Light microscopy of the enteric nervous system of horses with or without equine dysautonomia (grass sickness): its correlation with the motor effects of physostigmine.

The researchers in this study inspected the enteric nervous system (a network of nerves in the gut) in horses affected by acute or chronic grass sickness using light microscopy and studied the correlation between nerve damage and changes in gut motility in response to the drug physostigmine.

Experimental Methodology

• The researchers used light microscopy to examine the enteric nervous system in tissue sections from two parts of the digestive tract – the caudal flexure of the duodenum and the terminal ileum – in fifteen horses. These animals were divided into three groups: a control group of healthy horses, those diagnosed with acute grass sickness (AGS), and those with chronic grass sickness (CGS).
• They scored the extent of neuronal loss based on a subjective scoring system.
• To study gut motility, the research team measured the rate and frequency of tissue contractions isometrically in vitro (outside the body). They tested how the tissue responded to physostigmine, a drug known to stimulate gut motility by increasing the activity of cholinergic neurons.

Findings and Interpretations

• Compared to the healthy group, the horses with AGS showed the most severe loss of neurons, particularly those in the ileal submucous plexus (a network of nerves found in the intestinal wall).
• The proportion of neuron abnormalities in the AGS group was similar across all tested areas, whereas the number of abnormal cells in the CGS group was significantly higher in the duodenal region than in the ileal region.
• After exposure to physostigmine, the increase in the rate of contractions was observed to be the greatest in the AGS group, suggesting more severe functional damage.
• The longest latency period (the time it took for the tissue to respond to the drug) was also found in the AGS group, an indication of a lower number of active cholinergic neurons, the type that physostigmine stimulates.

Conclusions

• This research confirms that grass sickness in horses causes extensive damage to the enteric nervous system, more strongly in cases of AGS than CGS.
• The study also underlines a robust correlation between the functional responses of cholinergic neurons to physostigmine and the degree of neuronal degeneration.

These findings provide valuable insights into the pathological effects of grass sickness and could help identify potential treatment strategies for the disease based on the activity of cholinergic neurons.