injury (SNI trans ) and roughly half lost following a model which
contrastingly allows for n erve regenerations (SNI crush ).
Finally, we have observed that the vulnerability of Mrgprd 1
neurons extends to the in vitro setting and provide data to
support the hypothesis that loss is driven by a lack of
neurotrophic support following injury.
4.1. Neuronal loss
The question of whether DRG neurons die following traumatic
injury has been addressed by several groups over the last few
decades. Despite contrasting findings on the extent, timing, and
form that loss takes, most studies have observed frank loss of
DRG neurons. 6,38,46,53 However, more recent studies using
recombinase driver lines and novel machine-learning approaches
have cast doubt on this consensus.44,49 Our data strongly
support the loss hypothesis and suggest that approximately 60%
of axotomized afferents die within 2 weeks of SNI. The
discrepancy between our findings and other recent studies may
be partly explained by the sampling method used to estimate
neuronal numbers. For example, Schulte et al. 49 developed
a novel machine-learning approach and found no reduction in
neuron density across serial sections of rat DRG following SNI,
and they inferred from this that frank loss did not occur. Our
results are congruous, in that we also observed no reduction in
neuron density. However, we found a substantial loss in the total
neuron-containing volume of injured DRG, which underlies our
contrasting conclusion of frank loss. Of note, morphological
volumetric analysis and MRI have also previously demonstrated
volume loss in both rodent and human DRG following nerve
injury.35,65,66 These findings occur despite a major increase of
nonneuronal cells in the injured DRG 30 and support the notion
that the total DRG neuron number is decreased.
4.2. Selectivity of neuron loss
While definitively characterizing loss of molecularly defined
subpopulations was challenging before the advent of recombi-
nase driver lines, a consensus emerged that small-diameter
neurons are more vulnerable to nerve injury–induced loss. 50,53
Our data support this consensus and extend it to reveal that while
there is a generalized partial loss of C-fiber populations including
CGRP- and Trpm8-expressing neurons, Mrgprd-expressing
neurons are particularly sensitive to loss. This selective vulnera-
bility has been hinted at previously by the stark reduction in the
number of DRG neurons and their central terminals that bind IB4
and express canonical markers such as the P2X3 receptor
following nerve injury. 5,8,29,36 Type 1a glomeruli are also reduced
in lamina II, suggesting a structural loss of central terminals and
not simply a loss of IB4-binding.2 However, it was not clear
whether these data represented phenotypic changes in non-
peptidergic nociceptors or frank loss of neurons. We describe
neuron loss that is delayed (occurring .7 days postinjury) with
respect to histochemical and structural changes (occurring 1-
5 days postinjury 2,29), suggesting that these changes precede
and are not in themselves indicative of neuron loss.
The vulnerability of Mrgprd-expressing neurons is congruous
with recent subpopulation bulk RNA-seq data, which found that
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