Autoradiographic localisation of alpha-bungarotoxin-binding sites in the central nervous system

Fifty Years of Animal Toxin Research at the Shemyakin-Ovchinnikov Institute of Bioorganic Chemistry RAS

This review covers briefly the work carried out at our institute (IBCh), in many cases in collaboration with other Russian and foreign laboratories, for the last 50 years. It discusses the discoveries and studies of various animal toxins, including protein and peptide neurotoxins acting on the nicotinic acetylcholine receptors (nAChRs) and on other ion channels. Among the achievements are the determination of the primary structures of the α-bungarotoxin-like three-finger toxins (TFTs), covalently bound dimeric TFTs, glycosylated cytotoxin, inhibitory cystine knot toxins (ICK), modular ICKs, and such giant molecules as latrotoxins and peptide neurotoxins from the snake, as well as from other animal venoms. For a number of toxins, spatial structures were determined, mostly by 1H-NMR spectroscopy. Using this method in combination with molecular modeling, the molecular mechanisms of the interactions of several toxins with lipid membranes were established. In more detail are presented the results of recent years, among which are the discovery of α-bungarotoxin analogs distinguishing the two binding sites in the muscle-type nAChR, long-chain α-neurotoxins interacting with α9α10 nAChRs and with GABA-A receptors, and the strong antiviral effects of dimeric phospholipases A2. A summary of the toxins obtained from arthropod venoms includes only highly cited works describing the molecules' success story, which is associated with IBCh. In marine animals, versatile toxins in terms of structure and molecular targets were discovered, and careful work on α-conotoxins differing in specificity for individual nAChR subtypes gave information about their binding sites.

The Molecular Basis of Toxins' Interactions with Intracellular Signaling via Discrete Portals

An understanding of the molecular mechanisms by which microbial, plant or animal-secreted toxins exert their action provides the most important element for assessment of human health risks and opens new insights into therapies addressing a plethora of pathologies, ranging from neurological disorders to cancer, using toxinomimetic agents. Recently, molecular and cellular biology dissecting tools have provided a wealth of information on the action of these diverse toxins, yet, an integrated framework to explain their selective toxicity is still lacking. In this review, specific examples of different toxins are emphasized to illustrate the fundamental mechanisms of toxicity at different biochemical, molecular and cellular- levels with particular consideration for the nervous system. The target of primary action has been highlighted and operationally classified into 13 sub-categories. Selected examples of toxins were assigned to each target category, denominated as portal, and the modulation of the different portal’s signaling was featured. The first portal encompasses the plasma membrane lipid domains, which give rise to pores when challenged for example with pardaxin, a fish toxin, or is subject to degradation when enzymes of lipid metabolism such as phospholipases A₂ (PLA₂) or phospholipase C (PLC) act upon it. Several major portals consist of ion channels, pumps, transporters and ligand gated ionotropic receptors which many toxins act on, disturbing the intracellular ion homeostasis. Another group of portals consists of G-protein-coupled and tyrosine kinase receptors that, upon interaction with discrete toxins, alter second messengers towards pathological levels. Lastly, subcellular organelles such as mitochondria, nucleus, protein- and RNA-synthesis machineries, cytoskeletal networks and exocytic vesicles are also portals targeted and deregulated by other diverse group of toxins. A fundamental concept can be drawn from these seemingly different toxins with respect to the site of action and the secondary messengers and signaling cascades they trigger in the host. While the interaction with the initial portal is largely determined by the chemical nature of the toxin, once inside the cell, several ubiquitous second messengers and protein kinases/ phosphatases pathways are impaired, to attain toxicity. Therefore, toxins represent one of the most promising natural molecules for developing novel therapeutics that selectively target the major cellular portals involved in human physiology and diseases.

Effects of paraformaldehyde fixation on nicotinic acetylcholine receptor binding in adult and developing rat brain sections

In vitro receptor autoradiography requires unfixed tissue sections, but incubation and washing procedures often result in substantial tissue damage in sections from developing brain, hindering quantitative and qualitative analysis. Formaldehyde fixation greatly preserves morphology. However, fixation can interfere with pharmacological properties of receptors, increase in non-specific background labeling, or even destroy ligand binding sites. Two mild fixation protocols, 0.2% paraformaldehyde (pFA) and pFA vapor fixation, were compared for their ability to improve tissue morphology in postnatal day 7 (P7) brain slices and maintain binding of [125I]-epibatidine and [125I]-alpha-bungarotoxin to heteromeric and homomeric nicotinic acetylcholine receptors, respectively. Fixation greatly improved the ability of P7 brain slices to withstand incubation and washing procedures during binding, resulting in minimal or no loss of tissue after prior 0.2% pFA or vapor fixation, respectively. In adults, distribution pattern of [125I]-epibatidine was identical in fixed and unfixed slices, with no difference in total and non-specific labeling. Distribution of [125I]-alphaBTX labeling was similarly unaffected by 0.2% pFA fixation, but vapor fixation increased total and non-specific binding signal. Thus, mild fixations greatly improve tissue quality during receptor binding procedures and can preserve pharmacological properties of nicotinic acetylcholine receptors. However, different receptors or ligands might exhibit differential sensitivity to fixation protocols.

Distribution and development of nicotinic acetylcholine receptor subtypes in the optic tectum of Rana pipiens

Acetylcholine allows the elicitation of visually evoked behaviors mediated by the frog optic tectum, but the mechanisms behind its effects are unknown. Although nicotinic acetylcholine receptors (nAChRs) exist in the tectum, their subtype has not been assessed. By using quantitative autoradiography, we examined the binding of [(3)H]cytisine and [(125)I]alpha-bungarotoxin in the laminated tectum. In mammalian systems, these radioligands bind with high affinity to alpha4 nAChR subunits and alpha7 nAChR subunits, respectively. [(3)H]Cytisine demonstrated high specific binding in adult frogs in retinorecipient layer 9, intermediate densities in layer 8, and low binding in layers 1-7 of the tectum. [(3)H]Cytisine binding was significantly higher in the tecta of adults than in those of tadpoles. Lesioning the optic nerve for 6 weeks decreased [(3)H]cytisine binding in layers 8/9 by 70+/-1%, whereas 6-month lesions decreased binding by 76+/-3%. Specific binding of [(125)I]alpha-bungarotoxin in adults was present only at intermediate levels in tectal layers 8 and 9, and undetectable in the deeper tectal layers. However, the nucleus isthmi, a midbrain structure reciprocally connected to the tectum, exhibited high levels of binding. There were no significant differences in tectal [(125)I]alpha-bungarotoxin binding between tadpoles and adults. Six-week lesions of the optic nerve decreased tectal [(125)I]alpha-bungarotoxin binding by 33+/-10%, but 6-month lesions had no effect. The pharmacokinetic characteristics of [(3)H]cytisine and [(125)I]alpha-bungarotoxin binding in the frog brain were similar to those demonstrated in several mammalian species. These results indicate that [(3)H]cytisine and [(125)I]alpha-bungarotoxin identify distinct nAChR subtypes in the tectum that likely contain non-alpha7 and alpha7 subunits, respectively. The majority of non-alpha7 receptors are likely associated with retinal ganglion cell terminals, whereas alpha7-containing receptors appear to have a different localization.

A comparative non-radioactive in situ hybridization and immunohistochemical study of the distribution of alpha7 and alpha8 subunits of the nicotinic acetylcholine receptors in visual areas of the chick brain

The distribution of mRNA transcripts corresponding to the alpha7 and alpha8 subunits of the nicotinic acetylcholine receptors (nAChRs) was studied in selected structures of the chick visual system with non-radioactive in situ hybridization and immunohistochemical techniques. The results indicated that the alpha7 and alpha8 nAChR transcripts are widely distributed in the brain, exhibiting differential expression in some structures but also some degree of co-localization. The pattern of localization of alpha7 and alpha8 nAChR transcripts was highly correlated with immunohistochemical data, with very few instances of possible mismatches between the distribution of mRNAs and their corresponding proteins.

Synaptic potentials mediated via alpha-bungarotoxin-sensitive nicotinic acetylcholine receptors in rat hippocampal interneurons

Exogenous application of acetylcholine elicits inward currents in hippocampal interneurons that are mediated via alpha-bungarotoxin-sensitive nicotinic acetylcholine receptors, but synaptic responses mediated via such receptors have never been reported in mammalian brain. In the present study, EPSCs were evoked in hippocampal interneurons in rat brain slices by electrical stimulation and were recorded by using whole-cell voltage-clamp techniques. Nicotinic EPSCs were isolated pharmacologically, using antagonists to block other known types of ligand-gated ion channels, and then were tested with either alpha-bungarotoxin or methyllycaconitine, which are selective antagonists for nicotinic acetylcholine receptors that contain the alpha7 receptor subunit. Each antagonist proved highly effective at reducing the remaining synaptic current. Evoked alpha7-mediated nicotinic EPSCs also were desensitized by superfusion with 1 microM nicotine, had extrapolated reversal potentials near 0 mV, and showed strong inward rectification at positive potentials. In several interneurons, methyllycaconitine-sensitive spontaneous EPSCs also were observed that exhibited a biphasic decay rate very similar to that of the alpha7-mediated evoked response. These studies provide the first demonstration of a functional cholinergic synapse in the mammalian brain, in which the primary postsynaptic receptors are alpha-bungarotoxin-sensitive nicotinic acetylcholine receptors.

Effects of alpha-erabutoxin, alpha-bungarotoxin, alpha-cobratoxin and fasciculin on the nicotine-evoked release of dopamine in the rat striatum in vivo

Snake neurotoxins (NTX) have proven to be valuable tools for the characterisation of muscular nicotinic acetylcholine receptor structure and function. It is very likely that they could also be utilised to identify subtypes of neuronal nicotinic receptors controlling specific functions within the central nervous system. In this study we examined the effects of long alpha NTX (alpha-bungarotoxin, alpha-Bgt, and alpha-cobratoxin, alpha-Cbt) and short alpha NTX (alpha-erabutoxin a, alpha-Ebt) as well as the anticholinesterase toxin fasciculin-2 (FAS), on the nicotine-evoked release of dopamine (DA) in the striatum, using the in vivo push-pull technique. The short toxins alpha-Ebt and FAS blocked the extracellular increase of DA evoked by nicotine at 4.2 microM concentrations and alpha-Ebt was more potent, as reflected by the blockade at the lower dose of 0.42 microM. In contrast, the long toxins showed a different profile of action. Alpha-Cbt did not show any blockade of the nicotine-evoked release of DA at the doses studied while alpha-Bgt did block it only at the higher dose (4.2 microM) These results indicate that short neurotoxins show a stronger interaction with striatal nicotinic receptors subtypes controlling DA release when compared to the long ones. This interaction of short neurotoxin polypeptides and presynaptic receptors may permit the further elucidation of the particular nicotinic receptor populations responsible for the modulation of striatal DA release.

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

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

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.

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

Monoclonal antibodies against two alpha-bungarotoxin-binding subunits (alpha 7 and alpha 8) of the nicotinic acetylcholine receptors (nAChRs) were used as immunohistochemical probes to map their distribution in the chick diencephalon and mesencephalon. The distribution of the alpha 7 and alpha 8 nAChR subunits was compared to the distribution of immunoreactivity produced by a monoclonal antibody against the beta 2 structural subunit of the nAChRs. Structures that contained high numbers of alpha 7-like immunoreactive (LI) somata included the intergeniculate leaflet, nucleus intercalatus thalami, nucleus ovoidalis, organum paraventricularis, nucleus rotundus, isthmic nuclei, nucleus trochlearis, oculomotor complex, nucleus interstitio-pretecto-subpretectalis, stratum griseum centrale of the optic tectum, and nucleus semilunaris. Neuropil staining for alpha 7-LI was intense in the nucleus dorsomedialis hypothalami, nucleus geniculatus lateralis ventralis, griseum tecti, isthmic nuclei, nucleus lentiformis mesencephali, nucleus of the basal optic root, and stratum griseum et fibrosum superficiale of the tectum. High numbers of alpha 8-LI somata were found in the stratum griseum et fibrosum superficiale of the tectum and the nucleus interstitio-pretecto-subpretectalis, and intense neuropil staining for alpha 8-LI was found in the dorsal thalamus, nucleus geniculatus lateralis ventralis, lateral hypothalamus, griseum et fibrosum superficiale of the tectum. High numbers of beta 2-LI somata were found only in the nucleus spiriformis lateralis, whereas neuropil staining for beta 2-LI was intense in the nucleus geniculatus lateralis ventralis, nucleus suprachiasmaticus, nucleus lateralis anterior, nucleus habenularis lateralis, area pretectalis, griseum tecti, nucleus lentiformis mesencephalis, nucleus externus, and nucleus interpeduncularis, and in the stratum griseum centrale, stratum griseum et fibrosum superficiale, and stratum opticum of the tectum. These results indicate that there are major disparities in the localization of the alpha-bungarotoxin-binding alpha 7 and alpha 8 nAChR subunits and the beta 2 structural nAChR subunit in the chick diencephalon and mesencephalon. These nAChR subunits appear, however, to coexist in several regions of the chick brain.

A neuronal nicotinic acetylcholine receptor subunit (alpha 7) is developmentally regulated and forms a homo-oligomeric channel blocked by alpha-BTX

cDNA and genomic clones encoding alpha 7, a novel neuronal nicotinic acetylcholine receptor (nAChR) alpha subunit, were isolated and sequenced. The mature alpha 7 protein (479 residues) has moderate homology with all other alpha and non-alpha nAChR subunits and probably assumes the same transmembrane topology. alpha 7 transcripts transiently accumulate in the developing optic tectum between E5 and E16. They are present in both the deep and the superficial layers of E12 tectum. In Xenopus oocytes, the alpha 7 protein assembles into a homo-oligomeric channel responding to acetylcholine and nicotine. The alpha 7 channel desensitizes very rapidly, rectifies strongly above -20 mV, and is blocked by alpha-bungarotoxin. A bacterial fusion protein encompassing residues 124-239 of alpha 7 binds labeled alpha-bungarotoxin. We conclude that alpha-bungarotoxin binding proteins in the vertebrate nervous system can function as nAChRs.

Distinct regional expression of nicotinic acetylcholine receptor genes in chick brain

Four genes (alpha 2, alpha 3, alpha 4 and beta 2) have been reported as encoding subunits of the nicotinic acetylcholine receptor (nAChR) in chicken brain. The mRNAs transcribed from these genes have here been localised to particular regions using in situ hybridisation histochemistry. The beta 2 mRNA was clearly the most abundant transcript, being widely distributed throughout the chick brain. In the cerebellum, all four mRNA species were present, although they showed different cellular patterns of distribution. Only alpha 2 mRNA and beta 2 mRNA were found in significant amounts in the optic tectum. In the lateral spiriform nucleus, while alpha 2 mRNA, alpha 4 mRNA and beta 2 mRNA were all very abundant, the alpha 4 mRNA was localised to a subgroup of neurons containing alpha 2 mRNA and beta 2 mRNA. This represents the first evidence that individual cells may express two different nAChR alpha subunit genes in vivo. The distributions of the 4 mRNA species showed few common features. This suggests that other neuronal nAChR genes remain to be identified, and that these 4 genes are not generally expressed in the same cells to constitute a single macromolecular complex. The results therefore provide evidence for nAChR heterogeneity in the central nervous system.

The effect on vasopressin release of microinjection of cholinergic agonists into the paraventricular nucleus of conscious rats

Abstract We have studied in conscious unrestrained rats under basal conditions the effect of activation of muscarinic and nicotinic receptors in the paraventricular nucleus on vasopressin secretion and mean arterial blood pressure. The microinjection of oxotremorine (0.2, 2 or 20 ng), a specific muscarinic agonist, produced a substantial, dose-dependent, transient increase in the plasma vasopressin concentration. There was also a rise in mean arterial blood pressure and a bradycardia that followed the same time-course as the change in plasma vasopressin levels. The microinjection of nicotine (0.1, 1 or 10 HQ) into the paraventricular nucleus had only questionable effects on vasopressin release and mean arterial blood pressure; heart rate was unaffected. These findings suggest that muscarinic receptors may be of primary importance in the paraventricular nucleus in the Cholinergic stimulation of vasopressin release.

Immunohistochemical localization of choline acetyltransferase in the chicken mesencephalon

Choline acetyltransferase, a specific marker for cholinergic neurons, has been immunohistochemically localized in the mesencephalon and in the caudal diencephalon of the chicken. A complete series of transverse sections through the mesencephalon is presented. In the diencephalon, cholinergic fibers were found in the stria medullaris, the fasciculus retroflexus, and the ventral portion of the supraoptic decussation. The nucleus triangularis and the nucleus geniculatus lateralis, pars ventralis also contained cholinergic fibers. Small cholinergic cell bodies were found in the medial habenula. In the pretectum, cholinergic fibers innervated the nucleus lentiformis mesencephali and the tectal gray. The nucleus spiriformis lateralis also contained cholinergic fibers, while most of the cell bodies in the nucleus spiriformis medialis were cholinergic. In the mesencephalon, labelled fibers were found in the nucleus intercollicularis and in all layers of the optic tectum except the stratum opticum. The highest density of tectal cholinergic fibers was in the stratum griseum et fibrosum superficiale (SGFS), layer f. Radial cells located in SGFS, layer i were also cholinergic. In the isthmic nuclei, cholinergic fibers were found in the pars magnocellularis, while the pars parvicellularis and the nucleus semilunaris contained labelled cells. The oculomotor, Edinger-Westphal, trochlear, and trigeminal motor nuclei all had cholinergic cell bodies. Cholinergic axons were present in the oculomotor and trochlear nerves. In the tegmentum, cell bodies were labelled in the nucleus mesencephalicus profundus, pars ventralis, while the nucleus interpeduncularis had dense cholinergic innervation. Our localization of cholinergic cell bodies and fibers has been compared with earlier autoradiographic and anatomical studies to help define cholinergic systems in the avian brain. For example, the results indicate that the chicken may have a cholinergic habenulointerpeduncular system similar to that reported in the rat. Establishing the cholinergic systems within the avian midbrain is important for designing future neurophysiological and pharmacological studies of cholinergic transmission in this region.

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

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

Neosurugatoxin: CNS acetylcholine receptors and luteinizing hormone secretion in ovariectomized rats

Neosurugatoxin, a neurotoxin isolated from the Japanese ivory shell, inhibits ganglionic nicotinic acetylcholine receptors but not skeletal muscle nicotinic acetylcholine receptors. It has also been reported to inhibit (3H) L-nicotine binding to high-affinity agonist acetylcholine receptors in rat brain membrane preparations. In the present study, 10(-5) M neosurugatoxin inhibited the in vitro binding of (3H) L-nicotine to the medial habenular nucleus of frozen, coronal sections of rat brain as did 10(-5) M cytisine or nicotine and 10(-4) M dihydro-beta-erythroidine. Neosurugatoxin did not inhibit (125I) alpha-bungarotoxin binding to hypothalamic synaptosomal preparations or to frozen, coronal sections of rat brain. Injection of neosurugatoxin into the third ventricles of ovariectomized rats resulted in a significant decrease in the frequency of pulses of luteinizing hormone (LH) secretion but had no effect on the amplitude of pulses. A low dose (1 microgram/injection) of the nicotinic acetylcholine agent cytisine injected into the third ventricle had no significant effect on pulsatile LH secretion. Coadministration of cytisine could block the inhibitory effect of neosurugatoxin on LH secretion. It is suggested that neosurugatoxin is a useful antagonist to study the biological roles of a specific subclass of nicotinic acetylcholine receptors in mammalian brain and reproductive neuroendocrine functions.

kappa-Bungarotoxin: binding of a neuronal nicotinic receptor antagonist to chick optic lobe and skeletal muscle

kappa-Bungarotoxin, a snake venom kappa-neurotoxin, is a potent neuronal nicotinic receptor antagonist. kappa-Neurotoxins are structurally related to the long-type alpha-neurotoxins (including alpha-bungarotoxin), which often fail to block neuronal nicotinic transmission, but which are potent antagonists of nicotinic receptors found on vertebrate skeletal muscle. The binding of kappa-bungarotoxin has now been examined in homogenates of chick skeletal muscle and optic lobe. In muscle, kappa-bungarotoxin binds to nicotinic receptors with 200-fold lower affinity than does alpha-bungarotoxin. The weakest known alpha-neurotoxin, L.s. III, is found to be 6.5-fold more potent than kappa-bungarotoxin. These findings support the conclusion that kappa-neurotoxins are selective for neuronal nicotinic receptors. In the optic lobe, 125I-alpha-bungarotoxin and 125I-L.s. III. A second nicotinic site, detected with high affinity by both alpha-neurotoxins, is only weakly bound by kappa-bungarotoxin. No evidence for a unique 125I-kappa-neurotoxin site is observed. Furthermore, kappa-bungarotoxin does not recognize the high affinity L-[3H]nicotine binding site in chick optic lobe which is distinct from the alpha-neurotoxin binding sites. Three subtypes of nicotinic sites can thus be defined in chick optic lobe, although which of these subtypes is involved in nicotinic transmission in the lobe remains to be conclusively determined.

Distribution of nicotinic cholinergic receptors in the rat tel- and diencephalon: a quantitative receptor autoradiographical study using [3H]-acetylcholine, [alpha-125I]bungarotoxin and [3H]nicotine

The topographical distribution of [alpha-125I]bungarotoxin [125I]BTX, [3H]nicotine ([3H]Nic), [3H]acetylcholine ([3H]ACh) (in the presence of atropine) binding in rat tel- and diencephalon was investigated using a quantitative receptor autoradiographical technique. With the [3H]ACh and [3H]Nic radioligands, a strong labelling was observed in various thalamic nuclei, including the medial habenula, a moderate labelling in different areas of the cortex cerebri, the nucleus caudatus putamen, the nucleus accumbens and tuberculum olfactorium and a uniform weak labelling in the hypothalamus. When the binding data for [3H]Nic were plotted against binding data for [3H]ACh in various brain nuclei, a significant correlation was obtained. Considering [125I]BTX, the strongest labelling was observed in the lateral mammillary nucleus and the hilus gyrus dentatus of the hippocampal formation. A weak labelling occurred in areas such as the nucleus causatus putamen, the thalamus and the cerebral cortex. No significant correlation was therefore obtained between the degree of [125I]BTX binding in various brain nuclei and the degree of binding observed with [3H]Nic or [3H]ACh. The present results underline the view that the high-affinity [3H]Nic and [3H]ACh binding sites label the same cholinergic nicotinic receptor binding site, while [125I]BTX labels another subpopulation of nicotinic cholinergic receptors, predominantly found in discrete areas of the hypothalamus and the limbic cortex.

Quantitative light microscopic autoradiographic localization of cholinergic muscarinic receptors in the human brain: forebrain

The distribution of muscarinic cholinergic receptors in the human forebrain and cerebellum was studied in detail by quantitative autoradiography using N-[3H]methylscopolamine as a ligand. Only postmortem tissue from patients free of neurological diseases was used in this study. The highest densities of muscarinic cholinergic receptors were found in the striatum, olfactory tubercle and tuberal nuclei of the hypothalamus. Intermediate to high densities were observed in the amygdala, hippocampal formation and cerebral cortex. In the thalamus muscarinic cholinergic receptors were heterogeneously distributed, with densities ranging from very low to intermediate or high. N-[3H]Methylscopolamine binding was low in the hypothalamus, globus pallidus and basal forebrain nuclei, and very low in the cerebellum and white matter tracts. The localization of the putative muscarinic cholinergic receptors subtypes M1 and M2 was analysed in parallel using carbachol and pirenzepine at a single concentration to partially inhibit N-[3H]methylscopolamine binding. Mixed populations of both subtypes were found in all regions. M1 sites were largely predominant in the basal ganglia, amygdala and hippocampus, and constituted the majority of muscarinic cholinergic receptors in the cerebral cortex. M2 sites were preferentially localized in the diencephalon, basal forebrain and cerebellum. In some areas such as the striatum and substantia innominata there was a tendency to lower densities of muscarinic cholinergic receptors with increasing age. In general, we observed a slight decrease in M2 sites in elderly cases. Muscarinic cholinergic receptor concentrations seemed to be reduced following longer postmortem periods. The distribution of acetylcholinesterase was also studied using histochemical methods, and compared with the localization of muscarinic cholinergic receptors and other cholinergic markers. The correlation between the presence of muscarinic cholinergic receptors and the involvement of cholinergic mechanisms in the function of specific brain areas is discussed. Their implication in neurological diseases is also reviewed.

A quantitative and morphometric evaluation of 125I-alpha-bungarotoxin binding in the rat hypothalamus

The distribution of 125I-alpha-bungarotoxin (alpha-BTX), a putative nicotinic cholinergic receptor ligand was studied both in vitro and in vivo in the suprachiasmatic nucleus (SCN), anterior hypothalamic area (AHA), and supraoptic nucleus (SON) of the hypothalamus. For in vitro studies 20 micron frozen frontal sections containing SCN were incubated with either radioligand or, unlabeled alpha-BTX plus 125I alpha-BTX and tissues were processed for light microscopic autoradiography. Areas of cresyl violet stained SCN sections were measured using a Bioquant Analysis System and grain counts and distributions were determined. For in vivo investigations third ventricular infusion of either 125I alpha-BTX, or unlabeled alpha-BTX with 125I alpha-BTX was performed, and 24 hours later animals were perfused pericardially and 1 micron serial plastic sections of the SCN were processed for light microscopic autoradiography. Localization of silver grains in 1 micron serial sections was evaluated in a double blind study. In vitro and in vivo labeling patterns in the hypothalamus were the same and compared well with previously examined paraffin-processed tissues from animals which had received third ventricular infusions of the neurotoxin. We observed a distinctive and specific labeling pattern of the SCN. Grains tended to localize diffusely and uniformly in more rostral regions, but clustered densely in the dorsal and lateral mid-SCN, and dorsally in the mid-caudal SCN. Grains were localized in the SCN where larger neurons were found. In the most caudal regions of the SCN no labeling was observed. Tissues from unlabeled alpha-BTX plus 125I alpha-BTX in vitro or in vivo studies did not demonstrate grain counts above background levels.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of spinal cord lesions and rhizotomies on cholinergic and opiate receptor binding sites in rat spinal cord

Changes in the distribution of [3H]quinuclidinylbenzilate ([3H]QNB), [3H] alpha-bungaro-toxin ([3H]alpha-Btx) and [3H]etorphine binding sites were studied autoradiographically, and cholinacetyltransferase (ChAT) activity radioenzymatically, in the C6-7 segments of rats 1-20 days after combined dorsal and ventral C3-8 rhizotomies and spinal cord lesions at C3. After dorsal and ventral rhizotomies the number of [3H]QNB, [3H]alpha-Btx and [3H]etorphine binding sites were reduced ipsilaterally in the dorsal horn and those of [3H]QNB and [3H]alpha-Btx in the ventral horn. In the ventral horn ChAT activity was significantly reduced. After a unilateral spinal cord lesion at C3, ChAT activity was reduced in the ipsilateral ventral horn at C6-7 caudal to the lesions, whereas no change in receptor binding sites was observed.

Mecamylamine pretreatment counteracts cigarette smoke induced changes in hypothalamic catecholamine neuron systems and in anterior pituitary function

We have studied the effects of acute, intermittent exposure to tobacco smoke on discrete hypothalamic CA nerve terminal networks and on neuroendocrine function by means of quantitative histofluorimetrical determinations of catecholamine (CA) fluorescence in sections of rat brain and by radioimmunoassay procedures for hormones. Acute intermittent exposure to cigarette smoke induced a lowering of NA levels and increased NA turnover in discrete hypothalamic nerve terminal regions. This exposure also induced increases in DA turnover in the median eminence. The cigarette smoke lowered TSH, prolactin, LH and FSH serum levels, but induced an increase in serum corticosterone concentrations. To determine if the above mentioned changes in neuroendocrine function were nicotine mediated, a cholinergic nicotine-like blocking agent, mecamylamine, was administered prior to exposure to cigarette smoke. Pretreatment with mecamylamine (1.0 mg kg-1) counteracted the cigarette smoke induced changes in CA levels and turnover in all hypothalamic CA nerve terminal regions as well as the changes in serum levels of the pituitary hormones and corticosterone. It is suggested that acute intermittent exposure to cigarette smoke, via its nicotine component, lowers TSH, prolactin, LH and FSH secretion at least in part through activation of the tubero-infundibular DA neurons. Furthermore, the nicotine component of the cigarette smoke is suggested to induce the increase in corticosterone serum levels via increasing NA turnover in the paraventricular hypothalamic nucleus.

Mapping of cholinoceptive(nicotinoceptive)neurons in the lower brainstem: with special reference to the ventral surface of the medulla

The distribution of cholinoceptive neurons in the lower brainstem of the rat was investigated by means of a histochemical method for specific acetylcholinesterase. Nicotinoceptive neurons were characterized using an alpha-bungarotoxin-horseradish peroxidase conjugate for the detection of nicotinic acetylcholine receptors. For the first time a nearly complete mapping of the location of cholinoceptive (nicotinoceptive) neurons of the lower brainstem was achieved. Special attention was focused on the organization of the cholinoceptive neuronal matrix of the ventral surface of the medulla, where regulative centers for vasomotor and respiratory control are located.

Cholinergic, opioid and glycine receptor binding sites localized in human spinal cord by in vitro autoradiography. Changes in amyotrophic lateral sclerosis

Binding sites for the receptor ligands 3H-quinuclidinylbenzilate, 3H-alpha-bungarotoxin (3H-alpha-Btx), 3H-etorphine and 3H-strychnine were localized autoradiographically at cervical, thoracic and lumbar levels of spinal cords from post-mortem human control subjects and subjects with amyotrophic lateral sclerosis (ALS). The highest densities of muscarinic binding sites were found in the motor neuron areas and in the substantia gelatinosa, while the grey matter binding was very low within Clarke's column. Both 3H-alpha-Btx and opioid receptor binding sites were numerous within the substantia gelatinosa, while glycine receptor binding sites were more uniformly distributed within the spinal grey matter. In ALS cases, muscarinic receptor binding sites were markedly reduced in motor neuron areas and slightly reduced in the dorsal horn, while the other binding sites studied were relatively unchanged.

Mediation of passive avoidance learning by nicotinic hippocampo-entorhinal components in young rats

Young rats, 11, 16, and 20 days of age, received bilateral injections of three antinicotinic agents into the posteroventral hippocampo-subiculo-entorhinal area, and were trained to learn a cool-draft-stimulus, passive-avoidance task shortly after (17 min). Gallamine triethiodide had no action at low doses and provoked convulsions at higher concentrations. Pempidine tartrate produced age- and dose-dependent impairments of the passive avoidance, and was much more effective in younger groups (11 and 16 days) than at 20 days. alpha-bungarotoxin also induced dose-dependent deficits. These results, together with the mecamylamine-induced deficits already reported, suggest that nicotinic cholinergic synapses located in the posteroventral part of the hippocampal complex play a role in passive-avoidance learning in the young rat as soon as this type of conditioning is possible, but become relatively less important at older ages, when muscarinic mechanisms also become involved.

Autoradiographic localization of [3H]nicotine binding sites in the rat brain

In vitro autoradiography was used to visualize [3H]nicotine binding sites in the rat brain. Labeling was densest in the interpeduncular nucleus and medial habenula but was also detected in thalamic nuclei, areas related to sensory function, the cerebral cortex, and molecular layer of the dentate gyrus. Specific binding was sparse in the hypothalamus and caudate-putamen, and not detected in Ammon's horn of the hippocampus or in the periaqueductal grey matter. These findings may relate to the distribution of nicotine's cerebral loci of action.

Quantitative light microscopic autoradiographic localization of cholinergic muscarinic receptors in the human brain: brainstem

We have investigated the localization of muscarinic cholinergic receptors in the brainstem of eight patients free of neurological disease following quantitative autoradiography of microtome sections of postmortem tissue labeled in vitro with N-[3H]methyl scopolamine as a ligand. Receptor densities were quantified by microdensitometry with the aid of a computer assisted image analysis system. Our results reveal a heterogeneous distribution of receptor sites. High concentrations of muscarinic cholinergic receptor sites were associated with many nuclei and areas of the brainstem including the nucleus facialis (VII), hypoglossus (XII), ambiguus, the motor trigeminal nucleus (V), the nucleus solitarius, the nucleus of the lateral lemniscus, the superior and inferior colliculi, the sensory trigeminal nucleus (substantia gelatinosa), the pontine nuclei, the parabrachial nuclei, some tegmental nuclei and the periaqueductal gray matter. Very high concentrations of N-[3H]methyl scopolamine binding sites were also localized in the ventral tegmental area, the nucleus paranigralis and the nucleus ovalis. Receptor densities varied between individual brains although the relative distribution of the densities in the different nuclei was the same for all of the brains examined. Most of the brainstem nuclei containing muscarinic cholinergic receptors were enriched in high affinity agonist binding sites as shown by characteristic displacement of the ligand with carbachol. Exceptions were the substantia nigra, the nucleus olivaris inferior and the substantia gelatinosa of the fifth nerve. Receptor density values and pharmacological characteristics obtained in the cortex and basal ganglia in our cases are in good agreement with previously reported values in humans, using conventional biochemical methods. This indicates that procedures used in the autoradiographic technique are not detrimental to the pharmacological characteristics and densities of muscarinic cholinergic receptors. Our results thus clearly show the feasibility of using these techniques for the localization and quantification of muscarinic cholinergic receptors in human brain postmortem material. Furthermore, our findings indicate the potential involvement of the muscarinic cholinergic effect of acetylcholine in the normal function of many brainstem centers, including motor and sensory nuclei, visual and auditory relay nuclei and cardiovascular and respiratory-related nuclei.

Effects of gonadal steroids on the in vivo binding of [125I]alpha-bungarotoxin to the suprachiasmatic nucleus

The radioligand [125I]alpha-bungarotoxin (alpha-BTX) has been used to test receptor binding to putative nicotinic cholinergic receptors in the hypothalamus. Using light microscopic autoradiography following third ventricular infusion of the radioligand we have previously demonstrated that in normally cycling rats and in normal males, the suprachiasmatic nucleus (SCN) consistently binds the alpha-neurotoxin. In chronically (5 weeks) oophorectomized female, binding of [125I]alpha-BTX to the SCN is markedly diminished. The present series of experiments were designed to test the effects of gonadal steroids on the binding of [125I]alpha-BTX to the SCN. We first tested whether or not estradiol administered to ovariectomized females could duplicate the presence of the ovary. In females ovariectomized and immediately provided with a constant dose of estradiol-17 beta (E2)--1.0 cm silastic capsules for 5 weeks, (n = 4), the binding of the neurotoxin to the SCN was maintained. In females ovariectomized for 3 weeks and replaced with E2 for 2 weeks (n = 4), the binding of [125I]alpha-BTX to the SCN was restored. In chronically (4 weeks) ovariectomized females receiving E2 for 6 days (n = 2), the binding of the neurotoxin was partially restored. We next tested the effect of chronic (5 weeks) castration (n = 100) and observed that binding of [125I]alpha-BTX to the SCN of castrated males was like that of intact male controls (n = 6).(ABSTRACT TRUNCATED AT 250 WORDS)

Lack of stereoselectivity in ability of nicotine to release dopamine from rat synaptosomal preparations

Both the naturally occurring (-)-isomer and the synthetic (+)-isomer of nicotine caused release of 3H from a crude synaptosomal fraction of rat brain preincubated with [3H]dopamine. The isomers were equipotent in producing this response, which was concentration-dependent, a significant effect on the fractional release of dopamine being observed at 10(-4) M nicotine. The effect did not appear to be the result of synaptosomal damage, as levels of the intrasynaptosomal marker lactate dehydrogenase did not increase in the supernatant. Nicotine-induced release was inhibited by removal of external Ca2+ and by the presence in vitro of pempidine (230 microM). Neither hexamethonium (500 microM) in vitro nor the chronic administration of (-)-nicotine in vivo had any effect on the nicotine-induced release of [3H]dopamine. It is concluded that nicotine exerts this effect via a presynaptic nicotinic receptor of the "ganglionic" type, but that this receptor differs from that in the periphery by showing a relative lack of stereospecificity. There is no evidence of a functional "down regulation" in this receptor on chronic exposure to nicotine in vivo.

Binding of radiolabeled opiates to slide-mounted sections of molded minced rat brain: a novel method for conducting radioreceptor assays

To facilitate the quantitative study of the opiate receptor, we have developed a novel variation of the ligand binding technique. The binding of 3H-opiates to rat brain membranes is compared with the binding to slide-mounted sections of molded minced rat brain. The latter method is characterized by a high signal and a superior signal-to-noise ratio over a wide range of ligand concentrations. The results are discussed in reference to other ligand binding methodologies.

Immunohistochemical localization of monoclonal antibodies to the nicotinic acetylcholine receptor in chick midbrain

We used the indirect immunofluorescence method to determine the crossreactivity of a library of 57 monoclonal antibodies (mAbs) against each of the subunits of the nicotinic acetylcholine receptor (nAcChoR) isolated from Torpedo and Electrophorus electric organs or from fetal calf and human muscle, with specific neural elements in the midbrain of the chick. Out of 17 mAbs that recognized motor end plates on chick muscle, 14 produced a similar pattern of labeling in the midbrain: the neuronal perikarya and dendrites in the lateral spiriform nucleus (SpL) were intensely labeled, and there was moderate labeling of fibers in certain of the deeper layers of the optic tectum, which disappeared after the SpL was destroyed electrolytically. Two lines of evidence suggest that the mAbs may be crossreacting with nAcChoRs in the midbrain. First, all of the mAbs that stained the SpL also stained neuromuscular junctions in skeletal muscle, whereas none of the 40 mAbs that failed to stain end plates crossreacted with the SpL; second, in vitro immunological studies and blocking experiments on tissue sections (in which unlabeled mAbs were used to block the staining of a directly fluorescein-treated mAb) indicated the presence of mAbs specific for unique antigenic determinants on all four of the subunits (alpha, beta, gamma, and delta) from Torpedo nAcChoR in chick midbrain and muscle. On the other hand, the distribution of mAb staining in the optic tectum does not closely parallel that of either acetylcholinesterase staining or of 125I-labeled alpha-bungarotoxin binding; no toxin binding has been observed autoradiographically in the SpL, but the nucleus does contain moderately dense acetylcholinesterase staining. Take together, our observations suggest that there may be a cholinergic input to the SpL and that the projection fibers from the SpL to the optic tectum (which are also stained with an antiserum to [Leu]enkephalin) may contain presynaptic nAcChoRs. It is clear, however, that the distribution of the putative nAcChoRs, alpha-bungarotoxin binding sites, and acetylcholinesterase staining in the avian midbrain are quite different, although they do overlap to some degree in the deeper layers of the optic tectum.

Deficits in passive avoidance learning in young rats following mecamylamine injections in the hippocampo-entorhinal area

Young rats 11, 13, 16, and 20 days old were injected bilaterally with the nicotinic antagonist mecamylamine hydrochloride (5, 50, and 100 micrograms on each side) into the posteroventral hippocampo-entorhinal (VHE) area and trained on a "cool-draft-stimulus" passive avoidance task. The data showed impaired acquisition and reduced resistance to extinction. The deficits observed were age- and dose-dependent, rats being highly sensitive to the drug when 11 and 13 days old and decreasingly responsive up to day 20. The results may indicate that nicotinic cholinergic sites in the VHE area mediate passive avoidance learning in the young rat as soon as acquisition emerges. Muscarinic cholinergic mechanisms only develop later in this region, becoming progressively more important for passive avoidance behavior.

Role of dopamine receptors in the modulation of acetylcholine release in the rabbit hippocampus

Modulation of acetylcholine release was studied in slices of the rabbit hippocampus preincubated with 3H-choline and then continuously superfused with a medium containing 10 mumol/l hemicholinium-3. Electrical field stimulation of the superfused slices elicited an increase in tritium outflow, which was tetrodotoxin-sensitive and largely calcium-dependent. Stimulus-evoked acetylcholine release in the rabbit hippocampal slices was modulated by presynaptic muscarinic autoreceptors, as has been shown previously for the rat hippocampus. Drugs with affinity for alpha- and or beta-adrenoceptors did not affect the evoked overflow of tritium from rabbit hippocampal slices. In contrast, the dopamine receptor agonist apomorphine (0.1 or 1 mumol/l) and exogenous dopamine (1 or 10 mumol/l) significantly reduced the evoked outflow by about 10 or 20%, respectively. This effect was antagonized by haloperidol (0.01 mumol/l) but not by phentolamine (1 mumol/l). Attempts to enhance (using nomifensine 10 mumol/l) or reduce (using haloperidol, up to 1 mumol/l; or bretylium, 1 mmol/l for 5 min) endogenous dopaminergic transmission in the hippocampal slices did not affect stimulation evoked acetylcholine release. In conclusion, presynaptic dopamine receptors modulating acetylcholine release are present in the rabbit hippocampus, but they seem not to be of physiological significance.

Effects of ovariectomy on the binding of [125I]-alpha bungarotoxin (2.2 and 3.3) to the suprachiasmatic nucleus of the hypothalamus: an in vivo autoradiographic analysis

alpha-Bungarotoxin (alpha-BTX) has been used to label receptor binding sites on neural membranes. alpha-BTX fractions 2.2 and 3.3 were purified from Bungarus multicinctus by the method of Ravdin and Berg (1979) and we iodinated. There was no difference between these two fractions in their binding affinity or specificity of binding with hypothalamic synaptosomes. [125I] alpha-BTX 2.2S, 3.3 and commercially obtained [125I] alpha-BTX were injected into the third ventricle of ovariectomized female rats (n = 22), normally cycling rats (n = 12) or normal male rats (n = 4) and autoradiographic examination performed. Saline injected hypothalami (n = 4) or hypothalamus. Examination of serial sections from animals injected with each [125I] alpha-BTX showed that the supraoptic, periventricular, arcuate, premamillary and mamillary nuclei were consistently labeled. While the suprachiasmatic nucleus (SCN) in the intact females and males both showed high densities of alpha-BTX binding, the SCN in ovariectomized females showed little or no alpha-BTX binding. Thus, the labeling of the SCN in females without ovaries and ovarian hormones was markedly different from that of the intact males and females. Labeling patterns in castrate and intact animals may contribute to our understanding of gonadal steroid regulation of hypothalamic function.

Cholinergic pharmacology of mammalian hippocampal pyramidal cells

Responses of CAl pyramidal cells to cholinergic compounds were recorded with intracellular microelectrodes in guinea-pig hippocampal slices. Perfusion of slices with medium containing the muscarinic antagonists atropine or scopolamine (10(-7)-10(-6)M) blocked all actions of acetylcholine. Properties of control neurons and those from separate populations of neurons impaled in slices exposed to muscarinic blocking agents were compared. 1-2 h of perfusion with atropine-containing media significantly decreased membrane input resistance from 37.6 +/- 8.7 (S.D.) M omega (n = 74) to 21.9 +/- 7.7 (S.D.) M omega (n = 24) without producing significant changes in membrane potential. Muscarinic antagonists also reduced or eliminated the anomalous inward rectification normally seen in hippocampal pyramidal neurons. Exposure of slices to 10(-5)-10(-6)M eserine for about 1 h produced changes in neuronal membrane input resistance and potential and slow after hyperpolarizations similar to those elicited by application of acetylcholine. Bethanechol mimicked the actions of acetylcholine but was effective at lower concentrations and had longer lasting effects on afterhyperpolarizations. Nicotine produced an excitatory response in only one of 7 neurons. These experiments demonstrate that the actions of acetylcholine on hippocampal CAl neurons result from interaction with muscarinic receptors. Acetylcholine has modulatory effects on cell membrane properties which may be mediated through tonic release mechanisms.

Monoaminergic-cholinergic relationships and the chemical communication matrix of the substantia nigra and neostriatum

The historical development of histochemical methods for monoamines and chemicals involved with cholinergic function is reviewed. The use of these methods to elucidate neurochemical interactions in the substantia nigra and caudate-putamen complex is then discussed. Three hypotheses accounting for the localization of acetylcholinesterase within and/or on substantia nigra, pars compacta neurons are presented and evaluated: (a) to catabolize acetylcholine released from afferent cholinergic fibers, (b) to catabolize substance P released from some neostriato-nigral axon terminals, and/or (c) to serve as a communication link with nigral vasculature. Despite experimental evidence in favor of each of these possibilities, none have met with unqualified acceptance. Possible mechanisms and morphologic substrates accounting for dopaminergic-cholinergic, serotonergic-cholinergic, GABAergic-cholinergic, enkephalinergic-cholinergic, and cholinergic-cholinergic interactions in the caudate-putamen complex are discussed. These include synaptic and non-synaptic relationships, dendroaxonic information flow, and mutual regulatory processes.

Neurotransmitter receptor ligand binding and enzyme regional distribution in the pigeon visual system

The relative importance of acetylcholine, dopamine, endogenous opiates, gamma-aminobutyric acid (GABA), glutamate, glycine, noradrenaline, and serotonin as transmitters in the pigeon visual system was estimated by measuring the activity of choline acetyltransferase (ChAT), glutamic acid decarboxylase (GAD), and aromatic amino acid decarboxylase (AAD) as well as the binding of dihydroalprenolol, etorphine, kainic acid, muscimol, serotonin, spiroperidol, strychnine, and quinuclidinyl benzilate (QNB) in the tectum opticum, nucleus rotundus, ectostriatum, dorsolateral thalamus, and hyperstriatum (Wulst). As a nonvisual reference structure, the paleostriatal complex was included in the examination. The regional distribution of most of these parameters was very similar to data reported in the mammalian CNS supporting the hypothesis that the avian tectofugal and thalamofugal visual systems are homologous to the mammalian tecto-thalamo-cortical and retino-geniculo-striate pathways, respectively. On the basis of the low values of their parameters, some transmitters can be excluded as significant contributors in a number of structures. As a hypothesis for further investigations, the presence of cholinergic and serotoninergic systems in the Wulst, possibly originating in the dorsolateral thalamus and nucleus raphe, respectively, and of glycinergic and dopaminergic terminals in the paleostriatal complex is proposed.

Nicotinic acetylcholine receptor from chick optic lobe

An alpha-bungarotoxin-sensitive nicotinic cholinergic receptor from chick optic lobe has been completely purified. Its standard sedimentation coefficient is 9.1 S. The value near 12 S reported for the related component from other brain regions can be reproduced when the initial extraction is by Triton X-100 (rather than Lubrol PX), but other protein is then complexed with it. A single subunit of apparent molecular weight 54,000 is detected, and this subunit is specifically labeled by bromo-[3H]acetylcholine, but only after disulfide reduction. The same size subunit likewise is labeled in the protein (purified similarly) from the rest of the chick brain which can also bind alpha-bungarotoxin and nicotinic ligands. Immunological crossreactivity is demonstrated between both of these proteins with an antiserum to pure acetylcholine receptor from skeletal muscle. The acetylcholine receptor from chick optic lobe and the alpha-bungarotoxin-binding protein from the rest of the brain appear similar or identical by a series of criteria and are related to (but with differences from) peripheral acetylcholine receptors.

Fluorescent staining of neuromuscular junctions by using the antibody against acetylcholine receptors of Narke japonica, and double staining with the antibody and erabutoxin b

The neuromuscular junctions of various vertebrates were visualized by indirect immunofluorescence microscopy using antibody against purified acetylcholine receptors (AChRs) of the electric organ from Narke japonica. Further, by using rhodamine-labeled erabutoxin b (TMR-Eb), we showed that AChRs at the neuromuscular junctions of frog, chick and mouse muscle could not be doubly stained with the antibody and erabutoxin b. AChRs of snake muscle could not be stained with TMR-Eb, while they were stained with the antibody against AChR. Moreover, the antibody did not inhibit the binding of 3H-labeled erabutoxin b to the solubilized AChRs from the mouse muscle. These results indicate that, as far as the antibodies against Narke AChRs are concerned, most antibody-binding sites in the molecules of muscle AChR in situ are different from those responsible for binding of the snake neurotoxin.

Heart rate conditioning in the pigeon immobilized with alpha-bungarotoxin

Previous in vitro studies have yielded conflicting results concerning the physiological effects of alpha-bungarotoxin (alpha-BTX) at avian autonomic ganglia. In the present study heart rate change was classically conditioned in pigeons immobilized with alpha-BTX to assess the effects of this agent on autonomic function in the intact animal. When compared with non-immobilized controls, alpha-BTX had no significant effect on basal heart rate or on the rate of acquisition, magnitude or dynamics of conditioned heart rate change. Furthermore, a significant difference in performance between groups receiving conditioning and sensitization paradigms clearly indicated the establishment of conditioned responding. It is concluded that alpha-BTX has little or no effect on the cardiac innervation at doses sufficient to block neuromuscular transmission. The irreversible blockade of neuromuscular transmission by alpha-BTX without the side-effects produced by most other paralytics makes it an ideal choice as an immobilizing agent in acute preparations.

Imprinting. A study of cholinergic receptor sites in parts of the chick brain

The distribution of cholinergic receptor binding sites was studied in certain areas of the chick brain, especially that part (IMHV) of hyperstriatum ventrale which has previously been implicated in the imprinting process. There was relatively little binding of [125I]-alpha-bungarotoxin in this region, as shown using an autoradiographic technique. In contrast muscarinic receptor sites, with an affinity for [3H]-quinuclidinyl benzilate (QNB), are present in relatively high concentration in a medial forebrain sample containing IMHV and also in a visual projection area, the hyperstriatum accessorium (HA). To assess the effects of imprinting on the concentration of QNB binding sites 26 young chicks were exposed to an imprinting stimulus (a pulsing red light) for 20 min. Thirteen of the chicks were returned to the dark and the other 13 were trained for a further 100 min. There was no effect of further training on specific QNB binding in HA or in the medial forebrain sample. In undertrained chicks there was a positive correlation between QNB binding in the medial forebrain sample of the left hemisphere and that in the right; there was no such relationship in overtrained birds. Mean QNB binding in the right medial forebrain sample was greater than in the left by 21%. There was no such hemispheric asymmetry in HA.