Serum morphine levels in cats during dorsal horn neuron suppression by spinally administered morphine

The degree of serum uptake of morphine following spinal morphine administration was measured in cats. Total serum morphine levels (free plus conjugated) were determined during suppression of noxiously evoked wide dynamic range neuron activity by spinally administered morphine and compared with total serum morphine levels following i.v. administration of similar doses. The serum levels at 30 minutes after spinal application of morphine (a time at which there was significant suppression of noxiously evoked neuronal activity) were low; after 0.1 and 0.25 mg the levels were 6.5 (n=6) and 12.5 (n=5) ng/ml respectively. In contrast, intravenous administration of the same doses (0.1 and 0.25 mg) produced levels at 30 minutes of 24 and 36 ng/ml respectively, while an intravenous dose commonly used in earlier neurophysiologic studies (2 mg/kg) produced serum levels in excess of 400 ng/ml. These results indicate that although there is systemic uptake following spinal administration of morphine, the serum levels achieved are much lower than those following intravenous administration of a comparable dose, and are insufficient to explain the resultant suppression of wide dynamic range neurons.