Pamela T. Brown

Acute tolerance in morphine analgesia: continuous infusion and single injection in rats.

This study aimed to determine whether the decline of the analgesic effect of morphine with a continuous infusion or that after a single injection correlates with the changes in brain concentration of morphine. The analgesic effect of morphine and its brain and serum concentrations were determined with a continuous 8-h infusion at a constant rate and after a single subcutaneous injection of the agent. The analgesic effect was determined by measuring the threshold of motor response to noxious stimulation. Brain and serum concentrations of morphine were detected by radioimmunoassay with the use of 125I-labeled morphine. With the constant-rate (4 mg.kg-1.h-1, intravenous) morphine infusion, the peak of analgesia could not be maintained: the increase in the pain threshold at 2 h was 1,003 g and at 8h was 286 g (a decrease in analgesia by 72%, P less than 0.0002). At the same time, the brain morphine concentration tended to increase, to 278 ng/g at 2 h and 329 ng/g at 8 h. After the single morphine injection (6 mg/kg, subcutaneous), recovery from analgesia occurred at a much faster rate than did the decrease in morphine brain concentration; the decrease in pain threshold was 79% at 90 vs. 30 min after the injection (P less than 0.0001), and the corresponding decrease in brain concentration was 28% (NS). The absence of correlation between analgesia and morphine brain concentration both with the constant-rate morphine infusion and after the single injection suggests the development of acute tolerance, which is pharmacodynamic in nature.

Magnitude of Acute Tolerance to Opioids Is Not Related to Their Potency

It was suggested that for a given analgesic effect, more potent opioids may produce smaller degrees of tolerance than those with lower analgesic potency. The use of opioids with high analgesic potency to reduce the rate of tolerance development would be an important therapeutic consideration. This study tested the hypothesis that the degree of acute tolerance to the analgesic effect of opioids is inversely related to their potency. In the experiments on rats, the analgesic effects of morphine, alfentanil, and sufentanil given by a continuous 8-h infusion at a constant rate, were determined by measuring the threshold of motor response to noxious pressure on the tail. The comparative degree of acute tolerance was determined on the basis of the decline in the level of analgesia at the end of the infusion period. Morphine 4 mg.kg-1.h-1, alfentanil 0.45 mg.kg-1.h-1, and sufentanil 0.0085 mg.kg-1.h-1 caused approximately similar increases in the pain threshold. The peak of analgesia could not be maintained; it declined by 74 +/- 6% (P less than 0.0001) with morphine, 86 +/- 6% (P less than 0.0001) with alfentanil, and 92 +/- 2% (P less than 0.0001) with sufentanil. The results indicate that the infusion of alfentanil and sufentanil, which differ from morphine by higher analgesic potency (by 10-fold and more than 100-fold, respectively), results in a decline in the degree of analgesia during infusion similar to that of morphine. These data reject the hypothesis that the magnitude of acute tolerance to the analgesic action of opioid drugs following their systemic administration is inversely related to their potency.

Sedative and hypnotic midazolam-morphine interactions in rats

The midazolam-morphine interactions in relation to the sedative effect and in relation to the hypnotic effect were studied in rats. Two series of experiments (sedative and hypnotic) were performed. In the sedative series, doses that inhibited locomotor activity to 10% or more of the control level were determined when the agents were given singly or in combination. Dose-response curves were determined with a probit procedure. The ED50 values of both agents and their combination were compared with algebraic (fractional) and isobolographic analyses in one subseries of experiments. The effect of a small fixed dose of morphine (1/10 of ED50 value for the sedative effect) on the slope of the sedative dose-response curve for midazolam was determined in the other subseries. In the hypnotic series of experiments, doses (ED50) that blocked the righting reflex with drugs given separately and in combination were determined by a probit procedure and, as in the sedative series, compared with algebraic (fractional) and isobolographic analyses. Sedative interaction between midazolam and morphine was found to have a tendency for synergism (interaction coefficient of 1.56, P > 0.05) with decreased individual variability in the sedative response to the combination. Hypnotic midazolam-morphine interaction was highly synergistic with the interaction coeficient of 3.70 (P < 0.0001). A difference in the outcomes of rnidazolam-morphine interaction regarding sedation and hypnosis suggests that underlying mechanisms for these two effects are different; therefore, they should not be regarded as only increasing depths of the same action.

Pentobarbital-morphine Anesthetic Interactions in Terms of Intensity of Noxious Stimulation Required for Arousal

BackgroundPrevious reports suggest that the outcome (synergism, antagonism, summation) of opioid-barbiturate interactions may depend on the depth of anesthesia. One aim of the present study was to determine whether pentobarbital, alone and in combination with morphine, blocks awakening caused by noxious stimulation in a dose-related manner: the more intense the noxious stimulation, the more pentobarbital is required to suppress the response. A second aim of the study was to determine whether the pentobarbital-morphine anesthetic interaction depends on the depth of anesthesia measured in terms of intensity of noxious stimulation required for behavioral arousal (recovery of the righting reflex). MethodsExperiments were performed on rats, with the measure of anesthetic effect being suppression of the righting reflex. The noxious stimulus was pressure on the tail at four levels of intensity: 0.0, 0.25, 2.5, and 3.3 kg, generated with an Analgesy-Meter. Pentobarbital and morphine were injected intravenously via chronically implanted catheters. Dose-response curves for pentobarbital given alone and in combination with morphine were determined (by probit analysis) separately for each of the pressure levels. ResultsPentobarbital, alone and in combination with morphine, blocked awakening caused by noxious stimulation of different intensities In a dose-related fashion so that more anesthetic was required to block awakening with more intense stimulation. The pentobarbital ED50 values were: 12.0, 19.5, 22.7, and 24.3 mg/kg for 0.0, 0.25, 2.5, and 3.3 kg pressure, respectively. The addition of morphine (1 mg/kg) reduced the pentobarbital ED50 values for 0.0, 0.25, and 2.5 kg pressure by 34% (P < 0.0001), 39% (P < 0.0001), and 21% (P < 0.005), respectively. No change was seen in the pentobarbital ED50 value at the maximal (3.3 kg) pressure level. ConclusionsThe results suggest that the depth of anesthesia can be measured in terms of intensity of noxious stimulation required for arousal and that the outcome of barbiturate-opioid anesthetic interaction depends on the depth of anesthesia.

Diazepam--morphine hypnotic synergism in rats.

The effect of diazepam-morphine combination on the righting reflex was studied in rats. Doses of the drugs given alone and in combination that block righting reflex (RR ED50) were determined with a probit procedure. Brain concentrations following equieffective doses of the drugs administered separately and in combination were determined by radioimmunoassay. Equieffective intravenous doses and corresponding brain concentrations for the agents were compared with fractional (algebraic) and isobolographic analyses. Interaction between diazepam and morphine was found to be synergistic. It is not likely to be pharmacokinetic in nature.

Morphine-caffeine analgesic interaction in rats

The ability of caffeine to modify the effect of morphine on motor response to noxious stimulation was studied in 195 rat experiments. Motor reaction responses to noxious stimuli were studied in three series of experiments with three different techniques of mechanical tail stimulation. In each series of experiments, dose-response curves for morphine (probit analysis) were determined with and without the addition of caffeine (30 mg · kg−1). It was found that caffeine decreased morphine ED50 values in all three series of experiments, from 1.1 to 0.6 mg · kg−1 (P < 0.002), from 3.2 to 2.5 mg · kg−1 (P < 0.01), and from 13.2 to 6.2 mg · kg−1 (P < 0.002). When caffeine was used alone in a dose of 30 mg · kg−1, there were no significant changes in motor reaction responses with any of the three methods applied for the assessment of morphine-caffeine combinations. These data indicate that caffeine potentiates the inhibitory effect of morphine on motor response to noxious stimulation in rats. It has been suggested that the effect of morphine on the motor response to somatic noxious stimulation results primarily from activation of inhibitory control systems concerned with this response. Caffeine may modulate the antinociceptive effect of morphine by stimulating one of these systems.

Barbiturate-benzodiazepine interactions at the gamma-aminobutyric acidA receptor in rat cerebral cortical synaptoneurosomes.

Combinations of benzodiazepines (midazolam and diazepam) with barbiturates (pentobarbital and phenobarbital) exhibit synergistic (supra-additive) hypnotic interactions in rats. Because both benzodiazepines and barbiturates interact with the γ-aminobutyric acidA (GABAA) receptor complex, we have tested the hypothesis that these supra-additive hypnotic interactions are due to a synergistic effect on 36CI− conductance subsequent to binding at allosterically coupled sites on the GABAA receptor ionophore complex. Equilibrium binding and 36CI− flux measurements were performed under nearly identical conditions using rat brain cerebrocortical synaptoneuroomes. The benzodiazepines and barbiturates alone both allosterically enhance binding of [3H]muscimol to comparable, but modest, extents (range = 18%–32% enhancement). Isobolographic analysis reveals that combinations of benzodiazepines and barbiturates do in fact produce a synergistic enhancement of [3H]muscimol binding. Paradoxically, this effect is not translated into a synergistic enhancement of muscimolstimulated 36CI− flux. Because the positively cooperative interactions between benzodiazepines and barbiturates, as demonstrated both behaviorally and by binding measurements, are not reflected in enhanced CI− conductance, the mechanistic basis for hypnotic synergism may involve other non-GABAergic components.

Sympathetic Blockade Increases Tactile Sensitivity

To determine whether tactile sensitivity of the normal skin is altered by suppression of sympathetic efferent activity, the effect of stellate ganglion block and epidural sympathetic block on touch threshold was studied. The study was performed on ten individuals with various chronic pain syndromes. Tactile sensitivity was measured in the normal skin area with the use of von Frey filaments and a two-alternative forced-choice procedure with a staircase presentation of touch stimuli. With stellate ganglion block, touch threshold decreased on the side of the block by 48.8 ± 8.% (P = 0.002) without any significant change in the threshold on the healthy, nonblocked side (P = 0.003 for the difference between the sides). With epidural sympathetic block, touch threshold decreased to the same extent on the diseased and healthy sides, which were both affected by the block (46.2 ± 11.4%, P = 0.027 and 47.7 ± 12.5%, P = 0.032, respectively). The results show that sympathetic blockade increases tactile sensitivity. They also suggest that sympathetic efferent activity modulates the function of tactile receptors. It is hypothesized that the sympathetic modulation makes tactile receptors less sensitive to touch, less specific, and probably more prone to code tactile stimuli in such a way that the brain recognizes this code as pain.

Morphine and fentanyl anesthetic interactions with diazepam : relative antagonism in rats

The anesthetic effects of morphine-diazepam and fentanyl-diazepam combinations as characterized by abolition of the movement response to noxious stimulation were studied in rats to test the hypothesis of antagonistic interactions between the components of these combinations. Noxious pressure on the tail was used to induce the response. Dose-effect curves were constructed for the drugs given alone and in combination. With the use of probit procedure ED50 values for single drugs and their combinations were determined, and the interactions were analyzed with algebraic (fractional) and isobolographic methods. It was found that both morphine and fentanyl have a less than additive (antagonistic) interaction with diazepam. In combination the sum of fractional doses was higher than a single-drug fractional dose, 1.67 versus 1.00 (P < 0.051 for morphine-diazepam and 1.61 versus 1.00 (P < 0.05) for fentanyl-diazepam. The observed antagonism is a relative one that does not increase the requirement for one agent upon the addition of another agent.

Locomotor activity after recovery from hypnosis: midazolam-morphine versus midazolam.

This study was performed to test the hypothesis that sedation after recovery from pharmacologic hypnosis is less pronounced if hypnosis is induced with a midazolam-morphine combination compared with midazolam administered alone. Loss of the righting reflex was used as an index for the hypnotic effect and reduction of locomotor activity as an index for the sedative effect. One group of rats received midazolam (20 mg/kg IV) and another group an equipotent (in relation to the hypnotic effect) combination of midazolam (4 mg/kg TV) and morphine (1.3 mg/kg IV). The duration of loss of the righting reflex in the midazolam and midazolam-morphine groups was 30 ± 3 and 28 ± 2 min, respectively (mean ± SE). The difference between the groups in locomotor activity after recovery from hypnosis was very pronounced. The locomotor activity in the midazolam-morphine group at 1 and 2 h was seven and five times greater, respectively, than in the midazolam group (P < 0.005). The profound difference in locomotor activity for the two treatment groups was explained on the basis of the difference in the outcomes of midazolam-morphine interactions with regard to hypnosis (synergism) and sedation (summation). When the animals recovered from hypnosis, the synergism of the drug interaction ceased to be a contributing factor.