ATTENUATION OF MORPHINE-INDUCED ANTINOCICEPTION BY L-GLUTAMIC ACID ATTHE SPINAL SITE IN RATS

The present experimental study was planned to evaluate the effect of i ntrathecal administration of L-glutamic acid upon antinociception prod uced by intrathecal morphine in a prospective-controlled manner in con scious freely mobile Sprague-Dawley albino rats After chronic catheter ization of the spinal subarachnoid space, rats were randomly allocated into 12 treatment groups of ten each and the same number of rats serv ed as saline control for the comparison. L-glutamic acid (100 mmol), m orphine (1.2 mmol), ketamine (50 mmol) and saline (150 mmol) were inje cted intrathecally in 5 mu l volumes. Naloxone was injected in a dose of 1 mg . kg(-1) im. Immediately before and 15, 30 min, 1, 2 and 3 hr after injection, rats were subjected to a thermal noxious stimulus, us ing a tail-flick technoanalgesiometer and tail-flick latencies (TFL) w ere recorded. Intrathecal administration of L-glutamic acid attenuated the antinociceptive effect of intrathecal morphine with a decrease in TFL (1.4 +/- 0.3 sec; P < 0.0001) from 6.6 +/- 0.3 sec. Ketamine led to abolition of this effect (P < 0.01). In rats, pretreated with nalox one, there was restoration as well as augmentation of morphine-induced antinociception in the presence of L-glutamic acid with an increase i n TFL (9.0 +/- 0.4 sec; P < 0.0001). We conclude that there is modulat ion of opioid receptors by L-glutamic acid at the spinal site in rats.