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

μ-Opioid Receptor Gene A118 G Variants and Persistent Pain Symptoms Among Men and Women Experiencing Motor Vehicle Collision

The μ-opioid receptor 1 (OPRM1) binds endogenous opioids. Increasing evidence suggests that endogenous OPRM1 agonists released at the time of trauma may contribute to the development of posttraumatic musculoskeletal pain (MSP). In this prospective observational study, we evaluated the hypothesis that individuals with an AG or GG genotype at the OPRM1 A118 G allele, which results in a reduced response to opioids, would have less severe MSP 6 weeks after motor vehicle collision (MVC). Based on previous evidence, we hypothesized that this effect would be sex-dependent and most pronounced among women with substantial peritraumatic distress. European American men and women ≥ 18 years of age presenting to the emergency department after MVC and discharged to home after evaluation (N = 948) were enrolled. Assessments included genotyping and 6-week evaluation of overall MSP severity (0-10 numeric rating scale). In linear regression modeling, a significant A118 G Allele × Sex interaction was observed: an AG/GG genotype predicted reduced MSP severity among women with substantial peritraumatic distress (β = -.925, P = .014) but not among all women. In contrast, men with an AG/GG genotype experienced increased MSP severity at 6 weeks (β = .827, P = .019). Further studies are needed to understand the biologic mechanisms mediating observed sex differences in A118 G effects.

PERSPECTIVE

These results suggest a sex-dependent mechanism by which an emotional response to trauma (distress) contributes to a biologic mechanism (endogenous opioid release) that increases MSP in the weeks after stress exposure. These results also support the hypothesis that endogenous opioids influence pain outcomes differently in men and women.

Classical conditioning: The new hegemony

Converging data from different disciplines are showing the role of classical conditioning processes in the elaboration of human and animal behavior to be larger than previously supposed. Restricted views of classically conditioned responses as merely secretory, reflexive, or emotional are giving way to a broader conception that includes problem-solving, and other rule-governed behavior thought to be the exclusive province of either operant conditiońing or cognitive psychology. These new views have been accompanied by changes in the way conditioning is conducted and evaluated. Data from a number of seemingly unrelated phenomena such as relapse to drug abuse by postaddicts, the placebo effect, and the immune response appear to involve classical conditioning processes. Classical conditioning, moreover, has been found to occur in simpler and simpler organisms and recently even demonstrated in brain slices and in utero. This target article will integrate the several research areas that have used the classical conditioning process as an explanatory model; it will challenge teleological interpretations of the classically conditioned CR and offer some basic principles for testing conditioning in diverse areas.

Pavlovian conditioning of multiple opioid-like responses in mice

Conditional responses in rodents such as locomotion have been reported for drugs of abuse and similar to the placebo response in humans, may be associated with the expectation of reward. We examined several conditional opioid-like responses and the influence of drug expectation on conditioned place preference and concomitant conditional locomotion. Male C57BL/6J mice were conditioned with the selective mu opioid receptor agonist fentanyl (0.2mg/kg, i.p.) in a novel context and subsequently given a vehicle injection. In separate experiments, locomotor activity, Straub tail, hot plate sensitivity, and conditioned place preference (CPP) were measured. Mice exhibited multiple conditional opioid-like responses including conditional hyperlocomotion, a conditional pattern of opioid-like locomotion, Straub tail, analgesia, and place preference. Modulating drug expectation via administration of fentanyl to "demonstrator" mice in the home cage did not affect the expression of conditioned place preference or the concomitant locomotor activity in "observer" mice. In summary, Pavlovian conditioning of an opioid in a novel context induced multiple conditional opioid-like behaviors and provides a model for studying the neurobiological mechanisms of the placebo response in mice.

Cholecystokinin and endogenous opioid peptides: interactive influence on pain, cognition, and emotion

It is well documented that stressful life experiences contribute to the etiology of human mood disorders. Cholecystokinin (CCK) is a neuropeptide found in high concentrations throughout the central nervous system, where it is involved in numerous physiological functions. A role for CCK in the induction and persistence of anxiety and major depression appears to be conspicuous. While increased CCK has been associated with motivational loss, anxiety and panic attacks, an increase in mesocorticolimbic opioid availability has been associated with coping and mood elevation. The close neuroanatomical distribution of CCK with opioid peptides in the limbic system suggests that there may be an opioid-CCK link in the modulation and expression of anxiety or stressor-related behaviors. In effect, while CCK induces relatively protracted behavioral disturbances in both animal and human subjects following stressor applications, opioid receptor activation may change the course of psychopathology. The antagonistic interaction of CCK and opioid peptides is evident in psychological disturbances as well as stress-induced analgesia. There appears to be an intricate balance between the memory-enhancing and anxiety-provoking effects of CCK on one hand, and the amnesic and anxiolytic effects of opioid peptides on the other hand. Potential anxiogenic and mnemonic influences of site-specific mesocorticolimbic CCK and opioid peptide availability, the relative contributions of specific CCK and opioid receptors, as well as the time course underlying neuronal substrates of long-term behavioral disturbances as a result of stressor manipulations, are discussed.

Early onset of reduced morphine analgesia by ingestion of sweet solutions

Morphine analgesia can be reduced by prior exposure to food and flavored fluids. The early onset of reduced morphine-induced analgesia (RMA) was studied in 82 male Wistar rats after allowing them access to either a dextrose-saccharin solution or unflavored tap water for 6 or 3 h (Experiment 1, n = 40) or for 3 h, 90, or 45 min (Experiment 2, n = 42). Morphine (4 mg/kg) was injected subcutaneously at the end of the drinking period, and after 25 min a series of tail flick tests was conducted. Morphine produced strong analgesia in all rats that drank unflavored tap water; however, in rats that drank the flavored solution, the analgesic effect of morphine was significantly attenuated following exposures of 6 or 3 h, but not following exposures of 90 or 45 min. Similar quantities of flavored fluid were consumed by groups at all exposure durations; thus, RMA was determined by duration of exposure and not amount consumed. No analgesia attributable to flavor consumption per se was observed. The results suggest that RMA is mediated by endogenous opioid activity in the gustatory and analgesic systems by a mechanism akin to tolerance that requires about 3 h to operate.

Tolerance to morphine analgesia from brief exposure to a palatable solution

The onset of tolerance to morphine analgesia was studied in 34 female Wistar rats immediately after they drank a dextrose-saccharin cocktail or tap water for 6 or 24 hours. Tail flick tests conducted at the end of the drinking period showed no analgesia in cocktail or water groups. Morphine (3.5 mg/kg) was then injected subcutaneously, and after 20 min another series of tail flick tests was conducted. Morphine produced strong analgesia in rats that drank water, but only partial analgesia in rats that drank the cocktail. This reduction in morphine analgesia did not differ in comparisons between 6-hour and 24-hour cocktail groups. The results were interpreted as indicating that ingestion of the cocktail produced tolerance for morphine in the test of analgesia, and that such factors as novelty of flavor stimulation or stress from repeated testing were unlikely to yield the attenuation of analgesia that was observed. It was concluded that processes that produce tolerance become active in less than 6 hours.

Tonic pain time-dependently affects beta-endorphin-like immunoreactivity in the ventral periaqueductal gray matter of the rat brain

beta-Endorphin-like immunoreactivity (B-EP-LI) levels have been investigated in the ventral periaqueductal gray matter (vPAG) of rats killed 30, 60 or 120 min after the subcutaneous injection of dilute formalin (0.08 ml, 5%) in one fore- or hindpaw, or comparable handling. B-EP-LI was estimated by radioimmunoassay, using an anti-camel B-EP serum directed against the C-terminal portion of B-EP molecule. In both fore- and hindlimb groups vPAG B-EP-LI values were significantly increased 60 and 120 min after the injection relative to controls. Values from animals killed 120 min after formalin injection were higher than the ones at 30 and 60 min, forelimb effects being quantitatively more pronounced. The increase in B-EP-LI appeared distributed along the whole rostrocaudal extent of the region.

Analgesia or hyperalgesia following stress correlates with emotional behavior in rats

Analgesia following exposure to a variety of noxious and non-noxious stressors is well documented and is commonly referred to as stress-induced analgesia. Hyperalgesia following stress has also been reported. The present study shows that a mild stressor (15 min of vibration) produced increased tail-flick latencies (TFL) in some rats, but decreased latencies in other rats. The results of the individual subjects were reproduced in a later session: the rats showing increased TFL on day 1, responded with increased TFL on day 2. Rats showing decreased TFL on day 1 responded with decreased TFL on day 2. Whichever reaction occurred, analgesia or hyperalgesia, this correlated with the animal's behavior during the stress procedure. Analgesia was produced in quiet rats and hyperalgesia in hyperemotional ones. Various peripheral nerve stimulation procedures producing hyperemotional reactions also resulted in lowering of the pain threshold. The results of the present study show behavioral modulation of pain mechanisms.

Effects of centrifugal rotation on analgesia and conditioned flavor aversions

In the first experiment, 48 female Wistar rats were water deprived and given three conditioning days with saccharin-flavored water (1.5 g/liter) followed by 0, 5, 10 or 15 min of centrifugal rotation (150 rpm). Analgesia was measured by the tail flick test immediately after rotation. Over conditioning days, taste aversions developed. In general, taste aversion strength increased with duration of rotation. Analgesia also was in proportion to duration of rotation; however, over days tolerance developed in all rotated groups. In the second experiment 12 female Wistar rats were water deprived and given naloxone (20 mg/kg) or saline prior to 15 min of rotation. Rotation-induced analgesia was not affected by naloxone. It was concluded that somatic and gastrointestinal reactions to rotation are not served by a single mechanism.

Endogenous opiates: 1985

This paper is the eighth installment of our annual review of research involving the endogenous opiate peptides. It is restricted to the non-analgesic and behavioral studies of the opiate peptides published in 1985. The specific topics this year include stress, tolerance and dependence, eating, drinking and alcohol consumption, gastrointestinal and renal activity, mental illness, learning and memory, cardiovascular responses, respiration and thermoregulation, seizures and neurological disorders, activity, and some other selected topics.

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.