Stress-induced analgesia in humans: endogenous opioids and naloxone-reversible depression of pain reflexes

The effect of intravenous lidocaine on nociceptive processing in diabetic neuropathy

In a double-blind controlled design, 7 patients with painful diabetic neuropathy received lidocaine 5 mg/kg or saline intravenously over a period of 30 min. Thermal sensibility quantified by thermotest was not affected by lidocaine. In 3 of the patients nociceptive flexion reflex thresholds could be determined. The threshold was increased by lidocaine and returned to pre-infusion level within 10 days. Lidocaine also increased the threshold in 4 healthy subjects, but did not affect the Hoffmann reflex. These results suggest that lidocaine exerts its pain-relieving effect on the spinal level in diabetic neuropathy.

Naloxone in moderate dose does not aggravate chronic tension headache

Naloxone per se causes no pain in normal man, indicating that opioidergic antinociceptive systems are not tonically active, but this might not be the case in chronic pain conditions. The present investigation tested the hypothesis that pain in chronic headache is the result of insufficiently attenuated nociceptive impulses. Forty-seven patients suffering from chronic tension headache entered the present double-blind cross-over trial of naloxone 4 mg i.v. versus saline. Adverse effects were negligible. Patients scored headache pain on a 100 mm visual analog scale and change in headache on a 5-point verbal rating scale after 5, 15, 30, 60 and 90 min. Mean arterial blood pressure decreased 4.2 mm Hg (P less than 0.05) after naloxone compared to saline, but naloxone had no effect on headache (P = 0.96). A bimodal distribution of acute pain patients into placebo responders and non-responders has been reported, but our chronic pain patients showed a homogeneous placebo response. Review of the literature indicates that acute clinical pain and stimulation-induced analgesia in experimental pain has a naloxone-responsive component. Chronic pain does not appear to be influenced by naloxone in moderate doses.

Immunoreactive beta-endorphin levels in cerebrospinal fluid of children with acute lymphoblastic leukemia: relationship with glucocorticoid therapy and neurological complications

Neurological disorders, such as seizures, are not infrequently associated with anti-leukemic therapy. It has been hypothesized that a disrupted peptidergic transmission between neurons could be the cellular basis of the neurological dysfunction. Since endogenous opioids have been recently found to alter neuronal function and possess anticonvulsant properties, the cerebrospinal fluid (CSF) immunoreactive beta-endorphin levels in children with Acute Lymphoblastic Leukemia (ALL) during chemotherapy and cranial irradiation have been studied. Twenty-seven children, 2 at low, 20 at medium and 5 with high risk ALL, undergoing prophylactic treatment for meningeal leukemia, entered the study. Sequential lumbar punctures with introduction of MTX combined with oral prednisone therapy were performed; each lumbar puncture sample was collected and assayed for immunoreactive beta-endorphin. All the patients studied showed a biphasic profile of the peptide with the minimum levels reached during the induction (days 14-28) and the maximum levels detected at the end of the intensification chemotherapy (days 49-55). In the 3 groups the beta-endorphin decrease corresponded to the period of prednisone therapy; the increase was concomitant with the suspension of oral glucocorticoids. 3 patients showed tonic-clonic seizures which coincided with the lowest cerebrospinal fluid beta-endorphin levels and, in the follow-up, 13 out of 27 patients displayed EEG abnormalities. From these findings a relationship between cerebrospinal fluid beta-endorphin concentrations and neuronal excitability in patients with ALL can be suggested. It is also evidenced that oral glucocorticoid therapy has profound inhibitory effects on central beta-endorphin levels.

Naloxone does not alter the perception of pain induced by electrical and thermal stimulation of the skin in healthy humans

It has been hypothesized that, in the absence of acute or chronic pain, a tonically active system exists involving opioid peptides, which ensures a certain level of pain insensitivity. Although various studies have failed to support this concept, it has been reported that in conditions of both experimentally induced and clinical pain, high doses of the opioid antagonist naloxone induced a state of hyperalgesia and thus seemed to set off this hypothetical system. Lower doses were, however, without effect or even acted as analgesics. This study investigated the effect of 5 and 20 mg naloxone i.v., compared to placebo, on the perception of pain in healthy humans. Pain was induced by two methods, using electrical and thermal stimulation of the skin, which have previously been shown to be sensitive to the effects of opioid as well as of non-steroidal anti-inflammatory analgesics. Each of 12 males and 12 females participated in 3 experimental sessions, in which the treatments were administered double-blind according to a Latin square design. Threshold and tolerance to electrically induced pain and threshold to thermally induced pain were measured at 30 min intervals for 90 min before and 90 min after drug administration. Electrical stimuli were square wave constant current impulses of linearly increasing intensity; thermal stimuli were of constant intensity and variable duration. Threshold and tolerance to electrically induced pain were not altered by either dose of naloxone, whereas the threshold to thermally induced pain was significantly higher after both 5 and 20 mg naloxone than after placebo, the effects of the two naloxone doses not differing from each other.(ABSTRACT TRUNCATED AT 250 WORDS)

Correlation between plasma levels of ACTH and beta-endorphin in the first seven days of postnatal life

Plasma adrenocorticotropic hormone (ACTH) and beta-endorphin (beta-EP) concentrations were measured by radioimmunoassay in 122 newborns, born vaginally after spontaneous labor between the 38th and the 41st week of pregnancy. Blood samples were taken from umbilical cord in 10 newborns and from antecubital vein in the remaining 112 newborns, divided into 8 groups aged from 12 h to 7 days. The mean (+/- SE) ACTH concentrations in cord plasma were 81.87 +/- 10.16 pg/ml and decreased significantly (p less than 0.01) at the 24th h of life (49.09 +/- 6.93 pg/ml). Afterwards mean ACTH plasma concentrations fluctuated around the latter values. The mean (+/- SE) B-EP concentrations in cord plasma were 21.96 +/- 3.12 pmol/l and decreased significantly at the 24th h of life (13.43 +/- 2.08 pmol/l; p less than 0.01). From the 24th h to the 7th day the mean plasma concentrations of beta-EP were not significantly different. ACTH and beta-EP plasma levels were positively correlated (p less than 0.001) at delivery and during the first seven days of life.

Effects of heterotopic ischemic pain on muscular pain threshold and blink reflex in humans

Ischemic pain was induced in the left arm of 6 healthy volunteers using a modification of the submaximal effort tourniquet test. Its effects on both muscular pain threshold (determined in the vastus medialis muscle by means of electrical stimulation) and on the 2nd component of the blink reflex (R2) were investigated in the contralateral body side. Muscular pain threshold increased (peak value during ischemic pain) and maintained significantly higher values for 60 min. R2 response decreased while its latency increased; these variations appeared during ischemic pain and reached a maximum soon after the end of ischemia, still remaining significant for 15 min. The results support the suggestion that the analgesic effects of heterotopic noxious stimulation in humans are subserved by supraspinal inhibitory mechanisms.

The neuropsychiatry of post-traumatic stress disorder

The publication of DSM-III introduced the diagnosis Post-Traumatic Stress Disorder (PTSD), thus providing, for the first time, a framework for studying the consequences of extremely stressful events. Previously, traumatic neuroses had attracted a wide variety of labels - as wide as the experiences that produced them. Competing explanations in psychological and biological terms have characterised the approach to these disorders, and social and legal issues have added to the confusion. In recent years, psychosocial issues have tended to dominate the literature in relation to PTSD. While acknowledging the importance of such phenomenological and psychosocial approaches, this paper seeks to redress the balance by focusing on a biological perspective.

Implications of endogenous opioids in the informational aggression syndrome

Starting from the observation that the informational aggression syndrome (IAS) in animals resembles the opioid abstinence syndrome, the authors studied the implications of endogenous opioids in experimentally induced IAS. They induced an IAS in rats, by means of specific sound signals and examined the animals' behaviour by a test used in the study of the opioid abstinence syndrome, as well as their reaction to noxious stimuli by means of thermal stimulation. The results show the implications of endogenous opioids in the IAS. During the first stage, a larger amount of endogenous opioids is secreted; specific symptoms of the opioid abstinence syndrome develop during the second stage. These symptoms can be prevented with clonidine, that has been used in the treatment of the exogenous opioid abstinence syndrome.

Pentobarbital, in subanesthetic doses, depresses spinal transmission of nociceptive information but does not affect stimulation-produced descending inhibition in the cat

The present study evaluates the effect of systemic pentobarbital on the spinal transmission of nociceptive information and on stimulation-produced descending inhibition in the deeply anesthetized, paralyzed cat. Single neuronal responses to noxious skin heating were recorded extracellulary in the lumbar dorsal horn and found to be depressed by pentobarbital at subanesthetic doses (4.0, 8.0, 17.0 and 24.5 mg/kg) in a dose-dependent manner. At 0.5 and 1.5 mg/kg, depression by pentobarbital was positively correlated with the depth of the recording site in the spinal cord (laminae IV-VI), i.e., neurons in deeper laminae (V-VI) were attenuated, while neurons in lamina IV were unaffected. At all doses tested, pentobarbital failed to affect stimulation-produced descending inhibition from either the midbrain periaqueductal gray or the medullary nucleus raphe magnus. The present data furnish evidence for the antinociceptive potency of pentobarbital, they do not support the view that a 'partial pharmacological spinal cord transection' would attenuate stimulation-produced descending inhibition of nociceptive dorsal horn neurons.

Enkephalin release into the ventral tegmental area in response to stress: modulation of mesocorticolimbic dopamine

Enkephalin-containing neuronal fibers and perikarya, and opioid receptors are present in the A10 dopamine (DA) region, and many studies have implicated enkephalin as a neuromodulator of A10 DA neurons projecting to the prefrontal cortex and certain limbic nuclei. Footshock stress is known to activate the A10 DA neurons projecting to the prefrontal cortex and nucleus accumbens, and the present study was designed to evaluate the possibility that footshock-induced release of enkephalin into the A10 DA region may play a role in activating the DA neurons. Microinjection of the quaternary opioid antagonist, naltrexone methobromide (NMB), into the ventral tegmental area (VTA; subnucleus of the A10 DA region) significantly attenuated the increase in DA metabolism produced by exposure to footshock (0.2 mA; 200 ms on; 800 ms off for 20 min) in the prefrontal cortex and nucleus accumbens. Rats were exposed to footshock for 5, 10 or 20 min and a time-dependent decrease in the level of immunoreactive Met-enkephalin was measured in the midline A10 region, but not in the lateral A10 region. It has been shown that daily exposure to footshock enhances the motor stimulant effect of intra-VTA injection of the enkephalin analogue, [D-Ala2,Met]-enkephalinamide (DALA). Rats were pretreated with an intra-VTA injection of NMB prior to daily exposure to footshock, and it was found that NMB abolished the potentiating effect of daily footshock on subsequent intra-VTA injection of DALA. Taken together, these data indicate that footshock stress enhances the release of enkephalin into the A10 region, and that this enkephalin activates A10 DA neurons projecting to the prefrontal cortex and nucleus accumbens.

Effect of citalopram, amineptine, imipramine and nortriptyline on stress-induced (footshock) analgesia in rats

The influence of the oral administration of different doses of citalopram (5, 15 and 45 mg/kg), imipramine (15, 30, 45 and 60 mg/kg), nortriptyline (15, 45 and 60 mg/kg) and amineptine (45 mg/kg) on stress-induced analgesia has been studied in anaesthetized rats. None of the administered antidepressants seem to have appreciable analgesic activity when analgesia is tested by the tail-immersion method. Citalopram, imipramine and nortriptyline, but not amineptine, increase the analgesia induced by inescapable footshock delivered continuously for 2 min to rats. Citalopram is the most potent drug. Our results support the suggested importance of 5-HT and noradrenaline terminals, but not those of dopamine, in the mediation of the stress-induced analgesia and seem to support the hypothesis that the analgesic activity of antidepressants is partially related to their modulating effects on the endogenously released opioid peptides involved in the endogenous pain inhibitory systems.

Endorphins are related to pain perception in coronary artery disease

Plasma beta-endorphin levels were measured by radioimmunoassay before and after exercise in 25 patients with coronary artery disease. Eighteen patients were men and 7 were women; age range was 36 to 75 years (mean 60). All patients had angina pectoris, a positive treadmill test response or positive exercise radionuclide findings. The mean preexercise plasma endorphin level was 4.9 +/- 3.0 pmol/liter (range 0.7 to 13.5). The mean postexercise plasma endorphin level of 6.6 +/- 4.6 mol/liter (range 0 to 19.5) was significantly higher (p less than 0.05). A significant positive correlation was seen between postexercise endorphin levels and time to onset of angina (r = 0.4, p = 0.03). There were negative correlations between postexercise endorphin levels and occurrence (r = -0.4, p = 0.04) and duration of angina (r = -0.4, p = 0.05). No association was found for maximal heart rate-blood pressure product, workload, time to ST-segment depression or stress ejection fraction. A positive correlation was found between rest left ventricular ejection fraction and postexercise endorphin levels (r = 0.5, p = 0.02). In conclusion, in patients with exercise-induced myocardial ischemia, plasma beta-endorphin levels are increased after exercise; postexercise endorphin levels are related to timing and occurrence (presence or absence) of angina; and endorphins may alter the perception of pain caused by myocardial ischemia.

Experience with alcohol and the endogenous opioid system in ethanol analgesia

This study employed naloxone, an opiate antagonist, to explore whether a learned opioid response, mediated by drinking experience, accounts for ethanol and placebo analgesia. Cold pressor pain was evaluated before and after ethanol (0.5 g/kg), placebo, and no-alcohol control treatments (administered in randomized order) and again after double-blind administration (6 mg/kg) of naloxone to 11 men and saline to 9. A triple interaction of treatments, antagonist conditions, and drinking experience indicated that naloxone as compared to saline diminished ethanol and placebo analgesia among experienced drinkers but had opposite effects among the same men in the control treatment. Six men, who reported that the injection of naloxone had an effect on pain, had higher drinking experience scores than the five men who reported naloxone had no effect. The similar pattern of response to both the alcohol and the placebo treatments suggests that the opioid system response to alcohol is learned.

Analgesia and the cardiovascular changes evoked by stimulating neurones in the ventrolateral medulla in rats

In rats anaesthetised with Saffan, selective excitation of neurones in the ventrolateral medulla in the region of nucleus paragigantocellularis lateralis (PGL) by microinjection of the synaptic excitant, D,L-homocysteic acid, produced an increase in the latency of the tail flick response to noxious heat usually accompanied by an increase in blood pressure and heart rate. These findings are discussed in relation to the role of neurones in PGL in generating both tonic descending inhibition in the dorsal horn and sympathetic vasomotor tone as well as their involvement in a descending pathway that mediates the hypoalgesia which is a feature of certain stress-induced hypertensive states.

Methylnalorphinium fails to reverse naloxone-sensitive stress-induced analgesia in mice

Exposure of mice to cold-restraint stress markedly decreased the number of abdominal constrictions induced by IP acetic acid. Naloxone pretreatment significantly attenuated the antinociceptive effect of cold-restraint stress, suggesting a partial mediation by opioid mechanisms. Pretreatment with the quaternary opioid antagonist methylnalorphinium did not reverse analgesia in stressed mice. Also, nociception in both stressed and non-stressed mice was not modified by pretreatment with the selective alpha 2-adrenoceptor blocker yohimbine. The results suggest that cold-restraint stress promotes analgesia in mice which is mediated in part by opioid but not alpha 2-adrenoceptor mechanisms. Furthermore, the results do not substantiate a peripheral analgesic role for circulating opioids in this model of stress.

The ability of PK 8165, a quinoline derivative, to reduce responses to a stressful situation in a double-blind study in man

A double-blind and cross-over study was carried out in order to explore the effects of PK 8165 (a quinoline derivative) on the heart rate, respiratory rate and motor reflex responses produced by an experimental model of stress in 8 healthy volunteers. The stress was induced by repetitive sequences of anticipation of pain (stressful stimulus: S) spaced by resting periods (R). In a control session, the cumulative effects of S resulted, in all subjects, in a progressive increase in heart and respiratory rates; 5 subjects showed a cumulative facilitation in the H reflex (motor reflex response) while the 3 others exhibited a cumulative depression in this motor parameter as a function of repetition of S during the session. The three doses (50, 100, 150 mg) of PK 8165 produced a very significant dose-dependent reduction in these responses during both stressful periods and resting sequences. Furthermore, the baseline values of respiratory and especially heart rate were also significantly reduced in a dose-dependent fashion by PK 8165. In contrast, the treatment with placebo did not significantly modify these parameters, compared to control values. The functional implications of these data are discussed in terms of stress-induced activation of some CNS structures and of the possible mechanisms of the "anti-stress" effect of PK 8165.

Diazepam reduces stress-induced analgesia in humans

The analgesic effects of a repetitive stress induced by anticipation of pain (noxious footshock) were studied on both the threshold of a nociceptive flexion reflex and the corresponding pain sensation after a 4-day-treatment of diazepam vs placebo (cross-over and double-blind study) in normal volunteers. During diazepam, the stressor stimulus produced a weaker depression on both nociceptive reflex and pain sensation than that observed during placebo. Furthermore, the reversal effect by naloxone was much more marked during placebo than during diazepam. These data clearly suggest a possible moderating action of benzodiazepine brain type receptors upon the endogenous opiate systems involved in the phenomenon of stress-induced analgesia in humans.

The effect of the cold pressor test on vasopressin secretion in man

Stress has been thought to induce the release of arginine vasopressin (AVP). We evaluated this claim by studying the effects of a modified cold pressor test on plasma AVP, plasma cortisol, blood pressure, pulse rate, and a number of variables known to affect AVP secretion. In a cross-over study design, test and control values were obtained in seven male subjects. The pressor test was found to induce painful stress as evidenced by subjective reports and the objective findings of increased mean arterial pressure (13.9 +/- 3.1 mm Hg; p less than 0.004), pulse rate (9.2 +/- 2.8 beats/min; p less than 0.02), and plasma cortisol (3.5 +/- 0.8 micrograms/dl; p less than 0.005). In contrast, there were no significant changes in plasma AVP that could be attributed to the cold pressor test. There also were no changes in plasma osmolality, measured plasma solutes, hematocrit or body temperature. An unexpected finding was a premonitory drop in plasma AVP occurring just prior to the pressor test (2.5 +/- 2.0 pg/ml; p less than 0.04) and at the comparable time point in the control study (3.1 +/- 1.2 pg/ml; p less than 0.001). There were no changes in any of the other measured variables which could account for this drop. We conclude that the cold pressor test is not a stimulus to AVP release and that anticipation of stress may inhibit secretion of this hormone.

The effect of food and water deprivation on post-stress analgesia in mice and levels of beta-endorphin and dynorphin in blood plasma and hypothalamus

Pain sensitivity of food and/or water-deprived male mice was tested on a hotplate. The most pronounced analgesia ensued in animals given no food and water, and no food but water ad libitum, the least one in water-deprived mice. The magnitude of the rise in pain threshold depended on the duration of deprivation and was correlated with the increase in the blood plasma beta-endorphin level. In the hypothalamus beta-endorphin level increased after 72-h food deprivation only. The level of dynorphin remained unchanged. Naloxone (10 mg/kg) almost completely reversed food or water-deprivation induced analgesia.

Endogenous opiate peptides, stress reactivity, and risk for hypertension

Endogenous opiate peptides can regulate neuroendocrine and circulatory responses to behavioral stress and may be important in the pathogenic effects of sympathoadrenal reactivity. We tested this hypothesis by examining the effect of the opiate antagonist naloxone on blood pressure responses to behavioral stress in young adults with high, medium, or low casual blood pressures. Naloxone increased mean arterial pressure responses to stress in subjects with low casual pressure, but had no significant effect on responses in subjects with high casual pressure. These results suggest opioidergic inhibition of sympathetic nervous system responses may be deficient in persons at risk for essential hypertension.

Stress produces opioid-like effects on investigatory behavior

Stimulation of opioid systems with opiate agonists produce characteristic alterations in the investigatory behavior exhibited by rats in a novel environment. As numerous reports now indicate that opioid systems can be activated by exposure to stress, the following study examined whether exposure to stressors could produce opiate-like alterations of investigatory behavior. Naive rats were exposed to one of three stressors (restraint, tailpinch pressure, high intensity white noise) or to control procedures, and were observed in a novel environment. The frequency and duration of a wide range of behavioral activities were recorded. All three stressors were found to produce morphine-like alterations of investigatory behavior. The average time an animal spent per contact with stimuli in the environment was decreased significantly by stress, with greater reductions being associated with locomotor hypoactivity. The stress-induced reductions of investigatory behavior were blocked by very low doses of the opiate antagonist naloxone (0.1-0.25 mg/kg). These results are consistent with an activation of opioid systems underlying some of the changes in investigatory behavior produced by exposure to stress.

Studies on pain. Effects of morphine on a spinal nociceptive flexion reflex and related pain sensation in man

The nociceptive flexion reflex and the corresponding subjective pain score elicited by sural nerve stimulation were studied in 6 healthy volunteers. A significant correlation was found between the respective recruitment curves of the reflex and of the pain score as a function of stimulus intensity. Consequently, the reflex (Tr) and the pain (Tp) thresholds were found to be almost identical (mean: 10.6 and 10.3 mA, respectively). Similarly, the threshold of the maximal reflex response (Tmr) was very close to that of intolerable pain (Tip): 37.1 and 38.8 mA, respectively. These four parameters were studied before and after intravenous administration of morphine chlorhydrate (0.05, 0.1, 0.2 and 0.3 mg/kg) and subsequent administration of naloxone hydrochloride (0.02 mg/kg; i.v.). While 0.05 mg/kg morphine remained without any effect, higher doses produced an increase in the four thresholds (Tr, Tp, Tmr, Tip). Furthermore, a very significant linear relationship was found between the importance of the increase and the dose of morphine. Morphine also depressed in a dose-dependent fashion, the nociceptive reflexes elicited by a constant stimulation intensity (1.2-1.3 Tr). All these effects were immediately reversed by subsequent naloxone. During all the pharmacological situations, variations in Tr and Tp as well as in Tmr and Tip were found to be very significantly linearly related, indicating a close relationship between the effects of morphine on the nociceptive reflex and on the related pain sensation. These results suggest that, in our model involving a brief 'epicritic' nociceptive stimulus, the mechanisms of morphine-induced analgesia in man can be explained by a depressive effect on the nociceptive transmission directly at a spinal level.

Tonic pain inhibits phasic pain: evoked cerebral potential correlates in man

In a sample of 10 healthy volunteers phasic pain ratings and evoked cerebral potentials (EPs) elicited by brief electrical skin stimuli were investigated in periods before, during, and after contralateral tonic ischemia pain. In all subjects the phasic pain ratings and the late EP components P80-N150 and N150-P260 were depressed under concurrent tonic pain. The magnitude of the mean reduction (31%, 40%, and 26%) is comparable to morphine analgesia. The early EP components with latencies below 80 ms, which are considered to be correlates of mechanosensitivity, were not influenced. The findings of tonic pain inhibiting phasic pain are discussed on the basis of changes in attention as well as of pain-specific physiological mechanisms like diffuse noxious inhibitory control.

Ventrolateral medullary lesions block the antinociceptive and cardiovascular responses elicited by stimulating the dorsal periaqueductal grey matter in rats

In rats anaesthetised with Saffan (Glaxovet), inhibition of the tail flick reflex evoked by electrical stimulation in the dorsal periaqueductal grey matter (PAG) was accompanied by an increase in blood pressure, tachycardia, vasodilatation in hind limb muscle, an increase in respiration, pupillodilatation and widening of the palpebral fissure. Stimulation deeper in the PAG and in the tegmentum ventral to it produced analgesia but without this pattern of autonomic changes. The antinociceptive, cardiovascular and respiratory effects of PAG stimulation were abolished by bilateral lesions in the ventrolateral medulla in the area which lies ventromedial to the facial nucleus (i.e., in nucleus paragigantocellularis lateralis, RPGL). Lesions in nucleus raphe magnus (NRM), or bilateral lesions of nucleus reticularis gigantocellularis (RG) and paragigantocellularis (RPG) or a combination of a lesion in RPGL with one in NRM or the contralateral RG or RPG did not block the effects of stimulating in the dorsal PAG. These findings are discussed in relation to the role of the PAG in mediating behavioural responses to stress.

Inescapable shock, neurotransmitters, and addiction to trauma: toward a psychobiology of post traumatic stress

Chronic post traumatic stress has been described as a "physioneurosis" (Kardiner 1941), that is, a mental disorder with both psychological and physiological components. The behavioral sequelae of inescapable shock in animals and of massive psychic trauma in people show a striking parallel. Inescapable shock in animals leads to both transient catecholamine depletion and subsequent stress-induced analgesia. We postulate that the numbing and catatenoid reactions following trauma in humans correspond to the central nervous system (CNS) catecholamine depletion that follows inescapable shock in animals. We further explore the evidence for a human equivalent of "stress-induced analgesia" in animals, which is known to be mediated by endogenous opioids. Although reexposure to trauma may produce a paradoxical sense of calm and control due to endogenous opioid release, a cessation of traumtic stimulation will be followed by symptoms of opioid withdrawal and physiological hyperreactivity mediated by CNS noradrenergic hypersensitivity. This hyperreactivity can, in turn, be temporarily modified by reexposure to trauma. This factor could account for voluntary reexposure to trauma in many traumatized individuals and would provide a complementary formulation to the conventional psychodynamic concept of attempted mastery of the psychosocial meaning of the trauma.

Opiate modulation of glucose turnover in dogs

We examined the effect of opiate infusion and of opiate blockage on glucose turnover in the basal state, using isotope dilution techniques in trained conscious dogs (n = 5). After a primed-continuous infusion of 3-3H glucose to steady state specific activity (90 minutes), infusion of one of the following was given: D-met2 pro5 enkephalinamide (DMPE), a potent morphine-like opiate, 0.5 mus g/kg/min; naloxone, an opiate antagonist, 1.25 mg followed by 10 mus g/min; or saline control. Infusion of DMPE led to a fall in glucose from 92 +/- 3 to 87 +/- 3 mg/dL by 60 minutes (P less than 0.05), associated with a rise in glucose utilization (Rd) from 3.0 +/- 0.4 to 3.9 +/- 0.6 mg/kg/min by 30 minutes (P less than 0.05); a transient rise in glucose production (Ra; from 3.2 +/- 0.4 to 4.3 +/- 0.4 mg/kg/min; P less than 0.05). Changes in counterregulatory hormones did not account for these findings; insulin was unchanged during all infusions; glucagon showed small late rises at 75 minutes during both DMPE and naloxone infusion; cortisol rose by 30 and 15 minutes, respectively, of DMPE and naloxone infusion; epinephrine rose transiently after 5 minutes of naloxone but was unchanged during DMPE, and norepinephrine was unchanged throughout. Saline infusion had no effects on any of these indices. We conclude that a potent opiate with morphine-like effects (DMPE) can lower glucose in dogs by enhancing peripheral glucose utilization independently of hormonal changes.

The effects of naloxone on opiate and placebo analgesia in healthy volunteers

Two double blind cross-over studies were performed using a submaximal effort tourniquet test (SETT) in healthy volunteers to investigate the role of endogenous opioids in placebo analgesia. In the first study IV naloxone significantly inhibited analgesia, miosis and sedation produced by the opioid dipipanone 10 mg in 12 subjects. In the second naloxone, which did not produce hyperalgesia, failed to inhibit significant placebo analgesia in 12 subjects. The results do not support the involvement of endogenous opioids in ischemic limb pain or placebo analgesia under these conditions.

Intrinsic mechanisms of pain inhibition: activation by stress

Portions of the brain stem seem normally to inhibit pain. In man and laboratory animals these brain areas and pathways from them to spinal sensory circuits can be activated by focal stimulation. Endogenous opioids appear to be implicated although separate nonopioid mechanisms are also evident. Stress seems to be a natural stimulus triggering pain suppression. Properties of electric footshock have been shown to determine the opioid or nonopioid basis of stress-induced analgesia. Two different opioid systems can be activated by different footshock paradigms. This dissection of stress analgesia has begun to integrate divergent findings concerning pain inhibition and also to account for some of the variance that has obscured the reliable measurement of the effects of stress on tumor growth and immune function.

Stress analgesia: effects of PCPA, yohimbine, and naloxone

Evidence suggests that morphine analgesia depends on the integrity of monoaminergic transmitter systems. Some forms of stress analgesia seem to be related to morphine analgesia, while others are not. To assess whether opioid and non-opioid stress analgesia differ in their reliance on monoamine systems, the effects of parachlorophenylalanine (PCPA) and yohimbine on analgesia produced by prolonged intermittent and brief continuous footshock were examined on the hotplate test. The interaction of adrenergic and endorphinergic activity with serotonergic mechanisms following these stressors was also investigated by testing the effects of yohimbine and naloxone on rats with prior PCPA treatment. Yohimbine alone significantly reduced baseline hotplate latencies, while PCPA and naloxone did not. The two stressors differed in the effects produced by both naloxone and PCPA. Naloxone significantly reversed stress analgesia in the prolonged stress condition, but not the brief stress condition. PCPA significantly enhanced the antinociceptive effect of brief continuous shock, while leaving the response to prolonged intermittent shock unaffected. In contrast, yohimbine blocked the analgesic effects of prolonged stress. For the brief stress condition, naloxone reversed the elevated thresholds elicited in PCPA treated rats. Naloxone also reversed stress analgesia for PCPA treated rats exposed to prolonged intermittent stress. Yohimbine lowered the responses of PCPA treated rats subjected to brief continuous shock. These results support an interactive model of stress analgesia dependent upon serotonergic, adrenergic, and endorphinergic transmitter systems.

Endocrine effects of the cold pressor test: relationships to subjective pain appraisal and coping

Blood was drawn from 14 normal volunteers twice before, immediately after a 1-minute immersion of the nondominant hand in ice water (cold pressor test), and twice during recovery. Serum levels of beta-endorphin, cortisol, prolactin, growth hormone, and opioid activity were determined, and measures of subjective pain appraisal and coping styles were obtained. Cortisol was the only variable to show a significant increase as a function of noxious stimulation. Correlational analysis yielded relationships between neuroendocrine variables and subjective pain appraisal as well as coping styles, suggesting complex interactions between neuroendocrine and psychological processes in human pain.

The possible participation of a dopaminergic system in mutilating behavior in rats with forelimb deafferentation

Partial rhizotomy (section of dorsal roots C5 to Th 1 included) leads to abnormal mutilating behavior in the rat, presumably due to pain sensation in the deafferented limb. As dopamine (DA) has been shown to play a role in analgesia, destruction and stimulation of the ventral tegmental area (VTA) were used to check whether they induced an increase or a decrease in the mutilating behavior respectively. Destruction of DA neurons located in the VTA was performed by local administration of 6-hydroxydopamine whereas activation was achieved either by imposed electrical stimulation or by chronic oral administration of D-amphetamine sulfate. These 3 treatments accelerated the onset of the mutilating behavior and induced an extension of the mutilating wounds, D-amphetamine sulfate producing the most pronounced acceleration in their development. The results obtained after destruction of DA-VTA neurons fit well with the working hypothesis. The increase of the mutilating behavior rather than a decrease subsequent to the other treatments could have been caused by the hyperactivity and increase of chewing behavior induced by the less specific stimulating techniques.

The role of stimulus intensity and stress in opioid-mediated analgesia

Rats exposed to a Pavlovian conditioning paradigm developed naloxone-reversible analgesia only when the intensity of a noxious unconditioned stimulus was suprathreshold and the level of stress was augmented. The time course of the onset of this conditioned analgesia was reproduced by systemic administration of morphine. These findings suggest that both a minimal level of stimulus intensity and stress are necessary for the activation of endogenous opioid-mediated analgesia.

Co-localization of CRF-ir perikarya and ACTH-ir fibers in rat brain

The distribution of corticotropin-releasing factor immunoreactive (CRF-ir) perikarya and ACTH-ir fibers was examined immunocytochemically, in adjacent sections, in the forebrain and brainstem of the rat. Throughout the nervous system, a remarkable concordance of localization of these neuropeptide systems was noted. Both ACTH-ir fibers and CRF-ir perikarya were conjointly distributed within discrete hypothalamic, limbic and brainstem/autonomic regions previously demonstrated to contain opiate receptors and opiocortin (beta-endorphin, beta-LPH, ACTH) fibers. In view of the demonstrated interactions of CRF with the peripheral (pituitary) opiocortin system, these data suggest the possibility of a similar relationship of CRF and opiocortin systems in the central nervous system as well.

Pain sensitivity, mood and plasma endocrine levels in man following long-distance running: effects of naloxone

The effects of intense exercise on pain perception, mood, and plasma endocrine levels in man were studied under naloxone and saline conditions. Twelve long-distance runners (mean weekly mileage = 41.5) were evaluated on thermal, ischemic, and cold pressor pain tests and on mood visual analogue scales (VAS). Blood was drawn for determination of plasma levels of beta-endorphin-like immunoreactivity (BEir), growth hormone (GH), adrenocorticotrophic hormone (ACTH), and prolactin (PRL). These procedures were undertaken before and after a 6.3 mile run at 85% of maximal aerobic capacity. Subjects participated on two occasions in a double-blind procedure counterbalanced for drug order: on one day they received 2 i.v. injections of naloxone (0.8 mg in 2 ml vehicle each) at 20 min intervals following the run; on the other day, 2 equal volume injections of normal saline (2 ml). Sensory decision theory analysis of the responses to thermal stimulation showed that discriminability, P(A), was significantly reduced post-run under the saline condition, a hypoalgesic effect; response bias, B, was unaffected. Ischemic pain reports were significantly reduced post-run on the saline day, also a hypoalgesic effect. Naloxone reversed the post-run ischemic but not thermal hypoalgesic effects. Joy, euphoria, cooperation, and conscientiousness VAS ratings were elevated post-run; naloxone attenuated the elevation of joy and euphoria ratings only. Plasma levels of BEir, ACTH, GH, and PRL were significantly increased post-run. The results show that long-distance running produces hypoalgesia and mood elevation in man. The effects of naloxone implicate endogenous opioid neural systems as mechanisms of some but not all of the run-induced alterations in mood and pain perception.

Endogenous opioid peptides in intrapartum uterine blood

Endogenous opioid peptides have consistently been reported to exert significant influence on several parameters of reproductive physiology, including tolerance to pain, release of prolactin, pleasurable behavior patterns, and appetite satiety. Immunoreactive endorphins measured in several hundred samples of postpartum uterine blood had a mean value of 190 pg/ml and a range of 60 to 520 pg/ml. Individual values in 100 mothers who were delivered at the Northwest Hospital failed to correlate with doses of postpartum pain medications, infant feeding patterns, and recorded estimates of maternal bonding behavior. Postpartum "blues" may be related to stress-induced depletion of endorphinergic systems.

Plasma beta-endorphin, pain thresholds and anxiety levels across the human menstrual cycle

Nine normally cycling women and seven other women employing oral contraception were tested during five phases (menstrual, follicular, ovulatory, luteal and premenstrual) of their menstrual cycle. The procedure consisted of administration of an anxiety inventory and determination of pain detection and pain thresholds in response to electric shock and the cold pressor task. Venipunctures were also performed and the plasma of normally menstruating women later assayed for beta-endorphin. Analyses revealed that the variance but not the mean levels in peripheral beta-endorphin levels significantly differed (p less than 0.01) across the menstrual cycle with the greatest amount of variance found during the ovulatory phase and the least during the luteal phase. The high variance during the period around ovulation was due to several subjects having extremely elevated beta-endorphin levels which possibly may have resulted from the occurrence of ovulation. Furthermore, a significant positive correlation between anxiety levels and beta-endorphin levels was found only during the menstrual phase. The absence of findings concerning cyclic variation in pain thresholds is contrary to earlier reports and indicates that such a phenomenon may be dependent upon the paradigm employed.

Effect of endurance training on perceived exertion and stress hormones in women

Fifteen women (20- to 23-yr.-old), engaged in an intensive 6- to 8-wk. endurance running program, progressively increased distance from 20 miles during the first week to 50 miles during the fifth week and thereafter. Before (T1), during (T2), and after training (T3), submaximal treadmill runs of 1-hr. duration subdivided into three successive 20-min. segments were completed at approximately 60, 70, and 80% of maximal oxygen uptake, respectively. Ratings of perceived exertion (RPE) were differentiated to obtain local (L), central (C), and over-all (O) responses during these 20-min. segments. Subjects rated the effort during the final 30 sec. of each 5-min. interval. Upon completion of each exercise segment, blood samples were drawn for analysis of lactate (Hla), epinephrine (E), and norepinephrine (NE) to determine the relationship between the differentiated RPEs and these stress markers. Endurance training significantly lowered central and over-all ratings of perceived exertion between T1 and T3 runs but no change occurred in the L-RPE responses to muscular and joint strain. Significant correlations between the stress markers and RPE pooled across sessions were observed during the three treadmill sessions (Hla vs L-RPE, eta = 0.68; E vs C-RPE, eta = 0.54; and NE vs C-RPE, eta = 0.63). These findings indicate that central and over-all ratings of perceived exertion may be more readily influenced by intensive endurance training than local ratings. In addition, while lactate levels may be related to local ratings of perceived exertion, catecholamine levels appear to be associated with central ratings.