Behavioral effects of chronic morphine treatment: From tolerance to executive functions and interactions with N-methyl-D-aspartate receptor antagonists

Chronic exposure to opioids (e.g., chronic pain patients under opioid treatment or opioid abusers) can lead to adaptations at levels of receptor (enhanced desensitization), cell (up-regulated cyclic adenosine 39,59-monophosphate or cAMP pathway), neuronal network (increased glutamatergic or dopaminergic activities and activation of astrocytes) and synapses (inhibited long-term potentiation or LTP). Some of these changes such as increased neurotransmissions are short-term which last hours to days; however some of them are long-term such as synaptic remodeling which can last days to years. These changes may underlie neuroadaptations at behavioral level including development of tolerance and changes in cognitive functions. Tolerance refers to loss of effectiveness at initial dose and requiring increase in dose to produce the same level of physiological response. Development of tolerance is a characteristic feature of all opioids. In contrast, effects of chronic opioid exposure on alterations of cognitive functions are controversial. This dissertation is comprised of two main projects concerning these two behavioral adaptations. The first project assessed interactions between N-methyl-D-aspartic acid receptor (NMDAR) antagonists and antinociceptive or discriminative stimulus effects of morphine after acute and chronic treatment. Published studies suggest that NMDAR antagonists prevent development of tolerance to effects of mu-opioid receptor (MOR) agonists via blocking hyperactivation of NMDARs induced by chronic exposure to MOR agonists, and their potencies to inhibit morphine tolerance correspond to their relative affinities at NMDARs. However side-effects associated with high affinity NMDAR antagonists such as dizocilpine hinder the potential to use NMDAR antagonists as adjuncts to opioids in chronic pain management. Memantine is a channel blocker that binds to dizocilpine binding sites. As a lower affinity NDMAR antagonist, memantine is well tolerated, and has positive cognitive effects. To help evaluate the potential use of memantine as an adjunct to opioids in chronic pain treatment, we compared effects of memantine on tolerance to antinociceptive and stimulus effects of morphine to those of the more frequently studied dizocilpine. Sprague-Dawley rats were trained to discriminate 3.2 mg/kg morphine and saline under fixed ratio 15 (FR15) schedules of food delivery. Potency and maximal stimulus or rate-altering effects of cumulative doses of morphine were examined 30 min after pretreatment with dizocilpine or memantine, or after chronic co-treatment of dizocilpine or memantine with 10 mg/kg morphine, b.i.d., for 7 to 14 days. Effects of dizocilpine or memantine on antinociceptive effects of morphine were examined in a 55°C water tail-withdrawal assay with drug treatments parallel to those in discrimination studies. Acutely, 0.032 mg/kg dizocilpine potentiated the discriminative stimulus effects of morphine by 0.61-fold, 0.10 mg/kg dizocilpine potentiated antinociceptive effects of morphine by 2-fold, and 10 mg/kg memantine attenuated both effects (6.6- and 1.4-fold). Neither chronic dizocilpine nor memantine blocked development of tolerance to stimulus effects of morphine. In contrast, co-treatment with 0.1 mg/kg dizocilpine blocked tolerance to antinociceptive effects of low (0.1~3.2 mg/kg) but not higher doses of morphine, whereas 7.5 and 10 mg/kg memantine did not block tolerance. Results from this project indicated that memantine differed from dizocilpine in modifying stimulus and antinociceptive effects of morphine and may not be a beneficial adjunct to MOR agonists in chronic pain management. The second project developed a novel set-shifting and perseveration test for rats and assessed how chronic morphine treatment affected rats’ performance in this test. In pain patients and opioid abusers, effects of chronic opioids administration on cognitive functions are controversial, possibly due to the multiple contributing factors in clinical studies. These factors include: varied past and current poly-drug exposures in opioid abusers, different pain situations and different drug regimens for pain management, pre-existing or current cognitive status, and use of diverse behavioral tasks. Animal models of cognitive functions provide valuable tools to clarify the effects of chronic opioids. Whereas many assays of working memory for rodents are available, there is only one widely used behavioral assay of executive functions (a set of cognitive skills responsible for complex goal-oriented behaviors), the attentional set-shifting task (ASST, Birrell & Brown 2000). This assay is conceptually similar to the Wisconsin Card Sorting Test (WCST) that measures dimensions of executive functions, such as attentional selection and inhibition, in humans. The ASST lacks a major characteristic of the WCST, however, in that it does not allow analysis of the perseverative errors (repeated responses on inappropriate stimulus) that are increased in some populations who show impaired performance in the WCST (e.g., Alzheimer’s patients, alcoholics and opioid abusers). Therefore, we modified the ASST to a three-choice set-shifting and perseveration test (SSPT). Whereas the WCST uses color, shape and number of objects as stimulus dimensions, the SSPT utilizes odor (e.g., thyme) and digging medium (e.g., plastic beads). Successive sets of three bowls that differ in both odor and digging medium were presented, and on each trial, only one stimulus (an odor or digging medium) was paired with food. Rats were initially trained to dig in bowls for food and then tested with the SSPT. They need learn to select the baited bowl by either odor or medium, and to switch between cues in a series of discriminations: compound, intradimensional shift (correct stimulus switched within the same dimension) and extradimensional shift (correct stimulus switched between two dimensions) discriminations. In addition to “trials to criterion” used in the ASST, we were able to distinguish perseverative errors from non-perseverative errors. To reduce between- and within-subjects variances, we tested rats’ preference for 21 odors, and identified three odors preferred equally by rats and an odor intensity that was recognizable but not aversive. We then used these three odors and the intensity to evaluate chronic effects of morphine on performance of rats (N= 12/group) in the SSPT. Previous experiences in the test seemed to improve rats’ performance in their second test one week later, though current results were not significant. Daily treatment of morphine (10 mg/kg, -4 h) did not affect learning/acquisition measured in training sessions of simple discriminations. Neither acute (10 mg/kg, -4 h) nor chronic (10 mg/kg, b.i.d, 7 or 14 days) morphine treatment had significant effects on performance including perseverative errors in the SSPT. Results of this project provided a useful rodent analogue to the WCST. In rats with well-controlled drug exposure history, we failed to demonstrate that chronic morphine affect their executive functions under current laboratory conditions; however, further research using different conditions would help enhance our understanding of the relationship between chronic opioid exposure and cognitive functions.