Alpha-bungarotoxin binding and central nervous system nicotinic acetylcholine receptors

Incorporation of decellularized-ECM in graphene-based scaffolds enhances axonal outgrowth and branching in neuro-muscular co-cultures

Peripheral nerve and large-scale muscle injuries result in significant disability, necessitating the development of biomaterials that can restore functional deficits by promoting tissue regrowth in an electroactive environment. Among these materials, graphene is favored for its high conductivity, but its low bioactivity requires enhancement through biomimetic components. In this study, we extrusion printed graphene-poly(lactide-co-glycolide) (graphene) lattice scaffolds, aiming to increase bioactivity by incorporating decellularized extracellular matrix (dECM) derived from mouse pup skeletal muscle. We first evaluated these scaffolds using human-induced pluripotent stem cell (hiPSC)-derived motor neurons co-cultured with supportive glia, observing significant improvements in axon outgrowth. Next, we tested the scaffolds with C2C12 mouse and human primary myoblasts, finding no significant differences in myotube formation between dECM-graphene and graphene scaffolds. Finally, using a more complex hiPSC-derived 3D motor neuron spheroid model co-cultured with human myoblasts, we demonstrated that dECM-graphene scaffolds significantly improved axonal expansion towards peripheral myoblasts and increased axonal network density compared to graphene-only scaffolds. Features of early neuromuscular junction formation were identified near neuromuscular interfaces in both scaffold types. These findings suggest that dECM-graphene scaffolds are promising candidates for enhancing neuromuscular regeneration, offering robust support for the growth and development of diverse neuromuscular tissues.

Nicotinic acetylcholine receptors in neurological and psychiatric diseases

Neuronal nicotinic acetylcholine receptors (nAChRs) are ligand-gated ion channels that are widely distributed both pre- and post-synaptically in the mammalian brain. By modulating cation flux across cell membranes, neuronal nAChRs regulate neuronal excitability and the release of a variety of neurotransmitters to influence multiple physiologic and behavioral processes including synaptic plasticity, motor function, attention, learning and memory. Abnormalities of neuronal nAChRs have been implicated in the pathophysiology of neurologic disorders including Alzheimer's disease, Parkinson's disease, epilepsy, and Tourette´s syndrome, as well as psychiatric disorders including schizophrenia, depression, and anxiety. The potential role of nAChRs in a particular illness may be indicated by alterations in the expression of nAChRs in relevant brain regions, genetic variability in the genes encoding for nAChR subunit proteins, and/or clinical or preclinical observations where specific ligands showed a therapeutic effect. Over the past 25 years, extensive preclinical and some early clinical evidence suggested that ligands at nAChRs might have therapeutic potential for neurologic and psychiatric disorders. However, to date the only approved indications for nAChR ligands are smoking cessation and the treatment of dry eye disease. It has been argued that progress in nAChR drug discovery has been limited by translational gaps between the preclinical models and the human disease as well as unresolved questions regarding the pharmacological goal (i.e., agonism, antagonism or receptor desensitization) depending on the disease.

Therapeutic Targeting of α7 Nicotinic Acetylcholine Receptors

The α7-type nicotinic acetylcholine receptor is one of the most unique and interesting of all the members of the cys-loop superfamily of ligand-gated ion channels. Since it was first identified initially as a binding site for α-bungarotoxin in mammalian brain and later as a functional homomeric receptor with relatively high calcium permeability, it has been pursued as a potential therapeutic target for numerous indications, from Alzheimer disease to asthma. In this review, we discuss the history and state of the art for targeting α7 receptors, beginning with subtype-selective agonists and the basic pharmacophore for the selective activation of α7 receptors. A key feature of α7 receptors is their rapid desensitization by standard "orthosteric" agonist, and we discuss insights into the conformational landscape of α7 receptors that has been gained by the development of ligands binding to allosteric sites. Some of these sites are targeted by positive allosteric modulators that have a wide range of effects on the activation profile of the receptors. Other sites are targeted by direct allosteric agonist or antagonists. We include a perspective on the potential importance of α7 receptors for metabotropic as well as ionotropic signaling. We outline the challenges that exist for future development of drugs to target this important receptor and approaches that may be considered to address those challenges. SIGNIFICANCE STATEMENT: The α7-type nicotinic acetylcholine receptor (nAChR) is acknowledged as a potentially important therapeutic target with functional properties associated with both ionotropic and metabotropic signaling. The functional properties of α7 nAChR can be regulated in diverse ways with the variety of orthosteric and allosteric ligands described in this review.

Cholinergic activity is essential for maintaining the anterograde transport of Choline Acetyltransferase in Drosophila

Cholinergic activity is essential for cognitive functions and neuronal homeostasis. Choline Acetyltransferase (ChAT), a soluble protein that synthesizes acetylcholine at the presynaptic compartment, is transported in bulk in the axons by the heterotrimeric Kinesin-2 motor. Axonal transport of soluble proteins is described as a constitutive process assisted by occasional, non-specific interactions with moving vesicles and motor proteins. Here, we report that an increase in the influx of Kinesin-2 motor and association between ChAT and the motor during a specific developmental period enhances the axonal entry, as well as the anterograde flow of the protein, in the sensory neurons of intact Drosophila nervous system. Loss of cholinergic activity due to  Hemicholinium and Bungarotoxin treatments, respectively, disrupts the interaction between ChAT and Kinesin-2 in the axon, and the episodic enhancement of axonal influx of the protein. Altogether, these observations highlight a phenomenon of synaptic activity-dependent, feedback regulation of a soluble protein transport in vivo, which could potentially define the quantum of its pre-synaptic influx.

Recent advances in gene manipulation and nicotinic acetylcholine receptor biology

Pharmacological and immunological methods have been valuable for both identifying some native nicotinic acetylcholine receptor (nAChR) subtypes that exist in vivo and determining the neurobiological and behavioral role of certain nAChR subtypes. However, these approaches suffer from shortage of subtype specific ligands and reliable immunological reagents. Consequently, genetic approaches have been developed to complement earlier approaches to identify native nAChR subtypes and to assess the contribution of nAChRs to brain function and behavior. In this review we describe how assembly partners, knock-in mice and targeted lentiviral re-expression of genes have been utilized to improve our understanding of nAChR neurobiology. In addition, we summarize emerging genetic tools in nAChR research.

The road to discovery of neuronal nicotinic cholinergic receptor subtypes

The discovery that mammalian brain expresses the mRNAs for nine different nicotinic cholinergic receptor subunits (alpha2-alpha7, beta2-beta4) that form functional receptors when expressed in Xenopus laevis oocytes suggests that many different types of nicotinic cholinergic receptors (nAChRs) might be expressed in the mammalian brain., Using an historical approach, this chapter reviews some of the progress made in identifying the nAChR subtypes that seem to play a vital role in modulating dopaminergic function. nAChR subtypes that are expressed in dopamine neurons, as well as neurons that interact with dopamine neurons (glutamatergic, GABAergic), serve as the focus of this review. Subjects that are highlighted include the discovery of a low affinity alpha4beta2* nAChR, the identity of recently characterized alpha6* nAChRs, and the finding that these alpha6* receptors have the highest affinity for receptor activation of any of the native receptors that have been characterized to date. Topics that have been ignored in other recent reviews of this area, such as the discovery and potential importance of alternative transcripts, are presented along with a discussion of their potential importance.

Effects of nicotine on cerebral metabolism

Interest in identifying brain areas mediating the behavioural effects of nicotine led to autoradiographic studies on the distribution of cerebral metabolic responses to nicotine. The 2-deoxy-D-[1-14C] glucose method was used to map and quantitate nicotine's effects in the rat brain. The method allows simultaneous measurement of the regional cerebral metabolic rate(s) for glucose (rCMRglc), an index of functional activity, throughout the central nervous system. It provides information about sites of initial drug interactions, and of secondary effects propagated via afferents to remote areas. In rats given acute systemic (-)nicotine, stimulation occurs in brain areas which contain specific binding sites for [3H] nicotine, indicating that the sites are true receptors, linked to functional activity. Doses of nicotine that are discriminated by rats and that produce behavioural and physiological effects stimulate rCMRglc. The stimulation is transient and is antagonized by mecamylamine. Affected areas include limbic structures, components of the visual system, brainstem nuclei important in cardiovascular reflexes, and areas involved in motor function. The distribution of nicotine's in vivo effects on rCMRglc implicates various brain regions in the behavioural and physiological effects of nicotine. Future studies employing positron emission tomography will assess relations between nicotine's effects on mood and rCMRglc in man.

Involvement of alpha7 nicotinic acetylcholine receptors in activation of tyrosine hydroxylase and dopamine beta-hydroxylase gene expression in PC12 cells

Nicotine treatment increases intracellular free Ca(2+) concentration [Ca(2+)](i), stimulates catecholamine release, and elevates gene expression for the catecholamine biosynthetic enzymes tyrosine hydroxylase (TH) and dopamine beta-hydroxylase (DBH). However, the type of nicotinic acetylcholine receptors (nAChRs) mediating these events is unclear. The nAChR receptor antagonists alpha-bungarotoxin (alphaBTX) and methyllycaconitine greatly reduced the nicotine-triggered initial transient rise in [Ca(2+)](i) and prevented the second prolonged elevation of [Ca(2+)](i), suggesting the involvement of alpha7 nAChRs. Two specific alpha7 nicotinic agonists, 3-(2,4-dimethoxybenzilidene)anabaseine (DMXB) and E, E-3-(cinnamylidene)anabaseine (3-CA), were found to elicit a small, delayed increase in [Ca(2+)](i) with kinetics and magnitude similar to the second elevation observed with nicotine. This increase was inhibited by the inositol trisphosphate receptor antagonist xestospongin C. Exposure to 3-CA or DMXB for 6 or 24 h elevated TH and DBH mRNA levels two- to fourfold over control levels. These agonists were more effective than nicotine alone in increasing TH and DBH gene expression and significantly elevated [Ca(2+)](i) for up to 6 h. The increase in [Ca(2+)](i) or the elevation in TH mRNA by 3-CA was completely inhibited by alphaBTX. This study, for the first time, implicates stimulation of alpha7 nAChRs in the activation of TH and DBH gene expression.

Postnatal developmental regulation of neuronal nicotinic receptor subunit alpha 7 and multiple alpha 4 and beta 2 mRNA species in the rat

This study examined the postnatal development of neuronal nicotinic receptor (nAChR) alpha 7, alpha 4 and beta 2 subunit mRNA in the Sprague Dawley rat brain. The hippocampus, septum and cortex were removed on postnatal day 1 (P1), P7, P14, or P28 and analyzed by sex. Northern analysis of cortical and pooled hippocampal and septal total RNA with 32P-alpha-dCTP-labeled alpha 7, alpha 4 (recognizing alpha 4.1 and alpha 4.2 mRNA), and beta 2 nAChR cDNA probes identified three (2.4, 3.8 and 8.0 kb) alpha 4, four (3.5, 5.0, 7.5 and 10.0 kb) beta 2 and a single 5.7 kb alpha 7 mRNA species. Cortical alpha 4 mRNA peaked on P14 and remained high on P28, whereas hippocampal/septal alpha 4 mRNA was higher on P7 and P14 than on P1 and P28. Expression of cortical and hippocampal/septal beta 2 mRNAs decreased on P7, followed by a dramatic peak on P14. alpha 7 mRNA peaked on P7. Throughout development, 2.4 kb alpha 4 mRNA was more intense than 3.8 kb alpha 4 mRNA, whereas 5.0 kb beta 2 mRNA was the most intense cortical and hippocampal/septal beta 2 mRNA species. The alpha 4.1-specific cDNA probe detected similar-sized alpha 4 bands as the pan-specific alpha 4 cDNA probe, therefore precluding the identification of any band as alpha 4.2-specific. These results suggest that postnatal expression of alpha 4 and alpha 7 but not beta 2 mRNAs is brain region-specific, and that the contribution of multiple nAChR subunit mRNA species in development may vary.

Acetylcholine activates an alpha-bungarotoxin-sensitive nicotinic current in rat hippocampal interneurons, but not pyramidal cells

The effects of acetylcholine on both pyramidal neurons and interneurons in the area CA1 of the rat hippocampus were examined, using intracellular recording techniques in an in vitro slice preparation. In current-clamp mode, fast local application of acetylcholine (ACh) to the soma of inhibitory interneurons in stratum radiatum resulted in depolarization and rapid firing of action potentials. Under voltage-clamp, ACh produced fast, rapidly desensitizing inward currents that were insensitive to atropine but that were blocked by nanomolar concentrations of the nicotinic alpha7 receptor-selective antagonists alpha-bungarotoxin (alphaBgTx) and methyllycaconitine. Nicotinic receptor antagonists that are not selective for alpha7-containing receptors had little (mecamylamine) or no effect (dihydro-beta-erythroidine) on the ACh-induced currents. Glutamate receptor antagonists had no effect on the ACh-evoked response, indicating that the current was not mediated by presynaptic facilitation of glutamate release. However, the current could be desensitized almost completely by bath superfusion with 100 nM nicotine. In contrast to those actions on interneurons, application of ACh to the soma of CA1 pyramidal cells did not produce a detectable current. Radioligand-binding experiments with [125I]-alphaBgTx demonstrated that stratum radiatum interneurons express alpha7-containing nAChRs, and in situ hybridization revealed significant amounts of alpha7 mRNA. CA1 pyramidal cells did not show specific binding of [125I]-alphaBgTx and only low levels of alpha7 mRNA. These results suggest that, in addition to their proposed presynaptic role in modulating transmitter release, alpha7-containing nAChRs also may play a postsynaptic role in the excitation of hippocampal interneurons. By desensitizing these receptors, nicotine may disrupt this action and indirectly excite pyramidal neurons by reducing GABAergic inhibition.

alpha-Bungarotoxin, kappa-bungarotoxin, alpha-cobratoxin and erabutoxin-b do not affect [3H]acetylcholine release from the rat isolated left hemidiaphragm

Endplate preparations of the rat left hemidiaphragm were incubated with [3H]choline to label neuronal transmitter stores. Nerve evoked release of newly-synthesized [3H]acetylcholine was measured in the absence of cholinesterase inhibitors to investigate whether snake venom neurotoxins by blocking presynaptic nicotinic autoreceptors affect evoked transmitter release. Contractions of the indirectly stimulated hemidiaphragm were recorded to characterize the blocking effect of alpha-neurotoxins at the post-synaptic nicotinic receptors. Neither the long chain neurotoxins alpha-cobratoxin (1 microgram ml-1) and alpha-bungarotoxin (5 microgram ml-1) nor the short chain neurotoxin erabutoxin-b (0.1, 1 and 10 micrograms ml-1) affected the nerve-evoked release of [3H]acetylcholine. kappa-Bungarotoxin (1 and 5 micrograms ml-1), a toxin preferentially blocking neuronal nicotinic receptors, did also not affect evoked [3H]acetylcholine release, whereas (+)-tubocurarine (1 microM) under identical conditions reduced the release by about 50%. alpha-Bungarotoxin, alpha-cobratoxin and erabutoxin-b concentration-dependently (0.01-0.6 micrograms ml-1) inhibited nerve-evoked contractions of the hemidiaphragm. All neurotoxins except erabutoxin-b enhanced the basal tritium efflux immediately when applied to the endplate preparation or to a non-innervated muscle strip labelled with [3H]choline. This effect was attributed to an enhanced efflux of [3H]phosphorylcholine, whereas the efflux of [3H]choline and [3H]acetylcholine was not affected. It is concluded that the alpha-neurotoxins and kappa-bungarotoxin do not block presynaptic nicotinic receptors of motor nerves. These nicotinic autoreceptors differ from nicotinic receptors localized at the muscle membrane and at autonomic ganglia.

Nicotine phase-advances the circadian neuronal activity rhythm in rat suprachiasmatic nuclei explants

In vivo studies reported that cholinergic agents affect mammalian circadian rhythmicity. To study phase resetting properties of cholinergic compounds more directly, we carried out experiments in rat suprachiasmatic nuclei slices. Compounds were added to the perfusate for 1 h at specific phases of the circadian cycle. On the following day, the time of peak neuronal activity, a measure of the phase of the endogenous circadian pacemaker, was assessed by means of extracellular recording in the suprachiasmatic nuclei. The peak of neuronal activity occurred at circadian time 5.8 +/- 0.7 (mean +/- 95% confidence limits) in the control slice (circadian time 0: lights-on). Ten-micromolar carbachol had no effect on the phase of the circadian rhythm when given at circadian times 6 and 15, while at circadian time 21 a phase advance of one hour was observed. By contrast, 10 microM nicotine significantly phase advanced (> 1 h) the neuronal circadian rhythm at all but one experimental circadian phase. The circadian times of maximal nicotinic phase advances were 15 (+2.6 h) and 21 (+2.8 h). A concentration response curve for nicotine was generated and pharmacological blocking experiments were performed at circadian time 15. The estimated maximum response of nicotine was 3.4 h, and the estimated concentration for half maximal response was 5 microM. The Hill coefficient (= 1.08) indicated that the effects of nicotine may be explained by a single receptor occupancy model. Mecamylamine (20 microM) almost completely antagonized the nicotinic phase-advances, whereas tetrodotoxin (1 microM) or high Mg2+ (10 mM) did not significantly attenuate the nicotinic phase-advances.(ABSTRACT TRUNCATED AT 250 WORDS)

Sarcolemmal indentation in cardiomyopathy with mental retardation and vacuolar myopathy

Muscle biopsies from three patients with cardiomyopathy, mental retardation and increased serum creatine kinase levels revealed scattered fibers with tiny intracytoplasmic vacuoles containing basophilic and acid phosphatase-positive material and slightly increased amounts of PAS-positive granules. These findings are consistent with those seen in the so-called lysosomal glycogen storage disease with normal acid maltase. In addition to the vacuoles, there were occasional folds or indentations in the sarcolemma which were connected to the membrane enclosing the vacuoles. These membranes were well demonstrated histochemically by the nonspecific esterase and acetylcholinesterase stains. On electron microscopy, most of the vacuoles were bounded by membranes with basal lamina. The vacuolar membrane stained positively with antibodies raised to dystrophin, dystrophin-associated glycoproteins, laminin and type 4 collagen, and it was identical to the sarcolemma and its basal lamina. Therefore, the membrane abnormality which causes sarcolemmal folding is probably critical to understanding the pathomechanism of this disease.

A role for the nicotinic alpha-bungarotoxin receptor in neurite outgrowth in PC12 cells

The addition of nicotine decreased neuritic outgrowth in PC12 cells in culture. This effect occurs as early as one day after addition of nicotine to the culture medium in a concentration-dependent manner. The nicotine-induced decline in neurite outgrowth was prevented by d-tubocurarine (10(-4) M) indicating that the effect was mediated through a nicotinic receptor. alpha-Bungarotoxin (10(-8) M) was also able to inhibit the nicotine-induced decrease in process formation in a dose-dependent manner. The concentrations of alpha-bungarotoxin required to affect process outgrowth correlated with those required to inhibit radiolabelled alpha-bungarotoxin binding. alpha-Bungarotoxin had no effect on [3H]noradrenaline release, a functional response mediated through the alpha-bungarotoxin-insensitive neuronal nicotinic acetylcholine receptor, suggesting that alpha-bungarotoxin specifically interacts with the neuronal alpha-bungarotoxin receptor. The present results suggest a functional role for the neuronal nicotinic alpha-bungarotoxin receptor in neurite outgrowth.

Distribution of acetylcholine receptors in the central nervous system of adult locusts

A polyclonal antibody raised against nicotinic acetylcholine receptor protein from purified locust neuronal membrane was used to analyse the distribution of antigenic sites within the central nervous system of adult Schistocerca gregaria. Light microscopic examination showed that all principal neuropiles in the thoracic ganglia label with the antibody but that the major tracts and commissures do not. Analysis of this pattern of staining in the electron microscope reveals that the receptor is present on specific synaptic and extrajunctional neuronal membranes in the neuropile. Antigenic sites are also evident on the plasma membranes and within the cytoplasm adjacent to Golgi complexes of some neuronal somata, suggesting that these neurones synthesise nicotinic acetylcholine receptors. In addition to neuronal labelling, there is evidence that the receptor is also present on the membranes of three types of glial cells. The implications of this pattern of receptor distribution are discussed.

Exogenously administered alpha-bungarotoxin binds to embryonic chick spinal cord: implications for the toxin-induced arrest of naturally occurring motoneuron death

Administration of alpha-bungarotoxin and other curare-like drugs during embryogenesis arrests motoneuron death which normally occurs in the spinal cord from day 6 to day 10 of embryogenesis. The accepted explanation is that such motoneuron rescue is mediated by inhibition of neuromuscular transmission following the blockade of nicotinic cholinoceptors at the neuromuscular junction. In this study we investigated a further possibility, namely that motoneuron rescue might also involve the blockade of alpha-bungarotoxin-sensitive sites within the spinal cord. The kinetic profile of [125I]alpha-bungarotoxin binding was examined in the brachial and lumbar regions of chick spinal cord at embryonic day 15. Binding was specific and apparently saturable within the range 1-34 nM reaching a maximum after 45 min. Specific binding involved a single class of non-interacting sites with a KD of 8.0 nM and a Bmax of 106 +/- 12 fmol/mg of protein. Nicotine displaced specific [125I]alpha-bungarotoxin binding in a concentration-dependent manner. Furthermore, specific binding dissociated slowly in the absence of nicotine. Autoradiographs localizing [125I]alpha-bungarotoxin binding in embryonic spinal cord revealed that, at embryonic day 15, specific toxin binding sites could be detected throughout the gray matter. In contrast, at embryonic day 6, the ventral horn contained the majority of specific binding sites. Exogenously administered [125I]alpha-bungarotoxin reached and bound to nicotine-sensitive sites in the spinal cord at embryonic day 7. To conclude, these data demonstrate that central nicotine-sensitive sites which bind [125I]alpha-bungarotoxin in a saturable and specific manner were present at the beginning of the critical motoneuron death phase of neurogenesis and that they were accessible to exogenously administered toxin. It is proposed that the [125I]alpha-bungarotoxin binding characterized here is to a class of putative alpha-bungarotoxin-sensitive nicotinic cholinoceptors. These studies raise the possibility that alpha-bungarotoxin blockade of such putative nicotinic cholinoceptors within the spinal cord may contribute to toxin-induced arrest of naturally occurring motoneuron death.

Effects of mescaline and some of its analogs on cholinergic neuromuscular transmission

Mescaline (3,4,5-trimethoxyphenylethylamine; MES) and its analogs, anhalinine (ANH) and methylenemescaline trimer (MMT) were investigated, using sciatic-sartorius preparations of the frog and cortical tissue from the rat. The effects of MES and its analogs were examined with respect to muscle twitch, resting membrane potential and nicotinic receptor binding. Mescaline and its analogs (10-100 microM) blocked both directly and neurally evoked twitches but their effects on neurally evoked twitches were greater than those on directly evoked twitches. Mescaline, ANH and MMT decreased amplitude of the miniature endplate and endplate potentials, decreased acetylcholine (ACh) quantal content, hyperpolarized the resting membrane potential and prolonged duration of the action potential. They did not significantly displace the binding of [125I]-alpha-bungarotoxin (alpha-BTX) to nicotinic receptors, at concentrations which blocked neuromuscular transmission. These results suggest that MES and its analogs inhibit cholinergic neuromuscular transmission by blocking release of ACh; they also affect K+ conductance.

The fall and rise of neuronal alpha-bungarotoxin binding proteins

Although neuronal [125I]-alpha-bungarotoxin binding proteins are similar in many respects to muscle nicotinic acetylcholine receptors, their functional significance has eluded researchers for the past fifteen years. Over this period, their status became increasingly doubtful, as almost all attempts failed to demonstrate that alpha-bungarotoxin could block neuronal nicotinic responses. Recently, these enigmatic proteins have been cloned and expressed in oocytes, and have been examined afresh in their native state. As Paul Clarke explains, it is time to recognize neuronal alpha-bungarotoxin binding proteins as distinct members of the nicotinic acetylcholine receptor gene family, even if perhaps they do not function quite like other members.

Thymopoietin, a polypeptide ligand for the alpha-bungarotoxin binding site in brain: an autoradiographic study

Thymopoietin, a 48-49-amino acid polypeptide present in the thymus gland, was investigated as a potential ligand for the neuronal nicotinic alpha-bungarotoxin binding site in rat brain. Binding of [125I]alpha-bungarotoxin to whole rat brain sections was inhibited by thymopoietin in a concentration-dependent manner with an IC50 of 30.0 +/- 8.2 nM as compared to 1.1 +/- 0.3 nM for alpha-bungarotoxin. However, at concentrations of thymopoietin of up to 1 microM, [3H]nicotine binding to high affinity sites was not inhibited. Thysplenin, a polypeptide with considerable homology to thymopoietin did not affect [125I]alpha-bungarotoxin binding. These results suggest that thymopoietin selectively interacts with the nicotinic alpha-bungarotoxin binding site labelled by [125I]alpha-bungarotoxin rather than the neuronal nicotinic receptor(s) labelled by [3H]nicotine. Autoradiographic studies revealed that 1 microM thymopoietin almost completely inhibited [125I]alpha-bungarotoxin binding in all brain regions. Computer-assisted image analysis of displacement curves was performed on various brain areas rich in alpha-bungarotoxin binding, such as the dorsal endopiriform nucleus, fields 1 and 2 of Ammon's horn, the polymorph cell layer of the dentate gyrus and cortical layers 4 and 5. Thymopoietin inhibited [125I]alpha-bungarotoxin binding with similar potency in all these regions, suggesting that it interacted at the same site in the different brain areas. The IC50 values averaged over the six regions were 24.6 +/- 2.8 nM for thymopoietin and 1.2 +/- 0.2 nM for alpha-bungarotoxin. These results show that thymopoietin specifically interacted with the alpha-bungarotoxin site with a similar potency in different brain regions. It is suggested that thymopoietin represents a selective ligand for alpha-bungarotoxin binding sites in brain.

Species differences in diisopropylfluorophosphate-induced decreases in the number of brain nicotinic receptors

DBA and C3H mice were injected chronically with 2.0 mg/kg diisopropylfluorophosphate (DFP) every other day for 2 or 4 weeks. Although acetylcholinesterase (AChE) activity and muscarinic receptor numbers ([3H] quinuclidinyl benzilate (QNB) binding) were decreased in DFP-treated DBA and C3H mice, the number of nicotinic receptors (L-[3H]nicotine and alpha-[125I]bungarotoxin (BTX) binding) was unchanged by chronic DFP treatment. Sprague-Dawley rats injected chronically with lower doses of DFP than were used in mice exhibited a greater reduction in AChE activity, as well as accompanying decreases in [3H]QNB and [3H]nicotine binding. Neither species exhibited changes in alpha-[125I]BTX following chronic DFP injection. The effects of chronic DFP treatment on sensitivity to DFP and to nicotine were also assessed in the two mouse strains using a battery of behavioral and physiological tests that included rotarod performance, Y-maze crossing and rearing activity, heart rate, and body temperature. No tolerance to DFP was observed in either mouse strain after 2 weeks of treatment. Following 4 weeks of treatment, DFP-treated DBA mice exhibited modest tolerance to the effect of DFP on body temperature. C3H mice did not survive the 4-week treatment. Some evidence for reduced sensitivity to nicotine's effects was detected in the DFP-treated DBA mice, but cross-tolerance to nicotine was not observed in the DFP-injected C3H mice. Because chronic DFP treatment did not evoke a change in the number of brain nicotinic receptors, the reduced sensitivity to some of nicotine's effects seen in DBA mice must be due to some factor other than receptor downregulation.

Thymopoietin, a thymic polypeptide, potently interacts at muscle and neuronal nicotinic alpha-bungarotoxin receptors

Current studies suggest that several distinct populations of nicotinic acetylcholine (ACh) receptors exist. One of these is the muscle-type nicotinic receptors with which neuromuscular nicotinic receptor ligands and the snake toxin alpha-bungarotoxin interact. alpha-Bungarotoxin potently binds to these nicotinic receptors and blocks their function, two characteristics that have made the alpha-toxin a very useful probe for the characterization of these sites. In neuronal tissues, several populations of nicotinic receptors have been identified which, although they share a nicotinic pharmacology, have unique characteristics. The alpha-bungarotoxin-insensitive neuronal nicotinic receptors, which may be involved in mediating neuronal excitability, bind nicotinic agonists with high affinity but do not interact with alpha-bungarotoxin. Subtypes of these alpha-toxin-insensitive receptors appear to exist, as evidenced by findings that some are inhibited by neuronal bungarotoxin whereas others are not. In addition to the alpha-bungarotoxin-insensitive sites, alpha-bungarotoxin-sensitive neuronal nicotinic receptors are also present in neuronal tissues. These latter receptors bind alpha-bungarotoxin with high affinity and nicotinic agonists with an affinity in the microM range. The function of the nicotinic alpha-bungarotoxin receptors are as yet uncertain. Thymopoietin, a polypeptide linked to immune function, appears to interact specifically with nicotinic receptor populations that bind alpha-bungarotoxin. Thus, in muscle tissue where alpha-bungarotoxin both binds to the receptor and blocks activity, thymopoietin also potently binds to the receptor and inhibits nicotinic receptors-mediated function. In neuronal tissues, thymopoietin interacts only with the nicotinic alpha-bungarotoxin site and not the alpha-bungarotoxin-insensitive neuronal nicotinic receptor population. These observations that thymopoietin potently and specifically interacts with nicotinic alpha-bungarotoxin-sensitive receptors in neuronal and muscle tissue, together with findings that thymopoietin is an endogenously occurring agent, could suggest that this immune-related polypeptide represents a ligand for the alpha-bungarotoxin receptors. The function of thymopoietin at the alpha-bungarotoxin receptor is as yet uncertain; however, a potential trophic, as well as other roles are suggested.

Nicotinic and muscarinic modulation of 5-hydroxytryptamine (5-HT) release from porcine and canine small intestine

Strips of porcine and canine small intestine were incubated in vitro and the release of 5-hydroxytryptamine (5-HT) was determined by HPLC with electrochemical detection. The spontaneous outflow of 5-HT from the porcine and canine small intestine largely reflects calcium-dependent 5-HT secretion from enterochromaffin cells which are under a spontaneous neuronal, excitatory input as indicated by the inhibitory effect (30-40%) of tetrodotoxin. In both species, nicotine enhanced the release of 5-HT in a concentration-dependent manner by a maximum of about 50% at 100 microM. This effect was blocked by the nicotine receptor antagonist hexamethonium, but not by the subtype-selective nicotine receptor antagonist alpha-bungarotoxin. The effect of nicotine was rapidly desensitized. The presence of tetrodotoxin abolished the effect of nicotine on 5-HT release in canine tissue but not in porcine tissue. The presence of the muscarine receptor antagonist scopolamine prevented the effect of nicotine on 5-HT release from canine tissue, but significantly enhanced 5-HT release from porcine tissue. The muscarine receptor agonist oxotremorine inhibited 5-HT release from porcine tissue, but increased 5-HT release from canine tissue. However, in the presence of tetrodotoxin, oxotremorine enhanced 5-HT release in tissue from both species. In conclusion, activation of nicotine receptors induce the release of 5-HT from porcine and canine small intestine. In the dog, the effect of nicotine is mediated via the release of acetylcholine which then stimulates 5-HT release via muscarine receptors on the enterochromaffin cells. In the pig, the stimulatory effect of nicotine appears to be located directly on the enterochromaffin cells. In addition, activation of neuronal muscarine receptors in the porcine small intestine induced the release of a previously unidentified neurotransmitter which inhibited 5-HT release. Nicotine, via cholinergic interneurons, also appears to induce the release of this inhibitory neurotransmitter which opposes the direct stimulatory action of nicotine on 5-HT release.

Cholinergic nicotinic receptors in the vestibular epithelia

Receptor binding studies specific for nicotinic cholinergic receptors have been carried out on isolated vestibular epithelia of the frogs Rana catesbiana and Rana temporaria. Evidence is presented for the presence of nicotinic-like cholinergic receptors specifically associated with the sensory areas.

Purification and characterization of an alpha-bungarotoxin receptor that forms a functional nicotinic channel

Neither the structure nor the function of alpha-bungarotoxin (alpha Bgtx) binding molecules in the nervous system have yet been completely defined, although it is known that some of these molecules are related to cation channels and some are not. Using an improved method of affinity chromatography, we have isolated a toxin binding molecule from chicken optic lobe that contains at least three subunits with apparent Mr values of 52,000, 57,000, and 67,000. The Mr 57,000 subunit binds alpha Bgtx and seems to be present in two copies per receptor. The receptor is recognized by antibodies raised against the alpha Bgtx receptors of human neuroblastoma cells, fetal calf muscle, and chicken optic lobe but not by antibodies raised against Torpedo acetylcholine receptor, the serum of myasthenic patients, or monoclonal antibody, 35. 125I-labeled alpha Bgtx binding to the isolated receptor is blocked, with the same potency, by nicotinic agonists and antagonists, such as nicotine, neuronal bungarotoxin and, d-tubocurarine. When reconstituted in a planar lipid bilayer, the purified alpha Bgtx receptor forms cationic channels with a conductance of 50 pS. These channels are activated in a dose-dependent manner by carbamylcholine and blocked by d-tubocurarine.

Evidence for thymopoietin and thymopoietin/alpha-bungarotoxin/nicotinic receptors within the brain

Thymopoietin, a polypeptide hormone of the thymus that has pleiotropic actions on the immune, endocrine, and nervous systems, potently interacts with the neuromuscular nicotinic acetylcholine receptor. Thymopoietin binds to the nicotinic alpha-bungarotoxin (alpha-BGT) receptor in muscle and, like alpha-BGT, inhibits cholinergic transmission at this site. Evidence is given that radiolabeled thymopoietin similarly binds to a nicotinic alpha-BGT-binding site within the brain and does so with the characteristics of a specific receptor ligand. Thus specific binding to neuronal membranes was saturable, of high affinity (Kd = 8 nM), linear with increased tissue concentration, and readily reversible; half-time was approximately 5 min for association and 10 min for dissociation. Binding of 125I-labeled thymopoietin was displaced not only by unlabeled thymopoietin but also by alpha-BGT and the nicotinic receptor ligands d-tubocurarine and nicotine; various other receptor ligands (muscarinic, adrenergic, and dopaminergic) did not affect binding of 125I-labeled thymopoietin. Thymopoietin was shown by ELISA to be present in brain extracts, displacement curves of thymus and brain extracts being parallel to the standard thymopoietin curve, and Western (immuno) blot identified in brain and thymus extracts a thymopoietin-immunoreactive polypeptide of the same molecular mass as purified thymopoietin polypeptide. We conclude that thymopoietin and thymopoietin-binding sites are present within the brain and that the receptor for thymopoietin is the previously identified nicotinic alpha-BGT-binding site of neuronal tissue.

Cholinergic receptors in human brain: effects of aging and Alzheimer disease

A general review of cholinergic receptors in human brain is presented. The paper focuses upon changes in normal aging brain and in Alzheimer disease. Studies from five different approaches are reported: 1) molecular biology; 2) receptor binding studies; 3) studies with specific neurotoxins; 4) immunocytochemistry; and 5) PET scan. These studies document profound and characteristic differences between the normal aging and the pathological Alzheimer brain with regard to cholinergic receptor localization, distribution, and function.

Nicotinic acetylcholine receptor subtypes in human frontal cortex: changes in Alzheimer's disease

Molecular genetic and pharmacological studies have suggested that several subtypes of nicotinic acetylcholine receptors exist in the mammalian and avian brain. Combining 3H-(-)-nicotine, 125I-alpha-bungarotoxin, and 125I-kappa-bungarotoxin as ligands, we report here the first evidence for the existence in human frontal cortex of at least three different subtypes of nicotinic receptors. Autoradiographic analysis shows that specific 125I-kappa-bungarotoxin binding sites are concentrated mainly in several cortical layers. We also show that kappa-bungarotoxin, but not alpha-bungarotoxin decreases the evoked release of 3H-acetylcholine in rat cortical slices, indicating a likely presynaptic localization for some of the alpha-bungarotoxin-insensitive kappa-bungarotoxin sites in mammalian brain. The brains of patients with Alzheimer's disease show marked decreases in Bmax values for low-affinity 125I-kappa-bungarotoxin sites and both high- and low-affinity 3H-nicotine sites, whereas 125I-alpha-bungarotoxin sites are not significantly different in number from age-matched control brains. We conclude that Alzheimer's disease does not affect all subtypes of nicotinic receptors in the frontal cortex to the same extent.

Thymopoietin interacts at the alpha-bungarotoxin site of and induces process formation in PC12 pheochromocytoma cells

Thymopoietin, a polypeptide isolated from thymus and involved in immune regulation, potently inhibited [125I]alpha-bungarotoxin binding in both pheochromocytoma (PC12) cells in culture (IC50 of 3.9 nM) and in PC12 cell membranes (IC50 of 2.2 nM). The degree of inhibition produced by thymopoietin was similar to that observed with alpha-bungarotoxin; in contrast, nicotinic receptor ligands affected alpha-bungarotoxin binding only at micromolar concentrations, in agreement with previous work. Binding of thymopoietin was reversible. Studies with PC12 cell membranes suggested that the interaction between alpha-bungarotoxin and thymopoietin at the receptor was competitive. The effect of thymopoietin was subsequently assessed on various morphological characteristics of PC12 cells in culture. Exposure of the cells to the polypeptide resulted in neurite extension, which was evident as early as 1-2 days in culture and was maximal after 4-6 days; this response was observed with concentrations of thymopoietin as low as 10(-8) M. Nerve growth factor also induced neurite extension in PC12 cells; however, the effects of nerve growth factor were qualitatively and quantitatively distinct from those which occurred with thymopoietin. Moreover, a monoclonal antibody to nerve growth factor completely prevented the nerve growth factor-induced process formation without affecting the thymopoietin-induced response. On the other hand, alpha-bungarotoxin resulted in the formation of processes which appeared morphologically similar to those induced by thymopoietin, although alpha-bungarotoxin appeared less potent than the thymic polypeptide. The effect of thymopoietin appeared to be specific; thysplenin, a polypeptide with approximately 80% homology with thymopoietin, did not elicit process formation. The thymopoietin-induced effect was reversed upon removal of the polypeptide from the culture medium. These results show that thymopoietin, a polypeptide endogenous to mammalian systems, potently interacted at the alpha-bungarotoxin site in a neuronal cell line. Furthermore, thymopoietin could elicit process formation in PC12 cells, suggesting that it may be a neuronotrophic factor.

Neuronal nicotinic alpha-bungarotoxin receptors

Recent evidence has indicated that the nicotinic acetylcholine receptor and the nicotinic alpha-bungarotoxin (alpha-BGT) site may be distinct in neuronal tissues. With regard to function, the former receptor appears to be involved in mediating synaptic events; however, the role of the nicotinic alpha-BGT site in nervous tissue is currently not known. Since the binding of alpha-BGT exhibits such high affinity and selectivity for a specific receptor, this may implicate an involvement of the toxin binding site in some aspect of neuronal activity with the receptor possibly mediating functions other than nicotinic cholinergic transmission. A further hypothesis to explain the nature of the toxin binding site may be that the natural ligand for the alpha-BGT site is one other than acetylcholine, with acetylcholine acting as a modulator of the site. Current studies in our laboratory are exploring these possibilities by determining whether specific peptides and/or polypeptides can interact at the nicotinic alpha-BGT site in nervous tissue. Studies using both in vivo and in vitro approaches suggest that thymopoietin may serve a role as a modulator of the nicotinic alpha-BGT site in neuronal tissues.

Desensitization of central cholinergic mechanisms and neuroadaptation to nicotine

This review focuses on neuroadaptation to nicotine. The first part of the paper delineates some possible general mechanisms subserving neuroadaptation to commonly abused drugs. The postulated role of the mesocorticolimbic neuroanatomical pathway and drug-receptor desensitization mechanisms in the establishment of tolerance to, dependence on, and withdrawal from psychoactive drugs are discussed. The second part of the review deals with the pharmacological effects of nicotine at both pre- and postsynaptic locations within the central nervous system, and the still-perplexing upregulation of brain nicotine-binding sites seen after chronic nicotine administration. A special emphasis has been put on desensitization of presynaptic cholinergic mechanisms, and postsynaptic neuronal nicotinic-receptor function and its modulation by endogenous substances. A comparison with the inactivation process occurring at peripheral nicotinic receptors is also included. Finally, a hypothesis on the possible connections between desensitization of central cholinergic mechanisms and neuroadaptation to nicotine is advanced. A brief comment on the necessity of fully understanding the effects of nicotine on the developing nervous system closes this work.

Postnatal development of two nicotinic cholinergic receptors in seven mouse brain regions

The developmental profiles for binding of alpha-[125I]bungarotoxin and L-[3H]nicotine to putative nicotinic cholinergic receptors were determined in seven mouse brain regions. The overall pattern of development of alpha-bungarotoxin binding was similar in all of the regions. Neonatal binding values tended to be greater than those observed in adult brain regions. Maximal binding occurred within 10 days of birth and adult binding values were reached by 20 days of age. The patterns of development of nicotine binding in each of the seven brain regions differed according to region. Gross similarities in developmental profiles for nicotine binding were found among the more caudal and among the more rostral regions. In hindbrain and cerebellum, maximal nicotine binding was found at birth (5 days of age in cerebellum); binding declined approximately 4-fold by 20 days and remained relatively constant thereafter. In midbrain and hypothalamus, a less extensive decrease in nicotine binding occurred from birth to adulthood (midbrain, 25%; hypothalamus, 50%). Nicotine binding in hippocampus and cortex remained unchanged between birth and adulthood. The developmental pattern for nicotine binding in striatum differed from that found in the other brain regions. At 5 days of age, binding was about 65% of adult binding, which was reached at 30 days of age. In most of the brain regions the developmental profile for alpha-bungarotoxin binding was different from that of nicotine. This difference was especially notable in striatum, where adult nicotine binding was higher than neonatal nicotine binding, whereas adult alpha-bungarotoxin binding was lower than neonatal alpha-bungarotoxin binding.

Monoclonal antibodies against ?-bungarotoxin

Hybridomas secreting monoclonal antibodies against ?-bungarotoxin were produced from the fusions of mice lymphocytes from hyperimmune animals with two mice myeloma cell lines ((NSI/1 or Sp2/0). Several anti-?-bungarotoxin monoclonal antibodies were derived and characterized. One of these (spB57) belonged to the IgG(1) subclass and bound potently to ?-bungarotoxin in a radioimmunoassay. This effect was specific to the anti-?-bungarotoxin antibody; a control series of antibodies (against tyrosine hydroxylase, enkephalin, neurofilament and the nerve growth factor receptor) did not bind radiolabelled toxin. Furthermore, the anti-?-bungarotoxin antibody did not interact with other radiolabelled receptor ligands. Using autoradiographic techniques, spB57 was shown to block the binding of [(125)I]?-bungarotoxin to brain sections. Similarly, spB57 blocked radiolabelled toxin binding to brain membranes; again this was an effect specific to the anti-?-bungarotoxin antibody. The decrease in [(125)I]?-bungarotoxin binding suggested that spB57 specifically bound the toxin molecule such that it could no longer interact with its receptor. Since the ?-BGT site has the characteristics of a nicotinic receptor, the effect of the antibody was also tested on the inhibition of [(125)I]?-bungarotoxin binding by cholinergic ligands. SpB57 partially reversed the inhibition of ?-toxin binding observed with nicotinic agonists and d-tubocurarine, but not with other nicotinic antagonists nor with muscarinic receptor ligands. These effects appeared to be specific for spB57, as they occurred to a much lesser extent with two other anti-?-BGT mAbs, nsB8 and spB28. These results suggest that an antibody against the ?-toxin can affect the interaction of nicotinic receptor ligands at the ?-BGT site.

Phorbol esters and d-tubocurarine up-regulate alpha-bungarotoxin sites in chromaffin cells in culture via distinct mechanisms

Previous work had shown that nicotinic antagonists resulted in a marked up-regulation of alpha-bungarotoxin sites in chromaffin cells in culture. The present experiments were done to determine the intracellular mechanism(s) whereby nicotinic antagonists might mediate their effects on these receptors. Chromaffin cells were cultured for three days with various concentrations of 4 beta-phorbol 12-myristate 13-acetate, an agent which affects protein kinase C by mimicking the actions of diacylglycerol. The phorbol ester resulted in a dose-dependent increase in alpha-bungarotoxin binding which was maximal with 100 nM 4 beta-phorbol 12-myristate 13-acetate. This increase in binding appeared to be due to an increase in the maximal number of alpha-bungarotoxin sites. Time dependence studies showed that the effect of the phorbol was undetectable with incubations of 24 h or less and appeared to plateau by 72-96 h. A similar increase in toxin binding was also observed with 4 beta-phorbol 12,13-dibutyrate. On the other hand, an inactive analog of 4 beta-phorbol 12-myristate 13-acetate had no significant effect on binding. D-Sphingosine, an inhibitor of protein kinase C, was able to partially block the phorbol ester-induced increase in toxin binding while polymyxin B, another protein kinase C inhibitor, completely prevented the up-regulation of the alpha-bungarotoxin sites. Carbachol and nicotine prevented this enhancement of toxin binding in the presence of 4 beta-phorbol 12-myristate 13-acetate. Although the phorbol ester resulted in an increase in toxin binding, acetylcholine-evoked catecholamine secretion from chromaffin cells in culture was decreased, indicating a dissociation between the functional nicotinic acetylcholine receptor population and the alpha-bungarotoxin sites. To determine whether agents which affect protein kinase C can alter the up-regulation of alpha-bungarotoxin sites by d-tubocurarine, 4 beta-phorbol 12-myristate 13-acetate was added to the cells in combination with the nicotinic antagonist. The up-regulation of toxin binding sites induced by d-tubocurarine was additive with that induced by the phorbol and was not affected by polymyxin B. Thus, the results would suggest that there are at least two mechanisms by which alpha-bungarotoxin binding sites can be regulated. One is mediated via an interaction at nicotinic receptors, while the other occurs in response to phorbol esters and thus may be mediated by protein kinase C. Interestingly, although the molecular mechanisms resulting in alpha-bungarotoxin receptor up-regulation differ, both the d-tubocurarine- and the phorbol ester-induced increases were prevented by nicotinic receptor ligands.

Immunohisto- and cytochemical localization of cortical nicotinic cholinoceptors in rat and man

A monoclonal antibody (WF 6) raised against purified Torpedo nicotinic acetylcholine receptor was applied to study the cellular and subcellular receptor distribution in human and rat neocortex. In both species, immunostaining was most prominent in perikarya and dendrites of the projection neurons in layers III and V. In layer VI fusiform cells displayed immunoreactivity while in layers I, II and IV some round-shaped cells were immunostained. Subcellularly, immunoprecipitate was found in neuronal perikarya, dendrites and in the postsynaptic thickenings, indicating intracellular sites of synthesis, transport and membrane incorporation of receptor protein. The results suggest that WF 6-immunocytochemistry is a useful tool to label nicotinic cholinergic receptors rendering new information about the specific cell-type and subcellular receptor distribution hardly obtainable by using conventional receptor autoradiography.

Thymopoietin, a thymic polypeptide, specifically interacts at neuronal nicotinic alpha-bungarotoxin receptors

alpha-Bungarotoxin (alpha-BGT), a snake venom polypeptide, interacts potently and specifically with a nicotinic receptor population in neuronal tissue. However, the identity of this site is unclear, because, unlike at the neuromuscular junction and in electroplax, in nervous tissue the toxin does not block nicotinic cholinergic responses. Therefore, we sought endogenous compounds other than acetylcholine that could interact with the neuronal alpha-BGT site. In the present experiments, thymopoietin, a polypeptide isolated from the thymus, is shown to inhibit potently alpha-BGT binding to brain membranes in a dose-dependent manner (IC50 = 3.1 nM). This effect was not shared by a wide variety of other peptides, including thysplenin, a closely related polypeptide. Thymopoietin did not inhibit the binding of other radioligands known to interact with different populations of cholinergic receptors, such as [3H]nicotine and [3H]methylcarbachol, which bind to nicotinic receptors, or [3H]quinuclidinylbenzilate, which binds to muscarinic receptors. These results show that thymopoietin potently and specifically affects 125I-alpha-BGT binding to brain membranes and suggest that thymopoietin might be an endogenous ligand for alpha-BGT receptors in neuronal tissue.

Nicotinic binding sites in cerebral cortex and hippocampus in Alzheimer's dementia

Postmortem cerebral neocortical and hippocampal samples were taken from patients who died with dementia of the Alzheimer type (DAT) and individuals without diagnoses of neurological or psychiatric disease (control). Nicotinic binding was assayed with 20 nM [3H]acetylcholine [( 3H]ACh) in the presence of atropine, or with 4 nM (-)-[3H]nicotine ((-)-[3H]Nic). Binding of both ligands was lower in the following regions from DAT vs. control brains (P less than or equal to 0.05): superior, middle and inferior temporal gyri, orbital frontal gyrus, middle frontal gyrus, pre- and postcentral gyri, inferior parietal lobule, and hippocampal endplate. Values of the correlation coefficient (r's) for binding of the nicotinic cholinergic ligands in these regions ranged from 0.70 to 0.93 (P's less than 0.05), suggesting that [3H]ACh and (-)-[3H]Nic labeled the same sites in human brain. There was no difference in nicotinic binding in the presubiculum, comparing DAT and control samples (P greater than 0.05). Here too, correlations between binding of the two ligands were statistically significant in control and DAT groups (r's = 0.92, P's less than 0.05). Nicotinic binding measured with [3H]ACh, but not (-)-[3H]Nic, was significantly lower in the H2 (field of Rose) and H1-subiculum areas of DAT samples compared to control. Correlations between binding of the two ligands in these regions ranged from 0.21 to 0.34 for the two groups (P's greater than 0.05). The findings support a loss of neocortical and hippocampal nicotinic cholinergic binding sites in DAT.(ABSTRACT TRUNCATED AT 250 WORDS)

Variation of nicotinic binding sites among inbred strains

The specific binding of L-nicotine and alpha-bungarotoxin, two ligands which label different populations of putative nicotinic receptors, was determined in eight brain regions of 19 inbred mouse strains. The dissociation constants for L-nicotine (average = 2.26 nM) and alpha-bungarotoxin (average = 0.31 nM) did not vary significantly among the brain regions or strains. In contrast, significant variability among the maximal binding sites was observed between regions and among the strains within a region. Significant differences in L-nicotine binding were observed among the strains in midbrain, hindbrain, hippocampus, hypothalamus and colliculi, while little variability was noted in cortex or cerebellum. In general, those strains that had high L-nicotine binding in one region had high nicotine binding in the other regions. The strains clustered into two large groups: one group expressing relatively low binding and a second group expressing relatively high binding. Significant differences in alpha-bungarotoxin binding were observed in seven of the eight regions measured and, in general, those strains with high binding in one region tended to have high binding in the other regions. The strains clustered into three groups: those with low binding (DBA/1 and DBA/2), those with high binding (ST/b alone) and those with intermediate binding (the remaining 16 strains). The amount of binding of the two ligands did not correlate with each other. Comparison of nicotinic ligand binding with physiological response to nicotine suggests a relationship of L-nicotine binding with several responses observed after injection of low doses of nicotine and a relationship between alpha-bungarotoxin binding and nicotine-induced seizures.

Pharmacology of nicotine

In recent years progress in basic neuropsychopharmacology and clinical addiction research have allowed the conclusion that tobacco smoking essentially represents an addiction to nicotine. Parallel to this work, experimental research in biochemistry, physiology and pharmacology has provided detailed descriptions of the structure and function of the nicotinic receptor, the biologic mediator of the many actions of nicotine. This article reviews current knowledge of nicotinic mechanisms in the peripheral and central nervous systems as well as some implications for the notion of smoking as an addiction to nicotine. In particular this review will focus on the effects of nicotine on brain dopamine and noradrenaline systems since these neuronal systems appear to be crucially involved in the rewarding and stimulant effects of addictive drugs.

An autoradiographic analysis of cholinergic receptors in mouse brain

Autoradiographic techniques were used to localize cholinergic receptors in the central nervous system of female DBA mice. Nicotinic receptors were identified using [3H]-L-nicotine and alpha-[125I]-bungarotoxin (BTX); [3H]-quinuclidinyl benzilate (QNB) was used to examine muscarinic receptor binding. There was little overlap between the regional distribution of binding sites for these ligands. Nicotine binding was highest in thalamic nuclei, the superior colliculus and the interpeduncular nucleus. For BTX binding, high density receptor populations were identified in the hippocampus, caudate putamen, colliculi (superior and inferior) and various nuclei in the hypothalamus and hindbrain. Muscarinic receptors were distributed more uniformly than nicotinic receptors; the colliculi, hippocampus and cerebral cortex had the highest level of QNB binding. Species differences between rats and mice in terms of cholinergic receptor binding are discussed.

Pharmacological profile of nicotinic acetylcholine receptors in the rat prefrontal cortex: an electrophysiological study in a slice preparation

The specificity of nicotinic receptors in the neocortex has been questioned previously because: (i) electrophysiological responses to nicotine could not be blocked by nicotinic antagonists, and (ii) the effect of nicotine was not mimicked by acetylcholine. In the present study, the presence of functional nicotinic receptors in rat neocortex has been assessed in a slice preparation of prefrontal cortex, using evoked field potential and unit recordings. Nicotine and the nicotinic agonists, dimethylphenylpiperazinium, cytisine, acetylthiocholine, applied by iontophoresis, produced an increase in the negative wave of field potentials, reflecting an increased excitability of cortical neurons. This effect was blocked by the selective probe for neuronal nicotinic receptors Toxin F (1.4 microM in the perfusion medium) and by dihydro-beta-erythroidine (100 microM). Alpha-bungarotoxin, the blocker of skeletal muscle acetylcholine receptor had no effect. Iontophoretically applied acetylcholine, muscarine and pilocarpine, on the other hand, produced a decrease in the field potential amplitude, which was blocked by atropine and scopolamine (1-10 microM). In the presence of eserine (10 microM), the muscarinic effect of acetylcholine was dramatically altered, leading to the development of a nicotinic response sensitive to Toxin F. Thus, the physiological activation of nicotinic receptors in rat prefrontal cortex appears to require higher concentrations of acetylcholine than do muscarinic receptors. Our results show that: (i) the rat prefrontal cortex possesses functional nicotinic receptors with a pharmacological profile clearly distinct from muscle receptors, and (ii) a nicotinic effect of acetylcholine can be revealed when its degradation by acetylcholinesterase is inhibited.

Morphological and biochemical differentiation of the human medulloblastoma cell line TE671

Cells of the human medulloblastoma clonal line TE671 exhibit polymorphism when grown in vitro in serum-supplemented medium. Under these conditions, cell numbers double every 18 h during log phase growth. These tumorigenic precursors of cerebellar interneurons are not contact-inhibited and approach densities of one million cells per cm2. TE671 cells in proliferative growth express a class of nicotinic acetylcholine receptors that are fully sensitive to functional blockade by the neurotoxin alpha-bungarotoxin (Bgt). TE671 cells grown in medium containing dibutyryl cyclic adenosine monophosphate (dbcAMP) rapidly undergo a distinctive morphological transformation characterized by neurite extension and formation of cell-cell contacts. The rate of cell division and cell saturation densities are diminished coordinately with these treatments. Sodium fluoride and forskolin induce similar changes in cell division and morphology as does dbcAMP, and these effects are potentiated by aluminum and the phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine, respectively. The high-affinity binding of Bgt to TE671 cells also is reduced on exposure to dbcAMP in a time and dose-dependent manner. The results suggest that activation of adenylate cyclase and the concomitant elevation of intracellular cAMP levels may be involved in the morphological transformation of TE671 cells to a mature, neuronal phenotype and in changes in the level of expression of a subtype of human neuronal nicotinic receptors. These studies establish a unique, neural tube-derived model system for investigation of the mechanisms involved in these processes.

Binding pattern of alpha-bungarotoxin on horizontal cells of a marine teleost retina

A conjugate of alpha-bungarotoxin and a fluorescent marker (fluorescein isothiocyanate) has been used to localize "nicotinic" acetylcholine receptors on neurons in the outer plexiform layer of marine teleost retina. Toxin binding was confined to bipolar cell dendrites and to intermediate horizontal cells. The arrangement of labeled horizontal cells appears irregular in the whole retina, with a peak density in the ventral and dorsal quandrants. Alpha-bungarotoxin receptors on horizontal cells differ from those on bipolar cells and from those on dendrites in the inner plexiform layer in their sensitivity to agonists and antagonists such as d-tubocurarine and nicotine. They constitute a different type of "nicotinic" receptor that probably has a different function.

Characterization of N-[3H]methylcarbamylcholine binding sites and effect of N-methylcarbamylcholine on acetylcholine release in rat brain

The present experiments show that N-[3H]-methylcarbamylcholine ([3H]MCC) binds specifically and with high affinity to rat hippocampus, frontal cortex, and striatum. The highest maximal density of binding sites was apparent in frontal cortex and the lowest in hippocampus. [3H]MCC binding was potently inhibited by nicotinic, but not muscarinic, agonists and by the nicotinic antagonist dihydro-beta-erythroidine in all three brain regions studied. The effect of unlabeled MCC on acetylcholine (ACh) release from slices of rat brain was tested. The drug significantly enhanced spontaneous ACh release from slices of hippocampus and frontal cortex, but not from striatal slices. This effect of MCC to increase ACh release from rat hippocampus and frontal cortex was antagonized by the nicotinic antagonists dihydro-beta-erythroidine and d-tubocurarine, but not by alpha-bungarotoxin or by the muscarinic antagonist atropine. The MCC-induced increase in spontaneous ACh release from hippocampal and frontal cortical slices was not affected by tetrodotoxin. The results suggest that MCC might alter cholinergic transmission in rat brain by a direct activation of presynaptic nicotinic receptors on the cholinergic terminals. That this alteration of ACh release is apparent in hippocampus and frontal cortex, but not in striatum, suggests that there may be a regional specificity in the regulation of ACh by nicotinic receptors in rat brain.

Autoradiographic localization of alpha-bungarotoxin binding sites in the suprachiasmatic region of rat brain

High affinity alpha-bungarotoxin (alpha-BTX) binding sites of the hypothalamus in and near the suprachiasmatic nuclei (SCN) were mapped by in vitro macroautoradiographic analysis. Adult male rats were killed at specific circadian phases. Their brains were rapidly dissected out and frozen sections were made at a thickness of 16 micron. After having been mounted on slides, the sections were incubated with iodinated alpha-BTX (3 nM), washed and exposed to X-ray film. Analyses of binding were performed with the aid of a digital video densitometer system. Autoradiographic loci that bound alpha-BTX were traced and the image of the SCN histology from the adjacent section was superimposed. In this way the exact relationships of the topography of areas that bound alpha-BTX and those that contained SCN cell bodies (and other hypothalamic landmarks) could be observed. Non-specific binding was tested by incubation in the presence of 3 microM unlabelled alpha-BTX and was found to be very low and uniform throughout the sections. Hypothalamic areas that bound alpha-BTX included the SCN, supraoptic, periventricular, lateral and anterior hypothalamic nuclei. In the rostral SCN, alpha-BTX binding coincided with the nucleus proper. Caudally this relationship dissociated so that at mid-SCN alpha-BTX bound dorsally and laterally both within and outside the SCN and by the most caudal portion of the nucleus, alpha-BTX binding was entirely outside the SCN in a vertical band dorsal to the SCN. This topography suggests that alpha-BTX binding may be coincident with a major output pathway that courses dorsally and caudally from the SCN.

Mecamylamine reduces some EEG effects of nicotine chewing gum in humans

Spontaneous EEG was recorded in nine cigarette smokers who had been abstinent from tobacco for 12 hr. Subjects were treated with a capsule containing either centrally acting nicotine blocker, mecamylamine (10 mg), or placebo. At each of three 60-min intervals after the capsule was ingested, the subjects chewed two pieces of gum containing a total of 0, 4 or 8 mg of nicotine. Nicotine and mecamylamine dose combinations were randomized across subjects. Two three-minute periods of spontaneous EEG were recorded before the capsule and before and after gum chewing from bipolar electrode montages at the following positions: Cz-T5, Cz-T6, Cz-F7 and Cz-F8. During one period the subjects relaxed with eyes closed, in the other period they performed a math task with eyes open. When the drugs were given individually, mecamylamine decreased beta power and nicotine gum (4 and 8 mg) increased alpha frequency. Mecamylamine pretreatment prevented the increase in alpha frequency caused by the 4 mg gum dose but not the 8 mg dose. Alpha power was increased by the 8 mg gum dose and that increase was prevented by mecamylamine. Self-reported ratings of the "strength" of the gum were significantly diminished by mecamylamine pretreatment. The data are consistent with the results of earlier studies which indicate that the effects of tobacco administration and withdrawal are mediated by central actions of nicotine.

Genetic influences on acute responses to nicotine and nicotine tolerance in the mouse

Nineteen inbred mouse strains were tested for their relative sensitivity to nicotine's effects on respiratory rate, acoustic startle response, heart rate, Y-maze activity (crosses and rears) and body temperature. Separate animals were tested for their sensitivity to nicotine-induced seizures following IP injection or IV infusion. Dose-response curves were constructed for each measure. Large strain differences were obtained for all of these measures. Nicotine's effects on heart rate, Y-maze activity and body temperature segregated together into the various mouse strains whereas seizure sensitivity segregated independently which suggests that these responses are under separate genetic control. Evidence was obtained which suggests that nicotine-induced seizures are regulated, in part, by the number of hippocampal nicotinic receptors measured with alpha-bungarotoxin (BTX). Strain differences in the development of tolerance to nicotine were also observed. Four mouse strains were tested and one of these strains (C3H) did not exhibit tolerance to nicotine. The binding of (3H)nicotine and (125I)BTX increased in the brains of all four mouse strains. These changes may relate to tolerance in some mouse strains, but since C3H mice did not exhibit tolerance even though brain nicotinic receptors changed following chronic treatment, other explanations for the role of receptor changes in tolerance to nicotine must be sought.