Lack of a ceiling effect for intrathecal buprenorphine on C fibre mediated somatosympathetic reflexes.

We observed that buprenorphine 20 micrograms kg-1 i.v. in dogs caused an initial significant reduction in both A delta and C fibre mediated somatosympathetic reflexes evoked by tibial and radial nerve stimulation, to approximately 75% and 70% of control values. Larger doses (up to 100 micrograms kg-1 i.v.) had progressively less effect and the mean responses were depressed to only approximately 65% and 55% of control, indicating a ceiling effect. Buprenorphine 450 micrograms intrathecally (i.t.) completely abolished tibial C fibre reflexes, but 20% of A delta responses could not be eliminated with doses up to 1050 micrograms i.t. Fentanyl 100 micrograms kg-1 i.v. or 150 micrograms i.t. after buprenorphine i.v. or i.t., respectively, had little additional effect. This study confirms the limited ceiling effect of buprenorphine on nociceptive reflexes when administered systemically, and provides evidence that when administered i.t. in sufficient doses it abolishes the C responses (lack of ceiling effect for C responses), but the A delta responses show a plateau or ceiling effect.