Sustained potentiation of transmitter release by adrenaline and dibutyryl cyclic AMP in sympathetic ganglia

Sustained rise in ACh sensitivity of a sympathetic ganglion cell induced by postsynaptic electrical activities

Long-term alteration in synaptic efficacy found in several neurones of both vertebrates and invertebrates has been suggested as an important mechanism for learning and memory. In bullfrog sympathetic ganglia, acetylcholine (ACh) release from presynaptic nerve terminals is potentiated for a long time by adrenaline through a cyclic AMP system. We report here a new form of mechanism for long-term synaptic potentiation in sympathetic ganglia, which occurs postsynaptically in a Ca2+-dependent manner. Our results suggest that Ca2+ entry into a ganglion cell during repeated action potentials initiates a long-lasting mechanism for the enhancement of a nicotinic ACh action on the subsynaptic membrane. This, as well as the presynaptic mechanism, may contribute to neuronal plasticity in the peripheral autonomic nervous system.

Role of calcium in triggering the release of transmitters at the neuromuscular junction

Calcium ions have a key role in triggering the release of packaged transmitter at the amphibian neuromuscular junction and of the chromaffin granules at the adrenal medulla. It is suggested that (i) proteins on the vesicle and plasma membranes are of particular importance in promoting membrane fusion and exocytosis (ii) they may be divalent cation-stimulated ATPases, which form the calcium-binding sites or have a specific calcium-binding protein in close molecular apposition (iii) these ATPases in synaptic vesicles and chromaffin granules also generate a protonmotive force which is associated with the uptake of transmitter (iv) the osmotic properties of the vesicle may be important during fission, but it is not suggested that chemiosmotic effects are involved in Ca2+-triggered fusion (v) the action of calcium is markedly co-operative (vi) the adrenal medullary cell and the n.m.j. may differ in the Ca2+-binding site; there is evidence for the involvement of calmodulin in granule-plasmalemma fusion in the chromaffin cells, but not at present (surprisingly) for a role of this Ca2+-binding protein at the n.m.j. (vii) exocytosis requires MgATP (viii) phosphorylation of the ATPase may well be involved; phosphorylation via cAMP does not seem to be involved in fusion in either system (ix) the ATPase may undergo configurational changes during exocytosis and is markedly sensitive to the physical state of its phospholipid environment and to the oxidation of its -SH groups.

Depolarization of rat isolated superior cervical ganglia mediated by beta 2-adrenoceptors

Depolarizations of freshly-dissected isolated superior cervical ganglia of the rat were recorded extracellularly. The following sympathomimetic amines (in order of decreasing potency) produced depolarizations of up to 0.4 mV: isoprenaline, salbutamol, adrenaline, noradrenaline. Depolarizations were lost after overnight storage, leaving only hyperpolarizing responses. Depolarizations by isoprenaline were antagonized by (-)-propranolol (pA2 8.94 +/- 0.15), (+/-)-butoxamine (pA2 7.36 +/- 0.12) and (+/-)-practolol (pA2 5.14 +/- 0.13). They were not blocked by phentolamine (1 microM) or phenoxybenzamine (1 microM). Isoprenaline and salbutamol were antagonized with equal facility by practolol or butoxamine. In concentrations producing ganglionic depolarization, these compounds also produced a smaller depolarization of presynaptic elements in the ganglion, but not preganglionic trunk fibres. Presynaptic depolarization was blocked by 100 nM propranolol but not by 1 microM phentolamine. Isoprenaline and salbutamol increased the amplitude of the compound ganglionic action potential recorded following single preganglionic nerve stimuli when transmission had been rendered submaximal by adjusting the Ca/Mg ratio, but not in normal solution. Isoprenaline (0.1 microM) also increased the amount of [3H]-acetylcholine released by preganglionic stimulation in low Ca/high Mg solution. It is concluded that facilitatory adrenoceptors are present on pre- and postsynaptic elements in rat superior cervical ganglia, which resembles the 'beta 2' subclass of beta-receptors.

Cyclic adenosine 3',5'-monophosphate and beta-effects in rat isolated superior cervical ganglia

Isoprenaline (0.01-1 microM) increased the amount of cyclic adenosine 3',5'-monophosphate (cyclic AMP) in rat isolated superior cervical ganglia by up to 10 times after 10 min application. Cyclic AMP levels returned to control values after 20 min washing. Salbutamol, in concentrations (1-100 microM) that depolarized the ganglion and facilitated submaximal transmission, did not significantly raise ganglionic cyclic AMP levels. The action of isoprenaline was antagonized by butoxamine (apparent KI approximately equal to 0.14 microM) and weakly by practolol (apparent KI approximately equal to 9.1 microM). The effect of 0.1 microM isoprenaline was also inhibited 94% by 100 microM of the adenylate cyclase inhibitor, 9-(tetrahydro-2-furyl)adenine (SQ 22,536). Exogenous dibutyryl cyclic AMP did not replicate the effects of isoprenaline on ganglionic d.c. potentials or submaximal transmission. The phosphodiesterase inhibitors theophylline, isobutylmethylxanthine or 4-(3,4-dibutoxybenzyl)-2-imidazolidinone (Ro 20-1724) did not potentiate these electrical responses to isoprenaline. The adenylate cyclase inhibitor, SQ 22,536, did not inhibit the electrical responses to isoprenaline. It is concluded that available evidence does not support the view that the ganglion depolarization or facilitation of submaximal transmission in rat isolated ganglia produced by isoprenaline are likely to be mediated by cyclic AMP.

Calcium channel modulation by neurotransmitters, enzymes and drugs

Calcium channels in excitable membranes are of great importance for many cellular functions. Modulation of these channels by neurotransmitters and drugs regulates calcium influx into the cell and thereby alters the functional state of the cell. Recently it has become possible to measure properties of single calcium channels directly and to obtain evidence on mechanisms of their modulation.

Modulation of transmission in rat sympathetic ganglia by activation of presynaptic alpha- and beta-adrenoceptors

1 Conditions under which transmission in rat isolated superior cervical ganglia may be affected by activation of presynaptic alpha- and beta-adrenoceptors have been investigated by means of an extracellular recording method. 2 Clonidine caused a small hyperpolarization of the ganglia (mean EC50 approximately 2 nM) in unstimulated preparations; with continuous preganglionic stimulation at 0.2 Hz, clonidine markedly decreased the height of the compound action potential (mean EC50 approximately 18 nM). 3 Phentolamine (0.1-3 microM) per se increased the height of the compound action potential by up to 15%, and antagonized the inhibitory effects of adrenaline and clonidine. 4 Using a higher frequency of stimulation (0.5 Hz), the effect of phentolamine (1 microM) was unchanged, whereas the inhibitory effectiveness of adrenaline on the height of the compound action potential was reduced. 5 (+/-)-Propranolol (0.1 microM) did not affect the height of the compound action potential, whereas the inhibitory effects of high concentrations of adrenaline were enhanced. 6 During an infusion of clonidine (1 microM), adrenaline (1-100 microM) and, less effectively, noradrenaline (10-100 microM) increased the height of the compound action potential by up to 14%; these effects were antagonized by propranolol (0.1 microM). 7 In the presence of noradrenaline (10 and 30 microM) adrenaline (100 microM) caused a small (up to 5%) enhancement of the height of the compound action potential. 8 The results obtained are consistent with the existence of presynaptic alpha- and beta-adrenoceptors on preganglionic terminals. The alpha-adrenoceptor may be part of a trans-synaptic inhibitory feedback. mechanism; however the functional role of the facilitatory beta-adrenoceptor is not clear.

Reduction of long-term potentiation in the dentate gyrus of the rat following selective depletion of monoamines

1. Brief, high-frequency stimulation of the perforant path results in a long-term potentiation (l.t.p.) of the field response evoked in the dentate gyrus by single shocks to the perforant path. We have compared the magnitude and duration of l.t.p. in normal, anaesthetized rats with animals depleted of noradrenaline (NA), 5-hydroxytryptamine (5-HT), or both. 2. All animals were exposed to an identical sequence of eight high-frequency trains of increasing intensity given over a period of 140 min to the perforant path of one hemisphere. The potential evoked by test shocks to the perforant path was monitored in both hemispheres throughout this period and for a further 96 min after the last train. 3. Plots of the mean potentiation of the population e.p.s.p. as a function of time were computed for all animals in each group. L.t.p. in the NA-depleted group was about 50% of that in the non-depleted control group throughout the course of the experiment. L.t.p. in the 5-HT-depleted group was more severely affected; mean potentiation did not exceed 30% of that in the control group at any time. 4. The duration of l.t.p. was unaffected by NA depletion and reduced by 5-HT depletion. 5. The threshold for the intensity of high-frequency current pulses necessary to elicit l.t.p. was unaffected by NA depletion and raised by 5-HT depletion. 6. Short-term potentiation of the population e.p.s.p. was unaffected by either NA depletion or 5-HT depletion. 7. The effect of monoamine depletion on granule cell excitability was investigated. 5-HT depletion, but not NA depletion, induced an increase in the excitability of the granule cell population, in the sense that a population e.p.s.p. of a given size was associated with a larger population spike. 8. Long-term potentiation of granule cell excitability was not affected by NA depletion, but was reduced by 5-HT depletion. 9. These results show that monoamines can modulate long-term changes in synaptic function in the dentate gyrus, and suggest that 5-HT is more potent in this respect than NA.

Striatal dopamine release in vitro: a beta-adrenoceptor-regulated response not mediated through cyclic AMP

The effects of (-)isoproterenol (10(-6) M), dibutyryl cyclic AMP (10(-3) M), and the phosphodiesterase inhibitor 3-isobutyl-l-methylxanthine (IBMX) (10(-4) M) on in vitro [3H]dopamine ([3H]DA) efflux and synthesis were studied in rat striatal slices continuously superfused with [3H]tyrosine. The beta-adrenoceptor agonist (-)isoproterenol induced an immediate and significant facilitation of [3H]DA efflux but did not alter [3H]DA synthesis as measured by [3H]H2O formation. In contrast, both dibutyryl cyclic AMP and IBMX enhanced [3H]DA synthesis as well as efflux. The presence of IBMX in the superfusing medium did not potentiate the augmentation of [3H]DA efflux caused by (-)isoproterenol. Additionally, the blockade of [3H]DA synthesis by alpha-methyl-p-tyrosine (10(-4) M) completely prevented the action of dibutyryl cyclic AMP on [3H]DA efflux. However, under similar conditions, (-)isoproterenol was still able to increase [3H]DA efflux. The results suggest that (-)isoproterenol can modify striatal DA release through a mechanism not involving cyclic AMP.

Presynaptic muscarinic receptors inhibiting active acetylcholine release in the bullfrog sympathetic ganglion

1 The effects of bethanechol and atropine on the release of acetylcholine (ACh) from bullfrog sympathetic preganglionic nerve terminals were examined electrophysiologically. 2 Bethanechol (1 mM) caused no depolarization of sympathetic preganglionic nerve terminals, whereas carbachol or ACh in the same concentration induced marked depolarizations of these terminals. 3 Bethanechol (10 microM) depressed the amplitude of fast excitatory postsynaptic potentials (e.p.s.ps) recorded in Ca2+-high Mg2+ solution, without depolarizing ganglion cells. The quantal content measured from these fast e.p.s.ps by the variance method showed a significant reduction. 4 Amplitudes of both miniature e.p.s.ps and ACh-potentials induced by iontophoresis of ACh were not affected by addition of bethanechol (10 microM). 5 The depressant effect of bethanechol (10 microM) on fast e.ps.ps disappeared in the presence of atropine (3 microM). 6 Atropine (3 microM) increased the quantal content measured from fast e.p.s.ps recorded in low Ca2+-high Mg2+ solution. 7 The depressant effect of bethanechol (10 microM) on fast e.p.s.ps was unaffected by alpha-adrenoceptor blocking agents (phenoxybenzamine (10 microM) or phentolamine (10 microM). 8 These results suggest that presynaptic nerve terminals in bullfrog sympathetic ganglia possess a muscarinic receptor which inhibits active release of ACh.

Effects of histamine on acetylcholine release in bullfrog sympathetic ganglia

The effects of histamine on the release of acetylcholine (ACh) from bullfrog sympathetic preganglionic nerve terminals were examined by means of intracellular microelectrode techniques. Low concentrations of histamine (1, 3 muM) increased the amplitude of fast excitatory postsynaptic potentials (fast EPSPs) and ACh quantal content, while high concentrations (100, 300 muM) decreased the amplitude and content. Amplitudes of miniature EPSPs and ACh potentials were not affected by histamine (0.3-300 muM). The facilitatory effect of histamine on fast EPSPs disappeared in the presence of mepyramine, whereas the depressant effect of histamine on fast EPSPs disappeared in the presence of cimetidine. These results suggest that histamine has facilitatory and depressant actions on ACh release. The facilitatory action is probably mediated by the H1-receptor and the depressant action by the H2-receptor, both of which are located at the presynaptic nerve terminals of bullfrog sympathetic ganglia.

Modulation of nicotinic transmission by biogenic amines in bullfrog sympathetic ganglia

Studies of transmission in isolated paravertebral sympathetic ganglia of the bullfrog and at the sciatic-sartarius muscle synapse in the frog yielded evidence that biogenic amines such as catecholamines or 5-hydroxytryptamine can modulate transmission in sympathetic ganglia and at the neuromyal junction. These two transmitter substances are able to modulate transmission by affecting the amount of ACh release from presynaptic terminals and also by affecting the sensitivity of nicotinic Ach receptors of the subsynaptic membrane. Information is presented as to how these compounds exert their modulatory effects on these synapses.

Roles of cyclic nucleotides in the synaptic transmission in sympathetic ganglia of rabbits

1. Two kinds of slow postsynaptic potentials can be elicitable in the rabbit superior cervical ganglion: a dopaminergic hyperpolarizing (slow IPSP) one and a muscarinic depolarizing (slow EPSP) one, respectively. 2. Cyclic AMP may not be involved in the generation of slow IPSP, whereas it certainly mediates the dopamine-induced enduring modulatory enhancement of the slow EPSP response. 3. Cyclic GMP appears to mediate at least one of the components in the mechanism underlying the slow EPSP by generating a depolarization with no change in membrane conductance. 4. Cyclic GMP does additionally regulate the dopamine for cyclic AMP)-induced modulatory enhancement of the slow EPSP by antagonizing it in a uniquely time-dependent fashion.