Predator odor-evoked BOLD activation in the awake rat: modulation by oxytocin and V₁a vasopressin receptor antagonists

Gender-specific Single Transcript Level Atlas of Vasopressin and its Receptor (AVPR1a) in the Mouse Brain

Vasopressin (AVP), a nonapeptide synthesized predominantly by magnocellular hypothalamic neurons, is conveyed to the posterior pituitary via the pituitary stalk, where AVP is secreted into the circulation. Known to regulate blood pressure and water homeostasis, it also modulates diverse social behaviors, such as pair-bonding, social recognition and cognition in mammals including humans. Importantly, AVP modulates social behaviors in a gender-specific manner, perhaps, due to gender differences in the distribution in the brain of AVP and its main receptor AVPR1a. There is a corpus of integrative studies for the expression of AVP and AVPR1a in various brain regions, and their functions in modulating central and peripheral actions. In order to purposefully address sexually dimorphic and novel roles of AVP on central and peripheral functions through its AVPR1a, we utilized RNAscope to map Avp and Avpr1a single transcript expression in the mouse brain. As the most comprehensive atlas of AVP and AVPR1a in the mouse brain, this compendium highlights the importance of newly identified AVP/AVPR1a neuronal nodes that may stimulate further functional studies.

Where do we stand on fMRI in awake mice?

Imaging awake animals is quickly gaining traction in neuroscience as it offers a means to eliminate the confounding effects of anesthesia, difficulties of inter-species translation (when humans are typically imaged while awake), and the inability to investigate the full range of brain and behavioral states in unconscious animals. In this systematic review, we focus on the development of awake mouse blood oxygen level dependent functional magnetic resonance imaging (fMRI). Mice are widely used in research due to their fast-breeding cycle, genetic malleability, and low cost. Functional MRI yields whole-brain coverage and can be performed on both humans and animal models making it an ideal modality for comparing study findings across species. We provide an analysis of 30 articles (years 2011-2022) identified through a systematic literature search. Our conclusions include that head-posts are favorable, acclimation training for 10-14 d is likely ample under certain conditions, stress has been poorly characterized, and more standardization is needed to accelerate progress. For context, an overview of awake rat fMRI studies is also included. We make recommendations that will benefit a wide range of neuroscience applications.

Oxytocin receptor signaling contributes to olfactory avoidance behavior induced by an unpleasant odorant

Oxytocin (OXT) and its receptor (OXTR) regulate reproductive physiology (i.e. parturition and lactation), sociosexual behavior, learned patterns of behavior and olfactory behavior in social contexts. To characterize the function of OXTR in basic olfactory behavior, the present study compared the behavioral responses of homozygous, heterozygous and wild-type mice when these mice were confronted with an unpleasant odorant (butyric acid) in a custom-made Y-maze in the absence of a social context. Wild-type mice avoided the first encounter with the butyric acid odorant, whereas homozygous and heterozygous mice did not. However, both heterozygous and wild-type mice habituated when confronted with the butyric odorant again on the following 2 days. By contrast, homozygous mice failed to habituate and instead avoided the location of the odorant for at least 3 days. These data suggest that homozygous and heterozygous mice display abnormal olfactory responses to the presentation of an unpleasant odorant. Our studies demonstrate that OXTR plays a critical role in regulating olfactory behavior in the absence of a social context.

Summary: Homozygous mice exhibited abnormal olfactory behaviors, namely failure in the acute avoidance of butyric acid and in habituation behavior, in the absence of a social context.

The role of vasopressin in olfactory and visual processing

Neural vasopressin is a potent modulator of behaviour in vertebrates. It acts at both sensory processing regions and within larger regulatory networks to mediate changes in social recognition, affiliation, aggression, communication and other social behaviours. There are multiple populations of vasopressin neurons within the brain, including groups in olfactory and visual processing regions. Some of these vasopressin neurons, such as those in the main and accessory olfactory bulbs, anterior olfactory nucleus, piriform cortex and retina, were recently identified using an enhanced green fluorescent protein-vasopressin (eGFP-VP) transgenic rat. Based on the interconnectivity of vasopressin-producing and sensitive brain areas and in consideration of autocrine, paracrine and neurohormone-like actions associated with somato-dendritic release, we discuss how these different neuronal populations may interact to impact behaviour.

Habenula and interpeduncular nucleus differentially modulate predator odor-induced innate fear behavior in rats

Fear is an important behavioral system helping humans and animals to survive potentially dangerous situations. Fear can be innate or learned. Whereas the neural circuits underlying learned fear are already well investigated, the knowledge about the circuits mediating innate fear is still limited. We here used a novel, unbiased approach to image in vivo the spatial patterns of neural activity in odor-induced innate fear behavior in rats. We intravenously injected awake unrestrained rats with a 99m-technetium labeled blood flow tracer (99mTc-HMPAO) during ongoing exposure to fox urine or water as control, and mapped the brain distribution of the trapped tracer using single-photon emission computed tomography (SPECT). Upon fox urine exposure blood flow increased in a number of brain regions previously associated with odor-induced innate fear such as the amygdala, ventromedial hypothalamus and dorsolateral periaqueductal grey, but, unexpectedly, decreased at higher significance levels in the interpeduncular nucleus (IPN). Significant flow changes were found in regions monosynaptically connected to the IPN. Flow decreased in the dorsal tegmentum and entorhinal cortex. Flow increased in the habenula (Hb) and correlated with odor effects on behavioral defensive strategy. Hb lesions reduced avoidance of but increased approach to the fox urine while IPN lesions only reduced avoidance behavior without approach behavior. Our study identifies a new component, the IPN, of the neural circuit mediating odor-induced innate fear behavior in mammals and suggests that the evolutionarily conserved Hb-IPN system, which has recently been implicated in cued fear, also forms an integral part of the innate fear circuitry.

Stress, sex, and addiction: potential roles of corticotropin-releasing factor, oxytocin, and arginine-vasopressin

Stress sensitivity and sex are predictive factors for the development of neuropsychiatric disorders. Life stresses are not only risk factors for the development of addiction but also are triggers for relapse to drug use. Therefore, it is imperative to elucidate the molecular mechanisms underlying the interactions between stress and drug abuse, as an understanding of this may help in the development of novel and more effective therapeutic approaches to block the clinical manifestations of drug addiction. The development and clinical course of addiction-related disorders do appear to involve neuroadaptations within neurocircuitries that modulate stress responses and are influenced by several neuropeptides. These include corticotropin-releasing factor, the prototypic member of this class, as well as oxytocin and arginine-vasopressin that play important roles in affiliative behaviors. Interestingly, these peptides function to balance emotional behavior, with sexual dimorphism in the oxytocin/arginine-vasopressin systems, a fact that might play an important role in the differential responses of women and men to stressful stimuli and the specific sex-based prevalence of certain addictive disorders. Thus, this review aims to summarize (i) the contribution of sex differences to the function of dopamine systems, and (ii) the behavioral, neurochemical, and anatomical changes in brain stress systems.

Photoacoustic imaging of vascular hemodynamics: validation with blood oxygenation level-dependent MR imaging

PURPOSE

To noninvasively assess vascular hemodynamics with photoacoustic imaging (PAI) and blood oxygenation level-dependent (BOLD) magnetic resonance (MR) imaging in phantoms and in an animal model.

MATERIALS AND METHODS

In vivo studies were performed with institutional animal care and use committee approval. In vitro experiments were performed by using a tissue-mimicking phantom in multiple oxygenation conditions (n = 6) to compare PAI measurements and BOLD MR imaging measurements. PAI and T2-weighted spin-echo-based BOLD MR imaging were performed to assess tumor response to carbogen (95% O2, 5% CO2) in mice with head and neck tumors before (n = 11) and after (n = 9) treatment with a vascular disrupting agent (VDA). Two-tailed Pearson correlation analysis was performed to determine the correlation between the parameters measured with PAI and BOLD MR imaging in vitro. Two-tailed paired t tests were used to compare change in tumor hemoglobin oxygen saturation (sO2) levels and BOLD signal in response to carbogen. Changes in PAI and BOLD signal intensity before and after VDA treatment were analyzed for significance by using analysis of variance with repeated measures.

RESULTS

Phantom measurements yielded good correlation between photoacoustically derived sO2 levels and BOLD signal intensity (r = 0.937, P = .005) and partial pressure of oxygen (r = 0.981, P = .005). In vivo hemodynamic response to carbogen was characterized by a significant increase in tumor sO2 levels (P = .003) and BOLD signal (P = .001). When compared with pretreatment estimates, treatment with VDA resulted in a significant reduction in the tumor hemodynamic response to carbogen at PAI (P = .030).

CONCLUSION

Carbogen-based functional imaging with PAI and BOLD MR imaging enables monitoring of early changes in tumor hemodynamics after vascular targeted therapy.

Oxytocin and vasopressin modulation of the neural correlates of motivation and emotion: results from functional MRI studies in awake rats

Oxytocin and vasopressin modulate a range of species typical behavioral functions that include social recognition, maternal-infant attachment, and modulation of memory, offensive aggression, defensive fear reactions, and reward seeking. We have employed novel functional magnetic resonance mapping techniques in awake rats to explore the roles of these neuropeptides in the maternal and non-maternal brain. Results from the functional neuroimaging studies that are summarized here have directly and indirectly confirmed and supported previous findings. Oxytocin is released within the lactating rat brain during suckling stimulation and activates specific subcortical networks in the maternal brain. Both vasopressin and oxytocin modulate brain regions involved unconditioned fear, processing of social stimuli and the expression of agonistic behaviors. Across studies there are relatively consistent brain networks associated with internal motivational drives and emotional states that are modulated by oxytocin and vasopressin. This article is part of a Special Issue entitled Oxytocin and Social Behav.

Functional magnetic resonance imaging in rodents: an unique tool to study in vivo pharmacologic neuromodulation

When new compounds targeting the brain are developed, it is important to assess both the acute and chronic effects on brain functioning. This can be done non-invasively using a technique called functional magnetic resonance imaging (fMRI). This review discusses the possibilities of both stimulation-based and resting state fMRI to study pharmacological modulations of the rodent brain. Moreover, attention is given to the use of anesthetics which could importantly influence the outcome of both techniques.