Differential stress effects on responses to noxious stimuli as measured by tail-flick latency and squeak threshold in rats

TailTimer: A device for automating data collection in the rodent tail immersion assay

The tail immersion assay is a widely used method for measuring acute thermal pain in a way which is quantifiable and reproducible. It is non-invasive and measures response to a stimulus that may be encountered by an animal in its natural environment. However, quantification of tail withdrawal latency relies on manual timing of tail flick using a stopwatch, and precise temperatures of the water at the time of measurement are most often not recorded. These two factors greatly reduce the reproducibility of tail immersion assay data and likely contribute to some of the discrepancies present among relevant literature. We designed a device, TailTimer, which uses a Raspberry Pi single-board computer, a digital temperature sensor, and two electrical wires, to automatically record tail withdrawal latency and water temperature. We programmed TailTimer to continuously display and record water temperature and to only permit the assay to be conducted when the water is within ± 0.25°C of the target temperature. Our software also records the identification of the animals using a radio frequency identification (RFID) system. We further adapted the RFID system to recognize several specific keys as user interface commands, allowing TailTimer to be operated via RFID fobs for increased usability. Data recorded using the TailTimer device showed a negative linear relationship between tail withdrawal latency and water temperature when tested between 47-50°C. We also observed a previously unreported, yet profound, effect of water mixing speed on latency. In one experiment using TailTimer, we observed significantly longer latencies following administration of oral oxycodone versus a distilled water control when measured after 15 mins or 1 h, but not after 4 h. TailTimer also detected significant strain differences in baseline latency. These findings valorize TailTimer in its sensitivity and reliability for measuring thermal pain thresholds.

A reassessment of stress-induced "analgesia" in the rat using an unbiased method

An increased tail-flick latency to noxious heat during or after stress in the rodent is usually interpreted as a stress-induced reduction in pain sensitivity and often described as a form of stress-induced "analgesia." However, this measure is an indirect and flawed measure of the change in nociceptive threshold to noxious heat. A major confound of the latency measure is the initial temperature of the tail, which can drop down to room temperature during stress, the consequence of a marked sympathetically mediated vasoconstriction in the skin of the extremities. We addressed this issue with tail-flick tests during contextual fear using infrared thermography to monitor temperature changes and a CO2 laser to deliver the heat stimulus. The experiment revealed a 4.2°C increase of the nociceptive threshold, confirming a true antinociceptive effect. However, its contribution to the increased withdrawal latency was less than two-thirds (63.2%). Nearly one-third (32.2%) was due to the drop in tail temperature (4.4°C), which also slowed conduction along sensory fibers (2.2%, included in the 32.2%). The remaining 4.6% was due to an increase in decisional/motor latency. This new unbiased method establishes beyond doubt that a conditioned stress response is associated with true antinociception to noxious heat. It also confirms that stress-induced changes in skin temperature can be a major confound in tail-flick tests. The present study shows, for the first time, the exact contribution of these two components of the tail-flick latency for a stress response. Less than two-thirds of the increase in tail-flick latency to noxious heat, evoked by conditioned fear, reflects true antinociception. The remaining is due to skin vasoconstriction.

The renin angiotensin system and nociception in spontaneously hypertensive rats

This study investigated the effects of captopril, hydralazine and losartan on the locomotor activity, tailflick and hot plate latencies in spontaneously hypertensive rats (SHR) and their genetic controls the Wistar-Kyoto rat (WKY). The increased hot plate latencies normally exhibited by the spontaneously hypertensive rat were reduced or abolished by captopril (95 mg/kg/day p.o.) and losartan (18 mg/kg/day p.o.) treatment, but were unaffected by hydralazine (19 mg/kg/day p.o.). There were no observable effects of any of the drugs on tailflick latencies or locomotor activity. The results highlight a potential role for angiotensin II in analgesia that is independent of blood pressure change.

Different effects of amphetamine on reinforced variations versus repetitions in spontaneously hypertensive rats (SHR)

The spontaneously hypertensive rat (SHR) may serve as an animal model of human attention deficit hyperactivity disorder (ADHD). We compared performances of SHRs and Wistar-Kyoto normotensive controls rats (WKY) in two experiments. When rewarded for varying sequences of responses across two manipulanda, the SHRs were more likely to vary than the WKYs. On the other hand, when rewarded for repetitions of a small number of sequences, the WKYs were more likely to learn to repeat. Both of these results confirm previous findings. Injecting 0.75 mg/kg d-amphetamine facilitated learning by SHRs to repeat the required sequences, with amphetamine-injected SHRs learning as rapidly as saline-injected, control WKYs. On the other hand, amphetamine tended to increase variability in both strains when high levels of variations were required for reward, and to decrease it in both strains when low levels of variability were required. Thus, amphetamine may have different effects on reinforced repetitions vs. reinforced variations.

Spontaneously hypertensive rats (SHR) readily learn to vary but not repeat instrumental responses

When spontaneously hypertensive rats (SHR) and Wystar-Kyoto normotensive control rats (WKY) were rewarded in a 12-arm radial maze (Experiment 1), the SHRs varied their arm choices more, making fewer repetition errors than the WKYs. Similarly when rewards depended on variable sequences of responses on two levers in an operant chamber (Experiment 2), SHRs' sequences were more variable than those of WKYs. A requirement for response variability was then combined with a requirement to repeat selected responses in the radial maze (Experiment 3) and operant chamber (Experiment 4). WKYs learned to repeat more readily than the SHRs, whereas SHRs varied more readily. Thus, when subjects had to repeat responses, SHRs were at a disadvantage, but when variability was adaptive, SHRs excelled. The high variability of SHRs, together with their difficulty in learning to repeat, may have parallels in children diagnosed with attention deficit disorder with hyperactivity (ADDH).

Neurobiological background of pain and analgesia: the attempt at revaluation according to position of the organism's adaptive activity

The most adequate and successful way to understand the essence of any complex psychophysiological phenomenon, including pain, is obviously the study of its origin, its genesis, i.e., its biological background. Based on critical analysis of recent literature and our own electrophysiological, biochemical and pharmacological data we tried to overcome the difficulties and contradictions derived from the traditional reflex approach and analytical orientation in understanding the experimental investigation of pain-related problems and to determine the neurobiological background of pain and analgesia through the notion of the organism's adaptive activity. Interrelations between the notion of pain and other biological and psychological ideas, the place and functional significance of pain and endogenous analgesic mechanisms in the organization, maintenance and regulation of the organism's adaptive activity, characterization of the involvement of endogenous opioid peptides and monoamines in central processes associated with pain and analgesia, the essence and mechanisms of pain-depressing activity of the opiates are the main stages in our neurobiological consideration of the phenomenon of pain and its natural and pharmacological regulation.

Tail immersion test for the evaluation of a nociceptive reaction in mice. Methodological considerations

The effect of two commonly used methods to immobilize the animals, viz. tube restrainer and wrapping in a diaper (chux) on the tail flick latency in immersion test, was evaluated in mice using a stimulus temperature of 50 degrees C. The animals were immobilized either in the tube or chux briefly (25-30 sec) during the tail flick measurements. The basal tail flick latency was 2.8 +/- 0.2 in the tube restrained and 5.5 +/- 0.3 sec in chux restrained groups (p less than 0.001). The analgesic effect of morphine (1, 3, 4, 7, and 10 mg/kg) was significantly higher in the chux-restrained animals as indicated by the dose ratio of 2.16 for the 50% analgesic response in the chux versus tube restrained mice. The tail flick latency, 15 min after naloxone injection (1 and 3 mg/kg), expressed as % of predrug latency was significantly reduced in the chux- but not the tube-restrained group. The hyperalgesic effect of naloxone could not be detected in chux-restrained animals, when the water temperature was increased to 55 degrees C. The results demonstrate that the restraining procedure will influence the analgesic effects of test drugs in tail immersion test. Furthermore, the stimulus temperature appears to be an important variant that could influence the results in this test. The present results demonstrate the hyperalgesic effect of naloxone after systemic administration in the tail immersion test and supports the concept that tail flick response is tonically inhibited by endogenous opioid systems.

Analgesia or hyperalgesia following stress correlates with emotional behavior in rats

Analgesia following exposure to a variety of noxious and non-noxious stressors is well documented and is commonly referred to as stress-induced analgesia. Hyperalgesia following stress has also been reported. The present study shows that a mild stressor (15 min of vibration) produced increased tail-flick latencies (TFL) in some rats, but decreased latencies in other rats. The results of the individual subjects were reproduced in a later session: the rats showing increased TFL on day 1, responded with increased TFL on day 2. Rats showing decreased TFL on day 1 responded with decreased TFL on day 2. Whichever reaction occurred, analgesia or hyperalgesia, this correlated with the animal's behavior during the stress procedure. Analgesia was produced in quiet rats and hyperalgesia in hyperemotional ones. Various peripheral nerve stimulation procedures producing hyperemotional reactions also resulted in lowering of the pain threshold. The results of the present study show behavioral modulation of pain mechanisms.

Analgesia by low-frequency nerve stimulation mediated by low-threshold afferents in rats

Several authors claim that analgesia by low-frequency peripheral nerve stimulation (transcutaneously or through inserted needles) depends on the activation of small-diameter A delta afferents. In the present study, a marked increase in pain threshold assessed by the squeak threshold test (vocalization to electric shocks delivered to the tail) was obtained by 2 Hz sciatic or radial nerve low-intensity stimulation, using single pulses. Conduction velocities of the activated nerve fibers ranged between 33.6 and 46.4 m/sec, which is consistent with activation of large afferent A fibers. This analgesic effect was reversed by naloxone (3 mg/kg i.v.). Thus, low-threshold afferents may contribute to the analgesic effect in addition to the previously reported high-threshold induced effect. High-intensity stimulation (5-6 times the threshold for visible muscular twitches), activating small-diameter afferents (A delta fibers) resulted in a decrease in pain threshold. Hyperalgesia also resulted from low-intensity stimulation using trains of pulses. The analgesic or hyperalgesic effects were correlated with the animal's behavior during stimulation. Analgesia was obtained in calm rats, who went to sleep regularly. When the rats showed signs of being distressed, hyperalgesia resulted. These findings may explain the occasional failure to obtain reduction in pain in anxious human patients by peripheral stimulation.

Noradrenergic mechanisms in mediation of stress-induced hyperalgesia in rats

Exposure to a mild stressor (15 min of vibration) produced analgesia in some rats and hyperalgesia in other rats from the same batch treated in the same way. Rats which responded with decreased tail-flick latencies (TFL) showed signs of hyperemotionality during the stress procedure. Stress-induced hyperalgesia was abolished by the administration of diazepam (2.5 mg/kg i.p.) and clonidine (25 micrograms/kg i.p.). It is suggested that the reversal of hyperalgesia was due to the anxiolytic properties of the drugs. Yohimbine, an alpha 2-adrenoceptor antagonist (5 mg/kg i.p.), antagonized the effect of clonidine. The influence of clonidine on stress-induced hyperalgesia may be mediated by alpha 2-adrenoceptors.