Sensitivity to pain and c-Fos expression in brain structures in rats

Dynamics of Lateral Habenula-Ventral Tegmental Area Microcircuit on Pain-Related Cognitive Dysfunctions

Chronic pain is a health problem that affects the ability to work and perform other activities, and it generally worsens over time. Understanding the complex pain interaction with brain circuits could help predict which patients are at risk of developing central dysfunctions. Increasing evidence from preclinical and clinical studies suggests that aberrant activity of the lateral habenula (LHb) is associated with depressive symptoms characterized by excessive negative focus, leading to high-level cognitive dysfunctions. The primary output region of the LHb is the ventral tegmental area (VTA), through a bidirectional connection. Recently, there has been growing interest in the complex interactions between the LHb and VTA, particularly regarding their crucial roles in behavior regulation and their potential involvement in the pathological impact of chronic pain on cognitive functions. In this review, we briefly discuss the structural and functional roles of the LHb-VTA microcircuit and their impact on cognition and mood disorders in order to support future studies addressing brain plasticity during chronic pain conditions.

N-Methyl D-aspartate receptor subtype 2B/Ca2+/calmodulin-dependent protein kinase II signaling in the lateral habenula regulates orofacial allodynia and anxiety-like behaviors in a mouse model of trigeminal neuralgia

Trigeminal neuralgia (TN) is a peripheral nerve disorder often accompanied by abnormalities in mood. The lateral habenula (LHb) plays important roles in the modulation of pain and emotion. In the present study, we investigated the involvement of the LHb in the mechanisms underlying allodynia and anxiety induced by partial transection of the infraorbital nerve (pT-ION) in mice. Our results indicated that pT-ION induced persistent orofacial allodynia and anxiety-like behaviors, which were correlated with increased phosphorylation of N-Methyl D-aspartate receptor (NMDAR) subtype 2B (p-NR2B) and Ca²⁺/calmodulin-dependent protein kinase II (p-CaMKII) in LHb neurons. Bilateral inhibition of NMDARs and CaMKII in the LHb attenuated the allodynia and anxiety-like behavior induced by pT-ION. Furthermore, bilateral activation of NMDARs in the LHb increased the expression of p-NR2B and p-CaMKII and induced orofacial allodynia and anxiety-like behaviors in naive mice. Adeno-associated virus (AAV)-mediated expression of hM3D(Gq) in CaMKII⁺ neurons of the bilateral LHb, followed by clozapine-N-oxide (CNO) administration, also triggered orofacial allodynia and anxiety-like behaviors in naïve mice with successful virus infection in LHb neurons (verified based on immunofluorescence). In conclusion, these findings suggest that activation of NMDA/CaMKII signaling in the LHb contributes to the occurrence and development of TN and related anxiety-like behaviors. Therefore, suppressing the activity of CaMKII⁺ neurons in the bilateral LHb by targeting NMDA/CaMKII may represent a novel strategy for treating pain and anxiety associated with TN.

Lateral Habenula and Its Potential Roles in Pain and Related Behaviors

The lateral habenula (LHb) is a tiny structure that acts as a hub, relaying signals from the limbic forebrain structures and basal ganglia to the brainstem modulatory area. Facilitated by updated knowledge and more precise manipulation of circuits, the progress in figuring out the neural circuits and functions of the LHb has increased dramatically over the past decade. Importantly, LHb is found to play an integrative role and has profound effects on a variety of behaviors associated with pain, including depression-like and anxiety-like behaviors, antireward or aversion, aggression, defensive behavior, and substance use disorder. Thus, LHb is a potential target for improving pain management and related disorders. In this review, we focused on the functions, related circuits, and neurotransmissions of the LHb in pain processing and related behaviors. A comprehensive understanding of the relationship between the LHb and pain will help to find new pain treatments.

The lateral habenula is critically involved in histamine-induced itch sensation

Lateral habenula (LHb) is a brain region acting as a hub mediating aversive response against noxious, stressful stimuli. Growing evidences indicated that LHb modulates aminergic activities to induce avoidance behavior against nociceptive stimuli. Given overlapped neural circuitry transmitting pain and itch information, it is likely that LHb have a role in processing itch information. Here, we examined whether LHb is involved in itchy response induced by histamine. We found that histamine injection enhances Fos (+) cells in posterior portion within parvocellular and central subnuclei of the medial division (LHbM) of the LHb. Moreover, chemogenetic suppression of LHbM reduced scratching behavior induced by histamine injection. These results suggest that LHb is required for processing itch information to induce histaminergic itchy response.

Tacr3 in the lateral habenula differentially regulates orofacial allodynia and anxiety-like behaviors in a mouse model of trigeminal neuralgia

Trigeminal neuralgia (TN) is debilitating and is usually accompanied by mood disorders. The lateral habenula (LHb) is considered to be involved in the modulation of pain and mood disorders, and the present study aimed to determine if and how the LHb participates in the development of pain and anxiety in TN. To address this issue, a mouse model of partial transection of the infraorbital nerve (pT-ION) was established. pT-ION induced stable and long-lasting primary and secondary orofacial allodynia and anxiety-like behaviors that correlated with the increased excitability of LHb neurons. Adeno-associated virus (AAV)-mediated expression of hM4D(Gi) in glutamatergic neurons of the unilateral LHb followed by clozapine-N-oxide application relieved pT-ION-induced anxiety-like behaviors but not allodynia. Immunofluorescence validated the successful infection of AAV in the LHb, and microarray analysis showed changes in gene expression in the LHb of mice showing allodynia and anxiety-like behaviors after pT-ION. Among these differentially expressed genes was Tacr3, the downregulation of which was validated by RT-qPCR. Rescuing the downregulation of Tacr3 by AAV-mediated Tacr3 overexpression in the unilateral LHb significantly reversed pT-ION-induced anxiety-like behaviors but not allodynia. Whole-cell patch clamp recording showed that Tacr3 overexpression suppressed nerve injury-induced hyperexcitation of LHb neurons, and western blotting showed that the pT-ION-induced upregulation of p-CaMKII was reversed by AAV-mediated Tacr3 overexpression or chemicogenetic inhibition of glutamatergic neurons in the LHb. Moreover, not only anxiety-like behaviors, but also allodynia after pT-ION were significantly alleviated by chemicogenetic inhibition of bilateral LHb neurons or by bilateral Tacr3 overexpression in the LHb. In conclusion, Tacr3 in the LHb plays a protective role in treating trigeminal nerve injury-induced allodynia and anxiety-like behaviors by suppressing the hyperexcitability of LHb neurons. These findings provide a rationale for suppressing unilateral or bilateral LHb activity by targeting Tacr3 in treating the anxiety and pain associated with TN.

To scratch an itch: Establishing a mouse model to determine active brain areas involved in acute histaminergic itch

•

The specific histamine H4 receptor agonist ST-1006 induces acute itch in mice.

•

Histaminergic itch increases neuronal activity in the medial habenula.

•

Selective H4R activation in the skin increases neuronal activity in the medial habenula.

Background

Strategies to efficiently control itch require a deep understanding of the underlying mechanisms. Several areas in the brain involved in itch and scratching responses have been postulated, but the central mechanisms that drive pruritic responses are still unknown. Histamine is recognized as a major mediator of itch in humans, and has been the most frequently used stimulus as an experimental pruritogen for brain imaging of itch.

Objective

Histaminergic itch via histamine and the selective histamine H4 receptor (H4R) agonist, ST-1006, recruit brain nuclei through c-fos activation and activate specific areas in the brain.

Methods

An acute itch model was established in c-fos-EGFP transgenic mice using ST-1006 and histamine. Coronal brain sections were stained for c-fos immunoreactivity and the forebrain was mapped for density of c-fos + nuclei.

Results

Histamine and ST-1006 significantly increased scratching response in c-fos-EGFP mice compared to vehicle controls. Mapping c-fos immunostained brain sections revealed neuronal activity in the cortex, striatum, hypothalamus, thalamus, amygdala, and the midbrain.

Conclusions

Histaminergic itch and selective H4R activation significantly increased the density of c-fos + nuclei in the medial habenula (MHb). Thus, the MHb may be a new target to investigate and subsequently develop novel mechanism-based strategies to treat itch and possibly provide a locus for pharmacological control of pruritus.

Serum response factor mediates nociceptor inflammatory pain plasticity

Supplemental Digital Content is Available in the Text.

Serum response factor upregulates A-Kinase Anchoring Protein 79/150 expression in afferent sensory neurons through metabotropic glutamate receptor signaling.

Introduction:

Chronic metabotropic glutamate receptor activation in nociceptive afferents may upregulate A-Kinase Anchoring Protein 150 (AKAP150) expression and/or function.

Objectives:

To quantify transcriptional changes in AKAP150 expression and/or function after long-term mGluR5 agonist exposure, and identify transcriptional elements responsible.

Methods:

Dorsal root ganglia (DRG) were dissected from Sprague-Dawley rats and cultured for biochemical analysis of AKAP150 expression after prolonged mGluR5 agonist exposure. Serum response factor (SRF) expression was knocked down through siRNA in cultures to demonstrate significance to AKAP150 upregulation. Serum response factor was also knocked down in vivo through intrathecal injections of specifically targeted oligonucleotides to demonstrate significance to hyperalgesic priming behavior in persistent mechanical hypersensitivity.

Results:

Serum response factor and AKAP150 are coexpressed in TRPV1(+) DRG neurons in intact DRG. Prolonged mGluR5 agonist exposure increases SRF-dependent transcription and AKAP150 expression in a manner sensitive to protein kinase C inhibition and SRF knock down. Serum response factor in vivo knock down reduces mechanical hyperalgesic priming.

Conclusion:

Serum response factor transcription plays an important role in transcriptional upregulation of AKAP and hyperalgesic priming behavior, and may contribute to the increased role of AKAP150 in the transition from acute to chronic pain.

Enhanced innate fear and altered stress axis regulation in VGluT3 knockout mice

Glutamatergic neurons, characterized by vesicular glutamate transporters (VGluT1-3) provide the main excitation in the brain. Their disturbances have been linked to various brain disorders, which could be also modeled by the contextual fear test in rodents. We aimed to characterize the participation of VGluT3 in the development of contextual fear through its contribution to hypothalamic-pituitary-adrenocortical axis (HPA) regulation using knockout (KO) mice. Contextual fear conditioning was induced by foot shock and mice were examined 1 and 7 d later in the same environment comparing wild type with KO. Foot shock increased the immobility time without context specificity. Additionally, foot shock reduced open arm time in the elevated plus maze (EPM) test, and distance traveled in the open field (OF) test, representing the generalization of fear. Moreover, KO mice spent more time with freezing during the contextual fear test, less time in the open arm of the EPM, and traveled a smaller distance in the OF, with less entries into the central area. However, there was no foot shock and genotype interaction suggesting that VGluT3 does not influence the fear conditioning, rather determines anxiety-like characteristic of the mice. The resting hypothalamic CRH mRNA was higher in KO mice with reduced stressor-induced corticosterone elevations. Immunohistochemistry revealed the presence of VGluT3 positive fibers in the paraventricular nucleus of hypothalamus, but not on the hypophysis. As a summary, we confirmed the involvement of VGluT3 in innate fear, but not in the development of fear memory and generalization, with a significant contribution to HPA alterations. Highlights VGluT3 KO mice show innate fear without significant influence on fear memory and generalization. A putative background is the higher resting CRH mRNA level in their PVN and reduced stress-reactivity.

Enhancement in Tonically Active Glutamatergic Inputs to the Rostral Ventrolateral Medulla Contributes to Neuropathic Pain-Induced High Blood Pressure

Neuropathic pain increases the risk of cardiovascular diseases including hypertension with the characteristic of sympathetic overactivity. The enhanced tonically active glutamatergic input to the rostral ventrolateral medulla (RVLM) contributes to sympathetic overactivity and blood pressure (BP) in cardiovascular diseases. We hypothesize that neuropathic pain enhances tonically active glutamatergic inputs to the RVLM, which contributes to high level of BP and sympathetic outflow. Animal model with the trigeminal neuropathic pain was induced by the infraorbital nerve-chronic constriction injury (ION-CCI). A significant increase in BP and renal sympathetic nerve activity (RSNA) was found in rats with ION-CCI (BP, n = 5, RSNA, n = 7, p < 0.05). The concentration of glutamate in the RVLM was significantly increased in the ION-CCI group (n = 4, p < 0.05). Blockade of glutamate receptors by injection of kynurenic acid into the RVLM significantly decreased BP and RSNA in the ION-CCI group (n = 5, p < 0.05). In two major sources (the paraventricular nucleus and periaqueductal gray) for glutamatergic inputs to the RVLM, the ION-CCI group (n = 5, p < 0.05) showed an increase in glutamate content and expression of glutaminase 2, vesicular glutamate transporter 2 proteins, and c-fos. Our results suggest that enhancement in tonically active glutamatergic inputs to the RVLM contributes to neuropathic pain-induced high blood pressure.

Role of the Lateral Habenula in Pain-Associated Depression

Patients with chronic pain have significantly higher incidences of depression and anxiety than the average person. However, the mechanism underlying this link has not been elucidated in terms of how chronic pain causes significant mood changes and further develops into severe anxiety or depression. The serotonergic system in the raphe nuclei is an important component in both pain processing and the pathogenesis of depression. Since the lateral habenular nucleus (LHb) controls the raphe nuclei, it may participate in the regulation of pain-associated depression. Thus, the aim of the current study was to investigate the role of the LHb in this pathophysiological process. We used chronic constriction injury (CCI) of the sciatic nerve in rats as a model for neuropathic pain and assessed the changes potentially related to the mood disorders. The forced swim test (FST) and sucrose preference test (SPT) were performed to determine the behavioral changes 28 days after pain surgery. Expression of β calmodulin-dependent protein kinase type II (βCaMKII) in the LHb, cytochrome-c oxidase (COX) activity in the LHb and dorsal raphe nucleus (DRN) and serotonin (5-HT) levels in the DRN were measured. We found an increasing in LHb activity and βCaMKII expression, and a decrease in neuronal activity in the DRN and 5-hydroxyindoleacetic acid (5-HIAA)/5-HT ratios in the CCI rats. These effects were accompanied by the depression-like behaviors. Lesions in the LHb improved the pain threshold and depression-like behavior in the rats. These results suggest that the LHb may play a role in pain-associated depression by affecting the activity of 5-HT neurons in the DRN. Furthermore, we showed that increases in the LHb-DRN pathway activity were a common neurobiological mechanisms for pain and depression, which may explain the coexistence of pain and depression.

The possible mechanisms of analgesia produced by microinjection of morphine into the lateral habenula in the acute model of trigeminal pain in rats

This study aimed to assess the effect of intra-habenular injection of morphine on acute trigeminal pain in rats. Also here, we examined the involvement of raphe nucleus opioid and 5HT₃ receptors on the antinociceptive activity of intra habenular morphine to explore the possibility of existence of descending antinociceptive relay between the habenula and raphe nucleus. The numbers of eye wiping response elicited by applying a drop (40 μL) of NaCl (5 M) solution on the corneal surface were taken as an index of acute trigeminal nociception. Intra habenular microinjection of morphine at a dose of 2 μg was without effect, whereas at doses of 5 and 8 μg significantly produced antinociception. Microinjection of naltrexone (4 μg) and ondansetron (1 μg) into the dorsal raphe nucleus prior to intra-habenular saline did not produce any significant effect on corneal pain perception. Pretreatment of the raphe nucleus with ondansetron but not naltrexone prevented intra habenular morphine (8 μg) induced antinociception. Also, intra habenular injection of lidocaine (2%, 0.5 μL reduced corneal pain response. Moreover, intra-habenular microinjection of L-glutamic acid (1 and 2 μg/site) did not produce any analgesic activity in this model of pain. In conclusion, the present results suggest that the activation of the habenular μ opioid receptor by microinjection of morphine or inhibition of habenular neurons by microinjection of lidocaine produced an analgesic effect in the acute trigeminal model of pain in rats. The analgesic effect of intra habenular morphine was blocked by intra-dorsal raphe injection of serotonin 5-HT₃ antagonist.

Direct projection from the lateral habenula to the trigeminal mesencephalic nucleus in rats

Trigeminal mesencephalic nucleus (Vmes) neurons are primary afferents conveying deep sensation from the masticatory muscle spindles or the periodontal mechanoreceptors, and are crucial for controlling jaw movements. Their cell bodies exist in the brain and receive descending commands from a variety of cortical and subcortical structures involved in limbic (emotional) systems. However, it remains unclear how the lateral habenula (LHb), a center of negative emotions (e.g., pain, stress and anxiety), can influence the control of jaw movements. To address this issue, we examined whether and how the LHb directly projects to the Vmes by means of neuronal tract tracing techniques in rats. After injections of a retrograde tracer Fluorogold in the rostral and caudal Vmes, a number of neurons were labeled in the lateral division of LHb (LHbl) bilaterally, whereas a few neurons were labeled in the medial division of LHb (LHbm) bilaterally. After injections of an anterograde tracer, biotinylated dextranamine (BDA) in the LHbl, a small number of labeled axons were distributed bilaterally in the rostral and caudal levels of Vmes, where some labeled axonal boutons contacted the cell body of rostral and caudal levels of Vmes neurons bilaterally. After the BDA injection into the LHbm, however, no axons were labeled bilaterally in the rostral and caudal levels of Vmes. Therefore, the present study for the first time demonstrated the direct projection from the LHbl to the Vmes and the detailed projection patterns, suggesting that jaw movements are modulated by negative emotions that are signaled by LHbl neurons.

Acute effects of light on the brain and behavior of diurnal Arvicanthis niloticus and nocturnal Mus musculus

Photic cues influence daily patterns of activity via two complementary mechanisms: (1) entraining the internal circadian clock and (2) directly increasing or decreasing activity, a phenomenon referred to as "masking". The direction of this masking response is dependent on the temporal niche an organism occupies, as nocturnal animals often decrease activity when exposed to light, while the opposite response is more likely to be seen in diurnal animals. Little is known about the neural mechanisms underlying these differences. Here, we examined the masking effects of light on behavior and the activation of several brain regions by that light, in diurnal Arvicanthis niloticus (Nile grass rats) and nocturnal Mus musculus (mice). Each species displayed the expected behavioral response to a 1h pulse of light presented 2h after lights-off, with the diurnal grass rats and nocturnal mice increasing and decreasing their activity, respectively. In grass rats light induced an increase in cFOS in all retinorecipient areas examined, which included the suprachiasmatic nucleus (SCN), the ventral subparaventricular zone (vSPZ), intergeniculate leaflet (IGL), lateral habenula (LH), olivary pretectal nucleus (OPT) and the dorsal lateral geniculate (DLG). In mice, light led to an increase in cFOS in one of these regions (SCN), no change in others (vSPZ, IGL and LH) and a decrease in two (OPT and DLG). In addition, light increased cFOS expression in three arousal-related brain regions (the lateral hypothalamus, dorsal raphe, and locus coeruleus) and in one sleep-promoting region (the ventrolateral preoptic area) in grass rats. In mice, light had no effect on cFOS in these four regions. Taken together, these results highlight several brain regions whose responses to light suggest that they may play a role in masking, and that the possibility that they contribute to species-specific patterns of behavioral responses to light should be explored in future.

The role of lateral habenula-dorsal raphe nucleus circuits in higher brain functions and psychiatric illness

Serotonergic neurons in the dorsal raphe nucleus (DRN) play an important role in regulation of many physiological functions. The lateral nucleus of the habenular complex (LHb) is closely connected to the DRN both morphologically and functionally. The LHb is a key regulator of the activity of DRN serotonergic neurons, and it also receives reciprocal input from the DRN. The LHb is also a major way-station that receives limbic system input via the stria medullaris and provides output to the DRN and thereby indirectly connects a number of other brain regions to the DRN. The complex interactions of the LHb and DRN contribute to the regulation of numerous important behavioral and physiological mechanisms, including those regulating cognition, reward, pain sensitivity and patterns of sleep and waking. Disruption of these functions is characteristic of major psychiatric illnesses, so there has been a great deal of interest in how disturbed LHb-DRN interactions may contribute to the symptoms of these illnesses. This review summarizes recent research related to the roles of the LHb-DRN system in regulation of higher brain functions and the possible role of disturbed LHb-DRN function in the pathogenesis of psychiatric disorders, especially depression.

Short-term environmental enrichment exposure induces proliferation and maturation of doublecortin-positive cells in the prefrontal cortex

Previous studies have demonstrated that doublecortin-positive immature neurons exist predominantly in the superficial layer of the cerebral cortex of adult mammals such as guinea pigs, and these neurons exhibit very weak properties of self-proliferation during adulthood under physiological conditions. To verify whether environmental enrichment has an impact on the proliferation and maturation of these immature neurons in the prefrontal cortex of adult guinea pigs, healthy adult guinea pigs were subjected to short-term environmental enrichment. Animals were allowed to play with various cognitive and physical stimulating objects over a period of 2 weeks, twice per day, for 60 minutes each. Immunofluorescence staining results indicated that the number of doublecortin-positive cells in layer II of the prefrontal cortex was significantly increased after short-term environmental enrichment exposure. In addition, these doublecortin-positive cells co-expressed 5-bromo-2-deoxyuridine (a marker of cell proliferation), c-Fos (a marker of cell viability) and NeuN (a marker of mature neurons). Experimental findings showed that short-term environmental enrichment can induce proliferation, activation and maturation of doublecortin-positive cells in layer II of the prefrontal cortex of adult guinea pigs.

The Role of Habenula in Motivation and Reward

Located centrally along the dorsal diencephalic system, the habenula is divided into two structures: the medial and the lateral portions. It serves as an important relay between the forebrain and several hindbrain sites. In the last few years, a huge attention has been devoted to this structure, especially the lateral habenula (LHb), which seems to play an important role in emotion, motivation, and reward. Recent studies using techniques such as electrophysiology and neuroimaging have shown that the LHb is involved in motivational control of behavior. Its dysfunction is often associated with depression, schizophrenia, and mood disorder. This review focuses on providing a neuroanatomical and behavioral overview of some of the research previously done on the LHb. First, we describe the anatomical structure of the habenula and we explain how it is involved in reward and motivation. Then, we will discuss how this structure is linked to the limbic system, to finally provide a comparison between several studies that have used electrolytic lesions.

Common brain activations for painful and non-painful aversive stimuli

BACKGROUND

Identification of potentially harmful stimuli is necessary for the well-being and self-preservation of all organisms. However, the neural substrates involved in the processing of aversive stimuli are not well understood. For instance, painful and non-painful aversive stimuli are largely thought to activate different neural networks. However, it is presently unclear whether there is a common aversion-related network of brain regions responsible for the basic processing of aversive stimuli. To help clarify this issue, this report used a cross-species translational approach in humans (i.e. meta-analysis) and rodents (i.e. systematic review of functional neuroanatomy).

RESULTS

Animal and human data combined to show a core aversion-related network, consisting of similar cortical (i.e. MCC, PCC, AI, DMPFC, RTG, SMA, VLOFC; see results section or abbreviation section for full names) and subcortical (i.e. Amyg, BNST, DS, Hab, Hipp/Parahipp, Hyp, NAc, NTS, PAG, PBN, raphe, septal nuclei, Thal, LC, midbrain) regions. In addition, a number of regions appeared to be more involved in pain-related (e.g. sensory cortex) or non-pain-related (e.g. amygdala) aversive processing.

CONCLUSIONS

This investigation suggests that aversive processing, at the most basic level, relies on similar neural substrates, and that differential responses may be due, in part, to the recruitment of additional structures as well as the spatio-temporal dynamic activity of the network. This network perspective may provide a clearer understanding of why components of this circuit appear dysfunctional in some psychiatric and pain-related disorders.

Temporal changes in c-Fos activation patterns induced by conditioned fear

Mechanisms underlying shock-induced conditioned fear - a paradigm frequently used to model posttraumatic stress disorder, PTSD - are usually studied shortly after shocks. Some of the brain regions relevant to conditioned fear were activated in all the c-Fos studies published so far, but the overlap between the activated regions was small across studies. We hypothesized that discrepant findings were due to dynamic neural changes that followed shocks, and a more consistent picture would emerge if consequences were studied after a longer interval. Therefore, we exposed rats to a single session of footshocks and studied their behavioral and neural responses one and 28 days later. The neuronal activation marker c-Fos was studied in 24 brain regions relevant for conditioned fear, e.g. in subdivisions of the prefrontal cortex, hippocampus, amygdala, hypothalamic defensive system, brainstem monoaminergic nuclei and periaqueductal gray. The intensity of conditioned fear (as shown by the duration of contextual freezing) was similar at the two time-points, but the associated neuronal changes were qualitatively different. Surprisingly, however, Multiple Regression Analyses suggested that conditioned fear-induced changes in neuronal activation patterns predicted the duration of freezing with high accuracy at both time points. We suggest that exposure to electric shocks is followed by a period of plasticity where the mechanisms that sustain conditioned fear undergo qualitative changes. Neuronal changes observed 28 days but not 1 day after shocks were consistent with those observed in human studies performed in PTSD patients.

Mapping pain activation and connectivity of the human habenula

The habenula, located in the posterior thalamus, is implicated in a wide array of functions. Animal anatomical studies have indicated that the structure receives inputs from a number of brain regions (e.g., frontal areas, hypothalamic, basal ganglia) and sends efferent connections predominantly to the brain stem (e.g., periaqueductal gray, raphe, interpeduncular nucleus). The role of the habenula in pain and its anatomical connectivity are well-documented in animals but not in humans. In this study, for the first time, we show how high-field magnetic resonance imaging can be used to detect habenula activation to noxious heat. Functional maps revealed significant, localized, and bilateral habenula responses. During pain processing, functional connectivity analysis demonstrated significant functional correlations between the habenula and the periaqueductal gray and putamen. Probabilistic tractography was used to assess connectivity of afferent (e.g., putamen) and efferent (e.g., periaqueductal gray) pathways previously reported in animals. We believe that this study is the first report of habenula activation by experimental pain in humans. Since the habenula connects forebrain structures with brain stem structures, we suggest that the findings have important implications for understanding sensory and emotional processing in the brain during both acute and chronic pain.

Unmasking the mysteries of the habenula in pain and analgesia

The habenula is a small bilateral structure in the posterior-medial aspect of the dorsal thalamus that has been implicated in a remarkably wide range of behaviors including olfaction, ingestion, mating, endocrine and reward function, pain and analgesia. Afferent connections from forebrain structures send inputs to the lateral and medial habenula where efferents are mainly projected to brainstem regions that include well-known pain modulatory regions such as the periaqueductal gray and raphe nuclei. A convergence of preclinical data implicates the region in multiple behaviors that may be considered part of the pain experience including a putative role in pain modulation, affective, and motivational processes. The habenula seems to play a role as an evaluator, acting as a major point of convergence where external stimuli is received, evaluated, and redirected for motivation of appropriate behavioral response. Here, we review the role of the habenula in pain and analgesia, consider its potential role in chronic pain, and review more recent clinical and functional imaging data of the habenula from animals and humans. Even through the habenula is a small brain structure, advances in structural and functional imaging in humans should allow for further advancement of our understanding of its role in pain and analgesia.

The effect of CRF2 receptor antagonists on rat conditioned fear responses and c-Fos and CRF expression in the brain limbic structures

The influence of intracerebroventricular-administered selective corticotropin-releasing factor receptor 2 (CRF(2)) antagonists (antisauvagine-30, astressin-2B), on rat anxiety-like behavior, expression levels of c-Fos and CRF, and plasma corticosterone levels were examined in the present study. In fear-conditioned animals, both CRF receptor antagonists enhanced a conditioned freezing fear response and increased the conditioned fear-elevated concentration of serum corticosterone. Exogenously administered antisauvagine-30 increased the aversive context-induced expression of c-Fos in the 1 and 2 areas of the cingulate cortex (Cg1, Cg2), the central amygdala (CeA) and parvocellular neurons of the paraventricular hypothalamic nucleus (pPVN), and it enhanced the effect of conditioned fear in the secondary motor cortex (M2) and medial amygdala (MeA). Immunocytochemistry demonstrated an increase in CRF expression in the Cg1, M2 areas of the cortex, and pPVN, and it revealed the effect of conditioned fear in the CeA 35 min after antisauvagine-30 administration and 10 min after the conditioned fear test. Furthermore, astressin-2B, another CRF(2) receptor antagonist, enhanced expression of c-Fos and CRF in the CeA and pPVN, and revealed the effect of conditioned fear in the Cg1. These data support a model in which an excess in CRF(1) receptor activation, combined with reduced CRF(2) receptor signaling, may contribute to stronger expression of anxiety-like responses.

The localization of brain sites of anxiogenic-like effects of urocortin-2

The influence of intracerebroventricullary-administered urocortin-2, a selective corticotropin-releasing factor receptor 2 (CRF(2)) agonist, on rat anxiety-like behaviour, the expression of c-Fos and CRF, and plasma corticosterone levels was examined in the present study. When applied to animals exposed to the conditioned fear-induced context, urocortin-2 enhanced a conditioned freezing fear response. Urocortin-2 also significantly decreased rat exploratory activity in the open field test. Exogenous urocortin-2 increased the conditioned fear-induced expression of c-Fos in the central amygdala (CeA), and parvocellular neurons of the paraventricular hypothalamic nucleus (pPVN), and revealed the effect of conditioned fear in the medial amygdala (MeA). In the fear-conditioned animals, immunocytochemistry showed an increase in the density of CRF-related immunoreactive complexes in the lateral septum (LS), 35min after urocortin-2 administration and 10min after the conditioned fear test, compared with saline-pretreated fear-conditioned animals. These data suggest a role of urocortin-2 in the behavioural and immunocytochemical responses to stress, in which it strengthens the measures of anxiety-like responses.

Increased c-fos expression in the central nucleus of the amygdala and enhancement of cued fear memory in Dyt1 DeltaGAG knock-in mice

DYT1 dystonia is caused by a trinucleotide deletion of GAG (DeltaGAG) in DYT1, which codes for torsinA. A previous epidemiologic study suggested an association of DYT1 DeltaGAG mutation with early-onset recurrent major depression. However, another study reported no significant association with depression, but instead showed an association with anxiety and dystonia. In this study, we analyzed these related behaviors in Dyt1 DeltaGAG heterozygous knock-in mice. The knock-in mice showed a subtle anxiety-like behavior but did not show depression-like behaviors. The mutant mice also displayed normal sensorimotor gating function in a prepulse inhibition test. While normal hippocampus-dependent contextual fear memory and hippocampal CA1 long-term potentiation (LTP) were observed, the knock-in mice exhibited an enhancement in the formation of cued fear memories. Anatomical analysis indicated that the number of c-fos positive cells was significantly increased while the size of the central nucleus of the amygdala (CE) was significantly reduced in the knock-in mice. These results suggest that the Dyt1 DeltaGAG mutation increased the activity of the CE and enhanced the acquisition of the cued fear memory.

Mapping of c-Fos expression in the rat brain during the evolution of pentylenetetrazol-kindled seizures

c-Fos protein immunocytochemistry was used to map the brain structures recruited during the evolution of seizures that follows repeated administration of a subconvulsive dose (35mg/kg, ip) of pentylenetetrazol in rats. c-Fos appeared earliest in nucleus accumbens shell, piriform cortex, prefrontal cortex, and striatum (stages 1 and 2 of kindling in comparison to control, saline-treated animals). At the third stage of kindling, central amygdala nuclei, entorhinal cortex, and lateral septal nuclei had enhanced concentrations of c-Fos. At the fourth stage of kindling, c-Fos expression was increased in basolateral amygdala and CA1 area of the hippocampus. Finally, c-Fos labeling was enhanced in the dentate gyrus of the hippocampus only when tonic-clonic convulsions were fully developed. The most potent changes in c-Fos were observed in dentate gyrus, piriform cortex, CA1, lateral septal nuclei, basolateral amygdala, central amygdala nuclei, and prefrontal cortex. Piriform cortex, entorhinal cortex, prefrontal cortex, lateral septal nuclei, and CA3 area of the hippocampus appeared to be the brain structures selectively involved in the process of chemically induced kindling of seizures.

Cerebral activity during the anesthesia-like state induced by mesopontine microinjection of pentobarbital

Microinjection of pentobarbital into a restricted region of rat brainstem, the mesopontine tegmental anesthesia area (MPTA), induces a reversible anesthesia-like state characterized by loss of the righting reflex, atonia, antinociception, and loss of consciousness as assessed by electroencephalogram synchronization. We examined cerebral activity during this state using FOS expression as a marker. Animals were anesthetized for 50 min with a series of intracerebral microinjections of pentobarbital or with systemic pentobarbital and intracerebral microinjections of vehicle. FOS expression was compared with that in awake animals microinjected with vehicle. Neural activity was suppressed throughout the cortex whether anesthesia was induced by systemic or MPTA routes. Changes were less consistent subcortically. In the zona incerta and the nucleus raphe pallidus, expression was strongly suppressed during systemic anesthesia, but only mildly during MPTA-induced anesthesia. Dissociation was seen in the tuberomammillary nucleus where suppression occurred during systemic-induced anesthesia only, and in the lateral habenular nucleus where activity was markedly increased during systemic-induced anesthesia but not following intracerebral microinjection. Several subcortical nuclei previously associated with cerebral arousal were not affected. In the MPTA itself FOS expression was suppressed during systemic anesthesia. Differences in the pattern of brain activity in the two modes of anesthesia are consistent with the possibility that anesthetic endpoints might be achieved by alternative mechanisms: direct drug action for systemic anesthesia or via ascending pathways for MPTA-induced anesthesia. However, it is also possible that systemically administered agents induce anesthesia, at least in part, by a primary action in the MPTA with cortical inhibition occurring secondarily.

Analysis of c-fos and zif268 expression reveals time-dependent changes in activity inside and outside the lesion projection zone in adult cat area 17 after retinal lesions

Retinal lesions induce a topographic reorganization in the corresponding lesion projection zone (LPZ) in the visual cortex of adult cats. To gain a better insight into the reactivation dynamics, we investigated the alterations in cortical activity throughout area 17. We implemented in situ hybridization and real-time polymerase chain reaction to analyze the spatiotemporal expression patterns of the activity marker genes zif268 and c-fos. The immediate early gene (IEG) data confirmed a strong and permanent activity decrease in the center of the LPZ as previously described by electrophysiology. A recovery of IEG expression was clearly measured in the border of the LPZ. We were able to register reorganization over 2.5-6 mm. We also present evidence that the central retinal lesions concomitantly influence the activity in far peripheral parts of area 17. Its IEG expression levels appeared dependent of time and distance from the LPZ. We therefore propose that coupled changes in activity occur inside and outside the LPZ. In conclusion, alterations in activity reporter gene expression throughout area 17 contribute to the lesion-induced functional reorganization.

The effects of acute corticosterone administration on anxiety, endogenous corticosterone, and c-Fos expression in the rat brain

The effects of acute pretreatment of rats with corticosterone (5 and 20 mg/kg, s.c.) on emotional behavior, expression of c-Fos protein in brain structures, and serum concentration of corticosterone were studied to model the short-term glucocorticoid-dependent changes in brain functions. Corticosterone was administered 90 min before training of a conditioned fear reaction (a freezing response), and behavioral, hormonal and immunocytochemical effects were examined 1 day later, on the test day. Pretreatment of rats with corticosterone significantly attenuated the freezing reaction in the conditioned fear test. The effect of the corticosterone was accompanied by a selective enhancement of the aversive context-induced c-Fos expression in some brain structures: the parvocellular and magnocellular neurons of the paraventricular hypothalamic nucleus (pPVN and mPVN), the medial amygdala nucleus (MeA), and the cingulate cortex, area 1 (Cg1), as well as an increase in the concentration of aversive context-induced endogenous serum glucocorticoid, 1.5 h and 10 min after the test session, respectively. It is suggested that the behavioral effects of acute pretreatment of rats with corticosterone could be due to changes in the mnemonic processes in the brain, inhibition of brain corticotropin releasing factor (CRF) synthesis, or stimulation of GABA-A receptor modulating neurosteroids synthesis. It is hypothesized that the enhanced activity of Cg1, MeA, pPVN, and mPVN, and the hypothalamic-pituitary-adrenal axis with concomitant increased serum glucocorticoid concentration, might serve to facilitate active coping behavior in a threatening situation.

Fluoxetine attenuates the effects of pentylenetetrazol on rat freezing behavior and c-Fos expression in the dorsomedial periaqueductal gray

The aim of the study was to investigate the role of the periaqueductal gray (PAG) in anxiolytic-like actions of fluoxetine in animals treated with an anxiogenic drug, pentylenetetrazol (PTZ), and subjected to fear conditioning procedure. The data showed that PTZ given at the dose of 30 mg/kg 15 min before a retention trial significantly decreased freezing reaction (p<0.01), and potently enhanced rat locomotor activity (p<0.01), in comparison to the control group. These effects were reversed by prior (60 min) administration of fluoxetine (20 mg/kg). Simultaneously, PTZ significantly increased c-Fos expression in the dorsomedial periaqueductal gray (DMPAG), examined 2h after the retention trial, in comparison to the control group (p<0.01). Fluoxetine (20 mg/kg) administered 60 min before PTZ reversed this effect. PTZ given at the same dose and time interval in the open field test did not affect rat locomotor behavior. Importantly, fluoxetine pretreatment did not change PTZ concentration in brain tissue. Our experiment based on PTZ-enhanced aversive conditioning revealed that acutely administered fluoxetine antagonized PTZ-induced panic-like behavior, and this phenomenon was accompanied by inhibition of activity of DMPAG.

Midazolam inhibits neophobia-induced Fos expression in the rat hippocampus

The effect of midazolam on expression of c-Fos protein was examined in the rat hippocampus, following the open field test of neophobia. It was found that pretreatment of rats with midazolam, at the dose of 0.5 mg/kg, enhanced rat exploratory behavior, and inhibited neophobia related stimulation of c-Fos in the CA-1 and CA-3 areas of the hippocampus. The presented results provide new immunocytochemical data on the involvement of hippocampus in emotional processes related to neophobia, and indicate a possible site of action of benzodiazepines.