Chianfang G. Cherng

Activation of amygdaloid PKC pathway is necessary for conditioned cues-provoked cocaine memory performance

Drug-associated cues are critical in reinstating the drug taking behavior even during prolonged abstinence and thus are thought to be a key factor to induce drug craving and to cause relapse. Amygdaloid complex has been known for its physiological function in mediating emotional experience storage and emotional cues-regulated memory retrieval. This study was undertaken to examine the role of basolateral nuclei of amygdala and the intracellular signaling molecule in drug cues-elicited cocaine memory retrieval. Systemic anisomycin treatment prior to the retrieval test abolished the cues-provoked cocaine conditioned place preference (CPP) memory. Likewise, a similar blockade of cues-provoked cocaine CPP performance was achieved by infusion of anisomycin and cycloheximide into the basolateral nuclei of amygdala before the test. Intra-amygdaloid infusion of H89, a protein kinase A inhibitor, or U0126, a MEK inhibitor, did not affect retrieval of the cues-elicited cocaine CPP memory. In contrast, intra-amygdaloid infusion of NPC 15437, a PKC inhibitor, abolished the cues-elicited cocaine CPP expression, while left the memory per se intact. Intra-amygdaloid infusion of NPC 15437 did not seem to affect locomotor activity or exert observable aversive effect. Taken together, our results suggest that activation of PKC signaling pathway and probably downstream de novo protein synthesis in the basolateral nuclei of amygdala is required for the cues-elicited cocaine memory performance. However, temporary inhibition of this signaling pathway does not seem to affect cocaine CPP memory per se.

Long-term compulsive exercise reduces the rewarding efficacy of 3,4-methylenedioxymethamphetamine

Although exercise has been known to regulate brain plasticity, its impact on psychostimulant reward and the associated mesolimbic dopamine system remained scarcely explored. A psychostimulant, 3,4-methylenedioxymethamphetamine (MDMA), is currently a worldwide abused drug of choice. We decided to examine the modulating effects of long-term, compulsive treadmill exercise on the hedonic value of MDMA in male C57BL/6J mice. MDMA-induced conditioned place preference (CPP) was used as a behavioral paradigm to indicate the reward efficacy of MDMA. We observed that sedentary control mice all demonstrated reliable MDMA-induced CPP with our conditioning protocol. Interestingly, pre-exposure to a treadmill exercise decreased the later MDMA-induced CPP in a running period-dependent manner. Specifically, mice undergoing a 12-week treadmill running exercise did not exhibit any approaching bias toward the MDMA-associated compartment in this CPP paradigm. Twelve weeks of treadmill running did not alter peripheral metabolism of MDMA 30min following single intraperitoneal injection of MDMA (3mg/kg). We further used microdialysis technique to study the underlying mechanisms for the impaired MDMA reward produced by the12-week exercise pre-exposure. We found that acute MDMA-stimulated dopamine release in nucleus accumbens was abolished in the exercised mice, whereas an obvious elevation of accumbal dopamine release was observed in sedentary control mice. Finally, the 12-week exercise program did not alter the protein levels of primary dopamine receptors, vesicular or membrane transporters in this area. We conclude that the long-term, compulsive exercise is effective in curbing the reward efficacy of MDMA possibly via its direct effect on reversing the MDMA-stimulated dopamine release in nucleus accumbens.

Systemic treatment with protein synthesis inhibitors attenuates the expression of cocaine memory

It has been proposed that a memory trace enters a labile phase each time it is retrieved. A reactivated memory relies on de novo protein synthesis to be faithfully reconsolidated and restored. Thus, in theory, a long-lasting and pathological memory associated with drug use may be disrupted by inhibiting its reconsolidation through use of protein synthesis inhibitors administered immediately following the memory retrieval. However, effective and efficient strategies to reactivate drug memory remained elusive. This study was undertaken to examine the effects of systemic cycloheximide and anisomycin treatment on the reconsolidation and maintenance of a reactivated cocaine-conditioned place preference (CPP) in mice using several strategies designed to reactivate the previously acquired memory. We found that anisomycin (50 mg/kg/injection) and cycloheximide (15 mg/kg/injection) administered immediately after the reactivation of cocaine-CPP ameliorated subsequent expression and maintenance of this memory. Likewise, when anisomycin and cycloheximide were administered immediately after additional cocaine and saline conditioning trials, the reactivated memory engendered by those extra training trials was also diminished. However, a similar anisomycin dosing regimen failed to affect subsequent expression of cocaine-CPP when additional cocaine conditioning trial was used in the absence of additional saline trial. Finally, cocaine and saline administration to mice in their home cages with or without anisomycin treatment had no effect on later cocaine-CPP expression. Taken together, these findings suggest that systemic treatment with protein synthesis inhibitors immediately after the reactivation of cocaine-CPP effectively diminished the reconsolidation and maintenance of such a cocaine memory. More importantly, reactivation of cocaine-CPP could be achieved by presentation of cocaine-conditioned cues as well as by administering additional cocaine and saline conditioning trials in a balanced fashion.

Memantine abolishes the formation of cocaine-induced conditioned place preference possibly via its IL-6-modulating effect in medial prefrontal cortex

In this study, we decided to use low doses of memantine pretreatment to examine the roles of the immune function in cocaine-supported conditioning. Cocaine-induced conditioned place preference (CPP) was used to assess the hedonic value and/or reinforcing efficacy of cocaine and cocaine-supported conditioning. Systemic pretreatment with memantine (20, 2.0, 0.2, and 0.02 mg/kg/injection) 30 min before each cocaine and saline conditioning trial abolished the acquisition of cocaine-induced CPP in mice. Even a total of 0.12 mg/kg memantine pretreatment in three days was effective in diminishing cocaine-induced CPP. Three consecutive days of cocaine conditioning increased interleukin-6 (IL-6) but decreased tumor necrosis factor (TNF-α) levels in medial prefrontal cortex (mPFC) and nucleus accumbens (Acb). Interestingly, pretreatment with memantine at the lowest effective dose (0.02 mg/kg/injection) reversed cocaine conditioning-enhanced IL-6 and -decreased TNF-α levels in these brain regions. Nevertheless, such a memantine dosing regimen did not affect dopamine metabolism in mPFC and Acb. Single memantine (0.02 mg/kg) injection did not acutely affect mouse locomotor activity or cocaine-increased locomotor activity. Similar memantine dosing regimen was ineffective to affect the maintenance of cocaine-induced CPP. Finally, intra-mPFC infusion of recombinant IL-6, but not thalidomide, reversed memantine (0.02 mg/kg/injection × 6)-decreased cocaine-induced CPP. These results, taken together, suggest that cocaine conditioning-enhanced IL-6 in mPFC may be, in part, involved in the acquisition of cocaine-induced CPP. Moreover, an extremely low dose of memantine may decrease the acquisition of cocaine-induced CPP by reversing cocaine conditioning-increased IL-6 levels in mPFC.

Presence of conspecifics and their odor‐impregnated objects reverse stress‐decreased neurogenesis in mouse dentate gyrus

J. Neurochem. (2010) 112, 1138–1146.

D-cycloserine, sarcosine and D-serine diminish the expression of cocaine-induced conditioned place preference.

Reactivation of cocaine-associated memories plays a critical role in reinstating the cocaine-seeking behavior and causing relapse. Cocaine-induced conditioned place preference (CPP) was used as a behavioral paradigm indicative of cocaine-associated memory and repeated cocaine-free preference tests served as a behavioral procedure to retrieve such a memory in this study. Since D-cycloserine was reported to eradicate drug-associated memories, two other N-methyl-D-aspartate (NMDA) receptor agonists were assessed for their efficacy on facilitating the extinction of cocaine-induced CPP. Although D-cycloserine (30 mg/kg) abolished cocaine (10 mg/kg)-induced CPP, sarcosine (300 and 600 mg/kg) and D-serine (600 mg/kg) diminished the expression of such a cocaine memory. Sarcosine (600 mg/kg) and D-serine (600 mg/kg) did not affect the storage of this cocaine memory. It was of interest to note that D-cycloserine facilitated the extinction of cocaine-induced CPP in a fast and early-onset manner, while sarcosine and D-serine decreased cocaine-induced CPP expression in a delay-onset manner. D-cycloserine (30 mg/kg), D-serine (600 mg/kg) and sarcosine (600 mg/kg) did not affect the consolidation of cocaine (5 mg/kg)-induced CPP. Finally, sarcosine (at 600 mg/kg/day for 3 consecutive days) and D-serine (at 600 mg/kg/day for 3 consecutive days) did not produce observable aversive effect associated with their administration in a conditioned place aversion paradigm. Likewise, a similar dosing regimen of sarcosine or D-serine did not cause evident activity-impairing effect. In addition to D-cycloserine treatment, our results indicate that long-term treatment with D-serine and sarcosine may afford a therapeutic advance in suppressing the expression of cocaine-associated memory.

Medial prefrontal cortex and nucleus accumbens core are involved in retrieval of the methamphetamine-associated memory

Immunohistochemical Fos staining has proven to be a method to identify the neurons that are activated by stimulation. Although methamphetamine (MA)-conditioned place preference (CPP) memory was long-lasting, how this memory was established and retrieved remained unknown. We used the vehicle- and MA-conditioned environment (including cues and context) to reactivate the MA-CPP memory in mice. In the limbic system, Fos-positive neurons were examined following retrieval of the MA-CPP memory. We demonstrated that the current conditioning procedure produced reliable MA-CPP performance. Moreover, enhanced Fos expressions were found in the medial prefrontal cortex and the core of the nucleus accumbens after reactivation of the MA-CPP memory. Furthermore, familiarity with the environmental cues/context was found to significantly enhance Fos expressions in dorsal striatum and dentate gyrus. Nucleus accumbens shell, basolateral or lateral amygdala, in this regard, did not seem to be involved in retrieval of the MA-CPP memory. These results, taken together, suggest that the medial prefrontal cortex and the core of the nucleus accumbens are anatomical substrates responsible for reactivation of the MA-CPP memory.

Repeated co-administrations of alcohol- and methamphetamine-produced anxiogenic effect could be associated with the neurotoxicity in the dentate gyrus

To date, joint use of alcohol (EtOH) and methamphetamine (MA) represents a specific combination of polydrug abuse. Repeated administrations of EtOH, MA, and combined EtOH and MA were assessed for their effects on brain cell toxicity, cell mitosis and anxiety/depression-like behavior. We found that repeated co-administrations of EtOH and MA produced profound anxiogenic effects. Specifically, combined EtOH and MA decreased open arm exploratory responses in the elevated plus maze test. Moreover, combined EtOH and MA significantly decreased immobile responses in the tail suspension test. MA, EtOH, and their combination all reduced the number of NeuN-positive cells in amygdala (Amg), while neither of them altered the number of NeuN-positive cells in striatum (St) or prefrontal cortex (PFC). Combined EtOH and MA decreased the number of NeuN-positive cells in dentate gyrus (DG). EtOH, MA, and combined EtOH and MA all diminished comparable number of GFAP-positive cells in Amg, DG, and St. Neither of these treatment affected the number of GFAP-positive cells in PFC. EtOH, MA, and combined EtOH and MA generated comparable inhibiting effects on cell proliferation in the subventricular zone (SVZ) and DG. These results, taken together, suggest that repeated co-administrations of MA and EtOH may produce an observable anxiogenic effect. This combination-produced anxiogenic effect could be associated with neuronal loss in the dentate gurus. In contrast, such an anxiogenic effect is less likely related to this combination-caused glial toxicity in limbic regions or cell proliferation-inhibiting effect in the SVZ or DG.

Disruption of memory reconsolidation impairs storage of other, non-reactivated memory

Two hypotheses were tested in this study. First, blockade of neural activity by lidocaine immediately following the retrieval of a memory may impair the reconsolidation and subsequent expression of that memory. Second, a non-retrieved memory would not be affected by this lidocaine treatment. Since the basolateral nucleus of the amygdala (BLA) is involved in emotion-related memory, an intra-BLA lidocaine infusion was used immediately after the retrieval of two emotion-related memories, the step-through passive avoidance response (PA) and cocaine-induced conditioned place preference (CPP). Intra-BLA lidocaine infusion immediately after cocaine-induced CPP retrieval diminished CPP magnitude in retests. However, intra-BLA lidocaine infusion alone did not affect cocaine-induced CPP performance. Intra-BLA lidocaine infusion immediately after PA retrieval decreased PA performance in retests. Omission of PA retrieval procedure, intra-BLA lidocaine infusion did not affect subsequent PA performance. Surprisingly, intra-BLA lidocaine infusion immediately following the retrieval of PA or cocaine-induced CPP diminished both PA and cocaine-induced CPP performance in the retests. Finally, Fos-staining results revealed that a number of BLA neurons were activated by the retrieval of both cocaine-induced CPP and PA. We conclude that inactivation of neural activity in BLA immediately following retrieval of a fear or cocaine-conditioned memory can impair subsequent expression of both memories. More importantly, retrieval of a memory does not seem to be an absolute condition for rapidly changing the memory.

Disruption of conditioned drug memories

We first made a brief review on historical development of drug craving theories. A special emphasis was given on the proposal that conditioned drug memories can be a critical psychopathological basis to elicit compulsive drug taking, craving and subsequent relapse. We then discussed the different processes associated with drug learning and memory as well as recent findings pertaining to these processes, specifically using the conditioned place preference (CPP) paradigm as a model for assessing these conditioned drug memories. Over this decade, many factors have been identified to affect the acquisition, consolidation, expression and reconsolidation of such a drug-induced CPP memory in rodents. Since reactivation of the established drug-induced CPP memory and modification of the reactivated memory are two core components for developing potential treatments, a few theoretical and practical considerations in the CPP paradigm are provided and discussed accordingly.