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Juana Gallar, Baldemar Santiago, M Carmen Acosta, Carlos Belmonte; In vivo Functional Characterization of Trigeminal Neurons Innervating the Eye and Periocular Tissues. Invest. Ophthalmol. Vis. Sci. 2014;55(13):3645.
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© ARVO (1962-2015); The Authors (2016-present)
The aim of this work was to study in vivo the firing characteristics of the different functional classes of trigeminal primary sensory neurons innervating cornea, sclera, bulbar and palpebral conjunctiva, and eyelids.
Anesthetized Wistar male rats (300±25g) were placed in a stereotaxic frame and electrical activity of trigeminal ganglion (TG) neurons was recorded extracellularly with a tungsten electrode (2MΩ) introduced into the TG. Heart rate, expiratory CO2, SpO2 and rectal temperature were monitored. Electrical signals were amplified (1000x), filtered (300Hz, 10KHz) and recorded at 20KHz using an ADC interface and software for off-line analysis. Mechanical stimulation of the ipsilateral cornea and surrounding tissue was performed using a fine brush and calibrated von Frey filaments, mechanical threshold was measured and receptive field (RF) mapped. CO2 gas puffs and 20µl drops of 100µM menthol applied onto the ocular surface were used for chemical stimulation. Additionally, the corneal surface was subjected to variable conditions of humidity, and the receptive fields of the units were stimulated electrically (0.1-2ms, 15V) to measure the conduction velocity of the afferent fiber.
Neurons innervating the superior eyelid (11), the inferior eyelid (3), the cornea (5) and the conjunctiva (7) were identified. Neurons with RFs covering the eyelids were classified as mechanoreceptive by their response to punctate, stretching and pulling mechanical stimuli; their mechanical threshold varied from 0.07 to 1.56 mN in the hairy and shaved skin respectively. Neurons with corneal and conjunctival RFs showed ongoing activity, increasing their firing to decreased temperature, augmented evaporation, CO2 puffs and menthol drops, being classified as cold thermosensory neurons. Part of the corneal and conjunctival neurons responded only to punctate stimuli (threshold around 0.07mN).
These results proved the feasibility of using high impedance tungsten electrodes for stable, long-lasting recordings in vivo of the different functional classes of TG neurons innervating the ocular surface, thus opening the possibility of analyzing their activity under different experimental conditions.
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