Effect of analgesics on audible and ultrasonic pain-induced vocalization in the rat

Chemogenetic Manipulation of Amygdala Kappa Opioid Receptor Neurons Modulates Amygdala Neuronal Activity and Neuropathic Pain Behaviors

Neuroplasticity in the central nucleus of the amygdala (CeA) plays a key role in the modulation of pain and its aversive component. The dynorphin/kappa opioid receptor (KOR) system in the amygdala is critical for averse-affective behaviors in pain conditions, but its mechanisms are not well understood. Here, we used chemogenetic manipulations of amygdala KOR-expressing neurons to analyze the behavioral consequences in a chronic neuropathic pain model. For the chemogenetic inhibition or activation of KOR neurons in the CeA, a Cre-inducible viral vector encoding Gi-DREADD (hM4Di) or Gq-DREADD (hM3Dq) was injected stereotaxically into the right CeA of transgenic KOR-Cre mice. The chemogenetic inhibition of KOR neurons expressing hM4Di with a selective DREADD actuator (deschloroclozapine, DCZ) in sham control mice significantly decreased inhibitory transmission, resulting in a shift of inhibition/excitation balance to promote excitation and induced pain behaviors. The chemogenetic activation of KOR neurons expressing hM3Dq with DCZ in neuropathic mice significantly increased inhibitory transmission, decreased excitability, and decreased neuropathic pain behaviors. These data suggest that amygdala KOR neurons modulate pain behaviors by exerting an inhibitory tone on downstream CeA neurons. Therefore, activation of these interneurons or blockade of inhibitory KOR signaling in these neurons could restore control of amygdala output and mitigate pain.

Pharmacokinetics and Efficacy of a Long-lasting, Highly Concentrated Buprenorphine Solution in Rats

Buprenorphine (Bup) is an opioid analgesic that is commonly used in laboratory rodents to provide postoperative analgesia. However, dosing every 4 to 6 h is necessary to maintain an analgesic plasma concentration of the drug. A long lasting, highly concentrated veterinary formulation of Bup (LHC-Bup) has been used to provide prolonged analgesia in cats and nonhuman primates. In the current study, we evaluated the duration of efficacy of LHC-Bup to determine if this formulation would provide a similarly prolonged analgesia in rats. Drug concentrations were measured after subcutaneous injection of 0.5 mg/kg LHC-Bup in both male and female rats. Plasma levels were measured at 0.25, 0.5, 1, 2, 4, 8, 12, 24, 36, 48, and 72 h. Male and female rats had peak plasma levels of LHC-Bup at 90 ng/mL and 34 ng/mL, respectively, at 15 min after administration, with a steady decrease by 24 h to 0.7 ng/mL in males and 1.3 ng/mL in females. Mechanical pain tolerance was evaluated after LHC-Bup administration using a Randall-Selitto analgesiometer to assess paw withdrawal. Male rats had a significantly longer paw withdrawal time for up to 12 h after administration, and females had longer paw withdrawal times for up to 24 h. An experimental laparotomy model was then used to assess the clinical efficacy of LHC-Bup at 0.5 mg/kg. LHC-Bup treatment was associated with a greater total distance traveled, reduced time to retrieve a food treat, and reduced grooming from 3 to 12 h after surgery as compared with saline controls. Groups receiving LHC-Bup showed coprophagy whereas other rats did not. These results suggest that administering LHC-Bup at 0.5 mg/kg provides therapeutic plasma concentrations for 12 to 24 h after administration and analgesic efficacy for at least 12 h after dosing. As such, LHC-Bup is a suitable alternative to Bup-HCl.

Biological Functions of Rat Ultrasonic Vocalizations, Arousal Mechanisms, and Call Initiation

This review summarizes all reported and suspected functions of ultrasonic vocalizations in infant and adult rats. The review leads to the conclusion that all types of ultrasonic vocalizations subserving all functions are vocal expressions of emotional arousal initiated by the activity of the reticular core of the brainstem. The emotional arousal is dichotomic in nature and is initiated by two opposite-in-function ascending reticular systems that are separate from the cognitive reticular activating system. The mesolimbic cholinergic system initiates the aversive state of anxiety with concomitant emission of 22 kHz calls, while the mesolimbic dopaminergic system initiates the appetitive state of hedonia with concomitant emission of 50 kHz vocalizations. These two mutually exclusive arousal systems prepare the animal for two different behavioral outcomes. The transition from broadband infant isolation calls to the well-structured adult types of vocalizations is explained, and the social importance of adult rat vocal communication is emphasized. The association of 22 kHz and 50 kHz vocalizations with aversive and appetitive states, respectively, was utilized in numerous quantitatively measured preclinical models of physiological, psychological, neurological, neuropsychiatric, and neurodevelopmental investigations. The present review should help in understanding and the interpretation of these models in biomedical research.

Female Urine-induced Ultrasonic Vocalizations in Male C57BL/6J Mice as a Proxy Indicator for Postoperative Pain

Objectively recognizing postoperative pain in mice is challenging, making it difficult to determine an appropriate postoperative analgesic regimen. Adult male mice produce ultrasonic vocalizations after exposure to adult female urine (FiUSV). To determine if FiUSV can be used as a indicator of postoperative pain, FiUSV produced by male C57BL/6J mice were assessed for 5 d before and after vasectomy or sham surgery with or without sustained-release buprenorphine. Postoperative pain was assessed by monitoring vocalization using an ultrasonic microphone and by evaluating orbital tightness, posture, and piloerection at postoperative time points. Before vasectomy or sham surgery, 25 of 38 male mice produced FiUSV on 4 of 5 d (143 ± 93 FiUSV). Vasectomized mice without postoperative analgesia produced significantly fewer FiUSV (59 ± 26 FiUSV) compared with baseline (212 ± 102 FiUSV) at 4 h postoperatively, but returned to baseline by 28 h. Vasectomized mice treated with buprenorphine and sham-surgery mice had no change in FiUSV from baseline at any time point after surgery. Activity was decreased compared with baseline in vasectomized mice, regardless of receiving postoperative analgesia or not, but only at the 4-h time point. There were no differences in behavior scores between vasectomized mice and sham-surgery mice at any time point. These results show that FiUSV can be used to detect postoperative pain in male C57BL/6J mice after vasectomy.

Developing Improved Translational Models of Pain: A Role for the Behavioral Scientist

The effective management of pain is a longstanding public health concern. Although opioids have been frontline analgesics for decades, they also have well-known undesirable effects that limit their clinical utility, such as abuse liability and respiratory depression. The failure to develop better analgesics has, in some ways, contributed to the escalating opioid epidemic that has claimed tens of thousands of lives and has cost hundreds of billions of dollars in health-care expenses. A paradigm shift is needed in the pharmacotherapy of pain management that will require extensive efforts throughout biomedical science. The purpose of the present review is to highlight the critical role of the behavioral scientist to devise improved translational models of pain for drug development. Despite high heterogeneity of painful conditions that involve cortical-dependent pain processing, current models often feature an overreliance on simple reflex-based measures and an emphasis on the absence, rather than presence, of behavior as evidence of analgesic efficacy. Novel approaches should focus on the restoration of operant and other CNS-mediated behavior under painful conditions.

Characterization of rat ultrasonic vocalization in the orofacial formalin test: Influence of the social context

Rats emit ultrasonic vocalizations (USVs) about 22 kHz and 50 kHz sound frequency to communicate the presence of negative or positive emotional states, respectively. The calling behavior may be influenced by several factors, including environmental factors. Likewise, pain behavior can be modulated according to the social context, and also can be transferred to conspecifics through direct observation and/or social interaction. Herein we investigated if acute pain induction was related to changes in emission of aversive and appetitive calls and how different social contexts affected the nociceptive behavior and USVs. Our results demonstrated that orofacial formalin injection in rats induced aversive calls in addition to the nociceptive behavior, and both are reduced by systemic treatment with morphine (2.5 mg/kg). Exposure of formalin-injected rats to cagemates had no effect on the nociceptive behavior or calls emitted by the demonstrator, but the observer showed emotional contagion of pain. In contrast, exposure of formalin-injected rats to non-cagemates decreased the nociceptive behavior of the demonstrator, without affecting the calls emission. The emotional contagion was not detected in non-cagemates or in cagemates separated by a visual barrier. In conclusion, we suggest that familiarity and the visual contact contributes to emotional contagion of pain. USV analysis may represent an additional measure in the evaluation of the emotional aspect of orofacial pain, and for the study of pain modulation.

Female- and Intruder-induced Ultrasonic Vocalizations in C57BL/6J Mice as Proxy Indicators for Animal Wellbeing

Female urine-induced male mice ultrasonic vocalizations (FiUSV) are ultrasonic vocalizations produced by adult male mice after presentation of adult female urine, whereas intruder-induced ultrasonic vocalizations (IiUSV) are produced by resident adult female mice when interacting with an intruder female mouse. These affiliative behaviors may be reduced when mice have decreased wellbeing or are in pain and distress. To determine whether FiUSV and IiUSV can be used as proxy indicators of animal wellbeing, we assessed FiUSV produced by male C57BL/6J mice in response to female urine and IiUSV produced by female C57BL/6J mice in response to a female intruder at baseline and 1 and 3 h after administration of a sublethal dose of LPS (6 or 12.5 mg/kg IP) or an equal volume of saline. Behavior was assessed by evaluating orbital tightness, posture, and piloerection immediately after USV collection. We hypothesized that LPS-injected mice would have a decreased inclination to mate or to interact with same-sex conspecifics and therefore would produce fewer USV. At baseline, 32 of 33 male mice produced FiUSV (149 ± 127 USV in 2 min), whereas all 36 female mice produced IiUSV (370 ± 156 USV in 2 min). Saline-injected mice showed no change from baseline at the 1- and 3-h time points, whereas LPS-injected mice demonstrated significantly fewer USV than baseline, producing no USV at both 1 and 3 h. According to orbital tightness, posture, and piloerection, LPS-injected mice showed signs of poor wellbeing at 3 h but not 1 h. These findings indicate that FiUSV and IiUSV can be used as proxy indicators of animal wellbeing associated with acute inflammation in mice and can be detected before the onset of clinical signs.

Rat Ultrasonic Vocalizations and Behavioral Neuropharmacology: From the Screening of Drugs to the Study of Disease

Several lines of evidence indicate that rats emit ultrasonic vocalizations (USVs) in response to a wide range of stimuli that are capable of producing either euphoric (positive) or dysphoric (negative) emotional states. On these bases, recordings of USVs are extensively used in preclinical studies of affect, motivation, and social behavior. Rat USVs are sensitive to the effects of certain classes of psychoactive drugs, suggesting that emission of rat USVs can have relevance not only to neurobiology, but also to neuropharmacology and psychopharmacology. This review summarizes three types of rat USVs, namely 40-kHz USVs emitted by pups, 22-kHz USVs and 50-kHz USVs emitted by young and adult animals, and relevance of these vocalizations to neuropharmacological studies. Attention will be focused on the issues of how rat USVs can be used to evaluate the pharmacological properties of different classes of drugs, and how rat USVs can be combined with other behavioral models used in neuropharmacology. The strengths and limitations of experimental paradigms based on the evaluation of rat USVs will also be discussed.

Interleukin-33 mediates formalin-induced inflammatory pain in mice

Interleukin-33 (IL-33), a member of the IL-1 family, has attracted growing interest since its discovery in 2003. IL-33 has been implicated in many diseases, including arthritis, asthma, allergies, and cardiovascular and infectious diseases. However, few studies have investigated its role in the transmission and modulation of pain. The present study was designed to explore the possible roles of IL-33 and its receptor, ST2, in formalin-induced inflammatory pain in mice. We found that both subcutaneous (s.c., 300 ng) and intrathecal injection (i.t., 3 ng) of recombinant IL-33 (rIL-33) increased paw lifting and licking time not only in normal mice but also in formalin models. Administration of ST2 antibody, which blocked the IL-33/ST2 signaling, alleviated the formalin-induced spontaneous pain behavior. Moreover, the ST2(-/-) mice showed significantly decreased pain behavior, as well as reduced ultrasonic vocalization induced by formalin, compared with the wild-type group. Additionally, ST2 antibody alleviated the potentiating effects of rIL-33 on pain behavior in the formalin mice, indicating that IL-33 plays a role in pain modulation through its ST2 receptor. These data suggest IL-33 and its ST2 receptor mediate formalin-induced inflammatory pain, and as a result this cytokine and its receptor may be new targets for the development of analgesics.

An improved behavioural assay demonstrates that ultrasound vocalizations constitute a reliable indicator of chronic cancer pain and neuropathic pain

Background

On-going pain is one of the most debilitating symptoms associated with a variety of chronic pain disorders. An understanding of mechanisms underlying on-going pain, i.e. stimulus-independent pain has been hampered so far by a lack of behavioural parameters which enable studying it in experimental animals. Ultrasound vocalizations (USVs) have been proposed to correlate with pain evoked by an acute activation of nociceptors. However, literature on the utility of USVs as an indicator of chronic pain is very controversial. A majority of these inconsistencies arise from parameters confounding behavioural experiments, which include novelty, fear and stress due to restrain, amongst others.

Results

We have developed an improved assay which overcomes these confounding factors and enables studying USVs in freely moving mice repetitively over several weeks. Using this improved assay, we report here that USVs increase significantly in mice with bone metastases-induced cancer pain or neuropathic pain for several weeks, in comparison to sham-treated mice. Importantly, analgesic drugs which are known to alleviate tumour pain or neuropathic pain in human patients significantly reduce USVs as well as mechanical allodynia in corresponding mouse models.

Conclusions

We show that studying USVs and mechanical allodynia in the same cohort of mice enables comparing the temporal progression of on-going pain (i.e. stimulus-independent pain) and stimulus-evoked pain in these clinically highly-relevant forms of chronic pain.

Nocifensive behaviors components evoked by brief laser pulses are mediated by C fibers

Nocifensive behavior involves several response elements that have been used to assess neuropharmacological effects in different animal models of pain. Our previous analysis of laser-evoked nocifensive behaviors suggested that hierarchically organized responses in the nocifensive motor system are recruited in varying degrees by noxious stimuli of different intensities. Nocifensive behaviors can be differentially elicited and mediated by different classes of nociceptors. Thus, the aim of this study was to test the hypothesis that nocifensive behavioral elements elicited by brief laser pulse stimuli are mediated by C nociceptors. Laser-evoked cortical potentials and nocifensive behavior elements were recorded concurrently. As stimulus energy increased, rats exhibited a larger number of different responses and a greater frequency of each response element. Applying the neurotoxin, capsaicin, which selectively inhibits C fibers, to the sciatic nerves of rats, differentially blocked nocifensive behavioral components of flinch, withdrawal and licking but not non-nocifensive responses, namely movement and head turning. Based on these results we suggest that flinch, withdrawal and licking are mediated by C fibers, which are temporally associated with the nocifensive motor system as well as spinal and cortical evoked potentials. These results link hierarchically organized nocifensive responses and the afferent C fibers in the nocifensive motor system.

Evidence for mediation of nociception by injection of the NK-3 receptor agonist, senktide, into the dorsal periaqueductal gray of rats

RATIONALE

Ultrasound vocalizations (USVs) at approximately 22 kHz are usual components of the defensive response of rats. However, depending on the neural substrate that is activated, such as the dorsal periaqueductal gray (dPAG), USV emissions may be reduced. Activation of neurokinin-1 (NK-1)-mediated mechanisms of the dPAG causes analgesia, reduced 22 kHz USVs, and anxiogenic-like effects in rats exposed to the elevated plus maze (EPM). Involvement of other types of neurokinin receptors in this activation has not yet been evaluated.

OBJECTIVES

The present study examined whether local injections of the selective NK-3 agonist senktide (1-100 pmol/0.2 microL) into the dPAG can (1) cause anxiogenic effects in the EPM, (2) influence novelty-induced 22 kHz USVs, or (3) change nociceptive reactivity in the tail-flick test.

RESULTS

Senktide elicited a significant increase in exploratory behavior, an effect accompanied by hyperalgesia and an increase in the number of 22 kHz USVs. The nociceptive effects, increased locomotor activity, and USV emissions elicited by local injections of senktide (50 pmol/0.2 microL) were reduced by prior injections of the selective NK-3 receptor antagonist SB222200 (50 pmol/0.2 microL) into the dPAG.

CONCLUSIONS

These findings show that NK-3 receptors in the dPAG mediate nociceptive responses in this area, contrasting with the known fear-related processes mediated by NK-1 receptors in the dPAG. Both hyperalgesia and fear-related processes are accompanied by emissions of 22 kHz USVs.

Binaural Unmasking of Frequency-Following Responses in Rat Amygdala

Survival in natural environments for small animals such as rats often depends on precise neural coding of life-threatening acoustic signals, and binaural unmasking of species-specific pain calls is especially critical. This study investigated how species-specific tail-pain chatter is represented in the rat amygdala, which receives afferents from both auditory thalamus and auditory association cortex, and whether the amygdaloid representation of the chatter can be binaurally unmasked. The results show that chatter with a fundamental frequency (F0) of 2.1 kHz was able to elicit salient phase-locked frequency-following responses (FFRs) in the lateral amygdala nucleus in anesthetized rats. FFRs to the F0 of binaurally presented chatter were sensitive to the interaural time difference (ITD), with the preference of ipsilateral-ear leading, as well as showing features of binaural inhibition. When interaurally correlated masking noises were added and ipsilateral chatter led contralateral chatter, introducing an ITD disparity between the chatter and masker significantly enhanced (unmasked) the FFRs. This binaural unmasking was further enhanced by chemically blocking excitatory glutamate receptors in the auditory association cortex. When the chatter was replaced by a harmonic tone complex with an F0 of 0.7 kHz, both the binaural-inhibition feature and the binaural unmasking were preserved only for the harmonic of 2.1 kHz but not the tone F0. These results suggest that both frequency-dependent ascending binaural modulations and cortical descending modulations of the precise auditory coding of the chatter in the amygdala are critical for processing life-threatening acoustic signals in noisy and even reverberant environments.

Ultrasonic Rat Vocalizations During the Formalin Test: A Measure of the Affective Dimension of Pain?

The emission of ultrasonic vocalization (USV) by rats submitted to the formalin test has not yet been demonstrated. We performed two experiments to establish the formalin concentration to induce USV and the relationship of USV emission with motor behaviors and the effects of morphine and naloxone on USV during the formalin test. Male Wistar rats were used. In Experiment 1, 3 different groups of rats were subcutaneously injected with 5%, 10%, or 12.5% formalin in 1 of the anterior paws. Experiment 2 was intended to verify the effect of morphine 1, 2.5, or 5 mg/kg on USV during the 12.5% formalin test, whereas other groups of rats received naloxone 2 mg/kg with each one of the morphine doses to verify the specificity of opioid action. USV and motor behaviors were simultaneously measured in 5-min windows for 40 min, and early (0-5 min), interphase (5-20 min), and late (20-40 min) phases of the test were characterized. Vocalization was detected mostly during the interphase of the formalin test, mainly after formalin 12.5%. Morphine suppressed USV in a naloxone-reversible manner. This is a demonstration of USV during the formalin test, allowing the inclusion of an additional nonreflex behavioral measure to help characterize more clinically relevant integrated behavioral patterns in this rat model of pain.

Analgesia and hyperalgesia from CRF receptor modulation in the central nervous system of Fischer and Lewis rats

This study examines the contribution of central corticotropin-releasing factor (CRF) to pain behavior. CRF is the principal modulator of the hypothalamo-pituitary-adrenal (HPA) axis, in addition to acting on many other areas of the central nervous system. We compared nociceptive thresholds (heat and mechanical) and pain behavior in response to a sustained stimulus (formalin test) between Fischer and Lewis rats that have different HPA axis activity. Intracerebroventricular (i.c.v.) administration of CRF produced dose-dependent antinociception at a lower dose in Lewis (40 ng, paw pinch 71+/-0 g) compared to Fischer rats (200 ng, 112+/-3 g). The antinociceptive effect of CRF was mostly preserved in adrenalectomized Fischer rats. The i.c.v. administration of the CRF receptor antagonist, astressin, had a hyperalgesic effect, suggesting that CRF is tonically active. Lewis rats required higher doses of astressin (5 ng, paw pinch 51+/-1 g) to show nociceptive effects compared to Fischer rats (1 ng, 79+/-1 g). Only Lewis rats vocalized during mechanical stimulus, and this behavior was prevented by diazepam or morphine but was worsened by CRF, despite its antinociceptive property. In the formalin test, CRF and astressin had the largest effect on the interphase suggesting that they act on the endogenous pain inhibitory system. CRF also increased anxiety/fear-like behaviors in the forced swim and predator odor tests. Our results establish that central CRF is a key modulator of pain behavior and indicates that CRF effects on nociception are largely independent of its mood modulating effect as well as its control of the HPA axis.

Ultrasonic Vocalization Response Elicited in Adjuvant-induced Arthritic Rats as a Useful Method for Evaluating Analgesic Drugs

Adjuvant-induced arthritic (AIA) rats develop a severe chronic polyarthritis which shares some features in common with human rheumatoid arthritis. The purpose of the present study was to examine whether AIA rats emit ultrasonic vocalizations (USVs) when they are confronted with a healthy 'stimulus rat' in social interactions. We also examined the effects of three analgesic drugs (piroxicam, rofecoxib and ketoprofen) on USV responses using the same paradigm. In social interactions, AIA rats and intact controls emitted USVs in the 22-28 kHz range. Vocalization activities were significantly higher in AIA rats than those in intact controls. Moreover, the USVs of AIA rats were significantly inhibited by the three analgesic drugs. These results suggest that the USV responses elicited in AIA rats are useful for the evaluation of analgesic drugs.

Computerized analysis of audible and ultrasonic vocalizations of rats as a standardized measure of pain-related behavior

The behavioral assessment of experimental pain is essential for the analysis of pain mechanisms and the validation of therapeutic targets. Arthritic pain, in particular, is significantly associated with negative affective states and disorders. Here we present a standardized method for the quantitative analysis of audible and ultrasonic (25 +/- 4 kHz) vocalizations in awake rats as a measure of higher integrated behavior in a model of arthritic pain. A bat detector and a condenser microphone were used to record ultrasonic and audible vocalizations, respectively, in response to innocuous and noxious mechanical stimulation of the knee before and after induction of acute arthritis in one knee. A computerized system was used to analyze number and duration of the filtered signals. For the behavioral tests, the animal was placed in a customized recording chamber to ensure consistent stimulus application and stable recordings and to eliminate any movement-induced noise. Noxious stimuli produced stronger vocalizations than innocuous stimuli. Both audible and ultrasonic vocalizations to innocuous (allodynia) and noxious (hyperalgesia) stimuli increased after the induction of acute arthritis. These changes were accompanied by increased knee joint circumference, lowered hind limb withdrawal thresholds and reduced exploratory behavior in the same animals. The computerized analysis of audible and ultrasonic vocalizations is a valid, quantitative, reliable and convenient method to measure pain-related behavior.

Analysis of ultrasonic vocalisation does not allow chronic pain to be evaluated in rats

Most pain tests used for the assessment of drug analgesic activity in animal chronic pain models are based on the measurement of the response to an external acute stimulation (thermal, mechanical or electrical). But these stimuli are not related to the chronic pain experienced by the animal. Quantitative analysis of the spontaneous behaviour induced by the chronic pain state is needed. Several authors have suggested that ultrasonic vocalisations (USVs) emitted by rats in painful situations might reflect expression of affective pain. In a first study, we recorded spontaneous USVs in sub-chronic and chronic pain models: inflammation induced by carrageenan, arthritis induced by Freund's adjuvant and diabetes induced by streptozotocin. The USVs were analysed when naive Sprague-Dawley rats were alone and during non-agonistic interaction with a conspecific. When the rats were alone they did not emit any USV. During social interaction, no difference in either the frequency or the duration was observed between the emissions of healthy rats and rats in pain. In a third study, the influence of three parameters, degree of confrontation between the rats, age of the conspecific and housing conditions (isolated or collective) was studied in the arthritic rat model. Arthritic rats did not emit more USVs than controls in any of our experimental conditions. A fourth study showed that Aspirin (200 mg/kg) had no effect on the USVs, this data confirms the lack of direct relationship between USVs and experimental chronic pain in rats in our conditions.

[Critical analysis of animal models of acute pain. II]

OBJECTIVE

To analyse models of acute pain in experimental animals.

DATA SOURCES

References were obtained from computerised bibliographic data banks (Medline and others) and the authors' personal documents.

DATA SYNTHESIS

The majority of tests permit only a measurement of threshold, whereas clinical pain is almost always prolonged. The relationships between tests of acute pain and motor activity are reviewed from a number of standpoints; in particular we consider the influence, which postural adjustments of the animal may exert on motor responses in the limbs and the significance of flexor and extensor reflexes. In analysing the problem of the sensitivity of tests, we raise the following questions: 1) what type(s) of fibres underlie the observed responses and might these be different depending on whether one is stimulating a healthy or an inflamed tissue; 2) what significance do measurements of "latency" have when a stimulus is increasing; 3) how valid are the methods of analysing the results? The influence of species and the genetic line used in tests and the specificity and predictivity of tests are considered. Finally, we review those factors, which may distort behavioural measurements in animals, notably--pharmacokinetics, interactions between heterotopic stimuli, environmental factors and related psychophysiological/psychological considerations (subjective "undesirable" phenomena, learning phenomena). We pay particular attention to related physiological functions (thermoregulation, vasomotricity, blood pressure). These considerations lead us to re-position nociception within a much larger homeostatic framework which in addition to pain, includes phenomena such as anxiety and vegetative functions. They also suggest that we should define an "effective stimulus" as one, which activates nociceptive nerve terminals after a physical stimulus, has passed through a "peripheral lens" which regulates its intensity for reasons, which are physical, albeit of biological origin. Finally they remind us that the "system of pain" forms part of a whole set of subsystems--sensory, motor, vegetative, emotional, motivational--which scientific method, being reductionist by nature, cannot study in its entirety. However one must consider results of nociceptive tests within this general context.

CONCLUSION

It is only by taking the approach described in this review, that fundamental and clinical research can interact usefully.

Automated behavioural analysis in animal pain studies

The assessment of the effectiveness of analgesics is strongly based on observational data from behavioural tests. These tests are interesting and give a quantification of the effect of the drugs on the whole animal but their use is subject to several difficulties: (i) many results are difficult to analyse as they only correspond to the evaluation of a reflex response; (ii) the tests dealing with more integrated responses are also more difficult to use and closely depend on the experimenter's subjectivity. If automation is widely used in a lot of research fields, this is not the case in behavioural pharmacology. Yet, it can contribute to optimize the tests. The use of signal processing devices allows the automated (and thus objective) measurement of behavioural reactions to nociceptive stimulation (amplitude of a reflex, vocal emission intensity). Mechanical devices based on a computer-driven dynamic force detector allows the recording of some pain behaviours. Video image analysis allows the quantification of more complex behaviours (nociception-induced specific motor behaviours) as well as meaningful information during the same experimentation (exploratory behaviour, total motor activity, feeding behaviour). Moreover, these methods make it possible to obtain a more objective measurement, to reduce animal-experimenter interactions, to ease system use, and to improve effectiveness. The prospects to work in this field are multiple: continuation of the attempts at an automation of the behaviours specifically induced by chronic pain; development of real animal pain monitoring based on analysis of specific and non-specific behavioural modifications induced by pain. In this context, the automation of the behavioural analysis is likely to make possible real ethical progress thanks to an increase in the test's effectiveness and a real taking into account of animal's pain. Nevertheless, there are some limits due to characteristics of the behavioural expression of nociception and technological problems.