Clonidine intensifies memantine cutaneous analgesia in response to local skin noxious pinprick in the rat

Novel insights on the encapsulation mechanism of PLGA terminal groups on ropivacaine

Currently, the influences of free terminal groups (hydroxyl, carboxyl and ester) of PLGA on encapsulating active pharmaceutical ingredient are relatively ambiguous even though PLGA types were defined as critical quality attributes in vast majority of design of experiment process. In this study, emulsion method combined with premix membrane emulsification technique has been used to encapsulate ropivacaine (RVC), a small molecule local anesthetic in clinical. Based on the narrow particle size distribution, the influences and mechanisms of the terminal groups on properties of ropivacaine loaded microspheres have been investigated in detail. It was found that microspheres prepared by PLGA with hydroxyl or ester groups exhibited lower encapsulation efficiency but faster in vitro release rate than that of carboxyl groups. In the meanwhile, on microcosmic level analysis by quartz crystal microbalance with dissipation, atomic force microscope and confocal laser scanning microscopy, we attributed this distinction to the specific interaction between ropivacaine and different terminal groups. Subsequently, the reaction activation centers were verified by density functional simulation calculation and frontier molecular orbital theory at molecular level. Additionally, pharmacokinetics and pharmacodynamic research of infiltration anesthesia model were performed to compare sustained release ability, duration and intensity of the anesthetic effect in vivo. Finally, potential safety and toxicity were evaluated by the biochemical analysis. This study not only provides a novel mechanism of drug encapsulation process but also potential flexible selections in terms of various anesthesia indications in clinical.

Skin nociceptive block with pramoxine delivery by subcutaneous injection in rats

BACKGROUND

Pramoxine has been shown to produce spinal anesthesia, while cutaneous analgesia (peripheral) of pramoxine is not established. The experimental goal was to examine cutaneous antinociception produced by a local anesthetic (LA) pramoxine and compare this result with that of another well-known LA lidocaine.

METHODS

Cutaneous antinociception was evaluated by blockade of pinprick- induced cutaneous trunci muscle reflex (CTMR) on the skin of rat's back. After the dose-related curves were constructed, the quality and duration of drug's (lidocaine and pramoxine) cutaneous antinociception were compared.

RESULTS

We showed that pramoxine, as well as lidocaine produced skin antinociception in a dose-related fashion. The relative potency (ED₅₀ [50% effective dose] basis) was lidocaine (5.44 [4.67-6.35] μmol) greater than pramoxine (42.1 [38.8-45.7] μmol) (p < 0.01). On the basis of equianalgesic doses (ED₇₅, ED₅₀, and ED₂₅), pramoxine caused equivalent duration of cutaneous antinociception to lidocaine.

CONCLUSIONS

These preclinical data indicated that pramoxine elicits skin antinociception dose-relatedly. Pramoxine exhibits a potency less than that of lidocaine while they have a comparable duration of skin antinociceptive action.

Spinal sensory and motor blockade by intrathecal doxylamine and triprolidine in rats

OBJECTIVES

The aim of this experiment was mainly to examine the effects of intrathecally injected doxylamine and triprolidine, two antihistamine drugs spinal motor and sensory functions.

METHODS

After intrathecally injecting the rats with five different doses, the dose-response curves of spinal sensory and motor block with doxylamine and triprolidine were constructed. In comparison with the local anaesthetic mepivacaine, the quality and duration of spinal anaesthesia with doxylamine or triprolidine were conducted.

KEY FINDINGS

Doxylamine, mepivacaine and triprolidine elicited spinal motor and sensory (nociception and proprioception) blockades in a dose-dependent fashion. On the ED₅₀ (50% effective dose) basis, the rank order of drug potency was triprolidine > mepivacaine > doxylamine (P < 0.05) at provoking spinal motor, proprioceptive and nociceptive blockades. On the equianaesthetic doses (ED₂₅ , ED₅₀ and ED₇₅ ), the duration of spinal anaesthesia with doxylamine was longer (P < 0.01) than that with mepivacaine or triprolidine. Moreover, doxylamine or triprolidine displayed greater potency (ED₅₀ ) (P < 0.05) and duration (P < 0.05) of sensory block over motor block.

CONCLUSIONS

Doxylamine or triprolidine produces a dose-dependent effect of spinal motor and sensory block. Triprolidine with a better nociception-selective action over motor block has a better potency than mepivacaine or doxylamine. Doxylamine and triprolidine produce longer durations than mepivacaine.

Subcutaneous infiltration of doxylamine on cutaneous analgesia in rats

BACKGROUND

We aimed to evaluate the effect of doxylamine, a first generation antihistamine, as a local analgesic agent by comparing its effect to bupivacaine.

METHODS

After blocking the cutaneous trunci muscle reflex (CTMR) by subcutaneous injection of doxylamine, we assessed doxylamine's cutaneous analgesic effect in rats. The dose-related effect and duration of doxylamine on infiltrative cutaneous analgesia were compared with that of bupivacaine.

RESULTS

We demonstrated that doxylamine, as well as the local anesthetic bupivacaine produced the cutaneous analgesic effects in a dose-related fashion. At the equipotent dose (50% effective doses (ED₅₀)), the relative potency was bupivacaine (0.41 (0.36-0.48) mmol)> doxylamine (7.39 (6.91-7.91)mmol) (p<0.01). On an equipotent basis (ED₂₅, ED₅₀ and ED₇₅), subcutaneous doxylamine resulted in greater duration of action (p<0.01) than bupivacaine at producing cutaneous analgesia.

CONCLUSIONS

The result of this experiment indicated that doxylamine has the local anesthetic property less potent than bupivacaine, but its nociceptive block duration is longer than that of bupivacaine at an equianalgesic dose.

Adding Dopamine to Proxymetacaine or Oxybuprocaine Solutions Potentiates and Prolongs the Cutaneous Antinociception in Rats

BACKGROUND

We evaluated the interaction of dopamine-proxymetacaine and dopamine- oxybuprocaine antinociception using isobolograms.

METHODS

This experiment uses subcutaneous drug (proxymetacaine, oxybuprocaine, and dopamine) injections under the skin of the rat's back, thus simulating infiltration blocks. The dose-related antinociceptive curves of proxymetacaine and oxybuprocaine alone and in combination with dopamine were constructed, and then the antinociceptive interactions between the local anesthetic and dopamine were analyzed using isobolograms.

RESULTS

Subcutaneous proxymetacaine, oxybuprocaine, and dopamine produced a sensory block to local skin pinpricks in a dose-dependent fashion. The rank order of potency was proxymetacaine (0.57 [0.52-0.63] μmol/kg) > oxybuprocaine (1.05 [0.96-1.15] μmol/kg) > dopamine (165 [154-177] μmol/kg; P < .01 for each comparison) based on the 50% effective dose values. On the equianesthetic basis (25% effective dose, 50% effective dose, and 75% effective dose), the nociceptive block duration of proxymetacaine or oxybuprocaine was shorter than that of dopamine (P < .01). Oxybuprocaine or proxymetacaine coinjected with dopamine elicited a synergistic antinociceptive effect and extended the duration of action.

CONCLUSIONS

Oxybuprocaine and proxymetacaine had a higher potency and provoked a shorter duration of sensory block compared with dopamine. The use of dopamine increased the quality and duration of skin antinociception caused by oxybuprocaine and proxymetacaine.

Mexiletine co-injected with clonidine increases the quality and duration of cutaneous analgesia in response to skin pinpricks in the rat

The goal of the experimental design was to assess the cutaneous analgesic effect of mexiletine by co-injection with clonidine. The effect of nociceptive block was evaluated according to the inhibition of the cutaneous trunci muscle reflex (CTMR) in response to skin pinpricks in rats. The dose-related analgesic effect of mexiletine alone or mexiletine co-administrated with clonidine was constructed after subcutaneous injection. Subcutaneous injections of mexiletine elicited dose-related cutaneous analgesia. Compared with mexiletine (1.8μmol), adding clonidine to mexiletine (1.8μmol) solutions for skin nociceptive block potentiated and prolonged the action (p<0.01). Mexiletine (6μmol) combined with clonidine extended the duration of cutaneous analgesia when compared with mexiletine (6μmol) alone (p<0.01). Co-administration of clonidine increases the potency and extends the duration of cutaneous analgesia by mexiletine, and the minimal dose of clonidine to intensify the analgesic effect is 0.06μmol.

Clonidine as an adjuvant for propranolol enhances its effect on infiltrative cutaneous analgesia in rats

Clonidine prolongs duration of analgesia when used as an adjunct to local anesthetics for infiltrative cutaneous analgesia, and propranolol produces local anesthesia. The purpose of the experiment was to evaluate clonidine as an adjuvant for propranolol on the quality and duration of cutaneous analgesia. A rat model of cutaneous trunci muscle reflex (CTMR) in response to local skin pinprick was employed to evaluate the cutaneous analgesic effect of propranolol combined with clonidine. The long-lasting local anesthetic bupivacaine was used as control. Cutaneous analgesia elicited by propranolol and bupivacaine was dose-dependent, and both propranolol (9.0μmol) and bupivacaine (1.8μmol) produced 100% nociceptive blockade. On an 50% effective dose (ED50) basis, the relative potency was bupivacaine [0.48 (0.42-0.55) μmol] greater than propranolol [2.27 (1.98-2.54) μmol] (p<0.01). Subcutaneous saline and clonidine (0.12μmol) did not produce cutaneous analgesia. The mixture of an ineffective-dose clonidine (0.12μmol) and a drug (propranolol or bupivacaine) at ED50 or ED95 increased the potency and extended the duration at producing cutaneous analgesia. The resulting data demonstrated that propranolol is less potent than bupivacaine as an infiltrative anesthetic. Clonidine as an adjuvant for propranolol or bupivacaine has a significant peripheral action in increasing the depth and duration of action on infiltrative cutaneous analgesia.

Memantine elicits spinal blockades of motor function, proprioception, and nociception in rats

Although memantine blocks sodium currents and produces local skin anesthesia, spinal anesthesia with memantine is unknown. The purpose of the study was to evaluate the local anesthetic effect of memantine in spinal anesthesia and its comparison with a widely used local anesthetic lidocaine. After intrathecally injecting the rats with five doses of each drug, the dose-response curves of memantine and lidocaine were constructed. The potencies of the drugs and durations of spinal anesthetic effects on motor function, proprioception, and nociception were compared with those of lidocaine. We showed that memantine produced dose-dependent spinal blockades in motor function, proprioception, and nociception. On a 50% effective dose (ED50 ) basis, the rank of potency was lidocaine greater than memantine (P < 0.05 for the differences). At the equipotent doses (ED25 , ED50 , ED75 ), the block duration produced by memantine was longer than that produced by lidocaine (P < 0.05 for the differences). Memantine, but not lidocaine, displayed more sensory/nociceptive block than motor block. The preclinical data demonstrated that memantine is less potent than lidocaine, whereas memantine produces longer duration of spinal anesthesia than lidocaine. Memantine shows a more sensory-selective action over motor blockade.