Ultrastructural localization of acetylcholinesterase in substantia nigra: a comparison between rat and guinea pig

Evidence for a novel neuronal mechanism driving Alzheimer's disease, upstream of amyloid

This perspective offers an alternative to the amyloid hypothesis in the etiology of Alzheimer's disease (AD). We review evidence for a novel signaling mechanism based on a little-known peptide, T14. T14 could drive neurodegeneration as an aberrantly activated process of plasticity selective to interconnecting subcortical nuclei, the isodendritic core, where cell loss starts at the pre-symptomatic stages of the disease. Each of these cell groups has the capacity to form T14, which can stimulate production of p-Tau and β-amyloid, suggestive of an upstream driver of neurodegeneration. Moreover, results in an animal AD model show that antagonism of T14 with a cyclated variant, NBP14, prevents formation of β-amyloid, and restores cognitive function to that of wild-type counterparts. Any diagnostic and/or therapeutic strategy based on T14-NBP14 awaits validation in clinical trials. However, an understanding of this novel signaling system could bring much-needed fresh insights into the progression of cell loss underlying AD. HIGHLIGHTS: The possible primary mechanism of neurodegeneration upstream of amyloid. Primary involvement of selectively vulnerable subcortical nuclei, isodendritic core. Bioactive peptide T14 trophic in development but toxic in context of mature brain. Potential for early-stage biomarker to detect Alzheimer's disease. Effective therapeutic halting neurodegeneration, validated already in 5XFAD mice.

Characterization of a Bioactive Peptide T14 in the Human and Rodent Substantia Nigra: Implications for Neurodegenerative Disease

The substantia nigra is generally considered to show significant cell loss not only in Parkinson's but also in Alzheimer's disease, conditions that share several neuropathological traits. An interesting feature of this nucleus is that the pars compacta dopaminergic neurons contain acetylcholinesterase (AChE). Independent of its enzymatic role, this protein is released from pars reticulata dendrites, with effects that have been observed in vitro, ex vivo and in vivo. The part of the molecule responsible for these actions has been identified as a 14-mer peptide, T14, cleaved from the AChE C-terminus and acting at an allosteric site on alpha-7 nicotinic receptors, with consequences implicated in neurodegeneration. Here, we show that free T14 is co-localized with tyrosine hydroxylase in rodent pars compacta neurons. In brains with Alzheimer's pathology, the T14 immunoreactivity in these neurons increases in density as their number decreases with the progression of the disease. To explore the functional implications of raised T14 levels in the substantia nigra, the effect of exogenous peptide on electrically evoked neuronal activation was tested in rat brain slices using optical imaging with a voltage-sensitive dye (Di-4-ANEPPS). A significant reduction in the activation response was observed; this was blocked by the cyclized variant of T14, NBP14. In contrast, no such effect of the peptide was seen in the striatum, a region lacking the T14 target, alpha-7 receptors. These findings add to the accumulating evidence that T14 is a key signaling molecule in neurodegenerative disorders and that its antagonist NBP14 has therapeutic potential.

A novel process driving Alzheimer's disease validated in a mouse model: Therapeutic potential

Introduction

The neuronal mechanism driving Alzheimer's disease (AD) is incompletely understood.

Methods

Immunohistochemistry, pharmacology, biochemistry, and behavioral testing are employed in two pathological contexts—AD and a transgenic mouse model—to investigate T14, a 14mer peptide, as a key signaling molecule in the neuropathology.

Results

T14 increases in AD brains as the disease progresses and is conspicuous in 5XFAD mice, where its immunoreactivity corresponds to that seen in AD: neurons immunoreactive for T14 in proximity to T14‐immunoreactive plaques. NBP14 is a cyclized version of T14, which dose‐dependently displaces binding of its linear counterpart to alpha‐7 nicotinic receptors in AD brains. In 5XFAD mice, intranasal NBP14 for 14 weeks decreases brain amyloid and restores novel object recognition to that in wild‐types.

Discussion

These findings indicate that the T14 system, for which the signaling pathway is described here, contributes to the neuropathological process and that NBP14 warrants consideration for its therapeutic potential.

The multiple biological roles of the cholinesterases

It is tacitly assumed that the biological role of acetylcholinesterase is termination of synaptic transmission at cholinergic synapses. However, together with its structural homolog, butyrylcholinesterase, it is widely distributed both within and outside the nervous system, and, in many cases, the role of both enzymes remains obscure. The transient appearance of the cholinesterases in embryonic tissues is especially enigmatic. The two enzymes' extra-synaptic roles, which are known as 'non-classical' roles, are the topic of this review. Strong evidence has been presented that AChE and BChE play morphogenetic roles in a variety of eukaryotic systems, and they do so either by acting as adhesion proteins, or as trophic factors. As trophic factors, one mode of action is to directly regulate morphogenesis, such as neurite outgrowth, by poorly understood mechanisms. The other mode is by regulating levels of acetylcholine, which acts as the direct trophic factor. Alternate substrates have been sought for the cholinesterases. Quite recently, it was shown that levels of the aggression hormone, ghrelin, which also controls appetite, are regulated by butyrylcholinesterase. The rapid hydrolysis of acetylcholine by acetylcholinesterase generates high local proton concentrations. The possible biophysical and biological consequences of this effect are discussed. The biological significance of the acetylcholinesterases secreted by parasitic nematodes is reviewed, and, finally, the involvement of acetylcholinesterase in apoptosis is considered.

In vivo imaging of human cholinergic nerve terminals with (-)-5-(18)F-fluoroethoxybenzovesamicol: biodistribution, dosimetry, and tracer kinetic analyses

(-)-5-(18)F-fluoroethoxybenzovesamicol ((18)F-FEOBV) is a vesamicol derivative that binds selectively to the vesicular acetylcholine transporter (VAChT) and has been used in preclinical studies to quantify presynaptic cholinergic nerve terminals. This study presents, to our knowledge, the first-in-human experience with (18)F-FEOBV, including radiation dosimetry, biodistribution, tolerability and safety in human subjects, and brain kinetics and methods for quantitative analysis of (18)F-FEOBV.

METHODS

Whole-body (18)F-FEOBV scans were obtained in 3 healthy human volunteers. Seven additional subjects underwent dynamic brain imaging 0-120, 150-180, and 210-240 min after bolus injection of (18)F-FEOBV. Arterial blood sampling was performed with chromatographic identification of authentic (18)F-FEOBV to determine the arterial plasma input function. Analysis methods included nonlinear least-squares fitting of a 2-tissue-compartmental model, reference tissue modeling, and late single-scan imaging.

RESULTS

No pharmacologic or physiologic changes were observed after intravenous administration of up to 1.3 μg of (18)F-FEOBV. Radiation dosimetry estimates indicate that more than 400 MBq may be administered without exceeding regulatory radiation dose limits. Kinetic analysis showed brain uptake to be relatively high with single-pass extraction of 25%-35%. VAChT binding estimates varied by a factor of greater than 30 between the striatum and cortex. Coefficients of variation in k3 estimates varied from 15% to 30%. Volume of distribution measures yielded a dynamic range of approximately 15 but with little reduction in variability. Reference tissue approaches yielded more stable estimates of the distribution volume ratio (1 + BPND), with coefficients of variation ranging from 20% in the striatum to 6%-12% in cortical regions. The late static distribution of (18)F-FEOBV correlated highly with the distribution volume ratio estimates from reference tissue models (r = 0.993).

CONCLUSION

(18)F-FEOBV PET confirms that the tracer binds to VAChT with the expected in vivo human brain distribution. Both reference tissue modeling and late static scanning approaches provide a robust index of VAChT binding.

Co-localization of PRiMA with acetylcholinesterase in cholinergic neurons of rat brain: an immunocytochemical study

In the central nervous system, acetylcholinesterase (AChE) is present in a tetrameric form that is anchored to membranes via a proline-rich membrane anchor (PRiMA). Previously it has been found that principal cholinergic neurons in the brain express high concentrations of AChE enzymic activity at their neuronal membranes. The aim of this study was to use immunocytochemical methods to determine the distribution of PRiMA in these neurons in the rat brain. Confocal laser and electron microscopic investigations showed that PRiMA immunoreactivity is associated with the membranes of the somata, dendrites and axons of cholinergic neurons in the basal forebrain, striatum and pedunculopontine nuclei, i.e. the neurons that innervate forebrain and brainstem structures. In these neurones, PRiMA also co-localizes with AChE immunoreactivity at the plasma membrane. PRiMA label was absent from neighboring GABAergic neurons, and from other neurons of the brain known to express high levels of AChE enzymic activity including cranial nerve motor neurons and dopaminergic neurons of the substantia nigra zona compacta. A strong association of AChE with PRiMA at the plasma membrane is therefore a feature specific to principal cholinergic neurons that innervate the central nervous system.

Mefloquine Enhances Nigral γ-Aminobutyric Acid Release via Inhibition of Cholinesterase

Mefloquine, a widely used antimalarial drug, has many neuropsychiatric effects. Although the mechanisms underlying these side effects remain unclear, recent studies show that mefloquine enhances spontaneous transmitter release and inhibits cholinesterases. In this study, we examined the effect of mefloquine on GABA receptor-mediated, spontaneous inhibitory postsynaptic currents (sIPSCs) of dopaminergic neurons, mechanically dissociated from the substantia nigra pars compacta of rats aged 6 to 17 postnatal days. Mefloquine (0.1-10 microM) robustly and reversibly increased the frequency of sIPSCs with an EC50 of 1.3 microM. Mefloquine also enhanced the frequency of miniature inhibitory postsynaptic currents in the presence of tetrodotoxin but without changing their mean amplitude. This suggests that mefloquine acts presynaptically to increase GABA release. Mefloquine-induced enhancement of sIPSCs was significantly attenuated in medium containing low Ca2+ (0.5 mM) or following pretreatment with 1,2-bis (2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetrakis(acetoxymethyl ester (30 microM), a membrane-permeable Ca2+ chelator. In contrast, 100 microM Cd2+ did not alter the action of mefloquine. This suggests that mefloquine-induced facilitation of GABA release depends on extracellular and intraterminal Ca2+ but not on voltage-gated Ca2+ channels. Mefloquine-induced enhancement of sIPSCs was significantly attenuated in the presence of the anticholinesterase agent physostigmine or blockers of non-alpha7 nicotinic acetylcholine receptors. Taken together, these data suggest that mefloquine enhances GABA release through its inhibition of cholinesterase. This allows accumulation of endogenously released acetylcholine, which activates neuronal nicotinic receptors on GABAergic nerve terminals. The resultant increase of Ca2+ entry into these terminals enhances vesicular release of GABA. This action may contribute to the neurobehavioral effects of mefloquine.

Correlation between dopaminergic neurons, acetylcholinesterase and nicotinic acetylcholine receptors containing the α3- or α5-subunit in the rat substantia nigra

The aim of this study was to investigate the relationship between the cells possessing the alpha3 or alpha5 nicotinic acetylcholine receptor subunits and the enzyme acetylcholinesterase, with respect to tyrosine hydroxylase immunoreactive dopaminergic neurons in the rat substantia nigra. Most, but certainly not all, acetylcholinesterase immunoreactive cells were located in the pars compacta. In the substantia nigra pars compacta there were in turn two populations of acetylcholinesterase containing neurons: those that were tyrosine hydroxylase reactive and those that were not. Double label studies, that included an antibody immunoreactive against a common immunogen on alpha1 of muscle and alpha3 and alpha5 neuronal nicotinic acetylcholine receptor subunits, revealed that nearly all nicotinic receptor positive cells were also tyrosine hydroxylase neurons. However, a minority non-tyrosine hydroxylase population was alpha3- and/or alpha5-nAChR positive and these were always AChE-immunoreactive. In summary, there appears to be a close correlation between nicotinic receptors and acetylcholinesterase in the substantia nigra, irrespective of the transmitter phenotype in different neuronal subpopulations.

Tyrosine hydroxylase and acetylcholinesterase in the domestic pig mesencephalon: An immunocytochemical and histochemical study

The mesencephalon of the young domestic pig was studied by tyrosine hydroxylase (TH) immunocytochemistry and acetylcholinesterase (AChE) histochemistry with focus on the substantia nigra (SN), the ventral tegmental area (VTA), and related areas. The purpose was to obtain information on the organization of the mesencephalic, TH immunoreactive (TH-i), and dopaminergic areas of the pig, in order to provide the necessary background for the possible use of the pig as an alternative large animal experimental model for research on Parkinson's disease, including the use of encapsulated pig dopaminergic neurons for intracerebral xenotransplantation. Significant findings in the pig, compared to observations in other species, included the presence of prominent bundles of TH-i dendrites passing in a dorsoventral direction from pars compacta into pars reticulata at middle and caudal levels of the SN, and the presence of a distinct TH-i substantia nigra pars lateralis (SNL). Caudally in the pig mesencephalon, the retrorubral field (RRF) was found to be very extensive. The view of the RRF, SN, and VTA as parts of the same integrated system was indicated by the crisscrossing of TH-i dendrites at the transitions between these areas. Estimation of the number of TH-i neurons in the SN and the VTA showed that these nuclei were of equal size in the pig. Further, it was found that TH-i nerve cells were present in the midline between the VTA in the interfascicular and rostral linear groups. TH-i nerve cells were also present in the otherwise serotoninergic dorsal raphe nuclei, just as other TH-i cells formed a perirubral cell group. AChE-positive neurons were present in both SN and VTA, and appeared to have the same size and morphology as the TH-i neurons in these areas. Within both nuclei, there were local differences in the AChE staining density, but perhaps more significantly were some marked differences in the structure of the AChE-positive neuropil of the two areas. We anticipate that the present description of the cellular organization of the TH-i dopaminergic areas in the domestic pig ventral mesencephalon will be useful for the development of a nonprimate, large animal, experimental model of Parkinson's disease.

Expression of PRiMA in the mouse brain: membrane anchoring and accumulation of 'tailed' acetylcholinesterase

We analysed the expression of PRiMA (proline-rich membrane anchor), the membrane anchor of acetylcholinesterase (AChE), by in situ hybridization in the mouse brain. We compared the pattern of PRiMA transcripts with that of AChE transcripts, as well as those of choline acetyltransferase and M1 muscarinic receptors which are considered pre- and postsynaptic cholinergic markers. We also analysed cholinesterase activity and its molecular forms in several brain structures. The results suggest that PRiMA expression is predominantly or exclusively related to the cholinergic system and that anchoring of cholinesterases to cell membranes by PRiMA represents a limiting factor for production of the AChE tailed splice variant (AChET)-PRiMA complex, which represents the major AChE component in the brain. This enzyme species is mostly associated with cholinergic neurons because the pattern of PRiMA mRNA expression largely coincides with that of ChAT. We also show that, in both mouse and human, PRiMA proteins exist as two alternative splice variants which differ in their cytoplasmic regions.

Diffuse transmission by acetylcholine in the CNS

Recent immunoelectron microscopic studies have revealed a low frequency of synaptic membrane differentiations on ACh (ChAT-immunostained) axon terminals (boutons or varicosities) in adult rat cerebral cortex, hippocampus and neostriatum, suggesting that, besides synaptic transmission, diffuse transmission by ACh prevails in many regions of the CNS. Cytological analysis of the immediate micro-environment of these ACh terminals, as well as currently available immunocytochemical data on the cellular and subcellular distribution of ACh receptors, is congruent with this view. At least in brain regions densely innervated by ACh neurons, a further aspect of the diffuse transmission paradigm is envisaged: the existence of an ambient level of ACh in the extracellular space, to which all tissue elements would be permanently exposed. Recent experimental data on the various molecular forms of AChE and their presumptive role at the neuromuscular junction support this hypothesis. As in the peripheral nervous system, degradation of ACh by the prevalent G4 form of AChE in the CNS would primarily serve to keep the extrasynaptic, ambient level of ACh within physiological limits, rather than totally eliminate ACh from synaptic clefts. Long-lasting and widespread electrophysiological effects imputable to ACh in the CNS might be explained in this manner. The notions of diffuse transmission and of an ambient level of ACh in the CNS could also be of clinical relevance, in accounting for the production and nature of certain cholinergic deficits and the efficacy of substitution therapies.

The spontaneous release of acetylcholinesterase in rat substantia nigra is altered by local changes in extracellular levels of dopamine

Acetylcholinesterase release in the guinea-pig substantia nigra has been previously investigated 'on-line', using a sensitive chemiluminescent system. Since histological observations suggest that there is a difference in acetylcholinesterase distribution in the rat substantia nigra compared to that of the guinea-pig, the first aim of the present study was to use this chemiluminescent method to characterise acetylcholinesterase release in this brain region of the freely moving rat, and the second was explore the relationship between acetylcholinesterase release and dopamine systems in this region. Accordingly, acetylcholinesterase release in the rat substantia nigra was studied under basal conditions of spontaneous release and following the local administration of (a) elevated potassium ions (30, 45, 60'mM), (b) a stimulator of dopamine/acetylcholinesterase release-D-amphetamine (10(-7), 10(-6) and 10(-5) M), (c) an inhibitor of dopamine uptake-GBR12909 (10(-7), 10(-6) and 10(-5) M). Spontaneous release of acetylcholinesterase in this brain region of the rat appears to be comparable with that observed in the guinea-pig, despite the smaller number of acetylcholinesterase-containing neurones. Furthermore, not only elevated potassium ions, but D-amphetamine as well as GBR12909, all produced significant increases in the percentage spontaneous release of acetylcholinesterase. Thus, the release of acetylcholinesterase in this region may be triggered by levels of dopamine outside of the neurone.

Differential release of acetylcholinesterase in vivo, from the guinea pig substantia nigra compared to the caudate putamen following dopamine depletion

In the substantia nigra acetylcholinesterase may have a novel role unrelated to acetylcholine but linked instead to dopamine. Using a sensitive chemiluminescent system, we have investigated the effects of dopamine depletion on the vivo release of acetylcholinesterase in both the substantia nigra and the caudate putamen. Dopamine levels in the caudate putamen were significantly depleted compared to the non-lesioned side, using either of two different toxins for dopaminergic nigrostriatal cells: 6-hydroxydopamine ( 1 or 3 weeks prior to study) or N-methyl-4-phenyl-1,2,5,6-tetrahydropyridine (1 week prior to study). Spontaneous release of acetylcholinesterase from the substantia nigra was significantly reduced following all three pretreatments; however, in the caudate putamen a significant reduction in the spontaneous release of acetylcholinesterase, compared to controls, was only seen in animals studied 1 week after the administration of 6-hydroxydopamine. In all control groups, application of potassium ions (60 mM) evoked a significant release of acetylcholinesterase in the substantia nigra (p < 0.05) and this effect persisted in the surviving neurones following a partial lesion by neurotoxin pre-treatment. The results from this study are discussed in the light of a regulatory mechanism for acetylcholinesterase release from the striatum, which may come into operation depending on the extent of destruction of dopaminergic nigrostriatal neurones.

Non-classical actions of cholinesterases: role in cellular differentiation, tumorigenesis and Alzheimer's disease

The cholinesterases are members of the serine hydrolase family, which utilize a serine residue at the active site. Acetylcholinesterase (AChE) is distinguished from butyrylcholinesterase (BChE) by its greater specificity for hydrolysing acetylcholine. The function of AChE at cholinergic synapses is to terminate cholinergic neurotransmission. However, AChE is expressed in tissues that are not directly innervated by cholinergic nerves. AChE and BChE are found in several types of haematopoietic cells. Transient expression of AChE in the brain during embryogenesis suggests that AChE may function in the regulation of neurite outgrowth. Overexpression of cholinesterases has also been correlated with tumorigenesis and abnormal megakaryocytopoiesis. Acetylcholine has been shown to influence cell proliferation and neurite outgrowth through nicotinic and muscarinic receptor-mediated mechanisms and thus, that the expression of AChE and BChE at non-synaptic sites may be associated with a cholinergic function. However, structural homologies between cholinesterases and adhesion proteins indicate that cholinesterases could also function as cell-cell or cell-substrate adhesion molecules. Abnormal expression of AChE and BChE has been detected around the amyloid plaques and neurofibrillary tangles in the brains of patients with Alzheimer's disease. The function of the cholinesterases in these regions of the Alzheimer brain is unknown, but this function is probably unrelated to cholinergic neurotransmission. The presence of abnormal cholinesterase expression in the Alzheimer brain has implications for the pathogenesis of Alzheimer's disease and for therapeutic strategies using cholinesterase inhibitors.

Uptake of acetylcholinesterase by neurons in the substantia nigra

It is known that acetylcholinesterase is secreted by the dopaminergic neurons of the substantia nigra and has a subsequent action independent of cholinergic transmission. Although non-cholinergic actions of this protein have been demonstrated, the subsequent fate of acetylcholinesterase is unknown. One possibility is that acetylcholinesterase is taken up following secretion into the extracellular space. This hypothesis has been tested in vivo, in both conscious and anaesthetized guinea-pigs. Exogenous acetylcholinesterase (2-20 pM) was infused via a push-pull cannula implanted into either the substantia nigra or the surrounding extranigral regions: the amount subsequently recovered in the perfusate was then compared with control values. Only when the push-pull cannulae were implanted in the substantia nigra was there a marked decrease in the amount of acetylcholinesterase recovered; this selective retention was abolished when the perfusion medium was cooled to 4 degrees C or when the experiment was performed post mortem. Direct visualization of immunocytochemically identified nigral dopaminergic cells revealed co-localized deposits of labelled, exogenous acetylcholinesterase. Moreover, when exogenous acetylcholinesterase was boiled to prevent detection by the assay system and to eliminate any classical enzymatic action, an enhancement in perfusate levels of endogenous acetylcholinesterase was observed from nigral but not from extranigral sites, indicating that endogenous and exogenous acetylcholinesterases were in competition. These results suggest that, within the substantia nigra, secreted acetylcholinesterase may be subject to a temperature- and energy-dependent uptake mechanism.

The release of acetylcholinesterase in vivo is regulated by dopaminergic systems in the guinea-pig substantia nigra

Acetylcholinesterase (AChE) and dopamine are both stored and released from dendrites within the substantia nigra: however, it is as yet unknown whether the regulation of these two purported neuromodulators is in any way related. Using a sensitive chemiluminescent system to monitor AChE release 'on-line', the effects of inhibiting synthesis and storage of dopamine with alpha-methyl-p-tyrosine (AMPT: 250 mg/kg, i.p.) and reserpine (6 mg/kg, i.p.), respectively, have been studied. Both these agents significantly reduced nigral tissue dopamine levels by decreases of 83% and 63%, respectively; however, only AMPT had a significant effect in vivo on the spontaneous release of AChE compared to conscious control animals (66% decrease). Co-application of both AMPT and reserpine resulted in a significant decrease in the tissue dopamine content (95%) and in spontaneous release of AChE compared to conscious control guinea-pigs (72%); however, these effects were not significantly different from when AMPT was employed alone. Application of potassium ions (60 mM) or veratridine (100 microM) both evoked release of AChE in control animals: however, when expressed as a percentage of basal levels, this increase in release was not influenced by drug treatment or state of consciousness. These results suggest that de novo dopamine synthesis may at least in part, have an influential effect on release (and possibly storage) of AChE in the substantia nigra.

Radioimmunocytochemical distribution of neutral endopeptidase (enkephalinase E.C.3.4.24.11) at the ultrastructural level in the rat median eminence

Neutral endopeptidase (E.C.3.4.24.11) was visualized at the ultrastructural level in the external zone of the rat median eminence by using 125I-labelled IgG of a monoclonal serum. A precise analysis of the localization of the immunolabelling, which appears in the form of individual stray silver grains, was undertaken. Among the 1,045 grains counted, 82% were localized over membrane appositions involving nerve endings only and nerve endings plus tanycytes. The difference between the real and a randomly generated population of grains was statistically significant. Our results provide morphological arguments in support of the view of a paracrine action of neuropeptides present in the median eminence especially enkephalins but possibly, substance P, angiotensin, cholecystokinin and neurotensin. These neuropeptides are known to be inactivated by neutral endopeptidase. The action of these peptides may be exerted on nerve endings (autocrine or paracrine) but an intervention on tanycytes cannot be excluded.

Cloning and expression of a rat acetylcholinesterase subunit: generation of multiple molecular forms and complementarity with a Torpedo collagenic subunit

We obtained a cDNA clone encoding one type of catalytic subunit of acetylcholinesterase (AChE) from rat brain (T subunit). The coding sequence shows a high frequency of (G+C) at the third position of the codons (66%), as already noted for several AChEs, in contrast with mammalian butyrylcholinesterase. The predicted primary sequence of rat AChE presents only 11 amino acid differences, including one in the signal peptide, from that of the mouse T subunit. In particular, four alanines in the mouse sequence are replaced by serine or threonine. In northern blots, a rat AChE probe indicates the presence of major 3.2- and 2.4-kb mRNAs, expressed in the CNS as well as in some peripheral tissues, including muscle and spleen. In vivo, we found that the proportions of G1, G2, and G4 forms are highly variable in different brain areas. We did not observe any glycolipid-anchored G2 form, which would be derived from an H subunit. We expressed the cloned rat AChE in COS cells: The transfected cells produce principally an amphiphilic G1a form, together with amphiphilic G2a and G4a forms, and a nonamphiphilic G4na form. The amphiphilic G1a and G2a forms correspond to type II forms, which are predominant in muscle and brain of higher vertebrates. The cells also release G4na, G2a, and G1a in the culture medium. These experiments show that all the forms observed in the CNS in vivo may be obtained from the T subunit. By co-transfecting COS cells with the rat T subunit and the Torpedo collagenic subunit, we obtained chimeric collagen-tailed forms. This cross-species complementarity demonstrates that the interaction domains of the catalytic and structural subunits are highly conserved during evolution.

Membrane properties of identified mesencephalic dopamine neurons in primary dissociated cell culture

Dopamine (DA)-containing neurons in primary dissociated cell cultures derived from the embryonic mouse mesencephalon (day E13) were studied by histochemical and electrophysiological techniques. DA neurons exhibited two distinct morphologies, fusiform and multipolar, tended to reside in groups and organize dendrites into common fascicles. While these neurons expressed the cell-surface marker acetylcholinesterase, the presence of this enzyme could not be used to identify DA neurons unequivocally, since it was also observed in nondopaminergic cells. Neurons were therefore identified as DA by their distinct morphology, and this identification was validated with a double-labeling procedure that entailed the intracellular deposition of a fluorescent dye (Lucifer yellow or ethidium bromide), followed by processing for tyrosine hydroxylase immunocytochemistry. DA neurons identified in this manner were observed to have resting membrane potentials between -50 and -75 mV, input resistances of 50-360 M omega, and membrane time constants of 4.1-14.1 msec. Forty-seven percent of these cells displayed spontaneous activity that was irregular in nature and often contained bursts (burst length was between two and six action potentials). The DA neurons displayed a variety of ionic conductances, including (1) a Na+ conductance (gNa) that underlies the action potential, (2) Ca2+ conductances (gCa) that mediate the nonsomatic low- and high-threshold spikes observed, and (3) at least three K+ conductances (gK). Voltage-clamp analysis revealed several distinct transmembrane ionic currents, including (1) a large, rapidly inactivating tetrodotoxin-sensitive inward Na+ current (INa), (2) a 4-aminopyridine-sensitive, transient early outward K+ current that required a conditioning hyperpolarization of the membrane to be activated by a subsequent depolarization (A-current, IA), (3) a slowly developing inward current that was seen only after a conditioning hyperpolarization of the membrane and that was dependent on the presence of external Ca2+ ions (ICa), and (4) a late-onset, noninactivating K+ current. Between 25% and 54% of the late-onset K+ current was Ca(2+)-dependent and was not affected by tetraethylammonium ions. This current was termed IAHP. The remaining current was not sensitive to changes in the extracellular Ca2+ concentration but was blocked by external tetraethylammonium. This current was termed IK. The direct pressure application of DA (1-200 microM) onto the soma dose-dependently hyperpolarized these neurons; this effect was potentiated by the presence of the catecholamine reuptake blocker cocaine hydrochloride (10-200 microM). Under voltage-clamp conditions, DA was observed to increase IK significantly and had little effect on IAHP.(ABSTRACT TRUNCATED AT 400 WORDS)

Widespread release of peptides in the central nervous system: quantitation of tannic acid-captured exocytoses

Tannic acid methods have been applied to capture the exocytosis of peptide-containing granules from peptidergic neurons. The captured exocytoses have been quantitated to assess the proportion and amount of peptide released at different parts of the neuronal membrane. Examination of hypothalamic synaptic boutons shows that only about one-half of the peptidergic vesicles is exocytosed into the synaptic cleft and also that exocytosis also occurs from undilated peptidergic axons. Study of the magnocellular neurosecretory system reveals that all parts of their extensive terminal arborization appear to be equally capable to exocytose peptide. Only about one-half of their peptide is released from their nerve endings, which about the capillaries. The remainder is released much deeper in the lobules of secretory tissue where its principal target(s) could be the pituicytes and/or neurosecretory axons. Dendrites of magnocellular neurons are also capable of releasing peptide by exocytosis and dendrites could release sufficient oxytocin and vasopressin to account for the peptide known to be released into the hypothalamus. We conclude that peptidergic neurons release substantial amounts of peptides from all of their processes and that this must be taken into account when considering what functions those peptides might serve.

A noncholinergic action of acetylcholinesterase (AChE) in the brain: from neuronal secretion to the generation of movement

1. In various brain regions, there is a puzzling disparity between large amounts of acetylcholinesterase and low levels of acetylcholine. One such area is the substantia nigra. Furthermore, within the substantia nigra, a soluble form of acetylcholinesterase is released from the dendrites of dopamine-containing nigrostriatal neurons, independent of cholinergic transmission. These two issues have prompted the hypothesis that acetylcholinesterase released in the substantia nigra has an unexpected noncholinergic function. 2. Electrophysiological studies demonstrate that this dendritic release is a function, not of the excitability of the cell from which the acetylcholinesterase is released, but of the inputs to it. In order to explore this phenomenon at the behavioral level, a novel system has been developed for detecting release of acetylcholinesterase "on-line." It can be seen that release of this protein within the substantia nigra can reflect, but is not causal to, movement. 3. Once released, the possible actions of acetylcholinesterase can be studied at both the cellular and the behavioral level. Independent of its catalytic site, acetylcholinesterase has a "modulatory" action on nigrostriatal neurons. The functional consequences of this modulation would be to enhance the sensitivity of the cells to synaptic inputs. 4. Many basic questions remain regarding the release and action of acetylcholinesterase within the substantia nigra and, indeed, within other areas of the brain. Nonetheless, tentative conclusions can be formulated that begin, in a new way, to provide a link between cellular mechanisms and the control of movement.

A comparison of the subnuclear and ultrastructural distribution of acetylcholinesterase and choline acetyltransferase in the rat interpeduncular nucleus

The subnuclear and synaptic staining patterns for acetylcholinesterase (AChE) activity and choline acetyltransferase (ChAT) activity were studied in the rat interpeduncular nucleus (IPN) using histochemical and immunohistochemical methods. AChE reactivity was prominent in the neuropil of the rostral, lateral and dorsomedial subnuclei, whereas ChAT immunoreactivity was confined to axons and terminals in the rostral, intermediate and central subnuclei. AChE-positive somata were evident in all the subnuclear divisions of the IPN, and possessed reaction product in the rough endoplasmic reticulum and nuclear envelope. ChAT-positive somata were not present in the IPN. Characteristic axodendritic synapses in the rostral, intermediate and central subnuclei possessed ChAT immunoreactivity presynaptically, and AChE reactivity both pre- and postsynaptically. Other synaptic arrangements in the lateral subnucleus lacked ChAT-immunoreactive terminals, yet possessed prominent AChE reactivity. The results of the present study reveal that AChE reactivity and ChAT immunoreactivity are heterogeneously distributed among the subnuclear divisions of the rat IPN, and that AChE reactivity is present in both the cholinoceptive and noncholinoceptive subnuclei. Although neuronal colocalization of ChAT and AChE activity is not evident in the IPN, AChE-positive neurons are in receipt of putative cholinergic, as well as peptidergic, afferent inputs.

Pulsatile release of acetylcholinesterase from the substantia nigra following electrical stimulation of the striatum

The spatio-temporal characteristics of release of acetylcholinesterase from the in vivo substantia nigra can now be described using a chemiluminescent reaction. Electrical stimulation of the striatum caused an increase in release of acetylcholinesterase within the ipsilateral substantia nigra; however, this increase in release of protein was non-uniform and pulsatile. The findings suggest that release of acetylcholinesterase within the substantia nigra has a fine space-time resolution never before appreciated.

Organotypic slice cultures of dopaminergic neurons of substantia nigra

Morphological methods were used to study the plasticity of target-deprived mammalian dopaminergic (DA) neurons. Slices of substantia nigra (SN) were taken from the midbrain of rats aged one to twelve days, and cultured for one to two weeks. Localization of tyrosine hydroxylase (TH) was used to examine the distribution and shapes of DA neurons. Histochemical staining for acetylcholinesterase (AChE) was carried out to estimate both survival and biosynthesis of SN neurons. We found that some DA neurons can survive in vitro without their usual target neurons. This was demonstrated by injecting rhodamine-conjugated microspheres (RD) into the caudate putamen, a SN target area, at 6 to 8 days prior to culturing. RD-labeled cells survived in SN cultures and some of them were doubly labeled with AChE. TH neurons had different shapes and their axon terminals formed close contacts with adjacent nondopaminergic neurons. These findings suggest that a subset of DA neurons may switch targets, but the majority of them require target interactions with the caudate putamen for survival in vitro.

Acetylcholinesterase on the dendrites of central cholinergic neurons: an electron microscopical study in the ferret

A combination of choline acetyltransferase immunohistochemistry and acetylcholinesterase histochemistry was used to characterize the ultrastructural distribution of acetylcholinesterase in identified cholinergic and non-cholinergic neurons in the ferret brain. Previous studies have shown that most of the cholinergic input to the brain arises from choline acetyltransferase-positive neurons found in the neostriatum, basal forebrain and dorsolateral pontine tegmentum. In all these cells intense staining for acetylcholinesterase was localized within the cisternae of the rough endoplasmic reticulum, in the nuclear envelope and Golgi apparatus, and along the plasma membranes of the soma and dendrites. In contrast, the distribution of acetylcholinesterase in non-cholinergic neurons was restricted mainly to the cisternae of the endoplasmic reticulum and the nuclear envelope. Since previous studies have associated high levels of acetylcholinesterase staining with non-cholinergic neurons in the locus coeruleus and substantia nigra zona compacta, these areas were examined as well. The ultrastructural localization of acetylcholinesterase in the principal locus coeruleus neurons was as observed in typical non-cholinergic neurons. On the other hand, the distribution of acetylcholinesterase in the principal cells of the zona compacta of the substantia nigra was more like that found in cholinergic neurons. In conclusion, the subcellular distribution of acetylcholinesterase in the principal cholinergic neurons of the brain follows a characteristic pattern which, with one exception, is different from that of acetylcholinesterase-positive non-cholinergic neurons.

A possible pacemaker mechanism in pars compacta neurons of the guinea-pig substantia nigra revealed by various ion channel blocking agents

The membrane properties of pars compacta neurons in the in vitro guinea-pig substantia nigra have been studied in the presence of sodium, calcium and potassium channel blockers. The following properties, which have already been described for dopamine-containing substantia nigra zona compacta neurons were observed: high and low threshold calcium spikes; a calcium-activated potassium-mediated transient; inward rectification. Inward rectification was sensitive to caesium ions. An additional property was seen reminiscent of an "A" current, although resistant to 4-aminopyridine. It is suggested that this outward transient is in fact a calcium activated potassium conductance. Under certain conditions calcium-mediated rhythmic depolarizations were observed. It is suggested that at least two of the properties seen (outward rectification and low threshold calcium spike) could interact to provide the basis for a pacemaker mechanism in pars compacta neurons.

Electrophysiological evidence for the dendritic localization of a calcium conductance in guinea-pig substantia nigra neurones in vitro

Within the substantia nigra, anatomical, neurochemical and pharmacological findings strongly suggest that transmitter and protein are secreted from the dendrites of nigrostriatal neurones. This phenomenon may underlie a non classical modulatory cellular mechanism. Two conductances are generated in nigrostriatal neurones independent of somatic action potentials, that might mediate this modulation. However, these conductances have never been directly nor precisely located specifically within the dendrites. The aim of this study was to record the membrane properties of substantia nigra zona compacta neurones in response to selective sectioning of the population of long 'apical' dendrites i.e. the removal of the zona reticulata. Intracellular recordings from substantia nigra zona compacta neurones were made from mesencephalic slices of the guinea-pig brain maintained in vitro. In cells without the apical dendrites, the membrane potential, input resistance and mean firing frequency was not significantly different from the control neurones. However, removal of the substantia nigra zona reticulata virtually abolished one conductance in particular. This conductance, seen in control neurones, is a long lasting slow depolarization which is resistant to tetrodotoxin blockade of sodium channels: rather, it is mediated by the entry of calcium ions and is optimally deinactivated at a hyperpolarised membrane potential. Hence, this study strongly suggests that this conductance is generated exclusively in the 'apical' dendrites. It has been postulated that this long lasting calcium conductance is central to the modulation of nigrostriatal neuronal excitability. Thus, the 'apical' dendrites could play a specific and active role in the functioning of nigrostriatal neurones.

Release of acetylcholinesterase within the guinea-pig substantia nigra: effects of 5-hydroxytryptamine and amphetamine

A soluble form of acetylcholinesterase is released from the substantia nigra of the cat, rabbit and rat. The possibility of spontaneous and evoked release of acetylcholinesterase within the substantia nigra of a different species, the guinea-pig, was tested using push-pull cannulae perfusion in vivo. Microinfusion of either potassium or amphetamine into the substantia nigra caused an increase in the local release of acetylcholinesterase. Furthermore, similar application of the neurotransmitter 5-hydroxytryptamine also caused an increase in the local release of the enzyme. However, in the presence of the 5-hydroxytryptamine receptor antagonist, cinanserin, or calcium-free perfusing medium, 5-hydroxytryptamine no longer had any significant effects. These results demonstrate that release of acetylcholinesterase within the substantia nigra appears to be, qualitatively but not quantitatively, a consistent phenomenon in different species. Furthermore, within the substantia nigra, release of acetylcholinesterase may be a function of interactions between local transmitters. The possible neurochemical and ionic mechanisms underlying these interactions is discussed.

Acetylcholinesterase has a non-cholinergic neuromodulatory action in the guinea-pig substantia nigra

Acetylcholinesterase is released within the substantia nigra from the soma/dendrites of nigrostriatal neurons. Previous work suggests that this phenomenon is independent of cholinergic systems, but rather serves to modulate the sensitivity of dopamine-containing nigrostriatal cells to synaptic events. This hypothesis was tested directly in the anaesthetized guinea-pig. Micro-infusion of acetylcholinesterase into the substantia nigra led to an increase in spontaneous firing of nigrostriatal neurons. Furthermore, the pattern of firing evoked by stimulation of the striatum was markedly enhanced. By contrast, administration of butyrylcholinesterase had no effect. It thus appears that acetylcholinesterase has a modulatory action on the firing of nigrostriatal cells, independent of hydrolysis of acetylcholine.

On-line visualization of dendritic release of acetylcholinesterase from mammalian substantia nigra neurons

This study presents, to our knowledge, the first on-line measurement of acetylcholinesterase (AcChoEase) release from brain tissue. It is now well established that a soluble form of the enzyme is released from central nervous system neurons, and it has been proposed on indirect grounds that such release may occur not only from presynaptic terminals but also from the dendrites of dopamine-containing nigrostriatal neurons. We have used a chemiluminescent reaction to examine the real-time release of AcChoEase from the substantia nigra in vitro in brainstem slices. The light emission was captured by two fiber optic systems, one in direct contact with the brain slice from below and the second 4-mm above the slice, allowing simultaneous imaging of the emitted light and quantitative photometry. It was determined that the light signals are not due to the spontaneous hydrolysis of acetylcholine or the presence of free choline, but are caused by the enzymatic action of AcChoEase. Using this technique, it can be directly shown that AcChoEase is spontaneously released from the soma or dendrites of nigral neurons. The release of the enzyme, which is stored in the subcisternal dendritic compartment, is resistant to blockade of voltage-dependent sodium conductances, is calcium dependent, and can be increased by addition of potassium to the bathing solution. The procedure we describe here will make it possible to study the release of endogenous AcChoEase on a time-scale close to that over which it functions.

Does the substantia nigra have a cholinergic innervation?

We have studied the distribution of choline acetyltransferase-containing fibres in ferret substantia nigra using immunohistochemistry at the light microscopical level. Positive staining for choline acetyltransferase was not observed in any nigral cell bodies, but was found in a network of fine-calibre, varicose axons, more densely distributed in the zona compacta than the zona reticulata.

6-Hydroxydopamine-induced turning behaviour in the rat: the significance of acetylcholinesterase in cerebrospinal fluid

In the substantia nigra, acetylcholinesterase may have a novel function related not to cholinergic transmission, but to the homeostasis of dopaminergic nigrostriatal neurones. The initial aim of this study was thus to see whether, in the rat, release of the enzyme into cerebrospinal fluid would reflect turning behaviour following unilateral 6-hydroxydopamine lesions of varying severity. It was found that acetylcholinesterase levels, lower than those in the cerebrospinal fluid of control rats, were accompanied by marginal circling behaviour and a small loss of striatal dopamine: on the other hand, elevated acetylcholinesterase activity was observed in the cerebrospinal fluid of rats displaying vigorous turning behaviour and with large depletion of striatal dopamine. It has already been demonstrated that exogenous acetylcholinesterase, applied locally to nigral neurones, has both electrophysiological and behavioural effects reminiscent of dopamine agonists. Hence it is possible that exogenous acetylcholinesterase could modify turning behaviour resulting from unilateral striatal dopamine depletion. Purified acetylcholinesterase, administered by cisternal puncture, attenuated circling behaviour for up to 7 days. The possible mechanisms are discussed by which endogenous acetylcholinesterase in cerebrospinal fluid could serve as an index of dopamine depletion in the nigrostriatal pathway and how exogenous enzymes might alleviate the accompanying motor dysfunction.

Release of acetylcholinesterase in the rat nigrostriatal pathway: relation to receptor activation and firing rate

Acetylcholinesterase is released from both axon terminals and dendrites of nigrostriatal neurons. The relationship of this phenomenon to: (1) activation of receptors, and (2) firing rate, has been examined. In the first series of experiments, apomorphine or acetylcholine were infused into substantiae nigrae of urethane anaesthetized rats, via push-pull cannulae. In the ipsilateral striatum, the release of acetylcholinesterase was modified in a fashion reminiscent of the changes in firing rate induced by these drugs in nigrostriatal cells. Thus, application of acetylcholine into the substantia nigra induced an increase, while apomorphine induced a decrease in the release of acetylcholinesterase from the striatum. However, in the substantia nigra, the release of acetylcholinesterase did not follow this pattern: acetylcholine reduced local release, while apomorphine caused no change. In the second part of this study we studied the relationship between firing rate of nigrostriatal cells and the release of acetylcholinesterase. Two compounds known to block neuronal impulse flow were infused into the substantia nigra. These drugs were tetrodotoxin (a Na+ channel blocker), or gamma-hydroxybutyrate (a drug which blocks impulse flow specifically in dopaminergic cells). Both compounds reduced the release of acetylcholinesterase in the ipsilateral striatum. However, locally in the substantia nigra there was no decrease in release of the enzyme. In fact, following administration of gamma-hydroxybutyrate, there was a large Ca2+ dependent increase in release of acetylcholinesterase in the substantia nigra. These results suggest that release of acetylcholinesterase in the striatum may be linked to the discharge frequency of nigrostriatal neurons. On the other hand, release of acetylcholinesterase from the substantia nigra, which probably occurs from dendrites, is independent of Na+ mediated action potentials. This release may instead be associated with specific dendritic Ca2+ conductances.

Acetylcholinesterase activity and type C synapses in the hypoglossal, facial and spinal-cord motor nuclei of rats. An electron-microscope study

Using the electron-microscope technique of Lewis and Shute, we studied the localization of the acetylcholinesterase (AChE) activity in the hypoglossal, facial and spinal-cord motor nuclei of rats. The technique used selectively detects synapses with subsynaptic cisterns (type C synapses) as well as heavy deposits of reaction products in the rough endoplasmic reticulum, in fragments of the nuclear envelope, in some Golgi zones and on parts of the pericaryal plasma membrane, the axolemma and the dendritic membrane. In C synapses, AChE activity was located in the synaptic cleft and on the membrane of presynaptic boutons. Some C synapses exhibited distinct synaptic specialization in the form of multiple 'active zones'. These zones were characterized by dense presynaptic projections, short dilations of the synaptic cleft, and postsynaptic densities localized between the postsynaptic membrane and the outer membrane of the subsynaptic cistern. Within the postsynaptic densities, rows of rod- or channel-like structures were observed. The subsynaptic cisterns were continuous with the positive rough endoplasmic reticulum. The results are discussed in terms of the possible role of C synapses in the regulation of AChE synthesis in postsynaptic cholinergic neurons and/or in the regulation of AChE release into the extracellular space as well as in the establishment of new synaptic contacts.