Selective neurotoxic effects of nicotine on axons in fasciculus retroflexus further support evidence that this a weak link in brain across multiple drugs of abuse

Functional Adaptation in the Brain Habenulo-Mesencephalic Pathway During Cannabinoid Withdrawal

The mesolimbic reward system originating from dopamine neurons in the ventral tegmental area (VTA) of the midbrain shows a profound reduction in function during cannabinoid withdrawal. This condition may underlie aversive states that lead to compulsive drug seeking and relapse. The lateral habenula (LHb) exerts negative control over the VTA via the GABA rostromedial tegmental nucleus (RMTg), representing a potential convergence point for drug-induced opponent processes. We hypothesized that the LHb-RMTg pathway might be causally involved in the hypodopaminergic state during cannabinoid withdrawal. To induce Δ9-tetrahydrocannabinol (THC) dependence, adult male Sprague-Dawley rats were treated with THC (15 mg/kg, i.p.) twice daily for 6.5-7 days. Administration of the cannabinoid antagonist rimonabant (5 mg/kg, i.p.) precipitated a robust behavioral withdrawal syndrome, while abrupt THC suspension caused milder signs of abstinence. Extracellular single unit recordings confirmed a marked decrease in the discharge frequency and burst firing of VTA dopamine neurons during THC withdrawal. The duration of RMTg-evoked inhibition was longer in THC withdrawn rats. Additionally, the spontaneous activity of RMTg neurons and of LHb neurons was strongly depressed during cannabinoid withdrawal. These findings support the hypothesis that functional changes in the habenulo-mesencephalic circuit are implicated in the mechanisms underlying substance use disorders.

Hedgehog-interacting protein acts in the habenula to regulate nicotine intake

Hedgehog-interacting protein (HHIP) sequesters Hedgehog ligands to repress Smoothened (SMO)-mediated recruitment of the GLI family of transcription factors. Allelic variation in HHIP confers risk of chronic obstructive pulmonary disease and other smoking-related lung diseases, but underlying mechanisms are unclear. Using single-cell and cell-type-specific translational profiling, we show that HHIP expression is highly enriched in medial habenula (MHb) neurons, particularly MHb cholinergic neurons that regulate aversive behavioral responses to nicotine. HHIP deficiency dysregulated the expression of genes involved in cholinergic signaling in the MHb and disrupted the function of nicotinic acetylcholine receptors (nAChRs) through a PTCH-1/cholesterol-dependent mechanism. Further, CRISPR/Cas9-mediated genomic cleavage of the Hhip gene in MHb neurons enhanced the motivational properties of nicotine in mice. These findings suggest that HHIP influences vulnerability to smoking-related lung diseases in part by regulating the actions of nicotine on habenular aversion circuits.

Chronic infusions of mecamylamine into the medial habenula: Effects on nicotine self-administration in rats

The habenula is an epithalamic structure through which descending connections go from the telencephalon to the brainstem, putting it in a key location to provide feedback control over the ascending projections from the brainstem to the telencephalon. The medial habenula has a high concentration of nicotinic receptors. We assessed the role of medial habenular nicotinic receptors for nicotine self-administration (SA) in female young adult Sprague-Dawley rats. The rats had bilateral chronic infusion cannulae placed into the medial habenula nucleus. Each cannula was connected to a slow delivery osmotic minipump to chronically infuse mecamylamine (100 µg/side/day) or vehicle for four consecutive weeks. The rats were tested for nicotine SA for the first two weeks of mecamylamine infusion. Then, they had one week of enforced abstinence, during which they had no access to the nicotine SA. Finally, they had one week of resumed nicotine SA access. There was a significantly differential mecamylamine effects in animals with lower and higher pretreatment baseline nicotine SA. Rats with lower baseline nicotine SA levels showed a nearly significant mecamylamine-induced reduction in SA while those with higher baseline levels of SA showed a significant mecamylamine-induced increase in nicotine SA. This study determined that medial habenular nicotinic receptors are important for nicotine reinforcement. Baseline level of performance makes a crucial difference for the involvement of habenular mechanisms in nicotine reinforcement with nicotinic activation being important for maintaining nicotine self-administration for those with lower levels of baseline self-administration and the opposite effect with subjects with higher levels of baseline self-administration.

Addiction-related neuroadaptations following chronic nicotine exposure

The addiction-relevant molecular, cellular, and behavioral actions of nicotine are derived from its stimulatory effects on neuronal nicotinic acetylcholine receptors (nAChRs) in the central nervous system. nAChRs expressed by dopamine-containing neurons in the ventral midbrain, most notably in the ventral tegmental area (VTA), contribute to the reward-enhancing properties of nicotine that motivate the use of tobacco products. nAChRs are also expressed by neurons in brain circuits that regulate aversion. In particular, nAChRs expressed by neurons in the medial habenula (mHb) and the interpeduncular nucleus (IPn) to which the mHb almost exclusively projects regulate the "set-point" for nicotine aversion and control nicotine intake. Different nAChR subtypes are expressed in brain reward and aversion circuits and nicotine intake is titrated to maximally engage reward-enhancing nAChRs while minimizing the recruitment of aversion-promoting nAChRs. With repeated exposure to nicotine, reward- and aversion-related nAChRs and the brain circuits in which they are expressed undergo adaptations that influence whether tobacco use will transition from occasional to habitual. Genetic variation that influences the sensitivity of addiction-relevant brain circuits to the actions of nicotine also influence the propensity to develop habitual tobacco use. Here, we review some of the key advances in our understanding of the mechanisms by which nicotine acts on brain reward and aversion circuits and the adaptations that occur in these circuits that may drive addiction to nicotine-containing tobacco products.

Neuronal adaptations in the lateral habenula during drug withdrawal: Preclinical evidence for addiction therapy

The epithalamic lateral habenula (LHb) regulates monoaminergic systems and contributes to the expression of both appetitive and aversive behaviours. Over the past years, the LHb has emerged as a vulnerable brain structure in mental illnesses including addiction. Behavioural and functional evidence in humans and rodents provide substantial support for a role of LHb in the negative affective symptoms emerging during withdrawal from addictive substances. Multiple forms of cellular and synaptic adaptations that take hold during drug withdrawal within the LHb are causally linked with the emergence of negative affective symptoms. These results indicate that targeting drug withdrawal-driven adaptations in the LHb may represent a potential strategy to normalize drug-related behavioural adaptations. In the current review we describe the mechanisms leading to functional alterations in the LHb, as well as the existing interventions used to counteract addictive behaviours. Finally, closing this loop we discuss and propose new avenues to potentially target the LHb in humans in light of the mechanistic understanding stemming from pre-clinical studies. Altogether, we provide an overview on how to leverage cellular-level understanding to envision clinically-relevant approaches for the treatment of specific aspects in drug addiction.

Untangling the dorsal diencephalic conduction system: a review of structure and function of the stria medullaris, habenula and fasciculus retroflexus

The often-overlooked dorsal diencephalic conduction system (DDCS) is a highly conserved pathway linking the basal forebrain and the monoaminergic brainstem. It consists of three key structures; the stria medullaris, the habenula and the fasciculus retroflexus. The first component of the DDCS, the stria medullaris, is a discrete bilateral tract composed of fibers from the basal forebrain that terminate in the triangular eminence of the stalk of the pineal gland, known as the habenula. The habenula acts as a relay hub where incoming signals from the stria medullaris are processed and subsequently relayed to the midbrain and hindbrain monoaminergic nuclei through the fasciculus retroflexus. As a result of its wide-ranging connections, the DDCS has recently been implicated in a wide range of behaviors related to reward processing, aversion and motivation. As such, an understanding of the structure and connections of the DDCS may help illuminate the pathophysiology of neuropsychiatric disorders such as depression, addiction and pain. This is the first review of all three components of the DDCS, the stria medullaris, the habenula and the fasciculus retroflexus, with particular focus on their anatomy, function and development.

A little rein on addiction

Rewarding and aversive experiences influence emotions, motivate specific behaviors, and modify future action in animals. Multiple conserved vertebrate neural circuits have been discovered that act in a species-specific manner to reinforce behaviors that are rewarding, while attenuating those with an adverse outcome. A growing body of research now suggests that malfunction of the same circuits is an underlying cause for many human disorders and mental ailments. The habenula (Latin for "little rein") complex, an epithalamic structure that regulates midbrain monoaminergic activity has emerged in recent years as one such region in the vertebrate brain that modulates behavior. Its dysfunction, on the other hand, is implicated in a spectrum of psychiatric disorders in humans such as schizophrenia, depression and addiction. Here, I review the progress in identification of potential mechanisms involving the habenula in addiction.

The molecular mechanisms controlling morphogenesis and wiring of the habenula

The habenula is an evolutionarily conserved brain region comprising bilaterally paired nuclei that plays a key role in processing reward information and mediating aversive responses to negative stimuli. An important aspect underlying habenula function is relaying information between forebrain and mid- and hindbrain areas. This is mediated by its complex organization into multiple subdomains and corresponding complexity in circuit organization. Additionally, in many species habenular nuclei display left-right differences at the anatomical and functional level. In order to ensure proper functional organization of habenular circuitry, sophisticated molecular programs control the morphogenesis and wiring of the habenula during development. Knowledge of how these mechanisms shape the habenula is crucial for obtaining a complete understanding of this brain region and can provide invaluable tools to study habenula evolution and function. In this review we will discuss how these molecular mechanisms pattern the early embryonic nervous system and control the formation of the habenula, how they shape its asymmetric organization, and how these mechanisms ensure proper wiring of the habenular circuit. Finally, we will address unexplored aspects of habenula development and how these may direct future research.

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

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

The habenula in psychiatric disorders: More than three decades of translational investigation

The habenula is an epithalamic structure located at the center of the dorsal diencephalic conduction system, a pathway involved in linking forebrain to midbrain regions. Composed of a medial and lateral subdivisions, the habenula receives inputs from the limbic system and basal ganglia mainly through the stria medullaris (SM), and projects to midbrain regions through the fasciculus retroflexus (FR). An increasing number of studies have implicated this structure in psychiatric disorders associated with dysregulated reward circuitry function, notably mood disorders, schizophrenia, and substance use disorder. However, despite significant progress in research, the mechanisms underlying the relationship between the habenula and the pathophysiology of psychiatric disorders are far from being fully understood, and still need further investigation. This review provides a closer look at key findings from animal and human studies illustrating the role of the habenula in mood disorders, schizophrenia, and substance use disorder, and discusses the clinical potential of using this structure as a therapeutic target.

Hippocampal and striatal histomorphology following chronic nicotine administration in female and male rats

Background

Nicotine is a subject of continuous research because of its likely ameliorative effects on neurologic and neurodegenerative disorders.

Purpose

This study examined the effects of its chronic subcutaneous administration on hippocampal and striatal microstructure in both female and male rats.

Methods

Forty adult female and male Wistar rats were divided into 4 groups. Three experimental groups were administered nicotine via subcutaneous injections at doses of 0.25, 2 and 4 mg/kg body weight for 28 days. Control groups received normal saline. Following administration, routine processing of brain tissues was carried out. Sections obtained were stained using routine H&E methods for general histological appearance, Cresyl violet methods for nissl substances, and Bielschwolsky silver impregnation method for neuritic plaques and neurofibrillary tangles.

Results

The study showed significant increase in percentage of neurons showing degenerating features in the hippocampus and striatum of both female and male rats following the higher doses of nicotine treatment. Only female rats showed positive agyrophilic (black-coloured) aggregations in the hippocampal and striatal regions after nicotine treatment.

Conclusion

The present study indicates that higher dose of chronic nicotine administration may induce hippocampal and striatal neurodegenerative changes. However, further studies using more specific method for studying neurodegeneration within brain regions is recommended.

Adolescent, but not adult, binge ethanol exposure leads to persistent global reductions of choline acetyltransferase expressing neurons in brain

During the adolescent transition from childhood to adulthood, notable maturational changes occur in brain neurotransmitter systems. The cholinergic system is composed of several distinct nuclei that exert neuromodulatory control over cognition, arousal, and reward. Binge drinking and alcohol abuse are common during this stage, which might alter the developmental trajectory of this system leading to long-term changes in adult neurobiology. In Experiment 1, adolescent intermittent ethanol (AIE; 5.0 g/kg, i.g., 2-day on/2-day off from postnatal day [P] 25 to P55) treatment led to persistent, global reductions of choline acetyltransferase (ChAT) expression. Administration of the Toll-like receptor 4 agonist lipopolysaccharide to young adult rats (P70) produced a reduction in ChAT+IR that mimicked AIE. To determine if the binge ethanol-induced ChAT decline was unique to the adolescent, Experiment 2 examined ChAT+IR in the basal forebrain following adolescent (P28-P48) and adult (P70-P90) binge ethanol exposure. Twenty-five days later, ChAT expression was reduced in adolescent, but not adult, binge ethanol-exposed animals. In Experiment 3, expression of ChAT and vesicular acetylcholine transporter expression was found to be significantly reduced in the alcoholic basal forebrain relative to moderate drinking controls. Together, these data suggest that adolescent binge ethanol decreases adult ChAT expression, possibly through neuroimmune mechanisms, which might impact adult cognition, arousal, or reward sensitivity.

Differential expression and function of nicotinic acetylcholine receptors in subdivisions of medial habenula

Neuronal nAChRs in the medial habenula (MHb) to the interpeduncular nucleus (IPN) pathway are key mediators of nicotine's aversive properties. In this paper, we report new details regarding nAChR anatomical localization and function in MHb and IPN. A new group of knock-in mice were created that each expresses a single nAChR subunit fused to GFP, allowing high-resolution mapping. We find that α3 and β4 nAChR subunit levels are strong throughout the ventral MHb (MHbV). In contrast, α6, β2, β3, and α4 subunits are selectively found in some, but not all, areas of MHbV. All subunits were found in both ChAT-positive and ChAT-negative cells in MHbV. Next, we examined functional properties of neurons in the lateral and central part of MHbV (MHbVL and MHbVC) using brain slice patch-clamp recordings. MHbVL neurons were more excitable than MHbVC neurons, and they also responded more strongly to puffs of nicotine. In addition, we studied firing responses of MHbVL and MHbVC neurons in response to bath-applied nicotine. Cells in MHbVL, but not those in MHbVC, increased their firing substantially in response to 1 μm nicotine. Additionally, MHbVL neurons from mice that underwent withdrawal from chronic nicotine were less responsive to nicotine application compared with mice withdrawn from chronic saline. Last, we characterized rostral and dorsomedial IPN neurons that receive input from MHbVL axons. Together, our data provide new details regarding neurophysiology and nAChR localization and function in cells within the MHbV.

Biodegradable biomatrices and bridging the injured spinal cord: the corticospinal tract as a proof of principle

Important advances in the development of smart biodegradable implants for axonal regeneration after spinal cord injury have recently been reported. These advances are evaluated in this review with special emphasis on the regeneration of the corticospinal tract. The corticospinal tract is often considered the ultimate challenge in demonstrating whether a repair strategy has been successful in the regeneration of the injured mammalian spinal cord. The extensive know-how of factors and cells involved in the development of the corticospinal tract, and the advances made in material science and tissue engineering technology, have provided the foundations for the optimization of the biomatrices needed for repair. Based on the findings summarized in this review, the future development of smart biodegradable bridges for CST regrowth and regeneration in the injured spinal cord is discussed.

A reassessment of whether cortical motor neurons die following spinal cord injury

Over the past century, the question of whether the cells of origin of the corticospinal tract (CST) die following spinal cord injury (SCI) has been debated. A recent study reported an approximately 20% loss of retrogradely labeled cortical motoneurons following damage to their axons resulting from SCI at T9 (Hains et al. [2003] J. Comp. Neurol. 462:328-341). In follow-up studies, however, we failed to find any evidence of loss of CST axons in the medullary pyramid, which must occur if CST neurons die. Here, we seek to resolve the discrepancy by re-evaluating possible loss of CST neurons using the same techniques as Hains et al. (quantitative analysis of retrograde labeling and staining for cell death markers including TUNEL and Hoechst labeling of the nuclei). Following either dorsal funiculus lesions at thoracic level 9 (T9) or lateral hemisection at cervical level 5 (C5), our results reveal no evidence for a loss of retrogradely labeled neurons and no evidence for TUNEL staining of axotomized cortical motoneurons. These results indicate that CST cell bodies do not undergo retrograde cell death following SCI, and therefore targeting such cell death is not a valid therapeutic target.

Histopathological evaluation of the nervous system in National Toxicology Program rodent studies: a modified approach

This article outlines the changes and underlying rationale for modifications to the histopathological evaluation of the nervous system during toxicology and carcinogenesis studies conducted by the National Toxicology Program (NTP). In the past, routine evaluation of the nervous system was mostly limited to three sections of brain, and occasionally the spinal cord and peripheral nerves. Factors such as the increasing occurrence of human neurological diseases and associated economical cost burden, the role of unidentified environmental stressors in neurodegenerative disorders, multiple therapeutic drug-induced neuropathies noted in human clinical trials, and the exponential use of environmental chemicals with unknown neurotoxic potential necessitate a more extensive evaluation of the nervous system. The NTP has modified its protocol to include examination of key anatomic subsites related to neurodegenerative diseases such as Parkinson's disease. Modifications include four additional sections of the brain. Increasing the number of brain sections permits examination of a greater number of specific anatomic subsites with unique vulnerability. In addition, the spinal cord, peripheral nerves, trigeminal ganglion, and intestinal autonomic ganglia will be evaluated as needed. It is expected that this modified approach will increase the sensitivity of detecting neurotoxicants and neurocarcinogens important in human neurologic and neurodegenerative disorders.

Recommended Neuroanatomical Sampling Practices for Comprehensive Brain Evaluation in Nonclinical Safety Studies

Adequate tissue sampling is known to reduce the likelihood that the toxicity of novel biomolecules, chemicals, and drugs might go undetected. Each organ, and often specific structurally and functionally distinct regions within it, must be assessed to detect potential site-specific toxicity. Adequate sampling of the brain requires particular consideration because of the many major substructures and more than 600 subpopulations of generally irreplaceable cells with unique functions and vulnerabilities. All known neurotoxicants affect specific subpopulations (usually neurons) rather than damaging a certain percentage of cells throughout the brain; thus, all populations should be independently assessed for lesions. Historically, the affected neural cell subpopulation has not been predictable, but it is now clear that sampling selected populations (e.g., cerebral cortex, hippocampus, cerebellar folia) cannot forecast the health of other populations. This article reviews the neuroanatomical domains affected by several model neurotoxicants to illustrate the need for more comprehensive neurohistological evaluation during nonclinical development of novel compounds. The article also describes an easily executed, cost-effective method that uses a set number of evenly spaced coronal (cross) sections to accomplish this comprehensive brain assessment during nonclinical safety studies performed in rodents, dogs, and nonhuman primates.

Unexpected survival of neurons of origin of the pyramidal tract after spinal cord injury

There is continuing controversy about whether the cells of origin of the corticospinal tract (CST) undergo retrograde cell death after spinal cord injury (SCI). All previous attempts to assess this have used imaging and/or histological techniques to assess upper motoneurons in the cerebral cortex. Here, we address the question in a novel way by assessing Wallerian degeneration and axon numbers in the medullary pyramid of Sprague Dawley rats after both acute SCI, either at cervical level 5 (C5) or thoracic level 9 (T9), and chronic SCI at T9. Our findings demonstrate that only a fraction of a percentage of the total axons in the medullary pyramid exhibit any sign of degeneration at any time after SCI--no more so than in uninjured control rats. Moreover, design-based counts of myelinated axons revealed no decrease in axon number in the medullary pyramid after SCI, regardless of injury level, severity, or time after injury. Spinal cord-injured rats had fewer myelinated axons in the medullary pyramid at 1 year after injury than aged matched controls, suggesting that injury may affect ongoing myelination of axons during aging. We conclude that SCI does not cause death of the CST cell bodies in the cortex; therefore, therapeutic strategies aimed at promoting axon regeneration of the CST in the spinal cord do not require a separate intervention to prevent retrograde degeneration of upper motoneurons in the cortex.

Voxel-based analysis of postnatal white matter microstructure in mice exposed to immune challenge in early or late pregnancy

Maternal infection during prenatal life is a risk factor for neurodevelopmental disorders, including schizophrenia and autism, in the offspring. We and others have reported white mater microstructure abnormalities in prefrontal-striato-temporal networks in these disorders. In addition we have shown that early rather than late maternal immune challenge in the mouse model precipitates ventricular volume change and impairs sensorimotor gating similar to that found in schizophrenia. However, it is not known whether the timing of maternal infection has a differential impact upon white matter microstructural indices. Therefore this study directly tested the effect of early or late gestation maternal immune activation on post-natal white matter microstructure in the mouse. The viral mimic PolyI:C was administered on day 9 or day 17 of gestation. In-vivo diffusion tensor imaging (DTI) was carried out when the offspring reached adulthood. We describe a novel application of voxel-based analysis to evaluate fractional anisotrophy (FA). In addition we conducted a preliminary immunohistochemical exploration of the oligodendrocyte marker, 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNPase), to determine whether differences in myelination might contribute to any changes in FA observed. Our results provide experimental evidence that prenatal exposure to inflammation elicits widespread differences in FA throughout fronto-striatal-limbic circuits compared to control saline exposure. Moreover, FA changes were more extensive in the group exposed earliest in gestation.

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.

Ca2+ flux and signaling implications by nicotinic acetylcholine receptors in rat medial habenula

The fraction of inward current carried by Ca(2+) (FCa(2+)) through nicotinic acetylcholine receptors (nAChRs) on acutely isolated rat medial habenula (MHb) neurons was calculated from experiments that simultaneously monitored agonist-induced membrane currents and intracellular [Ca(2+)], measured with patch-clamp and indo-1 fluorescence, respectively. In physiological concentrations of extracellular Ca(2+) (2 mM) at -50 mV, the percentage of current carried by Ca(2+) was determined to be roughly 3-4%, which is in close agreement with measurements from other heteromeric nicotinic receptors expressed in peripheral tissue. Among factors that may have affected this measurement, such as Ca(2+) influx through voltage-gated Ca(2+) channels, the concentration of intracellular Ca(2+) buffer, and Ca(2+) sequestration and release from intracellular stores, only Ca(2+) uptake by mitochondria was shown to confound the analysis. Furthermore, we find that because of the high density of nAChRs on MHb cells, low concentrations of ACh (10 microM) and its hydrolysis product, choline (1 mM), can significantly elevate intracellular Ca(2+). Moreover, during persistent activation of nAChRs, the level of intracellular Ca(2+) is proportional to its extracellular concentration in the physiological range. Together, these findings support the suggestion that nAChRs may be capable of sensing low concentrations of diffusely released neurotransmitter and, in addition, transfer information about ongoing local synaptic activity by changes in extracellular Ca(2+).

Alteration of the in vivo nicotinic receptor density in ADNFLE patients: a PET study

Nicotinic acetylcholine receptors (nAChRs) are involved in a familial form of frontal lobe epilepsy, autosomal dominant nocturnal frontal lobe epilepsy (ADNFLE). In several ADNFLE families, mutations were identified in the nAChR alpha4 or beta2 subunit, which together compose the main cerebral nAChR. Electrophysiological assessment using in vitro expression systems indicated a gain of function of the mutant receptors. However the precise mechanisms by which they contribute to the pathogenesis of a focal epilepsy remain obscure, especially since alpha4beta2 nAChRs are known to be widely distributed within the entire brain. PET study using [18F]-F-A-85380, a high affinity agonist at the alpha4beta2 nAChRs, allows the determination of the regional distribution and density of the nAChRs in healthy volunteers and in ADNFLE patients, thus offering a unique opportunity to investigate some in vivo consequences of the molecular defect. We have assessed nAChR distribution in eight non-smoking ADNFLE patients (from five families) bearing an identified mutation in nAChRs and in seven age-matched non-smoking healthy volunteers using PET and [(18)F]-F-A-85380. Parametric images of volume of distribution (Vd) were generated as the ratio of tissue to plasma radioactivities. The images showed a clear difference in the pattern of the nAChR density in the brains of the patients compared to the healthy volunteers. Vd values revealed a significant increase (between 12 and 21%, P < 0.05) in the ADNFLE patients in the mesencephalon, the pons and the cerebellum when compared to control subjects. Statistical parametric mapping (SPM) was then used to better analyse subtle regional differences. This analysis confirmed clear regional differences between patients and controls: patients had increased nAChR density in the epithalamus, ventral mesencephalon and cerebellum, but decreased nAChR density in the right dorsolateral prefrontal region. In five patients who underwent an additional [(18)F]-fluorodeoxyglucose (FDG) PET experiment, hypometabolism was observed in the neighbouring area of the right orbitofrontal cortex. The demonstration of a regional nAChR density decrease in the prefrontal cortex, despite the known distribution of these receptors throughout the cerebral cortex, is consistent with a focal epilepsy involving the frontal lobe. We also propose that the nAChR density increase in mesencephalon is involved in the pathophysiology of ADNFLE through the role of brainstem ascending cholinergic systems in arousal.

Deletion of the beta 2 nicotinic acetylcholine receptor subunit alters development of tolerance to nicotine and eliminates receptor upregulation

RATIONALE

Chronic nicotine exposure induces both tolerance and upregulation of [3H]nicotine binding sites in rodent and human brain. However, the mechanism for chronic tolerance is unclear because a direct relationship between tolerance and receptor upregulation is not consistently observed.

OBJECTIVES

In the present experiments, the role of beta2* nicotinic acetylcholine receptors (nAChRs) on tolerance development and nAChR upregulation was examined following chronic nicotine treatment of beta2 wild-type (+/+), heterozygous (+/-), and null mutant (-/-) mice.

METHODS

Saline or nicotine (1, 2, or 4 mg/kg/h) was infused intravenously for 10 days. Locomotor activity and body temperature responses were measured before and after nicotine challenge injection to observe changes in nicotine sensitivity. [3H]Epibatidine binding was then measured in ten brain regions.

RESULTS

Beta2+/+ mice developed dose-dependent tolerance and upregulation of [3H]epibatidine binding sites. In contrast, beta2-/- mice, initially less sensitive to acute nicotine's effects, became more sensitive following treatment with the lowest chronic dose (1 mg/kg/h). Beta2-/- mice treated with 4.0 mg/kg/h nicotine were no longer supersensitive, indicating that tolerance developed at this higher dose. However, these changes in nicotine sensitivity occurred in the absence of any nAChR changes in either low- or high-affinity [3H]epibatidine sites. Responses of beta2+/- mice were intermediate between wild-type and mutant mice.

CONCLUSIONS

Upregulation of nAChRs in vivo requires the presence of the beta2 subunit. Changes in nicotine sensitivity occurred both in the presence (beta2+/+) and absence (beta2-/-) of beta2* nAChRs and suggest that mechanisms involving both beta2* and non-beta2* nAChR subtypes modulate adaptation to chronic nicotine exposure.

Attenuation of morphine withdrawal signs by intracerebral administration of 18-methoxycoronaridine

18-Methoxyroconaridine (18-MC), a synthetic derivative of ibogaine, reduces morphine self-administration and alleviates several signs of acute opioid withdrawal in rats. Although there is already well documented evidence of the mechanism mediating 18-MC's action to reduce the rewarding effects of morphine, nothing is known about the mechanism responsible for 18-MC's attenuation of opioid withdrawal. In vitro studies have demonstrated that 18-MC is a potent antagonist of alpha3beta4 nicotinic receptors (IC50=0.75 microM), which are predominantly located in the medial habenula and interpeduncular nuclei. Previous work indicating that alpha3beta4 nicotinic receptors mediate 18-MC's effects on drug self-administration prompted us to assess whether brain areas having high or moderate densities of alpha3beta4 receptors might be involved in 18-MC's modulation of opioid withdrawal. To test this possibility, 18-MC was locally administered into the medial habenula, interpeduncular nucleus and locus coeruleus of morphine-dependent rats; this treatment was followed by naltrexone to precipitate a withdrawal syndrome. Pretreatment with various doses of 18-MC into the locus coeruleus significantly reduced wet-dog shakes, teeth chattering, burying and diarrhea, while pretreatment into the medial habenula attenuated teeth chattering, burying, and weight loss. Some doses of 18-MC administered into the interpeduncular nucleus significantly ameliorated rearing, teeth chattering, and burying, while other doses exacerbated diarrhea and teeth chattering. The present findings suggest that 18-MC may act in all three nuclei to suppress various signs of opioid withdrawal.

Effects of acute administration of ethanol on cerebral glucose utilization in adult alcohol-preferring and alcohol-nonpreferring rats

Local cerebral glucose utilization (LCGU) rates, as determined by the [(14)C]-2-deoxyglucose (2-DG) technique, were examined after acute ethanol administration within selected brain regions of alcohol-preferring (P) and alcohol-nonpreferring (NP) rats. Adult male P and NP rats were injected with saline, 0.25 g/kg, or 1.0 g/kg ethanol, intraperitoneally (ip), 10 min before an intravenous bolus of [(14)C]2-DG (125 microCi/kg). Timed arterial blood samples were collected over 45 min and assayed for plasma glucose, ethanol, and [(14)C]2-DG levels. Image densities were determined using quantitative autoradiography and LCGU values calculated. Data were collected from several key limbic, basal ganglionic, cortical, and subcortical structures. Low-dose ethanol (0.25 g/kg) significantly decreased LCGU rates in several brain regions including the medial prefrontal cortex, olfactory tubercles, and the CA1 subregion of the hippocampus of P rats. Low-dose ethanol had no significant effects on LCGU rates in the NP rats. Moderate-dose ethanol (1.0 g/kg) also significantly lowered LCGU rates in many brain regions of P rats, including key limbic structures, such as the medial prefrontal cortex, olfactory tubercles, ventral tegmental area, basolateral nucleus of the amygdala, lateral septum, and ventral pallidum. Moderate-dose ethanol also significantly lowered LCGU rates in the medial prefrontal cortex as well as in the habenula of NP rats. All other regions were unaffected in the NP rats. These findings support the suggestion that certain central nervous system regions of P rats may be more sensitive than those of NP rats to the effects of low to intermediate doses of ethanol.

Habenula lesions cause impaired cognitive performance in rats: implications for schizophrenia

Cognitive impairment is a prominent feature of schizophrenia. Currently there is no well-accepted explanation of the aetiology of this disorder, but recent evidence indicates that dysfunction of the habenula may be involved. We therefore examined whether habenula lesions in Sprague-Dawley rats cause behavioural changes resembling those of schizophrenia. Rats received either habenula lesions, a sham operation or a small lesion of the overlying dorsal hippocampus as a check that effects observed were not due to incidental damage to this structure. As there are alterations of social behaviour, sensorimotor gating and cognition in schizophrenia, we examined comparable behaviours. Social interaction time was measured during a 5-min encounter with a novel juvenile conspecific. Prepulse inhibition of an acoustic startle response, as an index of sensorimotor gating, was measured with prepulses of various amplitudes, and spatial cognitive performance was assessed in the Morris water maze task. Histological analysis showed that habenula lesions substantially damaged both medial and lateral habenula bilaterally while largely sparing neighbouring structures. Assay of choline acetyltransferase (ChAT) in the interpeduncular nucleus terminal region of the habenulo-interpeduncular tract, showed marked reduction (by 80%) in habenula-lesioned animals. Habenula-lesioned rats, but not the control group with small dorsal hippocampus lesions, showed marked impairment of Morris maze performance compared to the sham-operated control group. Social interaction time and prepulse inhibition were not significantly altered in either lesion group. The results are consistent with a role of the habenula in cognition, and with the view that pathology of the habenula may contribute to the cognitive impairments of schizophrenia.

Anti-addictive actions of an iboga alkaloid congener: a novel mechanism for a novel treatment

18-Methoxycoronaridine (18-MC), a novel iboga alkaloid congener that decreases drug self-administration in several animal models, may be a potential treatment for multiple forms of drug abuse. In animal models, 18-MC reduced intravenous morphine, cocaine, methamphetamine and nicotine self-administration, oral alcohol and nicotine intake, and attenuated signs of opioid withdrawal, but had no effect on responding for a nondrug reinforcer (water) and produced no apparent toxicity [Brain Res. 719 (1996) 29; NeuroReport 11 (2000) 2013; Pharmacol. Biochem. Behav. 58 (1997) 615; Psychopharmacology (Berl.) 139 (1998) 274; NeuroReport 9 (1998) 1283; Ann. N. Y. Acad. Sci. 914 (2000) 369]. Consistent with a relationship among drug sensitization, mesolimbic dopamine, and drug-seeking behavior, 18-MC also blocked the sensitized dopamine responses to morphine and cocaine in the nucleus accumbens. An extensive series of receptor studies showed that 18-MC was most potent and somewhat selective as an antagonist at alpha3beta4 nicotinic receptors. Low-dose combinations of 18-MC with other drugs known to have this same action (e.g., mecamylamine, dextromethorphan, bupropion) decreased morphine, methamphetamine, and nicotine self-administration in rats at doses that were ineffective if administered alone. Together, the data support the hypothesis that diencephalic pathways having high densities of alpha3beta4 nicotinic receptors modulate mesocorticolimbic pathways more directly involved in drug reinforcement. Antagonists of alpha3beta4 nicotinic receptors may represent a totally novel approach to treating multiple addictive disorders, and 18-MC might be the first of a new class of synthetic agents acting via this novel mechanism and having a broad spectrum of activity.

Measuring nicotinic receptors with characteristics of alpha4beta2, alpha3beta2 and alpha3beta4 subtypes in rat tissues by autoradiography

Comparison of [125I]epibatidine and 5-[125I]iodo-3-(2-azetidinylmethoxy)pyridine ([125I]A-85380) autoradiography showed evidence for nicotinic receptor heterogeneity. To identify the receptor subtypes, we performed [125I]epibatidine autoradiography in the presence of cytisine or A-85380. By comparing these results with binding data from human embryonic kidney (HEK) 293 cells stably transfected with different combinations of rat nicotinic receptor subunits, we were able to quantify three distinct populations of [125I]epibatidine binding sites with characteristics of alpha4beta2, alpha3beta2 and alpha3beta4 receptors. Although the predominant subtype in rat brain was alpha4beta2, non-alpha4beta2 binding sites were prominent in many regions. In the habenulo-peduncular system, cerebellum, substantia gelatinosa, and many medullary nuclei, alpha3beta4-like binding accounted for more than 40% of [125I]epibatidine binding, and nearly all binding in superior cervical ganglion and pineal gland. Other regions enriched in alpha3beta4-like binding included locus ceruleus, dorsal tegmentum, subiculum and anteroventral thalamic nucleus. Regions enriched in alpha3beta2-like binding included the habenulo-peduncular system, many visual system structures, certain geniculate nuclei, and dopaminergic regions. The combination of autoradiography using a broad spectrum radioligand in the presence of selective competitors, and data from binding to defined receptor subtypes in expression systems, allowed us to quantify the relative populations of these three subtypes.

Nicotine produces selective degeneration in the medial habenula and fasciculus retroflexus

Nicotine's neurotoxic properties in rats were investigated by administering (-)-nicotine tartrate for 5 days either continuously in doses of 5.01, 5.72, 6.44, 7.13, 20.41 and 43.1 mg/kg/day via osmotic minipump or intermittently at 11.32 mg/kg/day via one daily subcutaneous injection. As assessed by silver staining, neurotoxicity was seen almost exclusively in the axons of the medial habenula and its output tract, the fasciculus retroflexus, in all treatment groups except the lowest dose. Within the habenula, the damage was noted in the ventral-medial-most portion of the nucleus which is thought to be dense with the alpha 4 beta 2 and/or alpha 3 beta 4 receptor subtypes. Past research has shown the medial habenula to be highly sensitive to the effects of nicotine, and these findings, in conjunction with related research using dopaminergic stimulants, indicate that the habenula may be a weak link in the neurotoxicity seen following stimulant drugs of abuse.