TC-2559 excites dopaminergic neurones in the ventral tegmental area by stimulating alpha4beta2-like nicotinic acetylcholine receptors in anaesthetised rats

Influence of age on nicotinic cholinergic regulation of blood flow in rat's olfactory bulb and neocortex

The olfactory bulb receives cholinergic basal forebrain inputs as does the neocortex. With a focus on nicotinic acetylcholine receptors (nAChRs), this review article provides an overview and discussion of the following findings: (1) the nAChRs-mediated regulation of regional blood flow in the neocortex and olfactory bulb, (2) the nAChR subtypes that mediate their responses, and (3) their activity in old rats. The activation of the α4β2-like subtype of nAChRs produces vasodilation in the neocortex, and potentiates olfactory bulb vasodilation induced by olfactory stimulation. The nAChR activity producing neocortical vasodilation was similarly maintained in 2-year-old rats as in adult rats, but was clearly reduced in 3-year-old rats. In contrast, nAChR activity in the olfactory bulb was reduced already in 2-year-old rats. Thus, age-related impairment of α4β2-like nAChR function may occur earlier in the olfactory bulb than in the neocortex. Given the findings, the vasodilation induced by α4β2-like nAChR activation may be beneficial for neuroprotection in the neocortex and the olfactory bulb.

Altered expression of DENND5B in patients with epilepsy and its regulation of seizures in mice

Epilepsy is a high incidence neurological disease, and its repeated attacks cause serious physical and psychological damage to the patient. Differentially expressed in normal and neoplastic cells (DENN) domain containing 5B (DENND5B) is a lipoprotein binding protein that mediates synaptic vesicle transport and regulates neuroplasticity and lipid metabolism. Nevertheless, the effect of DENND5B on seizures remains unclear. We aimed to investigate the association of DENND5B with epilepsy, detect its expression and distribution in the nervous system, and explore its role in epileptogenesis through western blot, immunofluorescence staining, and behavioral studies. In this experiment, two C57BL/6 mice models, which induced seizures by pentylenetetrazole and kainic acid, were established. We observed that the expression of DENND5B was reduced in the brains of patients with temporal lobe epilepsy, and its expression was also similarly decreased in both chronic epileptic mice. The findings strongly suggest that DENND5B may be associated with epileptic seizures. Results of immunofluorescence showed that DENND5B was mainly expressed in the hippocampal region and co-located with neurons but not with astrocytes. Next, we used lentivirus to induce both lentiviral vector-mediated overexpression and knockdown of DENND5B in mice to test the change of susceptibility and severity of seizures in the two chronic seizure models. Knockdown of DENND5B was found to promote epileptic seizures, increase chronic spontaneous recurrent epileptic seizures and epileptic discharge, and reduce the incubation period. However, overexpression of DENND5B showed the opposite effect. These results suggest that DENND5B overexpression decreased the behavioral phenotype of epileptic seizures, but DENND5B downregulation had the opposite effect. In summary, our findings suggest that DENND5B can regulate epileptic seizures and may provide a new target for antiepileptic therapy.

Neural circuits and nicotinic acetylcholine receptors mediate the cholinergic regulation of midbrain dopaminergic neurons and nicotine dependence

Midbrain dopaminergic (DA) neurons are governed by an endogenous cholinergic system, originated in the mesopontine nuclei. Nicotine hijacks nicotinic acetylcholine receptors (nAChRs) and interferes with physiological function of the cholinergic system. In this review, we describe the anatomical organization of the cholinergic system and the key nAChR subtypes mediating cholinergic regulation of DA transmission and nicotine reward and dependence, in an effort to identify potential targets for smoking intervention. Cholinergic modulation of midbrain DA systems relies on topographic organization of mesopontine cholinergic projections, and activation of nAChRs in midbrain DA neurons. Previous studies have revealed that α4, α6, and β2 subunit-containing nAChRs expressed in midbrain DA neurons and their terminals in the striatum regulate firings of midbrain DA neurons and activity-dependent dopamine release in the striatum. These nAChRs undergo modification upon chronic nicotine exposure. Clinical investigation has demonstrated that partial agonists of these receptors elevate the success rate of smoking cessation relative to placebo. However, further investigations are required to refine the drug targets to mitigate unpleasant side-effects.

Antinociceptive effects of novel epibatidine analogs through activation of α4β2 nicotinic receptors

The study of α4β2 nicotinic receptors has provided new indications in the treatment of pain. Efforts have been made to explore new α4β2 nicotinic receptor agonists, including TC-2559, as antinociceptive drugs. In this study, we discovered a set of novel epibatidine analogs with strong binding affinities to the α4β2 nicotinic receptors. Among these compounds, C-159, C-163, and C-9515 attenuated formalin-induced nociceptive responses in mice; C-9515 caused the most potent analgesic effect, which was blocked by mecamylamine, a non-selective nicotinic receptor antagonist. Furthermore, C-9515 potently inhibited chronic constriction injury (CCI)-induced neuropathic pain in rats, which was sensitive to DHβE, a selective α4β2 subtype antagonist, indicating that its analgesic effect was mediated by the activation of the α4β2 nicotinic receptors. In conclusion, the epibatidine analog C-9515 was found to be a potent α4β2 nicotinic receptor agonist with potent analgesic function, which demonstrated potential for the further exploration of its druggability.

Understanding the role α7 nicotinic receptors play in dopamine efflux in nucleus accumbens

Neuronal nicotinic acetylcholine receptors (NNRs) of the α7 subtype have been shown to contribute to the release of dopamine in the nucleus accumbens. The site of action and the underlying mechanism, however, are unclear. Here we applied a circuit modeling approach, supported by electrochemical in vivo recordings, to clarify this issue. Modeling revealed two potential mechanisms for the drop in accumbal dopamine efflux evoked by the selective α7 partial agonist TC-7020. TC-7020 could desensitize α7 NNRs located predominantly on dopamine neurons or glutamatergic afferents to them or, alternatively, activate α7 NNRs located on the glutamatergic afferents to GABAergic interneurons in the ventral tegmental area. Only the model based on desensitization, however, was able to explain the neutralizing effect of coapplied PNU-120596, a positive allosteric modulator. According to our results, the most likely sites of action are the preterminal α7 NNRs controlling glutamate release from cortical afferents to the nucleus accumbens. These findings offer a rationale for the further investigation of α7 NNR agonists as therapy for diseases associated with enhanced mesolimbic dopaminergic tone, such as schizophrenia and addiction.

α6β2* and α4β2* nicotinic acetylcholine receptors as drug targets for Parkinson's disease

Parkinson's disease is a debilitating movement disorder characterized by a generalized dysfunction of the nervous system, with a particularly prominent decline in the nigrostriatal dopaminergic pathway. Although there is currently no cure, drugs targeting the dopaminergic system provide major symptomatic relief. As well, agents directed to other neurotransmitter systems are of therapeutic benefit. Such drugs may act by directly improving functional deficits in these other systems, or they may restore aberrant motor activity that arises as a result of a dopaminergic imbalance. Recent research attention has focused on a role for drugs targeting the nicotinic cholinergic systems. The rationale for such work stems from basic research findings that there is an extensive overlap in the organization and function of the nicotinic cholinergic and dopaminergic systems in the basal ganglia. In addition, nicotinic acetylcholine receptor (nAChR) drugs could have clinical potential for Parkinson's disease. Evidence for this proposition stems from studies with experimental animal models showing that nicotine protects against neurotoxin-induced nigrostriatal damage and improves motor complications associated with l-DOPA, the "gold standard" for Parkinson's disease treatment. Nicotine interacts with multiple central nervous system receptors to generate therapeutic responses but also produces side effects. It is important therefore to identify the nAChR subtypes most beneficial for treating Parkinson's disease. Here we review nAChRs with particular emphasis on the subtypes that contribute to basal ganglia function. Accumulating evidence suggests that drugs targeting α6β2* and α4β2* nAChR may prove useful in the management of Parkinson's disease.

Partial agonists for α4β2 nicotinic receptors stimulate dopaminergic neuron firing with relatively enhanced maximal effects

BACKGROUND AND PURPOSE

Partial agonists selective for α4β2 nicotinic ACh receptors have been developed for smoking cessation as they induce weak activation of native α4β2* receptors and inhibit effect of nicotine. However, it is unclear whether at brain functions there is an existence of receptor reserve that allows weak receptor activation to induce maximum physiological effects. We assessed the extent of α4β2 partial agonist-induced increase of firing rate in dopaminergic neurons and evaluated the influence of receptor reserve.

EXPERIMENTAL APPROACH

The relative maximal effects and potencies of six nicotinic agonists were assessed on recombinant human α4β2 and α7 receptors expressed in mammalian cell lines by measuring calcium influx. Agonist-induced increase of the spontaneous firing rate of dopaminergic neurons was recorded using microelectrodes in the ventral tegmental area of rat brain slices.

KEY RESULTS

All α4β2 partial and full agonists increased the firing rate concentration-dependently. Their sensitivity to subtype-selective antagonists showed predominant activation of native α4β2* receptors. However, partial agonists with relative maximal effects as low as 33% on α4β2 receptors maximally increased the firing rate and induced additional depolarization block of firing, demonstrating that partial activation of receptors caused the maximum increase in firing rate in the presence of a receptor reserve.

CONCLUSIONS AND IMPLICATIONS

Partial α4β2 agonists induced relatively enhanced effects on the firing rate of dopaminergic neurons, and the effect was mainly attributed to the existence of native α4β2* receptor reserve. The results have implications in the understanding of physiological effects and therapeutic efficacies of α4β2 partial agonists.

Selective nicotinic receptor antagonists: effects on attention and nicotine-induced attentional enhancement

RATIONALE

The question of the subtype(s) of the nicotinic acetylcholine receptor (nAChR) mediating the attention-enhancing effects of nicotine is still unsettled. While early studies pointed towards subtypes other than the homomeric α7 nAChR, pro-cognitive effects of α7 nAChR agonists have since been demonstrated.

OBJECTIVES

This study tested whether the performance-enhancing effects of nicotine in a rodent model of attention could be reversed by the α4β2, α4β4, α3β2, and α2β2 nAChR antagonist dihydro-β-erythroidine (DHβE), or the α7 antagonist methyllycaconitine (MLA).

METHODS

In repeated tests, 12 rats trained to perform the 5-choice serial reaction time task were systemically injected with nicotine or vehicle in the presence of increasing doses of DHβE or MLA.

RESULTS

DHβE did not antagonize the attention-enhancing effects of nicotine reflected by measures of accuracy and omission errors, suggesting that its previously reported antagonism of nicotine effects on latency and anticipatory responses specifically reflected the stimulant effects of nicotine. MLA dose-dependently reversed the reduction in omission errors by nicotine. In the absence of nicotine, low doses of MLA (0.4 and 1.3 mg/kg) not previously tested on attention improved response accuracy, resulting in an inverted U-shape dose-response function.

CONCLUSIONS

nAChR subtypes involved in the performance-enhancing effects of nicotine appear to vary depending on the function assessed. Our findings suggest a greater involvement of α7 nAChRs in the effects of nicotine on attention than first suggested by preclinical studies, with different optimal receptor tones for aspects of stimulus detection and response readiness to task stimuli.

Enhanced inhibitory synaptic transmission in the spinal dorsal horn mediates antinociceptive effects of TC-2559

Background

TC-2559 is a selective α4β2 subtype of nicotinic acetylcholine receptor (nAChR) partial agonist and α4β2 nAChR activation has been related to antinociception. The aim of this study is to investigate the analgesic effect of TC-2559 and its underlying spinal mechanisms.

Results

1) In vivo bioavailability study: TC-2559 (3 mg/kg) had high absorption rate in rats with maximal total brain concentration reached over 4.6 μM within first 15 min after administration and eliminated rapidly with brain half life of about 20 min after injection. 2) In vivo behavioral experiments: TC-2559 exerts dose dependent antinociceptive effects in both formalin test in mice and chronic constriction injury (CCI) model in rats by activation of α4β2 nAChRs; 3) Whole-cell patch-clamp studies in the superficial dorsal horn neurons of the spinal cord slices: perfusion of TC-2559 (2 μM) significantly increased the frequency, but not amplitude of spontaneous inhibitory postsynaptic currents (sIPSCs). The enhancement of sIPSCs was blocked by pre-application of DHβE (2 μM), a selective α4β2 nicotinic receptor antagonist. Neither the frequency nor the amplitude of spontaneous excitatory postsynaptic currents (sEPSCs) of spinal dorsal horn neurons were affected by TC-2559.

Conclusions

Enhancement of inhibitory synaptic transmission in the spinal dorsal horn via activation of α4β2 nAChRs may be one of the mechanisms of the antinociceptive effects of TC-2559 on pathological pain models. It provides further evidence to support the notion that selective α4β2 subtype nAChR agonist may be developed as new analgesic drug for the treatment of neuropathic pain.

Activation of presynaptic alpha7 nicotinic receptors evokes an excitatory response in hippocampal CA3 neurones in anaesthetized rats: an in vivo iontophoretic study

BACKGROUND AND PURPOSE

alpha7 Nicotinic receptors have been suggested to play an important role in hippocampal learning and memory. However, the direct action of this receptor subtype on hippocampal pyramidal neurones in vivo has not yet been fully investigated. The availability of selective agonists for alpha7 receptors [AR-R17779 and (R)-(-)-5'-phenylspiro[1-azabicyclo[2.2.2] octane-3,2'-(3'H)furo[2,3-b]pyridine (PSAB-OFP)] has now allowed this role to be investigated.

EXPERIMENTAL APPROACH

Single-cell extracellular recordings were made from hippocampal CA3 pyramidal neurones in anaesthetized rats. The effects of nicotine, AR-R17779 and PSAB-OFP, applied either systemically or iontophoretically, were studied on the activity of these neurones.

KEY RESULTS

Intravenous injection of cumulative doses of nicotine and PSAB-OFP induced dose-related, significant increases in neuronal firing in the majority of neurones tested. This excitation could be inhibited by intravenous administration of methyllycaconitine (MLA), a selective alpha7 nicotinic receptor antagonist. Furthermore, iontophoretic application of nicotine, AR-R17779 and PSAB-OFP each evoked current-dependent excitation of most CA3 pyramidal neurones studied, and this excitation was antagonized by co-iontophoretic application of MLA. In addition, the excitation induced by iontophoretic application of nicotine, AR-R17779 or PSAB-OFP was also blocked by co-iontophoretic application of either 6,7-dinitroquinoxaline-2,3-dione (DNQX) or D(2)-2-amino-5-phosphonopentanoate (D-AP5), selective N-methyl-D-aspartic acid (NMDA) and non-NMDA receptor antagonists respectively.

CONCLUSIONS AND IMPLICATIONS

CA3 pyramidal neurones are modulated by activation of presynaptic alpha7 nicotinic receptors, which, at least in part, enhances glutamate release onto post-synaptic (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazole proprionic acid and NMDA receptors on these CA3 neurones. This mechanism probably contributes to the effects of nicotine on hippocampal learning and memory.

BDNF-TrkB signaling pathway mediates the induction of epileptiform activity induced by a convulsant drug cyclothiazide

Brain-derived neurotrophic factor (BDNF) and its receptor TrkB play an important function in neuronal development and synaptic plasticity. Recently we have established that cyclothiazide (CTZ) is a novel convulsant drug inducing robust epileptiform activity in hippocampal neurons both in vitro and in vivo. However, the molecular mechanisms underlying such convulsant action of CTZ are unknown. Here, we investigated potential roles of BDNF-TrkB signaling pathway in the CTZ-induction of epileptiform activity. In anaesthetized rats, CTZ dose-dependently induced epileptiform activity characterized by progressing of multiple peaks of population spikes, spontaneous spiking events, and synchronized epileptiform bursts. Pre-injection of a receptor tyrosine kinase inhibitor K252a or a specific antibody for TrkB receptors before intracerebroventricular injection of CTZ significantly suppressed the epileptiform activity induced by CTZ. Similarly, in cultured hippocampal pyramidal neurons, pre-treatment with CTZ together with K252a or TrkB-receptor antibody also inhibited the CTZ-induction of epileptiform activity. Furthermore, we demonstrated that acute application of K252a in hippocampal cultures inhibited epileptiform bursts and action potential firing. We conclude that activation of BDNF-TrkB signaling pathway is fundamentally important during the CTZ-induction of epileptiform activity both in vitro and in vivo.

Partial nicotinic acetylcholine (alpha4beta2) agonists as promising new medications for smoking cessation

OBJECTIVE

To review the pharmacology, clinical efficacy and safety of partial agonists of alpha4beta2 nicotinic acetylcholine receptor.

DATA SOURCES

Primary literature and review articles were obtained via a PUBMED search (1988-August 2006) using the key terms smoking cessation, partial agonist alpha4beta2 nicotinic acetylcholine receptor, varenicline, cytisine and SSR591813. Additional studies and abstracts were identified from the bibliographies of reviewed literature.

STUDY SELECTION AND DATA EXTRACTION

Studies and review articles related to varenicline, cytisine and the partial agonist alpha4beta2 nicotinic acetylcholine receptor were reviewed.

DATA SYNTHESIS

Smoking is widely recognized as a serious health problem. Smoking cessation has major health benefits. According to the US Public Health Services, all patients attempting to quit smoking should be encouraged to use one or more effective pharmacotherapy. Currently, along with nicotine replacement therapy, bupropion, nortriptyline and clonidine, are the mainstay of pharmacotherapy. More than (3/4) of patients receiving treatment for smoking cessation return to smoking within the first year. Nicotine, through stimulating alpha4beta2 nAChR, releases dopamine in the reward pathway. Partial agonist of alpha4beta2 nAChR elicits moderate and sustained release of dopamine, which is countered during the cessation attempts; it simultaneously blocks the effects of nicotine by binding with alpha4beta2 receptors during smoking. Recently, varenicline, a partial agonist at alpha4beta2 nAChR, has been approved by the FDA (Food and Drug Administration) for smoking cessation.

CONCLUSION

Partial agonist alpha4beta2 nAChR appears to be a promising target in smoking cessation. Varenicline of this group is approved for treatment of smoking cessation by the FDA in May 2006.

Cardiovascular effects of activation of central alpha7 and alpha4beta2 nAChRs: a role for vasopressin in anaesthetized rats

BACKGROUND AND PURPOSE

Central application of nicotine causes the release of vasopressin and affects blood pressure. Involvement of the 5 neuronal nicotinic receptor groups, alpha2(*)-alpha7(*) in these effects is unknown. The availability of selective agonists for alpha7 (PSAB-OFP) and alpha4beta2 (TC-2559) nACh receptors allowed their role to be investigated.

EXPERIMENTAL APPROACH

Recordings were made of arterial blood pressure, heart rate and renal sympathetic nerve activity in anaesthetized male rats with neuromuscular blockade and artificial respiration. Effects of the agonists, PSAB-OFP (1-10 micromol kg(-1)) and TC-2559 (1-10 micromol kg(-1)) on these variables given intracerebroventricularly (i.c.v.) and intracisternally (i.c.) in the presence or absence of the antagonists, DhbetaE (10 micromol kg(-1)) and MLA (0.5 micromol kg(-1)), for the appropriate nicotinic receptor subtypes, respectively, and a V(1) receptor antagonist, given i.v. or centrally, were investigated.

KEY RESULTS

Both agonists given i.c.v. caused a delayed rise in blood pressure and renal nerve activity which could be blocked only with the appropriate antagonist. The agonists had an earlier onset of action when given i.c., favouring the brainstem as the major site of action. The effects of these agonists were also attenuated by the V(1) receptor antagonist given i.v. and blocked when this antagonist was given centrally. Antagonists had no effect on baseline variables.

CONCLUSIONS AND IMPLICATIONS

Activation of alpha4beta2 and alpha7 receptors in the brainstem is mainly responsible for the cardiovascular effects of activating these receptors, which have a similar profile of action. These actions, although independent, are mediated by the central release of vasopressin.

Activation of rat ventral tegmental area dopamine neurons by endogenous kynurenic acid: a pharmacological analysis

Kynurenic acid (KYNA) is an endogenous NMDA receptor antagonist as well as a blocker of the alpha7* nicotinic receptor and mounting evidence suggests that the compound participates in the pathophysiology of schizophrenia. Previous studies have shown that elevated levels of endogenous KYNA are associated with an increased firing of midbrain dopamine (DA) neurons. In the present study, utilizing extracellular single unit cell recording techniques, the mechanism involved in this excitatory action of the compound was analyzed in male Sprague-Dawley rats. Administration of 4-chlorokynurenine (4-Cl-KYN; 25mg/kg, i.p.), which is converted to the selective NMDA glycine-site antagonist 7-chloro-kynurenic acid (7-Cl-KYNA), was found to increase firing rate and per cent burst firing activity of ventral tegmental area (VTA) DA neurons to the same magnitude as pretreatment of kynurenine (causing a 25-fold elevation in extracellular brain KYNA). Intravenous administration of the selective antagonist at the alpha7* nicotinic receptor methyllycaconitine (MLA; 1-4mg/kg) did not affect firing of VTA DA neurons, whereas intraperitoneal administration of this drug in a high dose (6mg/kg) was associated with a decreased firing rate and per cent burst firing activity. Administration of SDZ 220-581 (10mg/kg, i.v.), a competitive antagonist at the glutamate recognition-site of the NMDA receptor, was found to increase firing rate and per cent burst firing. Present results have potential implications for the treatment of schizophrenia, and indicate that the increased activity of VTA DA neurons following elevation of brain KYNA is mediated through glutamatergic rather than by nicotinergic mechanisms.

Central Nicotinic Receptors: Structure, Function, Ligands, and Therapeutic Potential

The growing interest in nicotinic receptors, because of their wide expression in neuronal and non-neuronal tissues and their involvement in several important CNS pathologies, has stimulated the synthesis of a high number of ligands able to modulate their function. These membrane proteins appear to be highly heterogeneous, and still only incomplete information is available on their structure, subunit composition, and stoichiometry. This is due to the lack of selective ligands to study the role of nAChR under physiological or pathological conditions; so far, only compounds showing selectivity between alpha4beta2 and alpha7 receptors have been obtained. The nicotinic receptor ligands have been designed starting from lead compounds from natural sources such as nicotine, cytisine, or epibatidine, and, more recently, through the high-throughput screening of chemical libraries. This review focuses on the structure of the new agonists, antagonists, and allosteric ligands of nicotinic receptors, it highlights the current knowledge on the binding site models as a molecular modeling approach to design new compounds, and it discusses the nAChR modulators which have entered clinical trials.

The alpha4beta2 nicotinic acetylcholine receptor agonist TC-2559 impairs long-term potentiation in the dentate gyrus in vivo

Nicotinic acetylcholine receptors (nAChR) are widely expressed throughout the nervous system, are involved in some fast excitatory neurotransmission, and play an important role in modulating the release of several neurotransmitters, including the major excitatory and inhibitory neurotransmitters, glutamate and GABA. We used a recently characterised alpha4beta2 nAChR subunit selective partial agonist, TC-2559, to study the effect of alpha4beta2 nAChR activation on synaptic plasticity in the medio-dorsal perforant pathway input to the dentate gyrus, in the intact nervous system in vivo. We show for the first time, that the selective activation of alpha4beta2 containing nAChR can reduce the level of long-term potentiation (LTP) induced by high frequency stimulation, an effect that was reversed by the selective antagonist, dihydro-beta-erythroidine (DbetaHE). This modulator role of nAChRs is in contrast to previous findings that used broad spectrum agonists, highlighting the complex actions of nicotine.

The Nemertine Toxin Anabaseine and Its Derivative DMXBA (GTS-21): Chemical and Pharmacological Properties

Nemertines are a phylum of carnivorous marine worms that possess a variety of alkaloidal, peptidic or proteinaceous toxins that serve as chemical defenses against potential predators. The hoplonemertines additionally envenomate their prey with a mixture of proboscis alkaloids delivered with the help of a calcareous stylet that punctures the skin of the victim. Anabaseine, the first of these alkaloids to be identified, stimulates a wide variety of animal nicotinic acetylcholine receptors (AChRs), especially the neuromuscular [e.g., α1₂β1γδ (embryogenic) or α1₂β1γɛ (adult)] and α7 AChRs that are inhibited by the snake peptide α-bungarotoxin. A synthetic derivative, 3-(2,4-Dimethoxybenzylidene)-Anabaseine (DMXBA; also called GTS-21), improves memory in experimental animals and humans and is currently in clinical trials to determine whether it can ameliorate cognitive problems associated with schizophrenia. Here we summarize present knowledge concerning the chemistry and mechanisms of action of these two substances (anabaseine and DMXBA) on AChRs, especially those found in the mammalian brain.