**4. Combined animal model of chronic social isolation and long-term daily treadmill running in rats and "cross stressor adaptation hypothesis"**

risk of cardiovascular diseases [21, 22]. In addition, the gene expression of splenic catecholamine biosynthetic enzymes is decreased after CSI. This might reduce catecholamine synthesis in the spleen and deplete the immunocompetent tissues of catecholamines which cause an impairment of immune response [11]. Key question in adaptive response to stress is how the addition stressor can elicit a variant or altered response depending on prior experience with the current or different stressor. A potentiation of the sympathoadrenal system activity in socially isolated rats upon exposure to novel acute stressors has been reported [23]. The additional acute immobilization does not affect the gene expression of catecholamine biosynthetic enzymes in both auricles of long-term socially isolated rats. This suggests that the response to stress depends on prior experience with stressors [24]. Data from literature indicate a possible adaptation of catecholamine-synthesizing system at the level of gene expression in the heart auricles of chronically socially isolated rats exposed to acute immobilization stress [24]. However, protein levels of catecholamine biosynthetic enzymes in both ventricles of socially isolated rats increased after additional acute stress [25]. With regard to the role of cardiac catecholamines in physiological and pathophysiological processes, it could be hypothesized that increased catecholamine synthesis in the ventricles after acute stress indicates sensitivity of the heart to subsequent stress [25].

286 Experimental Animal Models of Human Diseases - An Effective Therapeutic Strategy

It could be concluded that animal model of chronic social isolation (CSI) in rats is a good animal model in the research of neuroendocrine and immune functions in stress conditions. Also, the described results indicate the potential application of CSI animal models in under-

Physical exercise produces modulation of neuroendocrine and immune functions [26] and increases the activity of the antioxidant defense system [27]. Long-term treadmill running in rats is forced exercise which has the propensity to induce both psychological and physical stress [28].

Long-term treadmill running animal model (TR) consists of 11-week-old Wistar male rats that are exposed to long-term treadmill running. Long-term treadmill running is achieved by the rats' daily running on the treadmill for a period of 12 weeks. The treadmill running intensity is gradually increased from week to week and from the initial 10 min—10m/min up

Treadmill running may induce physiological adaptations, which can be reflected in increased plasma catecholamine concentration, as well as in the change of the synthesis of catecholamine biosynthetic enzymes in rats [31]. It is a very strong stressor, which activates the sympathoadrenomedullary system and increases the synthesis of splenic PNMT protein catalyzing the conversion of NA to A, which both can modulate the immune functions [31]. It is known that cardiovascular diseases, such as hypertension and heart failure, are often associated with sympathetic nervous system overreactivity [32, 33]. The increase of the noradrenaline biosynthetic enzyme expression in stellate ganglia, which causes the increase of plasma NA levels, due to chronic forced running, may play a role in the growing risk for cardiovascular diseases [22, 34].

incline [22, 29, 30]. Animals are being exposed to treadmill training

**3. Animal model of long-term treadmill running in rats**

standing of stress in humans.

to 20 min—20m/min at 00

5 days a week for 12 weeks [22].

Data from literature confirm that exercise has been widely used in the last years with therapeutic and preventive purposes in a series of pathophysiological conditions. Exercise training reduces the risk of developing diseases related to chronic stress. For example, a physically active lifestyle is associated with decreased risks of coronary heart disease and high blood pressure [35]. In addition, in humans, regular exercise has a beneficial impact on depression [36]. It is known that the theory of "cross stressor adaptation hypothesis" suggests that exercise training, as a stressor on the body, may alter responsiveness to other types of stressors [37]. Mueller [38] suggests that exercise training appears to reduce sympathoexcitation to a variety of centrally mediated sympathoexcitatory stimuli. Reduction in sympathoexcitation may contribute, in part, to the reduced incidence of cardiovascular disease in physically active individuals [38]. In addition, physical activity prevents splenic NA depletion, or spillover, typically observed in sedentary rats following periods of intense sympathetic drive [39]. Also, physical activity may prevent stress-induced suppression of splenic immunity by reducing sympathetic drive to the spleen during stress [40, 41].

Treatment of chronic social isolation and long-term daily treadmill running (CSITR) consists of exposing the individually housed Wistar male rats to the daily treadmill running during 12 weeks [42].

Understanding the mechanisms by which CSITR training alters control of the SNS in health and disease could be important for developing new strategies in the prevention and treatment of cardiovascular diseases. Treadmill exercise leads to a decreased gene transcription of catecholamine biosynthetic enzymes in stellate ganglia in stressful conditions. This may suggest the beneficial effects of treadmill exercise on cardiovascular system in stressed animals [22].
