Distinct circuits in anterior cingulate cortex encode safety assessment and mediate flexibility of fear reactions

Synchronicity of pyramidal neurones in the neocortex dominates isoflurane-induced burst suppression in mice

BACKGROUND

Anaesthesia-induced burst suppression signifies profound cerebral inactivation. Although considerable efforts have been directed towards elucidating the electroencephalographic manifestation of burst suppression, the neuronal underpinnings that give rise to isoflurane-induced burst suppression are unclear.

METHODS

Electroencephalography combined with micro-endoscopic calcium imaging was used to investigate the neural mechanisms of isoflurane-induced burst suppression. Synchronous activities of pyramidal neurones in the auditory cortex and medial prefrontal cortex and inhibitory neurones in the auditory cortex (including parvalbumin [PV], somatostatin [SST], and vasoactive intestinal peptide [Vip]) and subcortical regions (including the medial geniculate body, locus coeruleus, and thalamic reticular nucleus) were recorded during isoflurane anaesthesia. In addition, the effects of chemogenetic manipulation inhibitory neurones in the auditory cortex on isoflurane-induced burst suppression were studied.

RESULTS

Isoflurane-induced burst suppression was highly correlated with the synchronous activities of excitatory neurones in the cortex (∼65% positively and ∼20% negatively correlated neurones). Conversely, a minimal or absent correlation was observed with the neuronal synchrony of inhibitory interneurones and with neuronal activities within subcortical areas. Only activation or inhibition of PV neurones, but not SST or Vip neurones, decreased (P<0.0001) or increased (P<0.0001) isoflurane-induced neuronal synchrony.

CONCLUSIONS

Isoflurane-induced burst suppression might be primarily driven by the synchronous activities of excitatory pyramidal neurones in the cortex, which could be bidirectionally regulated by manipulating the activity of inhibitory PV interneurones. Our findings provide new insights into the neuronal mechanisms underlying burst suppression.

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.

Overwriting an instinct: Visual cortex instructs learning to suppress fear responses

Fast instinctive responses to environmental stimuli can be crucial for survival but are not always optimal. Animals can adapt their behavior and suppress instinctive reactions, but the neural pathways mediating such ethologically relevant forms of learning remain unclear. We found that posterolateral higher visual areas (plHVAs) are crucial for learning to suppress escapes from innate visual threats through a top-down pathway to the ventrolateral geniculate nucleus (vLGN). plHVAs are no longer necessary after learning; instead, the learned behavior relies on plasticity within vLGN populations that exert inhibitory control over escape responses. vLGN neurons receiving input from plHVAs enhance their responses to visual threat stimuli during learning through endocannabinoid-mediated long-term suppression of their inhibitory inputs. We thus reveal the detailed circuit, cellular, and synaptic mechanisms underlying experience-dependent suppression of fear responses.

Anterior cingulate cortex in complex associative learning: monitoring action state and action content

Environmental changes necessitate adaptive responses, and thus the ability to monitor one's actions and their connection to specific cues and outcomes is crucial for survival. The anterior cingulate cortex (ACC) is implicated in these processes, yet its precise role in action monitoring and outcome evaluation remains unclear. To investigate this, we developed a novel discrimination-avoidance task for mice, designed with clear temporal separation between actions and outcomes. Our findings show that ACC neurons primarily encode post-action variables over extended periods, reflecting the animal's preceding actions rather than the outcomes or values of those actions. Specifically, we identified two distinct subpopulations of ACC neurons: one encoding the action state (whether an action was taken) and the other encoding the action content (which action was taken). Importantly, increased post-action ACC activity was associated with better performance in subsequent trials. These findings suggest that the ACC supports complex associative learning through extended signaling of rich action-relevant information, thereby bridging cue, action, and outcome associations.

A distributed subcortical circuit linked to instrumental information-seeking about threat

Daily life for humans and other animals requires switching between periods of threat- and reward-oriented behavior. We investigated neural activity associated with spontaneous switching, in a naturalistic task, between foraging for rewards and seeking information about potential threats with 7T fMRI in healthy humans. Switching was driven by estimates of likelihood of threat and reward. Both tracking of threat and switching to a vigilant mode in which people sought more information about potential threats were associated with specific but distributed patterns of activity spanning habenula, dorsal raphe nucleus (DRN), anterior cingulate cortex, and anterior insula cortex. Different aspects of the distributed activity patterns were linked to monitoring the threat level, seeking information about the threat, and actual threat detection. A distinct pattern of activity in the same circuit and elsewhere occurred during returns to reward-oriented behavior. Individual variation in DRN activity reflected individual variation in the seeking of information about threats.

Functional segregation and dynamic integration of the corticotectal descending signal in rat

The superior colliculus (SC) receives inputs from various brain regions in a layer- and radial subregion-specific manner, but whether the SC exhibits subregion-specific dynamics remains unclear. To address this issue, we recorded the spiking activity of single SC neurons while photoactivating cortical areas in awake head-fixed Thy1-ChR2 rats. We classified 309 neurons that responded significantly into 8 clusters according to the response dynamics. Among them, neurons with monophasic excitatory responses (7-12 ms latency) that returned to baseline within 20 ms were commonly observed in the optic and intermediate gray layers of centromedial and centrolateral SC. In contrast, neurons with complex polyphasic responses were commonly observed in the deep layers of the anterolateral SC. Cross-correlation analysis suggested that the complex pattern could be only partly explained by an internal circuit of the deep gray layer. Our results indicate that medial to centrolateral SC neurons simply relay cortical activity, whereas neurons in the deep layers of the anterolateral SC dynamically integrate inputs from the cortex, SNr, CN, and local circuits. These findings suggest a spatial gradient in SC integration, with a division of labor between simple relay circuits and those integrating complex dynamics.

The superior colliculus: New insights into an evolutionarily ancient structure

The superior colliculus is a structure located in the dorsal midbrain with well conserved function and connectivity across species. Essential for survival, the superior colliculus has evolved to trigger rapid orientation and avoidance movements in response to external stimuli. The increasing recognition of the widespread connectivity of the superior colliculus, not only with brainstem and spinal cord, but also with virtually all brain structures, has rekindled the interest on this structure and revealed novel roles in the past few years. In this review, we focus on the most recent advancements in understanding its cellular composition, connectivity and function, with a particular focus on how the cellular diversity and connectivity arises during development, as well as on its recent role in the emergence of sensory circuits.

The zona incerta system: Involvement in Parkinson's disease

Parkinson's disease (PD) is characterized by degeneration of the nigrostriatal dopamine system, resulting in progressive motor and nonmotor symptoms. Although most studies have focused on the basal ganglia network, recent evidence suggests that the zona incerta (ZI), a subthalamic structure composed of 4 neurochemically defined regions, is emerging as a therapeutic target in PD. This review summarizes the clinical and animal studies that indicate the importance of ZI in PD. Human clinical studies have shown that subthalamotomy or deep brain stimulation (DBS) of the ZI alleviates muscle rigidity, bradykinesia, tremors and speech dysfunction in patients with PD. Researchers have also studied the impact of DBS of the ZI on nonmotor signs such as pain, anxiety, and depression. Animal studies combining optogenetics, chemogenetics, behavioral assays, and neural activity recordings reveal the functional roles of ZI GABAergic and glutamatergic neurons in locomotion, gait, and coordination of the symptoms of PD, all of which are discussed in this review. Controversies and possible future studies are also discussed.

Identification of GABAergic subpopulations in the lateral hypothalamus for home-driven behaviors in mice

Home information profoundly influences behavioral states in both humans and animals. However, how "home" is represented in the brain and its role in driving diverse related behaviors remain elusive. Here, we demonstrate that home bedding contains sufficient home information to modulate affective behaviors, including aversion responses, defensive aggression, and mating behaviors. These varied responses to home information are mediated by gama-aminobutyric acid (GABA)ergic neurons in the lateral hypothalamus (LHGABA). Inhibiting LHGABA abolishes, while activating mimics, the effects of home bedding on these behaviors across different contexts. Specifically, projections from LHGABA to the ventral tegmental area (VTA) mediate the relaxation of aversive emotion, while projections to the periaqueductal gray (PAG) initiate defensive concerns. Thus, our data suggest that home information in different contexts converges to activate distinct subgroups of the LHGABA, which, in turn, elicit appropriate affective behaviors in relieving aversion, fighting intruders, or enhancing mating through involving distinct downstream projections.

Empathic pain: Exploring the multidimensional impacts of biological and social aspects in pain

Empathic pain refers to an individual's perception, judgment, and emotional response to others' pain. This complex social cognitive ability is crucial for healthy interactions in human society. In recent years, with the development of multidisciplinary research in neuroscience, psychology and sociology, empathic pain has become a focal point of widespread attention in these fields. However, the neural mechanism underlying empathic pain remain a controversial and unresolved area. This review aims to comprehensively summarize the history, influencing factors, neural mechanisms and pharmacological interventions of empathic pain. We hope to provide a comprehensive scientific perspective on how humans perceive and respond to others' pain experiences and to provide guidance for future research directions and clinical applications. This article is part of the Special Issue on "Empathic Pain".

Targeting fear memories: Examining pharmacological disruption in a generalized fear framework

Labilization-reconsolidation, which relies on retrieval, has been considered an opportunity to attenuate the negative aspects of traumatic memories. A therapeutic strategy based on reconsolidation blockade is deemed more effective than current therapies relying on memory extinction. Nevertheless, extremely stressful memories frequently prove resistant to this process. Here, after inducing robust fear memory in mice through strong fear conditioning, we examined the possibility of rendering it susceptible to pharmacological modulation based on the degree of generalized fear (GF). To achieve this, we established an ordered gradient of GF, determined by the perceptual similarity between the associated context (CA) and non-associated contexts (CB, CC, CD, and CE) to the aversive event. We observed that as the exposure context became less similar to CA, the defensive pattern shifted from passive to active behaviors in both male and female mice. Subsequently, in conditioned animals, we administered propranolol after exposure to the different contexts (CA, CB, CC, CD or CE). In males, propranolol treatment resulted in reduced freezing time and enhanced risk assessment behaviors when administered following exposure to CA or CB, but not after CC, CD, or CE, compared to the control group. In females, a similar change in behavioral pattern was observed with propranolol administered after exposure to CC, but not after the other contexts. These results highlight the possibility of indirectly manipulating a robust contextual fear memory by controlling the level of generalization during recall. Additionally, it was demonstrated that the effect of propranolol on reconsolidation would not lead to a reduction in fear memory per se, but rather to its reorganization resulting in greater behavioral flexibility (from passive to active behaviors). Finally, from a clinical viewpoint, this would be of considerable relevance since following this strategy could make the treatment of psychiatric disorders associated with traumatic memory formation more effective and less stressful.

Effective Regulation of Auditory Processing by Parvalbumin Interneurons in the Tail of the Striatum

Parvalbumin (PV) interneurons in the auditory cortex (AC) play a crucial role in shaping auditory processing, including receptive field formation, temporal precision enhancement, and gain regulation. PV interneurons are also the primary inhibitory neurons in the tail of the striatum (TS), which is one of the major descending brain regions in the auditory nervous system. However, the specific roles of TS-PV interneurons in auditory processing remain elusive. In this study, morphological and slice recording experiments in both male and female mice revealed that TS-PV interneurons, compared with AC-PV interneurons, were present in fewer numbers but exhibited longer projection distances, which enabled them to provide sufficient inhibitory inputs to spiny projection neurons (SPNs). Furthermore, TS-PV interneurons received dense auditory input from both the AC and medial geniculate body (MGB), particularly from the MGB, which rendered their auditory responses comparable to those of AC-PV interneurons. Optogenetic manipulation experiments demonstrated that TS-PV interneurons were capable of bidirectionally regulating the auditory responses of SPNs. Our findings suggest that PV interneurons can effectively modulate auditory processing in the TS and may play a critical role in auditory-related behaviors.

Disentangling the identity of the zona incerta: a review of the known connections and latest implications

The zona incerta (ZI) is a subthalamic region composed by loosely packed neurochemically mixed neurons, juxtaposed to the main ascending and descending bundles. The extreme neurochemical diversity that characterizes this area, together with the diffuseness of its connections with the entire neuraxis and its hard-to-reach positioning in the brain caused the ZI to keep its halo of mystery for over a century. However, in the last decades, a rich albeit fragmentary body of knowledge regarding both the incertal anatomical connections and functional implications has been built mostly based on rodent studies and its lack of cohesion makes difficult to depict an integrated, exhaustive picture regarding the ZI and its roles. This review aims to provide a unified resource that summarizes the current knowledge regarding the anatomical profile of interactions of the ZI in rodents and non-human primates and the functional significance of its connections, highlighting the aspects still unbeknown to research.