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

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.

Competitive elimination of foreign motor innervation on autonomic neurones in the frog heart

1. Somatic motoneurones are capable of forming functional synapses when redirected to vagotomized autonomic neurones in the frog heart. We tested if regenerating vagus nerves could reinnervate ganglion cells in the presence of foreign hypoglossal innervation and, furthermore, whether hypoglossal innervation persisted when vagal axons regenerated to the heart. 2. Simulating the redirected hypoglossus nerve produced a parasympathetic-like cardiac inhibition in the absence of vagal regeneration. However, when the vagus nerve was allowed to regenerate to the heart, vagal cardio-inhibition was restored and hypoglossal inhibition disappeared. 3. Intracellular recordings showed that 71% of the cardiac ganglion cells were innervated by hypoglossal axons before vagal regeneration, but that this value fell to less than 9% over a period of 40 weeks during vagal regeneration. 4. If the vagus nerve was prevented from regenerating to the heart, hypoglossal innervation did not decline, indicating that elimination of the foreign motor innervation was dependent upon vagal reinnervation. 5. Although hypoglossal terminals formed synapses only on the axons of parasympathetic ganglion cells, regenerating vagal fibres re-established synaptic contact both on axons as well as on neuronal perikarya. 6. The data indicate that in the frog parasympathetic cardiac ganglion, extensive synaptic remodelling can take place during reinnervation and that previously established, inappropriate inputs can be functionally eliminated by regeneration of the native nerve supply.

The characteristics of synaptic currents and responses to acetylcholine of rat submandibular ganglion cells

1. Synaptic currents and responses to acetylcholine (ACh) have been recorded at 20 degrees C from rat submandibular ganglion cells by a two micro-electrode volatage-clamp technique.2. The peak amplitude (a(p)) of excitatory synaptic currents (e.s.c.s) was linearly related to membrane potential (E(m)), with a reversal potential close to - 10 mV. E.s.c.s decayed with a bi-exponential time course, the fast phase comprising just over half the total amplitude. The time constant (tau(f)) of the fast phase was 5-9 msec, while that of the slow phase (tau(s)) was 27-45 msec. The relative amplitudes of the two components remained constant at different membrane potentials, showing that the reversal potential was the same for both.3. Both tau(f) and tau(s) increased as the cell was hyperpolarized, the ratio tau(-80)/tau(-40) being about 1.6 for both fast and slow components.4. Increasing the calcium concentration from 2.5 to 7.5 mm increased the amplitude of both components by about 40% and also prolonged the synaptic currents 30-50%, its effect being slightly greater on tau(s) than on tau(f).5. In contrast to e.s.c.s, spontaneous or potassium-evoked miniature synaptic currents (m.s.c.s) showed a simple exponential decay with a time constant (tau(m.s.c.)) very similar to tau(f). tau(m.s.c.) showed the same sensitivity to membrane potential and calcium concentration as tau(f).6. In the presence of neostigmine (10 mum) e.s.c.s were prolonged, tau(f) about 3.5-fold and tau(s) about 2.5-fold. The decay remained bi-exponential, with little change in the relative amplitude or voltage-dependence of the two components. M.s.c.s were prolonged to a lesser extent (1.5-2-fold) and the voltage dependence of tau(m.s.c.) was unaffected by neostigmine.7. Reduction of the quantal content of the e.s.c. by low calcium-high magnesium solution did not affect the time course. The relative amplitudes, and the time constants of the two components were unchanged even with a 90% reduction of a(p).8. Voltage-jump studies, in which the cell was abruptly hyperpolarized by 20-40 mV during a response to ionophoretically applied ACh, showed a relaxation pattern consisting of two distinct exponential components, whose relative amplitudes varied considerably in different cells. The two rate constants tau(f.rel) and tau(s.rel) were somewhat shorter than tau(f) and tau(s) for e.s.c.s, the difference being generally less than two-fold.9. Measurements of ACh noise also revealed two kinetic components, the time constants of which corresponded closely to tau(f) and tau(s) for e.s.c.s. On the assumption that the two components represent channels of equal conductance, the single channel conductance, gamma, was calculated to be 31+/-3 pS, similar to that of endplate channels.10. It is concluded that the two kinetic components of e.s.c.s and ACh responses probably represent two distinct classes of ACh-operated ionic channels, whose mean lifetime differs about fivefold. The two types of channel show the same ionic selectivity and their mean lifetime varies in the same way with the membrane potential. The absence of a slow component in m.s.c.s suggests that the two types of channel are spatially separate in the membrane.

Localization of specialized noradrenaline receptors at neuromuscular junctions on arterioles of the guinea-pig

1. Noradrenaline was applied by ionophoresis to various positions on the surface of an arteriole, and any changes in membrane potential of the arteriolar smooth muscle were recorded. 2. At a proportion of positions noradrenaline produced membrane depolarization. 3. The positions where noradrenaline produced these membrane potential changes were restricted to regions close to the sympathetic nerves which innervated the arterioles.

The effect of changing interpulse intervals on the negative chronotropic response to repetitive bursts of vagal stimuli in the dog

The chronotropic responses to repetitive bursts of vagal stimulation were determined in open-chest, anesthetized dogs. Either 5 or 10 electrical pulses were included in each stimulus burst, and the interpulse interval (deltat) was varied over the range of 5 to 150 msec. As the frequency of the stimulus bursts was progressively changed, the sinoatrial (SA) nodal pacemaker cells became synchronized with the repetitive bursts of stimuli over a certain range of burst frequencies. The magnitude of this frequency range varied with deltat. For 5 and 10 pulses/burst, the values of deltat that produced the greatest magnitude of this frequency range were 30.2 and 24.3 msec, respectively. Also, over the range of values of deltat from 5 to 50 msec, the magnitude of the negative chronotropic effect of the vagal stimulus burst varied directly with deltat. It is likely that, as the interpulse interval is increased within the range of values, either more acetylcholine is released from the vagal nerve endings per pulse or there is less saturation of the receptors on the pacemaker cell membranes during each burst.

Hyperpolarizing potentials induced by Ca-mediated K-conductance increase in hamster submandibular ganglion cells

The mechanisms of three types of hyperpolarizing electrogenesis in hamster submandibular ganglion cells were analyzed with intracellular microelectrodes. These included (1) spike-induced hyperpolarizing afterpotential (S-HAP), (2) spontaneous transient hyperpolarizing potential (HP), and (3) the hyperpolarizing (H) phase of postsynaptic potential (PSP). Most of these hyperpolarizing potentials were due to conductance increases and reversed polarity at membrane potential (Em) between -70 and -85 mV, which was close to the K-equilibrium potential. The average resting potential of ganglion cells was -53 mV. Action potential overshoot increased slightly in high [Ca2+]0 and decreased in low [Ca2+]0. In most neurons action potentials were completely suppressed by 10(-7)-M tetrodotoxin (TTX). The S-HAP has an initial component due to delayed rectification and a late component. The late component is enhanced by increasing [Ca2+]0, or by applying Ca-ionophore (A23187), TEA, caffeine, or dibutyryl cyclic (DBc-) AMP; it is suppressed by decreasing [Ca2+]0, or by applying Mn2+. Perfusion with Cl--free saline reduced membrane potential slightly but did not modify the S-HAP. Depolarizing pulses also induced hyperpolarizing afterpotential (D-HAP), similar to the S-HAP. Spontaneous transient HPs occurred in some neurons at irregular intervals. HPs were insensitive to TTX but were suppressed by Mn2+. Caffeine induced low frequency rhythmic HPs in many neurons, often alternating with periods of repetitive spiking. The PSP was a monophasic depolarizing (D-) potential in some neurons, but in others the D-phase was followed by a small H-phase. Perfusion with A23187, caffeine or DBc-AMP increased the H-phase of the PSP. Perfusion with K+-free saline or treatment with 10(-5)M ouabain did not abolish the H-phase of PSPs. These membrane potential-dependent phenomena appear to be induced mainly by Ca-mediated K-conductance increases. This mechanism contributes to the regulation of low-frequency repetitive firing in submandibular ganglion cells.

Intracellular recording from visually identified motoneurons in rat spinal cord slices

Motoneurons were directly visualized with Nomarski optics in slices prepared from new born rat spinal cord. Intracellular recordings from these neurons showed spontaneous potentials, probably triggered by inter-neuronal activity. Action potentials could also be evoked by direct intracellular stimulation of the motoneurons. Iontophoretically applied L-glutamate caused a fast depolarization of the motoneuronal membrane. Considerable differences in local sensitivity to L-glutamate were found on the surface of the motoneuron.

Permeability changes produced by L-glutamate at the excitatory post-synaptic membrane of the crayfish muscle

1. Permeability changes produced by L-glutamate at the neuromuscular junction of the crayfish (Cambarus clarkii) were investigated by application of the drug iontophoretically to the voltage-clamped junction and measuring the resulting 'glutamate current'. 2. Reversal potentials were determined by measuring the glutamate current at different membrane potentials. They were +39-1 +/- 3-6 mV (mean +/- S.E. of mean) in normal solution and +16-5 +/- 2-0 mV in solutions made twice as hypertonic by the addition of sucrose. 3. Decreasing external Na+ concentration shifted the reversal potential in the negative direction; increased Na+ in the positive direction. 4. The relation between the amplitude of the glutamate current and extracellular Na+ concentration was approximately linear. 5. Alteration of the external K+ or Cl- concentration did not affect the amplitude or reversal potential of glutamate current. 6. In Na+-free solution the application of L-glutamate produced a small inward current at the resting potential and its amplitude was augmented by increasing the external Ca2+ concentration. 7. Increasing the Ca2+ concentration in the normal Na+ media produced no appreciable effect on the reversal potential but decreased the amplitude of glutamate current. 8. The results indicate that L-glutamate increases the membrane permeability mainly to Na+ and slightly to Ca2+. 9. The time course of glutamate current was shorter than that of the concentration calculated from the diffusion equation and it was simulated more closely by the square of the concentration.

The ionic basis of the resting potential and a slow depolarizing response in Rohon-Beard neurones of Xenopus tadpoles

1. Rohon-Beard cells in the spinal cord of Xenopus laevis tadpoles have been studied in animals 4-days to 2-weeks-old (Nieuwkoop & Faber, 1956, stages 45-49). These neurones have an unusually large resting membrane potential of -88 mV, in Ringer solution containing 3-0 mM K+. 2. Their resting potential (R..) depends on the concentration gradient of K+ across the cell membrane. These cells follow the prediction of the Nernst equation for a K+-selective electrode, down to external K+ concentrations as low as 1-0 mM (R.P. -118 mV). 3. The resting potentials of muscle cells in these animals exhibit the same dependence on external [K+], as has been shown previously. 4. Rohon-Beard cells can be driven antidromically, bu stimulation of the anterior end of the spinal cord with brief current pulses through a suction electrode. Antidromic action potentials fail to invade the cell body with repeated stimulation at 1Hz. 5. Even when impulses fail to invade Rohon-Beard somata, slow depolarizations can be produced by single shocks or trains of shocks which cause impulse activity in other neurones. The response can be observed to a single stimulus or to a train of stimuli. The magnitude of the depolarization is graded, depending on the number of stimuli and the frequency of stimulation. 6. Support is presented for the hypothesis that the slow depolarization in Rohon-Beard cells is mediated by the release of K+ into their environment by the impulse activity of neighbouring neurones. The slow depolarization increases in solutions containing 1-5 mM-K+, and decreases in solutions containing 6-0 mM-K+. The changes are in quantitative agreement with those anticipated by theory. 7. The slow depolarization is unlikely to be due to a conductance change produced by a synaptic transmitter, since hyperpolarization and depolarization of the Rohon-Beard cell with injected current do not change the amplitude of the response. Further, low Ca-high Mg solutions which block neuromuscular transmission do not block the response. 8. The possible role of the slow depolarizing response in the physiological activity of these neurones is discussed.

Visual identification of two kinds of nerve cells and their synaptic contacts in a living autonomic ganglion of the mudpuppy (Necturus maculosus)

1. Many of the nerve cells comprising the cardiac parasympathetic ganglion of the mudpuppy are spread out in a thin, transparent sheet of tissue, enabling one to see cellular details in living preparations with differential interference contrast optics. The aim of this study was twofold: to establish the morphology of the nerve cells and their synaptic connections by light and electron microscopy, and to determine which aspects of the ganglion's structure could be reliably identified in the living tissue. 2. There are two types of neurones in the ganglion: (a) principal cells that send post-ganglionic axons to cardiac muscle fibres, and (b) interneurones whose processes are confined to the ganglion. 3. Interneurones are distinguished from principal cells by the presence of numerous granular vesicles seen with the electron microscope, and by intense formaldehyde-induced fluorescence. The interneurones are thus similar to catecholamine-containing interneurones in autonomic ganglia of other vertebrates. 4. Principal cells are innervated by processes that terminate mainly on the cell body, forming up to forty-five synaptic boutons and covering, on the average, 5% of the perikaryal surface. The synaptic terminals are derived from three sources: (a) axons from the vagus nerves, (b) interneurones and (c) other principal cells. Vagal terminals contacting principal cells contain agranular vesicles typical of preganglionic cholinergic endings. At regions of contact between processes of interneurones and principal cells, the interneurones have granular vesicles focused at membrane specializations; in addition there are small areas of close plasma membrane apposition, probably gap junctions. Some of the contacts between principal cells are characterized by gap junctions; others are structurally similar to vagal endings but persist after vagal degeneration. 6. Interneurones are innervated by axons that make contact mainly with their processes. The axon terminals on processes of interneurones contain agranular vesicles similar to vagal terminals on principal cells. 7. In live preparations principal cells are distinguished from interneurones by their size and the appearance of their organelles. Synaptic contacts on principal cells could often be identified and, in some cases, large contacts from interneurones or those from other nearby principal cells could be traced back to their cell bodies of origin. The validity of these identifications was confirmed by subsequent electron microscopic examination of the same cells.

Some effects of 5-hydroxytryptamine, dopamine and noradrenaline on neurones in the submucous plexus of guinea-pig small intestine

1. Responses to the iontophoretic application of 5-hydroxytryptamine (5-HT), dopamine and noradrenaline were examined in neurones of the submucous plexus of guinea-pig small intestine. 2. Every neurone was excited by 5-HT. 3. In a proportion of cells, dopamine or noradrenaline caused an increase in membrane potential. This response was only observed in cells which received in inhibitory innervation. The responses closely resembled inhibitory synaptic potentials evoked by transmural stimulation. 4. Both inhibitory synaptic potentials and inhibitory responses to dopamine and noradrenaline were blocked by methysergide. 5. It seems possible that these two catecholamines may interact with similar receptors to those activated by inhibitory transmitter.

Metabolism of acetylcholine in the nervous system of Aplysia californica. II. Reginal localization and characterization of choline uptake

The choline required for synthesis of acetylcholine is derived exogenously by Aplysia ganglia. Under physiological conditions choline was taken up primarlily by neuropile and nerves and not by cholinergic cell bodies. In addition, compared with their contents of choline acetyltransferase, those components of nervous tissue which contain nerve terminals and axons synthesized acetylcholine far more efficiently. Choline was accumulated by high and low affinity uptake processes; the high affinity process appeared to be characteristic of cholinergic nuerons (Swartz, J. H., M. L. Eisenstadt, and H. Cedar.1975. J. Gen. Physiol. 65:255). The two uptake processes were similarly affected by temperature with a Q10 of 2.8. Both were dependent on a variety of ions in a complicated manner. High affinity uptake seemed to be more dependent on Na+, showed greater inhibition by ouabain, and was selectively inhibited by oxotremorine. We found that the functional state of neurons did not alter uptake of radioactive choline by either process, nor did it change the conversion to radioactive acetylcholine.

An analysis of the release of acetylcholine from preganglionic nerve terminals

1. A study has been made of the release of acetylcholine (ACh) from the preganglionic nerve terminals of the isolated superior cervical ganglion of the guinea-pig during short trains of impulses, using the amplitude of the excitatory post-synaptic potential (e.p.s.p.) as a measure of the ACh released by each impulse. 2. The time course of decay of facilitation following a single impulse could be described by two exponential components, with T1 = 200 msec, and T2 = 13-3 sec. The increase in ACh output at the beginning of stimulation at frequencies smaller than or equal to 2Hz was reasonably predicted in terms of summation of the individual facilitatory effects of each impulse in the train, but fell short of the prediction at higher frequencies. 3. The steady-state output of ACh during repetitive stimulation at frequencies between 0-5 and 20 Hz was lower than that predicted by summation of the facilitatory effects of each impulse, but reached the predicted level at frequencies smaller than or equal to 2Hz in raised Mg2+ concentrations. 4. Statistical analysis of the quantal content (m) of e.p.s.p.s evoked by each of the first five impulses in a train showed that Poisson statistics described release of ACh at the beginning of a train in most cases; when binomial statistics could be applied (two of seven axons studied), the increase in m was accompanied by an increase in the statistical parameter, n. 5. Analyses were also made of release during continuous stimulation; at the time when the steady state of release was reached, the statistical parameter, p, had also increased. Increased release of ACh at increased frequencies of stimulation was associated with increases in p in axons with p smaller than 0-5; however, in most axons (eleven of seventeen), p was greater than 0-5 in the steady state, and increases in m with frequency were due to increases in n.

The effects of physostigmine on synaptic transmission in the inferior mesenteric ganglion of guinea-pigs

1. Synaptic potentials were recorded with intracellular electrodes from cells in the inferior mesenteric ganglion of the guinea-pig.2. Half-widths of the synaptic potentials recorded fell into two groups: type L cells had long synaptic potentials (11.6-15.2 msec) and low thresholds (14.6 mV mean), type S cells had short synaptic potentials (6.1-9.3 msec) and high thresholds (29.9 mV mean).3. Physostigmine (1.2 x 10(-6)M) caused a significant increase in the half-width of both types of synaptic potential.4. Physostigmine caused a significant increase in the half-width of spontaneous synaptic potentials and an increase in their amplitude.5. Repetitive preganglionic stimulation, in the presence of physostigmine, led to a marked and prolonged depolarization in all cells. In most cells repetitive spontaneous firing of action potentials was then observed. This effect was blocked by atropine (1.4 x 10(-7)M).6. The effect of atropine on the half-width in a physostigmine-treated cell was inconsistent: although synaptic potentials in some cells were slightly shortened their half-widths were always greater than the control.7. It is concluded that cholinesterase plays a role in limiting the time course of the synaptic potential, by limiting the duration of action of acetylcholine.

Acetylcholine receptors

The idea that certain drugs and neurotransmitters produce their effects by combining with specific receptors was first clearly expressed by Langley (1905) on the basis of the selective and localized effect of nicotine on striated muscle fibres. In 1914, Langley published a paper in which the antagonism between ‘curari’ and nicotine was analysed and measured as the ratio by which the nicotine concentration had to be increased in order to produce a standard response in the presence of tubocurarine. It was clear that Langley had in mind the idea of competition between nicotine and curare for the receptor sites and it was surprising that he did not formulatethe theory quantitatively, particularly since Hill, working in Langley's laboratory in 1909, published a mathematical analysis of the action of nicotine on frog muscle giving kinetic and equilibrium equations based on the law of mass action, which could easily have been extended to give an account of competitive antagonism. Barger & Dale (1910) did not favour the idea of specific receptors for sympathomimetic amines.

A note on the interaction of spontaneous and evoked release at the frog neuromuscular junction

1. The interaction between spontaneous miniature end-plate potentials and evoked end-plate potentials was investigated at the frog neuromuscular junction using focal extracellular recording techniques.2. End-plate potentials evoked immediately after a spontaneous miniature potential were facilitated by up to 20%. The percentage facilitation was negatively correlated with the average quantal content of the end-plate potential.

The formation of synapses in mammalian sympathetic ganglia reinnervated with preganglionic or somatic nerves

1. A study has been made of the formation of synapses in the superior cervical ganglion of the guinea-pig, during reinnervation either with axons of the cervical sympathetic trunk, or with somatic axons of the nerve to the sternohyoid muscle.2. No significant changes in either the geometry or electrical parameters of sympathetic motorneurones were detected following denervation for periods of 3-6 weeks, or after reinnervation with either preganglionic or somatic axons.3. The post-ganglionic action potential reappeared about 4 weeks after preganglionic trunk section (eighteen of eighteen ganglia); its amplitude increased progressively and was almost normal by more than 10 weeks after nerve section. A very small response was detected from thirteen of eighteen ganglia after periods longer than 8 weeks after cross-reinnervation with somatic axons.4. Regenerated preganglionic or somatic nerve terminals were demonstrated around the ganglion cells using ZIO impregnation and electron-microscopy; the structure of these terminals was unchanged following regeneration into the ganglia, although many more synapses were formed by preganglionic terminals than by somatic terminals.5. The facilitation of evoked synaptic potentials which occurs during repetitive stimulation of preganglionic axons was retained following their regeneration, whereas most synapses formed on ganglion cells by regenerating somatic axons showed facilitation of transmitter release during trains of stimuli, rather than the normal depression.6. These observations suggest that the structure and electrical properties of adult mammalian autonomic motorneurones are not under neural control, but that these neurones do show some selectivity in the type of nerve which they will permit to form synapses on them.

The formation of synapses in mammalian striated muscle reinnervated with autonomic preganglionic nerves

1. A study has been made of the formation of synapses during reinnervation of the hemidiaphragm of adult rabbits with preganglionic fibres of the thoracic vagus, using histological, ultrastructural and electrophysiological techniques.2. Following reinnervation with preganglionic axons, silver-stained nerve terminals were found in association with cholinesterase-stained end-plates only in the region of the muscle corresponding to the original innervation band.3. The fine preganglionic axons retained their normal structure in striated muscle, but were found to synapse over discrete areas of dimensions not larger than those of the original end-plates.4. The regenerated varicose preganglionic nerve terminals were observed with the electronmicroscope in positions either overlying or in the vicinity of the old synaptic folds.5. Spontaneous potentials and evoked synaptic potentials were recorded only in the middle of the muscle fibres after vagus reinnervation.6. In a few cases, multiple synaptic potentials with similar time courses were recorded, suggesting that several axons had formed synapses at a single site on a muscle fibre.7. It has been shown that, during reinnervation of adult mammalian striated muscle fibres with nerves other than those of the somatic system, synapses are formed preferentially in the region of the old end-plates.

Quantal components of the inhibitory synaptic potential in spinal mononeurones of the cat

1. Monosynaptic i.p.s.p.s were produced in spinal motoneurones of the cat by stimulation of a pool of interneurones following chronic degeneration of descending tracts and primary afferent fibres in the lumbosacral cord.2. Monosynaptic i.p.s.p.s so evoked by supramaximal stimuli often showed a fluctuation in amplitude with occasional failures of response.3. When two successive stimuli were applied at a short interval, the mean amplitude of the second i.p.s.p.s was greater than that of the first. This facilitation was associated with a decrease in the number of failures, a decrease in the coefficient of variation of the amplitude distribution and an increase in the probability of occurrences of large i.p.s.p.s.4. A statistical analysis of the i.p.s.p. amplitude fluctuation showed that the monosynaptic i.p.s.p. is composed of discrete unit potentials evoked with a certain probability in a manner described by a binomial law.5. The application of strychnine decreased the mean amplitude of i.p.s.p.s with little change in the coefficient of variation of the i.p.s.p. amplitude distribution.6. It is concluded that the release of inhibitory transmitter occurs in quantal steps and that strychnine blocks primarily the post-synaptic receptors for the inhibitory transmitter.

An investigation of spontaneous potentials recorded from the smooth-muscle cells of the guinea-pig seminal vesicle

1. A spontaneous discharge of small potentials from smooth muscle cells of the guinea-pig seminal vesicle has been described. The mean amplitude, frequency and duration of the potentials were 0.9 mV, 12.2/min, and 92 msec, respectively. Occasionally, large potentials of 5-7 mV were recorded.2. The frequency of spontaneous potentials was enhanced by about twofold in 15 mM potassium-Krebs solution. Removal of calcium from the perfusing solution, or adding excess magnesium (12 mM) suppressed the rate of these potentials. Cobalt (2.5 mM) did not affect them but blocked excitatory junction potentials evoked by the hypogastric nerve stimulation.3. The spontaneous potentials were unaffected by atropine. Their rate and amplitude were reversibly reduced in the presence of high concentrations of phentolamine (10(-4) but not by 10(-5) g/ml.). Guanethidine enhanced the frequency initially (5 min), but subsequently the control rate was not appreciably changed, even after 20 min of guanethidine perfusion.4. After post-ganglionic denervation or reserpine treatment, the frequency of discharge of spontaneous potentials was reduced, without any effect on amplitude.5. The properties of the spontaneous potentials recorded from the smooth muscle cells of the seminal vesicle are similar to those reported for the vas deferens. They appear to be due to an interaction between noradrenaline released from sympathetic nerve endings and the smooth-muscle cell membrane. Present experiments cannot completely rule out the possibility that another transmitter in addition to noradrenaline may be released at this neuroaffector junction.

The onset and development of transmission in the chick ciliary ganglion

1. The onset and development of transmission has been studied electro-physiologically in the isolated chick ciliary ganglion from Stage 25 (Hamburger & Hamilton, 1951) until 28 days after hatching. Ultrastructure of the synapses was concomitantly investigated.2. Synaptic transmission began at Stage 26(1/2) and was 100% in both cell groups, ciliary and choroid, by Stage 33. It was initially chemical until Stage 41 when effective electrical coupling first appeared in the ciliary population. The proportion of electrically transmitting synapses increased to 80% by 1-2 days post-hatching.3. Few morphological synapses were present at Stage 33(1/2) when all ganglion cells were transmitting. A scarcity of synaptic vesicles persisted until late in embryonic development when all ciliary cells possessed calyces. At hatching the calyces were filled with synaptic vesicles.4. Initial synaptic contacts were by fine terminal branches often on the intricate processes of early ganglion cells. Calyces formed from Stage 36(1/2) and there was a concomitant retraction of ganglion cell processes, so that by Stage 40 all ciliary cells had simple calyces. The calyx was a transitory structure, which from the first week post-hatching began to break up into a cluster of boutons.5. Chemical post-synaptic potentials (PSPs) were at Stage 40 long (30 x the membrane time constant) and further prolonged by eserine. By Stage 43, PSPs had become markedly shortened and were unaffected by eserine. No simple explanation can be offered for the changes in PSP time course and sensitivity to anticholinesterases during development.6. Intracellular records from Stage 40 ciliary cells, which all possess calyces, showed 1-2 mV amplitude, diphasic, fast decaying electrical coupling potentials (CPs). Later in development the CPs became 20-40 mV amplitude, more slowly decaying and monophasic. This seemed to be correlated with faster presynaptic conduction velocities and myelination of the cell soma. Such changes in CPs may reflect a shift from capacitative to more resistive coupling and point to several factors contributing in varying degrees to the electrical transmission.7. Presynaptic fibres innervating ciliary cells were from the start of lower threshold and faster conduction velocity than those innervating ciliary cells, as occurred in the adult. It is concluded that these preganglionic fibres were probably specified by the time transmission starts and that they selectively innervated the proper post-synaptic cells.

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

1. The chemosensitivity of the surface of parasympathetic neurons in the interatrial septum of the heart of the frog has been explored by focal iontophoretic application of acetylcholine (ACh). 2. There is a specific highly chemosensitive area on the cell surface near synaptic boutons. ACh released close to visually identified boutons produced depolarizing responses which arise more rapidly and are larger than responses evoked at randomly chosen spots on the neuronal surface. 3. Individual synapses, or small numbers of them, can be desensitized by applied ACh without blocking synaptic transmission at nearby synapses on the same neuron. This shows that the method of ACh application resolves chemosensitivity in patches restricted to a few micrometres in diameter.