Involvement of central beta-adrenoceptors in the tachycardia induced by water immersion stress in rats

Chewing reduces sympathetic nervous response to stress and prevents poststress arrhythmias in rats

Reducing stress is important in preventing sudden death in patients with cardiovascular disease, as stressful events may cause autonomic imbalance and trigger fatal arrhythmias. Since chewing has been shown to inhibit stress-induced neuronal responses in the hypothalamus, we hypothesized that chewing could ameliorate stress-induced autonomic imbalance and prevent arrhythmias. To test this hypothesis, we analyzed changes in radiotelemetered electrocardiograms in rats that were allowed to chew a wooden stick during a 1-h period of immobilization stress. Chewing significantly reduced the occurrence of ventricular premature beats (VPBs) and complex ventricular ectopy after immobilization and prevented stress-induced prolongation of the QT interval of VPBs throughout the 10-h experimental period. It also prevented prolongation of the QRS complex and fluctuations in the QT interval in normal sinus rhythm beats preceding VPBs during both immobilization and in the poststress period. Fast Fourier transform-based spectral analysis of heart-rate variability further showed that chewing significantly inhibited the stress-induced increase in the power ratio of low-to-high frequency activity (LF/HF: a marker of sympathetic activity) during immobilization and in addition was associated with blunting of the stress-induced increase in plasma noradrenaline observed at the termination of immobilization. Similar suppressive effects on the occurrence of VPBs and the LF/HF were observed in rats that were administered the β-adrenergic blocker propranolol before immobilization. These results indicate that chewing can ameliorate sympathetic hyperactivity during stress and prevent poststress arrhythmias and suggest that chewing may provide a nonpharmacological and cost-effective treatment option for patients with a high risk of stress-induced fatal arrhythmia.

Role of brain adrenoceptors in the corticortopin-releasing factor-induced central activation of sympatho-adrenomedullary outflow in rats

We investigated the role played by catecholamine-dependent pathways in modulating the ability of centrally administered corticotropin releasing factor (CRF) to activate sympatho-adrenomedullay outflow, using urethane-anesthetized rats. The CRF (1.5 nmol/animal, i.c.v.)-induced elevations of both plasma noradrenaline and adrenaline were attenuated by phentolamine (a non-selective alpha adrenoceptor antagonist) [125 and 250 microg (0.33 and 0.66 micromol)/animal], Heat (a selective alpha(1) adrenoceptor antagonist) [10 and 30 microg (30 and 90 nmol)/animal, i.c.v.] and clonidine (a selective alpha(2) adrenoceptor agonist) [100 microg (0.375 micromol)/animal, i.c.v.]. On the other hand, the CRF (1.5 nmol/animal, i.c.v.)-induced elevation of both catecholamines was not influenced by RS 79948 (a selective alpha(2) adrenoceptor antagonist) [10 and 30 microg (7.2 and 72 nmol)/animal, i.c.v.]. Furthermore, the CRF (1.5 nmol/animal, i.c.v.)-induced elevation of noradrenaline was attenuated by sotalol (a non-selective beta adrenoceptor antagonist) [125 and 250 microg (0.4 and 0.8 micromol)/animal, i.c.v.], while that of adrenaline was not influenced by sotalol. These results suggest that centrally administered CRF-induced elevation of plasma noradrenaline is mediated by an activation of alpha(1) and beta adrenoceptors in the brain, and that of plasma adrenaline is mediated by an activation of alpha(1) adrenoceptors in the brain. Furthermore, central alpha(2) adrenoceptors are involved in modulating the CRF-induced elevation of both plasma catecholamines.

Central β-adrenoceptors mediate phasic and sustained components of hemodynamic responses to acute behavioral stress

Startle elicits a pattern of cardiovascular responses that is consistent within individual rats but varies between rats. We examined the hypothesis that central beta-adrenoceptors mediate differences in the hemodynamic responses to stress. Conscious rats exposed to cold water (1 cm deep, 1 min) had an initial phasic (startle) response (first 5 s) that varied considerably between rats. We designated those rats with an initial increase in cardiac output (CO) and systemic vascular resistance (SVR) as mixed responders while those with only an increase in SVR were vascular responders. Propranolol pretreatment (3 microg, icv) made the phasic changes in CO more negative, whereas isoproterenol (3 microg) made the CO response more positive in mixed responders and attenuated the increases in arterial pressure and SVR in vascular responders. Metoprolol (30 microg, icv) depressed the change in CO due to startle in mixed responders by decreasing heart rate. ICI 188,551 (25 microg, icv) did not alter the responses to startle but depressed the heart rate and CO responses and enhanced the pressor and SVR responses to sustained stress (1 min exposure to cold water). The results suggest that startle elicits hemodynamic responses that are primarily dependent on beta1-adrenoceptors but responses to sustained stress are dependent on beta2-adrenoceptors in the CNS.

Atrial supersensitivity to noradrenaline in stressed female rats

Stress can change the responses to catecholamines in many tissues. The aim of this study was to investigate the influence of the estrous cycle on the sensitivity of right atria to noradrenaline in female rats subjected to acute swimming stress. Female Wistar rats in proestrus, estrus, metestrus or diestrus were submitted to a 50 min-swimming session. Immediately after the exercise, the rats were killed and their right atria were mounted for isometric recording of the spontaneous beating rate. Concentration-effect curves to noradrenaline were obtained before and after the inhibition of neuronal uptake with phenoxybenzamine (10 microM) and of extraneuronal uptake with estradiol (5 microM). Acute swimming stress did not change the right atrial sensitivity to noradrenaline in rats in estrus, metestrus and diestrus. However, swimming stress produced supersensitivity to noradrenaline in proestrus (pD(2) control: 7.14 +/- 0.03 vs. pD(2) swimming: 7.55 +/- 0.04; p<0.05). This supersensitivity was still observed after uptake inhibition. When catecholamine uptake was inhibited, the concentration-effect curve to noradrenaline was shifted to the left 2.5-fold in the proestrus control group and 1.7-fold in the proestrus stress group (p<0.05). In conclusion, the estrous cycle influenced the acute stress-induced atrial supersensitivity to noradrenaline.