Margit Szikszay

Potentiation of thermoregulatory and analygesic effects of morphine by calcium antagonists

Diltiazem, a calcium slow channel blocker, greatly potentiated and prolonged the antinociceptive effect of morphine in rats. Hypothermia, the primary thermoregulatory effect of morphine, was also potentiated. Verapamil, another calcium blocker elicited corresponding changes in the analgetic and thermoregulatory effects of morphine. These findings seem to be in concert with the suggestions that opiate effects on thermoregulation and nociception are exerted via modulation of calcium fluxes across neural membranes.

Antinociceptive effects of intrathecal endomorphin-1 and -2 in rats

Endomorphin-1 and endomorphin-2 were recently postulated to be endogenous mu-opioid receptor agonists. We have investigated the antinociceptive and antihyperalgesic effects of intrathecally administered endomorphins in cumulative doses (0.1-100 microg) on acute and inflammatory pain sensations in awake rats. In the tail-flick test, both peptides caused a dose-dependent short-lasting antinociception, except at the highest dose, which caused motor impairment also. The dose-response curves revealed the development of acute tolerance (tachyphylaxis) to endomorphin. Similarly in the carrageenan-injected paw, the endomorphins (10 microg) exerted transient antinociceptive effects. These are the first data to demonstrate decreased responsivity in models of both acute and inflammatory pain after intrathecal administration of endomorphin-1 and -2 in awake rats.

The Effects of Ketamine and Its Enantiomers on the Morphine- or Dexmedetomidine-induced Antinociception after Intrathecal Administration in Rats

Background The spinal administration of some N-methyl-d-aspartate receptor antagonists results in antinociception and potentiates the effects of opioids and &agr;2-adrenoceptor agonists, but ketamine and its enantiomers have not been examined. The present study investigated the interactions of racemic ketamine, R (−)-ketamine and S (+)-ketamine with morphine and with dexmedetomidine. Methods Intrathecal catheters were implanted into male Wistar rats. Three days later, the acute nociceptive sensitivity was assessed using the tail-flick test. Analgesic latencies were converted to the percentage maximum possible effect. The dose that yielded 50% of the maximum possible effect (ED50) and dose–response and time–course curves were determined for the ketamines (30–300 &mgr;g), morphine (0.1–3.0 &mgr;g), dexmedetomidine (0.3–10.0 &mgr;g), and mixtures of two doses of ketamines (30 or 100 &mgr;g) with different doses of morphine or dexmedetomidine for fixed-dose analysis. Results Neither racemic ketamine nor its enantiomers alone had a significant effect on the tail-flick test, with the exception of the highest dose of racemic ketamine, which caused motor impairment. Morphine and dexmedetomidine each produced dose-dependent antinociception, with ED50 of 1.7 &mgr;g (95% confidence interval: 1.04–2.32) and 4.85 &mgr;g (3.96–5.79), respectively. A low dose (30 &mgr;g) of racemic ketamine or its enantiomers did not influence the ED50 of morphine significantly. Coadministration of 100 &mgr;g racemic ketamine or S (+)-ketamine, but not R (−)-ketamine, significantly enhanced and prolonged the antinociceptive effect of morphine. Both doses of racemic ketamine or its isomers significantly decreased the ED50 value for dexmedetomidine, although the higher dose of racemic or S (+)-ketamine had the highest potency. One-hundred micrograms of racemic ketamine or S (+)-ketamine also prolonged the effects of dexmedetomidine. Conclusions These data indicate that racemic ketamine and S (+)-ketamine, but not R (−)-ketamine, exhibit similar effectiveness in potentiating the antinociceptive effects of both morphine and dexmedetomidine.

Antinociceptive effect of the S(+)-enantiomer of ketamine on carrageenan hyperalgesia after intrathecal administration in rats

Ketamine exerts antinociceptive effects in many pain tests.We investigated the antinociceptive effect of intrathecally administered racemic ketamine and its S(+)- and R(-)-enantiomer on carrageenan-induced thermal hyperalgesia with a paw withdrawal test and acute pain (hot plate and tailflick) tests. Rats were prepared with a chronic lumbar intrathecal catheter to receive either saline or ketamine enantiomers in cumulative doses. None of the ketamines (10, 50, or 100 [micro sign]g) had any effect on the withdrawal latency of the contralateral, noninjected paw. In the injected paw, intrathecal saline did not alter carrageenan-induced thermal hyperalgesia, whereas intrathecally applied S(+)-, R(-)-, and racemic ketamine decreased thermal hyperalgesia. However, compared with saline, racemic ketamine had a higher efficacy than S(+)-ketamine, whereas R(-)-ketamine did not achieve statistical significance. Neither S(+)- nor R(-)-ketamine had a significant effect in the tailflick test (10, 100, or 500 [micro sign]g). In the hot plate test, only the largest dose of ketamine (500 [micro sign]g) caused a non-stereospecific, significant increase in hot plate latency; this dose caused supraspinal effects as well. The results demonstrate that the behavioral hyperalgesia associated with carrageenan-induced hindpaw inflammation in rats is attenuated by the intrathecal administration of racemic and S(+)-ketamine, but not R(-)-ketamine, which only displayed an insignificant trend toward a dose-response relationship. This finding warrants further studies to investigate a possible clinical advantage of preservative-free S(+)-ketamine over the currently used preservative containing racemic mixture. Implications: In rats, intrathecal S(+)-ketamine was effective for treating inflammatory pain. Although racemic ketamine has a greater efficacy, S(+)-ketamine is available as a preservative-free drug and might be of clinical interest for future neuraxial administration in different pain states. (Anesth Analg 1998;86:561-5)

Effect of intrathecal agmatine on inflammation-induced thermal hyperalgesia in rats

Agmatine, an endogenous ligand, interacts both with the alpha2-adrenoceptors and with the imidazoline binding sites. The effect of intrathecally administered agmatine on carrageenan-induced thermal hyperalgesia was investigated by means of a paw-withdrawal test in rats. The effect of agmatine on morphine-induced anti-hyperalgesia was also studied. Intrathecal agmatine in doses larger than 250 microg caused a decrease in the pain threshold, with vocalization and agitation lasting for several hours in all animals. Agmatine alone at 1-100 microg did not give rise to any change in the thermal withdrawal threshold in the contralateral non-inflamed paw. Agmatine pretreatment was found to dose-dependently attenuate the thermal hyperalgesia induced by intraplantar carrageenan. The effect of 100 microg agmatine was completely lost by 60 min, whereas the effect of 50 microg was of similar magnitude but exhibited a longer duration. Agmatine posttreatment had a slighter effect. Agmatine pretreatment (100 microg) together with 1 microg morphine (subeffective dose) has significantly higher anti-hyperalgesic effect then the individual compounds by themselves. These are the first data demonstrating the behavioral and anti-hyperalgesic effects of intrathecal agmatine. The results reveal important interactions between intrathecal agmatine and opioids in thermal hyperalgesia.

Enhancement of fentanyl analgesia by clonidine plus verapamil in rats

An investigation was made of the analgesic effects of the subcutaneous coad ministration of fentanyl, an opioid μ- agonist (15 μg/kg), clonidine, an α2-agonist (100 μg/kg), and verapamil, a calcium channel blocker (10 mg/kg) in rats. Nociceptive sensitivity was assessed with hot-plate and tail-flick techniques. None of the three drugs alone was associated with appreciable analgesic effects in the doses used. The simultaneous administration of the three drugs resulted in marked analgesia superior to that of all binary combinations of these drugs. Two-way analysis of variance showed statistically significant differences in hot-plate and tail-flick latencies after drug treatments (P < 0.001). The significant differences in the area under the time-response curve values (P < 0.001) might indicate not only an increased analgesic effect, but also a prolongation of anti- nociception. These results suggest the existence of hitherto unreported interactions between drugs involved in the production of analgesia.

Sensitization or tolerance to morphine effects after repeated stresses

Rats were subjected to prolonged footshock, intensive acoustic stress, cold water swim and restraint over a period of 10 days. The analgesic and thermoregulatory properties of morphine (2, 4 and 8 mg/kg, sc.) were tested on the 11th day. Analgesia assessment was performed by means of hot-plate (HP) and tail-flick (TF) tests, and body temperature (Tb) changes was measured. Prolonged footshock and acoustic stress increased the sensitivity to morphine, while repeated restraint lessened morphines effect. Cold water swim caused ambiguous consequences, facilitated the effect of a small dose of morphine, but reduced that of a large dose. It was concluded that the sensory components of the stressful exposure determine the effects of repeated stress on morphine sensitivity. Whereas painful interventions led to sensitization, and non-painful procedures result in tolerance to morphines effects. The finding that analgesic and thermoregulatory effects of morphine were simultaneously enhanced supports the contention that the mechanism of sensitization to opiates involves a site where pathways mediating opiate analgesia and thermoregulatory effects converge.

Calcium channels are involved in the hypnotic-anesthetic action of dexmedetomidine in rats

Our study examined whether calcium channels are involved in the anesthetic action of dexmedetomidine (100–300 μg/kg), a highly selective α2-adrenoceptor agonist. To investigate this, we studied the effects of verapamil (1.25 or 2.5 mg/kg), a calcium channel blocker, and BAY K8644 (0.5 or 1 mg/kg), a calcium channel agonist, on the hypnotic-anesthetic effect of dexmedetomidine in rats. Loss of the righting reflex was used to determine the presence of anesthesia, and its length in minutes was referred to as the duration of hypnosis. Verapamil significantly enhanced the duration of the hypnotic-anesthetic action of dexmedetomidine (P <0.05). In contrast, BAY K8644 caused a significantly increased onset of hypnosis (P <0.001) and attenuated the anesthetic property of dexmedetomidine. These results suggest that the hypnotic-anesthetic action of dexmedetomidine is influenced by the activation/gating of calcium channels.

An isobolographic analysis of the hypnotic effects of combinations of dexmedetomidine with fentanyl or diazepam in rats.

The effects of the combinations of dexmedetomidine-fentanyl and dexmedetomidine-diazepam on the righting reflex were studied in rats. The doses that block the righting reflex for the agents given alone and for their combinations were determined with a probit procedure and compared with an isobolographic analysis. The interactions between dexmedetomidine and fentanyl or diazepam were found to be synergistic. In the dexmedetomidine-diazepam combination studies, less than one-fourth of the single drug dose (for each of two agents) was needed to produce the required effect. These data confirm synergistic interactions between dexmedetomidine and fentanyl or diazepam in producing hypnotic-anesthetic action.

An ultrasonographic method for the evaluation of dexmedetomidine on micturition in intact rats

We describe a simple, noninvasive, nontraumatic, and reproducible ultrasonographic method to determine the effect of the alpha 2-adrenoceptor agonist dexmedetomidine in anesthetic dose (300 micrograms/kg subcutaneously) on the micturition reflex in intact rats. The bladder volumes were estimated by an ellipsoid equation. To validate the reliability of the method, an in vitro model assessment also was performed. The mean difference between estimated and instilled volumes were 27.3 microL (-21.61, 76.23). The highly significant correlation (r = 0.98, p < .001) indicates that the ultrasonography with the equation is a reliable tool. After the dexmedetomidine administration to intact rats, urine dribbling occurred at 30 +/- 4.8 min. The volume threshold for urination was 2100 +/- 100 microL. Although dribbling of the urine was observed almost continuously, significant differences were not observed between bladder volumes obtained at any time (from 10 to 100 min). This study indicates that the alpha 2-adrenoceptor agonist dexmedetomidine inhibits the micturition reflex in intact rats. The method described, which is both noninvasive and nonpainful, may therefore be widely used to quantify in small animals pharmacological effects on the urinary bladder.