The effect of endogenous opioids on blood pressure during stress

The role of CRF1 receptors for sympathetic nervous response to laparotomy in anesthetized rats

Corticotropin-releasing factor (CRF) is released in response to various types of stressors and mediates endocrine, autonomic, immune, and behavioral responses to stress through interaction with CRF1 and CRF2 receptors. To investigate the role of CRF1 receptors in physiological responses to surgical stress, we analyzed the effects of two different non-peptide selective CRF1 receptor antagonists (JTC-017 and CP-154,526) and a peptide non-selective CRF receptor antagonist (astressin) on laparotomy-induced sympathetic nervous responses in isoflurane-anesthetized rats. JTC-017, CP-154,526, and astressin similarly suppressed plasma ACTH elevation induced by laparotomy. JTC-017 and CP-154,526 significantly augmented plasma noradrenaline and adrenaline responses to laparotomy, while astressin showed no effect on these responses. Laparotomy-induced maximum increases in mean blood pressure and heart rate were augmented by JTC-017, but were not affected by astressin. The results suggested for the first time that there was a pathway to attenuate sympathetic nervous response to surgical stress through CRF1 receptors in the central nervous system.

Blood pressure and heart rate are increased by AF-DX 116, a selective M2 antagonist, in autonomic imbalanced and hypotensive rats caused by repeated cold stress

Rats exposed to SART (specific alternation of rhythm in temperature) stress, which are ideal animal models for vagotonia-type dysautonomia, show various changes in cardiac and circulatory systems. In this study, attention was directed to cholinergic function in the SART-stressed rat heart and the effects of AF-DX 116, a specific muscarinic M2 antagonist, on blood pressure and heart rate. The results were compared with those obtained for atropine and pirenzepine. In SART-stressed rats, systolic and diastolic blood pressures (SBP and DBP) were lower than in unstressed rats. Oral AF-DX 116 resulted in greater elevation of DBP than SBP in unstressed rats. In stressed rats, greater and more prolonged elevation of SBP than in unstressed rats was noted, particularly at higher doses. A dose-dependent SBP change in stressed rats, caused by intravenous AF-DX 116, was shifted upward in parallel with that in unstressed groups, unlike with oral administration. The positive chronotropic effect of this drug was smaller in stressed rats than in unstressed rats, in contrast to the pressor effect. SART-stressed rats may thus have an enhanced sympathetic tone in the heart, as well as changes in muscarinic M2 receptors at sympathetic nerve endings and at the heart muscle. The effects of AF-DX 116 on blood pressure and heart rate thus may arise from peripheral action and AF-DX 116 may be useful for treating hypotension related to autonomic imbalance of the vagotonia type.

Effects of naloxone, glycyrrhizic acid, dexamethasone and deoxycorticosterone in repetitive stress

1. The present study examined the effect of naloxone (NAL), glycyrrhizic acid (GCA), deoxycorticosterone (DOC) and dexamethasone (DEX) on daily repeated 2 h chronic restrained stress (RS) on the locomotor activity (LA) of rats tested in the open field arena to elucidate the possible roles of opioids, glucocorticoids and mineralocorticoids in response to stress. 2. Intact and adrenalectomized (ADX) rats were either injected with 0.1 mL of NAL (0.32 microgram/100 g BW), 2.4 mg/kg DOC or 120 micrograms/kg DEX or had 1.0 mg/mL GCA dissolved in their drinking water or normal saline (for the ADX group) dissolved in their drinking water. 3. In intact groups, treatment with NAL completely blocked the stress response and treatment with GCA, DOC and DEX partially prevented the stress response. Adaptation occurred on either days 4, 5, 6 or 7 for intact rats treated with DEX, DOC, GCA or control rats, respectively. All ADX control rats died following the first 2 h RS. Adrenalectomized rats treated with DEX or DOC adapted later compared with intact rats, while rats given either GCA or NAL were unable to block or adapt to chronic RS. 4. These findings demonstrate that the stress response is primarily mediated by endogenous opioids, in that it is blocked by NAL. Both mineralocorticoids and glucocorticoids, which can act centrally to inhibit endorphins, partially blocked the stress response. The effect of GCA in intact rats was similar to that of both DEX and DOC in intact rats. Adrenalectomized rats treated with GCA (despite their lack of endogenous corticosterone) showed a stress response that was significantly different from the other ADX groups, implying that GCA had effects independent of endogenous corticosterone.

Mineralocorticoid and glycyrrhizic acid block stress induced hypotension in rats

1. The aim of this study was to investigate the effect of repeated exposure to stress on tail blood pressure (TBP) of normal as well as GCA (glycyrrhizic acid) and steroid treated rats. Male Sprague-Dawley rats (250 g) were exposed to ether vapour to achieve light anaesthesia prior to TBP recording. Rats were injected with either normal saline or naloxone prior to exposure to stress. Tail blood pressure was recorded daily for 2 weeks. 2. We found that ether stress caused a transient drop in TBP in control as well as in dexamethasone (DEX) treated rats. The stress-induced fall in blood pressure was reduced by naloxone in control rats but not in DEX treated rats. However the transient drop in TBP following stress was not seen in either GCA or deoxycorticosterone (DOC) treated rats. 3. We conclude that first, the reduction in TBP was due to the release of endogenous opioids caused by stress. Second, DOC may block the release of such endogenous opioids, preventing the drop in TBP in response to stress, while DEX did not. Third, GCA caused a similar mineralocorticoid effect on reversing stress induced hypotension.

Restraint stress in biomedical research: an update

Since the publication of our initial review of restraint stress in 1986, much work has continued with this technique, either as a tool for the investigation of other pharmacological, physiological, or pathologic phenomena or with restraint stress itself serving as the object of the study. As we noted in 1986, the major use of restraint has been for the induction of stress responses in animals and, more specifically, for the investigation of drug effects, particularly as they affect typical stress-related pathology--gastrointestinal, neuroendocrine, and immunological agents have been extensively studied. In compiling this update on restraint stress and its effects, we noted an increasing emphasis on central nervous system mechanisms in peripheral disease, especially gastrointestinal disease. In particular, many CNS-active agents have been tested for their effects on gastric and duodenal lesion formation and gastric secretion, including antidepressants, antipsychotics, anxiolytics, noradrenergic, serotonergic, dopaminergic, and peptidergic compounds. Some of these agents are especially active in the gastrointestinal tract even when administered centrally, further solidifying the concept of a brain-gut axis. The present update includes studies of: methods and procedures, pre-restraint manipulations, post-restraint/healing effects, and drug effects. In addition, a current bibliography of reports that have employed restraint is included.

Stress and endogenous opioids: behavioral and circulatory interactions

Endogenous opioid peptides are the basis of a diverse system of complex neuroregulatory and endocrine mechanisms. While relatively quiescent in the resting state, these peptides are released during intense stimulation and modify, in a number of ways, circulatory homeostatic mechanisms. The endogenous opioids, primarily via endorphins and enkephalins, are capable of influencing circulatory responses to stress at the behavioral, the endocrinological, and the neural level. Recent research in humans and animals has described several roles for opioids in regulation of the circulatory stress response, and has also provided clues about the significance of opioid dysregulation in the pathophysiology of stress. Increased understanding of the basic mechanisms of stress and endogenous opioids will clarify the potential roles of opioids in important pharmacologic and behaviorally based therapeutics.

Effect of acetylcholine and morphine on bronchial smooth muscle contraction and its modulation by steroid hormones

1. The effects of corticosteroid pretreatment on acetylcholine (ACH)-induced contraction of bronchial smooth muscle (BSM) were studied. 2. ACH dose-response curves for dexamethasone (DM)- and corticosterone (B)-treated but not deoxycorticosterone (DOC)-treated BSM were significantly shifted to the right; this provides evidence that glucocorticoid treatment reduced the sensitivity of BSM to ACH. 3. Morphine enhanced BSM contraction in response to ACH by 20%. DM suppressed this enhancement. 4. These findings correlated well with the reduction of muscarinic receptor numbers in BSM by glucocorticoids in our previous study. In addition, glucocorticoids reduced the sensitivity of BSM to opioids.

Role of sympathoadrenomedullary system in cardiovascular response to stress in rats

Sympathetic nerve and/or adrenal medulla contributions to stress-induced cardiovascular responses were investigated by factoring out their influence using adrenal demedullation (DMED) and/or chemical sympathectomy with guanethidine (GUAN). Rats divided into 4 groups [sham-operated/saline (SHAM/SAL), SHAM/GUAN, DMED/SAL and DMED/GUAN] were injected i.p. over 4 weeks with either saline or GUAN (25 mg/kg/day). At the end of this treatment period, blood pressure (BP) and heart rate (HR) were monitored via carotid catheter prior to and during restraint in conscious rats. Treatments did not alter basal BP or HR when compared to controls. Restraint increased HR (delta 72 bpm) and systolic, diastolic and mean BP (delta approximately 20 mm Hg) in control animals. Restraint-induced HR change was significantly greater in DMED/SAL animals (delta 88 bpm), but less in SHAM/GUAN animals (delta 40 bpm) than in controls. DMED/GUAN was not different from SHAM/GUAN alone in altering HR response to stress, supporting the greater influence of sympathetic nerves over adrenal medulla in controlling HR. Chronic GUAN abolished normal pressor responses to restraint stress. DMED increased diastolic blood pressure response to stress. However, in DMED/GUAN rats, not only did stress fail to increase blood pressure but rather stress produced hypotension (delta - 34 mm Hg MAP), demonstrating the role of adrenal medulla in maintaining BP during stress. Differential effects of the various treatments on diastolic and systolic pressure suggest that the treatments had effects on peripheral vasculature. These results demonstrate that sympathetic nerves and adrenal medulla have important influences on cardiovascular function during stress and that in the absence of either, the other system may partially compensate.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential sensitivity to cocaine in spontaneously hypertensive and Wistar-Kyoto rats

Physiological, pharmacological and toxicological responses to two regimens of cocaine administration were compared between spontaneously hypertensive (SHR) and Wistar-Kyoto (WKY) rats. An initial experiment examined renal excretory and hemodynamic function in response to an acute volume load in anesthetized SHR and WKY following subacute cocaine treatment (20 mg/kg, s.c., twice a day for 9 days). Anticipated renal responses to volume loading were obtained but the responses of cocaine-treated SHR and WKY did not differ from vehicle-treated rats. A second group of experiments compared responses to continuous i.v. infusions of cocaine (1.25 mg/kg.min). In freely moving animals, no differences were noted between SHR and WKY in the increases in mean blood pressure (MBP) and heart rate (HR) produced during cocaine infusion. The elapsed time-to-onset of convulsions (Tc) elicited by cocaine was similar in both strains. However, when rats were subjected to restraint during the infusion period, pressor and tachycardic responses were observed to be significantly less in WKY than in SHR or in freely moving rats of either strain. Restraint also differentially affected rectal temperature (RT) responses to cocaine. Hypothermic responses to cocaine were observed in all WKY. Both hypothermic and hyperthermic responses were observed in SHR. A significant correlation was demonstrated between the Tc and the maximal change in RT produced during cocaine infusion. Division of SHR into two arbitrary groups was made, based on the direction of cocaine-induced change in RT. A significant (p less than 0.01) shortening of the Tc was obvious in SHR (8 of 15) in whom cocaine produced a hyperthermia. These animals were designated SHRH. The mean value for Tc in those SHR which demonstrated a lowering in RT (SHRL; 7 of 15) in response to cocaine was similar to that for WKY. Moreover, the SHRH evidenced significantly greater increases in HR, but not MBP, to cocaine infusion than did SHRL. The results indicate that restraint stress causes expression of a significant heterogeneity in the RT response of SHR to cocaine. The magnitude and direction of the RT responses are negatively correlated with sensitivity to the convulsive effects of cocaine in SHR. Stress may modify toxic responses to cocaine by interactions with body temperature homeostasis.

Endogenous opiates: 1987

This paper is the tenth installment of our annual review of the research during the past year involving the endogenous opiate system. It covers the nonanalgesia and behavioral studies of the opiate peptides published in 1987. The specific topics this year include stress; tolerance and dependence; eating; drinking; gastrointestinal and renal activity; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical activity; locomotor activity; sex, pregnancy, and development; immunology and cancer; and other behavior.