Naloxone-reversible analgesia induced by electrical stimulation of the habenula in the rat

Dynamics of Lateral Habenula-Ventral Tegmental Area Microcircuit on Pain-Related Cognitive Dysfunctions

Chronic pain is a health problem that affects the ability to work and perform other activities, and it generally worsens over time. Understanding the complex pain interaction with brain circuits could help predict which patients are at risk of developing central dysfunctions. Increasing evidence from preclinical and clinical studies suggests that aberrant activity of the lateral habenula (LHb) is associated with depressive symptoms characterized by excessive negative focus, leading to high-level cognitive dysfunctions. The primary output region of the LHb is the ventral tegmental area (VTA), through a bidirectional connection. Recently, there has been growing interest in the complex interactions between the LHb and VTA, particularly regarding their crucial roles in behavior regulation and their potential involvement in the pathological impact of chronic pain on cognitive functions. In this review, we briefly discuss the structural and functional roles of the LHb-VTA microcircuit and their impact on cognition and mood disorders in order to support future studies addressing brain plasticity during chronic pain conditions.

Thalamus: The 'promoter' of endogenous modulation of pain and potential therapeutic target in pathological pain

More recently, the thalamic mediodorsal (MD) and ventromedial (VM) nuclei have been revealed to be functioned as 'nociceptive discriminator' in discriminating noxious and innocuous peripheral afferents, and exhibits distinct different descending controls of nociception. Of particularly importance, the function of thalamic nuclei in engaging descending modulation of nociception is 'silent' or inactive during the physiological state as well as in condition exposed to insufficient noxious stimulation. Once initiation by sufficient noxious or innocuous C-afferents associated with temporal and spatial summation, the thalamic MD and VM nuclei exhibit salient, different effects: facilitation and inhibition, on noxious mechanically and heat evoked nociception, respectively. Based on series of experimental evidence, we here summarize a novel hypothesis involving thalamic MD and VM nuclei functioned as 'promoter' in initiating descending facilitation and inhibition of pain with specific spatiotemporal characteristics. We further hypothesize that clinical remedy in targeting thalamic VM nucleus by enhancing its activities in recruiting inhibition alone or decreasing thalamic MD nucleus induced facilitation may provide promising way in effectively control of pathological pain.

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.

Inactivation of endopeduncular nucleus impaired fear conditioning and hippocampal synaptic plasticity in rats

The internal globus pallidus (GPi) is one part of basal ganglion nucleuses which play fundamental role in motor function. Recent studies indicated that GPi could modulate emotional processing and learning, but the possible mechanism remains still unknown. In this study, the effects of endopeduncular nucleus (EP, a rodent homolog of GPi) on fear conditioning were tested in rats. GABA_A receptor agonist muscimol was bilaterally delivered into the EP 15 min before or immediately after fear conditioning in rats. We found that EP inactivation impaired the acquisition but not consolidation of fear memory in rats. Furthermore, the long-term potentiation (LTP) in hippocampal CA1 area was impaired, and the learning related phosphorylation of α-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid receptor (AMPAR) subunit 1 (GluA1) at the Ser845 site in hippocampus was decreased in muscimol treated group. These results demonstrated that dysfunction of EP impaired hippocampal dependent learning and memory in rats.

[The roles of habenula and related neural circuits in neuropsychiatric diseases]

The habenula is a small and bilateral nucleus above dorsal thalamus, which contains several different types of neurons. The habenula has extensive connections with the forebrain, septum and monoaminergic nuclei in the midbrain and brainstem. Habenula is known as an 'anti-reward' nucleus, which can be activated by aversive stimulus and negative reward prediction errors. Accumulating researchs have implicated that the habenula is involved in several behaviors crucial to survival. Meanwhile, the roles of the habenula in neuropsychiatric diseases have received increasing attention. This review summaries the studies regarding the roles of habenula and the related circuits in neuropathic pain, depression, drug addiction and schizophrenia, and discusses the possibility to use the habenula as a treatment target.

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.

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.

The role of lateral habenula-dorsal raphe nucleus circuits in higher brain functions and psychiatric illness

Serotonergic neurons in the dorsal raphe nucleus (DRN) play an important role in regulation of many physiological functions. The lateral nucleus of the habenular complex (LHb) is closely connected to the DRN both morphologically and functionally. The LHb is a key regulator of the activity of DRN serotonergic neurons, and it also receives reciprocal input from the DRN. The LHb is also a major way-station that receives limbic system input via the stria medullaris and provides output to the DRN and thereby indirectly connects a number of other brain regions to the DRN. The complex interactions of the LHb and DRN contribute to the regulation of numerous important behavioral and physiological mechanisms, including those regulating cognition, reward, pain sensitivity and patterns of sleep and waking. Disruption of these functions is characteristic of major psychiatric illnesses, so there has been a great deal of interest in how disturbed LHb-DRN interactions may contribute to the symptoms of these illnesses. This review summarizes recent research related to the roles of the LHb-DRN system in regulation of higher brain functions and the possible role of disturbed LHb-DRN function in the pathogenesis of psychiatric disorders, especially depression.

Reward and adversity processing circuits, their competition and interactions with dopamine and serotonin signaling

We propose that dorsal anterior cingulate cortex (dACC), anterior insula (AI) and adjacent caudolateral orbitofrontal cortex (clOFC), project to lateral habenula (LHb) and D2 loop of ventral striatum (VS), forming a functional adversity processing circuit, directed towards inhibitory avoidance and self-control. This circuit learns what is bad or harmful to us, evaluates and predicts risks - to stop us from selecting and going/moving for the bad or suboptimal choices that decrease our well-being and survival chances. Proposed role of dACC is to generate a WARNING signal when things are going (or might end) bad or wrong to prevent negative consequences: pain, harm, loss or failure. The AI signals about bad, low, noxious and aversive qualities, which might make us sick or cause discomfort. These cortical adversity processing regions activate directly and indirectly (via D2 loop of VS) the LHb, which then inhibits dopamine and serotonin release (and is reciprocally inhibited by VTA/SNc, DRN) to avoid choosing and doing things leading to harm or loss, but also to make us feel worse, even down when overstimulated. We propose that dopamine attenuates output of the adversity processing circuit, thus decreasing inhibitory avoidance and self-control, while serotonin attenuates dACC, AI, clOFC, D1 loop of VS, LHb, amygdala and pain pathway. Thus, by reciprocal inhibition, by causing dopamine and serotonin suppression - and by being suppressed by them, the adversity processing circuit competes with reward processing circuit for control of choice behaviour and affective states. We propose stimulating effect of dopamine and calming inhibitory effect of serotonin on the active avoidance circuit involving amygdala, linked to threat processing, anger, fear, self-defense and violence. We describe causes and roles of dopamine and serotonin signaling in health and in mental dysfunctions. We add new idea on ventral ACC role in signaling that we are doing well and inducing serotonin, when we gain/reach safety, comfort, valuable resources (social or biological rewards), affection and achieve goals.

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.

Lesions of the fasciculus retroflexus alter footshock-induced cFos expression in the mesopontine rostromedial tegmental area of rats

Midbrain dopamine neurons are an essential part of the circuitry underlying motivation and reinforcement. They are activated by rewards or reward-predicting cues and inhibited by reward omission. The lateral habenula (lHb), an epithalamic structure that forms reciprocal connections with midbrain dopamine neurons, shows the opposite response being activated by reward omission or aversive stimuli and inhibited by reward-predicting cues. It has been hypothesized that habenular input to midbrain dopamine neurons is conveyed via a feedforward inhibitory pathway involving the GABAergic mesopontine rostromedial tegmental area. Here, we show that exposing rats to low-intensity footshock (four, 0.5 mA shocks over 20 min) induces cFos expression in the rostromedial tegmental area and that this effect is prevented by lesions of the fasciculus retroflexus, the principal output pathway of the habenula. cFos expression is also observed in the medial portion of the lateral habenula, an area that receives dense DA innervation via the fr and the paraventricular nucleus of the thalamus, a stress sensitive area that also receives dopaminergic input. High-intensity footshock (120, 0.8 mA shocks over 40 min) also elevates cFos expression in the rostromedial tegmental area, medial and lateral aspects of the lateral habenula and the paraventricular thalamus. In contrast to low-intensity footshock, increases in cFos expression within the rostromedial tegmental area are not altered by fr lesions suggesting a role for non-habenular inputs during exposure to highly aversive stimuli. These data confirm the involvement of the lateral habenula in modulating the activity of rostromedial tegmental area neurons in response to mild aversive stimuli and suggest that dopamine input may contribute to footshock- induced activation of cFos expression in the lateral habenula.

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.

The habenula: from stress evasion to value-based decision-making

Surviving in a world with hidden rewards and dangers requires choosing the appropriate behaviours. Recent discoveries indicate that the habenula plays a prominent part in such behavioural choice through its effects on neuromodulator systems, in particular the dopamine and serotonin systems. By inhibiting dopamine-releasing neurons, habenula activation leads to the suppression of motor behaviour when an animal fails to obtain a reward or anticipates an aversive outcome. Moreover, the habenula is involved in behavioural responses to pain, stress, anxiety, sleep and reward, and its dysfunction is associated with depression, schizophrenia and drug-induced psychosis. As a highly conserved structure in the brain, the habenula provides a fundamental mechanism for both survival and decision-making.

Differential neuromodulation of acquisition and retrieval of avoidance learning by the lateral habenula and ventral tegmental area

Several studies suggest an opponent functional relationship between the lateral habenula (LHb) and the ventral tegmental area (VTA). Previous work has linked LHb activation to the inhibition of dopaminergic neurons during loss of reward, as well as to deficits in escape and avoidance learning. We hypothesized that a dopamine signal might underlie the negative reinforcement of avoidance responses and that LHb activation could block this signal and thereby cause avoidance deficits. To test this idea, we implanted stimulating electrodes in either the VTA or LHb of gerbils engaged in two-way active avoidance learning, a task that shows learning-associated dopamine changes and that is acquired faster following LHb lesions. We delivered brief electrical brain stimulation whenever the animal performed a correct response, i.e., when the successful avoidance of foot shock was hypothesized to trigger an intrinsic reward signal. During the acquisition phase, VTA stimulation improved avoidance performance, while LHb stimulation impaired it. VTA stimulation appeared to improve both acquisition and asymptotic performance of the avoidance response, as VTA-stimulated animals reached above-normal performance but reverted to normal responding when stimulation was discontinued. The effects of LHb stimulation during avoidance acquisition were long lasting and persisted even after stimulation was discontinued. However, when given after successful acquisition of avoidance behavior, LHb stimulation had no effect, indicating that LHb stimulation specifically impaired avoidance acquisition without affecting memory retrieval or motivation or ability to perform the avoidance response. These results demonstrate opponent roles of LHb and VTA during acquisition but not during retrieval of avoidance learning.

Evidence for structural abnormalities of the human habenular complex in affective disorders but not in schizophrenia

BACKGROUND

The habenular complex is composed of important relay nuclei linking the limbic forebrain to the midbrain and brain stem nuclei. Based on clinical observations, experiments with animals and theoretical considerations, it has been speculated that this brain area might be involved in psychiatric diseases (i.e. schizophrenia and depression). However, evidence in favour of this hypothesis is still lacking because the human habenular complex has rarely been studied with regard to mental illness.

METHOD

We examined habenular volumes in post-mortem brains of 17 schizophrenia patients, 14 patients with depression (six patients with major depression and eight patients with bipolar depression) and 13 matched controls. We further determined the neuronal density, cell number and cell area of the medial habenular nuclei of the same cohorts using a counting box and a computer-assisted instrument.

RESULTS

Significantly reduced habenular volumes of the medial and lateral habenula were estimated in depressive patients in comparison to normal controls and schizophrenia patients. We also found a reduction in neuronal cell number and cell area in depressive patients for the right side compared to controls and schizophrenia patients. No such changes were seen in schizophrenia.

CONCLUSIONS

Our anatomical data argue against prominent structural alterations of the habenular nuclei in schizophrenia but demonstrate robust alterations in depressive patients. We are currently applying immunohistochemical markers to better characterize neuronal subpopulations of this brain region in schizophrenia and depression.

Cell-dependent physiological synaptic action of morphine in the rat habenular nucleus: morphine both inhibits and facilitates excitatory synaptic transmission

Although several lines of evidence have suggested that the activity of thalamic neurons is modulated by opioids, the mechanism by which morphine in the thalamus regulates the release of excitatory neurotransmitters remains unclear. In the present study, we investigated the synaptic modulation of morphine to regulate excitatory synaptic transmission, probably glutamatergic transmission, in habenular nucleus (Hb) and centrolateral nucleus (CL) neurons in the rat thalamus. Using the whole-cell patch-clamp technique, we found dual modulation by morphine in Hb neurons: morphine caused either inhibition or facilitation of the miniature excitatory postsynaptic current (mEPSC) frequency in the Hb. In Hb neurons that showed a morphine-induced decrease in the mEPSC frequency, the mEPSC amplitude was also decreased in the presence of morphine. In contrast, the mEPSC amplitude was markedly increased in Hb neurons that showed a morphine-induced increase in the mEPSC frequency. We also observed a significant decrease in the mEPSC frequency with morphine in CL neurons without any change in the mEPSC amplitude, whereas morphine did not facilitate the mEPSC frequency in CL neurons. These results suggest that morphine may induce cell-dependent dual modulation of glutamatergic synaptic transmission in the Hb.

Post-synaptic action of morphine on glutamatergic neuronal transmission related to the descending antinociceptive pathway in the rat thalamus

Morphine is a prototypical mu-opioid receptor (MOR) agonist, and can directly inhibit pain transmission at both spinal and supraspinal levels. In the present study, we investigated the properties of thalamic neurons in an opioid-sensitive pain-modulating circuit. Application of morphine to cultured thalamic neurons evoked a potentiation of glutamate-induced peak currents, which was blocked by the MOR antagonist. Application of the protein kinase C inhibitor chelerythrine significantly inhibited the morphine-evoked enhancement of glutamate-induced currents. Immunoreactivity for MOR was observed with high density in the habenular nucleus (Hb) of the thalamus in rats, which was clearly co-localized with NMDA receptor subunit 1 (NRI). In this study, we show that microinjection of morphine into the Hb produced a dose-dependent increase in the tail-flick latency and enhanced the antinociceptive effect induced by the intra-Hb injection of glutamate. When fluoro-gold (FG) was used as a retrograde tracer, we found that FG-labeled neurons in the Hb after the microinjection of FG into the periaqueductal gray expressed both MOR and NR1. The present data suggest that the stimulation of MOR in the Hb may be involved in activation of the descending antinociceptive pathway through glutamatergic neurotransmission via the NMDA receptor.

Deep brain stimulation for the treatment of chronic, intractable pain

Deep brain stimulation (DBS) was first used for the treatment of pain in 1954. Since that time, remarkable advances have been made in the field of DBS, largely because of the resurgence of DBS for the treatment of movement disorders. Although DBS for pain has largely been supplanted by motor cortex and spinal cord stimulation during the last decade, no solid evidence exists that these alternative modalities truly offer improved outcomes. Furthermore, nuclei not yet fully explored are known to play a role in the transmission and modulation of pain. This article outlines the history of DBS for pain, pain classification, patient selection criteria, DBS target selection, surgical techniques, indications for DBS (versus ablative techniques), putative new DBS targets, complications, and the outcomes associated with DBS for pain.

Characterization of epibatidine binding to medial habenula: potential role in analgesia

The objective of the present study was to characterize a recently described binding site in the habenula, which has high affinity for [(3)H]epibatidine and low affinity for nicotine and acetylcholine. We report that the extension of this binding area in coronal and horizontal sections corresponds to the anatomical extension of the medial habenula. The affinity (K(D)) of the medial habenula receptors for [(3)H]epibatidine was estimated to be 0.5 nM using an autoradiographic saturation assay, whereas the affinity of the binding site for nicotine and acetylcholine was estimated to be 5 and 8 microM, respectively. The receptor density (B(max)) in the medial habenula was estimated to be about 1100 fmol/mg wet weight using [(3)H]epibatidine. The subunit composition of the "epibatidine receptor" was investigated by the ability of different compounds with affinity to various subtypes of nicotinic receptors to displace [(3)H]epibatidine bound to the receptor. The results suggest that the receptor contains alpha3 subunits but that it is unlikely to be an alpha3beta4 nicotinic receptor. Systemic administration of epibatidine has analgesic effects in rats. Here we report that 2 x 1 microl of 10 nM epibatidine, resulting in a 2 x 10-fmol dose, administered directly to the medial habenula by bilateral stereotactic injection had an analgesic effect measured in the hot-plate test. This dose of epibatidine increased hot-plate latency significantly, whereas 2 x 2 fmol of epibatidine or 2 x 10 fmol of nicotine were without effect. This leads us to suggest that the medial habenular epibatidine binding site might be a valuable target for the development of non-opiate analgesics.

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.

Covariation of activity in habenula and dorsal raphé nuclei following tryptophan depletion

Abnormal serotonergic function is implicated in the pathogenesis of affective disorders. We induced transient depressive relapses in volunteer patients by rapidly depleting plasma tryptophan, the precursor of serotonin (5-HT), and measured neural activity during different cognitive tasks using positron emission tomography (PET). Neural activity in several 5-HT-related brain areas, e.g., dorsal raphé, habenula, septal region, amygdala, and orbitofrontal cortex, covaried significantly with plasma levels of tryptophan and ratings of depressed mood. Task-specific responses in left amygdala and left anterior cingulate were attenuated by tryptophan depletion. We used these PET data to test the hypothesis that projections from the habenula modulate dorsal raphé activity and that this modulation is enhanced in patients experiencing a profound mood change following serotonergic challenge. A strong linear correlation (r(2) > 0.5) between habenula and raphé activity was observed in subjects with postdepletion ratings >/=10 on a modified Hamilton depression scale, whereas subjects experiencing milder changes in mood had weaker habenula-raphé coupling (r(2) < 0.5). These data support a model of the serotonergic system in which the habenula projection to the raphé represents a convergent feedback pathway that controls the release of 5-HT throughout the brain. In our experiment we were able to engage this system in patients who were sensitive to tryptophan depletion.

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.

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.

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.

Distribution of effects of the kappa-opioid agonist CI-977 on cerebral glucose utilization in rat brain

The effects of the kappa-opioid agonist CI-977 upon local cerebral glucose utilization have been examined in conscious, lightly restrained rats to gain insight into the potential adverse effects of this neuroprotective agent. Cerebral glucose utilization was assessed quantitatively in 45 anatomically discrete brain regions by means of [14C]2-deoxyglucose autoradiography. The i.v. administration of CI-977 (0.03-3 mg/kg) induced relatively homogeneous patterns of altered cerebral glucose utilization with moderate statistically significant reductions (approximately 25%) being observed in 29 brain regions, and a statistically significant increase (approximately 40%) in one brain region, the lateral habenular nucleus. Glucose use throughout the entire neocortex and inferior colliculus was particularly sensitive to reduction (approximately 35%) following CI-977 administration, although there was only a limited dose dependency to the response. Minimal alterations in glucose use were observed in 15 of the 45 brain regions, particularly in the lower brain stem (e.g. superior olives, cochlear nucleus and median raphe) and forebrain limbic regions (e.g. septal nucleus, nucleus accumbens and mediodorsal thalamus). These data demonstrate that CI-977 produces widespread, anatomically organized alterations in function-related glucose use which contrast those seen previously with the NMDA receptor antagonists, thereby suggesting that CI-977 may be intrinsically safer as an in vivo neuroprotective agent.

Transcranial electrostimulation effects on rat opioid and neurotransmitter levels

A specific form of Transcranial Electrostimulation Treatment (TCET) has been shown to induce analgesia, alleviate symptoms of opiate withdrawal and alter nociceptive responses in neurons in the midbrain and hypothalamus of rats. TCET consists of a 10Hz, charge balanced, 10 mu A current passed for 30 minutes between electrodes placed in the ears. Both serotonin (5HT) and endogenous opioids have been strongly implicated in TCET responses. This study directly measured brain levels of several neurotransmitters and their metabolites in anesthetized rats stimulated with either 10 mu A TCET or 0 mu A (Sham). Neurotransmitters measured in selected homogenized brain areas by high performance liquid chromatography were 5HT and its metabolite, 5-hydroxyindolacetic acid (5HIAA); norepinephrine (NE) and its metabolite, 3-methoxy-4-hydroxyphenethyleneglycol (MHPG); and dopamine (DA). Levels of NE and DA were significantly higher in the hypothalamic region of TCET rats than of control rats. The midbrains of TCET rats contained significantly elevated levels of DA, MHPG, 5HT and 5HIAA. In the hindbrain no significant differences were observed. Thus, TCET appears to cause an increase in the synthesis or release of 5HT, DA and NE in the midbrain and DA and 5HT in the hypothalamus. In a separate experiment, beta-endorphin-like immunoreactivity was measured in blood plasma taken from rats at intervals before, during and after a 30 minute TCET treatment, but no demonstrable TCET effect was observed. The lack of change in serum endorphin levels suggests that TCET-induced opioid activity may be confined to the central nervous system, a reasonable theory because the current passes only through the head.

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.

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.

Hypothalamic, dorsal raphe and external electrical stimulation modulate noxious evoked responses of habenula neurons

Extracellular recording techniques were used to investigate the effects of focal brain stimulation and external electrical stimulation on spontaneous activity and on noxious evoked responses in the habenular nucleus of anesthetized Sprague-Dawley rats. Two hundred and forty-one habenular neurons were tested to noxious and non-noxious stimuli. The habenular neurons exhibited three cell types according to their patterns of response to the noxious stimulus: 123 neurons (51%) responded to noxious stimulus by excitation and were classified as "nociceptive-on" cells; 56 neurons (23%) responded to the same noxious stimulus by decreasing their firing rate and were classified as "nociceptive-off" cells; and 62 neurons (26%) failed to respond to noxious stimulation and were classified as "non-nociceptive" cells. None of these 241 cells responded to non-noxious stimulus. One hundred and fifty-five, 160, 142 and 241 habenular neurons were tested following focal lateral hypothalamus stimulation, dorsal raphe stimulation, cerebellar stimulation and transcranial electrical stimulation alone and concomitant with noxious stimulation, respectively. The observations demonstrate that focal lateral hypothalamic, dorsal raphe and external (transcranial) electrical stimulation suppresses habenular noxious evoked responses while cerebellar electrical stimulation elicits no effect on the nociceptive-off cells and augmenting effects on the nociceptive-on cells. In addition, it was observed that low current (below threshold) external transcranial electrical stimulation was as effective in suppression of habenular noxious evoked responses as was focal brain electrical stimulation in the lateral hypothalamus and dorsal raphe.

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.

Antinociception and behavioral changes induced by carbachol microinjected into identified sites of the rat brain

The sites of the rat brain in which intracerebral administration of carbachol (0.4 microgram/0.5 microliter) elevates the nociceptive threshold to thermic (tail-flick test) and mechanical (calibrated-pinch test) noxious stimuli were examined. An extensive mapping (510 sites) ranging from AP + 10.5 to AP-0.1 mm revealed that antinociception was obtained from 119 sites (23%) widely scattered in the brain, and reached structures distant from, or within the immediate vicinity of the ventricular system. The effects from most placement were demonstrated using the tail-flick test, whereas a smaller proportion (approximately 13%) of sites was effective in reducing the response to mechanical stimuli only. Structures containing sensitive sites include the dorsal raphe nucleus, lateral border of the superior cerebellar peduncle, caudal portion of the superior colliculus, medial geniculate body, habenular complex, amygdala, temporal pole of the ventral hippocampus, rostral aspect of the dorsal hippocampus, lateral septal area, and triangular nucleus of the septum. Analysis of the distribution of responsive sites indicated that they are poorly superposed to the known distribution of opiate-sensitive areas. Most of the structures found to be responsive to carbachol are also known to possess cholinergic receptors and to evoke antinociception following focal electrical stimulation. In various placements, particularly in limbic structures, microinjection of carbachol evoked jumping to mechanical noxious stimulation, hyperexcitability to non-noxious stimuli, convulsive reactions, and other less frequent reactions. On few occasions, however, these changes were accompanied by antinociception.

Antinociception elicited by electrical or chemical stimulation of the rat habenular complex and its sensitivity to systemic antagonists

The effects of intraperitoneal administration of antagonists to morphine, norepinephrine, acetylcholine, dopamine and 5-hydroxytryptamine (5-HT) have been studied on the antinociceptive effect of electrical stimulation of the rat habenular complex (HbC). The antinociceptive effect of agonists microinjected into the HbC was also examined. A 15-s period of 53 microA rms sine-wave stimulation of the HbC significantly increased the latency of the tail-flick reflex to noxious heat for periods of up to 15 min. This effect was significantly attenuated by pretreating rats with naloxone (1 mg/kg) or phenoxybenzamine (5 mg/kg). Methysergide (5 mg/kg), haloperidol (5 mg/kg), atropine (1 mg/kg), and mecamylamine (1 mg/kg) had little effect on the antinociceptive effect of HbC stimulation. L-Glutamate (3.5 and 7.0 micrograms), morphine (1.0 and 5.0 micrograms), and carbachol (0.4 and 0.8 micrograms), but not 5-HT (5 micrograms), dopamine (5 micrograms) or norepinephrine (5 micrograms), induced a dose-dependent increase in the tail-flick latency when microinjected into the HbC. The effect of carbachol was significantly attenuated in rats previously treated with intraperitoneal administration of atropine or mecamylamine and fully depressed in rats previously treated with a combination of these two cholinergic antagonists. It is concluded that antagonists of opiate receptors and alpha-adrenoceptors, but not dopamine or cholinergic receptors, reduce the antinociceptive effects of HbC stimulation. These observations differ from the reported effects of these antagonists on the antinociception caused by stimulating the periaqueductal gray, but resemble the antinociception caused by stimulating the ventrolateral medulla and locus coeruleus.(ABSTRACT TRUNCATED AT 250 WORDS)

Brain stimulation-induced feeding alters regional opioid receptor binding in the rat: an in vivo autoradiographic study

Although opioid antagonists block feeding behavior in a variety of animal models, the number and identity of CNS regions in which the inferred endogenous opioid activity mediates feeding have yet to be established. Furthermore, it is not yet clear whether the opioid activity that sustains feeding is a concomitant of the appetitive motivational state or the consummatory response. In an effort to address these issues, an in vivo autoradiographic method was used to visualize CNS regional changes in opioid release during appetitively motivating electrical stimulation in the lateral hypothalamus (ESLH) and during consummatory behavior elicited by such stimulation. Regional decreases in [3H]diprenorphine [(3H]Dpr) binding, suggesting increased release of an endogenous opioid peptide, were observed in the medial prefrontal cortex, medial septum, gustatory cortex, zona incerta, mediodorsal thalamus, and hippocampus of rats receiving ESLH. Decreased binding in the latter 4 structures did not appear when animals were allowed to eat during ESLH, suggesting that the inferred opioid release is associated with appetitive behaviors elicited by ESLH which are suppressed when food is available and consummatory behavior predominates. When animals were allowed to eat during ESLH, [3H]Dpr binding in anterior cingulate cortex decreased substantially, suggesting that feeding behavior specifically triggers opioid release in this region. ESLH and feeding were found to increase [3H]Dpr binding in a number of CNS regions. Alternative explanations for increased binding, including inhibition of tonic opioid release, changes in cerebral blood flow, and opioid receptor up-regulation are discussed.

Habenulopetal catecholaminergic projections in the rat brain: a combined Fluoro-Gold/catecholamine fluorescence study

Habenulopetal catecholamine (CA)-containing neurons were determined using Fluoro-Gold retrograde tracing combined with a CA-fluorescence technique. A robust number of habenulopetal CA neurons were found mainly in the ipsilateral locus coeruleus (LC), while a considerable number of habenulopetal non-CA neurons were found in the LC, subcoeruleus, A5, A4 and A7 areas.

Intensity-dependent nociceptive responses from presumed dopaminergic neurons of the substantia nigra, pars compacta in the rat and their modification by lateral habenula inputs

The characteristics of nociceptive responses from presumed dopaminergic (DAergic) neurons in the SN were investigated in the anesthetized rat with extracellular recordings. 194 presumed DAergic neurons were recorded. A majority of these neurons (78%) were inhibited by intensive electrical stimulation performed at the tail (PNS) and 15% were excited. Both inhibitory and excitatory responses were intensity-dependent. Single shock stimulation of the lateral habenula (LHb) inhibited 89% of the tested DAergic neurons, most of which (83.8%) were also inhibited by PNS. LHb stimulation increased PNS-induced inhibition of DAergic neurons and electrical destruction of ipsilateral LHb depressed their nociceptive responses. Our results strongly suggest that DAergic neurons encode the nociceptive stimulation intensity and that the LHb shares a step in nociceptive projection to the SN.

Antinociception induced by stimulation of the habenular complex of the rat

The changes in the tail-flick latency to noxious heat stimulation and in the threshold for defensive/affective reactions to noxious pressure of the skin were studied following electrical stimulation of the habenular complex (HbC) and adjacent brain structures in the male rat. Single brief (15 s), low intensity (53 microA r.m.s.) stimulation of the HbC caused no significant increase in the locomotor activity or motor deficit but induced a potent and short-lasting antinociception as revealed by both algesimetric tests. Animals stimulated in the HbC also displayed poor avoidance learning in a conditioned place preference paradigm, thus suggesting that aversion is unlikely to determine antinociception. Rats daily stimulated in the HbC became tolerant to the antinociception induced by HbC stimulation or to a high systemic dose of morphine (6 mg/kg i.p.). These results indicate that stimulation of the HbC may increase the thresholds of spinally and supraspinally integrated reflexes, thus supporting the hypothesis that this nucleus may play a role in pain control, probably involving an opioid-dependent mechanism.

Microiontophoresis of cocaine, desipramine, sulpiride, methysergide, and naloxone in habenula and parafasciculus

The effects of microiontophoretically applied cocaine, desipramine (DES), sulpiride (SUL), methysergide (METH), and naloxone (NAL) on the responses of physiologically identified single neurons in the habenula (Hab) and parafasciculus thalami nucleus (PF) were examined in rats. Three cell types were identified in both nuclei on the basis of the responses obtained following noxious stimulation that were classified as "nociceptive-on," "nociceptive-off," and "nonnociceptive" cells. Administration of cocaine generally resulted in a decrease in the firing rate of nociceptive-on and nonnociceptive neurons in both Hab and PF. In contrast, cocaine generally induced an excitation in the baseline firing of the nociceptive-off cells. Cocaine application concomitant with noxious stimulation prevented the evoked responses of the nociceptive-on and the nociceptive-off cells. DES, when applied alone, was found to induce excitation in neuronal discharge of all three cell types in both sites. Combined application of cocaine with DES resulted in no observable change in discharge frequency for the nociceptive-on and nonnociceptive cells, while inducing an additive excitatory effect on the nociceptive-off cells. SUL, in contrast, induced no observable effect on baseline firing when given alone, yet consistently antagonized cocaine-induced effects on all three cell types. Finally, METH and NAL induced no effects on baseline firing or cocaine-induced modifications in neuronal discharge frequency.

Regulation of striatal serotonin release by the lateral habenula-dorsal raphe pathway in the rat as demonstrated by in vivo microdialysis: role of excitatory amino acids and GABA

Striatal extracellular levels of serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) were monitored with the microdialysis technique during electrical stimulation of the lateral habenula-dorsal raphe (LHb-NRD) pathway in halothane anaesthetized rats. A new double-loop probe, with an improved recovery factor, was implanted into the head of the caudate-putamen and perfused with Ringer solution containing 1 microM of the 5-HT uptake blocker indalpine. Samples were collected every 15 min and analyzed with HPLC coupled to fluorimetric detection. Low frequency stimulation of the LHb (1.5 and 3 Hz, 0.5 mA) produced no detectable changes in striatal indole levels, whereas 15 Hz stimulation induced a 70% increase in 5-HT release. This effect was most likely mediated by a direct LHb-NRD link, since it persisted after ibotenic acid lesions of the interpeduncular nucleus (which is the major projection area for the medial habenular nucleus), but was completely abolished after transection of the fasciculus retroflexus, which carries the axons of the LHb-NRD pathway. The possible identity of the transmitter operating in the LHb-NRD pathway was investigated by NRD injections of kynurenic acid, a potent blocker of excitatory amino acid transmission, and by NRD injections of the GABA antagonist bicuculline. Kynurenic acid (300 nl, 50 mM) did not by itself induce any detectable changes in spontaneous indole output, but completely blocked the effect of LHb stimulation. Injection of bicuculline (300 nl, 2 mM) increased the striatal 5-HT output by about 70%, and potentiated the effect of LHb stimulation by a further 50%. In none of the experiments performed in this study were there any significant changes in striatal 5-HIAA output. These data are compatible with the idea that excitatory amino acids in the LHb-NRD pathway are involved in the regulation of striatal 5-HT release, and that this influence is modulated by GABAergic synaptic activity at the level of the NRD.

Cells of the rat lateral habenula respond to high-threshold somatosensory inputs

Extracellular electrical recordings of single units in the anaesthetized rat demonstrate that about two thirds of the neurons in the lateral habenula respond to peripheral noxious stimuli in a way similar to that of the cells in the centrum-medianum complex which can be activated (with several peaks of various latencies) or inhibited. The firing pattern of these lateral habenula cells is either excitatory (75%) or inhibitory (24%) and is related to the intensity of the stimulus; their receptive field is large and bilateral. Most of these cells also respond to other kinds of noxious but not to non-noxious stimuli. The present study strongly suggests that the lateral habenula is a central target at the upper brainstem level for nociceptive inputs. This original finding is consistent with our previous observation of naloxone reversible analgesia induced by stimulation of the habenula and suggests an involvement of this structure in the processing of noxious inputs.

Neosurugatoxin: CNS acetylcholine receptors and luteinizing hormone secretion in ovariectomized rats

Neosurugatoxin, a neurotoxin isolated from the Japanese ivory shell, inhibits ganglionic nicotinic acetylcholine receptors but not skeletal muscle nicotinic acetylcholine receptors. It has also been reported to inhibit (3H) L-nicotine binding to high-affinity agonist acetylcholine receptors in rat brain membrane preparations. In the present study, 10(-5) M neosurugatoxin inhibited the in vitro binding of (3H) L-nicotine to the medial habenular nucleus of frozen, coronal sections of rat brain as did 10(-5) M cytisine or nicotine and 10(-4) M dihydro-beta-erythroidine. Neosurugatoxin did not inhibit (125I) alpha-bungarotoxin binding to hypothalamic synaptosomal preparations or to frozen, coronal sections of rat brain. Injection of neosurugatoxin into the third ventricles of ovariectomized rats resulted in a significant decrease in the frequency of pulses of luteinizing hormone (LH) secretion but had no effect on the amplitude of pulses. A low dose (1 microgram/injection) of the nicotinic acetylcholine agent cytisine injected into the third ventricle had no significant effect on pulsatile LH secretion. Coadministration of cytisine could block the inhibitory effect of neosurugatoxin on LH secretion. It is suggested that neosurugatoxin is a useful antagonist to study the biological roles of a specific subclass of nicotinic acetylcholine receptors in mammalian brain and reproductive neuroendocrine functions.