The modality‐specific contribution of peptidergic and non‐peptidergic nociceptors is manifest at the level of dorsal horn nociresponsive neurons

Key points Ablation of TRPV1+/peptidergic or of MrgprD+/non‐peptidergic nociceptors produces modality‐specific deficits in the behavioural responses to heat and mechanical stimuli, respectively. Noxious heat‐induced dorsal horn Fos expression is also eliminated, despite the heat responsiveness of the non‐peptidergic nociceptors. To assess whether this modality‐specific contribution is manifest at the level of individual spinal neurons, we made extracellular recordings from mouse dorsal horn after selective ablation of the two nociceptor populations. Intrathecal capsaicin, which ablated the TRPV1+ nociceptors, abolished responsiveness of superficial and deep dorsal horn neurons to noxious heat, with no change in response to noxious mechanical stimulation. Ablation of MrgprD+ afferents did not alter the response to noxious heat but reduced the firing of dorsal horn neurons in response to noxious mechanical stimulation. These findings argue strongly that TRPV1+ and MrgprD+ nociceptors provide modality‐specific contributions to the response properties of spinal cord neurons. Abstract We previously demonstrated that genetic and/or pharmacological ablation of the TRPV1+/peptidergic or the MrgprD+/non‐peptidergic subset of nociceptors produced selective, modality‐specific deficits in the behavioural responses to heat and mechanical stimuli, respectively. To assess whether this modality‐specific contribution is also manifest at the level of spinal cord neuron responsiveness, here we made extracellular recordings from lumbar dorsal horn neurons of the mouse in response to graded thermal and mechanical stimulation. We found that, following intrathecal injection of capsaicin to eliminate the central terminals of TRPV1+ nociceptors, neurons in the region of laminae I and V of the spinal cord lost responsiveness to noxious heat (whether generated by a contact heat probe or diode laser), with no change in their response to noxious mechanical stimulation. In contrast, ablation of MrgprD+ afferents did not alter the response to noxious heat, but reduced the firing of superficial dorsal horn nociceptive‐specific neurons in response to graded mechanical stimulation and decreased the relative number of wide dynamic range neurons that were exclusively mechanosensitive. Neither ablation procedure reduced the number of dorsal horn neurons that responded to noxious cold. These findings support the conclusion that TRPV1+ nociceptors are necessary and probably sufficient for noxious heat activation of dorsal horn neurons and that, despite their polymodal properties, TRPV1+ and MrgprD+ nociceptors provide modality‐specific contributions to the response properties of spinal cord neurons.