In the present study, we investigated the effects of a nitric oxide (NO) precursor, L-arginine, on the effect of different drugs, [trans-3, 4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl]-benzeneacetamid e hydrochloride] (U-50,488, a kappa-opioid receptor agonist); dPTyr(Me)AVP (a vasopressin receptor antagonist); dizocilpine (MK-801, a N-methyl-D-aspartate (NMDA) receptor antagonist), to block the development of morphine tolerance or NO release in Sprague-Dawley rat hippocampal slices (450 microm). Slices were continuously superfused with artificial cerebrospinal fluid (ACSF) or drugs at 1 ml/min. Nichrome wire electrodes were placed in the Schaffer-collateral pathway and used to deliver biphasic 0.2-ms pulses of 5-30 V (0.033 Hz). A glass microelectrode was placed in the CA1 area to record population spikes. The amount of NO released in the superfusate was measured as nitrite formation. When the slices were superfused with 10 microM morphine, the amplitude of population spikes increased 200%-300% in 30-40 min. However, this effect of morphine decreased, i.e., tolerance developed, after continuous superfusion of morphine for 2-6 h. On the other hand, the nitrite level was increased about 250% of the control level through 6 h of morphine superfusion. Co-superfusion of L-arginine with morphine could further increase the nitrite level and also facilitate the development of morphine tolerance. On the other hand, 3-Br-7-nitroindazole (a neuronal NO synthase inhibitor) decreased the nitrite level significantly and blocked the development of morphine tolerance. When either U-50,488 (200 nM) or dPTyr(Me)AVP (500 pM) or MK-801 (500 pM) was co-superfused with morphine (10 microM), the development of morphine tolerance was blocked significantly and the nitrite level decreased to 100%-150% of the control level. L-arginine (500 nM) significantly reversed the effect of these drugs to block the development of morphine tolerance or to decrease the nitrite level through 6 h of superfusion. These data suggest that NO may play a key role in the development of morphine tolerance. Drugs which suppress the synthesis or release of NO would be expected to block the development of morphine tolerance.