Subchronic lithium treatment increases the anxiolytic-like effect of mirtazapine on the expression of contextual conditioned fear

Sex difference affects fear extinction but not lithium efficacy in rats following fear-conditioning with respect to the hippocampal level of BDNF

In rodents, exposure to electrical shock and creating a strong fear memory using fear-conditioning model can induce PTSD-like behavior. In this study, we induced a fear-conditioning model in rats and investigated freezing (PTSD-like) behavior, 21 days after three shocks exposure (0.6 mA, 3 s, 30 seconds interval) in both male and female rats. Lithium was injected intraperitoneally (100 mg/kg) in three protocols: (1) 1 h after fear-conditioning (2) 1 h, 24 h, and 48 h after fear-conditioning (3), 1 h, 24 h, 48 h, 72 h, and 96 h after fear-conditioning. Extinction training (20 sounds without shocks, 75 dB, 3 s, 30 seconds interval) was performed in three protocols: (1) 1 h after fear-conditioning (one session), (2) 1 h, 24 h, and 48 h after fear-conditioning (three sessions), (3), 1 h, 24 h, 48 h, 72 h, and 96 h after fear-conditioning (five sessions). Forced swim test (FST) and hot plate were used to assess behavior. Results showed that lithium in all protocols had no effect on freezing behavior, FST, and pain subthreshold in all rats. Extinction training decreased freezing behavior, with more efficacy in females. In males, only 5-session training was effective, while in females all protocols were effective. Extinction training also altered pain perception and the results of FST, depending on the sessions and was different in males and females. Brain-derived neurotrophic factor (BDNF) mRNA level was increased in females following 3 and 5 sessions, and in males following 5 sessions extinction training. In conclusion, we suggested that there is a sex difference for the effect of extinction training on freezing behavior and BDNF mRNA level in a rat model of fear-conditioning.

Mirtazapine attenuates anxiety- and depression-like behaviors in rats during cocaine withdrawal

BACKGROUND

Anxiety and depression, key symptoms of the cocaine withdrawal syndrome in human addicts, are considered the main factors that precipitate relapse in chronic cocaine addiction. Preclinical studies have found that rodents exposed to different withdrawal periods show an increase in anxiety and depressive-like behavior. Mirtazapine - a tetracyclic medication - is used primarily to treat depression and, sometimes, anxiety. It has also successfully improved withdrawal symptoms in drug-dependent patients.

AIM

This study sought to determine whether chronic dosing of mirtazapine during cocaine withdrawal reduced depression- and anxiety-like behaviors that characterize cocaine withdrawal in animals.

METHODS

Cocaine pre-treated Wistar rats were subjected to a 60-day cocaine withdrawal period during which depression- and anxiety-like behaviors were evaluated in open field tests (OFT), the elevated plus-maze (EPM), the light-dark box test (LDT), the forced swimming test (FST) and spontaneous locomotor activity (SLA).

RESULTS

We found that chronic dosing with different doses of mirtazapine (30 and 60 mg/kg) decreased depression- and anxiety-like behaviors induced by different doses of cocaine (10, 20 and 40 mg/kg) during the 60-day cocaine withdrawal.

INTERPRETATION

Our results suggest that the pharmacological effect of mirtazapine on its target sites of action (α₂-adrenergic and 5-HT_2A and 5-HT₃ receptors) within the brain may improve depression- and anxiety-like behaviors for long periods.

CONCLUSION

Therefore, the findings support the use of mirtazapine as a potentially effective therapy to reduce anxiety and depressive-like behavior during cocaine withdrawal.

Mirtazapine exerts an anxiolytic-like effect through activation of the median raphe nucleus-dorsal hippocampal 5-HT pathway in contextual fear conditioning in rats

The functional role of serotonergic projections from the median raphe nucleus (MRN) to the dorsal hippocampus (DH) in anxiety remains understood poorly. The purpose of the present research was to examine the functional role of this pathway, using the contextual fear conditioning (CFC) model of anxiety. We show that intra-MRN microinjection of mirtazapine, a noradrenergic and specific serotonergic antidepressant, reduced freezing in CFC without affecting general motor activity dose-dependently, suggesting an anxiolytic-like effect. In addition, intra-MRN microinjection of mirtazapine dose-dependently increased extracellular concentrations of serotonin (5-HT) but not dopamine in the DH. Importantly, intra-DH pre-microinjection of WAY-100635, a 5-HT1A antagonist, significantly attenuated the effect of mirtazapine on freezing. These results, for the first time, suggest that activation of the MRN-DH 5-HT1A pathway exerts an anxiolytic-like effect in CFC. This is consistent with the literature that the hippocampus is essential for retrieval of contextual memory and that 5-HT1A receptor activation in the hippocampus primarily exerts an inhibitory effect on the neuronal activity.

Combined treatment with subchronic lithium and acute intracerebral mirtazapine microinjection into the median raphe nucleus exerted an anxiolytic-like effect synergistically

Although preclinical and clinical studies have established the efficacy of lithium augmentation of antidepressant drugs, the mechanism of action of lithium augmentation is not fully understood. Our previous study reported that subchronic lithium treatment enhanced the anxiolytic-like effect of systemic mirtazapine. In the present study, we examined the effect of subchronic lithium in combination with acute local intracerebral injection of mirtazapine on fear-related behaviors in a contextual fear conditioning test in rats to clarify the target brain region of lithium augmentation of mirtazapine. After conditioning by footshock, diet (food pellets) containing Li2CO3 at a concentration of 0.2% was administered for 7 days. Ten min before testing and 7 days after conditioning, mirtazapine (3μg/site) in a volume of 0.5µl was acutely injected into the median raphe nucleus (MRN), hippocampus or amygdala. The combination of subchronic lithium and acute mirtazapine microinjection into the MRN but not the hippocampus or the amygdala reduced fear expression synergistically. These results suggest that intra-MRN mirtazapine treatment with subchronic lithium exerts the anxiolytic-like effect through the facilitation of the MRN-5HT pathway.