Immunohistochemical localization of nicotinic acetylcholine receptor subunits in the mesencephalon and diencephalon of the chick (Gallus gallus)

Nicotinic acetylcholine receptors in separate brain regions exhibit different affinities for methyllycaconitine

The family of nicotinic acetylcholine receptors contains numerous subtypes. Since the subunit compositions of most native neuronal nicotinic receptors are unknown, an important method for distinguishing subtypes of functional neuronal receptors is based on pharmacological criteria, such as affinity for snake toxins. We have now examined the affinities of native chick nicotinic receptors for methyllycaconitine, a toxin purified from Delphinium. We find that methyllycaconitine is a potent antagonist at central nicotinic receptors located on Edinger-Westphal neurons, producing nearly complete functional blockade of nicotinic responses at 10 nM. In marked contrast, methyllycaconitine is 1000-fold less potent at blocking nicotinic responses in the lateral spiriform nucleus. Methyllycaconitine inhibits kappa-bungarotoxin-sensitive nicotinic receptors in ciliary ganglia at 0.5-1.0 microM. Radioligand binding studies also reveal heterogeneity in the affinity of the toxin for nicotinic receptors. Methyllycaconitine binds most avidly to [125I] alpha-bungarotoxin sites in brain (Ki = 5.4 nM), and is 200-fold less potent at muscle nicotinic receptors (IC50 = 1.1 microM). The least potent binding of the toxin is to [3H]nicotine sites in brain (Ki = 3.7 microM). Methyllycaconitine is thus a useful pharmacological tool for distinguishing certain subtypes of native nicotinic receptors. The relatively low affinity of the toxin for nicotinic receptors in the lateral spiriform nucleus is consistent with the known properties of these receptors, which include a high affinity for [3H]nicotine and a lack of sensitivity to alpha- and kappa-bungarotoxin. On the basis of high affinity for methyllycaconitine and insensitivity to alpha-bungarotoxin, the nicotinic receptors in the Edinger-Westphal nucleus are unlike any previously described nicotinic receptor subtype.

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.

Target-independent diversification and target-specific projection of chemically defined retinal ganglion cell subsets

In diverse vertebrate species, defined subsets of retinal ganglion cells (RGCs, the neurons that project from retina to brain) are distinguishable on the basis of their dendritic morphology, physiological properties, neurotransmitter content and synaptic targets. Little is known about when this diversity arises, whether diversification requires target-derived signals, and how subtype-specific projection patterns are established. Here, we have used markers for two chemically defined RGC subsets in chick retina to address these issues. Antibodies to substance P (SP) and the nicotine acetylcholine receptor (AChR) beta 2 subunit label two small (< 10%), mutually exclusive groups of RGCs in mature retina. SP and AChRs accumulate in distinct RGCs before retinotectal synapses have formed. Moreover, both populations of RGCs form in retinae that develop following tectal ablation or transplantation to the coelomic cavity. Thus, RGC subsets acquire distinct neurotransmitter phenotypes in the absence of extraretinal cues. In the mature optic tectum, SP- and AChR-positive RGC axonal arbors are confined to distinct retinorecipient (synaptic) laminae. In the developing tectum, SP- and AChR-positive axons are initially intermingled in a superficial fiber layer, but then enter and arborize in appropriate laminae soon after those laminae form. Importantly, SP-positive axons, which synapse in a superficial lamina, never extend into the deeper, AChR-positive lamina. Tectal interneurons rich in SP receptors are concentrated in the lamina to which SP-positive RGC axons project, and a set of cholinergic (choline acetyltransferase-positive) tectal projection neurons elaborate dendrites in the lamina to which AChR-positive RGC axons project. These populations of tectal neurons, which are likely targets of the RGC subsets, form in tecta that develop following enucleation. Thus, RGCs and their targets can diversify in each others absence. Accordingly, we propose that the lamina-selective connectivity we observe reflects the presence of complementary cues on RGC subsets and their laminar targets.

Laminar distribution and sources of catecholaminergic input to the optic tectum of the pigeon (Columbia livia)

A combined immunohistochemical and retrograde tracing approach was used to characterize the catecholaminergic innervation of the optic tectum (TeO), the major target of retinal projections in many avian species. Giemsa counterstaining was employed to determine precisely the laminar localization of immunoreactive fibers and presumptive terminals. The TeO of the pigeon is densely innervated by fibers immunoreactive for tyrosine hydroxylase (TH), which are most heavily distributed to the superficial layers of its dorsal and anterior portions. Within the dorsal-anterior tectum, TH-immunoreactive processes are particularly dense in retinorecipient layers 4 and 7 and in layer 5a. As in the mammalian superior colliculus, the bulk of the catecholaminergic innervation of the pigeon TeO reflects inputs, presumably noradrenergic, originating in the locus coeruleus and nucleus subcoeruleus. However, the catecholaminergic innervation of the pigeon TeO shows several features distinct from those reported for the mammalian superior colliculus. These include an input from a pretectal TH-positive cell group unknown in mammals and the presence of residual TH immunoreactivity after administration of the noradrenergic neurotoxin DSP-4. Moreover, the pattern of TH-immunoreactive fibers in pigeon TeO indicates more laminar and regional specialization within this structure than has been reported for the catecholaminergic innervation of the superior colliculus in mammals.

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.

Effects of retinal lesions upon the distribution of nicotinic acetylcholine receptor subunits in the chick visual system

Immunohistochemistry was used in this study to evaluate the effects of retinal lesions upon the distribution of neuronal nicotinic acetylcholine receptor subunits in the chick visual system. Following unilateral retinal lesions, the neuropil staining with an antibody against the beta 2 receptor subunit, a major component of alpha-bungarotoxin-insensitive nicotinic receptors, was dramatically reduced or completely eliminated in all of the contralateral retinorecipient structures. On the other hand, neuropil staining with antibodies against two alpha-bungarotoxin-sensitive receptor subunits, alpha 7 and alpha 8, was only slightly affected after retinal lesions. Decreased neuropil staining for alpha 7-like immunoreactivity was only observed in the nucleus of the basal optic root and layers 2-4 and 7 of the optic tectum. For alpha 8-like immunoreactivity, slight reduction of neuropil staining could be observed in the ventral geniculate complex, griseum tecti, nucleus lateralis anterior, nucleus lentiformis mesencephali, layers 4 and 7 of the tectum, and nucleus suprachiasmaticus. Taken together with previous data on the localization of nicotinic receptors in the retina, the present results indicate that the beta 2 subunit is transported from retinal ganglion cells to their central targets, whereas the alpha 7 and alpha 8 subunit immunoreactivity appears to have a central origin. The source of these immunoreactivities could be, at least in part, the stained perikarya that were observed to contain alpha 7 and alpha 8 subunits in all retinorecipient areas. In agreement with this hypothesis, the beta 2 subunit of the nicotinic acetylcholine receptors was not frequently found in perikarya of the same areas.

Depletion and recovery of the calcium-binding proteins calbindin and parvalbumin in the pigeon optic tectum following retinal lesions

Retinal lesions in pigeons produced a marked depletion of somata and neuropil staining for both calbindin-D28k and parvalbumin immunoreactivities in the contralateral optic tectum. Calbindin-like immunoreactivity reappeared in some tectal layers by 6 weeks postlesion, whereas paravalbumin-like immunoreactivity recovered almost completely after 5 weeks. These data indicate that the retinal input may control the expression of calbindin and parvalbumin in the pigeon optic tectum.

The distribution of tachykinin binding sites in the brain of an electric fish (Apteronotus leptorhynchus)

We mapped the distribution of tachykinin binding sites utilizing quantitative autoradiography of iodinated substance P and eledoisin as prototypic ligands for neurokinin-1 (NK1) and neurokinin-3 (NK3) receptors, respectively. The two ligands produced highly heterogenous and quantitatively different patterns of specific binding, suggesting that they revealed different tachykinin receptor subtypes. Although [125I]substance P and [125I]eledoisin binding were correlated in most brain regions, the binding of substance P was usually denser. [125I]substance P binding and substance P-like immunoreactivity were reasonably correlated in most brain areas, although discrepancies were found in some nuclei. Dense [125I]substance P binding was found in most areas of the subpallium and in parts of the pallium related to the olfactory system, as well as in the glomerular layer of the olfactory bulb. Moderate to dense binding of both ligands was observed in preoptic area, hypothalamus, habenula, parts of the thalamus and preglomerular complex. Especially noteworthy was the presence of [125I] substance P binding in the diencephalic prepacemaker nucleus, a region involved in the control of electroncommuncatory behavior. Substance P-like immunoreactivity is sexually dimorphic in certain diencephalic nuclei, including the prepacemaker nucleus (Weld and Maler, 1992); no obvious difference was seen between [125I]substance P or [125I]eledoisin binding in the brains of male versus female fish. In the mesencephalon striking laminar patterns of binding were seen in the torus semicircularis dorsalis and the optic tectum. Dense binding was also noted in the raphé nuclei, the locus ceruleus and the sensory nucleus of the vagus. Although binding of substance P in the electrosensory lateral line lobe and nucleus preeminentialis was light, it was distributed in a discrete fashion, suggesting a role of substance P in electrosensory processing.

Nicotinic acetylcholine receptors in the ground squirrel retina: localization of the beta 4 subunit by immunohistochemistry and in situ hybridization

Immunohistochemical and in situ hybridization techniques were used to localize the beta 4 subunit of the neuronal nicotinic acetylcholine receptors (nAChRs) in the ground squirrel retina. The beta 4 nAChR subunit was detected in both transverse and horizontal sections of the retina using a subunit-specific antiserum and the avidin-biotin complex technique. Two bands of labeled processes were seen in the inner plexiform layer, corresponding approximately to the laminae where the cholinergic cells arborize. Labeled cells were found in the ganglion cell layer and the inner third of the inner nuclear layer. The cells in the ganglion cell layer were medium- to large-sized and were frequently observed to give rise to axon-like processes. Most of the labeled neurons in the inner nuclear layer were small presumptive amacrine cells, but a few medium-to-large cells were also labeled. These could constitute a different class of amacrine cells or displaced ganglion cells. The latter possibility is supported by the existence of nAChR-containing displaced ganglion cells in the avian retina. In situ hybridization with a 35S-labeled cRNA probe revealed the expression of mRNA coding for the nAChR beta 4 subunit in the ganglion cell layer and the inner third of the inner nuclear layer. This finding confirmed the immunohistochemical data of the cellular localization of beta 4 nAChR subunit. These results indicate that the beta 4 nAChR subunit is expressed by specific subtypes of neurons on the ground squirrel retina.(ABSTRACT TRUNCATED AT 250 WORDS)

Distribution of choline acetyltransferase immunoreactivity in the pigeon brain

We have investigated the distribution of cholinergic perikarya and fibers in the brain of the pigeon (Columba livia). With this aim, pigeon brain sections were processed immunohistochemically by using an antiserum specific for chicken choline acetyltransferase. Our results show cholinergic neurons in the pigeon basal telencephalon, the hypothalamus, the habenula, the pretectum, the midbrain tectum, the dorsal isthmus,the isthmic tegmentum, and the cranial nerve motor nuclei. Cholinergic fibers were prominent in the dorsal telencephalon, the striatum, the thalamus, the tectum, and the interpeduncular nucleus. Comparison of our results with previous studies in birds suggests some major cholinergic pathways in the avian brain and clarifies the possible origin of the cholinergic innervation of some parts of the avian brain. In addition, comparison of our results in birds with those in other vertebrate species shows that the organization of the cholinergic systems in many regions of the avian brain (such as the basal forebrain, the epithalamus, the isthmus, and the hindbrain) is much like that in reptiles and mammals. In contrast, however, birds appear largely to lack intrinsic cholinergic neurons in the dorsal ("neocortex-like") parts of the telencephalon.

The retinal targets of centrifugal neurons and the retinal neurons projecting to the accessory optic system

In birds, neurons of the isthmo-optic nucleus (ION), as well as "ectopic" neurons, send axons to the retina, where they synapse on cells in the inner nuclear layer (INL). Previous work has shown that centrifugal axons can be divided into two anatomically distinct types depending on their model of termination: either "convergent" or "divergent" (Ramon y Cajal, 1889; Maturana & Frenk, 1965). We show that cytochrome-oxidase histochemistry specifically labels "convergent" centrifugal axons and target neurons which appear to be amacrine cells, as well as three "types" of ganglion cells: two types found in the INL (displaced ganglion cells) and one in the ganglion cell layer. Labeled target amacrine cells have distinct darkly labeled "nests" of boutons enveloping the somas, are associated with labeled centrifugal fibers, and are confined to central retina. Lesions of the isthmo-optic tract abolish the cytochrome-oxidase labeling in the centrifugal axons and in the target amacrine cells but not in the ganglion cells. Cytochrome-oxidase-labeled ganglion cells in the INL are large; one type is oval and similar to the classical displaced ganglion cells of Dogiel, which have been reported to receive centrifugal input; the other type is rounder. Rhodamine beads injected into the accessory optic system results in retrograde label in both types of cells, showing that two distinct types of displaced ganglion cells project to the accessory optic system in chickens. The ganglion cells in the ganglion cell layer that label for cytochrome oxidase also project to the accessory optic system. These have proximal dendrites that ramify in the outer inner plexiform layer.(ABSTRACT TRUNCATED AT 250 WORDS)

New subdivision schema for the avian torus semicircularis: neurochemical maps in the chick

Chemoarchitectonic subdivisions in the chicken torus semicircularis were mapped by means of acetylcholinesterase histochemistry and immunocytochemical labeling of leucine-enkephalin, choline acetyltransferase, neuropeptide Y, and calbindin/calretinin in adjacent sections. The torus semicircularis was found to consist of three main divisions: intercollicular area, toral nucleus, and preisthmic superficial area. All three appear variously subdivided. The intercollicular area is a mid-mesencephalic ventral periventricular region and appears subdivided into core and shell intercollicular regions. The toral nucleus is formed by a large caudal periventricular cytoarchitectonic complex, consisting of a periventricular lamina subdivided into core and shell regions, a pericentral, diffuse external nucleus, a central nucleus subdivided into core and shell regions, a caudomedial shell nucleus, a paracentral nucleus, and a posterior hiliar nucleus, apart from other minor parcellations. The preisthmic superficial area extends superficially at the caudomedial end of the toral nucleus, reaching the paramedian dorsal brain surface just rostral to the isthmo-optic nucleus. It is subdivided into core and shell regions. This previously unnoticed area is distinguished here from the intercollicular area and from the caudomedial shell and paracentral nuclei, all of which are frequently mixed in the literature under the concept "intercollicular nucleus." The revised terminology and subdivision for the avian torus clarifies many chemoarchitectonic and hodological mappings reported in the literature. It also suggests new research subjects and eliminates some causes of confusion.

The distribution of GABA-containing perikarya, fibers, and terminals in the forebrain and midbrain of pigeons, with particular reference to the basal ganglia and its projection targets

Immunohistochemical techniques were used to study the distributions of glutamic acid decarboxylase (GAD) and gamma-aminobutyric acid (GABA) in pigeon forebrain and midbrain to determine the organization of GABAergic systems in these brain areas in birds. In the basal ganglia, numerous medium-sized neurons throughout the striatum were labeled for GABA, while pallidal neurons, as well as a small population of large, aspiny striatal neurons, labeled for GAD and GABA. GAD+ and GABA+ fibers and terminals were abundant throughout the basal ganglia, and GABAergic fibers were found in all extratelencephalic targets of the basal ganglia. Most of these targets also contained numerous GABAergic neurons. In pallial regions, approximately 10-12% of the neurons were GABAergic. The outer rind of the pallium was more intensely labeled for GABAergic fibers than the core. The olfactory tubercle region, the ventral pallidum, and the hypothalamus were extremely densely labeled for GABAergic fibers, while GABAergic neurons were unevenly distributed in the hypothalamus. GABAergic neurons and fibers were abundant in the dorsalmost part of thalamus and the dorsal geniculate region, while GABAergic neurons and fibers were sparse (or lightly labeled) in the thalamic nuclei rotundus, triangularis, and ovoidalis. Further, GABAergic neurons were abundant in the superficial tectal layers, the magnocellular isthmic nucleus, the inferior colliculus, the intercollicular region, the central gray, and the reticular formation. GABAergic fibers were particularly abundant in the superficial tectal layers, the parvocellular isthmic nucleus, the inferior colliculus, the intercollicular region, the central gray, and the interpeduncular nucleus. These results suggest that GABA plays a role as a neurotransmitter in nearly all fore- and midbrain regions of birds, and in many instances the observed distributions of GABAergic neurons and fibers closely resemble the patterns seen in mammals, as well as in other vertebrates.

Bipolar cells of the chick retina containing alpha-bungarotoxin-sensitive nicotinic acetylcholine receptors

Two cDNA clones for nicotinic acetylcholine receptor (nAChR) subunits sensitive to alpha-bungarotoxin (alpha-Bgt) have been isolated, the so-called alpha-Bgt binding proteins alpha 1 (or alpha 7 nAChR subunit) and alpha 2 (or alpha 8 nAChR subunit). Immunohistochemical experiments have shown that both alpha 7 and alpha 8 subunits, as well as subunits insensitive to alpha-Bgt (beta 2 and alpha 3), are present in amacrine and ganglion cells of the chick retina. However, only the alpha 8 subunit was observed in presumptive bipolar cells. The present study investigated in detail the pattern of distribution of the bipolar cells containing the alpha 8 nAChR subunit and its relation to the pattern of distribution of bipolar cells immunoreactive to protein kinase C (PKC). Presumptive alpha 8- and PKC-like immunoreactive (alpha 8-LI and PKC-LI) bipolar cells were observed sending their dendrites to the outer plexiform layers and their axons to the inner plexiform layer. Whereas alpha 8-LI bipolar cells corresponded to 40-53% of the whole population of bipolar cells, PKC-LI bipolar cells represented only 6-8% of the same population. The soma sizes of the alpha 8-LI bipolar cells were slightly smaller (mean +/- S.D.; 4.9 +/- 0.8 microns) than the soma sizes of the PKC-LI bipolar cells (5.4 +/- 0.9 microns). Double-labeling experiments indicated that probably all PKC-LI bipolar cells also contain alpha 8-LI. This indicates that two distinct groups of cholinoceptive bipolar cells exist in the chick retina, one that contains PKC-LI, and another one that does not.

Localization of NADPH-diaphorase in the brain of the chicken

NADPH-diaphorase, an enzyme catalyzed reaction thought to reflect the activity of nitric oxide synthase in the mammalian nervous system, was mapped in the brain of the chicken. Intensely stained neurons and fibers were found in most parts of the telencephalon, in particular in the neostriatum, paleostriatum augmentatum, olfactory tubercle, lobus parolfactorius, hyperstriatum accessorium, and hyperstriatum ventrale. Medial to the nucleus taeniae, an accumulation of stained cells was observed that appeared to merge with a band of stained neurons located dorsal to the occipitomesencephalic tract. These are considered to belong to the nucleus interstitialis of the dorsal olfactory projection. Further caudally, neurons with different staining intensities were found in the lateral hypothalamic area, lateral mammillary nucleus, periventricular organ, ventral tegmental area, medial spiriform nucleus, optic tectum, isthmooptic nucleus, mesencephalic trigeminal nucleus, interpeduncular nucleus, and central gray of the mesencephalon. A particularly dense cluster of NADPH-diaphorase positive neurons was located in the locus coeruleus. It is proposed that these might represent cholinergic cells intermingled with catecholaminergic neurons, thus forming the avian counterpart of the tegmental cholinergic nuclei of mammals. Several NADPH-diaphorase reactive neurons were seen in the parabrachial nucleus and medial and dorsal vestibular nucleus, as well as scattered in the reticular formation. In the caudal medulla, intensely stained cells were grouped around the central canal. Therefore the pattern of expression of NADPH-diaphorase, and thus possibly of nitric oxide synthase, within the avian and mammalian brain might be largely conserved.

Homomeric and native alpha 7 acetylcholine receptors exhibit remarkably similar but non-identical pharmacological properties, suggesting that the native receptor is a heteromeric protein complex

Sucrose gradient analysis of chick acetylcholine receptor (AChR) alpha 7 subunits expressed in oocytes indicates that they form pharmacologically active homomers of the same size as native alpha 7 AChRs, a size compatible with a complex of five alpha 7 subunits. By immunoisolating the [35S]methionine-labeled alpha 7 subunits we also demonstrate that they do not appear to assemble with endogenous Xenopus AChR subunits. Pharmacological characterization of detergent-solubilized brain alpha 7 AChRs and alpha 7 homomers reveals that they have similar but nonidentical properties. The pharmacological difference is most accentuated for cytisine (approximately 50-fold). Thus, at least in E18 chicken brain, most or all of the native alpha 7 AChRs do not appear to be homomeric.

The fall and rise of neuronal alpha-bungarotoxin binding proteins

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

Neurons of the chick brain and retina expressing both alpha-bungarotoxin-sensitive and alpha-bungarotoxin-insensitive nicotinic acetylcholine receptors: an immunohistochemical analysis

Immunohistochemical methods were used to study the possible co-localization of two alpha-bungarotoxin-sensitive (alpha 7 and alpha 8) and two alpha-bungarotoxin-insensitive (beta 2 and alpha 3) subunits of the nicotinic acetylcholine receptors in neurons of the chick brain and retina. Several structures contained neurons that were doubly-labeled with antibodies against the alpha 7 subunit and the beta 2 subunit. These structures included, for example, the interpeduncular nucleus, nucleus spiriformis lateralis, optic tectum, pretectal visual nuclei, and the lateral hypothalamus. Double-labeling with antibodies against the alpha 7 and alpha 8 subunits was also seen in several regions, which included the interpeduncular nucleus, visual pretectum, lateral hypothalamus, dorsal thalamus, and the habenular complex. In the retina, many cells in the inner nuclear layer were observed to contain alpha 8 and alpha 3 subunits, whereas neurons in the ganglion cell layer were seen to contain alpha 7 and alpha 8 or, less frequently, alpha 7 and alpha 3 subunits. These results indicate that alpha-bungarotoxin-sensitive and alpha-bungarotoxin-insensitive subunits of the nicotinic receptors are co-expressed by neurons of the chick brain and retina.

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.