Mu receptors at discrete hypothalamic and brainstem sites mediate opioid peptide-induced increases in central sympathetic outflow

Neurogenic Hypotension and Bradycardia Modulated by Electroacupuncture in Hypothalamic Paraventricular Nucleus

Electroacupuncture (EA) stimulates somatic median afferents underlying P5-6 acupoints and modulates parasympathoexcitatory reflex responses through central processing in the brainstem. Although decreases in blood pressure and heart rate by the neural-mediated Bezold-Jarisch reflex responses are modulated by EA through opioid actions in the nucleus tractus solitarius and nucleus ambiguus, the role of the hypothalamus is unclear. The hypothalamic paraventricular nucleus (PVN) is activated by sympathetic afferents and regulates sympathetic outflow and sympathoexcitatory cardiovascular responses. In addition, the PVN is activated by vagal afferents, but little is known about its regulation of cardiopulmonary inhibitory hemodynamic responses. We hypothesized that the PVN participates in the Bezold-Jarisch reflex responses and EA inhibits these cardiopulmonary responses through the PVN opioid system. Rats were anesthetized and ventilated, and their heart rate and blood pressures were monitored. Application of phenylbiguanide every 10 min close to the right atrium induced consistent depressor and bradycardia reflex responses. Unilateral microinjection of the depolarization blockade agent kainic acid or glutamate receptor antagonist kynurenic acid in the PVN reduced these reflex responses. In at least 70% of the rats, 30 min of bilateral EA at P5-6 acupoints reduced the depressor and bradycardia responses for at least 60 min. Blockade of the CCK-1 receptors converted the non-responders into EA-responders. Unilateral PVN-microinjection with naloxone reversed the EA inhibition. Vagal-evoked activity of the PVN cardiovascular neurons was reduced by 30 min EA (P5-6) through opioid receptor activation. These data indicate that PVN processes inhibitory cardiopulmonary reflexes and participates in EA-modulation of the neural-mediated vasodepression and bradycardia.

Comparing the Antinociceptive Effects of Methamphetamine, Buprenorphine, or Both After Chronic Treatment and Withdrawal in Male Rats

Introduction:

Methamphetamine (Meth) and Buprenorphine (BUP) modulate pain perception. However, the antinociceptive effects of their interactions, which affect through different systems, are unclear in rats. This study aimed to compare the analgesic effects of Meth, BUP, and their coadministration, as well as the effect of withdrawal from these substances on nociception in male rats.

Methods:

In this experiment, 40 male Wistar rats (weight: 250–300 g) were categorized into four groups: control, Meth, BUP, or BUP+Meth. After seven days of treatments, the antinociceptive effects were assessed using the hot plate and the tail flick tests. The differences among the groups were analyzed with ANOVA and Tukey’s post hoc tests. P values less than 0.05 were considered significant.

Results:

Meth and BUP increased the reaction times during the hot plate and tail flick tests. The combination of Meth and BUP increased reaction time more than Meth or BUP alone.

Conclusion:

The significantly high reaction times in rats treated with Meth and BUP indicate that these substances have antinociceptive effects. In addition, Meth enhanced the antinociceptive effects of BUP. These synergistic effects might occur through the dopaminergic, serotonergic, and or adrenergic systems.

Morphine-induced supraventricular tachycardia in near-term fetus

Background

Fetal supraventricular tachycardia (SVT), characterized by fetal heart rate between 220 and 260 bpm, is a rare but most commonly encountered fetal cardiac arrhythmia in pregnancy that may be associated with adverse perinatal outcome.

Case presentation

We describe a 36/6 week near term fetus who presented morphine-induced SVT after maternal treatment of a renal colic. Following emergency cesarean section, the neonate had resolution of symptoms.

Conclusions

The pathophysiology of morphine-related SVT, previously documented in experimental animal models, and for the first time reported in the human fetus, is presented.

Different blood pressure responses to opioids in 3 rat hypertension models: role of the baseline status of sympathetic and renin-angiotensin systems

Opioids interact with sympathetic and renin-angiotensin systems in control of mean arterial pressure (MAP). Our earlier finding that biphalin, a synthetic enkephalin analogue, decreased MAP in anaesthetized spontaneously hypertensive rats (SHR) prompted us to further explore this action, to get new insights into pathogenesis of various forms of hypertension. Biphalin effects were studied in SHR, uninephrectomized rats on a high-salt diet (HS/UNX), and rats with angiotensin-induced hypertension (Ang-iH). Besides MAP, renal and iliac blood flows (RBF, IBF) and vascular resistances were measured. In anaesthetized and conscious SHR, biphalin (300 μg·h^-1·kg^-1 i.v.) decreased MAP by ∼10 and ∼20 mm Hg, respectively (P < 0.001). In anaesthetized HS/UNX and normotensive rats, MAP increased by ∼6-7 mm Hg (P < 0.02); without anaesthesia, only transient decreases occurred. MAP never changed in Ang-iH rats. Morphine (1.5 mg·h^-1·kg^-1 i.v.) decreased MAP in HS/UNX but only transiently so without anaesthesia; such anaesthesia dependence of response was also seen in normotensive rats. Ang-iH rats never responded to morphine. Hypotensive effect in SHR only depends primarily on the reduction by biphalin of vascular responsiveness to increased sympathetic stimulation; such increase is well documented for SHR. No MAP response to biphalin or morphine in Ang-iH could depend on angiotensin-induced alterations of the vascular wall morphology and function.

Interplay between RAS and opioids: opening the Pandora of complexities

Angiotensin and endogenous opioids are important bioactive neuropeptides, which are widely distributed in the brain and peripheral regions to produce diverse biological and neurobiological activities. An endogenous opioid system includes proopiomelanocortin-derived enkephalin, dynorphin and endorphin that act on their specific receptors such as delta (δ), kappa (κ) and mu (μ) receptors. Research evidence demonstrates significant positive as well as negative interactions between renin angiotensin system (RAS) and endogenous opioids in the brain and periphery. The diverse actions of Ang II are possibly mediated indirectly through endogenous opioids, while opioids are also shown to activate RAS components suggesting the up-regulation of each system in concern with each other. On the contrary, there are reports suggesting a negative correlation between RAS and opioid system. Research evidence also supports the notion that Ang II acts as anti-opioid peptide to decrease the actions of opioids. Moreover, opioids-induced decline in angiotensin release and functioning has also been reported. Co-administration of ACE inhibitors with opioids exhibits significant interactions possibly due to decreased metabolism of opioids leading to potentiation of their actions. The present review describes the complexities of positive and negative interactions between RAS and opioids along with possible mechanisms responsible for these interactions.

Surgery as a double-edged sword: a clinically feasible approach to overcome the metastasis-promoting effects of surgery by blunting stress and prostaglandin responses

Surgery remains an essential therapeutic approach for most solid malignancies, including breast cancer. However, surgery also constitutes a risk factor for promotion of pre-existing micrometastases and the initiation of new metastases through several mechanisms, including the release of prostaglandins and stress hormones (e.g., catecholamines and glucocorticoids). However, the perioperative period also presents an opportunity for cell mediated immunity (CMI) and other mechanisms to eradicate or control minimal residual disease, provided that the deleterious effects of surgery are minimized. Here, we discuss the key role of endogenous stress hormones and prostaglandins in promoting the metastatic process through their direct impact on malignant cells, and through their deleterious impact on anti-cancer CMI. We further discuss the effects of anesthetic techniques, the extent of surgery, pain alleviation, and timing within the menstrual cycle with respect to their impact on tumor recurrence and physiological stress responses. Last, we suggest an attractive perioperative drug regimen, based on a combination of a cyclooxygenase (COX)-2 inhibitor and a β-adrenergic blocker, which we found effective in attenuating immune suppression and the metastasis-promoting effects of surgery in several tumor models. This regimen is clinically applicable, and could potentially promote disease free survival in patients operated for breast and other types of cancer.

Acute morphine administration reduces white blood cells' capability to induce innate resistance against HSV-1 infection in BALB/c mice

OBJECTIVE

It has been reported that acute morphine administration modulates innate immune response to herpes simplex virus 1 (HSV-1) infection. In this study, the effect of acute morphine on innate resistance and its probable mechanisms in increasing the mortality rate during HSV-1 infection were investigated.

METHODS

Mice were infected with HSV-1 24 h prior to different doses of morphine or saline administration and the mortality rate was recorded. Spleen cells were obtained from morphine- or saline-treated mice, then natural killer (NK) cell activity and interferon-gamma (IFN-gamma) production were evaluated. The effect of morphine on white blood cells' capacity to induce protection against HSV-1 infection was evaluated by adoptive transfer of spleen cells to cyclophosphamide-treated mice that were previously infected with HSV-1. Furthermore, in a separate experiment, a different group of mice received corticosterone 24 h after HSV-1 infection.

RESULTS

Mortality rate in high-dose acute morphine-treated mice increased significantly compared to saline-treated mice (p = 0.035). NK cell cytotoxicity and IFN-gamma mRNA levels also showed a significant reduction compared to those of control groups (p < 0.001 and p = 0.014, respectively). Corticosterone administration reduces innate resistance against HSV-1 infection compared to saline-treated mice (p = 0.044). Furthermore, adoptive transfer of normal but not morphine-treated spleen cells induces resistance against HSV infection in cyclophosphamide-injected mice (p = 0.009).

CONCLUSIONS

The current study shows that acute morphine administration reduces white blood cells' capability to induce protection against HSV-1 infection via suppression of IFN-gamma production and NK cells activity. This may be due to the increase in corticosteroids. Further studies are needed to test the effect of acute morphine on other immune cells.

Neuropeptide Y Y1 receptors mediate morphine-induced reductions of natural killer cell activity

Morphine suppresses a number of immune parameters, such as natural killer (NK) cell activity and lymphocyte proliferation, by acting through mu-opioid receptors in the central nervous system. Prior studies have implicated the sympathetic nervous system in mediating the immunomodulatory effects of acute morphine treatment. However, the peripheral mechanism whereby morphine inhibits NK cell activity is not fully understood. The aim of the present study was to investigate the role of the sympathetic transmitter neuropeptide Y (NPY) in mediating morphine-induced immune alterations. The results showed that administration of the selective NPY Y1 receptor antagonist BIBP3226 blocked morphine's effect on splenic NK activity but did not attenuate the suppression splenocyte proliferative responses to Con-A or LPS. Furthermore, intravenous NPY administration produced a dose-dependent inhibition of splenic NK activity but did not suppress lymphocyte proliferation. Recent studies from our laboratory have demonstrated that morphine modulates NK activity through a central mechanism that requires the activation of dopamine D1 receptors in the nucleus accumbens. Results from the present study showed that microinjection of the D1 receptor agonist SKF-38393 into the nucleus accumbens shell induced a suppression of NK activity that was reversed by BIBP3226. Collectively, these findings demonstrate that NPY Y1 receptors mediate morphine's suppressive effect on NK activity and further suggest that opioid-induced increases in nucleus accumbens D1 receptor activation inhibit splenic NK activity via NPY released from the sympathetic nervous system.

The cardiovascular responses to mu opioid agonist and antagonist in conscious normal and obese rats

Beta-endorphin decreases blood pressure in normal rats but increases blood pressure in obese rats. Since beta-endorphins can bind both mu opioid and kappa-opioid receptors we investigated the effect of a mu specific receptor agonist, D-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin (DAMGO) and a mu specific antagonist, D-Phe-Cys-Trp-Arg-Thr-Pen-Thr-NH2 (CTAP) on cardiovascular responses in conscious control and obese rats. Rats were also implanted with telemetry transmitters and intracerebroventricular (ICV) cannulas for recording and peptide administration. The mu agonist, DAMGO, increased blood pressure (BP) in control rats. DAMGO also increased BP in obese rats but only at high concentrations. The heart rate responses paralleled the MAP responses. CTAP, the mu antagonist, paradoxically increased the MAP in both control and obese rats. The responsiveness to the mu agonist and antagonist was greater in controls. In other animals the brains were excised and the ventral medial hypothalamic area removed and mu receptor expression determined using PCR. The expression of mu opioid receptors was increased in obese rats. We conclude that the mu opioids can stimulate cardiovascular responses, but the excitatory responsiveness was not increased in conscious obese rats.

Influence of opioids in hypothalamic nuclei on cold thermogenesis of lean and obese LA/N-cp rats

An overactive endogenous opioid peptide system (EOP) in the hypothalamus of the obese rats could contribute to a subnormal metabolic response to cold stress. Specific mu, delta, kappa opioid receptor antagonists and naloxone were infused into cannulaes aimed at the paraventricular nucleus (PVN) of awake freely moving obese (LA/N-cp corpulent) and lean littermate rats. Metabolic responses were measured by indirect calorimetry during thermoneutrality (30 degrees C) and at 10 degrees C for 60 minutes each. When expressed relative to metabolic body size (kg(-.75)) obese rats had lower values (obese = 21.1 +/- 1.9 vs. lean = 27.9 +/- 2.7 ml x kg(-.75) x min, mean +/- s.d., p < 0.05) at 10 degrees C during saline infusion. EOP antagonist infusions at 30 degrees C had no effect on metabolic rate for either lean or obese animals. Mu (23.5 +/- 3.4 ml x kg x (-75) x min) and delta (23.0 +/- 2.0) antagonism and naloxone (25.0 +/- 2.3) significantly increased the metabolic response to cold in obese but not lean rats. These data suggest that certain subtypes of EOP receptors in or near PVN are overactive in obese rats. This overactive state may inappropriately inhibit the thermogenic response to cold stress in obesity.

Suppression of natural killer cell activity by morphine is mediated by the nucleus accumbens shell

Despite a wealth of data indicating that morphine modulates immune status by acting at mu-opioid receptors in the brain, there is little known about how the opioid system interacts with other neurotransmitter systems to modulate specific immune parameters. The aim of the present study was to investigate whether dopaminergic projections to the nucleus accumbens are involved in morphine-induced suppression of splenic natural killer (NK) cell activity. The results indicate that administration of the dopamine D1 antagonist SCH-23390 into the nucleus accumbens shell, but not core, blocked morphine's suppressive effect on NK activity in male Lewis rats. In support of these findings, the effect of morphine was also prevented by intra-accumbens microinfusions of the dopaminergic immunotoxin anti-DAT-saporin. Additionally, administration of the D1 agonist SKF-38393 into the nucleus accumbens shell produced reductions in splenic NK activity comparable to morphine, suggesting a critical role for D1 receptors in the modulation of NK activity. Collectively, these findings demonstrate that dopaminergic inputs to the nucleus accumbens are critically involved in opioid-induced immunosuppression and suggest that opioid-induced increases in D1 receptor activation may have adverse consequences on immune status.

Tonic inhibitory control exerted by opioid peptides in the paraventricular nuclei of the hypothalamus on regional hemodynamic activity in rats

1. Systemic and regional cardiovascular changes were measured following bilateral microinjection of specific and selective opioid-receptor antagonists into the paraventricular nuclei of the hypothalamus (PVN) of awake, freely moving rats. 2. PVN microinjection of increasing doses of the specific opioid antagonist naloxone - methiodide (1 - 5.0 nmol), or a selective mu-opioid receptor antagonist, beta-funaltrexamine (0.05 - 0.5 nmol), evoked important cardiovascular changes characterized by small and transient increases in heart rate (HR) and mean arterial pressure (MAP), vasoconstriction in renal and superior mesenteric vascular beds and vasodilation in the hindquarter vascular bed. 3. No significant cardiovascular changes were observed following PVN administration of the highly selective delta-opioid-receptor antagonist, ICI 174864 (0.1 - 1 nmol), or the selective kappa-opioid-receptor antagonist, nor-binaltorphine (0.1 - 1 nmol). 4. Most of the cardiovascular responses to naloxone (3 nmol) and beta-funaltrexamine (0.5 nmol) were attenuated or abolished by an i.v. treatment with a specific vasopressin V(1) receptor antagonist. 5. These results suggest that endogenous opioid peptides and mu-type PVN opioid receptors modulate a tonically-active central depressor pathway acting on systemic and regional haemodynamic systems. Part of this influence could involve a tonic inhibition of vasopressin release.

Effects of morphine on T-cell recirculation in rhesus monkeys

A 2-yr study on effects of morphine on lymphocyte circulation in rhesus monkeys (Macaca mulatta) showed that, over time, a well-maintained morphine-dependency caused biphasic depressive effects on circulating lymphocyte levels. Depression of T cell circulation by opiates actually was a relative effect. Morphine exposure basically stabilized T cell circulation in the context of concurrent increases in controls. Biphasic effects of morphine were attributable to distinctions in circulation kinetics of CD4+/CD62L (+ & -) T cells. That is, levels of CD4+/CD62L+ T cells were selectively depressed by opiates through the first 32wk after initiation of drug, and levels of CD4+/CD62L- T cells were selectively depressed thereafter. Regression analyses also showed that morphine stabilized lymphocyte recirculation. Circulating levels of resting and activated-memory types of T cells were positively correlated in opiate-exposed monkeys during the first 32wk after opiate exposure--an effect not seen with control monkeys. Considerations of changes in the types of experimental stressors extant during the study suggested that temporally differential effects of opiates on T cell recirculation were connected with changes in the stress environment and the ability of morphine to modulate these changes. Thus, morphine, and by inference the endogenous opioid system, are involved in homeostasis of lymphocyte recirculation, probably through effects on central mediation of the stress axis.

The effects of simultaneous administration of alpha(2) -adrenergic agents with L-NAME or L-arginine on the development and expression of morphine dependence in mice

Both alpha(2)-adrenoceptors and the L-arginine/nitric oxide (NO) pathway have been implicated in the modulation of morphine dependence. This study examined the effects of simultaneous administration of the alpha(2)-adrenoceptor agonist clonidine or the antagonist yohimbine together with the NO precursor L-arginine or the NO synthase (NOS) inhibitor NG-nitro-L-arginine methyl ester (L-NAME) on the induction and expression of morphine dependence as assessed by naloxone-precipitated withdrawal jumping and diarrhoea. Male NMRI mice weighing 20-30 g were used. In the induction phase, clonidine (0.01-0.1 mg/kg) intensified and yohimbine (0.5-2 mg/kg) attenuated the degree of morphine dependence. Yohimbine reversed the effect of clonidine. L-NAME (5 and 10 mg/kg) did not affect the development of morphine dependence, but significantly potentiated the effects of both subeffective (0.01 mg/kg) and effective (0.03 mg/kg) doses of clonidine. L-Arginine did not alter morphine dependence but inhibited the effect of clonidine. The effects of yohimbine in the induction phase were attenuated by L-NAME, but were not significantly affected by L-arginine. In the expression phase, clonidine attenuated and yohimbine intensified the signs of dependence. The effect of clonidine was inhibited by yohimbine. In the expression phase, L-NAME attenuated the withdrawal syndrome at 10 mg/kg and showed potentiation with clonidine in suppressing withdrawal signs. L-Arginine did not alter morphine dependence, but at 20 mg/kg inhibited and at 100 mg/kg potentiated the attenuating effect of clonidine on the expression of withdrawal syndrome. The effect of yohimbine on the expression phase was also attenuated by L-NAME, but was not significantly affected by L-arginine. In conclusion, alpha(2)-adrenergic and NO pathways seem to be functionally linked in the modulation of opioid dependence.

Rapid elevation of plasma interleukin-6 by morphine is dependent on autonomic stimulation of adrenal gland

Several studies have demonstrated that opioids regulate a number of immune cell functions either through direct mechanisms or through the modulation of central nervous system outputs. It has been previously shown that morphine increases serum interleukin-6 (IL-6) levels; however, the mechanism by which this effect is produced is unknown. In the present study, experiments were designed to address the potential role of central opioid receptors, peripheral autonomic ganglia, and activation of the adrenals in the elevation of plasma IL-6 after morphine administration. A rapid and significant (2-fold) increase in plasma IL-6 was observed after morphine administration (10 mg/kg s.c.) to rats. This effect of morphine peaked within 30 min and remained elevated for at least 2 h. Central microinjection of morphine (10 microg/2 microl i.c.v.) mimicked the effects of peripherally administered morphine and was completely blocked by naltrexone (10 mg/kg s.c.) pretreatment. Pretreatment with a ganglionic blocker, chlorisondamine (0.5 mg/kg i.p.), also blocked the elevation of IL-6 by morphine, suggesting a role of the autonomic nervous system. In adrenalectomized animals, morphine administration did not increase IL-6 levels, whereas in adrenal demedullated animals, the effect of morphine remained intact. Thus, the adrenal cortex may be a potential source of IL-6, because IL-6 mRNA has been localized in the adrenal gland. Collectively, these data suggest a unique mechanism by which stimulation of central opioid receptors results in the elevation of plasma IL-6 through autonomic activation specifically of the adrenal cortex.

Suppressive effects of remifentanil on hemodynamics in baro-denervated rabbits

PURPOSE

To elucidate mechanisms by which remifentanil, an ultra-short-acting mu-opioid receptor agonist, causes hypotension and bradycardia.

METHODS

Mean arterial pressure (MAP), heart rate (HR) and renal sympathetic nerve activity (RSNA) were measured and recorded after bolus injections of 1, 2 or 5 microg x kg(-1) of remifentanil in neuraxis intact (n=6 for each dose) and baro-denervated rabbits (n=6 for each dose). Arterial baroreflex sensitivity was assessed by depressor tests. An additional six baro-denervated animals received remifentanil, 5 microg x kg(-1) after pretreatment with naloxone, 40 microg x kg(-1).

RESULTS

All values were expressed in % change from baseline. In the neuraxis intact animals, MAP and HR were decreased briefly immediately after remifentanil injection. RSNA was increased dose-dependently: 137 +/- 8% (mean +/- SE), 170 +/- 14% (P < 0.05) and 225 +/- 29% (P < 0.05) after 1, 2 and 5 microg x kg(-1) remifentanil, respectively. RSNA was increased even after MAP and HR had returned to baseline values. The depressor tests revealed that remifentanil did not attenuate arterial baroreflex sensitivity. In the baro-denervated animals, MAP and HR decreased gradually to 77 +/- 3% (P < 0.05) and 94 +/- 1% (P < 0.05), respectively 300 sec after 5 microg x kg(-1) remifentanil. At that time, increased RSNA (159 +/- 9%, P < 0.05) had returned to baseline. Pretreatment with naloxone in the baro-denervated animals abolished these changes.

CONCLUSION

Remifentanil decreases HR and MAP by its central vagotonic effect and by stimulating peripheral mu-opioid receptors. These effects appear to be counteracted and masked by its central sympathotonic effect and by maintaining arterial baroreflex integrity.

Heroin modulates the expression of inducible nitric oxide synthase

The use of heroin (diacetylmorphine) is associated with a high incidence of infectious disease, and the immunologic alterations responsible for heroin-induced changes in resistance to infection have not been well characterized. The present study tests the hypothesis that expression of inducible nitric oxide synthase (iNOS) is modulated by the administration of heroin. The initial study using rats showed that heroin administration (0, 0.01, 0.1, or 1.0 mg/kg s.c.) results in a pronounced reduction in lipopolysaccharide (LPS)-induced expression of iNOS mRNA in spleen, lung, and liver tissue as measured by RT-PCR. Heroin also produced a reduction in the level of plasma nitrite/nitrate, the more stable end-product of nitric oxide degradation. In a subsequent study, administration of the opioid receptor antagonist, naltrexone (0.1 mg/kg) prior to the injection of heroin (1.0 mg/kg) blocked the heroin-induced reduction of iNOS expression and plasma nitrite/nitrate levels indicating that the effect is mediated via the opioid-receptor. This study provides the first evidence that heroin induces an alteration of iNOS expression, and suggests that a reduction in nitric oxide production may be involved in the increased incidence of infectious diseases amongst heroin users.

Influence of morphine treatment in pregnant rats on the mineralocorticoid activity of the adrenals in their neonates

Exposure of pregnant rats to morphine, from day 11 to day 18 of gestation, was previously reported to induce both an adrenal atrophy and hypoactivity of the glucocorticoid function in newborns at term, but did not affect, in vitro, the responsiveness of those glands to adrenocorticotrophin hormone (ACTH) concerning corticosterone release. Moreover, these effects were mediated by maternal hormones from the adrenal glands. In the present work, we investigated the effects of a prenatal morphine exposure on the mineralocorticoid activity of the adrenals in neonates. The first aim of the present study was to determine in these newborns 1) the adrenal and plasma aldosterone concentrations at birth time and during the early postnatal period 2) the plasma levels of Na+ and K+ at birth time, 3) the in vitro responsiveness of the newborn adrenals to angiotensin II (A(II)) and ACTH. The second aim of our study was to investigate the mineralocorticoid activity of the adrenals in newborns from adrenalectomized mothers treated with morphine during gestation. According to present data morphine given to intact mothers induced in newborns a severe adrenal atrophy but increased adrenal aldosterone content and plasma aldosterone level. However, prenatal morphine was unable to affect significantly Na+/K+ ratio in both mothers and newborns. In vitro, the adrenals of neonates from morphine-treated mothers were unresponsive to An and ACTH for promoting aldosterone release; in contrast, aldosterone secretion was significantly stimulated by high potassium levels (55 mEq). Maternal adrenalectomy performed one day before the beginning of morphine treatment prevented morphine-induced adrenal atrophy but was unable to affect significantly the adrenal mineralocorticoid function of the offspring. Such data suggest that a prenatal morphine exposure stimulated both aldosterone synthesis and release in neonates. However, this basal hyperfunction did not appear to be coupled with an enhanced adrenal responsivity to AII or ACTH. Prenatal morphine-induced hyperactivity of the mineralocorticoid function of the newborn adrenals, which drastically contrast with hypoactivity of the glucocorticoid one, was independent of adrenal factors from maternal origin.

Psychoneuroendocrine immunology: perception of stress can alter body temperature and natural killer cell activity

Psychoimmunology has been credited with using the mind as a way to alter immunity. The problem with this concept is that many of the current psychoimmunology techniques in use are aimed at alleviating stress effects on the immune system rather than at direct augmentation of immunity by the brain. Studies in animals provide a model that permits us to approach the difficulties associated with gaining an understanding of the CNS-immune system connection. A particular advantage of using animals over humans is that psychological and social contributions play a less prominent role for animals than for human subjects, since the animals are all inbred and reared under identical controlled conditions. If the insightful information provided by animal studies is correct, then psychotherapy for the treatment of diseases might be made more effective if some aspect of this knowledge is included in the design of the treatment. We emphasize conditioning as a regimen and an acceptable way to train the brain to remember an output pathway to raise immunity. We propose that a specific drug or perception (mild stress, represented by rotation, total body heating or handling) could substitute and kindle the same output pathway without the need for conditioning. If this view is correct, then instead of using conditioning, it may be possible to use an antigen to activate desired immune cells, and substitute a drug or an external environmental sensory stimulus (perception) to energize the output pathway to these cells. Alternatively, monitoring alterations of body temperature in response to a drug or perception might allow us to follow how effectively the brain is performing in altering immunity. Studies with animals suggest that there are alternative ways to use the mind to raise natural or acquired immunity in man.

Role of central mu-opioid receptors in the modulation of nitric oxide production by splenocytes

Previous studies have shown that administration of morphine results in alterations of splenic macrophage nitric oxide production. The present studies were conducted to determine the subtype of opioid receptor involved in the modulation of macrophage nitric oxide production. Moreover, the present work was directed at determining whether nitric oxide production is regulated through opioid receptors in the central nervous system (CNS) or via opioid receptors found directly on splenocytes. The study shows that intracerebroventricular (i.c.v.) administration of the mu-selective opioid agonist, DAMGO, to rats dose-dependently increases the production of nitric oxide by splenocytes stimulated with toxic shock syndrome toxin (TSST-1). The effect of DAMGO is blocked by prior i.c.v. administration of N-methylnaltrexone. In contrast, i.c.v. administration of the kappa-selective agonist, U69,593, and the delta-selective agonist, DPDPE, have no significant effect on the production of nitric oxide. Furthermore, the in vitro administration of DAMGO, DPDPE, or U69,593 to splenocytes cultures does not significantly alter the production of nitric oxide by splenocytes. In addition, the present work shows that elevation of nitric oxide production by i.c.v. administration of DAMGO produces functional changes in splenic lymphocytes. Collectively, these results indicate that mu-opioid receptors within the CNS are involved in the regulation of splenic nitric oxide production.

Involvement of mu-opioid receptors and alpha-adrenoceptors in the immunomodulatory effects of dihydroetorphine

The present study investigated the effects of acutely administered dihydroetorphine on mitogen-stimulated lymphocyte proliferation and lymphokine production in mice. These immune functions were significantly suppressed by dihydroetorphine at 24 microg/kg and 128 microg/kg in a dose-dependent fashion. This study further examined the involvement of micro-opioid receptors and alpha-adrenoceptors in the immunomodulatory effects of dihydroetorphine. The micro-opioid receptor antagonist, naloxone (4 mg/kg), and alpha-adrenoceptor antagonist, phentolamine (10 mg/kg), but not the beta-adrenoceptor antagonist, propranolol (10 mg/kg), effectively blocked dihydroetorphine-induced suppression of splenic lymphocyte proliferation and lymphokine production. These results demonstrate that dihydroetorphine has significant immunosuppressive effects in mice and the mechanisms of these effects may lie in its interactions with opioid receptors and adrenergic pathways.

Evidence for central opioid receptors in the immunomodulatory effects of morphine: review of potential mechanism(s) of action

This review will discuss studies demonstrating that activation of opioid receptors within the central nervous system alters various immune system parameters. Specifically, natural killer cell cytolytic activity and lymphocyte proliferative responses to mitogen appear to be modulated predominantly, if not exclusively, through central opioid receptors. The potential mechanisms by which central opioid receptors appear to modulate these peripheral immune functions will be examined by evaluating the role of both the hypothalamic-pituitary-adrenal (HPA) axis and the autonomic nervous system. The studies discussed below indicate that acute administration of morphine or related compounds appears to primarily alter peripheral immune function through the sympathetic nervous system, while more prolonged exposure to opioids alter the immune system predominantly by activation of the HPA axis. Finally, the potential clinical relevance of these observations are discussed in relationship to both the therapeutic use, as well as the abuse of opioid compounds.

Characterization of [125I]beta-endorphin binding sites in the rat caudal dorsomedial medulla

[125I]beta-endorphin bound to high affinity (Kd = 0.25 nM) receptors in the caudal dorsomedial medulla of rats with a Bmax of 97 fmol/mg protein. The relative potency for displacement of [125I]beta-endorphin binding was: beta-endorphin(1-31) > beta-endorphin(1-27) > DAMGO > naloxone > N-acetyl-beta-endorphin(1-31) > U50488 > DPDPE. The Bmax for [3H]DAMGO binding was 81 fmol/mg protein, indicating that most [125I]beta-endorphin binding corresponds to mu-opioid receptors. [3H]DAMGO binding was not influenced by lesioning noradrenergic nerve terminals in the caudal dorsomedial medulla. Our findings indicate that beta-endorphin interacts primarily with mu-opioid receptors in the caudal dorsomedial medulla. These receptors are not affected by noradrenergic denervation.

Psychoneuroendocrine immunology: site of recognition, learning and memory in the immune system and the brain

How the interaction between the brain and immune system takes place has not been clearly defined. Because multiple changes are occurring simultaneously in all organ systems (e.g., cardiovascular, gastrointestinal, reproductive, renal, respiratory, immune, CNS), how many single systems interacts with the brain becomes extraordinarily difficult to understand. The problem boils down to developing an approach that not only allows one to study the whole organism and define the mediators of the interacting systems, but also permit one to establish the connection and physiologic relevance of the responses that are being evaluated. Conditioning, a phenomenon made popular by the work of Pavlov (1906, 1927), may provide insight into the pathways of communication between the brain and possibly any organ system of the body. Conditioning allows one to separate the afferent from the efferent circuits. That is, signals from the immune system to the CNS (IS-->CNS) can be effectively separated from signals from the CNS to immune system (CNS-->IS). This permits one to study each pathway individually. Simple, single association trial models to condition fever, natural killer (NK) cell and cytotoxic lymphocyte (CTL) activities have been developed to evaluate the pathways. Single trial learning is not new. Pavlov has observed that "The electric buzzer set going before administration of food established a conditioned alimentary reflex after only a single combination," whereas the reverse order of presentation failed to condition the animal (Pavlov 1927 p. 27). Thus, conditioning can be used to train the brain to activate the immune system and other organ systems participating in the response. During the course of the conditioned response, presumably the CNS via the hypothalamus integrates in a cohesive orderly fashion all input and output signals and coordinates the responses made by the brain to the organ systems. The odor of camphor, the conditioned stimulus (CS) can be associated with the response produced by an unconditioned stimulus (US). The unconditioned stimuli used are poly I:C to raise fever and nonimmunospecific NK cell activity or alloantigens to raise immunospecific CTL activity. The unconditioned stimulus serves only as a means to activate the immune system and unbalance the homeostasis so that a transient but new bidirectional communication loop can be established between the immune system and the CNS (IS<-->CNS). The expression of the conditioned response (i.e., elevation of fever, NK cell, or CTL activity) induced with the CS (odor stimulus) is an outcome of neural activity (CNS-->IS). This infers that during conditioning, the signals generated by the CS and US imprints a neural pathway located within the central nervous system and leaves behind a CS/US memory of the association. The immune activity (NK cell or CTL activity) which is modulated indicate that the memory pathway was activated in the brain of the animal expressing the conditioned response. The immune cells that are modulated can be considered to be casual bystander cells. These cells however must be in the proper (ready) state of activation to receive salient signals from the brain. Along with changes in the indicator cell population, other complex physiological processes are altered by the brain via sympathetic and neuroendocrine pathways to raise the fever response. These observations suggest that the physiological changes which are being evaluated such as fever, NK cell or CTL activities or perhaps blood pressure, heart rate, fat metabolism, oxygen consumption serve only as indicators (readouts), and infer that the CNS has made a coordinated reply in response to the CS signal.

Effect of morphine on lipopolysaccharide-induced tumor necrosis factor-alpha production in vivo: involvement of the sympathetic nervous system

Morphine treatment modulates a variety of immunological parameters, including tumor necrosis factor-alpha (TNF-alpha) production by activated macrophages in vitro. The aim of our study was to clarify the effect of morphine on lipopolysaccharide (LPS)-induced TNF-alpha production in vivo. Plasma TNF-alpha levels of mice were determined by ELISA. Subcutaneous injection of morphine decreased LPS-induced TNF-alpha production throughout the response, an effect that was dose-dependent and reversible by naloxone. Blockade of the sympathetic transmission by chlorisondamine prevented the inhibitory effect of morphine on TNF-alpha production. It is concluded that (i) systemic administration of morphine inhibits LPS-induced TNF-alpha production in vivo via 'classic' opioid receptors; (ii) this effect requires intact sympathetic outflow. Since the increased incidence of bacterial and viral infections in opioid addicts is well documented, it is suggested that the inhibitory effect of morphine on TNF-alpha production might play a substantial role in the increased vulnerability of these individuals to certain infections.

Beta-endorphin inhibition of endogenous norepinephrine release from the A2 noradrenergic nucleus in vitro: role of mu opiate receptors and Na+ ion permeability

An in vitro approach was used to determine the opioid receptor subtype mediating beta-endorphin inhibition of endogenous norepinephrine release from the A2 nucleus in the caudal dorsomedial medulla of rats. The voltage-sensitive Na+ channel blocker tetrodotoxin was used to investigate the role of Na+-dependent action potentials in beta-endorphin inhibition of K+-evoked norepinephrine release. Human beta-endorphin(1-31) inhibited K+-evoked norepinephrine release in a concentration-dependent fashion. Activation of delta- and kappa-opioid receptors had no effect on endogenous norepinephrine release. The inhibitory effect of beta-endorphin was blocked in a concentration-dependent manner by the mu-opioid receptor antagonist CTOP (Cys2, Tyr3, Orn5, Pen7 amide). Tetrodotoxin (TTX) inhibited norepinephrine release evoked by 25 mM K+ in a concentration-dependent manner and blocked the inhibitory effects of beta-endorphin. These results indicate that beta-endorphin acts on mu-opioid receptors to inhibit K+-evoked norepinephrine release from A2 neurons and suggest that the receptors involved are not located on noradrenergic nerve terminals.

Potential role of the autonomic nervous system in the immunosuppressive effects of acute morphine administration

These studies investigated the role of the autonomic nervous system in mediating the immunosuppressive effect of morphine on blood lymphocyte proliferation in rats. To determine the contribution of the autonomic nervous system, rats were pretreated with the ganglionic blocker chlorisondamine (5 mg/kg) prior to morphine (7 mg/kg) administration. Ganglionic blockade with chlorisondamine completely antagonized the inhibitory actions of morphine, suggesting that intact ganglionic transmission was required for the inhibition to occur. Blockade of postganglionic parasympathetic neurotransmission with atropine methylbromide (1 mg/kg) or blockade of sympathetic neurotransmission with the alpha-adrenoceptor antagonist phentolamine (1 mg/kg) did not attenuate the suppressive effect of morphine. Blockade of beta-adrenoceptors with propranolol (2.5 mg/kg) resulted in partial antagonism, but this action was not shared by the peripherally acting beta-adrenoceptor antagonist nadolol (6 mg/kg). These results suggest that the inhibitory effect of morphine on blood lymphocyte proliferation may be mediated through activation of the autonomic nervous system; however, individual blockade of either the parasympathetic or sympathetic division of the autonomic nervous system was not sufficient to antagonize this immunosuppressive effect.

Opioid mechanisms controlling renal function

1. Over the past 50 years considerable evidence has been reported suggesting that endogenous opioids participate in the control of renal function. 2. Exogenous administration of opioids produces profound changes in the renal excretion of water and sodium. 3. Opioids produce changes in urine output and urine sodium excretion by multiple integrated neural and hormonal mechanisms within the periphery, central nervous system and kidneys. 4. Although opioid antagonist administration does not consistently reveal an action of endogenous opioid systems on renal function, this may result from the quiescent nature of the endogenous opioid system under basal conditions. 5. Manipulations that activate endogenous opioid systems have begun to reveal important, previously unrecognized mechanisms that control kidney function and can enhance renal tubular sodium reabsorption in normal and potentially pathological states.

Exogenous and endogenous opioids as biological response modifiers

Narcotic opioid compounds are among the most widely prescribed drug interventions for individuals suffering pain. Among the unwarranted side effects of respiratory depression, constipation, and physical dependence are the immunosuppressive qualities, particularly those which affect cell-mediated immunity. The immunosuppressive characteristics of opioid narcotics (e.g., morphine) have recently come into focus with the advent of acquired immune deficiency syndrome (AIDS) and the putative causative agent, human immunodeficiency virus type 1 (HIV-1). Specifically, a vast reservoir of HIV-1-infected individuals exists among drug abusers. Moreover, experimental evidence would suggest narcotic opioids may increase viral load in infected individuals by modifying the cellular machinery of activated leukocytes. Likewise, investigators have shown that opioids modify tumor growth and development. In this review, a comparison between endogenous opioid peptides and exogenous opiates on cell-mediated immunity and its relationship to viral infection and tumors is described.

Mechanisms of conditioned immunomodulation

This article presents some recent work from our laboratory indicating that multiple physiological systems play a role in conditioned immunomodulation. The first study shows that naltrexone, but not N-methylnaltrexone, blocks the suppressive effects of an aversive conditioned stimulus on Con-A-induced proliferation and natural killer cell activity of splenic lymphocytes. This finding indicates that central opioid activity is involved in the conditioned effects. The second study shows that the beta-adrenergic antagonists atenolol and ICI-118,551 block of the suppressive effects of an aversive conditioned stimulus on Con-A-induced proliferation, but have no effect on natural killer cell activity. This result indicates the involvement of the adrenergic system in a subset of the conditioned effects. Collectively, these experiments provide evidence that both the opioid system and the sympathetic nervous system are involved in conditioned immunomodulatory changes elicited by an aversive conditioned stimulus.

Regional haemodynamic effects of mu-, delta-, and kappa-opioid agonists microinjected into the hypothalamic paraventricular nuclei of conscious, unrestrained rats

1. The cardiovascular effects of bilateral injection into the hypothalamic paraventricular nuclei of selective mu-, delta-, and kappa-opioid receptor agonists were investigated in conscious, unrestrained Wistar Kyoto rats, chronically instrumented with pulsed Doppler flow probes for measurement of regional haemodynamics. 2. The selective mu-agonist [D-Ala2,MePhe4,Gly5ol]enkephalin (DAMGO), injected bilaterally into the hypothalamic paraventricular nuclei (0.01-1.0 nmol), caused increases in blood pressure, tachycardias, vasoconstriction in renal and superior mesenteric vascular beds and substantial vasodilatation in the hindquarter vascular bed. 3. The administration of increasing doses (0.01-5.0 nmol) of the selective delta-agonist [D-Phe2,5]enkephalin (DPDPE) or the selective kappa-agonist, U50488H into the paraventricular nuclei (PVN) had no significant effect on blood pressure, heart rate, or regional haemodynamics. 4. Together, the present results are further evidence of a role for opioid peptides, especially acting at mu-receptors in the PVN, in the central regulation of the cardiovascular system, whereas a role for opioid peptides, acting at delta- and kappa-receptors in the PVN, seems less obvious from the present results.

Opioid peptide inhibition of endogenous norepinephrine release from the A2 noradrenergic cell group in vitro

We investigated the potential influence of opioid and melanotropic peptides on endogenous norepinephrine (NE) release from the A2 noradrenergic cell group of rats using a static, fixed-volume incubation procedure. Norepinephrine release from slices of caudal dorsomedial medulla (CDMM) was evoked by high potassium concentrations (20 and 60 mM) in a Ca(2+)-dependent and dose-related manner. Treatment with the potent melanotropin agonist [Nle4,D-Phe7] alpha-MSH(NDP-MSH) had no effect on K(+)-induced NE release. In contrast, human beta-endorphin1-31 significantly reduced K(+)-stimulated NE release, but not in the presence of naloxone. The highly-selective mu-opioid agonist [D-Ala2, N-Me-Phe4, Gly-ol5]-enkephalin (DAMGO) also significantly reduced evoked NE release. The inhibitory effect of DAMGO was completely blocked by naloxone. Naloxone alone did not alter evoked NE release. The inhibitory effect of DAMGO was not enhanced by reducing the stimulatory concentration of K+. None of the peptides tested influenced basal NE release. These data indicate that melanotropin receptors do not regulate NE release in CDMM. In contrast, the opioid peptides DAMGO and beta-endorphin inhibit K(+)-stimulated release of endogenous NE. These data suggest a role for mu-opioid receptors in controlling NE release from A2 noradrenergic neurons.

Differential effects of prolonged restraint stress on proenkephalin gene expression in the brainstem

Enkephalins have been discovered in various regions of the brain involved in cardiovascular regulation. The primary source of plasma Met-enkephalin released in response to stress, appears to be from sympathetic nerves. However, levels of Met-enkephalin are 2-3-fold higher at 2 min versus 30 min of restraint stress. Therefore, the purpose of this investigation was 2-fold; to determine whether proenkephalin gene expression is altered in the brainstem during restraint stress, and whether the magnitude of the change is attenuated with prolonged stress. Proenkephalin mRNA levels were compared in the area postrema (AP), nucleus tractus solitarius (NTS), rostral (RVLM) and caudal ventrolateral medulla (CVLM) following 2 and 30 min of restraint stress. During 2 min restraint stress, there was an approximately 1 fold increase in proenkephalin gene expression in the NTS, CVLM and RVLM with a approximately 50% decrease in the AP. With 30 min restraint stress, the increase in proenkephalin gene expression was maintained in the CVLM and RVLM, however mRNA levels had returned to control levels in the NTS and were approximately 1-fold higher than control in the AP. If the increases of proenkephalin gene expression in the NTS, CVLM and RVLM reflect changes in enkephalinergic neuronal activity in those regions, the alterations in enkephalinergic neuronal activity may be an important regulator of blood pressure homeostasis.

Effects of intracerebroventricular clonidine on the hypothalamic noradrenaline and plasma corticosterone levels of opiate naive rats and after naloxone-induced withdrawal

The aim of this study was to determine whether hypothalamic noradrenergic neuronal activity contributes to the abstinence-induced hypersecretion of corticosterone during naloxone-induced withdrawal. With this purpose the effects of intracerebroventricular (i.c.v.) clonidine on hypothalamic noradrenaline (NA) and plasma corticosterone were studied in chronically placebo-treated rats (controls) and during naloxone-induced morphine withdrawal. In control rats, clonidine (10 micrograms) significantly increased plasma levels of corticosterone without changing the hypothalamic content of NA. Naloxone (1 mg/kg, s.c.) also increased plasma corticosterone levels and clonidine administered prior to naloxone, antagonized the effect of naloxone on plasma corticosterone. In chronically morphine-treated rats, naloxone treatment induced an increase in plasma corticosterone and reduced the hypothalamic NA content. Clonidine significantly prevented the reduction in the hypothalamic NA, without modifying plasma levels of corticosterone. The results show an interaction between opioid-receptors and alpha 2-adrenoceptors in the hypothalamus, and suggest that mechanisms other than hyperactivity of NA neurons contribute to the hypothalamus-pituitary-adrenocortical (HPA) axis hyperactivity during the opiate withdrawal.

L-type Ca2+ channel ligands modulate morphine effects on the hypothalamus-pituitary-adrenocortical axis in rats

The role of the L-type Ca2+ channel in the acute effects of morphine on the hypothalamo-pituitary-adrenocortical (HPA) system was studied by administration of the Ca2+ channel agonist, BAY K 8644, and the antagonists, verapamil and nimodipine, to rats. Morphine (30 mg/kg i.p.) induced an increase in corticosterone secretion 30 min after injection, which was correlated with a simultaneous change in hypothalamic noradrenaline (NA) and dopamine (DA) contents. Pretreatment with verapamil (10 or 20 mg/kg i.p.) or nimodipine (5 mg/kg i.p.) antagonized the HPA activation induced by morphine, blocking both the decrease in hypothalamic NA levels and the elevation in plasma corticosterone induced by the opioid. BAY K 8644 (2 mg/kg i.p.) potentiated the effects of morphine, decreasing the hypothalamic NA content and increasing the release of corticosterone. The Ca2+ channel antagonist, nimodipine, given alone induced a slight reduction in hypothalamic NA content but did not modify plasma corticosterone levels. Verapamil given alone did not alter HPA activity. Instead, the Ca2+ agonist decreased the hypothalamic catecholamine content and increased plasma corticosterone levels. These results indicate that Ca2+ influx is necessary for the expression of opioid actions on the HPA system, and suggest that the Ca2+ flux in hypothalamic neurons is functionally linked to activation of opioid receptors.

Pavlovian conditioning of morphine-induced alterations of immune status

The present study provides the first demonstration that alterations of immune status can be conditioned to environmental stimuli paired with morphine administration. In the first manipulation, male Lewis rats received 0, 2, 4, 8 or 16 conditioning sessions during which a subcutaneous injection of morphine (15 mg/kg) was paired with a distinctive environment. The results showed that subsequent re-exposure to the distinctive environment induced a decrease in the mitogenic responsiveness of splenic lymphocytes and a decrease in natural killer cell activity in animals which had received 2, 4, 8 and 16 conditioning sessions. In the second manipulation, using two conditioning sessions, a wider range of immune measures was assessed and control groups were included to ascertain whether the observed immune alterations were due to Pavlovian conditioning processes. The results showed that the environment which had been paired with morphine altered the mitogenic responsiveness of blood and splenic lymphocytes, decreased natural killer cell activity and decreased interleukin-2 production. In contrast, the conditioned environment did not have any effect on mitogenic responsiveness of lymphocytes derived from the mesenteric lymph nodes. Data from the control groups revealed that the compartment-specific immune alterations were the result of a Pavlovian conditioning process.

Evidence for involvement of catecholamines in the effect of morphine on ventricular automaticity in the rat

1. The present study examined the effects of morphine on ectopic automaticity induced by local injury in the isolated right ventricle of the rat. 2. Morphine (10(-7)-5 x 10(-5) M) induced a significant increase of ventricular rate similar to that produced by noradrenaline. The excitatory effect of morphine was not modified by the presence of naloxone (5 x 10(-5) M). The maximal effect obtained with morphine in the presence of naloxone was 60 +/- 7%, similar to that obtained with morphine alone (67 +/- 15%). The EC50 values for morphine in the absence (0.89 x 10(-7) M) and presence of naloxone (0.87 x 10(-7) M) were also similar. Apparently this effect is not mediated by postsynaptic opioid receptors. 3. The ventricular automaticity induced in isolated right ventricle of the rat was significantly decreased by the highest concentrations of naloxone (5 x 10(-5) and 10(-7) M). 4. Morphine (10(-9)-5 x 10(-5) M) did not significantly change ventricular automaticity in the presence of propranolol (5 x 10(-8) M) or in reserpinized rats (5 mg kg-1 i.p. 24 h before the experiments). The maximal increases induced by morphine in the presence of propranolol or in reserpinized rats were 5 +/- 0.8% and 16 +/- 14.7% respectively. These results were significantly different from the maximal increase obtained without propranolol or with non-reserpinized animals. It is possible that the effects of morphine on ventricular automaticity could be mediated by an indirect effect located presynaptically at the adrenergic nerve terminals through the release of catecholamines.

Role of opioid receptors in the long-lasting blood pressure depression after electric muscle stimulation in the hind leg of the rat

In a previous study, electrically induced contractions of the gastrocnemius muscle in conscious spontaneously hypertensive rats were shown to induce a blood pressure reduction of 15-20 mmHg lasting several hours. We showed in that study that endogenous opioid systems were involved. In this study, drugs with selective affinity for different opioid receptors were used to analyse further the involvement of endogenous opioid systems in the post-stimulatory drop in blood pressure in spontaneously hypertensive rats. Prestimulatory intracerebroventricular administration of beta-FNA (a mu-receptor antagonist) did not significantly influence the response at all, nor did a lower intravenous dose of naloxone reverse the post-stimulatory drop in blood pressure. High-dose naloxone (15 mg kg-1) increased post-stimulatory blood pressure by around 10 mmHg. About 50% of the drop thus remained after this treatment. A similar, partial reversal of the decreased blood pressure was seen after intravenous administration of a delta-receptor antagonist, ICI 154,129. However, the depressor response was completely reversed by a low intravenous dose of MR 2266 BS (a kappa-receptor antagonist). These results suggest that the reduction in blood pressure after muscle stimulation is mainly mediated by the opioid kappa-receptor. A certain involvement of the delta-receptor is also indicated.

Fluorescent histochemical localization of neutral endopeptidase-24.11 (enkephalinase) in the rat brainstem

Characterization of the distribution of the peptide-degrading enzyme neutral endopeptidase-24.11 (E.C. 3.4.24.11; NEP; enkephalinase) in the rat brainstem was examined by means of a unique fluorescent histochemical method. Enzyme staining was completely blocked by three potent NEP inhibitors (thiorphan, phosphoramidon, and JHF-26) at a concentration of 50 nM, supporting the specificity of this method to visualize sites of NEP activity selectively. At all levels of the brainstem, NEP was localized to cell bodies, cell processes or terminal-like fields and was localized to more than 90 distinct nuclei or subnuclei. In the mesencephalon these included the central gray, cuneiform n., dorsal and lateral tegmental n., inferior colliculus, interpeduncular n., lateral and medial geniculate n., central linear raphe n., mesencephalic n. of the trigeminal nerve, mammillary nuclei, occulomotor n., red n., superior colliculus, ventral n. of the lateral lemniscus, substantia nigra-ventral tegmental area, and the zona incerta. In the pons, NEP staining was restricted to fewer regions or nuclei, including the dorsal and ventral cochlear n., facial n., motor trigeminal n., principal sensory trigeminal n., parabrachial nuclei, pontine n., the oral and caudal pontine reticular n., pontine olivary nuclei, several pontine tegmental nuclei, pontine raphe nuclei, and the trapezoid n. In the cerebellum, staining was localized largely to the granule cell layer of the cerebellar cortex. Scattered staining was observed in the molecular cell layer. The medulla contained extensive NEP staining localized to nuclei that included the ambiguous n., dorsal motor n. of the vagus, hypoglossal n., inferior olivary n., prepositus hypoglossus n., solitary tract n., nuclei of the spinal tract of the trigeminal n., and the lateral, medial, and superior vestibular nuclei. Nuclei of the medullary reticular formation that were also richly stained for NEP included the raphe magnus n., raphe obscurus n., raphe pallidus n., dorsal, lateral, and ventral reticular nuclei of the medulla, and the gigantocellular, lateral paragigantocellular, linear, paramedian and parvicellular reticular nuclei. The widespread distribution of NEP in the brainstem suggests the existence of a number of functional systems, including the pathways involved in the mechanisms of pain and analgesia, which are potential targets of NEP inhibitors. In most regions, the distribution of NEP closely overlapped with that reported for the enkephalins, and showed a more restricted overlap with the reported distribution of substance P.

Morphine inhibition of lymphocyte activity is mediated by an opioid dependent mechanism

The effects of acutely-administered morphine on mitogen stimulated lymphocyte proliferation and natural killer cell cytolytic activity were investigated. Two hours after a subcutaneous injection of morphine (25 mg/kg), blood lymphocyte proliferation was found to be 70% depressed, compared to saline-injected controls. This effect was partially antagonized in animals pretreated with naltrexone (10 mg/kg) and was present only in blood lymphocytes, since proliferative responses of splenic lymphocytes were not significantly altered. The administration of morphine, however, did result in a 30-40% inhibition of cytolytic activity of natural killer cells, which was completely antagonized in naltrexone-pretreated animals. Naltrexone alone was found to have no effect on either proliferation of blood and splenic lymphocytes or the cytolytic activity of splenic lymphocytes. Although naltrexone had no effect on the activity of lymphocytes, animals treated with either naltrexone or morphine alone, or their combination, had 4- to 8-fold increases in corticosterone in plasma. These results demonstrate that the effect of morphine on immune cells was dependent on the tissue source of lymphocytes. Furthermore, the suppression of blood lymphocyte proliferation and splenic cytolytic activity of natural killer cells by morphine was opiate receptor-mediated, as indicated by the reversibility by naltrexone of the observed effects of morphine. Finally, the accompanying increase in circulating levels of corticosterone most likely did not contribute to these effects.

Opiate receptors and the endorphin-mediated cardiovascular effects of clonidine in rats: evidence for hypertension-induced mu-subtype to delta-subtype changes

Effects of opiate receptor antagonists on centrally mediated cardiovascular responses to clonidine and beta-endorphin were studied in urethane-anesthetized spontaneously hypertensive Okamoto-Aoki rats (SHR), normotensive Sprague-Dawley rats, and Sprague-Dawley rats made hypertensive with deoxycorticosterone pivalate/salt. Microinjection of 270 pmol of naloxone into the nucleus tractus solitarii (NTS) significantly inhibited the hypotensive and bradycardic response to 5 nmol of similarly administered clonidine in both SHR and normotensive Sprague-Dawley rats. In SHR, a similar inhibition was observed after the delta-opiate receptor antagonist ICI 174864, but not after the mu-receptor antagonist beta-funaltrexamine (both at 270 pmol, intra-NTS), whereas in normotensive Sprague-Dawley rats, beta-funaltrexamine, but not ICI 174864, was an effective inhibitor. The same pattern of differential inhibition was seen when clonidine was given i.v. and the opiate antagonists were given intracisternally in SHR and Sprague-Dawley rats. Intra-NTS microinjection of 280 fmol of beta-endorphin caused hypotension and bradycardia, and these effects were similarly inhibited by ICI 174864 in SHR and by beta-funaltrexamine in Sprague-Dawley rats. In Sprague-Dawley rats made hypertensive by chronic administration of deoxycorticosterone pivalate and salt, the hypotensive and bradycardic effects of intra-NTS clonidine were inhibited by ICI 174864, but not by beta-funaltrexamine, a pattern similar to that in SHR, but different from that in normotensive Sprague-Dawley rats. These results support the hypothesis that beta-endorphin release and subsequent stimulation of opiate receptors in the NTS are involved in the cardiovascular effects of clonidine in rats. These results further suggest, however, that hypertension regulates the subtype of opiate receptors mediating these effects.

Cerebrospinal fluid changes in experimental cardiac arrest (maximal stress)

Cardiac arrest produces a prompt and maximal increase of plasma catecholamines, with associated elevations of the hormones involved in the endocrine response to stress. To investigate the participation of the central nervous system (CNS) in the generation of the endocrine response, the catecholamines epinephrine and norepinephrine in cerebrospinal fluid (CSF) were measured before, during, and after cardiac arrest accompanied by cardiopulmonary resuscitation (CPR) in adrenalectomized (ADX) and sham-operated (SHAM) dogs. We also determined the activity of acetylcholine esterase (AChE), an intracellular enzyme released into the CSF after hypothalamic or caudate stimulation. During CPR, plasma epinephrine increased significantly in SHAM but not ADX dogs, increasing from (mean +/- SE) 480 +/- 171 to 29,800 +/- 14,200 pg/ml (P less than 0.05). Prearrest CSF norepinephrine was higher in ADX than SHAM dogs and increased in both groups with cardiac arrest, but the increase was significant only in SHAM animals; CSF epinephrine remained unchanged during or after cardiac arrest. CSF AChE activity increased during and after defibrillation; the difference with basal levels became significant when the peak postarrest values were considered (P less than 0.05). These results document biochemical changes occurring in the CNS during maximal stress represented by cardiac arrest. It is suggested that CSF norepinephrine and AChE activity elevations are markers for hypothalamic activation from the stress of cardiac arrest.