Jp Chambers

Depression of glutamatergic transmission by nociceptin in the neonatal rat hemisected spinal cord preparation in vitro

The present study explored the action of nociceptin, the putative endogenous ligand for the orphan opioid receptor (ORL1), on the rat hemisected spinal cord preparation. Electrical stimulation of a dorsal root evokes a glutamatergic population ventral root potential (DR‐VRP) in the corresponding ventral root. Low intensity stimulation evokes two A fibre‐mediated components; a compound action potential of motonuerones superimposed on a population e.p.s.p. (excitatory postsynaptic potential); at higher stimulus intensities sufficient to activate C fibres a more prolonged population e.p.s.p. is evoked. All three components were depressed by nociceptin in a concentration‐dependent manner with IC50 values (s.e.mean) of 119±2 nM (n = 4), 241±3 nM (n = 4) and 32±2 nM (n = 4), respectively. The depressant actions of nociceptin (30 nM and 300 nM) were not reversed by the opioid antagonist naloxone (1 μm). Nociceptin (100 nM and 300 nM) had no effect on the afferent volleys in the dorsal root. Nociceptin therefore appears to be acting as an inhibitory peptide at the spinal level through a naloxone‐insensitive opioid receptor.

Depression of A and C fibre‐evoked segmental reflexes by morphine and clonidine in the in vitro spinal cord of the neonatal rat

Population synaptic responses of motoneurones were recorded from a ventral root following electrical stimulation of the corresponding lumbar dorsal root in neonatal rat hemisected spinal cord preparations in vitro. Two levels of electrical stimulation were used to elicit dorsal root compound action potentials that contained either an A fibre component alone or both A and C fibre components. The effects of centrally acting analgesics and an N‐methyl‐d‐aspartate (NMDA) receptor antagonist were tested on synaptic responses produced by these two levels of stimulation. At stimulus intensities below four times threshold (T) there was no C fibre component in the dorsal root compound action potential. Responses to a single pulse at 3T (the low intensity excitatory postsynaptic potential (e.p.s.p.)), a train of five pulses at 2T (the train e.p.s.p.) and a single supramaximal pulse (the high intensity e.p.s.p.) were used to compare the depressant actions of morphine, clonidine and the competitive NMDA antagonist CGP40116 (d‐(E)‐2‐amino‐4‐methyl‐5‐phosphono‐pentenoic acid). The train e.p.s.p. (mean half‐time to decay 5±0.6 s, n=6) had a similar profile to the high intensity e.p.s.p. (mean half‐time to decay 6.8±0.7, n=8). The monosynaptic compound action potential of motoneurones (MSR) was resistant to all three drugs irrespective of the intensity of dorsal root stimulation. The low intensity e.p.s.p., the train e.p.s.p. and the high intensity e.p.s.p. were depressed by all three drugs. The EC50 values for depression by morphine were 79±1 nm (n=8) for the high intensity e.p.s.p. and 99±1 nm (n=4) for the low intensity e.p.s.p. The corresponding values for clonidine were 25±1 nm (n=8) and 9±1 nm (n=4) and those for CGP40116 were 860±1.3 nm (n=4) and 76±1.1 nm (n=4). The depressant profile of the NMDA antagonist, having the least depressant activity on the C fibre‐mediated response, was different from that of the two analgesics. CGP40116 (3 μm) depressed the high intensity e.p.s.p. to 62±8%, the low intensity e.p.s.p. to 22±4% and the train e.p.s.p. to 16±2% of control values. The depressant actions of morphine were fully reversed by naloxone (1 μm) and those of clonidine were fully reversed by atipamezole (1 μm). These results show that, in contrast to previous findings, activation of primary afferent C fibres in dorsal roots is not required for generation of morphine‐ or clonidine‐sensitive synaptic responses in ventral roots of this in vitro preparation.

Electroencephalographic responses of tramadol, parecoxib and morphine to acute noxious electrical stimulation in anaesthetised dogs.

This study compared the efficacy of different classes of analgesics in preventing the changes in electroencephalographic (EEG) indices of nociception in anaesthetised dogs, subjected to a standard electrical stimulus. In a crossover study, eight dogs received morphine (0.5mg/kg) or tramadol (3mg/kg) or parecoxib (1mg/kg) or 0.9% saline subcutaneously (SC) at the time of pre-anaesthetic medication. After induction with intravenous propofol, anaesthesia was maintained with halothane at a stable concentration between 0.85% and 0.95%. EEG was recorded in a three electrode montage, using SC needle electrodes, before and after electrical stimulation of dogs during anaesthesia. Post-stimulation median frequency (a reliable indicator of nociception) of the EEG increased significantly in tramadol, parecoxib and saline groups compared to that of morphine. Total EEG power decreased in all treatment groups following stimulation. These results indicate that the changes in EEG responses to noxious stimulation can be used for evaluating anti-nociceptive efficacy of analgesics.

Effects of tramadol, morphine or their combination in dogs undergoing ovariohysterectomy on peri-operative electroencephalographic responses and post-operative pain

Abstract AIM: To compare the peri-operative electroencephalogram (EEG) responses and post-operative analgesic efficacy of pre-operative morphine or tramadol with a combination of low-dose pre-operative morphine and post-operative tramadol, in dogs undergoing ovariohysterectomy. METHODS: Dogs undergoing routine ovariohysterectomy were treated with either pre-operative morphine (0.5 mg/kg S/C, n=8), or tramadol (3 mg/kg S/C, n=8), or pre-operative low-dose morphine (0.1 mg/kg S/C) and post-operative tramadol (3 mg/kg I/V, n=8). All dogs received routine pre-anaesthetic medication, and anaesthesia was induced with I/V thiopentone to effect and maintained with halothane in oxygen. Respiratory rate, heart rate, end-tidal halothane tension (EtHal) and end-tidal CO2 tension (EtCO2) were monitored throughout surgery. The EEG was recorded continuously in a three electrode montage. Median frequency (F50), total power (Ptot) and 95% spectral edge frequency (F95) of the EEG power spectra were compared during different 100-second periods of surgery: prior to and during skin incision, ligation of each ovarian pedicle, ligation of uterine body and skin closure. Post-operatively, pain was assessed using the short form of the Glasgow composite measure pain scale (CMPS-SF). RESULTS: There was no difference in F50 or Ptot of the EEG between baseline and noxious surgical events within each treatment group, or between the three groups (p>0.05). The mean F95 was higher during the first three periods of surgery for dogs administered tramadol and low-dose morphine than those that received 0.5 mg/kg morphine (p=0.001). Dogs that received low-dose morphine and tramadol had lower CMPS-SF pain scores after ovariohysterectomy than those that received either tramadol or morphine alone (p=0.001). There was no difference in pain scores between dogs in the latter two groups. CONCLUSION AND CLINICAL RELEVANCE: Tramadol and morphine administered pre-operatively provided an equal degree of post-operative analgesia in dogs after ovariohysterectomy. A combination of pre-operative low-dose morphine and post-operative tramadol produced better post-operative analgesia than either drug administered alone pre-operatively. Administration of analgesics pre- and post-operatively could result in improved post-operative well-being of ovariohysterectomised dogs.

A comparison of the organization of longitudinal and circular contractions during pendular and segmental activity in the duodenum of the rat and guinea pig

Background  Little is known of the spatiotemporal organization of pendular duodenal contractions.

Depression of NMDA receptor-mediated synaptic transmission by four α2 adrenoceptor agonists on the in vitro rat spinal cord preparation

α2‐Adrenoceptor agonists have a spinal site of analgesic action. In the current study the synaptic depressant actions of xylazine, detomidine, romifidine and dexmedetomidine have been compared on segmental reflexes containing NMDA receptor‐mediated components in the neonatal rat hemisected spinal cord preparation in vitro. Reflexes were evoked in the ventral root following either supramaximal electrical stimulation of the corresponding ipsilateral lumbar dorsal root to evoke the high intensity excitatory postsynaptic potential (e.p.s.p.) involving all primary afferent fibres, or low intensity stimulation to evoke the solely A fibre‐mediated low intensity e.p.s.p. The high intensity e.p.s.p. contains a greater NMDA receptor‐mediated component. Xylazine, romifidine, detomidine and dexmedetomidine all depressed both the high intensity e.p.s.p. and the low intensity e.p.s.p. giving respective EC50 values of 0.91±0.2 μM (n=12), 23.4±3 nM (n=12), 37.7±7 nM (n=8) and 0.84±0.1 nM (n=4) for depression of the high intensity e.p.s.p. and 0.76±0.1 μM (n=12), 22.0±3 nM (n=12), 24.9±6 nM (n=4) and 2.7±0.6 nM (n=4) for depression of the low intensity e.p.s.p., respectively. Unlike the other three drugs, the two values for dexmedetomidine, showing a greater selectivity for the high intensity e.p.s.p., are significantly different. Each of these depressant actions was reversed by the selective α2‐adrenoceptor antagonist atipamezole (1 μM). In contrast to previous reports of the actions of α2‐adrenoceptor agonists on the in vitro spinal cord preparation, at concentrations ten fold higher than the above EC50 values xylazine, romifidine, detomidine and dexmedetomidine depressed the initial population spike of motoneurons (MSR). This depression was not reversed by atipamezole. Comparison of the rank order of the present EC50 values for depression of the high intensity e.p.s.p. with potency ratios from in vivo analgesic tests in previous studies show a close correlation between the present in vitro tests and analgesic potency. There is no correlation between the present data and previously obtained affinities of the agonists at non‐adrenergic imidazoline binding sites. The current findings therefore suggest that xylazine, romifidine, detomidine and dexmedetomidine are exerting their central analgesic actions at the spinal level principally through α‐2‐adrenoceptors. All four agonists showed the same profile of selective depression of the NMDA receptor‐mediated component of reflexes similar to that reported previously for clonidine. However dexmedetomidine, unlike the other ligands, selectively depressed the high intensity e.p.s.p.

Components of electroencephalographic responses to slaughter in halothane-anaesthetised calves: effects of cutting neck tissues compared with major blood vessels.

Abstract AIM: To identify whether cutting neck tissues or cutting major blood vessels initiates the mechanisms responsible for electroencephalographic (EEG) responses to slaughter by ventral-neck incision without prior stunning in halothane-anaesthetised calves. METHODS: Calves were assigned to two groups, viz transection of neck tissues with intact blood circulation through the brain (n=10), or transection of the major blood vessels of the neck but not most other neck tissues (n=7). They were minimally anaesthetised with halothane, using an established anaesthesia protocol. The animals in the neck-tissue transection group had their carotid arteries and jugular veins exposed and cannulated proximal and distal to the proposed site of subsequent ventral-neck incision; this diverted blood fl ow through these vessels so that cerebral perfusion and drainage were preserved. In animals in the blood-vessel transection group, the carotid arteries and jugular veins were exposed bilaterally by surgical dissection. They were then transected without further damage to the remaining structures of the neck. Changes in the median frequency (F50), 95% spectral edge frequency (F95), total power of the EEG (Ptot), and arterial blood pressure were compared within each group before and after neck-tissue or blood-vessel transection, and between groups following treatments. RESULTS: Neck-tissue transection resulted in little overall change in the F50, an increase in the F95, and an initial increase in Ptot followed by a transient decrease and eventual return to pre-treatment values. There was between-animal variation in these EEG parameters. Transection of the major blood vessels of the neck resulted in a decrease in F50 in most animals; changes in F95 were highly variable, and there was a decrease in Ptot. CONCLUSIONS: The EEG responses seen following necktissue and blood-vessel transection were qualitatively distinct, and suggested that cutting neck tissues caused greater noxious sensory input than transection of only the major blood vessels of the neck. These observations support the conclusion that the EEG responses seen after ventral-neck incision in intact animals are primarily due to noxious stimulation as a result of incision of ventral-neck tissues and not mainly as a result of loss of blood flow through the brain.

Use of analgesic drugs for pain management in sheep

Abstract Awareness of pain and its effects is increasing within the veterinary profession, but pain management in food animals has been neglected. Sheep seldom receive analgesics despite various conditions, husbandry practice and experimental procedures being known to be painful, e.g. footrot, mastitis, vaginal prolapse, castration, vasectomy, penis deviation, and laparoscopy. The evidence supporting use of analgesic drugs in this species is reviewed here. Opioid agonists are of dubious efficacy and are short acting. α2-agonists such as xylazine are good, short-lived analgesics, but induce hypoxaemia. Non-steroidal anti-inflammatory drugs (NSAID) such as ketoprofen provide long-lasting analgesia, but not as marked as that from α2-agonists; they should be more widely used for inflammatory pain. Local anaesthetics reliably block pain signals, but may also induce motor blockade. Balanced analgesia using more than one class of drug, such as an α2 agonist (e.g. medetomidine) and N-methyl-D-aspartate antagonist (e.g. ketamine), with the combination selected for the circumstances, probably provides the best analgesia for severe pain. It should be noted that there are no approved analgesic drugs for use in sheep and therefore the use of such drugs in this species has to be off-label. This information may be useful to veterinary practitioners, biomedical researchers, and regulators in animal welfare to develop rational analgesic regimens which ultimately may improve the health and welfare of sheep in both farming and experimental conditions.

Characterisation of the contractile dynamics of the resting ex vivo urinary bladder of the pig.

To characterise the area and movements of ongoing spontaneous localised contractions in the resting porcine urinary bladder and relate these to ambient intravesical pressure (Pves), to further our understanding of their genesis and role in accommodating incoming urine.

Effects of tramadol or morphine in dogs undergoing castration on intra-operative electroencephalogram responses and post-operative pain

AIM: To compare the effects of pre-operatively administered tramadol with those of morphine on electroencephalographic responses to surgery and post-operative pain in dogs undergoing castration. METHODS: Dogs undergoing castration were treated with either pre-operative morphine (0.5 mg/kg S/C, n = 8) or tramadol (3 mg/kg S/C, n = 8). All dogs also received 0.05 mg/kg acepromazine and 0.04 mg/kg atropine S/C in addition to the test analgesic. Anaesthesia was induced with thiopentone administered I/V to effect and maintained with halothane in oxygen. Respiratory rate, heart rate, end-tidal halothane tension (EtHal) and end-tidal CO2 tension (EtCO2) were monitored throughout surgery. Electroencephalograms (EEG) were recorded continuously using a three electrode montage. Median frequency (F50), total power (Ptot) and 95% spectral edge frequency (F95) derived from EEG power spectra recorded before skin incision (baseline) were compared with those recorded during ligation of the spermatic cords of both testicles. Post-operatively, pain was assessed after 1, 3, 6 and 9 h using the short form of the Glasgow composite measure pain scale (CMPS-SF). RESULTS: Dogs premedicated with tramadol had higher mean F50 (12.2 (SD 0.2) Hz) and lower Ptot (130.39 (SD 12.1) µv2) compared with those premedicated with morphine (11.5 (SD 0.2) Hz and 161.8 (SD 15.1) µv2, respectively; p<0.05) during ligation of testicle 1. There were no differences in EEG responses between the two treatment groups during ligation of testicle 2 (p>0.05). The F95 of the EEG did not differ between the two groups during the ligation of either testicle (p > 0.05). Post-operatively, no significant differences in the CMPS-SF score were found between animals premedicated with tramadol and morphine at any time during the post-operative period. No dog required rescue analgesia. CONCLUSION AND CLINICAL RELEVANCE: Tramadol and morphine administered pre-operatively provided a similar degree of post-operative analgesia in male dogs at the doses tested.