Electrophysiological properties and chemosensitivity of acutely dissociated trigeminal somata innervating the cornea

Abstract

Adult rat sensory trigeminal ganglion neurons innervating the cornea (cTGNs) were isolated and identified following retrograde dye labeling with FM1-43. Using standard whole-cell patch clamp recording techniques, cTGNs could be subdivided by their action potential (AP) duration. Fast cTGNs had AP durations < 1 ms (40%) while slow cTGNs had AP durations > 1 ms and an inflexion on the repolarization phase of the AP. With the exception of membrane input resistance, the passive membrane properties of fast cTGNs were different from those of slow cTGNs (capacitance: 61 ± 4.5 pF vs. 42 ± 2.6 pF, resting membrane potential: −59 ± 0.7 mV vs. −53 ± 0.9 mV, for fast and slow cTGNs respectively). Active membrane properties also differed between fast and slow cTGNs. Slow cTGNs had a higher AP threshold (−25 ± 1.6 mV vs. −38 ± 0.8 mV), a larger rheobase (14 ± 1.9 pA/pF vs. 6.8 ± 1.0 pA/pF), and a smaller AP undershoot (−56 ± 1.7 mV vs. −67 ± 2.5 mV). The AP overshoot, however was similar between the two types of neurons (46 ± 3.1 mV vs. 48 ± 4 mV). Slow cTGNs were depolarized by capsaicin (1 μM, 80%) and 60% of their APs were blocked by TTX (100 nM). Fast cTGNs were unaffected by capsaicin and 100% of their APs were blocked by TTX. Similarly, cTGNs were also heterogeneous with respect to their responses to exogenous ATP and 5-HT.The current work shows that cTGNs have distinctive electrophysiological properties and chemosensitivity profiles. These characteristics may mirror the distinct properties of corneal sensory nerve terminals. The availability of isolated identified cTNGs constitutes a tractable model system to investigate the biophysical and pharmacological properties of corneal sensory nerve terminals. Keywordsnociceptor; pain; primary sensory neuron; autacoids; patch clampThe cornea is the most densely innervated structure in the body (Belmonte and Gallar, 1996). All sensory corneal nerve terminals (CNTs) are considered nociceptive because their activation by disparate stimuli exclusively elicits the perception of pain (Beuerman and Corresponding author: Dr. Daniel Weinreich, Department of Pharmacology and Experimental Therapeutics, University of Maryland, School of Medicine, 655 W. Baltimore Street, Baltimore, MD 21201, USA, dweinrei@umaryland.edu. Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. Tanelian, 1979; Chen et al., 1995). Two types of nociceptive nerve fibers can be distinguished in the cornea, C-fibers with action potential (AP) conduction velocities < 1m/s and Aδ-fibers with AP conduction velocities > 1m/s (MacIver and Tanelian, 1990). In the cornea, these...

Electrophysiological and Chemical Properties in Subclassified Acutely Dissociated Cells of Rat Trigeminal Ganglion by Current Signatures

“…When AP and AHP features were compared between TRG and DRG cell types, the difference of current injection protocols between the present TRG and earlier DRG studies should be noted. The long current injection used in the present study has frequently been used to generate AP and AHP in many other TRG in vitro studies (López de Armentia et al 2000;Park et al 2009;Takeda et al 2004aTakeda et al ,b, 2005Takeda et al , 2006Tsutsui et al 2008;Veiga Moreira et al 2007;Yoshida and Matsumoto 2005;Yoshida et al 2007). The durations of AP and AHP are thought to be prolonged and shortened, respectively, by the injected current compared with those of physiologically induced AP and AHP in vivo.…”

“…Based on the in vivo studies, dissociated TRG and DRG cells have been classified into two (F-and S-types) or four (types I-IV) cell groups by AP duration and whether they have a pronounced deflection (hump) on the falling phase (Cabanes et al 2003;Liu et al 2001;Park et al 2009). To identify nociceptive neurons, capsaicin and tetrodotoxin (TTX) are widely combined in the investigation of cell properties to classify cells (Liu et al 2001;Park et al 2009;Veiga Moreira et al 2007). However, the combination is not useful to discriminate capsaicin-insensitive and TTX-sensitive nociceptive cell groups.…”

TFOS DEWS II pain and sensation report

“…These investigations have been performed in dissociated TG neurons [26–28] and in neurons of intact TGs isolated in vitro [29,30] or recorded in vivo in anesthetized mice [31]. In summary, the electrophysiological studies have demonstrated that corneal sensory neurons are either thinly myelinated (Aδ-type) or unmyelinated (C-type) and have heterogeneous passive and active membrane properties [2].…”

Morphological and Functional Changes of Corneal Nerves and Their Contribution to Peripheral and Central Sensory Abnormalities

“…Functional studies have been performed in trigeminal neurons to better understand their activation and how various corneal nerve injuries may alter their modalities. In vivo electrophysiology works have assessed action potentials from single neurons and/or clusters of neurons in rodent TGs (Lopez de Armentia et al, 2000 ; Veiga Moreira et al, 2007 ; Hirata and Meng, 2010 ; Kurose and Meng, 2013 ; Hirata et al, 2015 ; Quallo et al, 2015 ) under normal and pathological conditions. The literature is not as rich as the one from DRG neurons, but it provides important information about changes in the neuronal properties of corneal neurons under physiological and DE conditions.…”