Subclassified acutely dissociated cells of rat DRG: Histochemistry and patterns of capsaicin-, proton-, and ATP-activated currents

We used a "current signature" method to subclassify acutely dissociated dor sal root ganglion (DRG) cells into nine subgroups. Cells subclassified by c urrent signature had uniform properties. The type 1 cell had moderate capsa icin sensitivity (25.9 pA/pF), powerful, slowly desensitizing (tau = 2,300 ms), ATP-activated current (13.3 pA/pF), and small nondesensitizing respons es to acidic solutions (5.6 pA/pF). Type 1 cells expressed calcitonin gene- related peptide immunoreactivity (CGRP-IR), manifested a wide action potent ial (7.3 ms), long duration afterhyperpolarization (57.0 ms), and were IB4 positive. The type 2 cell exhibited large capsaicin activated currents (134 .9 pA/pF) but weak nondesensitizing responses to protons (15.3 pA/pF). Curr ents activated by ATP and alpha beta -m-ATP (51.7 and 44.6 pA/pF, respectiv ely) had fast desensitization kinetics (tau = 214 ms) that were distinct fr om all other cell types. Type 2 cells were IB4 positive but did not contain either substance P (SP) or CGRP-IR. Similar to capsaicin-sensitive nocicep tors in vivo, the afterhyperpolarization of the type 2 cell was prolonged ( 54.7 ms). The type 3 cell expressed, amiloride-sensitive, rapidly desensiti zing (tau = 683 ms) proton-activated currents (127.0 pA/pF), and was insens itive to ATP or capsaicin. The type 3 cell was IB4 negative and contained n either CGRP nor SP-IR. The afterhyperpolarization (17.5 ms) suggested nonno ciceptive function. The type 4 cell had powerful ATP-activated currents (17 .4 pA/pF) with slow desensitization kinetics (tau = 2,813 ms). The afterhyp erpolarization was prolonged (46.5 ms), suggesting that this cell type migh t belong to a capsaicin-insensitive nociceptor population. The type 4 cell did not contain peptides. The type 7 cell manifested amiloride-sensitive, p roton-activated currents (45.8 pA/pF) with very fast desensitization kineti cs (tau = 255 ms) and was further distinct from the type 3 cell by virtue o f a nondesensitizing amiloride-insensitive component (6.0 pA/pF). Capsaicin and ATP sensitivity were relatively weak (4.3 and 2.9 pA/pF, respectively) . Type 7 cells were IB4 positive and contained both SP and CGRP-IR. They ex hibited an exceptionally long afterhyperpolarization (110 ms) that was sugg estive of a silent (mechanically insensitive) nociceptor. We concluded that presorting of DRG cells by current signatures separated them into internal ly homogenous subpopulations that were distinct from other subclassified ce ll types.