SEROTONERGIC pontomedullary neurons are not activated by antinociceptive stimulation in the periaqueductal gray

The 'in's and out's' of descending pain modulation from the rostral ventromedial medulla

The descending-pain modulating circuit controls the experience of pain by modulating transmission of sensory signals through the dorsal horn. This circuit's key output node, the rostral ventromedial medulla (RVM), integrates 'top-down' and 'bottom-up' inputs that regulate functionally defined RVM cell types, 'OFF-cells' and 'ON-cells', which respectively suppress or facilitate pain-related sensory processing. While recent advances have sought molecular definition of RVM cell types, conflicting behavioral findings highlight challenges involved in aligning functional and molecularly defined types. This review summarizes current understanding, derived primarily from rodent studies but with corroborating evidence from human imaging, of the role of RVM populations in pain modulation and persistent pain states and explores recent advances outlining inputs to, and outputs from, RVM pain-modulating neurons.

Interactive Mechanisms of Supraspinal Sites of Opioid Analgesic Action: A Festschrift to Dr. Gavril W. Pasternak

Almost a half century of research has elaborated the discoveries of the central mechanisms governing the analgesic responses of opiates, including their receptors, endogenous peptides, genes and their putative spinal and supraspinal sites of action. One of the central tenets of "gate-control theories of pain" was the activation of descending supraspinal sites by opiate drugs and opioid peptides thereby controlling further noxious input. This review in the Special Issue dedicated to the research of Dr. Gavril Pasternak indicates his contributions to the understanding of supraspinal mediation of opioid analgesic action within the context of the large body of work over this period. This review will examine (a) the relevant supraspinal sites mediating opioid analgesia, (b) the opioid receptor subtypes and opioid peptides involved, (c) supraspinal site analgesic interactions and their underlying neurophysiology, (d) molecular (particularly AS) tools identifying opioid receptor actions, and (e) relevant physiological variables affecting site-specific opioid analgesia. This review will build on classic initial studies, specify the contributions that Gavril Pasternak and his colleagues did in this specific area, and follow through with studies up to the present.

Activation of Brainstem Neurons During Mesencephalic Locomotor Region-Evoked Locomotion in the Cat

The distribution of locomotor-activated neurons in the brainstem of the cat was studied by c-Fos immunohistochemistry in combination with antibody-based cellular phenotyping following electrical stimulation of the mesencephalic locomotor region (MLR) – the anatomical constituents of which remain debated today, primarily between the cuneiform (CnF) and the pedunculopontine tegmental nuclei (PPT). Effective MLR sites were co-extensive with the CnF nucleus. Animals subject to the locomotor task showed abundant Fos labeling in the CnF, parabrachial nuclei of the subcuneiform region, periaqueductal gray, locus ceruleus (LC)/subceruleus (SubC), Kölliker–Fuse, magnocellular and lateral tegmental fields, raphe, and the parapyramidal region. Labeled neurons were more abundant on the side of stimulation. In some animals, Fos-labeled cells were also observed in the ventral tegmental area, medial and intermediate vestibular nuclei, dorsal motor nucleus of the vagus, n. tractus solitarii, and retrofacial nucleus in the ventrolateral medulla. Many neurons in the reticular formation were innervated by serotonergic fibers. Numerous locomotor-activated neurons in the parabrachial nuclei and LC/SubC/Kölliker–Fuse were noradrenergic. Few cholinergic neurons within the PPT stained for Fos. In the medulla, serotonergic neurons within the parapyramidal region and the nucleus raphe magnus were positive for Fos. Control animals, not subject to locomotion, showed few Fos-labeled neurons in these areas. The current study provides positive evidence for a role for the CnF in the initiation of locomotion while providing little evidence for the participation of the PPT. The results also show that MLR-evoked locomotion involves the parallel activation of reticular and monoaminergic neurons in the pons/medulla, and provides the anatomical and functional basis for spinal monoamine release during evoked locomotion. Lastly, the results indicate that vestibular, cardiovascular, and respiratory centers are centrally activated during MLR-evoked locomotion. Altogether, the results show a complex pattern of neuromodulatory influences of brainstem neurons by electrical activation of the MLR.

Animal Models of Peripheral Pain: Biology Review and Application for Drug Discovery

Pain is a complex constellation of cognitive, unpleasant sensory, and emotional experiences that primarily serves as a survival mechanism. Pain arises in the peripheral nervous system and pain signals synapse with nerve tracts extending into the central nervous system. Several different schemes are used to classify pain, including the underlying mechanism, tissues primarily affected, and time-course. Numerous animal models of pain, which should be employed with appropriate Institutional Animal Care and Use approvals, have been developed to elucidate pathophysiology mechanisms and aid in identification of novel therapeutic targets. The variety of available models underscores the observations that pain phenotypes are driven by several distinct mechanisms. Pain outcome measurement encompasses both reflexive (responses to heat, cold, mechanical and electrical stimuli) and nonreflexive (spontaneous pain responses to stimuli) behaviors. However, the question of translatability to human pain conditions and potential treatment outcomes remains a topic of continued scrutiny. In this review we discuss the different types of pain and their mechanisms and pathways, available rodent pain models with an emphasis on type of pain stimulations and pain outcome measures and discuss the role of pathologists in assessing and validating pain models.

Comparative Physiology of Nociception and Pain

The study of diverse animal groups allows us to discern the evolution of the neurobiology of nociception. Nociception functions as an important alarm system alerting the individual to potential and actual tissue damage. All animals possess nociceptors, and, in some animal groups, it has been demonstrated that there are consistent physiological mechanisms underpinning the nociceptive system. This review considers the comparative biology of nociception and pain from an evolutionary perspective.

Descending pain modulation and chronification of pain

PURPOSE OF REVIEW

Chronic pain is an important public health problem that negatively impacts quality of life of affected individuals and exacts an enormous socio-economic cost. Currently available therapeutics provide inadequate management of pain in many patients. Acute pain states generally resolve in most patients. However, for reasons that are poorly understood, in some individuals, acute pain can transform to a chronic state. Our understanding of the risk factors that underlie the development of chronic pain is limited. Recent studies have suggested an important contribution of dysfunction in descending pain modulatory circuits to pain 'chronification'. Human studies provide insights into possible endogenous and exogenous factors that may promote the conversion of pain into a chronic condition.

RECENT FINDINGS

Descending pain modulatory systems have been studied and characterized in animal models. Human brain imaging techniques, deep brain stimulation and the mechanisms of action of drugs that are effective in the treatment of pain confirm the clinical relevance of top-down pain modulatory circuits. Growing evidence supports the concept that chronic pain is associated with a dysregulation in descending pain modulation. Disruption of the balance of descending modulatory circuits to favour facilitation may promote and maintain chronic pain. Recent findings suggest that diminished descending inhibition is likely to be an important element in determining whether pain may become chronic. This view is consistent with the clinical success of drugs that enhance spinal noradrenergic activity, such as serotonin/norepinephrine reuptake inhibitors (SNRIs), in the treatment of chronic pain states. Consistent with this concept, a robust descending inhibitory system may be normally engaged to protect against the development of chronic pain. Imaging studies show that higher cortical and subcortical centres that govern emotional, motivational and cognitive processes communicate directly with descending pain modulatory circuits providing a mechanistic basis to explain how exogenous factors can influence the expression of chronic pain in a susceptible individual.

SUMMARY

Preclinical studies coupled with clinical pharmacologic and neuroimaging investigations have advanced our understanding of brain circuits that modulate pain. Descending pain facilitatory and inhibitory circuits arising ultimately in the brainstem provide mechanisms that can be engaged to promote or protect against pain 'chronification'. These systems interact with higher centres, thus providing a means through which exogenous factors can influence the risk of pain chronification. A greater understanding of the role of descending pain modulation can lead to novel therapeutic directions aimed at normalizing aberrant processes that can lead to chronic pain.

The role of dorsomedial and ventrolateral columns of the periaqueductal gray matter and in situ 5-HT₂A and 5-HT₂C serotonergic receptors in post-ictal antinociception

The periaqueductal gray matter (PAG) consists in a brainstem structure rich in 5-hydroxytryptamine (5-HT) inputs related to the modulation of pain. The involvement of each of the serotonergic receptor subtypes found in PAG columns, such as the dorsomedial (dmPAG) and the ventrolateral (vlPAG) columns, regarding post-ictal antinociception have not been elucidated. The present work investigated the participation of the dmPAG and vlPAG columns in seizure-induced antinociception. Specifically, we studied the involvement of serotonergic neurotransmission in these columns on antinociceptive responses that follow tonic-clonic epileptic reactions induced by pentylenetetrazole (PTZ), an ionophore GABA-mediated Cl(-) influx antagonist. Microinjections of cobalt chloride (1.0 mM CoCl2 /0.2 µL) into the dmPAG and vlPAG caused an intermittent local synaptic inhibition and decreased post-ictal antinociception that had been recorded at various time points after seizures. Pretreatments of the dmPAG or the vlPAG columns with the nonselective serotonergic receptors antagonist methysergide (5.0 µg/0.2 µL) or intramesencephalic microinjections of ketanserin (5.0 µg/0.2 µL), a serotonergic antagonist with more affinity to 5-HT2A/2C receptors, decreased tonic-clonic seizure-induced antinociception. Both dmPAG and vlPAG treatment with either the 5-HT2A receptor selective antagonist R-96544 (10 nM/0.2 µL), or the 5-HT2C receptors selective antagonist RS-102221 (0.15 µg/0.2 µL) also decrease post-ictal antinociception. These findings suggest that serotonergic neurotransmission, which recruits both 5-HT2A and 5-HT2C serotonergic receptors in dmPAG and vlPAG columns, plays a critical role in the elaboration of post-ictal antinociception.

The perception and endogenous modulation of pain

Pain is often perceived an unpleasant experience that includes sensory and emotional/motivational responses. Accordingly, pain serves as a powerful teaching signal enabling an organism to avoid injury, and is critical to survival. However, maladaptive pain, such as neuropathic or idiopathic pain, serves no survival function. Genomic studies of individuals with congenital insensitivity to pain or paroxysmal pain syndromes considerable increased our understanding of the function of peripheral nociceptors, and especially of the roles of voltage-gated sodium channels and of nerve growth factor (NGF)/TrkA receptors in nociceptive transduction and transmission. Brain imaging studies revealed a "pain matrix," consisting of cortical and subcortical regions that respond to noxious inputs and can positively or negatively modulate pain through activation of descending pain modulatory systems. Projections from the periaqueductal grey (PAG) and the rostroventromedial medulla (RVM) to the trigeminal and spinal dorsal horns can inhibit or promote further nociceptive inputs. The "pain matrix" can explain such varied phenomena as stress-induced analgesia, placebo effect and the role of expectation on pain perception. Disruptions in these systems may account for the existence idiopathic pan states such as fibromyalgia. Increased understanding of pain modulatory systems will lead to development of more effective therapeutics for chronic pain.

Changes in response properties of rostral ventromedial medulla neurons during prolonged inflammation: modulation by neurokinin-1 receptors

Activation of neurokinin-1 (NK-1) receptors in the rostral ventromedial medulla (RVM) can facilitate pain transmission in conditions such as inflammation, and thereby contribute to hyperalgesia. Since blockade of NK-1 receptors in the RVM can attenuate hyperalgesia produced by prolonged inflammation, we examined the role of NK-1 receptors in changes of response properties of RVM neurons following four days of hind paw inflammation with complete Freund's adjuvant. Recordings were made from functionally identified ON, OFF and NEUTRAL cells in the RVM. Spontaneous activity and responses evoked by a series of mechanical (10, 15, 26, 60, 100, and 180 g) and heat (34-50 °C) stimuli applied to the inflamed and non-inflamed hind paws were determined before and at 15 and 60 min after injection of the NK-1-antagonist L-733,060 or vehicle into the RVM. Prolonged inflammation did not alter the proportions of functionally-identified ON, OFF and NEUTRAL cells. ON cells exhibited enhanced responses to mechanical (60-100g) and heat (48-50 °C) stimuli applied to the inflamed paw, which were attenuated by L-733,060 but not by vehicle. Inhibitory responses of OFF cells evoked by mechanical stimuli applied to the inflamed paw were also inhibited by L-733,060, but responses evoked by stimulation of the contralateral paw were increased. Heat-evoked responses of OFF cells were not altered by L-733,060. Also, neither L-733,060 nor vehicle altered spontaneous ongoing discharge rate of RVM neurons. These data indicate that NK-1 receptors modulate excitability of ON cells which contribute to both mechanical and heat hyperalgesia, whereas NK-1 modulation of OFF cells contributes to mechanical hyperalgesia during prolonged inflammation.

Opioid receptor modulation of GABAergic and serotonergic spinally projecting neurons of the rostral ventromedial medulla in mice

The rostral ventromedial medulla (RVM) is an important site of opioid actions and forms part of an analgesic pathway that projects to the spinal cord. The neuronal mechanisms by which opioids act within this brain region remain unclear, particularly in relation to the neurotransmitters GABA and serotonin. In the present study, we examined serotonergic and GABAergic immunoreactivity, identified using immunohistochemistry for tryptophan hydroxylase (TPH) and glutamate decarboxylase (GAD), in combination with in vitro whole cell patch clamping to investigate the role of opioids on the mouse RVM with identified projections to the spinal cord. Tyr-d-Ala-Gly-N-Me-Phe-Gly-ol enkephalin (DAMGO) produced μ-opioid receptor-mediated outward currents in virtually all TPH-immunoreactive projecting neurons and GAD-immunoreactive nonprojecting neurons (87% and 86%). The other groups of RVM neurons displayed mixed responsiveness to DAMGO (40-68%). Deltorphin II and U-69593 produced δ- and κ-opioid receptor-mediated outward currents in smaller subpopulations of RVM neurons, with many of the δ-opioid responders forming a subpopulation of μ-opioid-sensitive GABAergic nonprojecting neurons. These findings are consistent with prior electrophysiological and anatomic studies in the rat RVM and indicate that both serotonergic and GABAergic RVM neurons mediate the actions of μ-opioids. Specifically, μ-opioids have a direct postsynaptic inhibitory influence over both GABAergic and serotonergic neurons, including those that project to the dorsal spinal cord.

Nociceptin/Orphanin FQ in PAG modulates the release of amino acids, serotonin and norepinephrine in the rostral ventromedial medulla and spinal cord in rats

High density Nociceptin/Orphanin FQ (N/OFQ) and its receptor (NOPr) have been found in the ventrolateral periaqueductal gray (vlPAG), a main output pathway involved in the descending pain-control system. Our previous study demonstrated that the microinjection of N/OFQ into the vlPAG markedly facilitated nociceptive responses of spinal dorsal horn neurons. The aim of the present work was to further provide evidence for the supraspinal mechanisms of action for N/OFQ-mediated nociceptive facilitation by examining the effect of N/OFQ in the vlPAG on neurotransmitter release in the descending pain-control system, including the nucleus raphe magnus (NRM), nucleus reticularis gigantocellularis (NGC) and dorsal horn of the spinal cord. The results showed that the microinjection of N/OFQ into the vlPAG produced robust decreases in 5-hydroxytryptamine (5-HT, serotonin), norepinephrine (NE), and gamma-aminobutyric acid (GABA), and increase in glutamate (Glu) release in the spinal dorsal horn. Spinal application of 5-HT, 2-Me-5-HT (5-HT(3) receptor agonist), muscimol (GABA(A) receptor agonist), and baclofen (GABA(B) receptor agonist) significantly blocked intra-vlPAG-induced facilitation on nociceptive responses. However, the extracellular concentrations of these neurotransmitters in the NRM and NGC exhibited diversity following intra-vlPAG of N/OFQ. In the NRM, intra-vlPAG injection of N/OFQ significantly decreased 5-HT, NE, and Glu, but increased GABA release. Differently, in the NGC, both NE and GABA releases were attenuated by intra-vlPAG of N/OFQ, whereas the concentration of 5-HT and Glu exhibited a trend to increase. These findings provide direct support for the hypothesis that intra-PAG of N/OFQ-induced facilitation of nociceptive responses is associated with the release of 5-HT, NE, and amino acids.

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.

Central serotonergic neurons are differentially required for opioid analgesia but not for morphine tolerance or morphine reward

Opioids remain the most effective analgesics despite their potential adverse effects such as tolerance and addiction. Mechanisms underlying these opiate-mediated processes remain the subject of much debate. Here we describe opioid-induced behaviors of Lmx1b conditional knockout mice (Lmx1bf/f/p), which lack central serotonergic neurons, and we report that opioid analgesia is differentially dependent on the central serotonergic system. Analgesia induced by a kappa opioid receptor agonist administered at the supraspinal level was abolished in Lmx1bf/f/p mice compared with their wild-type littermates. Furthermore, compared with their wild-type littermates Lmx1bf/f/p mice exhibited significantly reduced analgesic effects of mu and delta opioid receptor agonists at both spinal and supraspinal sites. In contrast to the attenuation in opioid analgesia, Lmx1bf/f/p mice developed tolerance to morphine analgesia and displayed normal morphine reward behavior as measured by conditioned place preference. Our results provide genetic evidence supporting the view that the central serotonergic system is a key component of supraspinal pain modulatory circuitry mediating opioid analgesia. Furthermore, our data suggest that the mechanisms of morphine tolerance and morphine reward are independent of the central serotonergic system.

Involvement of local orphanin FQ in the tolerance induced by repeated microinjections of morphine into ventrolateral periaqueductal gray in rats

The present study evaluated the role of ventrolateral periaqueductal gray (vlPAG)-located orphanin-FQ (OFQ) in the opioid tolerance induced by repeated microinjections of morphine (MOR) into vlPAG. Microinjection of MOR (5 microg/0.5 microl) into vlPAG caused antinociception as quantified with the tail flick and the hot plate tests. When MOR microinjection was repeated twice daily, the antinociceptive effect disappeared within 2 days (tolerance). However, if MOR microinjection was preceded by the OFQ receptor antagonist nocistatin (NST; 1 ng/0.5 microl), the microinjections of MOR did not induce tolerance. If NST microinjections were suspended, subsequent MOR microinjections induced tolerance. In MOR-tolerant rats, a single NST microinjection into vlPAG was enough to restore the antinociceptive effect of MOR. Furthermore, if OFQ (1 ng/0.5 microl) was microinjected into vlPAG, then a MOR microinjection administered 15 min later into vlPAG did not elicit antinociception. Finally, opioid tolerance induced by repeated systemic MOR injections (5 mg/kg, i.p.) was reversed by a single microinjection of NST into vlPAG. This emphasizes the central importance of vlPAG-located OFQ in the MOR tolerance.

Moving from an averaged to specific view of spinal cord pain processing circuits

Neurons in the superficial dorsal horn (SDH) of the spinal cord play a critical role in processing potentially painful or noxious signals from skin, muscle, and viscera. Many acute pain therapies are based on the notion that altering the excitability of SDH neurons can block or gate these signals and reduce pain. This same notion also underlies treatments for certain chronic pain states. Basic scientists are now beginning to identify a number of potential molecular targets for spinal cord-based pain therapies with a focus on ion channels and receptors that can alter neuronal excitability. The current challenge in pain research is to identify which are the most promising targets and how their manipulation alters pain processing. In this review, we propose that our understanding of spinal pain processing mechanisms and translation of these discoveries into pain therapies could be improved by 1) better appreciating and understanding neuronal heterogeneity in the SDH; 2) establishing connectivity patterns among SDH neuron types; and 3) testing and extending findings made in vitro to intact (in vivo) animal models. As this information becomes available, it will be possible to determine the precise distribution of potential therapeutic targets on various SDH neuron types within specific circuits known to be functionally important in spinal pain processing.

The rostral ventromedial medulla mediates the facilitatory effect of microinjected orphanin FQ in the periaqueductal gray on spinal nociceptive transmission in rats

Single unit extracellular recordings from spinal dorsal horn neurons were obtained with glass micropipettes in pentobarbital-anesthetized rats. A total of 50 wide dynamic range (WDR) neurons were studied in 25 rats. Microinjection of orphanin FQ (OFQ, 0.1 microg/0.1 microl) (a potent endogenous ligand of the opioid receptor-like receptor (ORL-1)) into the ipsilateral ventrolateral parts of periaqueductal gray (vlPAG) significantly increased C-response and post-discharge activity in most of the WDR neurons. Pre-microinjection of lidocaine (4%) into the nucleus raphe magnus (NRM) (0.5 microl), ipsilateral nucleus reticularis gigantocellularis (NGC) (0.6 microl), or nucleus gigantocellularis pars alpha (NGCalpha) and nucleus reticularis paragigantocellularis lateralis (NPGL) (0.5 microl) markedly reduced intra-vlPAG microinjection of OFQ-induced facilitatory effects on nociceptive responses of WDR neurons. Furthermore, if the NRM and ipsilateral NGC were simultaneously pre-microinjected with lidocaine, the intra-vlPAG microinjection of OFQ-induced facilitation on nociceptive responses of WDR neurons was eliminated. Also, a similar effect was observed when all the NRM, ipsilateral NGC, NGCalpha and NPGL were blocked with lidocaine. No significant effect on nociceptive responses of WDR neurons per se was found after blocking the NRM, ipsilateral NGC, NGCalpha/NPGL, or all the NRM, ipsilateral NGC, and NGCalpha/NPGL with lidocaine. These results indicate that (1) the facilitatory effect evoked by microinjection of OFQ into the vlPAG on nociceptive responses of WDR neurons in the spinal dorsal horn is primarily mediated by the NRM and ipsilateral NGC; (2) the NRM, ipsilateral NGC, and NGCalpha/NPGL do not mediate tonic descending inhibition of the spinal dorsal horn neurons.

Brainstem modulation of pain during sleep and waking

Moderately painful stimuli applied during sleep evoke motor and neural responses indicative of arousal, but seldom cause awakening. Different reactions occur in response to acute pain stimulation across behavioral states; pain reactions are modulated by the activity of serotonergic and non-serotonergic cells in the raphe magnus (RM). Serotonergic RM cells have state-dependent discharge and may inhibit simple motor withdrawal responses during waking. ON and OFF cells are non-serotonergic RM neurons thought to facilitate and inhibit pain, respectively. These cells display reciprocal spontaneous discharge patterns across the sleep-wake cycle, with ON cells most active during waking and OFF cells most active during sleep. We propose that they also play an important role in modulating the alertness evoked by any brief external stimulus, either noxious or innocuous. ON cells may facilitate alertness during waking and OFF cells suppress arousals during sleep. In the presence of chronic pain, both ON and OFF cell discharge appear to increase. The increase in ON cell discharge may contribute to enhancing pain sensitivity and alertness. Future research is needed to understand why sleep is so adversely affected in chronic pain patients, whereas sleep is minimally disrupted, even by acutely painful stimuli, in humans and animals without chronic pain.

Heterogeneous synaptic inputs from the ventrolateral periaqueductal gray matter to neurons responding to somatosensory stimuli in the rostral ventromedial medulla of rats

The ventrolateral cell column of the midbrain periaqueductal gray matter (vl-PAG) plays a major role in the attenuation of pain behaviour. It is established that this effect is exerted via modulation of neuronal activities in the rostral ventromedial medulla (RVM). Until recently it has been generally accepted that the vl-PAG exerts its modulatory effects upon RVM neurons through a direct monosynaptic pathway. However, recent data suggest that an additional indirect, di- or polysynaptic pathway may also exist. Using in vivo intracellular recordings we tested this hypothesis, by studying synaptic responses of somatosensory receptive RVM neurons evoked by electric stimulation of the vl-PAG in rats. RVM neurons were regarded as somatosensory receptive if they responded to electrical stimulation of the sciatic nerve. Most of the recorded RVM cells were excited by vl-PAG stimulation. Some of them responded with a short onset latency (3.6+/-0.9 ms) corresponding to monosynaptic excitation. All of these neurons were also excited by sciatic nerve stimulation at nociceptive intensities. In contrast to this, another proportion of the recorded RVM neurons responded with a four times longer (14.8+/-3 ms) onset latency to the vl-PAG stimulation, corresponding to polysynaptic modulation. All of these neurons were inhibited by sciatic nerve stimulation. The findings show that RVM neurons receive heterogeneous monosynaptic and polysynaptic inputs from the vl-PAG. The results also suggest that the monosynaptic and polysynaptic pathways modulate the activity of functionally distinct groups of RVM neurons.

Rostral ventromedial medulla neurons that project to the spinal cord express multiple opioid receptor phenotypes

The rostral ventromedial medulla (RVM) forms part of a descending pathway that modulates nociceptive neurotransmission at the level of the spinal cord dorsal horn. However, the involvement of descending RVM systems in opioid analgesia are a matter of some debate. In the present study, patch-clamp recordings of RVM neurons were made from rats that had received retrograde tracer injections into the spinal cord. More than 90% of identified spinally projecting RVM neurons responded to opioid agonists. Of these neurons, 53% responded only to the mu-opioid agonist D-Ala2, N-Me-Phe4, Gly-ol5 enkephalin, 14% responded only to the kappa-opioid agonist U-69593, and another group responded to both mu and kappa opioids (23%). In unidentified RVM neurons, a larger proportion of neurons responded only to mu opioids (75%), with smaller proportions of kappa- (4%) and mu/kappa-opioid (13%) responders. These RVM slices were then immunostained for tryptophan hydroxylase (TPH), a marker of serotonergic neurons. Forty-percent of spinally projecting neurons and 11% of unidentified neurons were TPH positive. Of the TPH-positive spinally projecting neurons, there were similar proportions of mu- (33%), kappa- (25%), and mu/kappa-opioid (33%) responders. Most of the TPH-negative spinally projecting neurons were mu-opioid responders (67%). These findings indicate that functional opioid receptor subtypes exist on spinally projecting serotonergic and nonserotonergic RVM neurons. The proportions of mu- and kappa-opioid receptors expressed differ between serotonergic and nonserotonergic neurons and between retrogradely labeled and unlabeled RVM neurons. We conclude that important roles exist for both serotonergic and nonserotonergic RVM neurons in the mediation of opioid effects.

Comparison of morphine and kainic acid microinjections into identical PAG sites on the activity of RVM neurons

The rostral ventromedial medulla (RVM) modulates nociception through changes in the activity of two classes of neuron, ON- and OFF-cells. The activity of these neurons is regulated, in part, by input from the periaqueductal gray (PAG). The objective of this study was to determine whether PAG-mediated antinociception is associated with excitation of both ON- and OFF-cells in the RVM. Microinjection of morphine into the ventrolateral PAG produced antinociception at 50% of the injection sites. This antinociception was associated with continuous activation of RVM OFF-cells and inhibition of both the spontaneous and reflex-related activity of RVM ON-cells. Microinjection of kainic acid into the same injection sites produced antinociception 92% (37/40) of the time. Although kainic acid directly excites PAG output neurons, the changes in ON- and OFF-cell activity associated with microinjection of kainic acid into the ventrolateral PAG were the same as when morphine was injected. That is, ON-cells were inhibited and OFF-cells were activated. These data indicate that the excitatory connection between the PAG and RVM is directed at RVM OFF-cells specifically. In addition, these data suggest that direct activation of PAG output neurons, as occurs with kainic acid, is much more likely to produce antinociception than disinhibition of output neurons as occurs following morphine administration.

Opponent interactions between serotonin and dopamine

Anatomical and pharmacological evidence suggests that the dorsal raphe serotonin system and the ventral tegmental and substantia nigra dopamine system may act as mutual opponents. In the light of the temporal difference model of the involvement of the dopamine system in reward learning, we consider three aspects of motivational opponency involving dopamine and serotonin. We suggest that a tonic serotonergic signal reports the long-run average reward rate as part of an average-case reinforcement learning model; that a tonic dopaminergic signal reports the long-run average punishment rate in a similar context; and finally speculate that a phasic serotonin signal might report an ongoing prediction error for future punishment.

Descending control of pain

Upon receipt in the dorsal horn (DH) of the spinal cord, nociceptive (pain-signalling) information from the viscera, skin and other organs is subject to extensive processing by a diversity of mechanisms, certain of which enhance, and certain of which inhibit, its transfer to higher centres. In this regard, a network of descending pathways projecting from cerebral structures to the DH plays a complex and crucial role. Specific centrifugal pathways either suppress (descending inhibition) or potentiate (descending facilitation) passage of nociceptive messages to the brain. Engagement of descending inhibition by the opioid analgesic, morphine, fulfils an important role in its pain-relieving properties, while induction of analgesia by the adrenergic agonist, clonidine, reflects actions at alpha(2)-adrenoceptors (alpha(2)-ARs) in the DH normally recruited by descending pathways. However, opioids and adrenergic agents exploit but a tiny fraction of the vast panoply of mechanisms now known to be involved in the induction and/or expression of descending controls. For example, no drug interfering with descending facilitation is currently available for clinical use. The present review focuses on: (1) the organisation of descending pathways and their pathophysiological significance; (2) the role of individual transmitters and specific receptor types in the modulation and expression of mechanisms of descending inhibition and facilitation and (3) the advantages and limitations of established and innovative analgesic strategies which act by manipulation of descending controls. Knowledge of descending pathways has increased exponentially in recent years, so this is an opportune moment to survey their operation and therapeutic relevance to the improved management of pain.

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.

Physiological and anatomic evidence for functional subclasses of serotonergic raphe magnus cells

Serotonergic cells in the medullary nucleus raphe magnus (RM) and adjacent nucleus reticularis magnocellularis (NRMC) project to the spinal cord where they are likely to modulate nociceptive transmission. Previous studies have suggested that these cells are physiologically and anatomically heterogeneous. In the present investigation, we examined whether subclasses of serotonergic RM and NRMC cells can be delineated based on their response to a visceral stimulus, and whether any such subclasses are morphologically distinct. Most RM and NRMC serotonergic cells tested (81 of 116) responded to retraction of the descending aorta into a polyethylene tube (the snare stimulus) with 57% of all cells tested excited and 13% inhibited. Responses of serotonergic cells to the snare outlasted the stimulus, were not reflective of evoked cardiovascular changes, and were observed in sino-aortic deafferented rats, evidence that the snare stimulus does not influence serotonergic cell discharge through activation of baroreceptors. Because serotonergic cells responsive to the snare were also responsive to mechanical brushing within the retroperitoneum, the snare is likely to change serotonergic cell discharge by means of the activation of mechanosensitive visceral afferents. Intracellular labeling of physiologically characterized serotonergic RM and NRMC cells showed that cells that were responsive to the snare stimulus had simpler axonal collateralization patterns than cells that were unresponsive to the snare stimulus. This association between morphological and physiological properties provides additional evidence that subpopulations of serotonergic cells exist and serve varied physiological functions.

The projections of the midbrain periaqueductal grey to the pons and medulla oblongata in rats

It is now established that stimulation of the ventrolateral midbrain periaqueductal grey (PAG) evokes inhibition of nociceptive spinal neurons, which results in analgesia and a powerful attenuation of pain behaviour. It is postulated that the PAG exerts this inhibitory effect on spinal nociceptive functions through the activation of descending serotonergic and noradrenergic pathways that arise from the rostral ventromedial medulla (RVM) and pontine noradrenergic nuclei. To investigate the neuroanatomical substrate of this functional link between the PAG and RVM, as well as the pontine noradrenergic nuclei in the rat, we labelled axons that project from the ventrolateral PAG to various regions of the pons and medulla oblongata using the anterograde tracing substance, Phaseolus vulgaris leucoagglutinin. We demonstrated that some of PAG efferents really do terminate in the RVM and pontine noradrenergic nuclei, but a substantial proportion of them project to the intermediate subdivision of the pontobulbar reticular formation. Combining the axonal tracing with serotonin- and tyrosine-hydroxylase-immunohistochemistry, we also found that, in contrast to previous results, PAG efferents make relatively few appositions with serotonin- and tyrosine-hydroxylase-immunoreactive neurons in the RVM and pontine noradrenergic nuclei; most of them terminate in nonimmunoreactive territories. The results suggest that the ventrolateral PAG may activate a complex pontobulbar neuronal assembly including neurons in the intermediate subdivision of the pontobulbar reticular formation, serotonin- and tyrosine-hydroxylase-immunoreactive and nonimmunoreactive neurons in the RVM and pontine noradrenergic nuclei. This pontobulbar neural circuitry, then, may mediate the PAG-evoked activities towards the spinal dorsal horn resulting in the inhibition of spinal nociceptive functions.

Contributions of the medullary raphe and ventromedial reticular region to pain modulation and other homeostatic functions

The raphe magnus is part of an interrelated region of medullary raphe and ventromedial reticular nuclei that project to all areas of the spinal gray. Activation of raphe and reticular neurons evokes modulatory effects in sensory, autonomic, and motor spinal processes. Two physiological types of nonserotonergic cells are observed in the medullary raphe and are thought to modulate spinal pain processing in opposing directions. Recent evidence suggests that these cells may modulate stimulus-evoked arousal or alerting rather than pain-evoked withdrawals. Nonserotonergic cells are also likely to modulate spinal autonomic and motor circuits involved in thermoregulation and sexual function. Medullary serotonergic cells have state-dependent discharge and are likely to contribute to the modulation of pain processing, thermoregulation, and sexual function in the spinal cord. The medullary raphe and ventromedial reticular region may set sensory, autonomic, and motor spinal circuits into configurations that are appropriate to the current behavioral state.

The discharge of a subset of serotonergic raphe magnus cells is influenced by baroreceptor input

In order to determine whether serotonergic cells in the medullary raphe magnus (RM) receive baroreceptor input, cells were tested for their responses to descending aortic occlusion, aortic nerve stimulation, or systemic phenylephrine administration in the lightly anesthetized rat. Serotonergic cells were identified physiologically by a quantitative analysis of their slow and steady discharge. Greater than 40% of the serotonergic RM cells tested responded to brief occlusion of the descending aorta at the level of the coeliac arteries, a stimulus that elevated blood pressure by about 30 mmHg. Similarly, about 40% of the serotonergic RM cells responded to stimulation of the aortic nerve, a nerve that contains primarily baroreceptor afferents from the aortic arch. Greater than 70% of RM serotonergic cells responded to phenylephrine administration which elevated blood pressure by an average of 50 mmHg. Serotonergic cell responses to all methods of baroreceptor activation were small in magnitude and were largely restricted in time to the stimulus duration. The results indicate that a subset of serotonergic cells in RM are influenced by baroreceptor activity.

Serotonergic Raphe magnus cells that respond to noxious tail heat are not ON or OFF cells

Pharmacological studies have suggested that serotonergic cells in RM contribute to both the inhibition and facilitation of spinal nociceptive transmission. Physiological studies in the medullary nucleus raphe magnus (RM) and adjacent nucleus reticularis magnocellularis have identified putative nociceptive-inhibitory OFF cells and nociceptive-facilitatory neurons ON cells by their responses to noxious thermal stimulation. The present study was designed to determine 1) whether any serotonergic RM cells respond to noxious thermal stimulation and 2) whether noxious heat-responsive serotonergic cells should be classified as ON or OFF cells. Serotonergic cells (n = 150) were identified by physiological criteria in anesthetized rats; 30 of 32 cells tested contained serotonin immunoreactivity. Noxious tail heat elicited a neuronal response in less than a quarter of the serotonergic cells. Most serotonergic cells that responded to tail heat were excited (n = 25), while a small minority of the cells tested were inhibited (n = 8). The tail heat-evoked responses of serotonergic cells were small in magnitude, averaging five to eight spikes in 10 s. Excitatory responses rarely persisted for more than 10 s, while inhibitory responses rarely persisted for more than 20 s. The tail heat-evoked responses of serotonergic cells were compared to those of non-serotonergic cells (n = 186). Non-serotonergic cells that responded to noxious tail heat had significantly greater response magnitudes, averaging 75-95 spikes in 10 s, than heat-responsive serotonergic cells. In addition, most heat-responsive non-serotonergic cells responded for at least 30 s after stimulus onset. These results demonstrate that the tail heat-evoked responses of serotonergic RM cells are qualitatively and quantitatively distinct from those of non-serotonergic ON and OFF cells. It is therefore unlikely that serotonergic RM cells, even the subpopulation that responds to noxious tail heat, share a physiological function with ON and OFF cells.

Supraspinal circuitry mediating opioid antinociception: antagonist and synergy studies in multiple sites

Supraspinal opioid antinociception is mediated by sensitive brain sites capable of supporting this response following microinjection of opioid agonists. These sites include the ventrolateral periaqueductal gray (vIPAG), the rostral ventromedial medulla (RVM), the locus coeruleus and the amygdala. Each of these sites comprise an interconnected anatomical and physiologically relevant system mediating antinociceptive responses through regional interactions. Such interactions have been identified using two pharmacological approaches: (1) the ability of selective antagonists delivered to one site to block antinociception elicited by opioid agonists in a second site, and (2) the presence of synergistic antinociceptive interactions following simultaneous administration of subthreshold doses of opioid agonists into pairs of sites. Thus, the RVM has essential serotonergic, opioid, cholinergic and NMDA synapses that are necessary for the full expression of morphine antinociception elicited from the vIPAG, and the vIPAG has essential opioid synapses that are necessary for the full expression of opioid antinociception elicited from the amygdala. Further, the vIPAG, RVM, locus coeruleus and amygdala interact with each other in synergistically supporting opioid antinociception.

Central mechanisms of pain modulation

Bulbospinal serotonergic neurons and two physiological classes of bulbospinal nonserotonergic cells interact to modulate pain transmission. Recent studies have begun to elaborate targets of descending pain modulation other than the well-studied flexion withdrawal pathways. Site-specific, naloxone-sensitive placebo analgesia, which is hard to reconcile with current models of descending pain modulation, presents an exciting challenge to the field.

The induction of pain: an integrative review

The highly disagreeable sensation of pain results from an extraordinarily complex and interactive series of mechanisms integrated at all levels of the neuroaxis, from the periphery, via the dorsal horn to higher cerebral structures. Pain is usually elicited by the activation of specific nociceptors ('nociceptive pain'). However, it may also result from injury to sensory fibres, or from damage to the CNS itself ('neuropathic pain'). Although acute and subchronic, nociceptive pain fulfils a warning role, chronic and/or severe nociceptive and neuropathic pain is maladaptive. Recent years have seen a progressive unravelling of the neuroanatomical circuits and cellular mechanisms underlying the induction of pain. In addition to familiar inflammatory mediators, such as prostaglandins and bradykinin, potentially-important, pronociceptive roles have been proposed for a variety of 'exotic' species, including protons, ATP, cytokines, neurotrophins (growth factors) and nitric oxide. Further, both in the periphery and in the CNS, non-neuronal glial and immunecompetent cells have been shown to play a modulatory role in the response to inflammation and injury, and in processes modifying nociception. In the dorsal horn of the spinal cord, wherein the primary processing of nociceptive information occurs, N-methyl-D-aspartate receptors are activated by glutamate released from nocisponsive afferent fibres. Their activation plays a key role in the induction of neuronal sensitization, a process underlying prolonged painful states. In addition, upon peripheral nerve injury, a reduction of inhibitory interneurone tone in the dorsal horn exacerbates sensitized states and further enhance nociception. As concerns the transfer of nociceptive information to the brain, several pathways other than the classical spinothalamic tract are of importance: for example, the postsynaptic dorsal column pathway. In discussing the roles of supraspinal structures in pain sensation, differences between its 'discriminative-sensory' and 'affective-cognitive' dimensions should be emphasized. The purpose of the present article is to provide a global account of mechanisms involved in the induction of pain. Particular attention is focused on cellular aspects and on the consequences of peripheral nerve injury. In the first part of the review, neuronal pathways for the transmission of nociceptive information from peripheral nerve terminals to the dorsal horn, and therefrom to higher centres, are outlined. This neuronal framework is then exploited for a consideration of peripheral, spinal and supraspinal mechanisms involved in the induction of pain by stimulation of peripheral nociceptors, by peripheral nerve injury and by damage to the CNS itself. Finally, a hypothesis is forwarded that neurotrophins may play an important role in central, adaptive mechanisms modulating nociception. An improved understanding of the origins of pain should facilitate the development of novel strategies for its more effective treatment.

Physiological survey of medullary raphe and magnocellular reticular neurons in the anesthetized rat

The present study was designed to provide a detailed and quantitative description of the physiological characteristics of neurons in the medullary raphe magnus (RM) and adjacent nucleus reticularis magnocellularis (NRMC) under anesthetized conditions. The background discharge and noxious stimulus-evoked responses of RM and NRMC neurons were recorded in rats lightly anesthetized with isoflurane. All cells that were isolated successfully were studied. After recording background discharge, the neuronal response to repeated noxious thermal and noxious mechanical stimulation of the tail was recorded. Most cells were identified as nonserotonergic by their irregular or rapid background discharge pattern. Because the spontaneous discharge of most RM nonserotonergic cells contained pauses and bursts, a comparison between the change in rate evoked by tail heat and the range of rate changes that occur spontaneously was used to classify cells. The mean responses of ON and OFF cells were more than four times the standard deviation of the changes in rate observed spontaneously. ON cells were excited in 86% of the tail heat trials tested. Similarly, OFF cells were inhibited in 97% of the noxious tail heat trials tested. The heat-evoked changes in ON and OFF cell discharge varied over more than two orders of magnitude and were greater in cells with greater rates of background discharge. The heat-evoked responses of and cells had durations of tens of seconds to minutes and were always sustained beyond the visible motor response. Most ON and OFF cells responded to noxious tail clamp in a manner that was similar to their response to noxious heat. More than half of the NEUTRAL cells that were unresponsive to noxious heat were responsive to noxious tail clamp. A minority of ON, OFF, and NEUTRAL cells responded to innocuous brush stimulation with weak, transient responses. Although many cells discharged too infrequently to be classified, units with physiological properties that were different from those described above were rare. In conclusion, most RM and NRMC cells belong to three nonserotonergic physiological cell classes that can be distinguished from each other by the consistency, not the magnitude, of their responses to repeated noxious thermal stimulation. Because most of the heat-evoked change in and cell discharge occurs after the conclusion of the initial motor withdrawal, ON and OFF cells are likely to principally modulate the response to subsequent noxious insults.

Activation of serotonergic neurons in the raphe magnus is not necessary for morphine analgesia

A wealth of pharmacological and behavioral data suggests that spinally projecting serotonergic cells mediate opioid analgesia. A population of medullary neurons, located within raphe magnus (RM) and the neighboring reticular nuclei, contains serotonin and is the source of serotonin in the spinal dorsal horn. To test whether serotonergic neurons mediate opioid analgesia, morphine was administered during recordings from medullary cells that were physiologically characterized as serotonergic (5HTp) by their slow and steady discharge pattern in the lightly anesthetized rat. Selected 5HTp cells (n = 14) were intracellularly labeled, and all contained serotonin immunoreactivity. The discharge of most 5HTp cells was not affected by an analgesic dose of systemic morphine. In a minority of cases, 5HTp cells either increased or decreased their discharge after morphine administration. However, morphine altered the discharge of some 5HTp cells in the absence of producing analgesia and conversely did not alter the discharge of most 5HTp cells in cases in which analgesia occurred. RM cells with irregular discharge patterns and excitatory or inhibitory responses to noxious tail heat were classified as ON and OFF cells, respectively. All ON and OFF cells that were intracellularly labeled (n = 9) lacked serotonin immunoreactivity. All ON cells were inhibited, and most OFF cells were excited by systemic morphine. Because 5HTp cells do not consistently change their discharge during morphine analgesia, they are unlikely to mediate the analgesic effects of morphine. Instead, nonserotonergic cells are likely to mediate morphine analgesia in the anesthetized rat. In light of the sensitivity of morphine analgesia to manipulations of serotonin, serotonin release, although neither necessary nor sufficient for opioid analgesia, is proposed to facilitate the analgesic effects of nonserotonergic RM terminals in the spinal cord.