Isolectin B₄-Positive and -Negative Nociceptors Are Functionally Distinct

Abstract

Small-diameter sensory neurons that are primarily nociceptors can be divided neurochemically into two populations: isolectin B₄ (IB₄)-positive nonpeptidergic neurons, and IB₄-negative peptidergic neurons. It has been shown that IB₄-positive neurons depend on glial-derived neurotrophic factor (GDNF), whereas IB₄-negative neurons depend on NGF for survival during postnatal development (Molliver et al., 1997). Furthermore, these two populations of nociceptors terminate in distinct regions of the superficial spinal cord. To date, however, no evidence exists that indicates whether these two groups of nociceptors have distinct functional roles in the process of nociception (Snider and McMahon, 1998). To search for functional differences, we performed whole-cell voltage and current-clamp recordings on acutely isolated adult mouse dorsal root ganglion neurons that were labeled with fluorescent IB₄. We found that IB₄-positive neurons have longer-duration action potentials, higher densities of TTX-resistant sodium currents, and smaller noxious heat-activated currents than IB₄-negative neurons. Furthermore, we show that NGF, but not GDNF, directly increases the number of neurons that respond to noxious heat. The different electrophysiological properties expressed by IB₄-positive and -negative small neurons, including their different heat sensitivities, indicates that they may relay distinct aspects of noxious stimuli both acutely and after injury in vivo.