Effect of narcotic analgesics on the striatal homovanillic acid content in mice; relation to antinociceptive effect.

The effects of various narcotic analgesics on striatal homovanillic acid (HVA) content, hot plate time and rectal temperature in mice were compared in relation to dose and time. The hypothermia induced by narcotic analgesics did not correlate with the striat"al HVA increase. Pentazocine, cyclazocine and thebaine had no effect on the hot plate time. The maximum prolongation of hot plate time induced by morphine, methadone or piminodine occurred before the highest HVA increase. The highest increase induced by narcotic analgesics in striatal HVA content was twice the original concentration. This occurred 2 hr after 40 mg/kg of morphine; 2 hr after 20 mg/kg of methadone; 1/2 hr after 20 mg/kg of piminodine; and 1 hr after 60 mg/kg of pentazocine. Cyclacozine (10 and 20 mg/kg) and thebaine (10 mg/kg) did not alter the HVA content. With the exception of pentazocine, those doses of narcotic analgesics that caused equal increases in striatal HVA content were also equianalgesic. These results suggest that there are similarities in the structural requirements for antinociceptive and striatal HVA-increasing effects of narcotic analgesics. The neuroleptic compound haloperidol (0.5 mg/kg) caused a fourfold increase in striatal HVA content making it twice as efficient as narcotic analgesics. This finding suggests that narcotic analgesics do not act on the same sites as neuroleptics when causing an increase in striatal HVA content.