Tachykinin receptors in the equine pelvic flexure

Distribution of the neurokinin-1 receptor in equine intestinal smooth muscle

REASON FOR PERFORMING STUDY

Tachykinins have profound effects on equine intestinal motility, but the distribution of the neurokinin receptors (NKRs) through which they act is unknown. This study reports the distribution of one of these receptors, the neurokinin-1 receptor (NK1R), in smooth muscle throughout the equine intestinal tract.

OBJECTIVES

To quantify the distribution of the NK1R, based upon mRNA expression, in smooth muscle of different regions of the equine intestinal tract.

METHODS

Nine regions of the intestinal tract were sampled in 5 mature horses. Total RNA was isolated from smooth muscle and reverse transcribed; NK1R mRNA was then quantified using real-time PCR.

RESULTS

NK1R mRNA was found at all levels of the sampled intestinal tract. The smooth muscle of the proximal small intestine and the ventral colon exhibited the highest level of NK1R mRNA expression in the equine intestinal tract.

CONCLUSIONS

Tachykinins probably affect intestinal contractility and propulsion in the proximal small intestine and in the ventral colon.

Characterisation of the response of equine digital arteries and veins to substance P

Substance P (SP), a potent vasodilator, has been detected in equine digital sensory-motor nerves. The aim of the study was to characterise the functional responses of equine digital blood vessels to exogenous SP. Pre-constricted equine digital arteries (EDA) and veins (EDV) vasodilated in a biphasic, endothelium- and concentration-dependent manner to SP. A nitric oxide (NO) synthase inhibitor Nomega-nitro-L-arginine methyl ester hydrochloride (L-NAME; 300 microm) inhibited both phases of the relaxation response curve of EDAs to SP by >70%. In EDVs, the first relaxant phase to SP was largely L-NAME-resistant, whereas the second phase was inhibited by 60%. Both L-NAME and a cyclo-oxygenase inhibitor (ibuprofen; 10 microm) were required to inhibit EDV relaxation to SP by > or =80%. Experiments determining the receptor mediated responses to physiological concentrations of SP (1 nm) revealed that the relaxant responses of both EDA and EDV were inhibited by a neurokinin-1 (NK1) receptor antagonist (CP-96 345; 10 nm). In conclusion, SP is an endothelium-dependent vasodilator of both EDA and EDV. NO is the predominant pathway activated in EDA, whereas both prostacyclin and NO pathways are involved in EDVs. NK1 receptors appear to mediate responses to low concentrations of SP.

Treatment with neurokinin-1 receptor antagonist reduces severity of inflammatory bowel disease induced by Cryptosporidium parvum

Inflammatory bowel disease (IBD) is a chronic, debilitating disorder of uncertain and perhaps multiple etiologies. It is believed to be due in part to disregulation of the immune system. Neuroimmune interactions may be involved in induction or maintenance of IBD. In the present study, we examined the potential role of a neurotransmitter, substance P, in a mouse model of IBD. We found that binding sites for substance P, and more specifically, neurokinin-1 receptors, were upregulated in intestinal tissue of mice with IBD-like syndrome. Dosing of mice with LY303870, a neurokinin-1 receptor antagonist, reduced the severity of IBD, and treatment of mice with preexisting IBD allowed partial healing of lesions. We hypothesize that blocking the binding of substance P to the neurokinin-1 receptor interrupts the inflammatory cascade that triggers and maintains intestinal lesions of IBD.

Evaluation of substance P as a neurotransmitter in equine jejunum

OBJECTIVE

To determine whether substance P (SP) functions as a neurotransmitter in equine jejunum.

SAMPLE POPULATION

Samples of jejunum obtained from horses that did not have lesions in the gastrointestinal tract.

PROCEDURE

Jejunal smooth muscle strips, oriented in the plane of the circular or longitudinal muscle, were suspended isometrically in muscle baths. Neurotransmitter release was induced by electrical field stimulation (EFS) delivered at 2 intensities (30 and 70 V) and various frequencies on muscle strips that were maintained at low tension or were under contraction. A neurokinin-1 receptor blocker (CP-96,345) was added to baths prior to EFS to interrupt SP neurotransmission. Additionally, direct effects of SP on muscle strips were evaluated, and SP-like immunoreactivity was localized in intestinal tissues, using indirect immunofluorescence testing.

RESULTS

Substance P contracted circularly and longitudinally oriented muscle strips. Prior treatment with CP-96,345 altered muscle responses to SP and EFS, suggesting that SP was released from depolarized myenteric neurons. Depending on orientation of muscle strips and stimulation variables used, CP-96,345 increased or decreased the contractile response to EFS. Substance P-like immunoreactivity was detected in the myenteric plexus and circular muscle layers.

CONCLUSIONS AND CLINICAL RELEVANCE

Substance P appears to function as a neurotransmitter in equine jejunum. It apparently modulates smooth muscle contractility, depending on preexisting conditions. Effects of SP may be altered in some forms of intestinal dysfunction. Altering SP neurotransmission in the jejunum may provide a therapeutic option for motility disorders of horses that are unresponsive to adrenergic and cholinergic drugs.

Distribution of substance P binding sites in equine airways

Autoradiography with [125I]-Bolton Hunter substance P ([I]-BHSP) was used to detect substance P binding sites in the equine lung. Specific [I]-BHSP binding sites were very dense over small bronchial vessels, tracheobronchial glands and airway epithelium in large and small airways. The density of [I]-BHSP binding sites over airway smooth muscle was much lower than in the preceding tissues. Competition with an excess of either a specific neurokinin 1 receptor agonist, or a specific neurokinin 2 receptor agonist indicated that most specific [I]-BHSP binding sites in the equine lung represent neurokinin 1 receptors. The receptor-mediated effects of substance P in the equine lung are most likely to involve regulation of vascular tone and airway secretions based upon the density of specific [I]-BHSP binding sites in these tissues. Activation of intrapulmonary afferent nerves containing Substance P by noxious stimuli such as inhaled allergens or irritants may lead to increased mucus secretion and decreased airway diameter due to vascular congestion.