The effect of old age on the disposition and antinociceptive response of morphine and morphine-6 beta-glucuronide in the rat

Respiratory Effects of Biased Ligand Oliceridine in Older Volunteers: A Pharmacokinetic-Pharmacodynamic Comparison with Morphine

BACKGROUND

Oliceridine is a G protein-biased µ-opioid, a drug class that is associated with less respiratory depression than nonbiased opioids, such as morphine. The authors quantified the respiratory effects of oliceridine and morphine in elderly volunteers. The authors hypothesized that these opioids differ in their pharmacodynamic behavior, measured as effect on ventilation at an extrapolated end-tidal Pco2 at 55 mmHg, V̇E55.

METHODS

This four-arm double-blind, randomized, crossover study examined the respiratory effects of intravenous 0.5 or 2 mg oliceridine and 2 or 8 mg morphine in 18 healthy male and female volunteers, aged 55 to 89 yr, on four separate occasions. Participants' CYP2D6 genotypes were determined, hypercapnic ventilatory responses were obtained, and arterial blood samples were collected before and for 6 h after treatment. A population pharmacokinetic-pharmacodynamic analysis was performed on V̇E55, the primary endpoint; values reported are median ± standard error of the estimate.

RESULTS

Oliceridine at low dose was devoid of significant respiratory effects. High-dose oliceridine and both morphine doses caused a rapid onset of respiratory depression with peak effects occurring at 0.5 to 1 h after opioid dosing. After peak effect, compared with morphine, respiratory depression induced by oliceridine returned faster to baseline. The effect-site concentrations causing a 50% depression of V̇E55 were 29.9 ± 3.5 ng/ml (oliceridine) and 21.5 ± 4.6 ng/ml (morphine), the blood effect-site equilibration half-lives differed by a factor of 5: oliceridine 44.3 ± 6.1 min and morphine 214 ± 27 min. Three poor CYP2D6 oliceridine metabolizers exhibited a significant difference in oliceridine clearance by about 50%, causing higher oliceridine plasma concentrations after both low- and high-dose oliceridine, compared with the other participants.

CONCLUSIONS

Oliceridine and morphine differ in their respiratory pharmacodynamics with a more rapid onset and offset of respiratory depression for oliceridine and a smaller magnitude of respiratory depression over time.

EDITOR’S PERSPECTIVE

Aging reduces the sensitivity to the reinforcing efficacy of morphine

The US geriatric population is growing and using more opioids than ever before. The purpose of this study was to determine whether aging influenced the reinforcing efficacy of morphine in male and female rats using a rodent intravenous self-administration paradigm. Male and female aged (20-24 months) and young (2-4 months) Wistar rats were tested at 2 doses of morphine (0.75 mg/kg/infusion and 0.25 mg/kg/infusion). During 10 days of self-administration, aged rats took significantly less morphine than their younger counterparts at the 0.25 mg/kg/infusion dose. Aged males also earned significantly fewer infusions on a progressive ration reinforcement schedule at this dose, suggesting that the reinforcing efficacy of morphine is decreased for this group at this dose. These effects dissipated when a separate group of animals had access to the 0.75 mg/kg/infusion dose for both sexes. Our results indicate that morphine is less reinforcing at lower doses in aged male, but not female rats. This research has potential clinical implications for the chronic treatments involving opioids in aged individuals.

The effects of aging on hydromorphone-induced thermal antinociception in healthy female cats

Introduction:

This study aimed to evaluate the effects of aging on hydromorphone-induced thermal antinociception in cats.

Methods:

In a prospective, randomized, blinded, controlled design, 10 healthy female cats received each of the following treatments intramuscularly: hydromorphone (0.1 mg/kg) and 0.9% saline (0.05 mL/kg) with a 1-week washout between treatments at 6, 9, and 12 months of age. Skin temperature and thermal thresholds (TTs) were recorded before and up to 12 hours after injection. Data were analyzed using a repeated-measures linear mixed model (α = 0.05).

Results:

After saline treatment, TT was not significantly different from baseline at any time point for any age group. After hydromorphone treatment, TT was significantly higher than baseline at 6 months for up to 1 hour, and at 9 and 12 months for up to 4 hours. Peak TT at 6, 9, and 12 months were 50.4 ± 2.7, 50.9 ± 2.0, and 53.6 ± 2.0°C at 0.5, 1, and 1 hours, respectively. Mean TT was significantly higher after hydromorphone treatment when compared with saline treatment at 9 and 12 months for up to 4 hours but not at 6 months. Magnitude of antinociception was consistently larger at 12 months when compared with 6 months of age. Hydromorphone provided a shorter duration and smaller magnitude of antinociception at 6 months when compared with 9 and 12 months.

Conclusion:

Pediatric cats may require more frequent dosing of hydromorphone than adults.

Age-dependent antinociception and behavioral inhibition by morphine

In current clinical practice, morphine is dosed in older patients based on patient-weight, with different calculations for adjustment. However, at present, neither clinical experience nor the literature offers a clear evidence base for the relationship between antinociception, behavioral effects and morphine administration in older patients. In this study, we compared the nociceptive response of 8 and 24 week old rats after subcutaneous administration of morphine per body weight and analyzed their behavior using an advanced multi-conditioning system. Residual morphine in all major tissues was determined. We observed prolonged morphine-induced antinociception in older rats compared to younger rats. Moreover, morphine significantly stimulated locomotor and rearing behavior 180 min after injection, which was significantly higher in the 8 week compared to 24 week old rats. Tissue analysis from animals extracted 240 min post-injection revealed a significantly higher concentration of residual morphine in the brains of older versus younger animals when standardized on tissue weight. However, this effect was not observed when residual morphine was standardized on protein content. Collectively, our data suggest that in older rats morphine exhibits higher antinociception and increased behavioral inhibition compared to younger animals. This effect is likely due to a significantly higher accumulation of morphine in the brain of older animals.

Thermal sensitivity across ages and during chronic fentanyl administration in rats

RATIONALE

Chronic pain is becoming a more common medical diagnosis and is especially prevalent in older individuals. As such, prescribed use of opioids is on the rise, even though the efficacy for pain management in older individuals is unclear.

OBJECTIVES

Thus, the present preclinical study assessed the effectiveness of chronic fentanyl administration to produce antinociception in aging rats (16, 20, and 24 months).

METHODS

Animals were tested in a thermal sensitivity procedure known to involve neural circuits implicated in chronic pain in humans. Sensitivity to heat and cold thermal stimulation was assessed during 28 days of fentanyl administration (1.0 mg/kg/day), and 28 days of withdrawal.

RESULTS

Fentanyl resulted in decreased thermal sensitivity to heat but not cold stimulation indicated by more time spent in the hot compartment relative to time spent in the cold or neutral compartments. Unlike previous findings using a hot-water tail withdrawal procedure, tolerance did not develop to the antinociceptive effects of fentanyl over a 28-day period of drug administration. The oldest animals were least sensitive, and the youngest animals most sensitive to the locomotor-stimulating effects of fentanyl. The effect on the antinociceptive response to fentanyl in the oldest group of rats was difficult to interpret due to profound changes in the behavior of saline-treated animals.

CONCLUSIONS

Overall, aging modifies the behavioral effects of opioids, a finding that may inform future studies for devising appropriate treatment strategies.

Opioid activation of toll-like receptor 4 contributes to drug reinforcement

Opioid action was thought to exert reinforcing effects solely via the initial agonism of opioid receptors. Here, we present evidence for an additional novel contributor to opioid reward: the innate immune pattern-recognition receptor, toll-like receptor 4 (TLR4), and its MyD88-dependent signaling. Blockade of TLR4/MD2 by administration of the nonopioid, unnatural isomer of naloxone, (+)-naloxone (rats), or two independent genetic knock-outs of MyD88-TLR4-dependent signaling (mice), suppressed opioid-induced conditioned place preference. (+)-Naloxone also reduced opioid (remifentanil) self-administration (rats), another commonly used behavioral measure of drug reward. Moreover, pharmacological blockade of morphine-TLR4/MD2 activity potently reduced morphine-induced elevations of extracellular dopamine in rat nucleus accumbens, a region critical for opioid reinforcement. Importantly, opioid-TLR4 actions are not a unidirectional influence on opioid pharmacodynamics, since TLR4(-/-) mice had reduced oxycodone-induced p38 and JNK phosphorylation, while displaying potentiated analgesia. Similar to our recent reports of morphine-TLR4/MD2 binding, here we provide a combination of in silico and biophysical data to support (+)-naloxone and remifentanil binding to TLR4/MD2. Collectively, these data indicate that the actions of opioids at classical opioid receptors, together with their newly identified TLR4/MD2 actions, affect the mesolimbic dopamine system that amplifies opioid-induced elevations in extracellular dopamine levels, therefore possibly explaining altered opioid reward behaviors. Thus, the discovery of TLR4/MD2 recognition of opioids as foreign xenobiotic substances adds to the existing hypothesized neuronal reinforcement mechanisms, identifies a new drug target in TLR4/MD2 for the treatment of addictions, and provides further evidence supporting a role for central proinflammatory immune signaling in drug reward.

Molecular and cellular mechanisms of the age-dependency of opioid analgesia and tolerance

The age-dependency of opioid analgesia and tolerance has been noticed in both clinical observation and laboratory studies. Evidence shows that many molecular and cellular events that play essential roles in opioid analgesia and tolerance are actually age-dependent. For example, the expression and functions of endogenous opioid peptides, multiple types of opioid receptors, G protein subunits that couple to opioid receptors, and regulators of G protein signaling (RGS proteins) change with development and age. Other signaling systems that are critical to opioid tolerance development, such as N-methyl-D-aspartic acid (NMDA) receptors, also undergo age-related changes. It is plausible that the age-dependent expression and functions of molecules within and related to the opioid signaling pathways, as well as age-dependent cellular activity such as agonist-induced opioid receptor internalization and desensitization, eventually lead to significant age-dependent changes in opioid analgesia and tolerance development.

Opioid use in the elderly

Pain treatment in the elderly is an important challenge to Western societies due to increasing numbers of old persons, their higher incidence of pain, and their greater susceptibility to adverse effects of pain medication. We provide an overview of the factors liable to influence opioid action in the elderly population. A major challenge for the physician prescribing opioids in the elderly is their greater risk of medication-associated problems. Thus, older patients suffer increased vulnerability to adverse drug effects and interactions, higher rates of polypharmacy, and more comorbidity. These problems are compounded by a relative lack of definitive published information. There is clearly a need for more research in this area. Aging affects opioid pharmacokinetics via altered body composition (distribution volumes) and organ function (liver=metabolism, kidney=excretion). Pharmacodynamics is affected via impaired neurotransmitter/peptide production and changed receptor affinities/populations. Older women may need less morphine analgesia postoperatively, while pain sensitivity tends to increase particularly in older men. However, the net effects of changes in opioid pharmacology with age on clinical opioid analgesia remain unclear, probably due to the significantly greater variability in body function with increasing age. Practical recommendations for opioid prescription in the elderly include meticulous review of indication for opioid use, not only initially but also at regular intervals thereafter. A policy of careful titration should be followed, with conservative choice of dosage on starting. Dosing intervals may need to be lengthened subsequently. Finally, it should be remembered that old persons do not necessarily need less opioid than younger ones.

Effects of morphine on thermal sensitivity in adult and aged rats

There are contradictory data regarding older individuals' sensitivity to pain stimulation and opioid administration. Adult (12-16 months; n = 10) and aged (27-31 months; n = 7) male F344xBN rats were tested in a thermal sensitivity procedure where the animal chooses to remain in one of two compartments with floors maintained at various temperatures ranging from hot (45°C) through neutral (30°C) to cold (15°C). Effects of morphine were determined for three temperature comparisons (ie, hot/neutral, cold/neutral, and hot/cold). Aged rats were more sensitive to cold stimulation during baseline. Morphine produced antinociception during hot thermal stimulation, but had no effect on cold stimulation. The antinociceptive (and locomotor-altering) effects of morphine were attenuated in aged rats. These data demonstrate age-related differences in baseline thermal sensitivity and responsiveness to opioids. Based on behavioral and physiological requirements of this procedure, it is suggested that thermal sensitivity may provide a relevant animal model for the assessment of pain and antinociception.

Reduction of opioid withdrawal and potentiation of acute opioid analgesia by systemic AV411 (ibudilast)

Morphine-induced glial proinflammatory responses have been documented to contribute to tolerance to opioid analgesia. Here, we examined whether drugs previously shown to suppress glial proinflammatory responses can alter other clinically relevant opioid effects; namely, withdrawal or acute analgesia. AV411 (ibudilast) and minocycline, drugs with distinct mechanisms of action that result in attenuation of glial proinflammatory responses, each reduced naloxone-precipitated withdrawal. Analysis of brain nuclei associated with opioid withdrawal revealed that morphine altered expression of glial activation markers, cytokines, chemokines, and a neurotrophic factor. AV411 attenuated many of these morphine-induced effects. AV411 also protected against spontaneous withdrawal-induced hyperactivity and weight loss recorded across a 12-day timecourse. Notably, in the spontaneous withdrawal study, AV411 treatment was delayed relative to the start of the morphine regimen so to also test whether AV411 could still be effective in the face of established morphine dependence, which it was. AV411 did not simply attenuate all opioid effects, as co-administering AV411 with morphine or oxycodone caused three-to-five-fold increases in acute analgesic potency, as revealed by leftward shifts in the analgesic dose response curves. Timecourse analyses revealed that plasma morphine levels were not altered by AV411, suggestive that potentiated analgesia was not simply due to prolongation of morphine exposure or increased plasma concentrations. These data support and extend similar potentiation of acute opioid analgesia by minocycline, again providing converging lines of evidence of glial involvement. Hence, suppression of glial proinflammatory responses can significantly reduce opioid withdrawal, while improving analgesia.

A warmer ambient temperature increases the passage of interleukin-1beta into the brains of old rats

We have demonstrated that after intraperitoneal lipopolysaccharide (LPS) injection, old rats mount fevers similar to those of young rats at an ambient temperature (Ta) of 31 degrees C, but not at 21 degrees C. The same is true for intraperitoneal or intravenous IL-1beta administration. The underlying mechanism responsible for blunted fever in old rats may be a deficiency in communication between the periphery and the brain. Possibly, peripheral cytokine actions are altered in old rats, such that the signal that reaches the brain is diminished. Here, we hypothesized that at standard laboratory temperatures, not enough IL-1beta is reaching the brain for fever to occur and that a warmer Ta would increase the influx of IL-1beta into the brain, enabling old rats to generate fever. Young (3-5 mo) and old (23-29 mo) Long-Evans rats were maintained for 3 days at either Ta 21 or 31 degrees C prior to intravenous injection with radiolabeled IL-1beta to measure passage across the blood-brain barrier. Young rats showed similar influx of IL-1beta into the brain at the two Tas, but old rats showed significant influx only at the warmer Ta. These data suggest that the lack of fever at a cool Ta may be due to a reduced influx of IL-1beta into the brain.

Age-related changes in the gastrointestinal tract: a functional and immunohistochemical study in guinea-pig ileum

It is known that there is an age-related increase in gastrointestinal diseases. However, there is a lack of studies dealing with the correlation between age-related changes in function and intrinsic innervation in the gastrointestinal tract. The purpose of this work was to study this subject in the guinea pig ileum, whose functional and structural features are well known in the young age. Ileal longitudinal muscle -- myenteric plexus (LMMP) preparations were obtained from 3-to 24-month-old guinea pigs. Both functional and immunohistochemical techniques were applied. The force of the contraction elicited by excitatory stimuli (electrical stimulation, acetylcholine, substance P, and opioid withdrawal) increased in parallel with an age-dependent reduction in the density of excitatory motor neurones to the longitudinal muscle, whereas other subpopulations of neurones, including inhibitory motor neurones, decreased much more slowly. Although the increase in responsiveness could be related to the age/weight-related increment in muscle bulk, some compensatory modifications to the lowered density of excitatory neurones could also be involved. On the other hand, the acute inhibitory response to morphine remained unaltered in old animals, whilst in vitro tolerance was lower. These results suggest that although age-dependent neuronal loss does not cause dramatic changes in intestinal motility, it is a factor that could contribute to disturbing normal responsiveness and, perhaps, underlie the higher frequency of gastrointestinal diseases encountered in the elderly.

Nociceptive threshold and analgesic response to morphine in aged and young adult rats as determined by thermal radiation and intracerebral electrical stimulation

The present experiment compared the nociceptive threshold and analgesic response to morphine in young (4-5 months) and aged (24 months) rats using peripheral thermal stimulation and intracerebral electrical stimulation. Responses to thermal stimuli were assessed using both the classical tail-flick procedure in which latency of response is the dependent variable and a new method in which threshold in calories of heat is the dependent variable. In the intracerebral nociceptive threshold procedure, electrical stimuli were delivered via an electrode implanted in the mesencephalic reticular formation (MRF), a pain pathway, and the animals were trained to terminate the stimulation by turning a cylindrical manipulandum embedded in one wall of the experimental chamber. For the classical tail-flick method, the aged rats required a greater intensity of stimulation to produce a basal response latency that was between 2.5 and 3.5 s. Using the new psychophysical method for determining the tail-flick threshold, the aged rats' basal thresholds were significantly higher than that of the young rats. However, the basal thresholds obtained by direct stimulation of the MRF failed to show a significant age effect, suggesting that the registration of pain is not different between young and aged rats. These age-related differences in baseline tail-flick response may be due to changes in the spinal reflex associated with aging. Although, there was no difference in the analgesic effects of morphine between young and aged rats using the latency of the tail-flick response, evidence for decreased analgesic response was seen using the tail-flick threshold measure and the intracerebral stimulation threshold method.

Morphine-6-glucuronide: Actions and mechanisms

Morphine-6-glucuronide (M6G) appears to show equivalent analgesia to morphine but to have a superior side-effect profile in terms of reduced liability to induce nausea and vomiting and respiratory depression. The purpose of this review is to examine the evidence behind this statement and to identify the possible reasons that may contribute to the profile of M6G. The vast majority of available data supports the notion that both M6G and morphine mediate their effects by activating the micro-opioid receptor. The differences for which there is a reasonable consensus in the literature can be summarized as: (1) Morphine has a slightly higher affinity for the micro-opioid receptor than M6G, (2) M6G shows a slightly higher efficacy at the micro-opioid receptor, (3) M6G has a lower affinity for the kappa-opioid receptor than morphine, and (4) M6G has a very different absorption, distribution, metabolism, and excretion (ADME) profile from morphine. However, none of these are adequate alone to explain the clinical differences between M6G and morphine. The ADME differences are perhaps most likely to explain some of the differences but seem unlikely to be the whole story. Further work is required to examine further the profile of M6G, notably whether M6G penetrates differentially to areas of the brain involved in pain and those involved in nausea, vomiting, and respiratory control or whether micro-opioid receptors in these brain areas differ in either their regulation or pharmacology.

Effects of morphine on brain-stimulation reward thresholds in young and aged rats

Mesolimbic opioid systems are altered with aging; however, the effects of these changes on the rewarding actions of opioids have not been examined. The present experiment assessed differences in the responsiveness of brain reward pathways in young and aged rats to the effects of morphine using the brain-stimulation reward (BSR) model. Aged (24 months) and young (5 months) male F344/BNF1 rats were stereotaxically implanted with a bipolar stainless steel electrode into the lateral hypothalamic (LH) region of the medial forebrain bundle. Thresholds were determined using the rate-independent psychophysical method. Each animal was tested after the administration of saline or morphine at 0.5, 1, 2.5, 5 and 10 mg/kg doses. A significant difference in the mean baseline threshold between aged (99.8+/-6 microA) and young rats (149.1+/-14 microA) was observed. Although in both groups morphine lowered the BSR threshold, there were no significant differences between the groups except at the 10-mg/kg dose, the difference did approach significance. This study indicates that there are baseline differences in the rewarding threshold in the two groups, that morphine lowers the threshold in young and aged animals and that the hedonic effects produced by morphine, for the most, part remain preserved in aged animals.

Increased sensitization to morphine-induced oral stereotypy in aged rats

Sensitization develops to the stereotypic biting behavior that appears with the repeated administration of high dose morphine to rats. Because there is evidence that this behavior is dopamine-mediated and that there are age-related changes in dopamine systems, we compared the development and expression of morphine-induced biting behavior in aged (24 months) and young rats (5 months). Animals were treated with four sensitizing 10 mg/kg doses of morphine or saline, followed by three weekly challenges with 4 mg/kg doses of morphine or saline. By the fourth sensitizing morphine dose and after the administration of each low dose challenge, the biting time was significantly greater for aged than for young morphine pre-treated rats. After the first weekly low dose challenge, the aged but not young animals expressed more biting than when they did after the last 10 mg/kg dose. These results indicate that sensitization to morphine-induced oral stereotypy is significantly greater in aged as compared to young rats. Age-related enhanced sensitivity to morphine-induced oral stereotypy might be related to age-induced increases in vulnerability to opioid-induced insults to the basal ganglia, and may be a model for certain diseases of this pathway.

Opioids and renal function

Opioids, both endogenous and exogenous, have a strong influence on the renal function through different mechanisms, producing changes in the renal excretion of water and sodium. Several studies have demonstrated that opioids influence renal function, according to the agonist profile used. Mu, kappa, and delta agonists produce different renal effects, although the mechanisms remain unclear. Experimental data have given the input for a possible therapeutic role of kappa agonists for some specific conditions, for example, in treating water retention or hyponatremia occurring in patients who have hepatic cirrhosis with ascites. On the other hand, changes in renal function might strongly condition the use of opioids in the clinical setting, and the knowledge of the relationship between opioids and renal function is mandatory for a tailored approach to accommodate the individual responses in terms of pain intensity, tolerance, and adverse effects experienced by these groups of patients. The influence of renal function when using different opioids in the clinical setting is reviewed, as well as problems related to transplantation, renal damage induced by opioid addiction, and problems related to the use of opioid antagonists in such conditions.

PERSPECTIVE

Endogenous opioids exert physiologic effects on renal function, and the use of opioids may have an influence on renal activity. Renal impairment has a serious impact on the clearance of most opioids used in the clinical setting. Biochemical and clinical monitoring is mandatory to prevent serious complications.

A retrospective study of the association between haematological and biochemical parameters and morphine intolerance in patients with cancer pain

BACKGROUND

Morphine is the strong opioid of choice for the treatment of moderate to severe cancer pain according to guidelines of the World Health Organization (WHO). However, a minority of patients do not receive the desired analgesic effect or suffer intolerable side effects from morphine, and are switched to alternative opioids.

METHODS

The aim of this retrospective study was to identify factors that might be associated with morphine intolerance. Data were analysed from 100 controls who tolerated morphine and 77 patients who were switched to an alternative opioid. We investigated whether currently logged data could fully explain the need to switch. Demographic details, cancer type (histological diagnosis) and markers related to organ function were included in an analysis of biochemical and haematological parameters.

RESULTS

Patients over 78 years (P = 0.03), or with a high white cell (P = 0.002) or high platelet count (P = 0.003), were more likely to switch. Although our numbers were small, patients with severe organ impairment were more likely to switch. However, a model including white cell count, platelet count, age, serum albumin and alkaline phosphatase, accurately separated switchers and controls in only 68% of cases. There was no significant difference between the two groups in terms of the numbers of patients having cytotoxic drugs in the two weeks prior to the haematological and biochemical analysis. Similarly, there were no significant differences in histological diagnoses between groups.

CONCLUSIONS

The white cell count was the strongest single effect observed and, as such, warrants further investigation. Further studies are needed in order to accurately define a model that will predict those patients likely to be intolerant of morphine.

Age differences in postoperative pain are scale dependent: a comparison of measures of pain intensity and quality in younger and older surgical patients

As the population ages, research into the assessment of postoperative pain in older patients is urgently needed. The reliability and validity of most pain scales for the assessment of acute postoperative pain in the elderly remain to be demonstrated. The present study reports the analysis of age-related patterns on three pain scales (McGill Pain Questionnaire, MPQ; Present Pain Intensity, PPI; and Visual Analog Scale, VAS) completed by younger (n=95, mean age=56.4+/-5.8 years) and older (n=105; mean age=66.8+/-2.7 years) men following radical prostatectomy. All patients received intravenous morphine via patient-controlled analgesia (PCA) throughout the study. On the first 2 postoperative days (POD), patients completed the pain scales and PCA opioid intake was recorded. An interaction was found between amount of opioid self-administered and POD. In both groups, less opioid was administered on POD 2 than POD 1, but the decrease over time was greater in younger than older men. On both PODs, older men self-administered less opioid than younger men. Age differences in pain were dependent on the pain scale used. Older men had significantly lower scores than younger men on the MPQ and PPI but there were no differences on the VAS. Several age differences in the psychometric properties of the scales were evident. On both PODs, the correlation between VAS and MPQ scores was significantly lower in the older than younger group. POD effect sizes did not differ between the scales or age groups suggesting that all three scales have comparable sensitivity within an age group. However, the different results between the scales for the effect of age suggests that the VAS is not sufficiently sensitive to detect age differences. Therefore, age differences in postoperative pain are better captured by verbal descriptions of pain qualities than non-verbal measures of intensity.

Impact of ageing on the antinociceptive effect of reference analgesics in the Lou/c rat

1. Research on the evolution of experimental pain perception and on the achievement of analgesia with ageing has led so far to contradictory results. 2. This study investigated in the rat the impact of ageing on the antinociceptive effect of reference analgesics, acetaminophen (50, 100, 200, 400 mg kg(-1) po), aspirin (50, 100, 200, 400 mg kg(-1) sc), clomipramine (5, 10, 20, 40 mg kg(-1) sc) and morphine (1.25, 2.5, 5, 10 mg kg(-1) sc). 3. Lou/c rats were chosen because they provide a model of healthy ageing and they do not develop obesity with age. Three groups of 40 rats each (mature (4 months), middle-aged (18 months) and old (26 months)), were treated with each drug at 14 days interval. Two tests were used: a thermal test (tail immersion in 48 degrees C water and measurement of reaction latency) and a mechanical test (paw pressure and measurement of struggle threshold). 4. Results confirm the increased mechanical sensitivity to pain and no change in thermal sensitivity for old rats compared to mature and middle-aged animals. They show a marked decrease in the effect of morphine with age and no age-related effect for acetaminophen, aspirin or clomipramine. Plasma levels of morphine and metabolites are not different in the three age groups. 5. It is likely that the influence of age on morphine analgesia is linked mainly to pharmacodynamic rather than pharmacokinetic changes.

Effect of morphine on caloric intake and macronutrient selection in male and female Lou/c/jall rats during ageing

Previous studies have showed a shift of preferences from carbohydrate to fat in the Lou/c/jall rat with advancing age when they are submitted to a self-selection procedure. Protein intake also decreased according to the age, earlier for males (after 16 months) than for females (29 months). The present study aimed at investigating the mechanism underlying these modifications. We analysed the effect of the reference mu agonist, morphine (5 mg/kg subcutaneous), on the caloric intake, body weight and macronutrient intake of 30 male and 30 female rats divided in four age groups: young adults (10), mature (17), old (24) and senescent rats (29 months). During the experiment, animals had the choice between separate sources of the three pure macronutrients. Morphine injection reduced total daily caloric intake and induced a decrease in body weight. The weight loss was age- and sex-related (males and old rats were more affected by the drugs). The injection of morphine evoked a triphasic influence on the chronology of the intake. A brief (1 h) hypophagia was followed by an hyperphagia (3 h) and a persistent hypophagia (8 h). No modification in the diet composition was observed. These results did not support a clear involvement of the opioid system concerning the modifications in macronutrient rates in diet previously observed across ageing.

Precipitated withdrawal following codeine administration is dependent on CYP genotype

The role of metabolic polymorphism in the development of physical dependence to codeine was assessed in cytochrome P450 2D2 (CYP2D2) deficient Dark Agouti and CYP2D2 intact Sprague-Dawley rats by assessment of the severity of naloxone precipitated withdrawal after codeine and morphine administration. Plasma morphine concentrations after codeine were significantly higher (P<0.01) in Sprague-Dawley than in Dark Agouti rats with metabolic ratios of 0.71 +/- 0.27 and 0.07 +/- 0.04, respectively. Withdrawal after codeine resulted in significantly greater hypothermia (3.5-4 degrees C, P<0.0001) in Sprague-Dawley animals compared to the other groups. Body weight loss was similar for all groups ranging from 6.2 +/- 0.4 to 8.2 +/- 0.6 g. When strain and treatment data were combined, a relationship between body temperature and plasma morphine concentration could be described by the inverse Hill equation (r(2)=0.76, EC(50)=556 +/- 121 ng/ml, n=2.9 +/- 1.5). These data indicate that dependence and withdrawal after codeine administration are dependent on its bioconversion to morphine.

Age differences in nociception and pain behaviours in the rat

Much remains to be learned about the effects of ageing on pain. Studies of life-span changes in nociception and pain behaviours in the rat are equivocal making it difficult to draw firm conclusions. This paper reviews the available data and finds that age differences in nociception may be dependent on the pain test employed. Specifically, reflexive responses to nociceptive stimuli do not change with age while there may be no change or a linear decrease with age on more highly organized tests of nociception. Interestingly, age differences in pain behaviours on models of tissue injury and inflammation may not be linear. It is shown that important changes that begin at mid-life in neuroanatomy, neurochemistry and endogenous pain inhibition may be associated with alterations in pain sensitivity. Several testable hypotheses which might encourage future research in this domain are developed throughout this paper.

Effect of uranyl nitrate-induced renal failure on morphine disposition and antinociceptive response in rats

1. The aims of the present study were to administer morphine (14.0 mumol/kg, s.c.) to male Hooded Wistar rats and to determine the effect of uranyl nitrate-induced renal failure on: (i) the antinociceptive effect of morphine; (ii) the pharmacokinetics of morphine and morphine-3-glucuronide (M3G); and (iii) the relationship between antinociceptive effect and the pharmacokinetics of morphine in plasma and brain. 2. Renal failure was induced by a single s.c. injection of uranyl nitrate and kinetic/dynamic studies were performed 10 days after its administration, when creatinine clearance was 17% of the control group. Antinociceptive effect was measured by the tail-flick method at various times up to 2 h post-drug administration. Concentrations of morphine and M3G in plasma and brain and concentrations of creatinine in urine and serum were determined by specific HPLC methods. 3. After morphine administration, the area under the antinociceptive effect-time curve was decreased by 44% in renal failure rats. There were no differences between control and renal failure rats in: (i) plasma morphine concentration-time curves; (ii) brain morphine concentration-time curves; and (iii) plasma M3G concentration-time curves. Morphine-6-glucuronide was not detected in any plasma or brain sample from rats administered morphine and no M3G was detected in brain. 4. For both control and renal failure rats, the relationships between antinociceptive effect and plasma morphine concentration were characterized by counterclockwise hysteresis loops, probably reflecting a delay for the relatively polar morphine to cross the blood-brain barrier. The relationship between antinociceptive effect and brain morphine concentration in control rats revealed no evidence of acute tolerance and was described by a sigmoidal function. In contrast, the relationship in renal failure rats was characterized by clockwise hysteresis, which is consistent with acute tolerance development.

The role of morphine glucuronides in cancer pain

Morphine metabolites are involved in various ways in determining the complex effects of morphine, both favourable and adverse, and may complicate the clinical use of morphine in the treatment of cancer pain. The production and effects of the principal morphine metabolites, morphine-3-glucuronide and morphine-6-glucuronide, in both normal and pathological states have been reviewed in the current literature. Therapeutic implications are also reviewed on the basis of experimental and clinical reports. The presence of these metabolites should be recognized in the chronic treatment of cancer pain with morphine, especially in the presence of renal impairment, and should be considered to have an important influence on opioid responsiveness, defined as a balance between the achievement of an optimal analgesia and the occurrence of adverse effects.