Rostral agranular insular cortex and pain areas of the central nervous system: a tract-tracing study in the rat

Pathological activity in mediodorsal thalamus of rats with spinal cord injury pain

Spinal cord injury (SCI) results not only in motor deficits, but produces, in many patients, excruciating chronic pain (SCI pain). We have previously shown, in a rodent model, that SCI causes suppression of activity in the GABAergic nucleus, the zona incerta (ZI), and concomitant increased activity in one of its main targets, the posterior nucleus of the thalamus (PO); the increased PO activity is correlated with the maintenance and expression of hyperalgesia after SCI. Here, we test the hypothesis that SCI causes a similar pathological increase in other thalamic nuclei regulated by the ZI, specifically the mediodorsal thalamus (MD), which is involved in the emotional-affective aspects of pain. We recorded single and multiunit activity from MD of either anesthetized or awake rats, and compared data from rats with SCI with data from sham-operated controls (anesthetized experiments) or with data from the same animals prelesion (awake experiments). Consistent with our hypothesis, MD neurons from rats with SCI show significant increases in spontaneous firing rates and in the magnitude and duration of responses to noxious stimuli. In a subset of anesthetized animals, similar changes in activity of MD neurons were produced by pharmacologically inactivating ZI in naive rats, suggesting that the changes in the MD after SCI are related to suppressed inhibition from the ZI. These data support our hypothesis that SCI pain results, at least in part, from a loss of inhibition to thalamic nuclei associated with both the sensory-discriminative and emotional-affective components of pain.

The role of medial prefrontal cortex in memory and decision making

Some have claimed that the medial prefrontal cortex (mPFC) mediates decision making. Others suggest mPFC is selectively involved in the retrieval of remote long-term memory. Yet others suggests mPFC supports memory and consolidation on time scales ranging from seconds to days. How can all these roles be reconciled? We propose that the function of the mPFC is to learn associations between context, locations, events, and corresponding adaptive responses, particularly emotional responses. Thus, the ubiquitous involvement of mPFC in both memory and decision making may be due to the fact that almost all such tasks entail the ability to recall the best action or emotional response to specific events in a particular place and time. An interaction between multiple memory systems may explain the changing importance of mPFC to different types of memories over time. In particular, mPFC likely relies on the hippocampus to support rapid learning and memory consolidation.

Stimulation of the dorsal periaqueductal gray enhances spontaneous recovery of a conditioned taste aversion

Due to its relevance to clinical practice, extinction of learned fears has been a major focus of recent research. However, less is known about the means by which conditioned fears re-emerge (i.e., spontaneously recover) as time passes or contexts change following extinction. The periaqueductal gray represents the final common pathway mediating defensive reactions to fear and we have reported previously that the dorsolateral PAG (dlPAG) exhibits a small but reliable increase in neural activity (as measured by c-fos protein immunoreactivity) when spontaneous recovery (SR) of a conditioned taste aversion (CTA) is reduced. Here we extend these correlational studies to determine if inducing dlPAG c-fos expression through electrical brain stimulation could cause a reduction in SR of a CTA. Male Sprague-Dawley rats acquired a strong aversion to saccharin (conditioned stimulus; CS) and then underwent CTA extinction through multiple non-reinforced exposures to the CS. Following a 30-day latency period after asymptotic extinction was achieved; rats either received stimulation of the dorsal PAG (dPAG) or stimulation of closely adjacent structures. Sixty minutes following the stimulation, rats were again presented with the saccharin solution as we tested for SR of the CTA. The brain stimulation evoked c-fos expression around the tip of the electrodes. However, stimulation of the dPAG failed to reduce SR of the previously extinguished CTA. In fact, dPAG stimulation caused rats to significantly suppress their saccharin drinking (relative to controls) - indicating an enhanced SR. These data refute a cause-and-effect relationship between enhanced dPAG c-fos expression and a reduction in SR. However, they highlight a role for the dPAG in modulating SR of extinguished CTAs.

The role of medial prefrontal cortex in memory and decision making

Some have claimed that the medial prefrontal cortex (mPFC) mediates decision making. Others suggest mPFC is selectively involved in the retrieval of remote long-term memory. Yet others suggests mPFC supports memory and consolidation on time scales ranging from seconds to days. How can all these roles be reconciled? We propose that the function of the mPFC is to learn associations between context, locations, events, and corresponding adaptive responses, particularly emotional responses. Thus, the ubiquitous involvement of mPFC in both memory and decision making may be due to the fact that almost all such tasks entail the ability to recall the best action or emotional response to specific events in a particular place and time. An interaction between multiple memory systems may explain the changing importance of mPFC to different types of memories over time. In particular, mPFC likely relies on the hippocampus to support rapid learning and memory consolidation.

Kainate receptor-mediated synaptic transmissions in the adult rodent insular cortex

Kainate (KA) receptors are expressed widely in the central nervous system and regulate both excitatory and inhibitory synaptic transmission. KA receptors play important roles in fear memory, anxiety, and pain. However, little is known about their function in synaptic transmission in the insular cortex (IC), a critical region for taste, memory, and pain. Using whole cell patch-clamp recordings, we have shown that KA receptors contribute to fast synaptic transmission in neurons in all layers of the IC. In the presence of the GABA(A) receptor antagonist picrotoxin, the NMDA receptor antagonist AP-5, and the selective AMPA receptor antagonist GYKI 53655, KA receptor-mediated excitatory postsynaptic currents (KA EPSCs) were revealed. We found that KA EPSCs are ~5-10% of AMPA/KA EPSCs in all layers of the adult mouse IC. Similar results were found in adult rat IC. KA EPSCs had a significantly slower rise time course and decay time constant compared with AMPA receptor-mediated EPSCs. High-frequency repetitive stimulations at 200 Hz significantly facilitated the summation of KA EPSCs. In addition, genetic deletion of GluK1 or GluK2 subunit partially reduced postsynaptic KA EPSCs, and exposure of GluK2 knockout mice to the selective GluK1 antagonist UBP 302 could significantly reduce the KA EPSCs. These data suggest that both GluK1 and GluK2 play functional roles in the IC. Our study may provide the synaptic basis for the physiology and pathology of KA receptors in the IC-related functions.

Capsaicin induces theta-band synchronization between gustatory and autonomic insular cortices

In the insular cortex, the primary gustatory area caudally adjoins the primary autonomic area that is involved in visceral sensory-motor integration. However, it has not been addressed whether neural activity in the gustatory insula (Gu-I) is coordinated with that in the autonomic insula (Au-I). We have demonstrated that TRPV1 activation in Gu-I induces theta-band synchronization between Gu-I and Au-I in rat slice preparations. Electron-microscopic immunohistochemistry revealed that TRPV1 immunoreactivity was much higher in Gu-I than in Au-I, and was mostly detected in dendritic spines receiving asymmetrical synapses. Whole-cell voltage-clamp recordings revealed that, in Gu-I, capsaicin-induced currents in layer 3 (L3) pyramidal cells (PCs) displayed no apparent desensitization, while those in layer 5 (L5) PCs displayed Ca(2+)-dependent desensitization, suggesting that L3 and L5 PCs respond differentially to TRPV1 activation. Voltage-sensitive dye imaging demonstrated that TRPV1 activation in Gu-I can alter an optical response with a monophasic and columnar temporospatial pattern evoked within Gu-I into an oscillatory one extending over Gu-I and Au-I. Power and cross-power spectral analyses of optical responses revealed theta-band synchronization between Gu-I and Au-I. Whole-cell current-clamp recordings demonstrated that such theta-band waves were mediated by sustained rhythmic firings at 4 and 8 Hz in L3 and L5 PCs, respectively. These results strongly suggested that theta-band oscillatory neural coordination between Gu-I and Au-I was induced by two distinct TRPV1-mediated theta-rhythm firings in L3 and L5 PCs in Gu-I. This network coordination induced by TRPV1 activation could be responsible for autonomic responses to tasting and ingesting spicy foods.

Topography of descending projections from anterior insular and medial prefrontal regions to the lateral habenula of the epithalamus in the rat

The epithalamic lateral nucleus of the habenula (LHb) plays a key role in regulating firing of dopamine and serotonin neurons in the midbrain and is thereby involved in various cognitive and affective behaviors. It is not yet clear, however, from where the LHb receives cognitive and affective information relevant to its regulation of the midbrain monoaminergic systems. The prefrontal cortex would be among the ideal sources. Here, using anterograde and retrograde tracer injections in the rat brain, we characterized the topography of the corticohabenular projections. Following injections of cholera toxin subunit B into the LHb, retrogradely labeled neurons were produced in the anterior insular, cingulate, prelimbic and infralimbic cortices. Consistent with this retrograde tracing, injections of biotinylated dextran amine (BDA) into these cortical regions labeled robust terminals in the LHb. Our quantification of the BDA-impregnated varicosities revealed that projections from the anterior insula terminated mainly in the intersection regions of the lateral and ventral two-thirds of the LHb, while projections from the cingulate cortex terminated mainly in the lateral two-thirds of the LHb. By comparison, BDA-labeled terminals originating from the medial prefrontal regions were contained mainly in the medial plus ventral one-third of LHb. Based on these data, we hypothesize that LHb provides a link for conveying cognitive and affective information from prefrontal and insular regions to the midbrain monoaminergic centers.

A role for the insular cortex in long-term memory for context-evoked drug craving in rats

Drug craving critically depends on the function of the interoceptive insular cortex, and may be triggered by contextual cues. However, the role of the insula in the long-term memory linking context with drug craving remains unknown. Such a memory trace probably resides in some neocortical region, much like other declarative memories. Studies in humans and rats suggest that the insula may include such a region. Rats chronically implanted with bilateral injection cannulae into the high-order rostral agranular insular cortex (RAIC) or the primary interoceptive posterior insula (pIC) were conditioned to prefer the initially aversive compartment of a 2-compartment place preference apparatus by repeatedly pairing it to amphetamine. We found a reversible but long-lasting loss (ca. 24 days) of amphetamine-conditioned place preference (CPP) and a decreased expression in the insula of zif268, a crucial protein in memory reconsolidation, when anisomycin (ANI) was microinjected into the RAIC immediately after the reactivation of the conditioned amphetamine/context memory. ANI infusion into the RAIC without reactivation did not change CPP, whereas ANI infusion into pIC plus caused a 15 days loss of CPP. We also found a 24 days loss of CPP when we reversibly inactivated pIC during extinction trials. We interpret these findings as evidence that the insular cortex, including the RAIC, is involved in a context/drug effect association. These results add a drug-related memory function to the insular cortex to the previously found role of the pIC in the perception of craving or malaise.

Nicotine-induced dendritic remodeling in the insular cortex

The insular cortex has emerged as a novel target for nicotine addiction research. One unresolved question about the insular cortex is whether its neurons exhibit nicotine-induced dendritic remodeling similar to other brain regions implicated in nicotine addiction. To test this question, Long-Evans rats were administered nicotine via osmotic pump for two weeks. Thirty-seven days following the end of nicotine dosing, rats were sacrificed for Golgi-Cox staining and pyramidal neurons from the rostral agranular insular cortex were digitally reconstructed in three dimensions. Results from morphometric analyses revealed an increased complexity of dendrites in the insular cortex following nicotine. Increases were found for both total dendrite length and number of bifurcations. Sholl analyses revealed these changes depended on the distance from the soma, with the most prominent changes distributed at distal points along the dendritic tree. A follow-up comparison of length and bifurcation measurements from Sholl analyses suggested that new dendritic branches, rather than growth of existing dendrites, most likely contributed to overall changes in complexity. No change in dendrite morphology was found for apical dendrites. Together, these results show the insular cortex is a target for neuroplasticity following nicotine exposure.

Pain affect in the absence of pain sensation: evidence of asomaesthesia after somatosensory cortex lesions in the rat

Multidimensional models of pain processing distinguish the sensory, motivational, and affective components of the pain experience. Efforts to understand underlying mechanisms have focused on isolating the roles of specific brain structures, including both limbic and non-limbic cortical areas, in the processing of nociceptive stimuli. The purpose of this study was to examine the role of the somatosensory cortex in both sensory and affective aspects of pain processing. It was hypothesized that animals with lesions of the hind limb area of the somatosensory cortex would demonstrate altered sensory processing (asomaesthesia, a deficit in the ability to detect and identify somatic sensation) in the presence of an inflammatory state when compared to animals with sham lesions. The level of pain affect produced by an inflammatory pain condition was not expected to change, as this region has not demonstrated a role in processing the affective component of pain. Seventy-nine adult female Sprague-Dawley rats were randomly assigned to receive bilateral lesions or a sham procedure. The results showed that somatosensory lesions to the hindlimb region altered responses to mechanical stimulation in the presence of experimentally-induced inflammation, but did not attenuate the inflammation-induced paw volume changes or the level of pain affect, as demonstrated by escape/avoidance behavior in response to mechanical stimulation. Overall, these results support previous evidence suggesting that the somatosensory cortex is primarily involved in the processing the sensory/discriminative aspect of pain, and the current study is the first to demonstrate the presence of pain affect in the absence of somatosensory processing.

Transneuronal tracing of airways-related sensory circuitry using herpes simplex virus 1, strain H129

Sensory input from the airways to suprapontine brain regions contributes to respiratory sensations and the regulation of respiratory function. However, relatively little is known about the central organization of this higher brain circuitry. We exploited the properties of the H129 strain of herpes simplex virus 1 (HSV-1) to perform anterograde transneuronal tracing of the central projections of airway afferent nerve pathways. The extrathoracic trachea in Sprague-Dawley rats was inoculated with HSV-1 H129, and tissues along the neuraxis were processed for HSV-1 immunoreactivity. H129 infection appeared in the vagal sensory ganglia within 24 h and the number of infected cells peaked at 72 h. Brainstem nuclei, including the nucleus of the solitary tract and trigeminal sensory nuclei were infected within 48 h, and within 96 h infected cells were evident within the pons (lateral and medial parabrachial nuclei), thalamus (ventral posteromedial, ventral posterolateral, submedius, and reticular nuclei), hypothalamus (paraventricular and lateral nuclei), subthalamus (zona incerta), and amygdala (central and anterior amygdala area). At later times H129 was detected in cortical forebrain regions including the insular, orbital, cingulate, and somatosensory cortices. Vagotomy significantly reduced the number of infected cells within vagal sensory nuclei in the brainstem, confirming the main pathway of viral transport is through the vagus nerves. Sympathetic postganglionic neurons in the stellate and superior cervical ganglia were infected by 72 h, however, there was no evidence for retrograde transynaptic movement of the virus in sympathetic pathways in the central nervous system (CNS). These data demonstrate the organization of key structures within the CNS that receive afferent projections from the extrathoracic airways that likely play a role in the perception of airway sensations.

Subjecting elite athletes to inspiratory breathing load reveals behavioral and neural signatures of optimal performers in extreme environments

Background

It is unclear whether and how elite athletes process physiological or psychological challenges differently than healthy comparison subjects. In general, individuals optimize exercise level as it relates to differences between expected and experienced exertion, which can be conceptualized as a body prediction error. The process of computing a body prediction error involves the insular cortex, which is important for interoception, i.e. the sense of the physiological condition of the body. Thus, optimal performance may be related to efficient minimization of the body prediction error. We examined the hypothesis that elite athletes, compared to control subjects, show attenuated insular cortex activation during an aversive interoceptive challenge.

Methodology/Principal Findings

Elite adventure racers (n = 10) and healthy volunteers (n = 11) performed a continuous performance task with varying degrees of a non-hypercapnic breathing load while undergoing functional magnetic resonance imaging. The results indicate that (1) non-hypercapnic inspiratory breathing load is an aversive experience associated with a profound activation of a distributed set of brain areas including bilateral insula, dorsolateral prefrontal cortex and anterior cingulated; (2) adventure racers relative to comparison subjects show greater accuracy on the continuous performance task during the aversive interoceptive condition; and (3) adventure racers show an attenuated right insula cortex response during and following the aversive interoceptive condition of non-hypercapnic inspiratory breathing load.

Conclusions/Significance

These findings support the hypothesis that elite athletes during an aversive interoceptive condition show better performance and an attenuated insular cortex activation during the aversive experience. Interestingly, differential modulation of the right insular cortex has been found previously in elite military personnel and appears to be emerging as an important brain system for optimal performance in extreme environments.

Expression of the dopaminergic D1 and D2 receptors in the anterior cingulate cortex in a model of neuropathic pain

BACKGROUND

The anterior cingulate cortex (ACC) has been related to the affective component of pain. Dopaminergic mesocortical circuits, including the ACC, are able to inhibit neuropathic nociception measured as autotomy behaviour. We determined the changes in dopamine D1 and D2 (D1R and D2R) receptor expression in the ACC (cg1 and cg2) in an animal model of neuropathic pain. The neuropathic group had noxious heat applied in the right hind paw followed 30 min. later by right sciatic denervation. Autotomy score (AS) was recorded for eight days and subsequently classified in low, medium and high AS groups. The control consisted of naïve animals.A semiquantitative RT-PCR procedure was done to determine mRNA levels for D1R and D2R in cg1 and cg2, and protein levels were measured by Western Blot.

RESULTS

The results of D1R mRNA in cg1 showed a decrease in all groups. D2R mRNA levels in cg1 decreased in low AS and increased in medium and high AS. Regarding D1R in cg2, there was an increase in all groups. D2R expression levels in cg2 decreased in all groups. In cg1, the D2R mRNA correlated positively with autotomy behaviour. Protein levels of D2R in cg1 increased in all groups but to a higher degree in low AS. In cg2 D2R protein only decreased discretely. D1R protein was not found in either ACC region.

CONCLUSIONS

This is the first evidence of an increase of inhibitory dopaminergic receptor (D2R) mRNA and protein in cg1 in correlation with nociceptive behaviour in a neuropathic model of pain in the rat.

Profiles of DARPP-32 in the insular cortex with schizophrenia: a postmortem brain study

In patients with schizophrenia, various physical disorders are sometimes discovered only when they have reached a later and more severe stage. This phenomenon is believed to be caused, at least in part, by an increase in pain threshold. The gamma-aminobutyric acid (GABA)-ergic and glutamatergic systems in the rostral agranular insular cortex (RAIC) are thought to be involved in the regulation of pain threshold. However, no postmortem studies of the cerebral cortex have previously been published. Dopamine and cAMP-regulated phosphoprotein 32 kD (DARPP-32), which is involved in the GABAergic and glutamatergic systems, is considered to be crucial for elucidating the pathogenesis of schizophrenia. Using specific antibodies, we conducted immunohistochemical examinations of the RAIC in 10 subjects from a healthy control group, and 11 subjects from a schizophrenia group. The sex, age, and postmortem interval (PMI) of the schizophrenia group were matched to those of the healthy control group. We revealed that the density of DARPP-32-immunoreactive (IR) neurons in the II and III layers of the RAIC was significantly decreased (p<0.05) in the schizophrenia group compared with the healthy control group. Our findings could partially explain the molecular basis of the pain threshold abnormalities found in patients with schizophrenia.

Periaqueductal gray c-Fos expression varies relative to the method of conditioned taste aversion extinction employed

A conditioned taste aversion (CTA) is acquired when an animal consumes a novel taste (CS) and then experiences the symptoms of poisoning (US). Following CTA training, animals will avoid the taste that was previously associated with malaise. This defensive reaction to a learned fear can be extinguished by repeated exposure to the CS alone (CS-only; CSO-EXT). However, following a latency period in which the CS is not presented, the CTA will spontaneously recover (SR). Through the use of an explicitly unpaired extinction procedure (EU-EXT) we have shown that we can speed up extinction and attenuate SR of the CTA. Here we compared and contrasted the ability of CSO and EU extinction procedures to affect c-Fos expression in the periaqueductal gray (PAG). Fluid-deprived Sprague-Dawley rats acquired a strong CTA [via 3 pairings of 0.3% oral saccharin (SAC; the CS) and 81mg/kg i.p. lithium chloride (LiCl; the US)] followed by extinction trials consisting of multiple exposures to either, (a) the CS every-other day (CSO-EXT), or (b) CS and US on alternate days (EU-EXT). A different group of rats did not receive multiple CS exposures and served as a "no extinction" (NE) control. Both extinction procedures resulted in ≥90% reacceptance of SAC (achieving asymptotic extinction). Some of the animals were sacrificed for c-Fos immunohistochemical analysis following asymptotic extinction. Other rats entered a 30-day latency period where they drank water only. These remaining animals were then tested for SR with a final exposure to SAC before being sacrificed for c-Fos immunohistochemistry. As reported previously, rats in the CS-only group exhibited a significant SR of the CTA. However, animals in the EU extinction group reached asymptotic extinction more rapidly than did CSO rats and they did not show SR of the CTA. As compared to rats that retained their CTA, both groups of extinguished rats showed suppression in the number of c-Fos-labeled neurons in all 4 longitudinal columns of the PAG. The number of c-Fos-labeled cells in the PAG was generally low but there was a reliable increase in c-Fos expression in dorsolateral PAG (dlPAG) following the SR test in the brains of rats that went through the EU-EXT procedure as compared with those that either went through the more-traditional CSO extinction procedure or experienced no extinction at all. The number of c-Fos-labeled neurons in the dlPAG was significantly correlated with the amount of SAC consumed at the SR test. Surprisingly, the brains of EU-extinguished rats and CSO extinguished rats did not differ in the number of c-Fos-labeled neurons in gustatory neocortex, medial prefrontal cortex, basolateral amygdala, or the central nucleus of the amygdala. Thus, behavioral differences in SR between the EU and CSO extinction animals were not represented by corresponding changes in the neural activity of several brain nuclei classically associated with extinction learning. However a detailed analysis of PAG c-Fos expression provided hints about some of the physiological changes evoked by these 2 extinction paradigms that produce very different behavioral outcomes. The findings are clinically relevant as we seek the development of treatments for deficits in fear extinction (e.g. PTSD, phobias).

Preferential networks of the mediodorsal nucleus and centromedian-parafascicular complex of the thalamus--a DTI tractography study

Distinct thalamic nuclei, like the mediodorsal (MD) nucleus and the centromedian/parafascicular complex (CM/Pf), are embedded in different basal ganglia-thalamocortical loops, which were shown to integrate cognitive and emotional aspects of human behavior. Despite well described connections on a microscopic scale, derived from tracing studies in animals, little is known about the intrinsic anatomical connections of these nuclei in humans. This lack of knowledge limits not only interpretation of functional imaging studies but also estimation of direct effects of deep brain stimulation which treats diseases as different as epilepsy or major depression. Therefore, non-invasive diffusion tensor imaging (DTI) studies are key to analyzing connectivity patterns and elaborate approaches to close this gap. For our study, we explored the structural connectivity of the MD thalamic nuclei and the CM/Pf complex towards five cortical and six subcortical regions by using a preferential fiber calculation. We found both thalamic nuclei to be preferentially associated to distinct networks: whereas the MD is preferentially connected to prefrontal and limbic cortical regions, the CM is linked to subcortical regions. The anterior insula was the only cortical region associated with the subcortical network of the CM and the cortical network of the MD comprised one subcortical hub, the caudate nucleus, suggesting an integrative role of these two regions. Adding to predescribed anatomical tract tracing connectivities in animal studies, our finding lends support to the existence of similar basal ganglia-thalamocortical circuits in humans and we could show a robust distinction of preferential connectivity for both thalamic nuclei.

Spatiotemporal dynamics of long-term potentiation in rat insular cortex revealed by optical imaging

Long-term potentiation (LTP) of the gustatory cortex (GC), a part of the insular cortex (IC) around the middle cerebral artery, is a key process of gustatory learning and memory, including conditioned taste aversion learning. The rostral (rGC) and caudal GC (cGC) process different tastes; the rGC responds to hedonic and the cGC responds to aversive tastes. However, plastic changes of spatial interaction of excitatory propagation between the rGC and cGC remain unknown. The present study aimed to elucidate spatiotemporal profiles of excitatory propagation, induced by electrical stimulation (five train pulses) of the rGC/cGC before and after LTP induction, using in vivo optical imaging with a voltage-sensitive dye. We demonstrated that tetanic stimulation of the cGC induced long-lasting expansion of the excitation responding to five train stimulation of the cGC, and an increase in amplitude of optical signals in the IC. Excitatory propagation after LTP induction spread preferentially toward the rostral IC: the length constant (λ) of excitation, obtained by fitting optical signals with a monoexponential curve, was increased to 121.9% in the rostral direction, whereas λ for the caudal, dorsal, and ventral directions were 48.9%, 44.2%, and 62.5%, respectively. LTP induction was prevented by pre-application of D-APV, an NMDA receptor antagonist, or atropine, a muscarinic receptor antagonist, to the cortical surface. In contrast, rGC stimulation induced only slight LTP without direction preference. Considering the different roles of the rGC and cGC in gustatory processing, these characteristic patterns of LTP in the GC may be involved in a mechanism underlying conversion of palatability.

Placing prediction into the fear circuit

Pavlovian fear conditioning depends on synaptic plasticity at amygdala neurons. Here, we review recent electrophysiological, molecular and behavioral evidence suggesting the existence of a distributed neural circuitry regulating amygdala synaptic plasticity during fear learning. This circuitry, which involves projections from the midbrain periaqueductal gray region, can be linked to prediction error and expectation modulation of fear learning, as described by associative and computational learning models. It controls whether, and how much, fear learning occurs by signaling aversive events when they are unexpected. Functional neuroimaging and clinical studies indicate that this prediction circuit is recruited in humans during fear learning and contributes to exposure-based treatments for clinical anxiety. This aversive prediction error circuit might represent a conserved mechanism for regulating fear learning in mammals.

Corticofugal direct projections to primary afferent neurons in the trigeminal mesencephalic nucleus of rats

Little is known about projections from the cerebral cortex to the trigeminal mesencephalic nucleus (Vmes) which contains the cell bodies of primary sensory afferents innervating masticatory muscle spindles and periodontal ligaments of the teeth. To address this issue, we employed retrograde (Fluorogold, FG) and anterograde (biotinylated dextranamine, BDA) tracing techniques in the rat. After injections of FG into the Vmes, a large number of neurons were retrogradely labeled in the prefrontal cortex including the medial agranular cortex, anterior cingulate cortex, prelimbic cortex, infralimbic cortex, deep peduncular cortex and insular cortex; the labeling was bilateral, but with an ipsilateral predominance to the injection site. Almost no FG-labeled neurons were found in the somatic sensorimotor cortex. After BDA injections into the prefrontal cortex, anterogradely labeled axon fibers and boutons were distributed bilaterally in a topographic pattern within the Vmes, but with an ipsilateral predominance to the injection site. The rostral Vmes received more preferential projections from the medial agranular cortex, while the deep peduncular cortex and insular cortex projected more preferentially to the caudal Vmes. Several BDA-labeled axonal boutons made close associations (possible synaptic contacts) with the cell bodies of Vmes neurons. The present results have revealed the direct projections from the prefrontal cortex to the primary sensory neurons in the Vmes and their unique features, suggesting that deep sensory inputs conveyed by the Vmes neurons from masticatory muscle spindles and periodontal ligaments are regulated with specific biological significance in terms of the descending control by the cerebral cortex.

Nocistatin excites rostral agranular insular cortex-periaqueductal gray projection neurons by enhancing transient receptor potential cation conductance via G(alphaq/11)-PLC-protein kinase C pathway

Rostral agranular insular cortex (RAIC) projects to periaqueductal gray (PAG) and inhibits spinal nociceptive transmission by activating PAG-rostral ventromedial medulla (RVM) descending antinociceptive circuitry. Despite being generated from the same precursor prepronociceptin, nocistatin (NST) and nociceptin/orphanin FQ (N/OFQ) produce supraspinal analgesic and hyperalgesic effects, respectively. Prepronociceptin is highly expressed in the RAIC. In the present study, we hypothesized that NST and N/OFQ modulate spinal pain transmission by regulating the activity of RAIC neurons projecting to ventrolateral PAG (RAIC-PAG). This hypothesis was tested by investigating electrophysiological effects of N/OFQ and NST on RAIC-PAG projection neurons in brain slice. Retrogradely labeled RAIC-PAG projection neurons are layer V pyramidal cells and express mRNA of vesicular glutamate transporter subtype 1, a marker for glutamatergic neurons. N/OFQ hyperpolarized 25% of RAIC-PAG pyramidal neurons by enhancing inwardly rectifying potassium conductance via pertussis toxin-sensitive G(alphai/o). In contrast, NST depolarized 33% of RAIC-PAG glutamatergic neurons by causing the opening of canonical transient receptor potential (TRPC) cation channels through G(alphaq/11)-phospholipase C-protein kinase C pathway. There were two separate populations of RAIC-PAG pyramidal neurons, one responding to NST and the other one to N/OFQ. Our results suggest that G(alphaq/11)-coupled NST receptor mediates NST excitation of RAIC-PAG glutamatergic neurons, which is expected to cause the supraspinal analgesia by enhancing the activity of RAIC-PAG-RVM antinociceptive pathway. Opposite effects of NST and N/OFQ on supraspinal pain regulation are likely to result from their opposing effects on RAIC-PAG pyramidal neurons.

Patterns of social-experience-related c-fos and Arc expression in the frontal cortices of rats exposed to saccharin or moderate levels of ethanol during prenatal brain development

Recent findings from our laboratory indicate that alterations in frontal cortex function, structural plasticity, and related social behaviors are persistent consequences of exposure to moderate levels of ethanol during prenatal brain development [24]. Fetal-ethanol-related reductions in the expression of the immediate early genes (IEGs) c-fos and Arc and alterations in dendritic spine density in ventrolateral and medial aspects of frontal cortex suggest a dissociation reminiscent of that described by Kolb et al. [38] in which these aspects of frontal cortex undergo reciprocal experience-dependent changes. In addition to providing a brief review of the available data on social behavior and frontal cortex function in fetal-ethanol-exposed rats, the present paper presents novel data on social-experience-related IEG expression in four regions of frontal cortex (Zilles LO, VLO, Fr1, Fr2) that are evaluated alongside our prior data from AID and Cg3. Social experience in normal rats was related to a distinct pattern of IEG expression in ventrolateral and medial aspects of frontal cortex, with generally greater expression observed in ventrolateral frontal cortex. In contrast, weaker expression was observed in all aspects of frontal cortex in ethanol-exposed rats, with the exception of an experience-related increase in the medial agranular cortex. Behaviors related to social investigation and wrestling/boxing were differentially correlated with patterns of activity-related IEG expression in the regions under investigation for saccharin- and ethanol-exposed rats. These observations suggest that recruitment and expression of IEGs in frontal cortex following social experience are potentially important for understanding the long-term consequences of moderate prenatal ethanol exposure on frontal cortex function, synaptic plasticity, and related behaviors.

Selective changes in inhibition as determinants for limited hyperexcitability in the insular cortex of epileptic rats

The insular cortex (IC) is involved in the generalization of epileptic discharges in temporal lobe epilepsy (TLE), whereas seizures originating in the IC can mimic the epileptic phenotype seen in some patients with TLE. However, few studies have addressed the changes occurring in the IC in TLE animal models. Here, we analyzed the immunohistochemical and electrophysiological properties of IC networks in non-epileptic control and pilocarpine-treated epileptic rats. Neurons identified with a neuron-specific nuclear protein antibody showed similar counts in the two types of tissue but parvalbumin- and neuropeptide Y-positive interneurons were significantly decreased (parvalbumin, approximately -35%; neuropeptide Y, approximately -38%; P < 0.01) in the epileptic IC. Non-adapting neurons were seen more frequently in the epileptic IC during intracellular injection of depolarizing current pulses. In addition, single-shock electrical stimuli elicited network-driven epileptiform responses in 87% of epileptic and 22% of non-epileptic control neurons (P < 0.01) but spontaneous postsynaptic potentials had similar amplitude, duration and intervals of occurrence in the two groups. Finally, pharmacologically isolated, GABA(A) receptor-mediated inhibitory postsynaptic potentials had more negative reversal potential (P < 0.01) and higher peak conductance (P < 0.05) in epileptic tissue. These data reveal moderate increased network excitability in the IC of pilocarpine-treated epileptic rats. We propose that this limited degree of hyperexcitability originates from the loss of parvalbumin- and neuropeptide Y-positive interneurons that is compensated by an increased drive for GABA(A) receptor-mediated inhibition.

Interoception in anxiety and depression

We review the literature on interoception as it relates to depression and anxiety, with a focus on belief, and alliesthesia. The connection between increased but noisy afferent interoceptive input, self-referential and belief-based states, and top-down modulation of poorly predictive signals is integrated into a neuroanatomical and processing model for depression and anxiety. The advantage of this conceptualization is the ability to specifically examine the interface between basic interoception, self-referential belief-based states, and enhanced top-down modulation to attenuate poor predictability. We conclude that depression and anxiety are not simply interoceptive disorders but are altered interoceptive states as a consequence of noisily amplified self-referential interoceptive predictive belief states.

Presynaptic Interneuron Subtype- and Age-Dependent Modulation of GABAergic Synaptic Transmission by β-Adrenoceptors in Rat Insular Cortex

β-Adrenoceptors play a crucial role in the regulation of taste aversion learning in the insular cortex (IC). However, β-adrenergic effects on inhibitory synaptic transmission mediated by γ-aminobutyric acid (GABA) remain unknown. To elucidate the mechanisms of β-adrenergic modulation of inhibitory synaptic transmission, we performed paired whole cell patch-clamp recordings from layer V GABAergic interneurons and pyramidal cells of rat IC aged from postnatal day 17 (PD17) to PD46 and examined the effects of isoproterenol, a β-adrenoceptor agonist, on unitary inhibitory postsynaptic currents (uIPSCs). Isoproterenol (100 μM) induced facilitating effects on uIPSCs in 33.3% of cell pairs accompanied by decreases in coefficient of variation (CV) of the first uIPSC amplitude and paired-pulse ratio (PPR) of the second to first uIPSC amplitude, whereas 35.9% of pairs showed suppressive effects of isoproterenol on uIPSC amplitude obtained from fast spiking (FS) to pyramidal cell pairs. Facilitatory effects of isoproterenol were frequently observed in FS–pyramidal cell pairs at ≥PD24. On the other hand, isoproterenol suppressed uIPSC amplitude by 52.3 and 39.8% in low-threshold spike (LTS)–pyramidal and late spiking (LS)–pyramidal cell pairs, respectively, with increases in CV and PPR. The isoproterenol-induced suppressive effects were blocked by preapplication of 100 μM propranolol, a β-adrenoceptor antagonist. There was no significant correlation between age and changes of uIPSCs in LTS–/LS–pyramidal cell pairs. These results suggest the presence of differential mechanisms in presynaptic GABA release and/or postsynaptic GABAA receptor-related assemblies among interneuron subtypes. Age- and interneuron subtype-specific β-adrenergic modulation of IPSCs may contribute to experience-dependent plasticity in the IC.

Differential brain activation to angry faces by elite warfighters: neural processing evidence for enhanced threat detection

BACKGROUND

Little is known about the neural basis of elite performers and their optimal performance in extreme environments. The purpose of this study was to examine brain processing differences between elite warfighters and comparison subjects in brain structures that are important for emotion processing and interoception.

METHODOLOGY/PRINCIPAL FINDINGS

Navy Sea, Air, and Land Forces (SEALs) while off duty (n = 11) were compared with n = 23 healthy male volunteers while performing a simple emotion face-processing task during functional magnetic resonance imaging. Irrespective of the target emotion, elite warfighters relative to comparison subjects showed relatively greater right-sided insula, but attenuated left-sided insula, activation. Navy SEALs showed selectively greater activation to angry target faces relative to fearful or happy target faces bilaterally in the insula. This was not accounted for by contrasting positive versus negative emotions. Finally, these individuals also showed slower response latencies to fearful and happy target faces than did comparison subjects.

CONCLUSIONS/SIGNIFICANCE

These findings support the hypothesis that elite warfighters deploy greater processing resources toward potential threat-related facial expressions and reduced processing resources to non-threat-related facial expressions. Moreover, rather than expending more effort in general, elite warfighters show more focused neural and performance tuning. In other words, greater neural processing resources are directed toward threat stimuli and processing resources are conserved when facing a nonthreat stimulus situation.

Glossopharyngeal Neuralgia Triggered by Non-Noxious Stimuli at Multiple Cephalic and Extracephalic Sites

Glossopharyngeal neuralgia (GN) triggered by non-noxious stimuli at multiple cephalic and extracephalic sites with positron emission tomography (PET) evidence for involvement of the upper brainstem has never been reported. We present such a patient, a 73-year-old man who since the age of 50 had suffered from GN with a high recurrence rate and very severe unilateral, non-familial GN episodes with very easy trigger zones widely extending beyond the n IX territory. Extensive neuroimaging and neurophysiological tests detected no precise underlying cause. PET scan revealed activation in the upper brainstem on extracephalic triggers. Single-fibre electromyography data will be discussed. We hypothesize that deficient inhibition as seen in trigeminal nociceptive reflexes on the level of brainstem interneurons, a functional lesion in the primary somatosensory cortex-sensory thalamic nuclei circuit and the dorsal column-thalamic pathway both activated by light touch may in part be involved in the extra- cephalic triggering.

Spatiotemporal dynamics of excitation in rat insular cortex: intrinsic corticocortical circuit regulates caudal-rostro excitatory propagation from the insular to frontal cortex

The insular cortex (IC), composing unique anatomical connections, receives multi-modal sensory inputs including visceral, gustatory and somatosensory information from sensory thalamic nuclei. Axonal projections from the limbic structures, which have a profound influence on induction of epileptic activity, also converge onto the IC. However, functional connectivity underlying the physiological and pathological roles characteristic to the IC still remains unclear. The present study sought to elucidate the spatiotemporal dynamics of excitatory propagation and their cellular mechanisms in the IC using optical recording in urethane-anesthetized rats. Repetitive electrical stimulations of the IC at 50 Hz demonstrated characteristic patterns of excitatory propagation depending on the stimulation sites. Stimulation of the granular zone of the IC (GI) and other surrounding cortices such as the motor/primary sensory/secondary sensory cortices evoked round-shaped excitatory propagations, which often extended over the borders of adjacent areas, whereas excitation of the agranular and dysgranular zones in the IC (AI and DI, respectively) spread along the rostrocaudal axis parallel to the rhinal fissure. Stimulation of AI/DI often evoked excitation in the dorsolateral orbital cortex, which exhibited spatially discontinuous topography of excitatory propagation in the IC. Pharmacological manipulations using 6,7-dinitroquinoxaline-2,3(1H,4H)-dione (DNQX), a non-NMDA receptor antagonist, D-2-amino-5-phosphonovaleric acid (D-APV), an NMDA receptor antagonist, and bicuculline methiodide, a GABA(A) receptor antagonist, indicate that excitatory propagation was primarily regulated by non-NMDA and GABA(A) receptors. Microinjection of lidocaine or incision of the supragranular layers of the rostrocaudally middle part of excitatory regions suppressed excitation in the remote regions from the stimulation site, suggesting that the excitatory propagation in the IC is largely mediated by cortical local circuits. These features of excitatory propagation in the AI/DI, that is the propagation along the rostrocaudal axis with less propagation in the ventro-dorsal direction, may play an important role for transmitting neural excitation arising from the limbic structures to the frontal and orbital cortices.

MRI structural brain changes associated with sensory and emotional function in a rat model of long-term neuropathic pain

In human conditions, chronic pain is associated with widespread anatomical changes in the brain. Nevertheless, little is known about the time course of these changes or the relationship of anatomical changes to perception and behaviour. In the present study, we use a rat model of neuropathic pain (spared nerve injury, SNI) and 7 T MRI to determine the longitudinal supraspinal changes associated with pain-like and anxiety-like behaviours. SNI rats and sham controls were scanned at seven time points, 1 week before surgery, 2 weeks after, and then once a month for 5 months. At each time point we performed behavioural tests, including thermal and mechanical sensitivity, and tests of locomotion and exploratory behaviour (open field and elevated plus maze). We found that SNI rats had early and sustained thermal and mechanical hyperalgesia, and developed anxiety-like behaviours several months after injury. Compared to sham controls, SNI rats had decreased frontal cortex volumes several months after surgery, coincident with the onset of anxiety-like behaviours. There was also decreased volume in retrosplenial and entorhinal cortices. We also explored areas that correlated with mechanical hyperalgesia and found that increased hyperalgesia was associated with decreased volumes in bilateral S1 hindlimb area, anterior cingulate cortex (ACC, areas 32 and 24), and insula. Overall, our results suggest that long-term neuropathic pain has widespread effects on brain anatomy related to the duration and magnitude of the pain.

The role of interoception and alliesthesia in addiction

This review presents a novel conceptualization of addiction, integrating the concepts of interoception (i.e., the CNS representation of visceral feelings) and alliesthesia (i.e., that rewarding properties of stimuli are dependent on the internal state of the individual) with existing theories. It is argued that the body state, as defined by the integration of interoceptive information, is a crucial arbiter of the risk for initiation of and transition to compulsive use of addictive compounds. Overall, individuals at risk for drug dependence are characterized by an altered internal bodily state that leads to a change in hedonic and incentive motivational properties of addictive drugs. Specifically, drug dependent individuals experience alliesthesia of interoceptive processing, leading to increased incentive motivational properties of the drug over time and thereby increasing the probability of subsequent use. This extension of previous theories of addiction to include interoception and alliesthesia is based upon a clearly delineated set of neural substrates mediating interoception, key elements of which also recently have been implicated in drug addiction. The model thereby provides new potential targets for interventions that are aimed at changing the internal state that puts the individual at risk for continued substance use.

Anatomical connections between brain areas activated during rectal distension in healthy volunteers: a visceral pain network

Diffusion Tensor Imaging (DTI) is a promising new imaging method allowing in vivo mapping of anatomical connections in the living human brain. We combined DTI with functional magnetic resonance imaging (fMRI) to investigate the anatomical relationships between areas involved in visceral sensations in humans. Non-painful and moderately painful rectal distensions were performed in 11 healthy women (38.4+/-3.1years). fMRI was used to analyse the changes in brain activity during both types of distension. Then, DTI was applied for tracking fibers between the main activated regions. Non-painful distension bilaterally activated the PreFrontal Cortex (PFC), the Anterior Cingulate Cortex (ACC) and the right insula. Painful distension bilaterally activated the primary (S1) and secondary (S2) somatosensory cortices, the motor cortex, the frontal inferior gyrus, the thalamus, the insula, the striatum and the cerebellum. DTI revealed direct connections between insula, and the four areas more frequently activated in this study, i.e. ACC, thalamus, S1, S2 and PFC. The combined use of fMRI and DTI in healthy subjects during rectal distension revealed a neural network of visceral sensory perception involving the insula, thalamus, somatosensory cortices, ACC and PFC.

Cerebral cortex modulation of pain

Pain is a complex experience encompassing sensory-discriminative, affective-motivational and cognitiv e-emotional components mediated by different mechanisms. Contrary to the traditional view that the cerebral cortex is not involved in pain perception, an extensive cortical network associated with pain processing has been revealed using multiple methods over the past decades. This network consistently includes, at least, the anterior cingulate cortex, the agranular insular cortex, the primary (SI) and secondary somatosensory (SII) cortices, the ventrolateral orbital cortex and the motor cortex. These cortical structures constitute the medial and lateral pain systems, the nucleus submedius-ventrolateral orbital cortex-periaqueductal gray system and motor cortex system, respectively. Multiple neurotransmitters, including opioid, glutamate, GABA and dopamine, are involved in the modulation of pain by these cortical structures. In addition, glial cells may also be involved in cortical modulation of pain and serve as one target for pain management research. This review discusses recent studies of pain modulation by these cerebral cortical structures in animals and human.

Nociceptive behavior in animal models for peripheral neuropathy: spinal and supraspinal mechanisms

Since the initial description by Wall [Wall, P.D., 1967. The laminar organization of dorsal horn and effects of descending impulses. J. Neurophysiol. 188, 403-423] of tonic descending inhibitory control of dorsal horn neurons, several studies have aimed to characterize the role of various brain centers in the control of nociceptive input to the spinal cord. The role of brainstem centers in pain inhibition has been well documented over the past four decades. Lesion to peripheral nerves results in hypersensitivity to mild tactile or cold stimuli (allodynia) and exaggerated response to nociceptive stimuli (hyperalgesia), both considered as cardinal signs of neuropathic pain. The increased interest in animal models for peripheral neuropathy has raised several questions concerning the rostral conduction of the neuropathic manifestations and the role of supraspinal centers, especially brainstem, in the inhibitory control or in the abnormal contribution to the maintenance and facilitation of neuropathic-like behavior. This review aims to summarize the data on the ascending and descending modulation of neuropathic manifestations and discusses the recent experimental data on the role of supraspinal centers in the control of neuropathic pain. In particular, the review emphasizes the importance of the reciprocal interconnections between the analgesic areas of the brainstem and the pain-related areas of the forebrain. The latter includes the cerebral limbic areas, the prefrontal cortex, the intralaminar thalamus and the hypothalamus and play a critical role in the control of pain considered as part of an integrated behavior related to emotions and various homeostatic regulations. We finally speculate that neuropathic pain, like extrapyramidal motor syndromes, reflects a disorder in the processing of somatosensory information.

Neural basis of reward and craving--a homeostatic point of view

Here, it is argued that the interoceptive system, which provides information about the subject's internal state and is integrated in the insular cortex, and not the subcortical ventral striatum, is the critical neural substrate for reward-related processes. Understanding the internal state of the individual, which is processed via this system, makes it possible to develop new interventions that are aimed at treating reward-dysfunction disorders, ie, substance and alcohol dependence. Although the ventral striatum is important for signaling the degree to which rewarding stimuli are predicted to occur, this system alone cannot account for the complex affective, cognitive, and behavioral phenomena that occur when individuals come into contact with potentially rewarding stimuli. On the other hand, the interoceptive system is able to make connections between all cortical, subcortical, and limbic systems to orchestrate a complex set of responses. Craving and urges are among the most notable responses, and may have important functions to preserve homeostasis.

Middle short gyrus of the insula implicated in pain processing

Different lines of evidence have suggested an involvement of the insular cortex in pain processing. Direct electrical stimulation (ES) of the human insular cortex during surgical procedure sometimes induces painful sensations and painful stimuli induce activation of the insular cortex as shown by functional neuroimaging. Invasive evaluation of epileptic patients by deep brain stereotactically implanted electrodes provides an opportunity to analyze responses induced by ES of the insular cortex in awake and fully conscious patients. For this study, we included 25 patients suffering from drug refractory focal epilepsy with at least one electrode stereotactically implanted in the insular cortex using an oblique approach (transfrontal or transparietal). Out of the 83 responses induced by insular ES, eight (9.6%) were reported by five patients as painful sensations. Four were restricted to the cephalic region and four were felt on the ipsilateral or bilateral upper limbs, the shoulders and the trunk (pinprick sensations). The eight stimulation sites were anatomically localized via image fusion between pre-implantation 3D MRI and post-implantation 3D CT scans revealing the electrode contacts. All sites inducing painful sensations were restricted to the upper portion of the middle short gyrus of the insula. The findings of this study suggest that middle short gyrus is involved in the processing of pain-producing stimuli.

Epileptiform synchronization in the rat insular and perirhinal cortices in vitro

The hippocampus plays a primary role in temporal lobe epilepsy, a common form of partial epilepsy in adults. Recent studies, however, indicate that extrahippocampal areas such as the perirhinal and insular cortices represent important participants in this epileptic disorder. By employing field potential recordings in the in vitro 4-aminopyridine model of temporal lobe epilepsy, we have investigated here the contribution of glutamatergic and GABAergic signaling to epileptiform activity in these structures. First, we provide evidence of epileptiform synchronicity between the perirhinal and insular cortices, and resolve some pharmacological and network mechanisms involved in sustaining the interictal- and ictal-like discharges recorded there. Second, we report that in the absence of ionotropic glutamatergic transmission, GABAergic networks produce synchronous potentials that spread between the perirhinal and insular cortices. Finally, we have established that such activity is modulated by activating micro-opioid receptors. Our findings support clinical and experimental evidence concerning the involvement of the perirhinal and insular cortex networks in temporal lobe epilepsy, and provide observations that may impact research focussing on the role of the insular cortex in nociception.

Dopamine receptors in the anterior insular cortex modulate long-term nociception in the rat

The rostral agranular insular cortex (RAIC) receives dopaminergic projections from the mesolimbic system, which has been involved in the modulation of nociceptive processes. In this study we determined the contribution of dopamine D(1) and D(2) receptors in the RAIC regarding nociception processing in a neuropathic pain model, as well as inflammatory articular nociception measured as pain-induced functional impairment in the rat (PIFIR). Microinjection of vehicle or substances into the RAIC was performed after the induction of nociception. The groups were treated with: a dopamine D(1) receptor antagonist (SCH-23390), a dopamine D(1) receptor agonist (SKF-38393), a dopamine D(2) receptor agonist (TNPA) and a dopamine D(2) receptor antagonist (spiperone). Chronic nociception, induced by denervation, was measured by the autotomy score in which onset and incidence were also determined. The SCH-23390 and TNPA groups showed a decrease in the autotomy score and a delay on the onset as compared to control, whereas the PIFIR groups did not show statistical differences. This work shows the differential role of dopamine receptors within the RAIC in which the activation of D(2) or the blockade of D(1) receptors elicit antinociception.