Morphine injected into the habenula and dorsal posteromedial thalamus produces analgesia in the formalin test

Potentiation of the lateral habenula-ventral tegmental area pathway underlines the susceptibility to depression in mice with chronic pain

Chronic pain often develops severe mood changes such as depression. However, how chronic pain leads to depression remains elusive and the mechanisms determining individuals' responses to depression are largely unexplored. Here we found that depression-like behaviors could only be observed in 67.9% of mice with chronic neuropathic pain, leaving 32.1% of mice with depression resilience. We determined that the spike discharges of the ventral tegmental area (VTA)-projecting lateral habenula (LHb) glutamatergic (Glu) neurons were sequentially increased in sham, resilient and susceptible mice, which consequently inhibited VTA dopaminergic (DA) neurons through a LHbGlu-VTAGABA-VTADA circuit. Furthermore, the LHbGlu-VTADA excitatory inputs were dampened via GABAB receptors in a pre-synaptic manner. Regulation of LHb-VTA pathway largely affected the development of depressive symptoms caused by chronic pain. Our study thus identifies a pivotal role of the LHb-VTA pathway in coupling chronic pain with depression and highlights the activity-dependent contribution of LHbGlu-to-VTADA inhibition in depressive behavioral regulation.

Cell-type specific molecular architecture for mu opioid receptor function in pain and addiction circuits

Opioids are potent analgesics broadly used for pain management; however, they can produce dangerous side effects including addiction and respiratory depression. These harmful effects have led to an epidemic of opioid abuse and overdose deaths, creating an urgent need for the development of both safer pain medications and treatments for opioid use disorders. Both the analgesic and addictive properties of opioids are mediated by the mu opioid receptor (MOR), making resolution of the cell types and neural circuits responsible for each of the effects of opioids a critical research goal. Single-cell RNA sequencing (scRNA-seq) technology is enabling the identification of MOR-expressing cell types throughout the nervous system, creating new opportunities for mapping distinct opioid effects onto newly discovered cell types. Here, we describe molecularly defined MOR-expressing neuronal cell types throughout the peripheral and central nervous systems and their potential contributions to opioid analgesia and addiction.

Habenula activation patterns in a preclinical model of neuropathic pain accompanied by depressive-like behaviour

Pain and depression are complex disorders that frequently co-occur, resulting in diminished quality of life. The habenula is an epithalamic structure considered to play a pivotal role in the neurocircuitry of both pain and depression. The habenula can be divided into two major areas, the lateral and medial habenula, that can be further subdivided, resulting in 6 main subregions. Here, we investigated habenula activation patterns in a rat model of neuropathic pain with accompanying depressive-like behaviour. Wistar rats received active surgery for the development of neuropathic pain (chronic constriction injury of the sciatic nerve; CCI), sham surgery (surgical control), or no surgery (behavioural control). All animals were evaluated for mechanical nociceptive threshold using the paw pressure test and depressive-like behaviour using the forced swimming test, followed by evaluation of the immunoreactivity to cFos—a marker of neuronal activity—in the habenula and subregions. The Open Field Test was used to evaluate locomotor activity. Animals with peripheral neuropathy (CCI) showed decreased mechanical nociceptive threshold and increased depressive-like behaviour compared to control groups. The CCI group presented decreased cFos immunoreactivity in the total habenula, total lateral habenula and lateral habenula subregions, compared to controls. No difference was found in cFos immunoreactivity in the total medial habenula, however when evaluating the subregions of the medial habenula, we observed distinct activation patterns, with increase cFos immunoreactivity in the superior subregion and decrease in the central subregion. Taken together, our data suggest an involvement of the habenula in neuropathic pain and accompanying depressive-like behaviour.

Lateral Habenula and Its Potential Roles in Pain and Related Behaviors

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

A diencephalic circuit in rats for opioid analgesia but not positive reinforcement

Mu opioid receptor (MOR) agonists are potent analgesics, but also cause sedation, respiratory depression, and addiction risk. The epithalamic lateral habenula (LHb) signals aversive states including pain, and here we found that it is a potent site for MOR-agonist analgesia-like responses in rats. Importantly, LHb MOR activation is not reinforcing in the absence of noxious input. The LHb receives excitatory inputs from multiple sites including the ventral tegmental area, lateral hypothalamus, entopeduncular nucleus, and the lateral preoptic area of the hypothalamus (LPO). Here we report that LHb-projecting glutamatergic LPO neurons are excited by noxious stimulation and are preferentially inhibited by MOR selective agonists. Critically, optogenetic stimulation of LHb-projecting LPO neurons produces an aversive state that is relieved by LHb MOR activation, and optogenetic inhibition of LHb-projecting LPO neurons relieves the aversiveness of ongoing pain.

Opioids are potent analgesics but also have addiction risk. Here a lateral preoptic area to lateral habenula connection is identified by which opioids relieve ongoing pain but do not produce reward in animals that do not have ongoing pain.

Fish as a model in social neuroscience: conservation and diversity in the social brain network

Mechanisms for fish social behaviours involve a social brain network (SBN) which is evolutionarily conserved among vertebrates. However, considerable diversity is observed in the actual behaviour patterns amongst nearly 30000 fish species. The huge variation found in socio-sexual behaviours and strategies is likely generated by a morphologically and genetically well-conserved small forebrain system. Hence, teleost fish provide a useful model to study the fundamental mechanisms underlying social brain functions. Herein we review the foundations underlying fish social behaviours including sensory, hormonal, molecular and neuroanatomical features. Gonadotropin-releasing hormone neurons clearly play important roles, but the participation of vasotocin and isotocin is also highlighted. Genetic investigations of developing fish brain have revealed the molecular complexity of neural development of the SBN. In addition to straightforward social behaviours such as sex and aggression, new experiments have revealed higher order and unique phenomena such as social eavesdropping and social buffering in fish. Finally, observations interpreted as 'collective cognition' in fish can likely be explained by careful observation of sensory determinants and analyses using the dynamics of quantitative scaling. Understanding of the functions of the SBN in fish provide clues for understanding the origin and evolution of higher social functions in vertebrates.

Mirtazapine, an α2 Antagonist-Type Antidepressant, Reverses Pain and Lack of Morphine Analgesia in Fibromyalgia-Like Mouse Models

Treatment of fibromyalgia is an unmet medical need; however, its pathogenesis is still poorly understood. In a series of studies, we have demonstrated that some pharmacological treatments reverse generalized chronic pain but do not affect the lack of morphine analgesia in the intermittent cold stress (ICS)-induced fibromyalgia-like pain model in mice. Here we report that repeated intraperitoneal treatments with mirtazapine, which is presumed to disinhibit 5-hydroxytriptamine (5-HT) release and activate 5-HT1 receptor through mechanisms of blocking presynaptic adrenergic α2 and postsynaptic 5-HT2 and 5-HT3 receptors, completely reversed the chronic pain for more than 4 to 5 days after the cessation of treatments. The repeated mirtazapine treatments also recovered the morphine analgesia after the return of nociceptive threshold to the normal level. The microinjection of small interfering RNA (siRNA) adrenergic α2a receptor (ADRA2A) into the habenula, which showed a selective upregulation of α2 receptor gene expression after ICS, reversed the hyperalgesia but did not recover the morphine analgesia. However, both reversal of hyperalgesia and recovery of morphine analgesia were observed when siRNA ADRA2A was administered intracerebroventricularly. As the habenular is reported to be involved in the emotion/reward-related pain and hypoalgesia, these results suggest that mirtazapine could attenuate pain and/or augment hypoalgesia by blocking the habenular α2 receptor after ICS. The recovery of morphine analgesia in the ICS model, on the other hand, seems to be mediated through a blockade of α2 receptor in unidentified brain regions. SIGNIFICANCE STATEMENT: This study reports possible mechanisms underlying the complete reversal of hyperalgesia and recovery of morphine analgesia by mirtazapine, a unique antidepressant with adrenergic α2 and serotonergic receptor antagonist properties, in a type of intermittently repeated stress (ICS)-induced fibromyalgia-like pain model. Habenula, a brain region which is related to the control of emotional pain, was found to play key roles in the antihyperalgesia, whereas other brain regions appeared to be involved in the recovery of morphine analgesia in the ICS model.

Circuits and functions of the lateral habenula in health and in disease

The past decade has witnessed exponentially growing interest in the lateral habenula (LHb) owing to new discoveries relating to its critical role in regulating negatively motivated behaviour and its implication in major depression. The LHb, sometimes referred to as the brain's 'antireward centre', receives inputs from diverse limbic forebrain and basal ganglia structures, and targets essentially all midbrain neuromodulatory systems, including the noradrenergic, serotonergic and dopaminergic systems. Its unique anatomical position enables the LHb to act as a hub that integrates value-based, sensory and experience-dependent information to regulate various motivational, cognitive and motor processes. Dysfunction of the LHb may contribute to the pathophysiology of several psychiatric disorders, especially major depression. Recently, exciting progress has been made in identifying the molecular and cellular mechanisms in the LHb that underlie negative emotional state in animal models of drug withdrawal and major depression. A future challenge is to translate these advances into effective clinical treatments.

Larger Numbers of Glial and Neuronal Cells in the Periaqueductal Gray Matter of μ-Opioid Receptor Knockout Mice

Background: μ-opioid receptor knockout (MOP-KO) mice display baseline hyperalgesia. We have recently identified changes in tissue volume in the periaqueductal gray matter (PAG) using magnetic resonance imaging voxel-based morphometry. Changes in the structure and connectivity of this region might account for some behavior phenotypes in MOP-KO mice, including hyperalgesia. Methods: Adult male MOP-KO and wild-type (WT) mice were studied. Immunohistochemistry was performed to detect microglia, astrocytes, and neurons in the PAG using specific markers: ionized calcium-binding adaptor molecule 1 (Iba-1) for microglia, glial fibrillary acidic protein (GFAP) for astrocytes, and the neuronal nuclei antigen (NeuN; product of the Rbfox3 gene) for neurons, respectively. Cell counting was performed in the four parallel longitudinal columns of the PAG (dorsomedial, dorsolateral, lateral, and ventrolateral) at three different locations from bregma (-3.5, -4.0, and -4.5 mm). Results: The quantitative analysis showed larger numbers of well-distributed Iba1-IR cells (microglia), NeuN-IR cells (neurons), and GFAP-IR areas (astrocytes) at all the anatomically distinct regions examined, namely, the dorsomedial (DM) PAG, dorsolateral (DL) PAG, lateral (L) PAG, and ventrolateral (VL) PAG, in MOP-KO mice than in control mice. Conclusions: The cellular changes in the PAG identified in this paper may underlie aspects of the behavioral alterations produced by MOP receptor deletion, and suggest that alterations in the cellular structure of the PAG may contribute to hyperalgesic states.

The medial habenula and interpeduncular nucleus circuitry is critical in addiction, anxiety, and mood regulation

Abstinence from chronic use of addictive drugs triggers an aversive withdrawal syndrome that compels relapse and deters abstinence. Many features of this syndrome are common across multiple drugs, involving both affective and physical symptoms. Some of the network signaling underlying withdrawal symptoms overlaps with activity that is associated with aversive mood states, including anxiety and depression. Given these shared features, it is not surprising that a particular circuit, the dorsal diencephalic conduction system, and the medial habenula (MHb) and interpeduncular nucleus (IPN), in particular, have been identified as critical to the emergence of aversive states that arise both as a result and, independently, of drug addiction. As the features of this circuit continue to be characterized, the MHb-IPN axis is emerging as a viable target for therapeutics to aid in the treatment of addiction to multiple drugs of abuse as well as mood-associated disorders. This is an article for the special issue XVth International Symposium on Cholinergic Mechanisms.

Translating the Habenula-From Rodents to Humans

The habenula (Hb) is a central structure connecting forebrain to midbrain regions. This microstructure regulates monoaminergic systems, notably dopamine and serotonin, and integrates cognitive with emotional and sensory processing. Early preclinical data have described Hb as a brain nucleus activated in anticipation of aversive outcomes. Evidence has now accumulated to show that the Hb encodes both rewarding and aversive aspects of external stimuli, thus driving motivated behaviors and decision making. Human Hb research is still nascent but develops rapidly, alongside with the growth of neuroimaging and deep brain stimulation techniques. Not surprisingly, Hb dysfunction has been associated with psychiatric disorders, and studies in patients have established evidence for Hb involvement in major depression, addiction, and schizophrenia, as well as in pain and analgesia. Here, we summarize current knowledge from animal research and overview the existing human literature on anatomy and function of the Hb. We also discuss challenges and future directions in targeting this small brain structure in both rodents and humans. By combining animal data and human experimental studies, this review addresses the translational potential of preclinical Hb research.

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

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

Specific regions display altered grey matter volume in μ-opioid receptor knockout mice: MRI voxel-based morphometry

BACKGROUND AND PURPOSE

μ Opioid receptor knockout (MOP-KO) mice display several behavioural differences from wild-type (WT) littermates including differential responses to nociceptive stimuli. Brain structural changes have been tied to behavioural alterations noted in transgenic mice with targeting of different genes. Hence, we assess the brain structure of MOP-KO mice.

EXPERIMENTAL APPROACH

Magnetic resonance imaging (MRI) voxel-based morphometry (VBM) and histological methods were used to identify structural differences between extensively backcrossed MOP-KO mice and WT mice.

KEY RESULTS

MOP-KO mice displayed robust increases in regional grey matter volume in olfactory bulb, several hypothalamic nuclei, periaqueductal grey (PAG) and several cerebellar areas, most confirmed by VBM analysis. The largest increases in grey matter volume were detected in the glomerular layer of the olfactory bulb, arcuate nucleus of hypothalamus, ventrolateral PAG (VLPAG) and cerebellar regions including paramedian and cerebellar lobules. Histological analyses confirm several of these results, with increased VLPAG cell numbers and increased thickness of the olfactory bulb granule cell layer and cerebellar molecular and granular cell layers.

CONCLUSIONS AND IMPLICATIONS

MOP deletion causes previously undescribed structural changes in specific brain regions, but not in all regions with high MOP receptor densities (e.g. thalamus, nucleus accumbens) or that exhibit adult neurogenesis (e.g. hippocampus). Volume differences in hypothalamus and PAG may reflect behavioural changes including hyperalgesia. Although the precise relationship between volume change and MOP receptor deletion was not determined from this study alone, these findings suggest that levels of MOP receptor expression may influence a broader range of neural structure and function in humans than previously supposed.

LINKED ARTICLES

This article is part of a themed section on Opioids: New Pathways to Functional Selectivity. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2015.172.issue-2.

Altered formalin-induced pain and Fos induction in the periaqueductal grey of preadolescent rats following neonatal LPS exposure

Animal and human studies have demonstrated that early pain experiences can produce alterations in the nociceptive systems later in life including increased sensitivity to mechanical, thermal, and chemical stimuli. However, less is known about the impact of neonatal immune challenge on future responses to noxious stimuli and the reactivity of neural substrates involved in analgesia. Here we demonstrate that rats exposed to Lipopolysaccharide (LPS; 0.05 mg/kg IP, Salmonella enteritidis) during postnatal day (PND) 3 and 5 displayed enhanced formalin-induced flinching but not licking following formalin injection at PND 22. This LPS-induced hyperalgesia was accompanied by distinct recruitment of supra-spinal regions involved in analgesia as indicated by significantly attenuated Fos-protein induction in the rostral dorsal periaqueductal grey (DPAG) as well as rostral and caudal axes of the ventrolateral PAG (VLPAG). Formalin injections were associated with increased Fos-protein labelling in lateral habenula (LHb) as compared to medial habenula (MHb), however the intensity of this labelling did not differ as a result of neonatal immune challenge. These data highlight the importance of neonatal immune priming in programming inflammatory pain sensitivity later in development and highlight the PAG as a possible mediator of this process.

Differential activation of the μ-opioid receptor by oxycodone and morphine in pain-related brain regions in a bone cancer pain model

BACKGROUND AND PURPOSE

Bone cancer pain is chronic and often difficult to control with opioids. However, recent studies have shown that several opioids have distinct analgesic profiles in chronic pain.

EXPERIMENTAL APPROACH

To clarify the mechanisms underlying these distinct analgesic profiles, functional changes in the μ-opioid receptor were examined using a mouse femur bone cancer (FBC) model.

KEY RESULTS

In the FBC model, the B(max) of [(3) H]-DAMGO binding was reduced by 15-45% in the periaqueductal grey matter (PAG), region ventral to the PAG (vPAG), mediodorsal thalamus (mTH), ventral thalamus and spinal cord. Oxycodone (10(-8) -10(-5)  M) and morphine (10(-8) -10(-5)  M) activated [(35) S]-GTPγS binding, but the activation was significantly attenuated in the PAG, vPAG, mTH and spinal cord in the FBC model. Interestingly, the attenuation of oxycodone-induced [(35) S]-GTPγS binding was quite limited (9-26%) in comparison with that of morphine (46-65%) in the PAG, vPAG and mTH, but not in the spinal cord. Furthermore, i.c.v. oxycodone at doses of 0.02-1.0 μg per mouse clearly inhibited pain-related behaviours, such as guarding, limb-use abnormalities and allodynia-like behaviour in the FBC model mice, while i.c.v. morphine (0.05-2.0 μg per mouse) had only partial or little analgesic effect on limb-use abnormalities and allodynia-like behaviour.

CONCLUSION AND IMPLICATIONS

These results show that μ-opioid receptor functions are attenuated in several pain-related regions in bone cancer in an agonist-dependent manner, and suggest that modification of the μ-opioid receptor is responsible for the distinct analgesic effect of oxycodone and morphine.

Mapping of reinforcing and analgesic effects of the mu opioid agonist endomorphin-1 in the ventral midbrain of the rat

INTRODUCTION

Agonists at the mu opioid receptor (MOR) are widely recognized for their effects on reward and pain. Although prior studies have attributed some of these effects to MORs on GABA neurons in the ventral tegmental area (VTA), recent studies have identified a region of particularly strong MOR immunostaining residing caudal to the VTA, in a region denoted the rostromedial tegmental nucleus (RMTg).

METHODS

Hence, we examined whether rats would self-administer small doses (50-250 pmol) of the selective MOR agonist endomorphin-1 (EM1) into the RMTg and adjacent sites. EM1 was chosen due to its short half-life, thus limiting drug spread, and due to its presence endogenously in brain neurons, including some afferents to the RMTg.

RESULTS

The highest rates of EM1 self-administration occurred within 0.5 mm of the RMTg center, in a region roughly 0.8-1.6 mm caudal to the majority of VTA DA neurons. In contrast, self-administration rates were much lower in the adjacent VTA, interpeduncular nucleus, central linear nucleus, or median raphe nucleus. Furthermore, EM1 infusions into the RMTg, but not surrounding regions, produced conditioned place preference, while EM1 infusions into the RMTg but not anterior VTA markedly reduced formalin-induced pain behaviors. EM1 effects were mimicked by infusions of the GABA agonist muscimol into the same region, consistent with EM1 having inhibitory actions on its target neurons.

CONCLUSION

These results implicate a novel brain region in modulating MOR influences on both appetitive and aversive behavior.

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

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

RSK2 signaling in medial habenula contributes to acute morphine analgesia

It has been established that mu opioid receptors activate the ERK1/2 signaling cascade both in vitro and in vivo. The Ser/Thr kinase RSK2 is a direct downstream effector of ERK1/2 and has a role in cellular signaling, cell survival growth, and differentiation; however, its role in biological processes in vivo is less well known. Here we determined whether RSK2 contributes to mu-mediated signaling in vivo. Knockout mice for the rsk2 gene were tested for main morphine effects, including analgesia, tolerance to analgesia, locomotor activation, and sensitization to this effect, as well as morphine withdrawal. The deletion of RSK2 reduced acute morphine analgesia in the tail immersion test, indicating a role for this kinase in mu receptor-mediated nociceptive processing. All other morphine effects and adaptations to chronic morphine were unchanged. Because the mu opioid receptor and RSK2 both show high density in the habenula, we specifically downregulated RSK2 in this brain metastructure using an adeno-associated-virally mediated shRNA approach. Remarkably, morphine analgesia was significantly reduced, as observed in the total knockout animals. Together, these data indicate that RSK2 has a role in nociception, and strongly suggest that a mu opioid receptor-RSK2 signaling mechanism contributes to morphine analgesia at the level of habenula. This study opens novel perspectives for both our understanding of opioid analgesia, and the identification of signaling pathways operating in the habenular complex.

Mapping pain activation and connectivity of the human habenula

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

Unmasking the mysteries of the habenula in pain and analgesia

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

Septal stimulation inhibits spinal cord dorsal horn neuronal activity

Deep brain stimulation (DBS) has been used for relieving chronic pain in patients that have been through other existing options. The septum has been one of the targets for such treatment. The purpose of this study was to determine the inhibitory effect of electrical stimulation in the medial septum diagonal band of broca (MSDB) on neuronal activity in the spinal cord of rats anesthetized with sodium pentobarbital. While unilaterally stimulating the MSDB, wide dynamic range neurons in the lumbar region of the spinal cord were recorded in response to graded mechanical stimulation of the hind paws (brush, pressure, and pinch). Stimulation was at 1, 5, 10, and 20V, at 100Hz, and 0.1ms duration. Significant bilateral reduction was observed in response to pressure (ipsilaterally: 0.90±0.05, 0.48±0.06*, 0.55±0.05*, 0.40±0.05*; and contralaterally: 0.70±0.06*, 0.59±0.08*, 0.75±0.05*, 0.49±0.07*) and pinch (ipsilaterally: 0.89±0.08, 0.46±0.05*, 0.54±0.04*, 0.50±0.05*; and contralaterally: 0.78±0.05, 0.61±0.07*, 0.64±0.04*, 0.53±0.06*). Data were expressed as a fraction of control. Significant changes were also found in responses to brush in certain groups (ipsilaterally: 1.08±0.08, 0.72±0.06*, 1.00±0.12, 0.65±0.06*; and contralaterally: 0.93±0.05, 0.77±0.07*, 0.98±0.05, 0.84±0.07). Further analysis suggested that 5V was adequate for achieving optimal inhibition. It is concluded that the MSDB can be used as alternative target for DBS in the treatment of pain.

The habenular nuclei: a conserved asymmetric relay station in the vertebrate brain

The dorsal diencephalon, or epithalamus, contains the bilaterally paired habenular nuclei and the pineal complex. The habenulae form part of the dorsal diencephalic conduction (DDC) system, a highly conserved pathway found in all vertebrates. In this review, we shall describe the neuroanatomy of the DDC, consider its physiology and behavioural involvement, and discuss examples of neural asymmetries within both habenular circuitry and the pineal complex. We will discuss studies in zebrafish, which have examined the organization and development of this circuit, uncovered how asymmetry is represented at the level of individual neurons and determined how such left–right differences arise during development.

The lateral habenula: no longer neglected

In this contribution to the CNS Spectrums neuroanatomy series, Stefanie Geisler, MD, discusses the lateral habenula (LHb). This nuclear complex is one of the areas of the brain that forms part of the cross-talk between limbic fore-brain and some important ascending modulatory pathways. Situated at the caudal end of the dorsal diencephalon and classically regarded as projecting largely to the brainstem, including the serotoninergic raphe nuclei, the LHb receives afferents from widespread forebrain areas. Therefore, the LHb is able to influence serotonin tone in the brain, and has long interested neuroanatomists as a potential limbic-motor interface. Nonetheless, the LHb was not much discussed outside neuroanatomical circles until recently, when it was discovered that its impact on the mesotelencephalic dopamine system is probably much greater than had been assumed. The LHb has become a hot topic. This article-addresses these developments and emphasizes the clinical relevance of this interesting brain structure.

A conductor hidden in the orchestra? Role of the habenular complex in monoamine transmission and cognition

Influences of the habenular complex on electrophysiological and neurochemical aspects of brain functioning are well known. However, its role in cognition has been sparsely investigated until recently. The habenular complex, composed of medial and lateral subdivisions, is a node linking the forebrain with midbrain and hindbrain structures. The lateral habenula is the principal actor in this direct dialogue, while the medial habenula mostly conveys information to the interpeduncular nucleus before this modulates further regions. Here we describe neuroanatomical and physiological aspects of the habenular complex, and its role in cognitive processes, including new behavioral, electrophysiological and imaging findings. Habenular complex lesions result in deficits in learning, memory and attention, some of which decline during repeated testing, while others become worse, consistent with multiple roles in cognition. The habenular complex is particularly responsive to feedback about errors. Electrophysiological studies indicate a role in metaplasticity, the modulation of neuroplasticity. These studies thus reveal important roles of the habenular complex in learning, memory and attention.

Functional magnetic resonance imaging studies of opioid receptor-mediated modulation of noxious-evoked BOLD contrast in rats

RATIONALE

Functional magnetic resonance imaging (fMRI) in rats can non-invasively identify brain regions activated by physiological stimuli and the effects of pharmacological intervention on these responses.

OBJECTIVES

This study was conducted to investigate the effects of systemic administration of the mu-opioid receptor agonist morphine on whole brain functional signal intensity in anaesthetised rats; to investigate whether pre-treatment with the opioid receptor antagonist naloxone blocks the effects of morphine; to determine whether pre-treatment with morphine attenuates noxious-evoked changes in whole brain functional signal intensity.

METHODS

Continuous whole brain fMRI scanning was used to study brain signal intensity prior to, and following, systemic administration of morphine (5 mg/kg, n=7), systemic administration of naloxone (1 mg/kg) and morphine (n=8). Effects of pre-treatment with saline (n=5) or morphine (5 mg/kg, n=5) on formalin (5%, intraplantar)-evoked changes in signal intensity were determined. Data were processed using SMP99 with fixed-effects analysis (p<0.05).

RESULTS

Morphine produced significant positive bilateral increases in signal intensity in the cingulate cortex, amygdala, thalamus, hypothalamus and PAG (p<0.05), and these effects were blocked by naloxone. Intraplantar injection of formalin produced a significant positive increase in signal intensity in the cingulate cortex, somatosensory cortex, amygdala, thalamus, hypothalamus and PAG (p<0.05). Morphine attenuated formalin-evoked increases in signal intensity in the PAG, amygdala, hypothalamus and cingulate cortex.

CONCLUSION

Our data demonstrate that morphine modulates noxious-evoked changes in signal intensity in discrete brain regions. fMRI studies in rats are able to identify specific brain regions involved in the pharmacological modification of physiologically evoked changes in regional brain activation.

Mapping of chemical trigger zones for reward

Addictive drugs are thought to activate brain circuitry that normally mediates more natural rewards such as food or water. Drugs activate this circuitry at synaptic junctions within the brain; identifying the junctions at which this occurs provides clues to the neurochemical and anatomical characteristics of the circuitry. One approach to identifying the junctions at which drugs interact with this circuitry is to determine if animals will lever-press for site-specific microinjections of addictive drugs. This approach has identified GABAergic, dopaminergic, glutamatergic, and cholinergic trigger zones within meso-corticolimbic circuitry important for natural reward function.

Independent time courses of supraspinal nociceptive activity and spinally mediated behavior during tonic pain

The behavioral response to acute tissue injury is usually characterized by different phases, but the brain mechanisms underlying changes in pain-related behavior over time are still poorly understood. We aimed to analyze time-dependent changes in metabolic activity levels of 49 forebrain structures in the formalin pain model, using the autoradiographic 2-deoxyglucose method in unanesthetized, freely moving rats. We examined rats during the first phase of pain-related reactions ('early' groups), or during the third recovery phase, 60 min later, when the supraspinally mediated behavioral responses were reduced ('late' group). In the early groups, metabolic rates were bilaterally increased over control values in the periaqueductal gray, zona incerta and in several thalamic nuclei (anteroventral, centrolateral, lateral dorsal, parafascicular, posteromedial, submedius, ventromedial, and ventrobasal complex), as well as in the habenulae and in the parietal, cingulate, antero-dorsal insular, and anterior piriform cortex. A contralateral, somatotopically specific activation was found in the putative hindlimb representation area of the somatosensory cortex. In the late group, noxious-induced activation declined in most structures. However, metabolic rates were higher than controls in the periaqueductal gray and zona incerta and in two other structures not previously active: the prerubral area/field of Forel and the arcuate hypothalamic nucleus. These findings provide a time-dependent functional map of nociceptive and anti-nociceptive forebrain circuits during tonic pain. The parallel decrease in licking behavior and forebrain activity, at times when spinally mediated limb flexion responses were still present, suggests that endogenous antinociceptive systems may differently modulate spinal and supraspinal nociceptive networks following acute tissue injury.

Antinociceptive effects of morphine injected into the nucleus parafascicularis thalami of the rat

The antinociceptive action of morphine microinjected into the nucleus parafascicularis thalami (nPf) on pain behaviors organized at different levels of the neuraxis was examined in the rat. Behaviors organized at spinal (spinal motor reflexes, SMRs), medullary (vocalizations during shock, VDSs), and forebrain (vocalization afterdischarges, VADs) levels were elicited by noxious tailshock. Morphine administered into nPf generated dose-dependent increases in thresholds of VDS and VAD, but failed to elevate SMR thresholds. Increases in vocalization thresholds were reversed in a dose-dependent manner by the microinjection of the mu-opiate receptor antagonist, methylnaloxonium, into nPf. Results are discussed in terms of the relative influence of nPf-administered morphine on nociceptive processing at spinal versus supraspinal levels of the neuraxis.

Amygdaloid-thalamic interactions mediate the antinociceptive action of morphine microinjected into the periaqueductal gray

The bilateral administration of the serotonin receptor antagonist methysergide (2.5 microg, 5 microg, and 10 microg) into either the central nucleus of the amygdala (ACe) or nucleus parafascicularis thalami (nPf) produced dose-dependent inhibition of the antinociceptive action of ventrolateral periaqueductal gray (vPAG)-administered morphine. Unilateral administration of these doses of methysergide into either the ACe or nPf had no effect on morphine-induced antinociception. However, the combined unilateral administration of these doses of methysergide into the ACe and nPf produced dose-dependent inhibition of morphine antinociception that was identical to that observed after its bilateral administration into either site. This latter finding is interpreted as evidence that a functional interaction between the ACe and nPf supports the antinociceptive action of morphine administered into the vPAG.

Bilateral behavioral and regional cerebral blood flow changes during painful peripheral mononeuropathy in the rat

A unilateral chronic constriction injury (CCI) of the sciatic nerve produced bilateral effects in both pain related behaviors and in the pattern of forebrain activation. All CCI animals exhibited spontaneous pain-related behaviors as well as bilateral hyperalgesia and allodynia after CCI. Further, we identified changes in baseline (unstimulated) forebrain activation patterns 2 weeks following CCI by measuring regional cerebral blood flow (rCBF). Compared to controls, CCI consistently produced detectable, well-localized and typically bilateral increases in rCBF within multiple forebrain structures in unstimulated animals. For example, the hindlimb region of somatosensory cortex was significantly activated (22%) as well as multiple thalamc nuclei, including the ventral medial (8%), ventral posterior lateral (10%) and the posterior (9%) nuclear groups. In addition, several forebrain regions considered to be part of the limbic system showed pain-induced changes in rCBF, including the anterior dorsal nucleus of the thalamus (23%), cingulate cortex (18%), retrosplenial cortex (30%), habenular complex (53%), interpeduncular nucleus (45%) and the paraventricular nucleus of the hypothalamus (30%). Our results suggest that bilateral somatosensory and limbic forebrain structures participate in the neural mechanisms of prolonged persistent pain produced by a unilateral injury.

The role of dopamine in the nucleus accumbens in analgesia

Opioid and psychostimulant drugs have long been used for the relief of chronic pain in the clinical situation. Animal studies confirm that these drugs alleviate persistent or tonic pain. Little is known, however, about the neural systems underlying the suppression of tonic pain except that they are different from those mediating the suppression of phasic (i.e., sharp and short-lasting) pain. Although spinal and brainstem-descending pain suppression mechanisms play a role in mediating the inhibition of tonic pain, it appears that this response is additionally mediated by the activation of mechanisms lying rostral to the brainstem. Recent studies suggest that the activation of mesolimbic dopamine (DA) neurons, arising from the cell bodies of the ventral tegmental area (VTA) and projecting to the nucleus accumbens (NAcc), plays an important role in mediating the suppression of tonic pain. Other studies suggest that this pain-suppression system involving the activation of mesolimbic DA neurons is naturally triggered by exposure to stress, through the endogenous release of opioids and substance P (SP) in the midbrain.

Forebrain mechanisms of nociception and pain: analysis through imaging

Pain is a unified experience composed of interacting discriminative, affective-motivational, and cognitive components, each of which is mediated and modulated through forebrain mechanisms acting at spinal, brainstem, and cerebral levels. The size of the human forebrain in relation to the spinal cord gives anatomical emphasis to forebrain control over nociceptive processing. Human forebrain pathology can cause pain without the activation of nociceptors. Functional imaging of the normal human brain with positron emission tomography (PET) shows synaptically induced increases in regional cerebral blood flow (rCBF) in several regions specifically during pain. We have examined the variables of gender, type of noxious stimulus, and the origin of nociceptive input as potential determinants of the pattern and intensity of rCBF responses. The structures most consistently activated across genders and during contact heat pain, cold pain, cutaneous laser pain or intramuscular pain were the contralateral insula and anterior cingulate cortex, the bilateral thalamus and premotor cortex, and the cerebellar vermis. These regions are commonly activated in PET studies of pain conducted by other investigators, and the intensity of the brain rCBF response correlates parametrically with perceived pain intensity. To complement the human studies, we developed an animal model for investigating stimulus-induced rCBF responses in the rat. In accord with behavioral measures and the results of human PET, there is a progressive and selective activation of somatosensory and limbic system structures in the brain and brainstem following the subcutaneous injection of formalin. The animal model and human PET studies should be mutually reinforcing and thus facilitate progress in understanding forebrain mechanisms of normal and pathological pain.

Subnuclear organization of the rat habenular complexes

The habenular complexes represent phylogenetically constant structures in the diencephalon of all vertebrates. Available evidence suggests that this area is engaged in a variety of important biological functions, such as reproductive behaviors, central pain processing, nutrition, sleep-wake cycles, stress responses, and learning. Based on Nissl-stained sections, one medial nucleus and two lateral nuclei (divisions) have been widely accepted in the rat. Cytochemical, hodologic, and functional studies suggest a considerably more complex subnuclear structure. To improve our knowledge of the precise structural composition of the habenular complexes, we have systematically investigated their fine ultrastructure in the rat. Based on the detailed analysis of complete series of large, semithin sections supplemented with electron photomicrographs of selected fields, clear criteria for the delineation of five distinct subnuclei of the medial and ten subnuclei of the lateral habenular complexes were elaborated for the first time. All 15 subnuclei were reconstructed, and their dimensions were determined. A medial and lateral stria medullaris were described. Different roots of the fasciculus retroflexus were differentiated within the medial and lateral habenular complexes. The topographical relationships with respect to the adjacent habenular areas as well as to the neighboring thalamic nuclei were identified and demonstrated. The new understanding of the subnuclear organization of the habenular complexes certainly will facilitate further functional investigations. Whether the newly identified subnuclei finally will be recognized as functionally distinct awaits ongoing immunocytochemical, hodologic, and functional studies.

Regional changes in forebrain activation during the early and late phase of formalin nociception: analysis using cerebral blood flow in the rat

This is the first neural imaging study to use regional cerebral blood flow (rCBF) in an animal model to identify the patterns of forebrain nociceptive processing that occur during the early and late phase of the formalin test. We measured normalized rCBF increases by an autoradiographic method using the radiotracer [99mTc]exametazime. Noxious formalin consistently produced detectable, well-localized and typically bilateral increases in rCBF within multiple forebrain structures, as well as the interpeduncular nucleus (Activation Index, AI = 66) and the midbrain periaqueductal gray (AI = 20). Structures showing pain-induced changes in rCBF included several forebrain regions considered part of the limbic system. The hindlimb region of somatosensory cortex was significantly activated (AI = 31), and blood flow increases in VPL (AI = 8.7) and the medial thalamus (AI = 9.0) exhibited a tendency to be greater in the late phase as compared to the early phase of the formalin test. The spatial pattern and intensity of activation varied as a function of the time following the noxious formalin stimulus. The results highlight the important role of the limbic forebrain in the neural mechanisms of prolonged persistent pain and provide evidence for a forebrain network for pain.

Morphine analgesia in the formalin test: reversal by microinjection of quaternary naloxone into the posterior hypothalamic area or periaqueductal gray

Bilateral microinjection of 5 nmol morphine into the posterior hypothalamic area (PHA), periaqueductal gray matter (PAG) or ventral tegmental area (VTA) elicits powerful suppression of nociceptive behaviors in the formalin test, an animal model of injury produced pain. The object of the present study was to determine whether analgesia in the formalin test (50 microl 2.5% formalin injected s.c. in one hindpaw) induced by systemically administered morphine requires opioid action at these sites, or other putative sites of opioid action. Morphine sulphate (6 mg/kg s.c.) produced almost complete analgesia in the second phase of the formalin test (30-50 min after formalin). Bilateral microinjection of the quaternary opioid antagonist naloxone methobromide (NxBr, 28 ng in 0.5 microl, 22 min after morphine) into the PHA completely abolished morphine analgesia, while NxBr into PAG partially reversed analgesia. Microinjection of NxBr into the VTA, central nucleus of the amygdala, habenula, striatum, nucleus accumbens or hypothalamic sites outside the PHA did not antagonize morphine analgesia, although microinjections into some of these sites appeared to reduce the cataleptogenic effects of morphine. The data indicate that the PHA and PAG are probably the primary sites of action of morphine in the formalin test.

The effect of morphine on responses of mediodorsal thalamic nuclei and nucleus submedius neurons to colorectal distension in the rat

In halothane-anesthetized rats, we characterized the responses of single neurons in the nuclei of medial thalamus (MT), specifically the mediodorsal thalamic nucleus (MD) and the nucleus submedius (Sm), to a noxious visceral stimulus (colorectal balloon distension, CRD), and studied the effects of intravenous morphine (Mor) on these responses using standard extracellular microelectrode recording techniques. 62 MD and 46 Sm neurons were isolated on the basis of spontaneous activity. 47 of the MD neurons (76%) responded to CRD, of which 70% had excitatory and 30% had inhibitory responses. 38 of the Sm neurons (83%) responded to CRD, of which 89% had excitatory and 11% had inhibitory responses. Responses of MD and Sm neurons excited by CRD were related significantly to distension pressure (20-100 mmHg), with maximum excitation occurring at 60 and 100 mmHg, respectively. MD neurons inhibited by CRD also had graded responses to graded CRD, with maximum inhibition occurring at 80 mmHg. The responses to noxious (pinch, heat) and nonnoxious (tap, brush) cutaneous stimuli were studied in 59 of the MD and 44 of the Sm neurons isolated. 22 of the MD neurons (37%) studied had cutaneous receptive fields, of which 59% were large and bilateral, 41% were small and usually contralateral receptive fields. 55% of these neurons were nociceptive-specific, 45% responded to both noxious and nonnoxious cutaneous stimulation. 29 of the Sm neurons (66%) studied had cutaneous receptive fields, of which 72% were large and usually bilateral, 14% were small and bilateral, 14% were small and contralateral receptive fields. 90% of these neurons were nociceptive-specific, 10% responded to both noxious and nonnoxious stimulation. No MD or Sm neurons responded exclusively to nonnoxious cutaneous stimulation. Mor (0.125, 0.25, 0.5 and 1 mg/kg I.V.) attenuated MD and Sm neuronal excitatory responses to CRD in a dose-dependent fashion, abolishing evoked activity with a dose of 0.5 mg/kg (p < 0.05) and 1 mg/kg (p < 0.05), respectively. Naloxone (0.4 mg/kg I.V.) reversed the effects of Mor. Mor and naloxone had no effects on spontaneous activity. These data support the involvement of MD and Sm neurons in visceral nociception, and are consistent with a role of Sm in affective-motivational, and MD in both sensory-discriminative and affective-motivational aspects of nociception.

Picrotoxin produces a "central" pain-like syndrome when microinjected into the somato-motor cortex of the rat

In this study, we report the possibility of producing marked electrocorticographic changes and "pain-like" reactions, when the GABAA antagonist picrotoxin is microinjected unilateraly into the rat somato-motor Sml cortex in the region of the hind paw. After the microinjection, we observed continuous seizure isolated spikes, spikes-and-waves, bursts, and pain-like reactions, almost exclusively confined to the hind paw. These reactions considered of lifting off the floor, licking of the paw palm or digits, biting, paw tremors, and a peculiar paw position that we called "turn-in" paw. We also noted other behaviors, such as "limping," "neglected" paw, or rearing. The "pain-like" character of these manifestations was suggested by the fact that similar qualitative and quantitative data occurred consequent to the administration of 2.5% diluted formalin into the palm of the hind paw in different rats. Bringing together the electrocorticographic events and the behavioral reactions produced by Sml picrotoxin indicated that there was no obvious correlation between the phenomena, except that the tremor was always associated with the bursts. Sensory denervation of the hind paw, produced by sciatic and saphenous nerve transections, did not significantly modify either the ictal activity or the behavior. Finally, microinjection of naloxone prior to picrotoxin did not change the cortical events, but greatly diminished the "pain-like" reactions. All these results favor the cortical microinjection of a GABAA receptor antagonist as a good rat model for studying pain of "central" origin. They emphasize the possible role of the Sml cortex in such a phenomenon, and the deficit of cortical GABAergic processing, which can include an opioid link.

Diabetes alters mu and kappa opioid binding in rat brain regions: comparison with effects of food restriction

Diabetic rats display changes in opioid pharmacology and brain regional levels of opioid peptides and prodynorphin mRNA. Previous investigations of opioid receptor binding, carried out in whole-brain homogenates, have, however, failed to detect changes. In the present study, quantitative autoradiography was used to measure mu and kappa opioid receptor binding in discrete brain regions of streptozotocin-treated diabetic rats. Measurement was limited to regions that previously displayed opioid binding changes in chronically food-restricted rats, since our primary aim is to identify brain mechanisms that mediate adaptive responses to persistent metabolic need and adipose depletion. Diabetics displayed strong trends or statistically significant changes which matched seven of the thirteen binding changes observed in food-restricted rats. In no case did diabetics display changes in the opposite direction. The two statistically significant changes common to food-restricted and diabetic rats are increased kappa binding in the medial preoptic area and decreased mu binding in the lateral habenula. The possible functional significance of these changes is discussed.

Simultaneous recording of spontaneous activities and nociceptive responses from neurons in the pars compacta of substantia nigra and in the lateral habenula

Using simultaneous extracellular single-unit recording in the pars compacta of the substantia nigra and in the lateral habenula of rats, 45 pairs of neurons responding to peripheral nociceptive stimulation were recorded. In 41 of these pairs, nigral dopaminergic neurons were inhibited by peripheral nociceptive stimulation, while lateral habenula neurons were excited. Moreover, in 14 pairs, when sweeps were triggered randomly by spontaneous spikes from lateral habenula neurons the spontaneous firing rate of the dopaminergic neurons during the first 250 ms after the sweep was much lower than rates after this time period. In this case, the sweep was often triggered by burst-firing of lateral habenula neurons. Our results indicate a cross-correlation between the spontaneous activities of these two nuclei, suggesting that the excitation of lateral habenula neurons induced by peripheral nociceptive stimulation might be directly responsible for inhibition of nigral dopaminergic neurons.

Lateral hypothalamus: site involved in pain modulation

The present study is an attempt to examine the neuronal circuitry of a supraspinal site engaged in pain modulation. Five physiological measures were postulated as the criteria for defining a central nervous system site engaged in the circuitry of pain modulation. The lateral hypothalamus met these five measures: (i) 81% of the lateral hypothalamus neurons (247/304) responded to noxious stimuli using a single cell recording procedure; (ii) stimulation of the periaqueductal gray-dorsal raphe area or the habenula modulated 98% and 87% of the lateral hypothalamus noxious-evoked activity; (iii) microiontophoretically applied morphine modulated 77% of the lateral hypothalamus noxious evoked activity; (iv) electrical stimulation of the lateral hypothalamus produced behavioral analgesia proportional to the stimulus intensity as assessed by the tail flick assay; and (v) morphine application into the lateral hypothalamus produced behavioral analgesia in a dose-response manner using the tail flick assay. In conclusion, the lateral hypothalamus can be considered one of the pain modulation sites.

The central nucleus of the amygdala contributes to the production of morphine antinociception in the formalin test

The rat paw formalin test is a model of prolonged pain due to mild tissue injury. There is some evidence suggesting that morphine does not produce antinociception in the formalin test via the brain-stem and spinal cord circuitry normally associated with antinociception. Furthermore, morphine appears to require an intact forebrain in order to function as an analgesic for formalin pain. In the 2 experiments reported here, we investigated the possibility that the central nucleus of the amygdala (Ce) contributes to the production of morphine antinociception (MA) in the formalin test. Nociception in this test occurs in 2 phases, with the 1st phase occurring 0-5 min after formalin injection and the 2nd phase beginning 10-15 min after injection and continuing for approximately 1 h. In Exp. 1, bilateral neurotoxic lesions of the Ce, but not lesions of the adjacent basolateral nucleus (BL), reliably attenuated MA (7 mg/kg morphine sulfate) during the 2nd phase of the formalin test without affecting baseline nociception. These results were obtained regardless of whether the rating scale method or flinch-frequency method of nociceptive scoring was used. During the 1st phase, Ce lesions reliably attenuated MA as measured by the flinch-frequency method, but not as measured by the rating scale method. In Exp. 2, Ce lesions also reliably attenuated the antinociception produced by 12 mg/kg morphine sulfate during the 2nd phase of the formalin test. Antinociception appeared to be almost completely re-instated, however, if the dose of morphine was raised to 20 mg/kg. The results indicate that neurons originating from the Ce contribute to the production of MA during the 2nd phase, and possibly the 1st phase, of the formalin test, especially at relatively lower doses of morphine. This suggests that in addition to coordinating conditioned antinociceptive responses, the amygdala may be a component of endogenous antinociceptive circuitry. These and other issues are discussed with reference to the spino-ponto-amygdaloid nociceptive pathway, and the proposed role of the amygdala in the mediation of defense reactions.

A comparison of the antinociceptive effects of opioid agonists in neonatal and adult rats in phasic and tonic nociceptive tests

Changes in the attitudes about neonatal pain and pain management have recently resulted in increases in the administration of opioids to neonates. Little is known, however, about the relative potencies of the various opioid agonists employed, especially in comparison to adult responses. The first objective in the present study was to compare the antinociceptive potency of four clinically relevant opioids in neonatal and adult rats. The second objective was to compare and contrast these agents in two different types of nociceptive tests: tonic (formalin-induced inflammation) and phasic (tail flick and hot plate). Our results indicate that the opioid agonists morphine, meperidine, and fentanyl, and the mixed agonist buprenorphine were all effective antinociceptive agents in both neonates and adults in each of the three tests employed, and that the relative potencies of these agents appeared to be similar in neonates and adults. In general, the pups were more sensitive to the antinociceptive agents when tested in the phasic nociceptive tests, and the drugs were more potent in the tonic test than either of the phasic tests.

The local cerebral metabolic effects of morphine in rats exposed to escapable footshock

The 2-deoxy-D-[1-14C]glucose (2-DG) method was used to examine the effects of morphine sulfate (MS) on local cerebral metabolic rates for glucose (LCMRglu) in male F-344 rats required to turn a wheel manipulandum in order to escape from nociceptive footshock. Four groups of rats were studied: control-saline, control-MS, footshock-saline and footshock-MS. All animals were administered MS (4 mg/kg, s.c.) or saline 7 days, 3 days and 10 min prior to the start of the 2-DG experiment. In agreement with its well-known effect on the emotional component of pain, MS administered to rats exposed to footshock caused a significant decrease in LCMRglu compared to footshock-saline rats in limbic structures such as the diagonal band of Broca, lateral septum, bed nucleus of the stria terminalis, horizontal limb of the diagonal band, habenular complex and medial amygdala. Additionally, two components of the midline thalamus with extensive connections with the limbic system, the paraventricular and paratenial thalamic nuclei, were similarly affected by morphine. Footshock caused an overall increase in cerebral metabolism as 52 of 73 measured structures demonstrated increases in activity compared to saline control; however, statistically significant effects in specific structures were limited. These results identify limbic and midline thalamic structures important in morphine-induced analgesia and indicate that footshock tends to have a generalized stimulatory effect on LCMRglu.

Morphine analgesia in the formalin test: evidence for forebrain and midbrain sites of action

A mapping study was performed to determine where in the rat brain morphine acts to produce analgesia in the formalin test, which is an animal model of prolonged pain associated with tissue injury. A single dose (5 nmol) of morphine was bilaterally microinjected into a wide range of brain areas throughout the midbrain and forebrain. Strong analgesia was elicited from the posterior hypothalamic area, the periaqueductal gray and ventral tegmental area. Other sites from which analgesia was elicited were the nucleus accumbens and a few sites in the retrorubral field and caudate-putamen. Analgesia from the periaqueductal gray or nucleus accumbens was accompanied by decreased locomotor activity and catalepsy, whereas analgesia from the posterior hypothalamic area or ventral tegmentum was accompanied by a noticeable increase in locomotor activity and rearing. Morphine into various thalamic nuclei had no effect. These results indicate that the primary sites of action of morphine in the formalin test are probably the posterior hypothalamic area and periaqueductal gray, with an additional contribution from regions innervated by tegmental dopamine cells.

The GABAA receptor antagonist picrotoxin induces a 'pain-like' behavior when administered into the thalamic reticular nucleus of the behaving rat: a possible model for 'central' pain?

In this study, we reported that the microinjection of the GABAA antagonist picrotoxin into the rat thalamic reticular nucleus produced a 'pain-like' behavior. This behavior was primarily characterized by repetitive lifting off the hindpaw from the floor contralateral to the injection site, sometimes accompanied by extension of the leg and maximal fingers separation. Surprisingly, these manifestations were not occurring when picrotoxin was applied to the ventrobasal complex itself, alternatively producing 'wet-dog' shakes. These data show that the local administration of picrotoxin is a relevant approach for studying pain of 'central' origin and complex GABAergic modulatory mechanisms within the thalamic sensory complex.

The habenula and pain: repeated electrical stimulation produces prolonged analgesia but lesions have no effect on formalin pain or morphine analgesia

Recent studies have found that electrical stimulation of the habenula or microinjection of morphine into it reduces pain in several pain tests. The present study explored additional properties of the habenula. Expt. 1 examined the influence of the duration of stimulation on the duration of poststimulation analgesia in the formalin test. Expt. 2 was carried out to determine whether destruction of the habenula would affect either baseline pain levels or analgesia produced by morphine administered systemically in the formalin test. The results showed that the duration of analgesia is related to the duration of electrical stimulation. However, habenular lesions did not affect baseline pain levels or morphine analgesia. These studies support earlier evidence that manipulation of the habenula can produce analgesia, but suggest that it is not tonically active in modulating pain or necessary for the analgesic effects of systemically administered morphine.

The formalin test: an evaluation of the method

The formalin test for nociception, which is predominantly used with rats and mice, involves moderate, continuous pain generated by injured tissue. In this way it differs from most traditional tests of nociception which rely upon brief stimuli of threshold intensity. In this article we describe the main features of the formalin test, including the characteristics of the stimulus and how changes in nociceptive behaviour may be measured and interpreted. The response to formalin shows an early and a late phase. The early phase seems to be caused predominantly by C-fibre activation due to the peripheral stimulus, while the late phase appears to be dependent on the combination of an inflammatory reaction in the peripheral tissue and functional changes in the dorsal horn of the spinal cord. These functional changes seem to be initiated by the C-fibre barrage during the early phase. In mice, the behavioural response in the late phase depends on the ambient temperature. We argue that the peripheral tissue temperature as well as other factors influencing the peripheral inflammation may affect the response, possibly confounding the results obtained with the test. Furthermore, we discuss the methods of recording the response and the value of observing more than one aspect of behaviour. Scoring of several behavioural variables provides a means of assessing motor or sensorimotor function as possible causes for changes in behaviour. In conclusion, the formalin test is a valuable addition to the battery of methods available to study nociception.