Cutaneous synergistic analgesia of bupivacaine in combination with dopamine in rats

Targeted local anesthesia: a novel slow-release Fe3O4-lidocaine-PLGA microsphere endowed with a magnetic targeting function

PURPOSE

Lidocaine microspheres can prolong the analgesic time to 24-48 h, which still cannot meet the need of postoperative analgesia lasting more than 3 days. Therefore, we added Fe3O4 to the lidocaine microspheres and used an applied magnetic field to attract Fe3O4 to fix the microspheres around the target nerves, reducing the diffusion of magnetic lidocaine microspheres to the surrounding tissues and prolonging the analgesic time.

METHODS

Fe3O4-lidocaine-PLGA microspheres were prepared by the complex-emulsion volatilization method to characterize and study the release properties in vitro. The neural anchoring properties and in vivo morphology of the drug were obtained by magnetic resonance imaging. The nerve blocking effect and analgesic effect of magnetic lidocaine microspheres were evaluated by animal experiments.

RESULTS

The mean diameter of magnetically responsive lidocaine microspheres: 9.04 ± 3.23 μm. The encapsulation and drug loading of the microspheres were 46.18 ± 3.26% and 6.02 ± 1.87%, respectively. Magnetic resonance imaging showed good imaging of Fe3O4-Lidocain-PLGA microspheres, a drug-carrying model that slowed down the diffusion of the microspheres in the presence of an applied magnetic field. Animal experiments demonstrated that this preparation had a significantly prolonged nerve block, analgesic effect, and a nerve anchoring function.

CONCLUSION

Magnetically responsive lidocaine microspheres can prolong analgesia by slowly releasing lidocaine, which can be immobilized around the nerve by a magnetic field on the body surface, avoiding premature diffusion of the microspheres to surrounding tissues and improving drug targeting.

Intrathecal dopamine and serotonin enhance motor and nociceptive blockades of lidocaine in rats

The study examined the effect of intrathecal injection of dopamine (serotonin) and/or lidocaine. Intrathecal injections of dopamine (serotonin or epinephrine), lidocaine, or their combination were carried out in male Sprague Dawley rats. Neurobehavioral examinations (motor and nociceptive reactions) were performed before and after spinal injection. Intrathecal serotonin (1.5 μmol), dopamine (2.5 μmol), epinephrine (1:40000), and lidocaine (0.75 μmol) produced 29%, 33%, 29%, and 54% nociceptive blockade, whereas serotonin (1.5 μmol), dopamine (2.5 μmol), or epinephrine (1:40000) produced a longer duration of nociceptive blockade than lidocaine (0.75 μmol) (P < 0.05). Serotonin (1.5 μmol), dopamine (1.25 and 2.5 μmol), or epinephrine (1:40000 and 1:80000) prolonged the duration and increased the potency of spinal motor and nociceptive blockades of lidocaine (50% effective dose, ED50) (P < 0.05). The motor and nociceptive blockades caused by lidocaine (ED50) plus dopamine (2.5 μmol) or lidocaine (ED50) plus epinephrine (1:40000) were more outstanding than lidocaine (ED50) plus serotonin (0.75 μmol) (P < 0.05). Our study provides evidence that intrathecal dopamine or serotonin produces spinal nociceptive blockade dose-dependently. Dopamine and serotonin are less potent than lidocaine in inducing spinal nociceptive blockade. When mixed with lidocaine solution, dopamine or serotonin improves spinal motor and nociceptive blockades. The motor and nociceptive blockade caused by lidocaine (ED50) plus dopamine (2.5 μmol) is similar to that caused by lidocaine (ED50) plus epinephrine (1:40000).

A comparative study on the effect of dopamine vs phenylephrine in improving the cutaneous analgesic effect of mexiletine in rats

BACKGROUND

The present study aimed to compare the effects of the combined administration of two adjuvants, dopamine and phenylephrine, on the cutaneous analgesic effect and duration of mexiletine in rats.

METHODS

Nociceptive blockage was evaluated by the inhibition of response to skin pinpricks in rats via the cutaneous trunci muscle reflex (CTMR). After subcutaneous injection, the analgesic activities of mexiletine in the absence and presence of either dopamine or phenylephrine were assessed. Each injection was standardized into 0.6 ml with a mixture of drugs and saline.

RESULTS

Subcutaneous injections of mexiletine successfully induced dose-dependent cutaneous analgesia in rats. The results revealed that rats injected with 1.8 μmol mexiletine exhibited 43.75% blockage (%MPE), while rats injected with 6.0 μmol mexiletine showed 100% blockage. Co-application of mexiletine (1.8 or 6.0 μmol) with dopamine (0.06, 0.60, or 6.00 μmol) elicited full sensory block (%MPE). Sensory blockage ranged from 81.25% to 95.83% in rats injected with mexiletine (1.8 μmol) and phenylephrine (0.0059 or 0.0295 μmol), and complete subcutaneous analgesia was observed in rats injected with mexiletine (1.8 μmol) and a higher concentration of phenylephrine (0.1473 μmol). Furthermore, mexiletine at 6.0 μmol completely blocked nociception when combined with any concentration of phenylephrine, while 0.1473 μmol phenylephrine alone exhibited 35.417% subcutaneous analgesia. The combined application of dopamine (0.06/0.6/6 μmol) and mexiletine (1.8/6 μmol) resulted in increased %MPE, complete block time, full recovery time, and AUCs compared to the combined application of phenylephrine (0.0059 and 0.1473 μmol) and mexiletine (1.8/6 μmol) (p < 0.001).

CONCLUSION

Dopamine is superior to phenylephrine in improving sensory blockage and enhancing the duration of nociceptive blockage by mexiletine.

Intrathecal pramipexole and selegiline for sensory and motor block in rats

BACKGROUND

The purpose of the study was to investigate spinal sensory and motor block by antiparkinsonian drugs (pramipexole and selegiline), and the combination of pramipexole and the local anesthetic lidocaine.

METHODS

Using a technique of spinal blockade in rats, the effects of pramipexole, selegiline, and coadministration of pramipexole and lidocaine on spinal blockades of motor and sensory function were investigated.

RESULTS

Under a concentration of 100 mM, pramipexole displayed more potent and had a longer duration of nociceptive, proprioceptive, and motor block than selegiline, whereas pramipexole and selegiline were less potent in comparison to lidocaine. Pramipexole produced spinal nociceptive, proprioceptive, and motor blocks in a dose-related manner. On the ED₅₀ (50% effective dose) basis, the rank-order potency on nociceptive, proprioceptive, and motor block was pramipexole < lidocaine. The spinal block duration of pramipexole was greater than lidocaine at every equipotent dose tested (ED₂₅, ED₅₀, and ED₇₅). Coadministration of lidocaine (ED₅₀ or ED₉₅) with pramipexole (4.5 μmol/kg) improved the effect (efficacy) and duration of the spinal block.

CONCLUSIONS

Pramipexole and selegiline were less potent than lidocaine to block sensory and motor responses. The duration of the spinal anesthetic effect of pramipexole was longer than lidocaine. At a non-effective dose, pramipexole increased the duration of efficacy of lidocaine.

Dopamine enhancement of dextrorphan-induced skin antinociception in response to needle pinpricks in rats

BACKGROUND

Dextrorphan with long-acting local anesthetic effects did not cause system toxicity as fast as bupivacaine, while catecholamines (i.e., epinephrine) with the vasoconstrictive characteristics enhanced the effects of local anesthetic drugs. The objective of the experiment was to examine the synergistic effect of local dopamine (a catecholamine) injection on cutaneous antinociception of dextrorphan.

METHODS

The panniculus reflex in response to skin stimulation with a needle was used as the primary endpoint when dextrorphan (1.50, 2.61, 5.46, 10.20 and 20.40 μmol) alone, dopamine (16.20, 32.40, 51.60, 60.00 and 81.60 μmol) alone, or dopamine + dextrorphan (a ratio of ED₅₀vs. ED₅₀) was injected subcutaneously on the rat's back. We used an isobolographic modelling approach to determine whether a synergistic effect would be observed.

RESULTS

We showed that dextrorphan, dopamine, or the mixture of dopamine and dextrorphan produced dose-related skin antinociception. The potency (ED₅₀, 50% effective dose) for cutaneous antinociception was dextrorphan [6.02 (5.93-6.14) μmol] greater than dopamine [48.91 (48.80-49.06) μmol] (p < 0.01). The duration of nociceptive inhibition induced by dopamine was longer than that induced by dextrorphan (p <  0.01) based on their equipotent doses (ED₂₅, ED₅₀, and ED₇₅). Enhancement and prolongation of skin antinociception occurred after co-administration of dopamine with dextrorphan.

CONCLUSIONS

When compared to dopamine, dextrorphan was more potent and had a shorter duration of skin nociceptive block. Dopamine produced a synergistic effect on dextrorphan-mediated antinociception, and prolonged dextrorphan's antinociceptive duration.

Phentolamine Reverses Epinephrine-Enhanced Skin Antinociception of Dibucaine in Rats

BACKGROUND

The objective of the experiment was to assess the antinociceptive effect of dibucaine, bupivacaine, and epinephrine. To assess the mechanism of action of the interaction between dibucaine and epinephrine, phentolamine, a nonselective α-adrenergic antagonist, was added to the mixture.

METHODS

We assessed sensory blockade with these drugs by injecting 0.6 mL of drug-in-saline in the dorsal thoracolumbar area of rats; pinprick of the "wheal" formed by the injectate was the area targeted for stimulation to elicit a cutaneous trunci muscle reflex. The sensory block of dibucaine was compared with that of bupivacaine or epinephrine. Drug-drug interactions were analyzed by isobologram. Phentolamine was added to investigate the antinociceptive effect of dibucaine coinjected with epinephrine.

RESULTS

We demonstrated that dibucaine, epinephrine, and bupivacaine produced dose-dependent skin antinociception. On the median effective dose (ED50) basis, the potency was higher for epinephrine (mean, 0.011 [95% confidence interval {CI}, 0.007-0.015] μmol) than for dibucaine (mean, 0.493 [95% CI, 0.435-0.560] μmol) (P < .01), while there were no significant differences between dibucaine and bupivacaine (mean, 0.450 [95% CI, 0.400-0.505] μmol). On the equipotent basis (75% effective dose, median effective dose, and 25% effective dose), sensory block duration provoked by epinephrine was greater (P < .01) than that provoked by dibucaine or bupivacaine. Coadministration of dibucaine with epinephrine produced a synergistic nociceptive block, whereas phentolamine blocked that synergistic block.

CONCLUSIONS

The preclinical data indicated that there is no statistically significant difference between the potency and duration of dibucaine and bupivacaine in this model. Epinephrine synergistically enhances the effects of dibucaine, while phentolamine partially blocked those effects. α-Adrenergic receptors play an important role in controlling synergistic analgesic effect of dibucaine combined with epinephrine.

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.

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.

Isobolographic analysis of the cutaneous antinociceptive interaction between bupivacaine co-injected with serotonin in rats

BACKGROUND

The aim of this experiment was to investigate a long-lasting local anesthetic bupivacaine combined with serotonin at inducing cutaneous antinociception.

METHODS

The skin antinociception, characterized by an inhibition of the cutaneous trunci muscle reflex (CTMR) following the pinprick on the dorsal skin of rats, was evaluated. The cutaneous antinociceptive effects of bupivacaine alone, serotonin alone, or bupivacaine co-injected with serotonin in a dose-dependent fashion were constructed, while the drug-drug interactions were evaluated by isobologram.

RESULTS

Subcutaneous serotonin, as well as the local anesthetic bupivacaine provoked dose-related cutaneous antinociception. On an equipotent basis (50% effective dose [ED₅₀]), the relative potency was bupivacaine (0.43 [0.37-0.50] μmol)>serotonin (1.27 [1.15-1.40] μmol) (p<0.01). At the equi-anesthetic doses (ED₇₅, ED₅₀ and ED₂₅), the duration of bupivacaine was similar to that of serotonin at producing cutaneous antinociceptive effects. Co-administration of bupivacaine and serotonin displayed a synergistic antinociception.

CONCLUSIONS

The preclinical data demonstrated that serotonin is less potent in eliciting cutaneous antinociceptive effects but has the similar duration of action, compared with bupivacaine. We also found a more significant depth of the sensory block with bupivacaine+serotonin than bupivacaine alone.

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.

Synergistic Effects of Serotonin or Dopamine Combined With Lidocaine at Producing Nociceptive Block in Rats

BACKGROUND

The purpose of this experiment was to investigate the interactions of the local anesthetic lidocaine combined with an agent (serotonin or dopamine) as infiltrative anesthetics.

METHODS

Cutaneous analgesia was characterized by the blockade of the cutaneous trunci muscle reflex following 6 skin pinpricks on the rat back. Serotonin or dopamine at producing cutaneous analgesia in a dosage-dependent fashion was compared with lidocaine. Drug-drug interactions were calculated using the isobolographic analysis.

RESULTS

We revealed that serotonin, dopamine, and lidocaine provoked dose-related cutaneous analgesic effects. On the 50% effective dose basis, the rank of drugs' potency was found to be serotonin (1.70 [1.56-1.85] μmol) > lidocaine (5.18 [4.67-5.75] μmol) > dopamine (43.0 [40.9-45.2] μmol) (P < 0.01). At doses equivalent to their 25%, 50%, and 75% effective doses, serotonin or dopamine elicited a longer duration of action than lidocaine (P < 0.01) on producing cutaneous analgesia. Coadministration of serotonin or dopamine with lidocaine produced a synergistic effect.

CONCLUSIONS

The preclinical data showed that serotonin and dopamine produce dose-related cutaneous analgesic effects as an infiltrative anesthetic. Serotonin has a better potency with a much longer duration of action compared with lidocaine at provoking cutaneous analgesia. Serotonin or dopamine as an adjuvant increases the quality of lidocaine in cutaneous analgesia.