Spinal sources of noxious visceral and noxious deep somatic afferent drive onto the ventrolateral periaqueductal gray of the rat

Electrophysiology as a Tool to Decipher the Network Mechanism of Visceral Pain in Functional Gastrointestinal Disorders

Abdominal pain, including visceral pain, is prevalent in functional gastrointestinal (GI) disorders (FGIDs), affecting the overall quality of a patient's life. Neural circuits in the brain encode, store, and transfer pain information across brain regions. Ascending pain signals actively shape brain dynamics; in turn, the descending system responds to the pain through neuronal inhibition. Pain processing mechanisms in patients are currently mainly studied with neuroimaging techniques; however, these techniques have a relatively poor temporal resolution. A high temporal resolution method is warranted to decode the dynamics of the pain processing mechanisms. Here, we reviewed crucial brain regions that exhibited pain-modulatory effects in an ascending and descending manner. Moreover, we discussed a uniquely well-suited method, namely extracellular electrophysiology, that captures natural language from the brain with high spatiotemporal resolution. This approach allows parallel recording of large populations of neurons in interconnected brain areas and permits the monitoring of neuronal firing patterns and comparative characterization of the brain oscillations. In addition, we discussed the contribution of these oscillations to pain states. In summary, using innovative, state-of-the-art methods, the large-scale recordings of multiple neurons will guide us to better understanding of pain mechanisms in FGIDs.

Detailed organisation of the human midbrain periaqueductal grey revealed using ultra-high field magnetic resonance imaging

The midbrain periaqueductal grey (PAG) is a critical region for the mediation of pain-related behavioural responses. Neuronal tract tracing techniques in experimental animal studies have demonstrated that the lateral column of the PAG (lPAG) displays a crude somatotopy, which is thought to be critical for the selection of contextually appropriate behavioural responses, without the need for higher brain input. In addition to the different behavioural responses to cutaneous and muscle pain - active withdrawal versus passive coping - there is evidence that cutaneous pain is processed in the region of the lPAG and muscle pain in the adjacent ventrolateral PAG (vlPAG). Given the fundamental nature of these behavioural responses to cutaneous and muscle pain, these PAG circuits are assumed to have been preserved, though yet to be definitively documented in humans. Using ultra-high field (7-Tesla) functional magnetic resonance imaging we determined the locations of signal intensity changes in the PAG during noxious cutaneous heat stimuli and muscle pain in healthy control participants. Images were processed and blood oxygen level dependant (BOLD) signal changes within the PAG determined. It was observed that noxious cutaneous stimulation of the lip, cheek, and ear evoked maximal increases in BOLD activation in the rostral contralateral PAG, whereas noxious cutaneous stimulation of the thumb and toe evoked increases in the caudal contralateral PAG. Analysis of individual participants demonstrated that these activations were located in the lPAG. Furthermore, we found that deep muscular pain evoked the greatest increases in signal intensity in the vlPAG. These data suggest that the crude somatotopic organization of the PAG may be phyletically preserved between experimental animals and humans, with a body-face delineation capable of producing an appropriate behavioural response based on the location and tissue origin of a noxious stimulus.

Evidence that increased cholecystokinin (CCK) in the periaqueductal gray (PAG) facilitates changes in Resident-Intruder social interactions triggered by peripheral nerve injury

Individual differences in the effects of a chronic neuropathic injury on social behaviours characterize both the human experience and pre-clinical animal models. The impacts of these changes to the well-being of the individual are often underappreciated. Earlier work from our laboratory using GeneChip® microarrays identified increased cholecystokinin (CCK) gene expression in the periaqueductal gray (PAG) of rats that showed persistent changes in social interactions during a Resident-Intruder encounter following sciatic nerve chronic constriction injury (CCI). In this study, we confirmed these gene regulation patterns using RT-PCR and identified the anatomical location of the CCK-mRNA as well as the translated CCK peptides in the midbrains of rats with a CCI. We found that rats with persistent CCI-induced changes in social behaviours had increased CCK-mRNA in neurons of the ventrolateral PAG and dorsal raphe nuclei, as well as increased CCK-8 peptide expression in terminal boutons located in the lateral and ventrolateral PAG. The functional significance of these changes was explored by microinjecting small volumes of CCK-8 into the PAG of uninjured rats and observing their Resident-Intruder social interactions. Disturbances to social interactions identical to those observed in CCI rats were evoked when injection sites were located in the rostral lateral and ventrolateral PAG. We suggest that CCI-induced changes in CCK expression in these PAG regions contributes to the disruptions to social behaviours experienced by a subset of individuals with neuropathic injury.

A novel visceral pain model of uterine cervix inflammation in rat

Treatment of visceral pain originating from the uterine cervix is a substantial clinical problem. The underlying mechanisms of such visceral pain remain unclear mainly due to a lack of reliable model. This study aimed to develop and evaluate the performance of a rat model of pain induced by uterine cervix inflammation. Rats were randomized to six groups according to the solution injected into the uterine cervix: normal saline, vehicle, capsaicin (0.3 mg, 0.6 mg, 0.9 mg), capsaicin 0.9 mg + morphine (n = 15 in each group). Spontaneous behaviors after cervical injection were recorded by a computerized video system and analyzed offline. An equation for calculating a novel pain score was derived from particular behaviors, based on Pearson's correlation analysis and regression analysis. c-Fos expression in the spinal cord was detected. The pain score and c-fos expression in the spinal cord were highest in the 0.9 mg capsaicin group and lowest in the normal saline and vehicle groups (P < 0.05). Intrathecal morphine significantly decreased the pain score (P < 0.05) and c-fos expression in the spinal cord (P < 0.05). Injection of capsaicin into the uterine cervix in rats could be a practical model of inflammatory cervical pain, which can be evaluated using our novel pain score. This model will provide further insight into the mechanism underlying visceral pain originating from the uterine cervix.

Cutaneous Hypersensitivity as an Indicator of Visceral Inflammation via C-Nociceptor Axon Bifurcation

Pain on the body surface can accompany disorders in the deep tissue or internal organs. However, the anatomical and physiological mechanisms are obscure. Here, we provided direct evidence of axon bifurcation in primary C-nociceptive neurons that innervate both the skin and a visceral organ. Double-labeled dorsal root ganglion (DRG) neurons and Evans blue extravasation were observed in 3 types of chemically-induced visceral inflammation (colitis, urocystitis, and acute gastritis) rat models. In the colitis model, mechanical hypersensitivity and spontaneous activity were recorded in vivo from double-labeled C-nociceptive neurons in S1 or L6 DRGs. These neurons showed significantly enhanced responses to both somatic stimulation and colorectal distension. Our findings suggest that the branching of C-nociceptor axons contribute to cutaneous hypersensitivity in visceral inflammation. Cutaneous hypersensitivity on certain locations of the body surface might serve as an indicator of pathological conditions in the corresponding visceral organ.

Effect of chronic pretreatment with 17β-estradiol and/or progesterone on the nociceptive response to uterine cervical distension in a rat model

It is widely known that visceral pain is more prevalent in women than in men, and this phenomenon is interpreted as a consequence of the gonadal hormone modulation of pain perception and transduction. Uterine cervical distension might cause obstetric and gynecologic pain with clinical relevance to visceral pain. In this study, we focused on the roles of 17β-estradiol and progesterone in visceral nociception with the use of a rat model of uterine cervical distension. Female ovariectomized rats were injected with 17β-estradiol (E₂) or progesterone (P₄) for 21 days, after which visceral pain-induced spinal c-fos expression and visceromotor reflex changes were compared between ovariectomized and hormone-substituted groups. We found that uterine cervical distension induced a drastic increase in spinal c-fos expression and visceromotor reflex activity, and ovariectomy inhibited the increase in c-fos expression induced by visceral pain; this inhibition was reversed by estrogen but not progesterone replacement. This study demonstrates that estrogen is involved in uterine cervical nociception, while progesterone plays less of a significant role.

A Neuroscience Perspective of Physical Treatment of Headache and Neck Pain

The most prevalent primary headaches tension-type headache and migraine are frequently associated with neck pain. A wide variety of treatment options is available for people with headache and neck pain. Some of these interventions are recommended in guidelines on headache: self-management strategies, pharmacological and non-pharmacological interventions. Physical treatment is a frequently applied treatment for headache. Although this treatment for headache is predominantly targeted on the cervical spine, the neurophysiological background of this intervention remains unclear. Recent knowledge from neuroscience will enhance clinical reasoning in physical treatment of headache. Therefore, we summarize the neuro- anatomical and—physiological findings on headache and neck pain from experimental research in both animals and humans. Several neurophysiological models (referred pain, central sensitization) are proposed to understand the co-occurrence of headache and neck pain. This information can be of added value in understanding the use of physical treatment as a treatment option for patients with headache and neck pain.

The brainstem network controlling blood pressure: an important role for pressor sites in the caudal medulla and cervical spinal cord

: Although medullary control of blood pressure (BP) has been extensively studied, the contribution of critical regions, such as pressor sites in the caudal medulla and upper cervical spinal cord and the lateral tegmental field, remains controversial and underappreciated. A series of pressor sites caudal to the caudal ventrolateral medulla (CVLM), including the caudal pressor area (CPA) and medullocervical pressor area, play an important role in control of BP. Activation and inhibition of these sites elicits pressor and depressor responses, respectively. Basal sympathetic tone is provided principally by the medullary lateral tegmental field and rostral ventrolateral medulla (RVLM). RVLM presympathetic neurons, which project to and drive preganglionic sympathetic somata in the intermediolateral cell column, are powerfully regulated by neurons in CVLM via tonic and phasic inhibition. The current state of knowledge is summarized thus: rostrocaudally organized columns of pressor sites caudal to CVLM extend to the upper cervical spinal cord; CPA pressor responses are RVLM-dependent; CPA mediates pressor responses by (first) inhibiting RVLM-projecting inhibitory CVLM units and (second) activating RVLM-projecting excitatory CVLM units; the chemoreflex is CPA-dependent; the baroreflex is CPA-independent; pressor responses to raphe obscurus stimulation are CPA-dependent; and medullocervical pressor area pressor responses are RVLM-independent, likely mediated by direct projections to the intermediolateral cell column. In this review, we seek to underscore and characterize the critical role played by the caudal medulla and upper cervical spinal cord in BP regulation and highlight important gaps in knowledge in interactions between the caudal medulla and other regions controlling BP, which may prove critical in revealing central mechanisms underlying pathophysiology of, and pharmacotherapeutic targets for, hypertension.

Pharmacological sensitivity of reflexive and nonreflexive outcomes as a correlate of the sensory and affective responses to visceral pain in mice

Pain encompasses both sensory and affective dimensions which can be differentially modulated by drugs. Here, we compare the pharmacological sensitivity of the sensory and affective responses using acetic acid-induced abdominal writhings (sensory-reflexive outcome) and acetic acid-induced depression of reward seeking behaviour (RSB, affective-nonreflexive outcome) to a highly palatable food in mice. We found that the expression of RSB critically depends on factors such as sex and previous knowledge and type of the food stimulus. Intraperitoneal administration of acetic acid (iAA) produced a long-lasting (beyond the resolution of writhing behaviour) and concentration-dependent decrease on both appetitive-approach and consummatory dimensions of RSB. Ibuprofen and diclofenac were much more potent in reversing AA-induced changes in RSB: latency to eat (ED₅₀ = 2 and 0.005 mg/kg, intraperinoneally, respectively) and amount consumed (ED₅₀ = 11 and 0.1 mg/kg) than in AA-induced writhing (ED₅₀ = 123 and 60 mg/kg). Morphine and duloxetine inhibited the writhing response (ED₅₀ = 0.8 and 6 mg/kg, respectively) but not the AA-induced changes in RSB. Caffeine was ineffective in both AA-induced writhing and RSB changes. Overall, this study characterized a preclinical mouse model of hedonic deficits induced by pain that can be used to assess affective responses as well as complementary classic reflexive approaches in the evaluation of candidate analgesics.

Changes in saccharin preference behavior as a primary outcome to evaluate pain and analgesia in acetic acid-induced visceral pain in mice

Reflex-based procedures are important measures in preclinical pain studies that evaluate stimulated behaviors. These procedures, however, are insufficient to capture the complexity of the pain experience, which is often associated with the depression of several innate behaviors. While recent studies have made efforts to evidence the suppression of some positively motivated behaviors in certain pain models, they are still far from being routinely used as readouts for analgesic screening. Here, we characterized and compared the effect of the analgesic ibuprofen (Ibu) and the stimulant, caffeine, in assays of acute pain-stimulated and pain-depressed behavior. Intraperitoneal injection of acetic acid (AA) served as a noxious stimulus to stimulate a writhing response or depress saccharin preference and locomotor activity (LMA) in mice. AA injection caused the maximum number of writhes between 5 and 20 minutes after administration, and writhing almost disappeared 1 hour later. AA-treated mice showed signs of depression-like behaviors after writhing resolution, as evidenced by reduced locomotion and saccharin preference for at least 4 and 6 hours, respectively. Depression-like behaviors resolved within 24 hours after AA administration. A dose of Ibu (40 mg/kg) – inactive to reduce AA-induced abdominal writhing – administered before or after AA injection significantly reverted pain-induced saccharin preference deficit. The same dose of Ibu also significantly reverted the AA-depressed LMA, but only when it was administered after AA injection. Caffeine restored locomotion – but not saccharin preference – in AA-treated mice, thus suggesting that the reduction in saccharin preference – but not in locomotion – was specifically sensitive to analgesics. In conclusion, AA-induced acute pain attenuated saccharin preference and LMA beyond the resolution of writhing behavior, and the changes in the expression of hedonic behavior, such as sweet taste preference, can be used as a more sensitive and translational model to evaluate analgesics.

Fear and the Defense Cascade: Clinical Implications and Management

Evolution has endowed all humans with a continuum of innate, hard-wired, automatically activated defense behaviors, termed the defense cascade. Arousal is the first step in activating the defense cascade; flight or fight is an active defense response for dealing with threat; freezing is a flight-or-fight response put on hold; tonic immobility and collapsed immobility are responses of last resort to inescapable threat, when active defense responses have failed; and quiescent immobility is a state of quiescence that promotes rest and healing. Each of these defense reactions has a distinctive neural pattern mediated by a common neural pathway: activation and inhibition of particular functional components in the amygdala, hypothalamus, periaqueductal gray, and sympathetic and vagal nuclei. Unlike animals, which generally are able to restore their standard mode of functioning once the danger is past, humans often are not, and they may find themselves locked into the same, recurring pattern of response tied in with the original danger or trauma. Understanding the signature patterns of these innate responses--the particular components that combine to yield the given pattern of defense-is important for developing treatment interventions. Effective interventions aim to activate or deactivate one or more components of the signature neural pattern, thereby producing a shift in the neural pattern and, with it, in mind-body state. The process of shifting the neural pattern is the necessary first step in unlocking the patient's trauma response, in breaking the cycle of suffering, and in helping the patient to adapt to, and overcome, past trauma.

Identification of neural networks that contribute to motion sickness through principal components analysis of fos labeling induced by galvanic vestibular stimulation

Motion sickness is a complex condition that includes both overt signs (e.g., vomiting) and more covert symptoms (e.g., anxiety and foreboding). The neural pathways that mediate these signs and symptoms are yet to identified. This study mapped the distribution of c-fos protein (Fos)-like immunoreactivity elicited during a galvanic vestibular stimulation paradigm that is known to induce motion sickness in felines. A principal components analysis was used to identify networks of neurons activated during this stimulus paradigm from functional correlations between Fos labeling in different nuclei. This analysis identified five principal components (neural networks) that accounted for greater than 95% of the variance in Fos labeling. Two of the components were correlated with the severity of motion sickness symptoms, and likely participated in generating the overt signs of the condition. One of these networks included neurons in locus coeruleus, medial, inferior and lateral vestibular nuclei, lateral nucleus tractus solitarius, medial parabrachial nucleus and periaqueductal gray. The second included neurons in the superior vestibular nucleus, precerebellar nuclei, periaqueductal gray, and parabrachial nuclei, with weaker associations of raphe nuclei. Three additional components (networks) were also identified that were not correlated with the severity of motion sickness symptoms. These networks likely mediated the covert aspects of motion sickness, such as affective components. The identification of five statistically independent component networks associated with the development of motion sickness provides an opportunity to consider, in network activation dimensions, the complex progression of signs and symptoms that are precipitated in provocative environments. Similar methodology can be used to parse the neural networks that mediate other complex responses to environmental stimuli.

Human brain stem structures respond differentially to noxious heat

Concerning the physiological correlates of pain, the brain stem is considered to be one core region that is activated by noxious input. In animal studies, different slopes of skin heating (SSH) with noxious heat led to activation in different columns of the midbrain periaqueductal gray (PAG). The present study aimed at finding a method for differentiating structures in PAG and other brain stem structures, which are associated with different qualities of pain in humans according to the structures that were associated with different behavioral significances to noxious thermal stimulation in animals. Brain activity was studied by functional MRI in healthy subjects in response to steep and shallow SSH with noxious heat. We found differential activation to different SSH in the PAG and the rostral ventromedial medulla (RVM). In a second experiment, we demonstrate that the different SSH were associated with different pain qualities. Our experiments provide evidence that brainstem structures, i.e., the PAG and the RVM, become differentially activated by different SSH. Therefore, different SSH can be utilized when brain stem structures are investigated and when it is aimed to activate these structures differentially. Moreover, percepts of first pain were elicited by shallow SSH whereas percepts of second pain were elicited by steep SSH. The stronger activation of these brain stem structures to SSH, eliciting percepts of second vs. first pain, might be of relevance for activating different coping strategies in response to the noxious input with the two types of SSH.

Tetrodotoxin-resistant fibres and spinal Fos expression: differences between input from muscle and skin

Nociceptive information from muscle and skin is differently processed at many levels of the central nervous system. In most articles on this issue, noxious stimuli were used that also excited non-nociceptive receptors. The effects of a pure nociceptive input from muscle or skin on spinal neurones are largely unknown. The aim of the study was to find out whether the Fos-protein expression in dorsal horn neurones induced by an exclusively nociceptive muscle input differs from that of the skin. Fos-proteins are transcription factors that regulate neuronal gene expression and induce neuroplastic effects that are involved in the development of chronic pain. A pure nociceptive input was achieved by tetrodotoxin (TTX) that is known to block all TTX-sensitive afferents and leave the TTX-resistant (TTX-r), presumably nociceptive, afferent fibres intact. We studied the c-Fos and FosB expression in the spinal cord following electrical stimulation of TTX-r afferent fibres in the gastrocnemius-soleus nerve (muscle) and compared it to the sural nerve (skin). In the spinal dorsal horn, the main effect of a TTX-r input from muscle was an increase in FosB (P < 0.05), but not in c-Fos expression (P = 0.51). In contrast, an input from the skin induced both FosB (P < 0.01) and c-Fos expression (P < 0.05). The data indicate that in the spinal, dorsal horn nociceptive input from skin and muscle has different effects on the Fos expression. The only effect of muscle input was an increase in FosB expression while skin input increased both c-Fos and FosB expression.

Ultrastructural analysis of rat ventrolateral periaqueductal gray projections to the A5 cell group

Stimulation of neurons in the ventrolateral periaqueductal gray (PAG) produces antinociception as well as cardiovascular depressor responses that are mediated in part by pontine noradrenergic neurons. A previous report using light microscopy has described a pathway from neurons in the ventrolateral PAG to noradrenergic neurons in the A5 cell group that may mediate these effects. The present study used anterograde tracing and electron microscopic analysis to provide more definitive evidence that neurons in the ventrolateral PAG form synapses with noradrenergic and non-catecholaminergic A5 neurons in Sasco Sprague-Dawley rats. Deposits of anterograde tracer, biotinylated dextran amine, into the rat ventrolateral PAG labeled a significant number of axons in the region of the rostral subdivision of the A5 cell group, and a relatively lower number in the caudal A5 cell group. Electron microscopic analysis of anterogradely-labeled terminals in both rostral (n=127) and caudal (n=70) regions of the A5 cell group indicated that approximately 10% of these form synapses with noradrenergic dendrites. In rostral sections, about 31% of these were symmetric synapses, 19% were asymmetric synapses, and 50% were membrane appositions without clear synaptic specializations. In caudal sections, about 22% were symmetric synapses, and the remaining 78% were appositions. In both rostral and caudal subdivisions of the A5, nearly 40% of the anterogradely-labeled terminals formed synapses with non-catecholaminergic dendrites, and about 45% formed axoaxonic synapses. These results provide direct evidence for a monosynaptic pathway from neurons in the ventrolateral PAG to noradrenergic and non-catecholaminergic neurons in the A5 cell group. Further studies should evaluate if this established monosynaptic pathway may contribute to the cardiovascular depressor effects or the analgesia produced by the activation of neurons in the ventrolateral PAG.

Differential ascending projections of temporomandibular joint-responsive brainstem neurons to periaqueductal gray and posterior thalamus of male and female rats

Several craniofacial pain conditions, including temporomandibular joint disorders (TMJDs), are more prevalent in women than men. The basis for sex differences in deep craniofacial pain is not known. The present study compared the magnitude of ascending projections from temporomandibular joint (TMJ)-responsive neurons in trigeminal brainstem with the ventrolateral periaqueductal gray (vlPAG) or posterior nucleus of the thalamus (Po) in males and female rats. Fluorogold (FG) was injected into vlPAG or Po, and TMJ-responsive neurons were identified by Fos-like immunoreactivity (Fos-LI) after mustard oil injection. TMJ-evoked Fos-LI was similar in males and females; however, significant differences in cell counts were seen for FG single-labeled and Fos/FG double-labeled neurons in trigeminal brainstem. After vlPAG injections, the number of FG-labeled neurons in trigeminal subnucleus interpolaris (Vi), ventral interpolaris/caudalis transition (vl-Vi/Vc), and dorsal paratrigeminal region (dPa5) was greater in females than males. The percentage of Fos/FG double-labeled neurons in vl-Vi/Vc and dPa5 after vlPAG injection also was greater in females than males. In contrast, after Po injections, males displayed a greater number of FG-labeled neurons in superficial laminae (Lam I/II) of trigeminal subnucleus caudalis (Vc) and upper cervical spinal cord (C(1-2)) and deeper laminae (Lam III/V) at C(1-2) than females. The percentage of Fos/FG double-labeled neurons in Lam I/II of Vc after Po injection also was greater in males than females. These data revealed significant sex differences in ascending projections from TMJ-responsive neurons in trigeminal brainstem. Such differences may influence the ability of males and females to recruit autonomic reflexes and endogenous pain control circuits relevant for TMJ nociception.

Periaqueductal gray afferents synapse onto dopamine and GABA neurons in the rat ventral tegmental area

The midbrain central gray (periaqueductal gray; PAG) mediates defensive behaviors and is implicated in the rewarding effects of opiate drugs. Projections from the PAG to the ventral tegmental area (VTA) suggest that this region might also regulate behaviors involving motivation and cognition. However, studies have not yet examined the morphological features of PAG axons in the VTA or whether they synapse onto dopamine (DA) or GABA neurons. In this study, we injected anterograde tracers into the rat PAG and used immunoperoxidase to visualize the projections to the VTA. Immunogold-silver labeling for tyrosine hydroxylase (TH) or GABA was then used to identify the phenotype of innervated cells. Electron microscopic examination of the VTA revealed axons labeled anterogradely from the PAG, including myelinated and unmyelinated fibers and axon varicosities, some of which formed identifiable synapses. Approximately 55% of these synaptic contacts were of the symmetric (presumably inhibitory) type; the rest were asymmetric (presumably excitatory). These findings are consistent with the presence of both GABA and glutamate projection neurons in the PAG. Some PAG axons contained dense-cored vesicles indicating the presence of neuropeptides in addition to classical neurotransmitters. PAG projections synapsed onto both DA and GABA cells with no obvious selectivity, providing the first anatomical evidence for these direct connections. The results suggest a diverse nature of PAG physiological actions on midbrain neurons. Moreover, as both the VTA and PAG are implicated in the reinforcing actions of opiates, our findings provide a potential substrate for some of the rewarding effects of these drugs.

Neural substrate of depression during migraine

Migraine headache is triggered by and associated with a variety of hormonal, emotional, nutritional and physiological changes. The perception of migraine headache is formed when nociceptive signals originating in the meninges are conveyed to the somatosensory cortex through the trigeminal ganglion, medullary dorsal horn and thalamus. We propose that different migraine triggers activate a wide variety of brain areas that impinge on parasympathetic neurons innervating the meninges. According to this hypothesis, migraine triggers such as stress activate multiple hypothalamic, limbic and cortical areas, all of which contain neurons that project to the preganglionic parasympathetic neurons in the superior salivatory nucleus (SSN). The SSN, in turn, activates postganglionic parasympathetic neurons in the sphenopalatine ganglion, resulting in vasodilation and local release of inflammatory molecules that activate meningeal nociceptors. We propose that trigeminovascular projections from the medullary dorsal horn to selective areas in the midbrain, hypothalamus, amygdala and basal forebrain are functionally positioned to produce migraine symptoms such as irritability, loss of appetite, fatigue, depression and the quest for solitude. The network of bidirectional trafficking by which the trigeminovascular system can activate the same brain areas that have triggered its own activity in the first place provides an attractive mechanism of perpetual feedback that drives a migraine attack for many hours and even days.

Acidic saline-induced primary and secondary mechanical hyperalgesia in mice

Most of our knowledge about chronic musculoskeletal pain is based on cutaneous pain models. To test the hypothesis that animals develop chronic muscular hyperalgesia following intramuscular acidic saline injections, primary hyperalgesia within the gastrocnemius muscle was analyzed compared to secondary cutaneous hyperalgesia in the hind paw that develops following intramuscular acid saline injection. Two acidic saline (pH 4) injections were administrated into the gastrocnemius of female CF-1 mice. The results indicate that mice developed a robust hypersensitivity bilaterally in primary (gastrocnemius muscle) secondary (cutaneous hind paw) sites that lasted up to 2 weeks. In addition, primary hyperalgesia correlated well with levels of Fos expression. Fos expression patterns in the spinal cord were different for primary secondary site stimulation. Hind-paw palpation stimulated ipsilateral Fos expression in the superficial spinal laminae at L4/L5 levels, bilaterally in deep laminae at L2-L5 spinal levels. In contrast, gastrocnemius compression stimulated widespread Fos expression in all regions of the ipsilateral dorsal horn within L2-L6 spinal segments. These findings indicate that acidic saline injection induces primary hyperalgesia in muscle that the patterns of Fos expression in response to primary vs secondary stimulation are strikingly different.

PERSPECTIVE

This study assesses primary site muscular pain, which is the main complaint of people with musculoskeletal conditions, and identifies spinal patterns activated by noxious mechanical stimuli to the gastrocnemius. This study demonstrates approaches to test nociception arising from muscle aids in our understanding of spinal processing of primary secondary site hyperalgesia.

Patterns of FOS expression in the spinal cord and periaqueductal grey matter of 6OHDA-lesioned rats

A less well-known feature of Parkinson disease is that up to 40% of patients experience distinct sensory disturbances, including hyperalgesia and chronic pain. There is a limited understanding of the neural mechanisms that generate these symptoms, however. This study explores the patterns of Fos expression (a well-known marker for changes in cell activity) in the spinal cord and periaqueductal grey matter (PaG), two major sensory (nociceptive) centers, of hemiParkinsonian rats. The medial forebrain bundle (mfb; major tract carrying dopaminergic nigrostriatal axons) was injected with either 6OHDA or saline (controls). A week later, some rats were subjected to mechanical stimulation (pinching) of the hindpaw for 2 h, whereas others received no stimulation. Thereafter, brains were processed using routine tyrosine hydroxylase (marker for dopaminergic cells) or Fos immunocytochemistry. In the PaG, there were many more Fos(+) cells in the 6OHDA-lesioned than in the Control group, in both the stimulation and, in particular, the non-stimulation cases. In the spinal cord, there were also more Fos(+) cells in the 6OHDA-lesioned than in the Control group, but in the stimulation cases only. Overall, the results show distinct changes in Fos expression in the spinal cord and PaG of 6OHDA-lesioned rats, suggesting a substrate for some of the abnormal sensory (nociceptive) circuits that may be evident in parkinsonian cases.

Convergence of cutaneous, musculoskeletal, dural and visceral afferents onto nociceptive neurons in the first cervical dorsal horn

The convergence of cutaneous, musculoskeletal, dural and visceral afferents onto nociceptive neurons in the first cervical dorsal horn was investigated in urethane/chloralose-anesthetized rats. Electrical stimulation was applied to facial, neck, shoulder and forepaw skin, cornea (COR), dura, second cervical (C2) nerve, hypoglossal nerve, temporomandibular joint, masseter (MAS) muscle and superior laryngeal nerve. In addition, acetic acid was injected intraperitoneally and microinjection of glutamate was applied to the tongue, MAS muscle, splenius cervicis muscle, dura and intrapericardial area. A total of 52 nociceptive neurons classified as wide dynamic range (n = 28) or nociceptive-specific (n = 24) was studied. All nociceptive neurons received afferent input from the skin and at least one COR, musculoskeletal, dural or visceral afferent source in the trigeminal (V) or cervical area but input from afferent sources caudal to the C2 innervation territory was sparse. The proportion of neurons responding to COR, dural, C2 nerve, hypoglossal nerve, temporomandibular joint, MAS muscle and superior laryngeal nerve stimulations was 87, 54, 85, 52, 73, 64 and 31%, respectively. Electrical stimulation of all tested sites showed a double logarithmic stimulus-response relation, and cluster analysis of the excitability to COR, musculoskeletal, dural and visceral stimulations revealed two groups of neurons, one mainly containing wide dynamic range neurons and one mainly containing nociceptive-specific neurons. These findings indicate that afferent convergence in first cervical dorsal horn nociceptive neurons may be limited to the craniofacial area and that they may play an important role in the integration of craniofacial and upper cervical nociceptive inputs.

New models for visceral pain

PURPOSE OF REVIEW

After the initial use of unspecific stimuli, scientists have recently put a lot of effort into the development of new visceral pain models. These models will help to study physiological and pharmacological questions closely related to clinically treated visceral pain. The aim of this review is to provide an overview of the new models published most recently and encourage further studies in this field.

RECENT FINDINGS

Four rodent and three human models have been introduced. Each of these models addresses specific types of visceral pain, related to the urogenital tract (n=3), to the gastrointestinal tract (n=3), and to visceral organs (n=1). Mechanical distension as a brief stimulus was used mainly, but chemical stimuli resulting in intermediate to prolonged duration of pain were also investigated.

SUMMARY

Developments in the scientific visceral pain field are encouraging. Growing interest in studying aetiology, physiology and pharmacology of visceral pain proves the clinical importance of this pain entity. However, these new models need further evaluation. Future studies will show which of these new models will become a standard scientific tool.

Central Modulation of Pain Evoked From Myofascial Trigger Point

OBJECTIVES

Low-intensity low-frequency electrostimulation delivered within a myofascial trigger point (MTP) has been used as intervention to deactivate MTPs. The therapeutic effect has been suggested to be due to peripheral mechanisms. However, nonpainful stimuli are also known to reduce simultaneous pain through central effects. The primary objective of the present study was to assess if central pain modulation occurs after intervention with low-intensity electrostimulation within an MTP. We hypothesized that intervention induces pain inhibition via the periaqueductal gray (PAG).

METHODS

Twenty-four patients with myofascial pain syndrome participated in the study. During functional magnetic resonance scanning, painful (high-intensity) intramuscular electrostimulation was delivered at random intervals (mean interstimulus interval=10.2 s) within an MTP of the upper left trapezius muscle. In-between scanning sessions, intervention (intramuscular electrostimulation, low-intensity, interstimulus interval=0.5 s) was applied to the same area. Patients were divided into responders and nonresponders according to their change in pressure pain thresholds relative to intervention. In addition to a whole brain search, a region of interest approach was also implemented to test the effect of intervention on PAG signal change.

RESULTS

The main findings were: (1) intervention modulated PAG activity to painful stimuli more in responders than in nonresponders, (2) change in PAG activity from the whole patient population correlated with change in pressure pain threshold, and (3) a network known to regulate affective qualities of the pain experience was (subsignificantly) engaged more in responders than in nonresponders.

DISCUSSION

The applied intervention most likely involves supraspinal pain control mechanisms related to both antinociception and regulation of pain affect.

Effects of neonatal maternal separation on neurochemical and sensory response to colonic distension in a rat model of irritable bowel syndrome

Early life stress has been implicated as a risk factor for irritable bowel syndrome (IBS). We studied the effect of neonatal maternal separation on the visceromotor response and the expression of c-fos, 5-HT, and its receptors/transporters along the brain-gut axis in an animal model of IBS. Male neonatal Sprague-Dawley rats were randomly assigned to a 3-h daily maternal separation (MS) or nonhandling (NH) on postnatal days 2-21. Colorectal balloon distention (CRD) was performed for assessment of abdominal withdrawal reflex as a surrogate marker of visceral pain. Tissues from dorsal raphe nucleus in midbrain, lumbar-sacral cord, and distal colon were harvested for semiquantitative analysis of c-fos and 5-HT. The expression of 5-HT expression, 5-HT3 receptors, and 5-HT transporter were analyzed by RT-PCR. Pain threshold was significantly lower in MS than NH rats. The abdominal withdrawal reflex score in response to CRD in MS rats was significantly higher with distension pressures of 40, 60, and 80 mmHg. In MS rats, the number of c-fos-like immunoreactive nuclei at dorsal horn of lumbar-sacral spinal cord increased significantly after CRD. 5-HT content in the spinal cord of MS rats was significant higher. In the colon, both 5-HT-positive cell number and 5-HT content were comparable between MS and NH groups before CRD. Post-CRD only MS rats had significant increase in 5-HT content. Protein and mRNA expression levels of 5-HT3 receptors and 5-HT transporter were similar in MS and NH rats. Neonatal maternal separation stress predisposes rats to exaggerated neurochemical responses and visceral hyperalgesia in colon mimicking IBS.

Effects on c-Fos expression in the PAG and thalamus by selective input via tetrodotoxin-resistant afferent fibres from muscle and skin

Nociceptive information from skin and muscle is differently processed at many levels of the central nervous system. However, with regard to nociceptive input from muscle to the thalamus, only few data are available. Here, we investigated the c-Fos expression in the thalamus and the periaqueductal grey matter (PAG) induced by electrical stimulation of tetrodotoxin-resistant (TTX-r), presumably nociceptive, afferent fibres. In addition, a comparison between the effects of TTX-r input from muscle and skin was made. In anaesthetised rats, a skin or a muscle nerve was stimulated electrically for 1h at an intensity supramaximal for unmyelinated fibres. To block TTX-sensitive afferents, TTX was applied to the sciatic nerve. c-Fos was visualized using DAB immunohistochemistry. Here we report for the first time that in the PAG and medial thalamus, the main effect of TTX-r input from muscle was a reduction in c-Fos expression, and that in some thalamic nuclei (e.g. posterior, reuniens, and central medial nuclei), significant differences in the number of c-Fos-positive cells were found after muscle and cutaneous input, respectively. The thalamic regions with the strongest effects of muscle input were the VL bilaterally and the VPL contralaterally (increase in c-Fos expression) as well as the rhomboid nucleus (decrease in c-Fos expression).

A novel pressor area at the medullo-cervical junction that is not dependent on the RVLM: efferent pathways and chemical mediators

Chemical stimulation of a region extending from the most caudal ventrolateral medulla into the upper cervical spinal cord evoked large sympathetically mediated pressor responses. These responses were not dependent on the integrity of the rostral ventrolateral medulla (RVLM) and may be mediated by glutamatergic neurons embedded in the white matter that project to the thoracic spinal cord. We term this new region the medullo-cervical pressor area (MCPA). This region is distinct from the caudal pressor area, because blockade of the RVLM with muscimol inhibited this pressor response but not that evoked from the MCPA. This is the first study to provide functional evidence for a cardiovascular role for neurons in the cervical spinal cord white matter that innervate sympathetic preganglionic neurons (Jansen and Loewy, 1997). Using retrograde tracing, in combination with immunohistochemistry and in situ hybridization, we identified two groups of spinally projecting neurons in the region. Approximately 50% of neurons in one group were excitatory because they contained vesicular glutamate transporter 1 (VGluT1)/VGluT2 mRNA, whereas the other contained a mixed population of neurons, some of which contained either VGluT1/VGluT2 or GAD67 (glutamic acid decarboxylase 67) mRNA. Despite the fact that activation of the MCPA causes potent sympathoexcitation, it does not act to restore arterial pressure after chemical lesion of the RVLM so that a role for this novel descending sympathoexcitatory region remains to be elucidated.

Unitary hypothesis for multiple triggers of the pain and strain of migraine

Migraine headache is triggered by and associated with a variety of hormonal, emotional, nutritional, and physiological changes. The perception of migraine headache is formed when nociceptive signals originating in the meninges are conveyed to the somatosensory cortex through the trigeminal ganglion, medullary dorsal horn, and thalamus. Is there a common descending pathway accounting for the activation of meningeal nociceptors by different migraine triggers? We propose that different migraine triggers activate a wide variety of brain areas that impinge on parasympathetic neurons innervating the meninges. According to this hypothesis, migraine triggers such as perfume, stress, or awakening activate multiple hypothalamic, limbic, and cortical areas, all of which contain neurons that project to the preganglionic parasympathetic neurons in the superior salivatory nucleus (SSN). The SSN, in turn, activates postganglionic parasympathetic neurons in the sphenopalatine ganglion, resulting in vasodilation and local release of inflammatory molecules that activate meningeal nociceptors. Are there ascending pathways through which the trigeminovascular system can induce the wide variety of migraine symptoms? We propose that trigeminovascular projections from the medullary dorsal horn to selective areas in the midbrain, hypothalamus, amygdala, and basal forebrain are functionally positioned to produce migraine symptoms such as irritability, loss of appetite, fatigue, depression, or the quest for solitude. Bidirectional trafficking by which the trigeminovascular system can activate the same brain areas that have triggered its own activity in the first place provides an attractive network of perpetual feedback that drives a migraine attack for many hours and even days.

Craniofacial inputs to upper cervical dorsal horn: implications for somatosensory information processing

The aim of this study was to characterize the properties of somatosensory neurons in the first 2 cervical spinal dorsal horns (C1 and C2 DHs) and compare them with those previously described for the rostral subnucleus caudalis (rVc). A total of 74 nociceptive neurons classified as wide-dynamic-range (WDR) or nociceptive-specific (NS), as well as 72 low-threshold mechanoreceptive (LTM) neurons, was studied in urethane/chloralose-anesthetized rats. The majority of LTM neurons were located in laminae III/IV and had a small mechanoreceptive field (RF) that included the posterior face and cervical tissues. In contrast, the nociceptive neurons were located in laminae I/II or V/VI, and the RF of each C1 and C2 DH nociceptive neuron included a part of the face and in 47% of them the RF included a region supplied by upper cervical afferents. There was a gradual caudal shift in the neuronal RF from nasal/intraoral tissues towards the neck as recording sites progressed from rVc to C1 and C2 DHs. In contrast to LTM neurons, many C1 and C2 DH nociceptive neurons received mechanosensitive convergent afferent inputs from cervical and craniofacial deep tissues (e.g., tongue muscles or temporomandibular joint), and over 50% could be activated by hypoglossal (XII) nerve electrical stimulation. We propose that C1 and C2 DHs represent part of the caudal extension of the Vc, and that Vc and C1 and C2 DHs may act together as one functional unit to process nociceptive information from craniofacial and cervical tissues, including that from deep craniofacial tissues.

Personality, affective response, and facial blood flow during brief cognitive tasks

Blood pressure, heart rate, and changes in facial and finger blood flow were monitored in 58 women during three laboratory tasks, i.e., reading out a neutral text, a personally relevant speech (including a silent preparation phase), and a tracking task. Participants rated the tasks as mild to moderate with regard to affect intensity. Significant effects of personality on affect states were demonstrated, indicating higher levels of anxiety, depression, and anger in the high-neuroticism group, and more curiosity in the high-extraversion group. Arterial pressure, heart rate, and facial blood flow increased during task performance in all three conditions. No relationship between facial blood flow changes and muscle activity measured by m. masseter electromyography (EMG) were demonstrated. Digital vasoconstriction occurred in parallel with facial vasodilatation during tracking and in the preparation phase before the speech. There were no moderator effects of neuroticism or extraversion on blood flow or other cardiovascular responses.

The hypotension evoked by visceral nociception is mediated by delta opioid receptors in the periaqueductal gray

This study tested the hypothesis that the ventrolateral column of the midbrain periaqueductal gray (vlPAG) region mediates the hypotension and bradycardia evoked by visceral nociception. To test this, the local anesthetic lidocaine (2%; 0.5 microl) was microinjected into the vlPAG of halothane-anesthetized rats bilaterally and visceral nociception was induced 2 min later by injecting 5% acetic acid (0.5 ml) intraperitoneally. Acetic acid injection caused an abrupt fall in arterial pressure (-12.2+/-2.1 mm Hg) and heart rate (-37+/-93 bpm) lasting approximately 15 min. Lidocaine injection into the vlPAG prevented the fall in arterial pressure and heart rate completely. Cobalt chloride (5 mM; 0.2 or 0.5 microl) injection into the vlPAG also prevented nociceptive hypotension but it did not affect the fall in heart rate significantly. Lidocaine pretreatment also inhibited the depressor response caused by intramuscular formalin (5%; 0.2 ml) administration, a model of deep somatic nociception, although it did not prevent the response completely. To determine if opioid receptors mediate the response, selective mu, delta or kappa opioid receptor antagonists were microinjected into the vlPAG 5 min before intraperitoneal (ip) acetic acid administration. Naltrindole, a delta receptor antagonist, inhibited the response significantly but mu and kappa antagonists were completely ineffective. Lidocaine and naltrindole had no effect when injected into the dorsolateral PAG and did not influence cardiovascular function when injected into the vlPAG of saline treated control animals. These data support the hypothesis that the vlPAG mediates the depressor response evoked by visceral nociception and indicate that delta opioid receptors participate in the response.

Organization of axonal projections from the anterolateral area of the bed nuclei of the stria terminalis

The anterolateral group of the bed nuclei of the stria terminalis (BSTalg) contains four distinct cell groups embedded within an undifferentiated anterolateral area (BSTal) that architectonically resembles a subjacent subcommissural zone (BSTsc). The overall distributions of axonal projections from various regions of the BSTal and from the BSTsc were determined with the Phaseolus vulgaris-leucoagglutinin (PHAL) anterograde tracer method and found to be identical. The BSTal and BSTsc share dense bidirectional connections, and also project heavily within the BST to the rhomboid and fusiform nuclei and the anteroventral and anterodorsal areas. They project less densely to the juxtacapsular, oval, magnocellular, ventral, and interfascicular BST nuclei. Outside the BST, brain areas receiving strong to moderate inputs from the BSTal and BSTsc fall into several functional groups: somatomotor system (nucleus accumbens, substantia innominata, ventral tegmental area, and retrorubral area and adjacent midbrain reticular nucleus), central autonomic control system (central amygdalar nucleus, dorsal lateral hypothalamic area, ventrolateral periaqueductal gray, parabrachial nucleus, and nucleus of the solitary tract), neuroendocrine system (paraventricular and supraoptic nuclei, hypothalamic visceromotor pattern generator network), and thalamocortical feedback loops (midline, medial, and intralaminar nuclei). The results indicate that the BSTal and BSTsc are parts of the same cell group (dorsal and ventral to the anterior commissure), which plays a role in coordinating visceral and somatic motor responses (during ingestive behaviors, for example), especially in response to noxious stimuli (learned anorexia associated with noxious stimuli). BSTal projections are distinct from those of the adjacent juxtacapsular, oval, fusiform, and rhomboid nuclei.

Differential contributions of the basolateral and central nuclei of the amygdala in the negative affective component of chemical somatic and visceral pains in rats

Pain is a multidimensional conscious experience including sensory and negative affective components. The neural systems of the sensory component of pain have been extensively studied, while those of the negative affective component are less clear. The amygdala is a forebrain structure composed of several distinct nuclei including the basolateral (BLA) and central (CeA) nuclei, and is thought to be a key neural substrate underlying emotional responses. Recently, we reported that intraplantar (i.pl.) injection of formalin as a chemical somatic noxious stimulus increased c-fos mRNA expression in the BLA, but not CeA, while intraperitoneal (i.p.) injection of acetic acid as a chemical visceral noxious stimulus induced it highly in the CeA, and hardly in BLA [Nakagawa et al. (2003) Neurosci. Lett., 344, 197-200]. In this study, we examined the effects of discrete bilateral excitotoxic lesions of the BLA or CeA on the sensory and negative affective components of the two types of pain in Sprague-Dawley rats. In the place-conditioning paradigm, both i.pl. formalin and i.p. acetic acid produced conditioned place aversion. The i.pl. formalin-induced conditioned place aversion was abolished by the lesion of the BLA or CeA, while the i.p. acetic acid-induced conditioned place aversion was abolished by the CeA-, but not BLA-lesion. On the other hand, the BLA- or CeA-lesion failed to reduce the i.pl. formalin- and i.p. acetic acid-produced nociceptive behaviours. Taken together, these results suggest that activation of distinct amygdaloid nuclei could differentially contribute to chemical somatic and visceral noxious stimuli-induced negative affective, but not sensory components of pains.

Somatic and visceral afferents to the 'vasodepressor region' of the caudal midline medulla in the rat

Previous research has found that the integrity of a restricted region of the caudal midline medulla (including caudal portions of nucleus raphé obscurus and nucleus raphé pallidus) was critical for vasodepression (hypotension, bradycardia, decreased cardiac contractility) evoked either by haemorrhage or deep pain. In this anatomical tracing study we found that the vasodepressor part of the caudal midline medulla (CMM) receives inputs arising from spinal cord, spinal trigeminal nucleus (SpV) and nucleus of the solitary tract (NTS). Specifically: (i) a spinal-CMM projection arises from neurons of the deep dorsal horn, medial ventral horn and lamina X at all spinal segmental levels, with approximately 60% of the projection originating from the upper cervical spinal cord (C1-C4); (ii) a SpV-CMM projection arises primarily from neurons at the transition between subnucleus caudalis and subnucleus interpolaris; (iii) a NTS-CMM projection arises primarily from neurons in ventrolateral and medial subnuclei. In combination, the specific spinal, SpV and NTS regions which project to the CMM receive the complete range of somatic and visceral afferents known to trigger vasodepression. The role(s) of each specific projection is discussed.

Cardiac nociception in rats: neuronal pathways and the influence of dermal neurostimulation on conveyance to the central nervous system

Neurostimulation for refractory angina pectoris is often advocated for its clinical efficacy. However, the recruited pathways to induce electroanalgesia are partially unknown. Therefore, we sought to study the effect of neurostimulation on experimentally induced cardiac nociception, using capsaicin as nociception-induced substance. Four different groups of male Wistar rats were pericardially infused with either saline or capsaicin with or without neurostimulation. Group StimCap was infused with capsaicin, and group StimVeh was infused with saline. Both groups were treated with neurostimulation. Group ShamCap was only infused with capsaicin without stimulation, whereas group ShamVeh was only infused with saline. Neuronal activation differences were assessed with cytochemical staining, revealing the cellular expression of c-fos. Pain behavior was registered on video and was quantitatively analyzed. In the StimCap and ShamCap groups, all animals exerted typical pain behavior, whereas in the StimVeh group only moderate changes in behavior were observed. Group ShamVeh animals were unaffected by the procedure. The upper thoracic spinal cord showed high numbers of c-fos-positive cells, predominantly in laminae III and IV in both StimCap and StimVeh groups. Almost no c-fos expression was noticed in groups ShamCap and ShamVeh in these sections of the spinal cord. In groups StimCap and ShamCap a significantly higher number of c-fos-positive cells in comparison with groups StimVeh and ShamVeh were noticed in the periambigus region, the nucleus tractus solitarius, and the paraventricular hypothalamus. In the paraventricular thalamus, periaqueductal gray, and central amygdala, no significant differences were noticed among the first three groups, and the c-fos concentration in these three groups was significantly higher than in group ShamVeh. It is concluded that neurostimulation does not influence capsaicin-induced cardiac nociceptive pain pulses to the central nervous system. Furthermore, capsaicin-induced cardiac pain and neurostimulation may utilize two different pathways.

Noxious activation of spinal or vagal afferents evokes distinct patterns of fos-like immunoreactivity in the ventrolateral periaqueductal gray of unanaesthetised rats

The consequences of a severe traumatic injury--deep pain and haemorrhage--usually evoke a passive emotional coping reaction characterised by: quiescence and immobility, decreased vigilance, hypotension and bradycardia. Results of studies utilising microinjections of excitatory amino acids suggest that passive coping reactions are mediated, at least in part, by activation of the midbrain, ventrolateral periaqueductal gray (vlPAG) region. Further, experiments in anaesthetised rats, using the expression of the immediate-early gene, c-fos, as a marker of neuronal activation, report that pain arising from muscles, joints or viscera selectively activates the vlPAG. Anaesthesia alone, however, evokes substantial Fos-like immunoreactivity (IR) within the vlPAG and this may have obscured any differences in patterns of Fos expression following noxious deep somatic versus noxious visceral activation. In these experiments, in unanaesthetised rats, the effects of noxious spinal versus noxious vagal primary afferent activation were re-examined and distinct rostrocaudal patterns of Fos-expression were observed. Specifically: (i) injection of algesic substances into muscle, which preferentially activates spinal afferents, evoked Fos expression predominantly within the caudal vlPAG; whereas, (ii) noxious manipulations whose effects are mediated by (cardiopulmonary) vagal activation evoked preferential Fos-expression within the rostral vlPAG. On the other hand, hypotensive haemorrhage evoked substantial Fos expression along the entire rostrocaudal extent of the vlPAG, a finding which fits with suggestions that haemorrhagic shock is triggered by a combination of: (i) spinally-relayed nociceptive signals originating from ischaemic tissue, and (ii) vagally-relayed signals reflecting poor cardiac filling.

Parallel circuits mediating distinct emotional coping reactions to different types of stress

All animals, including humans, react with distinct emotional coping strategies to different types of stress. Active coping strategies (e.g. confrontation, fight, escape) are evoked if the stressor is controllable or escapable. Passive coping strategies (e.g. quiescence, immobility, decreased responsiveness to the environment) are usually elicited if the stressor is inescapable and help to facilitate recovery and healing. Neural substrates mediating active versus passive emotional coping have been identified within distinct, longitudinal neuronal columns of the midbrain periaqueductal gray (PAG) region. Active coping is evoked by activation of either the dorsolateral or lateral columns of the PAG; whereas passive coping is triggered by activation of the ventrolateral PAG. Recent anatomical studies indicate that each PAG column receives a distinctive set of ascending (spinal and medullary) and descending (prefrontal cortical and hypothalamic) afferents. Consistent with the anatomy, functional studies using immediate early gene expression (c-fos) as a marker of neuronal activation have revealed that the preferential activation of a specific PAG column reflects (i) the type of emotional coping reaction triggered, and (ii) whether a physical or psychological stressor was used.

Responses and afferent pathways of superficial and deeper c(1)-c(2) spinal cells to intrapericardial algogenic chemicals in rats

Electrical stimulation of vagal afferents or cardiopulmonary sympathetic afferent fibers excites C(1)--C(2) spinal neurons. The purposes of this study were to compare the responses of superficial (depth <0.35 mm) and deeper C(1)--C(2) spinal neurons to noxious chemical stimulation of cardiac afferents and determine the relative contribution of vagal and sympathetic afferent pathways for transmission of noxious cardiac afferent input to C(1)--C(2) neurons. Extracellular potentials of single C(1)--C(2) neurons were recorded in pentobarbital anesthetized and paralyzed male rats. A catheter was placed in the pericardial sac to administer a mixture of algogenic chemicals (0.2 ml) that contained adenosine (10(-3) M), bradykinin, histamine, serotonin, and prostaglandin E(2) (10(-5) M each). Intrapericardial chemicals changed the activity of 20/106 (19%) C(1)--C(2) spinal neurons in the superficial laminae, whereas 76/147 (52%) deeper neurons responded to cardiac noxious input (P < 0.01). Of 96 neurons responsive to cardiac inputs, 48 (50%) were excited (E), 41 (43%) were inhibited (I), and 7 were excited/inhibited (E-I) by intrapericardial chemicals. E or I neurons responsive to intrapericardial chemicals were subdivided into two groups: short-lasting (SL) and long-lasting (LL) response patterns. In superficial gray matter, excitatory responses to cardiac inputs were more likely to be LL-E than SL-E neurons. Mechanical stimulation of the somatic field from the head, neck, and shoulder areas excited 85 of 95 (89%) C(1)--C(2) spinal neurons that responded to intrapericardial chemicals; 31 neurons were classified as wide dynamic range, 49 were high threshold, 5 responded only to joint movement, and no neuron was classified as low threshold. For superficial neurons, 53% had small somatic fields and 21% had bilateral fields. In contrast, 31% of the deeper neurons had small somatic fields and 46% had bilateral fields. Ipsilateral cervical vagotomy interrupted cardiac noxious input to 8/30 (6 E, 2 I) neurons; sequential transection of the contralateral cervical vagus nerve (bilateral vagotomy) eliminated the responses to intrapericardial chemicals in 4/22 (3 E, 1 I) neurons. Spinal transection at C(6)--C(7) segments to interrupt effects of sympathetic afferent input abolished responses to cardiac input in 10/10 (7 E, 3 I) neurons that still responded after bilateral vagotomy. Results of this study support the concept that C(1)-C(2) superficial and deeper spinal neurons play a role in integrating cardiac noxious inputs that travel in both the cervical vagal and/or thoracic sympathetic afferent nerves.

Central circuits mediating patterned autonomic activity during active vs. passive emotional coping

Animals, including humans, react with distinct emotional coping strategies to different sets of environmental demands. These strategies include the capacity to affect appropriate responses to "escapable" or "inescapable" stressors. Active emotional coping strategies--fight or flight--are particularly adaptive if the stress is escapable. On the other hand, passive emotional coping strategies-quiescence, immobility, decreased responsiveness to the environment-are useful when the stress is inescapable. Passive strategies contribute also to facilitating recovery and healing once the stressful event is over. Active vs. passive emotional coping strategies are characterised further by distinct patterns of autonomic change. Active strategies are associated with sympathoexcitation (hypertension, tachycardia), whereas passive strategies are associated with sympathoinhibitory patterns (hypotension, bradycardia). Distinct neural substrates mediating active vs. passive emotional coping have been identified within the longitudinal neuronal columns of the midbrain periaqueductal gray region (PAG). The PAG offers then a potentially useful point of entry for delineating neural circuits mediating the different forms of emotional coping and their associated patterns of autonomic activity. As one example, recent studies of the connections of orbital and medial prefrontal cortical (PFC) fields with specific PAG longitudinal neuronal columns are reviewed. Findings of discrete orbital and medial PFC projections to different PAG columns, and related PFC and PAG columnar connections with specific subregions of the hypothalamus, suggest that distinct but parallel circuits mediate the behavioural strategies and patterns of autonomic activity characteristic of emotional "engagement with" or "disengagement from" the external environment.