Associative and non-associative mechanisms of morphine analgesic tolerance are neurochemically distinct in the rat spinal cord

Opiate tolerance involves both associative and non-associative changes. However, procedures designed to distinguish between these two processes have rarely been employed when investigating the physiological basis of such plasticity. The present experiments assessed some of the mechanisms contributing to both associative and non-associative decreases in morphine analgesic potency following repeated IV morphine administration (4 days, 5 mg/kg per day). For one group of rats, testing for morphine analgesia (tailflick) occurred in a context that had been repeatedly paired with morphine administration. Another group of rats, exposed equally to the testing context, handling procedures and morphine, was tested for morphine analgesia in a context that was specifically unpaired with prior drug. Although both of these groups showed a decrease in the drug effect following repeated administrations, those rats tested in the morphine-associated context were significantly more tolerant than the unpaired group. We evaluated the spinal cord involvement of NMDA receptors, as well as the peptide neurotensin in these two types of tolerance. NMDA receptors appeared to mediate non-associative changes in drug potency, as rats tested in either context were less tolerant when morphine administration was preceded with the non-competitive NMDA antagonist, MK-801 (2.5 and 5 nmol). Spinal neurotensin antagonism with [D-Trp(11)]neurotensin (3 pmol) selectively abolished associative tolerance. These findings provide information about the mechanisms of opiate tolerance and support the distinction between associative and non-associative processes underlying these changes.