Effect of glutamic acid and ketamine on morphine-induced analgesic response in rats.

Rats were treated with intraperitoneal injections of morphine (10 mg/kg) followed by glutamic acid (20 mg/kg.) and ketamine. (5 mg/kg). Pain thresholds were recorded as tail flick latencies for a period of 23 days and the mean area under curves calculated. Glutamic acid and ketamine, partially blocked the analgesic effects of morphine. Two types of effects were observed. In 4 animals, there was a partial blockade of the response, and in 2 animals there was a complete blockade followed by reversal in both the groups. It is suggested that two different mechanisms one excitatory and one inhibitory may be operating for the interaction of NMDA receptors with the opioid analgesic systems for modulating nociceptive responses.