Developmental expression of heteromeric nicotinic receptor subtypes in chick retina

Cholinergic Receptors and Addiction

Human behavior can be controlled by physical or psychological dependencies associated with addiction. One of the most insidious addictions in our society is the use of tobacco products which contain nicotine. This addiction can be associated with specific receptors in the brain that respond to the natural neurotransmitter acetylcholine. These nicotinic acetylcholine receptors (nAChR) are ligand-gated ion channels formed by the assembly of one or multiple types of nAChR receptor subunits. In this paper, we review the structure and diversity of nAChR subunits and our understanding for how different nAChR subtypes play specific roles in the phenomenon of nicotine addiction. We focus on receptors containing β2 and/or α6 subunits and the special significance of α5-containing receptors. These subtypes all have roles in regulating dopamine-mediated neurotransmission in the mesolimbic reward pathways of the brain. We also discuss the unique roles of homomeric α7 nAChR in behavioral responses to nicotine and how our knowledge of nAChR functional diversity may help guide pharmacotherapeutic approaches for treating nicotine addiction. While nicotine addiction is a truly global problem, the use of areca nut (betel) products is also a serious addiction associated with public health issues across most of South Asia, impacting as many as 600 million people. We discuss how cholinergic receptors of the brain are also involved with areca addiction and the unique challenges for dealing with addiction to this substance.

Evidence of alpha 7 nicotinic acetylcholine receptor expression in retinal pigment epithelial cells

Some evidence suggests that retinal pigment epithelium (RPE) can express nicotinic acetylcholine receptors (nAChRs) as described for other epithelial cells, where nAChRs have been involved in processes such as cell development, cell death, cell migration, and angiogenesis. This study is designed to determine the expression and activity of α7 nAChRs in RPE cells. Reverse transcriptase (RT)-PCR was performed to test the expression of nicotinic α7 subunit in bovine RPE cells. Protein expression was determined by Western blot and by immunocytochemistry. Expression of nicotinic α7 subunits was also analyzed in cryostat sections of albino rat retina. Changes in protein expression were tested under hypoxic conditions. Functional nAChRs were studied by examining the Ca2+transients elicited by nicotine and acetylcholine stimulation in fura-2–loaded cells. Expression of endogenous modulators of nAChRs was analyzed by RT-PCR and Western blot in retina and RPE. Cultured bovine RPE cells expressed nicotinic receptors containing α7 subunit. RT-PCR amplified the expected specific α7 fragment. Western blotting showed expression at the protein level, with a specific band being found at 57 kDa in both cultured and freshly isolated RPE cells. Expression of nAChRs was confirmed for cultured cells by immunofluorescence. Immunohistochemistry confirmed α7 receptor expression in rat RPE retina. α7 receptor expression was down-regulated by long-term hypoxia. A small subpopulation of RPE cultured cells showed functional nAChRs, as evidenced by the selective response elicited by nicotine and acetylcholine stimulation. Expression of the endogenous nicotinic receptors’ modulator lynx1 was confirmed in bovine retina and RPE, and expression of lynx1 and other endogenous nicotinic receptor modulators (SLURP1 and RGD1308195) were also confirmed in rat retina. These results suggest that nAChRs could have a significant role in RPE, which may not be related to the traditional role in nerve transmission but could more likely be related to the nonneuronal cholinergic system in the eye.

Local and global calcium signals associated with the opening of neuronal alpha7 nicotinic acetylcholine receptors

Neuronal nicotinic acetylcholine receptors (nAChRs) are Ca(2+)-permeable ligand-gated channels widely expressed in the central and peripheral nervous system. One of the most Ca(2+) selective isoform is the homopentameric alpha7-nAChR implicated in schizophrenia. The activity of alpha7-nAChRs is usually recorded electrophysiologically, which limits the amount of information obtained. Here, we used fluorescence imaging to record Ca(2+) transients associated with activation of the alpha7-nAChR in neuroblastoma cells stably expressing human alpha7-nAChRs. Application of nicotine (50 microM) consistently evoked transient (30s), stereotyped Ca(2+) responses that were inhibited by the selective alpha7-nAChRs antagonists methyllycaconitine (MLA) and alpha-bungarotoxin, and greatly increased and prolonged by the allosteric modulator PNU-120596 (1 microM). Unexpectedly, brief (1-5s), repetitive Ca(2+) transients of sub-micrometric dimension were observed in filopodia of cells expressing alpha7-nAChR. PNU-120596 increased the frequency and slowed the decay kinetics of these miniature Ca(2+) elevations, which were insensitive to ryanodine, preserved during hyperpolarisation, and prevented by MLA, alpha-bungarotoxin, or Ca(2+) removal. Global Ca(2+) responses were also recorded in ganglion cells of embryo chicken retina during co-application of PNU-120596 and nicotine, together with whole-cell currents and brief current bursts. These data demonstrate that Ca(2+) signals generated by alpha7-nAChRs can be recorded optically both in cell lines and in intact tissues. The possibility to image miniature Ca(2+) signals enables to map the location of functional alpha7-nAChR channel clusters within cells and to analyze their single channel properties optically. Deciphering the rich pattern of intracellular Ca(2+) signals generated by the activity of the alpha7-nAChRs will reveal the physiological role of these receptor-channels.

Nicotinic acetylcholine receptor subunits in rhesus monkey retina

PURPOSE

The purpose of this study was to detect and establish the cellular localizations of nicotinic acetylcholine receptor (nAChR) subunits in Rhesus monkey retina.

METHODS

Retinas were dissected from the eyes of monkeys killed after unrelated experiments. RNA was extracted and analyzed by RT-PCR, using primers designed against human sequences of alpha3-alpha7, alpha9, and beta2-beta4 nAChR subunits. The RT-PCR products were separated by gel electrophoresis and sequenced. Frozen sections of postmortem fixed monkey eyes were immunolabeled with well-characterized and specific monoclonal antibodies against the alpha3, alpha4, alpha6, alpha7, beta2, or beta4 nAChR subunits and visualized with fluorescence labeling.

RESULTS

Products of the predicted size for the alpha3-alpha7, alpha9, and beta2-beta4 nAChR subunits were detected by RT-PCR in Rhesus monkey retina. Homology between transcripts from monkey retina and human nucleotide sequences ranged from 93 to 99%. Immunohistochemical studies demonstrated that neurons in various cell layers of monkey retina expressed alpha3, alpha4, alpha7, or beta2 nAChR subunits and cells with the morphology of microglia were immunoreactive for the alpha6 or beta4 nAChR subunits.

CONCLUSIONS

nAChR subunits are expressed in the monkey retina and localize to diverse retinal neurons as well as putative microglia. Besides mediating visual processing, retinal nAChRs may influence refractive development and ocular pathologies such as neovascularization.

Diversity of vertebrate nicotinic acetylcholine receptors

Nicotinic acetylcholine receptors (nAChRs) are pentameric neurotransmitter receptors. They are members of the Cys-loop family of ligand-gated ion channels which also include ionotropic receptors for 5-hydroxytryptamine (5-HT), gamma-aminobutyric acid (GABA) and glycine. Nicotinic receptors are expressed in both the nervous system and at the neuromuscular junction and have been implicated in several neurological and neuromuscular disorders. In vertebrates, seventeen nAChR subunits have been identified (alpha1-alpha10, beta1-beta4, gamma, delta and epsilon) which can co-assemble to generate a diverse family of nAChR subtypes. This review will focus on vertebrate nAChRs and will provide an overview of the extent of nAChR diversity based on studies of both native and recombinant nAChRs.

Heterogeneity and complexity of native brain nicotinic receptors

Neuronal cholinergic nicotinic receptors (nAChRs) are a heterogeneous class of cationic channels that are widely distributed in the nervous system that have specific functional and pharmacological properties. They consist of homologous subunits encoded by a large multigene family, and their opening is physiologically controlled by the acetylcholine neurotransmitter or exogenous ligands such as nicotine. Their biophysical and pharmacological properties depend on their subunit composition, which is therefore central to understanding receptor function in the nervous system and discovering new subtype-selective drugs. We will review rodent brain subtypes by discussing their subunit composition, pharmacology and localisation and, when possible, comparing them with the same subtypes present in the brain of other mammalian species or chick. In particular, we will focus on the nAChRs present in the visual pathway (retina, superior colliculus and nucleus geniculatus lateralis), in which neurons express most, if not all, nAChR subunits. In addition to the major alpha4beta2 and alpha7 nAChR subtypes, the visual pathway selectively expresses subtypes with a complex subunit composition. By means of ligand binding and immunoprecipitation and immunopurification experiments on tissues obtained from control and lesioned rats, and wild-type and nAChR subunit knockout mice, we have qualitatively and quantitatively identified, and pharmacologically characterised, the multiple complex native subtypes containing up to four different subunits.

Onset of synaptogenesis in the plexiform layers of the chick retina: A transmission electron microscopic study

The presently acknowledged onset of synaptogenesis in the chick retina from embryonic day 12 (E12) onward stands in contrast with the appearance of spontaneous electrical activity, of presynaptic proteins, or of neurotransmitters during early formation of the inner (E6-E8) and outer (E9) plexiform layers. Therefore, we investigated the chick retina from E6 to E12 at which age first synapses appear by transmission electron microscopy (TEM). The study provides evidence that synaptogenesis in the chick retina begins shortly after the plexiform layers have started to emerge. The first synapses are electrical synapses, which appear on E7, one day after the future inner plexiform layer emerged, and towards the end of E8 in the nascent outer plexiform layer. Conventional chemical synapses appear in both plexiform layers on E8, in the inner plexiform layer (stage 34) only a few hours earlier than in the outer plexiform layer (stage 35). The first synapses are formed close to the apex of the optic fissure and their frequency increases rapidly with age. The onset, the topography, and the developmental course of synaptogenesis correlate with the chronotopic course of maturation of retinal neurons and the age when spontaneous electrical activity occurs in the retina.

Brain nicotinic acetylcholine receptors: native subtypes and their relevance

Neuronal nicotinic acetylcholine receptors comprise a heterogeneous class of cationic channels that is present throughout the nervous system. These channels are involved both in physiological functions (including cognition, reward, motor activity and analgesia) and in pathological conditions such as Alzheimer's disease, Parkinson's disease, some forms of epilepsy, depression, autism and schizophrenia. They are also the targets of tobacco-smoking effects and addiction. Neuronal nicotinic acetylcholine receptors are pentamers of homomeric or heteromeric combinations of alpha (alpha2-alpha10) and beta (beta2-beta4) subunits, which have different pharmacological and biophysical properties and locations in the brain. The lack of subtype-specific ligands and the fact that many neuronal cells express multiple subtypes initially hampered the identification of the different native nicotinic acetylcholine receptor subtypes, but the increasing knowledge of subtype composition and roles will be of considerable interest for the development of new and clinically useful nicotinic acetylcholine receptor ligands.

Evaluation of the role of nicotinic acetylcholine receptor subtypes and cannabinoid system in the discriminative stimulus effects of nicotine in rats

Male Wistar rats were trained to discriminate (-)-nicotine (0.4 mg/kg) from saline under a two-lever, fixed-ratio 10 schedule of water reinforcement. During test sessions the following drugs were coadministered with saline (substitution studies) or nicotine (0.025-0.4 mg/kg; combination studies): the alpha4beta2 nicotinic acetylcholine receptor subtype antagonist dihydro-beta-erythroidine (DHbetaE), the non-selective nicotinic acetylcholine receptor subtype antagonist mecamylamine, the alpha7 nicotinic acetylcholine receptor subtype antagonist methyllycaconitine (MLA), the alpha4beta2 nicotinic acetylcholine receptor subtype agonist 5-iodo-3-(2(S)-azetidinylmethoxy)pyridine (5-IA), the cannabinoid CB1 receptor antagonist/partial agonist rimonabant, the cannabinoid CB2 receptor antagonist N-[(1S)-endo-1,3,3-trimethylbicyclo-[2.2.1]heptan-2-yl]5-(4-chloro-3-methyl-phenyl)-1-(4-methybenzyl)pyrazole-3-carboxamide (SR 144528), the cannabinoid CB1/2 receptor agonists (-)-cis-3-[2-hydroxy-4-(1,1-dimethylheptyl)-phenyl]-trans-4-(3-hydroxy-propyl)cyclohexanol (CP 55,940) or R(+)-[2,3-dihydro-5-methyl-3-[(morpholinyl)methyl]-pyrrolo[1,2,3-de]-1,4-benzoxazin-6-yl]-(1-naphthalenyl)-methanone mesylate (WIN 55,212-2), the endogenous cannabinoid agonist and non-competitive alpha7 nicotinic acetylcholine receptor subtype antagonist anandamide, the anandamide uptake and fatty acid amide hydrolase inhibitor N-(4-hydroxyphenyl)-5Z,8Z,11Z,14Z-eicosatetraenamide (AM-404), the fatty acid amide hydrolase inhibitor cyclohexylcarbamic acid 3'-carbamoyl-biphenyl-3-yl ester (URB 597), AM-404+anandamide or URB 597+anandamide. 5-IA (0.01 mg/kg) fully substituted for nicotine, while other drugs were inactive. In combination studies, DHbetaE and mecamylamine dose-dependently attenuated the discriminative stimulus effects of nicotine and the full substitution of 5-IA, while MLA, rimonabant, SR 144528, CP 55,940, WIN 55,212-2, and URB 597 did not alter the nicotine cue. Pretreatment with AM-404+anandamide or URB 597+anandamide weakly enhanced nicotine-lever responding. Our pharmacological analyses demonstrates that the expression of nicotine discrimination is under the control of nicotinic acetylcholine receptor subtypes composed of alpha4beta2 (but not of alpha7) subunits. Furthermore, we excluded the involvement of either cannabinoid CB1 and CB2 receptors or increases in the endocannabinoid tone in the nicotine discrimination.

Nicotinic cholinergic receptors in the rat cerebellum: multiple heteromeric subtypes

Nicotinic receptors (nAChRs) in the cerebellum have been implicated in the pathology of autism spectrum disorders (Lee et al., 2002; Martin-Ruiz et al., 2004). The subtypes of nAChRs in the cerebellum are not known in any detail, except that, in addition to the homomeric alpha7 subtype, there appears to be one or more heteromeric subtypes consisting of combinations of alpha and beta subunits. To begin to better understand the potential roles of these heteromeric nAChRs in cerebellar circuitry and their potential as targets for nicotinic drugs, we investigated their subunit composition. Using subunit-selective antibodies in sequential immunoprecipitation assays, we detected six structurally distinct heteromeric nAChR populations in the rat cerebellum. Among these were several subtypes that have not been encountered previously, including alpha3alpha4beta2 and alpha3alpha4beta4 nAChRs. This diversity suggests that nAChRs play multiple roles in cerebellar physiology.

Synaptically Released and Exogenous ACh Activates Different Nicotinic Receptors to Enhance Evoked Glutamatergic Transmission in the Lateral Geniculate Nucleus

The effects of activation of nicotinic acetylcholine receptors (nAChRs) on glutamatergic transmission in the ventral lateral geniculate nucleus (LGNv) were examined in chick brain slices. Whole cell recordings showed that monosynaptic postsynaptic currents (PSCs) evoked in LGNv neurons by optic tract stimulation were blocked by glutamate receptor antagonists. Exogenously applied nicotine (0.5 μM), choline (1 mM), or acetylcholine (ACh, 100 μM) markedly increased (>3-fold) these evoked PSCs. Potentiation by ACh was dose-dependent and did not desensitize during a 5-min application. In a second set of experiments, the effect of releasing endogenous ACh by stimulating the lateral portion of the LGNv through a separate conditioning electrode before optic tract stimulation was examined. Conditioning stimulation trains increased PSCs by an average of 5.2-fold, an effect dependent on both the intensity and number of conditioning pulses. This increase in PSC amplitude was most likely caused by released ACh activating α6- and/or α3-containing nAChRs because it was blocked by 100 nM α-conotoxin MII, 100 nM dihydro-β-erythroidine (DHβE), and 0.1–1.0 μM methyllycaconitine (MLA). In contrast, exogenously applied ACh increased PSC amplitude by activating a pharmacologically different population of nAChRs because this effect was inhibited by 100 nM α-bungarotoxin, 50 nM MLA, and a high concentration (30 μM) of DHβE, indicating that α7- and/or α8-containing receptors were involved. The results are consistent with a model whereby α6- and/or α3-containing nAChRs on retinal ganglion cell nerve terminals are located preferentially at cholinergic synapses, whereas α7- and/or α8-containing receptors are primarily extrasynaptic.

Nicotinic cholinergic receptors in the rat retina: simple and mixed heteromeric subtypes

Neuronal nicotinic acetylcholine receptors (nAChRs) were measured in the rat retina to determine the heteromeric subtypes. We detected seven nicotinic receptor subunit mRNA transcripts, alpha2-alpha4, alpha6, and beta2-beta4, with RNase protection assays. The density of heteromeric nAChR binding sites is approximately 3 times higher in the retina than in the cerebral cortex. Moreover, the density of the sites in the retina measured with [3H]epibatidine ([3H]EB) is approximately 30% higher than with 125I-3-(2(S)-azetidinylmethoxy)pyridine (A-85380) and more than twice that measured with [3H]cytisine or [3H](-)nicotine. These data suggest that the retina expresses multiple subtypes of nAChRs, including a large fraction of receptors containing the beta2 subunit and a smaller fraction containing the beta4 subunit. Consistent with this, in binding competition studies, nicotinic ligands fit a model for two affinity classes of binding sites, with the higher affinity sites representing 70 to 80% of the nAChRs in the retina. To determine the specific subtypes of nAChRs in the rat retina, we used subunit-specific antibodies in immunoprecipitation assays. Immunoprecipitation of [3H]EB-labeled nAChRs with antibodies specific to the beta2 and beta4 subunits indicated that approximately 80% of the receptors contained beta2 subunits and approximately 25% contained beta4 receptors, consistent with the binding pharmacology results. Sequential immunoprecipitation assays indicated that the rat retina contains multiple subtypes of nAChRs. The majority of the receptors measured seemed to be simple heteromeric subtypes, composed of a single type of alpha and a single type of beta subunit; but a significant fraction are mixed heteromeric subtypes, composed of two or more alpha and/or beta subunits.

Functional properties of neuronal nicotinic acetylcholine receptors in the chick retina during development

Acetylcholine (ACh) has been recognized for a long time as a major neurotransmitter in the retina, however, little is known about the contribution of acetylcholine receptors in synaptic processing. Moreover, even less information is available concerning their role during development. To address this question further, we examined the physiological and pharmacological properties of neuronal nicotinic acetylcholine receptors (nAChRs) in retinal ganglion cells from embryonic (E) 12-18-day-old Leghorn chicks. Patch-clamp recordings in whole-cell configuration revealed that at E12 approximately 21% of the ganglion cells responded to acetylcholine pulses with inward currents. The number of responsive cells progressively increased to 57% at E15 to reach up to 15 positive cells out of 15 cells tested at E18. Acetylcholine-evoked responses could be subdivided, according to their time course, into fast and slowly desensitizing. Taking advantage of the selectivity of the frog toxin epibatidine (Epi), that preferentially activates heteromeric neuronal nicotinic acetylcholine receptors, we compared the currents evoked by this toxin vs. the effects of acetylcholine. A further characterization of the receptor diversity during development was to assess their sensitivity to the alpha-conotoxin MII (alpha-CTX-MII), which has been shown to preferentially block alpha6- and alpha3beta2-containing receptors. These data demonstrate that ganglion cells of the chick retina express multiple receptor subtypes that progressively develop as a function of retina maturation.

Heterogeneity and Selective Targeting of Neuronal Nicotinic Acetylcholine Receptor (nAChR) Subtypes Expressed on Retinal Afferents of the Superior Colliculus and Lateral Geniculate Nucleus: Identification of a New Native nAChR Subtype α3β2(α5 or β3) Enriched in Retinocollicular Afferents

The activation of neuronal nicotinic acetylcholine receptors (nAChRs) has been implicated in the activity-dependent development and plasticity of retina and the refinement of retinal projections. Pharmacological and functional studies have also indicated that different presynaptic nAChRs can have a modulatory function in retinotectal synapses. We biochemically and pharmacologically identified the multiple nAChR subtypes expressed on retinal afferents of the superior colliculus (SC) and lateral geniculate nucleus (LGN). We found that the alpha6beta2(*) and alpha4(nonalpha6)beta2(*) nAChRs are the major receptor populations expressed in both SC and LGN. In addition, the LGN contains two minor populations of alpha2alpha6beta2(*) and alpha3beta2(*) subtypes, whereas the SC contains a relatively large population of a new native subtype, the alpha3beta2(alpha5/beta3) nAChR. This subtype binds the alpha-conotoxin MII with an affinity 50 times lower than that of the native alpha6beta2(*) subtype. Studies of tissues obtained from eye-enucleated animals allowed the identification of nAChRs expressed by retinal afferents: in SC alpha6beta2(*), alpha4alpha6beta2(*), and alpha3beta2(*) (approximately 45, 35, and 20%, respectively), in LGN, alpha4alpha6beta2(*), alpha6beta2(*), alpha4beta2(*), alpha2alpha6beta2(*), and alpha3beta2(*) (approximately 40, 30, 20, 5, and 5%, respectively). In both regions, more than 50% of nAChRs were not expressed by retinal afferents and belonged to the alpha4beta2(*) (90%) or alpha4alpha5beta2(*) (10%) subtypes. Moreover, studies of the SC tissues obtained from wild-type and alpha4, alpha6, and beta3 knockout mice confirmed and extended the data obtained in rat tissue and allowed a comprehensive dissection of the composition of nAChR subtypes present in this retinorecipient area.

Neuronal nicotinic receptors: from structure to pathology

Neuronal nicotinic receptors (NAChRs) form a heterogeneous family of ion channels that are differently expressed in many regions of the central nervous system (CNS) and peripheral nervous system. These different receptor subtypes, which have characteristic pharmacological and biophysical properties, have a pentameric structure consisting of the homomeric or heteromeric combination of 12 different subunits (alpha2-alpha10, beta2-beta4). By responding to the endogenous neurotransmitter acetylcholine, NAChRs contribute to a wide range of brain activities and influence a number of physiological functions. Furthermore, it is becoming evident that the perturbation of cholinergic nicotinic neurotransmission can lead to various diseases involving nAChR dysfunction during development, adulthood and ageing. In recent years, it has been discovered that NAChRs are present in a number of non-neuronal cells where they play a significant functional role and are the pathogenetic targets in several diseases. NAChRs are also the target of natural ligands and toxins including nicotine (Nic), the most widespread drug of abuse. This review will attempt to survey the major achievements reached in the study of the structure and function of NAChRs by examining their regional and cellular localisation and the molecular basis of their functional diversity mainly in pharmacological and biochemical terms. The recent availability of mice with the genetic ablation of single or double nicotinic subunits or point mutations have shed light on the role of nAChRs in major physiological functions, and we will here discuss recent data relating to their behavioural phenotypes. Finally, the role of NAChRs in disease will be considered in some details.

Influence of melatonin on the development of functional nicotinic acetylcholine receptors in cultured chick retinal cells

The influence of melatonin on the developmental pattern of functional nicotinic acetylcholine receptors was investigated in embryonic 8-day-old chick retinal cells in culture. The functional response to acetylcholine was measured in cultured retina cells by microphysiometry. The maximal functional response to acetylcholine increased 2.7 times between the 4th and 5th day in vitro (DIV4, DIV5), while the Bmax value for [125I]-alpha-bungarotoxin was reduced. Despite the presence of alpha8-like immunoreactivity at DIV4, functional responses mediated by alpha-bungarotoxin-sensitive nicotinic acetylcholine receptors were observed only at DIV5. Mecamylamine (100 microM) was essentially without effect at DIV4 and DIV5, while dihydro-ss-erythroidine (10-100 microM) blocked the response to acetylcholine (3.0 nM-2.0 microM) only at DIV4, with no effect at DIV5. Inhibition of melatonin receptors with the antagonist luzindole, or melatonin synthesis by stimulation of D4 dopamine receptors blocked the appearance of the alpha-bungarotoxin-sensitive response at DIV5. Therefore, alpha-bungarotoxin-sensitive receptors were expressed in retinal cells as early as at DIV4, but they reacted to acetylcholine only after DIV5. The development of an alpha-bungarotoxin-sensitive response is dependent on the production of melatonin by the retinal culture. Melatonin, which is produced in a tonic manner by this culture, and is a key hormone in the temporal organization of vertebrates, also potentiates responses mediated by alpha-bungarotoxin-sensitive receptors in rat vas deferens and cerebellum. This common pattern of action on different cell models that express alpha-bungarotoxin-sensitive receptors probably reflects a more general mechanism of regulation of these receptors.

Nicotinic Acetylcholine Receptor Subtypes Expression during Rat Retina Development and Their Regulation by Visual Experience

By acting through retinal nicotinic acetylcholine receptors (nAChRs), acetylcholine plays an important role in the development of both the retina and central visual pathways. Ligand binding and immunoprecipitation studies with subunit-specific antibodies showed that the expression of alphaBungarotoxin (alphaBgtx) and high-affinity epibatidine (Epi) receptors is regulated developmentally and increases until postnatal day 21 (P21). The increase in Epi receptors is caused by a selective increase in the subtypes containing the alpha2, alpha4, alpha6, beta2, and beta3 subunits. Immunopurification studies revealed three major populations of Epi receptors on P21: alpha6(*) receptors (26%), which contain the alpha6beta3beta2, alpha6alpha4beta3beta2, and alpha6alpha3/alpha2beta3beta2 subtypes; alpha4(non-alpha6)(*) receptors (60%), which contain the alpha2alpha4beta2 and alpha4beta2 subtypes; and (non-alpha4/non-alpha6)(*) receptors (14%), which contain the alpha2beta2/beta4 and alpha3beta2/beta4 subtypes. These three populations can be pharmacologically discriminated using alphaconotoxin MII, which binds the alpha6(*) population with high affinity. In situ hybridization showed that the transcripts for all of the subunits are heterogeneously distributed throughout retinal neurons at P21, with alpha3, alpha6, and beta3 transcripts preferentially concentrated in the ganglion cell layer, alpha5 in the inner nuclear layer, and alpha4 and beta2 distributed rather homogeneously. To investigate whether nAChR expression is affected by visual experience, we also studied dark-reared P21 rats. Visual deprivation had no effect on the expression of alphaBgtx receptors or the developmentally regulated Epi receptors containing the alpha2, alpha6, and/or beta3 subunits but significantly increased the expression of the Epi receptors containing the alpha4 and beta2 subunits. Overall, this study demonstrates that the retina is the rat neural region that expresses the widest array of nAChR subtypes. These receptors have a specific distribution, and their expression is finely regulated during development and by visual experience.

Ganglion cells from chick retina display multiple functional nAChR subtypes

We have examined the properties of nicotinic acetylcholine receptors in embryonic chick retinal ganglion cells. Ganglion cells, identified according to morphological and physiological criteria, displayed spontaneous or induced action potentials. In 94/99 cells acetylcholine pulses evoked responses. In current clamp mode, acetylcholine provoked membrane depolarization and triggered action potentials. Under voltage clamp conditions, acetylcholine evoked inward currents that were readily blocked by d-tubocurarine. Antagonists specific for homomeric (alpha-bungarotoxin) and heteromeric (dihydro-beta-erythroidine) receptors revealed that ganglion cells express multiple functional receptor subtypes. These findings demonstrate that ACh modulates the electrical activity of these cells and is likely to mediate synaptic transmission. The presence of multiple receptor subtypes may contribute to processing and transmission of information in the retina.