The spinal dorsal horn (SDH) comprises heterogeneous populations of interneurons and projection neurons, which form neuronal circuits crucial for processing primary sensory information. Sensory stimulation applied to spatially distinct skin points is transmitted to distinct area of the spinal dorsal horn, which enables spatial discriminative capability of sensory perception. It remains to be elucidated how SDH neuronal circuits contribute to the spatial tuning of sensory perception. Monitoring the pattern of neuronal activity of a large ensemble of SDH neurons would provide data for a comprehensive view about SDH circuits. In the present study, we performed in vivo calcium imaging of multiple SDH neurons in response to pinch stimulation toward different skin points.
We first searched for the point around the abdomen to which pinch stimulation efficiently activated many neurons in the pre-determined recording frame at the level of L1. We designated this point as "P1." Two additional points of the skin 1 cm caudal and 1 cm rostral to P1 were designated as "P2" and "P3," respectively. To examine the central projection field of primary sensory neurons responsible for these skin points, we retrogradely labeled primary sensory neurons with Cholera toxin B, by injecting it into P1, P2, and P3. The central projection field of the primary sensory neurons responsible for P1, P2, and P3 did not overlap and were about 2 mm apart from each other.
We then performed in vivo calcium imaging of SDH neurons following pinch stimulation at P1, P2, and P3, and extracted three-dimensional neuronal activity maps of these neurons. Each SDH neuron had differential responsiveness toward pinch stimulation at these 3 points. Although less efficient than P1 stimulation, P2 or P3 stimulation elicited calcium transients in quite a few SDH neurons at the L1 level. These results suggest that cutaneous pinch stimulation elicited activities of SDH neurons localized rostrocaudally far beyond the central projection field of primary sensory neurons responsible for the stimulated skin point.