The role of adrenocorticotropic hormone in inhibition of pain reaction in awake rats

The effect of hormones of hypothalamic-pituitary-adrenocortical system on pain sensitivity were studied in experiments on awake Sprague-Dawley males rats. Pain sensitivity was tested by tail flick reaction induced by thermal stimuli. Systemic glucocorticoids and ACTH injection increased the tail flick latency. The ACTH-induced analgesic effect was unaffected by deficiency of glucocorticoids production in pretreatment with pharmacological dose of cortisol but was fully eliminated by pretreatment with opiate antagonist naltrexone. These findings suggest that ACTH-induced analgesic effect is mediated by opiate receptors but not by glucocorticoids released in response to ACTH injection.     

The role of hormones of hypothalamic-pituitary-adrenocortical system in regulation of pain sensitivity

The review focuses on the role of hypothalamic-pituitary-adrenocortical system (HPAS) in regulation of pain sensitivity and discusses the mechanisms involved in this process. Analgesic effects of exogenous hormones of HPAS (corticotropin-releasing hormone (CRH), ACTH, glucocorticoids) have been shown in rats. It is mediated by both opioid and non-opioid mechanisms. Endogenous glucocorticoids produce development of analgesia mediated by non-opioid mechanisms. Analgesic effect of ACTH is mediated by both non-glucocorticoids mechanisms associated with endogenous glucocorticoids and opioid mechanisms. In contrast to ACTH, analgesic effect of CRH is mediated only by non-opioid mechanisms associated or dissociated with endogenous glucocorticoids. The neurons of midbrain periaqueductal gray matter may be involved in the analgesia induced by glucocorticoids.     

Effect of ascorbic acid on ACTH-induced cyclic AMP formation and steroidogenesis in isolated adrenal cells of vitamin E-deficient rats.

Isolated adrenal cells from Vitamin E-deficient and control rats were prepared by a trypsin digestion method. Cyclic adenosine 3',5'-monophosphate (cyclic AMP) formation was studied in response to adrenocorticotropin (ACTH) in the presence and absence of ascorbate by measuring the conversion of prelabeled adenosine 5'-triphosphate [14C]ATP to cyclic [14C]AMP. Ascorbate (0.5 mM) inhibited ACTH-induced cyclic [14C]AMP formation in adrenal cells isolated from Vitamin E-deficient rats but had no effect in the control cells. The inhibitory effect of ascorbate on ACTH-induced cyclic AMP formation in Vitamin E-deficient rats decreased as the concentration of ACTH increased. In Vitamin E-deficient rats ascorbate inhibited ACTH-induced cyclic [14C]AMP formation after 30 min of incubation. There was no further significant accumulation of cyclic [14C]AMP at 60 min or 120 min although in the absence of ascorbate cyclic [14C]AMP continued to be formed. The in vitro addition of alpha-tocopherol reduced the inhibition of ACTH-induced cyclic [14C]AMP formation by ascorbate in Vitamin E-deficient rats. These studies suggest that alpha-tocopherol and ascorbate may affect ACTH-induced cyclic AMP formation through interaction with the membrane-bound enzyme adenylate cyclase.     

Glucocorticoid control of steroidogenesis in isolated rat adrenocortical cells

The role of end-product glucocorticoids in the regulation of corticosteroidogenesis in isolated adrenocortical cells was investigated. Trypsin-isolated cells from male rat adrenal glands were incubated with or without corticotropin (ACTH) and with or without corticosterone. Endogenous corticosterone production was determined by radioimmunoassay at the end of incubation. Cessation of ACTH-induced corticosterone production was apparent after 2-4 h of incubation. The suppression occurred later with lower cell concentrations. Corticosterone production was partially restored after washing the suppressed cells. Supernatant fluid from suppressed cell suspensions also suppressed steroidogenesis of a fresh population of cells. However, the suppressing property of the supernatant fluid was abolished after the removal of corticosterone by charcoal-dextran treatment, suggesting that corticosterone or other steroids caused the suppression. Exogenous corticosterone induced suppression over a wide range of ACTH concentrations, but did not change the half-maximal steroidogenic concentration of ACTH, indicating that the suppression does not change the sensitivity of the cells to ACTH. Suppression occurred within 30-60 min after corticosterone had been added to the incubation medium either at the start of incubation or while steroidogenesis was in progress. Suppression varied directly with the concentration of exogenous corticosterone. These data indicate that glucocorticoids can directly and acutely suppress corticosteroidogenesis and thus control adrenocortical function in concert with other regulators such as ACTH and Ca2+.     

Central corticotropin-releasing factor (CRF) may attenuate somatic pain sensitivity through involvement of glucocorticoids

Corticotropin-releasing factor (CRF) is an important regulator of physiological functions and behavior in stress. Analgesia is one of the characteristics of stress reaction and CRF is involved in providing stress-induced analgesia, however, the underlying mechanisms remain to be determined. Exogenous CRF mimics stress effects on pain sensitivity and causes analgesic effect. The present study was performed to investigate the participation of endogenous glucocorticoids in analgesic effects induced by central administration of CRF in anesthetized rats. The participation of glucocorticoids was studied by pharmacological suppression of the hypothalamic-pituitary-adrenocortical (HPA) axis as well as an occupation of glucocorticoid receptors by its antagonist RU 38486. Since CRF administration causes the release of β-endorphin from the pituitary, the opioid antagonist naltrexone was used to determine the contribution of opioid-dependent mechanism to CRF-induced analgesia. An electrical current threshold test was applied for measurement of somatic pain sensitivity in anesthetized rats. Intracerebroventricular administration of CRF (2 μg/rat) caused analgesic effects (an increase of pain thresholds) and an increase in plasma corticosterone levels. Pretreatment with naltrexone did not change analgesic effects of central CRF as well as corticosterone levels in blood plasma. However, pharmacological suppression of the HPA axis leading to an inability of corticosterone release in response to CRF resulted in an elimination of CRF-induced analgesic effects. Pretreatment with RU 38486 also resulted in an elimination of CRF-induced effects. The data suggest that CRF-induced analgesic effects may be mediated by nonopioid mechanism associated with endogenous glucocorticoids released in response to central CRF administration.     

白细胞介素－2的中枢镇痛作用

本实验采用侧脑室给药,以钾离子透入法引起大鼠甩尾反应为指标,测定动物的痛阈,发现白细胞介素－２（ＩＬ－２）具有显著提高大鼠痛阈的作用,此作用能被抗ＩＬ－２单克隆抗体所阻断。纳洛酮能反转ＩＬ－２的镇痛作用,表明其作用机理与阿片受体有关。     

ACTH and corticosterone response to naloxone and morphine in normal, hypophysectomized and dexamethasone-treated rats

The effect of opiate receptors blocker naloxone on ACTH and corticosterone secretion in normal, dexamethasone-treated and hypophysectomized rats was studied. A dose-related increase in plasma corticosterone level was found at 45 min after s.c. injection of naloxone in a dose range of 0.25-2.0 mg kg-1. The rise in plasma corticosterone was preceded by a slight increase in plasma ACTH. Acute morphine administration in a relatively low dose (6 mg kg-1 s.c.) induced a significant rise in both plasma ACTH and corticosterone levels. Dexamethasone treatment was followed by low basal corticosterone level, by total inhibition of the stress response and response to morphine injection, while the response to ACTH administration was normal. Under these circumstances as well as in rats 6 days after hypophysectomy, naloxone failed to increase plasma corticosterone levels. It is concluded that a direct stimulation of corticosteroid biosynthesis in adrenal cortex is not involved in the mechanism of naloxone-induced activation of pituitary-adrenocortical function.     

Effect of ascorbic acid on ACTH-induced cyclic AMP formation and steroidogenesis in isolated adrrenal cells of vitamin E-deficient rats

Isolated adrenal cells from Vitamin E-deficient and control rats were prepared by a trypsin digestion method. Cyclic adenosine 3′,5′-monophosphate (cyclic AMP) formation was studied in response to adrenocorticotropin (ACTH) in the presence and absence of ascorbate by measuring the conversion of prelabeled adenosine 5′-triphosphate [¹⁴C]ATP to cyclic [¹⁴C]AMP. Ascorbate (0.5 mM) inhibited ACTH-induced cyclic [¹⁴C]AMP formation in adrenal cells isolated from Vitamin E-deficient rats but had no effect in the control cells. The inhibitory effect of ascorbate on ACTH-induce cyclic AMP formation in Vitamin E-deficient rats decreased as the concentration of ACTH increased. In Vitamin E-deficient rats ascorbate inhibited ACTH-induced cyclic [¹⁴C]AMP formation after 30 min of incubation. There was no further significant accumulation of cyclic [¹⁴C]AMP at 60 min or 120 min although in the absence of ascorbate cyclic [¹⁴C]AMP continued to be formed. The in vitro addition of α-tocopherol reduced the inhibition of ACTH-induced cyclic [¹⁴C]AMP formation by ascorbate in Vitamin E-deficient rats.These studies suggest that α-tocopherol and ascorbate may affect ACTH-induced cyclic AMP formation through interaction with membrane-bound enzyme adenylate cyclase.     

Naloxone does not antagonize PCP-induced stimulation of the pituitary-adrenal axis in the rat

Phencyclidine (PCP) has been shown to stimulate the pituitary-adrenal axis in the rat. The purpose of the present study was to determine whether opiate receptors are involved in this effect by testing whether pretreatment with the opiate antagonist naloxone can antagonize PCP-induced ACTH and corticosterone release. PCP (10.0 mg/kg) produced increases in plasma ACTH and corticosterone 60 min after s.c. administration. Pretreatment with naloxone (2.0 mg/kg s.c.) did not reduce the rise in plasma levels of ACTH or corticosterone produced by PCP. These results indicate that naloxone-sensitive opiate receptors are not involved in the PCP-induced stimulation of the pituitary-adrenal axis in rats.     

Thermoregulatory (core, surface and metabolic) responses of unrestrained rats to repeated POAH injections of beta-endorphin or adrenocorticotropin

Repeated preoptic-anterior hypothalamic (POAH) injections of saline and 10 or 25 micrograms/microliters of beta-endorphin or ACTH were given to groups of male Sprague-Dawley rats. One hr after the fifth injection of beta-endorphin or ACTH, each rat received a POAH injection of naloxone HCl (10 micrograms/microliters). Core (Tre-rectal) and surface (Tt-tail) temperatures, metabolic (VO2) and behavioral responses were recorded 30 min before and 60 min after each drug injection. The initial POAH injection of either dose of beta-endorphin produced a hyperthermia. Peak hyperthermia was reduced in the group given 10 micrograms/microliters of beta-endorphin repeatedly. TtS rose after each beta-endorphin injection but temporally lagged Tre increases. Metabolic rate (VO2) was increased with repeated POAH injections of beta-endorphin. Naloxone reduced the elevated Tre seen with beta-endorphin by increasing Tt's further and reducing VO2. POAH administration of ACTH evoked only a slight hyperthermic Tre response, but elevated TtS and VO2S, due to enhanced grooming and explorative behavior. With repeated ACTH injections, TreS did not change from those on the first day as TtS and VO2 remained enhanced. Naloxone reduced VO2 and TtS of the ACTH-treated rats but TreS still were unchanged. Results suggest that the hyperthermia of unrestrained rats given an acute as opposed to repeated POAH beta-endorphin injections is mediated by different effector mechanisms. With the doses used, the slight and unchanging TreS seen with ACTH occurred because this peptide increased heat production due to locomotor activation yet also exaggerated heat loss by vasodilating the peripheral vasculature.     

Analgesic effect of interferon-alpha via mu opioid receptor in the rat

Using the tail-flick induced by electro-stimulation as a pain marker, it was found that pain threshold (PT) was significantly increased after injecting interferon-alpha (IFN alpha) into the lateral ventricle of rats. This effect was dosage-dependent and abolished by monoclonal antibody (McAb) to IFN alpha. Naloxone could inhibit the analgesic effect of IFN alpha, suggesting that the analgesic effect of IFN alpha be related to the opioid receptors. Beta-funaltrexamine (beta-FNA), the mu specific receptor antagonist could completely block the analgesic effect of IFN alpha. The selective delta-opioid receptor antagonist, ICI174,864 and the kappa-opioid receptor antagonist, nor-BNI both failed to prevent the analgesic effect of IFN alpha. IFN alpha could significantly inhibit the production of the cAMP stimulated by forskolin in SK-N-SH cells expressing the mu-opioid receptor, not in NG108-15 cells expressing the delta-opioid receptor uniformly. The results obtained provide further evidence for opioid activity of IFN alpha and suggest that this effect is mediated by central opioid receptors of the mu subtype. The evidence is consistent with the hypothesis that multiple actions of cytokines, such as immunoregulatory and neuroregulatory effects, might be mediated by distinct domains of cytokines interacting with different receptors.     

Lack of inhibition of ACTH-induced aldosterone stimulation by des-asp1-,ileu8-angiotensin II in man.

In 5 normal men an intravenous injection of 0.5 mg of synthetic 1-24 ACTH caused a significant increase in plasma aldosterone and a simultaneous intravenous infusion of 600 ng/kg/min of des-asp1-, ileu8-angiotensin II (AIIIA) did not inhibit this increase. Since this dose of AIIIA is known to inhibit an angiotensin II-induced increase in plasma aldosterone in normal men, the present results suggest that the ACTH-induced aldosterone stimulation is mediated by an adrenocortical receptor which is different from angiotensin II receptors.     

L-364,718 Potentiates Electroacupuncture Analgesia Through Cck-A Receptor of Pain-Related Neurons in the Nucleus Parafascicularis

Electroacupuncture (EA) has been successfully used to alleviate pain produced by various noxious stimulus. Cholecystokinin-8 (CCK-8) is a neuropeptide involved in the mediation of pain. We have previously shown that CCK-8 could antagonize the analgesic effects of EA on pain-excited neurons (PENs) and pain-inhibited neurons (PINs) in the nucleus parafascicularis (nPf). However, its mechanism of action is not clear. In the present study, we applied behavioral and neuroelectrophysiological methods to determine whether the mechanisms of CCK-8 antagonism to EA analgesia are mediated through the CCK-A receptors of PENs and PINs in the nPf of rats. We found that focusing radiant heat on the tail of rats caused a simultaneous increase in the evoked discharge of PENs or a decrease in the evoked discharge of PINs in the nPf and the tail-flick reflex. This showed that radiant heat could induce pain. EA stimulation at the bilateral ST 36 acupoints in rats for 15 min resulted in an inhibition of the electrical activity of PEN, potentiation of the electrical activity of PIN, and prolongation in tail-flick latency (TFL), i.e. EA stimulation produced an analgesic effect. The analgesic effect of EA was antagonized when CCK-8 was injected into the intracerebral ventricle of rats. The antagonistic effect of CCK-8 on EA analgesia was reversed by an injection of CCK-A receptor antagonist L-364,718 (100 ng/μl) into the nPf of rats. Our results suggest that the pain-related neurons in the nPf have an important role in mediating EA analgesia. L-364,718 potentiates EA analgesia through the CCK-A receptor of PENs and PINs in the nPf.     

The effect of exposure to positive space charge on aversive responses to noxious stimuli in rats

Exposure of rats to positive atmospheric ions for four days or longer results in an increase in aversive response times (tail flick and hot-plate). While these effects are similar to those produced after opiate administration, they could not be inhibited by naloxone treatment. Prompt reversal of the positive ion analgesic effect, however was brought about by treatment with parachlorophenylalanine which lowers brain serotonin levels. The data suggest that the action of positive ions on the pain inhibitory system is dependent upon an intact serotonin pathway and that the effects on this system are mediated via central rather than peripheral neural mechanisms.     

Analysis of the dual mechanism of ACTH release by arginine vasopressin and its analogs in conscious rats

Plasma ACTH and/or corticosterone levels were measured in conscious rats 30 min after subcutaneous administration of arginine vasopressin (AVP), oxytocin (OT) and various analogs with a large range of activity on the vasopressor (V1), antidiuretic (V2) or oxytocic receptors. The comparison of their dose-response curves indicated that two different mechanisms are involved in the release of ACTH by neurohypophysial peptides and their analogs. AVP itself and a specific vasopressor agonist (Phe2, Orn8, OT) displayed a similar, high slope dose-response curve. Non-vasopressor analogs, such as dDAVP were characterized by a low slope dose-response curve. Furthermore, dDAVP potentiated CRF and neither its own ACTH-releasing action nor its potentiation of CRF were sensitive to previous VI- or V2-receptor blockade. These results, together with other available data, are interpreted as indicative of the existence of two mechanisms of action for ACTH release by AVP and its analogs in vivo: an indirect action via endogenous CRF release, mediated by a VI receptor mechanism, and a direct action on the pituitary, shared by dDAVP and other non-vasopressor analogs, with receptor characteristics different to both the V1 and the V2 classical types.     

Analgesic and Antipyretic Activities of a Novel Tetrapeptide in Rats

The efficacy of a novel tetrapeptide sequence, FLPS (Phe-Leu-Pro-Ser), to alleviate severe pain associated with surgical incision is demonstrated in the Brennan model, a model used for developing new drugs for postoperative pain in humans. The tetrapeptide (100 mg/kg dose) administered by subdermal injection completely alleviated post-incisional pain in rats using the hindpaw withdrawal as an endpoint response. When the tetrapeptide (0.15 mg/paw) was topically applied to the vicinity of the surgical wound, it also alleviated pain. Statistically significant increases in pain threshold (assessed by von Frey filaments pressed against the surgical wound, 15–20 min after dosing) were observed on the day of surgery and on the third day post-surgery. Up to a 0.5°C decrease in body temperature under basal conditions and yeast-provoked pyrexia was observed at doses that alleviate pain. The tetrapeptide does not exhibit any significant anti-edema activity in carrageenan-induced hindpaw edema, and does not affect human recombinant cyclooxygenase-2 activity, indicating that the analgesic property of the tetrapeptide is unlikely to be mediated through inflammatory pathways. The tetrapeptide at 10 μM, a dose that is sufficient to increase the pain threshold in rats, does not compete with naloxone for the opioid receptors in membrane preparations from rat brain, indicating that it does not mediate its effect through the opioid receptors. It also does not bind to the vanilloid receptor, indicating that peripheral vanilloid receptors are not involved in pain relief by the tetrapeptide.     

Increased ACTH and cortisol secretion after interleukin-alpha injection in the common marmoset (Callithrix jacchus jacchus)

We have studied the effect of intravenous injection of interleukin-1 (dose range: from 0.25 to 4.5 microg/kg of body weight) on plasma ACTH and cortisol levels in the marmoset, a primate paradygm of peripheral glucocorticoid resistance. Blood sampling were collected and body temperature recorded 0, 15, 30, 60, 120, 180, 240 and 300 min after injection. Interleukin-1 stimulated secretion of ACTH in a dose-dependent fashion. Maximal secretion occurred 120 min after injection, and lasted up to 240 min. Plasma ACTH levels returned to baseline 300 min after interleukin-1 injection. Plasma cortisol levels were related to ACTH levels. Body temperature elevation, which occurred 10-15 min after injection was dose-dependent, and lasted 3 h. Results suggest that the pyrogenic effect of interleukin is associated, in the marmoset, with integrated activation of the hypothalamic-pituitary-adrenal axis. In light of the proneness of marmosets to hyperimmune disorders, our data are consistent with the hypothesized central biological role of IL-1, as well as the pathophysiological relevance of the neuro-endocrine-immune cross-talk during the acute phase response.     

Secretion of aldosterone and corticosterone by the adrenals of Brattleboro rats in response to ACTH administration

The secretions of aldosterone and corticosterone in response to administration of 0,5 mUI of (1,24) ACTH (synacthène-Ciba) were measured in the adrenal venous blood of 15 Brattleboro female rats genetically lacking vasopressin and in 15 Long-Evans female rats, pretreated with dexamethasone. The secretions of aldosterone and corticosterone increased according to a similar profile in the two groups of animals: maximum values were 20-30 min. after ACTH injection; however the steroidogenic secretion of the adrenal cortex was always about 50% less in the Brattleboro female rats than in Long-Evans female rats. This result suggests mainly that vasopressin may be involved in the mechanisms which control the in vivo production of aldosterone by the adrenal glomerulosa cells.     

Calcitonin induced increase in ACTH, beta-endorphin and cortisol secretion

The response of ACTH, beta-endorphin and cortisol to calcitonin administration was investigated in 8 subjects with recent fractures of the vertebrae due to postmenopausal or senile osteoporosis (Ost) and in seven normal healthy controls (NC). A significant increase of the three hormones was observed in 13 subjects. The maximum increase was observed between 15 and 60 min.: the cortisol level (microgram/100 ml) rose from 14.3 +/- 1.9 to 24.8 +/- 3.2 (P less than 0.05) in Ost and from 7.7 +/- 0.6 to 21.7 +/- 1.7 (P less than 0.001) in NC, the beta-endorphin (pmol/l) from 5.8 +/- 0.6 and to 21.2 +/- 1.3 in OST (P less than 0.001) and from 5.9 +/- 0.4 to 21.9 +/- 4.5 (P less than 0.01) in NC and the ACTH levels (pg/ml) from 21.3 +/- 5.7 to 61.7 +/- 3.6 (P less than 0.001) in OST and from 30.0 +/- 6.2 to 58.8 +/- 7.5 (P less than 0.05) in NC. The results indicate a possible role of calcitonin in modulating the anterior pituitary function. It also suggests that the analgesic effect of calcitonin might be mediated by the increase of beta-endorphin. The possibility that this analgesic effect of calcitonin is due to its direct binding to the opiate receptors was excluded in the present study by in vitro binding assay.