Distribution of alpha-bungarotoxin binding sites in the central nervous system and peripheral organs of the rat

Methyllycaconitine: a non-radiolabeled ligand for mapping α7 neuronal nicotinic acetylcholine receptors - in vivo target localization and biodistribution in rat brain

INTRODUCTION

Reduction of cerebral cortical and hippocampal α7 neuronal nicotinic acetylcholine receptor (nAChR) density was observed in the Alzheimer's disease (AD) and other neurodegenerative diseases. Mapping the subtypes of nAChRs with selective ligand by viable, quick and consistent method in preclinical drug discovery may lead to rapid development of more effective therapeutic agents. The objective of this study was to evaluate the use of methyllycaconitine (MLA) in non-radiolabeled form for mapping α7 nAChRs in rat brain.

METHODS

MLA pharmacokinetic and brain penetration properties were assessed in male Wistar rats. The tracer properties of MLA were evaluated in rat brain by dose and time dependent differential regional distribution studies. Target specificity was validated after blocking with potent α7 nAChR agonists ABBF, PNU282987 and nicotine. High performance liquid chromatography combined with triple quad mass spectral detector (LC-MS/MS) was used to measure the plasma and brain tissue concentrations of MLA.

RESULTS

MLA has shown rapid brain uptake followed by a 3-5 fold higher specific binding in regions containing the α7 nAChRs (hypothalamus - 1.60 ng/g), when compared to non-specific regions (striatum - 0.53 ng/g, hippocampus - 0.46 ng/g, midbrain - 0.37 ng/g, frontal cortex - 0.35 ng/g and cerebellum - 0.30 ng/g). Pretreatment with potent α7 nAChR agonists significantly blocked the MLA uptake in hypothalamus. The non-radiolabeled MLA binding to brain region was comparable with the α7 mRNA localization and receptor distribution reported for [(3)H] MLA in rat brain.

DISCUSSION

The rat pharmacokinetic, brain penetration and differential brain regional distribution features favor that MLA is suitable to use in preclinical stage for mapping α7 nAChRs. Hence, this approach can be employed as an essential tool for quicker development of novel selective ligand to map variation in the α7 receptor densities, as well as to evaluate potential new chemical entities targeting neurodegenerative diseases.

Cholinergic modulation of slow cortical rhythm in urethane-anesthetized rats

Slow cortical rhythm (SCR) is characterized by rhythmic cycling of active (UP) and silent (DOWN) states in cortical cells. In urethane anesthesia, SCR appears as alternation of almost isoelectrical EEG periods and low-frequency, high-amplitude large shifts with superimposed high-frequency activity in the local field potentials (LFPs). Dense cholinergic projection reaches the cortex from the basal forebrain (BF), and acetylcholine (ACh) has been demonstrated to play a crucial role in the regulation of cortical activity. In the present experiments, cholinergic drugs were administered topically to the cortical surface of urethane-anesthetized rats to examine the direct involvement of ACh and the BF cholinergic system in the SCR. SCR was recorded by a 16-pole vertical electrode array from the hindlimb area of the somatosensory cortex. Multiple unit activity (MUA) was recorded from layer V to VI in close proximity of the recording array. Neither a low dose (10 mM solution) of the muscarinic antagonist atropine or the nicotinic agonist nicotine (1 mM solution) had any effect on SCR. In contrast, the higher dose (100 mM solution) of atropine, the cholinergic agonist carbachol (32 mM solution), and the cholinesterase inhibitor physostigmine (13 mM solution) all decreased the number of UP states, delta power (0-3 Hz) and MUA. These results suggest that cholinergic system may influence SCR through muscarinic mechanisms during urethane anesthesia. Cholinergic activation obstructs the mechanisms responsible for local or global synchronization seen during SCR as this rhythm was disrupted or aborted. Muscarinic antagonism can evoke similar changes when high dose of atropine is applied.

Potentiation of evoked calcitonin gene-related peptide release from oral mucosa: a potential basis for the pro-inflammatory effects of nicotine

Inflammation of the buccal mucosa, gingiva and periodontal tissues is a significant problem in users of nicotine-containing tobacco products; however, the potential role of nicotine in the development of this inflammation is unclear. In many tissues, nicotine, acting through nicotinic acetylcholine receptors (nAChRs), has been shown to increase the release of the pro-inflammatory mediator calcitonin gene-related peptide (CGRP) thereby potentially contributing to neurogenic inflammation. The purpose of the present studies was to determine the effects of nicotine and other nAChR agonists on capsaicin-evoked immunoreactive CGRP (iCGRP) release from rat buccal mucosa and to identify a potential cellular basis for these effects. Using a previously validated model of in vitro superfusion, we show that the nAChR agonists nicotine (EC50 557 micro m), epibatidine (EC50 317 pm) and cytisine (EC50 4.83 nm) potentiated capsaicin-evoked iCGRP release in a concentration-dependent manner by 123, 70 and 76%, respectively. The expression and distribution patterns of the mRNA transcripts encoding the alpha3, alpha4 and alpha6 nAChR subunits and their colocalization with CGRP and the capsaicin receptor VR1 were examined in rat trigeminal ganglion using combined in situ hybridization and immunohistofluorescence. Of all trigeminal neurons counted, mRNA encoding the alpha3, alpha4 and alpha6 subunits was found, respectively, in 14.45, 9.2 and 19.21% of neurons. The cell body diameter of most neurons containing any nAChR subunit was in the 30-40 micro m range with slightly fewer in the 20-30 micro m range. Co-localization of these alpha subunit transcripts with either CGRP or VR1 immunoreactivity ranged from approximately 5 to 7% for alpha4 and over 8% for alpha3 to 18% for alpha6. These data support the hypothesis that nicotinic agents, acting at nAChRs contained on primary sensory neurons, are capable of directly modulating the stimulated release of iCGRP. In the case of users of nicotine-containing tobacco products, this modulation could contribute to inflammatory processes within the oral cavity.

Individual variation in the expression profiles of nicotinic receptors in the olfactory bulb and trigeminal ganglion and identification of alpha2, alpha6, alpha9, and beta3 transcripts

Nicotine evokes dose-dependent and often variable chemosensory responses in animals and humans. Earlier observations that nicotine binds to some nicotinic acetylcholine receptor (nAChR) subtypes in the olfactory bulb (OB) and trigeminal ganglion (TG) led us to investigate the complete nAChR expression profile in each tissue and to determine whether inter-individual differences exist in male and female rats. Total RNA was extracted from individual samples of dissected OB and TG and analyzed by a sensitive reverse transcription-polymerase chain reaction (RT-PCR) assay to determine the messenger RNA profiles of ten transcripts encoded by the alpha2, alpha3, alpha4, alpha5, alpha6, alpha7, alpha9, beta2, beta3, and beta4 nAChR genes. We found that (a) in the OB, all animals expressed alpha2, alpha3, alpha4, alpha5, alpha7, beta2, and beta4 subunit mRNAs, whereas alpha6, beta3, and alpha9 transcripts were expressed in only 17, 28, and 33% of the animals, respectively, and (b) in the TG, all animals expressed alpha2, alpha3, alpha6, alpha7, beta2, and beta4 subunit mRNAs, whereas alpha9, beta3, alpha4, and alpha5 transcripts were expressed in 4, 38, 88, and 92% of the animals, respectively. These results also identified new subunits that are expressed in each tissue (alpha2, alpha6, alpha9, and beta3) and demonstrated that individual rats may have different tissue-specific expression profiles for alpha4, alpha5, alpha6, alpha9, and beta3 transcripts. Such variations are likely to be reflected in the composition of functional receptor subtypes in the rat OB and TG that have different activation and desensitization characteristics to acetylcholine and nicotine.

Characterisation of the binding of [3H]methyllycaconitine: a new radioligand for labelling alpha 7-type neuronal nicotinic acetylcholine receptors

Methyllycaconitine (MLA), a norditerpenoid alkaloid isolated from Delphinium seeds, is one of the most potent non-proteinacious ligands that is selective for alpha bungarotoxin-sensitive neuronal nicotinic acetylcholine receptors (nAChR). [3H]MLA bound to rat brain membranes with high affinity (Kd = 1.86 +/- 0.31 nM) with a good ratio of specific to non-specific binding. The binding of [3H]MLA was characterised by rapid association (t 1/2 = 2.3 min) and dissociation (t 1/2 = 12.6 min) kinetics. The radioligand binding displayed nicotinic pharmacology, consistent with an interaction with alpha bungarotoxin-sensitive nAChR. The snake alpha-toxins, alpha bungarotoxin and alpha cobratoxin, displaced [3H]MLA with high affinity (Ki = 1.8 +/- 0.5 and 5.5 +/- 0.9 nM, respectively), whereas nicotine was less potent (Ki = 6.1 +/- 1.1 microM). The distribution of [3H]MLA binding sites in crudely dissected rat brain regions was identical to that of [125I] alpha bungarotoxin binding sites, with a high binding site density in hippocampus and hypothalamus, but low density in striatum and cerebellum. [3H]MLA also labelled a sub-population of binding sites which are not sensitive to the snake alpha toxins, but which did not differ significantly from the major population with respect to their other pharmacological properties or regional distribution. [3H]MLA, therefore, is a novel radiolabel for characterising alpha 7-type nAChR. A good signal to noise ratio and rapid binding kinetics provide advantages over the use of radiolabelled alpha bungarotoxin for rapid and accurate equilibrium binding assays.

Neural nicotinic acetylcholine responses in solitary mammalian retinal ganglion cells

Using the patch-clamp technique, whole-cell recordings from solitary rat retinal ganglion cells in culture have established the nicotinic nature of the acetylcholine responses in these central neurons. Currents produced by acetylcholine (5-20 mumol/l) or nicotine (5-20 mumol/l) reversed in polarity near -5 mV and were unaffected by atropine (10 mumol/l). Agonist-induced currents were blocked by low doses (2-10 mumol/l) of the classical 'ganglionic' antagonists hexamethonium and mecamylamine, as well as by d-tubocurarine and dihydro-beta-erythroidine (the latter two do not discriminate clearly between ganglionic and neuromuscular junction receptors). Treatment with the potent neuromuscular blocking agent alpha-bungarotoxin (10 mumol/l) did not affect the cholinergic responses of these cells, while toxin F (0.2 mumol/l), a neural nicotinic receptor antagonist, readily abolished acetylcholine-induced currents. Thus, the experiments performed to date show that the nicotinic responses of retinal ganglion cells in the central nervous system share the pharmacology of autonomic ganglion cells in the peripheral nervous system. The ionic current carried by the nicotinic channels was selective for cations, similar to that described for nicotinic channels in other tissues. In addition, single-channel currents elicited by acetylcholine were observed in whole-cell recordings with seals greater than 5 G omega as well as in occasional outside-out patches of membrane. These acetylcholine-activated events, which had a unitary conductance of 48 pS and a reversal potential of 0 mV, represent the ion channels that mediate the neural nicotinic responses observed in these experiments on retinal ganglion cells.

Cholinergic systems and multiple cholinergic receptors in ocular tissues

Acetylcholine (ACh), choline acetyltransferases and cholinesterases occur in cornea, iris-ciliary body complex and retina of several vertebrates. In cornea, ACh may serve as a sensory transmitter as well as a local hormone, the function of which is not delineated. The function of ACh as the parasympathetic neurotransmitter at the iris and ciliary body is well established. The muscarinic receptors on the iris smooth muscle are similar to the muscarinic receptors (M2 type in two way classification) at other smooth muscles towards their interaction with agonists and antagonists. Binding studies using radiolabeled antagonists and their displacement by agonists indicate that muscarinic receptors in membranes of iris-ciliary body complex are heterogeneous indicating more than one subtype of muscarinic receptors. A subtype other than M2 receptors may occur at the presynaptic sites of parasympathetic nerves, which have yet to be investigated using specific agonists and antagonists. Cholinergic markers, choline acetyltransferase and acetylcholinesterase, differ quantitatively and qualitatively in retinas of different species. However, amacrine cells are cholinergic in all vertebrate species. Although they make up 1% of retinal neurons, they influence the activity of a majority of ganglion cells. Cholinergic effects in ganglia are mediated through nicotinic and muscarinic receptors. Both of these types of cholinergic receptors are heterogeneous. They have yet to be investigated for their subtypes using specific agonists and antagonists. Although the role of cholinergic retinal neurons in the processing of visual information is not known, their input to ganglion cells generally increases the rate of spontaneous activity or the number of action potentials in light-evoked responses. Thus, the cholinergic input seems to modify the overall neuronal input to the ganglion cells from the receptive fields. Endothelial cells of blood vessels contain muscarinic receptors, which are activated by ACh to cause relaxation. Although retinal blood vessels provide recognizable characteristic signs in diabetes mellitus and hypertensive disease, no information is available on the muscarinic receptors of these vessels.

Nature of nicotine binding to rat brain P2 fraction

(-)-Nicotine may bind to as many as 5 sites in the rat brain P2 preparation: A very high affinity site (KD approximately 2.2 X 10(-11) M); a positive cooperativity site; a high affinity site (KD approximately 5.2 X 10(-9) M); a low affinity site (KD approximately 4.5 X 10(-5) M) and a very low affinity site. The curvilinear nature of both Scatchard plots and kinetic curves indicates the presence of multiple binding sites. Evidence for a positive cooperativity site includes: (1) The configuration of Scatchard plots (at low concentrations) of saturation as well as inhibition curves for (-)- and (+)-nicotine. (2) The Hill number of 1.37 for the binding of low concentrations of (+/-)-[3H]nicotine. (3) Selectivity among cholinergic drugs for producing positive cooperativity. (4) Markedly different specificities of drugs for the positive cooperativity site. Thus while only (+)- and (-)-nicotine interacted with the very high affinity site, acetylcholine, atropine, mecamylamine, lobeline, carbachol, (+)-nicotine and (-)-nicotine enhanced the binding of (+/-)-[3H]nicotine and cytisine, anabasine, cotinine and choline selectively inhibited binding at the high affinity site. Several lines of evidence indicate that there is stereospecificity. (+)-Nicotine was more potent than (-)-nicotine in inducing positive cooperativity whereas (-)-nicotine was 80 times more potent than (+)-nicotine in inhibiting binding at the high affinity site. Further, the specificity of the binding sites can be altered by changing the concentration of the buffer which gives additional evidence for the lability of the nicotine binding site. Although the pharmacologic significance of the different binding sites has not been determined, these data taken together indicate that (+/-)-[3H]nicotine binds with specificity to multiple sites in the rat brain P2 preparation with a complexity not addressed heretofore.

Experimental febrile convulsions in the developing rat: effects on the cholinergic system

The effects of hyperthermia-induced convulsions (HCs) on nicotinic and muscarinic receptor sites, choline acetyltransferase (ChAT), and acetylcholinesterase (AChE) in the rat were investigated. A series of 10 convulsions, evoked between 5 and 16 days of age, had marked effects on the development of the cholinergic system in the cerebellum and frontal cortex, but not in the hippocampus or hypothalamus. The concentration of muscarinic receptor sites in the cerebellum of the HC group was similar to that in controls at 2 days, greater than in controls at 8 days, but less than in controls at 55 days after the last convulsion. ChAT and AChE activities were increased at 2 days, similar at 8 days, and less than those in controls at 55 days. A decrease in muscarinic receptors and a decline in ChAT activity in the frontal cortex of the HC group were observed at 55 days after the last convulsion. The concentration of nicotinic receptor sites did not distinguish HC from control groups. A simple relationship between the experimental febrile convulsion and the cholinergic system was not found. The greatest effects were noted at 55 days after the last HC, which suggests that these may reflect secondary and possibly transsynaptic influences of the convulsion on cholinergic activity.

Cholinergic receptor site binding, choline acetyltransferase, and acetylcholinesterase activity in the forebrain and brainstem of the Dahl rat model of essential hypertension

Muscarinic and nicotinic receptor site binding and the activity of choline acetyltransferase (ChAT) and acetylcholinesterase (AChE) in the forebrain and brainstem of Dahl salt-sensitive (DS) and salt-resistant (DR) rats was investigated. The DS line had a greater density of muscarinic sites in the cortex, hypothalamus, and medulla. Hypertensive DS rats had a greater density of sites than normotensive DS rats. ChAT activity was also higher in the cortex and hypothalamus of the DS line than the DR line. No significant differences were found in the activity of AChE or the concentration of nicotinic sites. These results suggest that the central muscarinic cholinergic system may participate in the pathogenesis of hypertension in the DS rat. The data indicate that central cholinergic activity is possibly greater in the DS than the DR rat and that this may help to explain the enhanced pressor response in the DS line after pharmacological activation of the central cholinergic system.

Localization of synaptic and nonsynaptic nicotinic-acetylcholine receptors in the goldfish retina

The localization of nicotinic-cholinergic receptors in the inner plexiform layer (IPL) of goldfish retina was studied by electron microscopic analysis of the binding pattern of a conjugate or horseradish peroxidase and alpha bungarotoxin (HRP-alpha BTx). Specific HRP reaction product (blockade by 1mM curare) was found at both synaptic and nonsynaptic sites. Synaptic binding sites for HRP-alpha BTx, which accounted for only 16% of the total specific reaction product sites, always involved an amacrine process as the presynaptic element, whereas amacrine, ganglion, and bipolar cells could be post-synaptic elements at labeled synapses. Only 17.5% of the total number of amacrine synapses were labeled by HRP-alpha BTx. Labeled synapses showed the same distribution in the IPL as unlabeled synapses: bimodal for amacrine-to-bipolar synapses with peak concentrations at the 20% and 80% layers and unimodal for amacrine-to-nonbipolar synapses with a peak concentration at the 60% layer. Nonsynaptic binding sites for HRP-alpha BTx (84% of total) were seen on the dendrites of ganglion, amacrine, and bipolar cells. The distribution of the nonsynaptic sites in the IPL largely accounts for the trilaminar binding pattern of 125I-alpha BTx as observed in light microscopic autoradiographs. If, as appears likely, the distribution of synapses is the relevant variable in determining the sites of neuronal interaction for a given transmitter system, then this study further illustrates the importance of distinguishing synaptic from nonsynaptic binding when using receptor-ligand probes to localize sites of chemical synaptic transmission.

Nicotine binding sites and their localization in the central nervous system

The resolution of racemic nicotine to provide optically pure (+)-nicotine and the synthesis of radiolabeled nicotine with high specific activity have facilitated the study of nicotine binding in brain. The actions of the stereoisomers of nicotine on the central nervous system are qualitatively similar in most tests but (-)-nicotine is more potent than the unnatural (+)-isomer by 10-fold or greater. Binding of radiolabeled nicotine to brain has both saturable and nonsaturable components. Only saturable binding is affected by incubation conditions such as time, temperature, pH and ion concentration. Excess concentrations of the stereoisomers are equally effective in displacing (-)-[3H]-nicotine from brain homogenates. Nevertheless, a direct comparison of (+)-[3H]-nicotine and (-)-[3H]-nicotine binding shows that the latter has a KD three times lower than the former. (-)-[3H]-Nicotine is bound to the greatest degree in hypothalamus and hippocampus, areas that also exhibited the most stereoselectivity for nicotine. However, differences in the binding affinities of the two isomers were far less than the pharmacological stereospecificity observed.

The effect of kainic acid on cholinergic enzymes and receptors in the amygdala complex of the rat

Kainic acid (KA) was injected into the amygdala (AM) complex of the rat and its effect on the cholinergic enzymes, choline acetyltransferase (CAT) and acetylcholinesterase (AChE), and the binding of the muscarinic ligand, [3H]quinuclidinyl-benzilate (QNB) and the nicotinic ligand [125I]alpha-bungarotoxin (aBuTX) was investigated. Ka produced a loss of approximately 35% of the CAT activity in the AM. However, no effect on AChE activity was observed. A 30--50% decrease in the number of muscarinic and nicotinic receptor sites was also found. CAT, AChE and QNB binding in the AM contralateral to the injection did not change. However, the binding of aBuTX was found to decrease by approximately 40%. The present results suggest that the loss of CAT activity in the AM after treatment with KA is due to the destruction of cholinergic neurons within the AM. The lack of effect on AChE suggests that the major cholinergic input to the AM is not affected by KA. In addition, the loss of nicotinic receptors in the contralateral AM may reflect anterograde degeneration of terminals that have nicotinic sites located on them, or may be secondary to the elicitation of intense seizure activity evoked by the KA.

Histological localization of binding sites of alpha-bungarotoxin and of antibodies specific to acetylcholine receptor in goldfish optic nerve and tectum

Goldfish optic nerve as well as ganglion cell neurites grown in culture selectively bind rhodamine-labeled alpha-bungarotoxin following tissue fixation. Binding is competed for by unlabeled bungarotoxin, by carbamylcholine and tubocurarine, but not by atropine. In cross-sections, the label is seen confined to axonal bundles. The binding is not detectable without prior fixation and is very faint in brain sections, even after fixation. To further establish the nature of the binding, immunocytochemical studies were performed, taking advantage of a high cross-reactivity found between goldfish brain and antibodies against eel acetylcholine receptor (AChR). Antigenic sites were detected by an indirect unlabeled antibody complexed to horseradish peroxidase. Anti-AChR antibody binding to optic nerve and neurites in culture correlated with that seen with alpha-bungarotoxin. Binding of anti-AChR was observed in the brain, and was reduced in the denervated tectum following unilateral optic nerve crush or enucleation. The results are discussed in relation to functions of receptor proteins in the retinotectal system.

Stability of muscarinic-cholinergic receptor activity in the deafferented retinotectal pathway

The high-affinity antagonist, 3-quinuclidinyl benzilidate (QNB), was used to analyze muscarinic-cholinergic receptor activity in the optic tectum of goldfish and optic lobe of developing chicks and adult pigeons after deafferentation. After deafferentation no significant loss of total or specific muscarinic receptor binding activity was observed in contrast to prior experiments where there was a marked and rapid loss of nictonic-cholinergic receptor binding activity, as measured by alphabungarotoxin binding. The relative stability of the muscarinic site as opposed to the instability of the nicotinic site in these experiments is discussed.

Activity of choline acetyltransferase and acetylcholinesterase in the goldfish optic tectum after disconnection

Activities of choline acetyltransferase (CAT) and acetylcholinesterase (AChE) were investigated in the goldfish optic tectum after disconnection of the optic afferents. Permanent disconnection was achieved by eye removal, and optic nerve crush produced a temporary disconnection until regeneration. There was a rapid loss in total activity per tectum for both enzymes under the two disconnection conditions. At longer intervals after optic nerve crush the levels of total activity for both enzymes returned toward control levels, as regeneration of the nerve proceeded. Total activity for both enzymes remained depressed after eye removal, however. Variable results were obtained in specific activity data, expressed per unit protein, although there was a 10% loss in specific activity of CAT at early intervals after eye removal. The data are interpreted as consistent with the possibility that at least a fraction of the axons in the retinotectal pathway of goldfish are cholinergic, and parallel our previous observations showing similar rapid losses of nicotinic-cholinergic receptor activity in this system.

Rapid loss of nicotine-cholinergic receptor binding activity in the deafferented avian optic lobe

The levels of alpha-bungarotoxin (alpha-BuTX) sensitive receptor sites were investigated in the optic lobe after optic deafferentation in the neonatal and adult chicken. Within two days a 30% loss of alpha-BuTX binding sites per optic lobe is observed in the neonatal chick after enucleation. The results are similar with the adult chicken in experiments where the receptor binding activity is measured in the optic lobe and in the optic tectum after enucleation. The possibility that acetylcholine is a neurotransmitter in the vertebrate retinotectal pathway is discussed.

Recovery of tectal nicotinic-cholinergic receptor sites during optic nerve degeneration in goldfish

The concentration of cholinergic nicotinic-like sites as measured by alphabungarotoxin (alphaBuTX) binding, decreased in the goldfish (Carassius auratus) optic tectum after optic nerve disconnection. Initially, the rate of loss of sites is greater than the rate of tissue or protein degradation in experiments where disconnection was achieved either by unilateral optic nerve crush or by enucleation of one eye. When the crushed optic nerve is allowed to regenerate and form behaviorally potent connections, the number and concentration of these sites appears restored. Pharmacological studies indicate that the alphaButTX binding site in the goldfish optic tectum has a drug binding profile similar to that seen at central or peripheral alphaBuTX sites in other species.

Some observations on the binding patterns of alpha-bungarotoxin in the central nervous system of the rat

Patterns of alpha-bungarotoxin (alphaBuTX) binding within the brain of the rat have been studied following one of the two procedures: (1) the intraventricular injection of 125I-labeled toxin followed by a survival period of 1--8 days before aldehyde perfusion, or (2) the incubation of fresh cryostat sections of brain tissue in dilute solutions of radioactive toxin. Appropriate controls with nicotine, curare, atropine and native alphaBuTX established that specific nature of the binding. The principal observations were that toxin binding sites are predominantly associated with central areas of the brain in direct receipt of sensory inputs (the main and accessory olfactory bulbs, superior colliculus, ventral lateral geniculate nucleus, cochlear nuclei, the substantia gelatinosa of the spinal cord and spinal trigeminal nucleus, the principal sensory nucleus of the trigeminal, and the dorsal column nuclei) and with limbic areas of the brain (hippocampus, amygdala, olfactory tubercle, medial mammillary nucleus, and the dorsal tegmental nucleus of Gudden). Toxin was not found to bind to cranial motor nuclei with the exception of the dorsal motor nucleus of the vagus and the nucleus ambiguus. The discrete distribution of clusters of silver grains within the granule layer of cerebellar folia I, IX, and X is described as well as the heavy labeling of the inferior and accessory olivary nuclei. In many areas of the brain silver grains were found to overlie cell bodies. It is suggested that this may reflect the presence of both membrane-bound toxin and internalized ligand following initial binding to a membrane receptor site. An attempt was made to correlate the localization of toxin binding sites with the terminal distributions of previously described cholinergic pathways. There appears to be a reasonably good agreement between the distribution of toxin receptors and proposed sites of cholinergic transmission within the hippocampus, interpeduncular nucleus and cerebellum. In most other cases however, lack of data precluded such correlations. The anatomical relationship of alphaBuTX binding activity and neurotransmitters other than acetylcholine is also considered.