Met-enkephalin, injected during the early phase of stress, attenuates stress-induced increases in noradrenaline release in rat brain regions

The long-term impact of early life pain on adult responses to anxiety and stress: Historical perspectives and empirical evidence

Approximately 1 in 6 infants are born prematurely each year. Typically, these infants spend 25 days in the Neonatal Intensive Care Unit (NICU) where they experience 10-18 painful and inflammatory procedures each day. Remarkably, pre-emptive analgesics and/or anesthesia are administered less than 25% of the time. Unalleviated pain during the perinatal period is associated with permanent decreases in pain sensitivity, blunted cortisol responses and high rates of neuropsychiatric disorders. To date, the mechanism(s) by which these long-term changes in stress and pain behavior occur, and whether such alterations can be prevented by appropriate analgesia at the time of insult, remains unclear. Work in our lab using a rodent model of early life pain suggests that inflammatory pain experienced on the day of birth blunts adult responses to stress- and pain-provoking stimuli, and dysregulates the hypothalamic pituitary adrenal (HPA) axis in part through a permanent upregulation in central endogenous opioid tone. This review focuses on the long-term impact of neonatal inflammatory pain on adult anxiety- and stress-related responses, and underlying neuroanatomical changes in the context of endogenous pain control and the HPA axis. These two systems are in a state of exaggerated developmental plasticity early in postnatal life, and work in concert to respond to noxious or aversive stimuli. We present empirical evidence from animal and clinical studies, and discuss historical perspectives underlying the lack of analgesia/anesthetic use for early life pain in the modern NICU.

Stress-induced epigenetic regulation of κ-opioid receptor gene involves transcription factor c-Myc

Exposure to stress is associated with adverse emotional and behavioral responses. Whereas the κ-opioid receptor (KOR) system is known to mediate some of the effects, it is unclear whether and how stress affects epigenetic regulation of this gene. Because the KOR gene can use two promoters (Pr1 and Pr2) and two polyadenylation signals (PA1 and PA2), it is also interesting whether and how these distinct regulatory mechanisms are differentially modulated by stress. The current study examined the effects of stress on these different regulatory mechanisms of the KOR gene. Results showed that stress selectively increased the expression of KOR mRNA isoforms controlled by Pr1 and terminated at PA1 in specific brain areas including the medial-prefrontal cortex, hippocampus, brainstem, and sensorimotor cortex, but not in the amygdala or hypothalamus. These effects correlated with altered epigenetic state of KOR Pr1 chromatin, as well as elevation and increased recruitment of the principal transcription factor c-Myc, which could activate Pr1. Stress-induced modulation of Pr1 was further validated using glutamate-sensitive murine hippocampal cell line, HT22. The results revealed a common molecular mechanism underlying the effect of stress on selected chromatin regions of this gene at the cellular level and in the context of whole animal and identified a critical role for c-Myc in stress-triggered epigenetic regulation of the KOR gene locus. This study sheds light on the mechanisms of stress-induced epigenetic regulation that targets specific chromatin segments and suggests certain KOR transcripts and its principal transcription factor c-Myc as potential targets for brain-area-specific intervention.

Host Brain Regulation of Fetal Locus Coeruleus Neurons Grafted to the Hippocampus in 6-Hydroxydopamine-Treated Rats. An Intracerebral Microdialysis Study

Release properties of intrahippocampal transplants of noradrenergic neurons were monitored by microdialysis in awake and halothane-anaesthetized rats. Fetal locus coeruleus neurons were implanted as a cell suspension into hippocampi deprived of their innate noradrenalin (NA) innervation by intraventricular 6-hydroxydopamine treatment. Dialysis probes of the loop type were implanted into the dorsal hippocampus 1 - 2 days before each experiment, i.e. 7 - 11 months after grafting. Age-matched intact and lesion-only animals served as controls. Microscopic analysis showed a graft-derived tyrosine hydroxylase immunoreactive, presumably noradrenergic, fibre network throughout the dorsal hippocampal formation, surrounding the probe site. The innervation density varied from sub- to supranormal. The grafts restored baseline NA release in the graft-reinnervated hippocampus to near-normal levels both in awake and halothane-anaesthetized animals. Potassium chloride (100 mM) in the perfusion fluid induced a dramatic increase in NA release that was similar in magnitude in the grafted and intact hippocampi. A NA uptake blocker (desipramine) added to the perfusion fluid at 5 microM induced a similar increase in NA output in the grafted and intact hippocampi, and the output was substantially reduced by tetrodotoxin, added at 1 microM in the presence of uptake blockade. Electrical stimulation of the lateral habenular nucleus (15 Hz, 0.5 mA) in halothane-anaesthetized rats induced a significant increase in NA output both in the intact and grafted hippocampi. This effect was abolished by transection of the fasciculus retroflexus, which carries the efferent projections of the habenular complex. Behavioural activation through handling induced a consistent increase in NA release only in the intact animals, but in a few grafted rats (which also responded to habenular stimulation) the NA output was clearly elevated by handling. Forced immobilization induced a significant increase in NA output both in the intact and grafted hippocampi, but in the grafted ones the response was somewhat smaller and more transient. In the same set of animals, swimming in warm water (25 - 30 degrees C) induced a sharp increase in NA output in the intact animals, whereas only one of the grafted rats responded by increased NA output. The results indicate that the locus coeruleus grafts, despite their ectopic location, can become functionally integrated with the host brain, and that the activity of the transplanted noradrenergic neurons can, under some circumstances, be modulated from the host brain in response to environmental challenges.

Noradrenaline systems in the hypothalamus, amygdala and locus coeruleus are involved in the provocation of anxiety: basic studies

A variety of stressful events, including emotional stress, cause a marked increase in noradrenaline release in several brain regions, and especially in the hypothalamus, amygdala and locus coeruleus, in the rat brain. These findings suggest that an increased noradrenaline release could be closely related to the provocation of negative emotions such as anxiety and/or fear. In order to confirm this hypothesis, we carried out several studies. Diazepam, a typical benzodiazepine anxiolytic, significantly attenuated not only the immobilization stress-induced increase in noradrenaline release in the three rat brain regions but also the emotional changes of these animals, and these effects were antagonized by flumazenil, a benzodiazepine antagonist. Naloxone and opioid agents, such as morphine, beta-endorphin and [Met(5)]-enkephalin, significantly enhanced and attenuated the stress-induced increase in noradrenaline release in these regions and the stress-induced emotional change, respectively. Two stressful events which predominantly involve emotional factors, i.e., psychological stress and conditioned fear, caused significant increases in noradrenaline release selectively in these three brain regions and these increases were also significantly attenuated by pretreatment with diazepam in a flumazenil reversible manner. Yohimbine, an alpha(2)-adrenoceptor antagonist which caused a marked increase in noradrenaline release in the several brain regions, had an anxiolytic action in the two behavioral tests involving anxiety, i.e., the conditioned defensive burying test and the modified forced swim test. beta-Carbolines, which possess anxiogenic properties, significantly increased noradrenaline release in the hypothalamus, amygdala and locus coeruleus. Taken together, these findings suggest that the increased release of noradrenaline in the hypothalamus, amygdala and locus coeruleus is, in part, involved in the provocation of anxiety and/or fear in animals exposed to stress, and that the attenuation of this increase by benzodiazepine anxiolytics acting via the benzodiazepine receptor/GABAA receptor/chloride ionophore supramolecular complex may be the basic mechanism of action of these anxiolytic drugs.

Vulnerability to stressor-induced disturbances in self-stimulation from the dorsal and ventral A10 area: differential effects of intraventricular D-Ala2-Met5-enkephalinamide, D-Ala2, N-Me-Phe4, Gly-Ol5-enkephalin, and D-Pen2, D-Pen5-enkephalin administration

D-Ala2-Met5-enkephalinamide (DALA) (1.0 microg/microl) was administered intraventricularly to mice responding for electrical stimulation from the dorsal or ventral aspects of the VTA immediately prior to footshock (Experiment 1). Predictably, footshock reduced self-stimulation from the dorsal but not the ventral VTA immediately, 24, and 168 h following the stressor. Intraventricular DALA administration effected a partial attenuation of stressor-induced self-stimulation reductions from the dorsal VTA immediately and 24 h poststressor. Deficits appeared among DALA-Shocked mice responding for brain stimulation from the ventral VTA during comparable test intervals. The long-term depressant influence of footshock on self-stimulation from the dorsal VTA was abolished among DALA-treated mice and DALA-associated reductions in self-stimulation from the ventral A10 region among stressed mice were not evident 1 week later. Administration of D-Ala2, N-Me-Phe4, Gly-Ol5-enkephalin (DAGO) (0.01 microg/microl) or D-Pen2, D-Pen5-enkephalin (DPDPE) (1.0 microg/microl) intraventricularly prior to footshock effected an immediate and a delayed antagonism, respectively, of the stressor on self-stimulation from the dorsal VTA, which persisted for 1 week. Prophylactic administration of 0.001 microg/microl DAGO or 0.01 microg/microl DPDPE prior to the stressor failed to influence self-stimulation from the ventral VTA (Experiment 2). Administration of 0.01 microg/microl DAGO or 1.0 microg/microl DPDPE among mice responding for brain stimulation from the dorsal VTA following footshock produced a weak therapeutic effect immediately poststressor, but effected protracted amelioration of footshock-induced reductions of self-stimulation from the dorsal VTA (Experiment 3). Taken together, mu, delta, and mu-delta activation influenced self-stimulation differentially from the dorsal and ventral VTA according to the temporal order of opioid peptide challenge relative to stressor imposition. These data are discussed with respect to stressors, motivational alterations, and the putative modulating influence of endogenous enkephalin activity in subareas of the VTA.

Stress versus depression

1. Exhaustive evidence is quoted showing that uncontrollable (uncoping) stress provoked in experimental mammals leads to depletion of central noradrenergic activity+ adrenomedullary-cortical gland hyperactivity. These physiological disorders cause the typical neuroendocrine peripheral profile: a) raised catecholamines (CA) in plasma [noradrenaline (NA)+adrenaline (Ad)+dopamine (DA), b) reduced NA/Ad ratio in plasma and c) raised plasma cortisol. 2. Exhaustive evidence is quoted which indicates that severely ill humans show peripheral neuroendocrine profile similar to that found in mammals submitted to uncontrollable stress situation. Further, the NA/Ad ratio does not increase but decreases during orthostasis and exercise stress challenges, as well as oral glucose stress (tolerance) test. 3. Exhaustive evidence is quoted which indicates that endogenous depressed subjects show a neuroendocrine profile opposite to that observed in stressed mammals and severely ill humans. This profile consists of central NA (neural sympathetic) hyperactivity+ adrenomedullary glands hyporresponsivity. These disorders are reflected in a three to ten fold increase of the NA/Ad ratio in plasma. 4. Exhaustive evidence is also quoted showing that dysthymic depressed patients show low plasma catecholamines+low NA/Ad plasma ratio (< 2) during supine-resting condition, it is normalized at orthostasis and exercise periods. 5. It is quoted evidence showing that whereas platelet serotonin is increased in dysthymics, the same is reduced in both endogenous depressed and stressed mammals as well as severely ill humans. 6. It is quoted evidence showing that free serotonin in plasma is greatly raised in uncoping stressed mammals and severely ill humans. The same parameter is normal or slightly increased in dysthymic and endogenous depressed humans. These findings are consistent with the increased platelet aggregability observed in "uncontrollable" stressed mammals and in severely ill, but not depressed patients. 7. It is also quoted evidence showing that whereas parasympathetic activity is absent in uncontrollable stressed mammals and severely ill humans, the same is increased in both types of depressed humans. 8. According to the above, the authors postulate the existence of 3 distinct central+ peripheral neuroendocrine profiles for endogenous depression, dysthymic depression and maladaptation to stress syndrome. These different profiles should lead researchers to attempt different therapeutical approach. 9. In view of the fact that the authors found much clinical overlap among the three syndromes (endogenous depression, dysthymic depression and severely ill patients), they believe that a differential diagnosis should be based on neurochemical, neuroendocrine, physiologic, metabolic and neuropharmacological grounds. 10. The experimentally induced uncontrollable stress (behavioral despair) syndrome in mammals should not be used as a valid model of human depressive syndrome.

Effects of immobilization stress on hippocampal monoamine release: modification by mivazerol, a new alpha 2-adrenoceptor agonist

Mivazerol is a new and selective alpha 2-adrenoceptor agonist which has demonstrated anti-ischemic effects, both in animals and in patients with myocardial ischemia. In the present study, mivazerol was evaluated for its ability to inhibit the release of catecholamines and serotonin (5-HT) in the hippocampus of freely moving rats, and also was compared to clonidine. In vivo microdialysis in combination with high-performance liquid chromatography (HPLC) was employed. Intravenous administration of mivazerol (8.0 micrograms/kg) had no effect on basal outflow of norepinephrine (NE), dopamine (DA) and 3,4-dihydroxyphenylacetic acid (DOPAC). In contrast, clonidine (8.5 micrograms/kg, i.v.) attenuated the basal release of DOPAC, which has been proposed to reflect NE biosynthesis, suggesting that clonidine has an inhibitory effect on NE synthesis. In addition, both mivazerol and clonidine decreased the spontaneous release of 5-HT, which provided further evidence that alpha 2-adrenoceptors in the hippocampus modulate 5-HT. Sixty-min immobilization stress significantly increased the release of NE (177 +/- 28%), DA (209 +/- 46%) and DOPAC (337 +/- 72%). Mivazerol (2.5, 8.0 and 25 micrograms/kg, i.v.) completely prevented the immobilization stress-induced enhancement of NE, DA and DOPAC, which was equi-effective to clonidine at a dose of 8.5 micrograms/kg, i.v. These findings demonstrate that mivazerol has a profound modulatory effect on stress-induced neurotransmitter release in the hippocampus, at dose levels reported to protect against myocardial ischemia.

Differential effects of prolonged restraint stress on proenkephalin gene expression in the brainstem

Enkephalins have been discovered in various regions of the brain involved in cardiovascular regulation. The primary source of plasma Met-enkephalin released in response to stress, appears to be from sympathetic nerves. However, levels of Met-enkephalin are 2-3-fold higher at 2 min versus 30 min of restraint stress. Therefore, the purpose of this investigation was 2-fold; to determine whether proenkephalin gene expression is altered in the brainstem during restraint stress, and whether the magnitude of the change is attenuated with prolonged stress. Proenkephalin mRNA levels were compared in the area postrema (AP), nucleus tractus solitarius (NTS), rostral (RVLM) and caudal ventrolateral medulla (CVLM) following 2 and 30 min of restraint stress. During 2 min restraint stress, there was an approximately 1 fold increase in proenkephalin gene expression in the NTS, CVLM and RVLM with a approximately 50% decrease in the AP. With 30 min restraint stress, the increase in proenkephalin gene expression was maintained in the CVLM and RVLM, however mRNA levels had returned to control levels in the NTS and were approximately 1-fold higher than control in the AP. If the increases of proenkephalin gene expression in the NTS, CVLM and RVLM reflect changes in enkephalinergic neuronal activity in those regions, the alterations in enkephalinergic neuronal activity may be an important regulator of blood pressure homeostasis.

Histaminergic neurons mediate restraint stress-induced increases in the activity of noradrenergic neurons projecting to the hypothalamus

The role of histamine in mediating restraint stress-induced increases in the activity of noradrenergic neurons projecting to the hypothalamus was evaluated in male rats. Noradrenergic neuronal activity was estimated by measuring concentrations of the norepinephrine metabolite 3-methoxy-4-hydroxyphenylethyleneglycol (MHPG) in the paraventricular and medial preoptic nuclei which contain terminals of these neurons. Placement of rats within restraining tubes rapidly increased MHPG but not norepinephrine concentrations in the paraventricular and medial preoptic nuclei. Depletion of neuronal histamine by alpha-fluoromethylhistidine and antagonism of H1 receptors by mepyramine attenuated, whereas blockade of H2 receptors by zolantidine did not prevent the stress-induced increases in MHPG concentrations. Neither mepyramine nor zolantidine affected MHPG concentrations in hypothalamic regions of nonstressed rats. These results indicate that histaminergic neurons contribute to the stress-induced increase the activity of noradrenergic neurons projecting to the hypothalamus via an action at H1 receptors.

Proenkephalin gene expression is altered in the brain of spontaneously hypertensive rats during the development of hypertension

Enkephalins have been discovered in various regions of the brain involved in cardiovascular regulation. Sympathoadrenal hyperactivity and altered baroreflex activity have been implicated in the development of hypertension. The purpose of this investigation was to determine whether proenkephalin gene expression is altered in the arterial baroreceptor reflex region of the brain and in neurons involved in regulating sympathetic outflow, during the development of hypertension. Proenkephalin mRNA levels were compared, using in situ hybridization, in 4- and 14 week-old spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats (WKY). Systolic blood pressure was measured by tail-cuff impedance plethysmography. There were no differences in blood pressure at 4 weeks, however by 14 weeks resting systolic blood pressure was approximately 40% higher in SHR (162.5 +/- 1.6 vs. 117.3 +/- 1.5 mmHg). Proenkephalin gene expression in the nucleus tractus solitarius (NTS), caudal (CVLM) and rostral ventrolateral medulla (RVLM) was lower (approximately 67, 50, and 55%, respectively) in the SHR at 14 weeks. However, in the locus coeruleus (LC), anterior (AH) and lateral hypothalamus (LH), proenkephalin mRNA was significantly increased (approximately 50, 100 and 100%, respectively) in the SHR. The decrease in proenkephalin mRNA in the NTS, CVLM, and RVLM may attenuate arterial baroreceptor reflex activity, while the increase in proenkephalin mRNA in the LC, AH and LH may increase sympathetic tone by inhibiting the activity of sympathodepressor preganglionic neurons in the intermediolateral cell column of the spinal cord.

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.

Hemodynamic stress activates locus coeruleus neurons of unanesthetized rats

The effects of hypotensive stress elicited by nitroprusside infusion on discharge activity of noradrenergic locus coeruleus (LC) neurons of unanesthetized rats were characterized. Nitroprusside (75 micrograms/30 microliters/min, 15 min IV infusion) decreased mean arterial pressure of unanesthetized rats by 50 +/- 2 mmHg (n = 5). Simultaneous recordings of LC spontaneous discharge revealed an increase in discharge rate (197 +/- 87%) that was associated with hypotension. A lower concentration of nitroprusside (10 micrograms/30 microliters/min) that decreased blood pressure of halothane-anesthetized rats by 55 +/- 2 mmHg was much less effective in producing hypotension and did not increase LC discharge when administered to unanesthetized rats. Prior administration of the CRF antagonist, alpha helical CRF9-41 (50 micrograms, ICV) greatly attenuated LC activation by nitroprusside. These findings demonstrate that LC activation elicited by nitroprusside is dependent on the magnitude of hypotension. The present results also demonstrate that nitroprusside is a less potent hemodynamic challenge in unanesthetized rats. Finally, LC activation associated with nitroprusside administration to unanesthetized rats is mediated to a large extent by CRF, confirming findings in anesthetized rats.

Psychological stress-induced increases in noradrenaline release in rat brain regions are attenuated by diazepam, but not by morphine

By measuring levels of noradrenaline (NA) and its major metabolite, 3-methoxy-4-hydroxyphenylethyleneglycol sulfate (MHPG-SO4) in the hypothalamus, amygdala and locus coeruleus region, we investigated the effects of diazepam 5.0 mg/kg, morphine 6.0 mg/kg, or naloxone at 5.0 or 10 mg/kg injected SC immediately before stress exposure, on increases in NA release caused by psychological stress. Psychological stress, wherein rats were exposed to emotional responses which were displayed by other electrically shocked rats, significantly increased MHPG-SO4 levels in the three brain regions examined and elevated plasma corticosterone levels. Both increases in brain MHPG-SO4 levels and elevations of plasma corticosterone levels induced by stress were attenuated significantly by diazepam but neither by morphine nor by naloxone. MHPG-SO4 levels in the hypothalamus and amygdala in the morphine-stress group were significantly higher than those in the saline-stress group. These findings suggest that psychological stress, in which an emotional factor is predominantly involved, causes increases in NA release in these brain regions examined and that these increases are attenuated only by diazepam, in contrast to the previous report, where increases in brain NA release caused by immobilization stress are attenuated not only by diazepam but also by morphine and are enhanced by naloxone.

Influence of chronic opioid delta receptor antagonism on blood pressure development and tissue contents of catecholamines and endogenous opioids in spontaneously hypertensive rats

Opioid delta receptors seem to be involved in blood pressure regulation of spontaneously hypertensive rats (SHR), possibly by an interaction with the sympathetic nervous system. In the present study the effect of four weeks' chronic delta receptor antagonism with ICI 154 129 on development of blood pressure was evaluated in young SHR. Contents of adrenaline and noradrenaline and the opioid peptides beta-endorphin and leucine-enkephalin were measured in brain stem, mid brain, hypothalamus, and adrenal glands. After four weeks' treatment, systolic blood pressure was lower when compared with control SHR. During chronic delta antagonism, concentrations of adrenaline were higher in hypothalamus, mid brain and adrenal glands, contents of noradrenaline were higher in hypothalamus and adrenal glands than in control rats, contents of opioid peptides were not altered with the exception of an increase of beta-endorphin concentration in the hypothalamus. The changes in concentrations of catecholamines following chronic delta antagonism may reflect an alteration of sympathetic activity and could contribute to the retardation of blood pressure development.

Stress-ethanol interaction: involvement of endogenous opioid mechanisms

Stress effects vary with different environmental situations or stress intensities. The effects of restraint stress on locomotion and or corticosterone were examined. Rats were restrained for 0, 15, 30, 60, 90 or 120 min, subsequent locomotion was measured for 10 min. Rats were also sacrificed for corticosterone determinations. Restraint stress affects both variables. Locomotion was recorded in rats pretreated with naltrexone or vehicle prior to restraint of 15 or 60 min. Naltrexone influenced the effects of stress differentially. It did not affect the results following 15 min of restraint but suppressed locomotion after 60-min restraint to a level comparable to that found after 15 min. Treatment with ethanol (1.0, 1.5, 2.0 g/kg) prior to 15 or 60 min of restraint resulted in the prevention of hypomotility induced by 15-min stress. It also interacted nonadditively with 15-min stress on corticosterone release. No such interaction occurred with 60 min stress. Also, naltrexone made it possible to block the effect of ethanol on restraint-induced hypomotility. Results describe stress as a nonunitary concept. Its effects tend to vary with its duration. The differential interaction of stress with naltrexone and ethanol depending on its duration supports the above notion. Results further suggest recruitment of opioid systems in long duration stress (60 min).

Endogenous opiates: 1989

This paper is the twelfth installment of our annual review of the research published during 1989 involving the behavioral, nonanalgesic, effects of the endogenous opiate peptides. The specific topics this year include stress; tolerance and dependence; eating; drinking; gastrointestinal and renal functions; mental illness; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical-related activity; locomotor activity; sex, development, pregnancy, and aging; immunological responses; and other behavior.