Modulation of expression of fear by oxytocin signaling in the central amygdala: From reduction of fear to regulation of defensive behavior style

Oxytocin receptors in the nucleus accumbens shell are necessary for the onset of maternal behavior

In rodents, oxytocin (Oxt) contributes to the onset of maternal care by shifting the perception of pups from aversive to attractive. Both Oxt receptor knockout (Oxtr -/-) and forebrain-specific Oxtr knockout (FB/FB) dams abandon their first litters, likely due to a failure of the brain to 'switch' to a more maternal state. Whether this behavioral shift is neurochemically similar in virgin females, who can display maternal behaviors when repeatedly exposed to pups, or what neuroanatomical substrate is critical for the onset of maternal care remains unknown. To understand similarities and differences in Oxtr signaling in virgin pup-sensitized Oxtr FB/FB as opposed to post-parturient Oxtr -/- and Oxtr FB/FB dams, maternal behavior (pup-sensitized females only) and immediate early gene activation were assessed. Pup-sensitized Oxtr FB/FB females retrieved pups faster on day one of testing and had reduced c-Fos expression in the dorsal lateral septum as compared to virgin pup-sensitized Oxtr +/+ females. This differs from what was observed in post-parturient Oxtr -/- and Oxtr FB/FB dams, where increased c-Fos expression was observed in the nucleus accumbens (NAcc) shell. Based on these data, we then disrupted Oxtr signaling in the NAcc shell or the posterior paraventricular thalamus (pPVT) (control region) of female Oxtr floxed mice using a Cre recombinase expressing adeno-associated virus. Knockout of the Oxtr only in the NAcc shell prevented the onset of maternal care post-parturient females. Our data suggest that a pup-sensitized brain may differ from a post-parturient brain and that Oxtr signaling in the NAcc shell is critical to the onset of maternal behavior.

Oxytocin and social learning in socially anxious men and women

OBJECTIVE

This study extended a classic self-referential learning paradigm by investigating the effects of intranasally-administered oxytocin in high and low socially anxious participants during social learning, as a function of social anxiety levels and sex.

METHODS

In a randomized double-blinded design, 160 participants were either given intranasal oxytocin (24 I.U.) or placebo. Subsequently, while lying in an MR scanner, participants were shown neutral faces that were paired with positively, neutrally, or negatively valenced self-referential sentences, during which we measured self-reported arousal and sympathy of the facial stimuli, pupil dilation, and changes in the brain-oxygen-level dependent signal. Four-factor mixed analyses of variance with the between-subjects factors group (high socially anxious vs. low socially anxious), substance (oxytocin vs. placebo), and sex (male vs. female) and the within-subjects factor sentence valence (positive vs. neutral vs. negative) were conducted for each measure, respectively.

RESULTS

Administration of intranasal oxytocin yielded an increase in sympathy ratings in high socially anxious compared to low socially anxious individuals and decreased arousal ratings for positively-conditioned faces in low socially anxious participants. As an objective physiological measure of arousal, pupil dilation mirrored the behavioral results. Oxytocin effects on neural activation in the insula interacted with anxiety levels and sex: low socially anxious individuals yielded lower activation under oxytocin than placebo; the converse was observed in high socially anxious individuals. This interaction also differed between sexes, as men yielded higher activation levels than women. These findings were more prominent for positively- and negatively-conditioned faces. Within the amygdala, high socially anxious men yielded higher activation than high socially anxious women in the left hemisphere, and low socially anxious men yielded higher activation than low socially anxious women from positively- and negatively-conditioned faces, though no influence of oxytocin was detected.

CONCLUSION

These results suggest oxytocin-induced behavioral, physiological, and neural changes as a function of social learning in socially low and high anxious individuals. These findings challenge the amygdalocentric view of the role of emotions in social learning, instead contributing to the growing body of findings implicating the insula therein, revealing an interaction between oxytocin, sex, and emotional valence. Such discoveries raise an interesting set of questions regarding the computational goals of regions such as the insula in emotional learning and how neural activity can play a diagnostic or prognostic role in social anxiety, potentially leading to new treatment opportunities that may combine oxytocin and neurofeedback differentially for men and women.

The modulation of emotional and social behaviors by oxytocin signaling in limbic network

Neuropeptides can exert volume modulation in neuronal networks, which account for a well-calibrated and fine-tuned regulation that depends on the sensory and behavioral contexts. For example, oxytocin (OT) and oxytocin receptor (OTR) trigger a signaling pattern encompassing intracellular cascades, synaptic plasticity, gene expression, and network regulation, that together function to increase the signal-to-noise ratio for sensory-dependent stress/threat and social responses. Activation of OTRs in emotional circuits within the limbic forebrain is necessary to acquire stress/threat responses. When emotional memories are retrieved, OTR-expressing cells act as gatekeepers of the threat response choice/discrimination. OT signaling has also been implicated in modulating social-exposure elicited responses in the neural circuits within the limbic forebrain. In this review, we describe the cellular and molecular mechanisms that underlie the neuromodulation by OT, and how OT signaling in specific neural circuits and cell populations mediate stress/threat and social behaviors. OT and downstream signaling cascades are heavily implicated in neuropsychiatric disorders characterized by emotional and social dysregulation. Thus, a mechanistic understanding of downstream cellular effects of OT in relevant cell types and neural circuits can help design effective intervention techniques for a variety of neuropsychiatric disorders.

Oxytocin Release: A Remedy for Cerebral Inflammaging

Oxytocin facilitates reproduction both by physiological and behavioral mechanisms. Oxytocinergic neurons emerging from the hypothalamus release oxytocin from the pituitary gland to the blood by axonal discharge to regulate reproductive organs. However, at the same time, oxytocin is secreted into neighboring areas of the hypothalamus from the dendrites of these neurons. Here, the peptide acts by autocrine and paracrine mechanisms to influence other neuroendocrine systems. Furthermore, oxytocinergic neurons project to many different locations in the brain, where they affect sensory processing, affective functions, and reward. Additional to its regulatory role, significant anti-inflammatory and restoring effects of oxytocin have been reported from many invivo and in-vitro studies. The pervasive property of the oxytocin system may enable it generally to dampen stress reactions both peripherally and centrally, and protect neurons and supportive cells from inadequate inflammation and malfunctioning. Animal experiments have documented the importance of preserving immune- and stem cell functions in the hypothalamus to impede age-related destructive processes of the body. Sexual reward has a profound stimulating impact on the oxytocinergic activity, and the present article therefore presents the hypothesis that frequent sexual activity and gratigying social experiance may postpone the onset of frailty and age-associated diseases by neural protection from the bursts of oxytocin. Furthermore, suggestions are given how the neuroplastic properties of oxytocin may be utilized to enhance sexual reward by learning processes in order to further reinforce the release of this peptide.

Control of Behavioral Arousal and Defense by a Glutamatergic Midbrain-Amygdala Pathway in Mice

In response to external threatening signals, animals evolve a series of defensive behaviors that depend on heightened arousal. It is believed that arousal and defensive behaviors are coordinately regulated by specific neurocircuits in the central nervous system. The ventral tegmental area (VTA) is a key structure located in the ventral midbrain of mice. The activity of VTA glutamatergic neurons has recently been shown to be closely related to sleep–wake behavior. However, the specific role of VTA glutamatergic neurons in sleep–wake regulation, associated physiological functions, and underlying neural circuits remain unclear. In the current study, using an optogenetic approach and synchronous polysomnographic recording, we demonstrated that selective activation of VTA glutamatergic neurons induced immediate transition from sleep to wakefulness and obviously increased the amount of wakefulness in mice. Furthermore, optogenetic activation of VTA glutamatergic neurons induced multiple defensive behaviors, including burrowing, fleeing, avoidance and hiding. Finally, viral-mediated anterograde activation revealed that projections from the VTA to the central nucleus of the amygdala (CeA) mediated the wake- and defense-promoting effects of VTA glutamatergic neurons. Collectively, our results illustrate that the glutamatergic VTA is a key neural substrate regulating wakefulness and defensive behaviors that controls these behaviors through its projection into the CeA. We further discuss the possibility that the glutamatergic VTA-CeA pathway may be involved in psychiatric diseases featuring with excessive defense.

Toxoplasma gondii Infection Causes an Atypical Abundance of Oxytocin and Its Receptor in the Female Rat Brain

Infection with the protozoan Toxoplasma gondii causes loss of innate fear of cat odors in both male and female rats. This behavioral change is presumed to reflect a parasitic manipulation that increases transmission of the parasite from its intermediate to definitive host. The host behavioral change in male rats is dependent on gonadal steroids. In contrast, the loss of fear in female rats is not accompanied by greater gonadal steroids and cannot be rescued by gonadectomy. This disparity suggests that proximate mechanisms of the post infection host behavioral change in rats are sexually dimorphic. Here, we report that female rats infected with Toxoplasma gondii exhibit greater abundance of messenger RNA for oxytocin and oxytocin receptors in the paraventricular nucleus of the hypothalamus and posterodorsal medial amygdala, respectively. Brain oxytocin is critical for sex-typical social and sexual behaviors in female rodents. The change in oxytocin and its receptor could potentially alter activity in the social salience circuits, leading to a reduction in defensive behaviors and an increase in approach to ambivalent environmental cues. Our results argue that sexually dimorphic neural substrates underpin sexually monomorphic host behavioral change in this host–parasite association.

Intranasal oxytocin compensates for estrus cycle-specific reduction of conditioned safety memory in rats: Implications for psychiatric disorders

Stress and anxiety disorder patients frequently fail to benefit from psychotherapies which often consist of inhibitory fear learning paradigms. One option to improve the therapy outcome is medication-enhanced psychotherapy. Research in humans and laboratory rodents has demonstrated that oxytocin (OT) reduces fear and facilitates fear extinction. However, the role of OT in conditioned safety learning, an understudied but highly suitable type of inhibitory fear learning, remains to be investigated. The present study aimed at investigating the effect of intranasal OT on conditioned safety. To test this, Sprague Dawley rats (♂n = 57; ♀n = 72) were safety conditioned. The effects of pre-training or pre-testing intranasal OT on conditioned safety and contextual fear, both measured by the acoustic startle response, and on corticosterone plasma levels were assessed. Furthermore, the involvement of the estrous cycle was analyzed. The present data show that intranasal OT administration before the acquisition or recall sessions enhanced conditioned safety memory in female rats while OT had no effects in male rats. Further analysis of the estrus cycle revealed that vehicle-treated female rats in the metestrus showed reduced safety memory which was compensated by OT-treatment. Moreover, all vehicle-treated rats, regardless of sex, expressed robust contextual fear following conditioning. Intranasal OT-treated rats showed a decrease in contextual fear, along with reduced plasma corticosterone levels. The present data demonstrate that intranasal OT has the capacity to compensate deficits in safety learning, along with a reduction in contextual fear and corticosterone levels. Therefore, add-on treatment with intranasal OT could optimize the therapy of anxiety disorders.

The Role of Central Serotonin Neurons and 5-HT Heteroreceptor Complexes in the Pathophysiology of Depression: A Historical Perspective and Future Prospects

Serotonin communication operates mainly in the extracellular space and cerebrospinal fluid (CSF), using volume transmission with serotonin moving from source to target cells (neurons and astroglia) via energy gradients, leading to the diffusion and convection (flow) of serotonin. One emerging concept in depression is that disturbances in the integrative allosteric receptor–receptor interactions in highly vulnerable 5-HT1A heteroreceptor complexes can contribute to causing major depression and become novel targets for the treatment of major depression (MD) and anxiety. For instance, a disruption and/or dysfunction in the 5-HT1A-FGFR1 heteroreceptor complexes in the raphe-hippocampal serotonin neuron systems can contribute to the development of MD. It leads inter alia to reduced neuroplasticity and potential atrophy in the raphe-cortical and raphe-striatal 5-HT pathways and in all its forebrain networks. Reduced 5-HT1A auto-receptor function, increased plasticity and trophic activity in the midbrain raphe 5-HT neurons can develop via agonist activation of allosteric receptor–receptor interactions in the 5-HT1A-FGFR1 heterocomplex. Additionally, the inhibitory allosteric receptor–receptor interactions in the 5-HT1AR-5-HT2AR isoreceptor complex therefore likely have a significant role in modulating mood, involving a reduction of postjunctional 5-HT1AR protomer signaling in the forebrain upon activation of the 5-HT2AR protomer. In addition, oxytocin receptors (OXTRs) play a significant and impressive role in modulating social and cognitive related behaviors like bonding and attachment, reward and motivation. Pathological blunting of the OXTR protomers in 5-HT2AR and especially in 5-HT2CR heteroreceptor complexes can contribute to the development of depression and other types of psychiatric diseases involving disturbances in social behaviors. The 5-HTR heterocomplexes are novel targets for the treatment of MD.

Social communication of affective states

Social interactions promote the communication of explicit and implicit information between individuals. Implicit or subconscious sharing of cues can be useful in conveying affective states. Knowing the affective state of others can guide future interactions, while an inability to decipher another's affective state is a core feature of autism spectrum disorder. The precise neural circuitry and mechanisms involved in communicating affective states are not well understood. Over the past few years, a number of important observations in rodent models have increased our knowledge of the neural processes for social communication of affective state. Here we highlight these contributions by first describing the rodent models used to investigate social communication of affect and then summarising the neural circuitry and processes implicated by these rodent models. We relate these findings to humans as well as to the current global context where social interactions have been modified by the Covid-19 pandemic.

Molecular, biochemical and behavioural evidence for a novel oxytocin receptor and serotonin 2C receptor heterocomplex

The complexity of oxytocin-mediated functions is strongly associated with its modulatory effects on other neurotransmission systems, including the serotonin (5-hydroxytryptamine, 5-HT) system. Signalling between oxytocin (OT) and 5-HT has been demonstrated during neurodevelopment and in the regulation of specific emotion-based behaviours. It is suggested that crosstalk between neurotransmitters is driven by interaction between their specific receptors, particularly the oxytocin receptor (OTR) and the 5-hydroxytryptamine 2C receptor (5-HTR_2C), but evidence for this and the downstream signalling consequences that follow are lacking. Considering the overlapping central expression profiles and shared involvement of OTR and 5-HTR_2C in certain endocrine functions and behaviours, including eating behaviour, social interaction and locomotor activity, we investigated the existence of functionally active OTR/5-HTR_2C heterocomplexes. Here, we demonstrate evidence for a potential physical interaction between OTR and 5-HTR_2Cin vitro in a cellular expression system using flow cytometry-based FRET (fcFRET). We could recapitulate this finding under endogenous expression levels of both receptors via in silico analysis of single cell transcriptomic data and ex vivo proximity ligation assay (PLA). Next, we show that co-expression of the OTR/5-HTR_2C pair resulted in a significant depletion of OTR-mediated Gαq-signalling and significant changes in receptor trafficking. Of note, attenuation of OTR-mediated downstream signalling was restored following pharmacological blockade of the 5-HTR_2C. Finally, we demonstrated a functional relevance of this novel heterocomplex, in vivo, as 5-HTR_2C antagonism increased OT-mediated hypoactivity in mice. Overall, we provide compelling evidence for the formation of functionally active OTR/5-HTR_2C heterocomplexes, adding another level of complexity to OTR and 5-HTR_2C signalling functionality. This article is part of the special issue on Neuropeptides.

Selective sub-nucleus effects of intra-amygdala oxytocin on fear extinction

There is growing evidence that the neuropeptide oxytocin (OT) modulates fear and extinction in humans and rodents through actions in corticolimbic circuits including the central amygdala (CeA). Prior studies have, however, been limited to subjects that exhibit intact basal extinction, rather than extinction-impaired populations that could potentially therapeutically benefit from viable OT-targeting treatments. Here, we assessed the effects of pre-extinction training infusion of OT into the CeA, or basolateral amygdala (BLA), on extinction in an inbred mouse strain (S1) model of impaired extinction. We found that intra-CeA OT, at a dose of 0.01 μg, enabled extinction memory formation, as evidenced by lesser freezing as compared to vehicle-infused controls on a drug-free retrieval test. Conversely, infusion of a higher, 1.0 μg OT dose, markedly reduced freezing and increased grooming during extinction training and produced elevated freezing on drug-free retrieval. Infusion of the 0.01 μg dose into the BLA was without behavioral effects. Together, our data show that OT acts in a dose-dependent manner within the CeA to promote extinction in otherwise extinction-deficient mice. These findings provide further support for the potential utility of OT as an adjunctive treatment to extinction-based therapies for trauma and anxiety disorders.