Intrinsic mechanisms of pain inhibition: activation by stress

Effects of footshock stress and morphine on natural killer lymphocytes in rats: studies of tolerance and cross-tolerance

Exposure to a form of footshock stress known to cause opioid-mediated analgesia suppresses the cytotoxic activity of natural killer (NK) cells in rats. This suppression is blocked by the opioid antagonist, naltrexone and is mimicked by morphine administration, suggesting mediation by opioid receptors. Supporting this hypothesis, we now report that the morphine-induced suppression of NK activity shows tolerance after 14 daily injections. The NK-suppressive effect of stress, however, shows neither tolerance with repetition nor cross-tolerance in morphine-tolerant rats.

Cimetidine penetrates brain and inhibits non-opiate footshock-induced analgesia

The inhibition of hindpaw (non-opiate) footshock-induced analgesia (HP-FSIA) by cimetidine, the histamine H2-receptor antagonist, was characterized in rats, and the drug's presence in brain was demonstrated. Cimetidine (100 mg/kg, IP) inhibited HP-FSIA when administered 30 min before testing, but was inactive when testing began sooner (15 min) or later (1-4 hr) than this time. Lower doses (20 mg/kg) were also ineffective when given 30 min before testing, whereas higher doses (200 mg/kg) effectively inhibited the response. Increasing the footshock current from 4 mA (which elicited cimetidine-sensitive analgesia) to higher currents (5 and 6 mA) yielded cimetidine-insensitive analgesia. Administration of isotopically labeled cimetidine (100 mg/kg, IP, 30 min) yielded whole brain cimetidine levels of 1.95 nmols/g, respectively, with a brain/blood ratio of 0.017. These findings confirm a limited penetration of brain by cimetidine, and show that large peripheral doses of cimetidine are required to block brain H2-receptors. The specific dose and time requirements for cimetidine to inhibit the HP-FSIA are probably attributable to the brain drug levels that can be achieved after peripheral administration.

Opioid and non-opioid stress analgesia from cold water swim: importance of stress severity

We have previously reported that stress severity plays an important role in determining the neurochemical basis of stress-induced analgesia from inescapable footshock. Increasing severity (duration or intensity of continuous footshock) causes a shift in mediation of the resultant analgesia from opioid to non-opioid. In this study, we find that stress severity plays a similar role in analgesia from cold water swim. More severe swims (longer duration or lower water temperature) produce stress analgesia insensitive to the opiate antagonist, naltrexone, whereas less severe swims produce analgesia significantly attenuated by this drug.

Opposing actions of cimetidine on naloxone-sensitive and naloxone-insensitive forms of footshock-induced analgesia

The effects of the opiate antagonist naloxone (10 mg/kg) and the histamine H2-antagonist cimetidine (100 mg/kg; both administered i.p.) were studied on the analgesia elicited by 3 currents of continuous-scrambled AC footshock (FSIA). Repeated analgesic measurements were made in each animal by use of the radiant heat tail-flick test. As shown by others, naloxone effectively inhibited the FSIA produced by 3 min of 2.0 mA, but had no effect on the responses elicited by higher currents (2.5 and 3.5 mA) of the same duration. Cimetidine significantly reduced the naloxone-insensitive FSIA after 3.5 mA, had no effect on that produced by 2.5 mA and potentiated the naloxone-sensitive analgesia elicited by 2.0 mA. These findings add to existing data supporting a role for brain histamine as a mediator of naloxone-insensitive analgesia, and also suggest the possibility that histamine may mediate hyperalgesic responses.

Clinical efficacy of antihistaminics as analgesics

This paper reviews the clinical information on antihistaminic agents as analgesics and as analgesic adjuvants. The evidence indicates a direct analgesic effect of various antihistaminics. In clinical studies, diphenhydramine, hydroxyzine, orphenadrine and pyrilamine have been shown to produce analgesia as simple entities but chlorpheniramine has not and results with phenyltoloxamine have been equivocal when tested alone. Analgesic adjuvant effects of several antihistaminics have been reported. Clinically, orphenadrine and phenyltoloxamine have shown adjuvant effects with acetaminophen and aspirin. The mechanism of action remains speculative. The most recent trends in the classification of histamine receptors and how these receptors may interact with pain modulation are also considered.

Two opioid forms of stress analgesia: studies of tolerance and cross-tolerance

We have previously reported that stress analgesia sensitive to and insensitive to opiate antagonists can be differentially produced in rats by varying the severity or temporal pattern of inescapable footshock. In these studies, we give further evidence for the opioid and non-opioid bases of these paradigms of stress analgesia. We find that naloxone-sensitive analgesia demonstrates tolerance with repeated stress and cross-tolerance with morphine, whereas naloxone-insensitive analgesia demonstrates neither of these characteristics. Moreover, different forms of opioid, but not non-opioid, stress analgesia manifest cross-tolerance with each other. These data are discussed in terms of the similarities and differences between two forms of opioid stress analgesia.

Naloxone-reversible stress-induced feeding and analgesia in the slug Limax maximus

Exposure to tail-pinch stress increased the thermal nociceptive thresholds and food intakes of the slug, Limax maximus. These stress-induced "analgesic" and feeding responses, which were similar to the behaviors observed after treatment with exogenous opiates, were blocked by the opiate antagonist, naloxone. These results indicate that exposure to stress increases endogenous opioid activity in slugs and induces various behavioral and physiological responses in a manner analogous to that reported in mammals.

Long-term hyperalgesia in rats induced by neonatal administration of vasopressin antiserum

Vasopressin antiserum was given to two day old rats and the nociceptive thresholds were evaluated three months later. The rats were hypersensitive to pain when electrical current, but not heat, was used as the noxious stimulus. These animals were also insensitive to cold-water swim, a non-opioid form of stress analgesia. The vasopressin content in the pituitary or in the hypothalamus was not however modified by the neonatal treatment. The present results suggest a physiological role for vasopressin in non-opioid pain inhibitory systems.

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.

Effects on long-term sensitivity to pain and morphine of stress induced in the newborn rat by pain or manipulation

Four times daily from postnatal day 1 to 15, rats were stressed either by being removed from the maternity cage (manipulation stress, MS) or by being placed on a hotplate at 55 degrees C (pain stress, PS). When 70 days old, they were examined for sensitivity to pain and to the analgesic effect of morphine, and for brain opiate receptors. Pain sensitivity of MS and PS rats was not significantly different from that of controls. The analgesic activity of morphine, assessed by the hotplate test at 49 degrees C, was significantly reduced in MS rats, while in PS rats it was similar to that in controls. 3H-dihydromorphine binding studies performed on whole brain synaptic membranes showed a reduction in the maximum number of binding sites in both MS and PS rats; on the other hand, the affinity constant was higher in PS rats, while in MS rats it was similar to that of controls. These data show that the repeated stress of removal from the mother during the first 15 days of life induce a reduction in the number of brain opiate receptors with reduced activity of morphine, while in rats exposed to repeated removal stress associated with painful stimuli the reduction in the number of brain opiate receptors seems to be counterbalanced by their higher affinity.

Acute non-opioid analgesia in defeated male mice

Exposure to repeated attack induces a long-lasting analgesia in male mice. Although this reaction has been linked to the special biological significance of defeat, earlier research has confounded defeat and exposure to further attack. In the present studies, DBA/2 intruder mice were individually placed into the home cages of aggressive conspecifics and removed immediately upon display of the species-characteristic upright submissive posture. Under these test conditions, intruders did indeed show a profound analgesia. However, in marked contrast to the antinociceptive effects of repeated attack, this reaction was of short duration (less than 10 min), was not blocked by naloxone (1-10 mg/kg, IP) and did not show cross-tolerance either to or from morphine (5 mg/kg, IP). These findings are discussed in relation to multiple endogenous pain inhibitory systems and their possible adaptive significance in murine social behaviour.

Differential effects of scopolamine on 3 forms of stress analgesia

We have previously reported that non-opioid stress analgesia and two forms of opioid stress analgesia can be differentially produced in rats by varying the severity or temporal pattern of inescapable footshock. In this study, we investigated the role of muscarinic cholinergic mechanisms in mediating these 3 forms of stress analgesia. Whereas the muscarinic anticholinergic drug, scopolamine, had no effect on either non-opioid stress analgesia or opioid stress analgesia from 1 min of continuous 2.5-mA footshock, it significantly attenuated opioid analgesia from 20 min of intermittent footshock at this same intensity. The data are discussed in reference to other similarities and differences between these two forms of opioid stress analgesia.

Stimulation-produced and stress-induced analgesia: cross-tolerance between opioid forms

Electrical stimulation of medial brainstem sites produces potent analgesia in rats that is either opioid- or non-opioid-mediated depending on the specific brain region stimulated. Footshock stress also causes opioid and non-opioid forms of analgesia in rats depending on the exact parameters of footshock administered. We now report that opioid, but not non-opioid, stress analgesia demonstrates cross-tolerance with opioid, but not non-opioid, stimulation-produced analgesia. This finding suggests that opioid forms of stimulation-produced and stress-induced analgesia share a common substrate.

FMRFamide, a putative endogenous opiate antagonist: evidence from suppression of defeat-induced analgesia and feeding in mice

Social conflict and defeat in mice leads to an activation of endogenous opiate systems. The effects of intracerebroventricular administration of the peptide FMRFamide (Phe-Met-Arg-Phe-NH2) and the opiate antagonist naloxone, on aggressive encounters, defeat-induced analgesia and defeat-induced feeding were examined in male mice. Both substances reduced the number of bites required to cause defeat in subordinate mice during aggressive encounters, as well as suppressing the subsequent defeat-induced analgesia. Administration of FMRFamide or naloxone also reduced defeat-induced feeding. These results indicate that FMRFamide (or FMRFamide-like neuropeptides) may function as endogenous opioid antagonists.

Differential effects of dPTyr(Me)AVP, a vasopressin antagonist, upon foot shock analgesia

Acute exposure to either prolonged intermittent foot shock (PIFS) or brief continuous foot shock, (BCFS) decreases the sensitivity of rats to noxious stimuli, but differ in their mechanisms of actions. Since the peptide vasopressin (VP) has been implicated in analgesic and stress-related processes, the present study examined whether antagonism of central VP receptors with dPTyr(Me)AVP would alter the analgesic responses following PIFS or BCFS. While intracerebroventricular administration of dPTyr(Me)AVP, a V1 receptor antagonist, significantly attenuated the analgesic response to PIFS, it potentiated the analgesic response to BCFS. It should be noted that the form of PIFS employed in the present study was not blocked by naloxone. These results are discussed in terms of multiple forms of pain-inhibitory systems that may utilize collateral inhibition as a means of providing selective activation.

Systemic morphine enhances spread of sensory analgesia during postoperative epidural bupivacaine infusion

Eight patients undergoing abdominal surgery received epidural analgesia with 0.5% plain bupivacaine at a fixed dose rate (8 ml/h) for postoperative pain relief. Mean sensory level of analgesia (pin prick) was assessed hourly and regressed from a mean preoperative level of T3.7 (+/- 0.3 SEM) to T10 (+/- 0.7) at an average of 8.9 +/- 1.4 h post skin incision (range 4-16 h). Simultaneously, pain scores (4-point scale) increased from zero to 2.1 +/- 0.2. When analgesia regressed greater than or equal to 5 segments, administration of 10 mg morphine intravenously led to a pronounced cephalad increase in sensory analgesia; the initial level of analgesia was achieved in every case despite unchanged bupivacaine infusion. Simultaneously, pain score decreased to 0.3 +/- 0.3. This synergistic effect of systemic morphine on the extent of neural blockade with epidural bupivacaine may have an important role in improving postoperative pain relief.

Involvement of peripheral and central catecholamine systems in neural-immune interactions

In this review, we have attempted to delineate the current state of knowledge of the relationships between the immune system and one chemically specific component of the nervous system, the noradrenergic system, both in the brain and the periphery. We have discussed recent work describing the presence of noradrenergic innervation in lymphoid tissues in the major lymphatic organs. Our findings demonstrate clearly that the regions in which lymphocytes (mainly T cells) reside, and through which they recirculate, receive direct sympathetic neural input. The immune system can, therefore, be considered 'hard-wired' to the brain. The evidence for receptors on cells of the immune system capable of receiving signals from the brain is discussed. The significance of this 'hard-wiring' to the function of the immune system is considered, both with regard to the effect of its disruption on immune responses, and to the direct and indirect effects of sympathetic neurotransmitter substances on lymphocytes and their behavior in vitro and in vivo. Finally, our detailed analysis of changes occurring in central noradrenergic pathways as a result of stimulation of the immune system leads to an emerging picture of feedback loops from the immune system to the brain. Such circuits employ endocrine, and probably autonomic, outflow to modulate and regulate immune responses.

Social conflict analgesia: studies on naloxone antagonism and morphine cross-tolerance in male DBA/2 mice

It has recently been reported that male mice exhibit pronounced analgesia in response to attack from aggressive conspecifics. Although several studies indicate that this reaction can be blocked by opiate antagonist pretreatment, unequivocal evidence of opioid involvement is very much more limited. In the present study, the phenomenon of conflict analgesia has been studied in male DBA/2 intruder mice following exposure to a criterion level of attack from aggressive BKW residents. Our findings indicate that this analgesia is blocked and reversed by naloxone unaltered by methyl naloxone, except at high doses (75 mg/kg) and fully cross-tolerant with morphine. This profile confirms and extends earlier findings with B6AF1 mice, indicating that the opioid mediation of this biologically-relevant form of environmental analgesia is not strain specific.

Stress-induced analgesia in the mouse: strain comparisons

Inescapable footshock is capable of differentially activating opioid- and non-opioid-mediated mechanisms of stress-induced analgesia in the rat, depending on the temporal and intensive parameters of its administration. In this study we compared the effects of opioid and non-opioid stress analgesia in two strains of mice, one known to be deficient in central opioid binding sites (CXBK) and one normal in this regard (C57BL/6BY). We found that although the C57 strain showed robust opioid and non-opioid stress analgesia, the CXBK strain only showed stress analgesia of the non-opioid type.

Endogenous opiates: 1984

This paper is the seventh in an annual series of reviews of research involving the endogenous opiate peptides, each installment being restricted to work published during the previous year. As in the past three years, the review this year is limited to non-analgesic and behavioral studies of the opiate peptides. The specific topics this year include: stress, tolerance and dependence, consummatory responses, gastric and renal activity, alcohol, mental illness, learning and memory, cardiovascular responses, respiratory effects, thermoregulation, seizures and neurological disorders, activity, and miscellaneous other topics.

Opioid systems and feeding in the slug, Limax maximus: similarities to and implications for mammalian feeding

Substantial evidence is accumulating to implicate opioid systems in the regulation of behavioral and physiological functions in invertebrates in a manner analogous to that observed in vertebrates. This communication reviews opiate involvement in the mediation of the ingestive behaviors of the terrestrial slug, Limax maximus. The similarities to and implications for opioid modulation of mammalian feeding are considered.