1. Agmatine, the proposed endogenous ligand for imidazoline receptors, has been shown to attenuate tolerance to morphine-induced antinociception (Kolesnikov el al., 1996). The main aim of this study was to assess if idazoxan, an alpha2-adrenoceptor antagonist that also interacts with imidazoline receptors, could also modulate opioid tolerance in rats and to establish which type of imidazoline receptors (or other receptors) are involved. 2. Antinociceptive responses to opioid drugs were determined by the tail-flick test. The acute administration of morphine (10 mg kg(-1), i.p., 30 min) or pentazocine (10 mg kg(-1), i.p., 30 min) resulted in marked increases in tail-flick latencies (TFLs). As expected, the initial antinociceptive response to the opiates was lost after chronic (13 days) treatment (tolerance). When idazoxan (10 mg kg(-1), i.p.) was given chronically 30 min before the opiates it completely prevented morphine tolerance and markedly attenuated tolerance to pentazocine (TFLs increased by 71-143% at day 13). Idazoxan alone did not modify TFLs. 3. The concurrent chronic administration (10 mg kg(-1), i.p., 13 days) of 2-BFI, LSL 60101, and LSL 61122 (valldemossine), selective and potent I2-imidazoline receptor ligands, and morphine (10 mg kg(-1), i.p.), also prevented or attenuated morphine tolerance (TFLs increased by 64 172% at day 13). This attenuation of morphine tolerance was still apparent six days after discontinuation of the chronic treatment with LSL 60101-morphine. The acute treatment with these drugs did not potentiate morphine-induced antinociception. These drugs alone did not modify TFLs. Together, these results indicated the specific involvement of I2-imidazoline receptors in the modulation of opioid tolerance. 4. The concurrent chronic (13 days) administration of RX821002 (10 mg kg(-1), i.p.) and RS-15385-197 (1 mg kg(-1), i.p.), selective alpha2-adrenoceptor antagonists, and morphine (10 mg kg(-1), i.p.), did not attenuate morphine tolerance. Similarly, the concurrent chronic treatment of moxonidine (1 mg kg(-1), i.p.), a mixed I(1)-imidazoline receptor and alpha2-adrenoceptor agonist, and morphine (10 mg kg(-1), i.p.), did not alter the development of tolerance to the opiate. These results discounted the involvement of alpha2-adrenoceptors and I(1)-imidazoline receptors in the modulatory effect of idazoxan on opioid tolerance. 5. Idazoxan and other imidazol(ine) drugs fully inhibited [3H]-(+)-MK-801 binding to N-methyl-D-aspartate (NMDA) receptors in the rat cerebral cortex with low potencies (Ki: 37-190 microM). The potencies of the imidazolines idazoxan, RX821002 and moxonidine were similar, indicating a lack of relationship between potency on NMDA receptors and ability to attenuate opioid tolerance. These results suggested that modulation of opioid tolerance by idazoxan is not related to NMDA receptors blockade. 6. Chronic treatment (13 days) with morphine (10 mg kg(-1), i.p.) was associated with a marked decrease (49%) in immunolabelled neurofilament proteins (NF-L) in the frontal cortex of morphine-tolerant rats, suggesting the induction of neuronal damage. Chronic treatment (13 days) with idazoxan (10 mg kg(-1)) and LSL 60101 (10 mg kg(-1)) did not modify the levels of NF-L proteins in brain. Interestingly, the concurrent chronic treatment (13 days) of idazoxan or LSL 60101 and morphine, completely reversed the morphine-induced decrease in NF-L immunoreactivity, suggesting a neuroprotective role for these drugs. 7. Together, the results indicate that chronic treatment with I2-imidazoline ligands attenuates the development of tolerance to opiate drugs and may induce neuroprotective effects on chronic opiate treatment. Moreover, these findings offer the I2-imidazoline ligands as promising therapeutic coadjuvants in the management of chronic pain with opiate drugs.