Patricia L. Cmielewski

Antinociceptive and motor effects of intrathecal morphine combined with intrathecal clonidine, noradrenaline, carbachol or midazolam in rats

&NA; This study investigated antinociceptive effects of intrathecal morphine combined with intrathecal clonidine, noradrenaline, carbachol or midazolam in rats. Each animal received intrathecally, on 3 separate occasions (i) 2 &mgr;g morphine (M), (ii) a dose (D) of one of the non‐opioid drugs, and (iii) a combination, Symbol(M + D), consisting of 1 &mgr;g morphine plus half the dose of the non‐opioid drug. Antinociceptive effects were assessed by the hot‐plate and tail‐flick tests over the duration of drug action. All non‐opioid drugs studied led to dose‐related antinociceptive effects when given alone. Addition of morphine caused a left shift in the dose‐response curves of all the non‐opioid drugs, indicating at least some degree of additive effects. Effects were considered supra‐additive when the effect of the combination, Symbol(M + D), was significantly greater than both the effect of 2 &mgr;g morphine and the dose of non‐opioid. Evidence of supra‐additive antinociceptive effects was obtained only with the clonidine‐morphine combination. Figure. No caption available Figure. No caption available

Influence of polarity on dose-response relationships of intrathecal opioids in rats

&NA; Dose‐response curves were constructed for intrathecal morphine (M), oxymorphone (OM), hydromorphone (HM), diamorphine (DM), 14‐hydroxydihydromorphine (OHM), oxycodone (OC), hydrocodone (HC) and fentanyl (F). Intrathecal catheters were placed in 69 rats under halothane/N2O anaesthesia. After recovery, baseline hot plate and tail flick latencies were measured, and a dose of opioid was given. Hot plate and tail flick latencies were assessed at 5, 15, 30, 60, 90, 120 min and then hourly until they returned to within 25% of baseline. Response latencies were converted to per cent of maximum possible effect (% MPE) and the area under the % MPE × time curve was taken as the response. This measure includes information about both potency and duration of action. Each rat received 3 opioids and saline at intervals of 2–3 days. On a fifth occasion, the animals first treatment was repeated. Each opioid was studied over an 8‐fold dose range. Results of both hot plate and tail flick were best described by a model including log(dose), a component due to development of tolerance over the 5 experimental days, and an among‐rat variation term. In the hot plate test, doses equieffective in producing a response (AUC) over the dose range studied were in the order OHM < OM < HM < M < F < DM < HC < OC. Slopes of the log(dose)‐response curves were similar for all drugs except OHM, which had a steeper slope. A model is proposed in which hot plate and tail flick latencies are prolonged while CSF concentrations of a drug are above its minimum effective concentration, and drug is cleared from the CSF by a first‐order process, possibly uptake into the spinal cord and removal via the blood. This model predicts that log(dose)‐response curves will be linear, as was observed, with slopes inversely proportional to the rate constant for clearance from CSF. According to this model the steeper slope of the OHM log(dose)‐response may be interpreted as indicating slower clearance from CSF. OHM has the lowest octanol/pH 7.4 buffer distribution coefficient (0.34) of all opioids studied, possibly leading to a lower rate of uptake into the spinal cord.

Assessment of antinociceptive drug effects in the presence of impaired motor performance.

The hot-plate (HP) and tail-flick (TF) tests are widely used to assess analgesic activity of drugs. These tests do not directly measure the intensity of the noxious stimulus perceived by the animal, but only the animals response to it, and so may be affected by non-analgesic drugs. Sedatives and muscle relaxants, for example, may impair the ability to respond and hence be wrongly considered to have analgesic activity. We examined response of rats in the HP (55 degrees C, cutoff time 25 sec) and TF (cutoff time 5 sec) tests following administration of pentobarbitone, diazepam or pancuronium. These drugs all impaired motor performance as assessed by reduction in mean rotarod performance times to 6-32% of predrug values. However, HP and TF latencies were not appreciably prolonged. We also found that pancuronium did not alter effects of morphine on HP or TF latencies, despite reduction in rotarod performance to 38% of predrug values. Our results support the validity of HP and TF tests as analgesic assays even in the presence of substantial impairment of motor performance.

The pathology of liver injury induced by the chronic administration of alcohol and low-dose carbon tetrachloride in porton rats

We have previously established a model for micronodular cirrhosis by feeding Wistar rats alcohol, in the Lieber–DeCarli liquid diet, and exposing them to ‘low‐dose’ carbon tetrachloride (CCl4) vapour for 10 weeks. This study reports the spectrum of liver pathology seen in male Porton rats exposed to ‘low‐dose’ CCl4 vapour 5 nights/week, 6 h/night while being fed alcohol (300 kcal/L) in the Lieber‐DeCarli diet. Micronodular cirrhosis developed in all animals after 5–7 weeks of treatment. The simultaneous administration of silymarin, a putative hepatoprotective agent, in the liquid diet, did not alleviate or prevent the chronic liver injury. The histopathological features of the liver injury are described, with particular emphasis on the presence of small epithelial cells (‘progenitor or stem cell), which appear to be playing a role in liver regeneration.

Behavioural effects of norpethidine, a metabolite of pethidine, in rats.

This study investigated behavioural effects of the toxic pethidine metabolite, norpethidine, in rats and its interactions with reserpine, apomorphine and physostigmine. Following intraperitoneal administration, brain concentrations of norpethidine reached a plateau after 20-40 min and remained elevated for 2 h. In the dose range 0.06-0.18 mmol/kg, norpethidine induced myoclonic jerks, a characteristic splayed posture, and episodes of exaggerated shivering. Forward locomotion, grooming, yawning and rearing were suppressed. Seizures and reverse locomotion occurred occasionally. Administration of reserpine 1 h prior to norpethidine, or of apomorphine or physostigmine 15 min after norpethidine, did not alter the norpethidine-induced behaviours; neither did norpethidine block the effects of apomorphine or physostigmine. The characteristic profile of behaviours induced by norpethidine make this toxicant readily amenable to animal studies. Our results indicate that its mechanism of action is unlikely to involve dopaminergic or cholinergic pathways.

Development of tolerance to antinociceptive effects of an intrathecal morphine/clonidine combination in rats

Previous animal studies have shown the antinociceptive effects of intrathecal clonidine and intrathecal morphine to be synergistic. This study investigated the intrathecal administration of multiple doses of this drug combination to examine the rate of development of tolerance and to determine whether there was any toxic effect on the spinal cord.Rats with indwelling intrathecal catheters were given saline, morphine (2.5–7.5 μg), clonidine (17.5 μg), or clonidine (17.5 μg) plus morphine (1 μg) intrathecally twice daily for 41/2 days (total of 9 doses). Hot plate and tail flick tests were conducted after the first, fifth and ninth doses. After the ninth dose animals were killed and their spinal cords were removed for histological examination.Tolerance developed to the antinociceptive effects of the drug combination, but at a slower rate than to morphine alone. No evidence of toxicity or injury to the spinal cord was observed other than changes which could be ascribed to the presence of the catheter.

Leakage of fluid administered epidurally to rats into subcutaneous tissue.

&NA; Epidural catheters were implanted in rats under halothane/nitrous oxide anaesthesia. Contrast medium (Iopamidol®) was injected via the catheter under fluoroscopic control 24–48 h after implantation. In 15 of 20 rats contrast could be seen leaking out of the epidural space, usually after only 25 &mgr;l was administered. Leakage was associated with diminished antinociceptive response to morphine administered via the catheter. Both leakage and decreased response to morphine could be largely prevented by applying a drop of Supa‐Glue® over the site of entry of the catheter to the epidural space at the time of catheter implantation. Investigators using epidurally cannulated rats should document that leakage does not occur or discard results from rats showing evidence of leakage.

Use of artificial sweeteners to promote alcohol consumption by rats

Summary Cirrhosis may be reliably produced in rats by exposing them intermittently to low levels of carbon tetrachloride vapour while feeding alcohol in the Lieber‐DeCarli liquid diet. Providing the alcohol in drinking water that has been sweetened with sucrose is a cheaper and more convenient method but it does not yield reliable results. This study aimed to determine whether alcohol in drinking water sweetened with artificial sweeteners would give adequate alcohol intake to achieve the desired hepatic effects. Rats were fed alcohol (8% v/v) in drinking water sweetened with sucrose (5% w/v) (n = 12), or with one of the artificial sweeteners aspartame (0.025%), saccharin (0.025%) or cyclamate (0.05%) (n = 8 per agent). During the alcohol treatment the animals were exposed to carbon tetrachloride vapour, 40 ppm, six hours per night for five nights per week, over a period of 14 weeks. All groups achieved good alcohol intakes of 5–6 g/kg/day. Only one rat, in the aspartame group, became cirrhotic; all the others had varying degrees of fibrosis which did not differ significantly among the treatments. Although it was not effective in reliably achieving cirrhosis, sweetening the alcohol solution with artificial sweeteners led to reasonable alcohol intakes with resultant hepatic fibrosis, and without the high carbohydrate intake which occurs when sucrose is used.

Leakage of epidurally administered fluid into subcutaneous tissue

AIMS OF INVESTIGATION; This study aimed to determine whether the antinociceptive effects of intrathecal (i.t.) morphine and i.t. clonidine are su ra-additive when the drugs are given in combination. METHODS: Intrathecal cannulae were imp anted in male Porton rats under halothane anesthesia. P Two weeks later? rats received each of three treatments in random sequence, at 2-3 day intervals. Treatment 1 was 1.t. morphine, 2 I.cg; Treatment 2 was i.t. clonidine at one of 10 dose levels in the ran e O50 pg, and Treatment 3 was 1 pg morphine plus half the dose of clonidine received in Treatment 2. !rh is design allows a non-parametric, within-animal test of supra-additivity. Six to seven rats were used at each clomdine dose level. Following drug administration, latency in the hot-plate test (SSOC, 25 s cutoff) and tail-flick test (5 s cutoff) were measured at intervals until they returned to within 25% of baseline values. Area under the latency (as % maximum possible effect) vs time curve (AUC) was taken as the response. A supra-additive effect was defined as response to Treatment 3 being greater than responses to both Treatments 1 and 2. RESULTS: MO hine had a significant antinocice AUC units, mean f S ‘g _, n = 63) and tail flick 7353 f 4 s tive effect in both the hot-plate (4440 f 4432

Lack of supra-additivity of antinociceptive effects of intrathecal morphine and intrathecal clonidine in rats

AIMS OF INVESTIGATION; This study aimed to determine whether the antinociceptive effects of intrathecal (i.t.) morphine and i.t. clonidine are su ra-additive when the drugs are given in combination. METHODS: Intrathecal cannulae were imp anted in male Porton rats under halothane anesthesia. P Two weeks later? rats received each of three treatments in random sequence, at 2-3 day intervals. Treatment 1 was 1.t. morphine, 2 I.cg; Treatment 2 was i.t. clonidine at one of 10 dose levels in the ran e O50 pg, and Treatment 3 was 1 pg morphine plus half the dose of clonidine received in Treatment 2. !rh is design allows a non-parametric, within-animal test of supra-additivity. Six to seven rats were used at each clomdine dose level. Following drug administration, latency in the hot-plate test (SSOC, 25 s cutoff) and tail-flick test (5 s cutoff) were measured at intervals until they returned to within 25% of baseline values. Area under the latency (as % maximum possible effect) vs time curve (AUC) was taken as the response. A supra-additive effect was defined as response to Treatment 3 being greater than responses to both Treatments 1 and 2. RESULTS: MO hine had a significant antinocice AUC units, mean f S ‘g _, n = 63) and tail flick 7353 f 4 s tive effect in both the hot-plate (4440 f 4432