Dorsal periaqueductal gray post-stimulation freezing is counteracted by neurokinin-1 receptor antagonism in the central nucleus of the amygdala in rats

Defense or death? A review of the neural mechanisms underlying sensory modality-triggered innate defensive behaviors

Defense or death presents a canonical dilemma for animals when encountering predators. Threatening sensory cues provide essential information that signals predator presence, driving the evolution of a spectrum of defensive behaviors. In rodents, these behaviors, as described by the classic "predatory imminence continuum" model, range from risk assessment and freezing to rapid escape responses. During the pre-encounter phase, risk assessment and avoidance responses are crucial for monitoring the environment with vigilance and cautiousness. Once detected during the post-encounter phase or physically attacked during the circa-strike phase, multiple sensory systems are rapidly activated, triggering escape responses to increase the distance from the threat. Although there are species-specific variations, the brain regions underpinning these defensive strategies, including the thalamus, hypothalamus, and midbrain, are evolutionarily conserved. This review aims to provide a comprehensive overview of the universal innate defensive circuit framework to enrich our understanding of how animals respond to life-threatening situations.

D2-like receptor activation by intranasal dopamine attenuates fear responses induced by electrical stimulation of the dorsal periaqueductal grey matter, but fails to reduce aversion to pit vipers and T-maze performance

BACKGROUND

Panic-like reactions elicited by electrical stimulation of the dorsal periaqueductal grey matter (ES-dPAG) seem to be regulated by dopamine (DA). We showed that DA applied intranasally (IN) increased escape-behaviour thresholds induced by ES-dPAG of rats, indicating a panicolytic-like effect.

AIMS

We investigated whether IN-DA increases escape-response thresholds induced by ES-dPAG by acting on D₂-like receptors, and whether IN-DA affects escape responses elicited by the presence of a potential predator and by open space and height of the elevated T-maze (ETM) as well as motor performance in the open field (OF) test.

METHODS

Wistar rats exposed to ES-dPAG were treated with Sulpiride (SUL, 40 mg/kg, D₂-like receptor antagonist) previously IN-DA (2 mg/kg). Independent groups of rats treated with IN-DA were submitted to prey versus snake paradigm (PSP), ETM and OF.

RESULTS

Anti-aversive effects of the IN-DA were reduced by SUL pretreatment in the ES-dPAG test. IN-DA did not affect the escape number in the PSP nor the escape latencies in the ETM as well as motor performance in the OF.

CONCLUSIONS/INTERPRETATION

The IN-DA effects in reducing unconditioned fear responses elicited by ES-dPAG seem to be mediated by D₂-like receptors. The lack of effects on panic-related responses in the ETM and PSP may be related to the possibility of avoiding the danger inherent to these models, a defence strategy not available during ES-dPAG. These findings cannot be attributed to motor performance. The decision-making responses to avoid dangerous situations can be orchestrated by supra-mesencephalic structures connected by non-dopaminergic inputs.

Dorsal periaqueductal gray ensembles represent approach and avoidance states

Animals must balance needs to approach threats for risk assessment and to avoid danger. The dorsal periaqueductal gray (dPAG) controls defensive behaviors, but it is unknown how it represents states associated with threat approach and avoidance. We identified a dPAG threatavoidance ensemble in mice that showed higher activity farther from threats such as the open arms of the elevated plus maze and a predator. These cells were also more active during threat avoidance behaviors such as escape and freezing, even though these behaviors have antagonistic motor output. Conversely, the threat approach ensemble was more active during risk assessment behaviors and near threats. Furthermore, unsupervised methods showed that avoidance/approach states were encoded with shared activity patterns across threats. Lastly, the relative number of cells in each ensemble predicted threat avoidance across mice. Thus, dPAG ensembles dynamically encode threat approach and avoidance states, providing a flexible mechanism to balance risk assessment and danger avoidance.

Intranasal dopamine attenuates fear responses induced by electric shock to the foot and by electrical stimulation of the dorsal periaqueductal gray matter

PURPOSE

Intranasally applied dopamine (IN-DA), which likely reaches the brain via nasal-brain pathways and bypasses the blood-brain barrier, has been found to increase extracellular DA and bind to the DA2 transporter in the striatum. Recent studies suggest that DA plays a significant role in the processing of signaled and unconditioned aversive stimulation, including evidence that may attenuate responses to painful input. The purpose of this study was to examine the effects of IN-DA on fear-related behaviors induced by electric shock to the foot or by electrical stimulation of the dorsal periaqueductal gray matter (dPAG).

METHODS

DA hydrochloride suspended in a viscous castor oil gel (1 or 2 mg/kg) was applied (IN-DA) in a volume of 5 μL into the nostrils of adult Wistar male rats in order to evaluate its effects on (a) freezing induced by electric shock to the foot and (b) thresholds of freezing and escape and duration of post-stimulation freezing induced by electrical stimulation of the dPAG.

RESULTS

IN-DA attenuated freezing induced by electric shock to the foot in the three test trials, indicating that it reduced long-term fear responses. IN-DA also increased the threshold of dPAG stimulation-induced escape responses and reduced post-stimulation freezing.

CONCLUSIONS

IN-DA, which has previously been shown to facilitate learning and to have antidepressive-like effects, attenuated unconditioned fear responses elicited by peripheral and intramesencephalic (dPAG) stimulation and reduced long-term conditioned fear responses.

Role of the dorsal periaqueductal gray in posttraumatic stress disorder: mediation by dopamine and neurokinin

In susceptible individuals, exposure to intensely traumatic life events can lead to the development of posttraumatic stress disorder (PTSD), including long-term dysregulation of the contextual processing of aversive stimuli, the overgeneralization of learned fear, and impairments in the ability to learn or respond to safety signals. The neuropathophysiological changes that underlie PTSD remain incompletely understood. Attention has focused on forebrain structures associated with fear processing. Here we consider evidence from human and animal studies that long-lasting changes in functional connectivity between the midbrain periaqueductal gray (dPAG) and amygdala may be one of the precipitating events that contribute to PTSD. Long-lasting neuroplastic changes in the dPAG can persist after a single aversive stimulation and are pharmacologically labile. The early stage (at least up to 24 h post-stimulation) involves neurokinin-1 receptor-mediated events in the PAG and amygdala and is also regulated by dopamine, both of which are mainly involved in transferring ascending aversive information from the dPAG to higher brain structures, mainly the amygdala. Changes in the functional connectivity within the dPAG-amygdala circuit have been reported in PTSD patients. We suggest that further investigations of plasticity and pharmacology of the PAG-amygdala network provide a promising target for understanding pathophysiological circuitry that underlies PTSD in humans and that dopaminergic and neurokininergic drugs may have a potential for the treatment of psychiatric disorders that are associated with a dysfunctional dPAG.

Blocking constitutive activity of GHSR1a in the lateral amygdala facilitates acquisition of conditioned taste aversion

Ghrelin is a circulating peptide hormone promoting feeding and regulating energy metabolism in human and rodents. Ghrelin functions by binding to its receptor, the growth hormone secretagogue receptor 1a (GHSR1a), which are widely distributed throughout the brain including the amygdala, a brain region important for regulating valenced behavior, such as aversion. Interestingly, GHSR1a was once characterized by highly constitutive, ligand-independent activity. However, the physiological importance of such ligand-independent signaling on aversive memory processing has not been tested yet. Here, we applied [D-Arg¹, D-Phe⁵, D-Trp^7,9, Leu¹¹]-Substance P (D-SP), a full inverse agonist for GHSR1a, into the lateral amygdala (LA) and investigated the effect of blocking GHSR1a constitutive activity on conditioned taste aversion (CTA) in rats. We found that intra-LA infusion of a single low dose of D-SP (8ng/0.5μl/side) facilitates CTA acquisition. Moreover, pre-administration of a high dose of D-SP into the LA abolishes the suppressive effect of exogenous ghrelin on CTA acquisition. In contrast, pre-administration of the same dose of D-SP does not affect the suppression of substance P, a potent neurokinin-1 (NK1) receptor ligand, on CTA. Therefore, our data indicated that the spontaneous or basal activity of GHSR1a signaling in the LA might interfere with CTA memory formation. D-SP decreases the constitutive activity of GHSR1a and thus facilitates CTA. Altogether, our present findings along with previous results support the idea that ghrelin/GHSR1a signaling in the LA circuit blocks conditioned taste aversion.

Behavioral sensitization induced by dorsal periaqueductal gray electrical stimulation is counteracted by NK1 receptor antagonism in the ventral hippocampus and central nucleus of the amygdala

A single threatening experience may change the behavior of an animal in a long-lasting way and elicit generalized behavioral responses to a novel threatening situation that is unrelated to the original aversive experience. Electrical stimulation (ES) of the dorsal periaqueductal gray (dPAG) produces a range of defensive reactions, characterized by freezing, escape, and post-stimulation freezing (PSF). The latter reflects the processing of ascending aversive information to prosencephalic structures, including the central nucleus of the amygdala (CeA), which allows the animal to evaluate the consequences of the aversive situation. This process is modulated by substance P (SP) and its preferred receptor, neurokinin 1 (NK1). The ventral hippocampus (VH) has been associated with the processing of aversive information and expression of emotional reactions with negative valence, but the participation of the VH in the expression of these defensive responses has not been investigated. The VH is rich in NK1 receptor expression and has a high density of SP-containing fibers. The present study examined the role of NK1 receptors in the VH in the expression of defensive responses and behavioral sensitization that were induced by dPAG-ES. Rats were implanted with an electrode in the dPAG for ES, and a cannula was implanted in the VH or CeA for injections of vehicle (phosphate-buffered saline) or the NK1 receptor antagonist spantide (100 pmol/0.2 μL. Spantide reduced the duration of PSF that was evoked by dPAG-ES, without changing the aversive freezing or escape thresholds. One and 7 days later, exploratory behavior was evaluated in independent groups of rats in the elevated plus maze (EPM). dPAG-ES in rats that received vehicle caused higher aversion to the open arms of the EPM compared with rats that did not receive dPAG stimulation at both time intervals. Injections of spantide in the VH or CeA prevented the proaversive effects of dPAG-ES in the EPM only 1 day later. These findings suggest that NK1 receptors are activated in both the VH and CeA during the processing of aversive information that derives from dPAG-ES. As previously shown for the CeA, SP/NK1 receptors in the VH are recruited during PSF that is evoked by dPAG-ES, suggesting that a 24-h time window is susceptible to interventions with NK1 antagonists that block the passage of aversive information from the dPAG to higher brain areas.

Kindling epileptogenesis and panic-like behavior: Their bidirectional connection and contribution to epilepsy-associated depression

Anxiety is one of the most common comorbidities of epilepsy, which has major detrimental effects on the quality of life. Generalized anxiety disorder (GAD) associated with epilepsy has been receiving most attention. However, several other forms of anxiety reportedly present in patients with epilepsy, including panic disorder (PD). In this study, using an animal model of limbic epilepsy, we examined the interplay between epilepsy and panic-like behavior (PLB). Further, considering the high degree of comorbidity between depression on the one hand, and both epilepsy and PD on the other hand, we studied whether and how the presence of PLB in animals with epilepsy would affect their performance in depression-relevant tests. Fifty-day-old male Wistar rats were subjected to repeated alternating electrical stimulations of the basolateral amygdala (BLA) to induce kindling of limbic seizures, and the dorsal periaqueductal gray (DPAG) to induce panic-like episodes. Seizure susceptibility and panic reaction threshold were examined before the first and 24h after the last stimulation. At the end of the stimulations, the rats were examined in depression-relevant tests: saccharin preference test (SPT) for anhedonia and forced swimming test (FST) for despair/hopelessness. With regard to kindling, BLA+DPAG stimulation induced more profound increase of seizure susceptibility than BLA stimulation alone (evident as the reduction of the afterdischarge threshold and the increase of the afterdischarge duration). With regard to PLB, the BLA+DPAG stimulation exacerbated the severity of panic-like episodes, as compared with the DPAG stimulation alone. Basolateral amygdala stimulation alone had no effects on panic-like reactions, and DPAG stimulation alone did not modify kindling epileptogenesis. Combined stimulation of BLA and DPAG induced depressive-like behavioral impairments. This is the first experimental study showing bidirectional, mutually exacerbating effect of epilepsy and PLB, and the precipitation of depressive-like state by the epilepsy-PLB comorbidity.

Periaqueductal Gray Neuronal Activities Underlie Different Aspects of Defensive Behaviors

Defense is a basic survival mechanism when animals face danger. Previous studies have suggested that the midbrain periaqueductal gray (PAG) is essential for the generation of defensive reactions. Here we showed that optogenetic activation of neurons in the PAG in mice was sufficient to induce a series of defensive responses (including running, freezing, and avoidance). However, the endogenous neural dynamics of the PAG underlying defensive behaviors still remain elusive. Using chronic extracellular recording, we recorded the spiking activities of PAG neurons in freely behaving mice exposed to natural threats (rats). We observed that there exist distinct neuronal subsets within the PAG participating in respective detection (risk assessment) and response (flight) aspects of defensive behaviors. Our results demonstrate the important role of PAG neuronal activities in the control of different aspects of defensive behaviors, and provide novel insights for investigating defense from an electrophysiological perspective.

SIGNIFICANCE STATEMENT

Defense is crucial for animals' survival in nature. Here, using optogenetic stimulation and in vivo recording in behaving mice reacting to threats, we explored the role of the midbrain periaqueductal gray (PAG) in defense. We show that optogenetic activation of PAG neurons is sufficient to elicit different aspects of defensive responses. Consistently, the present study provides in vivo evidence demonstrating that activity of the population of dorsal PAG neurons is activated during defense. Also, different subpopulations of units recorded in the dorsal PAG participate in distinct aspects of defensive behaviors. These findings help us understand the role of the PAG in animal behavior at the single neuron level.

Inhibition of substance P-induced defensive behavior via neurokinin-1 receptor antagonism in the central and medial but not basolateral nuclei of the amygdala in male Wistar rats

RATIONALE

The production of unconditioned defensive behaviors has been related to the amygdala, a key component of the encephalic aversion system. Microinjection of the neuropeptide substance P (SP) in the amygdala elicits defensive behaviors via the activation of type 1 neurokinin (NK-1) receptors. However, no studies have investigated whether intra-amygdala SP/NK-1 mechanisms can elicit other types of defensive responses, such as antinociception and ultrasonic vocalizations (USVs).

METHODS

The present study investigated the effects of SP-induced activation of the neurokininergic system in three main nuclei of the amygdala-basolateral (BLA), central (CeA), and medial (MeA) nuclei-in rats that were subjected to the elevated plus maze (EPM), tail-flick test, and USV recording. The effects of SP in these amygdaloid nuclei were challenged with combined injections of the NK-1 receptor antagonist spantide.

RESULTS

The present study showed that SP injections in the CeA and MeA but not BLA exerted anxiogenic-like effects. In contrast to the CeA, the anxiogenic-like effects of SP in the MeA were not dependent on NK-1 mechanisms. In the tail-flick test, SP microinjections produced antinociceptive effects only in the MeA through NK-1 receptor activation. No USV emissions were detected after the SP microinjections.

CONCLUSIONS

The present study showed that NK-1 receptors in the CeA and MeA but not BLA are involved in defensive reactions to conditions of fear. The present results may provide a better understanding of the neurochemical mediation of fear states.

Parasubthalamic and calbindin nuclei in the posterior lateral hypothalamus are the major hypothalamic targets for projections from the central and anterior basomedial nuclei of the amygdala

The parasubthalamic nucleus (PSTN) and the ventrally adjacent calbindin nucleus (CbN) form a nuclear complex in the posterior lateral hypothalamic area (LHA), recently characterized as connected with the central nucleus of the amygdala (CEA). The aim of the present work is to analyze in detail the projections from the amygdala into the PSTN/CbN, also focusing on pathways into the LHA. After fluorogold injections into the PSTN/CbN, the medial part of the CEA (CEAm) appears to be the main supplier of projections from the CEA. Other amygdalar nuclei contribute to the innervation of the PSTN/CbN complex, including the anterior part of the basomedial nucleus (BMAa). Injections of the anterograde tracer, Phaseolus vulgaris leucoagglutinin (PHAL), into the CEAm and BMAa revealed that projections from the CEAm follow two pathways into the LHA: a dorsal pathway formed by axons that also innervate the paraventricular hypothalamic nucleus, the anterior perifornical LHA and the PSTN, and a ventral pathway that runs laterally adjacent to the ventrolateral hypothalamic tract (vlt) and ends in the CbN. By contrast, the BMAa and other telencephalic structures, such as the fundus striatum project to the CbN via the ventral pathway. Confirming the microscopic observation, a semi-quantitative analysis of the density of these projections showed that the PSTN and the CbN are the major hypothalamic targets for the projections from the CEAm and the BMAa, respectively. PSTN and CbN receive these projections through distinct dorsal and ventral routes in the LHA. The ventral pathway forms a differentiated tract, named here the ventrolateral amygdalo-hypothalamic tract (vlah), that is distinct from, but runs adjacent to, the vlt. Both the vlt and the vlah had been previously described as forming an olfactory path into the LHA. These results help to better characterize the CbN within the PSTN/CbN complex and are discussed in terms of the functional organization of the network involving the PSTN and the CbN as well as the CEA and the BMAa.