Figure 1. Effects of intramedullary microinjection of SP on RVM ON cells and itch and pain behavior.
(A) SP or saline was microinjected while recording from single ON cells. (B) Peristimulus-time histogram of ON cell response to pinch before (left) and 5 min after local microinjection of SP (right). (C) Normalized firing rate of ON cells following local microinjection of saline (blue, n=8) or SP (red, n=8) at time indicated by arrow. ON cells showed a significant increase in evoked firing following SP injection compared with saline (*; two-way ANOVA, F1, 14 = 8.020, p=0.0133, bolded lines: mean responses; error bars SEM). Male mice were used in these experiments. (D) Lesion sites from the RVM ON cell recordings. Numbers to right indicate Bregma coordinates. (E) An implanted intramedullary microinjection cannula allowed assessment of itch and pain behavior after injection of SP into RVM. (F, G) Graphs plot the number of scratch bouts elicited by intradermal injection of histamine (F, n=9, M = (106.2; 59.72), t=3.4113, DF = 8, 95% CI [15.06, 77.83], p=0.0092) or chloroquine (G, n=9, M = (102.9; 22.00), t=2.847, DF = 8, 95% CI [15.39, 146.5], p=0.0216) for each mouse (red dots and lines), and mean scratch bouts (black line; error bars: SEM), following intramedullary microinjection of saline or SP. Experiments with saline and SP microinjections were conducted at least 7 days apart. Microinjection of SP significantly attenuated histamine- and chloroquine-evoked scratching (F, G). (H) Mechanical withdrawal thresholds were reduced by intramedullary SP, n=10, M = (4.855; 3.253), t=8.96, DF = 8, 95% CI [1.198, 2.007], p = <0.0001 (I) Thermal withdrawal latency was not significantly affected by intramedullary SP, n=8, M = (5.271; 4.691), t=0.9784, DF = 7, 95% CI [–0.8216, 1.981], p=0.3605. (G, F) students t-test *p<0.05, **p<0.01, ***p<0.001. n=5–7 males, 3 females/ group.