Optogenetic evaluation of the ability of different cutaneous C-fiber afferents to evoke aversive behaviors
Most cutaneous C-fibers, including both peptidergic and non-peptidergic subtypes are presumed to be nociceptors and respond to noxious input in a graded manner. However, mechanically sensitive, non-peptidergic C-fibers also respond to mechanical input in the innocuous range, and so the degree to which they contribute to nociception remains unclear. To address this gap, we investigated the function of non-peptidergic afferents using the MrgprdCre allele. In real time place aversion studies, we found that low frequency optogenetic activation of MrgrpdCre lineage neurons was not aversive in naive mice, but became aversive after spared nerve injury (SNI). To address the underlying mechanisms of this allodynia, we recorded from lamina I spinoparabrachial (SPB) neurons using the semi-intact ex vivo preparation. Following SNI, innocuous brushing of the skin gave rise to abnormal activity in lamina I SPB neurons, consisting of an increase in the proportion of recorded neurons that responded with excitatory post synaptic potentials or action potentials. This increase was likely due, at least in part, to an increase in the proportion of lamina I (LI) SPB neurons that received input upon optogenetic activation of MrgprdCre lineage neurons. Intriguingly, in SPB neurons there was a significant increase in the EPSC latency from MrgprdCre lineage input following SNI, consistent with the possibility that the greater activation post SNI could be due to the recruitment of a new polysynaptic circuit. Together, our findings suggest MrgprdCre lineage neurons can provide mechanical input to the dorsal horn that is non-noxious before injury but becomes noxious afterwards due the engagement of a previously silent polysynaptic circuit in the dorsal horn.