Intrathecal substance P-saporin in the dog: distribution, safety, and spinal neurokinin-1 receptor ablation.

BACKGROUND: Neurokinin-1 receptors (NK1-rs) located on superficial dorsal horn neurons are essential for integration of nociceptive input. Intrathecal injection of substance P-saporin (SP-SAP) leads to local loss of spinal NK1-r (+) neurons suggesting its potential as a therapeutic agent for chronic pain. The authors determined, in a canine model, effects of lumbar intrathecal SP-SAP. METHODS: Distribution of SP-SAP and Saporin was determined in plasma, lumbar cerebrospinal fluid, and tissue. Safety of intrathecal SP-SAP was determined in four groups (six dogs each) administered 0 (0.9% saline), 1.5, 15, or 150 µg SP-SAP through lumbar intrathecal catheters. Behavioral, physiologic, and biochemical variables were assessed. Spinal tissues were collected at 7 and approximately 90 days, or earlier if significant morbidity developed, and analyzed for NK1-r (+) neuron loss and histopathology. RESULTS: SP-SAP and Saporin were detectable in lumbar cerebrospinal fluid for up to 4 and 24 h, respectively. Animals receiving intrathecal saline, 1.5, or 15 µg of SP-SAP showed no persistent neurologic deficits. Three animals receiving 150 µg of SP-SAP developed pelvic limb paraparesis and were euthanized prematurely. Immunohistochemistry and in situ hybridization cell counts confirmed a significant reduction in NK1-r (+) in superficial dorsal horn neurons from lumbar spinal cord after intrathecal administration of 15 and 150 µg of SP-SAP. A significant loss of NK1-r neurons in the lumbar ventral horn occurred only with 150-µg SP-SAP. CONCLUSION: Intrathecal 15-µg SP-SAP reduced dorsal, but not ventral, NK1-r (+) neurons at the spinal level of delivery with minimal side effects, whereas 150-µg SP-SAP resulted in motor neuron toxicity.

Role of meningeal mast cells in intrathecal morphine-evoked granuloma formation.

BACKGROUND: Intrathecal morphine forms granulomas that arise from the adjacent arachnoid membrane. The authors propose that these inflammatory cells exit the meningeal vasculature secondary to meningeal mast cell degranulation. METHODS: Three sets of experiments were accomplished in dogs: (1) ex vivo meningeal mast cell degranulation (histamine release was measured ex vivo from canine dura incubated with opiates); (2) in vivo cutaneous mast cell degranulation (flare areas on the dog abdomen were measured after subcutaneous opiates); and (3) in vivo granuloma pharmacology. Dogs with lumbar intrathecal catheters received infusion of intrathecal saline or intrathecal morphine. Intrathecal morphine dogs received (1) no other treatment (control); (2) twice-daily subcutaneous naltrexone; (3) intrathecal co-infusion of cromolyn; or (4) twice-daily subcutaneous cromolyn for the 24- to 28-day study course. RESULTS: Morphine but not fentanyl evoked dural histamine release, which was blocked by cromolyn but not naloxone. Wheal/flare was produced by subcutaneous morphine, methadone, hydromorphone, but not fentanyl, and was unaffected by naltrexone but prevented by cromolyn. Granulomas occurred in all dogs receiving intrathecal morphine (15 of 15); subcutaneous naltrexone had no effect on granulomas (six of six) but was reduced by concurrent intrathecal cromolyn (zero of five) or twice-daily subcutaneous cromolyn (one of five). CONCLUSIONS: The pharmacology of cutaneous/dural mast cell degranulation and intrathecal granulomas are comparable, not mediated by opioid receptors, and reduced by agents preventing mast cell degranulation. If an agent produces cutaneous mast cell degranulation at concentrations produced by intrathecal delivery, the agent may initiate granulomas.

Alfentanil: correlations between absence of effect upon subcutaneous mast cells and absence of granuloma formation after intrathecal infusion in the dog.

BACKGROUND: We hypothesize that intrathecal (IT) granulomas arising from the IT infusion of several opiates may result from the degranulation of meningeal mast cells (MC). Given functional covariance between cutaneous and meningeal MC, we propose that opioids that do not degranulate cutaneous MC will not produce a granuloma. An opioid meeting this criteria is the phenylpiperadine alfentanil HCl. METHODS: Three experiments were accomplished in dogs. 1) Cutaneous MC degranulation. Flare areas on the dog abdomen were measured after intradermal alfentanil, morphine, or compound 48-80. 2) Dose ranging of analgesic effects of IT alfentanil infusion. Dogs with lumbar IT catheters received continuous infusion for 24 hours of different concentrations (1-20 mg/mL/d) of alfentanil and analgesic effects were assessed. 3) Granuloma inducing effects. Dogs received IT alfentanil (20 mg/mL/d; N = 5; 22-28 days) or morphine (12 mg/mL/d; N = 3; 22-30 days) and spinal cord harvested for histopathology after 22-30 days of infusion. RESULTS: 1) Intradermal morphine (10 mg/mL) and compound 48-80 (1 mg/mL) but not alfentanil at concentrations up to 20 mg/mL produced a cutaneous flare. IT alfentanil infusion produced increases in thermal escape latency at concentrations as low as 2 mg/mL/day. A significant depression of arousal was noted in the dogs receiving 20 mg/mL. Over the 22- to 30-day infusion period, morphine (12 mg/mL/day) resulted in granulomas in all three animals examined whereas IT alfentanil at 20 mg/mL/day failed to initiate a granuloma in any animal. CONCLUSIONS: These results support the hypothesis linking MC degranulation and IT granulomas.