Functional heterogeneity of calretinin-expressing neurons in the mouse superficial dorsal horn: implications for spinal pain processing

Neurons in the superficial dorsal horn (SDH) of the spinal cord play an important role in nociceptive, thermal, itch and light touch sensations. Excitatory interneurons comprise similar to 65% of all SDH neurons but surprisingly few studies have investigated their role in spinal sensory processing. Here we use a transgenic mouse to study putative excitatory SDH neurons that express the calcium binding protein calretinin (CR). Our immunocytochemical, morphological and electrophysiological analysis identified two distinct populations of CR-expressing neurons, which we termed 'Typical' and 'Atypical'. Typical CR-expressing neurons comprised similar to 85% of the population and exhibited characteristic excitatory interneuron properties including delayed firing discharge, large rapid A-type potassium currents, and central, radial or vertical cell morphologies. Atypical neurons exhibited properties consistent with inhibitory interneurons, including tonic firing or initial bursting discharge, I-h currents, and islet cell morphology. Although both Typical and Atypical CR-expressing neurons responded to noxious peripheral stimulation, the excitatory drive onto Typical CR-expressing neurons was much stronger. Furthermore, Atypical CR-expressing cells comprise at least two functionally distinct subpopulations based on their responsiveness to noxious peripheral stimulation and neurochemical profile. Together our data suggest CR expression is not restricted to excitatory neurons in the SDH. Under normal conditions, the contribution of 'Typical' excitatory CR-expressing neurons to overall SDH excitability may be limited by the presence of A-type potassium currents, which limit the effectiveness of their strong excitatory input. Their contribution may, however, be increased in pathological situations where A-type potassium currents are decreased. By contrast, 'Atypical' inhibitory neurons with their excitable phenotype but weak excitatory input may be more easily recruited during increased peripheral stimulation.