Tetrodotoxin-resistant impulses in single nociceptor nerve terminals in guinea-pig cornea

Abstract

1. Extracellular recording techniques have been used to study nerve impulses in single sensory nerve terminals in guinea-pig cornea isolated in vitro. 2. Nerve impulses occurred spontaneously and were evoked by electrical stimulation of the ciliary nerves. 3. The nerve impulses were identified as originating in polymodal receptors, mechano-receptors or 'cold' receptors. All three types are believed to be nociceptors. 4. Tetrodotoxin (TTX, 1 microM) blocked nerve impulses evoked by electrical stimulation of the ciliary nerves. However, ongoing and/or naturally evoked nerve impulses persisted in the presence of TTX in all three types of receptors. Lignocaine (lidocaine; 1 mM) blocked all electrical activity. 5. TTX-resistant sodium channels therefore play a major role in generating the action potentials that signal pain to the brain.

Figure 1. Recording from the corneal epithelium

A, schematic diagram of recording set-up and photomicrograph showing the location of the recording electrode (scale bar, 1 mm). B, a single nerve terminal impulse (NTI) evoked by stimulation of the ciliary nerves. The upper part shows 50 overlaid traces recorded during a train of stimuli at 1 Hz and the lower part shows the average of these traces (SA, stimulation artefact). C, frequency distribution of conduction velocities for all single NTIs recorded. D, confocal micrograph of nerve terminals in the guinea-pig cornea. Most nerve terminals approach the surface of the epithelium at right angles and appear as single dots. On average, 2 terminals lie beneath the opening of a 50 μm pipette (circle).

Figure 2. Spontaneous and evoked NTIs

A, upper part shows overlaid traces in which electrical stimulation of the ciliary nerves evoked a stimulus locked NTI and lower part shows traces in which the occurrence of a spontaneous NTI just before or after the stimulus artefact caused failure of the electrically evoked NTI (SA, stimulation artefact). B, averaged evoked (upper) and spontaneous (lower) NTIs recorded in the same attachment as in A. C and D, averages of electrically evoked (upper traces) and spontaneously occurring (lower traces) NTIs recorded from a mechano-nociceptor (C) and a polymodal receptor (D). The scale bars in C also apply in D.

Figure 4. Effects of TTX (1 μm for 30 min)

A-D, averaged electrically evoked (A and C) and spontaneously occurring (B and D) NTIs recorded before (thin line) and in the presence of TTX (thick line) from a mechano-nociceptor (A and B) and a polymodal receptor (C and D). E and F, the effects of capsaicin (0.1 μm) on the frequency of occurrence of NTIs recorded from a polymodal receptor before (E) and during (F) application of TTX. G and H, the effects of temperature changes (upper curve) on the frequency of occurrence of NTIs recorded from a cold-sensitive receptor before (G) and during (H) application of TTX. Histograms of ongoing activity have bin widths of 1 s.

Figure 3. Identification of subtypes of nociceptor

A and B, the effects of capsaicin (Aa and Ba) and mechanical stimulation (Ab and Bb) on the frequency of NTIs in a polymodal receptor (A) and a mechano-nociceptor (B). Ca, the effects of temperature on the frequency of NTIs in a cold-sensitive receptor. Cb, organ bath temperature during the period of recording shown in Ca. In A, B and C the abscissa scale in panel b also applies in panel a. Histograms of ongoing activity have bin widths of 1 s.