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

Dynamic Clamp Analysis of Synaptic Integration in Sympathetic Ganglia

Advances in modern neuroscience require the identification of principles that connect different levels of experimental analysis, from molecular mechanisms to explanations of cellular functions, then to circuits, and, ultimately, to systems and behavior. Here, we examine how synaptic organization of the sympathetic ganglia may enable them to function as use-dependent amplifiers of preganglionic activity and how the gain of this amplification may be modulated by metabotropic signaling mechanisms. The approach combines a general computational model of ganglionic integration together with experimental tests of the model using the dynamic clamp method. In these experiments, we recorded intracellularly from dissociated bullfrog sympathetic neurons and then mimicked physiological synapses with virtual computer-generated synapses. It thus became possible to analyze the synaptic gain by recording cellular responses to complex patterns of synaptic activity that normally arise in vivo from convergent nicotinic and muscarinic synapses. The results of these studies are significant because they illustrate how gain generated through ganglionic integration may contribute to the feedback control of important autonomic behaviors, in particular to the control of the blood pressure. We dedicate this paper to the memory of Professor Vladimir Skok, whose rich legacy in synaptic physiology helped establish the modern paradigm for connecting multiple levels of analysis in studies of the nervous system.

The sequence of events that underlie quantal transmission at central glutamatergic synapses

The properties of synaptic transmission were first elucidated at the neuromuscular junction. More recent work has examined transmission at synapses within the brain. Here we review the remarkable progress in understanding the biophysical and molecular basis of the sequential steps in this process. These steps include the elevation of Ca2+ in microdomains of the presynaptic terminal, the diffusion of transmitter through the fusion pore into the synaptic cleft and the activation of postsynaptic receptors. The results give insight into the factors that control the precision of quantal transmission and provide a framework for understanding synaptic plasticity.

Estimating use-dependent synaptic gain in autonomic ganglia by computational simulation and dynamic-clamp analysis

Biological gain mechanisms regulate the sensitivity and dynamics of signaling pathways at the systemic, cellular, and molecular levels. In the sympathetic nervous system, gain in sensory-motor feedback loops is essential for homeostatic regulation of blood pressure and body temperature. This study shows how synaptic convergence and plasticity can interact to generate synaptic gain in autonomic ganglia and thereby enhance homeostatic control. Using a conductance-based computational model of an idealized sympathetic neuron, we simulated the postganglionic response to noisy patterns of presynaptic activity and found that a threefold amplification in postsynaptic spike output can arise in ganglia, depending on the number and strength of nicotinic synapses, the presynaptic firing rate, the extent of presynaptic facilitation, and the expression of muscarinic and peptidergic excitation. The simulations also showed that postsynaptic refractory periods serve to limit synaptic gain and alter postsynaptic spike timing. Synaptic gain was measured by stimulating dissociated bullfrog sympathetic neurons with 1-10 virtual synapses using a dynamic clamp. As in simulations, the threshold synaptic conductance for nicotinic excitation of firing was typically 10-15 nS, and synaptic gain increased with higher levels of nicotinic convergence. Unlike the model, gain in neurons sometimes declined during stimulation. This postsynaptic effect was partially blocked by 10 microM Cd2+, which inhibits voltage-dependent calcium currents. These results support a general model in which the circuit variations observed in parasympathetic and sympathetic ganglia, as well as other neural relays, can enable functional subsets of neurons to behave either as 1:1 relays, variable amplifiers, or switches.

Long-term potentiation of nicotinic synaptic transmission in rat superior cervical ganglia produced by phorbol ester and tetanic stimulation

The long-term potentiation of nicotinic synaptic transmission induced by both active phorbol ester (4beta-phorbol-12,13-dibutyrate, PdBu) and tetanic trains of preganglionic stimulation was studied in single neurons of the superior cervical ganglion (SCG) of the rat using intracellular recording techniques. PdBu significantly increased the mean amplitude of both the unitary evoked fast excitatory postsynaptic potentials (EPSPs) and the fast excitatory postsynaptic currents (EPSCs) to 17.0+/-3.3 mV (control 8.4+/-1.9 mV, n=5) and 2.8+/-0.4 nA (control 0.8+/-0.1 nA, n=10), respectively. There was no significant change in either the resting membrane potential, input resistance, or the threshold for the initiation of an action potential. The response to exogenously applied acetylcholine (ACh) was also not changed following exposure to PdBu. In low-calcium, high-magnesium solutions, PdBu significantly increased the quantal content of EPSPs approximately threefold from a control of 0.9+/-0.2 (n=5) to 2.6+/-0.6 (n=5). The quantal content of EPSCs was also increased to 1.3+/-0.2 (control 0.5+/-0.1, n=10). PdBu increased the frequency of miniature EPSPs (mEPSPs) to 196+/-47% (n=6) of control, while the amplitude, rise time, rate of rise, and decay of mEPSPs were not significantly changed. Tetanic stimulation significantly increased the amplitude of the unitary synaptic EPSPs and EPSCs without significantly changing the resting membrane potential, input resistance, threshold for initiation of an action potential, or the response to exogenously applied ACh. Tetanic stimulation significantly increased quantal content of EPSPs and EPSCs threefold. The results obtained with tetanically induced LTP are similar to the results obtained with phorbol ester-induced LTP in these ganglion neurons. These results suggest that both tetanically induced and phorbol ester-induced LTP, in the rat, share similar mechanisms which involve, at least in part, activation of PKC-dependent mechanisms to increase quantal release from sympathetic preganglionic axon terminals.

Effects of trachynilysin, a protein isolated from stonefish (Synanceia trachynis) venom, on frog atrial heart muscle

The effects of trachynilysin (TLY), a protein toxin isolated from stonefish (Synanceia trachynis) venom, were studied on the electrical and mechanical activities of frog atrial fibres. TLY (1 microg/ml) hyperpolarized the membrane, shortened the action potential (AP) duration (APD), exerted a negative inotropic effect and elicited contracture. These effects did not develop in the presence of atropine. TLY shortened the APD of fibres isolated from a frog completely paralyzed with botulinum type A toxin, in the presence of Ca2+ but not when Ca2+ was replaced by Sr2+. TLY increased the basal and the peak of the fluorescence ratio of stimulated fibres loaded with fura-2. Confocal laser scanning microscopy revealed the existence of a diffuse innervation in atrial tissue. Our results suggest that TLY enhances the release of acetylcholine from atrial cholinergic nerve terminals and activates indirectly muscarinic receptors leading to a shortening of APD. They also show that the mechanical effects induced by TLY are due to an increase of the Ca2+ influx and to a rise in intracellular Ca2+ levels which leads to (i) a slowing of the Na+/Ca2+ exchange activity, which accounts for the contracture and (ii) the activation of a Ca2+-dependent K+ current involved in the APD shortening.

Secondary nicotinic synapses on sympathetic B neurons and their putative role in ganglionic amplification of activity

The strength and number of nicotinic synapses that converge on secretomotor B neurons were assessed in the bullfrog by recording intracellularly from isolated preparations of paravertebral sympathetic ganglia 9 and 10. One input to every B neuron invariably produced a suprathreshold EPSP and was defined as the primary nicotinic synapse. In addition, 93% of the cells received one to four subthreshold inputs that were defined as secondary nicotinic synapses. This contradicts the prevailing view, which has long held that amphibian B neurons are singly innervated. More important, the results revealed that B cells provide the simplest possible experimental system for examining the role of secondary nicotinic synapses on sympathetic neurons. Combining the convergence data with previous estimates of divergence indicates that the average preganglionic B neuron forms connections with 50 ganglionic B neurons and that the majority of these nicotinic synapses are secondary in strength. Secondary EPSPs evoked by low-frequency stimulation ranged from 0.5 to 10 mV in amplitude and had an average quantal content of 1. Nonetheless, secondary synapses could trigger action potentials via four mechanisms: spontaneous fluctuations of EPSP amplitude, two-pulse facilitation, coactivation with other secondary synapses, and coactivation with a slow peptidergic EPSP. The data were used to formulate a stochastic theory of integration, which predicts that ganglia function as amplifiers of the sympathetic outflow. In this two-component scheme, primary nicotinic synapses mediate invariant synaptic gain, and secondary nicotinic synapses mediate activity-dependent synaptic gain. The model also provides a common framework for considering how facilitation, metabotropic mechanisms, and preganglionic oscillators regulate synaptic amplification in sympathetic ganglia.

Stimulant-induced exocytosis from neuronal somata, dendrites, and newly formed synaptic nerve terminals in chronically decentralized sympathetic ganglia of the rat

Loss of preganglionic neurones underlies the autonomic failure of human multiple system atrophy. In rat sympathetic ganglia decentralization leads to new synapse formation. We explored whether these synapses are functional, and whether chronically decentralized neurones respond normally to activation, in terms of exocytosis. Potassium depolarization and cholinergic agonists were applied to freshly excised rat superior cervical sympathetic ganglia, preganglionically denervated with prevented reinnervation 5 months earlier. Ganglia were incubated and stimulated in the presence of tannic acid, which stabilizes released vesicle cores for subsequent electron microscopy. In denervated ganglia exocytosis was observed from newly formed synaptic nerve terminals, and from nonsynaptic surfaces of neurone somata and dendrites. The results demonstrated that the new intraganglionic synapses, which are mostly catecholaminergic, can function and that chronically decentralized sympathetic neurones remain capable of stimulant-induced exocytosis from somata and dendrites. The maximal release upon potassium depolarization did not differ significantly between denervated and contralateral ganglia. Relative to this, the exocytotic responses of decentralized somata and dendrites to nicotine resembled those of contralateral ganglia. Responses to muscarine were significantly less in denervated than in contralateral ganglia, indicating inhibition in dendrites. Responses to carbachol suggested interactions between nicotinic and excitatory muscarinic effects. Nerve terminals in denervated ganglia showed high basal release. Their responses to muscarine and carbachol resembled those of the decentralized neurones, from which most may originate. Their response to nicotine evidenced inhibition. Their actions, coupled with nonsynaptic effects of soma-dendritic exocytosis, might modulate responses of the decentralized neurone population to other surviving inputs. This modulation could be influential in disease-induced decentralization in man.

Contribution of active zone subpopulation of vesicles to evoked and spontaneous release

Our previous work on Drosophila synapses has suggested that two vesicle populations possessing different recycling pathways, a fast pathway emanating from the active zone and a slower pathway emanating from sites away from the active zone, exist in the terminal. The difference in recycling time between these two pathways has allowed us to create a synapse that possesses the small, active zone subpopulation without the larger, nonactive zone population. Synapses were depleted using the temperature-sensitive endocytosis mutant, shibire, which reversibly blocks vesicle recycling at the restrictive temperature. In the depleted state, both the excitatory junction potential (EJP) and spontaneous release are abolished. After shibire-induced depletion, the active zone population begins to reform within 30 s at the permissive temperature, whereas the nonactive zone population does not begin to reform until approximately 10-15 min later. Evoked release recovered at approximately the same time as the active zone population. During the time when the active zone population existed in the terminal without the nonactive zone population, enough transmitter release was available to sustain a normal evoked response for many minutes at frequencies above those produced during normal activity (flight) by this motor neuron. When only the active zone population existed in the terminal, the frequency of spontaneous release was greatly attenuated and possessed abnormal release characteristics. Spontaneous release recovered its predepletion frequency and release characteristics only after the nonactive zone population was reformed.

Development of the quantal properties of evoked and spontaneous synaptic currents at a brain synapse

In many studies of central synaptic transmission, the quantal properties of miniature synaptic events do not match those derived from synaptic events evoked by action potentials. Here we show that at mossy fiber-granule cell (MF-gc) synapses of mature cerebellum, evoked excitatory postsynaptic currents (EPSCs) are multiquantal, and their amplitudes vary in discrete steps, whereas miniature (m)EPSCs are monoquantal or multiquantal with quantal parameters identical to those of the EPSCs. In contrast, at immature MF-gc synapses, EPSCs are multiquantal, but their amplitudes do not vary in discrete steps, whereas most mEPSCs seem to be monoquantal with a broad and skewed amplitude distribution. The results demonstrate that quantal variance decreases during synaptic development. They also directly confirm the quantal hypothesis of neurotransmission at a mature brain synapse.

Cholinesterase affects dynamic transduction properties from vagal stimulation to heart rate

Recent investigations in our laboratory using a Gaussian white noise technique showed that the transfer function representing the dynamic properties of transduction from vagus nerve activity to heart rate had characteristics of a first-order low-pass filter. However, the physiological determinants of those characteristics remain to be elucidated. In this study, we stimulated the vagus nerve according to a Gaussian white noise pattern to estimate the transfer function from vagal stimulation to the heart rate response in anesthetized rabbits and examined how changes in acetylcholine kinetics affected the transfer function. We found that although increases in the mean frequency of vagal stimulation from 5 to 10 Hz did not change the characteristics of the transfer function, administration of neostigmine (30 microg . kg-1 . h-1 iv), a cholinesterase inhibitor, increased the dynamic gain from 8.19 +/- 3.66 to 11.7 +/- 4.88 beats . min-1 . Hz-1 (P < 0.05), decreased the corner frequency from 0.12 +/- 0.05 to 0.04 +/- 0.01 Hz (P < 0.01), and increased the lag time from 0.17 +/- 0.12 to 0.27 +/- 0.08 s (P < 0.05). These results suggest that the rate of acetylcholine degradation at the neuroeffector junction, rather than the amount of available acetylcholine, plays a key role in determining the dynamic properties of transduction from vagus nerve activity to heart rate.

On the origin of skewed distributions of spontaneous synaptic potentials in autonomic ganglia

The histograms of spontaneous synaptic potentials at synapses in autonomic ganglia are described by distributions consisting of mixtures of Gaussians, rather than by single Gaussian distributions. The possible origin of these mixed distributions is investigated, using Monte-Carlo simulations of the action of spontaneously released units of transmitter. A single unit of acetylcholine of fixed size, released from an active zone with receptor patches both beneath and adjacent to the zone, does not give rise to the observed histograms. But if the unit is of variable size, consisting of integer multiples of smaller units, and release is from an active zone onto either the receptor patch beneath, or in addition onto adjacent patches, then the histogram is well described by a mixture of Gaussians. However, this explanation is unlikely to be correct as present evidence suggests that in most cases the released unit of transmitter saturates the postsynaptic receptor patch beneath the active zone. The final case considered is where a unit of transmitter is spontaneously released from an active zone, simultaneously with a unit in an adjacent zone less than one micron away. The histogram of potentials then conforms to those observed even when there are differences in the sizes of the receptor patches. It is suggested that this kind of release could provide an explanation for distributions of spontaneous potentials that are mixtures of Gaussians.

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.

Calcium channel currents in acutely dissociated intracardiac neurons from adult rats

With the use of the whole cell patch-clamp technique, multiple subtypes of voltage-activated calcium channels, as indicated by measuring Ba2+ currents, were pharmacologically identified in acutely dissociated intracardiac neurons from adult rats. All tested neurons that were held at -80 mV displayed only high-voltage-activated (HVA) Ca2+ channel currents that were completely blocked by 100 microM CdCl2. The current density of HVA Ca2+ currents was dependent on the external Ca2+ concentration. The Ba2+ (5 mM) currents were half-activated at -16.3 mV with a slope of 5.6 mV per e-fold change. The steady-state inactivation was also voltage dependent with half-inactivation at -33.7 mV and a slope of -12.1 mV per e-fold change. The most effective L-type channel activator, FPL 64176 (2 microM), enhanced the Ba2+ current in a voltage-dependent manner. When cells were held at -80 mV, the saturating concentration (10 microM) of nifedipine blocked approximately 11% of the control Ba2+ current. The major component of the Ca2+ channels was N type (63%), which was blocked by a saturating concentration (1 microM) of omega-conotoxin GVIA. Approximately 19% of the control Ba2+ current was sensitive to omega-conotoxin MVIIC (5 microM) but insensitive to low concentrations (30 and 100 nM) of omega-agatoxin IVA (omega-Aga IVA). In addition, a high concentration (1 microM) of omega-Aga IVA occluded the effect of omega-conotoxin MVIIC. Taken together, these results indicate that the omega-conotoxin MVIIC-sensitive current represents only the Q type of Ca2+ channels. The current that was insensitive to nifedipine and various toxins represents the R-type current (7%), which was sensitive to 100 microM NiCl2. In conclusion, the intracardiac neurons from adult rats express at least four different subtypes (L, N, Q, and R) of HVA Ca2+ channels. This information is essential for understanding the regulation of synaptic transmission and excitability of intracardiac neurons by different neurotransmitters and neural regulation of cardiac functions.

Different types of ganglion cell in the cardiac plexus of guinea-pigs

1. Intracellular recordings were made from the parasympathetic ganglion cells that lie in the epicardium of the left atrium of guinea-pig heart near the interatrial septum. 2. Three distinct types of neurone were identified on the basis of their electrophysiological properties. In one group of neurones, S cells, somatic action potentials were followed by brief after-hyperpolarizations. In the other two sets of neurones, somatic action potentials were followed by prolonged after-hyperpolarizations. The neurones with prominent after-hyperpolarization were further subdivided: one group of neurones, P cells, showed inward rectification at membrane potentials near the resting membrane potential whilst neurones in the other group, SAH cells, did so only at more negative potentials. 3. In the group of neurones that displayed inward rectification at potentials near rest, rectification resulted from the activation of an inward current, which resembled the hyperpolarization-activated inward current present in cardiac muscle pacemaker cells. 4. The three different types of neurone received different patterns of synaptic input. Each SAH cell received a synaptic excitatory connection from the vagus which in most cells released sufficient transmitter to initiate an action potential in that cell; several SAH cells also received a separate connection, which could be activated by local stimulation. Although most S cells failed to receive a synaptic input from the vagus, all of those tested received an excitatory synaptic input which could be activated by local stimulation. Virtually all P cells failed to receive a synaptic input from the vagus; in addition, local stimulation failed to initiate synaptic potentials in P cells. 5. When the structure of cardiac ganglion cells was determined, by loading the cells with either biocytin or neurobiotin, it was found that most cells lacked extensive dendritic processes. S cells were invariably monopolar, most P cells were dipolar or pseudodipolar, whereas many SAH cells were multipolar. 6. In many neurones an on-going discharge of action potentials was detected in the absence of obvious stimulation. In S and SAH cells, the action potentials resulted from an on-going discharge of excitatory synaptic potentials. However, when a spontaneous discharge of action potentials was detected in P cells a discharge of excitatory synaptic potentials was not detected. 7. The results are discussed in relation to the idea that the three different types of cell may have different functions and that some of the cells may be organized in such a way as to permit the local handling of neuronal information within the heart.

Probabilistic secretion of quanta: spontaneous release at active zones of varicosities, boutons, and endplates

The amplitude-frequency histogram of spontaneous miniature endplate potentials follows a Gaussian distribution at mature endplates. This distribution gives the mean and variance of the quantum of transmitter. According to the vesicle hypothesis, this quantum is due to exocytosis of the contents of a single synaptic vesicle. Multimodal amplitude-frequency histograms are observed in varying degrees at developing endplates and at peripheral and central synapses, each of which has a specific active zone structure. These multimodal histograms may be due to the near synchronous exocytosis of more than one vesicle. In the present work, a theoretical treatment is given of the rise of intraterminal calcium after the stochastic opening of a calcium channel within a particular active zone geometry. The stochastic interaction of this calcium with the vesicle-associated proteins involved in exocytosis is then used to calculate the probability of quantal secretions from one or several vesicles at each active zone type. It is shown that this procedure can account for multiquantal spontaneous release that may occur at varicosities and boutons, compared with that at the active zones of motor nerve terminals.

Two-photon scanning photochemical microscopy: mapping ligand-gated ion channel distributions

The locations and densities of ionotropic membrane receptors, which are responsible for receiving synaptic transmission throughout the nervous system, are of prime importance in understanding the function of neural circuits. It is shown that the highly localized liberation of "caged" neurotransmitters by two-photon absorption-mediated photoactivation can be used in conjunction with recording the induced whole-cell current to determine the distribution of ligand-gated ion channels. The technique is potentially sensitive enough to detect individual channels with diffraction-limited spatial resolution. Images of the distribution of nicotinic acetylcholine receptors on cultured BC3H1 cells were obtained using a photoactivatable precursor of the nicotinic agonist carbamoylcholine.

Resting membrane potential and potassium currents in cultured parasympathetic neurones from rat intracardiac ganglia

1. Whole-cell K+ currents contributing to the resting membrane potential and repolarization of the action potential were studied in voltage-clamped parasympathetic neurones dissociated from neonatal rat intracardiac ganglia and maintained in tissue culture. 2. Rat intracardiac neurones had a mean resting membrane potential of -52 mV and mean input resistance of 850 M omega. The current-voltage relationship recorded during slow voltage ramps indicated the presence of both leakage and voltage-dependent currents. The contribution of Na+, K+ and Cl- to the resting membrane potential was examined and relative ionic permeabilities PNa/PK = 0.12 and PCl/PK < 0.001 were calculated using the Goldman-Hodgkin-Katz voltage equation. Bath application of the potassium channel blockers, tetraethylammonium ions (TEA; 1 mM) or Ba2+ (1 mM) depolarized the neurone by approximately 10 mV. Inhibition of the Na(+)-K+ pump by exposure to K(+)-free medium or by the addition of 0.1 mM ouabain to the bath solution depolarized the neurone by 3-5 mV. 3. In most neurones, depolarizing current pulses (0.5-1 s duration) elicited a single action potential of 85-100 mV, followed by an after-hyperpolarization of 200-500 ms. In 10-15% of the neurones, sustained current injection produced repetitive firing at maximal frequency of 5-8 Hz. 4. Tetrodotoxin (TTX; 300 nM) reduced, but failed to abolish, the action potential. The magnitude and duration of the TTX-insensitive action potential increased with the extracellular Ca2+ concentration, and was inhibited by bath application of 0.1 mM Cd2+. The repolarization rate of the TTX-insensitive action potential was reduced, and after-hyperpolarization was replaced by after-depolarization upon substitution of internal K+ by Cs+. The after-hyperpolarization of the action potential was reduced by bath application of Cd2+ (0.1 mM) and abolished by the addition of Cd2+ and TEA (10 mM). 5. Depolarization-activated outward K+ currents were isolated by adding 300 nM TTX and 0.1 mM Cd2+ to the external solution. The outward currents evoked by step depolarizations increased to a steady-state plateau which was maintained for > 5 s. The instantaneous current-voltage relationship, examined under varying external K+ concentrations, was linear, and the reversal (zero current) potential shifted in accordance with that predicted by the Nernst equation for a K(+)-selective electrode. The shift in reversal potential of the tail currents as a function of the extracellular K+ concentration gave a relative permeability, PNa/PK = 0.02 for the delayed outward K+ channel(s).(ABSTRACT TRUNCATED AT 400 WORDS)

Synaptic potentials produced in jaw-closer and jaw-opener motoneurons by palatal stimulation

Excitation and inhibition of temporal and digastric motoneurons (Temp. and Dig. Mns) during transient jaw closing, the so-called jaw-closing reflex, were studied in cats. Application of diffuse pressure stimulation to the posterior palatal surface produced the jaw-closing reflex and it was found that mechanosensory inputs from the posterior palatal mucosa produce depolarizing potentials on the Temp. Mns responsible for jaw closure during the jaw-closing reflex. We have demonstrated that in one-third of 27 explored Temp. Mns the initial bursts of spikes were elicited before the onset of jaw closure, suggesting that these cells contribute to initiate jaw closure during the jaw-closing reflex. The remaining cells probably contributed to maintain the occlusal phase. Furthermore, it was found that mechanosensory inputs from the posterior palatal mucosa produce a hyperpolarization-depolarization sequence in the Dig. Mns responsible for the jaw-closing reflex. In addition, when pressure stimulation was applied to the anterior palatal mucosa, sustained jaw opening was elicited and an increase of firing frequency of Dig. Mns occurred 40 ms before the onset of jaw opening and continued for 80 ms.

Gross and microscopic anatomy of the vagal innervation of the rat heart

Intrinsic cardiac ganglia and their vagal innervation are described from gross and microscopic dissections and functional studies in the anesthetized, open-chest, adult rat. Dissecting microscope sketches of the ventral and dorsal aspects of the rat heart provide gross descriptions of the anatomical course of the vagal cardiac nerves. Histological sectioning of adipose tissue packets surrounding the terminal endings of vagal branches distributed to the roots of the great cardiac vessels (aorta, pulmonary artery, precaval veins) revealed clusters of autonomic ganglia. These packets or "fat pads" were located: (1) along the dorsal surface of the right precava and extending medially toward the aortic root, (2) deep to the aortic arch, (3) in the angle between the root of the left precava and the pulmonary artery on the superior-dorsal surface of the left atrium, and (4) in the rostro-dorsal interatrial septum. Vagal distributions of small terminal branches were traced to each of these pads, which contained numerous autonomic ganglia. Electrical excitation of right or left cervical vagus elicited varying degrees of sinus slowing, slowing of A-V conduction, and suppression in atrial contractile force. Very small quantities (0.5 mg in 10 microliters saline) of the ganglionic blocking agent, hexamethonium (C6) were injected selectively into a single fat pad, followed by repetition of right or left vagal stimulation, with careful analysis of changes in heart rate (paced and unpaced), A-V conduction, and contractile force.

Physiological properties of newly formed synapses between sympathetic preganglionic neurons and sympathetic ganglion neurons

We have examined the physiological properties of transmission at newly formed synapses between sympathetic preganglionic neurons and sympathetic ganglion neurons in vitro. Chick neurons were labeled with fluorescent carbocyanine dyes before they were placed into culture (Honig and Hume, 1986), and were studied by making intracellular recordings during the first 2 weeks of coculture. Evoked monosynaptic excitatory postsynaptic potentials (EPSPs) were not observed until 48 h of coculture. Beyond this time, the frequency with which connected pairs could be found did not vary greatly with time. With repetitive stimulation, the evoked monosynaptic EPSPs fluctuated in amplitude from trial to trial and showed depression at frequencies as low as 1 Hz. To gain further information about the quantitative properties of transmission at newly formed synapses, we analyzed the pattern of fluctuations of delayed release EPSPs. In mature systems, delayed release EPSPs are known to represent responses to single quanta, or to the synchronous release of a small number of quanta. For more than half of the connections we studied, the histograms of delayed release EPSPs were extremely broad. This result suggested that either quantal responses are drawn from a continuous distribution that has a large coefficient of variation or that there are several distinct size classes of quantal responses. The pattern of fluctuations of monosynaptic EPSPs was consistent with both of these possibilities, and was inconsistent with the possibility that monosynaptic EPSPs are composed of quantal subunits with very little intrinsic variation. Although variation in the size of responses to single quanta might arise in a number of ways, one attractive explanation for our results is that the density and type of acetylcholine receptors varies among the different synaptic sites on the surface of developing sympathetic ganglion neurons.

Acetylcholine-evoked currents in cultured neurones dissociated from rat parasympathetic cardiac ganglia

1. The properties of acetylcholine (ACh)-activated ion channels of parasympathetic neurones from neonatal rat cardiac ganglia grown in tissue culture were examined using patch clamp recording techniques. Membrane currents evoked by ACh were mimicked by nicotine, attenuated by neuronal bungarotoxin, and unaffected by atropine, suggesting that the ACh-induced currents are mediated by nicotinic receptor activation. 2. The current-voltage (I-V) relationship for whole-cell ACh-evoked currents exhibited strong inward rectification and a reversal (zero current) potential of -3 mV (NaCl outside, CsCl inside). The rectification was not alleviated by changing the main permeant cation or by removal of divalent cations from the intracellular or extracellular solutions. Unitary ACh-activated currents exhibited a linear I-V relationship with slope conductances of 32 pS in cell-attached membrane patches and 38 pS in excised membrane patches with symmetrical CsCl solutions. 3. Acetylcholine-induced currents were reversibly inhibited in a dose-dependent manner by the ganglionic antagonists, mecamylamine (Kd = 37 nM) and hexamethonium (IC50 approximately 1 microM), as well as by the neuromuscular relaxant, d-tubocurarine (Kd = 3 microM). Inhibition of ACh-evoked currents by hexamethonium could not be described by a simple blocking model for drug-receptor interaction. 4. The amplitude of the ionic current through the open channel was dependent on the extracellular Na+ concentration. The direction of the shift in reversal potential upon replacement of NaCl by mannitol indicates that the neuronal nicotinic receptor channel is cation selective and the magnitude suggests a high cation to anion permeability ratio. The cation permeability (PX/PNa) followed the ionic selectivity sequence Cs+ (1.06) greater than Na+ (1.0) greater than Ca2+ (0.93). Anion substitution experiments showed a relative anion permeability, PCl/PNa less than or equal to 0.05. 5. The nicotinic ACh-activated channels described mediate the responses of postganglionic parasympathetic neurones of the mammalian heart to vagal stimulation.

Adenosine triphosphate-evoked currents in cultured neurones dissociated from rat parasympathetic cardiac ganglia

1. The excitatory response of cultured neurones of rat parasympathetic cardiac ganglia to extracellular adenosine 5'-triphosphate (ATP) was examined using the whole-cell isolated membrane patch recording configurations of the patch clamp technique. The short latency between ATP application and activation of the membrane current (less than 20 ms) suggests a direct coupling between purinergic receptor and ion channel. The response was maintained during exposure to ATP suggesting that receptor desensitization is not a factor in current decay. 2. The current-voltage (I-V) relationship for macroscopic ATP-evoked currents showed strong inward rectification in the presence and absence of external divalent cations and a reversal potential of +10 mV (NaCl outside, CsCl inside). Unitary ATP-activated currents in cell-attached membrane patches exhibited a linear (ohmic) I-V relationship with a slope conductance of approximately 60 pS. 3. The order of agonist potency for the purinergic receptor-mediated response was 2-methylthioATP = ATP greater than ADP greater than AMP greater than adenosine = alpha,beta-methylene ATP greater than beta,gamma-methylene ATP, a sequence consistent with a P2y receptor subtype. ATP-evoked currents were attenuated by alpha,beta-methylene ATP (IC50 approximately 10 microM) and reversibly inhibited in a dose-dependent manner by Reactive Blue 2 (Kd = 1 microM). 4. The amplitude of the ATP-evoked current was dependent on the extracellular Na+ concentration. The direction of the shift in reversal potential when NaCl was replaced with mannitol indicated that the purinergic receptor channel is cation selective. The cation permeability relative to Na+ followed the ionic selectivity sequence Ca2+ (1.48) greater than Na+ (1.0) greater than Cs+ (0.67). Anions were not measurably permeant. 5. ATP and ACh-evoked responses in rat intracardiac neurones are mediated by distinct receptor channels. The ATP-activated channels in cardiac neurones may contribute to non-cholinergic, non-adrenergic neurotransmission and mediate, in part, the vagal innervation of the mammalian heart.

Quantal analysis of inhibitory synaptic transmission in the dentate gyrus of rat hippocampal slices: a patch-clamp study

1. Synaptically connected neurones were identified in the granule cell layer of slices of 17- to 21-day-old rat hippocampus. Whole-cell current recording using the patch-clamp technique revealed synaptic currents ranging from less than 10 to 200 pA in symmetrical Cl- conditions, at a holding potential of -50 mV. These currents were blocked by 2 microM-bicuculline, indicating that they result from the activation of postsynaptic gamma-aminobutyric acid receptor (GABAA-receptor) channels. 2. Addition of tetrodotoxin (TTX, 1 microM) resulted in the loss of most currents of more than 40 pA in amplitude. Currents which disappeared after TTX treatment were assumed to be the result of spontaneous presynaptic action potentials. The currents seen in the absence of TTX are referred to as spontaneously occurring inhibitory postsynaptic currents (IPSCs); those remaining in the presence of TTX were defined as miniature IPSCs. 3. Similar currents were observed when recording in the whole-cell configuration while extracellular stimulation was applied to a nearby neurone. These currents were also completely blocked by 2 microM-bicuculline and by 0.5 microM-TTX. They were thus defined as stimulus-evoked IPSCs. 4. The half rise time of both miniature and stimulus-evoked IPSCs was fast (less than 1 ms). The time course of decay of both miniature IPSCs and stimulus-evoked IPSCs could be well fitted with the sum of two exponentials. At a membrane potential of -50 mV, the mean decay time constants of the two components were 2.0 +/- 0.38 and 54.4 +/- 18 ms (mean +/- S.D.) for miniature IPSCs (six cells) and 2.2 +/- 1.3 and 66 +/- 20 ms (three cells) for stimulus-evoked IPSCs. 5. Stimulus-evoked IPSCs varied in amplitude from less than ten to hundreds of picoamperes. In eight of eleven cells histograms of IPSC amplitudes showed several clear peaks which, when fitted with the sum of Gaussian curves, were found to be equidistant. This is consistent with the view that stimulus-evoked IPSC amplitudes vary in a quantal fashion. The quantal size varied between 7 and 20 pA, at a membrane potential of -50 mV. 6. Decreasing the Ca2+ and increasing the Mg2+ concentration in the extracellular solution decreased the number of peaks in the IPSC amplitude histogram but did not affect the size of the quantal event.(ABSTRACT TRUNCATED AT 400 WORDS)

Characteristics of miniature inhibitory postsynaptic currents in CA1 pyramidal neurones of rat hippocampus

1. Recordings were made in vitro from chloride-loaded CA1 rat hippocampal pyramidal neurones in the presence of tetrodotoxin (TTX) to examine miniature inhibitory postsynaptic currents (IPSCs). 2. Most spontaneous synaptic events recorded before TTX was applied, and all events that were resolved in the presence of TTX, were blocked by the GABAA receptor antagonist bicuculline. 3. At 25 degrees C, averaged miniature IPSCs had time to peak of about 3 ms and in most cases decayed with a single time constant close to 25 ms. 4. With a driving force for chloride ions between 70 and 80 mV, the mean miniature IPSC amplitude was 19.6-27.9 pA, yielding a conductance of 258-326 pS. The mean amplitude of unitary IPSCs recorded before TTX was applied was in the range of 31-73 pA. 5. When intervals between miniature IPSCs were compared with an exponential distribution, there was an excess of events at intervals shorter than 5 ms. Some individual events appeared to represent the nearly simultaneous release of two inhibitory quanta. 6. Miniature IPSC amplitude distributions were better fitted with the sum of two Gaussians than with one Gaussian. The variance in amplitude of a single quantal event exceeded that of the baseline noise. 7. Comparison of the conductance changes corresponding to the first Gaussian distribution with single GABA channel data suggests that one inhibitory quantum opens twelve to twenty chloride channels and that GABA molecules bind once to a postsynaptic receptor.

Differential effects of denervation on acetylcholinesterase activity in parasympathetic and sympathetic ganglia of the frog, Rana pipiens

The transsynaptic regulation of acetylcholinesterase (AChE) was studied by recording the changes in enzymatic activity following denervation in two types of autonomic ganglia in the frog, Rana pipiens. Opposite effects on AChE were found in the parasympathetic cardiac ganglion and in the sympathetic lumbar ganglion; denervation produced a significant increase in AChE activity in cardiac ganglia but a significant decrease in lumbar ganglia. The relative effects of denervation on intracellular and total AChE were examined by selectively inhibiting extracellular AChE with echothiophate, a poorly lipid-soluble cholinesterase inhibitor. Denervation resulted in a significant increase in intracellular AChE in cholinergic cardiac ganglia but had no effect on intracellular AChE activity in adrenergic lumbar ganglia. Histochemical studies revealed little change in extracellular AChE staining upon denervation in the cardiac ganglion, whereas in the lumbar ganglia there was a loss of AChE-specific reaction product. These results raise the possibility that the transsynaptic control of AChE activity by innervation in the frog is influenced by the transmitter synthetic properties of the postsynaptic ganglion cells.

Muscarinic modulation of calcium current in neurones from the interatrial septum of bull-frog heart

1. The effects of activation of muscarinic receptors on the voltage-dependent calcium current, ICa, in parasympathetic neurones were examined. 2. Neurones were enzymatically isolated from the interatrial septum of bull-frog (Rana catesbeiana) heart, and were maintained in short-term (1-6 day) tissue culture. ICa was recorded from the cells using whole-cell patch-clamp methods (Clark, Tse & Giles, 1990). 3. External application of 2 nM to 10 microM acetylcholine (ACh) reduced the amplitude and slowed the time course of activation of ICa. These effects were dependent on membrane potential; they were most pronounced at potentials near the peak of the current-voltage relation for ICa (i.e. +10 to +15 mV), whereas at more-negative potentials (i.e. -15 to -25 mV) the effects on both amplitude and time course were relatively small. 4. Atropine (1 microM) completely blocked the action of 1 microM-ACh, indicating that the effects of ACh on ICa were mediated by activation of muscarinic receptors. 5. Other muscarinic agonists, such as carbamylcholine (0.1-10 microM), DL-muscarine (0.1-2.5 microM) and oxotremorine (5 microM), had similar effects on ICa to ACh. 6. A guanine nucleotide-binding protein (G-protein) is involved in this muscarinic inhibition of ICa. Inclusion of the non-hydrolysable guanosine triphosphate analogue guanosine 5'-O-(3-thiotriphosphate) (GTP-gamma-S; 200 microM) in the intracellular solutions mimicked the effects of ACh, and application of external ACh in the presence of internal GTP-gamma-S produced smaller changes in ICa than in control conditions. Inclusion of another non-hydrolysable analogue, guanosine 5'-O-(2-thiodiphosphate) (GDP-beta-S; 0.5-5 mM), blocked the inhibitory effect of ACh on ICa. 7. The G-protein involved in the inhibition of ICa was sensitive to pertussis toxin (islet-activating protein; IAP). The inhibition of ICa by carbamylcholine (5 microM) was reduced by about 90% after incubating cells for 12-15 h in culture medium containing 200 ng/ml IAP. 8. The possible roles of cyclic AMP or cyclic GMP-dependent protein kinases, or protein kinase C, in the muscarinic inhibition of ICa were tested, but these enzymes appear not to be directly involved.

Electrophysiology of parasympathetic neurones isolated from the interatrial septum of bull-frog heart

1. Whole-cell voltage-clamp techniques were used to study the voltage-dependent membrane conductances in parasympathetic neurones enzymatically isolated from the interatrial septum of bull-frog heart and maintained in short-term (1-10 day) tissue culture. 2. The resting potential of the isolated neurones averaged -55.4 +/- 1.1 mV (+/- S.E.M., n = 11). Action potentials evoked in the isolated cells by brief (1-2 ms) current injections were similar to those recorded from neurones in the 'intact' septum. The amplitude of action potentials of isolated neurones averaged about 113 mV, with a peak depolarization of +32.8 +/- 2.8 mV and after-hyperpolarization of -80.0 +/- 2.8 mV. 3. The pattern of membrane currents recorded using voltage clamp with 'normal' external (containing 110 mM-Na+) and internal (110 mM-K+) solutions consisted of a rapidly activating and inactivating inward current followed by a slower, sustained outward current. 4. The inward components of current were isolated by using an internal solution in which Cs+ and TEA+ (tetrathylammonium) ions replaced K+. Depolarizations from holding potentials of -50 to -70 mV produced inward currents which had an initial transient phase followed by a maintained, or very slowly inactivating, component. The current-voltage relation for the initial transient phase reached a peak at membrane potentials near 0 mV, while the maintained phase, measured, for example, at the end of 50 ms voltage-clamp steps, had its peak near +10 mV. 5. The transient component of inward current was carried primarily by Na+ ions, as replacement of Na+ by TEA+ in the external solution abolished the transient. This current was thus identified as a voltage-dependent Na+ current, INa. The maintained component was greatly attenuated by removing 80-90% of the external Ca2+ ions, and it was abolished by divalent cations such as Cd2+ (0.2-0.4 mM), Ni2+ (0.5 mM) and La3+ (10-100 microM). This maintained component was thus a voltage-dependent calcium current, ICa. 6. About 80% of INa recorded in the presence of low (0.2-0.5 mM) external Ca2+ and 2 microM-LaCl3 was blocked by tetrodotoxin (TTX) with an apparent Kd of about 8 nM. The remaining 20% of INa was resistant to block by 2-10 microM-TTX. However, the 'TTX-resistant' component of INa was blocked by Cd2+ (0.2-0.4 mM). 7. The voltage-dependent calcium current, ICa, measured in saline in which Na+ was replaced by N-methyl-D-glucamine, activated near -40 mV and reached a peak near +10 to +15 mV.(ABSTRACT TRUNCATED AT 400 WORDS)

Intracellular recordings from pancreatic ganglia of the cat

1. The anatomy, morphology, and electrophysiology of parasympathetic ganglia of cat pancreas were studied in vitro. 2. Pancreatic ganglia existed as an interconnected plexus of small ganglia (ten to fifty cells) lying in the interlobular connective tissue. Occasionally smaller ganglia (four to ten cells) were observed lying on or within nerve trunks. 3. Electron micrographs revealed the presence of neurones and satellite cells as well as unmyelinated axons and nerve terminals. Nerve terminals contained small clear vesicles and/or large, dense-cored vesicles. 4. Intracellular recording of electrical activity revealed the presence of two types of ganglion cells. Type I ganglion cells exhibited resting membrane potentials that ranged from -40 to -63 mV and input resistances that ranged from 8 to 168 M omega. They responded to intracellular depolarizing current with action potentials, and received synaptic inputs which when activated caused fast and slow depolarizing responses. Type I cells were considered to be ganglionic neurones. Type II ganglion cells had higher resting membrane potentials that ranged from -61 to -83 mV, lower input resistances that ranged from 5 to 83 M omega and were electrically unexcitable. Repetitive stimulation of preganglionic nerves evoked a slow depolarization that was frequency dependent. Type II cells were considered to be satellite cells. 5. Stimulation of nerve trunks both central and peripheral to the ganglia evoked multiple, subthreshold, fast EPSPs in all type I cells tested. Fast EPSPs were blocked by the nicotinic antagonist hexamethonium. 6. Antidromic potentials were also observed following stimulation of either central or peripheral nerve trunks but never both. 7. In type I cells repetitive stimulation of both central and peripheral nerve trunks resulted in a slow, synaptically mediated depolarization which persisted during superfusion with nicotinic and muscarinic receptor antagonists. 8. Periods of low-frequency, spontaneous fast EPSPs and action potentials were observed in all type I cells tested. 9. It was concluded that parasympathetic neurones in cat pancreatic ganglia receive convergent fast and slow synaptic inputs from central and possibly peripheral sources and may function in vivo as sites of integration. The occurrence of spontaneous synaptic potentials in pancreatic ganglia suggests the possibility of intrinsic neural control of pancreatic function.

Rectification of synaptic and acetylcholine currents in the mouse submandibular ganglion cells

1. Synaptic currents and responses to acetylcholine (ACh) were recorded from mouse submandibular ganglion (SMG) cells under whole-cell voltage clamp. 2. The peak amplitude of excitatory synaptic currents (ESCs) as well as the currents evoked by the ionophoretic application of ACh followed a unique non-linear current-voltage (I-V) relation. The chord conductance of the whole-cell currents decreased with depolarization of the membrane potential and became virtually 0 at 50 mV. 3. The decay of ESCs was described by two exponential functions. Both the fast (tau f) and slow (tau s) time constants were sharply decreased at depolarizing potentials beyond -40 mV, being insensitive to hyperpolarizing potentials more than -50 mV. 4. Single ACh receptor channels were characterized by the whole-cell current noise analysis. The single-channel currents followed Ohm's law at negative membrane potentials but tended to reach a plateau at positive membrane potentials. The mean slope conductance measured between -40 and -20 mV was 28.5 pS. 5. The product of the number of functional channels (N) and the probability of a channel being open (p) showed a steep voltage dependence. The value of Np at 20 mV was only 31% of that at -20 mV. 6. The noise power spectrum was best fitted by a double-Lorentzian function. Both the fast (tau f) and slow (tau s) time constants were sharply decreased by depolarizations beyond -20 mV. being less sensitive to membrane potentials more negative than -30 mV. 7. The non-linear I-V relation of ESCs was attributed in part to the voltage dependence of p and in part to the voltage dependence of the single-channel conductance (gamma) of ACh receptor channels.

Nicotinic excitation of rat ventral tegmental neurones in vitro studied by intracellular recording

1. Intracellular recordings were made from presumed dopamine-containing neurones in the ventral tegmental area (VTA) in rat brain slices. 2. Nicotine (10-100 microM) and acetylcholine (ACh) depolarized the neurones. The depolarization caused by ACh was typically biphasic; both components were increased by neostigmine (0.1-10 microM), but only the slower component was blocked by scopolamine (1-10 microM). 3. The nicotinic action of ACh, studied in the presence of neostigmine and scopolamine, persisted in the presence of tetrodotoxin (1 microM) and cobalt (2-5 mM). 4. ACh or carbachol (30 microM) caused inward currents in neurones voltage-clamped near the resting potential. These currents reversed polarity at around -4 mV, were blocked by hexamethonium (1-100 microM) in a voltage-dependent manner, and showed desensitization with prolonged or repeated agonist applications. 5. Depolarizations caused by ACh and carbachol were reduced in slices pretreated with kappa-bungarotoxin, but were not changed by alpha-bungarotoxin. 6. These responses to ACh and nicotine resemble those previously described on autonomic ganglion cells. The direct action on VTA neurones may contribute to the positive reinforcement associated with nicotine consumption.

The effects of vagal stimulation and applied acetylcholine on the sinus venosus of the toad

1. The effects of vagal stimulation and applied acetylcholine were compared on the isolated sinus venosus preparation of the toad, Bufo marinus. 2. The effects of applied acetylcholine and of low-frequency, or short bursts of high-frequency vagal stimulation were abolished by hyoscine. 3. When intracellular recordings were made from muscle cells of the sinus venosus, it was found that applied acetylcholine caused bradycardia and a cessation of the heart beat which was associated with membrane hyperpolarization and a reduction in the duration of the action potentials. Much of the effect of acetylcholine can be attributed to it causing an increase in potassium conductance, gK. 4. When slowing was produced by low-frequency vagal stimulation, only a small increase in maximum diastolic potential was detected. During vagal arrest the membrane potential settled to a potential positive of the control maximum diastolic potential. 5. In the presence of barium, much of the bradycardia associated with vagal stimulation persisted. Although the bradycardia produced by added acetylcholine also persisted in the presence of barium, the effects of acetylcholine that could be attributed to an increase in gK were abolished. 6. Addition of caesium ions produced bradycardia with membrane potential changes similar to those seen during vagal stimulation. 7. The results are discussed in relation to the idea that neuronally released acetylcholine reduces inward current flow during diastole. In contrast applied acetylcholine as well as reducing inward current flow during diastole also increases outward current flow by increasing gK.

Bouton ultrastructure and synaptic growth in a frog autonomic ganglion

Postmetamorphic growth in the frog, Xenopus laevis, is accompanied by an increase both in the size of autonomic neurons in the heart and in the number of synaptic boutons that contact their surface. To determine whether the properties of individual boutons change as their number increases, serial-section electron microscopy was used to examine bouton ultrastructure at the end of metamorphosis and in the adult. The area of bouton contact, number of active zones per bouton, active zone size, percent of bouton area occupied by active zone, and vesicle density were examined. No differences were found between the two bouton populations for any of the parameters examined. These results support the hypothesis that boutons are structural units of synaptic growth, whereby the total area of synaptic contact increases through the addition of boutons without a change in their morphological properties.

Spontaneous release of multiquantal miniature excitatory junction potentials induced by a Drosophila mutant

1. Intracellular recordings were made from muscle fibre No. 6 of the dorsal longitudinal flight muscle (DLM) of Drosophila melanogaster in both wild-type flies and the temperature-sensitive paralytic mutant, shibirets-1 (shi). 2. Continuous recordings of the miniature excitatory junction potentials (MEJPs) in this fibre were made as the temperature was changed from 19 to 29 degrees C, and back to 19 degrees C. In shi flies, synapses become depleted of vesicles at 29 degrees C due to a temperature-dependent blockage in the recycling process, while transmitter release proceeds normally. When the temperature is lowered to 19 degrees C, recycling is allowed to proceed and recovery of the full complement of synaptic vesicles gradually occurs in about 20 min. 3. It was observed that the MEJP amplitude distribution in shi flies was unimodal at 19 degrees C prior to heating (as was wild-type), but during recovery from 8 min exposure to 29 degrees C became multimodal, with peaks at roughly integral multiples of the original peak prior to heating. This effect was never seen in wild-type flies. 4. Also, during recovery, the MEJP did not occur randomly, but rather occurred in a clustered fashion. 5. It is concluded that during recovery from depletion in shi neuromuscular junctions, a condition exists which causes the synchronization of spontaneous release, causing multiquantal MEJPs or clustering of MEJPs, depending on the degree of synchronization. 6. The possible role of Ca2+ in this phenomenon is discussed.

Membrane and cytoplasmic structure at synaptic junctions in the mammalian central nervous system

Application of rapid freezing, freeze substitution fixation, and freeze fracture techniques to the study of synaptic junctions in the mammalian central nervous system has revealed new aspects of synaptic structure that are consistent with and partially explicate advances in synaptic biochemistry and physiology. In the axoplasm adjacent to the presynaptic active zone, synaptic vesicles are linked to large spectrin-like filamentous proteins by shorter proteins that resemble synapsin I in morphology. This mesh of presynaptic filamentous proteins serves to concentrate synaptic vesicles in the vicinity of the active zone. The affinity with which the vesicles are bound by the mesh is probably modulated by the extent of phosphorylation at specific sites on the constituent filamentous proteins, and changes in the binding affinity result in changes in transmitter release. The structural organization of the postsynaptic density in Purkinje cell dendritic spines consists of very fine strands with adherent, heterogeneous globular proteins. Some of these globular proteins probably correspond to protein kinases and their substrates. The postsynaptic density, positioned at the site of the maximal depolarization caused by synaptic currents, apparently serves as a supporting framework for a variety of proteins, which respond to and transduce postsynaptic depolarization. At least two classes of filamentous protein fill the cytoplasm of spines with a complex mesh, which presumably contributes to maintenance of the spine shape. Membrane bound cisterns are a ubiquitous feature of Purkinje cell dendritic spines. Studies of rapidly frozen tissue with electron probe microanalysis and elemental imaging reveal that these cisterns take up and sequester calcium, which is derived from the extracellular space, and which probably enters the spine as part of the synaptic current.

Actions of acetylcholine in the guinea-pig and cat medial and lateral geniculate nuclei, in vitro

1. The mechanisms of action of acetylcholine (ACh) in the medial (m.g.n.) and dorsal lateral geniculate (l.g.n.d.) nuclei were investigated using intracellular recordings techniques in guinea-pig and cat in vitro thalamic slices. 2. Application of ACh to neurones in guinea-pig geniculate nuclei resulted in a hyperpolarization in all neurones followed by a slow depolarization in 52% of l.g.n.d. and 46% of m.g.n. neurones. Neither the hyperpolarization nor the slow depolarization were eliminated by blockade of synaptic transmission and both were activated by acetyl-beta-methylcholine and DL-muscarine and blocked by scopolamine, indicating that these responses are mediated by direct activation of muscarinic receptors on the cells studied. 3. The ACh-induced hyperpolarization was associated with an increase in apparent input conductance (Gi) of 4-13 nS. The reversal potential of the ACh-induced hyperpolarization varied in a Nernstian manner with changes in extracellular [K+] and was greatly reduced by bath application of the K+ antagonist Ba2+ or intracellular injection of Cs+. These findings show that the muscarinic hyperpolarization is mediated by an increase in K+ conductance. 4. The ACh-induced slow depolarization was associated with a decrease in Gi of 2-15 nS, had an extrapolated reversal potential near EK, and was sensitive to [K+]o, indicating that this response is due to a decrease in K+ conductance. 5. In contrast to effects on guinea-pig geniculate neurones, applications of ACh to cat l.g.n.d. and m.g.n. cells resulted in a rapid depolarization in nearly all cells, followed in some neurones by a hyperpolarization and/or a slow depolarization. The rapid excitatory response was associated with an increase in membrane conductance, had an estimated reversal potential of -49 to -4 mV and may be mediated by nicotinic receptors. The hyperpolarization and slow depolarization were similar to those of the guinea-pig in that they were associated with an increase and decrease, respectively, of Gi, and were mediated by muscarinic receptors. 6. The muscarinic hyperpolarization interacted with the intrinsic properties of the thalamic neurones to inhibit single-spike activity while promoting the occurrence of burst discharges. The muscarinic slow depolarization had the opposite effect; it brought the membrane potential into the range where burst firing was blocked and single-spike firing predominated. Depending upon the membrane potential, the rapid excitatory response of cat geniculate neurones could activate either a burst or a train of action potentials.(ABSTRACT TRUNCATED AT 400 WORDS)

Inhibitory synaptic potentials recorded from mammalian neurones prolonged by blockade of noradrenaline uptake

1. Intracellular recordings of membrane potential and membrane current were made from neurones of the rat nucleus locus coeruleus and the guinea-pig submucous plexus. These neurones exhibit inhibitory post-synaptic potentials (i.p.s.p.s) which result from noradrenaline acting on alpha 2-adrenoceptors to cause an increase in potassium conductance. 2. Cocaine (0.2-30 microM) reversibly increased the duration of the i.p.s.p. or inhibitory post-synaptic current (i.p.s.c.) in locus coeruleus neurones and submucous plexus neurones by approximately 750% and 350% respectively. The concentrations of cocaine causing half-maximal prolongation of the synaptic current were 3 microM in locus coeruleus and 0.5 microM in submucous plexus. The prolongation was due entirely to a slower rate of decay of the synaptic response. 3. Cocaine (10 microM) produced a maintained hyperpolarization (2-10 mV) or outward current (20-120 pA) in locus coeruleus neurones; in submucous plexus neurones cocaine increased the amplitude and duration of spontaneous i.p.s.p.s. 4. Outward currents produced by superfusion with noradrenaline were increased by cocaine with maximum effects being observed at 10-30 microM-cocaine. The maximum leftward shift in the relation between outward current or membrane hyperpolarization and noradrenaline concentration was 18- to 100-fold in locus coeruleus neurones and 4-fold in submucous plexus neurones. The concentrations of cocaine which caused a half-maximal increase in sensitivity to superfused noradrenaline were similar in both tissues, being 4 microM in locus coeruleus and 2 microM in submucous plexus. 5. These results show that neuronal uptake of noradrenaline released from adrenergic nerves plays a significant role in determining the time course of synaptic potentials mediated by noradrenaline.

Kinetics of acetylcholine activated ion channels in chick ciliary ganglion neurones grown in tissue culture

The acetylcholine activated conductance of chick ciliary ganglion neurones grown in tissue culture was studied by the patch clamp method. Single channel currents at 30 degrees C had a conductance of 38-42 pS, a reversal potential near + 10 mV and an average open lifetime of 1.08 ms (range 0.74 - 1.54 ms) at the resting potential. The presence of a single component in the distributions of amplitudes and open lifetimes, and also in the noise spectrum of voltage clamp currents, suggests that acetylcholine channels have uniform characteristics in these cells. Evidence of a desensitised state of the receptor was obtained from the distribution of gap intervals and the decline of voltage clamp current. These properties are similar to those of acetylcholine channels at the vertebrate neuromuscular junction. However, two important differences were found. (a) The acetylcholine concentrations used here were 10-25 times higher than those required to produce a similar degree of channel activation at the endplate. (b) When the membrane was hyperpolarised the mean open lifetime of the channel showed no change or a slight reduction.

Muscarinic inhibition of sympathetic C neurones in the bullfrog

1. The muscarinic inhibitory post-synaptic potential (i.p.s.p.) in sympathetic C neurones has been characterized in an isolated preparation of bullfrog paravertebral chain ganglia. Interactions between the i.p.s.p. and two other synaptic potentials have also been examined. 2. A single presynaptic stimulus to a C cell produces a nicotinic excitatory post-synaptic potential (e.p.s.p.) followed by a muscarine i.p.s.p. The latency of the i.p.s.p. is 50 msec or longer and the response lasts for seconds. C cells receive multiple cholinergic innervation but the thresholds for activation of the e.p.s.p. and i.p.s.p. are inseparable. Trains of 50 or more presynaptic stimuli produce a non-cholinergic e.p.s.p. which follows the nicotinic e.p.s.p. and i.p.s.p. and which lasts for tens of seconds. 3. The i.p.s.p. produced by a single presynaptic stimulus can be 30 mV in amplitude. However, in most cells, a short train of stimuli applied at an optimal frequency of 10 Hz is required to produce a large i.p.s.p. 4. The i.p.s.p. is blocked by atropine but is not affected by catecholamine antagonists. 5. Ionophoretically applied acetylcholine (ACh) mimics the i.p.s.p. in its latency, time course and amplitude. In addition, the i.p.s.p. and the muscarinic response to ACh reverse polarity at the same membrane potential: -102 mV in normal Ringer solution. The i.p.s.p. reversal potential shifts by 55 mV/decade change in extracellular K+ concentration and is insensitive to the Cl- gradient. 300 microM-Ba2+ totally blocks the muscarinically activated conductance in a reversible manner. 6. Action potentials, when initiated by a supramaximal nicotinic e.p.s.p. or by an antidromic impulse, are not blocked by the i.p.s.p. 7. Near resting potential (-50 to -60 mV), C cells can fire repetitively. The non-cholinergic slow e.p.s.p. is often accompanied by oscillations in membrane potential and firing of action potentials. This repetitive firing of C cells, which appears to be enhanced by the non-cholinergic e.p.s.p., is strongly inhibited by the i.p.s.p. The inhibition can be mimicked by injection of very small hyperpolarizing currents (e.g. 25 pA). Interactions between the i.p.s.p. and the non-cholinergic e.p.s.p. can generate phasic bursting patterns in C cells. 8. The mechanism underlying the i.p.s.p. and the consequences of these findings for ganglionic integration are discussed.

Cholinergic transmission in cat parasympathetic ganglia

1. Intracellular electrical recording techniques were used to study the ionic mechanisms of cholinergic synaptic transmission in cat vesical pelvic ganglia (v.p.g.). 2. Orthodromic nerve stimulation as well as ionophoretic application of acetylcholine (ACh) resulted in, first, a fast excitatory post-synaptic potential (f.e.p.s.p.) and secondly, a slow inhibitory post-synaptic potential (s.i.p.s.p). These distinct post-synaptic responses were direct actions of ACh and not mediated through an interneurone. In addition, a slow excitatory post-synaptic potential (s.e.p.s.p.) was observed in 44% of the cells. 3. The f.e.p.s.p., mediated via nicotinic receptors, had a reversal potential of -10 mV and resembled the conventional rapid depolarization in other ganglia. The s.i.p.s.p., mediated by muscarinic receptors, had a reversal potential of about -100 mV and resulted from an increase in potassium conductance. 4. The slow muscarinic hyperpolarization could be observed in the absence of antagonists and it was elicited at stimulus frequencies in the physiological range (2-10 Hz). the s.i.p.s.p. induced orthodromically or ionophoretically inhibited firing in spontaneously active neurones. These observations suggest that the muscarinic hyperpolarization may occur under physiological conditions and has sufficient magnitude to be inhibitory to neuronal activity.

Somatic motor axons can innervate autonomic neurones in the frog heart

1. The effects of sympathetic and parasympathetic stimulation on the heart rate in frogs were tested after hearts were reinnervated with a somatic motor nerve. When frogs were vagotomized and hypoglossal axons were redirected to the heart for 8 or more weeks, stimulating the redirected hypoglossus nerve produced a parasympathetic-like inhibition of the heart. Stimulating sympathetic rami of the anastomosed hypoglossus nerve produced cardiac acceleration.2. Individual parasympathetic neurones received synaptic input from hypoglossal terminals. The excitatory post-synaptic potentials evoked by hypoglossal stimulation were much smaller than those evoked by vagal stimulation in control or vagal-reinnervated ganglia. However, hypoglossal axons innervated most (71%) of the ganglion cells and this level of innervation persisted for at least 60 weeks.3. Hypoglossal axons formed networks of varicose terminals within cardiac ganglia and established axo-axonic synapses with parasympathetic neurones. Hypoglossal terminals did not reinnervate the neuronal perikarya, in contrast to vagal axons in control or vagal-reinnervated ganglia.4. Axo-axonic synapses from redirected hypoglossal axons were identified in cardiac ganglia by bathing isolated hearts in horseradish peroxidase (HRP) and stimulating the redirected nerve. Electron micrographs showed that axo-axonic synapses contained HRP-labelled presynaptic vesicles.5. The source of foreign innervation in experimental cardiac ganglia was confirmed to be hypoglossal motoneurones (a), by comparing the conduction velocity of the redirected presynaptic axons (1.32 m/sec) with regenerating vagal preganglionic fibres (< 0.3 m/sec), and (b), by retrograde HRP-labelling of large motoneurones in the hypoglossal nucleus after applying peroxidase to the axons which had grown into the heart.

Spontaneous and evoked excitatory junction potentials in rat tail arteries

1. The frequency, amplitude and time course of spontaneous excitatory junction potentials (s.e.j.p.s) and their relationship to the time course and amplitude of evoked excitatory junction potentials (e.j.p.s) were examined. 2. The frequency and amplitude of s.e.j.p.s varied dramatically between cells. There was good correlation between their rise and decay times. 3. The amplitude and time course of e.j.p.s also varied between cells. E.j.p.s with large amplitudes and fast time courses were recorded in cells with high s.e.j.p. frequencies. 4. Active responses propagated only for very limited distances. 5. The frequency of s.e.j.p.s decreased after reserpine and 6-hydroxydopamine (6-OHDA) treatments, suggesting that s.e.j.p.s were related to spontaneous release of noradrenaline from nerve terminals.