Evaluation of substance P as a neurotransmitter in equine jejunum.

The research article explores the role of substance P (SP) as a neurotransmitter in equine jejunum, revealing its potential involvement in modulating smooth muscle contractility. It also suggests that altering SP neurotransmission may provide therapeutic options for motility disorders in horses unresponsive to adrenergic and cholinergic drugs.

Methods

• The researchers obtained samples of jejunum from horses with no gastrointestinal tract lesions.
• The jejunal smooth muscle strips were suspended isometrically in muscle baths. These baths helped to investigate the role of SP in muscle contraction.
• To stimulate the release of neurotransmitters, they used electrical field stimulation (EFS) of two different intensities (30 and 70 V) and varied frequencies on muscle strips, which were either maintained at low tension or under contraction.
• A neurokinin-1 receptor blocker (CP-96,345) was added to the baths before EFS to interrupt SP neurotransmission. This was done to identify the role of SP in muscle contraction.
• The direct effects of SP on muscle strips were assessed, and SP-like immunoreactivity was evaluated in intestinal tissues using indirect immunofluorescence testing. This technique allowed the researchers to visually detect SP on a microscopic level in the tissue samples.

Results

• SP was found to contract circularly and longitudinally oriented muscle strips implying its role in muscular contractions.
• The use of CP-96,345 resulted in altered muscle responses to both SP and EFS. This suggests the release of SP from depolarized myenteric neurons, indicating its role as a neurotransmitter.
• Depending on the muscle strips’ orientation and the variables used in stimulation, CP-96,345 either increased or decreased the contractile response to EFS revealing that SP can modulate smooth muscle contraction.
• SP-like immunoreactivity was detected in the myenteric plexus and circular muscle layers, supporting its identified role in these tissues.

Conclusions

• The outcomes of the research strongly suggest that SP functions as a neurotransmitter in the equine jejunum.
• SP was found to modulate smooth muscle contractility, with its effects varying based on preexisting conditions.
• The results also suggest that in certain forms of intestinal dysfunction, the effects of SP might be altered which may influence the course of the disease or the therapeutic approach.
• The discovery of SP’s role in the jejunum could provide potential new therapeutic options for motility disorders in horses, particularly those that are unresponsive to adrenergic and cholinergic drugs.