Alpha 4 nicotinic acetylcholine receptor subunit links cholinergic to brainstem monoaminergic neurotransmission

Acute Administration of Desformylflustrabromine Relieves Chemically Induced Pain in CD-1 Mice

Neuronal nicotinic acetylcholine receptors are cell membrane-bound ion channels that are widely distributed in the central nervous system. The α4β2 subtype of neuronal nicotinic acetylcholine receptor plays an important role in modulating the signaling pathways for pain. Previous studies have shown that agonists, partial agonists, and positive allosteric modulators for the α4β2 receptors are effective in relieving pain. Desformylflustrabromine is a compound that acts as an allosteric modulator of α4β2 receptors. The aim of this study was to assess the effects of desformylflustrabromine on chemically induced pain. For this purpose, the formalin-induced pain test and the acetic acid-induced writhing response test were carried out in CD-1 mice. Both tests represent chemical assays for nociception. The results show that desformylflustrabromine is effective in producing an analgesic effect in both tests used for assessing nociception. These results suggest that desformylflustrabromine has the potential to become a clinically used drug for pain relief.

Antinociceptive effect of tebanicline for various noxious stimuli-induced behaviours in mice

Tebanicline (ABT-594), an analogue of epibatidine, exhibits potent antinociceptive effects and high affinity for the nicotinic acetylcholine receptor in the central nervous system. We assessed whether tebanicline exerts an effect on various noxious stimuli and mediates the nicotine receptor or opioid receptor through stimulation. The antinociceptive effects of tebanicline were determined by noxious chemical, thermal and mechanical stimuli-induced behaviours in mice. Tebanicline had dose-dependent analgesic effects in formalin, hot-plate and tail-pressure tests. By contrast, the antinociceptive effect of tebanicline was not demonstrated in the tail-flick assay. Pre-treatment with mecamylamine, a nicotinic acetylcholine receptor antagonist, blocked the effects of tebanicline in formalin, tail-pressure and hot-plate tests. Moreover, pre-treatment with naloxone, an opioid receptor antagonist, only partially inhibited the effects of tebanicline in formalin and tail-pressure tests. Tebanicline produced antinociception in persistent chemical (formalin), acute thermal (hot-plate, but not tail-flick) and mechanical (tail-pressure) pain states. Moreover, tebanicline stimulated the nicotinic acetylcholine receptor and opioid receptor.

Nicotinic and muscarinic cholinergic receptors are recruited by acetylcholine-mediated neurotransmission within the locus coeruleus during the organisation of post-ictal antinociception

Post-ictal antinociception is characterised by an increase in the nociceptive threshold that accompanies tonic and tonic-clonic seizures (TCS). The locus coeruleus (LC) receives profuse cholinergic inputs from the pedunculopontine tegmental nucleus. Different concentrations (1μg, 3μg and 5μg/0.2μL) of the muscarinic cholinergic receptor antagonist atropine and the nicotinic cholinergic receptor antagonist mecamylamine were microinjected into the LC of Wistar rats to investigate the role of cholinergic mechanisms in the severity of TCS and the post-ictal antinociceptive response. Five minutes later, TCS were induced by systemic administration of pentylenetetrazole (PTZ) (64mg/kg). Seizures were recorded inside the open field apparatus for an average of 10min. Immediately after seizures, the nociceptive threshold was recorded for 130min using the tail-flick test. Pre-treatment of the LC with 1μg, 3μg and 5μg/0.2μL concentrations of both atropine and mecamylamine did not cause a significant effect on seizure severity. However, the same treatments decreased the post-ictal antinociceptive phenomenon. In addition, mecamylamine caused an earlier decrease in the post-ictal antinociception compared to atropine. These results suggest that muscarinic and mainly nicotinic cholinergic receptors of the LC are recruited to organise tonic-clonic seizure-induced antinociception.

The Effects of Methylphenidate on Resting-State Functional Connectivity of the Basal Nucleus of Meynert, Locus Coeruleus, and Ventral Tegmental Area in Healthy Adults

Background: Methylphenidate (MPH) influences catecholaminergic signaling. Extant work examined the effects of MPH on the neural circuits of attention and cognitive control, but few studies have investigated the effect of MPH on the brain's resting-state functional connectivity (rsFC).

Methods: In this observational study, we compared rsFC of a group of 24 healthy adults who were administered an oral 45 mg dose of MPH with a group of 24 age and gender matched controls who did not receive MPH. We focused on three seed regions: basal nucleus of Meynert (BNM), locus coeruleus (LC), and ventral tegmental area/substantia nigra, pars compacta (VTA/SNc), each providing cholinergic, noradrenergic and dopaminergic inputs to the cerebral cortex. Images were pre-processed and analyzed as in our recent work (Li et al., 2014; Zhang et al., 2015). We used one-sample t-test to characterize group-specific rsFC of each seed region and two-sample t-test to compare rsFC between groups.

Results: MPH reversed negative connectivity between BNM and precentral gyri. MPH reduced positive connectivity between LC and cerebellum, and induced positive connectivity between LC and right hippocampus. MPH decreased positive VTA/SNc connectivity to the cerebellum and putamen, and reduced negative connectivity to left middle occipital gyrus.

Conclusion: MPH had distinct effects on the rsFC of BNM, LC, and VTA/SNc in healthy adults. These new findings may further our understanding of the role of catecholaminergic signaling in Attention Deficit Hyperactivity Disorder (ADHD) and Parkinson's disease and provide insights into the therapeutic mechanisms of MPH in the treatment of clinical conditions that implicate catecholaminergic dysfunction.

Nicotine Administration Attenuates Methamphetamine-Induced Novel Object Recognition Deficits

BACKGROUND

Previous studies have demonstrated that methamphetamine abuse leads to memory deficits and these are associated with relapse. Furthermore, extensive evidence indicates that nicotine prevents and/or improves memory deficits in different models of cognitive dysfunction and these nicotinic effects might be mediated by hippocampal or cortical nicotinic acetylcholine receptors. The present study investigated whether nicotine attenuates methamphetamine-induced novel object recognition deficits in rats and explored potential underlying mechanisms.

METHODS

Adolescent or adult male Sprague-Dawley rats received either nicotine water (10-75 μg/mL) or tap water for several weeks. Methamphetamine (4 × 7.5mg/kg/injection) or saline was administered either before or after chronic nicotine exposure. Novel object recognition was evaluated 6 days after methamphetamine or saline. Serotonin transporter function and density and α4β2 nicotinic acetylcholine receptor density were assessed on the following day.

RESULTS

Chronic nicotine intake via drinking water beginning during either adolescence or adulthood attenuated the novel object recognition deficits caused by a high-dose methamphetamine administration. Similarly, nicotine attenuated methamphetamine-induced deficits in novel object recognition when administered after methamphetamine treatment. However, nicotine did not attenuate the serotonergic deficits caused by methamphetamine in adults. Conversely, nicotine attenuated methamphetamine-induced deficits in α4β2 nicotinic acetylcholine receptor density in the hippocampal CA1 region. Furthermore, nicotine increased α4β2 nicotinic acetylcholine receptor density in the hippocampal CA3, dentate gyrus and perirhinal cortex in both saline- and methamphetamine-treated rats.

CONCLUSIONS

Overall, these findings suggest that nicotine-induced increases in α4β2 nicotinic acetylcholine receptors in the hippocampus and perirhinal cortex might be one mechanism by which novel object recognition deficits are attenuated by nicotine in methamphetamine-treated rats.

Neuronal nicotinic receptors as analgesic targets: it's a winding road

Along with their well known role in nicotine addiction and autonomic physiology, neuronal nicotinic receptors (nAChRs) also have profound analgesic effects in animal models and humans. This is not a new idea, even in the early 1500s, soon after tobacco was introduced to the new world, its proponents listed pain relief among the beneficial properties of smoking. In recent years, analgesics that target specific nAChR subtypes have shown highly efficacious antinociceptive properties in acute and chronic pain models. To date, the side effects of these drugs have precluded their advancement to the clinic. This review summarizes the recent efforts to identify novel analgesics that target nAChRs, and outlines some of the key neural substrates that contribute to these physiological effects. There remain many unanswered mechanistic questions in this field, and there are still compelling reasons to explore neuronal nAChRs as targets for the relief of pain.

α4β2* neuronal nicotinic receptor ligands (agonist, partial agonist and positive allosteric modulators) as therapeutic prospects for pain

α4β2* neuronal nicotinic acetylcholine receptor are ligand-gated ion channels and widely expressed throughout the central and peripheral nervous system. α4β2* neuronal nicotinic acetylcholine receptor play crucial role in pain signaling via modulation of multiple neurotransmitters like acetylcholine, dopamine, γ-amino butyric acid (GABA) and norepinephrine. Both spinal and supraspinal pathways are involved in the mechanisms by which α4β2* neuronal nicotinic acetylcholine receptor ligands modulate the neuropathic and inflammatory pain. Selective α4β2* neuronal nicotinic acetylcholine receptor ligands are being developed for the treatment of neuropathic and inflammatory pain as they show considerable efficacy in a wide range of preclinical pain models. Agonists/partial agonists of α4β2* neuronal nicotinic acetylcholine receptor show efficacy in animal models of pain and their anti-nociceptive properties are blocked by nicotinic antagonists. Positive allosteric modulators are being developed with the aim to increase the potency or therapeutic window of agonists/partial agonists. Accumulating evidences suggest that anti-nociceptive effects of nicotinic acetylcholine receptor ligands may not be mediated solely by α4β2* neuronal nicotinic acetylcholine receptor. We have also reviewed the stage of clinical development of various α4β2* neuronal nicotinic acetylcholine receptor ligands.

Nicotinic excitation of serotonergic projections from dorsal raphe to the nucleus accumbens

Tobacco use is a major public health problem, and although many smokers report that they want to quit, only a small percentage succeed. Side effects associated with nicotine withdrawal, including depression, anxiety, and restlessness, certainly contribute to the low success rate. The dorsal raphe nucleus (DRN) is a serotonergic center with many functions, including control of mood and emotional state. We investigated the effect of nicotine on DRN neurons that project to the nucleus accumbens (NAc), an area involved in reward-related behaviors. Using a retrograde labeling method, we found that 75% of DRN-NAc projection neurons are serotonergic. In coronal slices that include the DRN, whole cell recordings were conducted on neurons identified by fluorescent backlabeling from NAc or randomly selected within the nucleus. Nicotine increased action potential firing rates in a subset of DRN neurons. Voltage-clamp recording revealed nicotinic acetylcholine receptor (nAChR)-mediated inward currents that contribute to the nicotine-induced excitation. Nicotinic receptors also indirectly affect excitability by modulating synaptic inputs to these neurons. Nicotine enhanced excitatory glutamatergic inputs to a subset of DRN-NAc projection neurons, while inhibitory γ-aminobutyric acid (GABA)ergic inputs were modulated either positively or negatively in a subset of these neurons. The net effect of nAChR activation is enhancement of serotonergic output from DRN to the NAc, which may contribute to the effects of nicotine on mood and affect.

Prenatal nicotine-exposure alters fetal autonomic activity and medullary neurotransmitter receptors: implications for sudden infant death syndrome

During pregnancy, exposure to nicotine and other compounds in cigarette smoke increases the risk of the sudden infant death syndrome (SIDS) two- to fivefold. Serotonergic (5-HT) abnormalities are found, in infants who die of SIDS, in regions of the medulla oblongata known to modulate cardiorespiratory function. Using a baboon model, we tested the hypothesis that prenatal exposure to nicotine alters 5-HT receptor and/or transporter binding in the fetal medullary 5-HT system in association with cardiorespiratory dysfunction. At 87 (mean) days gestation (dg), mothers were continuously infused with saline (n = 5) or nicotine (n = 5) at 0.5 mg/h. Fetuses were surgically instrumented at 129 dg for cardiorespiratory monitoring. Cesarean section delivery and retrieval of fetal medulla were performed at 161 (mean) dg for autoradiographic analyses of nicotinic and 5-HT receptor and transporter binding. In nicotine-exposed fetuses, high-frequency heart rate variability was increased 55%, possibly reflecting increases in the parasympathetic control of heart rate. This effect was more pronounced with greater levels of fetal breathing and age. These changes in heart rate variability were associated with increased 5-HT(1A) receptor binding in the raphé obscurus (P = 0.04) and increased nicotinic receptor binding in the raphé obscurus and vagal complex (P < 0.05) in the nicotine-exposed animals compared with controls (n = 6). The shift in autonomic balance in the fetal primate toward parasympathetic predominance with chronic exposure to nicotine may be related, in part, to abnormal 5-HT-nicotine alterations in the raphé obscurus. Thus increased risk for SIDS due to maternal smoking may be partly related to the effects of nicotine on 5-HT and/or nicotinic receptors.

Medullary serotonin defects and respiratory dysfunction in sudden infant death syndrome

Sudden infant death syndrome (SIDS) is defined as the sudden and unexpected death of an infant less than 12 months of age that occurs during sleep and remains unexplained after a complete autopsy, death scene investigation, and review of the clinical history. It is the leading cause of postneonatal mortality in the developed world. The cause of SIDS is unknown, but is postulated to involve impairment of brainstem-mediated homeostatic control. Extensive evidence from animal studies indicates that serotonin (5-HT) neurons in the medulla oblongata play a role in the regulation of multiple aspects of respiratory and autonomic function. A subset of SIDS infants have several abnormalities in medullary markers of 5-HT function and genetic polymorphisms impacting the 5-HT system, informing the hypothesis that SIDS results from a defect in 5-HT brainstem-mediated control of respiratory (and autonomic) regulation. Here we review the evidence from postmortem human studies and animal studies to support this hypothesis and discuss how the pathogenesis of SIDS is likely to originate in utero during fetal development.

The development of nicotinic receptors in the human medulla oblongata: inter-relationship with the serotonergic system

Maternal cigarette smoking during pregnancy adversely affects fetal development and increases the risk for the sudden infant death syndrome (SIDS). In SIDS we have reported abnormalities in the medullary serotonergic (5-HT) system, which is vital for homeostatic control. In this study we analyzed the inter-relationship between nicotinic receptors (nAChRs), to which nicotine in cigarette smoke bind, and the medullary 5-HT system in the human fetus and infant as a step towards determining the mechanisms whereby smoking increases SIDS risk in infants with 5-HT defects. Immunohistochemistry for the alpha4 nAChR subunit and 5-HT neurons was applied in fetal and infant medullae (15-92 postconceptional weeks, n=9). The distribution of different nAChRs was determined from 39-82 postconceptional weeks (n=5) using tissue autoradiography for 3H-nicotine, 3H-epibatidine, 3H-cytisine, and 125I-bungarotoxin; the findings were compared to laboratory 5-HT1A and 5-HT transporter binding data, and 5-HT neuronal density. Alpha4 immunoreactivity was ubiquitously expressed in medullary nuclei related to homeostatic functions from 15 weeks on, including rhombic lip germinal cells. At all ages, alpha4 co-localized with 5-HT neurons, indicating a potential site of interaction whereby exogenous nicotine may adversely affect 5-HT neuronal development and function. Binding for heteromeric nAChRs was highest in the inferior olive, and for homomeric nAChRs, in the vagal complex. In the paragigantocellularis lateralis, 5-HT1A receptor binding simultaneously increased as alpha7 binding decreased across infancy. This study indicates parallel dynamic and complex changes in the medullary nicotinic and 5-HT systems throughout early life, i.e., the period of risk for SIDS.

Nicotinic acetylcholine receptor subunit mRNA expression in adult and developing rat medullary catecholamine neurons

Nicotinic acetylcholine receptors (nAChRs) mediate numerous visceral functions via medullary catecholamine (CA) neurons found in the nucleus tractus solitarius (NTS), dorsal motor nucleus of the vagus (DMV), and ventrolateral medulla (VLM). However, the nAChR subtypes involved are not known. We have therefore characterized expression of nine nAChR subunit mRNAs in adult and developing rat medullary CA nuclei using combined isotopic/nonisotopic in situ hybridization. Tyrosine hydroxylase (TH) mRNA, the CA-synthesizing enzyme, was used as a marker for CA neurons, because these nuclei consist of heterogeneous populations of cells. Subunit mRNA expression varied within and between nuclei, along the rostrocaudal axis, between cell types, and across development. All CA neurons expressed beta2 mRNA, whereas alpha2 mRNA was completely absent. alpha6 And beta3 mRNA expression were restricted mainly to the VLM. alpha4, alpha5, And alpha7 mRNA expression was significantly greater in the rostral than in the caudal VLM. alpha3 And beta4 mRNAs were highly expressed in the dorsal region of the NTS, whereas dense alpha7 mRNA expression was restricted to the DMV and ventral NTS. The remaining subunit mRNAs were detected to some degree in both DMV and NTS. Except for alpha4 mRNA, which peaked prenatally, expression levels of subunit transcripts in the NTS and DMV were lower during development compared with adults. In the VLM, alpha3, alpha4, and alpha5 mRNAs expression peaked perinatally, whereas alpha6 and beta3 levels increased with age. These variations in nAChR subunit mRNA expression suggest that different receptor subtypes may produce function-specific regulation of medullary CA systems.

Serotoninergic dorsal raphe neurons possess functional postsynaptic nicotinic acetylcholine receptors

Very few neurons in the telencephalon have been shown to express functional postsynaptic nicotinic acetylcholine receptors (nAChRs), among them, the noradrenergic and dopaminergic neurons. However, there is no evidence for postsynaptic nAChRs on serotonergic neurons. In this study, we asked if functional nAChRs are present in serotonergic (5-HT) and nonserotonergic (non-5-HT) neurons of the dorsal raphe nucleus (DRN). In rat midbrain slices, field stimulation at the tegmental pedunculopontine (PPT) nucleus evoked postsynaptic currents (eEPSCs) with different components in DRN neurons. After blocking the glutamatergic and GABAergic components, the remaining eEPSCs were blocked by mecamylamine and reduced by either the selective alpha7 nAChR antagonist methyllycaconitine (MLA) or the selective alpha4beta2 nAChR antagonist dihydro-beta-eritroidine (DHbetaE). Simultaneous addition of MLA and DHbetaE blocked all eEPSCs. Integrity of the PPT-DRN pathway was assessed by both anterograde biocytin tracing and antidromic stimulation from the DRN. Inward currents evoked by the direct application of acetylcholine (ACh), in the presence of atropine and tetrodotoxin, consisted of two kinetically different currents: one was blocked by MLA and the other by DHbetaE; in both 5-HT and non-5-HT DR neurons. Analysis of spontaneous (sEPSCs) and evoked (eEPSCs) synaptic events led to the conclusion that nAChRs were located at the postsynaptic membrane. The possible implications of these newly described nAChRs in various physiological processes and behavioral events, such as the wake-sleep cycle, are discussed.

Acute nicotine reduces and repeated nicotine increases spontaneous activity in male and female Lewis rats

The Lewis (LEW) strain of rat appears more sensitive to nicotine than other strains in self-administration, conditioned place preference, and drug discrimination behavioral studies. The present study sought to further evaluate the behavioral effects of chronic nicotine treatment in the LEW strain by assessing spontaneous activity, which has consistently revealed sensitization to chronic nicotine administration in Sprague Dawley (SD) rats. High active and low active male and female LEW rats (N=8 per group) were treated twice daily with either nicotine (0.4 mg/kg, sc) or vehicle for 14 consecutive days. Regardless of baseline activity level or sex, spontaneous activity was significantly decreased, compared to saline-treated rats, after a single nicotine injection. However, spontaneous activity increased in both low- and high-activity rats (both sexes) over the two weeks of nicotine administration to levels that were significantly higher than saline-treated rats. Based on these findings, acute and chronic nicotine administration had greater suppressive and enhancing effects on spontaneous activity in LEW rats compared to other strains of rats previously studied. These results further clarify the behavioral sensitivity of the LEW strain of rat to nicotine exposure and lend credence to the role of genetics in the individual susceptibility to nicotine dependence.

Alpha4 containing nicotinic receptors are positioned to mediate postsynaptic effects on 5-HT neurons in the rat dorsal raphe nucleus

Nicotinic acetylcholine receptors containing the alpha4 and beta2 subunits constitute the most abundant high-affinity binding site of nicotine in the brain and are critical for the addictive qualities of nicotine. 5-HT neurotransmission is thought to be an important contributor to nicotine addiction. Therefore in this study it was examined how alpha4-containing receptors are positioned to modulate the function of 5-HT neurons using ultrastructural analysis of immunolabeling for the alpha4 receptor subunit in the dorsal raphe nucleus (DR), a primary source of forebrain 5-HT in the rat. Of 150 profiles labeled for the alpha4 subunit, 140 or 93% consisted of either soma or dendrites, these were often small-caliber (distal) dendrites <1.5 microm in diameter (63/150 or 42%). The majority (107/150 or 71%) of profiles containing labeling for alpha4 were dually labeled for the synthetic enzyme for 5-HT, tryptophan hydroxylase (TPH). Within dendrites immunogold labeling for alpha4 was present on the plasma membrane or near postsynaptic densities. However, labeling for alpha4 was commonly localized to the cytoplasmic compartment often associated with smooth endoplasmic reticulum, plausibly representing receptors in transit to or from the plasma membrane. Previous studies have suggested that nicotine presynaptically regulates activity onto 5-HT neurons, however alpha4 immunolabeling was detected in only 10 axons in the DR or 7% of profiles sampled. This finding suggest that alpha4 containing receptors are minor contributors to presynaptic regulation of synaptic activity onto 5-HT neurons, but rather alpha4 containing receptors are positioned to influence 5-HT neurons directly at postsynaptic sites.

Electrophysiologic effects of systemic and locally infused epibatidine on locus coeruleus neurons

We evaluated the electrophysiologic response of locus coeruleus neurons to the systemic and local infusion of epibatidine. Rats were anesthetized with 2% halothane and single-unit locus coeruleus discharge was recorded after administration of systemic (2.5, 5 and 10 microg/kg subcutaneously) and intracoerulear (0.03-0.01-0.001 microg) epibatidine. The subcutaneous epibatidine activated locus coeruleus neurons only at the highest dose (10 microg/kg). The 2.5-5 microg/kg doses, previously shown to induce analgesia, did not activate locus coeruleus neurons. The intracoerulear infusion of epibatidine induced excitement of locus coeruleus neurons at every tested dose. Higher doses (0.03 and 0.01 microg) excited 100% of the recorded neurons. A significantly lower number of neurons (50% and 43% respectively) were excited when lower doses (0.005-0.001 microg) were used (P=0.035). The intracoerulear infusion of mecamylamine (1 microg) significantly reduced neuronal discharge rate (45%) and blocked the effects of epibatidine. The intra-dorsal raphe infusion of 0.03 microg epibatidine induced significant excitation of locus coeruleus neurons. These data show that the administration of epibatidine induces excitation of locus coeruleus neurons, which is mediated by nicotinic receptors. This activation occurs after systemic and selective local administration of epibatidine. The response of locus coeruleus neurons to systemic and locally administered epibatidine is dose-related.

Modulators in concert for cognition: modulator interactions in the prefrontal cortex

Research on the regulation and function of ascending noradrenergic, dopaminergic, serotonergic, and cholinergic systems has focused on the organization and function of individual systems. In contrast, evidence describing co-activation and interactions between multiple neuromodulatory systems has remained scarce. However, commonalities in the anatomical organization of these systems and overlapping evidence concerning the post-synaptic effects of neuromodulators strongly suggest that these systems are recruited in concert; they influence each other and simultaneously modulate their target circuits. Therefore, evidence on the regulatory and functional interactions between these systems is considered essential for revealing the role of neuromodulators. This postulate extends to contemporary neurobiological hypotheses of major neuropsychiatric disorders. These hypotheses have focused largely on aberrations in the integrity or regulation of individual ascending modulatory systems, with little regard for the likely possibility that dysregulation in multiple ascending neuromodulatory systems and their interactions contribute essentially to the symptoms of these disorders. This review will paradigmatically focus on neuromodulator interactions in the PFC and be further constrained by an additional focus on their role in cognitive functions. Recent evidence indicates that individual neuromodulators, in addition to their general state-setting or gating functions, encode specific cognitive operations, further substantiating the importance of research concerning the parallel recruitment of neuromodulator systems and interactions between these systems.

Muscarinic receptors and alpha2-adrenoceptors interact to modulate the respiratory rhythm in mouse neonates

The respiratory rhythm generator (RRG) is modulated by several endogenous substances, including acetylcholine (ACh) and noradrenaline (NA) that interact in several modulatory processes. To know whether ACh and NA interacted to modulate the RRG activity, we used medullary "en bloc" and slice preparations from neonatal mice where the RRG has been shown to receive a facilitatory modulation from A1/C1 neurons, via a continuous release of endogenous NA and activation of alpha2 adrenoceptors. Applying ACh at 25 microM activated the RRG but ACh had no effects at 50 microM. Applying the ACh receptor agonists nicotine and muscarine facilitated and depressed the RRG, respectively. After yohimbine pre-treatment that blocked the alpha2 facilitation, the nicotinic facilitation was not altered, the muscarinic depression was reversed and ACh 50 microM significantly facilitated the RRG. After L-tyrosine pre-treatment that potentiated the alpha2 facilitation, the muscarinic depression was enhanced. Thus, ACh regulates the RRG activity via nicotinic and muscarinic receptors, the muscarinic receptors interacting with alpha2 adrenoceptors.

Prenatal nicotine exposure alters the responses to subsequent nicotine administration and withdrawal in adolescence: Serotonin receptors and cell signaling

Offspring of women who smoke during pregnancy are themselves more likely to take up smoking in adolescence, effects that are associated with a high rate of depression and increased sensitivity to withdrawal symptoms. To evaluate the biological basis for this relationship, we assessed effects on serotonin (5-hydroxytryptamine, 5HT) receptors and 5HT-mediated cellular responses in rats exposed to nicotine throughout prenatal development and then given nicotine in adolescence (postnatal days PN30-47.5), using regimens that reproduce plasma nicotine levels found in smokers. Evaluations were then made during the period of adolescent nicotine treatment and for up to one month after the end of treatment. Prenatal nicotine exposure, which elicits damage to 5HT projections in the cerebral cortex and striatum, produced sex-selective changes in the expression of 5HT(1A) and 5HT2 receptors, along with induction of adenylyl cyclase (AC), leading to sensitization of heterologous inputs operating through this signaling pathway. Superimposed on these effects, the AC response to 5HT was shifted toward inhibition. By itself, adolescent nicotine administration, which damages the same pathways, produced similar effects on receptors and the 5HT-mediated response, but a smaller overall induction of AC. Animals exposed to prenatal nicotine showed a reduced response to nicotine administered in adolescence, results in keeping with earlier findings of persistent desensitization. Our results indicate that prenatal nicotine exposure alters parameters of 5HT synaptic communication lasting into adolescence and changes the response to nicotine administration and withdrawal in adolescence, actions which may contribute to a subpopulation especially vulnerable to nicotine dependence.

Estradiol increases alpha7 nicotinic receptor in serotonergic dorsal raphe and noradrenergic locus coeruleus neurons of macaques

Acetylcholine, acting on presynaptic nicotinic receptors (nAChRs), modulates the release of neurotransmitters in the brain. The rat dorsal raphe nucleus (DR) and the locus coeruleus (LC) receive cholinergic input and express the alpha7nAChR. In previous reports, we demonstrated that estradiol (E) administration stimulates DR serotonergic and LC noradrenergic function in the macaque. In addition, it has been reported that E induces the expression of the alpha7nAChR in rats. We questioned whether E increased the expression of the alpha7nAChR in the macaque DR and LC. We utilized double immunostaining to study the effect of a simulated preovulatory surge of E on the expression of the alpha7nAChR in the DR and the LC and to determine whether alpha7nAChR colocalizes with serotonin and tyrosine hydroxylase (TH) in macaques. There was no difference in the number of alpha7nAChR-positive neurons between ovariectomized (OVX) controls and OVX animals treated with a silastic capsule containing E (Ecap). However, supplemental infusion of E for 5-30 hours to Ecap animals (Ecap + inf) significantly increased the number of alpha7nAChR-positive neurons in DR and LC. In addition, supplemental E infusion significantly increased the number of neurons in which alpha7nAChR colocalized with serotonin and TH. These results constitute an important antecedent for study of the effects of nicotine and ovarian steroid hormones in the physiological functions regulated by the DR and the LC in women.

The locus coeruleus nucleus as a site of action of the antinociceptive and behavioral effects of the nicotinic receptor agonist, epibatidine

The mechanisms and sites of action of epibatidine-induced antinociception and side effects are poorly understood. The present study tested the hypothesis that the locus coeruleus is a site of action of epibatidine. Behavioral responses of rats to hindpaw formalin injection were compared after direct administration of epibatidine into the locus coeruleus (LC), and after subcutaneous administration. Different groups of rats were injected with formalin into the rear paw after administration of either ACSF, epibatidine (0.01, 0.06, 0.12, and 0.3mug) into the locus coeruleus or epibatidine (2.5-5mug/kg) subcutaneously. Assessment of pain-related behavior was done by evaluating the incidence of favoring, lifting and licking of the injected paw in the different groups. Abnormal motor behavior was also recorded. Infusion of epibatidine into LC induced analgesia, which was reversed by prior infusion of mecamylamine into LC. Epibatidine into the locus coeruleus resulted in a significant lower pain score in the second phase of the formalin test compared to control rats and was as effective as subcutaneous epibatidine. The analgesic effects of epibatidine were regionally selective in that the administration of epibatidine outside the locus coeruleus area was not analgesic. The every tested dose of epibatidine administered into the locus coeruleus also produced freezing behavior immediately after injection, which was relatively short-lived compared to the analgesic effect. Freezing was inhibited by administration of mecamylamine into the LC. Together the results implicate the LC as a target for the analgesic effects of epibatidine.

The cysteine-rich with EGF-like domains 2 (CRELD2) protein interacts with the large cytoplasmic domain of human neuronal nicotinic acetylcholine receptor alpha4 and beta2 subunits

Using a yeast two-hybrid screening we report the isolation of a novel human protein, hCRELD2beta, that interacts specifically with the large cytoplasmic regions of human nicotinic acetylcholine receptor (nAChR) alpha4 and beta2 subunits, both in yeast cells and in vitro. This interaction is not detected with nAChR alpha7 and alpha3 subunits. The hCRELD2 gene encodes for multiple transcripts, likely to produce multiple protein isoforms. A previously reported one has been renamed as CRELD2alpha. Isoforms alpha and beta are expressed in all tissues examined and have the same N-terminal and central regions but alternative C-terminal regions. Both isoforms interact with the alpha4 subunit. Within this subunit the interaction was localized to the N-terminal region of the large cytoplasmic loop. The CRELD2beta protein is present at the endoplasmic reticulum where colocalized with alpha4beta2 nAChRs upon cell transfection. Immunohistochemistry experiments demonstrated the presence of CRELD2 in the rat brain at sites where alpha4beta2 receptors have been previously detected. Labeling was restricted to neuronal perikarya. Finally, CRELD2 decreases the functional expression and impairs membrane transport of alpha4beta2 nAChRs in Xenopus leavis oocytes, without affecting alpha3beta4 and alpha7 nAChR expression. These results suggest that CRELD2 can act as a specific regulator of alpha4beta2 nAChR expression.

The organization of the brainstem and spinal cord of the mouse: relationships between monoaminergic, cholinergic, and spinal projection systems

Information regarding the organization of the CNS in terms of neurotransmitter systems and spinal connections in the mouse is sparse, especially at the level of the brainstem. An overview is presented of monoaminergic and cholinergic systems in the brainstem and spinal cord that were visualized immunohistochemically in inbred C57BL/6 and outbred CD-1 mice. This information is complemented with data on spinal cord-projecting systems that were characterized using retrograde tracing, spinal hemisections, and double labeling techniques. Attention is given to differences in these systems related to spinal levels. The data are discussed with reference to studies in the rat, and to standardized information as provided in the atlas of the mouse brain. Although the overall organization of these systems in the mouse is largely similar to those in the rat, species differences are present in relative location, size and/or connectivity of cell groups. For example, catecholaminergic neurons in the (ventro)lateral pons (A5 and A7 cell groups) in the mouse project to the spinal cord mainly via contralateral, and not ipsilateral, pathways. The data further supplement information as provided in standardized brainstem sections of the C57BL/6 mouse [Paxinos, G., Franklin, K.B.J., 2001. The mouse brain in stereotaxic coordinates. Academic Press, San Diego], especially with respect to the size and/or location of the catecholaminergic retrorubral field (A8 group), A5, A1, and C1 cell groups, and the serotonergic B4 group, reticulotegmental nucleus (B9 group), lateral paragigantocellular nucleus and raphe magnus nucleus (B3 group). Altogether this study provides a comprehensive overview of the spatial relationships of neurochemically and anatomically defined neuronal systems in the mouse brainstem and spinal cord.

Alpha 7 nicotinic receptor subunit is present on serotonin neurons projecting to hippocampus and septum

The serotonergic transmitter system regulates hippocampal activity through its raphe projection to hippocampus and medial septum/diagonal band of Broca complex (MS/DBB), and most likely also indirectly through its interaction with the cholinergic neurotransmitter system. Nicotine, e.g., enhances hippocampal serotonin release probably through presynaptic nicotinic receptors. We investigated the possible presence of the alpha 7-nicotinic subunit on serotonergic neurons projecting to hippocampus and MS/DBB. By retrograde neuronal tracing, hippocampal serotonergic neurons were identified and with double fluorescence immunostaining and Alexa-488 bound alpha-bungarotoxin the presence of active alpha 7 receptor on their soma was determined. Most of the retrogradely labeled serotonin neurons contained the alpha 7 subunit. A low degree of colocalization between alpha-bungarotoxin and serotonin-positive neurons suggest that the alpha 7 subunit may be transported anterogradely to the serotonergic axonal terminals.

Alpha-7 and alpha-4 nicotinic receptor subunit immunoreactivity in genioglossus muscle motoneurons

In the present study, immunohistochemistry combined with retrograde labeling techniques were used to determine if hypoglossal motoneurons (HMNs), retrogradely labeled after cholera toxin B subunit (CTB) injection to the genioglossus muscle in rats, show immunoreactivity for alpha-7 and alpha-4 subunits of nicotinic acetylcholine receptors (nAChRs). CTB-positive HMNs projecting to the genioglossus muscle were consistently labeled throughout the rostrocaudal extent of the hypoglossal nuclei with the greatest labeling at and caudal to area postrema. Alpha-7 subunit immunoreactivity was found in 39.44+/-5.10% of 870 CTB-labeled motoneurons and the alpha-4 subunit in 51.01+/-3.71% of 983 CTB-positive neurons. Rostrally, the number of genioglossal motoneurons demonstrating immunoreactivity for the alpha-7 subunit was 45.85+/-10.04% compared to 34.96+/-5.11% at and caudal to area postrema (P>0.1). The number of genioglossal motoneurons that showed immunoreactivity for the alpha-4 subunit was 55.03+/-4.83% at and caudal to area postrema compared to 42.98+/-3.90% in rostral areas (P=0.074). These results demonstrate that nAChR immunoreactivity is present in genioglossal motoneurons and suggest a role for alpha-7 and alpha-4 subunits containing nAChRs in the regulation of upper airway patency.

The dorsal raphe nucleus as a site of action of the antinociceptive and behavioral effects of the alpha4 nicotinic receptor agonist epibatidine

The mechanisms and sites of action of epibatidine-induced antinociception and side effects are poorly understood. The present study tested the hypothesis that the serotonergic dorsal raphe nucleus is a site of action of epibatidine. Behavioral responses of rats to hindpaw formalin injection were compared after direct administration of epibatidine into the dorsal raphe and after subcutaneous administration. Different groups of rats were injected with formalin into the rear paw after administration of either epibatidine (0.01, 0.015, 0.03, and 0.06 microg) in the dorsal raphe or epibatidine (2.5-5 microg/kg) subcutaneously. Assessment of pain related behavior was done evaluating the incidence of favoring, lifting, and licking of the injected paw in the different groups. Abnormal behavior (freezing) was also recorded. Epibatidine was at least 100 times more potent when administered into the dorsal raphe nucleus versus systemically, implicating this nucleus as a site of action of the analgesic effects of epibatidine. Thus, epibatidine (0.015, 0.03, and 0.06 microg) in the dorsal raphe resulted in a significant lower pain score in the second phase of the formalin test compared with control rats and was as effective as subcutaneous epibatidine. The analgesic effects of epibatidine were regionally selective in that administration of epibatidine within the periaqueductal gray area but outside the dorsal raphe area was not analgesic. The highest doses of intraraphe epibatidine (i.e., 0.03-0.06 microg) also produced "freezing" behavior immediately after injection, which was relatively short-lived compared with the analgesic effect. Together, the results implicate the dorsal raphe nucleus as a target for the analgesic and perhaps anxiogenic effects of epibatidine.

Nicotine and epibatidine triggered prolonged rise in calcium and TH gene transcription in PC12 cells

The effect of epibatidine on regulation of [Ca2+]i and tyrosine hydroxylase (TH) transcription was examined. Epibatidine triggers a biphasic rise in [Ca2+]i in PC12 cells similar to that observed with nicotine. There was an immediate transient increase in [Ca2+]i and a subsequent sustained second elevation. In contrast to nicotine, the epibatidine-triggered increase in [Ca2+]i was independent of activation of alpha7 nicotinic acetylcholine receptors, as it was not altered by either methyllycaconitine or alpha-bungarotoxin. The second [Ca2+]i elevation involves calcium release from intracellular stores and is inhibited by dantrolene or xestospongin C. Epibatidine, like nicotine, elevated TH promoter driven reporter transcription, mostly mediated by the cyclic-AMP responsive motifs. Elevation in TH promoter activity requires Ca2+ and cAMP since it is inhibited by 1,2-bis(o-Aminophenoxy)ethane-N,N,N',N'-tetraacetic Acid Tetra (acetoxymethyl ester) (BAPTA-AM) or 2',5'-dideoxyadenosine (DDA). The results reveal that epibatidine can elevate [Ca2+]i in an alpha7 independent manner and nevertheless induce TH transcription.

Expression of alpha 4 and alpha 7 nicotinic receptors in the brainstem of female rabbits after coitus

Coital signaling in the female rabbit involves sequential events in the brainstem and hypothalamus, resulting in a massive release of hypothalamic gonadotropin-releasing hormone (GnRH) that peaks within 1-2 h after mating. The neural connections between coitus and GnRH release involves norepinephrine (NE) and acetylcholine (ACh) since administration of antagonists against NE (dibenamine or phentolamine) or ACh (atropine, alpha-bungarotoxin (alpha-BTX) or scopolamine) blocks or attenuates ovulating events. Moreover, hypothalamic NE release and brainstem tyrosine hydroxylase (TH, the rate-limiting enzyme for NE synthesis) expression in the noradrenergic areas increase prior to, or in concert with, the preovulatory GnRH surge. How ACh is involved in the control of ovulation in the rabbit is lesser known. In the present study, the number of brainstem neurons expressing TH, alpha4 and alpha7 subunits of the nicotinic ACh receptor (nAChR) before and after coitus was determined by immunocytochemistry. Compared to non-mated female rabbits, the number of alpha4, alpha7 and TH single-labeled neurons as well as alpha4/TH and alpha7/TH double-labeled neurons increased in the A1, A2 and A6 brainstem noradrenergic areas at 1 h, but not 2 h, after coitus. The results suggest that the participation of ACh in the control of coitus-induced ovulation may include activation of alpha4beta2 and alpha7 nAChRs in neurons within or adjacent to the brainstem noradrenergic areas in female rabbits.