CCL2 facilitates spinal synaptic transmission and pain via interaction with presynaptic CCR2 in spinal nociceptor terminals.

Previous studies have shown that CCL2 may cause chronic pain, but the exact mechanism of central sensitization is unclear. In this article, we further explore the presynaptic role of CCL2. Behavioral experiments show that intervertebral foramen injection CCR2 antagonists into dorsal root ganglion (DRG) can inhibit the inflammatory pain caused by CCL2 in spinal cord. We raised the question of the role of presynaptic CCR2 in the spinal dorsal horn. Subsequent electron microscopy experiments showed that CCR2 was expressed in the presynaptic CGRP terminal in the spinal dorsal horn. CCL2 can enhance presynaptic calcium signal. Whole-cell patch-clamp recordings showed that CCL2 can enhance NMDAR-eEPSCs through presynaptic effects, and further application of glutamate sensor method proved that CCL2 can act on presynaptic CCR2 to increase the release of presynaptic glutamate. In conclusion, we suggest that CCL2 can directly act on the CCR2 on presynaptic terminals of sensory neurons in the spinal dorsal horn, leading to an increase in the release of presynaptic glutamate and participate in the formation of central sensitization.

Synaptic connectivity of urinary bladder afferents in the rat superficial dorsal horn and spinal parasympathetic nucleus.

That visceral sensory afferents are functionally distinct from their somatic analogues has been known for a long time but the detailed knowledge of their synaptic connections and neurotransmitters at the first relay nucleus in the spinal cord has been limited. To provide information on these topics, we investigated the synapses and neurotransmitters of identified afferents from the urinary bladder to the superficial laminae of the rat spinal dorsal horn (DH) and the spinal parasympathetic nucleus (SPN) by tracing with horseradish peroxidase, quantitative electron microscopical analysis, and immunogold staining for GABA and glycine. In the DH, most bladder afferent boutons formed synapses with 1-2 postsynaptic dendrites, whereas in the SPN, close to a half of them formed synapses with 3-8 postsynaptic dendrites. The number of postsynaptic dendrites and dendritic spines per bladder afferent bouton, both measures of synaptic divergence and of potential for synaptic plasticity at a single bouton level, were significantly higher in the SPN than in the DH. Bladder afferent boutons frequently received inhibitory axoaxonic synapses from presynaptic endings in the DH but rarely in the SPN. The presynaptic endings were GABA- and/or glycine-immunopositive. The bouton volume, mitochondrial volume, and active zone area, all determinants of synaptic strength, of the bladder afferent boutons were positively correlated with the number of postsynaptic dendrites. These findings suggest that visceral sensory information conveyed via the urinary bladder afferents is processed differently in the DH than in the SPN, and differently from the way somatosensory information is processed in the spinal cord.

Identifying local and descending inputs for primary sensory neurons.

Primary pain and touch sensory neurons not only detect internal and external sensory stimuli, but also receive inputs from other neurons. However, the neuronal derived inputs for primary neurons have not been systematically identified. Using a monosynaptic rabies viruses-based transneuronal tracing method combined with sensory-specific Cre-drivers, we found that sensory neurons receive intraganglion, intraspinal, and supraspinal inputs, the latter of which are mainly derived from the rostroventral medulla (RVM). The viral-traced central neurons were largely inhibitory but also consisted of some glutamatergic neurons in the spinal cord and serotonergic neurons in the RVM. The majority of RVM-derived descending inputs were dual GABAergic and enkephalinergic (opioidergic). These inputs projected through the dorsolateral funiculus and primarily innervated layers I, II, and V of the dorsal horn, where pain-sensory afferents terminate. Silencing or activation of the dual GABA/enkephalinergic RVM neurons in adult animals substantially increased or decreased behavioral sensitivity, respectively, to heat and mechanical stimuli. These results are consistent with the fact that both GABA and enkephalin can exert presynaptic inhibition of the sensory afferents. Taken together, this work provides a systematic view of and a set of tools for examining peri- and extrasynaptic regulations of pain-afferent transmission.

Oligodendrocyte ablation triggers central pain independently of innate or adaptive immune responses in mice.

Mechanisms underlying central neuropathic pain are poorly understood. Although glial dysfunction has been functionally linked with neuropathic pain, very little is known about modulation of pain by oligodendrocytes. Here we report that genetic ablation of oligodendrocytes rapidly triggers a pattern of sensory changes that closely resemble central neuropathic pain, which are manifest before overt demyelination. Primary oligodendrocyte loss is not associated with autoreactive T- and B-cell infiltration in the spinal cord and neither activation of microglia nor reactive astrogliosis contribute functionally to central pain evoked by ablation of oligodendrocytes. Instead, light and electron microscopic analyses reveal axonal pathology in the spinal dorsal horn and spinothalamic tract concurrent with the induction and maintenance of nociceptive hypersensitivity. These data reveal a role for oligodendrocytes in modulating pain and suggest that perturbation of oligodendrocyte functions that maintain axonal integrity can lead to central neuropathic pain independent of immune contributions.