A comparison of some pharmacological effects of naloxone and N-methylnaloxone in mice.

The effects of N-methylnaloxone following subcutaneous and intracerebroventricular administrations on nociception were investigated using the hot plate technique. Unlike naloxone, subcutaneous administration of N-methylnaloxone did not enhance the nociceptive reactions. In contrast, intracerebroventricular injection of N-methylnaloxone produced antinociception and tremor. Compared with naloxone, N-methylnaloxone was very weak in precipitating the signs of abstinence in mice rendered acutely dependent on morphine. Two factors, poor penetration into the central nervous system and steric hindrance, might render N-methylnaloxone very weak and hence both these factors must be taken into consideration while analyzing the effects following quaternary derivatives of opioid antagonists.