Somatosensory-evoked potentials (SEPs) are affected differently by mu- and kappa-selective opioids in the dog.

Alfentanil, a mu-opioid receptor ligand, induced a dose-related suppression of the early N50 peak and an increase in latency of the late N140 peak. The highest dose blocked all potentials. These effects were reversible by naloxone. The kappa-ligand bremazocine, induced a dose-related increase in latency of the late P150 peak which was, however, reversed by the specific kappa-antagonist Mr2266, not by naloxone. Continuous blockade of the mu-receptor with low-dose naloxone topped with cumulative doses of nalbuphine, a mixed narcotic analgesic, resulted in a Mr 2266-reversible increase in latency of the late N100 peak. Influences of the opioids on the SEP may account for their difference in clinical effects.