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Adenosine inhibited three Ca2+‐dependent potentials recorded intracellularly from post‐ganglionic neurones of the rat superior cervical ganglion. A shoulder on the falling phase of the action potential elicited in normal Locke solution, a hyperpolarizing after‐potential (h.a.p.) that follows the spike, and a regenerative Ca2+ spike elicited in Locke solution containing TTX and TEA were all reversibly inhibited by adenosine analogues in a dose‐dependent fashion. The maximum rate of rise of the Ca2+ spike (dV/dt) was markedly reduced suggesting that the underlying mechanism of adenosine action is inhibition of the Ca2+ conductance mechanism and thus, the voltage‐sensitive Ca2+ current. I/V curves in low Ca2+, high Mg2+, TTX, TEA, and Co2+ to block the Ca2+ current show no change in resistance in the presence of 2‐chloroadenosine. The actions of adenosine were nearly eliminated in the presence of 1 mM‐theophylline, an adenosine receptor antagonist. The order of agonist potency on the inhibition of the h.a.p. was: N‐6‐[L‐phenylisopropyl] adenosine (L‐PIA) greater than 2‐chloroadenosine greater than adenosine greater than cyclic AMP = 5' AMP. The concentration of L‐PIA which produced a half‐maximal effect (EC50) was 0.5 microM and that for cyclic AMP was 100 microM. Dipyridamole, an adenosine uptake blocker, potentiated the effects of low concentrations of adenosine and shifted the dose‐response curve for adenosine towards that of 2‐chloroadenosine (EC50 = 1 microM). These results are consistent with the concept of an external adenosine receptor, but we are unable to assign a receptor subtype. Cyclic AMP mimicked the effects of adenosine, but these effects were eliminated by adenosine deaminase. Our results suggest that the electrogenic effects of bath‐applied cyclic AMP may result from the metabolism of cyclic AMP to adenosine by ganglionic tissue. We conclude that adenosine activates a receptor on the neuronal cell surface to inhibit the voltage‐dependent Ca2+ current.

Brief tetanic stimulation of the preganglionic nerve induced a persistent potentiation of nicotinic synaptic transmission in the rat superior cervical sympathetic ganglion. Quantitative measurements of the post‐tetanic increase in synaptic efficacy revealed two distinct time courses. The early, rapidly decaying component, termed post‐tetanic potentiation (p.t.p.), had a decay time constant of 2‐3 min, as reported elsewhere. The duration of the more persistent component, called long‐term potentiation (l.t.p.), was extremely temperature dependent, lasting much longer at 32 degrees C than at 22 degrees C. In half of the experiments performed at 32 degrees C, l.t.p. showed no detectable decay over the course of 1 h or more after a brief tetanic stimulation. Other experiments were conducted at 22 degrees C. The induction of l.t.p. was dependent on the extracellular [Ca2+]. Transient elevation of the extracellular [K+] also produced a long‐term enhancement of synaptic efficacy, and this effect was also Ca2+ dependent. The tetani that were effective in inducing l.t.p. (5‐20 Hz for 5‐20 s) were well within the physiological range of preganglionic activity. The magnitude and time course were related to frequency and duration of stimulation. The occurrence of l.t.p. was restricted to those preganglionic fibres that were tetanically stimulated. This lack of heterosynaptic or generalized effects was demonstrated by splitting the preganglionic nerve into two branches that could be independently tested and conditioned. Physiological activation of muscarinic or nicotinic receptors apparently does not play an essential role in causing ganglionic l.t.p., which is expressed as an enhancement of nicotinic transmission. A muscarinic antagonist (2 microM‐atropine) did not block l.t.p. Preganglionic stimulation induced l.t.p. even when a high concentration of a nicotinic antagonist (3 mM‐hexamethonium) was present during the tetanic stimulation. Furthermore, bath application of a cholinergic agonist (100‐1000 microM‐carbachol) could not substitute for tetanic stimulation in provoking l.t.p. Activation of adrenergic receptors also appeared not to play an essential role. Neither a beta‐adrenergic antagonist (10 microM‐sotolol or 1 microM‐propranolol) nor an alpha‐adrenergic antagonist (1 microM‐phentolamine) had any significant effect on the magnitude or duration of l.t.p. The results indicate that ganglionic l.t.p. is a Ca2+‐ and temperature‐dependent process that can be created independently of the activation of nicotinic, muscarinic or adrenergic receptors.

The release of endogenous acetylcholine (ACh) from the in vitro rat superior cervical ganglion was measured by assaying the bathing medium. Simultaneously, synaptic transmission in the ganglion was assessed by recording post‐ganglionic compound action potentials. A brief period of tetanic preganglionic stimulation (20 Hz for 20 s) induced a long‐term potentiation of the post‐ganglionic compound action potential. The same tetanic stimulation also consistently induced a long‐term potentiation of stimulated ACh release. Spontaneous (non‐stimulated) ACh release was not enhanced after tetanic stimulation. The content of ACh in the ganglion was not measurably increased after tetanic stimulation. These results suggest that the long‐term increase in synaptic efficacy is due, at least in part, to an increase in the amount of ACh released by the afferent impulse.

1. Cyclic nucleotide levels and compound action potential magnitudes were measured in frog sciatic nerves following exposure to carbachol, isoprenaline and cyclic nucleotide related substances. 2. The resting cyclic AMP level was 2‐4 p‐mole/mg protein and the cyclic GMP level was 0‐27 p‐mole/mg protein in desheathed nerves. 3. Isoprenaline (100 micrometer) caused a twofold increase in cyclic AMP without affecting cyclic GMP levels. Carbachol (100 micrometer) caused a twofold increase in cyclic GMP without affecting cyclic AMP levels. 4. The phosphodiesterase inhibitor theophylline (5 mM) augmented both cyclic AMP and cyclic GMP. 5. The magnitude of the resting or compound action potential was not affected by isoprenaline, carbachol, or phosphodiesterase inhibitors. 6. The cyclic nucleotides and their butyryl derivatives did not affect the magnitude of the resting or compound action potential, either when applied alone or in the presence of a phosphodiesterase inhibitor. 7. In contrast to sympatic tissue we conclude that hormone mediated cyclic nucleotide metabolism in peripheral nerve is unrelated to control of axonal excitability.

1. Intracellular recordings from post‐ganglionic neurones of the rat superior cervical ganglion revealed two non‐synaptic potentials dependent upon Ca2+, a hyperpolarizing afterpotential (h.a.p.) and a tetrodotoxin (TTX)‐insensitive spike. 2. The h.a.p. followed regeneration discharge of the membrane potential in normal and TTX‐containing Locke solution. 3. The h.a.p. appeared to arise from an increased K+ conductance because it was associated with a decrease in input resistance, reversed at ‐90 mV, and was proportional in magnitude to the extracellular K+ concentration. 4. Tetraethylammonium (TEA) and 4‐aminopyridine (4‐AP) apparently antagonized a voltage‐sensitive K+ conductance because they broadened the action potential. However, these substances reduced only slightly the peak amplitude and earliest phases of the h.a.p. 5. The TTX‐insensitive spike was most apparent when TEA was present and was invariably followed by an h.a.p. with a magnitude proportional to that of the spike. 6. The magnitude of the h.a.p. and the TTX‐insensitive spike was directly proportional to the external Ca2+ concentration and was antagonized by Co2+ and Mn2+ in a dose‐dependent fashion. 7. In normal Locke solution, Ba2+ antagonized the h.a.p. and allowed the neurone to sustain discharge during prolonged depolarization. In Locke solution containing TTX and TEA, Ba2+ reduced the magnitude of the h.a.p. but greatly increased the duration of the TTX‐insensitive spike. 8. The h.a.p. was not significantly affected by altering external Cl‐ concentration and the TTX‐insensitive spike was not reduced by altering external Na+ concentration. 9. It is concluded that the post‐ganglionic neurone supports a regenerative Ca2+ conductance mechanism which in turn triggers an increased K+ conductance. The h.a.p. appears to result from outward K+ current in both a Ca2+ and voltage‐dependent fashion.

1. Post‐ganglionic neurones of the rat superior cervical ganglion were studied in vitro (21‐26 degrees C) using single intracellular micro‐electrode methods. 2. Three Ca2+‐dependent potentials were studied: the shoulder on the normal action potential, the hyperpolarizing afterpotential (h.a.p.), and th Ca2+ spike. 3. Bath‐applied noradrenaline reversibly inhibited these Ca2+‐dependent potentials. The EC50 for inhibition of peak h.a.p. amplitude was about 1 microM. The order of catetholamine potency was: L‐adrenaline > L‐noradrenaline > D‐noradrenaline congruent to dopamine > DL‐isoprenaline. Phentolamine (10 microM), an alpha‐blocker, but not MJ‐1999 (10 microM), a beta‐blocker, antagonized the action of noradrenaline. 4. Noradrenaline (10 microM) hyperpolarized most neurones (1‐6 mV) studied, with no detectable change in resting membrane conductance. 5. Superfusion with low external Ca2+ and high Mg2+ mimicked the effect of noradrenaline. Either procedure alone antagonized the h.a.p. conductance increase but did not alter the h.a.p. reversal potential. However, in the presence of low Ca2+, high Mg2+, the remaining action potential and h.a.p. were not further reduced by noradrenaline. 6. The Ca2+‐dependent shoulder of the action potential did not appear dependent upon GK. Noradrenaline and low Ca2+ antagonized the shoulder when enhanced by TEA+ or Ba2+. 7. Both the rate of rise and amplitude of the Ca2+ spike were antagonized by noradrenaline. 8. We propose that activation of an alpha‐adrenoceptor inhibits a voltage‐sensitive Ca2+ conductance (GCa(V)), thereby reducing the inward Ca2+ current which may generate the noraml action potential shoulder and the rising phase of the Ca2+ spike. Reduction of Ca2+ current would also reduce the Ca2+‐dependent portion of outward K+ current underlying the h.a.p.

1. Nicotinic fast excitatory postsynaptic potentials (fast EPSPs) were recorded intracellularly from postganglionic neurones in the isolated rat superior cervical ganglion. 2. An hours‐long potentiation of the fast EPSP could be induced by brief tetanic stimulation of the preganglionic nerve (5 Hz for 5 s to 20 Hz for 20 s). While long‐term potentiation (LTP) can be detected in every ganglion by extracellular techniques, LTP was induced in only two‐thirds of the nicotinic synaptic responses. 3. Muscarinic blockade with atropine did not prevent LTP of the fast EPSP. 4. LTP of the fast EPSP did not correlate with changes in input resistance nor cell potential, as recorded in the soma. 5. The formation of nicotinic LTP appeared to depend upon stimulation of the nerve terminals. Non‐synaptic tetanic depolarization of the postganglionic neurone, effected by injecting depolarizing current pulses through the intracellular microelectrode, was not sufficient. LTP could be induced by synaptic tetani in two‐thirds of the same neurones. 6. The response to exogenous 1,1‐dimethyl‐4‐phenylpiperazinium (DMPP), a selective nicotinic agonist, was not increased during nicotinic synaptic LTP. This was true whether DMPP was applied by pressure‐ejection from an extracellular micropipette during intracellular recording, or by brief superfusion during sucrose‐gap recording of postganglionic responses. 7. Responses to exogenous acetylcholine and carbachol were increased during nicotinic LTP when these non‐selective cholinergic agonists were applied by pressure‐ejection during intracellular recording. However, the potentiation of the fast EPSP was always at least twofold greater than the potentiation of the response to these exogenous agonists. 8. Potentiation of the responses to acetylcholine and carbachol may have been due to long‐term enhancement of muscarinic responses. Thus, no postsynaptic basis for nicotinic LTP was uncovered in these studies.

The effect of repetitive stimulation on synaptic transmission was studied in the isolated superior cervical ganglion of the rabbit under conditions of reduced quantal content. Excitatory postsynaptic potentials (EPSP) were recorded with the sucrose gap technique to obtain estimates of transmitter release. Four components of increased transmitter release, with time constants of decay similar to those observed at the frog neuromuscular junction at 20 degrees C, were found in the ganglion at 34 degrees C: a first component of facilitation, which decayed with a time constant of 59 +/- 14 ms (mean +/- SD); a second component of facilitation, which decayed with a time constant of 388 +/- 97 ms; augmentation, which decayed with a time constant of 7.2 +/- 1 s; and potentiation, which decayed with a time constant of 88 +/- 25 s. The addition of 0.1-0.2 mM Ba2+ to the Locke solution increased the magnitude but not the time constant of decay of augmentation. Ba2+ had little effect on potentiation. The addition of 0.2-0.8 mM Sr2+ to the Locke solution appeared to increase the magnitude of the second component of facilitation. Sr2+ had little effect on augmentation or potentiation. These selective effects of Ba2+ and Sr2+ on the components of increased transmitter release in the rabbit ganglion are similar to the effects of these ions at the frog neuromuscular junction. Although the effects of Ba2+ and Sr2+ are similar in the two preparations, the magnitudes of augmentation and the second component of facilitation after a single impulse were about 6-10 times greater in the rabbit ganglion than at the frog neuromuscular junction. These results suggest that the underlying mechanisms in the nerve terminal that give rise to the components of increased transmitter release in the rabbit ganglion and frog neuromuscular junction are similar but not identical.

Norepinephrine-3H (NE-3H) and its metabolite normetanephrine-3H were released from the superfused rabbit SCG incubated in tyrosine-3H in response to preganglionic stimulation. Neither dopamine-3H nor its metabolite, 3-methoxytyrosine could be detected in the superfusate from “control” or “nerve stimulated” tissues. Nerve stimulated NE-3H release a) was calcium dependent, b) had a stimulus threshold similar to that of C fibers, c) was not blocked by either 50 micromolar curare or 100 micromolar atropine, d) was abolished by chronic decentralization and e) was elicited by stimulation of both the cervical sympathetic and internal carotid nerves. These results suggest that there is a here-to-for undescribed noradrenergic pathway which originates elsewhere in the sympathetic chain but terminates in the superior cervical ganglion.