These experiments revealed that the long-term maintenance of mnemonic associations between the rewarding effects of methamphetamine and the methamphetamine-paired context can be disrupted by 10 once-daily injections of 5mg/kg mirtazapine. This disruption does not occur when mirtazapine is administered only on the last day of the 10 day period (day 17). Neither a single nor 10 once-daily injections of mirtazapine altered motor activity during the CPP test on day 20 (three days after the last mirtazapine injection); thus, the place preference results do not reflect a change in the capacity of rats to successfully execute the task. The findings suggest that processes that participate in memory maintenance and/or expression are vulnerable to repeated mirtazapine administration. Focus was on the behavioral consequences of mirtazapine measured three days after the last mirtazapine injection. Additional studies are needed to ascertain if the mirtazapine treatments can induce longer acting (or shorter acting) effects. The three-day period was imposed to allow for mirtazapine to be eliminated and we did not observe any long-lasting effects on spontaneous motor activity during the CPP test. Thus, the persistent consequences of repeated mirtazapine administration on memory maintenance and/or expression are likely not due to mirtazapine-induced motor consequences observed during the CPP test.
Maintenance of previously acquired CPP is not a static process. Rather, maintenance of behaviors induced by repeated methamphetamine likely reflects continual modifications at molecular, cellular, and circuit levels that differ depending on the time (hours, days, and weeks) after the last exposure [1
]. Indeed, successful conversion of experiences into short-term memories, short-term memories into long-term memories, and then to successfully express those memories requires activation of an array of receptors and protein kinases in a time-dependent manner [34
]. The temporal nature of these events presents opportunities that can be exploited to disrupt the mechanisms critical for the maintenance of methamphetamine-induced CPP. For example, consolidation of newly acquired memories occurs over several days [34
]. We have observed that with a two-day methamphetamine-conditioning protocol (day 1 methamphetamine-pairing & day 2 saline-pairing), a single mirtazapine injection 24h after the last conditioning session inhibits CPP tested on the following day [15
]. Given the close temporal nature of the conditioning and mirtazapine treatment, this protocol does not allow separation of mirtazapine effects on consolidation versus maintenance of the methamphetamine-context memory. The current study was designed to evaluate the effects of mirtazapine after the methamphetamine-induced CPP was established. Thus, the 10 day mirtazapine treatment was initiated three days after conditioning which allowed for examination of maintenance independent of consolidation.
This study revealed that 10 injections of mirtazapine were sufficient to inhibit the maintenance of previous established methamphetamine-induced CPP, whereas a single injection (corresponding to the last day of the repeated treatment) was not. One possible interpretation of these results is that adaptations that occur only after repeated mirtazapine treatment are required for mirtazapine inhibition of methamphetamine-induced CPP (i.e., duration-dependent effects). Similar to the current study, we previously demonstrated that 15 injections of mirtazapine inhibit the maintenance of motor sensitization induced by five once-daily treatments of 2.5mg/kg methamphetamine [3
]. Moreover, acute vs
. repeated mirtazapine administration have differential effects on several behavioral assessments in rats [41
]. Brain adaptations to repeated administration of mirtazapine (7 to 28 days) include transcription of genes involved in receptor expression/function, signal transduction, and neuronal structure [17
] that may impact neuronal function. Thus, behavioral consequences of repeated mirtazapine treatment measured in the current study may be due to long-term adaptations that occur as a consequence of repeated mirtazapine treatment. It also is possible that the 10-day mirtazapine administration encompassed a critical window of vulnerability which was necessary to disrupt the maintenance of methamphetamine-induced CPP (i.e., time-dependent effect). Although the last day of the 10-day mirtazapine treatment did not prove to be the critical treatment time, this does not preclude the possibility that a different time frame encompassed by the 10-day treatment is particularly sensitive to the effects of mirtazapine. For example, our lab has previously demonstrated that a single injection of mirtazapine administered one day after conditioning and tested for preference one day later was sufficient to disrupt methamphetamine-induced CPP established with a single methamphetamine-pairing conditioning protocol [15
]. Thus, it may be that the period of time soon after conditioning is critical especially in light of the consolidation processes which are known to occur at this time [34
]. Further studies are necessary to determine the contribution of these two possibilities in the current outcomes. These outcomes are corroborated by those where repeated mirtazapine was able to mitigate behavioral sensitization to chronic methamphetamine [3
]. Thus, the collective findings underscore that the effectiveness of repeated mirtazapine is independent of the methamphetamine protocol employed (i.e., CPP or behavioral sensitization), a feature that is required for clinical efficacy.
The pharmacology that underlies the effects of mirtazapine is complex. It acts as an antagonist with high affinity at histamine H1
adrenergic, and serotonin 5-HT2A/2C
]. An indirect consequence of mirtazapine administration is activation of the 5-HT1A
], which appears to become more robust with chronic treatments [18
]. These receptors are known to influence both the behavioral effects of psychostimulants and mnemonic processes as reviewed below. While the effects of selective antagonists for receptor targets of mirtazapine on the expression
of psychostimulant-induced behaviors have received scientific attention, the studies have yielded conflicting results. Both increases [47
] and decreases [48
] in psychostimulant-induced motor activity, motor sensitization, and self-administration are observed. These divergent reports make it difficult to attribute the expression results of the current study to a particular receptor target of mirtazapine. While no studies have examined the effects of specific receptors targets of mirtazapine in the maintenance of psychostimulant-induced behaviors, a role of the 5-HT1A
receptor (an indirect effect of mirtazapine) in the maintenance
of mnemonic processes has been explored. Administration of a 5-HT1A
receptor agonist after
training (as mirtazapine was given in the current study) inhibits the maintenance of spatial and fear-conditioned memories [60
]. Thus, in the current study mirtazapine may have inhibited the maintenance of methamphetamine-induced associative learning via
a number of different receptor systems and further studies with repeated administration of selective ligands are needed to determine the receptor(s) responsible for the observed behavioral inhibition by mirtazapine in the current study.
Dopamine/serotonin releasers may reduce relapse and alleviate negative aspects of withdrawal [14
]. It follows that mirtazapine administered to human addicts results in positive clinical outcomes with a reduction in withdrawal severity from amphetamine [65
] and methamphetamine [66
] and promotes abstinence in both opiate and stimulant abusers [67
]. While not all studies demonstrate favorable clinical outcomes [68
], there is ample data to warrant further investigation of mirtazapine as a treatment for methamphetamine abuse and cue-elicited relapse prevention.