A comparison of the effects of alfentanil applied to the spinal cord and intravenous alfentanil on noxiously evoked activity of dorsal horn neurons in the cat spinal cord.

The purpose of this study was to examine the effects of alfentanil applied to the surface of the spinal cord and the effects of intravenously administered alfentanil on noxiously evoked activity of dorsal horn neurons. Extracellular single neuron recordings were obtained from wide dynamic range neurons in 26 decerebrate cats with transected spinal cords. Spinally administered alfentanil, 25 micrograms or 50 micrograms, caused 36 and 86% suppression of noxiously evoked activity, respectively. Maximum mean suppression was achieved at 24 and 21 min after 25 micrograms, and 50 micrograms, respectively. Intravenous naloxone, 0.1 mg, when tested, completely reversed the suppression. Spontaneous recovery to control values occurred within 2 hr. Intravenously administered alfentanil, 12.5 micrograms/kg or 25 micrograms/kg, produced suppression of 43 and 89%, respectively, with maximum mean suppression observed at the 6- and 3-min time points, respectively. Complete recovery after intravenous administration was seen within 30 min. This study, using a sensitive neurophysiologic assay, demonstrates the important differences in onset and duration of drug effects that must be considered when comparing the responses of spinal cord neurons to intravenously administered narcotics and narcotics applied directly to the surface of the spinal cord.