Differential expression of the neuronal acetylcholine receptor alpha 2 subunit gene during chick brain development

Organophosphate Insecticide Toxicity in Neural Development, Cognition, Behaviour and Degeneration: Insights from Zebrafish

Organophosphate (OP) insecticides are used to eliminate agricultural threats posed by insects, through inhibition of the neurotransmitter acetylcholinesterase (AChE). These potent neurotoxins are extremely efficacious in insect elimination, and as such, are the preferred agricultural insecticides worldwide. Despite their efficacy, however, estimates indicate that only 0.1% of organophosphates reach their desired target. Moreover, multiple studies have shown that OP exposure in both humans and animals can lead to aberrations in embryonic development, defects in childhood neurocognition, and substantial contribution to neurodegenerative diseases such as Alzheimer's and Motor Neurone Disease. Here, we review the current state of knowledge pertaining to organophosphate exposure on both embryonic development and/or subsequent neurological consequences on behaviour, paying particular attention to data gleaned using an excellent animal model, the zebrafish (Danio rerio).

Opioid receptor-mediated changes in the NMDA receptor in developing rat and chicken

The use of opioids during pregnancy has been associated with neurodevelopmental toxicity in exposed children, leading to cognitive and behavioural deficits later in life. The N-methyl-D-aspartate receptor (NMDAR) subunit GluN2B plays critical roles in cerebellar development, and methadone has been shown to possess NMDAR antagonist effect. Consequently, we wanted to explore if prenatal opioid exposure affected GluN2B subunit expression and NMDAR function in rat and chicken cerebellum. Pregnant rats were exposed to methadone (10 mg/kg/day) or buprenorphine (1 mg/kg/day) for the whole period of gestation, using an osmotic minipump. To further examine potential effects of prenatal opioid exposure in a limited time window, chicken embryos were exposed to a 20 mg/kg dose of methadone or morphine on embryonic days 13 and 14. Western blot analysis of cerebella isolated from 14 days old rat pups exposed to buprenorphine showed significantly lower level of the GluN2B subunit, while the opioid exposed chicken embryo cerebellar GluN2B expression remained unaffected at embryonic day 17. However, we observed increased NMDA/glycine-induced calcium influx in cerebellar granule neurone cultures from opioid exposed chicken embryos. We conclude that prenatal opioid exposure leads to opioid receptor-dependent reduction in the postnatal expression of GluN2B in rat cerebella, and increase in NMDA-induced calcium influx in chicken embryo cerebella.

Mouse Fgf8-Cre-LacZ lineage analysis defines the territory of the postnatal mammalian isthmus

The isthmus is recognized as the most rostral segment of the hindbrain in non-mammalian vertebrates. In mammalian embryos, transient Fgf8 expression defines the developing isthmic region, lying between the midbrain and the first rhombomere, but there has been uncertainty about the existence of a distinct isthmic segment in postnatal mammals. We attempted to find if the region of early embryonic Fgf8 expression (which is considered to involve the entire extent of the prospective isthmus initially) might help to identify the boundaries of the isthmus in postnatal animals. By creating an Fgf8-Cre-LacZ lineage in mice, we were able to show that Fgf8-Cre reporter expression in postnatal mice is present in the same nuclei that characterize the isthmic region in birds. The 'signature' isthmic structures in birds include the trochlear nucleus, the dorsal raphe nucleus, the microcellular tegmental nuclei, the pedunculotegmental nucleus, the vermis of the cerebellum, rostral parts of the parabrachial complex and locus coeruleus, and the caudal parts of the substantia nigra and VTA. We found that all of these structures were labeled with the Fgf8-Cre reporter in the mouse brain, and we conclude that the isthmus is a distinct segment of the mammalian brain lying caudal to the midbrain and rostral to rhombomere 1 of the hindbrain.

Developmental alterations of the septohippocampal cholinergic projection in a lissencephalic mouse model

LIS1 is one of principal genes related with Type I lissencephaly, a severe human brain malformation characterized by abnormal neuronal migration in the cortex. The LIS1 gene encodes a brain-specific 45kDa non-catalytic subunit of platelet-activating factor (PAF) acetylhydrolase-1b (PAFAH1b), an enzyme that inactivates the PAF. We have studied the role of Lis1 using a Lis1/sLis1 murine model, which has deleted the first coding exon from Lis1 gene. Homozygous mice are not viable but heterozygous have shown a delayed corticogenesis and neuronal dysplasia, with enhanced cortical excitability. Lis1/sLis1 embryos also exhibited a delay of cortical innervation by the thalamocortical fibers. We have explored in Lis1/sLis1 mice anomalies in forebrain cholinergic neuron development, which migrate from pallium to subpallium, and functionally represent the main cholinergic input to the cerebral cortex, modulating cortical activity and facilitating attention, learning, and memory. We hypothesized that primary migration anomalies and/or disorganized cortex could affect cholinergic projections from the basal forebrain and septum in Lis1/sLis1 mouse. To accomplish our objective we have first studied basal forebrain neurons in Lis1/sLis1 mice during development, and described structural and hodological differences between wild-type and Lis1/sLis1 embryos. In addition, septohippocampal projections showed altered development in mutant embryos. Basal forebrain abnormalities could contribute to hippocampal excitability anomalies secondary to Lis1 mutations and may explain the cognitive symptoms associated to cortical displasia-related mental diseases and epileptogenic syndromes.

Ethanol inhibits neuritogenesis induced by astrocyte muscarinic receptors

In utero alcohol exposure can lead to fetal alcohol spectrum disorders, characterized by cognitive and behavioral deficits. In vivo and in vitro studies have shown that ethanol alters neuronal development. We have recently shown that stimulation of M(3) muscarinic receptors in astrocytes increases the synthesis and release of fibronectin, laminin, and plasminogen activator inhibitor-1, causing neurite outgrowth in hippocampal neurons. As M(3) muscarinic receptor signaling in astroglial cells is strongly inhibited by ethanol, we hypothesized that ethanol may also inhibit neuritogenesis in hippocampal neurons induced by carbachol-stimulated astrocytes. In the present study, we report that the effect of carbachol-stimulated astrocytes on hippocampal neuron neurite outgrowth was inhibited in a concentration-dependent manner (25-100 mM) by ethanol. This effect was because of the inhibition of the release of fibronectin, laminin, and plasminogen activator inhibitor-1. Similar effects on neuritogenesis and on the release of astrocyte extracellular proteins were observed after the incubation of astrocytes with carbachol in the presence of 1-butanol, another short-chain alcohol, which like ethanol is a competitive substrate for phospholipase D, but not by tert-butanol, its analog that is not a substrate for this enzyme. This study identifies a potential novel mechanism involved in the developmental effects of ethanol mediated by the interaction of ethanol with cell signaling in astrocytes, leading to an impairment in neuron-astrocyte communication.

Modulation of neuritogenesis by astrocyte muscarinic receptors

Astrocytes have been shown to release factors that have promoting or inhibiting effects on neuronal development. However, mechanisms controlling the release of such factors from astrocytes are not well established. Astrocytes express muscarinic receptors whose activation stimulates a robust intracellular signaling, although the role of these receptors in glial cells is not well understood. Acetylcholine and acetylcholine receptors are present in the brain before synaptogenesis occurs and are believed to be involved in neuronal maturation. The present study was undertaken to investigate whether stimulation of muscarinic receptors in astrocytes would modulate neurite outgrowth in hippocampal neurons. Rat hippocampal neurons, co-cultured with rat cortical astrocytes previously exposed to the cholinergic agonist carbachol, displayed longer neurites. The effect of carbachol in astrocytes was due to the activation of M3 muscarinic receptors. Exposure of astrocytes to carbachol increased the expression of the extracellular matrix proteins fibronectin and laminin-1 in these cells. This effect was mediated in part by an increase in laminin-1 and fibronectin mRNA levels and in part by the up-regulation of the production and release of plasminogen activator inhibitor-1, an inhibitor of the proteolytic degradation of the extracellular matrix. The inhibition of fibronectin activity strongly reduced the effect of carbachol on the elongation of all the neurites, whereas inhibition of laminin-1 activity reduced the elongation of minor neurites only. Plasminogen activator inhibitor-1 also induced neurite elongation through a direct effect on neurons. Taken together, these results demonstrate that cholinergic muscarinic stimulation of astrocytes induces the release of permissive factors that accelerate neuronal development.

Postnatal expression of alpha2 nicotinic acetylcholine receptor subunit mRNA in developing cortex and hippocampus

Nicotinic acetylcholine receptors (nAChRs) are pentameric ligand-gated cation channels composed of alpha and beta subunits. nAChR subunit expression is highly regulated during development. Previous studies have revealed increased expression of alpha3, alpha5, alpha7, and beta4 subunit mRNAs and alpha7 binding sites during hippocampal and cortical development. Here, we examined the expression of alpha2 subunit mRNA in rat cortex and hippocampus using highly sensitive radioactive in situ hybridization. alpha2 Subunit mRNA expression was first detected at P3 in cortex and hippocampus. During postnatal development the distribution of alpha2 subunit mRNA expression was spatially similar to the one found in adult, exhibiting highly restricted expression in scattered cells mostly in cortical layer V and retrosplenial cortex, and in scattered cells in CA1/CA3 stratum oriens and CA3 stratum radiatum. However, the expression intensity and number of alpha2 positive cells strongly increased to reach peak levels in both cortex and hippocampus at P7 and decreased thereafter to moderate to low to levels. Double in situ hybridization revealed that most, but not all, alpha2 mRNA expression was located in non-pyramidal GAD-positive cortical and hippocampal interneurons. Thus, similar to other nAChR subunits, alpha2 mRNA expression is transiently upregulated during postnatal development and nAChRs containing alpha2 subunits could regulate GABAergic activity during a critical period of network formation.

A single M1 residue in the beta2 subunit alters channel gating of GABAA receptor in anesthetic modulation and direct activation

General anesthetics allosterically modulate the activity of neuronal gamma-aminobutyric acid, type A (GABAA), receptors. Previous mutational studies from our laboratory and others have shown that the regions in transmembrane domain 1 (M1) and pre-M1 of alpha and beta subunits in GABA receptors are essential for positive modulation of GABA binding and function by the intravenous (IV) general anesthetics. Mutation of beta2Gly-219 to Phe corresponded in rho nearly eliminated the modulatory effect of IV anesthetics in alpha1/beta2/gamma2S combination. However, the general anesthetics retained the ability to directly open the channel of mutant G219F, and the apparent affinity for GABA was increased, and desensitization rate was reduced. In this study, we made additional single mutations such as 219 Ser, Cys, Ile, Asp, Arg, Tyr, and Trp. The larger side chains of the replacement residues produced the greatest reduction in enhancement of GABA currents by IV anesthetics at clinical concentrations (Trp > Tyr = Phe > Arg > Asp > Ile > Cys > Ser = wild type). Compared with a 2-3-fold response in wild type, pentobarbital and propofol enhanced less than 0.5-fold; etomidate and alphaxalone modulation was reduced from more than 4- to 1-fold in G219F, G219Y, and G219W. A linear correlation was observed between the volume of the residue at position 219 and the loss of modulation. An identical correlation was found for the effect of modulation on left-shift in the GABA EC50 value; furthermore, the same rank order of residues, related to size, was found for reduction in the maximal direct channel-gating by pentobarbital (1 mm) and etomidate (100 mum) and for increased apparent affinity for direct gating by the IV anesthetics.

The diversity of subunit composition in nAChRs: evolutionary origins, physiologic and pharmacologic consequences

Nicotinic acetylcholine receptors are made up of homologous subunits, which are encoded by a large multigene family. The wide number of receptor oligomers generated display variable pharmacological properties. One of the main questions underlying research in molecular pharmacology resides in the actual role of this diversity. It is generally assumed that the observed differences between the pharmacology of homologous receptors, for instance, the EC(50) for the endogenous agonist, or the kinetics of desensitization, bear some kind of physiologic relevance in vivo. Here we develop the quite challenging point of view that, at least within a given subfamily of nicotinic receptor subunits, the pharmacologic variability observed in vitro would not be directly relevant to the function of receptor proteins in vivo. In vivo responses are not expected to be sensitive to mild differences in affinities, and several examples of functional replacement of one subunit by another have been unravelled by knockout animals. The diversity of subunits might have been conserved through evolution primarily to account for the topologic diversity of subunit distribution patterns, at the cellular and subcellular levels. A quantitative variation of pharmacological properties would be tolerated within a physiologic envelope, as a consequence of a near-neutral genetic drift. Such a "gratuitous" pharmacologic diversity is nevertheless of practical interest for the design of drugs, which would specifically tackle particular receptor oligomers with a defined subunit composition among the multiple nicotinic receptors present in the organism.

Neurotransmitter regulation of neural development: acetylcholine and nicotinic receptors

Several neurotransmitter systems have been related to developmental processes during the past decade. In this review, we discuss the evidence that the nicotinic acetylcholine receptors could have an additional function during development that may be unrelated to their role in cholinergic neurotransmission in the vertebrate brain. Both temporal expression data and in vitro and in vivo studies with nicotinic agonists and antagonists have provided direct support for a role of nicotinic receptors in neural developmental processes such as neurite outgrowth and differentiation. A similar picture has emerged for other neurotransmitter and receptor systems as well, which generates a new view of neural processes during both development and mature life.

Distribution of choline acetyltransferase-immunoreactive structures in the lamprey brain

The distribution of cholinergic neurons and fibers was studied immunohistochemically in the brain of two species of lampreys (Petromyzon marinus and Lampetra fluviatilis), by using an antiserum against choline acetyltransferase (ChAT). The results obtained in both species were similar, but there appeared some interspecies differences. In the forebrain, cholinergic cells were present in the striatum, preoptic region, paraventricular nucleus, pineal and parapineal organs, habenula, and pretectum. The cranial nerve motoneurons (III, IV, V, VI, VII, IX, and X), the first and second spino-occipital nerves (so), and the ventral horn of the spinal cord showed a strong ChAT immunoreactivity. Additional cholinergic neurons were observed: the mesencephalic M5 nucleus of Schober, two different cell populations in the isthmic region, the efferent component of the eighth nerve, putative preganglionic parasympathetic cells, cells in the solitary tract nucleus, and the rhombencephalic reticular formation. Cholinergic fibers were widely distributed in the brain. Comparison with previous studies in other vertebrates suggests that major cholinergic pathways, like tectal innervation from the isthmic region, are also present in lampreys. Of particular interest was the prominent projection to the neurohypophysis from cholinergic neurons in the preoptic region and paraventricular nucleus. Present data were analyzed within the segmental paradigm, as was previously done in other vertebrates. Our results reveal that the organization of many cholinergic systems in the lamprey as, for example, in the striatal, preoptic, and isthmic regions, comprises features of the anamniote brain that remain common to all living amniotes studied so far, thus being conservative to a surprisingly high degree. Therefore, the distribution of ChAT-immunoreactive structures in the lamprey brain is, in general, comparable to that previously described in other vertebrate species.

Expression of cholinergic system molecules during development of the chick nervous system

There are suggestions of the participation of nicotinic acetylcholine receptors (nAChRs), the acetylcholine degradation enzyme, acetylcholinesterase (AChE), and the acetylcholine synthesizing enzyme, choline acetyltransferase (ChAT), in the development of the nervous system. In this study, we aimed at comparing the development of some subunits of the nAChRs, AChE, and ChAT in the chick nervous system by standard immunohistochemical methods. The expression of all molecules investigated here appeared very early in ganglia (embryonic day 3.5-4), persisting into posthatching, except for ChAT, which is not detected after hatching in ganglia. A differential development was observed for nAChR subunits, with these receptors appearing around embryonic day 6 in some sites. The time-course of development of different nAChR subunits revealed several instances of transient expression (such as in the cerebellum), increasing expression (such as in the nucleus spiriformis lateralis), and diminishing expression into posthatching stages (such as in the oculomotor and throclear nuclei). Expression of AChE and ChAT also starts around embryonic day 6 in some structures and follows mainly increasing time-courses in the chick brain. The results of this study reveal a developmentally regulated expression of cholinergic system-related molecules in the chick nervous system and characterize differential time-courses of expression for nAChR subunits, AChE, and ChAT during development.

Localization of nAChR subunit mRNAs in the brain of Macaca mulatta

We present here a systematic mapping of nAChR subunit mRNAs in Macaca mulatta brain. A fragment, from the transmembrane segments MIII to MIV of Macaca neuronal nAChR subunits was cloned, and shown to exhibit high identity (around 95%) to the corresponding human subunits. Then, specific oligodeoxynucleotides were synthesized for in situ hybridization experiments. Both alpha4 and beta2 mRNA signals were widely distributed in the brain, being stronger in the thalamus and in the dopaminergic cells of the mesencephalon. Most brain nuclei displayed both alpha4 and beta2 signals with the exception of some basal ganglia regions and the reticular thalamic nucleus which were devoid of alpha4 signal. alpha6 and beta3 mRNA signals were selectively concentrated in the substantia nigra and the medial habenula. The strongest signals for alpha3 or beta4 mRNAs were found in the epithalamus (medial habenula and pineal gland), whereas there were no specific alpha3 or beta4 signals in mesencephalic dopaminergic nuclei. alpha5 and alpha7 mRNA signals were found in several brain areas, including cerebral cortex, thalamus and substantia nigra, although at a lower level than alpha4 and beta2. The distribution of alpha3, alpha4, alpha5, alpha6, alpha7, beta2, beta3 and beta4 subunit mRNAs in the monkey is substantially similar to that observed in rodent brain. Surprisingly, alpha2 mRNA signal was largely distributed in the Macaca brain, at levels comparable with those of alpha4 and beta2. This observation represents the main difference between rodent and Macaca subunit mRNA distribution and suggests that, besides alpha4beta2*, alpha2beta2* nAChRs constitute a main nAChR isoform in primate brain.

Chick optic lobe contains a developmentally regulated alpha2alpha5beta2 nicotinic receptor subtype

The most widely expressed neuronal nicotinic acetylcholine receptor subtype in chick brain is that containing the alpha4 and beta2 subunits. However, immunoprecipitation and localization studies have shown that some brain areas also contain the alpha2 and/or alpha5 subunits, whose role in the definition of receptor properties is still intriguing. Using subunit-specific polyclonal antibodies, we found that the optic lobe is the chick central nervous system region that expresses the highest level of alpha2-containing receptors. Immunoprecipitation studies of these immunopurified alpha2-containing receptors labeled with the nicotinic agonist [(3)H]epibatidine showed that almost all of them contained the beta2 subunit and that more than 66% contained the alpha5 subunit. Western blot analyses of the purified receptors confirmed the presence of the alpha2, alpha5, and beta2 subunits and the absence of the alpha3, alpha4, alpha6, alpha7, alpha8, beta3, and beta4 subunits. The alpha2-containing receptors are developmentally regulated: their expression increases 25 times from embryonic day 7 to posthatching day 1 in the optic lobe, compared with an increase of only 5-fold in the forebrain. The alpha2-containing optic lobe receptors bind [(3)H]epibatidine (K(d) = 29 pM) and a number of other nicotinic agonists with very high affinity and have a pharmacological profile very similar to that of the alpha4beta2 subtype. They form functional cationic channels when reconstituted in lipid bilayers, with pharmacological and biophysical properties different from those of the alpha4beta2 subtype. These channels are activated by nicotinic agonists in a dose-dependent manner and are blocked by the nicotinic antagonist d-tubocurarine.

Effects of cholinergic deafferentation upon the expression of the alpha2 subunit of the nicotinic acetylcholine receptors in the chick lateral spiriform nucleus

Lesion, immunohistochemical, and immunoblotting methods were used to evaluate the effects of cholinergic deafferentation upon the expression of the alpha2 subunit of the nicotinic acetylcholine receptors in the lateral spiriform nucleus (SpL) of the chick brain. The expression of the alpha2 subunit in the SpL showed biphasic changes after lesion of its cholinergic source (nucleus semilunaris), with an increase after 2 days postlesion and a decrease after 3-7 days. Our results could represent a correlate of the phenomena of nicotinic receptor up- and down-regulation, induced by removal of the cholinergic input.

Nicotinic acetylcholine receptor gene expression in developing chick autonomic ganglia

The developmental expression patterns of ten genes encoding nicotinic acetylcholine receptor subunits were analyzed using Northern blots and in situ hybridization in chick peripheral ganglia of neural crest, placodal and dual embryonic origin. The superior cervical and ciliary ganglia were investigated in detail because they accumulated relatively abundant transcripts of the alpha3, beta4, alpha5 and alpha7 genes. In the superior cervical ganglion, these four mRNA species had similar developmental time-courses. They appeared at embryonic day 8 (E8), increased steadily until E16 and maintained a rather high plateau level until E18. In the ciliary ganglion, alpha7 transcripts were already abundant at E6, increased until E10, and considerably decreased thereafter. High-resolution in situ hybridization showed that alpha7 transcripts were present in all cell types of the E6 ciliary ganglion, whereas they were restricted to large neuronal somas at E16. Transfections with a reporter gene under the control of the alpha7 promoter demonstrated that a sharp developmental divide occurred at E11-12, after which stage the promoter was activatable in neurons exclusively.

Nicotine, brain nicotinic receptors, and neuropsychiatric disorders

Neuronal nicotinic acetylcholine receptors (nAChRs) represent a large family of ligand-gated cation channels with diverse structures and properties. In contrast to the muscular nAChRs, the physiological functions of neuronal nAChRs are not well defined to date. Behavioral studies indicate that brain nAChRs participate in complex functions such as attention, memory, and cognition, whereas clinical data suggest their involvement in the pathogenesis of certain neuropsychiatric disorders (Alzheimer's and Parkinson's diseases, Tourette's syndrome, schizophrenia, depression, etc.). For the majority of these disorders, the use of nAChRs' agonists may represent either a prophylactic (especially for Alzheimer's and Parkinson's diseases) or a symptomatic treatment. The possible mechanisms underlying these beneficial effects as well as the characteristics and potential therapeutic use of new, subtype-selective nAChRs agonists are presented.

Expression of a neuronal nicotinic acetylcholine receptor in insect and mammalian host cell systems

Different mammalian and insect somatic host cell systems were tested in their ability to express, fold, and assemble alpha7-type neuronal acetylcholine receptor (AChR) both at the transcriptional and translational level. For this purpose we employed clonal cell lines derived from the neural crest, such as PC12 cells from a rat adrenal pheochromocytoma, and GH3 cells isolated from a rat pituitary tumor, as well as non-neuronal cells such as NIH-3T3 fibroblasts from embryonic NIH Swiss mouse and Sf9 cells from ovary tissue of the Spodoptera frugiperda butterfly. Total RNA, isolated from either transfected or non-transfected PC12, GH3 or 3T3 cells, or recombinant AcNPV-infected and mock-infected Sf9 cells was analyzed by Northern blot. PC12 cells, which endogenously express alpha7 AChR, and all its heterologous alpha7-transfectant clones, exhibited variable but generally high amounts of a single transcript. GH3 and NIH-3T3 transfectant clones and recombinant AcNPV-infected Sf9 cells expressed variable levels of alpha7-mRNA, with a single transcript that co-migrated with the 28S rat rRNA. Only the neural crest-derived cell lines appeared to functionally express the alpha7 AChR, as measured by their [125I]alpha-bungarotoxin binding ability. The results suggest that heterologous expression of alpha7 is regulated not at the transcriptional, but at the postranslational level and that not all host cell systems appear to express the cellular factors needed for the correct postranslational modifications leading to mature and functional alpha7 AChR. Furthermore, the results suggest that tightly controlled expression mechanisms have evolved in parallel with this ancient cholinergic sequence.

Fast excitatory nicotinic transmission in the chick lateral spiriform nucleus

The lateral spiriform nucleus (SpL) in the chick mesencephalon contains functional nicotinic receptors and receives a cholinergic fiber projection. We now use double-label immunohistochemistry to demonstrate that choline acetyltransferase-immunopositive fibers in the SpL and in the cholinergic fiber tract lateral to the nucleus are associated with fibers expressing the alpha5 and/or alpha3 nicotinic receptor subunits as determined by mAb35 immunoreactivity. This morphological evidence suggests that there might be synapses between the cholinergic fibers and the dendrites of SpL neurons. Whole-cell recordings from SpL neurons in current-clamp mode revealed EPSPs evoked by stimulation of the cholinergic fiber tract lateral to the SpL. These EPSPs increased in amplitude in the presence of bicuculline. Further addition of the nicotinic antagonist dihydro-beta-erythroidine (DHbetaE) to the buffer significantly attenuated them. Almost all of the remaining EPSP was blocked by 6,7-dinitroquinoxaline-2,3-dione. In the presence of an antagonist cocktail that isolated the nicotinic responses, a fast, monosynaptic nicotinic EPSP or EPSC was evoked. In some neurons, the nicotinic EPSP resulted in the generation of an action potential. The nicotinic nature of the evoked response was confirmed by blockade of the EPSPs or EPSCs with nicotinic antagonists, including DHbetaE, D-tubocurare, and mecamylamine. The nicotinic response was insensitive to low concentrations (10-100 nM) of methyllycaconitine, indicating that typical alpha7-containing receptors were not involved. The results demonstrate that endogenously released acetylcholine generates EPSPs that can elicit action potentials by acting at postsynaptic nicotinic receptors on SpL neurons.

Functional properties of the neuronal nicotinic acetylcholine receptor beta3 promoter in the developing central nervous system

Within the chick central nervous system, expression of the beta3 nicotinic acetylcholine receptor gene is restricted to a subset of retinal neurons, the majority of which are ganglion cells. Transient transfection in retinal neurons and in neural and non-neural cells from other regions of the chick embryo allowed the identification of the cis-regulatory domain of the beta3 gene. Within this domain, a 75-base pair fragment located immediately upstream of the transcription start site suffices to reproduce the neuron-specific expression pattern of beta3. This fragment encompasses an E-box and a CAAT box, both of which are shown to be key positive regulatory elements of the beta3 promoter. Co-transfection experiments into retinal, telencephalic, and tectal neurons with plasmid reporters of beta3 promoter activity and a number of vectors expressing different neuronal (ASH-1, NeuroM, NeuroD, CTF-4) and non-neuronal (MyoD) basic helix-loop-helix transcription factors indicate that the cis-regulatory domain of beta3 has the remarkable property of discriminating accurately between related members of the basic helix-loop-helix protein family. The sequence located immediately 3' of the E-box participates in this selection, and the E-box acts in concert with the nearby CAAT box.

Brain nicotinic receptors: structure and regulation, role in learning and reinforcement

The introduction, in the late sixties, of the concepts and methods of molecular biology to the study of the nervous system had a profound impact on the field, primarily through the identification of its basic molecular components. These structures include, for example, the elementary units of the synapse: neurotransmitters, neuropeptides and their receptors, but also ionic channels, intracellular second messengers and the relevant enzymes, cell surface adhesion molecules, or growth and trophic factors [21,78,81, 52,79]. Attempts to establish appropriate causal relationships between these molecular components, the actual organisation of neural networks, and a defined behavior, nevertheless, still must overcome many difficulties. A first problem is the recognition of the minimum levels of organisation, from the molecular, cellular, or multicellular (circuit) to the higher cognitive levels, that determine the given physiological and/or behavioral performance under investigation. A common difficulty (and potential source of errors of interpretation) is to relate a cognitive function to a network organization which does not possess the required structural complexity and vice-versa. Another problem is to distinguish, among the components of the system, those which are actually necessary and those which, taken together, suffice for a given behavior to take place. Identification of such a minimal set of building blocks may receive decisive insights from the elaboration of neurally plausible formal models that bring together, within a single and coherent 'artificial organism', the neuronal network, the circulating activity, and the behavior they determine (see [42,43,45,72,30]). In this communication, we shall attempt, still in a preliminary fashion, to bring together: (1) our recent knowledge on the molecular biology of brain nicotinic receptors (nAChRs) and their allosteric properties and (2) integrated behaviors, such as cognitive learning, investigated for instance with delayed-response or passive avoidance tasks that are likely to involve nAChRs in particular at the level of reinforcement (or reward) mechanisms (see [18,29,135]).

Nicotinic receptor subtypes in the developing chick brain: appearance of a species containing the alpha4, beta2, and alpha5 gene products

Increasing evidence suggests nicotinic receptors regulate developmental events in the nervous system. We used [3H]epibatidine and 125I-alpha-bungarotoxin, together with subunit-specific monoclonal antibodies, to distinguish and quantify nicotinic receptor subtypes in developing chick brain. The results show that more than three fourths of the epibatidine-binding receptors at both early and late embryonic stages contain alpha4 and beta2 subunits, representing receptors previously distinguished by high affinity nicotine binding. A fraction of these also contain the alpha5 gene product, which is consistent with studies on transfected cells showing that the alpha4, beta2, and alpha5 gene products coassemble to produce epibatidine-binding receptors. A small portion of the receptors contain alpha3 and beta4 subunits, assembled in part with either alpha4 or beta2 subunits. The most abundant nicotinic receptors, however, at both early and late embryonic stages are those having high affinity for alpha-bungarotoxin rather than epibatidine. Most contain alpha7 subunits, whereas about half contain alpha8 subunits as well. The sharpest developmental increase between embryonic days 8 and 17/18 occurs with receptors containing alpha5 subunits, whereas receptors containing alpha3 or beta4 subunits undergo no specific increase. The three major receptor species (containing alpha4 and beta2 but not alpha5 subunits; alpha7 subunits; or alpha7 and alpha8 subunits) each increase approximately 3-fold during the same period. The results indicate greater receptor complexity than appreciated previously; they provide information about the rules governing subunit assembly in neuronal nicotinic receptors and draw attention to the role of alpha5 subunits in late development.

GABA(A) receptor subunit expression in intrastriatal striatal grafts comparison between normal developing striatum and developing striatal grafts

Expression of the alpha1, alpha2 and beta2/3 GABA(A) receptor subunits in maturing cell-suspension striatal grafts and in normal developing striatum was studied by immunocytochemistry. During normal postnatal development, the alpha1 subunit was present in the striatum only at very low density, while the alpha2 and beta2/3 subunits were present with a patchy distribution, in some patches at high density. Double-staining techniques indicated that DARPP-32 (a marker of striatal projection neurons) was not colocalized with alpha1, but was present in some beta2/3-positive areas and all alpha2-positive areas. In striatal grafts, alpha1 immunoreactivity was first detected 2 weeks post-grafting (p.g.), and by 3-10 weeks p.g. the pattern was similar to that observed in mature grafts (1 year p.g.), in which alpha1-immunopositive patches surrounding DARPP-32-positive (i.e. striatum-like) areas are observed. Alpha2 and beta2/3 immunoreactivity was observed within the first week p.g., and by 3-10 weeks p.g. was similar to that observed in mature grafts (i.e. immunoreactivity throughout the graft but with patches of different intensity). During graft maturation there was a marked decline in alpha2 immunoreactivity in DARPP-32-negative areas, as is observed during normal development of the globus pallidus and ventral pallidum. Interestingly, alpha1- and beta2/3-positive fibers (perhaps mostly dendrites) entered DARPP-32-positive patches from DARPP-32-negative areas. This study indicates that the time course of expression of GABA(A) receptor subunits in grafted striatal neurons, closely matches that of morphological maturation of the transplant, that of the development of functional synaptic activity and that of GABA(A) receptor subunit immunoreactivity in normal developing striatum. Our results also suggest that there are significant interactions between DARPP-32-positive and DARPP-32-negative areas with respect to the expression of GABA(A) receptors, and support the suggestion that miniature 'striatopallidal systems' may develop within grafts; such interactions may be important for the functional integration of striatal grafts with the host brain.

The molecular biology of neuronal nicotinic acetylcholine receptors

The molecular cloning of genes encoding neuronal nicotinic acetylcholine receptors (nAChRs) has made possible a better understanding of the pharmacology and toxicology of cholinergic compounds. Neuronal nAChRs are related in structure to the nAChRs present at the neuromuscular junction. They are composed of multiple subunits designated either alpha and beta. Eight alpha and three beta subunit genes have been cloned. The alpha subunits contain the ligand binding sites, whereas beta subunits are structural subunits that contribute to the function of the receptor. A large number of nAChRs can be formed from different combinations of alpha and beta subunits. Different combinations of alpha and beta subunits can produce receptors in vitro with distinct ion conducting properties. Each subunit gene is expressed in a distinct pattern in the nervous system. The expression of at least some of the nAChR subunit genes is regulated during development and by cell-cell interactions. Each neuronal nAChR subtype has a distinct pharmacology. Both alpha and beta subunits contribute to the pharmacological properties of each subtype. The expression of multiple nAChR subtypes may allow for precise control of neurotransmission mediated by acetylcholine in diverse populations of neurons.

Presumptive presynaptic nicotinic acetylcholine receptors in the chick tectum: effects of lesions of the lateral spiriform nucleus

There are indications that nicotinic acetylcholine receptor subunits in the superficial layers of the chick tectum (Cajal's layers 1-7) may be transported from the retina. However, nicotinic receptor subunits are detectable by immunohistochemistry in all layers of the optic tectum. In this study, we performed unilateral electrolytic lesions of the lateral spiriform nucleus, which projects to the deep layers of the tectum and contains high amounts of nicotinic receptors in its perikarya. Following lesions of the lateral spiriform nucleus, both the alpha 5 and the beta 2 subunits were markedly depleted in the neuropil of the deep layers of the ipsilateral optic tectum (layers 8-13). No changes were observed in somata that contain either subunit. The present results suggest that most of the nicotinic acetylcholine receptor subunits in the chick optic tectum occur in axonal systems and could then constitute presynaptic receptors.

Differential regulation of neuronal nicotinic acetylcholine receptor subunit gene promoters by Brn-3 POU family transcription factors

The regulatory region of the neuronal nicotinic acetylcholine (nACh) receptor alpha 2 subunit gene is activated by the Brn-3b POU family transcription factor but not by the closely related factors Brn-3a and Brn-3c. This pattern of regulation has not previously been observed for other neuronally expressed genes, several of which, such as those encoding alpha-internexin or SNAP-25, are activated by Brn-3a and Brn-3c but repressed by Brn-3b. The alpha 3 nACh receptor subunit gene is also shown to be activated by Brn-3a but is repressed by Brn-3b and Brn-3c. In contrast, the Brn-3 POU family transcription factors have no effects on either the alpha 7 or beta 4 nACh receptor subunit genes. The actions of Brn-3b on the alpha 2 subunit are thus in contrast to the inhibitory actions of Brn-3b on several promoters that are activated by Brn-3 alpha. The different actions of the Brn-3 POU factors on the range of nACh receptor genes tested suggests that the novel stimulation of the alpha 2 subunit by Brn-3b is specific to this subunit and not a general feature of nACh receptor genes.

Single-channel recording in brain slices reveals heterogeneity of nicotinic receptors on individual neurons within the chick lateral spiriform nucleus

We examined the functional properties of central nicotinic acetylcholine receptors at the single-channel level using tight-seal, voltage-clamp techniques. Single-channel currents were recorded from cell-attached patches on lateral spiriform neurons in chick brain slices. These neurons are known to express functional nicotinic receptors that are insensitive to the antagonists alpha-bungarotoxin and kappa-bungarotoxin, and which exhibit a high affinity for nicotine and other nicotinic agonists. Single-channel openings were observed in 84% of patches (n = 118) when the nicotinic agonists acetylcholine (1-100 microM), carbamylcholine (3-100 microM), or nicotine (3-10 microM) were present in the patch pipette. In contrast, single-channels were markedly reduced in number or entirely absent when the nicotinic antagonist dihydro-beta-erythroidine was present along with acetylcholine (n = 7) or when no agonist was present in the pipette (n = 22). Single-channel openings displayed inward rectification at depolarized potentials, and were dependent on extracellular sodium. Between 1 and 30 microM acetylcholine, a dose-response relationship was observed between agonist concentration and single-channel open probability during the first minute following seal formation. Multiple classes of single nicotinic channels, with calculated mean slope conductances of 15, 31, 40, and approximately 70 pS, were observed in membrane patches on different neurons within the lateral spiriform nucleus, and even within single patches on individual neurons. We conclude that neurons within the lateral spiriform nucleus express functionally heterogeneous nicotinic receptors and that in some neurons different nicotinic receptor subtypes are present in close proximity to each other on the same cell surface.

Promoter elements conferring neuron-specific expression of the beta 2-subunit of the neuronal nicotinic acetylcholine receptor studied in vitro and in transgenic mice

Several genes encoding subunits of the neuronal nicotinic acetylcholine receptors have been cloned and regulatory elements involved in the transcription of the alpha 2 and alpha 7-subunit genes have been described. Yet, the detailed mechanisms governing the neuron-specific transcription and the spatio-temporal expression pattern of these genes remain largely uninvestigated. The beta 2-subunit is the most widely expressed neuronal nicotinic receptor subunit in the nervous system. We have studied the structural and regulatory properties of the 5' sequence of this gene. A fragment of 1163 bp of upstream sequence is sufficient to drive the cell-specific transcription of a reporter gene in both transient transfection assays and in transgenic mice. Deletion analysis and site-directed mutagenesis of this promoter reveal two negative elements and one positive element. The positively-acting sequence includes one functional E-box. One of the repressor elements is located in the transcribed region and is the NRSE/RE1 sequence already described in promoters of neuronal genes. In this paper, we describe the neuron-specific promoter of the gene encoding the neuronal nicotinic acetylcholine receptor beta 2-subunit.

Expression of the alpha 4 neuronal nicotinic acetylcholine receptor subunit in the developing mouse hippocampus

Neurotransmitters such as acetylcholine can control neuritogenesis of hippocampal cells. The timing of its receptors expression consequently may influence synaptogenesis and neuronal activity in the developing hippocampus. We investigated the mRNA expression of the nicotinic acetylcholine-gated ion channel receptor (nAChR) alpha 4 subunit in the embryonic and postnatal hippocampal formation. Although its expression level is low in the adult hippocampus, this protein constitutes the major nAChR subunit in the central nervous system. We carried out in-situ hybridization experiments to determine whether or not the alpha 4 AChR subunit mRNA distributions show evidence of regional and developmental regulation during hippocampal maturation. Our studies reveal that alpha 4 AChR mRNA expression was low at the embryonic stage, but increased transiently during postnatal development reaching a maximum during the second week of life and decreasing thereafter, to a minimum at adulthood. In hippocampal regions, the peak values of alpha 4 AChR expression were between 400 and 800% of adult alpha 4 messenger levels. In the postnatal hippocampus, most of the cells from the pyramidal layer of the CA3 and CA2 areas displayed a strong hybridization signal for the alpha 4 AChR subunit. In the hilus and the CA1 regions, the localization of the alpha 4 transcripts seemed to be restricted to some interneurons and pyramidal cells, respectively. Moderate and uniform in-situ hybridization signals were observed in granular cells from the dentate gyrus. The transient profile of alpha 4 expression suggests that nAChRs may participate in the early postnatal maturation of hippocampal circuity.

The neuronal nicotinic acetylcholine receptor alpha 2 subunit gene promoter is activated by the Brn-3b POU family transcription factor and not by Brn-3a or Brn-3c

The regulatory region of the neuronal nicotinic acetylcholine receptor alpha 2 subunit gene, which contains six copies of the octamer-related sequence CCCCATGCAAT, is activated by the Brn-3b POU family transcription factor but not by the closely related factors Brn-3a and Brn-3c. This effect is in contrast to the previously documented inhibitory effect of Brn-3b on octamer-containing promoters that are activated by Brn-3a and Brn-3c. Activation of the alpha 2 gene by Brn-3b requires that both the POU domain and other N-terminal sequences are derived from Brn-3b and is dependent on the intactness of the alpha 2 gene regulatory region, being lost in truncated derivatives containing one, two, or four copies of the octamer-related sequence. Surprisingly, however, these truncated derivatives are activated by Brn-3c. These effects are discussed in terms of both the influence of the target sequence and its context in the promoter on activation by the various forms of Brn-3 as well as of the processes that restrict expression of the alpha 2 subunit gene to a few cells in the nervous system.

Expression of neuronal acetylcholine nicotinic receptor alpha 4 and beta 2 subunits during postnatal development of the rat brain

The expression of the alpha 4 and beta 2 subunits of neuronal nicotinic acetylcholine receptors (nAChRs) was studied in developing rat brain using in situ hybridization. The levels of both transcripts were already high at birth in cerebral cortex, medial habenula, CA1/CA3 regions of the hippocampus and several thalamic nuclei. In general, the beta 2 subunit showed a higher density of hybrids than the alpha 4. Beta 2 expression did not change with age in the medial habenula, medial geniculate nucleus or in the hippocampus whereas it decreased in the cortex. The developmental pattern of the hybridization signal for alpha 4 was different according to the brain area considered. The expression of the two transcripts showed a biphasic pattern in some thalamic nuclei: the lowest levels occurring during the first and second postnatal weeks respectively, and the highest levels during the second and fourth postnatal weeks. The ontogenetic profile of the expression of the alpha 4 subunit in the thalamic nuclei coincided with that of [3H]-L-nicotine binding sites. These findings suggest that the two subunits of nAChRs are independently regulated in most of the brain areas examined, and that in some regions, such as the thalamus, the ontogenetic variations reported for the alpha 4 subunit correlate with those observed for the [3H]-L-nicotine binding sites.

Characterization of nicotinic acetylcholine receptors expressed in primary cultures of cerebellar granule cells

Nicotinic acetylcholine receptors (nAChRs), like other calcium permeable channel receptors, may play a crucial role during neuronal development. We have characterized nAChRs in developing mouse cerebellar granule cells in primary culture. L-[3H]Nicotine, [3H]cytisine and [125I]alpha-bungarotoxin binding experiments revealed the presence of a single class of saturable and specific high affinity binding sites for each ligand. The expression of these nicotinic binding sites followed a developmental pattern reaching a maximum during the establishment of excitatory amino acid synaptic contacts. Immunolabeling with monoclonal antibodies to nAChR subunits revealed the presence of alpha 4 and beta 2 subunits in most neurons. Moreover, some neuronal cells displayed a somatic as well as a neuritic localization for the alpha 7 subunit as shown by [125I]alpha-bungarotoxin autoradiography. The reverse transcription-polymerase chain reaction (RT-PCR) detected the presence of mRNAs for alpha 3, alpha 4, alpha 5, alpha 7, beta 2 and beta 4 nAChR subunits. Non-neuronal cells did not express nAChRs, as shown by [3H]nicotine and [125I]alpha-bungarotoxin binding, immunocytochemistry and PCR. Maximum Ca2+ influx elicited by nicotine, and partly sensitive to alpha-bungarotoxin, was observed around 10-14 days after plating. This correlated with the time period at which the highest number of nicotine binding sites was detected. Sensitivity to several NMDA receptor antagonists as well as to removal of endogenous glutamate by pyruvate transaminase treatment revealed a glutamatergic component in the nicotine stimulated calcium influx. The time-dependent specific nAChR expression and the potential association between nAChRs and NMDA receptor activation suggest that nAChRs may regulate glutamatergic activity during synaptogenesis in cerebellar granule cells.

Molecular evolution of the nicotinic acetylcholine receptor: an example of multigene family in excitable cells

An extensive phylogenetic analysis of the nicotinic-acetylcholine-receptor subunit gene family has been performed by cladistic and phenetic methods. The conserved parts of amino acid sequences have been analyzed by CLUSTAL V and PHYLIP software. The structure of the genes was also taken in consideration. The results show that a first gene duplication may have occurred before the appearance of Bilateria. Three subfamilies then appeared: I--the neuronal alpha-bungarotoxin binding-site subunits (alpha 7, alpha 8); III--the neuronal nicotinic subunits (alpha 2-alpha 6, beta 2-beta 4), which also contain the muscle acetylcholine-binding subunit (alpha 1); and IV--the muscle non-alpha subunits (beta 1, gamma, delta, epsilon). The Insecta subunits (subfamily II) could be orthologous to family III and IV. Several tissular switches of expression from neuron to muscle and the converse can be inferred from the extant expression of subunits and the reconstructed trees. The diversification of the neuronal nicotinic subfamily begins in the stem lineage of chordates, the last duplications occurring shortly before the onset of the mammalian lineage. Such evolution parallels the increase in complexity of the cholinergic systems.

Innervation and target tissue interactions differentially regulate acetylcholine receptor subunit mRNA levels in developing neurons in situ

Neurons engage in two distinct types of cell-cell interactions: they receive innervation and establish synapses on target tissues. Regulatory events that influence synapse formation and function on developing neurons are largely undefined. We show here that nicotinic acetylcholine receptor (AChR) subunit transcript levels are differentially regulated by innervation and target tissue interactions in developing chick ciliary ganglion neurons in situ. Using ganglia that have developed in the absence of pre- or postganglionic tissues and quantitative RT-PCR, we demonstrate that alpha 3 and beta 4 transcript levels are increased by innervation and target tissue interactions. In contrast, alpha 5 transcript levels are increased by innervation, but target tissues have little effect. Whole-cell ACh-induced currents, used to estimate the number of functional AChRs, change in correlation with alpha 3 and beta 4, but not alpha 5, transcript levels. A model is proposed in which the changes in AChR subunit expression regulate levels of synaptic activity, which is a critical determinant of synapse stabilization and elimination, and neuronal cell death.

Negative regulatory elements upstream of a novel exon of the neuronal nicotinic acetylcholine receptor alpha 2 subunit gene

The expression of the nicotinic acetylcholine receptor alpha 2 subunit gene is highly restricted to the Spiriform lateralis nucleus of the Chick diencephalon. As a first step toward understanding the molecular mechanism underlying this regulation, we have investigated the structural and regulatory properties of the 5' sequence of this gene. A strategy based on the ligation of an oligonucleotide to the first strand of the cDNA (SLIC) followed by PCR amplification was used. A new exon was found approximately 3kb upstream from the first coding exon, and multiple transcription start sites of the gene were mapped. Analysis of the flanking region shows many consensus sequences for the binding of nuclear proteins, suggesting that the 1 kb flanking region contains at least a portion of the promoter of the gene. We have analysed the negative regulatory elements present within this region and found that a silencer region located between nucleotide -144 and +76 is active in fibroblasts as well as in neurons. This silencer is composed of six tandem repeat Oct-like motifs (CCCCATGCAAT), but does not bind any member of the Oct family. Moreover these motifs were found to act as a silencer only when they were tandemly repeated. When two, four or five motifs were deleted, the silencer activity of the motifs unexpectedly became an enhancer activity in all cells we have tested.

Molecular analysis of the Wnt-1 proto-oncogene in Ambystoma mexicanum (axolotl) embryos

To analyze Wnt-1 expression during neurulation in urodele embryos, we have isolated a Wnt-1 cDNA clone, Awnt-1, from an Ambystoma mexicanum (axolotl) neurula-stage cDNA library. Awnt-1 codes for a protein of 369 amino acids rich in cysteine residues, is preceded by a hydrophobic leader peptide sequence and contains four possible sites for N-linked glycosylation. The temporal expression profile of Awnt-1 was analyzed by reverse transcription-polymerase chain reaction (RT-PCR). Awnt-1 expression in the axolotl embryo is biphasic. Awnt-1 transcripts are found in early blastulae until gastrulation, are barely detectable during gastrulation, and are present again from neurulation until late embryogenesis. Transcripts are present before the midblastula transition, indicating that they might be of maternal origin. To localize Awnt-1 expression in embryos during the first phase of expression, early gastrulae were dissected by cutting along the animal-vegetal and future dorso-ventral axes and analyzed by RT-PCR. At the early gastrula stage Awnt-1 transcripts appear to be located in the future ventral region of the embryo. Hatching larvae no longer express Awnt-1. PCR reactions performed using cDNA library-phage DNA templates derived from whole neurulae versus embryos with the neuroectoderm removed suggest that, in the neurula, Awnt-1 transcripts are located in the neuroectoderm. This suggest that, as is the case for Wnt-1 in other vertebrates, Awnt-1 may be involved in neurogenesis. These results suggest that Wnt-1 has earlier roles in development than has been considered until now.

Chicken neuronal acetylcholine receptor alpha 2-subunit gene exhibits neuron-specific expression in the brain and spinal cord of transgenic mice

Transgenic mice carrying the complete structural gene of the alpha 2 subunit of the chicken neuronal nicotinic acetylcholine receptor (nAChR) and 7 kilobase pairs (kbp) of 5' upstream and 3 kbp of 3' downstream sequences have been generated. The transgene was stably integrated in transgenic lines and transmitted to their progeny. Avian transgene expression was predominant in the central nervous system as detected by specific alpha 2-subunit cDNA amplification. Moreover, in at least two independent mouse lines, its expression appeared to be neuron-specific and reproducibly restricted to subregions in the brain and spinal cord, as revealed by in situ hybridization histochemistry. Most cranial motor nuclei were positive, and several of the alpha 2-subunit transgene-expressing structures corresponded to cholinergic areas in rodents. This study reveals that regulatory mechanisms giving rise to neuronal-specific gene expression have been conserved at least in part between birds and mammals.

Developmental switch in the expression of NMDA receptors occurs in vivo and in vitro

The properties of many ligand-gated ion channels are altered during development. We have characterized a developmental switch in the sensitivity of NMDA receptors to the novel antagonist ifenprodil using ligand binding assays with rat brain membranes and voltage-clamp recording of Xenopus oocytes expressing NMDA receptors after injection of RNA from rat brain and from cloned subunits of the receptor. In neonatal rat brain, NMDA receptors have a uniformly high affinity for ifenprodil. During postnatal development, a second population of receptors having a 100-fold lower affinity for ifenprodil is expressed and represents 50% of NMDA receptors in adult rat brain. This developmental change also occurred in cortical neurons maintained in primary culture. Ifenprodil potently inhibited responses of homomeric NR1 and heteromeric NR1/NR2B receptors but not NR1/NR2A receptors expressed in oocytes, suggesting that inclusion of different NR2 subunits in native NMDA receptors can control the sensitivity to ifenprodil.

GABAA receptor messenger RNA expression in the deep cerebellar nuclei of Purkinje cell degeneration mutants is maintained following the loss of innervating Purkinje neurons

Recent studies have suggested that innervation modulates GABAA receptor gene expression in the rodent cerebellum. To examine this question, the expression and levels of GABAA receptor subunit messenger RNAs in the deep cerebellar nuclei of Purkinje cell degeneration mice and littermate controls were examined by quantitative in situ hybridization histochemistry. In the Purkinje cell degeneration mutant, the selective postnatal degeneration of Purkinje neurons disrupts GABAergic input from the cerebellar cortex to the deep nuclei. Despite this loss of Purkinje cells, virtually all large neurons of the deep cerebellar nuclei of Purkinje cell degeneration animals expressed the alpha 1, beta 2, and gamma 2 subunit messenger RNAs. These subunit messenger RNAs were observed at all experimental times from postnatal day 24 to postnatal day 90, a period ranging from the onset of behavioral abnormalities in the mutant to the completion of Purkinje cell loss. At no time were additional beta subunit messenger RNAs, normally absent from the deep cerebellar nuclei in control mice, detected in this region of the mutant. Quantitative analysis of the hybridization signals over individual neurons revealed that Purkinje cell loss differentially affected the expression of GABAA receptor subunit messenger RNAs. While the levels of the beta 2 and gamma 2 subunit messenger RNAs in individual neurons were comparable in mutants and controls at all ages, differences in alpha 1 subunit messenger RNA expression were observed. At postnatal day 24, the level of alpha 1 subunit mRNA in individual neurons of the mutant was only 60% that found in the control.(ABSTRACT TRUNCATED AT 250 WORDS)

Localization of 3H-nicotine, 125I-kappa-bungarotoxin, and 125I-alpha-bungarotoxin binding to nicotinic sites in the chicken forebrain and midbrain

We have previously localized cholinergic cell bodies and fibers within the midbrain of the chicken with choline acetyltransferase immunohistochemistry. In a continuing effort to characterize the central cholinergic system, the present study examines the distribution of various nicotinic acetylcholine receptors in the forebrain and midbrain of the chicken. The binding of 3H-nicotine, 125I-kappa-bungarotoxin, and 125I-alpha-bungarotoxin was localized by film autoradiography in adjacent sections of the adult chicken brain, allowing a comparison of the distribution of different classes of nicotinic binding sites within the brain. Although all three ligands were often co-localized, there were areas that bound 3H-nicotine but not the 125I-neurotoxins, or vice versa. Very high densities of all three ligands were found in the hyperstriatum ventrale; the nucleus geniculatus lateralis, pars ventralis; the griseum tectale; the nucleus dorsolateralis anterior thalami; the nucleus lentiformis mesencephali, pars lateralis and pars medialis; the periventricular organ; and the stratum griseum et fibrosum superficiale, layer f of the optic tectum. The nucleus spiriformis lateralis had the highest levels of 3H-nicotine binding in the chicken brain, but it did not bind either of the two snake neurotoxins. On the other hand, high levels of both 125I-alpha-bungarotoxin and 125I-kappa-bungarotoxin binding were found in the nucleus semilunaris and the nucleus ovoidalis, but these areas contained little or no 3H-nicotine binding. No unique 125I-kappa-bungarotoxin sites, unrecognized by 125I-alpha-bungarotoxin, were identified by the low resolution autoradiography performed in this study. In general, nicotinic receptors were found in areas that have been reported to contain cholinergic cell bodies or fibers. Comparison of our results with the expression of neuronal nicotinic receptor subunits, as determined by in situ hybridization, suggests that many of the high affinity 3H-nicotine sites are localized presynaptically, as, for example, in the retinorecipient nuclei and the nucleus interpeduncularis. The lack of 125I-kappa-bungarotoxin binding in the presence of alpha-bungarotoxin indicates that the chicken brain has only very low levels of a unique kappa-bungarotoxin site. This is in marked contrast to chicken, frog, and rat autonomic ganglia, where a unique kappa-neurotoxin-sensitive receptor has been identified and shown to mediate nicotinic neurotransmission.

Target-dependent regulation of retinal nicotinic acetylcholine receptor and tubulin RNAs during optic nerve regeneration in goldfish

A fundamental issue in central nervous system development regards the effect of target tissue on the differentiation of innervating neurons. We address this issue by characterizing the role the retinal ganglion cell target, i.e., the optic tectum, plays in regulating expression of tubulin and nicotinic acetylcholine receptor genes in regenerating retinal ganglion cells. Tubulins are involved in axonal growth, whereas nicotinic acetylcholine receptors mediate communication across synapses. Retinal ganglion cell axons were induced to regenerate by crushing the optic nerve. Following crush, there was a rapid increase in alpha-tubulin RNAs (3 days), which preceded the increase in nicotinic acetylcholine receptor RNAs (10-15 days). Both classes of RNAs approached control levels by the time retinotectal synapses and functional recovery were restored (4-6 weeks). If the optic nerve was repeatedly crushed or its target ablated, tubulin RNAs remained elevated, and the increase in receptor RNAs that would otherwise be seen 2 weeks after a single nerve crush did not occur. The interaction of retinal ganglion cell axons with their targets in the optic tectum appears, then, to exert a suppressive effect on the RNA encoding a cytoskeletal protein, tubulin, and an inductive effect on RNAs encoding nicotinic acetylcholine receptors involved in synaptic communication.

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.

Region-specific expression of messenger RNAs encoding GABAA receptor subunits in the developing rat brain

The distribution and levels of messenger RNAs encoding the alpha 1, beta 1, beta 2, beta 3, and gamma 2 subunits of the GABAA receptor in the developing and adult rat brain were investigated using quantitative in situ hybridization histochemistry and subunit-specific probes. Regional localization of the subunit messenger RNAs was determined with film autoradiography and expression in identified neuronal cell populations was examined using higher resolution techniques. Each of the GABAA receptor subunit messenger RNAs exhibits a distinct pattern of localization in the developing and adult brain. Of the subunits examined, the alpha 1, beta 2, and gamma 2 are the most abundant and are found in many brain regions, including the olfactory bulb, cortex, hippocampus, thalamic nuclei, and inferior colliculus. In addition, these subunit messenger RNAs are prominent in the cerebellum where virtually all cells of the deep cerebellar nuclei and Purkinje cell layer are labeled. The levels of most of the subunit messenger RNAs, with the exception of that encoding the beta 1 subunit, increase during postnatal development. While the alpha 1, beta 2, and gamma 2 subunit messenger RNAs rise in parallel in many regions and identified cell populations, different subsets of receptor subunit messenger RNAs are co-ordinately expressed at other sites. The greatest increases in subunit messenger RNA levels occur in the cerebellar cortex during the second postnatal week, a period coincident with cerebellar maturation. The co-distribution of different GABAA receptor subunit messenger RNAs in various regions of the developing and adult nervous systems supports the hypothesis that multiple receptor compositions exist. Moreover, that different subunit messenger RNAs exhibit coordinate changes in expression in different regions and cell populations suggests that receptor gene expression is modulated by cell type-specific signals. The temporal changes in subunit messenger RNA levels in the cerebellum raise the possibility that synaptogenesis may play a role in receptor gene regulation in this brain region.