Hemodynamic response pattern predicts susceptibility to stress-induced elevation in arterial pressure in the rat

Cardiovascular Reactivity to a Novel Stressor: Differences on Susceptible and Resilient Rats to Social Defeat Stress

Prolonged and heightened responses to stress are known factors that influence the development of mood disorders and cardiovascular diseases. Moreover, the coping strategies related to the experience of adverse events, i.e., resilience or the susceptibility to stress, are determinants for the individual risk of developing such diseases. Susceptible rats to the social defeat stress (SDS), identified by the social interaction test (SIT), show behavioral and cardiovascular alterations after SDS exposure that are not found in resilient rats. However, it is not elucidated yet how the cardiovascular system of susceptible and resilient phenotypes responds to a new stressor after SDS exposure. Thus, using the SDS exposure followed by the SIT, we evaluated heart rate, blood pressure (BP), tail skin temperature, and circulating corticosterone responses to an acute session of restraint stress in susceptible and resilient rats to SDS. Susceptible rats showed resting tachycardia and exaggerated BP response to restraint stress, while resilient rats did not present such alterations. In contrast, both phenotypes showed increased plasma corticosterone and a drop in tail skin temperature to restraint stress, which was similar to that observed in control animals. Our results revealed an increased cardiovascular reactivity in response to a new stressful stimulus in susceptible rats, which might be related to a greater risk for the development of cardiovascular diseases.

Blood pressure regulation in stress: focus on nitric oxide-dependent mechanisms

Stress is considered a risk factor associated with the development of various civilization diseases including cardiovascular diseases, malignant tumors and mental disorders. Research investigating mechanisms involved in stress-induced hypertension have attracted much attention of physicians and researchers, however, there are still ambiguous results concerning a causal relationship between stress and long-term elevation of blood pressure (BP). Several studies have observed that mechanisms involved in the development of stress-induced hypertension include increased activity of sympathetic nervous system (SNS), glucocorticoid (GC) overload and altered endothelial function including decreased nitric oxide (NO) bioavailability. Nitric oxide is well known neurotransmitter, neuromodulator and vasodilator involved in regulation of neuroendocrine mechanisms and cardiovascular responses to stressors. Thus NO plays a crucial role in the regulation of the stress systems and thereby in the BP regulation in stress. Elevated NO synthesis, especially in the initial phase of stress, may be considered a stress-limiting mechanism, facilitating the recovery from stress to the resting levels via attenuation of both GC release and SNS activity as well as by increased NO-dependent vasorelaxation. On the other hand, reduced levels of NO were observed in the later phases of stress and in subjects with genetic predisposition to hypertension, irrespectively, in which reduced NO bioavailability may account for disruption of NO-mediated BP regulatory mechanisms and accentuated SNS and GC effects. This review summarizes current knowledge on the role of stress in development of hypertension with a special focus on the interactions among NO and other biological systems affecting blood pressure and vascular function.

Role of the autonomic nervous system and baroreflex in stress-evoked cardiovascular responses in rats

Restraint stress (RS) is an experimental model to study stress-related cardiovascular responses, characterized by sustained pressor and tachycardiac responses. We used pharmacologic and surgical procedures to investigate the role played by sympathetic nervous system (SNS) and parasympathetic nervous system (PSNS) in the mediation of stress-evoked cardiovascular responses. Ganglionic blockade with pentolinium significantly reduced RS-evoked pressor and tachycardiac responses. Intravenous treatment with homatropine methyl bromide did not affect the pressor response but increased tachycardia. Pretreatment with prazosin reduced the pressor and increased the tachycardiac response. Pretreatment with atenolol did not affect the pressor response but reduced tachycardia. The combined treatment with atenolol and prazosin reduced both pressor and tachycardiac responses. Adrenal demedullation reduced the pressor response without affecting tachycardia. Sinoaortic denervation increased pressor and tachycardiac responses. The results indicate that: (1) the RS-evoked cardiovascular response is mediated by the autonomic nervous system without an important involvement of humoral factors; (2) hypertension results primarily from sympathovascular and sympathoadrenal activation, without a significant involvement of the cardiac sympathetic component (CSNS); (3) the abrupt initial peak in the hypertensive response to restraint is sympathovascular-mediated, whereas the less intense but sustained hypertensive response observed throughout the remaining restraint session is mainly mediated by sympathoadrenal activation and epinephrine release; (4) tachycardia results from CSNS activation, and not from PSNS inhibition; (5) RS evokes simultaneous CSNS and PSNS activation, and heart rate changes are a vector of both influences; (6) the baroreflex is functional during restraint, and modulates both the vascular and cardiac responses to restraint.

Physiological consequences of repeated exposures to conditioned fear

Activation of the stress response evokes a cascade of physiological reactions that may be detrimental when repeated or chronic, and when triggered after exposure to psychological/emotional stressors. Investigation of the physiological mechanisms responsible for the health damaging effects requires animal paradigms that repeatedly evoke a response to psychological/emotional stressors. To this end, adult male Sprague Dawley rats were repeatedly exposed (2X per day for 20 days) to a context that they were conditioned to fear (conditioned fear test, CFT). Repeated exposure to CFT produced body weight loss, adrenal hypertrophy, thymic involution, and basal corticosterone elevation. In vivo biotelemetry measures revealed that CFT evokes sympathetic nervous system driven increases in heart rate (HR), mean arterial pressure (MAP), and core body temperature. Extinction of behavioral (freezing) and physiological responses to CFT was prevented using minimal reinstatement footshock. MAP responses to the CFT did not diminish across 20 days of exposure. In contrast, HR and cardiac contractility responses declined by day 15, suggesting a shift toward vascular-dominated MAP (a pre-clinical marker of CV dysfunction). Flattened diurnal rhythms, common to stress-related mood/anxiety disorders, were found for most physiological measures. Thus, repeated CFT produces adaptations indicative of the health damaging effects of psychological/emotional stress.

α1 and α2-adrenoceptors in the medial amygdaloid nucleus modulate differently the cardiovascular responses to restraint stress in rats

Medial amygdaloid nucleus (MeA) neurotransmission has an inhibitory influence on cardiovascular responses in rats submitted to restraint, which are characterized by both elevated blood pressure (BP) and intense heart rate (HR) increase. In the present study, we investigated the involvement of MeA adrenoceptors in the modulation of cardiovascular responses that are observed during an acute restraint. Male Wistar rats received bilateral microinjections of the selective α1-adrenoceptor antagonist WB4101 (10, 15, and 20 nmol/100 nL) or the selective α2-adrenoceptor antagonist RX821002 (10, 15, and 20 nmol/nL) into the MeA, before the exposure to acute restraint. The injection of WB4101 reduced the restraint-evoked tachycardia. In contrast, the injection of RX821002 increased the tachycardia. Both drugs had no influence on BP increases observed during the acute restraint. Our findings indicate that α1 and α2-adrenoceptors in the MeA play different roles in the modulation of the HR increase evoked by restraint stress in rats. Results suggest that α1-adrenoceptors and α2-adrenoceptors mediate the MeA-related facilitatory and inhibitory influences on restraint-related HR responses, respectively.

Paraventricular nucleus modulates autonomic and neuroendocrine responses to acute restraint stress in rats

The paraventricular nucleus of the hypothalamus (PVN) has been implicated in several aspects of neuroendocrine and cardiovascular control. The PVN contains parvocellular neurons that release the corticotrophin release hormone (CRH) under stress situations. In addition, this brain area is connected to several limbic structures implicated in defensive behavioral control, as well to forebrain and brainstem structures involved in cardiovascular control. Acute restraint is an unavoidable stress situation that evokes corticosterone release as well as marked autonomic changes, the latter characterized by elevated mean arterial pressure (MAP), intense heart rate (HR) increases and decrease in the tail temperature. We report the effect of PVN inhibition on MAP and HR responses, corticosterone plasma levels and tail temperature response during acute restraint in rats. Bilateral microinjection of the nonspecific synaptic blocker CoCl(2) (1 mM/100 nL) into the PVN reduced the pressor response; it inhibited the increase in plasma corticosterone concentration as well as the fall in tail temperature associated with acute restraint stress. Moreover, bilateral microinjection of CoCl(2) into areas surrounding the PVN did not affect the blood pressure, hormonal and tail vasoconstriction responses to restraint stress. The present results show that a local PVN neurotransmission is involved in the neural pathway that controls autonomic and neuroendocrine responses, which are associated with the exposure to acute restraint stress.

Angiotensin II and CRF receptors in the central nucleus of the amygdala mediate hemodynamic response variability to cocaine in conscious rats

Stress or cocaine evokes either a large increase in systemic vascular resistance (SVR) or a smaller increase in SVR accompanied by an increase in cardiac output (designated vascular and mixed responders, respectively) in Sprague-Dawley rats. We hypothesized that the central nucleus of the amygdala (CeA) mediates this variability. Conscious, freely-moving rats, instrumented for measurement of arterial pressure and cardiac output and for drug delivery into the CeA, were given cocaine (5 mg/kg, iv, 4-6 times) and characterized as vascular (n=15) or mixed responders (n=10). Subsequently, we administered cocaine after bilateral microinjections (100 nl) of saline or selective agents in the CeA. Muscimol (80 pmol), a GABA(A) agonist, or losartan (43.4 pmol), an AT(1) receptor antagonist, attenuated the cocaine-induced increase in SVR in vascular responders, selectively, such that vascular responders were no longer different from mixed responders. The corticotropin releasing factor (CRF) antagonist, alpha-helical CRF(9-41) (15.7 pmol), abolished the difference between cardiac output and SVR in mixed and vascular responders. We conclude that greater increases in SVR observed in vascular responders are dependent on AT(1) receptor activation and, to a lesser extent on CRF receptors. Therefore, AT(1) and CRF receptors in the CeA contribute to hemodynamic response variability to intravenous cocaine.

The paraventricular nucleus of the hypothalamus is involved in cardiovascular responses to acute restraint stress in rats

The paraventricular nucleus of the hypothalamus (PVN) has been implicated in several aspects of cardiovascular control. Stimulation of the PVN evokes changes in blood pressure and heart rate. Additionally, this brain area is connected to several limbic structures implicated in behavioral control, as well as to forebrain and brainstem structures involved in cardiovascular control. This evidence indicates that the PVN may modulate cardiovascular correlates of behavioral responses to stressful stimuli. Acute restraint is an unavoidable stressor that evokes marked and sustained cardiovascular changes, which are characterized by elevated mean arterial pressure (MAP) and an intense heart rate (HR) increase. We report on the effect of inhibition of PVN synapses on MAP and HR responses evoked by acute restraint in rats. Bilateral microinjection of the nonspecific synaptic blocker cobalt (CoCl(2), 1 mM/100 nl) into the PVN did not change the HR response or the initial peak of the MAP response to restraint stress, but reduced the area under the curve of the MAP response. Moreover, bilateral microinjection of cobalt in areas surrounding the PVN did not change the cardiovascular response to restraint. These results indicate that synapses in the PVN are involved in the neural pathway that controls blood pressure changes evoked by restraint.

Angiotensin and NMDA receptors in the median preoptic nucleus mediate hemodynamic response patterns to stress

The brain renin-angiotensin system plays an important role in the regulation of arterial pressure in response to stress, in part due to activation of AT1 receptors in the hypothalamic median preoptic nucleus (MnPO) by endogenous angiotensin II (ANG II). N-methyl-d-aspartate (NMDA) receptors are also involved in the angiotensinergic signaling pathway through the MnPO. We investigated whether AT1 and NMDA receptors in the MnPO are responsible for variable hemodynamic response patterns to stress. Cocaine or startle with cold water evoked a pressor response in Sprague-Dawley rats due, in some rats [vascular responders (VR)], to a large increase in systemic vascular resistance (SVR) and, in other rats [mixed responders (MR)], to small increases in SVR and cardiac output (CO). Microinjection of the GABAA agonist muscimol into the MnPO to block synaptic transmission attenuated the cocaine- or stress-induced increase in SVR and the decrease in CO seen in VR without altering either response in MR. Likewise, administration of either an AT1 receptor antagonist, losartan, or an NMDA receptor antagonist, MK-801, attenuated the increase in SVR and the decrease in CO in VR in response to either cocaine (losartan and MK-801) or startle with cold water (losartan) without altering either response in MR. We propose that the MnPO is responsible for greater SVR responses in VR and that AT1 and NMDA receptors play an important role in greater SVR responses in VR. These data provide additional support for the critical role of the MnPO in cardiovascular responses to stress.

Central angiotensin II receptors mediate hemodynamic response variability to stressors

We examined whether ANG II receptors in the central nervous system mediate hemodynamic responses to pharmacological (cocaine) and behavioral (cold water) stressors. After administration of cocaine (5 mg/kg iv), rats were classified as vascular responders (VR) if their pressor response was due entirely to an increase in systemic vascular resistance (SVR) despite a decrease in cardiac output (CO). Cocaine elicited a pressor response in mixed responders (MR) that was dependent on small increases in both SVR and CO. ANG II (30 ng/5 μl icv, 5 min before cocaine) augmented the decrease in CO in VR and prevented the increase in CO in MR. Administration of [Sar1,Thr8]ANG II (20 μg/5 μl icv; sarthran) before cocaine attenuated the decrease in CO and the large increase in SVR in VR so that they were no longer different from MR. Losartan (20 μg icv) or captopril (50 μg icv) preceding cocaine administration also attenuated the decrease in CO and the large increase in SVR seen in VR only. The role of angiotensin was not specific for cocaine, because ANG II (icv) pretreatment before startle with cold water (1 cm deep) enhanced the decrease in CO and the increase in SVR in both MR and VR, whereas losartan (icv) pretreatment before startle attenuated the decrease in CO and the increase in SVR in VR so that they were no longer different from MR. These data suggest that central ANG II receptors mediate the greater vascular and cardiac responsiveness in vascular responders to acute pharmacological and behavioral stressors.

Role of the medial prefrontal cortex in cardiovascular responses to acute restraint in rats

The medial prefrontal cortex (mPFC) modulates neurovegetative and behavioral responses, being involved in memory, attention, motivational and executive processes. There is evidence indicating that mPFC modulates cardiovascular correlates of behavioral responses to stressful stimuli. Acute restraint is an unavoidable stress situation that evokes marked and sustained cardiovascular changes, characterized by elevated blood pressure (BP) and intense heart rate (HR) increase. We presently report effects of mPFC pharmacological manipulations on BP and HR responses evoked by acute restraint in rats. Bilateral microinjection of 200 nl of the unspecific synaptic blocker CoCl2 (1 mM) in the mPFC prelimbic area (PL) increased HR response to acute restraint, without significant effect on the BP response. This result indicates that PL synaptic mechanisms have an inhibitory influence on restraint-evoked HR changes. Injections of the non-selective glutamatergic receptor antagonist kynurenic acid (0.02 M) or the selective N-methyl-d-aspartic acid (NMDA) receptor glutamatergic antagonist (LY235959) (0.02 M) caused effects similar to cobalt, suggesting that local glutamatergic neurotransmission and NMDA receptors mediate the PL inhibitory influence on restraint-related HR responses. Pretreatment with the non-non-N-methyl-D-aspartic acid glutamatergic antagonist glutamatergic antagonist glutamatergic receptor antagonist NBQX (0.02 M) did not affect restraint-related cardiovascular responses, reinforcing the idea that NMDA receptors mediate PL-related inhibitory influence. Pretreatment with the glutamatergic-receptor antagonists did not affect baseline BP or HR values. I.v. pretreatment with the quaternary ammonium anticholinergic drug homatropine methyl bromide (0.2 mg/kg) also increased the restraint-related HR response to values similar to those observed after treatment with kynurenic acid or LY235959, thus, suggesting that PL inhibitory influence on restraint-evoked heart rate increase could be related to increased parasympathetic activity. This dose of homatropine had no significant effects on baseline BP or HR values. Results suggest a PL inhibitory influence on restraint-evoked HR increase. They also indicate that local NMDA receptors involved in parasympathetic activation mediate PL inhibitory influence on restraint-evoked HR increase.

Contribution of baroreflex sensitivity and vascular reactivity to variable haemodynamic responses to cocaine in conscious rats

1. Baroreflex function is critical for short-term arterial pressure regulation and decreased baroreflex responsivity may predict a predisposition to hypertension and sudden cardiac death. In the present study, we assessed whether baroreflex sensitivity (BRS) and/or vascular reactivity covary with haemodynamic responsiveness to cocaine in vascular and mixed responders. 2. We assessed the heart rate index of BRS in resting animals. We examined dose-response relationships to pressor and depressor agents to determine cardiovascular reactivity. Subsequently, rats were given cocaine (5 mg/kg, i.v.) to classify them as vascular or mixed responders. Vascular responders (n=16) were defined as those rats with a substantial (>8%) decrease in cardiac output in response to cocaine owing to a larger increase in systemic vascular resistance. The remaining rats (n=8) were mixed responders because they had smaller increases in vascular resistance and little change or an increase in cardiac output. 3. The BRS determined with angiotensin (Ang) II, but not with phenylephrine, was impaired in mixed responders compared with vascular responders. At equipressor doses, there were significantly greater reductions in cardiac output in vascular responders compared with mixed responders in response to phenylephrine or AngII. Methacholine produced greater decreases in heart rate in vascular responders, suggesting greater muscarinic responsivity. 4. We conclude that differences in vascular reactivity to AngII may contribute to differences in haemodynamic response profiles to cocaine in individual rats. More importantly, the differences in vascular responsivity and BRS do not appear to be primary determinants of haemodynamic response variability.

Effects of Psychological Stress on Autonomic Control of Heart in Rats

The aim of this study was to examine the effects of psychological stress on autonomic control of the heart in rats. For this purpose, we evoked anxiety-like or fear-like states in rats by means of classical conditioning and examined changes in autonomic nervous activity using an implanted telemetry system and power spectral analysis of heart rate variability. Anxiety-like states resulted in a significant increase in heart rate (HR), low frequency (LF) power, and LF/HF ratio, with no change in high frequency (HF) power. Fear-like states resulted in a significant increase in HR and a significant decrease in HF power with no significant change in both LF power and LF/HF ratio, although LF/HF ratio increased slightly. These results suggest that autonomic balance becomes predominant in sympathetic nervous activity in both anxiety-like and fear-like states. These changes in rats correspond to changes which are relevant to cardiovascular diseases in humans under many kinds of psychological stress. Therefore, the experimental design of this study is a useful experimental model for investigating the effects of psychological stress on autonomic control of the heart in humans.

Central alpha-adrenergic receptors and corticotropin releasing factor mediate hemodynamic responses to acute cold stress

Behavioral stress is likely to contribute to the development of hypertension in susceptible individuals. We reported that hemodynamic response patterns to acute startle vary and that those patterns predict the predisposition of rats to sustained stress-induced elevations in arterial pressure. Since considerable evidence suggests that central catecholamines and corticotropin releasing factor (CRF) contribute to the regulation of arterial pressure and the development of hypertension, we investigated the role of central alpha-adrenergic receptors and CRF in mediating different hemodynamic response patterns to acute cold water stress in conscious rats. Rats were instrumented for arterial pressure, heart rate and cardiac output determination and for intracerebroventricular (icv) administration of selective antagonists. After acclimation to a water tight cage, ice water (1 cm deep) was rapidly added then drained 1 min later. Although the early startle response to cold water stress elicited a pressor response in all rats, the hemodynamic response pattern varied between rats. Vascular responders (n=19) had an initial considerable increase in systemic vascular resistance and a decrease in cardiac output. In contrast, mixed responders (n=11) had a smaller increase in vascular resistance and an increase in cardiac output. Pretreatment with phentolamine (30 microgram/5 microliter, icv, n=8), prazosin (10 microgram/5 microliter, icv, n=12) or alpha-helical CRF(9-41) (10 microgram/5 microliter, icv, n=9) prevented the decrease in cardiac output elicited by acute cold water stress in vascular responders without affecting mixed responders. Yohimbine (3 microgram/5 microliter, icv, n=8) pretreatment did not alter hemodynamic responses. Therefore, we conclude that central alpha(1)-adrenoceptors and CRF mediate the specific hemodynamic response patterns to acute startle and may be responsible for the predisposition to develop hypertension in vascular responders.

Hemodynamic response profile predicts susceptibility to cocaine-induced toxicity

Cocaine evokes pressor responses due either to a large increase in systemic vascular resistance despite a decrease (>8%) in cardiac output (vascular responders) or to small increases in both cardiac output and vascular resistance (mixed responders) in conscious rats. These studies were designed to determine (1) if the hemodynamic response pattern to cocaine correlates with relative sensitivity to toxicity and (2) if altering the hemodynamic response pattern to cocaine using propranolol enhances toxicity. Rats were instrumented for determination of cardiac output and arterial pressure. After recovery, rats were classified as vascular or mixed responders to cocaine (5 mg/kg, i.v., four to six trials). Two weeks later, cocaine was infused (1.5 mg/kg/min) until death after pretreatment with saline or propranolol (1 mg/kg). Saline-pretreated mixed responders (n=6) had greater tolerance to cocaine toxicity compared to vascular responders (n=11). Furthermore, saline-pretreated vascular responders were less sensitive than propranolol-pretreated vascular responders (n=9) to cocaine toxicity. Therefore, we propose that the initial hemodynamic response pattern to cocaine predicts sensitivity to cocaine toxicity. In addition, propranolol, a drug that enhances the increase in vascular resistance to cocaine, also increases toxicity to cocaine in vascular responders.

Hemodynamic response patterns to acute behavioral stressors resemble those to cocaine

Hemodynamic responses to cocaine vary greatly between animals, and the variability is related to the incidence of cocaine-induced cardiomyopathies and hypertension. The variability in cardiac output and systemic vascular resistance responses to cocaine in individuals is correlated with the responses to acute startle (air jet). This experiment was designed to determine whether responses to cocaine and to air jet are related to those evoked by a conditioned stimulus (tone preceding foot shock) and to an unconditioned stimulus (cold water). We verified the relationship in hemodynamic response patterns between cocaine and cold stress using selective receptor antagonists. Rats were instrumented with a pulsed Doppler flow probe on the ascending aorta for determination of cardiac output and with an arterial cannula for recording arterial pressure and heart rate. After recovery, some rats were tested multiple times with four different stimuli: air jet (6 trials), 15-s tone preceding 1-s foot shock (12 trials), cold water exposure (1 cm deep for 1 min, 4-12 trials), and cocaine (5 mg/kg iv, 4-6 trials) while hemodynamic parameters were recorded. Each stimulus was capable of eliciting a pressor response that was associated with variable changes in cardiac output. The cardiac output response to cocaine was correlated with the initial responses to each stressor in individual rats. Responses evoked by cold stress were most similar to those elicited by cocaine. Furthermore, nicardipine (25 microg/kg iv) or atropine methylbromide (0.5 mg/kg iv) pretreatment prevented the cardiac output differences to acute cold stress, as noted after cocaine administration. On the other hand, propranolol (1 mg/kg iv) exacerbated both the decrease in cardiac output and the stress-induced increase in systemic vascular resistance as previously reported with cocaine. Therefore, the initial response to cold water exposure is a reliable method of evoking characteristic hemodynamic response patterns that, as seen with cocaine, may provide a suitable model for identifying the causes for predilection to stress-induced cardiovascular disease.