Response characteristics of nasal trigeminal nociceptors in Gallus domesticus

Evaluation of Poultry Stunning with Low Atmospheric Pressure, Carbon Dioxide or Nitrogen Using a Single Aversion Testing Paradigm

Simple Summary

The use of gas stunning for poultry in the abattoir is considered preferable from a welfare and ethical perspective since it reduces the need for stressful handling and birds do not need to be separated from each other. Stunning with low atmospheric pressure is thought to be less stressful than the widely used carbon dioxide; however, there are no published studies directly comparing their aversiveness. Here we trained broiler breeders to indicate aversion to either carbon dioxide, low atmospheric pressure or the inert gas nitrogen, by relinquishing a food reward to seek a preferable environment. We found that exposure to carbon dioxide resulted in the rapid cessation of feeding, whereas with low atmospheric pressure and nitrogen, birds continued to eat for longer. We further found that carbon dioxide exposure resulted in more aversion behaviours, such as headshaking and gasping. These findings suggest that both low atmospheric pressure and nitrogen offer a welfare refinement to gas stunning with carbon dioxide in poultry.

Abstract

Low atmospheric pressure stunning (LAPS) has been suggested for use in poultry under 4 kg in the abattoir as a more humane alternative to carbon dioxide (CO₂). However, there are currently no studies offering a direct comparison of the aversion between methods. Here, we trained adult female broiler breeders to relinquish a food reward by moving to another area of the gas chamber in response to aversive stimuli. They were then stunned and subsequently killed using single exposure to either CO₂, N₂, LAPS or medical air as a control. Birds exposed to CO₂ relinquished the food reward the quickest and exhibited gasping and headshaking more than the other groups. LAPS resulted in the quickest time to loss of posture (LOP) and birds in the N₂ group took the longest. Birds exposed to N₂ displayed the longest duration of ataxia of any group; however, they did not show any wing-flapping prior to LOP, unlike the LAPS and CO₂. Collectively these data demonstrate that both LAPS and N₂ are less aversive to poultry than CO₂ and may offer a significant welfare refinement for poultry killed for meat production.

The Mammalian Diving Response: Inroads to Its Neural Control

The mammalian diving response (DR) is a remarkable behavior that was first formally studied by Laurence Irving and Per Scholander in the late 1930s. The DR is called such because it is most prominent in marine mammals such as seals, whales, and dolphins, but nevertheless is found in all mammals studied. It consists generally of breathing cessation (apnea), a dramatic slowing of heart rate (bradycardia), and an increase in peripheral vasoconstriction. The DR is thought to conserve vital oxygen stores and thus maintain life by directing perfusion to the two organs most essential for life-the heart and the brain. The DR is important, not only for its dramatic power over autonomic function, but also because it alters normal homeostatic reflexes such as the baroreceptor reflex and respiratory chemoreceptor reflex. The neurons driving the reflex circuits for the DR are contained within the medulla and spinal cord since the response remains after the brainstem transection at the pontomedullary junction. Neuroanatomical and physiological data suggesting brainstem areas important for the apnea, bradycardia, and peripheral vasoconstriction induced by underwater submersion are reviewed. Defining the brainstem circuit for the DR may open broad avenues for understanding the mechanisms of suprabulbar control of autonomic function in general, as well as implicate its role in some clinical states. Knowledge of the proposed diving circuit should facilitate studies on elite human divers performing breath-holding dives as well as investigations on sudden infant death syndrome (SIDS), stroke, migraine headache, and arrhythmias. We have speculated that the DR is the most powerful autonomic reflex known.

Thermal nociceptive threshold testing detects altered sensory processing in broiler chickens with spontaneous lameness

Lameness is common in commercially reared broiler chickens but relationships between lameness and pain (and thus bird welfare) have proved complex, partly because lameness is often partially confounded with factors such as bodyweight, sex and pathology. Thermal nociceptive threshold (TNT) testing explores the neural processing of noxious stimuli, and so can contribute to our understanding of pain. Using an acute model of experimentally induced articular pain, we recently demonstrated that TNT was reduced in lame broiler chickens, and was subsequently attenuated by administration of Non-Steroidal Anti-Inflammatory Drugs (NSAIDs). This study extended these findings to a large sample of commercial broilers. It examined factors affecting thermal threshold (Part 1) and the effect of an NSAID drug (meloxicam, 5 mg/kg) and of an opioid (butorphanol; 4 mg/kg) (Part 2). Spontaneously lame and matched non-lame birds (n=167) from commercial farms were exposed to ramped thermal stimulations via a probe attached to the lateral aspect of the tarsometatarsus. Baseline skin temperature and temperature at which a behavioural avoidance response occurred (threshold) were recorded. In Part 1 bird characteristics influencing threshold were modelled; In Part 2 the effect of subcutaneous administration of meloxicam or butorphanol was investigated. Unexpectedly, after accounting for other influences, lameness increased threshold significantly (Part 1). In Part 2, meloxicam affected threshold differentially: it increased further in lame birds and decreased in non-lame birds. No effect of butorphanol was detected. Baseline skin temperature was also consistently a significant predictor of threshold. Overall, lameness significantly influenced threshold after other bird characteristics were taken into account. This, and a differential effect of meloxicam on lame birds, suggests that nociceptive processing may be altered in lame birds, though mechanisms for this require further investigation.

The mammalian diving response: an enigmatic reflex to preserve life?

The mammalian diving response is a remarkable behavior that overrides basic homeostatic reflexes. It is most studied in large aquatic mammals but is seen in all vertebrates. Pelagic mammals have developed several physiological adaptations to conserve intrinsic oxygen stores, but the apnea, bradycardia, and vasoconstriction is shared with those terrestrial and is neurally mediated. The adaptations of aquatic mammals are reviewed here as well as the neural control of cardiorespiratory physiology during diving in rodents.

Characterisation of chemosensory trigeminal receptors in the rainbow trout, Oncorhynchus mykiss: responses to chemical irritants and carbon dioxide

Trigeminally innervated, mechanically sensitive chemoreceptors (M) were previously identified in rainbow trout, Oncorhynchus mykiss, but it is not known whether these receptors are responsive only to noxious, chemical irritants or have a general chemosensory function. This study aimed to characterise the stimulus–response properties of these receptors in comparison with polymodal nociceptors (P). Both P and M gave similar response profiles to acetic acid concentrations. The electrophysiological properties were similar between the two different afferent types. To determine whether the receptors have a nociceptive function, a range of chemical stimulants was applied to these receptors, including non-noxious stimuli such as ammonium chloride, bile, sodium bicarbonate and alarm pheromone, and potentially noxious chemical irritants such as acetic acid, carbon dioxide, low pH, citric acid, citric acid phosphate buffer and sodium chloride. Only irritant stimuli evoked a response, confirming their nociceptive function. All receptor afferents tested responded to carbon dioxide (CO2) in the form of mineral water or soda water. The majority responded to 1% acetic acid, 2% citric acid, citric acid phosphate buffer (pH 3) and 5.0 mol l–1 NaCl. CO2 receptors have been characterised in the orobranchial cavity and gill arches in fish; however, this is the first time that external CO2 receptors have been identified on the head of a fish. Because the fish skin is in constant contact with the aqueous environment, contaminants with a low pH or hypercapnia may stimulate the nociceptive system in fish.

Nasal tactile sensitivity in allergic rhinitis

CONCLUSION

These preliminary data show a decrease in nasal tactile sensitivity and point out interesting aspects of the nasal chronic inflammatory condition in allergic rhinitis.

OBJECTIVES

The aim of this study was to evaluate the effects of allergic rhinitis on nasal tactile sensitivity during the intercritical period.

METHODS

A total of 70 patients aged between 18 and 67 years (average 42 years), with a positive history of allergy caused by seasonal outdoor allergens, were included (group A). Patient outcome was assessed by the nasal monofilament test: a set of 20 Semmes-Weinstein monofilaments was used to detect nasal sensitivity for both nasal cavities. The sensitivity threshold was recorded as the minimum monofilament size with which patients could detect at least two of three stimuli.

RESULTS

When compared to the control group (group B), subjects in group A required a significantly (p < 0.05) higher stimulus to trigger a touch response in the monofilament test, for both the inferior (195.1 ± 0.39 mg vs 67.7 ± 0.19 mg) and middle turbinate (108.7 ± 0.23 mg vs 67.7 ± 0.19 mg).

Avian chemoreception: an electrophysiological approach

The first detailed physiological evidence for olfactory and trigeminal chemoreception in an avian species is provided by a series of investigations in the chicken (Gallus domesticus). Initial work indicated that the activity of avian olfactory bulb neurones closely resembles that of other vertebrates, exhibiting variable spontaneous temporal firing patterns with mean firing rates between those reported for mammals and reptiles. Application of odors directly to the olfactory epithelium showed that like mammals, avian olfactory bulb neurones respond in the form of inhibition and excitation with accompanying changes in temporal firing pattern. When exposed to a range of concentrations of a single odor, all responsive neurones exhibited an ability to discriminate small step-changes in concentration producing clear stimulus response relationships. Avian trigeminal chemoreception was also investigated by examining the responses of single mucosal receptors in the nasal cavity and palate. Slowly and rapidly adapting nasal mechanoreceptors were identified, some of which exhibited chemical sensitivity when exposed to ammonia gas, acetic acid vapor or carbon dioxide. These results demonstrate that polymodal nociceptors are present in avian nasal mucosa and represent the first attempt in any species to quantify the responses of single trigeminal receptors to a range of concentrations of noxious airborne chemicals. Collectively, the findings demonstrate how an electrophysiological approach can improve our understanding of the underlying sensory physiology relating to avian perception of the chemical environment.

Behavioral correlates of olfactory and trigeminal gaseous stimulation in chickens, Gallus domesticus

Despite growing evidence of avian olfactory abilities, there are few reports of behavioral correlates of chemosensory stimulation in birds. The present study aimed to determine how the behavioral responses of hens to selected gases might be affected by input from different chemosensory systems. We also hoped to relate electrophysiological thresholds previously measured in our laboratory to behavioral evidence of perception. Immediate behavioral responses to ascending series of short (7 s) pulses of olfactory (hydrogen sulphide), trigeminal (carbon dioxide) and combining (ammonia) gaseous stimulants were measured in 12 partially restrained adult hens (Gallus domesticus) using a purpose built gas delivery system. The concentration ranges applied matched those used in our previous electrophysiological studies of olfactory bulb responses. Consistent and specific behavioral responses were observed in response to stimulation with each gas. While significant increases in mandibulation (a distinctive rapid bout of bill movements), interruption of ongoing behavior, orientation to the end of the stimulus, and struggling were seen during stimulation with all three gases, orientation towards the stimulus occurred only in response to the olfactory stimulants, ammonia and hydrogen sulphide. The only gas to elicit significant avoidance was hydrogen sulphide, while gasping and headshaking were elevated in response to stimulation with carbon dioxide. Approximate threshold values for some types of behavior related reasonably well to receptor thresholds determined electrophysiologically, but perception may have occurred at lower concentrations without overt behavioral consequences. The diversity of the behavioral responses observed supports the notion that the gases selectively stimulated different sensory pathways.

Putative nociceptor responses to mechanical and chemical stimulation in skeletal muscles of the chicken leg

Electrophysiological responses of nociceptive sensory afferent fibres in the skeletal muscle of the chicken (Gallus domesticus) were examined using mechanical and chemical stimulation. The activity of single nociceptive afferent fibres was recorded from micro-dissected filaments of the fibular and lateral tibial nerves, which innervate the fibularis longus and lateral gastrocnemius muscles. Seventeen putative nociceptive fibres were identified by mechanical stimulation (muscle compression). Conduction velocities (CVs) ranged from 2.8 to 11.3 m/s (mean 5.8; S.E.M.+/-0.9 m/s). Response thresholds to tissue compression ranged from 38 to 126 kPa (mean 81; S.E.M.+/-4 kPa). Increases in pressure intensity, above individual fibre thresholds (x2 moderate; x3 noxious), produced intensity dependent increases in discharge rates. Fibres exhibited slowly adapting, irregular discharges lasting the duration of the stimulus and showed no spontaneous activity in the absence of mechanical stimulation. Intramuscular injection of acetic acid (1% v/v in isotonic saline; pH 2.8) in to the receptive field area stimulated discharge activity in 13 of the 17 (76%) pressure sensitive fibres. Acid injection resulted in prolonged irregular single or intermittent clustered discharges, which continued beyond the 15-min recording period. This study demonstrates the existence of nociceptive sensory fibres in chicken skeletal muscle that are able to respond to and encode acute tissue threatening and subjectively painful stimuli. The physiological characteristics of these nociceptive afferents are consistent with mammalian group III skeletal muscle nociceptors. These findings support the suggestion of a common, acute nociceptive response function in skeletal muscle in avians and other vertebrate classes.

Mechano-chemical nociceptors in the avian trigeminal mucosa

This study characterised and compared the properties of chemically sensitive nociceptors in the avian nasal and oral epithelia recorded from either microelectrode stabs of the trigeminal ganglion (nasal mucosa) or microdissected twigs of the nasopalatine nerve (palatine mucosa) in urethane anaesthetized hens. A total of 146 slowly and rapidly adapting nasal and palate mechanically sensitive receptors were identified exhibiting von Frey thresholds to mechanical stimulation ranging from 0.2 to 8.3 g (mean 3.21+/-2.28 g, n=39) in the nasal cavity and <0.1 to 15 g (mean 2.17+/-2.69 g, n=62) in the palate. Receptive fields were situated throughout the nasal cavity and palate. A subset of receptors (classified as polymodal nociceptors) exhibited chemical sensitivity when exposed to noxious levels of ammonia gas, and some polymodal afferents also responded to acetic acid vapour or carbon dioxide exposure. Responses to ammonia were characterised by delayed onset, rapid prolonged discharge patterns usually outlasting the chemical stimulation. Clear concentration-response relationships were apparent for chemically receptive afferents in both the nasal and oral cavities, although curve shape varied considerably between receptors. Statistical comparisons of the responses of polymodal nociceptors in each mucosal region revealed no significant difference between their mechanical thresholds, but nasal afferents exhibited lower chemical (ammonia) thresholds and higher maximal firing rates. These response characteristics highlight distinct sensitivities to potentially painful chemical stimulation between epithelial areas. These effects may be due to the inherent properties of the nociceptors in each region but are also likely to reflect the role of the perireceptor environment in mediating peripheral nociception.

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.

Adaptation responses of single avian olfactory bulb neurones

Adaptation in the avian olfactory bulb (OB) was examined by recording the activity of single OB neurones in the chicken (Gallus domesticus) during prolonged (120 s) odour exposure (20 ppm ammonia). The activity of each neurone before, during and after odour stimulation was investigated with analysis examining changes in firing rate and pattern. Of the 73 OB neurones recorded, 22 responded to ammonia stimulation with either inhibition or excitation of firing (both 50%). Their responses then either adapted (lessening of the response, 32%) or sensitized (heightening of the response, 68%) during the odour stimulus period. Both inhibited and excited units underwent adaptation or sensitization and their responses were consistent during odour stimulation. These results demonstrate the role of OB neurones in adaptive responses of the avian olfactory system during prolonged odour stimulation.