Plasma cholinesterase activity: A benchmark for rapid detection of pesticide poisoning in an avian scavenger

Poisoning due to exposure to organophosphate and carbamate pesticides is a common threat for many wildlife species, especially for scavengers such as vultures. The Griffon vulture population (Gyps fulvus), for instance, is deteriorating in the Eastern Mediterranean, and is considered to be critically endangered in Israel, where 48 out of 107 (45 %) known injury/mortality cases in 2010-2021 were caused by poisoning. Lack of specific clinical indications, together with levels of organophosphate or carbamate pesticides too low to detect, challenge the ability to diagnose and treat such poisoning events. The activity of cholinesterase (ChE) in plasma has the potential to serve as an effective biomarker for monitoring exposure to anticholinesterase pesticides in live vultures. Yet, the applicability of this approach has been limited by intra- and inter-species variations in ChE basal levels. The present study aims to provide a benchmark for ChE activity levels in healthy Griffons and their intra-species variation. Blood samples from free-roaming (n = 231) and captive (n = 63) Griffons were collected during routine monitoring, and ChE levels were determined using a colorimetric method. We established that the ChE in the plasma of Griffons reflects mostly acetylcholinesterase as the dominant form. ChE levels in healthy Griffons are 0.601 ± 0.011 U/ml (mean ± SE), while Griffons with suspected or confirmed pesticide poisoning display much lower levels of ChE activity (typically <0.3 U/ml). We also characterized the age dependence of ChE activity, as well as differences among groups from different locations or origins. Our study provides a rapid diagnostic tool for the detection of exposure to organophosphate and carbamate pesticides that should facilitate the lifesaving treatment and the conservation of this species. Moreover, our protocols can be adapted to other species and geographical areas, addressing pesticide poisoning worldwide and contributing to the protection of endangered species and their ecological functions (e.g. sanitation by scavengers).

Preface: Cholinergic mechanisms: This is the Preface for the special issue "Cholinergic Mechanisms"

This special issue of the Journal of Neurochemistry, entitled "Cholinergic Mechanisms," presents 15 reviews and two original papers, which have been selected to cover the broad spectrum of topics and disciplines presented at the XVIth International Symposium on Cholinergic Mechanisms (ISCM-XVI), ranging from the molecular and the cellular to the clinical and the cognitive mechanisms of cholinergic transmission. The authors discuss recent developments in the field, for instance, the association of cholinergic transmission with a number of important neurological and neuromuscular diseases in the central and peripheral nervous systems.

Rhythmogenic networks are potently modulated by activation of muscarinic acetylcholine receptors in the rodent spinal cord

Electrical stimulation of the spinal cord is a potent means for activating mammalian stepping in the absence of the descending control from the brain. Previously, we have shown that stimulation of pain delivering (Aδ) sacrocaudal afferents (SCA) has a powerful capacity to activate the sacral and lumbar rhythmogenic networks in the neonatal rodent spinal cord. Relatively little is known about the neural pathways involved in activation of the locomotor networks by Aδ afferents, on their mechanism of action and on the possibility to modulate their activity. We have shown that elevation of the endogenous level of acetylcholine at the sacral cord by blocking cholinesterase could modulate the SCA-induced locomotor rhythm in a muscarinic receptor-dependent mechanism. Here, we review these and more recent findings and report that controlled stimulation of SCA in the presence of muscarine is a potent activator of the locomotor network. The possible mechanisms involved in the muscarinic modulation of the locomotor rhythm are discussed in terms of the differential projections of sacral relay neurons, activated by SCA stimulation, to the lumbar locomotor rhythm generators, and to their target motoneurons. Altogether, our studies show that manipulations of cholinergic networks offer a simple and powerful means to control the activity of locomotor networks in the absence of supraspinal control.

Fine Localization of Acetylcholinesterase in the Synaptic Cleft of the Vertebrate Neuromuscular Junction

Acetylcholinesterase (AChE) is concentrated at cholinergic synapses, where it is a major factor in controlling the duration of transmitter action. The concentration and localization of AChE within the synaptic cleft are in keeping with the functional requirements of the particular type of synapse. The densities of synaptic AChE at various neuromuscular junctions (NMJs) had been evaluated by quantitative EM-autoradiography using radiolabeled probes. Yet, fundamental issues concerning the precise distribution and location of the enzyme in the cleft remained open: whether and to what extent synaptic AChE is associated with pre- or postsynaptic membranes, or with synaptic basal lamina (BL), and whether it occurs only in the primary cleft (PC) or also in postjunctional folds (PJFs). Nanogold-conjugates of fasciculin, an anticholinesterase polypeptide toxin, were prepared and used to label AChE at NMJs of mouse and frog muscles. Selective intense labeling was obtained at the NMJs, with gold-labeled AChE sites distributed over the BL in the PC and the PJFs. Quantitative analysis demonstrated that AChE sites are almost exclusively located on the BL rather than on pre- or postsynaptic membranes and are distributed in the PC and down the PJFs, with a defined pattern. This localization pattern of AChE is suggested to ensure full hydrolysis of acetylcholine (ACh) bouncing off receptors, thus eliminating its unnecessary detrimental reattachment.

Ascending pathways that mediate cholinergic modulation of lumbar motor activity

Deciphering neuronal pathways that reactivate spinal central pattern generators (CPGs) and modulate the activity of spinal motoneurons in mammals in the absence of supraspinal control is important for understanding of neural control of movement and for developing novel therapeutic approaches to improve the mobility of spinal cord injury patients. Previously, we showed that the sacral and lumbar cholinergic system could potently modulate the locomotor CPGs in newborn rodents. Here, we review these and our more recent studies of sacral relay neurons with lumbar projections to the locomotor CPGs and to lumbar motoneurons and demonstrate that sacral and lumbar cholinergic components have the capacity to control the frequency of the locomotor CPGs and at the same time the motor output of the activated lumbar motoneurons during motor behavior. A model describing the suggested ascending sacro-lumbar connectivity involved in modulation of the locomotor rhythm by sacral cholinergic components is proposed and discussed. This is an article for the special issue XVth International Symposium on Cholinergic Mechanisms.

Electric eel acetylcholinesterase: a multisubunit enzyme containing a collagen tail

Molecular forms of acetylcholinesterase (AChE) in fresh electric organ tissue are elongated structures in which a multisubunit head containing the catalytic sites is attached to a fibrous tail. The principal form, 18S AChE, is of MW ca. 1,100,000 and aggregates reversibly at low ionic strength. Trypsin converts it to an 11S globular tetramer devoid of the tail and lacking the capacity to aggregate reversibly in low salt. Amino acid analysis, collagenase and pepsin digestion and immunological techniques were utilized to demonstrate that the fibrous tail of the elongated forms of AChE is a collagen triple helix. The distal portion of the tail contains a region responsible for the capacity for aggregation at low ionic strength. This latter property may be related to the postulated role of the tail in anchoring AChE to the fibrillar matrix of the basal lamina.

The sacral networks and neural pathways used to elicit lumbar motor rhythm in the rodent spinal cord

Identification of neural networks and pathways involved in activation and modulation of spinal central pattern generators (CPGs) in the absence of the descending control from the brain is important for further understanding of neural control of movement and for developing innovative therapeutic approaches to improve the mobility of spinal cord injury patients. Activation of the hindlimb innervating segments by sacrocaudal (SC) afferent input and by specific application of neurochemicals to the sacral networks is feasible in the isolated spinal cord preparation of the newborn rat. Here we review our recent studies of sacral relay neurons with lumbar projections and evaluate their role in linking the sacral and thoracolumbar (TL) networks during different motor behaviors. Our major findings show that: (1) heterogeneous groups of dorsal, intermediate and ventral sacral-neurons with ventral and lateral ascending funicular projections mediate the activation of the locomotor CPGs through sacral sensory input; and (2) rhythmic excitation of lumbar flexor motoneurons, produced by bath application of alpha-1 adrenoceptor agonists to the sacral segments is mediated exclusively by ventral clusters of sacral-neurons with lumbar projections through the ventral funiculus.

The motor output of hindlimb innervating segments of the spinal cord is modulated by cholinergic activation of rostrally projecting sacral relay neurons

Cholinergic networks have been shown to be involved in generation and modulation of the locomotor rhythmic pattern produced by the mammalian central pattern generators. Here, we show that changes in the endogenous levels of acetylcholine in the sacral segments of the isolated spinal cord of the neonatal rat modulate the locomotor-related output produced by stimulation of sacrocaudal afferents in muscarinic receptor-dependent mechanisms. Cholinergic components we found on sacral relay neurons with lumbar projections through the ventral and lateral funiculi are suggested to mediate this ascending cholinergic modulation. Our findings, possible mechanisms accounting for them, and their potential implications are discussed.

Changes in acetylcholinesterase in experimental autoimmune myasthenia gravis and in response to treatment with a specific antisense

Controlled regulation of synaptic nicotinic acetylcholine receptors (AChRs) and acetylcholinesterase (AChE), together with maintenance of a dynamic balance between them, is a requirement for proper function of cholinergic synapses. In the present study we assessed whether pathological changes in AChR perturb this balance, and whether such changes can be corrected. We studied the influence of AChR loss, caused by experimental autoimmune myasthenia gravis (EAMG), on muscle AChE, as well as the reciprocal effect of an antisense targeted towards AChE on both AChR and AChE at the neuromuscular synapse. The extensor digitorum longus (EDL) muscles of EAMG Lewis rats were isolated, and AChE levels and isoform compositions were examined. Although AChE levels in the muscles of healthy and EAMG rats were similar, marked changes were observed in isoform composition. Healthy EDL muscles contained globular (G(1,2) , G(4) ) and asymmetric (primarily A(12) ) isoforms. G(1,2) -AChE was significantly reduced in EAMG muscles, whereas both G(4) - and A(12) -AChE remained unchanged. Treatment of EAMG rats with the antisense EN101 resulted in decreased total muscle AChE, with recovery in G(1,2) and reduction in A(12) -AChE. AChE/AChR ratios were determined at the neuromuscular junctions (NMJ). The decrease in AChR levels that occurred as the disease progressed resulted in a dramatic increase in this ratio, and a significant recovery towards normal ratios occurred after EN101 treatment. This improvement was primarily due to increased synaptic AChR content. Our findings emphasise the tight connection between AChR and AChE at the myasthenic NMJ, and the importance of the AChE/AChR ratio in maintaining the required cholinergic balance.

Serum cholinesterases are differentially regulated in normal and dystrophin-deficient mutant mice

The cholinesterases, acetylcholinesterase (AChE), and butyrylcholinesterase (BChE) (pseudocholinesterase), are abundant in the nervous system and in other tissues. The role of AChE in terminating transmitter action in the peripheral and central nervous system is well understood. However, both knowledge of the function(s) of the cholinesterases in serum, and of their metabolic and endocrine regulation under normal and pathological conditions, is limited. This study investigates AChE and BChE in sera of dystrophin-deficient mdx mutant mice, an animal model for the human Duchenne muscular dystrophy (DMD) and in control healthy mice. The data show systematic and differential variations in the concentrations of both enzymes in the sera, and specific changes dictated by alteration of hormonal balance in both healthy and dystrophic mice. While AChE in mdx-sera is elevated, BChE is markedly diminished, resulting in an overall cholinesterase decrease compared to sera of healthy controls. The androgen testosterone (T) is a negative modulator of BChE, but not of AChE, in male mouse sera. T-removal elevated both BChE activity and the BChE/AChE ratio in mdx male sera to values resembling those in healthy control male mice. Mechanisms of regulation of the circulating cholinesterases and their impairment in the dystrophic mice are suggested, and clinical implications for diagnosis and treatment are considered.

Extracellular matrix components involved in neuromuscular transmission and regeneration

The portion of a skeletal muscle fibre's basal lamina sheath that lies in the synaptic cleft at the neuromuscular junction contains a high concentration of certain molecules that distinguish it from non-junctional portions of the sheath. Among the molecules are acetylcholinesterase, which terminates the action of the transmitter, acetylcholine, on the postsynaptic membrane, and factors that direct differentiation at neuromuscular junctions regenerating after trauma. In this communication the evidence that acetylcholinesterase and synapse differentiation factors are associated with synaptic cleft basal lamina is reviewed and the results of current experiments aimed at characterizing these extracellular matrix molecules are described.

125I-labeled fasciculin 2: a new tool for quantitation of acetylcholinesterase densities at synaptic sites by EM-autoradiography

Radio-iodinated fasciculin 2 (Fas2), a polypeptide anticholinesterase toxin from Mamba venom, was used as a new probe for localizing and quantifying acetylcholinesterase (AChE) at mouse neuromuscular junctions (NMJs) by quantitative electron microscope autoradiography. We demonstrate that 125I-Fas2 binds very specifically to the NMJs of mouse sternomastoid muscles, with very little binding to other regions in the muscles. Junctional AChE-site densities obtained from the autoradiograms were similar to those previously obtained for the same muscles using 3H-DFP. The use of 125I-Fas2 with EM-autoradiography is simpler and provides higher resolution and sensitivity, as well as considerably lower non-specific binding than previously attainable with 3H-DFP. The advantages and limitations of this procedure are discussed.

Transplantation of quail collagen-tailed acetylcholinesterase molecules onto the frog neuromuscular synapse

The highly organized pattern of acetylcholinesterase (AChE) molecules attached to the basal lamina of the neuromuscular junction (NMJ) suggests the existence of specific binding sites for their precise localization. To test this hypothesis we immunoaffinity purified quail globular and collagen-tailed AChE forms and determined their ability to attach to frog NMJs which had been pretreated with high-salt detergent buffers. The NMJs were visualized by labeling acetylcholine receptors (AChRs) with TRITC-alpha-bungarotoxin and AChE by indirect immunofluorescence; there was excellent correspondence (>97%) between the distribution of frog AChRs and AChE. Binding of the exogenous quail AChE was determined using a species-specific monoclonal antibody. When frog neuromuscular junctions were incubated with the globular G4/G2 quail AChE forms, there was no detectable binding above background levels, whereas when similar preparations were incubated with the collagen-tailed A12 AChE form >80% of the frog synaptic sites were also immunolabeled for quail AChE attached. Binding of the A12 quail AChE was blocked by heparin, yet could not be removed with high salt buffer containing detergent once attached. Similar results were obtained using empty myofiber basal lamina sheaths produced by mechanical or freeze-thaw damage. These experiments show that specific binding sites exist for collagen-tailed AChE molecules on the synaptic basal lamina of the vertebrate NMJ and suggest that these binding sites comprise a "molecular parking lot" in which the AChE molecules can be released, retained, and turned over.

Acetylcholinesterase density and turnover number at frog neuromuscular junctions, with modeling of their role in synaptic function

Acetylcholinesterase (AChE) density at the neuromuscular junction of frog cutaneous pectoris muscle was determined by electron microscope autoradiography and biochemistry to be approximately 600 sites micron-2 of postsynaptic area, approximately 4-fold lower than all previous reports (mouse), whereas the hydrolytic turnover number was 9,500 s-1, well within the range (2,000-16,000 s-1) for AChE from other species. Monte Carlo computer simulations of miniature endplate currents showed that for vertebrate neuromuscular junctions with different morphologies, an AChE density of only approximately 400 sites microns-2 and a turnover number of only approximately 1,000 s-1 are sufficient for normal quantal currents. Above these critical lower limits, miniature endplate currents were essentially insensitive to AChE density and turnover number values up to 5,000 sites microns-2 and 16,000 s-1, respectively.

Globular and asymmetric acetylcholinesterase in the synaptic basal lamina of skeletal muscle

The aim of this study was to characterize the molecular forms of acetylcholinesterase (AChE) associated with the synaptic basal lamina at the neuromuscular junction. The observations were made on the neuromuscular junctions of cutaneous pectoris muscles of frog, Rana pipiens, which are similar to junctions of most other vertebrates including mammals, but are especially convenient for experimentation. By measuring relative AChE activity in junctional and extrajunctional regions of muscles after selective inactivation of extracellular AChE with echothiophate, or of intracellular AChE with DFP and 2-PAM, we found that > 66% of the total AChE activity in the muscle was junction-specific, and that > 50% of the junction-specific AChE was on the cell surface. More than 80% of the cell surface AChE was solubilized in high ionic strength detergent-free buffer, indicating that most, if not all, was a component of the synaptic basal lamina. Sedimentation analysis of that fraction indicated that while asymmetric forms (A12, A8) were abundant, globular forms sedimenting at 4-6 S (G1 and G2), composed > 50% of the AChE. It was also found that when muscles were damaged in various ways that caused degeneration of axons and muscle fibers but left intact the basal lamina sheaths, the small globular forms persisted at the synaptic site for weeks after phagocytosis of cellular components; under certain damage conditions, the proportion of globular to asymmetric forms in the vacated basal lamina sheaths was as in normal junctions. While the asymmetric forms required high ionic strength for solubilization, the extracellular globular AChE could be extracted from the junctional regions of normal and damaged muscles by isotonic buffer. Some of the globular AChE appeared to be amphiphilic when examined in detergents, suggesting that it may form hydrophobic interactions, but most was non-amphiphilic consistent with the possibility that it forms weak electrostatic interactions. We conclude that the major form of AChE in frog synaptic basal lamina is globular and that its mode of association with the basal lamina differs from that of the asymmetric forms.

G2-acetylcholinesterase is presynaptically localized in Torpedo electric organ

In Torpedo electric organ, much of the acetylcholinesterase (AChE) is a globular dimer (G2), anchored to the plasma membrane via covalently attached phosphatidylinositol and selectively solubilized by a bacterial phosphatidylinositol-specific phospholipase C. While the structure of this form of the enzyme is well-established, the ultrastructural localization of G2-AChE is still unclear. Selective solubilization with phosphatidylinositol-specific phospholipase C was, therefore, combined with immunocytochemistry at the electron microscope level, in order to localize G2-AChE in electric organ of Torpedo ocellata. Thin sections of electric organ were labelled with antibodies raised against Torpedo AChE, followed by gold-conjugated second antibodies, before or after exposure to the phospholipase. For comparison, the location of AChE was examined using histochemical methods. We show that (1) immunolabelling is concentrated in the synaptic clefts between nerve terminals and the innervated face of the electrocyte; (2) this labelling co-localizes with AChE histochemical reaction products; and (3) prior exposure to the phospholipase causes a decrease in AChE-associated labelling. Quantitative analysis of immunolabelling in the synaptic clefts shows that the phospholipase treatment had reduced primary labelling at or adjacent to the presynaptic membrane. Together with our earlier biochemical and immunofluorescent evidence, these results support our previous assignment of a neuronal and synaptic localization for G2-AChE in Torpedo electric organ.

Acetylcholinesterase from the motor nerve terminal accumulates on the synaptic basal lamina of the myofiber

Acetylcholinesterase (AChE) in skeletal muscle is concentrated at neuromuscular junctions, where it is found in the synaptic cleft between muscle and nerve, associated with the synaptic portion of the myofiber basal lamina. This raises the question of whether the synaptic enzyme is produced by muscle, nerve, or both. Studies on denervated and regenerating muscles have shown that myofibers can produce synaptic AChE, and that the motor nerve may play an indirect role, inducing myofibers to produce synaptic AChE. The aim of this study was to determine whether some of the AChE which is known to be made and transported by the motor nerve contributes directly to AChE in the synaptic cleft. Frog muscles were surgically damaged in a way that caused degeneration and permanent removal of all myofibers from their basal lamina sheaths. Concomitantly, AChE activity was irreversibly blocked. Motor axons remained intact, and their terminals persisted at almost all the synaptic sites on the basal lamina in the absence of myofibers. 1 mo after the operation, the innervated sheaths were stained for AChE activity. Despite the absence of myofibers, new AChE appeared in an arborized pattern, characteristic of neuromuscular junctions, and its reaction product was concentrated adjacent to the nerve terminals, obscuring synaptic basal lamina. AChE activity did not appear in the absence of nerve terminals. We concluded therefore, that the newly formed AChE at the synaptic sites had been produced by the persisting axon terminals, indicating that the motor nerve is capable of producing some of the synaptic AChE at neuromuscular junctions. The newly formed AChE remained adherent to basal lamina sheaths after degeneration of the terminals, and was solubilized by collagenase, indicating that the AChE provided by nerve had become incorporated into the basal lamina as at normal neuromuscular junctions.

Immunocytochemical localization of phosphatidylinositol-anchored acetylcholinesterase in excitable membranes of Torpedo ocellata

In Torpedo electric organ much of the acetylcholinesterase is a 'globular' dimer (G2), anchored to the plasma membrane via covalently attached phosphatidylinositol and solubilized by a bacterial phosphatidylinositol-specific phospholipase C. This suggested that selective solubilization with phosphatidylinositol-specific phospholipase C, coupled with immunocytochemistry, might be used to localize G2 acetylcholinesterase in excitable tissues of Torpedo. Cryostat sections of electric organ, electromotor nerve, electric lobe and back muscle from Torpedo ocellata were labelled, using three different antibody preparations to Torpedo acetylcholinesterase, followed by a fluorescent second antibody, before and after exposure to the phospholipase. Sites of innervation on electrocytes and myofibers were labelled selectively, as were motor and electromotor nerves. In all these cases labelling was substantially diminished by prior exposure to the phospholipase. The results support our previous assignment, based on biochemical evidence, for a neuronal and synaptic localization of the G2 acetylcholinesterase in Torpedo. Electric lobe acetylcholinesterase appears insensitive to the phospholipase treatment and lacks certain epitopes present in both electric organ and electromotor nerve enzyme. This suggests that substantial processing of the G2 form occurs concomitantly with its movement from the electric lobe into the electromotor nerve.

A membrane-associated dimer of acetylcholinesterase from Xenopus skeletal muscle is solubilized by phosphatidylinositol-specific phospholipase C

The susceptibility to phosphatidylinositol-specific phospholipase C of the membrane associated acetylcholinesterase (AChE) forms of Xenopus laevis skeletal muscle was examined. This treatment released almost all the detergent-soluble AChE species from muscle homogenates. Sucrose gradient analysis showed that the released acetylcholinesterase form corresponds to a hydrophilic G2 dimer, indicating that this dimer has a glycolipid anchoring domain which contains phosphatidylinositol.

Basal lamina directs acetylcholinesterase accumulation at synaptic sites in regenerating muscle

In skeletal muscles that have been damaged in ways which spare the basal lamina sheaths of the muscle fibers, new myofibers develop within the sheaths and neuromuscular junctions form at the original synaptic sites on them. At the regenerated neuromuscular junctions, as at the original ones, the muscle fibers are characterized by junctional folds and accumulations of acetylcholine receptors and acetylcholinesterase (AChE). The formation of junctional folds and the accumulation of acetylcholine receptors is known to be directed by components of the synaptic portion of the myofiber basal lamina. The aim of this study was to determine whether or not the synaptic basal lamina contains molecules that direct the accumulation of AChE. We crushed frog muscles in a way that caused disintegration and phagocytosis of all cells at the neuromuscular junction, and at the same time, we irreversibly blocked AChE activity. New muscle fibers were allowed to regenerate within the basal lamina sheaths of the original muscle fibers but reinnervation of the muscles was deliberately prevented. We then stained for AChE activity and searched the surface of the new muscle fibers for deposits of enzyme they had produced. Despite the absence of innervation, AChE preferentially accumulated at points where the plasma membrane of the new muscle fibers was apposed to the regions of the basal lamina that had occupied the synaptic cleft at the neuromuscular junctions. We therefore conclude that molecules stably attached to the synaptic portion of myofiber basal lamina direct the accumulation of AChE at the original synaptic sites in regenerating muscle. Additional studies revealed that the AChE was solubilized by collagenase and that it remained adherent to basal lamina sheaths after degeneration of the new myofibers, indicating that it had become incorporated into the basal lamina, as at normal neuromuscular junctions.

Real-time optical imaging of naturally evoked electrical activity in intact frog brain

A major obstacle to understanding the function and development of the vertebrate brain is the difficulty in monitoring dynamic patterns of electrical activity in the millesecond time domain; this has limited investigations of such phenomena as modular organization of functional units, seizure activities and spreading depression. The use of voltage-sensitive dyes and the recent development of the use of an array of photodiodes has provided a unique technique for monitoring the dynamic patterns of electrical activity in real time from a variety of invertebrate or vertebrate neuronal preparations including the rat cortex. In the present study, this technique has been used to investigate the intact optic tectum of the frog. We demonstrate that optical measurements can be used for real-time imaging of spatio-temporal patterns of neuronal responses and for identification of functional units evoked by natural visual stimuli. We report also the structure of the new voltage-sensitive probe that facilitates the in vivo applications of this technique.

Improved fluorescent probes for the measurement of rapid changes in membrane potential

To improve the quality of fluorescent voltage-sensitive probes twenty new styryl dyes were synthesized. Some of the new probes are significantly better than any used in the past. A signal-to-noise ratio of 90 root mean square (rms) noise was obtained for an optical recording of action potentials from neuroblastoma cells maintained in monolayer culture. The fluorescence fractional change of the optical signal is as large as 14%/100 mV. Photodynamic damage and bleaching are much less significant with the new probes. These fluorescent probes can be used to measure small and rapid changes in membrane potential from single cells maintained in monolayer cultures, from single cells in invertebrate ganglia, from their arborization, and from other preparations. The optical measurement can be made with a standard fluorescent microscope equipped with DC mercury illumination. Guidelines for the design of even better fluorescent probes and more efficient instruments are suggested.