The correlation of the thermal and mechanical antinociceptive activity of pethidine hydrochloride with plasma concentrations of the drug in sheep

Effect of Meperidine on Equine Blood Histamine, Tryptase, and Immunoglobulin-E Concentrations

Objectives: To evaluate changes in immunological parameters following subcutaneous (SC) and intramuscular (IM) administration of meperidine in horses through quantitative analysis of plasma tryptase, histamine, and IgE levels.

Methods: Six adult horses were enrolled in a prospective randomized crossover design. Horses were administered one treatment per day, with a seven day washout period: (a) meperidine 1 mg/kg IM, saline 6 mL SC; (b) saline 6 mL IM, meperidine 1 mg/kg SC; (c) saline 6 mL SC, saline 6 mL IM. Blood samples were obtained for plasmatic histamine (baseline, 5, 10, 15, 30, and 60 min) via LC-MS/MS and plasmatic tryptase (baseline, 15, 30, 60, 120, and 240 min) quantification with enzyme-linked immunoabsorbent assays. Serum immunoglobulin E (IgE) concentrations prior to any meperidine treatment and 7–14 days following the first meperidine treatment were evaluated with enzyme-linked immunoabsorbent assays. Histamine and tryptase concentrations were evaluated with a mixed-effect analysis of variance. The levels of IgE at baseline (before the administration of the first dose of meperidine) were compared with the IgE values at 60 min following the second meperidine administration with the Paired t test. Biopsies of localized injection site reactions from subcutaneous meperidine administration were collected from two horses.

Results: No statistically significant elevations from baseline in histamine (p = 0.595), tryptase (p = 0.836), or IgE (p = 0.844) were found in any of the horses in this study. There were no differences between treatment groups. Administration of SC meperidine caused a localized vasculitis and thrombosis with regional edema and hemorrhage.

Conclusion: No evidence of anaphylactoid or anaphylactic type reactions occurred following IM or SC meperidine administration.

Pharmacokinetics and antinociceptive effects of tramadol and its metabolite O-desmethyltramadol following intravenous administration in sheep

Although sheep are widely used as an experimental model for various surgical procedures there is a paucity of data on the pharmacokinetics and efficacy of analgesic drugs in this species. The aims of this study were to investigate the pharmacokinetics of intravenously (IV) administered tramadol and its active metabolite O-desmethyltramadol (M1) and to assess the mechanical antinociceptive effects in sheep. In a prospective, randomized, blinded study, six healthy adult sheep were given 4 and 6 mg/kg tramadol and saline IV in a cross-over design with a 2-week wash-out period. At predetermined time points blood samples were collected and physiological parameters and mechanical nociceptive threshold (MNT) values were recorded. The analytical determination of tramadol and M1 was performed using high performance liquid chromatography. Pharmacokinetic parameters fitted a two- and a non-compartmental model for tramadol and M1, respectively. Normally distributed data were analysed by a repeated mixed linear model. Plasma concentration vs. time profiles of tramadol and M1 were similar after the two doses. Tramadol and M1 plasma levels decreased rapidly in the systemic circulation, with both undetectable after 6 h following drug administration. Physiological parameters did not differ between groups; MNT values were not statistically significant between groups at any time point. It was concluded that although tramadol and M1 concentrations in plasma were above the human minimum analgesic concentration after both treatments, no mechanical antinociceptive effects of tramadol were reported. Further studies are warranted to assess the analgesic efficacy of tramadol in sheep.

Antinociceptive effects of tramadol and acepromazine in cats

Effects of tramadol and acepromazine on pressure and thermal thresholds were examined in eight cats. After baseline measurements, subcutaneous (SC) tramadol 1 mg/kg, acepromazine 0.1 mg/kg, tramadol 1 mg/kg with acepromazine 0.1 mg/kg, or saline 0.3 ml were given. Serial measurements were made for 24 h. Mean thermal thresholds did not change significantly [analysis of variance (ANOVA)] from baseline. The maximum thermal threshold increase above baseline was 2.8±2.8°C at 6 h ( P>0.05) after tramadol; it was above the 95% confidence interval (CI) at 0.75, 3 and 6 h. Pressure thresholds increased above baseline from 0.25 to 2 h after acepromazine ( P<0.05) and from 0.5 to 3 h after the combination ( P<0.05), with a maximum increase of 132±156 mmHg 0.25 h after acepromazine and 197±129 mmHg 0.5 h after the combination. Pressure thresholds were above the 95% CI from 0.25 to 2 h after acepromazine and from 0.5 to 3 h after the combination. SC tramadol at 1 mg/kg in cats had limited effect on thermal and pressure nociception, but this was enhanced by acepromazine. Acepromazine alone had pressure antinociceptive effects.

Development and validation of a new model of inflammation in the cat and selection of surrogate endpoints for testing anti-inflammatory drugs

In laboratory animals many models of inflammation have been developed for preclinical evaluation of the pharmacological profiles of nonsteroidal anti-inflammatory drugs (NSAIDs). In contrast, in species of veterinary interest, including the cat, NSAIDs have been studied mainly using dose-titration or dose-confirmation studies in clinical subjects. This is due to the scarcity of appropriate animal models and to the associated lack of quantitative validated endpoints describing the magnitude and time course of drug response. Determination of pharmacokinetic/pharmacodynamic (PK/PD) relationships provides a powerful approach for the selection of effective and safe dosage regimens. In this study, a paw inflammation model in the cat was developed for the preclinical evaluation of NSAIDs using PK/PD modelling. Subcutaneous injection of 500 mg kaolin in the paw produced a well-defined and reproducible inflammatory response that lasted 4-5 days. Several endpoints were assessed for their clinical relevance and for their metrological performance (accuracy and reproducibility). Body temperature, lameness scoring, locomotion tests and possibly skin temperature were the most appropriate endpoints for testing the antipyretic, analgesic and anti-inflammatory effects of NSAIDs in the cat.

An analgesic evaluation of isoxsuprine in horses

Isoxsuprine is used clinically to treat navicular disease and laminitis in horses. Although it is thought to increase digital and laminar blood flow, isoxsuprine's mechanism of action remains controversial, and analgesia has been suggested recently as such possible mechanism. This research investigated the analgesic potential of isoxsuprine in healthy horses submitted to a mechanical nociceptive test. Isoxsuprine (1.2 mg/kg), xylazine (1.1 mg/kg), distilled water : ethanol 95% (2 : 1, v/v, 20 ml) and saline (0.9%, 20 ml) were injected intravenously, and nociceptive thresholds were measured over 90 min. Only xylazine significantly increased nociceptive thresholds, confirming that alpha(2)-adrenoceptor agonists produce analgesia in horses. Our results do not support an analgesic mechanism of action for isoxsuprine in horses, suggesting that other mechanisms might account for the clinical efficacy of this drug or that mechanical nociceptive testing may not be sufficiently sensitive to demonstrate an analgesic effect for this drug.

Use of thermal threshold response to evaluate the antinociceptive effects of butorphanol in cats

OBJECTIVE

To characterize the antinociceptive actions of several doses of butorphanol by use of a thermal threshold testing device specifically designed for cats.

ANIMALS

6 domestic shorthair cats.

PROCEDURE

The study was a masked, randomized, crossover design. Thermal thresholds were measured by use of a thermal threshold-testing device specifically developed for cats. A small probe containing a heater element and temperature sensor was held with consistent contact against a shaved area of the cat's skin with an elasticized band. Skin temperature was recorded before each test, prior to activation of the heater. On detection of a response (eg, the cat flinched, turned, or jumped), the stimulus was terminated and the threshold temperature recorded. Three baseline measurements were recorded before IV injection of 0.1, 0.2, 0.4, or 0.8 mg of butorphanol/kg. Each cat received all doses in a randomized order at least 1 week apart. The investigator was unaware of the treatment received. Thermal thresholds were measured every 15 minutes for 6 hours.

RESULTS

Mean+/-SD pretreatment threshold temperature for all cats was 40.8+/-2.2 degrees C. There were no dose-related differences among treatments. There was a significant increase in threshold values for all treatments from 15 to 90 minutes after injection. Mydriasis was detected in all cats after treatment with butorphanol and dysphoric behavior was frequently exhibited.

CONCLUSIONS AND CLINICAL RELEVANCE

Results obtained by use of a thermal stimulus indicated that the duration of antinociceptive action of butorphanol was 90 minutes and there was no dose-response relationship in cats.

A laser-based method to measure thermal nociception in dairy cows: short-term repeatability and effects of power output and skin condition

To validate a laser-based method to measure thermal nociception in dairy cows (e.g., for the use in studies on stress-induced analgesia), we performed three experiments to observe the behavioral responses to a computer-controlled CO2 laser beam applied to the skin on the caudal aspect of the metatarsus. In Exp. 1, effects of power output (0, 1.3, 1.8, 2.2, 2.4 and 2.6 W) on nociceptive responses were examined using 18 dairy cows kept and tested in tie stalls. Increasing the power output affected the latencies to respond (decreasing latencies, P < or = 0.01), types of response (less nonresponding and more kicking, P < 0.0001), and behavior during (increasing frequency of tail flicking, P = 0.003) and between single laser exposures (increasing frequency of kicking, P = 0.02). Therefore, behavioral responses to a laser stimulus seem to be a valid measure of nociception in dairy cows. Repeatability within 15 min was investigated in Exp. 2 using n = 36 dairy cows kept and tested in tie stalls and a power output of 1.8 W. The variables' latency to move the exposed leg and frequency of tail flicking during laser exposure showed the highest level of repeatability (0.50 and 0.38, respectively). However, retesting at t = 15 min led to increased responses in terms of shorter latencies to respond (P < 0.05), increased kicking (P = 0.05), and tail flicking (P = 0.02), which probably can be explained by sensitization. Effects of power output (1.0 vs. 1.8 W) and skin condition (naked vs. intact) were examined in Exp. 3 on 11 group-housed dairy cows, tested just outside their home pen. Increasing the power output and shaving off hair led to increased responses as seen by shorter latencies to respond (P < 0.0001), less nonresponding (P < 0.0001), and increased kicking (P = 0.0003), as well as reduced intra- and interindividual variability (P < or = 0.04). In conclusion, the results of these experiments suggest that behavioral responses to laser stimulation are a valid and reliable measure of nociception in dairy cows, especially when applied on naked skin, both in the home environment and just outside a group pen. The fact that repeated testing in itself at t = 15 min led to increased responses means that the test will be a conservative measure of stress-induced analgesia.

Antinociceptive activity of midazolam in sheep

The purpose of this study was to examine the effects of midazolam on the nociceptive threshold responses in sheep. The intravenous administration of midazolam (0.1-0.3 mg/kg) produced a significant dose-dependent elevation of the mechanical and thermal nociceptive thresholds. The intravenous administration of flumazenil (20 micrograms/kg) markedly attenuated the antinociceptive activity of midazolam in the mechanical nociceptive test, whereas intravenous naloxone (0.2 mg/kg) had no significant effect on midazolam-mediated analgesia. The intrathecal administration of midazolam (1 mg), via chronically implanted cervical subarachnoid catheters, produced a significant elevation in the mechanical threshold responses. These results indicate that midazolam has antinociceptive actions in the sheep and suggest that this effect is, at least partially, mediated at the spinal level.

Effect of flunixin meglumine on the thresholds to mechanical stimulation in healthy and lame sheep

The antinociceptive effect of flunixin meglumine was assessed in healthy and lame sheep by using a noxious mechanical stimulus. Sheep suffering from the chronically painful condition, footrot, have previously been shown to have lower thresholds to noxious mechanical stimuli than healthy animals. In the present study, 22 sheep suffering from footrot did not have a lower mean mechanical threshold than 25 matched healthy animals, but it was significantly greater than that recorded from eight experimental sheep (5.0 [2.5], 4.9 [2.1] and 3.0 [1.0] Newtons, respectively). Doses of 1.0 or 2.0 mg kg-1 of flunixin meglumine had no effect on the thresholds to noxious mechanical stimulation in either experimental sheep tested over six hours, or in lame sheep tested over a period of 30 minutes. The repeated administration of flunixin to sheep suffering from footrot over a period of three days reduced their thresholds to noxious mechanical stimulation.

Pharmacokinetics and milk residues of butorphanol in dairy cows after single intravenous administration

The pharmacokinetics of butorphanol tartrate were investigated following intravenous administration of 0.25 mg/kg of body weight to six healthy non-lactating Jersey cows. Three lactating Holstein cows also received 0.045 mg of butorphanol/kg of body weight intravenously to determine the extent and duration of drug transfer into milk. A radioimmunoassay technique was used to measure butorphanol concentrations in plasma and milk. The disposition of butorphanol following intravenous administration was characterized by rapid and extensive distribution followed by a slower elimination phase. Apparent volume of distribution was 4.178 +/- 1.145 (mean +/- SD) 1/kg, mean elimination half-life was 82 min, and clearance was 34.6 +/- 7.7 ml/min/kg. Trace quantities of butorphanol were detected in the cow's milk for up to 36 h following administration. These pharmacokinetic data were compared with pharmacokinetic and pharmacodynamic data for butorphanol in other species and for three other potent opioids in related ruminant species.

The influence of surgery and anaesthesia on the pharmacokinetics of pethidine in the horse

The plasma concentration of pethidine was measured in 16 horses, after its administration intravenously (i.v.) at a dose rate of 1 mg/kg bodyweight (bwt). In eight animals studied before surgery, the plasma levels of the drug decreased in a bi-exponential manner with a distributive half life of 3 mins and an elimination half life of 57.7 mins. Total body clearance was 17.7 ml/kg bwt/min. The remaining horses were investigated immediately after a period of anaesthesia and surgery and in these animals the drug exhibited smaller volumes of distribution (V1 cand Vdarea) and a significantly lower clearance rate of 12.8 ml/kg bwt/min in these animals. Correlation between the plasma concentration of pethidine and information published previously on the duration of action of the agent in the horses suggests that plasma levels of approximately 0.4 microgram/ml are required to produce analgesia in this species.

Further studies on the antinociceptive activity and respiratory effects of buprenorphine in sheep

The thermal and mechanical analgesic profile of buprenorphine at a dose rate of 1.5 micrograms/kg i.v. was investigated in five sheep. This dose produced significant analgesia for 40 min against the thermal stimulus, but no mechanical antinociception. A higher dose rate of 12 micrograms/kg also failed to produce antinociception to a mechanical stimulus. In addition, the effect of the drug (6 micrograms/kg) on respiratory gas tensions was determined and no significant changes were observed.

Receptor changes in the spinal cord of sheep associated with exposure to chronic pain

There is evidence that post-injury hypersensitivity is partly due to changes in the central nervous system. Sheep with foot rot were used to investigate the effect of chronic pain on some receptors thought to be involved in spinal nociceptive processing systems (alpha 2 adrenoceptor and mu and delta opioid receptors). Saturation binding studies showed a variable distribution of [3H] clonidine (alpha 2 adrenoceptor agonist) in the spinal cord of normal sheep. The number of receptors (Bmax) present in areas thought to be involved in nociceptive processing, laminae I and II and lamina X, increased to 131% and 169% of control sheep values respectively in animals exposed to chronic pain. The affinity of the receptors (KD), however, remained unchanged at approximately 2 nM. There was less [3H]DAGO (mu opioid agonist) and [3H]DPDPE (delta opioid agonist) binding in the sheep spinal cord. Both opioid receptor types being mainly located in the superficial dorsal horn. The [3H]DPDPE binding was unchanged in the sheep with foot rot, whilst the number (Bmax), but not the affinity, of the [3H]DAGO binding sites increased in laminae I and II in lame animals to 130% of the control sheep values. Hence, in animals in chronic pain, the number of alpha 2 adrenoceptors and mu opioid receptors increased mainly in areas of the sheep spinal cord associated with nociception.

The effect of chronic clinical pain on thermal and mechanical thresholds in sheep

Threshold responses were measured to a thermal skin test and a mechanical pressure test in two groups of conscious unrestrained sheep. The first group of sheep were healthy adult females and formed a control sample, the second group were also adult females, but were all suffering from a condition known as footrot. Footrot is a chronic infective lesion affecting usually one foot which appears to cause severe pain in its worst manifestation. These sheep were assessed for the severity of the lesion and degree of lameness and were divided into high and low severity subgroups. Footrot did not alter the threshold to the thermal test but the mechanical pressure threshold was significantly reduced in both footrot sub-groups compared to controls. A local anaesthetic block of the affected foot restored values to close to the control level. After treatment of the affected foot, the mechanical threshold in the low severity sub-group was returned to normal, but in the high severity sub-group it was still significantly reduced compared to the control animals. However, when retested 3 months later these values had returned to the normal control levels.