Central sensitization as a result of surgical pain: investigation of the pre-emptive value of pethidine for ovariohysterectomy in the rat.

The development of central hypersensitivity as a result of a routine surgical procedure, midline ovariohysterectomy, was investigated in rats using the paw pressure test (PPT) and tail-flick latency (TFL) tests of spinal reflex activity. In addition, the modulating effect of pre-emptive versus post-operative administration of pethidine (a short-acting pure mu-opioid agonist) on the development of central hypersensitivity was studied. Initial experiments indicated that pethidine (15 mg/kg, i.m.) gave sub-maximal increases in thresholds for 60 min, and also that the administration of an anaesthetic did not unduly prolong the action of pethidine. Subsequently, 24 female Wistar rats were allocated to 1 of 4 treatment protocols: (1) anaesthesia without analgesics; (2) anaesthesia and surgery (midline ovariohysterectomy) without analgesics; (3) anaesthesia and surgery with pre-operative administration of pethidine; (4) anaesthesia and surgery with post-operative administration of pethidine. Thirty-five minutes after the end of anaesthesia thermal and mechanical nociceptive thresholds were measured at stepwise increasing intervals for 480 min. Changes were expressed as percentage changes from baseline (PPT) or deviation from expected values (TFL). Area under the threshold versus time response curves (AUCs) were also calculated for the following time sectors: 30-90, 90-150, 150-270, 270-390 and 390-510 min post-anaesthetic. Results of the TFL testing did not indicate the development of any significant hyperalgesia in any groups, but the results of the PPT did. In the time sectors 150-270 and 270-390 min post-anaesthetic, the AUCs in rats subjected to anaesthesia and surgery with either post-operative administration of pethidine or surgery with no analgesic drug administration, were significantly lower than the AUCs in rats given analgesics pre-operatively or those subjected to general anaesthesia alone (P < 0.01), Mann-Whitney). In summary, it appears that pethidine, in this protocol, prevented the development of surgically induced hyperalgesia when it was given pre-emptively.

The development of tolerance to ketamine in rats and the significance of hepatic metabolism.

1. A decrease in sleeping time in rats pretreated with ten daily doses of ketamine compared to controls is shown. 2. This decrease in sleeping time is associated with a more rapid decrease in circulating and brain levels of ketamine and its N-demethylated metabolite and higher levels of the subsequent oxidation metabolite in the pretreated animals. 3. Metabolism of ketamine to its N-demethylated metabolite by liver homogenates in vitro was more rapid when the livers were obtained from ketamine pretreated rats. 4. Microsomal preparations from rat liver were capable of metabolizing ketamine to its N-demethylated metabolite and this metabolite to the subsequent oxidation metabolite in vitro. The Vmax and Km for the first reaction calculated from loss of substrate were 433 mol mg-1 protein h-1 and 0.133 mM respectively and 199 nmol mg-1 protein h-1 and 0.121 mM for the second reaction. 5. The results indicate that tolerance to ketamine in rats is associated with increased hepatic metabolism which can also be demonstrated in vitro in liver homogenates.

The spinal antinociceptive activity of the alpha 2-adrenoceptor agonist, xylazine in sheep.

1. The intrathecal administration of xylazine (100 micrograms), via a chronic indwelling, cervical intrathecal catheter, produced a marked elevation of the mechanical nociceptive thresholds in the sheep. This antinociceptive effect was abolished by the prior intrathecal administration of the alpha 2-adrenoceptor antagonist, idazoxan. 2. The intrathecal administration of the selective alpha 2-antagonists, idazoxan (100 micrograms) and RX811059 (33 micrograms), significantly attenuated the antinociceptive activity of intravenous xylazine, with a 60-65% reduction in the area under the antinociceptive curve. The intrathecal administration of the antagonists alone had no significant effect on nociceptive thresholds. 3. Examination of the distribution of tritiated idazoxan (25 microCi in 100 microliters) indicated that the site of action of the drug was limited to the cervical spinal cord after intrathecal administration. 4. These studies demonstrate that a significant proportion of the antinociceptive effect of systemically administered xylazine is mediated by spinal alpha 2-adrenoceptors.

Investigation of the antinociceptive activity of buprenorphine in sheep.

1. Buprenorphine given intravenously (6 micrograms kg-1) was examined for its antinociceptive activity in unrestrained sheep using devices to measure thermal and mechanical thresholds. 2. The plasma levels of buprenorphine following intravenous injection over the time period of the antinociceptive testing were measured using a radioimmunoassay. 3. Buprenorphine produced a clear antinociceptive effect lasting for up to three and a half hours when measured by the thermal threshold test, but no detectable antinociception in the mechanical test. 4. The plasma levels of buprenorphine indicated that the drug was rapidly distributed in a manner not dissimilar to that reported in man, although individual animals showed a wide variation in some parameters. 5. When plasma levels of the drug were high (less than 700 pg ml-1) during the first sixty minutes, no antinociceptive activity in the thermal test could be detected, which may be due to the slow receptor kinetics shown by this drug.

A technique for recording from spinal neurones in awake sheep.

A technique is described for implanting a chamber on 1 or 2 vertebrae of the spinal column of the sheep. This chamber protrudes permanently through the dorsal skin of the back and is covered by a light bandage. Between recording sessions the chamber houses an inner cap sealing the hole that gives access to the cord. During recording sessions this cap is removed and a miniature manipulator inserted instead. This manipulator can accept a motor drive that holds a glass-coated tungsten microelectrode. The drive has a hole through which an arthroscope tube can be passed so that insertion of the electrode can be performed under visual control. Extracellular recordings have been made of single spinal neurones for up to 4 h in animals that are drug-free, untrained and only lightly restrained. Recording sessions can be repeated on a daily basis for several weeks until the dura and/or arachnoid becomes too thickened to permit electrode penetrations. Animals remain healthy and their behaviour remains normal throughout this time.

Behavioural analysis of changes in nociceptive thresholds produced by remoxipride in sheep and rats.

The antinociceptive potential of remoxipride was investigated in sheep and rats with concurrent motor function assessments. Previous studies of sheep given intravenous remoxipride have revealed increases in mechanical nociceptive thresholds. Here, further investigation in sheep demonstrated elevated thermal nociceptive thresholds with no effect on subjectively assessed sedation or motor impairment scores. However, in rats, the dose of remoxipride (100 mg/kg i.p.) required to produce nociceptive thresholds similar to those elicited by morphine (30 mg/kg i.p.), itself reduced rotarod performance. Medetomidine (200 micrograms/kg i.p.) evoked sedation without influencing rotarod performance or antinociception. The antinociceptive, motor deficit and cataleptogenic actions of remoxipride were similar to those induced by two other dopamine antagonists, haloperidol (5 mg/kg) and raclopride (16 mg/kg i.p). Tocainide (100 mg/kg i.p.) induced thermal antinociception with normal rotarod performance and no catalepsy suggesting that Na+ channel blockade by remoxipride is not responsible for the changes in nociceptive thresholds. This study emphasizes the importance of motor function assessment during acute antinociceptive testing.

Investigation into the antinociceptive potential of remoxipride administered intrathecally in sheep.

Systemic administration of remoxipride, a dopamine (D2) antagonist, to sheep has previously been shown to generate an antinociceptive action without producing a significant motor impairment. The present study examined whether a spinal locus of action was responsible for this action of remoxipride. Remoxipride (17.7 mg) administered intrathecally via chronically indwelling catheters produced a greatly variable but significant (p<0.05) increase in nociceptive thresholds as judged by a focused mechanical stimulus (blunt pin) applied to the forelimb of four sheep. However, this dose of remoxipride induced a marked forelimb motor impairment as judged by a subjective visual analogue scoring system. Conversely, intrathecal xylazine (100 and 200 microg), an alpha-adrenergic agonist with antinociceptive properties, did not produce forelimb weakness although the higher dose (200 microg) produced significant sedation. In vitro autoradiography was performed on cervical spinal cord sections taken from sheep. Remoxipride displaced [3H] YM-09151-2, a selective D2 antagonist, from densely-labelled areas in the superficial layer of the dorsal horn, lamina X and ventral horn. Even though there are possible anatomical substrates within the spinal cord for both an antinociceptive and motor disturbance action of remoxipride, the behavioural data suggest that the spinal cord is unlikely to be the primary site of antinociceptive action for systemically-administered doses of remoxipride.

Evaluation of the actions and use of alphaxolone/alphadolone (CT1341) in sheep.

CT1341 (alphaxolone/alphadolone) (Saffan; Glaxo) at various dose rates was investigated for use as an intravenous anaesthetic agent in sheep and it was found that at least 1.65 mg/kg was required to induce anaesthesia. In an experimental group of animals the effect of CT1341 2.2 mg/kg, on PaO2, PaCO2, paH and mean arterial blood pressure was measured. The drug caused a mild respiratory acidosis accompanied by a slight decrease in PaCO2 tensions. Mean arterial blood pressure decreased by 50 per cent, 30 seconds after injection but this fall lasted for only 10 minutes. Prior administration of either atropine sulphate or mepyramine maleate failed to modify this hypotensive effect. Injection of the vehicle, Cremophor EL, alone had no effect on mean arterial blood pressure. Intracerebroventricular administration of CT1341 while producing anaesthesia failed to produce any hypotension. The drug was used for the induction of anaesthesia in 19 sheep. A mean dose of 3.0 mg/kg was found to be most useful clinically. In a further three sheep an infusion of a dilute solution (0.234 mg/kg per minute) was used to maintain anaesthesia. The most notable clinical feature in sheep was the fast, complete recovery. It is concluded that this agent is an extremely useful drug for the induction and maintenance of anaesthesia in sheep.

Influence of premedication with xylazine on the distribution and metabolism of intramuscularly administered ketamine in cats.

The pharmacokinetics and metabolism of intramuscularly administered ketamine (25 mg/kg) were determined in eight cats (group A). The effect of prior administration of the sedative agent xylazine hydrochloride (1 mg/kg intramuscularly) on the uptake distribution and metabolism of the drug was also investigated in another 10 cats (group B). It was found that xylazine significantly prolonged the duration of ketamine anaesthesia. In addition, xylazine not only prolonged the plasma half life of ketamine but significantly delayed the production of the primary metabolite of ketamine, norketamine. Studies on the urinary excretion of ketamine and norketamine in the cats in group B indicated that very little of the drug (less than 2.6 per cent) is excreted by this route in the initial 90 minutes after injection.

Pharmacokinetics of pethidine administered intramuscularly and intravenously to dogs over 10 years old.

The pharmacokinetics of pethidine administered postoperatively both intravenously and intramuscularly was investigated in dogs aged over 10 years. When the drug was given intravenously (2 mg kg-1), plasma levels declined in a biexponential manner, with an elimination half-life of 62.7 minutes. Following its intramuscular administration at the same dose rate, absorption was very slow and two distinct rates of absorption were observed. Maximum plasma concentrations were not achieved until 33 minutes after drug administration. The elimination of the intramuscularly administered drug was also slow with a t 1/2 beta' of 145.9 minutes. Thus, it seems that in elderly animals pethidine has a long elimination half-life and a slowed rate of absorption. However, the total body clearance of the drug does not seem to be affected by age.

Analgesic activity and respiratory effects of butorphanol in sheep.

The analgesic drug butorphanol tartrate has proved useful clinically in horses and dogs but its analgesic profile had not yet been investigated in sheep. This study was initiated to determine the thermal and mechanical antinociceptive activity of butorphanol (at the dose rates 0.05, 0.1 and 0.2 mg kg-1) in sheep. The drug produced significant analgesia in the thermal test system, the duration of which was dose related but no significant elevation in mechanical pressure thresholds could be detected. In a further set of experiments the dose rate was increased to 0.4 mg kg-1 and mechanical testing was repeated. There was still no clinically significant elevation in pressure thresholds. At a dose rate of 0.2 mg kg-1 the drug had no detectable effect on respiratory blood gas tensions. Behavioural changes were severe if a dose rate of 0.2 mg kg-1 was exceeded.

Measurement of mechanical thresholds, plasma cortisol and catecholamines in control and lame cattle: a preliminary study.

The threshold response to a mechanical nociceptive stimulus was significantly lower on the lame hind limb of lame cows than on the same limb of sound cows. There were no significant differences between the concentrations of cortisol, noradrenaline, adrenaline or dopamine in the blood plasma of the sound and lame cows.

Effects of clinically occurring chronic lameness in sheep on the concentrations of plasma noradrenaline and adrenaline.

Plasma adrenaline (AD) and noradrenaline (NA) concentrations were measured by high performance liquid chromatography with electrochemical detection in blood samples from control and lame sheep. The lame sheep suffered from naturally occurring foot rot and showed behavioural characteristics normally associated with chronic pain. The lame sheep were scored both for impairment of gait and pathology of the foot and divided into mild and severely affected groups. Both the mildly and severely lame group showed a significant increase in plasma AD and plasma NA which tended to persist even after clinical resolution of the condition. The measurement of plasma AD and NA may provide information which can be used to assess animals experiencing chronic pain, when taken in conjunction with other parameters, such as nociceptive thresholds and plasma hormone levels.

Effect of chronic pain associated with lameness on plasma cortisol concentrations in sheep: a field study.

Plasma cortisol concentrations were measured in two groups of sheep taken from 29 flocks in north Devon. The first group were healthy adult females and the second group were adult females suffering from footrot in one forefoot. These sheep were assessed for the severity of the lesion and the level of lameness and assigned a score. The plasma cortisol concentration was significantly higher in the lame sheep than in the healthy sheep and remained so for up to three months after the apparent resolution of the clinical lesion. There was no correlation between the severity of the footrot and the concentration of plasma cortisol.

Pharmacokinetics of intravenously administered ketamine in the horse.

The metabolism and distribution of ketamine and its two major metabolites (norketamine and dehydronorketamine) was investigated in 10 horses undergoing airway surgery. Following premedication with xylazine (1.1 mg kg-1 intravenously) anaesthesia was induced by the rapid injection of ketamine at a dose of 2.2 mg kg-1 intravenously. Anaesthesia was maintained with halothane vaporized in oxygen and nitrous oxide (50:50). Serially collected blood samples were analysed by a sensitive gas liquid chromatographic technique. Plasma ketamine concentrations declined biexponentially with a rapid initial distribution phase (t1/2 2.89 +/- 0.25 minutes) being followed by a slower elimination phase (t1/2 65.84 +/- 3.46 minutes). Norketamine was found in the plasma of all the horses, but only at low concentrations while there was very little dehydronorketamine detected. It seems that recovery from ketamine anaesthesia in the horse depends mainly on rapid redistribution of the drug from the central compartment and this explains the abrupt recovery from ketamine anaesthesia often observed in the horse.

Analgesic effects of detomidine in thoroughbred horses with chronic tendon injury.

This study was undertaken to assess the analgesia provided by detomidine (20 micrograms kg-1 intravenously) in thoroughbred horses. Pain thresholds to a mechanical noxious stimulus were measured before and after a period of mild chronic pain in one foreleg. Detomidine was a good analgesic in control animals; their pain thresholds were significantly elevated for about 60 minutes. After injury, the injured leg had a significantly lower pain threshold and the intensity and duration of analgesia provided by detomidine were significantly reduced. The analgesia in the opposite (sound) leg was also reduced, indicating that there were both central and peripheral aspects to this increased sensitivity to painful stimuli. Detomidine deserves to be considered as a potent analgesic in the horse rather than a sedative with analgesic side effects.

Paradoxical excitement following the intravenous administration of azaperone in the horse.

The rapid intravenous administration of the butyrophenone tranquilliser, azaperone, at a dose rate of 0.29-0.57 mg/kg body weight resulted in the immediate onset of excitement and ataxia of varying degree in over half the animals. The severity of the reaction appeared to be related to the size of the animal. Other side effects such as salivation, sweating, muscle tremor and vocalisation were also observed. The possible causes of this paradoxical reaction to the tranquilliser are discussed.

Associations between locomotion, claw lesions and nociceptive threshold in dairy heifers during the peri-partum period.

The locomotion of 15 heifers was examined at fortnightly intervals over a 4 month peri-partum period. Measurements were made of the development of gait abnormalities, thermal and mechanical nociceptive thresholds, and severity and size of sole lesions observed in the hind claws. All heifers developed lesions at, or shortly after, parturition, and in seven animals this induced marked lameness. Abnormalities of gait were related more to the severity than to the size of the lesion. Lameness was associated with a significant increase in sensitivity to mechanical noxious stimuli applied to the lame leg but not to a thermal stimulus applied to the ear. This study demonstrated interactions between lameness, claw lesions and the development of hyperalgesia in heifers during the post-partum period.

Quantitative analysis of cyanogen bromide-cleaved peptides for the assessment of type I: type II collagen ratios in equine articular repair tissue.

Cyanogen bromide was used to solubilise and specifically fragment purified equine Type I and II collagen and equine articular surface repair tissue. The resultant peptides were separated by sodium dodecyl sulphate-polyacrylamide gel electrophoresis and quantified by densitometric scanning. Measurement of the relative amounts of the peptides alpha 2(I) CB3, 5 and alpha 1(II)CB10 provided an accurate method of establishing the ratio of Type I to Type II collagen in mixtures of purified equine collagens. The method was sensitive to 6% Type II collagen when the band areas were corrected for peptide molecular weight and the number of chains in the parent tropocollagen molecule which contain that particular peptide. Use of this technique showed that repair tissue in full thickness osteochondral defects in the dorso-distal margins of the intermediate carpal bones of ponies did not contain detectable amounts of Type II collagen 11 weeks after defect induction.

Influence of intra-articular sodium hyaluronate and polysulphated glycosaminoglycans on the biochemical composition of equine articular surface repair tissue.

The influence of repeated intra-articular injections of sodium hyaluronate and polysulphated glycosaminoglycan on the repair of full-thickness osteochondral defects was examined in the midcarpal joints of ponies. The study showed no significant difference between treated and control groups with regard to total collagen content, uronic acid content or the relative proportions of Type I and Type II collagen in the repair tissue, indicating that the drugs did not affect the biochemical composition of the repair tissue 11 weeks after defect induction.