- 6.8 microU/ml (mean +/- SEM)]. Diastolic, but not systolic, blood pressure in hypothyroid patients over 50 years was higher than in euthyroid patients of corresponding age groups. The prevalence of hypertension was higher in hypothyroid patients when hypertension was defined as the systolic and/or diastolic blood pressure above 160/95 mm Hg (14.8% vs 5.5%; p less than 0.01). Correlations between diastolic, but not systolic, blood pressure and either the blood level of triiodothyronine (T3) or T4 was significant (r = - 0.174, p less than 0.01, and r = 0.208, p less than 0.01, respectively) when data from both euthyroid and hypothyroid patients were combined. Adequate thyroid hormone replacement therapy for an average 14.8 months in 14 patients resulted in a normalization of thyroid function and a reduction of blood pressure (p less than 0.01). In four who showed no change in thyroid function due to inadequate replacement therapy, blood pressure remained elevated. These results suggest a close association between hypertension and hypothyroidism.

Morphological findings in spontaneously hypertensive rats in comparison with findings after experimental renal hypertension

The morphological findings in spontaneously hypertensive rats (SHR) were compared with the findings after experimental renal hypertension. In addition the effect of an arteriosclerogenic diet was investigated. Cardiac hypertrophy occurred in all types of hypertension. During the course of renal hypertension there was also hypertrophy of the tunica media of the arterial vessels of the heart, kidneys, mesenterium, and orta. Degenerative changes in the vascular walls occurred when nephrectomized rats were given a diet containing 2% cholesterol and 1% cholic acid. This produced intima lipoidosis after seven to eleven weeks. The intensity seemed to be dependent on the degree of hypertension and the duration of the experiment.

Role of the thyroid gland in the development and maintenance of spontaneous hypertension in rats

We studied the vascular mechanisms involved in the prevention of the development of hypertension following thyroidectomy. Ablation of the thyroid gland of 4-week-old spontaneously hypertensive (SH) rats inhibited the development of hypertension and reduced the sensitivity of aortic strips to the vasoconstrictors phenylephrine, norepinephrine, and potassium chloride and the vasodilator isoproterenol. Daily injections of a replacement dose of L-thyroxine caused a complete recurrence of hypertension in these rats. This was accompanied by complete recovery of the aortic sensitivity to the vasconstrictors and isoproterenol. In 10-week-old SH rats thyroidectomy prevented a further increase of blood pressure but did not reverse the hypertention. Here, the aortic sensitivity to vasoactive substances also was reduced but to a lesser extent than in the SH rats thyroidectomized at 4 weeks of age. Hypertension was not obviously associated with hyperfunction of the thyroid gland. Furthermore, we found that at 4 weeks of age, during the prehypertensive period, SH rats have a significantly lower (42%) serum thyroxine level than age-matched normotensive Kyoto Wistar, American Wistar, and Sprague-Dawley rats. However, at 6 and 9 weeks, the serum thyroxine levels of SH rats are similar to those of the normotensive rats. In conclusion, we propose that the reduced sensitivity to endogenous vasoconstrictors in arteries of SH rats following juvenile ablation of the thyroid gland may prevent the development of hypertension in these rats. Moreover, the low serum thyroxine level in SH rats during the prehypertensive period may explain why young SH rats do not develop hypertension before 6 weeks of age.