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...