Fig 1.
CONSORT diagram illustrating the flow of participants through each phase of the randomized crossover trial.
Fig 2.
Schematic diagram of Experiment (1) in humans.
Tactile and mechanical pain thresholds were determined using von Frey (vF) hairs applied for 5 s, while thermal pain threshold was measured using a thermal stimulator (THERMAL STIMULATOR®, Dia-medical Co., Tokyo, Japan) heated at 1 °C/min from 32 °C to a maximum of 47 °C. All thresholds were measured at baseline and again at the same site and times following subcutaneous injection of local anesthetic (LA).
Table 1.
Demographic characteristics of human volunteers.
Fig 3.
Changes in tactile and pain thresholds in human volunteers following subcutaneous bupivacaine and levobupivacaine injections.
(A, B) Tactile detection threshold (TDT) (A) and mechanical pain threshold (MPT) (B) measured using von Frey hairs. (C) Thermal pain threshold (TPT) measured by a thermal stimulator. The TDT was significantly higher (tactile response less sensitive) following bupivacaine injection at concentrations of 0.025% and 0.0625% compared to equal-dose levobupivacaine (A). At the same time, MPT and TPT did not differ between LAs at any concentrations tested (B, C). Arrows indicate the cutoffs (B, C), and the dotted line indicates the baseline level (A, B). The Friedman test, followed by Dunn’s post hoc test, was utilized for within-group comparisons, and the Mann–Whitney U test was employed for between-group comparisons. FDR-adjusted p-values < 0.05 indicated significant between-group differences. #: significant differences compared to baseline. *: significant differences between bupivacaine and levobupivacaine.
Fig 4.
Both bupivacaine and levobupivacaine strongly suppressed the spiking responses of wide dynamic-range neurons to noxious stimuli in anesthetized rats.
(A) Typical spike responses of wide-dynamic-range (WDR) neurons to noxious stimuli (pinch and 10 g von Frey hair) at baseline and after subcutaneous administration of bupivacaine or levobupivacaine (both 0.05%). (B) Values represent the median percentage change following subcutaneous administration of bupivacaine (n = 5), levobupivacaine (n = 5), or saline (n = 3), compared with the number of spiking responses to noxious stimuli at the preadministration baseline. Baseline responses across the groups were analyzed using the Friedman test and evaluated using Dunn’s post hoc tests. The Mann–Whitney U test assessed changes induced by anesthesia between the two groups. An FDR-adjusted p-value of < 0.05 from the Mann–Whitney U test indicated significant differences between the groups. #: significant differences vs. baseline. *: significant differences between bupivacaine and levobupivacaine.
Fig 5.
Bupivacaine suppressed the spiking response of wide-dynamic-range neurons to innocuous stimuli more potently than levobupivacaine in anesthetized rats.
(A) Typical responses of wide-dynamic-range (WDR) neurons to innocuous stimuli (brush and 4 g von Frey hair) at baseline and after subcutaneous injection of bupivacaine and levobupivacaine (A). (B) Values represent the median percent change in spiking responses to innocuous stimuli following subcutaneous administration of bupivacaine (n = 5), levobupivacaine (n = 5), or saline (n = 3) compared with measurements taken before drug administration (baseline). The suppression of firing was stronger after administration of bupivacaine. The baseline responses of different groups were analyzed using Friedman and Dunn’s post hoc tests. The Mann–Whitney U test assessed between-group differences in anesthesia-induced changes. A significant difference was identified when the FDR-adjusted p-value from the Mann–Whitney U test was < 0.05. #: significant differences vs. baseline. *: significant differences between bupivacaine and levobupivacaine.
Fig 6.
Bupivacaine suppressed the spiking response of low-threshold neurons to innocuous stimuli more potently than levobupivacaine in anesthetized rats.
(A) Typical responses of low-threshold (LT) neurons to innocuous stimuli (brush and 4 g von Frey hair) at baseline and after subcutaneous administration of bupivacaine and levobupivacaine. (B) Values reflect the median percent changes after subcutaneous bupivacaine (n = 5), levobupivacaine (n = 5), or saline (n = 3) administration relative to the number of spiking responses to innocuous stimuli predrug administration (baseline). Firing was more suppressed after administering bupivacaine. Baseline responses for each group were evaluated using Friedman and Dunn’s post hoc tests. The Mann–Whitney U test analyzed changes induced by anesthesia between the two groups. A Mann–Whitney U test FDR-adjusted p-value of < 0.05 indicated significant differences between the groups. #: significant differences vs. baseline. *: significant differences between bupivacaine and levobupivacaine.