The response pattern of noradrenaline release to repeated stress in the hypothalamic paraventricular nucleus differs according to the form of stress in rats

Lesions of the ventral ascending noradrenergic bundles decrease the stress response to occlusal disharmony in rats

Occlusal disharmony induced by placing an acryl cap on the lower incisors of rats is perceived as chronic stress. This chronic stress activates corticotropin-releasing hormone (CRH) neurons in the hypothalamic paraventricular nucleus (PVN), resulting in stimulation of the hypothalamic-pituitary-adrenal (HPA) axis. The ventral ascending noradrenergic bundles (V-NAB) from the brainstem innervate the PVN. To investigate the relationship between the response of the HPA axis and the V-NAB, we examined changes in extracellular noradrenaline (NA) in the PVN and plasma corticosterone, the final output of the HPA axis, following occlusal disharmony in rats injected with 6-hydroxydopamine (6-OHDA), a specific catecholamine neurotoxin. 6-OHDA microinjection into the V-NAB reduced the magnitude of the responses of extracellular NA in the PVN and the plasma corticosterone to occlusal disharmony. Our results suggest that V-NAB to the PVN are involved in occlusal disharmony-induced activation of the HPA axis.

The effects of single and repeated exposure to 2.45 GHz radiofrequency fields on c-Fos protein expression in the paraventricular nucleus of rat hypothalamus

This study investigated the effects of microwave radiation on the PVN of the hypothalamus, extracted from rat brains. Expression of c-Fos was used to study the pattern of cellular activation in rats exposed once or repeatedly (ten times in 2 weeks) to 2.45 GHz radiation in a GTEM cell. The power intensities used were 3 and 12 W and the Finite Difference Time Domain calculation was used to determine the specific absorption rate (SAR). High SAR triggered an increase of the c-Fos marker 90 min or 24 h after radiation, and low SAR resulted in c-Fos counts higher than in control rats after 24 h. Repeated irradiation at 3 W increased cellular activation of PVN by more than 100% compared to animals subjected to acute irradiation and to repeated non-radiated repeated session control animals. The results suggest that PVN is sensitive to 2.45 GHz microwave radiation at non-thermal SAR levels.

Occlusal disharmony increases stress response in rats

Repeated or chronic stress is known to produce structural and functional changes in the rat brain, and in particular, alter the response of the hypothalamic -- pituitary -- adrenal (HPA) axis to subsequent new stress. Occlusal disharmony via placement of acryl cap on the lower incisors of rats is perceived as chronic stress. To determine the response of the HPA axis to subsequent new stress in rats with occlusal disharmony, we measured plasma corticosterone levels in these rats after subjecting them to new stress. Plasma corticosterone levels in rats with and without incisal cap increased and reached a peak 30 min after exposure to the new stress. However, a later decrease in plasma corticosterone levels from peak levels was found in rats with incisal cap compared with rats without incisal cap. This finding suggests that occlusal disharmony alters the response of the HPA axis to subsequent new stress.

Effects of stress on catecholamine stores in central and peripheral tissues of long-term socially isolated rats

Both the peripheral sympatho-adrenomedullary and central catecholaminergic systems are activated by various psycho-social and physical stressors. Catecholamine stores in the hypothalamus, hippocampus, adrenal glands, and heart auricles of long-term socially isolated (21 days) and control 3-month-old male Wistar rats, as well as their response to immobilization of all 4 limbs and head fixed for 2 h and cold stress (4 degrees C, 2 h), were studied. A simultaneous single isotope radioenzymatic assay based on the conversion of catecholamines to the corresponding O-methylated derivatives by catechol-O-methyl-transferase in the presence of S-adenosyl-l-(3H-methyl)-methionine was used. The O-methylated derivatives were oxidized to 3H-vanilline and the radioactivity measured. Social isolation produced depletion of hypothalamic norepinephrine (about 18%) and hippocampal dopamine (about 20%) stores and no changes in peripheral tissues. Immobilization decreased catecholamine stores (approximately 39%) in central and peripheral tissues of control animals. However, in socially isolated rats, these reductions were observed only in the hippocampus and peripheral tissues. Cold did not affect hypothalamic catecholamine stores but reduced hippocampal dopamine (about 20%) as well as norepinephrine stores in peripheral tissues both in control and socially isolated rats, while epinephrine levels were unchanged. Thus, immobilization was more efficient in reducing catecholamine stores in control and chronically isolated rats compared to cold stress. The differences in rearing conditions appear to influence the response of adult animals to additional stress. In addition, the influence of previous exposure to a stressor on catecholaminergic activity in the brainstem depends on both the particular catecholaminergic area studied and the properties of additional acute stress. Therefore, the sensitivity of the catecholaminergic system to habituation appears to be tissue-specific.

Novel stressors affected catecholamine stores in socially isolated normotensive and spontaneously hypertensive rats

Catecholamines in some central (hypothalamus and hippocampus) and peripheral tissues (adrenal glands and heart auricles) of long-term socially isolated normotensive and spontaneously hypertensive rats exposed to novel immobilization stress were determined by a simultaneous single isotope radioenzymatic assay. Long-term isolation (21 days) produced depletion of hypothalamic norepinephrine (NE) stores and hippocampal dopamine (DA) stores in both normotensive and spontaneously hypertensive rats. Acute immobilization stress (2 h) significantly decreased NE and DA stores in hypothalamus and hippocampus of naive normotensive and spontaneously hypertensive rats controls. However, novel immobilization stress applied to normotensive rats previously subjected to long-term isolation produced no changes in catecholamine levels in hypothalamus, while resulting in somewhat higher depletion of NE stores in hypothalamus of spontaneously hypertensive rats treated in the same way. Novel immobilization stress decreased NE and DA stores in hippocampus of normotensive but was without effect on NE and DA stores of spontaneously hypertensive rats. Social isolation did not affect catecholamine stores in peripheral tissues but novel immobilization stress produced a significant decrease in catecholamine content. The results suggest that some central and peripherals tissues of spontaneously hypertensive rats and normotensive rats differ with regard to catecholamine content and that there are certain differences in their responsiveness to stress.

Effects of acute, repeated and chronic variable stress on in vivo tyrosine hydroxylase activity and on alpha(2)-adrenoceptor sensitivity in the rat brain

We assessed the effects of a single tail pinch and two chronic stress regimes, repeated and variable, on in vivo tyrosine hydroxylase activity and on alpha2-adrenoceptor sensitivity in two brain regions. After administering a 3,4-dihydroxyphenylalanine (DOPA) decarboxylase inhibitor, tyrosine hydroxylase activity, measured as the accumulation of DOPA, and noradrenaline (NA) content were determined by using high-performance liquid chromatography. A single tail pinch for 5 min induced an enhancement of DOPA content in hippocampus (28%) and hypothalamus (67%) which was still present 24 h later. This increase could account for the lack of changes in NA content in both regions after the application of this stressor. However, tyrosine hydroxylase activity was unmodified 24 h after exposure to both repeated (5 min of tail pinch, twice daily, for 14 days) and chronic variable stress (one of 5 different stressors, once daily, for 14 days) although there was an enhancement of NA levels in hippocampus (45 and 54%, respectively) and hypothalamus (24.5 and 36%, respectively). The sensitivity of the alpha2-adrenoceptors which regulate [3H]-NA release in hippocampal and hypothalamic synaptosomes was not modified by the acute or chronic stress protocols assayed. The results show that both paradigms of chronic stress had similar effects on the noradrenergic indices evaluated.

Rapid habituation of hippocampal serotonin and norepinephrine release and anxiety-related behaviors, but not plasma corticosterone levels, to repeated footshock stress in rats

Prior stress exposure is known to alter the activation response to a subsequent stressor. In the present study, we examined neurochemical, neuroendocrinological, and behavioral correlates of short-term adaptation to homotypic stressors administered 60 min apart. An initial electric footshock significantly induced extracellular levels of both serotonin (5-HT) and norepinephrine (NE) in the rat hippocampus (650% and 200% above baseline, respectively), as measured by in vivo microdialysis. A rapid habituation in this response was evident in the inability of a second footshock to evoke similar increases. In contrast, the hypothalamic-pituitary-adrenal (HPA) response was augmented further after the second shock session: plasma corticosterone (CORT) levels were 18.1, 316.5, and 441.6 mg/ml in nonstressed, one-footshock-, or two-footshock-treated rats, respectively. In a social interaction paradigm, rats subjected to a single footshock showed several fear- and anxiety-related behaviors such as increases in freezing and decreases in rearing and active approach for social interaction. Exposure to a second footshock completely blocked the freezing response and restored rearing behavior without affecting the disruption in social interactions. Taken together, these data raise the possibility that neurochemical and neuroendocrine adaptations to short-term homotypic stressors differentially contribute to expression of different fear and anxiety-like responses in the rat.

Stressor specificity of central neuroendocrine responses: implications for stress-related disorders

Despite the fact that many research articles have been written about stress and stress-related diseases, no scientifically accepted definition of stress exists. Selye introduced and popularized stress as a medical and scientific idea. He did not deny the existence of stressor-specific response patterns; however, he emphasized that such responses did not constitute stress, only the shared nonspecific component. In this review we focus mainly on the similarities and differences between the neuroendocrine responses (especially the sympathoadrenal and the sympathoneuronal systems and the hypothalamo-pituitary-adrenocortical axis) among various stressors and a strategy for testing Selye's doctrine of nonspecificity. In our experiments, we used five different stressors: immobilization, hemorrhage, cold exposure, pain, or hypoglycemia. With the exception of immobilization stress, these stressors also differed in their intensities. Our results showed marked heterogeneity of neuroendocrine responses to various stressors and that each stressor has a neurochemical "signature." By examining changes of Fos immunoreactivity in various brain regions upon exposure to different stressors, we also attempted to map central stressor-specific neuroendocrine pathways. We believe the existence of stressor-specific pathways and circuits is a clear step forward in the study of the pathogenesis of stress-related disorders and their proper treatment. Finally, we define stress as a state of threatened homeostasis (physical or perceived treat to homeostasis). During stress, an adaptive compensatory specific response of the organism is activated to sustain homeostasis. The adaptive response reflects the activation of specific central circuits and is genetically and constitutionally programmed and constantly modulated by environmental factors.

Effect of novel stressors on gene expression of tyrosine hydroxylase and monoamine transporters in brainstem noradrenergic neurons of long-term repeatedly immobilized rats

Responses of central noradrenergic (NE) neurons to stressors like immobilization (IMO), cold exposure, insulin-induced hypoglycemia, and cellular glucoprivation caused by 2-deoxy-D-glucose (2-DG) were investigated in intact and long-term repeatedly immobilized (LTR, 2 h daily IMO for 41 days) rats. Expression of tyrosine hydroxylase (TH), norepinephrine transporter (NET) and vesicular monoamine transporter (VMAT2) genes were determined by using in situ hybridization histochemistry in brainstem A1, A2, A5 and locus coeruleus (LC) neurons. TH mRNA levels were increased by single IMO or 2-DG administration in all areas studied. Cold was effective only in LC and A2 neurons while insulin had no effect. LTR immobilization elevated TH mRNA levels in all investigated cell groups. These elevations were equally high to those elicited by a single IMO in each noradrenergic group, except the LC where LTR IMO was less effective than the single IMO. The levels of NET and VMAT2 mRNAs were elevated only in the A1 and A2 cell groups of LTR IMO rats. A newly applied IMO in LTR rats did not alter TH, NET, and VMAT2 mRNA levels in any NE cell group investigated. Novel stressors like cold and 2-DG exaggerated the increased TH mRNA levels only in the LC of LTR IMO rats, unlike in the other NE cell groups. The present data indicate that repeated exposure of rats to homotypic stressor induces an adaptation of NE neurons, whereas single exposure of such animals to heterotypic novel stressor produces an exaggerated response of the system at the level of TH (in LC) and NET (in A1, A2) gene expression.

Swim stress alters the behavioural response of mice to GABA-related and some GABA-unrelated convulsants

To elucidate the relationship between stress and seizures, the effect of a single swim stress on the convulsive signs and death produced by several GABA-related and GABA-unrelated convulsants, and the effect of repeated swim stress on picrotoxin-induced convulsions was studied. Mice were subjected to swim stress (10 min swimming at 18-19 degrees C), and the i.v. infusion of convulsants started 15 min thereafter. The latency to the onset of several convulsant signs and death was measured, and the doses of convulsants producing convulsions and death were calculated. Additional experiments included mice swimming at room temperature, and those which were stressed repeatedly (twice a day for four consecutive days, plus one stressful procedure on the fifth day). Swim stress increased the dose needed to produce convulsant signs and death after bicuculline, picrotoxin, pentylenetetrazole, strychnine and 4-aminopyridine, while kainic acid-induced convulsions were not affected. Using picrotoxin infusion, the effect of swimming in room temperature water was less than the effect of swimming in 18-19 degrees C water. In addition, the effect of repeated stress was less than the effect of acute stress on picrotoxin-induced convulsions. The results demonstrate that acute swim stress lowers the convulsive potency of GABA-related and some GABA-unrelated convulsants. Repeatedly stressed animals develop tolerance to anticonvulsive effect of swim stress.

Potent serotonin (5-HT)(2A) receptor antagonists completely prevent the development of hyperthermia in an animal model of the 5-HT syndrome

The serotonin (5-HT) syndrome is the most serious side effect of antidepressants, and it often necessitates pharmacotherapy. In the present study, the efficacy of several drugs was evaluated in an animal model of the 5-HT syndrome. When 2 mg/kg of clorgyline, a type-A monoamine oxidase inhibiting antidepressant, and 100 mg/kg of 5-hydroxy-L-tryptophan, a precursor of 5-HT, were administered intraperitoneally to rats to induce the 5-HT syndrome, the rectal temperature of the rats increased to more than 40 degrees C, and all of the animals died by 90 min after the drug administration. The noradrenaline (NA) levels in the anterior hypothalamus, measured by microdialysis, increased to 15.9 times the preadministration level. Pretreatment with propranolol (10 mg/kg), a 5-HT(1A) receptor antagonist as well as a beta-blocker, and dantrolene (20 mg/kg), a peripheral muscle relaxant, did not prevent the death of the animals, even though these two drugs suppressed the increase in rectal temperature to some extent. Chlorpromazine and cyproheptadine prevented the lethality associated with the 5-HT syndrome only at high doses. By contrast, pretreatment with ritanserin (3 mg/kg) and pipamperone (20 mg/kg), both potent 5-HT(2A) receptor antagonists, completely prevented the increase in rectal temperature and death of the animals, and the hypothalamic NA levels in these two groups increased less than that in the other groups. These results suggest that potent 5-HT(2A) receptor antagonists are the most effective drugs for treatment of the 5-HT syndrome, and that NA hyperactivity occurs in the 5-HT syndrome.

Selective corticotropin-releasing factor type 1 receptor antagonist blocks conditioned fear-induced release of noradrenaline in the hypothalamic paraventricular nucleus of rats

The effects of conditioned fear on the release of noradrenaline in the hypothalamic paraventricular nucleus (PVN) and the involvement of corticotropin-releasing factor (CRF) receptor type 1 (CRFR1) in conditioned fear-induced changes in noradrenaline release were examined by intracerebral microdialysis in rats. Conditioned fear was produced by placing animals into a box where they had previously been exposed to a 5-min period of electric footshock, 135 min prior to the start of experiment. Conditioned fear for 20 min produced a significant increase in the release of noradrenaline in the PVN. Intraperitoneal preadministration of a selective nonpeptidic CRFR1 antagonist, CRA1000, completely blocked the conditioned fear-induced release of noradrenaline. These results suggest that CRFR1 is involved in the release of noradrenaline in the hypothalamic PVN induced by conditioned fear.

Treadmill exercise training augments brain norepinephrine response to familiar and novel stress

In a test of hypothalamic-pituitary-adrenal (HPA) cortical and hypothalamic-pituitary-gonadal (HPG) interaction during familiar and novel stress, we previously reported that treadmill exercise training led to blunted plasma adrenocorticotrophin (ACTH) response to acute treadmill running but a hyper-responsiveness of ACTH after novel immobilization. In this follow-up analysis, we examined whether those results might be plausibly explained by a similar effect of treadmill exercise training on increased levels of norepinephrine (NE) in hypothalamic and limbic brain regions which synergize to modulate the release of ACTH during stress. Ovariectomized Sprague-Dawley rats that had been exercise trained by treadmill running or remained sedentary for 6 weeks received intramuscular injections of estradiol benzoate (Eb) or sesame oil on each of 3 days prior to 15 min of familiar treadmill running or novel immobilization. Treadmill exercise training, regardless of Eb treatment or type of stress, increased NE levels in the paraventricular (PVN), arcuate, medial preoptic, and ventromedial areas of the hypothalamus and protected against depletion of NE in the locus coeruleus, amygdala, and hippocampus. We conclude that treadmill exercise training has a hyperadrenergic effect in brain areas that modulate hypothalamic regulation of ACTH release during stress that is independent of HPA-HPG interaction and novelty of the stressor. To help elucidate these findings, the effects of treadmill exercise training on A1-A2 nuclei which innervate the PVN and their relationship with the limbic and hypothalamic responses we report require study.

Occlusal disharmonies modulate central catecholaminergic activity in the rat

Occlusal disharmonies have classically been thought to be involved in the etiopathogenesis of bruxism, as have, more recently, alterations in central neurotransmission, particularly dopaminergic neurotransmission. However, the connection between these two factors has still not been established. In this study, we assessed the effects of diverse occlusal disharmonies, maintained for either 1 day or 14 days, on neurochemical indices of dopaminergic and noradrenergic activity in the striatum, frontal cortex, and hypothalamus of the rat. The in vivo activity of tyrosine hydroxylase, determined as the accumulation of 3,4-dihydroxyphenylalanine (DOPA), 30 min after the administration of 3-hydroxybenzylhydrazine, a DOPA decarboxylase inhibitor, and dopamine and noradrenaline contents were quantified by high-performance liquid chromatography with electrochemical detection. The wearing of an acrylic cap on both lower incisors for 1 day induced a significant increase in DOPA accumulation in the regions analyzed, with parallel increases in dopamine levels in the hypothalamus and dopamine and noradrenaline in the frontal cortex. After the cap was maintained for 14 days, DOPA accumulation tended to return to control values, except in the left striatum, thereby causing an imbalance between hemispheres. In contrast, 1 or 14 days after the lower left and the upper right incisors were cut, less pronounced changes in catecholaminergic neurotransmission were found in the brain areas studied. Moreover, the cutting of one lower incisor did not modify either DOPA accumulation or dopamine and noradrenaline contents in the striatum or hypothalamus. These results provide experimental evidence of a modulation of central catecholaminergic neurotransmission by occlusal disharmonies, being dependent on the nature of the incisal alteration and on the time during which it was maintained.

Chronic variable stress induces supersensitivity of central alpha2-adrenoceptors which modulate the jaw-opening reflex in the rat

In a previous study, we found that the sensitivity of central postsynaptic alpha2-adrenoceptors which modulate, in an inhibitory way, the activity of the jaw-opening reflex (JOR) is reduced after chronic repeated stress (tail pinch) in the rat. The aim of this study was to assess the effects of exposure to a chronic variable stress regime on these adrenoceptors. To do this, the digastric electromyographic responses elicited by orofacial electrical stimulation after the intravenous administration of cumulative doses (x3.3) of the alpha2-adrenoceptor agonist, clonidine (0.1-10000 microgram/kg), were recorded. As expected, in unmanipulated control rats, clonidine inhibited the reflex, in a dose-dependent manner, until abolition (ED50 = 17.3 +/- 2.2 microgram/kg). Single tail pinch did not significantly alter the ability of clonidine to abolish the reflex. However, chronic variable stress led to an enhancement of the inhibitory effect of clonidine on the amplitude of JOR, resulting in a shift to the left of the dose-response curve in comparison with that of the control group (ED50 was reduced by 37%, P = 0.032), without affecting either the estimated maximum effect for the agonist or the slope of the inhibitory function. This in vivo result indicates that chronic variable stress leads to an increased sensitivity of central alpha2-adrenoceptors which modulate JOR, in contrast to the desensitization of these adrenoceptors found after repeated exposure to the same stressor.

Effect of acute or repeated stress on behavior and brain norepinephrine system in Wistar-Kyoto (WKY) rats

WKY rats develop more restraint-induced gastric ulcers and exhibit more depressive behavior compared to other rat strains. Exposure to novel stressors for 21 days exacerbates depressive behavior in WKY rats and alters beta-adrenoceptors (beta-ARs) and norepinephrine transporter (NET) sites in several limbic brain regions when compared to Sprague-Dawley rats. The present study examined whether these effects would be elaborated following an acute stressor and whether WKY rats would demonstrate adaptation after repeated stress. Rats were subjected to a 2-h supine restraint stress for either one or eight consecutive daily sessions. Open-field behavioral data were collected immediately after the daily stress sessions. Brains were sectioned for autoradiographic analysis of 125I-pindolol binding to beta-ARs and 3H-nisoxetine binding to NET sites in discrete brain regions. Acute 1-day stress resulted in a significant drop in body weight and an inhibition of behaviors in the open field. These effects were also sustained following 7 days of chronic restraint stress. In contrast, while acute stress had no effect on NET binding sites or beta-ARs, repeated stress decreased NET sites in the amygdala, hypothalamus, and locus coeruleus with little effect on beta-ARs in the brain regions examined.

Intracerebroventricular administration of neuropeptide Y inhibits release of noradrenaline in the hypothalamic paraventricular nucleus caused by manual restraint in the rat through an opioid system

Intracerebroventricular injection of 1.5 micrograms neuropeptide Y (NPY) had no effect on basal release of noradrenaline (NA) in the hypothalamic paraventricular nucleus (PVN), measured by intracerebral microdialysis in the rat. However, it blocked the increase in NA release caused by manual restraint but not that by tail-pinch, and the effect was blocked by naloxone (1.0 mg/kg body weight). Thus, NPY attenuates NA release in the PVN by a painless stressor, such as manual restraint, through an opioid system.