ADMINISTRATION OF AP5, A GLUTAMATE ANTAGONIST, UNMASKS GLYCINE ANALGESIC ACTIONS IN THE RAT

The effect of intrathecal (IT) injection of glycine alone or in combination with 2-amino-5-phosphonopentanoate (AP5) on two nociceptive tests-the vocalization threshold to tail-shock (VTTS) and the tail-flick latency (TFL)-was studied in ovariectomized Sprague-Dawley rats. IT injection of 400-mu-g glycine induced a nonsignificant decrease, that is, in comparison with saline, in both nociceptive thresholds. IT AP5 (10-mu-g) provoked a slight but significant increase in both nociceptive thresholds within the first 15 min postinjection. Combination of both glycine (400-mu-g) and AP5 (10-mu-g) produced marked and prolonged analgesia in both tests, which was significantly different from that obtained with AP5 alone. The results suggest that IT glycine acting through the strychnine-sensitive Gly1 receptor produces analgesia provided its effect on the Gly2 receptor linked to the NMDA receptor is prevented by an antagonist.