Paul D. Thut

The effects of different antiemetic agents on morphine-induced emesis in ferrets

There is interest in the development of antiemetics other than dopamine receptor antagonists for the treatment of postoperative nausea and vomiting. A ferret model of morphine-induced emesis may have wider application in evaluating newer agents than the apomorphine dog model. This study describes the conditions for morphine-induced emesis in ferrets and evaluates five antiemetics that are prototypical of three different mechanisms. The average numbers of vomiting and retching episodes induced by morphine (0.1-2.5 mg/kg s.c.) were distributed as a bell-shaped curve. Maximum number of vomits occurred at 0.3 mg/kg (11.8 +/- 2.1 vomits; 45 +/- 12.5 retches). Antiemetics or vehicle were given i.v. 5 min prior to morphine while each ferret was maintained under isoflurane-O2 anesthesia. Ondansetron, a 5-HT3 receptor antagonist, reduced vomiting episodes by 47% and 70% (3 and 10 mg/kg). Granisetron, a 5-HT3 receptor antagonist was inactive at doses of 0.1, 1.0, 3.0 and 10 mg/kg. Metoclopramide reduced vomiting episodes by 48% and 82% (3 and 10 mg/kg). Droperidol reduced vomiting episodes by 84% at 3 mg/kg. Naloxone reduced vomiting episodes by 91% and 43% at doses of 0.1 and 1.0 mg/kg. In most cases, prolonged latency times to the first episodes accompanied the reduction in total numbers of episodes. The significant reduction of morphine-induced emesis in the ferret by ondansetron, metoclopramide and droperidol is consistent with the reduction of postoperative emesis in man by these compounds when morphine was a component of the anesthetic regimen. These results suggested that the morphine ferret model may be useful for evaluating compounds having the potential for preventing and treating postoperative vomiting.

The effect of chronic alcoholism on wound healing.

Abstract This study showed that the initiation of granuloma formation in alcoholic mice was retarded but eventually reached normal levels. The delay appears to be related to a failure of cells to migrate into the sponge as rapidly as they did in control animals. Thus, there is clear evidence to support the clinical impression of poor wound healing in alcoholics. Although repair is eventually accomplished, the initial decrease in the rate of this process may be significant enough to put the alcoholic patient at risk.

Spinal nerve injury increases the percentage of cold-responsive DRG neurons.

We tested the hypothesis that cold allodynia, observed following nerve injury reflects change(s) in the cold responsiveness of sensory neurons. To test this hypothesis we assessed the impact of the spinal nerve ligation (SNL) model of nerve injury on the responses of cutaneous sensory neurons to cooling in vitro. Nerve injury induced a significant increase in the incidence of cold responsive cutaneous neurons in uninjured but not injured ganglia. Because an increase in the percentage of cold responsive neurons in uninjured ganglia should increase the total neuronal response to cooling of peripheral tissue, these findings suggest that cold allodynia reflects, at least in part, a change in sensory neurons.

The effect of 5,6-dihydroxytryptamine on post-decapitation convulsions

Abstract Previous data (1) have shown that L-DOPA increases the duration of the clonic phase of post-decapitation convulsions (PDC) in mice. It was suggested that this effect is produced by depleting 5-hydroxytryptamine (5-HT) in the inhibitory bulbospinal pathways and thus enhancing reflex activity in the spinal cord. If this were true then L-DOPA administration should not influence clonic PDC in animals whose 5-HT pathways were destroyed. We therefore tested the effects of L-DOPA on mice 3 weeks after pretreatment with the 5-HT neurotoxin, 5,6-dihydroxytryptamine (5, 6-DHT) (50 μg/kg, intracerebroventricularly). All mice were given the peripheral decarboxylase inhibitor, Ro 4-4602. 5,6-DHT halved the brain 5-HT levels and significantly increased the duration of clonic PDC. The administration of L-DOPA (320 mg/kg i.p.) to 5,6 DHT treated mice did not produce any further significant increases in duration. The administration of 5-hydroxytryptophan (5-HTP) (100 mg/kg, i.v.) to 5,6-DHT treated mice, however, increased 5-HT to above control levels and reduced convulsions to control levels. Administration of both 5-HTP and L-DOPA to 5,6-DHT treated mice resulted in 5-HT levels and convulsion times which were also not significantly different from the controls. These data give additional indication that intact 5-HT nerve terminals are necessary for L-DOPA to prolong the duration of clonic PDC.

The role of dopamine and serotonin in the prolongation of post-decapitation convulsions in mice.

Abstract L-DOPA (320 mg/kg, i.p.) increased the duration of the clonic phase of post-decapitation convulsions (PDC) by 60% in mice pretreated with the peripheral decarboxylase inhibitor, Ro 4-4602 (50 mg/kg, i.p.). Assays of brains at the time of decapitation showed a 300% increase in dopamine (DM), an 80% reduction in serotonin (5-HT) and no change in norepinephrine (NE) levels. The effect of L-DOPA on PDC was not blocked by haloperidol (0.5 – 5.0 mg/kg), a blocker of DM receptors, nor by diethyldithiocarbamate (400 mg/kg) an inhibitor of NE synthesis. Parachlorophenylalanine (300 mg/kg × 3 days) produced an 80% reduction in 5-HT and a prolongation of PDC similar to that observed after L-DOPA. Prolongation of PDC was also seen after the 5-HT antagonists methysergide (5 mg/kg) and cinanserin (10 mg/kg), but not after cyproheptadine (10 mg/kg). The 5-HT precursor, 5-hydroxytryptophan (100 mg/kg), produced no change in PDC when used alone but inhibited L-DOPAs prolongation of PDC. The results suggest that L-DOPA acts by depleting 5-HT in bulbospinal pathways and thus enhancing reflex activity in the spinal cord.

A RABBIT TOOTH-PULP ASSAY TO QUANTIFY EFFICACY AND DURATION OF ANTINOCICEPTION BY LOCAL ANESTHETICS INFILTRATED INTO MAXILLARY TISSUES

The rabbit tooth-pulp assay is well established as a method for measuring the efficacy and potency of parenteral analgesic drugs. We describe a method for administration of local anesthetic drugs into the maxillary arch and subsequent measurements of antinociceptive action. It was possible to use two different methods of ED50 estimation and to provide measures of the potency, efficacy, and duration of local anesthetic drugs. These measurements corresponded with in vitro estimates of potency and duration and with intrinsic observations of the clinical actions.

Imipramine-fentanyl antinociception in a rabbit tooth pulp model

Antinociception of imipramine (I) and its effect in combination with fentanyl (F) was evaluated in rabbits using electrically-induced lick chew responses via tooth pulp stimulation as the model of nociception. Acute i.v. injections of I elicited a graded dose response comparable to i.v. morphine (M) with I ED 50 = 4.35 mg/kg (2.31-8.14, 95% CL) and M ED 50 = 1.81 mg/kg (1.11-3.90), with no differences in the slopes between the two curves. The lethal dose of I was 10 mg/kg. An i.v. dose of I twice the ED 50 elicited an antinociceptive effect of more than 50% maximum possible effect (MPE) for 90 minutes with peak effect of 82% MPE occurring at 15 minutes. These effects of I were not reversed by a morphine-reversal dose of naloxone (0.1 mg/kg i.v.) but were reversed with a ten fold dose of naloxone. F ED 50 values (mcg/kg) were lowered from 11.35 to 2.70, 0.74 and 0.33 with increasing pretreatment doses of I (1.0, 2.1 and 3.2 mg/kg). These magnitudes of potency increases of F were 4.2, 15.3 and 34.4 fold respectively. A single i.v. ED 50 dose of I extended the time to 50% MPE of an ED 90 dose of F from 26 minutes to 77 minutes; of a 2 X ED 50 dose of F from 17 minutes to 28 minutes. Data points for three different combinations of I and F fell significantly within the synergistic field of an ED 50 isobologram and a polynomial equation described the curve best fitting the data points. F alone (i.v. ED 50 dose) increased the PaCO2 values to 74% above controls and three different combinations with I showed no increases in PaCO2 values above controls. I alone did not significantly cause any change in PaCO2 values from controls.

Effect of L-DOPA analogues in Sidman avoidance performance in mice.

Abstract Mice were trained to avoid foot shock by turning a drum mounted in the wall of a behavioral chamber. L-DOPA (178 to 320 mg/kg i.p.) and D-DOPA (320 to 1000 mg/kg, i.p.) but not L-3-0-methyl-DOPA (178 to 560 mg/kg, i.p.) significantly reduced the number of responses made by the animals. Pretreatment with Ro. 4-4602 (50 mg/kg, i.p.), a peripheral DOPA-decarboxylase inhibitor, enhanced the depressant effect of L-DOPA but not that of D-DOPA. Inhibition of central DOPA-decarboxylase (Ro. 4-4602, 500 mg/kg, i.p.) partially reduced the depressant effect of L-DOPA but not that of D-DOPA These results suggest that only part of the depressant action of L-DOPA is due to its central decarboxylation.