Nitric oxide-mediated defensive and antinociceptive responses organised at the anterior hypothalamus of mice are modulated by glutamatergic inputs from area 24b of the cingulate cortex

Nitric oxide-signalling affects panic-like defensive behaviour and defensive antinociception neuromodulation in the prelimbic cerebral cortex

RATIONALE

The prelimbic division (PrL) of the medial prefrontal cortex (mPFC) is a key structure in panic.

OBJECTIVES

To evaluate the role of nitric oxide (NO) in defensive behaviour and antinociception.

METHODS

Either Nω-propyl-L-arginine (NPLA) or Carboxy-PTIO was microinjected in the PrL cortex, followed by hypothalamic treatment with bicuculline. The exploratory behaviours, defensive reactions and defensive antinociception were recorded. Encephalic c-Fos protein was immunolabelled after escape behaviour.

RESULTS

NPLA (an inhibition of nNOs) decreased panic-like responses and innate fear-induced antinociception. The c-PTIO (a membrane-impermeable NO scavenger) decreased the escape behaviour. PrL cortex pre-treatment with c-PTIO at all doses decreased defensive antinociception. c-Fos protein was labelled in neocortical areas, limbic system, and mesencephalic structures.

CONCLUSION

The NPLA and c-PTIO in the PrL/mPFC decreased the escape behaviour and defensive antinociception organised by medial hypothalamic nuclei. The oriented escape behaviour recruits neocortical areas, limbic system, and mesencephalic structures. These findings suggest that the organisation of defensive antinociception recruits NO-signalling mechanisms within the PrL cortex. Furthermore, the present findings also support the role of NO as a retrograde messenger in the PrL cortex during panic-like emotional reactions.

The anterior cingulate cortex and its interface with the dorsal periaqueductal grey regulating nitric oxide-mediated panic-like behaviour and defensive antinociception

The anterior cingulate cortex (ACC) Cg1 (24b) area modulates glutamate-mediated unconditioned fear and antinociception organised by hypothalamus. However, it remains unknown whether 24b area also modulates these latter defensive responses through connections with the dorsal periaqueductal grey matter (dPAG), a midbrain structure implicated in the genesis of innate fear-induced defence. The aim of this work is to examine the correlation between the behavioural effects of intra-ACC microinjections of vehicle, NMDA (1 nmol) or lidocaine (2%) with Fos protein expression and nitrergic activity in the dPAG of male C57BL/6 mice that were threatened by snakes. In addition, the 24b area-dPAG pathways were also characterised by neural tract tracing procedures. Finally, the effect of dPAG pretreatment with the neuronal nitric oxide synthase inhibitor N(omega)-propyl-l-arginine (NPLA; 0.2, 0.4 or 0.8 nmol) 10 min before 24b area treatment with NMDA on behavioural and nociceptive responses of threatened mice was studied. The activation of 24b area N-methyl-d-aspartic acid receptors facilitated escape and freezing rather than risk assessment, and enhanced Fos expression and nitrite levels in dPAG, while lidocaine decreased escape and risk assessment as well as Fos and nitrergic activity in dPAG. In addition, dPAG pretreatment with NPLA suppressed intra-24b NMDA-facilitated panicogenic effects while increased nociception. Infusions of an antegrade neurotracer into 24b area showed axonal fibres surrounding both dorsomedial and dorsolateral PAG perikarya. Neurons were identified in 24b area after deposits of a retrograde neurotracer into dPAG. Our findings suggest that the ACC/24b area modulates innate defensive responses through the recruitment of dPAG nitrergic neurons.

Alpha1- and Beta-norepinephrinergic receptors of dorsomedial and ventromedial hypothalamic nuclei modulate panic attack-like defensive behaviour elicited by diencephalic GABAergic neurotransmission disinhibition

Gamma-aminobutyric acid (GABA) disinhibition in medial hypothalamus (MH) nuclei of rats elicits some defensive reactions that are considered panic attack-like behaviours. Recent evidence showed that the norepinephrine-mediated system modulates fear-related defensive behaviours organised by MH neurons at least in part via noradrenergic receptors recruitment on midbrain tegmentum. However, it is unknown whether noradrenergic receptors of the MH also modulate the panic attack-like reactions. The aim of this work was to investigate the distribution of noradrenergic receptors in MH, and the effects of either α1-, α2- or β-noradrenergic receptors blockade in the MH on defensive behaviours elaborated by hypothalamic nuclei. Defensive behaviours were evaluated after the microinjection of the selective GABAA receptor antagonist bicuculline into the MH that was preceded by microinjection of either WB4101, RX821002, propranolol (α1-, α2- and β-noradrenergic receptor selective antagonists, respectively), or physiological saline into the MH of male Wistar rats. The α1-, α2- and β-noradrenergic receptors were found in neuronal perikarya of all MH nuclei, and the α2-noradrenergic receptor were also found on glial cells mainly situated in the ventrolateral division of the ventromedial hypothalamic nucleus. The α1- and β-noradrenergic receptors blockade in the MH decreased defensive attention and escape reactions elicited by the intra-MH microinjections of bicuculline. These findings suggest that, despite the profuse distributions of α1-, α2- and β-noradrenergic receptors in the MH, both α1- and β-noradrenergic receptor- rather than α2-noradrenergic receptor-signalling in MH are critical for the neuromodulation of panic-like behaviour.

GABAergic and glutamatergic inputs to the medulla oblongata and locus coeruleus noradrenergic pathways are critical for seizures and postictal antinociception neuromodulation

We investigated the participation of the nucleus of the tractus solitarius (NTS) in tonic‒clonic seizures and postictal antinociception control mediated by NMDA receptors, the role of NTS GABAergic interneurons and noradrenergic pathways from the locus coeruleus (LC) in these phenomena. The NTS-lateral nucleus reticularis paragigantocellularis (lPGi)-LC pathway was studied by evaluating neural tract tracer deposits in the lPGi. NMDA and GABAergic receptors agonists and antagonists were microinjected into the NTS, followed by pharmacologically induced seizures. The effects of LC neurotoxic lesions caused by DSP-4, followed by NTS-NMDA receptor activation, on both tonic‒clonic seizures and postictal antinociception were also investigated. The NTS is connected to lPGi neurons that send outputs to the LC. Glutamatergic vesicles were found on dendrites and perikarya of GABAergic interneurons in the NTS. Both tonic‒clonic seizures and postictal antinociception are partially dependent on glutamatergic-mediated neurotransmission in the NTS of seizing rats in addition to the integrity of the noradrenergic system since NMDA receptor blockade in the NTS and intrathecal administration of DSP-4 decrease the postictal antinociception. The GABAA receptor activation in the NTS decreases both seizure severity and postictal antinociception. These findings suggest that glutamatergic inputs to NTS-GABAergic interneurons, in addition to ascending and descending noradrenergic pathways from the LC, are critical for the control of both seizures and postictal antinociception.

Panicolytic-like effects of environment enrichment on male mice threatened by Bothrops jararaca lancehead pit vipers

Environment enrichment (EE) is a well-known eustress model showing beneficial effects in different psychiatric diseases, but its positive properties in panic disorders are not yet established. The confrontation between prey and predator in complex arenas has been validated as a putative panic attack model. The principal aim of this work was to investigate the role of the EE on panic-like defensive responses elicited by mice threatened by venomous snakes. After 6 weeks of exposure either to an enriched or standard environments, 36 male mice were habituated in a complex polygonal arena for snakes containing an artificial burrow and elevated platforms for escape. The animals were confronted by Bothrops jararaca for 5 min, and the following antipredatory responses were recorded: defensive attention, stretched attend posture, flat back approach, prey versus predator interaction, oriented escape behavior, time spent in a safe place, and number of crossings. Mice threatened by snakes displayed several antipredatory reactions as compared to the exploratory behavior of those animals submitted to a nonthreatening situation (toy snake) in the same environment. Notably, EE causes anxiolytic- and panicolytic-like effects significantly decreasing the defensive attention and time spent in safe places and significantly increasing both prey versus predator interaction and exploratory behavior. In conclusion, our data demonstrate that EE can alter the processing of fear modulation regarding both anxiety- and panic-like responses in a dangerous condition, significantly modifying the decision-making defensive strategy.

Acute restraint stress regulates brain DNMT3a and promotes defensive behaviors in male rats

Depending on its duration and severity, stress may contribute to neuropsychiatric diseases such as depression and anxiety. Studies have shown that stress impacts the hypothalamic-pituitary-adrenal (HPA) axis, but its downstream molecular, behavioral, and nociceptive effects remain unclear. We hypothesized that a 2-hour single exposure to acute restraint stress (ARS) activates the HPA axis and changes DNA methylation, a molecular mechanism involved in the machinery of stress regulation. We further hypothesized that ARS induces anxiety-like and risk assessment behavior and alters nociceptive responses in the rat. We employed biochemical (radioimmunoassay for corticosterone; global DNA methylation by enzyme immunoassay and western blot for DNMT3a expression in the amygdala, ventral hippocampus, and prefrontal cortex) and behavioral (elevated plus maze and dark-light box for anxiety and hot plate test for nociception) tests in adult male Wistar rats exposed to ARS or handling (control). All analyses were performed 24 h after ARS or handling. We found that ARS increased corticosterone levels in the blood, increased the expression of DNMT3a in the prefrontal cortex, promoted anxiety-like and risk assessment behaviors in the elevated plus maze, and increased the nociceptive threshold observed in the hot plate test. Our findings suggest that ARS might be a helpful rat model for studying acute stress and its effects on physiology, epigenetic machinery, and behavior.

Unravelling the dorsal periaqueductal grey matter NMDA receptors relevance in the nitric oxide-mediated panic‑like behaviour and defensive antinociception organised by the anterior hypothalamus of male mice

RATIONALE

Previous studies suggested that the dorsal column of the periaqueductal grey matter (dPAG) can be a target of neural pathways from hypothalamic nuclei involved in triggering fear-related defensive responses. In turn, evidence is provided suggesting that microinjection of the nitric oxide (NO) donor SIN-1 into the anterior hypothalamus (AH) of mice evokes panic-like behaviours and fear-induced antinociception. However, it is unknown whether the dPAG of mice mediates these latter defensive responses organised by AH neurons.

OBJECTIVES

This study was designed to examine the role of dPAG in mediating SIN-1-evoked fear-induced defensive behavioural and antinociceptive responses organised in the AH of mice.

METHODS

First, neural tract tracing was performed to characterise the AH-dPAG pathways. Then, using neuropharmacological approaches, we evaluated the effects of dPAG pretreatment with either the non-selective synaptic blocker cobalt chloride (CoCl₂; 1 mM/0.1 μL) or the competitive N-methyl-D-aspartate (NMDA) receptor antagonist LY235959 (0.1 nmol/0.1 μL) on defensive behaviours and antinociception induced by microinjections of SIN-1 in the AH of male C57BL/6 mice.

RESULTS

AlexaFluor488-conjugated dextran-labelled axonal fibres from AH neurons were identified in both dorsomedial and dorsolateral PAG columns. Furthermore, we showed that pre-treatment of the dPAG with either CoCl₂ or LY235959 inhibited freezing and impaired oriented escape and antinociception induced by infusions of SIN-1 into the AH.

CONCLUSIONS

These findings suggest that the panic-like freezing and oriented escape defensive behaviours, and fear-induced antinociception elicited by intra-AH microinjections of SIN-1 depend on the activation of dPAG NMDA receptors.

Lifespan changes in cannabinoid 1 receptor mRNA expression in the female C57BL/6J mouse brain

Many brain functions that underlie behavior, cognition, and emotions vary with age, as does susceptibility to neuropsychological disorders. The expression of specific genes that are involved in these functions, such as the genes encoding for oxytocin, its receptors, and apolipoprotein D, varies with age across different brain regions. The cannabinoid 1 receptor (CB₁ R) is one of the most widely spread G-protein coupled receptors in the central nervous system and is increasingly recognized for its important contribution to various brain functions. Although changes in CB₁ R expression with age have been reported in the male mouse brain, they have not been well investigated in the female brain. Here, we used fluorescence in situ hybridization to target CB₁ R mRNA in the whole brains of female C57BL/6J mice aged 4, 6, 12, 52 (12 months) and 86 weeks (20 months), and quantified CB₁ R-positive cells in 36 brain regions across the whole brain. The results showed that CB₁ R-positive cells number changed with age. Specifically, CB₁ R expression increased with age in some subregions of the cortex, decreased with age in the lateral septal area, and reached its lowest level at 52 weeks in the thalamus, hypothalamus, and hindbrain subregions. Cluster analysis revealed that some brain regions shared similar temporal characteristics in CB₁ R-positive cell number across the lifespan. Our results provide evidence that investigation of the neural basis of age-related characteristics of female brain functions is not only warranted but required.

Prefrontal cortical circuits in anxiety and fear: an overview

Pathological anxiety is among the most difficult neuropsychiatric diseases to treat pharmacologically, and it represents a major societal problem. Studies have implicated structural changes within the prefrontal cortex (PFC) and functional changes in the communication of the PFC with distal brain structures in anxiety disorders. Treatments that affect the activity of the PFC, including cognitive therapies and transcranial magnetic stimulation, reverse anxiety- and fear-associated circuit abnormalities through mechanisms that remain largely unclear. While the subjective experience of a rodent cannot be precisely determined, rodent models hold great promise in dissecting well-conserved circuits. Newly developed genetic and viral tools and optogenetic and chemogenetic techniques have revealed the intricacies of neural circuits underlying anxiety and fear by allowing direct examination of hypotheses drawn from existing psychological concepts. This review focuses on studies that have used these circuit-based approaches to gain a more detailed, more comprehensive, and more integrated view on how the PFC governs anxiety and fear and orchestrates adaptive defensive behaviors to hopefully provide a roadmap for the future development of therapies for pathological anxiety.

Environmental Enrichment Facilitates Anxiety in Conflict-Based Tests but Inhibits Predator Threat-Induced Defensive Behaviour in Male Mice

INTRODUCTION

Environmental enrichment (EE) is a useful and sophisticated tool that improves rodents' well-being by stimulating social behaviour and cognitive, motor, and sensory functions. Exposure to EE induces neuroplasticity in different brain areas, including the limbic system, which has been implicated in the control of anxiety and fear. However, the effects of EE on ethologically relevant naturalistic behaviours, such as those displayed by prey in the presence of predators, remain largely unexplored.

MATERIAL AND METHODS

In the present study, we investigated anxiety- and panic attack-like behaviours in a predator (cat)-prey confrontation paradigm and compared them with those in classical assays, such as the elevated plus-maze (EPM), marble-burying, and open field tests (OFTs), using C57BL/6J male mice housed in enriched or standard environments for 6 weeks.

RESULTS

We observed that EE exposure caused enhancement of the levels of anxiety-like behaviours in the EPM and OFTs, increasing risk assessment (an anxiety-related response), and decreasing escape (a panic attack-like response) behaviours during exposure to the predator versus prey confrontation paradigm.

CONCLUSION

Taken together, our findings suggest that enriched external environments can modify the processing of fear- and anxiety-related stimuli in dangerous situations, changing the decision-making defensive strategy.

Ventrolateral periaqueductal gray matter integrative system of defense and antinociception

Defensive responses are neurophysiological processes crucial for survival during threatening situations. Defensive immobility is a common adaptive response, in rodents, elaborated by ventrolateral periaqueductal gray matter (vlPAG) when threat is unavoidable. It is associated with somatosensory and autonomic reactions such as alteration in the sensation of pain and rate of respiration. In this study, defensive immobility was assessed by chemical stimulation of vlPAG with different doses of NMDA (0.1, 0.3, and 0.6 nmol). After elicitation of defensive immobility, antinociceptive and respiratory response tests were also performed. Results revealed that defensive immobility was followed by a decrease in the nociceptive perception. Furthermore, the lowest dose of NMDA induced antinociceptive response without eliciting defensive immobility. During defensive immobility, respiratory responses were also disturbed. Interestingly, respiratory rate was increased and interspersed with prolonged expiratory phase of breathing. These findings suggest that vlPAG integrates three different defensive behavioral responses, contributing to the most effective defensive strategies during threatening situations.

The regulatory role of nitric oxide in morphine-induced analgesia in the descending path of pain from the dorsal hippocampus to the dorsolateral periaqueductal gray

BACKGROUND

Nitric oxide (NO) levels in brain nuclei, such as the hippocampus and brainstem, are involved in morphine analgesia, but the relationship between the dorsal hippocampus (dH) and the dorsolateral periaqueductal gray matter (dlPAG) needs to be clarified, which is our goal.

METHODS

Wistar rats were simultaneously equipped with a stereotaxic device with unilateral guide cannula at dH and dlPAG. After recovery, they were divided into control and experimental groups. Formalin (50 μL of 2.5%) was inoculated into the left hind paw of rat. Morphine (6 mg/kg) was administered intraperitoneally (i.p.) 10 min before formalin injection. L-Arginine (0.25, 0.5, 1 and 2 μg/rat), and L-NAME (0.25, 0.5, 1 and 2 μg/rat), unrelatedly or with the respect in the order of injection were used in the nuclei before morphine injection (i.p.). Activation of the neuronal NO synthase (nNOS) in the brains of all animals was measured using NADPH-diaphorase, a selective biochemical marker of nNOS.

RESULTS

Morphine reduced inflammatory pain in the early and late stages of the rat formalin test. The morphine response was attenuated by before injection of single L-arginine but not L-NAME in the two target areas. However, the acute phase result was stopped due to L-NAME pretreatment. When L-NAME was injected into dlPAG before injecting L-arginine at dH, the morphine response did not decrease at all, indicating a modulatory role of NO in dlPAG, which was confirmed by NADPH-d staining.

CONCLUSIONS

High levels of NO in dlPAG may regulate pain process in downward synaptic interactions.