Association of a MIND Diet with Brain Structure and Dementia in a French Population

BACKGROUND

Adherence to the Mediterranean-DASH Intervention for Neurodegenerative Delay (MIND) diet, which combines higher consumption of vegetables, berries, nuts, whole grains, olive oil, fish, beans and poultry, with lower consumption of meat, sugars and saturated fats, is a promising strategy to prevent dementia. However, evidence in populations with non-US food culture, especially from Europe, is limited.

OBJECTIVES

To evaluate the association of a French-adapted MIND diet score with gray matter volumes, white matter microstructure and incident dementia.

DESIGN AND SETTING

This longitudinal study included participants from the population-based Three-City Bordeaux cohort (≥65 years), with a follow-up from June 2001 to February 2018.

PARTICIPANTS

Dementia-free participants at dietary assessment, in 2001-2002, who underwent systematic detection of incident dementia (over up to 7 visits). A subset of the cohort was included in an ancillary MRI study in 2010-2011.

MEASUREMENTS

A French-adapted MIND diet score (range, 0-15) was computed from a 148-item Food Frequency Questionnaire and a 24-hour recall administered at home. Incident dementia and its subtypes were adjudicated by an expert committee; and gray matter volumes and white matter microstructure were assessed by 3D-T1 MRI and diffusion-MRI.

RESULTS

Among 1,412 participants (mean age, 75.8 [SD, 4.8]; 63% women), followed for a median of 9.7 years (maximum 16.3 years), 356 (25.2%) developed incident dementia. In multivariable-adjusted Cox model, a higher French MIND diet score was associated with lower risks of dementia and AD (hazard ratios for 1-point of score = 0.89 [95% confidence interval, 0.83-0.95] and 0.88 [0.81-0.96], respectively). In Tract-Based Spatial Statistics analysis of 175 participants included in the MRI sub-study, a higher MIND diet score was associated with lower diffusivity values in the splenium of the corpus callosum (P < .05 after Family-Wise Error-correction). In contrast, there was no significant association of the adapted MIND diet score with gray matter volumes in Voxel-Based Morphometry analysis.

CONCLUSION

In this cohort of French older adults, higher adherence to the French MIND diet was associated with a lower dementia risk and with preserved white matter microstructure. These results provide further evidence for a role of the MIND diet in the prevention of dementia.

The age-related positivity effect: forgetting the negative and/or remembering the positive? An inter-task study

A growing number of studies have shown that when compared to younger adults, older adults are better at recalling positive information than negative information. However, it is not yet clear whether this age-related positivity effect relies on a greater ability to recall positive information or on a decreased ability to recall negative information. We therefore aimed to study the specific mechanisms underlying the age-related positivity effect using different memory tasks. We used an emotional word memory paradigm including immediate free recall, recognition, and delayed free recall tasks. Forty-five young adults (m = 20.0 years) and 45 older adults (m = 69.2 years) participated, all of whom were native French speakers. Thirty-six French low-arousal words (12 positve, 12, negative, 12 neutral) were selected from an emotional lexical database (Gobin et al. 2017) and divided into three equal groups of positive, neutral and negative terms. For the recognition task, 36 new words were selected. The results show that the age-related positivity effect specifically depended on a decrease in negativity preference (i.e., the comparison between negative and neutral words) in older adults, in comparison with younger adults, both in the immediate and delayed free recall tasks. In these tasks, younger adults recalled more negative than neutral words, whereas there was no difference in older adults. During the recognition task, no age-related positivity effect was observed. The results also show that, for the immediate recall task, the greater the memory ability of older adults, the lower their negativity preference. This correlation was not significant in the delayed recall task. These results suggest that, when compared with younger adults, older adults disengage from processing negative words that require costly cognitive processes. A low negativity preference indicates that memory abilities are well-maintained. The results are discussed within the framework of socio-emotional selectivity theory.

Toward a unified analysis of cerebellum maturation and aging across the entire lifespan: A MRI analysis

Previous literature about the structural characterization of the human cerebellum is related to the context of a specific pathology or focused in a restricted age range. In fact, studies about the cerebellum maturation across the lifespan are scarce and most of them considered the cerebellum as a whole without investigating each lobule. This lack of study can be explained by the lack of both accurate segmentation methods and data availability. Fortunately, during the last years, several cerebellum segmentation methods have been developed and many databases comprising subjects of different ages have been made publically available. This fact opens an opportunity window to obtain a more extensive analysis of the cerebellum maturation and aging. In this study, we have used a recent state‐of‐the‐art cerebellum segmentation method called CERES and a large data set (N = 2,831 images) from healthy controls covering the entire lifespan to provide a model for 12 cerebellum structures (i.e., lobules I‐II, III, IV, VI, Crus I, Crus II, VIIB, VIIIA, VIIIB, IX, and X). We found that lobules have generally an evolution that follows a trajectory composed by a fast growth and a slow degeneration having sometimes a plateau for absolute volumes, and a decreasing tendency (faster in early ages) for normalized volumes. Special consideration is dedicated to Crus II, where slow degeneration appears to stabilize in elder ages for absolute volumes, and to lobule X, which does not present any fast growth during childhood in absolute volumes and shows a slow growth for normalized volumes.

Frailty and Brain-Muscle Correlates in Older People With Type 2 Diabetes: A structural-MRI Explorative Study

OBJECTIVES

Muscle alterations, mainly functional alterations are frequently observed in older people with type 2 diabetes (T2DM). Sarcopenia may be one mechanism of transition to frailty in these people. Thus, we aim to explore the characteristics of muscle and its association with cerebral grey matter volumes within this group.

METHODS

Single center study nested within the international MID-Frail (a randomized clinical trial to evaluate the effectiveness of a multi-modal intervention in older people with T2DM on frailty and quality of life) trial participants underwent both brain and muscle T1 MRI, nutritional and functional assessments. Muscle areas were measured in rectus femoris (RF). Relationships between MRI grey matter volumes and muscle areas or function tests were described using positive and negative regressions.

RESULTS

Twenty-six subjects (7 female, mean age 78.2 y, SD 5.0), 6 frail and 20 pre-frail were explored in this sub-study. Frail subjects had lower Mini Nutritional Assessment (MNA), Short Physical Performance Battery (SPPB), hip flexor strength than pre-frail ones but similar BMI and balance. Total SPPB was positively related with hip flexor strength and maximal RF area. Balance SPPB sub-score was unrelated to strength or RF area. MNA score was correlated with hip flexor strength and to global grey matter but not to SPPB. Hip flexor strength was correlated with grey matter areas involved in motor control. Walking time was negatively and rising chair sub-score was positively associated with grey matter volumes of motor areas.

CONCLUSIONS

Sarcopenia features were more frequent in frail than prefrail subjects and were associated with decrease in grey matter volumes involved in motor control.

Towards a unified analysis of brain maturation and aging across the entire lifespan: A MRI analysis

There is no consensus in literature about lifespan brain maturation and senescence, mainly because previous lifespan studies have been performed on restricted age periods and/or with a limited number of scans, making results instable and their comparison very difficult. Moreover, the use of nonharmonized tools and different volumetric measurements lead to a great discrepancy in reported results. Thanks to the new paradigm of BigData sharing in neuroimaging and the last advances in image processing enabling to process baby as well as elderly scans with the same tool, new insights on brain maturation and aging can be obtained. This study presents brain volume trajectory over the entire lifespan using the largest age range to date (from few months of life to elderly) and one of the largest number of subjects (N = 2,944). First, we found that white matter trajectory based on absolute and normalized volumes follows an inverted U-shape with a maturation peak around middle life. Second, we found that from 1 to 8-10 y there is an absolute gray matter (GM) increase related to body growth followed by a GM decrease. However, when normalized volumes were considered, GM continuously decreases all along the life. Finally, we found that this observation holds for almost all the considered subcortical structures except for amygdala which is rather stable and hippocampus which exhibits an inverted U-shape with a longer maturation period. By revealing the entire brain trajectory picture, a consensus can be drawn since most of the previously discussed discrepancies can be explained. Hum Brain Mapp 38:5501-5518, 2017. © 2017 Wiley Periodicals, Inc.

Detection of Alzheimer's disease signature in MR images seven years before conversion to dementia: Toward an early individual prognosis

Finding very early biomarkers of Alzheimer's Disease (AD) to aid in individual prognosis is of major interest to accelerate the development of new therapies. Among the potential biomarkers, neurodegeneration measurements from MRI are considered as good candidates but have so far not been effective at the early stages of the pathology. Our objective is to investigate the efficiency of a new MR-based hippocampal grading score to detect incident dementia in cognitively intact patients. This new score is based on a pattern recognition strategy, providing a grading measure that reflects the similarity of the anatomical patterns of the subject under study with dataset composed of healthy subjects and patients with AD. Hippocampal grading was evaluated on subjects from the Three-City cohort, with a followup period of 12 years. Experiments demonstrate that hippocampal grading yields prediction accuracy up to 72.5% (P < 0.0001) 7 years before conversion to AD, better than both hippocampal volume (58.1%, P = 0.04) and MMSE score (56.9%, P = 0.08). The area under the ROC curve (AUC) supports the efficiency of imaging biomarkers with a gain of 8.4 percentage points for hippocampal grade (73.0%) over hippocampal volume (64.6%). Adaptation of the proposed framework to clinical score estimation is also presented. Compared with previous studies investigating new biomarkers for AD prediction over much shorter periods, the very long followup of the Three-City cohort demonstrates the important clinical potential of the proposed imaging biomarker. The high accuracy obtained with this new imaging biomarker paves the way for computer-based prognostic aides to help the clinician identify cognitively intact subjects that are at high risk to develop AD.

Cerebellum involvement in post-stroke mood: a combined ecological and MRI study

This study evaluated a new approach combining magnetic resonance imaging and the experience sampling method in the understanding of post-stroke mood pathophysiology. Findings revealed that emotional cognition after stroke may be related to phenotypic characteristics such as cerebellar volume, thereby suggesting that this combined approach could provide new insights into the pathophysiology of post-stroke mood disorders as well as other forms of comorbidity.

A study of the role of neuro-glial remodeling in the oxytocin system at lactation

Under conditions of strong secretion of neurohypophysial hormone, such as during parturition, lactation and dehydration, the hypothalamic oxytocin-system displays a remarkable morphological plasticity such that astrocytic coverage of its neurones diminishes, their surfaces become directly juxtaposed and contacted by an increased number of synapses. A growing body of evidence indicates that these anatomical changes have an impact on glutamatergic neurotransmission in the supraoptic nucleus, and may be therefore of physiological consequence. We here evaluated the consequences of the inhibition of such plasticity on the overall activity of the oxytocin system during lactation. Remodeling was prevented by performing hypothalamic microinjections in gestating rats of endoneuraminidase, an enzyme that removes polysialic acid from the neural cell adhesion molecule. Our earlier studies established that the presence of polysialic acid is a prerequisite for remodeling of the oxytocin system in the supraoptic and paraventricular nuclei. In dams in which polysialic acid was absent in all magnocellular nuclei after bilateral endoneuraminidase injections, parturition was normal and neither the frequency nor the amplitude of suckling-induced reflex milk ejections was different from vehicle-treated dams. The weight gain of pups was also normal as was water intake by the dams. We then assessed the electrical activity of antidromically identified magnocellular neurones in the polysialic acid-free supraoptic nucleus of isoflurane-anesthetized lactating rats. Basal and bursting activity characteristic of oxytocin neurones before each reflex milk ejection was not significantly different from that recorded in the supraoptic nucleus of rats with normal levels of polysialic acid. Our results indicate that neuro-glial remodeling, despite its role on fine modulation of oxytocin neuronal activity, is not essential to parturition and lactation.

The effects of RB101, a mixed inhibitor of enkephalin-catabolizing enzymes, on carrageenin-induced spinal c-Fos expression are completely blocked by beta-funaltrexamine, a selective mu-opioid receptor antagonist

We have demonstrated that pre-administered RB101 (40 mg/kg, i.v.), a mixed inhibitor of enkephalin-catabolizing enzymes, decreased spinal c-Fos expression induced 1 h and 30 min after intraplantar (i.pl.) carrageenin (41% reduction, p<0.01). These effects were completely blocked by pre-administered beta-funaltrexamine (10 mg/kg, i.v., 24 h prior to stimulation), a selective long-lasting mu-opioid receptor antagonist. In conclusion, these results clearly demonstrate that the effects of endogenous enkephalins on noxiously evoked spinal c-Fos expression are essentially mediated via mu-opioid receptors.

Effects of NMDA receptor antagonists on morphine tolerance: a c-Fos study in the lumbar spinal cord of the rat

This study investigated the contribution of NMDA receptors to the development of tolerance to the antinociceptive properties of morphine at the level of the spinal cord dorsal horn. The expression of c-Fos protein following intraplantar (i.pl.) injection of carrageenin (6 mg/150 microl of saline) was used. In naive rats, acute intravenous (i.v.) administration of morphine (3 mg/kg) decreased the total number per section of Fos-Like-Immunoreactive (Fos-LI) neurons by 51%, observed at 2 h after injection of carrageenin. In tolerant rats, acute morphine did not significantly modify the total number of Fos-like immunoreactive neurons/section. In rats receiving chronic morphine and chronic injections of the non-competitive ((+)-MK 801 maleate: (5R,10S)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,1 0-imine) or the competitive (LY 235959: [3S-(3alpha,4a alpha,6beta,8a alpha)]-Decahydro-6-(phosphonomethyl)-3-isoquinolinecarboxylic+ ++ acid) NMDA receptor antagonists, only partial tolerance to the acute effects of morphine were observed (decrease of 42% and 38%, respectively). Administration of an antagonist at the strychnine-insensitive glycine site of the NMDA receptor ((+)-HA-966: R(+)-3-Amino-1-hydroxypyrrolidin-2-one) did not affect the development of morphine tolerance. These findings suggest that compounds attenuating the actions of the NMDA receptor via blockade of the glycine modulatory site may be substantially different from those acting at the ion channel of the NMDA receptor complex. This in vivo experiment in freely moving animals demonstrates for the first time an attenuation of tolerance at the cellular level.

Effects of U-69,593, a kappa-opioid receptor agonist, on carrageenin-induced peripheral oedema and Fos expression in the rat spinal cord

In an attempt to study the anti-inflammatory and the antinociceptive effects of a kappa1-opioid receptor agonist (U-69,593: trans-3,4-dichloro-N-methyl-N-[7-(1-pyrrolidinyl)cycloexil]benzene acetamide methanesulfonate), we used a combination of the measurement of peripheral oedema (with a calliper) and Fos immunodetection in the carrageenin model of inflammation. The intraplantar injection of carrageenin-induced the development of a peripheral oedema, associated with an increase in Fos-like immunoreactivity at the level of the dorsal horn of the spinal cord. U-69,593 administered intravenously (i.v.) 10 min before carrageenin administration over the dose range 0.75, 1.5 and 3 mg/kg, reduced both paw and ankle oedema in a non dose-dependent manner. The maximal decrease was observed at the highest dose and did not exceed 21% and 20% for the paw and the ankle respectively. These effects were kappa-opioid receptor specific since the anti-inflammatory effect of 1.5 mg/kg i.v. of U-69,593 was antagonised by a specific kappa-opioid receptor antagonist nor-binaltorphimine. Pre-treatment with U-69,593 strongly decreased the number of Fos-like Immunoreactive neurones of the spinal cord in a dose-dependent, antagonist reversible manner; maximal effect was 65%. The disparate results between the anti-inflammatory effects and the depressive effects on Fos expression suggest that anti-inflammatory effects of kappa-opioid receptor agonist are of minor importance for the antinociceptive effects of this compound.

Effects of opioid receptor antagonists on the effects of i.v. morphine on carrageenin evoked c-Fos expression in the superficial dorsal horn of the rat spinal cord

This study performed in freely moving rats evaluated the ability of specific opioid receptor antagonists to reverse the inhibitory effects of morphine on carrageenin-induced c-Fos expression in the spinal cord. Our study focused on the superficial dorsal horn (laminae I-II), which is the main termination site of nociceptive primary afferent fibers and is rich in opioid receptors. In order to replicate clinical routes of administration, all agents were administered intravenously (i.v.). As previously demonstrated, pre-administered i.v. morphine (3 mg/kg) produced a marked decrease (58+/-5%) in the number of Fos-LI neurones measured at 2 h after intraplantar (i.pl.) carrageenin (6 mg/150 microl) and yet was without influence on peripheral oedema. This decrease in c-Fos expression was completely blocked by combined administration of morphine with the mu-opioid receptor antagonist, [D-Phe-Cys-Tyr-D-Orn-Thr-Pen-Thr-NH2] (CTOP-1+1 mg/kg). Naltrindole (NTI-1+1 mg/kg), a delta-opioid receptor antagonist partially blocked the effects of systemic morphine, so that the inhibitory effects of morphine after NTI injection are now 40+/-4%. However, this effect of NTI was weak since the depressive effects of morphine were still highly significant (p<0.001). In contrast, nor-binaltorphimine (nor-BNI-1+1 mg/kg), a kappa-opioid receptor antagonist, had no significant effect on the effects of morphine. These results indicate the major contribution of mu-opioid receptors to the antinociceptive effects of systemic morphine at the level of the superficial dorsal horn. The observed effect of NTI is not necessarily related to a direct action of morphine on delta-opioid receptors and some possible actions of this antagonist are discussed.

Are there long-term changes in the basal or evoked Fos expression in the dorsal horn of the spinal cord of the mononeuropathic rat?

The long-term changes in Fos like-immunoreactivity (Fos-LI) in the dorsal horn of the spinal cord following various peripheral nerve lesions remain controversial. This study considers such an approach with chronic constriction injury rats (CCI: loose ligations of the sciatic nerve), at 2 weeks after the surgery, when changes in spontaneous and evoked behaviour were clearly described. All rats used for Fos studies displayed allodynia to mechanical stimulation (decrease of 32% of the vocalization threshold to paw pressure). In CCI rats, which displayed 'spontaneous pain-related behaviour', the number of Fos-LI neurones, in the absence of any intentional stimulation, was very low and comparable with that observed in normal and sham-operated rats (<10 neurones/40 microm section). Thus, in this model, the expression of Fos protein is not a reliable index of spontaneous pain. Surprisingly, despite the fact that in this model numerous anatomical studies described a dramatic loss of large and unmyelinated primary afferent fibers, we were unable to detect changes in the number and distribution of Fos-LI evoked by various modalities of peripheral noxious stimulation (noxious thermal stimuli, noxious mechanical stimuli and carrageenin induced inflammation). For example, the stimulus-response curves for the number of Fos-LI neurones evoked by a series of heat stimuli (40, 45, 48, 52, 55 degrees C) were almost superimposable for CCI, sham-operated and normal rats. In contrast, stroking of the nerve-injured paw induced a significant expression of Fos-LI in the superficial laminae (I-II) of the dorsal horn of CCI rats (19.5 +/- 3/sections, P = 0.027) which was greater than that observed in sham-operated (6.5 +/- 3/sections) or in normal rats (3.5 +/- 2/section). These modifications may reflect mechanical allodynia observed in behavioural studies and could be related to A beta fibers, which are known to be severely affected after the constriction of the nerve. These results suggest that this approach could be useful to study, at the cellular level, in freely moving rats, some pharmacological aspects of neuropathic pain.

Development of tolerance to the antinociceptive effect of systemic morphine at the lumbar spinal cord level: a c-Fos study in the rat

The development of tolerance to the antinociceptive effects of morphine was investigated in rats using carrageenin-induced spinal c-Fos expression. We took advantage of this technique to especially study, at the cellular level, in freely moving animals, the development of tolerance based on the visualization of dorsal horn spinal cord neurons which play a major role in nociceptive processes. Two hours after intraplantar injection of carrageenin (6 mg/150 microliter of saline), c-Fos-like immunoreactivity (FLI) was observed predominantly in the superficial and deep laminae of the dorsal horn in segments L4 and L5 of the spinal cord. In naive rats, acute intravenous morphine (3 mg/kg, i.v.) reduced the number of superficial and deep FLI neurons; 49% and 59% reduction respectively (p<0.0001 for both). In morphine-pretreated rats (daily administration of subcutaneous morphine: 1, 3, 5, 10, 20 or 40 mg/kg once a day for 4 days), antinociceptive tolerance tested on day 5 by acute morphine (3 mg/kg, i.v.) was manifest in those groups pretreated with the highest doses of morphine (10, 20 or 40 mg/kg). From regression analysis, it appeared that tolerance to the antinociceptive effect of morphine developed progressively as a function of the chronic morphine dose used on neurons involved in spinal nociceptive processes (superficial and deep dorsal horn neurons). Similarly, in rats pretreated with 10 mg/kg of morphine over 1, 2, 3 or 4 days, tolerance progressively developed, for both spinal neuronal populations, as a function of the duration of the pretreatment. These results are discussed in the context of the several possible sites of action of morphine.

Peripheral component in the enhanced antinociceptive effect of systemic U-69,593, a kappa-opioid receptor agonist in mononeuropathic rats

The contribution of a peripheral action of the kappa-opioid receptor agonist U-69,593 (trans-3,4-dichloro-N-methyl-N-[7-(1-pyrrolidinyl) cycloexil] benzene-acetamide methanesulfonate) in the augmented antinociceptive effect of this substance was investigated in a well-established rat model of peripheral unilateral neuropathy (chronic constriction of the common sciatic nerve). Relatively low dose of systemic U-69,593 (0.75 mg/kg intravenous (i.v.)) and intraplantar (i.pl.) low doses of specific antagonists of kappa-(nor-binaltorphimine) or mu-(D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2: CTOP) opioid receptors were used. Vocalization thresholds to paw pressure were used as a nociceptive test. The i.pl. injection of nor-binaltorphimine (10-15 microg injected into the nerve-injured hind paw) had no effect on the antinociceptive effect of U-69,593. Higher doses (20-30 microg i.pl. nor-binaltorphimine) significantly reduced the effect of U-69,593 on this paw but not on the contralateral paw, an effect which plateaued at 30 microg. By contrast, the i.pl. injection of CTOP (1 microg into the nerve-injured paw) had no effect on U-69,593 antinociception, whereas it reduced the effect of systemic morphine in these animals. The doses of nor-binaltorphimine used, injected into the contralateral paw or i.v., failed to modify the antinociceptive effects of U-69,593 on either paw. These results provide evidence for a peripheral component in the enhanced antinociceptive effect of systemic U-69,593 in this model of neuropathic pain.

Chronic treatment with systemic morphine induced tolerance to the systemic and peripheral antinociceptive effects of morphine on both carrageenin induced mechanical hyperalgesia and spinal c-Fos expression in awake rats

The effects of intravenous (3 mg/kg i.v.) and intraplantar (50 micrograms/50 microliters i.pl.) morphine were investigated on spinal c-Fos expression induced 2 h after intraplantar carrageenin (6 mg/150 microliters of saline) and on carrageenin (2 mg/150 microliters of saline) induced mechanical hyperalgesia, at day 4, in both naive and chronic morphine treated (80 mg/kg/day s.c. on days 1, 2 and 3) rats. In naive rats, i.v. and i.pl. morphine significantly decreased spinal c-Fos expression (64 +/- 4% and 44 +/- 4% reduction of control carrageenin c-Fos expression, P < 0.0001 for both, respectively) and mechanical hyperalgesia (maximal increase: 326 +/- 29%, P < 0.0001 and 87 +/- 5%, P < 0.005 of control carrageenin paw pressure vocalisation threshold (VTPP), respectively), which only developed in the carrageenin injected paw. Both treatments were ineffective in chronic morphine treated rats (92 +/- 9% and 106 +/- 6% of control carrageenin c-Fos expression; 33 +/- 17% and 30 +/- 15% increase of control carrageenin VTPP, respectively). Furthermore, only i.v. morphine increased the VTPP in the contralateral paw, in naive rats (maximal increase: 90 +/- 8%, P < 0.0001 of control carrageenin VTPP), its effects being significantly less pronounced than for the inflamed paw (P < 0.0001). These studies based on spinal c-Fos expression as an indirect marker of spinal nociceptive processes and on behavioural experiments clearly revealed that chronic treatment with systemic morphine induced tolerance to both its systemic and peripheral effects.

The antinociceptive activity of kappa- but not delta-opioid receptor agonists is maintained in morphine-tolerant neuropathic rats

The antinociceptive effect of the preferential mu-opioid receptor agonist morphine (1 mg/kg i.v.), the delta-opioid receptor agonists, DTLET ([D-Thr2,Leu5]enkephalin-Thr) (3 and 6 mg/kg i.v.) and BUBUC ([D-Cys(StBu)2,Leu5]enkephalin-Thr(OtBu) (3 mg/kg i.v.), and the kappa-opioid receptor agonist U-69,593 (trans-3,4-dichloro-N-methyl-N-[7-(1-pyrrolidinyl)cyclohexil]benze neacetamide methanesulfonate) (0.25, 0.5 and 0.75 mg/kg i.v.) was evaluated in mononeuropathic (chronic constriction of the common sciatic nerve) rats. The rats were pretreated s.c. with 10 mg/kg of morphine, or saline, twice daily from day 12 to day 16 after the surgery. In morphine-pretreated rats, the antinociceptive effect of morphine on the vocalization threshold to paw pressure was greatly reduced, as compared to the saline-pretreated group. The antinociceptive effect of DTLET and BUBUC had also disappeared in the morphine-pretreated rats. By contrast, the potent antinociceptive effect of U-69,593 was not affected by the morphine pretreatment. Furthermore, the effect of U-69,593 was reversed by the specific kappa-opioid receptor antagonist nor-binaltorphimine (1 and 2 mg/kg i.v.). These results suggest that in mononeuropathic rats, morphine pretreatment results in cross-tolerance to delta- but not to kappa-opioid receptor agonists.

Further evidence for a peripheral component in the enhanced antinociceptive effect of systemic morphine in mononeuropathic rats: involvement of kappa-, but not delta-opioid receptors

The contribution of a peripheral action of morphine in the augmented antinociceptive effect of this substance was re-evaluated in a well established rat model of peripheral unilateral mononeuropathy (chronic constriction of the common sciatic nerve), using a relatively low dose of systemic morphine (1 mg/kg i.v.) and local low doses of specific antagonists of kappa- (nor-binaltorphimine) or delta-(naltrindole) opioid receptors. Vocalization thresholds to paw pressure were used as a nociceptive test. Escalating doses of nor-binaltorphimine (10-30 micrograms injected locally into the nerve injured paw) significantly and dose dependently reduced the effect of morphine on this paw but not on the contralateral paw, an effect which plateaued at 30 micrograms. By contrast, the local injection of naltrindole (30-40 micrograms into the nerve injured paw) had no effect on morphine analgesia. The doses of opioid receptor antagonists used, injected i.v., in the contralateral paw, or alone in the nerve injured paw had no significant effect. These results suggest that the peripheral effect of systemic morphine in this model of neuropathic pain could be mediated not only by mu- but also by kappa-opioid receptors.