Pinch-current injection defines two discharge profiles in mouse superficial dorsal horn neurones, in vitro

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Neurones in the superficial dorsal horn (SDH) are a major target for nociceptive afferents and play an important role in pain processing. One approach to understanding the role of SDH neurones has been to study their action potential (AP) discharge in spinal cord slices during injection of depolarizing step-currents. Four or five neurone subpopulations are typically identified based on AP discharge, with various roles proposed for each in pain processing. During noxious peripheral stimulation in vivo, however, SDH neurones are activated via synaptic inputs. This produces a conductance change with different somato-dendritic distributions and temporal characteristics to that provided by a somatic step-current injection. Here we introduce an alternative approach to studying SDH neurone discharge under in vitro conditions. We recorded voltage-clamp responses in SDH neurones, in vivo, during noxious mechanical stimulation of the hindpaw (1 s pinch, ∼100 g mm−2). From these recordings a representative ‘pinch-current’ was selected and subsequently injected into SDH neurones in spinal cord slices (recording temperature 32°C). Pinch-current-evoked discharge was compared to that evoked by rectangular step-current injections. Pinch- and step-current-evoked AP discharge frequency was highly correlated (r2= 0.61). This was also true for rheobase current comparisons (r2= 0.61). Conversely, latency to discharge and discharge duration were not correlated when step- and pinch-current responses were compared. When neurones were grouped according to step-current-evoked discharge, five distinct patterns were apparent (tonic firing, initial bursting, delayed firing, single spiking, and reluctant firing). In contrast, pinch-current responses separated into two clear patterns of activity (robust and resistant firing). During pinch-current injection, tonic-firing and initial-bursting neurones exhibited robust AP discharge with similar characteristics. In contrast, single-spiking and reluctant-firing neurones were resistant to AP discharge. Delayed-firing neurones exhibited pinch-current responses that were transitional between those of tonic-firing/initial-bursting and single-spiking/reluctant-firing neurones. Injection of digitally filtered pinch-currents indicated that transient current fluctuations are necessary for robust repetitive discharge in initial-bursting neurones. These data suggest the functional significance of the diverse step-current-evoked firing patterns, previously reported in SDH neurones remains to be fully understood. When a ‘facsimile’ current profile or pinch-current is used in place of step-currents, AP discharge diversity is much reduced.

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