CRH and the noradrenergic system mediate the antinociceptive effect of central interleukin-1 alpha in the rat

Brain corticotropin-releasing factor signaling: Involvement in acute stress-induced visceral analgesia in male rats

BACKGROUND

Water avoidance stress (WAS) induces a naloxone-independent visceral analgesia in male rats under non-invasive conditions of monitoring. The objective of the study was to examine the role of brain CRF signaling in acute stress-induced visceral analgesia (SIVA).

METHODS

Adult male Sprague-Dawley rats were chronically implanted with an intracerebroventricular (ICV) cannula. The visceromotor response (VMR) to graded phasic colorectal distension (CRD: 10, 20, 40, 60 mm Hg, 20 seconds, 4 minutes intervals) was monitored using manometry. The VMR to a first CRD (baseline) was recorded 5 minutes after an ICV saline injection, followed 1 hour later by ICV injection of either CRF (30, 100, or 300 ng and 1, 3, or 5 μg/rat) or saline and a second CRD, 5 minutes later. Receptor antagonists against CRF₁ /CRF₂ (astressin-B, 30 μg/rat), CRF₂ (astressin₂ -B, 10 μg/rat), oxytocin (tocinoic acid, 20 μg/rat), or vehicle were injected ICV 5 minutes before CRF (300 ng/rat, ICV) or 15 minutes before WAS (1 hour).

KEY RESULTS

ICV CRF (100 and 300 ng) reduced the VMR to CRD at 60 mm Hg by -36.6% ± 6.8% and -48.7% ± 11.7%, respectively, vs baseline (P < 0.001), while other doses had no effect and IP CRF (10 µg/kg) induced visceral hyperalgesia. Astressin-B and tocinoic acid injected ICV induced hyperalgesia and prevented the analgesic effect of ICV CRF (300 ng/rat) and WAS, while astressin₂ -B only blocked WAS-induced SIVA.

CONCLUSIONS & INFERENCES

These data support a role for brain CRF signaling via CRF₂ in SIVA in a model of WAS and CRD likely mediated by the activation of brain oxytocin pathway.

Distribution of axon diameters in cortical white matter: an electron-microscopic study on three human brains and a macaque

The aim of this study was to obtain information on the axonal diameters of cortico-cortical fibres in the human brain, connecting distant regions of the same hemisphere via the white matter. Samples for electron microscopy were taken from the region of the superior longitudinal fascicle and from the transitional white matter between temporal and frontal lobe where the uncinate and inferior occipitofrontal fascicle merge. We measured the inner diameter of cross sections of myelinated axons. For comparison with data from the literature on the human corpus callosum, we also took samples from that region. For comparison with well-fixed material, we also included samples from corresponding regions of a monkey brain (Macaca mulatta). Fibre diameters in human brains ranged from 0.16 to 9 μm. Distributions of diameters were similar in the three systems of cortico-cortical fibres investigated, both in humans and the monkey, with most of the average values below 1 μm diameter and a small population of much thicker fibres. Within individual human brains, the averages were larger in the superior longitudinal fascicle than in the transitional zone between temporal and frontal lobe. An asymmetry between left and right could be found in one of the human brains, as well as in the monkey brain. A correlation was also found between the thickness of the myelin sheath and the inner axon diameter for axons whose calibre was greater than about 0.6 μm. The results are compared to white matter data in other mammals and are discussed with respect to conduction velocity, brain size, cognition, as well as diffusion weighted imaging studies.

Role of the calcium channel in blastocyst implantation: a novel contraceptive target

The proinflammatory blastocyst implantation cascade involves important mediators like prostaglandins (PG). The influx of calcium via the calcium channel acts as a trigger for the activation of the PG synthesis pathway. Hence, it was hypothesized that calcium channel blockers that are known to possess anti-inflammatory activity may interfere with normal implantation. Pregnant Swiss albino mice (Mus musculus) were treated with diltiazem (1) 4 mg/kg, po on days 1-6 of pregnancy, n=6/day) or (2) at the implantation site (25 microg/animal) via intrauterine injection in the right horn at 5:00 pm on day 4. The intact uterus was used to assay lipid peroxidation and superoxide dismutase activity as markers of membrane fluidity or to observe the day 15 fetus. Oral diltiazem treatment in therapeutic dosage before and during the implantation period did not cause any change in normal uterine milieu during the window of implantation. When injected into the uterine lumen 12-14 h before the average implantation time, however, a complete failure in implantation was observed. Thus, the site specific action of diltiazem may be blocking prostaglandin synthesis, hence causing implantation failure. Oral diltiazem treatment did not mimic this action, indicating that although orally safe in pregnancy in therapeutic dosage, calcium channel blockers may provide a new and yet unknown target in female contraceptive research.

The role of hypothalamo-hypophyseal-adrenocortical system hormones in controlling pain sensitivity

The present review addresses analysis of data demonstrating the role of the hypothalamo-hypophyseal-adrenocortical axis (HHACA) in controlling pain sensitivity. Experiments on rats have demonstrated the analgesic effects of exogenous hormones of all components of the HHACA - corticotropin-releasing hormone (CRH), adrenocorticotropic hormone (ACTH), and glucocorticoids - in the same models, and have also shown that the opioid and non-opioid mechanisms contribute to the development of the analgesia induced by these hormones. Endogenous glucocorticoids are involved in the development of analgesia mediated by non-opioid mechanisms. Along with the non-opioid mechanisms associated with endogenous glucocorticoids, the analgesic effect of ACTH can be mediated by the opioid mechanism. Unlike the situation with ACTH, the analgesic effect of CRH is mediated exclusively by non-opioid mechanisms, one of which is associated with HHACA hormones, while the other, appearing only on systemic administration, is not associated with these hormones. The actions of glucocorticoids on pain are mediated by neurons in the central gray matter of the midbrain.

Effects of cytokines and infections on brain neurochemistry

Administration of cytokines to animals can elicit many effects on the brain, particularly neuroendocrine and behavioral effects. Cytokine administration also alters neurotransmission, which may underlie these effects. The most well studied effect is the activation of the hypothalamo-pituitary-adrenocortical (HPA) axis, especially that by interleukin-1 (IL-1). Peripheral and central administration of IL-1 also induces norepinephrine (NE) release in the brain, most markedly in the hypothalamus. Small changes in brain dopamine (DA) are occasionally observed, but these effects are not regionally selective. IL-1 also increases brain concentrations of tryptophan, and the metabolism of serotonin (5-HT) throughout the brain in a regionally nonselective manner. Increases of tryptophan and 5-HT, but not NE, are also elicited by IL-6, which also activates the HPA axis, although it is much less potent in these respects than IL-1. IL-2 has modest effects on DA, NE and 5-HT. Like IL-6, tumor necrosis factor-α (TNFα) activates the HPA axis, but affects NE and tryptophan only at high doses. The interferons (IFN's) induce fever and HPA axis activation in man, but such effects are weak or absent in rodents. The reported effects of IFN's on brain catecholamines and serotonin have been very varied. However, interferon-γ, and to a lesser extent, interferon-α, have profound effects on the catabolism of tryptophan, effectively reducing its concentration in plasma, and may thus limit brain 5-HT synthesis.Administration of endotoxin (LPS) elicits responses similar to those of IL-1. Bacterial and viral infections induce HPA activation, and also increase brain NE and 5-HT metabolism and brain tryptophan. Typically, there is also behavioral depression. These effects are strikingly similar to those of IL-1, suggesting that IL-1 secretion, which accompanies many infections, may mediate these responses. Studies with IL-1 antagonists, support this possibility, although in most cases the antagonism is incomplete, suggesting the existence of multiple mechanisms. Because LPS is known to stimulate the secretion of IL-1, IL-6 and TNFα, it seems likely that these cytokines mediate at least some of the responses, but studies with antagonists indicate that there are multiple mechanisms. The neurochemical responses to cytokines are likely to underlie the endocrine and behavioral responses. The NE response to IL-1 appears to be instrumental in the HPA activation, but other mechanisms exist. Neither the noradrenergic nor the serotonergic systems appear to be involved in the major behavioral responses. The significance of the serotonin response is unknown.

Time- and dose-dependent effects of corticotropin releasing factor on cerebral glucose metabolism in rats

The time course and the relation to dose of locomotor activity and of the regional cerebral metabolic rates for glucose (rCMRglc) were measured in freely moving Sprague-Dawley rats after intracerebroventricular administration of ovine corticotropin releasing factor (oCRF). Motor activity was determined using a familiar photocage cell. rCMRglc was measured, using the quantitative autoradiographic [(14)C]2-deoxyglucose procedure, in 73 brain regions at 10, 30, 90 and 180 min after administration of oCRF 10 microg and at 90 min after oCRF 0.1, 1 and 100 microg. oCRF 10 microg increased motor activity in a sustained fashion and increased rCMRglc with different time courses throughout brain regions. In cerebellar regions rCMRglc increases peaked at 90 min and were sustained up to 180 min. In non-cerebellar regions rCMRglc increases peaked at 90 min but declined thereafter. At lower doses (0.1 and 1 microg) oCRF increased rCMRglc in fewer brain regions (1 and 5 regions affected, average increases 1% and 7%) including cerebellar areas and brainstem sensory nuclei and decreased rCMRglc in medial prefrontal cortex. At the highest dose (100 microg) oCRF induced large and widespread rCMRglc increases in cerebellar, brainstem, hypothalamic, limbic and neocortical areas (40 brain regions affected, average increase 32%). The findings indicate that cerebellar areas and brainstem nuclei are highly sensitive to oCRF and may mediate oCRF autonomic and behavioral effects.

Suppression of morphine withdrawal syndrome by interleukin-2 and its gene

The naloxone-precipitated withdrawal syndrome in mice and rats after intrathecal injection of recombinant human interleukin-2 protein (rIL-2) or its gene was studied. The results showed that rIL-2 could significantly decrease the number of jumps in mice. In rats, rIL-2 significantly suppressed irritating, diarrhea, weight loss, abnormal posture and salivation. Tendencies towards reductions in teeth chewing and dog-shaking were also observed. Furthermore, pcDNA3-IL-2 (8 microg DNA) had a similar effect as 1x10 IU rIL-2 protein on inhibition of morphine withdrawal syndrome in mice, and the expression of rIL-2 protein in spinal cord could be detected for 6 days. These findings provided further evidence for the neuroregulatory function of an immunological molecule such as IL-2.

Role of corticotropin-releasing factor and its receptor in nociceptive modulation in the central nucleus of amygdala in rats

Corticotropin-releasing factor (CRF) plays important physiological functions in the central nervous system. The present study was performed to investigate the role of CRF and CRF receptor in nociceptive modulation in the central nucleus of amygdala (CeA) of rats. The hindpaw withdrawal latency (HWL) to noxious thermal and mechanical stimulation increased significantly after intra-CeA administration of 0.1 and 0.01 nmol of CRF, but not 0.001 nmol, indicating that CRF induces antinociceptive effects in the CeA of rats. The antinociceptive effect may be due to the dose of CRF was attenuated by intra-CeA administration of 0.1 nmol alpha-hCRF9-41, a selective CRF receptor antagonist, suggesting that the CRF-induced antinociception is mediated by the CRF receptors in the CeA. Furthermore, the HWL to both thermal and mechanical stimulation decreased significantly after intra-CeA administration of alpha-hCRF9-41 alone, suggesting an involvement of endogenous CRF in the CeA in nociceptive modulation. The present study demonstrated that both exogenous and endogenous CRF plays an antinociceptive effect in the CeA, the effect is mediated by CRF receptor.

Hypothalamic mechanisms of pain modulatory actions of cytokines and prostaglandin E2

A decrease and subsequent increase in nociceptive threshold in the whole body are clinical symptoms frequently observed during the course of acute systemic infection. These biphasic changes in nociceptive reactivity are brought about by central signal substances induced by peripheral inflammatory messages. Systemic administration of lipopolysaccharide (LPS) or interleukin-1 beta (IL-1 beta), an experimental model of acute infection, may mimic the biphasic changes in nociception, hyperalgesia at small doses of LPS, and IL-1 beta and analgesia at larger doses. Our behavioral and electrophysiological studies have revealed that IL-1 beta in the brain induces hyperalgesia through the actions of prostaglandin E2 (PGE2) on EP3 receptors in the preoptic area and its neighboring basal forebrain, whereas the IL-1 beta-induced analgesia is produced by the actions of PGE2 on EP1 receptors in the ventromedial hypothalamus. An intravenous injection of LPS (10-100 micrograms/kg) produced hyperalgesia only during the period before fever develops and was abolished by microinjection of NS-398 (an inhibitor of cyclooxygenase 2) into the preoptic area, but not into the other areas in the hypothalamus. The hyperalgesia induced by the cytokines PGE2 and LPS may explain the systemic hyperalgesia clinically observed in the early phase of infectious diseases, which probably warns the organisms of infection before the full development of sickness symptoms. The switching of nociception from hyperalgesia to analgesia accompanied by sickness symptoms may reflect changes in the host's strategy for fighting microbial invasion as the disease progresses.

The role of corticotropin-releasing factor in pain and analgesia

Corticotropin-releasing factor (CRF) is a peptide that is released from the hypothalamus and in widespread areas of the brain following exposure to stressors. It is considered to be a mediator of many of the effects of stress, and its analgesic properties have been demonstrated in many studies. However, for primarily methodological reasons, the effects of CRF in the central nervous system have been neglected whereas the peripheral effects of CRF have been overemphasized. We present evidence that: (1) CRF can act at all levels of the neuraxis to produce analgesia; (2) the release of beta-endorphin does not explain the analgesia following intravenous or intracranial CRF administration; (3) inflammation must be present for local CRF to evoke analgesia and (4) the analgesic effects of CRF show specificity for prolonged pain. These findings suggest that CRF may have a significant role in chronic pain syndromes associated with hypothalamic-pituitary-adrenal axis abnormalities. Furthermore, CRF may represent a new class of analgesics that merits further study. Implications for the relationship between stress and pain are discussed.

Effects of cytokines on cerebral neurotransmission. Comparison with the effects of stress

Table 2 summarizes the reported responses of the HPA axis, as well as catecholamines and indoleamines to the cytokines discussed above. Cytokine administration to animals can elicit a number of effects on the brain, including neuroendocrine and behavioural effects, and also alters the metabolism of neurotransmitters. The most well documented effect is the activation by interleukin-1 (IL-1) of the hypothalamo-pituitary-adrenocortical (HPA) axis, which is accompanied by a stimulation of cerebral noradrenaline (NA) metabolism, probably reflecting increased NA secretion. IL-1 also stimulates indoleamine metabolism, most prominently increasing tryptophan concentrations, and increasing the metabolism of serotonin (5-hydroxytryptamine, 5-HT). IL-6 induces a short-lived activation of the HPA axis, and has effects on tryptophan and 5-HT similar to those of IL-1. Tumour necrosis factor alpha (TNF alpha) has effects on the HPA axis similar to those of IL-6, but affects NA and tryptophan only at high doses. Interferon alpha had no effects on the parameters studied. The effects of IL-1 are remarkably similar to those observed following administration of endotoxin (lipopolysaccharide, LPS), and infections, such as influenza virus. They also resemble quite closely the responses that are observed to stressors commonly studied in laboratory animals, such as electric shock or restraint. The major differences are: that the NA response to shock or restraint is very uniform throughout the brain, whereas that to IL-1, LPS or infection is significantly greater in the hypothalamus; and, responses in dopaminergic (DA) systems are normally observed to shock or restraint, with especially prominent responses in the limbic cortex, whereas DA responses are rarely observed in response to IL-1 and immune stimuli, and when they do occur, the mesocortical system is not selectively affected. The neurochemical responses to cytokines may underlie some of the endocrine and behavioural responses. The NA response to IL-1 is apparently related to the HPA activation, but not the hypophagia. The significance of the indoleaminergic responses is not known.

Neuroendocrine and behavioral effects of CRH blockade and stress in male rats

Our previous data have shown that restraint (RT), a mild nonpainful stressor, acutely impairs nonsocial and social behavior in male rats. Corticotropin-releasing hormone (CRH) is a regulator of these behavioral responses. To evaluate whether CRH mediates the neuroendocrine and behavioral alterations present 24 h after restraint stress, we administered the CRH antagonist alpha-helical CRH(9-41) (alpha-hCRH) intracerebroventricularly to male rats and we compared its effects with those of saline. Twenty-four hours after treatment, nonsocial behaviors were significantly decreased by alpha-hCRH, this effect being independent of RT. Among social behaviors, only introductory activity showed significant differences as a result of both RT and alpha-hCRH. The concentrations of ACTH in the plasma and those of beta-endorphin in the anterior and neurointermediate lobes of the pituitary were affected by alpha-hCRH treatment. The effect on ACTH was simply related to the administration of the alpha-hCRH, while for beta-endorphin, significant interactions between alpha-hCRH and RT were found. On the whole, these results point to the role played by CRH in the control of neuronal mechanisms involved in the stress-induced effects.

Effect of IL-1alpha on the release of norepinephrine in rat hypothalamus

The increased release of norepinephrine (NE) in the brain as part of the 'acute phase response' has been postulated to result from a direct action of IL-1 on the hypothalamus. To test whether the effects of IL-1alpha were direct, we carried out in vivo experiments using microdialysis and measured NE release in the hypothalamus using high pressure liquid chromatography (HPLC). Two groups of male Sprague Dawley rats were either injected intraperitoneally with 1 ml of IL-1alpha (2 microg/ml) or had IL-1alpha 2 microl (100 ng/ml) infused directly into the medial hypothalamus. A significant increase in extracellular hypothalamic NE was observed in the animal group treated with IL-1alpha intraperitoneally and not in the controls or the group treated with IL-1alpha intracerebrally. One-way ANOVA showed a significant effect of drug and route of administration with the ip IL-1alpha treated group, differing from all other groups (vehicle ip, IL-1alpha ic, and vehicle ic). Therefore these findings suggest that some aspects of IL-1alpha actions on the HPA may be indirect requiring other intermediate steps or mediators outside the central nervous system.

Interleukin-1 and nociception in the rat

It has recently become accepted that several cytokines may affect peripheral and central nervous system functions. Consistently with these findings, accumulating evidence points toward an important role for interleukin- in the modulation of nociceptive information. Here we review the observations collected after the administration of this cytokine by intracerebroventricular, intrathecal or peripheral route in rats. Taken together, these data suggest that IL-1 can differently affect pain responsivity depending on the dose and the site of action, and clearly demonstrate that this immune factor is deeply involved in the modulation of neuronal functions.

GM-CSF affects hypothalamic neurotransmitter levels in mice: involvement of interleukin-1

We studied the effects of granulocyte-macrophage colony stimulating factor (GM-CSF) on the brain levels of several neurotransmitters in mice. Administration of GM-CSF (5.0 and 10 microg, i.p.) significantly reduced the hypothalamic levels of glutamine, glutamic acid, GABA and aspartic acid. GM-CSF (5.0 microg, i.p.) also induced a significant reduction of norepinephrine and serotonin levels in the hypothalamus, without affecting dopamine levels. The hippocampal levels of neurotransmitters were not modified by GM-CSF administration. The peripheral administration of a specific interleukin-1 receptor antagonist (IL-1ra, 50 microg, i.p.) blocked the effects of GM-CSF. These results confirm our previous behavioural data suggesting that GM-CSF is able to exert neuromodulatory actions.