Habenula as a relay in the descending pathway from nucleus accumbens to periaqueductal grey subserving antinociception

The lateral habenula nucleus regulates pruritic sensation and emotion

Itch is a complex aversive sensory and emotional experience. As a most upsetting symptom in many dermatological and systemic diseases, it lacks efficient treatments. The lateral habenula nucleus (LHb) encodes negative emotions in the epithalamus and has been implicated in pain and analgesia. Nevertheless, the role of the lateral habenula nucleus in the pruritic sensation and emotion remains elusive. Here we defined the crucial role of glutamatergic neurons within the lateral habenula nucleus (Glu^LHb) in itch modulation in mice. We established histamine-dependent and histamine-independent models of acute pruritus, as well as the acetone-ether-water (AEW) model of chronic pruritus. We first assessed the effects of pruritogen injection on neural activation in both medial and lateral divisions of LHb in vitro. We then demonstrated that the population activity of Glu^LHb neurons was increased during the acute itch and chronic itch-induced scratching behaviors in vivo. In addition, electrophysiological data showed that the excitability of Glu^LHb neurons was enhanced by chronic itch. Chemogenetic suppression of Glu^LHb neurons disrupted both acute and chronic itch-evoked scratching behaviors. Furthermore, itch-induced conditioned place aversion (CPA) was abolished by Glu^LHb neuronal inhibition. Finally, we dissected the LHb upstream brain regions. Together, these findings reveal the involvement of LHb in processing both the sensational and emotional components of pruritus and may shed new insights into itch therapy.

Pain-related cortico-limbic plasticity and opioid signaling

Neuroplasticity in cortico-limbic circuits has been implicated in pain persistence and pain modulation in clinical and preclinical studies. The amygdala has emerged as a key player in the emotional-affective dimension of pain and pain modulation. Reciprocal interactions with medial prefrontal cortical regions undergo changes in pain conditions. Other limbic and paralimbic regions have been implicated in pain modulation as well. The cortico-limbic system is rich in opioids and opioid receptors. Preclinical evidence for their pain modulatory effects in different regions of this highly interactive system, potentially opposing functions of different opioid receptors, and knowledge gaps will be described here. There is little information about cell type- and circuit-specific functions of opioid receptor subtypes related to pain processing and pain-related plasticity in the cortico-limbic system. The important role of anterior cingulate cortex (ACC) and amygdala in MOR-dependent analgesia is most well-established, and MOR actions in the mesolimbic system appear to be similar but remain to be determined in mPFC regions other than ACC. Evidence also suggests that KOR signaling generally serves opposing functions whereas DOR signaling in the ACC has similar, if not synergistic effects, to MOR. A unifying picture of pain-related neuronal mechanisms of opioid signaling in different elements of the cortico-limbic circuitry has yet to emerge. This article is part of the Special Issue on "Opioid-induced changes in addiction and pain circuits".

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.

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.

Evidence and explanation for the involvement of the nucleus accumbens in pain processing

The nucleus accumbens (NAc) is a subcortical brain structure known primarily for its roles in pleasure, reward, and addiction. Despite less focus on the NAc in pain research, it also plays a large role in the mediation of pain and is effective as a source of analgesia. Evidence for this involvement lies in the NAc’s cortical connections, functions, pharmacology, and therapeutic targeting. The NAc projects to and receives information from notable pain structures, such as the prefrontal cortex, anterior cingulate cortex, periaqueductal gray, habenula, thalamus, etc. Additionally, the NAc and other pain-modulating structures share functions involving opioid regulation and motivational and emotional processing, which each work beyond simply the rewarding experience of pain offset. Pharmacologically speaking, the NAc responds heavily to painful stimuli, due to its high density of μ opioid receptors and the activation of several different neurotransmitter systems in the NAc, such as opioids, dopamine, calcitonin gene-related peptide, γ-aminobutyric acid, glutamate, and substance P, each of which have been shown to elicit analgesic effects. In both preclinical and clinical models, deep brain stimulation of the NAc has elicited successful analgesia. The multi-functional NAc is important in motivational behavior, and the motivation for avoiding pain is just as important to survival as the motivation for seeking pleasure. It is possible, then, that the NAc must be involved in both pleasure and pain in order to help determine the motivational salience of positive and negative events.

Dopaminergic mechanisms in periaqueductal gray-mediated antinociception

As important as perceiving pain is the ability to modulate this perception in some contextual salient situations. The periaqueductal gray (PAG) is perhaps the most important site of endogenous pain modulation; however, little is known about dopaminergic mechanisms underlying PAG-mediated antinociception. In this study, we used a pharmacological approach to evaluate this subject. We found that µ-opioid receptor-induced antinociception (DAMGO, 0.3 μg) from PAG was blocked by the coadministration of either D1-like or D2-like dopaminergic antagonists (SCH23390, 2, 4, and 6 μg or raclopride, 2 and 4 μg, respectively) both in the tail-flick and in the mechanical paw-withdrawal test. A selective D2-like receptor agonist (piribedil, 6 and 12 μg into the PAG) induced antinociception in the mechanical paw-withdrawal test, but not in the tail-flick test. This effect was blocked by the coadministration of its selective antagonist (raclopride 4 μg), as well as by either a GABAA agonist (muscimol, 0.1 μg) or an opioid receptor antagonist (naloxone, 0.5 μg). A selective D1-like receptor agonist (SKF38393, 1, 5, and 10 μg into the PAG) induced a poor and transient antinociceptive effect, but when combined with piribedil, a potentiated antinociceptive effect emerged. None of these treatments affected locomotion in the open-field test. These findings suggest that µ-opioid antinociception from the PAG depends on dopamine acting on both D1-like and D2-like receptors. Selective activation of PAG D2-like receptors induces antinociception mediated by supraspinal mechanisms dependent on inhibition of GABAA and activation of opioid neurotransmission.

Inhibition of the Prefrontal Projection to the Nucleus Accumbens Enhances Pain Sensitivity and Affect

Cortical mechanisms that regulate acute or chronic pain remain poorly understood. The prefrontal cortex (PFC) exerts crucial control of sensory and affective behaviors. Recent studies show that activation of the projections from the PFC to the nucleus accumbens (NAc), an important pathway in the brain's reward circuitry, can produce inhibition of both sensory and affective components of pain. However, it is unclear whether this circuit is endogenously engaged in pain regulation. To answer this question, we disrupted this circuit using an optogenetic strategy. We expressed halorhodopsin in pyramidal neurons from the PFC, and then selectively inhibited the axonal projection from these neurons to neurons in the NAc core. Our results reveal that inhibition of the PFC or its projection to the NAc, heightens both sensory and affective symptoms of acute pain in naïve rats. Inhibition of this corticostriatal pathway also increased nociceptive sensitivity and the aversive response in a chronic neuropathic pain model. Finally, corticostriatal inhibition resulted in a similar aversive phenotype as chronic pain. These results strongly suggest that the projection from the PFC to the NAc plays an important role in endogenous pain regulation, and its impairment contributes to the pathology of chronic pain.

AMPAkines Target the Nucleus Accumbens to Relieve Postoperative Pain

BACKGROUND

AMPAkines augment the function of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors in the brain to increase excitatory outputs. These drugs are known to relieve persistent pain. However, their role in acute pain is unknown. Furthermore, a specific molecular and anatomic target for these novel analgesics remains elusive.

METHODS

The authors studied the analgesic role of an AMPAkine, CX546, in a rat paw incision (PI) model of acute postoperative pain. The authors measured the effect of AMPAkines on sensory and depressive symptoms of pain using mechanical hypersensitivity and forced swim tests. The authors asked whether AMPA receptors in the nucleus accumbens (NAc), a key node in the brain's reward and pain circuitry, can be a target for AMPAkine analgesia.

RESULTS

Systemic administration of CX546 (n = 13), compared with control (n = 13), reduced mechanical hypersensitivity (50% withdrawal threshold of 6.05 ± 1.30 g [mean ± SEM] vs. 0.62 ± 0.13 g), and it reduced depressive features of pain by decreasing immobility on the forced swim test in PI-treated rats (89.0 ± 15.5 vs. 156.7 ± 18.5 s). Meanwhile, CX546 delivered locally into the NAc provided pain-relieving effects in both PI (50% withdrawal threshold of 6.81 ± 1.91 vs. 0.50 ± 0.03 g; control, n = 6; CX546, n = 8) and persistent postoperative pain (spared nerve injury) models (50% withdrawal threshold of 3.85 ± 1.23 vs. 0.45 ± 0.00 g; control, n = 7; CX546, n = 11). Blocking AMPA receptors in the NAc with 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo[f]quinoxaline-2,3-dione inhibited these pain-relieving effects (50% withdrawal threshold of 7.18 ± 1.52 vs. 1.59 ± 0.66 g; n = 8 for PI groups; 10.70 ± 3.45 vs. 1.39 ± 0.88 g; n = 4 for spared nerve injury groups).

CONCLUSIONS

AMPAkines relieve postoperative pain by acting through AMPA receptors in the NAc.

Corticostriatal Regulation of Acute Pain

The mechanisms for acute pain regulation in the brain are not well understood. The prefrontal cortex (PFC) provides top-down control of emotional processes, and it projects to the nucleus accumbens (NAc). This corticostriatal projection forms an important regulatory pathway within the brain’s reward system. Recently, this projection has been suggested to control both sensory and affective phenotypes specifically associated with chronic pain. As this projection is also known to play a role in the transition from acute to chronic pain, we hypothesized that this corticostriatal circuit can also exert a modulatory function in the acute pain state. Here, we used optogenetics to specifically target the projection from the PFC to the NAc. We tested sensory pain behaviors with Hargreaves’ test and mechanical allodynia, and aversive pain behaviors with conditioned place preference (CPP) test. We found that the activation of this corticostriatal circuit gave rise to bilateral relief from peripheral nociceptive inputs. Activation of this circuit also provided important control for the aversive response to transient noxious stimulations. Hence, our results support a novel role for corticostriatal circuitry in acute pain regulation.

Targeted disruption of the orphan receptor Gpr151 does not alter pain-related behaviour despite a strong induction in dorsal root ganglion expression in a model of neuropathic pain

BACKGROUND

Gpr151 is an orphan GPCR whose function is unknown. The restricted pattern of neuronal expression in the habenula, dorsal horn of the spinal cord and dorsal root ganglion plus homology with the galanin family of receptors imply a role in nociception.

RESULTS

Real-time quantitative RT-PCR demonstrated a 49.9±2.9 fold highly significant (P<0.001) increase in Gpr151 mRNA expression in the dorsal root ganglion 7days after the spared nerve injury model of neuropathic pain. Measures of acute, inflammatory and neuropathic pain behaviours were not significantly different using separate groups of Gpr151 loss-of-function mutant mice and wild-type controls. Galanin at concentrations between 100nM and 10μM did not induce calcium signalling responses in ND7/23 cells transfected with Gpr151.

CONCLUSIONS

Our results indicate that despite the very large upregulation in the DRG after a nerve injury model of neuropathic pain, the Gpr151 orphan receptor does not appear to be involved in the modulation of pain-related behaviours. Further, galanin is unlikely to be an endogenous ligand for Gpr151.

Activation of corticostriatal circuitry relieves chronic neuropathic pain

Neural circuits that determine the perception and modulation of pain remain poorly understood. The prefrontal cortex (PFC) provides top-down control of sensory and affective processes. While animal and human imaging studies have shown that the PFC is involved in pain regulation, its exact role in pain states remains incompletely understood. A key output target for the PFC is the nucleus accumbens (NAc), an important component of the reward circuitry. Interestingly, recent human imaging studies suggest that the projection from the PFC to the NAc is altered in chronic pain. The function of this corticostriatal projection in pain states, however, is not known. Here we show that optogenetic activation of the PFC produces strong antinociceptive effects in a rat model (spared nerve injury model) of persistent neuropathic pain. PFC activation also reduces the affective symptoms of pain. Furthermore, we show that this pain-relieving function of the PFC is likely mediated by projections to the NAc. Thus, our results support a novel role for corticostriatal circuitry in pain regulation.

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.

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.

Evidence for the participation of cocaine- and amphetamine-regulated transcript peptide (CART) in the fluoxetine-induced anti-hyperalgesia in neuropathic rats

Cocaine- and amphetamine-regulated transcript peptide (CART) has a role in chronic pain, and also in the actions of selective serotonin reuptake inhibitors (SSRIs) employed in the treatment of neuropathic pain. Herein, we test the hypothesis that CART may mediate the anti-hyperalgesic effect of the SSRI, fluoxetine, in neuropathic rats. Sciatic nerve in the right hind paw of rat was ligated to induce neuropathic pain, and the paw withdrawal latency was evaluated using Hargreaves apparatus. Fluoxetine [5-25mg/kg, intraperitoneal (ip)] or CART (54-102) [0.1-1.5μg/rat, intracerebroventricular (icv)] dose-dependently attenuated the hyperalgesic response observed in neuropathic rats, indicating anti-nociceptive properties of each agent. The anti-hyperalgesic effect of fluoxetine was potentiated by the subeffective dose of CART, and attenuated by CART-antibody (1:500 dilution; 5μl/rat, icv); CART-antibody had no effect per se. Isobolographic analysis showed a significant synergism between fluoxetine and CART, and antagonism between fluoxetine and CART-antibody. Immunocytochemical labeling with monoclonal antibodies against CART showed drastic increase in CART-immunoreactive fibers in the ventrolateral periaqueductal gray (VLPAG; 116%), dorsal subdivision of dorsal raphe nucleus (DRD; 176%), and locus coeruleus (LC; 733%) of neuropathic animals. Fluoxetine treatment significantly reduced the immunoreactivity in these areas. However, CART-immunoreactive cells and fibers in the arcuate nucleus did not respond to neuropathy or fluoxetine treatments. We suggest that the CART innervation of DRD, LC and VLPAG may be involved in the (i) central processing of neuropathic pain and (ii) fluoxetine-induced anti-hyperalgesic effect in neuropathic pain.

Functional interaction between medial thalamus and rostral anterior cingulate cortex in the suppression of pain affect

The medial thalamic parafascicular nucleus (PF) and the rostral anterior cingulate cortex (rACC) are implicated in the processing and suppression of the affective dimension of pain. The present study evaluated the functional interaction between PF and rACC in mediating the suppression of pain affect in rats following administration of morphine or carbachol (acetylcholine agonist) into PF. Vocalizations that occur following a brief noxious tailshock (vocalization afterdischarges) are a validated rodent model of pain affect, and were preferentially suppressed by injection of morphine or carbachol into PF. Vocalizations that occur during tailshock were suppressed to a lesser degree, whereas, spinal motor reflexes (tail flick and hindlimb movements) were only slightly suppressed by injection of carbachol into PF and unaffected by injection of morphine into PF. Blocking glutamate receptors in rACC (NMDA and non-NMDA) by injecting D-2-amino-5-phosphonovalerate (AP-5) or 6-cyano-7-nitroquinoxaline-2,3-dione disodium (CNQX) produced dose-dependent antagonism of morphine-induced increases in vocalization thresholds. Carbachol-induced increases in vocalization thresholds were not affected by injection of either glutamate receptor antagonist into rACC. The results demonstrate that glutamate receptors in the rACC contribute to the suppression of pain affect produced by injection of morphine into PF, but not to the suppression of pain affect generated by intra-PF injection of carbachol.

Contribution of the periaqueductal gray to the suppression of pain affect produced by administration of morphine into the intralaminar thalamus of rat

The parafascicular nucleus (nPf) of the intralaminar thalamus is implicated in the processing of pain affect in both animals and humans. Administration of morphine into nPf results in preferential suppression of the affective reaction to noxious tail shock in rats. The involvement of the ventrolateral periaqueductal gray in mediating the antinociceptive action of morphine injected into nPf was evaluated. Vocalizations that occur after tail shock offset (vocalization afterdischarges) are a validated rodent model of pain affect and were preferentially suppressed by injection of morphine into nPf. Vocalizations that occur during tail shock were suppressed to a lesser degree, whereas spinal motor reflexes (tail flick and hind limb movements) were unaffected by injection of morphine into nPf. Inactivation of the vPAG via the microinjection of muscimol (GABA(A) agonist) produced dose-dependent antagonism of morphine-induced increases in vocalization thresholds. The results demonstrate that a functional link between the nPf and vPAG in generating the antinociceptive action of morphine injected into nPf.

PERSPECTIVE

Microinjection of morphine into nucleus parafascicular preferentially suppressed rats' affective reaction to noxious stimulation. This affective analgesia was reversed by inactivation of the ventrolateral periaqueductal gray. Understanding the neurobiology underlying the suppression of pain affect will provide insights into new treatments for pain and its associated affective disorders.

Modulation of central nociceptive coding by acupoint stimulation

It is universally accepted that acupuncture or acupoint stimulation can produce analgesic effect on patients with painful disorders. The past decades has seen remarkable progress in exploring the central mechanisms of acupuncture-induced pain relief, including the neurotransmitter release and expression of particular receptors and genes in the spinal cord and the brain stem regions. Development of new techniques makes it possible to record and image the brain network patterns underlying pain perception and modulation, and to investigate the role of higher-level brain areas in mediating acupuncture analgesia. This review will present the current understanding of the neural network that is implicated in the modulation of pain by acupuncture.

mu/delta Cooperativity and opposing kappa-opioid effects in nucleus accumbens-mediated antinociception in the rat

We previously demonstrated that noxious peripheral stimulation (e.g. subdermal capsaicin injection in the hind paw) produces antinociception that is mediated by opioid receptors in nucleus accumbens. The current study used the trigeminal jaw-opening nociceptive reflex responses in the rat to assess the role of intra-accumbens mu-, delta- and kappa-opioid receptors in the antinociceptive effect of noxious stimulation and intra-accumbens opioid agonism. Whilst intra-accumbens injection of either the mu-receptor-selective antagonist Cys2,Tyr3,Orn5,Pen7amide (CTOP) or the delta-receptor-selective antagonist naltrindole blocked capsaicin-induced antinociception, neither the selective mu-agonist [D-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin (DAMGO; 150 or 300 ng) nor the selective delta-agonist D-Pen2,5-enkephalin (DPDPE; 150 or 300 ng) alone induced antinociception. Simultaneous injection of DAMGO and DPDPE (150 ng each), however, produced significant antinociception. Capsaicin-induced antinociception was not blocked by the selective kappa-receptor antagonist nor-binaltorphimine, but was blocked by the kappa-agonist U69,593. U69,593 also antagonized the antinociceptive effect of the DAMGO/DPDPE combination. Thus, in nucleus accumbens, mu- and delta- but not kappa-opioid receptors contributed to capsaicin-induced antinociception; selective activation of individual receptor subtypes was insufficient, but coactivation of mu- and delta-opioid receptors induced antinociception, and kappa-receptors appeared to play an antianalgesic role in nucleus accumbens.

Involvement of CGRP and CGRP1 receptor in nociception in the nucleus accumbens of rats

The present study was performed to investigate the role of calcitonin gene-related peptide (CGRP) and its antagonist CGRP8-37 on nociception in the nucleus accumbens of rats. Hindpaw withdrawal latencies (HWLs) to noxious stimulation induced by hot plate and Randall Selitto tests were measured. The HWL to both thermal and mechanical stimulation increased significantly after intra-nucleus accumbens administration of 0.5 or 1 nmol of CGRP, but not 0.1 nmol, indicating that CGRP plays an anti-nociceptive effect in the nucleus accumbens of rats. The anti-nociceptive effect induced by intra-nucleus accumbens administration of 1 nmol of CGRP was blocked significantly by following intra-nucleus accumbens administration of 1 nmol of CGRP8-37, a selective antagonist of CGRP1 receptor. Furthermore, the HWLs to both thermal and mechanical stimulation decreased significantly after intra-nucleus accumbens administration of 0.02, 0.1 and 0.5 nmol of CGRP8-37 alone. The hyperalgesic effect of intra-nucleus accumbens administration of CGRP8-37 lasted for more than 60 min after the injection, suggesting that CGRP1 receptor is involved in anti-nociception in the nucleus accumbens of rats. The results indicate that CGRP and CGRP1 receptor have important roles in nociceptive modulation in the nucleus accumbens of rats.

Involvement of neuropeptide Y and Y1 receptor in antinociception in nucleus raphe magnus of rats

The nociceptive response latencies increased significantly after intra-nucleus raphe magnus administration of 0.1 or 0.4 nmol of neuropeptide Y, but not 0.04 nmol, in rats. The neuropeptide Y-induced increases in hindpaw withdrawal latency were reversed by following injection of 0.42 nmol of the Y1 antagonist, NPY(28-36). The results indicate that NPY plays an antinociceptive role in nucleus raphe magnus in rats, which is mediated by the Y1 receptor. Furthermore, the neuropeptide Y-induced increases in hindpaw withdrawal latency were attenuated by following intra-nucleus raphe magnus injection of 6 nmol of the opioid antagonist naloxone, indicating that there is an interaction between NPY and opioids in nucleus raphe magnus.

Dopamine reuptake inhibition in the rostral agranular insular cortex produces antinociception

We provide evidence for an antinociceptive effect of dopamine in the rat cerebral cortex that is mediated through descending nociceptive inhibition of spinal neurons. Injection of the dopamine reuptake inhibitor GBR-12935 in the rostral agranular insular cortex (RAIC), a cortical area that receives a dense dopaminergic projection and is involved in descending antinociception (Burkey et al.,1996), resulted in dose-dependent inhibition of formalin-induced nociceptive behavior, without any alteration of motor function. Injection of the dopamine reuptake inhibitor in the surrounding cortical areas had no effect on nociceptive behaviors. GBR-12935 also produced a reduction in noxious stimulus-induced c-fos expression in nociceptive areas of the spinal dorsal horn, suggesting that dopamine in the RAIC acts in part through descending antinociception. Electrophysiological recording from single wide dynamic range-type spinal dorsal horn neurons confirmed the descending nociceptive inhibitory effect. GBR-12935 in the RAIC significantly reduced neuronal responses evoked by noxious thermal stimulation of the skin, an effect that was reversed by local administration of the selective D1 receptor antagonist SCH-23390. Finally, administration of SCH-23390 alone in the RAIC decreased paw withdrawal latencies from noxious heat, suggesting that dopamine acts tonically in the cortex to inhibit nociception.

Stimulant-induced psychosis, the dopamine theory of schizophrenia, and the habenula

While one of the original underpinnings of the dopamine theory of schizophrenia was the paranoid psychosis which often develops during the binges or speed runs of chronic amphetamine addicts (and, more recently, in cocaine addicts), neurochemical studies of such drug abusers or from animals given continuous stimulants in an effort to model stimulant psychoses have not played a major role in the further evolution of this theory. One clear persisting alteration produced by continuous amphetamine is a neurotoxicity to dopaminergic innervations in caudate. Yet continuous cocaine administration apparently does not induce a similar neurotoxicity and this makes this effect a poor candidate for an underpinning of stimulant psychoses. However, it has recently been found that both continuous amphetamine and cocaine induce a strong pattern of degeneration which is highly confined to the lateral habenula and its principal output pathway, fasciculus retroflexus. This finding has led to a reconsideration of the role of these structures in psychoses. The habenula, as the chief relay nucleus of the descending dorsal diencephalic system (consisting of stria medullaris, habenula and fasciculus retroflexus), is an important link between limbic and striatal forebrain and lower diencephalic and mesencephalic centers. Studies of glucose utilization have consistently shown the habenula to be highly sensitive to dopamine agonists and antagonists. Lesions of habenula produce a wide variety of behavioral alterations. The dorsal diencephalic system has major and predominantly inhibitory connections onto dopamine-containing cells and it mediates part of the negative feedback from dopamine receptors onto dopamine cell bodies. It represents one of the major inputs in brain to the raphe nuclei and has anatomical and functional connections to modulate important functions such as sensory gating through thalamus, pain gating through central gray and raphe and motor stereotypies and reward mechanisms through substantia nigra and the ventral tegmental area. It is argued that alterations in these pathways are ideal candidates for producing the behaviors which occur during psychosis and that future considerations of the circuitry underlying psychoses need to include this highly important but relatively neglected system.

Arousal following intra-preoptic area administration of naltrexone, ICI 174864 or nor-BNI in hibernating ground squirrels

Experiments were performed from January 1 to 30 January, 1989. Drugs were administered to the preoptic area of hibernating ground squirrels through a chronically implanted cannula on the 2nd day of a torpor bout. When naltrexone (an antagonist of opioid receptor) was injected, part (naltrexone, 0.5 or 1 microgram/microliter/32 min) or all (naltrexone, 2 micrograms/microliters/32 min) of the animals' body temperature was increased and they aroused from hibernation within 20 h after the injection. Further experiments showed that intra-preoptic area perfusion of 1 nmol of ICI 174864 (an antagonist of delta receptor) or nor-BNI (an antagonist of kappa receptor), but not beta-FNA (an antagonist of mu receptor), were able to increase the body temperature of hibernating ground squirrels and arouse them from hibernation in 20 h after the injection. These results indicate that (1) opioid peptides in the preoptic area may be involved in the mechanisms of hibernation; (2) activation of delta and kappa receptors in the preoptic area is indispensable for the maintenance of hibernation in ground squirrels.

Demonstration of habenular neurons which receive afferent fibers from the nucleus accumbens and send their axons to the midbrain periaqueductal gray

After injecting Phaseolus vulgaris leucoagglutinin (PHA-L) and wheat germ agglutinin conjugated with horseradish peroxidase (WGA-HRP), respectively, into the medial part of the nucleus accumbens (NA) and the midbrain periaqueductal gray including the dorsal raphe nucleus (PAG/DR) on one side of single rat brains, we observed that axon terminals labeled anterogradely with PHA-L made possible synaptic contact with neurons in the habenula (Hb) which were retrogradely labeled with WGA-HRP. These Hb neurons were distributed in the medial part of the lateral Hb. The results indicate that the medial part of the lateral Hb relays neuronal information from the medial part of the NA to the PAG/DR.

Further studies on interactions between periaqueductal gray, nucleus accumbens and habenula in antinociception

Previous findings from this laboratory with the intracerebral microinjection technique suggested that the periaqueductal gray (PAG), nucleus accumbens, and habenula might constitute a unidirectional loop to play their roles in pain modulation. In the present study we demonstrate that intra-habenular injection of naloxone antagonizes the analgesia elicited by morphine injected into the periaqueductal gray (PAG) and that intra-accumbens injection of naloxone is capable of attenuating the analgesic effects of morphine injected into the habenula. These results indicate that the relationships between these nuclei may be more complex than the putative unidirectional loop.

Relevance of the habenular complex to neuropsychiatry: a review and hypothesis

Since the initial observation by Brown (1914) that electrical stimulation applied to the habenular efferent bundle in the chimpanzee evoked a pattern of respiration which closely resembled the act of laughter, the habenular complex has remained a mysterious structure. The anatomy of the habenular complex is well delineated (Jones, 1985) forming a major component of the dorsal diencephalic conduction system. Data derived mainly from animal experimentation over the past decade point to the fact that the habenular complex functions as an important link between the limbic forebrain and the midbrain-extrapyramidal motor system. The elucidation of the functions of the habenular complex may thus significantly increase the current insight into the understanding of the interaction between behavioral and motor functions. Clearly, such information would be of great relevance for further understanding of neuropsychiatric disorders such as schizophrenia, Parkinson's disease, Tardive dyskinesia, and Tourette's syndrome in which behavioral and motor impairments are interfaced. This review summarizes anatomical, functional, and pharmacological aspects of the habenular complex and discusses its potential contribution to the pathophysiology of selected neuropsychiatric and movement disorders.