Effects of narcotics, including morphine, on visual evoked potential in rats

The side effects of narcotics, including morphine, on the visual system are still unclear; therefore, the present study was undertaken to examine the effects of narcotics on the visual system at each antinociceptive dose by using the evoked potential (VEP) in rats. Morphine (2 or 5 mg/kg) caused a significant increase in the amplitude of early and late VEP components (P1–N1, N1–P2, P3–N3 and N3–P4). Fentanyl (0.02 mg/kg) also showed a significant increase in the amplitude of late VEP components (P3–N3, N3–P4). The effects of morphine and fentanyl on VEP components were antagonized by naloxone (1 mg/kg). On the other hand, (±)-pentazocine (20 mg/kg) reduced the amplitude of the late VEP component (N3–P4), and this effect was not antagonized by naloxone. Butorphanol showed no significant changes in early and late VEP components.In conclusion, morphine stimulated the retino-geniculate-cortex pathway and the thalamus-cortical circuit through the opioid receptors, and fentanyl stimulated the thalamus-cortical circuit through the opioid receptors. It can therefore be assumed that VEP is a useful tool for examining the side effects of drugs, including narcotics, on the visual system.