Fluoxetine acts concomitantly on dorsal and ventral hippocampus to Trk-dependently modulate the extinction of fear memory

LF-DBS of the ventral striatum shortens persistence for morphine place preference and modulates BDNF expression in the hippocampus

BACKGROUND

Deep brain stimulation (DBS) of the ventral capsule/ventral striatum (VC/VS) represents a promising therapy for treatment-refractory patients with substance-use disorders. We previously found that low-frequency (LF) DBS aimed to the VC/VS during extinction training strengthens the extinction memory for morphine seeking under a partial extinction protocol.

OBJECTIVES/HYPOTHESIS

In this study, animals were tested in a full extinction protocol to determine whether LF-DBS applied during extinction facilitates extinction while preventing drug reinstatement, and study the molecular mechanisms underlying the effects of LF-DBS, METHODS/RESULTS: We used a full extinction CPP paradigm combined with LF-DBS to assess behavior. Western blots for the pro-extinction molecule, brain-derived neurotrophic factor (BDNF) were then performed in corticomesolimbic regions of the brain. Lastly, to determine whether changes in BDNF expression elicited by LF-DBS were specific to the VS/NAc afferents from the hippocampus, amygdala, and medial prefrontal cortex, we performed BDNF-like immunohistochemistry, combined with the retrograde tracer cholera toxin B (CtB).

RESULTS

We showed a significant reduction in the number of days required to fully extinguish morphine CPP in animals exposed to LF-DBS during extinction training accompanied by a significant increase in BDNF expression in the hippocampus. However, LF-DBS applied during extinction did not prevent drug reinstatement. Lastly, no changes in BDNF/CtB double-labeled cells were found in VS/NAc projecting cells after one-day exposure to LF-DBS.

CONCLUSION(S)

These data suggest that LF-DBS can facilitate extinction of morphine CPP by decreasing drug seeking through potential synaptic plasticity changes in the hippocampus to strengthen extinction memories.

GADD45B in the ventral hippocampal CA1 modulates aversive memory acquisition and spatial cognition

AIMS

This study was designed to investigate the role of growth arrest and DNA damage-inducible β (GADD45B) in modulating fear memory acquisition and elucidate its underlying mechanisms.

MAIN METHODS

Adeno-associated virus (AAV) that knockdown or overexpression GADD45B were injected into ventral hippocampal CA1 (vCA1) by stereotactic, and verified by fluorescence and Western blot. The contextual fear conditioning paradigm was employed to examine the involvement of GADD45B in modulating aversive memory acquisition. The Y-maze and novel location recognition (NLR) tests were used to examine non-aversive cognition. The synaptic plasticity and electrophysiological properties of neurons were measured by slice patch clamp.

KEY FINDINGS

Knockdown of GADD45B in the vCA1 significantly enhanced fear memory acquisition, accompanied by an upregulation of long-term potentiation (LTP) expression and intrinsic excitability of vCA1 pyramidal neurons (PNs). Conversely, overexpression of GADD45B produced the opposite effects. Notably, silencing the activity of vCA1 neurons abolished the impact of GADD45B knockdown on fear memory development. Moreover, mice with vCA1 GADD45B overexpression exhibited impaired spatial cognition, whereas mice with GADD45B knockdown did not display such impairment.

SIGNIFICANCE

These results provided compelling evidence for the crucial involvement of GADD45B in the formation of aversive memory and spatial cognition.

Reproductive experience alters the effects of diazepam and fluoxetine on anxiety-like behaviour, fear extinction, and corticosterone levels in female rats

OVERVIEW

Reproductive experience (pregnancy and motherhood) leads to long-term changes in the neurobiological and hormonal features of anxiety in rats and humans. The aim of this study was to examine whether reproductive experience alters the effects of two pharmacological treatments for anxiety, a benzodiazepine (diazepam) and a selective serotonin reuptake inhibitor (fluoxetine), on animal models of anxiety.

METHODS

In Experiment 1, virgin (n = 47) and age-matched mother (n = 50) rats at 1-month post-weaning were injected with diazepam (1.3 mg/kg or 1.7 mg/kg, i.p.) or vehicle, in the proestrus (high estradiol/progesterone/allopregnanolone) or metestrus (low estradiol/progesterone/allopregnanolone) phase of the estrous cycle 30 min prior to the elevated plus maze (EPM). In Experiment 2, virgin (n = 25) and mother rats (n = 20) were administered fluoxetine (10 mg/kg) or vehicle for 2 weeks prior to being tested on a Pavlovian fear conditioning and extinction protocol, and the EPM.

RESULTS

Replicating past research, in virgin rats, the low dose of diazepam produced anxiolytic-like effects in proestrus, but only the high dose was anxiolytic-like in metestrus. In contrast, in mother rats, both doses of diazepam were anxiolytic-like irrespective of estrous phase. Fluoxetine produced anxiogenic-like effects in virgin rats during fear extinction and the EPM, but had no behavioural effects in mothers. In contrast, fluoxetine increased plasma corticosterone levels measured 30-min post-EPM in mothers, but not virgin rats.

CONCLUSIONS

Reproductive experience alters the dose responsivity and efficacy of common anti-anxiety medications in female rats. These findings highlight the importance of considering reproductive status in studies on anxiety and its treatment.

Diazepam promotes active avoidance extinction associating with increased dorsal CA3 and amygdala activity

Active avoidance (AA) is an adaptive response to potentially harmful situations while maladapted avoidance that does not extinguish is one of the core symptoms of anxiety and post-traumatic stress disorder. However, the neural mechanisms of AA extinction and its relationship to anxiety remain unclear. We examined AA extinction during three extinction training sessions in two-way active avoidance paradigm and tested the effect of anxiolytic on AA extinction. Then we performed a meta-analysis of rodent studies, identified anxiolytic diazepam facilitates AA acquisition, and tested the same treatment in AA extinction. Diazepam-treated rats significantly reduced avoidance in the first two extinction training, compared with the saline-treated rats, and the reduction in avoidance remained in the third drug-free session. Then we explored extinction associated hippocampal and amygdala activity in saline-and diazepam-treated rats using c-Fos immunostaining following the last extinction session. The density of c-Fos positive cells was higher in dorsal CA3 of the diazepam group than in that of saline-treated animals, and was also higher in the central and basolateral amygdala regions of diazepam-treated rats than in that of saline-treated animals. Combined, these results suggest anxiolytics promotes AA extinction associated with dorsal CA3 and amygdala activity changes.

The immediate effect of overnutrition and fluoxetine treatment during the critical period of development on the hippocampus

It is well known that overnutrition, overweight, and obesity in children can modulate brain mechanisms of plasticity, monoaminergic systems, and mitochondrial function. The immediate effect of overnutrition during the developmental period has not been thoroughly examined in rats until the present. This study sought to evaluate the impact on adult rats of early life overfeeding and fluoxetine treatment from post-natal day 1 (PND1) to post-natal day 21 (PND21) relative to mitochondrial function, oxidative balance, and expression of specific monoaminergic genes in the hippocampus. The following were evaluated: mitochondrial function markers, oxidative stress biomarkers, dopamine-and serotonin-related genes, and BDNF mRNA levels. Overfeeding during the lactation period deregulates cellular metabolism and the monoaminergic systems in the hippocampus. Strikingly, serotonin modulation by fluoxetine treatment protected against some of the effects of early overnutrition. We conclude that overfeeding during brain development induce detrimental effects in mitochondria and in the genes that regulate homeostatic status that can be the molecular mechanisms related to neurological diseases.

Brain-derived neurotrophic factor expression in serotonergic neurons improves stress resilience and promotes adult hippocampal neurogenesis

The neurotrophin brain-derived neurotrophic factor (BDNF) stimulates adult neurogenesis, but also influences structural plasticity and function of serotonergic neurons. Both, BDNF/TrkB signaling and the serotonergic system modulate behavioral responses to stress and can lead to pathological states when dysregulated. The two systems have been shown to mediate the therapeutic effect of antidepressant drugs and to regulate hippocampal neurogenesis. To elucidate the interplay of both systems at cellular and behavioral levels, we generated a transgenic mouse line that overexpresses BDNF in serotonergic neurons in an inducible manner. Besides displaying enhanced hippocampus-dependent contextual learning, transgenic mice were less affected by chronic social defeat stress (CSDS) compared to wild-type animals. In parallel, we observed enhanced serotonergic axonal sprouting in the dentate gyrus and increased neural stem/progenitor cell proliferation, which was uniformly distributed along the dorsoventral axis of the hippocampus. In the forced swim test, BDNF-overexpressing mice behaved similarly as wild-type mice treated with the antidepressant fluoxetine. Our data suggest that BDNF released from serotonergic projections exerts this effect partly by enhancing adult neurogenesis. Furthermore, independently of the genotype, enhanced neurogenesis positively correlated with the social interaction time after the CSDS, a measure for stress resilience.

Neurogenic Interventions for Fear Memory via Modulation of the Hippocampal Function and Neural Circuits

Fear memory helps animals and humans avoid harm from certain stimuli and coordinate adaptive behavior. However, excessive consolidation of fear memory, caused by the dysfunction of cellular mechanisms and neural circuits in the brain, is responsible for post-traumatic stress disorder and anxiety-related disorders. Dysregulation of specific brain regions and neural circuits, particularly the hippocampus, amygdala, and medial prefrontal cortex, have been demonstrated in patients with these disorders. These regions are involved in learning, memory, consolidation, and extinction. These are also the brain regions where new neurons are generated and are crucial for memory formation and integration. Therefore, these three brain regions and neural circuits have contributed greatly to studies on neural plasticity and structural remodeling in patients with psychiatric disorders. In this review, we provide an understanding of fear memory and its underlying cellular mechanisms and describe how neural circuits are involved in fear memory. Additionally, we discuss therapeutic interventions for these disorders based on their proneurogenic efficacy and the neural circuits involved in fear memory.

Modulating role of serotonergic signaling in sleep and memory

Serotonin is an important neurotransmitter with various receptors and wide-range effects on physiological processes and cognitive functions including sleep, learning, and memory. In this review study, we aimed to discuss the role of serotonergic receptors in modulating sleep-wake cycle, and learning and memory function. Furthermore, we mentioned to sleep deprivation, its effects on memory function, and the potential interaction with serotonin. Although there are thousands of research articles focusing on the relationship between sleep and serotonin; however, the pattern of serotonergic function in sleep deprivation is inconsistent and it seems that serotonin has not a certain role in the effects of sleep deprivation on memory function. Also, we found that the injection type of serotonergic agents (systemic or local), the doses of these drugs (dose-dependent effects), and up- or down-regulation of serotonergic receptors during training with various memory tasks are important issues that can be involved in the effects of serotonergic signaling on sleep-wake cycle, memory function, and sleep deprivation-induced memory impairments. This comprehensive review was conducted in the PubMed, Scopus, and ScienceDirect databases in June and July 2021, by searching keywords sleep, sleep deprivation, memory, and serotonin.