Age differences in the response to the formalin test in rats

Pain and Itch Processing in Aged Mice

Most reports agree that aging negatively impacts pain processing and that the prevalence of chronic pain increases significantly with age. To improve current therapies, it is critical that aged animals be included in preclinical studies. Here we compared sensitivities to pain and itch-provoking stimuli in naïve and injured young and aged mice. Surprisingly, we found that in the absence of injury, aged male and female mice are significantly less responsive to mechanical stimuli and, in females, also to noxious thermal (heat) stimuli. In both older male and female mice, compared to younger (6-month-old mice), we also recorded reduced pruritogen-evoked scratching. On the other hand, after nerve injury, aged mice nevertheless developed significant mechanical hypersensitivity. Interestingly, however, and in contrast to young mice, aged mice developed both ipsilateral and contralateral postinjury mechanical allodynia. In a parallel immunohistochemical analysis of microglial and astrocyte markers, we found that the ipsilateral to the contralateral ratio of nerve injury-induced expression decreased with age. That observation is consistent with our finding of contralateral hypersensitivity after nerve injury in the aged but not the young mice. We conclude that aging has opposite effects on baseline versus postinjury pain and itch processing. PERSPECTIVE: Aged male and female mice (22-24 months) are less sensitive to mechanical, thermal (heat), and itch-provoking stimuli than are younger mice (6 months).

Pain and aging: A unique challenge in neuroinflammation and behavior

Chronic pain is one of the most common, costly, and potentially debilitating health issues facing older adults, with attributable costs exceeding $600 billion annually. The prevalence of pain in humans increases with advancing age. Yet, the contributions of sex differences, age-related chronic inflammation, and changes in neuroplasticity to the overall experience of pain are less clear, given that opposing processes in aging interact. This review article examines and summarizes pre-clinical research and clinical data on chronic pain among older adults to identify knowledge gaps and provide the base for future research and clinical practice. We provide evidence to suggest that neurodegenerative conditions engender a loss of neural plasticity involved in pain response, whereas low-grade inflammation in aging increases CNS sensitization but decreases PNS sensitivity. Insights from preclinical studies are needed to answer mechanistic questions. However, the selection of appropriate aging models presents a challenge that has resulted in conflicting data regarding pain processing and behavioral outcomes that are difficult to translate to humans.

Analgesic Characteristics of NanoBEO Released by an Airless Dispenser for the Control of Agitation in Severe Dementia

Chronic pain is one of the most common causes of the need for clinical evaluation, acquiring more importance in the elderly with cognitive impairment. Reduced self-reporting capabilities cause unrelieved pain contributing to the development of agitation. Safe and effective pain treatment can afford the management of agitation without the serious increase in death risk associated with neuroleptics. To this aim, the essential oil of bergamot (BEO), proven by rigorous evidence to have strong preclinical anti-nociceptive and anti-allodynic properties, has been engineered (NanoBEO, patent EP 4003294) to allow randomized, double-blind, placebo-controlled trials (BRAINAID, NCT04321889). The present study: (1) assesses the analgesic effects of a single therapeutic dose of NanoBEO, as supplied by an airless dispenser for clinical translation, in models of inflammatory, neuropathic, and sensitization types of pain relevant to clinic; (2) provides a dose-response analysis of the efficacy of NanoBEO on scratching behavior, a typical behavioral disturbance occurring in dementia. A single therapeutic dose of NanoBEO confirms efficacy following thirty minutes pre-treatment with capsaicin and on the central sensitization phase induced by formalin. Moreover, it has an ID50 of 0.6312 mg and it is efficacious on static and dynamic mechanical allodynia. Altogether, the gathered results strengthen the potential of NanoBEO for clinical management of pain and agitation.

Sex differences in pain-induced modulation of corticotropin-releasing hormone neurons in the dorsolateral part of the stria terminalis in mice

We earlier reported female-biased, sex-specific involvement of the dorsolateral bed nucleus of the stria terminalis (dl BST) in the formalin-induced pain response in rats. The present study investigated pain effects on mice behaviors. Because the dl BST is densely populated with corticotropin-releasing hormone (CRH) neurons, we examined sex differences in these parameters for the dl BST CRH neurons in male and female mice of a mouse line for which the CRH gene promoter (corticotropin-releasing factor [CRF]-Venus ΔNeo) controls the expression of the modified yellow fluorescent protein (Venus). Approximately 92% of Venus-positive cells in the dl BST were also CRH mRNA-positive, irrespective of sex. Therefore, the cells identified using Venus fluorescence were regarded as CRH neurons. A female-biased sex difference was observed in pain-induced behaviors during the interphase (5-15 min after formalin injection) but not during the later phase (phase 2, 15-60 min) in wild-type mice. In CRF-Venus ΔNeo mice, a female-biased difference was observed in either the earlier phase (phase 1, 0-5 min) or the interphase, but not in phase 2. Patch-clamp recordings taken using an acute BST slice obtained from a CRF-Venus ΔNeo mouse after formalin injection showed miniature excitatory postsynaptic currents (mEPSCs) and miniature inhibitory postsynaptic currents (mIPSCs). Remarkably, the mEPSCs frequency was higher in the Venus-expressing cells of formalin-injected female mice than in vehicle-treated female mice. Male mice showed no increase in mEPSC frequency by formalin injection. Formalin injection had no effect on mEPSC or mIPSC amplitudes in either sex. Pain-induced changes in mEPSC frequency in putative CRH neurons were phase-dependent. Results show that excitatory synaptic inputs to BST CRH neurons are temporally enhanced along with behavioral sex differences in pain response, suggesting that pain signals alter the BST CRH neurons excitability in a sex-dependent manner.

Age and Sex Differences in Acute and Osteoarthritis-Like Pain Responses in Rats

In this study, we investigated age and sex differences in acute and chronic pain in rats. Groups of young (3-6 months) and aged (20-24 months) male and female Fischer 344 rats were used to assess basal thermal and mechanical thresholds, capsaicin-induced acute nocifensive responses and c-Fos expression in the spinal cord, and monoiodoacetate (MIA)-induced knee osteoarthritis (OA)-like pain responses. There was a significant sex, but not age, effect on thermal threshold on the hindpaw and mechanical threshold on the knee joint. No significant age and sex differences in capsaicin-induced nocifensive and c-Fos responses were observed. MIA induced a greater peak reduction of weight-bearing responses in aged males than young rats. Aged females developed the most profound weight-bearing deficit. With knee joint sensitivity as a primary outcome measure, MIA induced more pronounced and longer-lasting hyperalgesia in older rats, with aged female rats showing the worst effect. These data suggest that age may not have significant effect on acute nociceptive processing, but it significantly impacts OA-like pain, making aged rats, especially females, more vulnerable to chronic pain conditions. These preclinical models should provide important tools to investigate basic mechanisms underlying the impact of age and sex in chronic pain conditions.

Effects of Aging on Formalin-Induced Pain Behavior and Analgesic Activity of Gabapentin in C57BL/6 Mice

Improved living conditions have induced an increase of lifespan often accompanied by comorbidities, responsible for pain, and by cognitive impairment and dementia, impairing communication capabilities. In most cases, the elderly do not receive pain relief because of underdiagnosis and of aging-induced changes of systems affecting nociceptive response. Unrelieved pain is involved in the development of behavioral symptoms, as agitation, representing a difficult challenge in this fragile population. Aged C57BL/6 mice and amyloid precursor protein (APP) mice display behavioral disturbances that mimic behavioral and psychological symptoms of dementia (BPSD). Therefore, this original study focuses on the influence of aging on nociception to provide insight into the occurrence of BPSD. We have investigated how aging can affect nociception after formalin administration and gabapentin effect in C57BL/6 mice, since it represents one of the treatments of choice for chronic neuropathic pain. Based on our results, changes of nociceptive behavior in response to an algogen stimulus occur during aging. Formalin-induced behavioral pattern in older C57BL/6 mice presents a temporal shift and an increase in the peak amplitudes. Our data show that the effectiveness of gabapentin is influenced by the age of the animal; though preliminary, the latter provide evidence upon which formalin test induced long-lasting mechanical allodynia might be a reliable as rapid and viable persistent pain model. The disclosed differences in effectiveness of gabapentin according to age can form the rational basis to deepen the study of pain treatment in the elderly.

Differential aging-related changes in neurophysiology and gene expression in IB4-positive and IB4-negative nociceptive neurons

Despite pain prevalence altering with age, the effects of aging on the properties of nociceptors are not well understood. Nociceptors, whose somas are located in dorsal root ganglia, are frequently divided into two groups based on their ability to bind isolectin B4 (IB4). Here, using cultured neurons from 1‐, 3‐, 5‐, 8‐, 12‐, and 18‐month‐old mice, we investigate age‐dependent changes in IB4‐positive and IB4‐negative neurons. Current‐clamp experiments at physiological temperature revealed nonlinear changes in firing frequency of IB4‐positive, but not IB4‐negative neurons, with a peak at 8 months. This was likely due to the presence of proexcitatory conductances activated at depolarized membrane potentials and significantly higher input resistances found in IB4‐positive neurons from 8‐month‐old mice. Repetitive firing in nociceptors is driven primarily by the TTX‐resistant sodium current, and indeed, IB4‐positive neurons from 8‐month‐old mice were found to receive larger contributions from the TTX‐resistant window current around the resting membrane potential. To further address the mechanisms behind these differences, we performed RNA‐seq experiments on IB4‐positive and IB4‐negative neurons from 1‐, 8‐, and 18‐month‐old mice. We found a larger number of genes significantly affected by age within the IB4‐positive than IB4‐negative neurons from 8‐month‐old mice, including known determinants of nociceptor excitability. The above pronounced age‐dependent changes at the cellular and molecular levels in IB4‐positive neurons point to potential mechanisms behind the reported increase in pain sensitivity in middle‐aged rodents and humans, and highlight the possibility of targeting a particular group of neurons in the development of age‐tailored pain treatments.

Suppressive effects of morphine injected into the ventral bed nucleus of the stria terminalis on the affective, but not sensory, component of pain in rats

Pain is a complex experience with both sensory and affective components. Clinical and preclinical studies have shown that the affective component of pain can be reduced by doses of morphine lower than those necessary to reduce the sensory component. Although the neural mechanisms underlying the effects of morphine on the sensory component of pain have been investigated extensively, those influencing the affective component remain to be elucidated. The bed nucleus of the stria terminalis (BNST) has been implicated in the regulation of various negative emotional states, including aversion, anxiety and fear. Thus, this study aimed to clarify the role of the ventral part of the BNST (vBNST) in the actions of morphine on the affective and sensory components of pain. First, the effects of intra-vBNST injections of morphine on intraplantar formalin-induced conditioned place aversion (CPA) and nociceptive behaviors were investigated. Intra-vBNST injections of morphine reduced CPA without affecting nociceptive behaviors, which suggests that intra-vBNST morphine alters the affective, but not sensory, component of pain. Next, to examine the effects of morphine on neuronal excitability in type II vBNST neurons, whole-cell patch-clamp recordings were performed in brain slices. Bath application of morphine hyperpolarized type II vBNST neurons. Thus, the suppressive effects of intra-vBNST morphine on pain-induced aversion may be due to its inhibitory effects on neuronal excitability in type II vBNST neurons. These results suggest that the vBNST is a key brain region involved in the suppressive effects of morphine on the affective component of pain.

Central amygdala activation of extracellular signal-regulated kinase 1 and age-dependent changes in inflammatory pain sensitivity in mice

Aging populations are more sensitive to noxious stimuli as a result of altered somatosensory systems. In these experiments, we examined pain-like behaviors in young, middle-aged, and old mice during peripheral inflammation to determine if the same sensitivity exists in preclinical animal models. Immediately following injury, middle-aged and old mice exhibited more spontaneous pain-like behaviors than young mice, matching pain prevalence in clinical populations. Middle-aged and old mice also developed persistent mechanical hypersensitivity in the injured paw. Furthermore, old mice developed mechanical hypersensitivity in the noninjured paw suggesting age-dependent changes in central nociceptive systems. To address this end, pain-related protein expression was examined in the central nucleus of the amygdala, a limbic brain region that modulates somatic pain. Following injury, increased phosphorylation of extracellular signal-regulated kinase 1, a protein with known nociceptive functions, was observed in the right central nucleus of the amygdala of old mice and not middle-aged or young animals. These findings suggest that age-dependent changes in supraspinal nociceptive systems may account for increased pain-like behaviors in aging populations.

Age-Dependent Changes in the Inflammatory Nociceptive Behavior of Mice

The processing of pain undergoes several changes in aging that affect sensory nociceptive fibers and the endogenous neuronal inhibitory systems. So far, it is not completely clear whether age-induced modifications are associated with an increase or decrease in pain perception. In this study, we assessed the impact of age on inflammatory nociception in mice and the role of the hormonal inhibitory systems in this context. We investigated the nociceptive behavior of 12-month-old versus 6–8-week-old mice in two behavioral models of inflammatory nociception. Levels of TRP channels, and cortisol as well as cortisol targets, were measured by qPCR, ELISA, and Western blot in the differently aged mice. We observed an age-related reduction in nociceptive behavior during inflammation as well as a higher level of cortisol in the spinal cord of aged mice compared to young mice, while TRP channels were not reduced. Among potential cortisol targets, the NF-κB inhibitor protein alpha (IκBα) was increased, which might contribute to inhibition of NF-κB and a decreased expression and activity of the inducible nitric oxide synthase (iNOS). In conclusion, our results reveal a reduced nociceptive response in aged mice, which might be at least partially mediated by an augmented inflammation-induced increase in the hormonal inhibitory system involving cortisol.

Development of monosodium acetate-induced osteoarthritis and inflammatory pain in ageing mice

Most conditions associated with ageing result from an age-related loss in the function of cells and tissues that maintain body homeostasis. In osteoarthritis (OA) patients, an inadequate response to stress or joint injury can lead to tissue destruction which can result in chronic pain. Here, we evaluated the development of monoiodoacetate (MIA)-induced OA in 3-, 15- and 22-month-old mice and assessed the pain-like behaviours and the spinal microglial changes associated with MIA administration. We observed that in aged mice, nocifensive behaviour was significantly attenuated in comparison to young adults despite similar knee joint pathology. Specifically referred mechanical allodynia associated with the MIA initial inflammatory phase (0-10 days) was significantly attenuated in 22-month-old mice. In contrast, the late phase of MIA-induced mechanical allodynia was comparable between age groups. Significant increase of microglia cell numbers was detected in 3, but not 15- and 22-month-old spinal cords. Furthermore, in the zymosan model of acute inflammation, mechanical allodynia was attenuated, and microglial response was less robust in 22 compared to 3-month-old mice. This study suggests that nocifensive responses to damaging stimuli are altered with advancing age and microglial response to peripheral damage is less robust.

Altered nociceptive, endocrine, and dorsal horn neuron responses in rats following a neonatal immune challenge

The neonatal period is characterized by significant plasticity where the immune, endocrine, and nociceptive systems undergo fine-tuning and maturation. Painful experiences during this period can result in long-term alterations in the neurocircuitry underlying nociception, including increased sensitivity to mechanical or thermal stimuli. Less is known about the impact of neonatal exposure to mild inflammatory stimuli, such as lipopolysaccharide (LPS), on subsequent inflammatory pain responses. Here we examine the impact of neonatal LPS exposure on inflammatory pain sensitivity and HPA axis activity during the first three postnatal weeks. Wistar rats were injected with LPS (0.05mg/kg IP, Salmonella enteritidis) or saline on postnatal days (PNDs) 3 and 5 and later subjected to the formalin test at PNDs 7, 13, and 22. One hour after formalin injection, blood was collected to assess corticosterone responses. Transverse spinal cord slices were also prepared for whole-cell patch clamp recording from lumbar superficial dorsal horn neurons (SDH). Brains were obtained at PND 22 and the hypothalamus was isolated to measure glucocorticoid (GR) and mineralocorticoid receptor (MR) transcript expression using qRT-PCR. Behavioural analyses indicate that at PND 7, no significant differences were observed between saline- or LPS-challenged rats. At PND 13, LPS-challenged rats exhibited enhanced licking (p<.01), and at PND 22, increased flinching in response to formalin injection (p<.05). LPS-challenged rats also displayed increased plasma corticosterone at PND 7 and PND 22 (p<.001) but not at PND 13 following formalin administration. Furthermore, at PND 22 neonatal LPS exposure induced decreased levels of GR mRNA and increased levels of MR mRNA in the hypothalamus. The intrinsic properties of SDH neurons were similar at PND 7 and PND 13. However, at PND 22, ipsilateral SDH neurons in LPS-challenged rats had a lower input resistance compared to their saline-challenged counterparts (p<.05). These data suggest neonatal LPS exposure produces developmentally regulated changes in formalin-induced behavioural responses, corticosterone levels, and dorsal horn neuron properties following noxious stimulation later in life. These findings highlight the importance of immune activation during the neonatal period in shaping pain sensitivity later in life. This programming involves both spinal cord neurons and the HPA axis.

The effects of age on pain sensitivity: preclinical studies

OBJECTIVE

Preclinical studies of pain and aging represent an area of research where considerations of age, strain, gender, and method of behavioral assessment are but some of the challenges that must be addressed. The results of studies related to the impact of age on pain sensitivity have ranged from increased to decreased sensitivity to no change. Examining the design of these studies one discovers that cross-sectional designs using animals of different ages have been used to evaluate age-related effects in normal animals as well as animals with inflammatory and neuropathic pain conditions. In the present review a summary of these studies is presented along with a discussion of potential mechanisms responsible for changes that have been described.

OUTCOME MEASURES

The dominant method of behavioral assessment in the majority of studies involving rodents has been reflex-based strategies that unfortunately do not reveal the same effects of experimental manipulations known to affect pain sensitivity in humans. A comparison of results obtained with reflex-based methods versus those obtained with cortically dependent operant methods reveals significant differences.

CONCLUSIONS

Increases in pain sensitivity under different experimental conditions have been suggested to result from age-related anatomical, physiological, and biochemical changes as well as compensatory changes in homeostatic mechanisms and intrinsic plasticity of somatosensory pathways involved in the processing and perception of pain. Other factors that may contribute to the impact of age on pain sensitivity include dysregulation of the hypothalamic-pituitary-adrenal axis and changes in autonomic function that occur with advancing age. In the future translational research in the field of pain and aging will need to focus on establishing clinically relevant animal models and assessment strategies to evaluate the causal relationships between the biological changes associated with advancing age and the varied behavioral changes in pain sensitivity.

Low formalin concentrations induce fine-tuned responses that are sex and age-dependent: a developmental study

The formalin test is increasingly applied as a model of inflammatory pain using high formalin concentrations (5–15%). However, little is known about the effects of low formalin concentrations on related behavioural responses. To examine this, rat pups were subjected to various concentrations of formalin at four developmental stages: 7, 13, 22, and 82 days of age. At postnatal day (PND) 7, sex differences in flinching but not licking responses were observed with 0.5% formalin evoking higher flinching in males than in females. A dose response was evident in that 0.5% formalin also produced higher licking responses compared to 0.3% or 0.4% formalin. At PND 13, a concentration of 0.8% formalin evoked a biphasic response. At PND 22, a concentration of 1.1% evoked higher flinching and licking responses during the late phase (10–30 min) in both males and females. During the early phase (0–5 min), 1.1% evoked higher licking responses compared to 0.9% or 1% formalin. 1.1% formalin produced a biphasic response that was not evident with 0.9 or 1%. At PND 82, rats displayed a biphasic pattern in response to three formalin concentrations (1.25%, 1.75% and 2.25%) with the presence of an interphase for both 1.75% and 2.25% but not for 1.25%. These data suggest that low formalin concentrations induce fine-tuned responses that are not apparent with the high formalin concentration commonly used in the formalin test. These data also show that the developing nociceptive system is very sensitive to subtle changes in formalin concentrations.

Pain management in older people

At times providing pain relief in elderly patients can prove troublesome. Their tolerance and perception of pain can differ from that of younger patients, while the incidence of pain is above that found in those of less advanced years.

Conventional approaches to providing pain relief can be successful, but the tolerance to the side-effects of those drugs used to provide pain relief can be less. Furthermore, polypharmacy can have implications for the range of analgesic drugs that can be considered. Fortunately there are an increasing range of medicinal products with reduced potential for side-effects that can be considered when treating older patients with pain.

The ageing gut: diminished symptom response to a standardized nutrient stimulus

BACKGROUND The prevalence of dyspepsia and the severity of reflux symptoms decreases with advancing age. We postulate that advancing age influences sensory function and this will be associated with a diminished symptom response to a standardized meal stimulus.Our aim was to assess the influence of age on visceral sensory function. METHODS Baseline gastrointestinal symptoms and anxiety and depression were assessed in 53 healthy volunteers using validated questionnaires. After an 8-h fast, subjects received 200 mL of a standardized enteral feeding solution every 5 min up to a cumulative volume of 800 mL. After each 200 mL drink, five key symptoms were assessed (fullness, abdominal pain, retrosternal/abdominal burning, nausea and regurgitation) using a standardized instrument on visual analogue scales (0-100). The cumulative symptom score across all symptoms was calculated. KEY RESULTS Fullness was the most prominent symptom reported (79.8 +/- 9.5) followed by nausea (14.9 +/- 4.9) and pain (9.8 +/- 4.5); these three items accounted for more than 90% of the overall symptom load. The cumulative pain and nausea scores during a standardized nutrient challenge were significantly and inversely correlated with age (r = -0.43, P = 0.002 and r = -0.28, P = 0.045). Subjects >60 years of age reported significantly lower pain and nausea scores (0.9 +/- 0.9, 4.5 +/- 3.9) than did subjects <40 years (22 +/- 11.9, P = 0.002; 29.3 +/- 12, P = 0.043). CONCLUSIONS & INFERENCES Symptom responses to a standardized nutrient challenge, in particular pain and nausea, are inversely correlated with age.

Formalin-induced nociceptive behavior and c-Fos expression in middle-aged female rats

The impact of the estrous cycle on the nociceptive response in middle-aged female rats was assessed using the formalin test and c-Fos immunoreactivity as a marker of neural activation. Young (2-month-old) and middle-aged (11-month-old) rats were examined, dividing the middle-aged rats into two groups based on their estrous cycle: regular 4-day estrous cycle and irregular estrous cycle. The right hind paw was subcutaneously injected with 50microl of 2% formalin or saline as a control. Behavioral changes were observed for 1h. Cycling rats were used during proestrus. Middle-aged female rats had a significantly higher score for nociceptive behavior compared to young rats, irrespective of estrous cyclicity, which suggests that aging, not the ability to maintain estrous cyclicity, causes hypersensitivity to the formalin injection. Immunohistochemical analysis found that the brain response to formalin injection was also more sensitive in middle-aged rats than young rats; a significant increase in the number of c-Fos immunoreactive cells was found in the ventral portion of the lateral septum of middle-aged rats injected with formalin compared to young and middle-aged rats injected with saline, irrespective of estrous cyclicity. Based on these results, we conclude that the sensitivity to painful stimuli in middle-aged female rats, which are in a neuroendocrine state similar to pre- and peri-menopausal women, is associated with age and not affected by reproductive ability.

Pain perception in the elderly patient

In the elderly patient, the barriers to effective treatment of pain are substantial. Even the perception of pain may differ from that in those of less advanced years. Of course, many other factors impinge on the presence of, and treatment of, pain in elderly patients. Issues of physical accessibility to treatment, cost of drugs, the presence of coexisting illness, the use of concomitant medication, and even the ability to understand the complaints of the patient who has cognitive impairment are only some of those factors that contribute to the complexity of the situation.

Effects of aging on current vocalization threshold in mice measured by a novel nociception assay

BACKGROUND

Age-related changes in nociception have been extensively studied in the past decades. However, it remains unclear whether in addition to the increased incidence of chronic illness, age-related changes in nociception contribute to increased prevalence of pain in the elderly. Although a great deal of evidence suggests that nociception thresholds increase with aging, other studies yield disparate results. The aim of this investigation was to longitudinally determine the effect of aging on nociception.

METHODS

The authors developed a nociception assay for mice using electrical stimuli at 2,000, 250, and 5 Hz that reportedly stimulate Abeta, Adelta, and C sensory nerve fibers, respectively. A system was designed to automate a method that elicits and detects pain-avoiding behavior in mice. Using a Latin square design, the authors measured current vocalization thresholds serially over the course of mice's life span.

RESULTS

For 2,000-Hz (Abeta), 250-Hz (Adelta), and 5-Hz (C) electrical stimuli, current vocalization thresholds first decreases and then increases with aging following a U-shaped pattern (P < 0.001). In addition, average current vocalization thresholds at youth and senescence are significantly higher than those at middle age for the 250-Hz (Adelta) and 5-Hz (C fiber) electrical stimulus (P < 0.05).

CONCLUSIONS

Using a novel and noninjurious nociception assay, the authors showed that over the life span of mice, current vocalization threshold to electrical stimuli changes in a U-shaped pattern. The findings support the notion that age-related changes in nociception are curvilinear, and to properly study and treat pain, the age of subjects should be considered.

Age-related impact of neuropathic pain on animal behaviour

The number of old and very old persons is increasing and there is evidence that aging coincides with chronic painful conditions. Pain induces behavioural disorders that have been so far poorly identified in old and even less in very old animals. The aim of this study was to: (1) compare the evolution of pain in senescent animals (37-39 months) to old (20-22 months) and young (4-6 months) Lou/cjall rats after a chronic constriction of the sciatic nerve; (2) evaluate pain during four weeks after surgery with an experimental and an observational approach to determine how the response to noxious stimuli correlates with recorded spontaneous behaviour. Results showed that senescent animals are less sensitive to neuropathic pain than old or young rats while senescent/old rats are more sensitive to acute pain. The correlation between observational and experimental pain scores stresses the reliability of non-invasive measures for pain evaluation in senescent populations. The dichotomy between neuropathic and acute pain perceptions with age needs to be further investigated and would help to better understand the reasons of this uneven pain perception and expression with age.

Sex differences in the responses of spinal wide-dynamic range neurons to subcutaneous formalin and in the effects of different frequencies of conditioning electrical stimulation

The purpose of this study was to investigate sex-related differences in nociception elicited by s.c. injection of different concentrations (1-5%) of formalin. S.c. formalin-induced biphasic (early and late phases) persistent nociception was assessed by extracellularly recording the spontaneous activities of single spinal dorsal horn wide-dynamic range neurons in anesthetized male and female rats. The nociceptive responses of the dorsal horn wide-dynamic range neurons following s.c. injection of 5%, but not 1% and 2.5%, formalin in female rats were significantly stronger than the responses obtained in male rats. However, these concentration-dependent differences with respect to different sexes existed only in the late, but not the early, phase of formalin-induced nociception in intact, not spinal rats. The 5% formalin-induced late phase nociception in male rats was significantly depressed by 15 min of repeated conditioning electrical stimulation at a frequency of 5 Hz as well as 50 Hz during and after the period of conditioning electrical stimulation (intensity: 1 mA; pulse duration: 1 ms). In contrast, the inhibitory effect of 50 Hz conditioning electrical stimulation on the 5% formalin-elicited late phase response in female rats was markedly greater in magnitude and longer in duration than that of 5 Hz conditioning electrical stimulation. No significant depressive effects of 5 Hz conditioning electrical stimulation on formalin-induced nociception were found in female rats, indicating that the distinct effects of conditioning electrical stimulation at different frequencies are different in animals of opposite sexes. In conclusion, s.c. administration of different concentrations of formalin shows a distinct sex-related difference in its late tonic nociception of spinal nociceptive sensory neurons. Sex differences in formalin-induced tonic nociception are stimulus intensity dependent and related to the modulation from the supraspinal regions. S.c. formalin-induced late phase nociception in female rats is only sensitive to depression at a frequency of 50 Hz, but not 5 Hz, of conditioning electrical stimulation. This suggests that the involvement of the central mechanisms in the antinociceptive effects of conditioning electrical stimulation may be different at various frequencies of stimulation.

Involvement of Dorsal Column Nucleus Neurons in Nociceptive Transmission in Aged Rats

To clarify the functional role of the dorsal column nucleus (DCN) in nociception in rats with advancing age, single neuronal activity and substance P–like immunoreactivity (SP-LI) of the gracile nucleus (GN) were studied in aged rats (29 to 34 mo old) and adult rats (9 to 12 mo old). A total of 122 neurons [aged: 34 wide-dynamic-range (WDR), two nociceptive-specific (NS), and 32 low-threshold mechanical (LTM) neurons; adult: 22 WDR and 32 LTM neurons] were recorded from GN. For WDR neurons, the latency to antidromic activation of the ventral posterior lateral nucleus of the thalamus showed no difference between the aged and adult rats. Sciatic nerve stimulation with C-fiber intensity induced responses of GN with significantly longer latency in aged rats than in adults, whereas there was no difference in the response latency to A-fiber intensity stimulation. Background activity and afterdischarges were significantly higher in the aged rats than those in the adult rats. Responses to noxious mechanical and thermal stimuli were significantly greater in the aged rats during application of graded stimuli. There were no significant differences in responses to nonnoxious mechanical stimulus, mechanical response threshold, and the size of the receptive fields between neurons in the aged and adult rats. The area occupied by SP-LI fibers in the GN and the size of SP-LI dorsal root ganglia neurons were significantly larger in aged rats than in adults. The present findings suggest that the hyperexcitability of GN neurons could be involved in abnormal noxious pain sensations with advancing age.

Reduced serotonin synthesis during early embryogeny changes effect of subsequent prenatal stress on persistent pain in the formalin test in adult male and female rats

The considerable evidence supporting a role for serotonin (5-HT) in the embryonic formation of CNS, mediation of prenatal stress, and pain processing is reviewed. Long-term influences of prenatal 5-HT depletion as well as its combination with prenatal stress effects on tonic nociceptive system in 90-day-old Wistar rats were studied in the formalin test. Pregnant dams were injected with para-chlorophenylalanine (pCPA, 400 mg/kg/2 ml, ip), producing 5-HT depletion during the early period of fetal serotonergic system development. The adult offspring from pCPA-treated dams revealed changes in behavioral indices of persistent pain (flexing + shaking and licking) in the formalin test (2.5%, 50 microl) that were accompanied by irreversible morphological alterations in the dorsal raphe nuclei. In the other series of experiments, the role of 5-HT in the mediation of prenatal stress on the behavioral indices of persistent pain was investigated in the adult offspring from dams with 5-HT depletion followed by restraint stress. Stress during the last embryonic week caused much more increase in flexing + shaking and licking in the second tonic phase of the response to formalin in offspring from pCPA- than saline-treated (control) dams. The former was characterized by alterations in the durations of the interphase, the second phase, and the whole behavioral response too. In offspring from pCPA-treated dams, sex dimorphism was revealed in tonic pain evaluated by licking. Together with our previous results in juvenile rats demonstrating the necessity of definite level of prenatal 5-HT for normal development of tonic nociceptive system, the present pioneering findings obtained in adult rats indicate that prenatal 5-HT depletion causes long-term morphological abnormalities in the dorsal raphe nuclei accompanied by alterations in behavioral indices of tonic pain. Early prenatal 5-HT depletion increases vulnerability of tonic nociceptive circuits to the following prenatal stress.

Analgesic effects of dietary caloric restriction in adult mice

Nociception was studied in male mice, mostly of the C57BL/6 strain, during continuous or prolonged restriction of caloric intake (60% of ad-libitum) from midlife to senescence (up to 105 weeks). Restricted mice showed fewer licking or biting responses 20-60 min after hind paw injection of 5% formalin at 46 and 70 weeks, but not at 93 weeks. Also, they showed longer response latencies around 46 weeks of age in the 52 degrees C hot-plate test, which partial tail amputation failed to affect, although it did produce at least 2 weeks of chronic neuropathic hypersensitivity in ad libitum controls. Injection of collagen subcutaneously at 36-42 weeks led to chronic hyperalgesia in the DBA/1 but not the C57BL/6 strain, measured weekly by the barely nociceptive 50 degrees C hot-plate test to minimize damage. This collagen-induced arthritic hyperalgesia was then gradually and reversibly blocked during 9-15 weeks of caloric restriction starting at 53-58 weeks. In longitudinal trials on normal mice, performed every 2-4 weeks between 42 and 105 weeks with the 50 degrees C hot-plate, caloric restriction led to altered latencies (higher relative to controls) only in the last 10-20 weeks, perhaps because it delayed the onset of age-related peripheral neuropathies. In conclusion, long-term caloric restriction leads to significant hypoalgesia in pre-senescent mice subjected to above-threshold pain of widely different durations, the effect disappearing at later ages unless spontaneous neuropathies become influential. A reduction in cumulative food intake thus appears to generate antinociceptive signals in adult male mice, perhaps serving specifically to promote riskier behavior during prolonged food shortages.

Underlying mechanisms of impaired visceral pain perception in older people

There is increasing recognition that aging can have a profound effect on the presentation of illness. Older patients with diseases of visceral organs are much more likely than younger adults to present atypically. Examples are the frequent absence of pain in older patients with conditions such as myocardial infarction, peptic ulcer disease, and pneumothorax. Recent developments have helped elucidate the complex processes involved in signaling information from the effects of noxious stimuli on visceral organs, but understanding of why older patients with visceral disease are more likely to present without pain is still rudimentary. Much of the previous experimental evidence on age-related differences relates to somatic rather than visceral sources of pain. As a result, it may not have direct comparability with transmission of information on visceral damage or noxious stimulation. This article reviews the published pathophysiological data on sensory transmission from visceral organs. Where possible, this is correlated with other published clinical studies on age-related differences in visceral pain perception. Areas in which experimental evidence is absent are also highlighted. Finally suggestions are made as to how newer experimental and neuroimaging techniques may help to increase understanding of this complex subject and its resulting clinical applicability.

Effects of aging on hyperalgesia and spinal dynorphin expression in rats with peripheral inflammation

The aging process is associated with various morphological and biochemical changes in the nervous system that may affect the processing of noxious inputs. This study showed greater hyperalgesia and up-regulation of spinal dynorphin (DYN) expression in aging than in young adult rats during CFA-induced peripheral inflammation. These data indicate that nociception is regulated differently in aging individuals, a fact that should be considered when selecting treatment strategies for aging populations with persistent pain.

Gonadectomy induces site-specific differences in nociception in rats

High prevalence of chronic orofacial pain in women and its relationship with ovarian states suggest that ovarian hormones may be involved in the control of orofacial nociception. Since the interaction between ovarian hormones and nociception seems more evident in the orofacial area than in many other parts of the body, a possible site specificity of an ovarian hormone effect on nociception was tested in rats. Two nociceptive tests were applied to three groups of male rats (n=46) and three groups of female rats (n=46), that were gonadectomised (n=17), sham-operated (n=15) or intact (n=14). Each rat in each group received a local subcutaneous injection of formalin in the upper lip and in the hindpaw. Upper lip injection resulted in an increased occurrence of upper lip rubbing for more than 45 min and hindpaw injection resulted in an increased occurrence of hindpaw licking for about 1h. The duration of the nociceptive behaviours was measured at 3 months after surgery. No significant difference was found between intact and sham-operated animals. A significant increase (54%) in the upper lip rubbing but not the hindpaw licking was observed in gonadectomised females. No difference was observed in castrated males for upper lip rubbing, but a tendency towards an increased duration (102%) of hindpaw licking was noted. The depletion in gonadal hormones was confirmed 3 months after gonadectomy and after the sacrifice of the animals, by the observed decline in the bone mineral density measured on the femur of 40 rats belonging to the six groups. A role of ovarian hormones was also suggested after immunostaining of oestrogen receptors in the lamina II of Caudalis subnucleus of the trigeminal sensory complex and cervical (C1-C2) spinal dorsal horn. The number of cells expressing oestrogen receptors displayed a small (13.6%) but significant (P=0.037) increase in ovariectomised compared with sham-operated rats. These results suggest that the lack of ovarian hormones induces a site-specific increase in the sensitivity to orofacial nociceptive stimulation, and that an up-regulation of oestrogen receptors in the Caudalis subnucleus and C1-C2 dorsal horn may be one of the factors involved in this effect.

The effects of aging on thermal hyperalgesia and tactile-evoked allodynia using two models of peripheral mononeuropathy in the rat

The effects of aging on the behavioral manifestations of neuropathic and inflammatory pain were investigated using two models of peripheral nerve injury. The left sciatic nerve of young and aged Fischer 344 FBNF1 hybrid rats (4-6 and 24-26 months old, respectively) was ligated using either the chronic constriction injury (CCI) model of Bennett and Xie or the partial sciatic nerve ligation (PSNL) model of Seltzer et al. A plantar analgesic meter was used to assess age-related differences in CCI- or PSNL-induced thermal hyperalgesia, and nerve injury-induced tactile-evoked allodynia was assessed with von Frey filaments. Aged animals subjected to the PSNL procedure developed a more vigorous and longer lasting thermal hyperalgesic response than did aged rats post-CCI. The CCI model incorporates a more prominent peripheral inflammatory component than the PSNL model. These data support the notion that the peripheral inflammatory response is diminished in aged rats.

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.

Age-related alterations in responses of nucleus basalis magnocellularis neurons to peripheral nociceptive stimuli

The present study investigated the effects of peripheral noxious stimuli on the spontaneous activity of the nucleus basalis magnocellularis (nbM) neurons in young, adult and old rats. Single unit extracellular recordings from the nbM neurons were obtained with glass micropipettes in urethane-anesthetized rats. A total of 104 units were antidromically identified as nbM-cortical neurons. Noxious but not innocuous mechanical stimulation elicited responses in 72% of the 104 neurons. Most of them were excited. The receptive fields were usually very large and bilateral. Most of the neurons also responded to noxious thermal, chemical and electrical stimuli. No marked differences were observed in the incidence of neurons having different spontaneous firing rates, firing patterns and response type among the three age groups. However, the latency of responses evoked by noxious thermal or electrical stimulation and the threshold of excitatory responses evoked by electrical stimulation were increased with aging. The duration and peak-firing rate of excitatory responses evoked by noxious thermal, chemical or electrical stimulation were decreased in old rats. These findings indicate that there might be some functional changes in the nbM neurons and its projection pathway with aging, which impair their responsive ability to peripheral nociceptive stimuli.

Endogenous opiates: 1999

This paper is the twenty-second installment of the annual review of research concerning the opiate system. It summarizes papers published during 1999 that studied the behavioral effects of the opiate peptides and antagonists, excluding the purely analgesic effects, although stress-induced analgesia is included. The specific topics covered this year include stress; tolerance and dependence; learning, memory, and reward; eating and drinking; alcohol and other drugs of abuse; sexual activity, pregnancy, and development; mental illness and mood; seizures and other neurologic disorders; electrical-related activity; general activity and locomotion; gastrointestinal, renal, and hepatic function; cardiovascular responses; respiration and thermoregulation; and immunologic responses.

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.