Differential chemosensitivity of synaptic and extrasynaptic areas on the neuronal surface membrane in parasympathetic neurons of the frog, tested by microapplication of acetylcholine

Emerging Pharmacological Properties of Cholinergic Synaptic Transmission: Comparison between Mammalian and Insect Synaptic and Extrasynaptic Nicotinic Receptors

Acetylcholine (ACh) is probably the oldest signalling neurotransmitter which appeared in evolution before the nervous system. It is present in bacteria, algae, protozoa and plants. In insects and mammals it is involved in cell-to-cell communications in various neuronal and non-neuronal tissues. The discovery of nicotinic acetylcholine receptors (nAChRs) as the main receptors involved in rapid cholinergic neurotransmission has helped to understand the role of ACh at synaptic level. Recently, several lines of evidence have indicated that extrasynaptically expressed nAChRs display distinct pharmacological properties from the ones expressed at synaptic level. The role of both nAChRs at insect extrasynaptic and/or synaptic levels has been underestimated due to the lack of pharmacological tools to identify different nicotinic receptor subtypes. In the present review, we summarize recent electrophysiological and pharmacological studies on the extrasynaptic and synaptic differences between insect and mammalian nAChR subtypes and we discuss on the pharmacological impact of several drugs such as neonicotinoid insecticides targeting these receptors. In fact, nAChRs are involved in a wide range of pathophysiological processes such as epilepsy, pain and a wide range of neurodegenerative and psychiatric disorders. In addition, they are the target sites of neonicotinoid insecticides which are known to act as nicotinic agonists causing severe poisoning in insects and mammals.

Chick ciliary ganglion neurons contain transcripts coding for acetylcholine receptor-associated protein at synapses (rapsyn)

A peripheral membrane protein of approximately 43 kDa (rapsyn) clusters muscle nicotinic acetylcholine receptors (AChRs), but molecules relevant to clustering neuronal AChRs have not been identified. Here, we have detected rapsyn transcripts in the chick nervous system, localized rapsyn mRNA in ciliary ganglion (CG) neurons, which are known to cluster AChRs, and identified three rapsyn cDNAs derived from the ganglion. Our initial Northern blots, performed using a mouse probe, revealed rapsyn-like transcripts in chick muscle and brain. To develop species-specific probes, we prepared a chick rapsyn cDNA construct, Ch43K.1, that encodes a protein having extensive homology to mouse rapsyn. Using primers designed to anneal near the 5' and 3' boundaries of Ch43K.1, three prominent cDNAs were amplified from chick muscle templates by reverse transcriptase based-PCR. Products of similar size were also amplified using cDNA prepared from neuronal tissues expected to contain clustered AChRs (CG and brain), whereas none were detected using templates from tissues not displaying clustered AChRs (sensory ganglia and liver). In situ hybridization confirmed that rapsyn mRNA is expressed both in chick muscle fibers and in CG neurons. Sequencing the three cDNAs amplified from CG templates revealed the largest to be Ch43K.1, whereas the smaller two may represent splice variants. These findings suggest that multiple rapsyn-like molecules are involved in clustering the distinct AChRs expressed by muscle fibers and neurons.

The role of acetylcholinesterase in denervation supersensitivity in the frog cardiac ganglion

1. The sensitivity of normal and denervated cardiac ganglion cells to the cholinergic agonists acetylcholine and carbamylcholine (carbachol) were compared in the frog, Rana pipiens. Acetylcholine and carbachol bind to the same acetylcholine receptors, but, unlike acetylcholine, carbachol is resistant to hydrolysis by acetylcholinesterase. 2. Sensitivity was assessed by the peak depolarization elicited in response to a sustained pulse of ligand emitted from a pipette positioned 10 microns from the ganglion cell surface. This technique allows the sensitivity of the entire cell to be recorded with a single measurement. 3. The acetylcholine sensitivity of normal cardiac ganglion cells was increased by inhibiting extracellular acetylcholinesterase with echothiophate. 4. Denervation increased the sensitivity of cardiac ganglion cells to acetylcholine but not to carbachol. 5. Following the inhibition of extracellular acetylcholinesterase with echothiophate, sensitivity to acetylcholine was similar in normal and in denervated ganglion cells. 6. The increased sensitivity to acetylcholine of cardiac ganglion cells following denervation is caused by a reduction in the hydrolysis of the transmitter by acetylcholinesterase rather than by changes in the number and/or properties of acetylcholine receptors.

Parallel processing and selection of the responses to serotonin during reinnervation of an identified leech neuron

In an attempt to define the mechanism of synaptic specificity, we have been studying pairs of identified leech neurons isolated in tissue culture. The cultured neurons reform specific synapses when paired with appropriate partners in the absence of other cell types. In recent studies, we have examined in detail the reformation of a serotoninergic synapse between the Retzius cell and one of its targets, the pressure sensitive (P) cell. The P cell in vivo and its soma in vitro have two types of responses to serotonin (5-HT). From voltage clamp analysis of cultured P cells, we demonstrated the parallel activation of chloride (gCls) and monovalent cation (gCations) channels coupled to distinct receptor subtypes and gated by separate second messengers. Only gCls was activated by 5-HT released from the presynaptic Retzius cell both in vivo and in vitro. This demonstrates the remarkable specificity of the reformation of this synapse in culture since only the correct 5-HT receptor subtype is activated. An 80% reduction of gCations was observed in P cells that had failed to be innervated by Retzius cells in culture, suggesting that gCations may be lost prior to synapse formation. Retzius cells depleted of 5-HT also reduced gCations in the paired P cells and incubating single P cells in 5-HT did not reduce gCations. In addition, aldehyde-fixed Retzius cells were able to selectively reduce gCations when paired with P cells. We conclude that the loss of gCations was due to contact between the neurons. The early clearing of counter-effective receptor subtypes may be a prelude to synapse formation.

Motor neurons contain agrin-like molecules

Molecules antigenically similar to agrin, a protein extracted from the electric organ of Torpedo californica, are highly concentrated in the synaptic basal lamina of neuromuscular junctions in vertebrate skeletal muscle. On the basis of several lines of evidence it has been proposed that agrin-like molecules mediate the nerve-induced formation of acetylcholine receptor (AChR) and acetylcholinesterase (AChE) aggregates on the surface of muscle fibers at developing and regenerating neuromuscular junctions and that they help maintain these postsynaptic specializations in the adult. Here we show that anti-agrin monoclonal antibodies selectively stain the cell bodies of motor neurons in embryos and adults, and that the stain is concentrated in the Golgi apparatus. We also present evidence that motor neurons in both embryos and adults contain molecules that cause the formation of AChR and AChE aggregates on cultured myotubes and that these AChR/AChE-aggregating molecules are antigenically similar to agrin. These findings are consistent with the hypothesis that agrin-like molecules are synthesized by motor neurons, and are released from their axon terminals to become incorporated into the synaptic basal lamina where they direct the formation of synapses during development and regeneration.

Molecular events in synaptogenesis: nerve-muscle adhesion and postsynaptic differentiation

The clustering of acetylcholine receptors (AChR) in the postsynaptic membrane of newly innervated muscle fibers is one of the earliest events in the development of the vertebrate neuromuscular junction. Here, we describe two hypotheses that can account for AChR clustering in response to innervation. The "trophic factor" hypothesis proposes that the neuron releases a soluble factor that interacts with the muscle cell in a specific manner and that this interaction results in the local accumulation of AChR. The "contact and adhesion" hypothesis proposes that the binding of the nerve to the muscle cell surface is itself sufficient to induce AChR clustering, without the participation of soluble factors. We present a model for the molecular assembly of AChR clusters based on the contact and adhesion hypothesis. The model involves the sequential assembly of three distinct membrane domains. The first domain to form serves to attach microfilaments to the cytoplasmic surface of the muscle cell membrane at sites of muscle-nerve adhesion. The second domain to form is clathrin-coated membrane; it serves as a site of insertion of additional membrane elements, including AChR. Upon insertion of AChR into the cell surface, a membrane skeleton assembles by anchoring itself to the AChR. The skeleton, composed in part of actin and spectrin, binds and immobilizes significant numbers of AChR, thereby forming the third membrane domain of the AChR cluster. We make several predictions that should distinguish this model of AChR clustering from one that invokes soluble, trophic factors.

Effects of preganglionic denervation and postganglionic axotomy on acetylcholine receptors in the chick ciliary ganglion

The regulation of nicotinic acetylcholine receptors (AChRs) in chick ciliary ganglia was examined by using a radiolabeled anti-AChR mAb to quantitate the amount of receptor in ganglion detergent extracts after preganglionic denervation or postganglionic axotomy. Surgical transection of the preganglionic input to the ciliary ganglion in newly hatched chicks caused a threefold reduction in the total number of AChRs within 10 d compared with that present in unoperated contralateral control ganglia. Surgical transection of both the choroid and ciliary nerves emerging from the ciliary ganglion in newly hatched chicks to establish postganglionic axotomy led to a nearly 10-fold reduction in AChRs within 5 d compared with unoperated contralateral ganglia. The declines were specific since they could not be accounted for by changes in ganglionic protein or by decreases in neuronal survival or size. Light microscopy revealed no gross morphological differences between neurons in operated and control ganglia. A second membrane component of cholinergic relevance on chick ciliary ganglion neurons is the alpha-bungarotoxin (alpha-Bgt)-binding component. The alpha-Bgt-binding component also declined in number after either postganglionic axotomy or preganglionic denervation, but appeared to do so with a more rapid time course than did ganglionic AChRs. The results imply that cell-cell interactions in vivo specifically regulate both the number of AChRs and the number of alpha-Bgt-binding components in the ganglion. Regulation of these neuronal cholinergic membrane components clearly differs from that previously described for muscle AChRs.

Single channels activated by acetylcholine in rat superior cervical ganglion

1. The elementary currents flowing through single channels opened by acetylcholine were recorded in rat superior cervical ganglion neurones using patch-clamp methods. Acetylcholine (30 microM) was included in the patch electrode (cell-attached recordings) or applied by ionophoresis (outside-out configuration). All measurements were made at 23-25 degrees C and mostly at -110 mV. 2. Channel openings appeared both as single events and as bursts of events. One population of the currents observed had a conductance of 20.0 +/- 0.2 pS (mean +/- S.E. of mean, n = 4). A second population had a conductance of about 50 pS, occurred more rarely, and was not included in further analysis. 3. Four channel closed time periods and two channel open time periods were found from the distributions of closed and open times. It was found that shorter channel openings (about 0.2 ms) appeared in isolation, whereas longer openings (duration 1.3 +/- 0.2 ms, n = 4) appeared as bursts of openings separated by the shortest channel closed time periods (about 0.15 ms). The next shortest closed time (about 2 ms) apparently corresponds to the lifetime of the channel not activated by acetylcholine. The two longer closed times (about 80 ms and 1 s) may reflect desensitization. The mean burst duration was 8.5 +/- 1.2 ms (n = 4), giving about six openings per burst. 4. Because the time constant of decay of the excitatory post-synaptic current is more similar to the burst duration than to the duration of individual single openings, it is suggested that acetylcholine released from presynaptic nerves may result in a burst of openings rather than a single opening. 5. On the basis of the above assumption, the rate constants were calculated for a sequential model in which acetylcholine binds to the receptor (forward rate k + 1 = 2.3 X 10(7) M-1 s-1; reverse rate k-1 = 1235 s-1) which then undergoes a conformational change to an open state (forward rate beta = 6293 s-1; reverse rate alpha = 894 s-1). 6. When heptamethonium (30 microM) was added to the solution in the patch electrode, the burst duration was markedly shortened, but there was no change in the closed time between two openings within the burst. This effect was voltage-dependent, which suggests that heptamethonium binds to the channel after it is opened by acetylcholine.

Disruption and reformation of the acetylcholine receptor clusters of cultured rat myotubes occur in two distinct stages

We have examined the redistribution of acetylcholine receptor (AChR) intramembrane particles (IMPs) when AChR clusters of cultured rat myotubes are experimentally disrupted and allowed to reform. In control myotubes, the AChR IMPs are evenly distributed within the AChR domains of cluster membrane. Shortly after addition of azide to disrupt clusters, IMPs become unevenly scattered, with some microaggregation. After longer treatment, IMPs are depleted from AChR domains with no further change in IMP distribution. Contact domains of clusters are relatively poor in IMPs both before and after cluster dispersal. Upon visualization with fluorescent alpha-bungarotoxin, some AChR in azide-treated samples appear as small, bright spots. These spots do not correspond to microaggregates seen in freeze-fracture replicas, and probably represent receptors that have been internalized. The internalization rate is insufficient to account completely for the loss of IMPs from clusters, however. During reformation of AChR clusters upon removal of azide, IMP concentration in receptor domains increases. At early stages of reformation, IMPs appear in small groups containing compact microaggregates. At later times, AChR domains enlarge and IMPs within them assume the evenly spaced distribution characteristic of control clusters. These observations suggest that the disruption of clusters is accompanied by mobilization of AChR from a fixed array, allowing AChR IMPs to diffuse away from the clusters, to form microaggregates, and to become internalized. Cluster reformation appears to be the reverse of this process. Our results are thus consistent with a two-step model for AChR clustering, in which the concentration of IMPs into a small membrane region precedes their rearrangement into evenly spaced sites.

Lack of nicotinic supersensitivity in frog sympathetic neurones following denervation

The sensitivity of bull-frog sympathetic neurones to nicotinic, cholinergic agonists has been studied in both normal (control) and surgically denervated ganglia. Using gross extracellular recording, the sensitivity to acetylcholine (ACh) increased 18-fold following denervation, while that to carbachol (CCh) was unchanged. Normal ganglia showed a similar sensitivity increase after inhibition of cholinesterase. This suggests that the rise in ACh sensitivity is due to reduced cholinesterase activity, not to true supersensitivity. There was no significant difference in resting membrane potential or input resistance between normal and denervated neurones. Neurones denervated for 7-50 days showed no significant change in sensitivity to ACh or CCh applied iontophoretically at a distance of 10 micron from the cell surface. In control ganglia, localized iontophoretic application of ACh revealed an uneven distribution of sensitivity which is attributed to the localization of receptors to synaptic areas. Fourteen days after denervation, the geometric mean sensitivity to focally applied ACh was not significantly different from that found in control ganglia. The variation in sensitivity to focally applied ACh at randomly chosen sites on denervated neurones was as great as that found in control ganglia. It is concluded that denervation does not cause frog sympathetic neurones to become supersensitive to ACh. The apparent increase in nicotinic ACh sensitivity observed using extracellular recording from whole ganglia is due not to a change in the number or distribution of ACh receptors, but to a decrease in cholinesterase activity.

Distribution of receptors for acetylcholine and 5-hydroxytryptamine on identified leech neurones growing in culture

The spatial distribution of receptors on identified leech neurones removed from the C.N.S. and grown in culture has been studied by applying acetylcholine (ACh) and 5-hydroxytryptamine (5-HT) ionophoretically and by pressure. Two cells were selected: a neurone called anterior Pagoda (Ap), that shows responses to ACh, and the pressure sensory neurone (P cell), upon which 5-HT synapses form in culture. ACh receptors of Ap neurones in culture had properties similar to those of their counterparts in situ. Thus, ACh responses of Ap cells were mediated by Cl- and were blocked by curare and alpha-bungarotoxin. The cell bodies of these neurones in culture had low (10 mV/nC) and uniform sensitivity to ACh over the surface of the soma. When a sprout grew out from the Ap cell, a region of increased sensitivity appeared at its base, with a gradient of sensitivity decreasing toward the tip of the neurite. Characteristically, the base was 3-5 times more sensitive to ACh than the soma or the growth cone. Cells with multipolar processes developed a similar pattern of sensitivity for each sprout. P sensory neurones in culture showed similar distributions of sensitivity to 5-HT and ACh. Experiments made with voltage clamp suggested that the non-uniform responses to transmitter represent true differences in sensitivity. Together these findings suggest that the receptors for ACh and 5-HT have a greater density at the base of each neurite compared to that of the soma and the tip.

Enhanced chemosensitivity of chick parasympathetic neurones in co-culture with myotubes

1. The influence of target interaction upon the electrophysiological properties of dissociated ciliary ganglion cells was investigated by testing the sensitivity of the neuronal somal membrane to ionophoretically applied acetylcholine (ACh). Variations in the percentage of cells responsive to the transmitter were measured with time in culture. 2. Twenty-four hours after plating, all cells respond to an ionophoretic pulse of ACh with a depolarization. However, 1 week after plating (between 7 and 14 days) most of the neurones are unresponsive, and highly responsive cells (greater than 100 mV peak depolarization/nC) are extremely rare. At even later times in culture, neurones sensitive to the transmitter are again more frequent. 3. When neurones are plated onto pre-formed pectoral myotubes, however, ACh sensitivity is maintained throughout a 3 week culture period. Neuromuscular junctions are formed by the neurones, and when sufficient neurones are present, all the muscle fibres tested show evidence of functional synaptic transmission. Chemosensitivity to ACh is not maintained by neurones in muscle-free microcultures are present on the same cover-slip. 4. Interneuronal synaptic contacts, defined by ultrastructural criteria, are formed in cultures of neurones alone, but evidence of widespread functional synaptic interaction between cells was not found at 7-14 days in culture. 5. It is concluded that the maintenance of ACh sensitivity of cultured ciliary ganglion cells is enhanced by the presence of muscle in co-culture. The interneuronal synaptic contacts observed are apparently not as potent a stimulus as co-culture with muscle for the full expression of the cholinergic phenotype under these culture conditions.

The appearance and development of chemosensitivity in Rohon-Beard neurones of the Xenopus spinal cord

1. We have examined the onset and subsequent development of chemosensitivity in Rohon-Beard neurones from the Xenopus spinal cord. These cells become sensitive to bath-applied gamma-aminobutyric acid (GABA) around stage 25 (early tailbud, about 1 d old), and remain so at least until stage 49 (9 d old). In contrast, a number of other neurotransmitter candidates tested caused no potential or conductance change during the same period.2. We examined ionophoretic dose-response relations of the cells at stage 26, a couple of hours after the first acquisition of GABA sensitivity. Sensitivities as high as 450 mV/nC were recorded. Comparable sensitivities were recorded between stages 46-49 (5-9 d old).3. Measurements of ionophoretic sensitivities and input resistances during several periods from stage 26 to maturity show that the underlying conductance change for a given GABA dose is likely to increase steadily during this time. A ;sensitivity index' (ionophoretic sensitivity/input resistance) was calculated, which is low at stage 26, higher at intermediate stages (stages 31-42), and highest for mature cells (stages 46-49; 5-9 d of development).4. The reversal potential of the ionophoretic GABA response is the same at stage 26 (-30 mV) as it is in mature cells. Ion substitution experiments show that Na(+) and K(+), but not Cl(-) or Ca(2+), are involved in the response.5. GABA responses at stage 26 are pharmacologically similar to those of mature cells. The responses are blocked by 10 muM-picrotoxin or curare, and muscimol is an agonist in concentrations as low as 1 muM.6. GABA responses at stage 26 desensitize in a manner similar to that seen for mature cells, either with prolonged bath application of GABA or with repetitive ionophoretic application.7. Nearly half of the cells tested at stage 26 respond to glycine, in concentrations as low as 5 muM. This sensitivity is absent by 3(1/2) d of development.8. The responses of Rohon-Beard neurones to GABA are similar to those of other cells in that they involve a conductance increase, are mimicked by muscimol, and are blocked by picrotoxin. These responses are different in that they do not involve Cl(-) and are blocked by low concentrations of curare.9. Many of the characteristics of GABA receptors, i.e. the reversal potential, desensitization, and pharmacology, are constant during development. However, the sensitivity of the cells to GABA and the spectrum of transmitters to which they are sensitive appear to change.

Reinnervation of denervated parasympathetic neurones in cardiac ganglia from Rana pipiens

1. The sequence of events during reinnervation of the cardiac ganglion in the frog following interruption of the vagosympathetic nerve supply was studied with both electrophysiological and morphological techniques. 2. When cardiac ganglia were denervated by crushing the vagosympathetic nerve supply to the heart all synaptic endings on parasympathetic ganglion cells degenerated. Vacated post-synaptic densities were detected on denervated neurones for periods of at least 7 weeks. 3. The earliest signs of reinnervation were subthreshold responses evoked by stimulating the regenerating vagosympathetic trunks 2 1/2-3 weeks after crushing the cardiac branches of the vagus nerves. Analysis of the reversal potentials of these responses indicated that these synapses were distant from the cell body. 4. At slightly longer times (4-5 weeks), regenerating synapses could be recognized on post-ganglionic axons; no synapses were detected on the neuronal perikarya at these times. 5. By 6-7 weeks following denervation, vagal synapses reinnervated neuronal perikarya as well as post-ganglionic axons. At the same time, vacated post-synaptic densities declined in number. Furthermore, vagal stimulation at this stage evoked large, suprathreshold post-synaptic potentials. 6. These studies indicate that post-ganglionic axons are the initial sites for reinnervation of parasympathetic neurones in the heart. Only some time later are neuronal perikarya reinnervated and ganglionic transmission completely restored.

Synaptic localization of alpha-bungarotoxin binding which blocks nicotinic transmission at frog sympathetic neurons

Sympathetic neurons receive direct synaptic input from cholinergic terminal boutons of preganglionic nerve fibers. The distribution of acetylcholine receptors at these synapses is not precisely known. This study shows that alpha-bungarotoxin, which binds specifically to nicotinic receptors on skeletal muscle, also may be useful for localizing postsynaptic nicotinic receptors on principal neurons in the paravertebral sympathetic ganglia of the bullfrog. alpha-Bungarotoxin (1-5 microM) produces a block of nicotinic (fast) excitatory postsynaptic potentials that is fully reversed after 5-8 hr of washing. Dihydro-beta-erythroidine, a nicotinic antagonist, reduces the half-time of recovery from the toxin block to one-third of the control value, presumably by competing for the same receptor sites. Furthermore, the response to applied carbachol is reduced by the toxin, indicating that the block of synaptic transmission is due to a decreased response of the postsynaptic membrane. Peroxidase-labeled alpha-bungarotoxin is localized to small (0.2- to 0.5-micrometers diameter) patches beneath synaptic boutons. Peroxidase reaction product is restricted to regions of the synaptic cleft just opposite the active zones of the presynaptic terminal. In addition, peroxidase-labeled antibodies against Torpedo acetylcholine receptor bind exclusively to these same synaptic regions; evidently these patches are the areas at which nicotinic receptors are concentrated at synaptic contacts on sympathetic neurons.

Further evidence for peptidergic transmission in sympathetic ganglia

We previously proposed that, in sympathetic ganglia of the bullfrog, a peptide which resembles luteinizing hormone-releasing factor (LH-RF, luliberin) functions as the transmitter for the late slow excitatory postsynaptic potential (epsp), a signal that may last 5-10 min. To test this hypothesis further, we have compared the physiological andpharmacological effects of LH-RF with those of the natural transmitter and have found a close parallel. (i) LH-RF, when ejected with a brief pulse of pressure through a micropipette near a ganglion cell, produces a depolarizing response lasting for minutes. (ii) The LH-RF-induced response is associated with changes in input resistance similar to thoe during a late slow epsp. (iii) The amplitudes of the LH-RF-induced response and the late slow epsp vary in parallel as the membrane potential is shifted over a wide range. (iv) Both responses increase the excitability of ganglion cells. (v) The two responses interact with the cholinergic epsps in a parallel manner: they cause diminution of the slow epsp but not of the fast epsp. (vi) Both responses are blocked by an analog of LH-RF that antagonizes the effects of LH-RF in the rat.

Distribution of muscarinic acetylcholine receptors and presynaptic nerve terminals in amphibian heart

At many synapses, neurotransmitter receptor molecules in the postsynaptic membrane are selectively concentrated at a site directly opposite the presynaptic nerve terminal. In this paper, I examine acetylcholine (ACh) receptor distribution in cardiac muscle in relatin to the distribution of presynaptic axonal varicosities. The density of varicosities, stained with zinc iodide and osmium, ranges from 0.7/100 micrometer 2 in ventricle to 1.9/100 micrometer 2 in sinus venosus. It is estimated that < 3% of the muscle surface is apposed to presynaptic varicosities. ACh receptors, however, are randomly distributed on the muscle surface and not concentrated in patche. ACh receptor distribution was determined by iontophoretic application of ACh and mapping of ACh sensitivity and by [3H]QNB (quinuclidinyl benzilate) binding and autoradiography [3H]QNB binds with > 90% specificity to a single, saturable, high-affinity (Kd = 11.1 pM at 21 degrees C) class of binding sites. QNB binding sites are thought to correspond to ACh receptors, because muscarinic agonists compete for [3H]QNB binding and produce a hyperpolarization in the sinus venosus with the same order of potency. The concentrations of QNB binding sites in the sinus and atria are about twice those found in ventricle. The receptor density corresponds to the density of innervation measured by zinc iodide and osmium staining. Autoradiographic experiments show that [3H]QNB binding sites are distributed randomly over the entire surface of the muscle. This distribution of ACh receptors in cardiac muscle has important implications for the function of the cardiac neuroeffector junction.

Amino acid pharmacology of mammalian central neurones grown in tissue culture

1. Spinal and cerebellar-brainstem areas of fetal mouse were dissociated and grown in tissue culture until large enough to permit stable intracellular recording. 2. The tissue-cultured neurones, growing as a monolayer and accessible under direct vision using phase contrast optics, allowed precise placement of intracellular recording and extracellular ionophoretic pipettes. 3. Ionophoresis of GABA and glutamate revealed a non-uniform distribution of responses over the cell surface, with a lack of spatial coincidence in sensitivity between the two. GABA inhibited and glutamate excited all cells tested. 4. GABA responses evoked at the cell body and on nearby process membrane were almost uniformly hyperpolarizing, while those at some peripheral process membrane were either hyperpolarizing, depolarizing or a combination of both events. All responses were associated with an increase in membrane slope conductance. 5. Membrane polarization showed that all hyperpolarizing events extrapolated to about the same inversion potential, which averaged about 9 mV more negative than resting potential (n = 95 cells). The depolarizing phases of responses evoked at peripheral membranes extrapolated to about 0 mV (n = 5 cells). 6. The hyperpolarization and increase in membrane conductance of GABA responses at the cell body were dependent on Cl- ions and the inversion potential of the response was dependent on the Cl- ion concentration gradient. The inversion potentials of GABA, glycine and beta-alanine responses were identical. 7. When matched in magnitude for evoked conductance increase, glycine responses decayed more rapidly than GABA. Glycine and beta-alanine voltage responses both decayed faster than GABA responses of comparable size. 8. In about half the cells tested sustained or rapidly repeated application of GABA and glycine transformed hyperpolarizing responses into depolarizations which were associated with a maintained conductance increase. Results from conditioning-test experiments with pairs of GABA and glycine responses suggest that the reversal of response polarity is due to a rapid redistribution of Cl- ions. 9. The limiting slope of log-log dose-response curves for GABA-induced conductance averaged about 2, while those for glutamate-induced depolarizations averaged about 1. The results suggest that two molecules of GABA and one molecule of glutamate participate in the respective post-synaptic responses. 10. The observation indicate that mammalian C.N.S. tissue grown in culture is a suitable model to study C.N.S. membrane pharmacology with increasing precision.

Physiological properties of dissociated muscle fibres obtained from innervated and denervated adult rat muscle

1. Adult rat flexor digitorum brevis muscles were dissociated by treatment with collagenase and trituration. Several hundred isolated fibres were obtained from each muscle. 2. Most isolated fibres appeared to be intact as judged by some morphological and physiological criteria, although resting membrane potentials were about -60 mV, which is somewhat lower than normal. 3. A small percentage of the muscle fibres were branched. 4. Acetylcholine sensitivity was measured iontophoretically. The sensitivity fell abruptly outside the margin of the end-plate. Extrajunctional sensitivty was detected on all fibres, and declined smoothly away from the end-plate to an undetectable level over a distance of about 200 micron. On a few fibres, ACh sensitivity was mapped circumferentially from the end-plate. It appeared to decline with distance in a manner similar to the longitudinal sensitivity gradient. 5. Fibres dissociated from muscles denervated a week earlier were sensitive to ACh everywhere on their surfaces.

Action potentials in the rat chromaffin cell and effects of acetylcholine

1. Electrophysiological properties of the rat chromaffin cell were studied using intracellular recording techniques. 2. The resting potential in the chromaffin cell was -49 +/- 6 mV (mean +/- S.D., n = 14) in standard saline containing 10 mM-Ca whereas that in Na-free saline was -63 +/- 9 mV (n = 17). At rest, the membrane has a substantial Na permeability. 3. Action potentials were evoked by passing current through the recording electrode. In standard saline the major fraction of the action potential disappeared either upon omission of external Na ions from standard saline or addition of 1 muM tetrodotoxin (TTX). We conclude that action potentials in the chromaffin cell are due mainly to an increase in the permeability of the membrane to Na ions. 4. Small but significant regenerative action potentials were observed in Na-free saline, and when Ca in Na-free saline was replaced by Ba, prolonged action potentials occurred. We conclude that action potentials in the chromaffin cell also have a Ca component. 5. Iontophoretic application of acetylcholine (ACh) produced a transient membrane depolarization in standard saline. 6. Spontaneous action potentials were recorded extracellularly by microsuction electrodes. They occurred at a rate of 0-05-0-1/sec in almost all cells. 7. When the perfusion fluid contained 3 x 10(-7) M to 10(-4) M ACh the spike frequency increased up to about 2/sec. This stimulatory effect of ACh was blocked by 10(-7) M atropine but not by 10(-3) M hexamethonium nor by 10(-5) M-d-tubocurarine. 8. The importance of Ca entry during action potentials for catecholamine secretion is discussed

Analysis of Mauthner cell responses to iontophoretically delivered pulses of GABA, glycine and L-glutamate

1. The intracellularly recorded responses of goldfish Mauthner neurones to iontophoretically applied pulses of amino acids have been analysed: their time courses have been compared with each other, and with those predicted from diffusion theory.2. The rise time of the response to GABA is slower than that to glycine or L-glutamate. The response curves of the latter substances were very similar, and unlike that of GABA were markedly affected by increasing the distance of pipette-tip from the membrane. The results suggest that the time course of the responses to glycine and L-glutamate are determined mainly by free diffusion in the brain tissue (at least within about 200 mum of the cell), while that to GABA must be rate-limited by other factors, e.g. drug-receptor activation time.3. The possibility that the responses are influenced by some desensitizing process was investigated by applying a second (test) drug pulse during the response to a prior conditioning one. In the case of glycine and of L-glutamate there was no attenuation of the response to a second pulse at any time. With GABA, however, the second response was reduced during the period of the conditioning response; the reduction was progressively less marked the later the test pulse occurred. A similar effect with GABA was seen when glycine was used as the test pulse. The responses to long-maintained drug pulses also indicated that for GABA, but not for glycine or glutamate, there seems to be some desensitizing process present.4. Calculated time courses of responses to brief pulses of glycine and of L-glutamate (based upon diffusion theory) differed somewhat from the observed curves, largely during the falling phase. However, when the calculations were based upon second-order reactions (two molecules of drug per receptor) the diffusion model gave results very like the observed ones.5. Possible implications of these results for the role these three amino acids may have as neuro-transmitters are mentioned.

Synaptic transmission and its duplication by focally applied acetylcholine in parasympathetic neurons in the heart of the frog

Synaptic transmission has been analysed in parasympathetic nerve cells that lie in the transparent interatrial septum of the heart of the frog. Using Nomarski interference optics, one can see much cellular detail, including synaptic boutons in living preparations. 1. On each ganglion cell, the 10 to 20 synaptic boutons are usually derived from a single vagal nerve fibre. These fibres branch extensively to innervate a number of septal ganglion cells. 2. The chemical transmitter, acetylcholine (ACh), liberated by a presynaptic impulse survives for up to 40 ms, setting up an excitatory postsynaptic potential (e.p.s.p.) which triggers one and sometimes two action potentials in the postsynaptic cell. The e.p.s.p. is made up of quantal components, as at the neuromuscular junction. 3. Nerve-evoked e.p.s.p.s can be well matched in amplitude and time course by iontophoretic application of ACh to selected areas of the neuronal membrane. In particular, the miniature e.p.s.p., which is due to the focal release of a small quantity of transmitter, was accurately mimicked by iontophoretic application of ACh. By grading the amount of ACh released from an electrode one could also duplicate the wide variety of nerve-evoked postsynaptic discharges of ganglion cells. 4. The permeability changes initiated in the postsynaptic membrane by applied ACh and the synaptic transmitter appear identical, since the ionic fluxes for both responses have the same equilibrium potential. Also, the receptors which react with the synaptic transmitter are desensitized by applied ACh. 5. Cholinesterase inhibitors (Tensilon and Eserine) have a variable action on different cells, with respect both to nerve-evoked and Ach evoked potentials. The reasons for this variation are unclear, and need further study. 6. Miniature e.p.s.p.s resemble analogous potentials at nerve-muscle junctions and other synapses. A significant proportion of the min e.p.s.p.s is released as multiple units. This proportion is increased in high Ca2+, while single units alone occur in a low Ca2+-high Mg2+ environment. 7. The experiments provide information about the release of ACh from nerve terminals and its action on the postsynaptic membrane of neurons. They are in good agreement with analogous studies on skeletal neuromuscular junctions