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* Danish Institute of Agricultural Sciences, Department of Animal Health and Welfare, Research Center Foulum, DK-8830 Tjele, Denmark;
and
Swiss Federal Institute of Technology, Zurich, Institute of Animal Science, Physiology and Animal Husbandry, 8603 Schwerzenbach, Switzerland;
and
Federal Agricultural Research Center, Institute of Animal Welfare and Animal Husbandry, 29223 Celle, Germany;
and
The Royal Veterinary and Agricultural University, Department of Animal Science Health, 1870 Frederiksberg C, Denmark
1 Correspondence:
(phone: 45-89-99-13-28; fax: 45-89-99-15-00; E-mail:
mettes.herskin{at}agrsci.dk).
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Abstract |
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 Top Abstract IntroductionMaterial and MethodsResultsDiscussionImplicationsLiterature Cited |
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0.01), types of response (less nonresponding and more kicking, P < 0.0001), and behavior during (increasing frequency of tail flicking, P = 0.003) and between single laser exposures (increasing frequency of kicking, P = 0.02). Therefore, behavioral responses to a laser stimulus seem to be a valid measure of nociception in dairy cows. Repeatability within 15 min was investigated in Exp. 2 using n = 36 dairy cows kept and tested in tie stalls and a power output of 1.8 W. The variables’ latency to move the exposed leg and frequency of tail flicking during laser exposure showed the highest level of repeatability (0.50 and 0.38, respectively). However, retesting at t = 15 min led to increased responses in terms of shorter latencies to respond (P 0.05), increased kicking (P = 0.05), and tail flicking (P = 0.02), which probably can be explained by sensitization. Effects of power output (1.0 vs. 1.8 W) and skin condition (naked vs. intact) were examined in Exp. 3 on 11 group-housed dairy cows, tested just outside their home pen. Increasing the power output and shaving off hair led to increased responses as seen by shorter latencies to respond (P < 0.0001), less nonresponding (P < 0.0001), and increased kicking (P = 0.0003), as well as reduced intra- and interindividual variability (P 0.04). In conclusion, the results of these experiments suggest that behavioral responses to laser stimulation are a valid and reliable measure of nociception in dairy cows, especially when applied on naked skin, both in the home environment and just outside a group pen. The fact that repeated testing in itself at t = 15 min led to increased responses means that the test will be a conservative measure of stress-induced analgesia.
Key Words: Behavior • Dairy Cows • Heat • Laser Radiation • Pain
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Introduction |
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The nociceptive measures in cattle include radiant thermal stimulation (Pinheiro Machado et al., 1998; Veissier et al., 2000), contact thermal stimulation (Whay et al., 1997), and mechanical stimulation (Ley et al., 1996; Whay et al., 1997).
The use of radiant heat via laser has been recommended for measuring nociceptive thresholds because it elicits specific activity without contamination from mechano-sensitive receptors (Arendt-Nielsen and Bjerring, 1988), and it permits a higher degree of control with spatial and temporal variables than traditional methods using radiant heat (Haimi-Cohen et al., 1983) and is therefore more precisely replicable (Fan et al., 1995).
Laser technique has been used successfully to measure nociceptive thresholds in cattle (Schwartzkopf-Genswein et al., 1997; Rushen et al., 1999; Veissier et al., 2000). There has been some validation of laser technique in calves (Veissier et al., 2000), but before the method can be used to examine nociceptive thresholds in dairy cows and the effects of acute stress thereupon, there is a need for more knowledge about several methodological aspects.
This paper presents data from three experiments examining whether: 1) the method is valid as a measurement of nociceptive threshold in dairy cows; 2) there is an acceptable repeatability within 15 min (the duration of most experimental acute stressors); and 3) skin condition (intact vs. naked) affects nociceptive threshold and its variability in dairy cows.
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Material and Methods |
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The cows were tethered with neck-bar ties in stalls measuring 180 x 120 cm with mats of hard rubber material and chopped straw added daily. Each cow had ad libitum access to water from a water bowl. The cows were fed twice daily from a fodder truck with a total mixed ration (DM percentage of diet: 28% grass silage, 26% whole-crop barley silage, 15% ground barley, 30% concentrates, and 1% minerals) at 0700 and 1430 h. Sufficient food was given to allow ad libitum intake. The cows were milked twice daily according to the daily routines in the barn (at 0430 and 1530 h). The experiment was performed in August 2000, with outdoor temperatures (measured at noon) of 16.9 ± 0.7°C and a relative humidity of 65.5 ± 4.4%.
Experiment 2.
Thirty-six Danish Friesian cows from the resident herd at the Research Center Foulum were used as experimental animals. The cows were 145 ± 22 d postpartum (range: 15 to 401 d) with a mean lactation number of 1.7 ± 0.2 (range: 1 to 5).
The cows were kept, fed, and milked as in Exp. 1. The experiment was performed in December 2000, with outdoor temperatures (measured at noon) of 7.1 ± 0.4°C and a relative humidity of 90.5 ± 1.5%.
Experiment 3.
Eleven Swiss Friesian cows from the resident herd at the Research Station Chamau of the Swiss Federal Institute of Technology were used as experimental animals. The cows were 121 ± 17 d postpartum (range: 47 to 189 d) with a mean lactation number of 2.5 ± 0.3 (range: 1 to 5).
The cows were kept as a group in a pen equipped with cubicles and access to an outdoor run. They were fed four times daily from a fodder truck with a total mixed ration (DM percentage of diet: 6% hay, 38% corn silage, and 56% grass silage) at 0600, 1045, 1415, and 1700 h. Sufficient food was given to allow ad libitum intake. Furthermore, the cows were fed concentrate according to the individual lactation state from a transponder-controlled self-feeder and had ad libitum access to water from a water trough. The cows were milked twice daily according to the daily routines in the barn (at 0500 and 1500 h). The experiment was performed in October 2001 with barn temperatures (measured at noon) of 14.6 ± 0.3°C and a relative humidity of 93.8 ± 1.4%.
Experimental Design
Experiment 1.
Effects of power output on nociceptive responses were examined in a 6 x 6 Latin square design (Lehner, 1996) with six different power outputs (0, 1.3, 1.8, 2.2, 2.4, and 2.6 W) and six test days (Monday, Wednesday, and Friday in two successive weeks). Each cow received all six power outputs in a randomized order and with at least 48 h between tests. The individual animals were tested in the same order each test day between 0800 and 1000.
Experiment 2.
Repeatability within 15 min was investigated by the performance of one test of nociception at t = 0 min followed by another test at t = 15 min. For both tests, a power output of 1.8 W was used. During and between tests the cows were kept in their home stalls. The tests were performed between 1000 and 1200. Each cow was tested on one day only, but in order to test all 36 cows, 4 d within the same week were used.
Experiment 3.
To investigate effects of power output and skin condition, we performed a crossover experiment with power output (1.0 vs. 1.8 W) and skin condition (intact hair vs. naked skin) as main factors. The cows were tested twice, separated by at least 72 h. In the first test, half the cows received 1.0 W and the other half 1.8 W. Within each power output, the cows were tested on both naked and hairy skin, half of them starting on the naked and vice versa. In the second test, the cows received the other power output and started on the other skin condition.
Laser Equipment
The nociceptive responses of the dairy cows were assessed by examining their reactions to a laser beam applied to the skin on the caudal part of the metatarsus of the hind leg. An adjustable 10-W computer-controlled (by modifying the voltage input to the control box using a digital-to-analog card mounted in a PC) CO2 laser with a wavelength of 10.6 µm and a beam diameter of 0.6 cm was used as the heat source (model 48-1, Synrad, Mukilteo, WA). Attached to the CO2 laser was a visible, cold He laser pointer (Bantex, Denmark), which was used to aim.
Experimental Procedure
Experiments 1 and 2.
At least 72 h before the initial test, the hair on each hind leg was trimmed from the tarsal joint to the pastern joint leaving approximately 0.5 cm of hair.
Two persons dressed like the usual caretakers carried out the test of nociception. On the test day, a trolley with the computerized laser was placed on the aisle, approximately 2 m behind the cows. The hind legs were brushed quickly in order to remove manure. After brushing, the cow was allowed a 2-min adaptation period before start of laser stimulation. If the cow was lying down, she was forced to get up before brushing. Each test consisted of six single laser exposures, three on each hind leg, in a balanced order. The laser was turned off as soon as the cow responded either by moving, lifting, or kicking the leg. In case of no response, the maximum exposure time was 20 s for Exp. 1 and 25 s for Exp. 2. The six single laser exposures were applied as quickly as possible, however, avoiding stimulation of the same spot of skin more than once and allowing the cow to stand quiet before each new exposure. In Exp. 1 the mean interval between single laser exposures was 21 ± 1 s (514 observations), whereas it was 17 ± 1 s in Exp. 2 (359 observations). If the cow started to urinate, defecate, or any other spontaneous movement not induced by laser stimulation (e.g., initiation of eating), the laser was turned off and the exposure was repeated when she was standing still again.
All behavioral observations were done by direct observation and data organized using specially designed computer software (written in VB-DOS) based on simple key presses by the operator. An ethogram of behavior recorded in the experiments is shown in Table 1
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Three persons carried out the test of nociception. On the day of testing, each cow was caught in the group and led by a halter to a testing area just outside the group pen, where she was tied at a height of 100 cm with a rope of approximately 30 cm. During testing, the cows had visual, auditory, and olfactory contact with group mates. A trolley with the computerized laser was placed in the test area, approximately 2 m behind the cow. The hind legs were brushed quickly in order to remove manure. After brushing, the cow was allowed a 2-min adaptation period before the start of laser stimulation. In case of no response, the maximum exposure time was 25 s. Otherwise, the laser was turned off as soon as the cow responded either by moving, lifting, or kicking the leg. The test procedure and the behavioral recording were the same as in Exp. 1 and 2. However, in Exp. 3, each session of nociceptive testing consisted of 12 single exposures: six on naked and six on hairy skin.
Immediately before testing on naked or hairy skin, the skin temperature at the area of testing was measured using infrared thermography (Thermotracer 6T62, NEC San-ei Instruments Ltd., FLIR AG, Kriens, Switzerland). Skin temperature on naked areas was 32.5 ± 0.4°C (range 29.2 to 35.2°C), whereas the hairy skin was 28.5 ± 0.5°C (range 23.7 to 32.2°C).
Statistical Analysis
The behavioral variables used in the experiments are shown in Table 2.
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2-values, and the P-value. Continuous variables were compared using one-way repeated-measures ANOVA (SigmaStat, Jandel, Inc., Richmond, CA), both on the comparison of power outputs and when comparing single laser exposures within power output. Post-hoc comparisons of each pair of power outputs were made using the Student-Neuman-Keuls method (SigmaStat, Jandel, Inc.).
Variation within animal was estimated by mean interquartile ranges (IQR) between the six single laser exposures applied to each cow within each power output. The following variables were used: 1) latency to move the exposed leg (s); 2) latency to flick the tail (s); 3) latency to the first reaction (s); 4) frequency of tail flicking per second during laser exposure; and 5) type of response (rated on a scale from 0 to 3). The variables from the intervals between single laser exposures (kicking, licking leg, or licking body) were omitted from IQR-estimation due to very low representation in the data. In this way, the mean IQR was estimated for each cow and subjected to one-way repeated measures ANOVA (SigmaStat, Jandel, Inc.) in order to compare the effects of different power outputs on the within-animal variation.
Differences in responses to laser exposure between right and left hind leg were examined for the latency to move the exposed leg as well as the frequency of tail flicking during laser exposure using the Wilcoxon Signed Rank test (SigmaStat, Jandel, Inc.), comparing the three exposures applied to each hind leg for each of the six power outputs and for all observations summed.
Experiment 2.
In order to incorporate the relatedness between responses at t = 0 and 15 min, a paired t-test was used for comparison (SigmaStat, Jandel, Inc.).
The repeatability of the behavioral responses within 15 min was estimated using the PROC MIXED (Littell et al., 1996) procedure of SAS (SAS Inst. Inc.) in order to reduce sources of variability other than the individual measures. The model included repetition (1, 2) as a class variable, the summed interval between single exposures (mean: 88 s; range: 34 to 276 s) as a covariate, and the identity of the cows as a random effect. Repeatability was calculated using the covariance parameter estimates for cow identity (cov [cow]) and residual (cov [error]). Repeatability = cov(cow)/[cov (cow) + cov (error)].
Experiment 3.
The four different treatments were compared using one-way repeated measures ANOVA (SigmaStat, Jandel, Inc.) and presented as means ± SE, as well as F (dftreatment, dferror), and the P-value.
Due to non-normality, the differences between the six single exposures were analyzed by Friedman repeated-measures ANOVA (SigmaStat, Jandel, Inc.) and represented by the P-value.
Variation within animal was estimated by mean IQR between the six single laser exposures applied to each cow within treatment. The following variables were used: 1) latency to move exposed leg (s); 2) type of response (measured on a scale from 0 to 3); and 3) frequency of tail flicking per second during laser exposure. Variables obtained in the intervals between single exposures were omitted from IQR estimation due to very low representation in the data. In this way, the mean IQR was estimated for each cow and subjected to Wilcoxon signed rank test (SigmaStat, Jandel, Inc.) to compare combinations of power output and skin condition.
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Results |
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; Table 3).
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2 = 22.60; P < 0.0001), with increasing power output leading to decreasing latency to first response. Tail flicking was the first response in 87% of the 103 tests of nociception and was affected by power output as well (2 = 6.17; P = 0.01). Again, increasing power output led to decreasing latency. For both the power output of 0 and 1.3 W, 44% of the dairy cows had a median latency to move the leg of at least 20 s (maximal observation time). Only 6% of the cows did not respond to 1.8 W, whereas all cows responded when the power was 2.2, 2.4, or 2.6 W. The latency to move the exposed leg was affected by power output (2 = 26.81; P < 0.0001), showing that increasing power output led to decreasing latency to move the leg. Power output affected all types of behavioral responses except moving the leg without lifting the hoof (F5,80 = 28.28; P < 0.0001 for the proportion of nonresponding, F5,80 = 6.87; P < 0.0001 for the proportion of lifting the hoof and F5,80 = 11.81; P < 0.0001 for the proportion of kicking in response to laser stimulation). Behavior during exposure was affected by power output as well: F5,80 = 3.98; P = 0.003 for the frequency of tail flicking per second of exposure. Finally, the behavior between exposures was affected by power output for the frequency of licking leg (F5,80 = 2.98; P = 0.02) and the frequency of kicking (F5,80 = 2.78; P = 0.02), whereas the frequency of licking body was not affected by power output. The summed interval between the six single laser exposures was not affected by power output. For all tests, the mean interval was 106 ± 4 s.
Each Single Laser Exposure.
Comparison of each of the six single laser exposures during the tests of nociception for each of the six power outputs revealed no significant differences from first to sixth laser exposure for any of the power outputs. No differences were found between the two hind legs—not for the six power outputs separately or for all of the observations summed. The median latency to move the exposed leg was 12 (7 to 20) s vs. 10 (6 to 19) s for left vs. right leg. The median frequency of tail flicking/s during laser exposure was 0.1 (0 to 0.2) vs. 0.11 (0 to 0.2) for left vs. right hind leg.
Within-Animal Variation.
For all variables, the mean IQR was affected by power output; however, it was affected in an inconsistent way (see Table 4). Generally, for all three latencies, the IQR decreased with increasing power output (F5,79 = 2.24; P = 0.06, F5,79 = 4.74; P = 0.0008 and F5,79 = 7.87; P < 0.0001 for the latency to move the exposed leg, the latency to flick the tail, and the latency to first response, respectively). The IQR for the frequency of tail flicking during exposure increased for increasing power output (F5,79 = 2.43; P = 0.04), whereas the IQR for the type of response showed an inverted U-shape in relation to the power output (F5,79 = 3.84; P = 0.004).
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). Latencies to respond decreased (P = 0.003, P = 0.05, and P = 0.03 for the latency to move the exposed leg, the latency to flick the tail, and the latency to first response, respectively). The type of response changed as well, showing less moving the exposed leg without lifting the hoof (P = 0.03), less lifting the exposed leg (P = 0.06), and more kicking (P = 0.05). The percentage of non-responding cows, however, was not affected by the repetition. Finally, retesting at t = 15 min led to increased frequency per second of tail flicking during laser exposure (P = 0.02), as well as decreased licking the hind legs in the pauses between exposures (P = 0.06).
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Experiment 3: Effects of the Combination of Skin Condition and Power Output on the Complete Test of Nociception
The combination of shaving off hair and increasing the power output had major effects on the behavior of the dairy cows (Table 6). The latency to move the exposed leg was affected by treatment (F3,26 = 32.3; P < 0.0001). The type of response was affected by treatment as well (F3,26 = 17.55; P < 0.0001 and F3,26 = 9.08; P = 0.0003 for the percentage of nonresponding and the percentage of kicking, respectively).
Single Laser Exposure.
No effects of the combination of power output and skin condition were observed on the differences between the six single laser exposures within each test of nociception.
Within-Animal Variation.
Treatment affected the mean IQR for the latency to move the exposed leg (F3,26 = 5.87; P = 0.003; Table 7). The IQR was lower for cows tested with 1.8 W on naked skin than for cows tested with either 1.8 W on hairy skin or 1.0 W on naked skin. Finally, IQR for the type of responding was affected by treatment as well, showing that testing with 1.8 W on naked skin led to less variability than using 1.0 W on hairy skin (F3,26 = 3.13; P = 0.04).
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Discussion |
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The results of Exp. 1 show that increasing the power output leads to faster and increased responses as shown by shorter latency to respond, less nonresponding, increased responding by kicking, as well as an increased occurrence of licking the exposed leg between single exposures. There are some behavioral studies on the validity of nociceptive tests, observed as increased response to an increasing intensity of nociceptive stimulation (e.g., increasing the dosage of pain-inducing substances [Rosland et al., 1990; Coderre et al., 1993], increasing the intensity of pain-inducing electric shocks [Morris et al., 1997; Ong et al., 1997], or increasing the temperature of contact thermal stimulations [Tjølsen et al., 1991; Perkins et al., 1992]). The present results with a power output of zero, thereby sham-testing the cows, correspond with results from other nociceptive assays (Nolan et al., 1988). However, effects of a full range of nociceptive treatments (from sham to intense nociceptive stimulation) on both reflexive and complex dairy cow behavior measured by laser technique has not been presented until now. Veissier et al. (2000) compared the effects of different laser power outputs on nociceptive responses of calves and found similar results on validity for both the latency to a leg response and the latency to first response. Fan et al. (1995) used laser technique for nociceptive stimulation on rat feet and studied a wide range of behaviors. As in the results of Exp. 1, Fan et al. (1995) found that responding is not only quickened, but also changed (from body movements to foot tapping in rats) when power output is increased. In the present experiment, tail flicking was often observed as the initial response, whereas responding by kicking and leg licking after laser exposure was not observed for power outputs lower than 1.8 W. Fan et al. (1995) suggested that the pain motor system consists of a hierarchically organized pool of motor responses, each with different threshold, and concluded that the reaction pattern changes in a probabilistic fashion, such that the higher the noxious stimulation, the more components will be evoked.
In general, it is recommended to include complex behavior (e.g., leg licking in the formalin test [Dubuisson and Dennis, 1977]) in studies of pain behavior (Chapman et al., 1985; Dubner and Ren, 1999) in order to include the supraspinal sensory processing involved in the perception of pain. In contrast, reflexive behavior (e.g., tail flicking in response to laser stimulation on the rump of sows [Jarvis et al., 1997] or on the legs of cattle [Schwartzkopf-Genswein et al., 1997]) only express spinal neural activity and is therefore not a measure of perceived pain (Dubner and Ren, 1999). During validation of a nociceptive laser test in calves, Veissier et al. (2000) stated that the observation of complex behavior between single laser exposures did not improve the validity or reliability of the test. In the present study, registrations were made of both reflexive withdrawal behavior and more complex behavior, and also here the reflexive behaviors seemed comparable to the complex ones concerning the validity of the test. However, the complex behavioral variables, such as leg licking between single exposures, showed much lower repeatability. When the nociceptive laser test is used to study stress-induced changes in nociceptive thresholds, it might still be advantageous to include observations of complex behavior since central nervous system functions involved in the perception of pain, which are only expressed via complex behavioral responding and not reflexive behavior, might be important to understand relations between acute stress and nociception.
The observed curvilinear relationship between power output and behavioral responses, with the largest changes in behavioral responding per unit power output around 1.8 W, suggests that the sensitivity of the test is higher at this intensity of stimulation than both at lower and higher intensities. Studies on stress and nociception in farm animals have shown that acute stress can lead to hypoalgesia (e.g., after nose slinging in pigs [Rushen and Ladewig, 1991], and social isolation in dairy cows [Rushen et al., 1999]). In contrast, exposure to chronic pain can lead to hyperalgesia in sheep (Ley et al., 1989) and cows (Ley et al., 1996; Whay et al., 1997; 1998). In experiments where the nociceptive laser test is used to measure nociceptive changes, it would be advantageous to use a power output of about 1.8 W, because in this intensity range, relatively small changes in power output lead to significant behavioral changes.
A general problem with nociceptive assays, however, is a very high intra- as well as interindividual variation (Taiwo et al., 1989). High variability has also been observed during highly controlled studies using laser stimulation of human skin (Arendt-Nielsen and Bjerring, 1988), where the variability is attributed to differences in receptor density as well as optical or physical skin properties. In the present experiments, a high degree of variability was observed as well, in the latencies to respond, the type of response, and in the complex behavior between single laser exposures and within and between individual cows. The results of Exp. 1 and 2, however, show that removal of hair or increasing the power output, can lead to reduced variability, especially within animals, while the combination of a relatively high power output and naked skin led to the lowest observed variability, thereby increasing the sensitivity of the test. Our observation of increasing intensity of stimulation leading to reduced variability has also been shown when laser technique was used for nociceptive testing in calves (Veissier et al., 2000) and for other types of nociceptive stimulation as well (Guy and Abbott, 1992).
Using a relatively large number of animals (n = 36), Exp. 2 showed that latency to a leg response has the highest short-term repeatability. Veissier et al. (2000) used 24 calves and were not able to show any differences in the repeatability between latency to tail flick or latency to first response and latency to a leg response. It is evident from Exp. 1 that in tied dairy cows, tail flicking occurs spontaneously at a much higher rate than leg movements, especially kicking; therefore, it is not surprising that variables based on tail flicking behavior have a lower repeatability than variables based on leg responses.
In Exp. 2, it was shown that testing at t = 0 followed by retesting at t = 15 min, between which the cows were standing undisturbed in the home environment, led to increased responses in terms of shorter latencies to respond, increased kicking, and tail flicking. This might be due to sensitization. Experiment 2 was supposed to investigate effects of testing itself on responding after short-term stimulation (e.g., from an acute stressor, why retesting was done after 15 min, a duration that is often used for acute stressors in cattle). Rushen et al. (1999) subjected 12 dairy cows to 15 min of social isolation and showed that a control treatment including transport to an isolation chamber as well as saline injection did not affect the latency to respond to laser stimulation at t = 15 min. The differences between these two results can be explained by the nociceptive thresholds of the control animals in the experiment of Rushen et al. (1999) being affected by the transport/injection, thereby counteracting the effects of repeated testing itself. Veissier et al. (2000) repeated nociceptive testing by laser technique as well, either after 60 min or 24 h. They showed that calves responded faster to laser exposure at t = 60 min than at t = 0, but that responding at t = 24 h did not differ from the initial test. Peripheral nociceptors can show sensitization after repeated heating of the receptive fields (Fitzgerald, 1979), but there are large differences between types of receptors (Treede et al., 1995), types of skin (Campbell and Meyer, 1983), and also between animal species, where research primarily has focused on monkeys (Thalhammer and LaMotte, 1982). There are no available data on the sizes or densities of heat-receptive fields from bovine limbs, but data from other mammalian species suggest that the present laser beam probably has been overlapping receptive fields of several nociceptors (Lynn and Carpenter, 1982; Treede et al., 1990). In the present experiments, attempts were made to avoid exposing the same spot of skin twice in order to avoid sensitization or habituation. However, due to the relatively low level of fixation of the animals combined with the six stimulations on each leg in Exp. 2, it is possible that the same receptive field has been hit both at t = 0 and t = 15 min. The observed faster and increased responding at t = 15 min might therefore be due to sensitization either at the level of nociceptors or via associative learning (McFarland, 1981). Regardless of the underlying mechanisms, the effects of testing itself observed at t = 15 min suggest that nociceptive testing using laser technique will be a conservative measure of stress-induced analgesia in dairy cows because possible effects of stress have to counteract effects of the repeated testing itself.
In conclusion, the results of the present experiments suggest that behavioral responses to laser stimulation are a valid and reliable measure of nociception in dairy cows, both in the home environment and just outside a group pen. The highest sensitivity is obtained by using power outputs around 1.8 W. The method seems to have an acceptable short-term repeatability, especially for the latency to a leg response, but the sensitivity can be further increased by the use of naked skin. The fact that repeated testing in itself at t = 15 min led to increased responding means that the test of nociception using laser technique will be a conservative measure of stress-induced analgesia.
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