This thesis describes investigations of small intestinal motility in horses with
and without equine grass sickness using an in vitro technique. Equine grass sickness is
a disease of horses of unknown aetiology, characterised by dysfunction of the
autonomic nervous system, primarily affecting the alimentary tract. Examination of
autonomic ganglia taken from horses affected with grass sickness shows evidence of
neuronal degeneration and ultimately depletion of cell numbers.
The in vitro technique implemented used strips of intestinal smooth muscle cut
parallel to the longitudinal muscle layer from the duodenum and ileum. Tissue was
taken from control horses and those affected with the three clinical forms of grass
sickness (acute [AGS], subacute [SAGS] and chronic grass sickness [CGS]). Motility
patterns were measured isometrically using strain gauge transducers and recorded
onto a Washington ink writing oscillograph. Contraction rate and amplitude,
alterations in tone (baseline) and the latency before a response to pharmacological
agents were recorded.
The characteristics of the background contractions were established. In the
control group the duodenal preparations had a significantly higher contractile rate
than ileal preparations (P<0.05). There was no significant difference in contractile
amplitude between the two regions. The contractile rate was reduced in grass sickness
cases, although not always significantly. The effect of storage for 24 hours at 4"C was
investigated to determine whether stored control tissue would subsequently behave
like fresh grass sickness tissue: it was concluded that this was not the case.
Physostigmine was used to establish the viability of enteric cholinergic
neurones and to test their capacity to release endogenous acetylcholine. All muscle
strips from both control horses and those affected with grass sickness showed
significant increases in the rate of contractions following physostigmine addition
(P<0.05 or less). The latency before a response to physostigmine in the AGS and
SAGS groups was significantly larger than for the control groups in both regions of
the gut (PcO.OOl for duodenal tissue, P<0.05 for ileal tissue). The effect of storage on
responses to physostigmine confirmed that neuronal cell death takes place during this
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storage period. Grass sickness tissue could no longer produce a significant increase in
contractile rate following physostigmine. Dose response curves to bethanecol were
constructed for control, AGS and CGS tissue. For the AGS group there was a
leftward shift of the dose response curve and a reduced ED50 value, which suggested
that the tissue has become supersensitive (denervation hypersensitivity). However,
there were no significant differences between the tissue sensitivity of the CGS and the
control groups.
Cisapride is a prokinetic drug which has been used with some clinical success
in the treatment of selected chronic grass sickness cases. Experiments using cisapride
indicated there was an increase in the contractile rate in control and CGS duodenal
muscle strips (P<0.06) and in control, AGS and CGS groups in ileal regions (P<0.06).
Cisapride also caused an increase in the amplitude of contractions in ileal muscle strips
taken from control horses (P<0.06).
In agreement with other workers, it was found that in vitro equine small
intestine contracts oa addition of noradrenaline or adrenaline. This contractile
response was found to be due to excitatory a2 receptors on the smooth muscle
membrane. Grass sickness affected tissue responded similarly to control tissue in the
duodenal region, however, in certain AGS and SAGS cases no contractile response to
noradrenaline could be achieved until tissue was pretreated with the (3 antagonist
propranolol. As cold storage had no significant effect on the contractile response to
noradrenaline it would suggest the response was independent of nervous elements
A subjective histological scoring method was applied on sections of small
intestine adjacent to those used in pharmacological experiments, to investigate
neuronal cell number, size and the proportion of abnormal cells in the myenteric and
submucous plexuses. There was a significantly lower score for neuronal cell number
for the AGS and CGS groups compared with control values (P<0.05). The evidence
suggested that in CGS cases the largest enteric neurones were preferentially affected
by grass sickness (P<0.05). In the ileal AGS group, the proportion of abnormal cells
was significantly greater than both the control and CGS ileal groups (P<0.05). In the
duodenum there was no significant difference between the proportion of abnormal
cells between the AGS and CGS groups, although they were both awarded
significantly higher scores than the control group (P<0.05).
