Response of sensory units with unmyelinated fibres to mechanical, thermal and chemical stimulation of the cat's cornea.

Abstract

SUMMARY1. In the cat anaesthetized with sodium pentobarbitone, electrical activity was recorded from single unmyelinated sensory fibres innervating the cornea.2. Based on their response to mechanical (calibrated aesthesiometer), chemical (10 mm acetic acid or 616 mm NaCl) and thermal (cooling from 35 to 5°C; heating to 51°C) stimuli, corneal unmyelinated fibres were classified as polymodal (71 %) or 'cold' nociceptors (29 %).3. Polymodal units responded to mechanical indentation of the cornea and developed fatigue after repeated stimulation. They were excited by temperatures over 37°C and exhibited sensitization to repeated heating.4. Corneal polymodal units were also activated by topical application of 10 mM acetic acid and hypertonic NaCl (616 mM). Capsaicin (0 33 mM) elicited a discharge of impulses that was followed by inactivation to mechanical, chemical and thermal stimuli.5. 'Cold' nociceptors had small receptive fields, preferentially located at the periphery of the cornea. They were excited by small temperature decreases of the corneal surface in a range between 30 and 8°C, but were not responsive to noxious heat.6. 'Cold' nociceptors encoded temperature changes between 35 and 23 'C. The discharge was proportional to the velocity of the temperature drop; sustained temperatures were not signalled by changes in static frequency values. 'Cold' nociceptive fibres responded to hypertonic NaCl (616 mM) and weakly to 10 mM acetic acid. Capsaicin (0 33 mM) first excited and then inactivated 'cold' nociceptors.7. Thermoreceptive fibres were found in the episclera. They fired in bursts and responded to small temperature decreases, but were insensitive to irritant chemicals and capsaicin.

Contribution of TRPM8 Channels to Cold Transduction in Primary Sensory Neurons and Peripheral Nerve Terminals

“…Thereafter, we used BCTC to explore the contribution of TRPM8 to cold-evoked electrical activity in the soma and peripheral nerve terminals of trigeminal ganglion (TG) neurons. Peripheral studies were performed in the cornea, a tissue with a dense innervation of cold thermoreceptors (Gallar et al, 1993;Carr et al, 2003). We conclude that, in the soma of the large majority of cultured trigeminal neurons, activation of TRPM8 channels is the major player in establishing cold-evoked responses.…”

Calcium responses of chicken trigeminal ganglion neurons to methyl anthranilate and capsaicin

“…Moreover, the responses of thermonociceptive neurons of both taxa were antagonized by capsazepine, an antagonist of the vanilloid receptor, VR1 (Marin-Burgin et al, 2000;Liu and Simon, 2000). Finally, polymodal nociceptors, an important class of neurons that respond to both noxious thermal and mechanical stimuli, are found in both taxa (Necker and Reiner, 1980;Gallar et al, 1993). However, although mammalian polymodal nociceptors are responsive to sub-micromolar concentrations of CAP (Liu et al, 1996), neural sensitivity to similar concentrations of CAP has not been observed in birds (Wood et al, 1988).…”

Relative Sensitivity of the Ocular Trigeminal, Nasal Trigeminal and Olfactory Systems to Airborne Chemicals

“…Most of the suggested alternatives to the Draize test involve in vitro assays (Booman et al, 1989), though in vivo assays have also been proposed (e.g., Kennah II et al, 1989). One in vivo approach explored in animals has consisted of recording electrophysiological responses to instilled compounds from the sensory nerve fibers innervating the eye (Belmonte et al, 1991;Beuerman et al, 1992;Gallar et al, 1993).…”