Introduction: Cinnamaldehyde (CA) elicits itch sensation in humans. We investigated if CA elicits scratching behavior in mice and determined the roles for TRPV1, TRPA1, and TRPV4. Materials and Methods: Scratching behavior elicited by intradermal injection of CA was assessed in wildtype (WT) mice and knockout (KO) mice lacking TRPV1, TRPA1, TRPV4, or deficient in mast cells. We also assessed scratching and wet dog shakes elicited by low-threshold mechanical stimulation of skin treated topically with CA or vehicle. Using calcium imaging we tested if CA activates dorsal root ganglion (DRG) neurons of each genotype. Results: Intradermal cheek injection of CA elicited dose-dependent hindlimb scratch bouts, with fewer forelimb wipes and facial groom bouts that were not dose-dependent. CA elicited significantly fewer scratch bouts in TRPV1 and TRPV4 KO mice, but not TRPA1KOs, compared with WTs. There were no sex differences across genotypes. The histamine H1 antagonist cetirizine did not affect CA-evoked scratching, which was normal in mast cell deficient mice, indicating lack of histamine involvement. Scores for alloknesis were significantly greater following topical application of CA compared with vehicle. Post-CA alloknesis scores were significantly higher in TRPV4KOs of both sexes and in female TRPV1 and TRPA1KOs, compared with WTs. Low threshold mechanical stimuli also elicited significantly more wet dog shakes in mice treated topically with 20% CA, with significantly fewer in TRPV1, TRPA1, and TRPV4KOs compared with WTs. In calcium imaging studies, CA excited 24% of WT DRG cells, significantly fewer (11.5%) in cells from TRPV4KOs, and none in TRPA1KOs. Responses of cells of all genotypes exhibited significant sensitization to repeated CA stimulation. Sensitization was significantly enhanced by IL-4, which itself excited 16% of WT DRG cells and none from TRPA1KOs. Discussion: The results indicate that TRPA1 is dispensable for CA-evoked scratching, which depends partly on TRPV1 and TRPV4.

Plaque psoriasis is a chronic inflammatory skin disease that affects a substantial proportion of the world population. This disorder is characterized by scaly, thick skin, intense ongoing itch, and itch from light touch (such as clothing contacting skin, called “alloknesis”). Imiquimod is a topical treatment for basal cell carcinomas and warts that has been used to create a mouse model of plaque psoriasis. Imiquimod-treated male, but not female, wildtype B6 mice showed significant increases in spontaneous scratching, while both sexes exhibited increased alloknesis, indicative of chronic itch. TRPV1 and TRPA1 knockout (KO) mice all exhibited numeric increases in spontaneous scratching which were significant for TRPV1KO mice and TRPA1KO males. Female TRPV1KO and TRPA1KO mice exhibited imiquimod-induced increases in alloknesis scores that did not significantly differ from wildtypes, while alloknesis scores in imiquimod-treated male TRPV1KO and TRPA1KO mice were significantly lower compared with wildtypes, suggesting that these ion channels are necessary for the development of alloknesis in males but not females in this model. Curiously, none of the groups exhibited any significant overall change in chloroquine-evoked scratching following imiquimod treatment, indicating that hyperknesis does not develop in this mouse model. Overall, the data indicate that there are sex differences in this mouse model of psoriasis, and that TRPV1 and TRPA1 ion channels have a small role in promoting the development of itch sensitization. This contrasts with the far greater role these channels play in the manifestation of skin changes in psoriatic dermatitis.

The Escherichia coli system of Cairns and Foster employs a lac frameshift mutation that reverts rarely (10−9/cell/division) during unrestricted growth. However, when 108 cells are plated on lactose medium, the nongrowing lawn produces ∼50 Lac+ revertant colonies that accumulate linearly with time over 5 days. Revertants carry very few associated mutations. This behavior has been attributed to an evolved mechanism (“adaptive mutation” or “stress-induced mutagenesis”) that responds to starvation by preferentially creating mutations that improve growth. We describe an alternative model, “selective inbreeding,” in which natural selection acts during intercellular transfer of the plasmid that carries the mutant lac allele and the dinB gene for an error-prone polymerase. Revertant genome sequences show that the plasmid is more intensely mutagenized than the chromosome. Revertants vary widely in their number of plasmid and chromosomal mutations. Plasmid mutations are distributed evenly, but chromosomal mutations are focused near the replication origin. Rare, heavily mutagenized, revertants have acquired a plasmid tra mutation that eliminates conjugation ability. These findings support the new model, in which revertants are initiated by rare pre-existing cells (105) with many copies of the F’lac plasmid. These cells divide under selection, producing daughters that mate. Recombination between donor and recipient plasmids initiates rolling-circle plasmid over-replication, causing a mutagenic elevation of DinB level. A lac+ reversion event starts chromosome replication and mutagenesis by accumulated DinB. After reversion, plasmid transfer moves the revertant lac+ allele into an unmutagenized cell, and away from associated mutations. Thus, natural selection explains why mutagenesis appears stress-induced and directed.

The rostral ventromedial medulla (RVM) is important in descending modulation of spinal nociceptive transmission, but it is unclear if the RVM also modulates spinal pruriceptive transmission. RVM ON cells are activated by noxious algesic and pruritic stimuli and are pronociceptive. Many RVM-spinal projection neurons express the neurokinin-1 receptor (Tacr1), and ON-cells are excited by local administration of substance P (SP). We hypothesized that Tacr1-expressing RVM ON cells exert an inhibitory effect on itch opposite to their pronociceptive action. Intramedullary microinjection of SP significantly potentiated RVM ON cells and reduced pruritogen-evoked scratching while producing mild mechanical sensitization. Chemogenetic activation of RVM Tacr1-expressing RVM neurons also reduced acute pruritogen-evoked scratching. Optotagging experiments confirmed RVM Tacr1-expressing neurons to be ON cells. We conclude that Tacr1-expressing ON cells in RVM play a significant role in the modulation of pruriceptive transmission.

Cyclin D2 knockout mice show decreased levels of endogenous dentate neurogenesis. We investigated whether transplanted dentate progenitor cells from wild-type mice respond in vivo to an enriched environment and whether they improve deficient dentate neurogenesis through a neurotrophic effect. Adult cyclin D2 knockout mice were transplanted with passaged adult progenitor cells and kept in an enriched environment or under standard housing conditions in isolation. After 1 week, animals living in an enriched environment underwent water maze testing. Progenitor cells grown on a laminin/poly-d-lysine monolayer expressed Sox2 and nestin and could be differentiated in vitro into neurons and astrocytes. After transplantation into the dentate gyrus, cells preferentially survived along the laminin-rich ependymal lining of the basal cistern or basal membrane of capillaries. A subpopulation of transplanted cells migrated into the interstitial space of the hippocampus and was not associated with laminin. Environmental enrichment led to a significant increase in the survival of transplanted progenitor cells on laminin in the dentate gyrus after 2 weeks. However, animals did not show an enhanced performance in the Morris water maze, and transplantation failed to exert a neurotrophic effect on endogenous neurogenesis after 2 weeks. However, a major limitation of the study is the short-term period of investigation, which may have been insufficient to capture functional effects. In conclusion, initial survival of transplanted neural progenitor cells was dependent on the presence of laminin and was significantly enhanced by environmental enrichment. Further studies are needed to address whether an enriched environment continues to promote graft survival over longer periods of time.