Involvement of hypothalamic pituitary adrenal axis on the effects of nifedipine in the development of morphine tolerance in rats

It has been shown that nifedipine, as a calcium channel blocker, can attenuate the development of tolerance to the antinociceptive effect of morphine; however, the role of HPA axis on this action has not been elucidated. We examined the effect of nifedipine on morphine analgesic tolerance in intact and adrenalectomized (ADX) rats and on HPA activity induced by morphine. Adult male rats were rendered tolerant to morphine by daily injection of morphine (15 mg/kg i.p.) for 8 days. To determine the effect of nifedipine on the development of morphine tolerance, nifedipine (1, 2 and 5 mg/kg i.p.) was injected concomitant with morphine. The tail-flick test was used to assess the nociceptive threshold, before and 30 min after morphine administration in days 1, 3, 5 and 8. Our results showed that despite the demonstration of tolerance in both ADX and sham operated rats, nifedipine in ADX rats prevented morphine tolerance development at a lower dose (2 mg/kg) than in sham operated rats, however corticosterone replacement prevented nifedipine effect in ADX rats. Acute administration of morphine produced significant increase in plasma corticosterone level, and with repeated injection, a tolerance to this neurosecretory effect was developed. Nifedipine (5 mg/kg) attenuated the acute effect of morphine, but could not block its neurosecretory tolerance.     

Involvement of hypothalamic pituitary adrenal axis on the nifedipine-induced antinociception and tolerance in rats

Nifedipine, a calcium channel blocker, can modulate the nociceptive threshold. However, the underlying mechanism, especially the role of HPA axis, on this effect has still not been elucidated. In the present study we investigated the analgesic effect of nifedipine in intact and adrenalectomized (ADX) male rats and we also measured the effect of nifedipine on HPA function. The Tail-Flick test was used to assess the nociceptive threshold before and 15, 30, 60, 90, and 120 min after drug administration. Corticosterone level was measured by radioimmunoassay as a marker of HPA function. Our results showed that in intact and sham operated animals, administration of 10 mg/kg nifedipine induces an antinociceptive effect. But at the dosage of 2 and 5 mg/kg animals do not exhibit this effect. With repeated injections, its analgesic effect was decreased, a phenomenon prevented by adrenalectomy. Acute administration of nifedipine produced significant decrease in plasma corticosterone level. In ADX animals, had a potent antinociceptive effect nifedipine at high dosage (10 mg/kg) as well as at lower dosage (5 mg/kg) that reversed with corticosterone replacement. In conclusion, the results of our study show that the elimination of HPA function through adrenalectomy potentiates the antinociceptive effect of nifedipine and attenuates its analgesic tolerance. Both effects are reversed by corticosterone replacement.     

Nifedipine potentiates antinociceptive effects of morphine in rats by decreasing hypothalamic pituitary adrenal axis activity

It has been shown that nifedipine, as a calcium channel blocker can potentiate the antinociceptive effect of morphine; however, the role of Hypothalamic-Pituitary-Adrenal (HPA) axis on this action has not been elucidated. We examined the effect of nifedipine on morphine-induced analgesia in intact and adrenalectomized (ADX) rats and on HPA activity induced by morphine. To determine the effect of nifedipine on morphine analgesia, nifedipine (2 mg/kg i.p.) that had no antinociceptive effect, was injected concomitant with sub-effective dose of morphine (1 and 2 mg/kg). The tail-flick test was used to assess the nociceptive threshold, before and 15, 30, 60, 90, 120 and 180 min after drug administration. Our results showed that, nifedipine could potentiate the antinociceptive effect of morphine and this effect of nifedipine in ADX was greater than sham operated rats which, was reversed by corticosterone replacement. Nifedipine has an inhibitory effect on morphine -induced corticosterone secretion. Thus, the data indicate that the mechanism underlying the potentiation of morphine analgesia by nifedipine involves mediation, at least in part, by attenuating the effect of morphine on HPA axis.     

ACTH 联合槐杞黄对大鼠下丘脑-垂体-肾上腺轴的影响

目的 研究促肾上腺皮质激素（ACTH）联合槐杞黄对大鼠下丘脑-垂体-肾上腺（HPA）轴的影响。 方法 50 只大鼠按随机数字表法分为空白对照组（A 组）、泼尼松模型组（B 组）、槐杞黄组（C 组）、ACTH 组（D 组）和联合治疗组（E 组）, 每组 10 只。 B~E 组采用醋酸泼尼松水溶液 12.5 mg/（kg·d）连续灌胃 4 周建立 HPA 轴抑制模型, A 组以蒸馏水 10 mL/（kg· d）灌胃作为对照。 C、E 组每次在醋酸泼尼松灌胃后 30 min 加用槐杞黄颗粒 5 g/（kg· d）灌胃。 实验第 3 周时, D、E 组加用 ACTH 200 µg/（kg·d）皮下注射。 分别于实验开始前、实验 2 周、4 周时测定各组血清皮质醇水平。 实验结束后处死动物并摘取垂体和肾上腺, 称质量后计算脏器指数, HE 染色观察垂体和肾上腺的病理情况。 结果 实验 2 周后, B、C、D、E 组的血清皮质醇水平较 A 组明显降低（P < 0.05）, 提示造模成功。 实验 4 周时, C、 D、E 组的血清皮质醇水平较 B 组均明显升高（P < 0.05）, 且各治疗组之间 E 组>D 组>C 组（P < 0.05）; 同时 3 组的垂体和肾上腺的质量及脏器指数均较 B 组升高（P < 0.05）。 HE 染色显示各组垂体远侧部未见明显改变; B 组肾上腺皮质束状带结构变薄且紊乱, C、D、E 组则出现不同程度的增生, 以 E 组最为明显。 结论 ACTH 联合槐杞黄可促进肾上腺皮质束状带增生及皮质醇的分泌, 减轻糖皮质激素对大鼠 HPA 轴的抑制作用。     

Single dose tolerance to the analgesic effect of clonidine and cross-tolerance between morphine and clonidine

A single dose of clonidine developed tolerance to its analgesic effect. The tolerance reached its peak acutely on the 2nd day and lasted more than 5 days. Neither the analgesic effect nor the development of tolerance was modified by the pretreatment with naloxone. On the 2nd day, clonidine tolerant animals were also tolerant to morphine, but morphine tolerant animals, after a single dose of morphine on the 1st day, were not tolerant to clonidine. On the 5th day, however, clonidine tolerant animals were tolerant to morphine, and vice versa. Thus, the interaction between morphine and clonidine was "one-way" on the 2nd day, and cross-tolerance was only demonstrated on the 5th day. With a treatment with clonidine plus naloxone on the 1st day, the development of cross-tolerance to morphine was completely suppressed on the 2nd day but not on the 5th day. These results confirmed our previous finding that acute and delayed tolerance are different in nature, and the development of tolerance to morphine and clonidine are partially underlaid with a common mechanism which is not mediated by opioid receptors.     

Lack of cross-tolerance between morphine and autoanalgesia

The acquisition of autoanalgesia (behaviorally-induced antinociception) was investigated in morphine-tolerant and non-tolerant rats. Tolerance to morphine did not affect analgesia acutely-elicited by a brief (15 sec) schedule of footshock. Similarly, analgesia elicited by classically conditioned fear was not attenuated by morphine tolerance. These data suggest that endorphins may not be the principle mediators of autoanalgesic phenomena.     

Analgesic cross-tolerance between morphine and opioid peptides

The analgesic effect, of intracerebroventricular administration of morphine, ketocyclazocine, [D-ala²]-methionine enkephalinamide (DAM), [D-ala²-D-leu⁵]-enkephalin. (DADLE), leuenkephalin, metenkephalin, and -endorphin on acetic acid-induced abdominal writhing (AAW) was investigated in naive and morphine-tolerant mice. It was found that the relative potencies of a series of opioids are different in naive and morphine-tolerant groups. In naive animals, the order of potency (ED₅₀, nmol) was -endorphin > morphine=DAM > DADLE > ketocyclazocine=leuenkephalin=metenkephalin. The morphine-tolerant animals were cross-tolerant to ketocyclazocine and to all the peptides studied; DAM and -endorphin exhibited the highest degree of tolerance. In morphine-tolerant animals, the order of potency was morphine=DADLE=-endorphin > DAM=ketocyclazocine =metenkephalin > leuenkephalin. The results indicate that endogenous opioid systems may be affected by tolerance development to morphine.     

Novel approaches to the treatment of depression by modulating the hypothalamic - pituitary - adrenal axis

Many patients with major depression show evidence of over-activation of the hypothalamic - pituitary - adrenal axis (HPA), as evidenced by hypercortisolism and adrenal hyperplasia. Such over-activity is associated with increased corticotropin releasing factor (CRF) reactivity in the CSF and blunted release of ACTH in response to CRF infusion. Recent evidence suggests a switch from CRF to AVP regulation of the axis during depression, with depressed patients showing enhanced response to ddAVP infusion. The HPA provides multiple potential sites for antidepressant development. The use of glucocorticoid antagonists, cortisol synthesis inhibitors, CRF and AVP antagonists have been suggested. Copyright 2001 John Wiley & Sons, Ltd.     

Opioid mydriasis: cross-tolerance between morphine and enkephalins

The occurrence of cross-tolerance between morphine and met-enkephalin, and between morphine and DADL (D-Ala-D-Leu-enkephalin) in causing mydriasis in mice was studied. Morphine-tolerant mice treated with met-enkephalin or DADL intracerebroventricularly (ICV) showed marked reduction of the pupillary effect of the endopioids. Maximal mydriasis in tolerant animals was only about 30% for met-enkephalin and 50% for DADL, compared to levels in nontolerant animals. These results are among the first to demonstrate cross-tolerance between morphine and enkephalins in intact animals and may suggest involvement of multiple opiate receptor systems in producing mydriasis.     

Development of tolerance to the analgesic effect of clonidine in rats cross-tolerance to morphine

Summary The site of action of naloxone to induce jumping in morphine tolerant/dependent rats appears to be dissociated from structures where apomorphine initiates its action to reinduce jumping in previously withdrawn animals. These findings suggest that dopaminergic pathways do not directly affect the neuronal circuit involved in withdrawal jumping behavior, but instead exert a facilitatory influence on neurons that become supersensitive during the state of withdrawal. Thus, an increased response to apomorphine during naloxone-precipitated opiate withdrawal does not necessarily imply a specific supersensitivity of the dopaminergic system.     

Effects of heat-stress on behavior and the pituitary adrenal axis in rats

Three experiments were performed in order to analyse the behavioral and biochemical correlates of four different intensities of the same stressor. In experiment 1, rats were exposed to heat stress (hot-plate) of varying temperatures for 30 seconds. Activity was recorded in an open field immediately after stress for 30 minutes. The data revealed that the milder temperatures increased (21, 47, 52°C), while the higher temperature (57°C) decreased activity. Experiment 2 assessed the pituitary-adrenal response to the different temperatures by measuring levels of plasma corticosterone 30 minutes after stress. The four levels of hot-plate temperatures induced differential levels of corticosterone which may best be described as an inverted U-shaped function, with only the extreme temperature (57°) inducing a significant elevation in levels of the steroid. Experiment 3 further manipulated the pituitary adrenal axis by administering dexamethasone 25 hr and 1 hr before stress and ACTH 15 min before stress. Both affected activity levels by depressing locomotion regardless of the stress intensity. These results are compared to other studies that have addressed the question of stress-induced activation and it is suggested that stress is not a unitary concept, but interacts with the performance of certain behaviors to produce both facilatory or inhibitory results.     

The development of cross-tolerance between morphine and nicotine in mice.

In the present study, cross-tolerance between nicotine and morphine in mice has been investigated. Mice were treated subcutaneously with three doses of morphine (12.5, 25 and 50 mg/kg) once daily, for 3 days in order to produce tolerance to morphine and nicotine antinociception. Tolerance only developed in the high dose group. On the 4th day, the antinociceptive effect of three test doses of morphine (3, 6 and 9 mg/kg) or nicotine (0.01, 0.05 and 0.1 mg/kg) were assessed. Tolerance to the responses of both drugs were observed. Intraperitoneal administration of nicotine (2 mg/kg) three times a day for a period of 12 days, also induced tolerance to the antinociceptive effects of both morphine and nicotine. When animals were tested on the 13th day, the antinociceptive responses of morphine or nicotine were reduced. Another group of animals was treated with low doses of morphine daily (12.5 or 25 mg/kg) plus nicotine (2 mg/kg) three times daily for 3 days. In this group of animals, the antinociception to either morphine or nicotine was tested. Combination of both drugs caused an increase in tolerance to either drug. It is concluded that there is cross-tolerance between the two drugs.     

Effects of morphine and morphine withdrawal on brainstem neurons innervating hypothalamic nuclei that control the pituitary-adrenocortical axis in rats

Different data support a role for brainstem noradrenergic inputs to the hypothalamic paraventricular nucleus (PVN) in the control of hypothalamus - pituitary - adrenocortical (HPA) axis. However, little is known regarding the functional adaptive changes of noradrenergic afferent innervating the PVN and supraoptic nucleus (SON) during chronic opioid exposure and upon morphine withdrawal. Here we have studied the expression of Fos after administration of morphine and during morphine withdrawal in the rat hypothalamic PVN and SON. Fos production was also studied in brainstem regions that innervate hypothalamic nuclei: the nucleus of solitary tract (NTS - A2) and the ventrolateral medulla (VLM - A1) and combined with immunostaining for tyrosine hydroxylase (TH) for immunohistochemical identification of active neurons during morphine withdrawal. Male rats were implanted with s.c. placebo or morphine (tolerant/dependent) pellets for 7 days. On day 8 rats received an injection of saline i.p., morphine i.p., saline s.c. or naloxone s.c. Acute morphine administration produced an increase in Fos expression at hypothalamic nuclei and in the brainstem regions, and tolerance developed towards this effect. Precipitated morphine withdrawal induced marked Fos immunoreactivity within the PVN and SON. Concomitantly, numerous neurons in the brainstem were stimulated by morphine withdrawal. Moreover, catecholaminergic-positive neurons in the brainstem showed a significant increase in Fos expression in response to morphine withdrawal. These findings demonstrate that chronic activation of opioid receptors results in altered patterns of immediate-early genes (IEG) expression in the PVN and SON, which occurs concurrently with an increased activity of their inputs from the brainstem.     

Interaction between morphine and reinforcing lateral hypothalamic stimulation

The interaction between morphine and lateral hypothalamic self-stimulation in rats was investigated in three experiments. In nonaddicted animals injections of moderate doses of morphine (8 mg/kg) resulted in a short-lasting increase in the self-stimulation threshold. Injections of low doses (2 mg/kg) did not alter the self-stimulation threshold significantly. In addicted animals self-stimulation thresholds were significantly lower 2 h compared to 22 h after injection of 200 mg/kg morphine hydrochloride (calculated as the salt). A 16 mg/kg injection of morphine in non-addicted rats suppressed self-stimulation. However, when the animals were administered noncontingent priming stimulation and were hand-shaped toward the lever whenver they left it, they continued to leverpress throughout the entire 90-min postinjection testing period. The animals that were neither primed nor shaped (and therefore remained unstimulated), however, showed a significantly better recovery when tested 90 min after the injection; i.e., their response rate was higher than that of the primed and shaped animals, which were engaged in bar-pressing throughout the entire testing procedure.     

Involvement of NCAM and FGF receptor signaling in the development of analgesic tolerance to morphine

Green odor is present in many green leaves, vegetables, and fruits and is composed of four 6-carbon straight-chain alcohols, n-hexanol, (E)-2-hexenol, (Z)-3-hexenol, and (E)-3-hexenol, and four aldehydes, n-hexanal, (E)-2-hexenal, (Z)-3-hexenal, and (E)-3-hexenal. It has been reported that certain green odor compounds enhance dopamine release from rat brain striatal slices and rat pheochromocytoma cells (PC12 cells). It is well known that intracellular Ca(2+) levels regulate dopamine release. The amount of dopamine released by n-hexanal-treated PC12 cells decreased in cells pretreated with a membrane-permeable Ca(2+) chelator. In this study, the effect of n-hexanal on dopamine release in the brain striatum of living rats was studied using an in vivo brain microdialysis system. Local stimulation with n-hexanal diluted in Ringer's solution to 0.01%, 0.05%, or 0.1% enhanced dopamine release in a concentration-dependent manner. The amount of dopamine released with 0.01% n-hexanal administration significantly declined when either extracellular or intracellular Ca(2+) levels decreased. Furthermore, the extracellular dopamine concentration increased with perfusion of nomifensine, an inhibitor of dopamine uptake into cells. When nomifensine was co-perfused with n-hexanal into the striatum, extracellular dopamine release increased further. Accordingly, the concentration of dopamine metabolite and the ratio of dopamine metabolite to dopamine decreased after treatment with n-hexanal. These responses were similar to those seen with KCl stimulation. These data suggest that n-hexanal stimulates dopamine release but does not inhibit dopamine uptake in the brain striatum of living rats, and that dopamine release associated with n-hexanal is regulated by both extracellular and intracellular Ca(2+) levels.     

Synergistic affective analgesic interaction between delta-9-tetrahydrocannabinol and morphine

Evidence for an analgesic interaction between delta-9-tetrahydrocannabinol (Delta(9)-THC) and morphine was sought using an experimental pain model applied to normal volunteers. The study incorporated a double blinded, four treatment, four period, four sequence, crossover design. Subjects received Delta(9)-THC 5 mg orally or placebo and 90 min later morphine 0.02 mg/kg intravenously or placebo. Fifteen minutes later subjects rated the pain associated with the application of thermal stimuli to skin using two visual analog scales, one for the sensory and one for the affective aspects of pain. Among sensory responses, neither morphine nor Delta(9)-THC had a significant effect at the doses used, and there was no significant interaction between the two. Among affective responses, although neither morphine nor Delta(9)-THC had a significant effect, there was a positive analgesic interaction between the two (p = 0.012), indicating that the combination had a synergistic affective analgesic effect. The surprisingly limited reported experimental experience in humans does not support a role for Delta(9)-THC as an analgesic or as an adjunct to cannabinoid analgesia, except for our finding of synergy limited to the affective component of pain. Comparison of our results with those of others suggests that extrapolation from experimental pain models to the clinic is not likely to be a straight-forward process. Future studies of Delta(9)-THC or other cannabinoids in combination with opiates should focus upon clinical rather than experimental pain.     

Function of the hypothalamic adrenal axis in patients with fibromyalgia syndrome undergoing mud-pack treatment.

Fibromyalgia (FM) is a nonarticular rheumatological syndrome associated with diverse clinical and psychological features. One of the major complaints in FM is reduced pain tolerance, especially in tender points (TP) for which patients derive significant benefit from nonsteroidal antiinflammatory drugs or corticosteroids. Patients with FM also have altered reactivity of the hypothalamic pituitary adrenal (HPA) axis where the predominant feature is reduced containment of the stress response system through diminished adrenocortical output and feedback resistance. Our results show that mud packs together with antidepressant treatment are able to influence the HPA axis, stimulating increased levels of adrenocorticotropic hormone, cortisol and beta-endorphin serum levels. The discharge of corticoids in the blood and the increase in beta-endorphin serum levels are followed by a reduction in pain symptoms, which is closely related to an improvement in disability, depression and quality of life. It seems that the synergic association between a pharmacological treatment (trazodone) and mud packs acts by helping the physiological responses to achieve homeostasis and to rebalance the stress response system. To clarify and optimize the effectiveness of this synergic association, studies involving a larger number of FM patients and a different pharmacological treatment are needed.     

Involvement of kappa opioid receptors in formalin-induced inhibition of analgesic tolerance to morphine in mice

This study examined the role of kappa opioid receptors (KOR) in the mechanism underlying tolerance to the analgesic effects of morphine induced by chronic pain. The analgesic effect of morphine (10 mg kg(-1)), estimated by the tail-flick test in mice, gradually decreased during repeated daily morphine treatment. A significant decrease in the analgesic effect of morphine was seen on the fifth day of repeated morphine treatment compared with the first day. Chronic pain was induced by subcutaneous administration of 2% formalin into the dorsal part of the left hind paw, which significantly inhibited development of tolerance to morphine analgesia. The effect of formalin-induced pain on inhibition of morphine tolerance was reversed by the KOR antagonist nor-binaltorphimine. Furthermore, an antisense oligodeoxynucleotide, but not a missense oligodeoxynucleotide, against KOR completely suppressed the inhibitory effect of formalin-induced pain on morphine tolerance. Naltrindole, an antagonist of delta opioid receptor, did not affect chronic-pain-induced tolerance to morphine. Our findings show that the inhibitory effect of chronic pain on analgesic tolerance to morphine is mediated by KOR rather than delta opioid receptors.