Chronic Fluoxetine Impairs the Effects of 5-HT1A and 5-HT2C Receptors Activation in the PAG and Amygdala on Antinociception Induced by Aversive Situation in Mice

The Influence of Fluoxetine on the Sensorimotor Function of the Sciatic Nerve in Wistar Rats after Axonotmesis: An Experimental Animal Model

BACKGROUND

Fluoxetine, a serotonin reuptake inhibitor antidepressant, raises extracellular serotonin levels and promotes angiogenesis and neurogenesis. Numerous animal models have shown its beneficial effects on recovery from peripheral nerve injury.

PURPOSE

The primary objective of this study was to analyze the influence of fluoxetine on the sensory-motor function recovery of the sciatic nerve in Wistar rats after axonotmesis.

STUDY DESIGN, SETTING, AND SAMPLE

This study utilized an experimental rat model, conducted in the laboratory at the Federal University of Pernambuco. The sample consisted of 40 male Wistar rats.

PREDICTOR VARIABLE

The primary predictor variable was the fluoxetine exposure. The animals were randomly divided into 4 groups (control and 3 experimental groups), with 10 animals in each group. They were injected subcutaneously with saline or fluoxetine 5, 10, and 20 mg/kg/day, respectively.

MAIN OUTCOME VARIABLES

The main outcome variables were postoperative motor and sensory sciatic nerve function. Sensory nerve function was measured using the withdrawal reflex by thermostimulation. Motor nerve function was measured using the Sciatic Nerve Recovery Index and the Static Sciatic Nerve Index (SSI).

COVARIATES

None.

ANALYSES

Descriptive statistical analysis was performed using mean and SD. Analaysis of variance (ANOVA) was used for comparisons between the groups and Dunnett's multiple comparisons test was used in case of significant differences between the groups. Statistical Analysis System was the software used for statistical analyses.

RESULTS

During the study, 15 animals were lost (3 in the control group and 4 in each experimental group), with no specific cause identified. On day 35, the latency time of the withdrawal reflex was significantly different, with decreased pain perception in the 5 mg/kg/day fluoxetine group (3.80 ± 1.20, P < .05). On day 14, the Sciatic Nerve Recovery Index showed greater deficits in the l0 and 20 mg/kg/day groups (-65.67 ± 7.20 and -63.57 ± 11.59, respectively) compared to the control group (P < .05). The SSI also showed a delay in recovery with the 10 mg/kg/day dose (-62.50 ± 6.72, P < .05).

CONCLUSION

The daily treatment with fluoxetine failed to bring any improvement to motor or sensory recuperation after injury to the sciatic nerve in Wistar rats.

Zingerone alleviates inflammatory pain by reducing the intrinsic excitability of anterior cingulate cortex neurons in a mice model

Background

Zingiber officinale Roscoe has been shown to possess analgesic properties. Zingerone (ZO), a bioactive compound derived from Zingiber officinale Roscoe, exhibits a range of pharmacological effects, including anti-inflammatory, anti-cancer, antioxidant, antibacterial, and anti-apoptotic activities. However, the analgesic properties of zingerone remain unclear.

Methods

Complete Freund's adjuvant (CFA) was administered to the left hind paw of C57BL/6 mice to induce a model of inflammatory pain. The analgesic effects of zingerone were assessed using the Von Frey and Hargreaves tests. In vivo fiber photometry and whole-cell patch clamp techniques were employed to investigate the potential mechanisms.

Results

Both acute and long-term treatment with zingerone resulted in a significant increase in mechanical and thermal pain thresholds in mice experiencing CFA-induced inflammatory pain. Mechanical stimulation led to a pronounced increase in calcium levels within the anterior cingulate cortex (ACC) neurons of the inflammatory pain model, which was alleviated by zingerone administration. Furthermore, zingerone was found to modify synaptic transmission to ACC neurons and decrease their intrinsic excitability by prolonging the refractory period of these neurons.

Conclusion

Zingerone demonstrates potential for alleviating CFA-induced inflammatory pain by reducing the intrinsic excitability of ACC neurons in a mouse model.

The unique role of fluoxetine in alleviating depression and anxiety by regulating gut microbiota and the expression of vagus nerve-mediated serotonin and melanocortin-4 receptors

Fluoxetine is a selective serotonin reuptake inhibitor (SSRI) widely used for depression, but its potential effects on gut microbiota regulation and vagus nerve-mediated serotonin receptor expression have not been well studied. We investigated changes in the gut microbiome regulated by fluoxetine and vagus nerve-mediated expression of several serotonin (5-HT) receptor types associated with anxiety and depression. Oral administration of fluoxetine alleviated lipopolysaccharide (LPS)-induced depressive and anxiety behaviors, increased 5-HT1A, 2 C, and melanocortin 4 (MC4) receptor expression, and the composition of Lactobacillus in mice's gut microbiome. In contrast, in the vagotomized group, fluoxetine did not modulate behaviors and receptor expression. Increased Lactobacillus composition was found to correlate significantly with behavioral test results. The importance of Lactobacillus growth to the efficacy of fluoxetine was confirmed by the effectiveness of fluoxetine, which was reduced by co-administering antibiotics. To determine the additional impact of the gut microbiome, we isolated Limosilactobacillus reuteri and Ligilactobacillus murinus, which were increased in the fluoxetine-treated group and administrated. The results showed that administration of each strain improved anxious or depressive behavior, as did fluoxetine, and vagotomy eliminated these effects. These results suggest that fluoxetine administration increases the proportion of Lactobacillus in the gut, which modulates 5-HT1A, 2 C, and MC4 receptor expression through the enteric nervous system and improves depression.

Neuroanatomical and neurochemical substrates mediating fear-induced antinociception: A systematic review of rodent preclinical studies

Fear-induced antinociception (FIA), an instinctive defensive response producing pain suppression in stressful and/or dangerous situations, has been the subject of extensive research to elucidate the mechanisms involved in triggering and controlling pain during emotional disorders. In this systematic review, we synthesized pre-clinical studies that demonstrated the neural hodology and the neurochemical bases of FIA in laboratory animals. The literature search in PubMed, Web of Science, Science Direct, and Scopus, from inception up to July 2022, retrieved 797 articles from which 50 studies were included in this review. This review highlights key encephalic regions implicated in the modulation of FIA, such as the prefrontal cortex, the amygdaloid complex, the hippocampus, the hypothalamus, the corpora quadrigemina, the periaqueductal gray matter, and some reticular formation nuclei. FIA-related neural pathways, neurotransmitters and neuromodulators such as glutamatergic, serotonergic, norepinephrine, GABAergic, nitrergic, opioidergic and endocannabinoid connections across these encephalic regions were also addressed. Understanding these neural circuits and molecular neural mediation sheds light on the complex interplay between fear, anxiety, and pain modulation, offering potential avenues for therapeutic interventions targeting pain management in the context of heightened emotional states.

ShuYu capsule alleviates emotional and physical symptoms of premenstrual dysphoric disorder: Impact on ALLO decline and GABAA receptor δ subunit in the PAG area

Premenstrual dysphoric disorder (PMDD) is a severe subtype of premenstrual syndrome in women of reproductive age, with its pathogenesis linked to the heightened sensitivity of type A γ -aminobutyric acid receptors (GABAAR) to neuroactive steroid hormone changes, particularly allopregnanolone (ALLO). While a low dose of fluoxetine, a classic selective serotonin reuptake inhibitor, is commonly used as a first-line drug to alleviate emotional disorders in PMDD in clinical settings, its mechanism of action is related to ALLO-GABAA receptor function. However, treating PMDD requires attention to both emotional and physical symptoms, such as pain sensitivity. This study aims to investigate the efficacy of ShuYu capsules, a traditional Chinese medicine, in simultaneously treating emotional and physical symptoms in a rat model of PMDD. Specifically, our focus centres on the midbrain periaqueductal grey (PAG), a region associated with emotion regulation and susceptibility to hyperalgesia. Considering the underlying mechanisms of ALLO-GABAA receptor function in the PAG region, we conducted a series of experiments to evaluate and define the effects of ShuYu capsules and uncover the relationship between the drug's efficacy and ALLO concentration fluctuations on GABAA receptor function in the PAG region. Our findings demonstrate that ShuYu capsules significantly improved oestrous cycle-dependant depression-like behaviour and reduced stress-induced hyperalgesia in rats with PMDD. Similar to the low dose of fluoxetine, ShuYu capsules targeted and mitigated the sharp decline in ALLO, rescued the upregulation of GABAAR subunit function, and activated PAG neurons in PMDD rats. The observed effects of ShuYu capsules suggest a central mechanism underlying PMDD symptoms, involving ALLO_GABAA receptor function in the PAG region. This study highlights the potential of traditional Chinese medicine in addressing both emotional and physical symptoms associated with PMDD, shedding light on novel therapeutic approaches for this condition.

Emphasizing the Crosstalk Between Inflammatory and Neural Signaling in Post-traumatic Stress Disorder (PTSD)

Post-traumatic stress disorder (PTSD) is a chronic incapacitating condition with recurrent experience of trauma-related memories, negative mood, altered cognition, and hypervigilance. Agglomeration of preclinical and clinical evidence in recent years specified that alterations in neural networks favor certain characteristics of PTSD. Besides the disruption of hypothalamus-pituitary-axis (HPA) axis, intensified immune status with elevated pro-inflammatory cytokines and arachidonic metabolites of COX-2 such as PGE2 creates a putative scenario in worsening the neurobehavioral facet of PTSD. This review aims to link the Diagnostic and Statistical Manual of mental disorders (DSM-V) symptomology to major neural mechanisms that are supposed to underpin the transition from acute stress reactions to the development of PTSD. Also, to demonstrate how these intertwined processes can be applied to probable early intervention strategies followed by a description of the evidence supporting the proposed mechanisms. Hence in this review, several neural network mechanisms were postulated concerning the HPA axis, COX-2, PGE2, NLRP3, and sirtuins to unravel possible complex neuroinflammatory mechanisms that are obscured in PTSD condition.

Pharmacological strategies for appetite modulation in eating disorders: a narrative review

BACKGROUND

A substantial increase in the prevalence of eating disorders has been noticed over the past decades. Priority in the treatment of eating disorders is justifiably given to psychosocial interventions. However, it is also well known that centrally acting drugs can significantly affect appetite and food consumption.

AIM

To narratively review the available neurobiological data on the mechanisms of central regulation of eating behavior as a rationale to summarize pharmacological strategies for appetite modulation in eating disorders.

METHODS

The authors have carried out a narrative review of scientific papers published from January 2013 to March 2023 in the PubMed and Web of Science electronic databases. Studies were considered eligible if they included data on the neurobiological mechanisms of appetite regulation or the results of clinical trials of centrally acting drugs in eating disorders. Relevant studies were included regardless of their design. Descriptive analysis was used to summarize the obtained data.

RESULTS

The review included 51 studies. The available neurobiological and clinical data allowed us to identify the following pharmacological strategies for appetite modulation in eating disorders: serotonergic, catecholaminergic, amino acidergic and peptidergic. However, implementation of these data into clinical practice difficult due to an insufficient number of good-quality studies, which is particularly relevant for adolescents as there is a research gap in this population.

CONCLUSION

The progress in neurobiological understanding of the mechanisms of central regulation of appetite opens opportunities for new pharmacotherapeutic approaches aimed at changing the patterns of eating behavior. Obviously, treatment of eating disorders is a much broader problem and cannot be reduced to the correction of eating patterns. Nevertheless, at certain stages of treatment, drug-induced modulation of appetite can play an important role among multi-targeted biological and psychosocial interventions. Translation of neurobiological data into clinical practice requires a large number of clinical studies to confirm the long-term efficacy and safety of pharmacotherapeutic approaches and to develop personalized algorithms for the treatment of various forms of eating disorders in different age groups.

Defensive and Emotional Behavior Modulation by Serotonin in the Periaqueductal Gray

Serotonin 5-hydroxytryptamine (5-HT) is a key neurotransmitter for the modulation and/or regulation of numerous physiological processes and psychiatric disorders (e.g., behaviors related to anxiety, pain, aggressiveness, etc.). The periaqueductal gray matter (PAG) is considered an integrating center for active and passive defensive behaviors, and electrical stimulation of this area has been shown to evoke behavioral responses of panic, fight-flight, freezing, among others. The serotonergic activity in PAG is influenced by the activation of other brain areas such as the medial hypothalamus, paraventricular nucleus of the hypothalamus, amygdala, dorsal raphe nucleus, and ventrolateral orbital cortex. In addition, activation of other receptors within PAG (i.e., CB1, Oxytocin, µ-opioid receptor (MOR), and γ-aminobutyric acid (GABAA)) promotes serotonin release. Therefore, this review aims to document evidence suggesting that the PAG-evoked behavioral responses of anxiety, panic, fear, analgesia, and aggression are influenced by the activation of 5-HT1A and 5-HT2A/C receptors and their participation in the treatment of various mental disorders.

Ameliorating effect offluoxetine on tamoxifen-induced memory loss: The role of corticolimbic NMDA receptors and CREB/BDNF/cFos signaling pathways in rats

Tamoxifen-induced cognitive dysfunction may lead to fluoxetine consumption in patients with breast cancer. Since the brain mechanisms are unclear in tamoxifen/fluoxetine therapy, the blockade effect of hippocampal/amygdala/prefrontal cortical NMDA receptors was examined in fluoxetine/tamoxifen-induced memory retrieval. We also assessed the corticolimbic signaling pathways in memory retrieval under the drug treatment in adult male Wistar rats. Using the Western blot technique, the expression levels of the cAMP response element-binding protein (CREB), brain-derived neurotrophic factor (BDNF), and cFos were evaluated in the corticolimbic regions. The results showed that pre-test administration of fluoxetine (3 and 5 mg/kg, i.p.) improved tamoxifen-induced memory impairment in the passive avoidance learning task. Pre-test bilateral microinjection of D-AP5, a selective NMDA receptor antagonist, into the dorsal hippocampal CA1 regions and the central amygdala (CeA), but not the medial prefrontal cortex (mPFC), inhibited the improving effect of fluoxetine on tamoxifen response. It is important to note that the microinjection of D-AP5 into the different sites by itself did not affect memory retrieval. Memory retrieval increased the signaling pathway of pCREB/CREB/BDNF/cFos in the corticolimbic regions. Tamoxifen-induced memory impairment decreased the hippocampal/PFC BDNF level and the amygdala level of pCREB/CREB/cFos. The improving effect of fluoxetine on tamoxifen significantly increased the hippocampal/PFC expression levels of BDNF, the PFC/amygdala expression levels of cFos, and the ratio of pCREB/CREB in all targeted areas. Thus, NMDA receptors' activity in the different corticolimbic regions mediates fluoxetine/tamoxifen memory retrieval. The corticolimbic synaptic plasticity changes likely accompany the improving effect of fluoxetine on tamoxifen response.

Fluoxetine modifies circadian rhythm by reducing melatonin content in zebrafish

Fluoxetine (FLX), a selective serotonin reuptake inhibitor (SSRI), increases the serotonin levels in the brain to treat depression. Antidepressants have been demonstrated to modulate circadian rhythm, but the underlying mechanisms by which antidepressants regulate circadian rhythm require more research. This study aimed to investigate the role of FLX on circadian rhythm by analyzing the movement behavior and internal circadian oscillations in zebrafish. The results showed that the expression of clock genes clock1a and bmal1b was significantly down-regulated, and the amplitude reduction and phase shift were observed after FLX treatment. Furthermore, FLX exposure inhibited the expression of aanat2, which led to a decrease in nocturnal melatonin secretion. aanat2^-/- larvae showed disrupted circadian rhythm. These findings may help reveal the effect of FLX exposure on the circadian rhythm and locomotor activity. It may provide theoretical data for the clinical application of FLX.

5-HT3 receptor within the amygdaloid complex modulates pain hypersensitivity induced by empathy model of cohabitation with a partner in chronic pain condition in mice

Cohabitation with a partner undergoing chronic pain induces pain hypersensitivity. Among a lot of other neurochemical pathways, the serotonin (5-HT) role, specifically the 5-HT₃ receptor (5-HT₃R), in the amygdala has never been evaluated in this model. Here we studied the effects of the amygdala's chemical inhibition, its neuronal activation pattern, and 5-HT, 5-HIAA, and 5-HT turnover within the amygdala. Furthermore, the systemic and intra-amygdala 5-HT₃R activation and blockade in mice that cohabited with a conspecific subjected to chronic constriction injury were investigated. Male Swiss mice were housed in partners for 28 days. The dyads were divided into two groups on the 14^th day: cagemate nerve constriction (CNC) and cagemate sham (CS). On the 24^th day, cagemates underwent a stereotaxic surgery (when necessary) and, on the 28^th day, they were evaluated on the writhing test. The amygdala inactivation promotes pain-hypersensitivity behaviors in groups and dyads; cohabitation with a partner with chronic pain did not change FosB-labeled cells in the amygdala's nucleus and increases 5-HT turnover in cagemates. Systemic and intra-amygdala 5-HT₃R activation attenuated and enhanced the number of writhes, respectively. In contrast, 5-HT₃R blockade reduced hypersensitivity pain response. Results suggest the involvement of amygdala serotonergic signaling via 5-HT₃R in empathy-like behavior.

Role of 5-HT1A and 5-HT2C receptors of the dorsal periaqueductal gray in the anxiety- and panic-modulating effects of antidepressants in rats

Antidepressant drugs are first-line treatment for panic disorder. Facilitation of 5-HT_1A receptor-mediated neurotransmission in the dorsal periaqueductal gray (dPAG), a key panic-associated area, has been implicated in the panicolytic effect of the selective serotonin reuptake inhibitor fluoxetine. However, it is still unknown whether this mechanism accounts for the antipanic effect of other classes of antidepressants drugs (ADs) and whether the 5-HT interaction with 5-HT_2C receptors in this midbrain area (which increases anxiety) is implicated in the anxiogenic effect caused by short-term treatment with ADs. The results showed that previous injection of the 5-HT_1A receptor antagonist WAY-100635 in the dPAG blocked the panicolytic-like effect caused by chronic systemic administration of the tricyclic AD imipramine in male Wistar rats tested in the elevated T-maze. Neither chronic treatment with imipramine nor fluoxetine changed the expression of 5-HT_1A receptors in the dPAG. Treatment with these ADs also failed to significantly change ERK1/2 (extracellular-signal regulated kinase) phosphorylation level in this midbrain area. Blockade of 5-HT_2C receptors in the dPAG with the 5-HT_2C receptor antagonist SB-242084 did not change the anxiogenic effect caused by a single acute injection of fluoxetine or imipramine in the Vogel conflict test. These results reinforce the view that the facilitation of 5-HT_1A receptor-mediated neurotransmission in the dPAG is a common mechanism involved in the panicolytic effect caused by chronic administration of ADs. On the other hand, the anxiogenic effect observed after short-term treatment with these drugs does not depend on 5-HT_2C receptors located in the dPAG.