Direct Evidence for Involvement of Substance P in Tear Secretion After Antidromic Electrical Stimulation of Rat’s Trigeminal Ganglion

Abstract

Abstract: : Purpose: In our previous studies we showed that antidromic stimulation of rat’s trigeminal ganglion influences tear secretion. The aim of this study was to present direct evidences that sensory neuropeptides (primarily substance P) released from nerve endings are responsible for these effects. Methods: Following unilateral electrical stimulation of the trigeminal ganglion tears were collected at both sides and processed for volume measurements and gel electrophoresis combined with luminol chemiluminescence with immunostaining to IgA and lysozyme. Animals were pretreated with physiologic solution (group A); with hexamethonium to block retrograde synaptic transmission in ganglia (group B); with SR140333, a selective neurokinin–1 (NK–1) receptor antagonist to block substance P mediated responses (group C). Effects of inadequate electrode position or incidental lesion of trigeminal ganglion was examined by placing the electrode in false position, or no stimulation at a correct position (Group D). Results: In group A following stimulation with increasing number of electrical impulses (300–600–1200–2400 pulses) secreted tear increased to 17.6±0.93 mm (r=0.69, p=0.0007) on the stimulated side. Hexamethonium pretreatment had no inhibiting effect (17.13±1.11 mm, p>0.05), but SR140333 pretreatment inhibited the increase in tear secretion significantly (8.75±1.09 mm, p<0.0001), however tear secretion remained significantly elevated comparing to the control side (4.00±0.38 mm). In group D no increase in tear secretion was measured (4.25±0.31mm, p>0.05). In group A,B and C similar electrophoretic patterns were observed on both sides and chemiluminescence showed increase in the amount of lysozyme and IgA (p<0.05). Conclusions: Ganglion blockage didn’t influence the increased tear secretion providing direct evidence of the exclusive involvement of peripheral sensory pathways of the trigeminal nerve. The strong correlation between the number of impulses and tear secretion suggests that the process is due to the release of neuropeptides from sensory nerve endings. The significantly inhibited tear secretion after antagonizing substance P effect on NK–1 receptors provides direct in vivo evidence for involvement of substance P in tear secretion, although other sensory neurotransmitters may also be involved.

Keywords: cornea: tears/tear film/dry eye • neuropeptides • conjunctiva