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

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.

Naphazoline and oxymetazoline are superior to epinephrine in enhancing the cutaneous analgesia of lidocaine in rats

This study observed the cutaneous analgesic effect of adrenergic agonists when combined with lidocaine. We aimed at the usefulness of four adrenergic agonists and epinephrine as analgesics or as tools to prolong the effect of local anesthetics using a model of cutaneous trunci muscle reflex (pinprick pain) in rats. We showed that subcutaneous four adrenergic agonists and epinephrine, as well as the local anesthetic bupivacaine and lidocaine, developed a concentration-dependent cutaneous analgesia. The rank order of the efficacy of different compounds (ED₅₀ ; median effective dose) was epinephrine [0.013 (0.012-0.014) μmol] > oxymetazoline [0.25 (0.22-0.28) μmol] > naphazoline [0.42 (0.34-0.53) μmol] = bupivacaine [0.43 (0.37-0.50) μmol] > xylometazoline [1.34 (1.25-1.45) μmol] > lidocaine [5.86 (5.11-6.72) μmol] > tetrahydrozoline [6.76 (6.21-7.36) μmol]. The duration of full recovery caused by tetrahydrozoline, oxymetazoline, or xylometazoline was greater (P < 0.01) than that induced via epinephrine, bupivacaine, lidocaine, or naphazoline at equianesthetic doses (ED₂₅ , ED₅₀ , and ED₇₅ ). Co-administration of lidocaine (ED₅₀ ) with four adrenergic agonists or epinephrine enhanced the cutaneous analgesic effect. We observed that four adrenergic agonists and epinephrine induce analgesia by themselves, and such an effect has a longer duration than local anesthetics. Co-administration of lidocaine with the adrenergic agonist enhances the analgesic effect, and the cutaneous analgesic effect of lidocaine plus naphazoline (or oxymetazoline) is greater than that of lidocaine plus epinephrine.

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.

Chlorpheniramine produces cutaneous analgesia in rats

BACKGROUND

This study sought to assess the cutaneous (peripheral) analgesic effects of antihistamine chlorpheniramine, compared with the long-lasting local anesthetic bupivacaine.

METHODS

After chlorpheniramine and bupivacaine were subcutaneously injected under the dorsal skin of the rats, the cutaneous analgesia effect was quantitatively evaluated by scoring the number to which the animal failed to react (cutaneous trunci muscle reflex). The quality and duration of chlorpheniramine and bupivacaine on infiltrative cutaneous analgesia were compared.

RESULTS

We revealed that subcutaneous chlorpheniramine, as well as the local anesthetic bupivacaine elicited cutaneous analgesia in a dosage-dependent manner. Based on their ED₅₀s (50% effective doses), the relative potency was found to be chlorpheniramine [1.13 (1.05-1.22) μmol] < bupivacaine [0.52 (0.46-0.58) μmol] (p < 0.01). When comparing the ED₂₅s, ED₅₀s and ED₇₅s, full recovery time induced by chlorpheniramine was longer (p < 0.01) than that induced by bupivacaine.

CONCLUSIONS

Our preclinical data demonstrated that both chlorpheniramine and bupivacaine dose-dependently provoked the cutaneous analgesic effects. Chlorpheniramine with a more prolonged duration was less potent than bupivacaine in inducing cutaneous analgesia.

Subcutaneous brompheniramine for cutaneous analgesia in rats

Brompheniramine as an antihistamine blocked sodium channels, and local anesthetics by blocking sodium channels produced the local anesthetic effects. The authors aimed to assess local anesthetic quality and duration of brompheniramine when compared to the local anesthetic mepivacaine. After rats were shaved and injected subcutaneously on the dorsal skin, the panniculus reflex, induced via applying a noxious pinprick to the skin (injected area), was scored. The dose-response curve and nociceptive block duration of brompheniramine were constructed and compared with mepivacaine. The cutaneous analgesic effects in both brompheniramine and mepivacaine groups were concentration-dependent. On the basis of the amount required to produce a 50% block effect (ED₅₀, 50% effective dose), the drug's potency was brompheniramine (0.89 [0.82-0.96] μmol) better than mepivacaine (2.45 [2.17-2.76] μmol) (P < 0.01). Full recovery time of brompheniramine was more prolonged than mepivacaine's (P < 0.01) on infiltrative cutaneous analgesia when comparing ED₂₅s, ED₅₀s and ED₇₅s. Our preclinical data demonstrated that subcutaneous brompheniramine induces dose-relatedly analgesic effects, and brompheniramine induces prolonged analgesic duration when compared with mepivacaine. Brompheniramine also provokes better cutaneous analgesia than mepivacaine.

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.