Effect of nitrous oxide alone or its combination with fentanyl on spinal reflexes in cats.

The effects of nitrous oxide alone and combination of nitrous oxide and fentanyl on spinal reflexes were studied in cats with transected spinal cord at the thoracic level. Nitrous oxide 33, 50 and 75% in oxygen depressed the amplitude of monosynaptic reflexes in a dose-dependent manner, but exerted little effect on polysynaptic reflexes. The addition of fentanyl 2.5 microgram kg-1 during the administration of 75% nitrous oxide in oxygen depressed monosynaptic reflexes further. These results suggest that nitrous oxide or its combination with fentanyl has a depressant action on the transmission of monosynaptic reflexes in the spinal cord, but nitrous oxide might exert less effect on the spinal interneurones which mediate polysynaptic reflexes.