When the increased activity of the HPA axis is documented, we still have to interpret the results in the context of environmental influences on HPA axis activity and individual differences, especially when no longitudinal data are available from the same animals, such as in epidemiological studies. First, environmental factors may have a strong influence on HPA axis activity, as described earlier.go to link
Neuroendocrine control of fertility | Centre for Applied Anatomy | University of Bristol
The study of the influence of temperature and humidity is rather impressive in this respect Marple et al. Second, large individual variations in HPA axis activity have been documented in pigs. In a pioneering series of studies, Hennessy and collaborators have shown that the adrenal response to ACTH is an individual characteristic, reproducible across successive testing, and inheritable Hennessy et al. The intensity of the adrenal response to ACTH was found to be negatively correlated with body weight and growth rate Hennessy and Jackson, Several lines of evidence confirm the genetic influences on HPA axis activity.
Large differences can be found between porcine breeds Bergeron et al. Individual variations can also arise from environmental influences, either during pregnancy and early post-natal life or as a result of previous experience. These early influences have been extensively studied in laboratory animals and offer new approaches to control for emotional reactivity, including neuroendocrine responses Meaney et al. In conclusion, the HPA axis is primarily involved in the regulation of energy fluxes in the body, and is therefore sensitive to various environmental stimuli challenging the energy balance of the body, such as nycthemeral influences, food intake and temperature regulation.
Furthermore, plasma cortisol levels are exquisitely sensitive to a whole range of stimuli, which do not need to be intense to maximally activate cortisol release, and altogether known as stressors. Therefore, monitoring plasma cortisol levels may give valuable information about the status of the animal with regard to its internal and external environment but must be interpreted with caution in terms of welfare.
For long lasting stimulations, dynamic testing of the HPA axis stimulation or suppression tests are necessary to demonstrate the resetting of the system as an adaptation to sustained stimulation, and the data have to be interpreted in the context of environmental influences and individual variation of genetic or acquired origin.
The autonomic nervous system. The ANS regulates the function of all the internal organs of the body including the cardiovascular, respiratory and digestive systems, and energy fluxes. In stress responses, the orthosympathetic part catecholaminergic of the system is primarily involved, although the parasympathetic cholinergic part may also be activated, for instance in the effect of stress on the digestive tract.
Noradrenaline is released at the nerve endings of the sympathetic nervous system in the target organs, and both adrenaline and noradrenaline are released in the general circulation from the medullary part of adrenal glands. The differential activity of both catecholamines on their receptors and the differential distribution of the different receptor types and subtypes in tissues shape the specificity of their actions. Cannon published in As a response to cold for instance, noradrenaline is preferentially released.
It induces peripheral vasoconstriction energy saving mechanism and energy mobilization, primarily from fat tissues. As a response to hypoglycemia on the other hand, adrenaline is released from the adrenal medulla as an effective mechanism to release glucose from liver and muscle glycogen.
In case of acute, non specific challenge, such as in response to a strong emotional stimulus, a general activation occurs with an increase of blood pressure and cardiac pulse rate, respiratory frequency, energy mobilization with an increase in plasma free fatty acids and glucose. Assessment of acute sympathetic responses. There are several limitations to this approach. Plasma levels of catecholamines are extremely sensitive to handling and the response can be detected within seconds, since the transmission of information, via neurons, is extremely fast, as compared to the HPA axis for instance there is a time lag of approx.
It is therefore illusory to measure basal catecholamine levels in plasma collected by direct venous puncture, and chronic catheter must be implanted for experimental purposes. On the other hand, the assay techniques to measure catecholamines in blood necessitate specific equipment and know-how and are not easily accessible to any biology laboratory.
This approach is therefore limited to physiological investigations. However, considering the time scale of the processes involved in welfare studies, this resolution may be largely sufficient.
PHYSIOLOGICAL and REPRODUCTIONAL ASPECTS OF ANIMAL PRODUCTION
In many cases, acute sympathetic responses have been studied by monitoring its physiological effects such as heart rate, blood pressure, plasma glucose and free fatty acid levels. The interest of such a parameter like heart rate is that it can be continuously monitored at distance by remote transmission of the physiological signal or stored by simple portable devices. It therefore allows a very sharp second to second analysis of the response to complex stimuli like transportation for instance. Therefore data will have to be interpreted in this context.
On the other hand, metabolic variables are frequently used in studies related to handling of animals before slaughter mixing, transportation, duration and conditions of lairage, duration of food withdrawal. Plasma glucose and free fatty acids levels reflect the balance between the mobilization of energy stores and the use of energetic metabolites, primarily by muscular activity.
Lactic acid levels reflect the intensity of anaerobic metabolism. These metabolic parameters are frequently associated with the measurement of circulating activity of enzymes of intracellular origin, such as transaminases and creatine kinase CK that reflect cell suffering. CK has been largely used to detect stress susceptibility in pigs Guise et al. Metabolic adaptation, stress, welfare and ANS. The data collected recently in the study of the response of piglets to early weaning illustrates several critical aspects of the use of physiological parameters to assess welfare Hay et al.
The selection of hyperprolific sows results sometimes in an excessive number of piglets that can be saved, when there is no possibility of adoption by other dams, by early weaning at 5 days after ingestion of colostrum. One question is whether this practice is acceptable in terms of animal welfare since these young animals are not prepared to ingest significant levels of solid food before the 3rd-4th week when the piglet is fed by the dam. Indeed, early weaning induces an early and long-lasting reduction in growth rate. Urine was collected from the piglets before weaning and at regular intervals over the two following weeks.
Cortisol levels were elevated in urine the day after weaning but were back to control levels at day 5 after weaning. This activation of the HPA axis by weaning has been shown in several studies using plasma cortisol levels, whatever the age at weaning. Since it is only short lasting, we could conclude that, although early weaning induces a typical stress response, stress is short lasting and the animal is adapted after a few days.
However, the measurement of catecholamine levels in urine gives a completely different view of adaptation processes. Indeed, early weaning induces an early and profound reduction of the levels of noradrenaline, that do not return to control levels before the end of the second week after weaning, as well as a delayed measurable on the ninth day after weaning but sustained reduction of adrenaline levels.
How can we interpret these changes? By considering the metabolic roles of catecholamines. As shown before, noradrenaline produces heat by burning lipids.
The noradrenergic system is therefore activated by cold to keep core temperature constant, and by excess caloric intake to regulate body weight. In case of food shortage, the switch off of the noradrenergic system is an energy saving mechanism Stefanovic et al. Indeed, the reduction of metabolic heat production is compensated for by behavioural adaptation mechanisms since the early weaned piglets spend more time under the heating lamp than control animals Orgeur et al. On the other hand, adrenaline is able to mobilize glycogen stores and its sustained release during the first days after weaning is therefore adapted to the reduction of food intake in order to maintain blood glucose levels.
What may be conclusions with regard to the assessment of welfare? That cortisol gives a limited and therefore biased view of adaptive processes. Although cortisol excretion levels were back to control levels a few days after early weaning, the profound and long lasting changes in catecholamine excretion levels are indicative of a sustained taxing of adaptive processes, primarily related to the deficit of food intake. Since this experiment was designed primarily to study molecular characteristics of muscles in relationship with meat quality, the animals were managed with as little stress as possible.
They were not mixed from different pens, the truck was driven smoothly, and the animals were handled gently. Urine was collected on farm basal conditions , upon the arrival in the slaughterhouse in the evening after 10 hours of transportation from the breeding farm and the next morning before slaughter. Data for the Large White and Duroc pigs are shown in figure 5 and compared to the values measured in urine collected from the bladder after slaughter in a group of approx.
First, these data illustrate the breed differences in neuroendocrine functioning both basal and in response to stimulation. Second, although urinary cortisol levels are in the same range after transportation in optimal conditions and after slaughter of animals handled more conventionally, large differences are measured for adrenaline and more so for noradrenaline that is hardly increased by smooth handling and transportation of the animals.
These two examples show that taking into consideration the ANS response to challenges widens our understanding of adaptive processes. The importance of metabolic factors. Although we showed previously their influence in the regulation of HPA axis activity, they are even more important for the functioning of the ANS see also Hay et al. One noticeable point to stress out is the specificity of metabolic influences on adrenaline and noradrenaline.
It shows that the ANS is not activated as a whole single system but differentially according to the specific action of adrenaline and noradrenaline. The differential sensitivity of cortisol, adrenaline and noradrenaline as measures of stress. The noradrenaline response appears to be the least sensitive, adrenaline being intermediate. Therefore monitoring the excretion of catecholamines gives not only qualitative but also quantitative information about the adaptive response.
The knowledge of allostatic processes in the ANS is rather limited in pigs. A few studies only have been done in pigs Stanton and Mueller, ; Roberts et al. Introduction 2. Neuroendocrine systems 3. The hypothalamus, located at the base of the diencephalon, is in a good position to integrate information about the state of the organism from diverse sources. It receives information about the external environment from forebrain sensory systems and about noxious stimuli from nociceptors ascending in the spinal cord and brainstem Chapter The hypothalamus is activated by subcortical circuits that process emotional responses such as fear, reward, and disgust eg, from ingesting noxious food , and receives information Forgot Password?
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