Chronic pain presents a major unmet clinical problem. The development of more effective treatments is hindered by our limited understanding of the neuronal circuits underlying sensory perception. Here, we show that parvalbumin (PV)-expressing dorsal horn interneurons modulate the passage of sensory information conveyed by low-threshold mechanoreceptors (LTMRs) directly via presynaptic inhibition and also gate the polysynaptic relay of LTMR input to pain circuits by inhibiting lamina II excitatory interneurons whose axons project into lamina I. We show changes in the functional properties of these PV interneurons following peripheral nerve injury and that silencing these cells unmasks a circuit that allows innocuous touch inputs to activate pain circuits by increasing network activity in laminae I-IV. Such changes are likely to result in the development of tactile allodynia and could be targeted for more effective treatment of mechanical pain.

Corticospinal tract (CST) neurons innervate the deep spinal dorsal horn to sustain chronic neuropathic pain. The majority of neurons targeted by the CST are interneurons expressing the transcription factor c-Maf. Here, we used intersectional genetics to decipher the function of these neurons in dorsal horn sensory circuits. We find that excitatory c-Maf (c-MafEX) neurons receive sensory input mainly from myelinated fibers and target deep dorsal horn parabrachial projection neurons and superficial dorsal horn neurons, thereby connecting non-nociceptive input to nociceptive output structures. Silencing c-MafEX neurons has little effect in healthy mice but alleviates mechanical hypersensitivity in neuropathic mice. c-MafEX neurons also receive input from inhibitory c-Maf and parvalbumin neurons, and compromising inhibition by these neurons caused mechanical hypersensitivity and spontaneous aversive behaviors reminiscent of c-MafEX neuron activation. Our study identifies c-MafEX neurons as normally silent second-order nociceptors that become engaged in pathological pain signaling upon loss of inhibitory control.

Neurexins (NRXNs) are key presynaptic cell adhesion molecules that regulate synapse formation and function via trans-synaptic interaction with postsynaptic ligands. Here, we generate cerebellar granule cell (CGC)-specific Nrxn triple-knockout (TKO) mice for complete deletion of all NRXNs. Unexpectedly, most CGCs die in these mice, and this requirement for NRXNs for cell survival is reproduced in cultured CGCs. The axons of cultured Nrxn TKO CGCs that are not in contact with a postsynaptic structure show defects in the formation of presynaptic protein clusters and in action-potential-induced Ca2+ influxes. These cells also show impaired secretion of depolarization-induced, fluorescence-tagged brain-derived neurotrophic factor (BDNF) from their axons, and the cell-survival defect is rescued by the application of BDNF. These results suggest that CGC survival is maintained by autocrine neurotrophic factors and that NRXNs organize the presynaptic protein clusters and the autocrine neurotrophic-factor secretory machinery independent of contact with postsynaptic ligands.

Although the contribution of macrophages to metastasis is widely studied in primary tumors, the involvement of macrophages in tumor-draining lymph nodes (LNs) in this process is less clear. We find CD169+ macrophages as the predominant macrophage subtype in naive LNs, which undergo proliferative expansion in response to tumor stimuli. CD169+ LN macrophage depletion, using an anti-CSF-1R antibody or clodronate-loaded liposomes, leads to increased metastatic burden in two mouse breast cancer models. The expansion of CD169+ macrophages is tightly connected to B cell expansion in tumor-draining LNs, and B cell depletion abrogates the effect of CD169+ macrophage absence on metastasis, indicating that the CD169+ macrophage anti-metastatic effects require B cell presence. These results reveal a protective role of CD169+ LN macrophages in breast cancer metastasis and raise caution for the use of drugs aiming at the depletion of tumor-associated macrophages, which might simultaneously deplete macrophages in tumor-draining LNs.

Although the importance of reactive astrocytes during CNS pathology is well established, the function of astroglia in adult CNS homeostasis is less well understood. With the use of conditional, astrocyte-restricted protein synthesis termination, we found that selective paralysis of GFAP(+) astrocytes in vivo led to rapid neuronal cell loss and severe motor deficits. This occurred while structural astroglial support still persisted and in the absence of any major microvascular damage. Whereas loss of astrocyte function did lead to microglial activation, this had no impact on the neuronal loss and clinical decline. Neuronal injury was caused by oxidative stress resulting from the reduced redox scavenging capability of dysfunctional astrocytes and could be prevented by the in vivo treatment with scavengers of reactive oxygen and nitrogen species (ROS/RNS). Our results suggest that the subpopulation of GFAP(+) astrocytes maintain neuronal health by controlling redox homeostasis in the adult CNS.

Some neurons have the ability to enhance output voltage to input current with a preferred frequency, which is called resonance. Resonance is thought to be a basis for membrane potential oscillation. Although ion channels responsible for resonance have been reported, the precise mechanisms by which these channels work remain poorly understood. We have found that resonance is reduced but clearly present in the inferior olivary neurons of Cav3.1 T-type voltage-dependent Ca(2+) channel knockout (KO) mice. The activation of Cav3.1 channels is strongly membrane potential dependent, but less frequency dependent. Residual resonance in Cav3.1 KO mice is abolished by a hyper-polarization-activated cyclic nucleotide-gated (HCN) channel blocker, ZD7288, and is partially suppressed by voltage-dependent K(+) channel blockers. Resonance is inhibited by ZD7288 in wild-type mice and impaired in HCN1 KO mice, suggesting that the HCN1 channel is essential for resonance. The ZD7288-sensitive current is nearly sinusoidal and strongly frequency dependent. These results suggest that Cav3.1 and HCN1 channels act as amplifying and resonating conductances, respectively.

Medium spiny neurons (MSNs) of mice show opposing activities upon the initiation of a food-seeking lever press task. Ventromedial striatal (VMS)-MSNs are inhibited but ventrolateral striatal (VLS)-MSNs are activated; these activities mediate action selection and action initiation, respectively. To understand what input shapes the opposing MSN activities, here, we monitor cortical input activities at the cell population level and artificially reverse them. We demonstrate that the ventral hippocampus (vHP) and the insular cortex (IC) are major inputs to the VMS and VLS, both projections show silencing at the trial start time, and the vHP-VMS and IC-VLS pathways form functionally coupled input-output units during the task. Of note, the upstream IC silencing is converted to the downstream VLS-MSN activation. We find biased localization of striatal parvalbumin-expressing interneurons (PV INs) and verify PV IN-dependent feedforward architecture in the VLS. Our results reveal a distinct mode of cortico-striatal signal conveyance via feedforward disinhibition in behaving animals.