Effects of caudolateral neostriatal ablations on pain-related behaviour in the chicken

Do laying hens with keel bone fractures experience pain?

The European ban on battery cages has forced a change towards the use of non-cage or furnished cage systems, but unexpectedly this has been associated with an increased prevalence of keel bone fractures in laying hens. Bone fractures are acutely painful in mammals, but the effect of fractures on bird welfare is unclear. We recently reported that keel bone fractures have an effect on bird mobility. One possible explanation for this is that flying becomes mechanically impaired. However it is also possible that if birds have a capacity to feel pain, then ongoing pain resulting from the fracture could contribute to decreased mobility. The aim was to provide proof of concept that administration of appropriate analgesic drugs improves mobility in birds with keel fracture; thereby contributing to the debate about the capacity of birds to experience pain and whether fractures are associated with pain in laying hens. In hens with keel fractures, butorphanol decreased the latency to land from perches compared with latencies recorded for these hens following saline (mean (SEM) landing time (seconds) birds with keel fractures treated with butorphanol and saline from the 50, 100 and 150 cm perch heights respectively 1.7 (0.3), 2.2 (0.3), p = 0.05, 50 cm; 12.5 (6.6), 16.9 (6.7), p = 0.03, 100 cm; 20.6 (7.4), 26.3 (7.6), p = 0.02 150 cm). Mobility indices were largely unchanged in birds without keel fractures following butorphanol. Critically, butorphanol can be considered analgesic in our study because it improved the ability of birds to perform a complex behaviour that requires both motivation and higher cognitive processing. This is the first study to provide a solid evidential base that birds with keel fractures experience pain, a finding that has significant implications for the welfare of laying hens that are housed in non-cage or furnished caged systems.

Welfare implications of avian osteoporosis

Cage layer fatigue was first noticed after laying hens began to be housed in cages in the mid-20th century. Hens producing eggs at a high rate were most susceptible to the disease. Early research revealed that cage layer fatigue was associated with osteoporosis and bone brittleness. Severe osteoporosis leads to spontaneous bone fractures commonly in the costochondral junctions of the ribs, the keel, and the thoracic vertebrae. Vertebral fracture may damage the spinal cord and cause paralysis. Osteoporosis appears to be inevitable in highly productive caged laying hens. The condition can be made worse by metabolic deficiency of calcium, phosphorus, or vitamin D. Hens in housing systems that promote physical activity tend to have less osteoporosis and rarely manifest cage layer fatigue. Genetic selection may produce laying hens that are less prone to bone weakness. The welfare implications of osteoporosis stem from pain, debility, and mortality associated with bone fracture. The chicken has well-developed neural and psychological systems specialized to respond to pain associated with trauma and inflammation. Although studies on the chicken have not focused on pain due to bone fracture, physiological and behavioral similarities to other species allow inference that a hen experiences both acute and chronic pain from bone fracture. There is little information on osteoporosis in commercial caged layer flocks, however, evidence suggests that it may be widespread and severe. If true, most caged laying hens suffer osteoporosis-related bone fracture during the first laying cycle. Osteoporosis also makes bone breakage a serious problem during catching and transport of hens prior to slaughter. Estimates of mortality due to osteoporosis in commercial caged layer flocks are few, but range up to a third of total mortality. Many of these deaths would be lingering and attended by emaciation and possibly pain. Osteoporosis-related bone breakage during processing has reduced the marketability of spent caged laying hens, contributing to the need to develop humane on-farm killing methods to support alternative means of spent hen disposition. Overall, the evidence indicates that cage layer osteoporosis is a serious animal welfare problem. A determined effort must be made to make the laying hen no longer susceptible to the harmful effects of excessive bone loss.

Characterization of the chick carrageenan response

The present study characterized carrageenan inflammatory nociception in the 7-day-old domestic chick. The time course effects of foot withdrawal latency to a thermal stimulus and edema were examined over a 6-h period following an intraplantar carrageenan (0.0-1.0%) injection. Carrageenan-induced hyperalgesia and edema had a similar course of action, enduring for approximately 6 h, with a peak effect at approximately 2 h post carrageenan injection. Carrageenan inflammation was produced in a robust concentration dependent manner. Carrageenan hyperalgesia was induced at all concentrations tested and no carrageenan concentration effects were discerned. In a subsequent series of experiments we challenged the carrageenan inflammation model with systemic administration of the opioid agonist morphine, the nonsteroidal anti-inflammatory drug naproxen or the steroidal antiinflammatory drug dexamethasone. Morphine produced a dose dependent attenuation of carrageenan hyperalgesia but had no effect upon carrageenan inflammation. Naproxen produced a moderate attenuation of carrageenan inflammation and hyperalgesia. Dexamethasone dramatically attenuated both carrageenan hyperalgesia and inflammation. Collectively, these experiments characterize the chick carrageenan response and demonstrate the potential of the chick carrageenan inflammation model as a less expensive adjunct model of inflammatory nociception.

Mechanothermal nociceptors in the scaly skin of the chicken leg

Electrophysiological recordings were made from single sensory mechanothermal nociceptive afferent fibres in dissected nerve filaments of the parafibular nerve innervating the scaly skin of the lower leg of the chicken. Two classes of mechanothermal nociceptors were identified consisting of 34 C fibres (conduction velocities 0.45-1.5 m/s, mean 1.08) and nine A-delta fibres (3-15 m/s, mean 6.34). The C fibre afferents had receptive fields which were circular or elliptical in shape and ranged in size from 1 mm in diameter to 4 x 3 mm. Thresholds to mechanical stimulation in the C fibre afferents ranged from 0.3 to 33 g (median 1.5 g) and thermal thresholds were in the range 39-61 degrees C (median 49.4 degrees C). Stimulus-response curves to thermal and/or mechanical stimulation were recorded from 28 C fibre afferents and subjected to a linear regression analysis to determine whether they were best fitted by a linear, log or power function. The results were variable and no single function provided the best fit for all the responses. Of the fibres tested with both stimulus modalities (n=17), only 12 fibres showed the same best fit for both stimuli; in the others the best fit regression lines differed between stimuli. The response of the A-delta fibres to mechanical and thermal stimulation was very similar to the C fibres but the small number of A-delta fibres precluded any detailed statistical analysis. Comparison of the physiological properties of the C fibres in the leg with those previously identified in the beak showed that those in the leg had significantly lower thermal thresholds, but higher mechanical thresholds. The possible functional significance of these differences is discussed. These findings are also discussed in a comparative context to identify similarities and differences between mechanothermal nociceptors in birds and other vertebrates, relating these to their possible evolutionary and functional significance.

Attentional Shifts Alter Pain Perception in the Chicken

In humans, psychological manipulations such as hypnosis, behavioural modifications, relaxation training and cognitive behaviour therapy have all been used to reduce pain intensity. One thing these treatments have in common is selective attention. Work on attention-based cognitive coping strategies has shown that they have potentially useful analgesic qualities in pain therapy. In animals, there have been few studies on the effects of attentional shifts on pain perception. There is extensive literature on stress-induced analgesia and it is likely that, in some of the experiments, attention could be an important variable. This paper will present some of our recent work on selective attention and pain perception using the sodium urate model of gouty arthritis. Birds are naturally prone to articular gout and the model we have developed mimics acute gouty attacks in a single joint. Experimental sodium urate arthritis produces a tonically painful inflammation lasting for at least 3h during which time the animals show pain-related behaviours. Changes in motivation can reduce these pain-related behaviours and it has been hypothesized that these motivational changes act by way of altering the attention of the animal away from pain. The motivational changes investigated included nesting, feeding, exploration and social interactions. The degree of pain suppression ranged from marked hypoalgesia to complete analgesia and as such demonstrates a remarkable ability to suppress tonic pain. These shifts in attention not only reduced pain but also significantly reduced peripheral inflammation. These results are discussed in terms of the limited capacity models of attention.

Parallel analyses of nociceptive neurones in rat superior colliculus by using c-fos immunohistochemistry and electrophysiology under different conditions of anaesthesia

Multiple sensory inputs to the superior colliculus (SC) play an important role in guiding head and eye movements toward or away from biologically significant stimuli. Much is now known about the visual, auditory, and somatosensory response properties of SC neurones that mediate these behavioural reactions. Rather less is known about the responses of SC neurones to noxious stimuli, and thus far, most of this information has been obtained in anaesthetised animals. Therefore, the purpose of the present study was to use the c-fos immunohistochemical technique and standard extracellular electrophysiology as parallel measures of nociceptive activity in the SC under different conditions of anaesthesia. In unanaesthetised animals, experimental and control treatments induced a qualitatively similar pattern of Fos-like immunoreactivity (FLI) in the SC, which was quantitatively related to the severity or biologic salience of the treatment; thus, baseline control < control injections of saline < a nonpainful stressor (immobilisation) < noxious injections of formalin. Compared with baseline levels, urethane and avertin anaesthesia induced FLI expression in the SC intermediate layers, although the FLI response to both noxious stimulation and control conditions was differentially suppressed in different layers of the SC by anaesthesia. Parallel electrophysiologic recordings found that anaesthesia was associated with high levels of spontaneous activity in the SC intermediate layers, often in neurones which were also nociceptive. High rates of background spike activity were also induced in the SC intermediate layers by noxious stimulation in chronically recorded awake animals. Although these results point to some differences between the nociceptive responses of SC neurones in anaesthetised and unanaesthetised animals, both data sets support the view that there are different populations of nociceptive neurones in the rodent SC that may be related to different adaptive functions of pain.