Abstract
Lambs castrated and tail-docked (c+td) by tight rubber ring experience severe acute
pain lasting up to 3 hours. The use of behaviour to assess the severity of this pain has
been validated. Within 5-7 days of application of rubber rings to the neck of the
scrotum and the tail, chronic inflammatory lesions form at the sites of the rings.
These lesions take 6-7 weeks to heal and can become severely infected. The presence
of these chronic inflammatory lesions has been associated with the infrequent
expression of the behaviours used to assess acute pain from c+td. It has been
proposed that the occurrence of these behaviours is indicative of chronic
inflammatory pain, but the use of behaviour has not been validated for the
assessment of this pain. The aim of this thesis was to test the following hypotheses:
1. Lambs undergoing castration and tail-docking by tight rubber ring
experience chronic inflammatory pain for up to six weeks, in association
with chronically inflamed lesions;
2. The chronic inflammatory pain experienced by lambs is sufficient to induce
changes in their behaviour;
3. Quantification of these behavioural changes constitutes a valid measure of
the chronic inflammatory pain experienced by rubber ring castrated and taildocked lambs.
Two methods were used to determine the presence of chronic inflammatory pain.
The analgesic self-administration paradigm has been used as a means of investigating
the experience of chronic inflammatory pain in rats suffering from adjuvant induced
arthritis (AA). Similarly, in the studies reported in this thesis, the ability of RR c+td
lambs to self-administer a NSAID was used to determine the presence of chronic
inflammatory pain. Up-regulation of the synthesis of AVP and reduction in the
synthesis of CRH in the parvocellular portion of the hypothalamic paraventricular
nucleus (pPVN) has also been detected in rats with AA. Using in situ hybridisation
histochemistry, evidence of such changes in the control of the HPA axis were sought
in the present studies, to confirm the presence of chronic inflammatory pain in RR
c+td lambs. Throughout the self-administration and neurohistochemical studies, the
severity of chronic inflammatory lesions and the expression of potential 'pain' behaviours were recorded to determine the significance of these measures with
respect to chronic inflammatory pain. Evidence of a relationship between lesion
severity and the magnitude of the change in behaviour was also sought.
The studies reported in this thesis further support the relationship between
assessments of lesion severity and quantified changes in behaviour that have been
associated with acute pain. The validity of the use of subtle changes in posture and
changes in feeding motivation to quantify chronic inflammatory pain was supported
as the consumption of analgesic creep feed resulted in elimination of the abnormal
expression of these behaviours. Some evidence that c+td lambs select more analgesic
feed was also present in these studies, although the responses were not reliable
within and between individuals. No evidence was found of changes in the expression
of hormones in the HPA axis that are characteristic of chronic inflammatory pain in
rats.
Definitive evidence of the experience of chronic inflammatory pain was not provided
by either the self-administration paradigm or the neurohistochemical studies. The use
of changes in behaviour to assess chronic inflammatory pain must still be validated,
but it is proposed that the incidence of abnormal lying and changes in feeding
motivation are reliably associated with the severity of chronic inflammatory pain.
It is possible that the unchanged expression of AVP and CRH mRNA in the pPVN is
a reflection of differences in the mechanisms of control of the chronically activated
HPA axis in the rat and the sheep. It is also considered likely that lambs were unable
to learn about the pain relieving properties of the analgesic creep feed, because the
study did not provide the appropriate opportunity to learn the association. It is
suggested that lambs may be able to control their experience of pain by adopting
changes in posture and through activation of local endogenous analgesic
mechanisms, thus avoiding the need for activation of the pPVN or other central
mechanisms and precluding the need for the lamb to select an analgesic treatment.
The evidence supports the assertion that chronic inflammatory lesions from RR c+td
are painful, a conclusion that can be used to further oppose of the use of RRs
commercially. The potential use of RR c+td lesions as a model of chronic
inflammatory pain was demonstrated.
