Assessment of canine sensory function by using sine-wave electrical stimuli paradigm.

The paradigm of sine-wave electrical stimuli has been used for sensory neurological assessment in humans. In the present study, we applied the paradigm to the dog for the quantitative assessment of sensory function. Sine-wave electrical current stimuli at frequencies of 2000, 250, and 5Hz were delivered to bipolar electrodes attached to the skin surface of the hind paws. The stimulation intensity was gradually increased, and the minimum intensity required to elicit the lifting behavior in the stimulated paw was determined as current threshold (CT) for each of the three frequencies. Dogs consistently showed the lifting behavior at CTs without showing aversive behaviors such as vocalization and wriggling. The baseline CTs (mean+/-SEM, n=12) were 4430+/-110microA for CT2000, 2215+/-173microA for CT250, and 2305+/-152microA for CT5. The CTs immediately increased after bolus intravenous injection of fentanyl at 10microg/kg, although the significant increase disappeared within 1h. The time course for the CTs was parallel to that of plasma fentanyl concentration. In conclusion, the present study applied the paradigm of transcutaneous sine-wave electrical stimuli to the dog, and used the hind paw lifting as endpoint behavior. This paradigm is simple, non-invasive, useful in the assessment of sensory function, and can be adapted to investigate the pharmacokinetics/pharmacodynamics relation of drugs. Further studies are needed to give the conclusive interpretation of the endpoint behavior.

Toxicology profile of N-methyl-D-aspartate antagonists delivered by intrathecal infusion in the canine model.

BACKGROUND: Intrathecal N-methyl-d-aspartate antagonists have antihyperalgesic efficacy. The authors examined toxicity in a canine model of chronic lumbar intrathecal infusion. METHODS: Dogs (10-16 kg) were prepared with lumbar intrathecal catheters connected to vest-mounted pumps (100 microl/h). In phase 1, stepwise incrementations in infusion concentration were performed at 48- to 72-h intervals to determine an infusion dose with minimal but detectable behavioral effects. In phase 2, the dose/concentration defined in phase 1 was infused for 28 days. Behavioral function during infusion and histopathology at sacrifice was assessed. Drugs examined were 2-amino-5-phosphono-valorate (AP5), MK801, memantine, amitriptyline, S-methadone, and saline. RESULTS: In the phase 1 dose ranging, the minimum effect doses for the several agents were as follows: AP5, 1 mg/day; amitriptyline, 1 mg/day; ketamine, 10 mg/day; MK801, 1 mg/day; and memantine, 4 mg/day. In phase 2, infusion of these doses typically resulted in mild hind limb weakness by 3-5 days after initiation of infusion, which progressed over the 28-day infusion interval. In a limited number of animals, a similar effect was observed with S-methadone. Histopathologically, vehicle-infused animals displayed a minor local catheter reaction. With the drug treatments, a gradient of increasing pathology from cervical to lumbar segments was noted. Pathology ranged from local demyelination to necrotizing lesions of spinal parenchyma near the catheter tip. All drugs given at their respective doses produced pathology scores significantly worse than saline controls. CONCLUSIONS: These drugs given for 28 days at acutely tolerable doses lead to spinal pathology. These data suggest a reevaluation of the use of these agents in chronic spinal delivery.

Development of a canine nociceptive thermal escape model.

Acute nociceptive models which have been validated for large animal species are limited, yet nociceptive assessment in non-rodent species is important in analgesic drug development where larger animals may be necessary because of the technical requirements of the study. Here we report development and validation of a canine hind paw thermal escape model and the effect of analgesics on withdrawal latencies. Individual focused projection bulbs were used as left and right voltage-adjusted thermal stimuli placed below a glass plate in a specifically designed canine holding apparatus. After acclimation, dogs were lightly restrained in a fabric sling while standing on the glass plate. The anterior center of the metatarsal pad of the left and right hind paw was positioned on the glass over each light, and duration of stimulation tolerance timed. For every trial, the escape latency from lamp actuation to paw withdrawal was recorded twice for each hind paw. The mean population baseline withdrawal latency of 9.3+/-1.7s (mean+/-S.D., n=12 dogs) was shown to be repeatable between paws, within and between individual animals, and between test days. This latency corresponded to a glass surface temperature of 49.5 degrees C. A cut-off time of 20s (corresponding to a glass surface temperature of 56.5 degrees C) was set to prevent tissue damage. Intravenous administration (mg/kg) of morphine (1.0), hydromorphone (0.2), butorphanol (0.4), fentanyl (0.01), and dexmedetomidine (0.01) significantly (p<0.05) increased withdrawal latency from baseline within 15-30 min of administration while buprenorphine (0.03) produced a delayed, modest but significant latency increase. Rank order of opioid analgesic duration was morphine=hydromorphone>butorphanol>bupenorphine>fentanyl=saline. A dose-effect curve for hydromorphone was generated and corresponded to previously described dose-effect relationships in other species. The non-analgesic tranquilizer acepromazine (0.1mg/kg) produced mild sedation, but no significant increase in latency from that of saline. The model yielded a clear distinction between analgesia and sedation for all agents tested. These studies provide validation of a canine thermal escape model and have demonstrated the efficacy of clinically relevant doses of analgesics in elevating escape latencies. This model will facilitate quantification of the effects of parenterally and neuraxially administered analgesics in dogs.

Time course and role of morphine dose and concentration in intrathecal granuloma formation in dogs: a combined magnetic resonance imaging and histopathology investigation.

BACKGROUND: Intrathecal morphine infusion leads to intrathecal granulomas. In dogs, the authors examined time course of granuloma formation and the role of concentration in granuloma development. METHODS: Dogs were prepared with lumbar intrathecal catheters and vest-mounted pumps. To define the time course of granuloma formation, serial magnetic resonance imaging was performed in animals receiving 10 or 31 days of morphine infusion (12.5 mg/ml at 40 microl/h). At these times, morphine was removed from the infusate, and further magnetic resonance images were acquired over 14-35 additional days. To assess dose versus concentration, dogs received 28-day infusions of vehicle, 12 mg morphine/day as 12.5 mg/ml at 40 microl/h, or 1.5 mg/ml at 334 microl/h (12 mg/day) for 28 days. Additional dogs received 3 mg/day as 12.5 mg/ml at 10 mul/h. RESULTS: Serial magnetic resonance images in dogs receiving morphine (12.5 mg/ml at 40 microl/h) revealed pericatheter-enhancing tissues as early as 3 days with a prominent signal by 10 days. Removal of morphine reduced the mass volume within 7 days. At a fixed infusion rate, the incidence of granuloma formation with the continuous intrathecal infusion of morphine ranged from 0 in vehicle-treated dogs to 100% in dogs treated with 12.5 mg/ml at 40 microl/h (12 mg/day). Infusion of 12 mg/day at 1.5 mg/ml (334 microl/h) resulted in granuloma in one of four animals. The authors found that infusion of morphine in different concentrations at a fixed rate resulted in a dose-dependent increase in concentration, with the granuloma-producing, dose-yielding lumbar cerebrospinal fluid morphine concentrations around 40 microg/ml. CONCLUSIONS: Serial magnetic resonance imaging showed a rapid formation and regression of the masses initiated by intrathecal morphine infusion. These masses are dependent on local concentration.

Opiate pharmacology of intrathecal granulomas.

BACKGROUND: Chronic intrathecal morphine infusion produces intradural granulomas. The authors examined a variety of opioids infused intrathecal for analgesic activity and toxicity. METHODS: Two sets of experiments were undertaken in dogs with chronic intrathecal catheters: (1) Six-hour intrathecal infusions were used to determine the full analgesic dose and the maximum tolerated dose. (2) To establish toxicity, the maximum tolerated dose was given for up to 28 days by continuous intrathecal infusion. Drugs examined were morphine sulfate, hydromorphone, D/L-methadone, L-methadone, D-methadone, fentanyl, [d-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin (DAMGO), naloxone, or saline. RESULTS ANALGESIA AND TOLERABILITY: Six-hour intrathecal infusion of agonists resulted in a time-dependent increase in thermal escape latency. At higher concentrations, dose-limiting motor dysfunction and sedation occurred, and hypersensitivity occurred. The concentrations, in mg/ml, for full analgesic dose/maximum tolerated dose were as follows: morphine, 0.9/12.0; hydromorphone, 1.0/3.0; D/L-methadone, 2.8/3; L-methadone, 1.0/> 1.0; fentanyl, 0.3/2.0; DAMGO, 0.1/> 2.0; D-methadone, > 1/> 1; naloxone, > 10/> 10. SPINAL PATHOLOGY: Chronic intrathecal infusion of the maximum tolerated dose revealed 100% intradural granuloma formation after morphine, hydromorphone, L-methadone, and naloxone. DAMGO induced a mass in only a single animal (one of three). D/L- and D-methadone produced intradural granulomas but were also associated with parenchymal necrosis. Saline and fentanyl animals displayed no granulomas. CONCLUSIONS: Intrathecal opiate-induced granulomas are not strictly dependent on opioid receptor activation. Therefore, opiates at equianalgesic doses present different risks for granuloma formation. Importantly, D/L- and D-methadone also resulted in parenchymal necrosis, an affect associated with the N-methyl-D-aspartate antagonist action of the D-isomer.

Intrathecal ketorolac in dogs and rats.

This study was conducted to assess spinal safety of the cyclo-oxygenase inhibitor ketorolac in dogs and rats. Beagle dogs were prepared with lumbar intrathecal catheters and received continuous spinal infusions of 5 mg/ml ketorolac (N = 6), 0.5 mg/ml ketorolac (N = 8), or saline vehicle (N = 6) at 50 microl/h (1.2 ml/day) for 28 days. No systematic drug or dose-related changes were observed in motor function, heart rate, or blood pressure. Histological examination revealed a mild pericatheter reaction in all groups with no drug or dose related effect upon spinal pathology at the lumbar site of highest drug concentration. Cisternal CSF protein was elevated for all treatment groups at necropsy, and cisternal glucose was within normal range for all treatment groups, though three dogs displayed decreases in cisternal glucose. Significant reductions in hematocrit were noted, and increased incidence of gastric bleeding at necropsy was observed in animals receiving ketorolac. Intrathecal ketorolac kinetics revealed a biphasic clearance: t1/2 s = 10.3 and 53 min, respectively. After initiation of infusion (0.5 mg and 5 mg/ml/50 microl/h), lumbar CSF concentrations of ketorolac were 3.8 and 52.7 microg/ml, respectively. Bolus and continuous infusion of intrathecal ketorolac resulted in significant reduction of lumbar CSF PGE2 concentrations. In rats, with intrathecal catheters, four daily bolus deliveries of saline or ketorolac (5 mg/ml/10 microl) had no effect upon spinal histology or upon spinal cord blood flow. These data indicate that intrathecal ketorolac in two species at the dose/concentrations employed does not induce evident spinal pathology but diminishes spinal prostaglandin release.

Chronically infused intrathecal morphine in dogs.

BACKGROUND: Despite the extensive use of intrathecal morphine infusion for pain, no systematic safety studies exist on its effects in high concentrations. The authors assessed the effects of morphine and clonidine given 28 days intrathecally in dogs. METHODS: Beagles with lumbar intrathecal catheters received solutions delivered by a vest-mounted infusion pump. Six groups (n = 3 each) received infusions (40 microl/h) of saline or 1.5, 3, 6, 9, or 12 mg/day of morphine for 28 days. Additional groups received morphine at 40 microl/h (1.5 mg/day) plus clonidine (0.25-1.0 mg/day) or clonidine alone at 100 microg/h (4.8 mg/day). RESULTS: In animals receiving 9 or 12 mg/day morphine, allodynia was observed shortly after initiation of infusion. A concentration-dependent increase in hind limb dysfunction evolved over the infusion interval. Necropsy revealed minimal reactions in saline animals. At the higher morphine concentrations (all dogs receiving 12 mg/day), there was a local inflammatory mass at the catheter tip that produced significant local tissue compression. All animals with motor dysfunction displayed masses, although all animals with masses did not show motor dysfunction. The mass, arising from the dura-arachnoid layer, consisted of multifocal accumulations of neutrophils, monocytes, macrophages, and plasma cells. Inflammatory cells and endothelial cells displayed significant IL1beta, TNFalpha, iNOS, and eNOS immunoreactivity. No evidence of bacterial or fungal involvement was detected. There were no other changes in spinal morphologic characteristics. In four other groups of dogs, clonidine alone had no effect and in combination with morphine reduced the morphine reaction. CONCLUSIONS: The authors found that high intrathecal morphine concentrations lead to aseptic intrathecal inflammatory masses. The lack of effect of clonidine and the possible suppressive effects of clonidine on the local reaction suggest the utility of such coadministration.