Effects of a quaternary lidocaine derivative, QX-314, on the respiratory activity in brainstem-spinal cord preparation from newborn rats

Transient receptor potential vanilloid 1 (TRPV1)-independent actions of capsaicin on cellular excitability and ion transport

Capsaicin is a naturally occurring alkaloid derived from chili pepper that is responsible for its hot pungent taste. Capsaicin is known to exert multiple pharmacological actions, including analgesia, anticancer, anti-inflammatory, antiobesity, and antioxidant effects. The transient receptor potential vanilloid subfamily member 1 (TRPV1) is the main receptor mediating the majority of the capsaicin effects. However, numerous studies suggest that the TRPV1 receptor is not the only target for capsaicin. An increasing number of studies indicates that capsaicin, at low to mid µM ranges, not only indirectly through TRPV1-mediated Ca²⁺ increases, but also directly modulates the functions of voltage-gated Na⁺ , K⁺ , and Ca²⁺ channels, as well as ligand-gated ion channels and other ion transporters and enzymes involved in cellular excitability. These TRPV1-independent effects are mediated by alterations of the biophysical properties of the lipid membrane and subsequent modulation of the functional properties of ion channels and by direct binding of capsaicin to the channels. The present study, for the first time, systematically categorizes this diverse range of non-TRPV1 targets and discusses cellular and molecular mechanisms mediating TRPV1-independent effects of capsaicin in excitable, as well as nonexcitable cells.

The 14-day repeated-dose toxicity studies of a fixed-dose combination, QXOH/levobupivacaine, via subcutaneous injection in mice

QXOH-LB, a fixed-dose combination (35 mM QXOH and 10 mM levobupivacaine) has been shown to induce a long duration of local anesthesia in animal efficacy testing, which indicates potential for postoperative pain management. In this study, we evaluated the potential toxicity of QXOH-LB in NIH mice under the Guidance on the repeated-dose toxicity published by the China Food and Drug Administration. Mice (n = 30 per sex per group) were subcutaneously injected 5, 10, 20 mg/kg QXOH-LB, 5, 10, 20 mg/kg QXOH, and 5 mg/kg levobupivacaine (LB) once a day for 14 days with sacrifice of main study animals; remaining mice (n = 10 per sex per group) were monitored for an additional 4-week recovery period. Mice in the 10 and 20 mg/kg QXOH, and 20 mg/kg QXOH-LB died, which was considered due to excessive respiratory inhibition. The doses of 10 mg/kg QXOH-LB and 5 mg/kg QXOH were well tolerated without any clinical signs of toxicity. Therefore, the no-observed-adverse-effect level (NOAEL) of QXOH-LB and QXOH was considered to be 10 and 5 mg/kg/day, respectively. In the dose range from 5 to 20 mg/kg, the exposure of QXOH and LB in QXOH-LB was equal to each agent used alone at the same dose in NIH mice. There was no gender difference on exposure and no evidence of accumulation.

Quaternary Lidocaine Derivatives: Past, Present, and Future

Local anesthetics have the advantage of complete analgesia with fewer side effects compared to systemic analgesics. However, their clinical use is limited due to their short duration of action. Thus, local anesthetics with fast onset, long duration of action, selective nociceptive block, and low local and systemic toxicity are highly desirable. In the past electrophysiological studies, quaternary lidocaine derivatives (QLDs) showed these characteristics. Here, we review electrophysiological properties of QLDs and their pharmacodynamic characteristics to shed light on potential problems.

Effects of a TRPV1 agonist capsaicin on respiratory rhythm generation in brainstem-spinal cord preparation from newborn rats

The heat-sensitive transient receptor potential vanilloid 1 (TRPV1) channels are expressed in the peripheral and central nervous systems. However, there is no report on how the activation of TRPV1 causes the modulation of neuronal activity in the medullary respiratory center. We examined effects of capsaicin, a specific agonist of TRPV1 channels, on respiratory rhythm generation in brainstem-spinal cord preparation from newborn rats. Capsaicin induced a biphasic response in the respiratory rhythm (a transient decrease followed by an increase in the C4 rate). The second-phase excitatory effect (but not the initial inhibitory effect) in the biphasic response was partly blocked by capsazepine or AMG9810 (TRPV1 antagonists). Capsaicin caused strong desensitization. After its washout, the strength of C4 burst inspiratory activity was augmented once per four to five respiratory cycles. The preinspiratory and inspiratory neurons showed tonic firings due to membrane depolarization during the initial inhibitory phase. In the presence of TTX, capsaicin increased the fluctuation of the membrane potential of the CO₂-sensitive preinspiratory neurons in the parafacial respiratory group (pFRG), accompanied by slight depolarization. The C4 inspiratory activity did not stop, even 60-90 min after the application of 50/100 μM capsaicin. Voltage-sensitive dye imaging demonstrated that the spatiotemporal pattern of the respiratory rhythm generating networks after application of capsaicin (50 μM, 70-90 min) was highly similar to the control. A histochemical analysis using TRPV1 channel protein antibodies and mRNA demonstrated that the TRPV1 channel-positive cells were widely distributed in the reticular formation of the medulla, including the pFRG. Our results showed that the application of capsaicin in the medulla has various influences on the respiratory center: transient inhibitory and subsequent excitatory effects on the respiratory rhythm and periodical augmentation of the inspiratory burst pattern. The effects of capsaicin were partially blocked by TRPV1 antagonists but could be also induced at least partially via the non-specific action. Our results also suggested a minor contribution of the TRPV1 channels to central chemoreception.