Lactoseries carbohydrate antigen, Gal beta 1-4GlcNAc-R, is expressed by a subpopulation of capsaicin-sensitive rat sensory neurons

1. The membrane properties of dorsal root ganglion (DRG) cells expressing the lactoseries carbohydrate antigen Gal beta 1-4GlcNAc-R were studied and compared with those of DRG cells lacking this antigen. Acutely dissociated rat DRG cells that ex pressed Gal beta 1-4GlcNAc-R on their outer cell membranes were detected with the use of a primary monoclonal mouse IgM antibody (A5), directed against Gal beta 1-4GlcNAc-R, and a fluorescent secondary antibody (fluorescein-conjugated goat anti-mouse IgM). We found 12.8 mu g/ml of A5 to be a saturating concentration of primary antibody that labeled similar to 19% of the DRG cells. A battery of membrane properties including action potential (AP) duration; sensitivity to capsaicin; expression of H current (I-H). A current (I-A), and Ca2+ current subtypes (L, N, and T); and inhibition of high-threshold Ca2+ currents by serotonin (5HT) or 8-hydroxy-2-(di-N-propylamino)-tetralin (8-OH-DPAT) was measured in DRG cells labeled (A5(+)) and unlabeled (A5(-)) by a saturating concentration of A5. 2. There was a significant difference in the number of capsaicin-sensitive DRG cells and a significant difference in the magnitude of the capsaicin-induced inward current in A5(+) versus A5(-) DRG cells. Of 35 A5(+) cells tested, 33 were sensitive to 1 mu M capsaicin, which produced an inward current averaging 4 +/- 0.46 (SE) nA (n = 33). In contrast, only 12 of 33 A5(-) cells were sensitive to 1 mu M capsaicin, which produced an inward current averaging 1.2 +/- 0.52 nA (n = 12). 3. There were also significant differences between A5(+) and A5(-) cells regarding average AP duration, N- and T-type Ca2+ current amplitude, and number of cells that expressed I-H and I-A A5(+) cells had significantly larger N-type Ca2+ currents and expressed I-A more frequently than A5(-) cells. Conversely, A5(-) cells had significantly longer AP duration and larger T-type Ca2+ currents, and expressed I-H more frequently compared with A5(+) cells. 4. A5(+) and A5(-) cells differed regarding the inhibition of high-threshold Ca2+ currents by maximal concentrations of 5HT(1A) agonists (10 mu M 5HT or 1 mu M 8-OH-DPAT). Inhibition of Ca2+ currents in A5(+) cells by 1 mu M 8-OH-DPAT (n = 15) or 10 mu M 5HT (n = 18) averaged 4 +/- 0.9%. In contrast, inhibition of Ca2+ currents in A5(-) cells by 10 mu M 5HT (n = 33) averaged 20 +/- 3.8%. 5. Cells for which sufficient data were collected were categorized as type 1, 2, 3, or 4 on the basis of sensitivity to capsaicin and expression of I-H, I-A, and T-type Ca2+ current amplitude, and the distribution of A5(+) and A5(-) cells among the various groups was observed. The categories were defined as follows: type 1 (capsaicin sensitive, no I-H or I-A); type 2 (capsaicin sensitive, significant I-A); type 3 (capsaicin insensitive, T-type Ca2+ currents <1 nA, significant I-H but no I-A); and type 4 (capsaicin insensitive, T-type Ca2+ currents >2.4 nA). On the basis of this criteria, 6 of 15 type 1 cells and all type 2 cells (n = 19) were A5(+). All type 3 cells (n = 8) and all type 4 cells (n = 11) were A5(-). 6. As indicated above, the expression of the Gal beta 1-4GlcNAc-R antigen differentiated two subgroups of DRG cells in the type 1 category (A5(+), n = 6 and A5(-), n = 9). These two groups varied regarding the sensitivity of Ca2+ currents to maximally effective concentrations of 5HT(1A) agonists. In type 1 A5(+) DRG cells, high-threshold Ca2+ currents were not significantly inhibited by 1 mu M 8-OH-DPAT (average inhibition = 1.2 +/- 0.8%, n = 6). However, in type 1 A5(-) cells, high-threshold Ca2+ currents were reduced 47 +/- 6.0% (n = 9) by 10 mu M 5HT. 7. The several significant differences in membrane properties between A5(-) and A5(+) DRG cells suggest that the Gal beta 1-4GlcNAc-R antigen is expressed by a distinct subset of DRG cells, consisting predominately of type 1 and type 2 cells. The observation that most A5(+) DRG cells were capsaicin sensitive suggests that the Gal beta 1-4GlcNAc-R antigen is expressed primarily by nociceptors. This idea is consistent with a previous study in which investigators observed the Gal beta 1-4GlcNAc-R antigen on a subpopulation of DRG neurons with afferent terminals in lamina I and II of the spinal cord, an important relay area for nociceptive information.