Mechanisms of the Analgesic Effect of Corticotropin-Releasing Factor in Conscious Rats

We report here our studies of the contribution of the hypothalamo-hypophyseal-adrenocortical system (HHACS) and opioid system to the analgesic effect of corticotropin-releasing factor (CRF) in conscious rats exposed to thermal stimuli. Two methodological approaches were used: 1) blockade of the functional activity of the HHACS by administration of hydrocortisone at a pharmacological dose one week before experiments; 2) blockade of glucocorticoid receptors with the glucocorticoid receptor antagonist RU38486. The contribution of the opioid system was studied by blockade of opioid receptors with their antagonist naltrexone. Blockade of opioid receptors completely eliminated the analgesic effect of CRF, blockade of the functional activity of the HHACS decreased it, and blockade of glucocorticoid receptors increased the effect. These data provide evidence that the analgesic effect of CRF in conscious rats exposed to a thermal stimulus is mediated by an opioid-associated mechanism. The actions of opioids on pain sensitivity may be modulated by glucocorticoid hormones.     

Analgesic Actions of Corticotropin-Releasing Factor (CRF) on Somatic Pain Sensitivity: Involvement of Glucocorticoid and CRF-2 Receptors

The aim of the present work was to study the involvement of glucocorticoid receptors and corticotropinreleasing factor type 2 receptors (CRF-2 receptors) in mediating the analgesic effects of CRF on somatic pain sensitivity. The involvement of glucocorticoid and CRF-2 receptors in the development of analgesia evoked by systemic administration of CRF was studied by blockade of these receptors by their specific antagonists RU 38486 and astressin 2-B, respectively, in anesthetized rats. Pain sensitivity was tested before and 30 min after administration of CRF in terms of the threshold of the pain reaction induced by stimulation of the rat’s tail with an electric current. Blockade of glucocorticoid receptors induced partial suppression of the analgesic action of CRF, while blockade of CRF-2 receptors produced complete suppression of the analgesic effect. These results provide evidence that glucocorticoid and CRF-2 receptors are involved in mediating the analgesic effects of CRF.     

Analgesic effect of corticotropin-releasing hormone: Involvement of the hypothalamic-pituitary-adrenocortical axis into its realization

Involvement of hypothalamic-pituitary-adrenocortical axis into the analgesic effect of corticotropin-releasing hormone after its systemic administration was studied in experiments on rats. Pharmacological blockade of the hypothalamic-pituitary-adrenocortical axis decreased the duration and degree of the analgesic effect of corticotropin-releasing hormone. This analgesic effect can be mediated via two pathways: related to hormones of the hypothalamic-pituitary-adrenocortical axis and independent of these hormones. __________ Translated from Byulleten’ Eksperimental’noi Biologii i Meditsiny, Vol. 141, No. 2, pp. 144–146, February, 2006     

Prenatal dexamethasone exposure affects anxiety-like behaviour and neuroendocrine systems in an age-dependent manner

Prenatal stress has been reported to alter the development of the central nervous system functions. This alteration is thought to be partly caused by increased fetal exposure to glucocorticoid. To clarify how prenatal stress affects neuroendocrine systems and behaviour in an age-dependent manner, we administered a synthetic glucocorticoid, dexamethasone, as a stressor to pregnant rats at gestational days 16–21 and examined the developmental changes in behaviour, hypothalamic corticotropin-releasing factor mRNA expression, corticosterone response and glucocorticoid receptor expression in male offspring. Prenatal dexamethasone exposure decreased corticotropin-releasing factor mRNA in the hypothalamus and disturbed the plasma corticosterone response to restraint stress in the offspring at postnatal week 4 (PW4). In contrast, it was not until PW10 that increased anxiety-like behaviour emerged in the dexamethasone-exposed offspring. In association with the acquisition of increased anxiety-like behaviour at PW10, glucocorticoid receptor expression was decreased in the amygdala in dexamethasone-exposed offspring at PW7 and PW10. Thus, our longitudinal analysis suggests that prenatal exposure to glucocorticoid hampers neuroendocrinological development in the offspring during early life, and that this disturbance results in the induction of increased anxiety-like behaviour in adulthood.     

促肾上腺皮质激素对慢性痛大鼠痛行为和海马内BDNF、CRF水平的影响

目的与方法:采用痛行为评分方法、免疫组化和原位杂交技术,观察促肾上腺皮质激素(ACTH)对完全福氏佐剂所致的关节炎大鼠海马内脑源性神经营养因子(BDNF)及其功能性受体trkB和促肾上腺皮质激素释放激素(CRH)水平的影响。结果:关节炎大鼠的痛行为评分显著高于正常对照组,同时注射侧对侧海马内BDNF免疫活性(IR)神经元、CRHmRNA阳性神经元和BDNF/CRHmRNA双染神经元数在注射佐剂后 2 4h显著高于正常对照组,而腹腔注射ACTH(2 5IU/kg或 12 5IU/kg)后,上述指标显著低于关节炎组;摘除双侧肾上腺后,腹腔注射ACTH的关节炎大鼠对侧海马内BDNF-IR、CRHmRNA阳性神经元和BDNF/CRHmRNA双染神经元数明显高于未摘除肾上腺的关节炎组。结论:海马内的BDNF和CRH参与慢性痛的调制,ACTH能抑制海马内BDNF和CRH的升高而产生镇痛作用,肾上腺对ACTH发挥其功能起决定性作用.     

Suppression by glucocorticoid of the immunoreactivity of corticotropin-releasing factor and vasopressin in the paraventricular nucleus of rat hypothalamus

The effect of glucocorticoid on the production of corticotropin-releasing factor (CRF) and vasopressin in the paraventricular nucleus of the hypothalamus (PVH) was examined immunocytochemically. Intraperitoneal administration of dexamethasone sulfate in a dose of 0.1 mg/day suppressed the immunoreactivity of CRF and vasopressin in the medial parvocellular divisions of the PVH of the rat subsequent to bilateral adrenalectomy. In the magnocellular divisions, suppression of vasopressin-immunoreactivity was not observed. These results suggest that the vasopressin in the medial parvocellular divisions plays a distinct role from that in the magnocellular divisions, the former having functional significance in the hypothalamo-hypophysio-adrenal axis.     

Expression of c-fos in regions of the basal limbic forebrain following intracerebroventricular corticotropin-releasing factor in unstressed or stressed male rats.

Corticotropin-releasing factor has an integrative role on the behavioral, endocrine and autonomic responses to stress. Immediate-early gene (c-fos) expression was used to determine patterns of neural activity in the limbic system following i.c.v. infusion of corticotropin-releasing factor. Either 250 or 1000 pmol corticotropin-releasing factor infused into the lateral ventricle of precannulated and handled male rats resulted in marked c-fos expression 60 or 120 min later in localized regions of the basal forebrain, including the ventrolateral septum, the dorsal and medial parvicellular divisions of the paraventricular nucleus, the central nucleus of the amygdala, and dorsal bed nucleus of the stria terminalis. Pre-infusion of alpha-helical corticotropin-releasing factor (2500 pmol), a competitive corticotropin-releasing factor antagonist of corticotropin-releasing factor, had no effect on immediate-early gene expression alone but reduced that elicited by exogenous i.c.v. corticotropin-releasing factor (250 pmol)--in some areas to control levels. Fifteen minutes of restraint stress, a situation in which corticotropin-releasing factor is released endogenously, also activated c-fos expression in a pattern that resembled corticotropin-releasing factor infusions but was not identical. There was enhanced expression in the dorsal and medial areas of the paraventricular nucleus, but not its magnocellular region, and increased expression in the ventrolateral septum; however, there was no detectable response on the central amygdala. Preinfusion of alpha-helical corticotropin-releasing factor (2500 pmol) had no significant effect on stress-induced c-fos expression in the ventrolateral septum or paraventricular nucleus. This suggests that corticotropin-releasing factor release may form only a part of the central neurochemical response to restraint stress. Rats given i.c.v. corticotropin-releasing factor (250 pmol) before restraint stress showed additive effects on c-fos in the ventrolateral septum but not in the paraventricular nucleus; the central nucleus of the amygdala reacted as if corticotropin-releasing factor alone had been infused. Corticosterone levels were raised by both stress and corticotropin-releasing factor, but pretreatment with alpha-helical corticotropin-releasing factor reduced them after either procedure, which correlates with c-fos expression in the paraventricular nucleus and ventrolateral septum. These results show that corticotropin-releasing factor induces a specific pattern of c-fos expression in localized regions of the amygdala, hypothalamus and septum, which may indicate a corresponding pattern of neural activation. Restraint, one form of stress, activates c-fos in a similar but not identical manner, suggesting that corticotropin-releasing factor may not be the only neuropeptide involved in the response to this stressor.(ABSTRACT TRUNCATED AT 400 WORDS)     

Differential alpha-1 and alpha-2 adrenergic effects on hypothalamic corticotropin-releasing factor and plasma adrenocorticotropin

There is presently no consensus as to the nature of the catecholaminergic influence on the regulation of corticotropin-releasing factor. The potential role that the alpha-adrenergic system plays was investigated by measuring hypothalamic corticotropin-releasing factor-like immunoreactivity and plasma adrenocorticotropin, following manipulation of alpha-1 and alpha-2 adrenergic receptor activation. Administration of the alpha-1 agonist methoxamine did not significantly alter either plasma adrenocorticotropin or hypothalamic corticotropin-releasing factor. Administration of the alpha-2 agonist clonidine resulted in a 24-fold increase in plasma adrenocorticotropin and a significant decrease in median eminence corticotropin-releasing factor, consistent with its release. Corticotropin-releasing factor in the remainder of the hypothalamus was not altered. Concurrent administration of clonidine with the selective alpha-2 antagonist yohimbine prevented the clonidine-induced changes in plasma adrenocorticotropin and hypothalamic corticotropin-releasing factor, consistent with the clonidine effect being mediated through alpha-2 receptors. Concurrent administration of clonidine with methoxamine did not prevent these effects, suggesting that the effect of clonidine was not mediated through presynaptic inhibition of noradrenergic adrenergic neurotransmission. Inhibition of protein synthesis by anisomycin induced changes in corticotropin-releasing factor and adrenocorticotropin which were not altered by combined treatment with methoxamine or clonidine. These data suggest differential roles for alpha-1 and alpha-2 systems in the regulation of corticotropin-releasing factor. Results from alpha-2 adrenergic activation were consistent with stimulation of corticotropin-releasing factor release, an effect mediated by a postsynaptic alpha-2 mechanism.(ABSTRACT TRUNCATED AT 250 WORDS)     

The corticotropin-releasing factor stimulation test. An aid in the evaluation of patients with Cushing's syndrome

We investigated the effect of exogenous corticotropin-releasing factor on plasma levels of ACTH and cortisol in 13 patients with ACTH-secreting pituitary adenomas (Cushing's disease) and in 9 patients with other forms of Cushing's syndrome. In all patients with Cushing's disease, ovine corticotropin-releasing factor, given intravenously as a bolus injection (1 microgram per kilogram of body weight), caused a further increase in the already elevated levels of ACTH and cortisol. Successful transphenoidal adenomectomy was followed as early as one week after surgery by normalization or near-normalization of the ACTH and cortisol responses to corticotropin-releasing factor. On the other hand, patients with the ectopic ACTH syndrome, who also had high basal plasma concentrations of ACTH and cortisol, had no ACTH or cortisol responses to corticotropin-releasing factor. This difference in responsiveness between these two patient groups cannot be explained on the basis of different metabolic clearance rates of exogenous corticotropin-releasing factor, as shown by similar disappearance curves of immunoreactive corticotropin-releasing factor from plasma. Patients with Cushing's syndrome of adrenal origin who were hypercortisolemic during testing had undetectable plasma levels of ACTH and no ACTH or cortisol responses to corticotropin-releasing factor. We conclude that stimulation of the pituitary-adrenal axis with corticotropin-releasing factor may be useful in differentiating pituitary from ectopic causes of Cushing's syndrome.     

Progesterone Suppression of Pregnancy Lymphocytes Is not Mediated by Glucocorticoid Effect

ABSTRACT: This study investigated whether the suppressive effect of progesterone on pregnancy lymphocytes is mediated by specific progesterone receptors. The effects of a competitive progesterone antagonist (RU486) and a specific glucocorticoid receptor blocker (RU43044) were tested on the release of a blocking factor by progesterone-treated pregnancy lymphocytes. RU 486 tested at an equal concentration as progesterone significantly inhibited the production of the blocking factor, while RU 43044 was without effect. These data suggest that in pregnancy, lymphocyte progesterone acts on specific progesterone receptors and glucocorticoid binding sites are not involved.     

Intracerebroventricularly administered corticotropin-releasing factor releases somatostatin through a cholinergic,vagal pathway in freely fed rats

The aim of this study was to investigate whether corticotropin-releasing factor influences the plasma levels of somatostatin, gastrin or cholecystokinin when administered intracerebroventricularly to rats, and if such an effect could be vagally mediated, and dependent on the animals feeding states. Anaesthetized, freely fed rats were given 5 μl intracerebroventricular injections of corticotropin-releasing factor in four doses; 10 pmol-1.28 nmol. Immediately following death, trunk blood was collected for subsequent peptide analysis with radioimmunoassay (RIA). The three higher doses of corticotropin-releasing factor elevated the plasma levels of somatostatin (P < 0.01) after 20 min but left the plasma levels of gastrin and cholecystokinin unchanged. Intraperitoneal injections of 60 and 320 pmol of corticotropin-releasing factor did not influence the somatostatin levels. Further, intracerebroventricular injections of 60 pmol of corticotropin-releasing factor produced a peak increase in somatostatin after 20 min (P < 0.01). After 60 min the somatostatin levels were still increased (P < 0.05). Gastrin and cholecystokinin remained unaltered at these timepoints. Intracerebroventricular administration of 10 nmol of a-helical corticotropin-releasing factor 9–41 attenuated the basal levels of somatostatin and blocked the corticotropin-releasing factor-induced rise in somatostatin. Bilateral truncal vagotomy, as well as pretreatment with atropine (0.05 mg kg^-1, subcutaneously) abolished the effects of corticotropin-releasing factor on somatostatin. In animals which were food-deprived for 24 h, corticotropin-releasing factor did not influence somatostatin, gastrin or cholecystokinin. Pretreatment with cholecystokinin did not potentiate corticotropin-releasing factor-induced somatostatin release in food-deprived rats. These findings suggest that corticotropin-releasing factor acting within the central nervous system may regulate gastrointestinal functions partially through a cholinergic, vagally mediated release of somatostatin in freely fed, but not in food-deprived rats.     

Differential distribution of urocortin- and corticotropin-releasing factor-like immunoreactivities in the rat brain.

Urocortin, a novel 40 amino acid neuropeptide, is a member of the corticotropin-releasing factor family. With 45% homology to corticotropin-releasing factor, urocortin binds with similar affinity to the corticotropin-releasing factor- and corticotropin-releasing factor-2 receptors and may play a role in modulating many of the same systems as corticotropin-releasing factor. To assess whether urocortin and corticotropin-releasing factor are localized in the same regions of the brain, we compared the distribution of urocortin- and corticotropin-releasing factor-like immunoreactivities in the rat central nervous system. Polyclonal antibodies to rat corticotropin-releasing factor and rat urocortin were generated and utilized to map the distribution of corticotropin-releasing factor- and urocortin-like immunoreactivities throughout the rat forebrain and brainstem. Characterization of the antibodies by radioimmunoassay showed no cross-reactivity with related peptides. Male Sprague-Dawley rats were treated with colchicine for 18-24 h. Following colchicine treatment, the rats were perfused with paraformaldehyde-lysine-periodate fixative and their brains removed. Serial coronal sections were taken throughout the rat brain and processed for either corticotropin-releasing factor- or urocortin-like immunoreactivity. Urocortin-like immunoreactivity shows a discrete localization within several regions including the supraoptic nucleus, the median eminence, Edinger-Westphal nucleus and the sphenoid nucleus. This is in contrast to the more abundant corticotropin-releasing factor-like immunoreactivity. Regions containing high levels of corticotropin-releasing factor immunoreactivity include the lateral septum, paraventricular nucleus of the hypothalamus, median eminence and locus coeruleus. There are a few regions that contain both urocortin-immunoreactive and corticotropin-releasing factor-immunoreactive cells, such as the supraoptic nucleus and the hippocampus. Therefore, urocortin and corticotropin-releasing factor appear to have different distribution patterns which may be indicative of their respective physiological functions.     

Effect of 6-hydroxydopamine on neuropeptide Y and corticotropin-releasing factor expression in rat amygdala

The influence of dopaminergic denervation on neuropeptide Y and corticotropin-releasing factor-containing neurons in the amygdala was investigated in rats by examining the effects of a selective, unilateral 6-hydroxydopamine lesion of mesencephalic dopaminergic neurons in both the substantia nigra and the ventral tegmental area on these peptides and their messenger RNA expression, observed eight to 10 days after the lesion. The studies were conducted by immunocytochemical and in situ hybridization methods. Neuropeptide Y or corticotropin-releasing factor-immunoreactive neurons were counted in sections of the amygdala under a microscope, and the messenger RNA expression was measured as optical density units in autoradiograms. A significant increase in both neuropeptide Y and corticotropin-releasing factor messenger RNA expression was found in the amygdala on the lesioned side in comparison with the contralateral one, as well as with the ipsilateral side of vehicle-injected controls. Immunohistochemical studies showed that the number of neuropeptide Y-immunoreactive neurons increased in the whole amygdala on the lesioned side. At the same time, the number of corticotropin-releasing factor-immunoreactive neurons grouped in the central amygdaloid nucleus declined, and so did the staining intensity. The obtained results indicate that dopaminergic denervation stimulates the synthesis of neuropeptide Y and corticotropin-releasing factor in rat amygdala, but the peptide levels are differently regulated, which points to a diverse release of these peptides.     

Corticotropin-releasing factor innervation of the locus coeruleus region: distribution of fibers and sources of input.

Electrophysiologic studies support the hypothesis that corticotropin-releasing factor, the neurohormone that initiates adrenocorticotropin release during stress, also serves as a neurotransmitter in the pontine noradrenergic nucleus, the locus coeruleus. To elucidate the circuitry underlying proposed corticotropin-releasing factor neurotransmission in the locus coeruleus, the present study utilized immunohistochemical techniques to characterize corticotropin-releasing factor innervation of rat locus coeruleus and pericoerulear regions. Corticotropin-releasing factor-like immunoreactive fibers were identified in the locus coeruleus of colchicine- and non-colchicine-treated rats. However, corticotropin-releasing factor innervation of pericoerulear regions rostral and lateral to the locus coeruleus was more dense than that of the locus coeruleus proper. Double-labeling studies utilizing antisera directed against corticotropin-releasing factor and tyrosine hydroxylase indicated that corticotropin-releasing factor-like immunoreactive fibers overlap with tyrosine hydroxylase-like immunoreactive processes of locus coeruleus neurons, particularly in rostral medial and lateral regions. A group of corticotropin-releasing factor-like immunoreactive neurons was localized just lateral to the locus coeruleus and numerous corticotropin-releasing factor-like immunoreactive neurons were visualized just ventral to the rostral pole of the locus coeruleus in a region corresponding to Barrington's nucleus. None of these corticotropin-releasing factor-like immunoreactive neurons were tyrosine hydroxylase-positive. To determine the source of corticotropin-releasing factor-like immunoreactive fibers in the locus coeruleus, injections of the retrograde tracer [wheat germ agglutinin conjugated to inactivated (apo) horseradish peroxidase coupled to gold particles] were made into the locus coeruleus and sections were processed for corticotropin-releasing factor-like immunoreactivity.(ABSTRACT TRUNCATED AT 250 WORDS)     

Blocking corticotropin-releasing factor-2 receptors, but not corticotropin-releasing factor-1 receptors or glucocorticoid feedback, disrupts the development of conditioned defeat

Several neuroendocrine signals of the hypothalamic-pituitary-adrenal (HPA) axis are released following exposure to stressful events. It has long been proposed that the signals in this cascade each act to modify ongoing and future behavior. In this study we investigated whether blocking glucocorticoid synthesis, corticotropin-releasing factor (CRF)-1 receptors, or CRF-2 receptors during social defeat would alter subsequent behavioral responses. We used a conditioned defeat model in Syrian hamsters in which social defeat results in a dramatic shift from territorial aggression to increased submissive and defensive behavior in future social encounters. We found that intracerebroventricular administration of anti-sauvagine-30, a CRF-2 receptor antagonist, prior to social defeat training reduced the acquisition of conditioned defeat. In contrast, the acquisition of conditioned defeat was not altered by the CRF-1 receptor antagonist CP-154,526 or the glucocorticoid synthesis inhibitor metyrapone. Our results suggest that CRF, and perhaps related neuropeptides such as urocortins, act at CRF-2 receptors to promote the development of defeat-induced changes in social behavior, whereas signaling at CRF-1 and glucocorticoid receptors plays a negligible role in this process.     

Heart rate dynamics and behavioral responses during acute emotional challenge in corticotropin-releasing factor receptor 1-deficient and corticotropin-releasing factor-overexpressing mice

The role of corticotropin-releasing factor in autonomic regulation of heart rate, heart rate variability and behavior responses was investigated in two genetic mouse models: corticotropin-releasing factor receptor 1-deficient mice, and corticotropin-releasing factor-transgenic mice overexpressing corticotropin-releasing factor. Heart rate was recorded by radio-telemetry during novelty exposure and auditory fear conditioning. Locomotor activity and freezing served as behavioral indices. Locomotor activity and heart rate were invariably increased in response to novelty exposure in both corticotropin-releasing factor receptor 1-deficient mice and littermate wild-type controls. The heart rate responses during retention of conditioned auditory fear and the exponential relationship between heart rate and heart rate variability were unaffected by genotype. Moreover, conditioned fear responses inferred from multiple behavioral measures including freezing did not differ between corticotropin-releasing factor receptor 1-deficient and corticotropin-releasing factor receptor 1 wild-type control mice. Corticotropin-releasing factor-transgenic mice exhibited markedly reduced locomotor activity during novelty exposure when compared with littermate wild-type controls. Baseline and novelty-driven heart rate was slightly elevated in corticotropin-releasing factor-transgenic mice, whereas the novelty-induced increase of heart rate was not different between genotypes. In contrast, corticotropin-releasing factor-transgenic mice did not display a heart rate response indicative of conditioned auditory fear. It is concluded that corticotropin-releasing factor receptor 1-deficiency does not affect heart rate adjustment and behavioral responses to acute fearful stimuli. The resiliency of behavioral and cardiovascular patterns elevation argues against the involvement of corticotropin-releasing factor receptor 1 in acute emotional regulation on these two functional levels despite an absent corticosterone elevation in corticotropin-releasing factor receptor 1-deficient mice. It is hypothesized that the lack of a conditioned heart rate response in corticotropin-releasing factor-transgenic mice is attributable to an impairment of cognitive function. The results are compared with those of corticotropin-releasing factor receptor 2-deficient mice, and the role of the corticotropin-releasing factor system in cardiovascular regulation is discussed.     

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. __________ Translated from Rossiiskii Fiziologicheskii Zhurnal imeni I. M. Sechenova, Vol. 93, No. 11, pp. 1252–1262, November, 2007.     

Mechanisms of the Effects of Adrenocorticotropic Hormone on Pain Sensitivity in Rats

Experiments on anaesthetized male Sprague-Dawley rats were performed to study the effects of adrenocorticotropic hormone (ACTH) on pain sensitivity. Systemic administration of ACTH to animals with normal hormone production induced rapidly developing (starting at 3 min) and prolonged (30 min) increases in pain response thresholds. Blockade of opiate receptors led to suppression of the initial stage of the analgesic effect of ACTH: the response was seen only from 15 to 30 min. In animals with deficient glucocorticoid production, the duration of the analgesic action of ACTH decreased to 15 min. Analgesia was completely eliminated by the combination of suppression of glucocorticoid production and blockade of opiate receptors. The analgesic effect of ACTH was mediated by two mechanisms: 1) a rapidly-acting (from 3 to 15 min) mechanism associated with opiate receptors and not related to glucocorticoids, and 2) a delayed (from 15 to 30 min) mechanism associated with glucocorticoids but not opiate receptors.