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

Nitric oxide, cAMP and the biphasic muscarinic modulation of ACh release at the lizard neuromuscular junction

In this study, we characterized the pharmacology and physiology of the automodulation of ACh release at the lizard neuromuscular junction (NMJ). The activation of muscarinic ACh receptors generated a biphasic modulation of synaptic transmission. Muscarine-induced activation of M3 receptors (0-12 min) decreased release, whereas M1 activation (> 12 min) enhanced release. Both phases of the biphasic effect are dependent on nitric oxide. However, cAMP acting via protein kinase A is also necessary for the M1 effect. In summary, we present a novel biphasic role for muscarine and implicate M3 receptors in the inhibition and M1 receptors in the enhancement of transmitter releaseat the cholinergic lizard NMJ.

Long-term potentiation of transmitter exocytosis expressed by Ca2+-induced Ca2+ release from thapsigargin-sensitive Ca2+ stores in preganglionic nerve terminals

We have studied whether Ca(2+)-induced Ca(2+) release (CICR) is involved in the mechanism of long-term potentiation (LTP) at nicotinic synapses of bullfrog sympathetic ganglia. Fast excitatory postsynaptic potentials (fast EPSPs) were recorded in a low-Ca(2+), high-Mg(2+) solution and quantal analysis was applied. The conditioning stimulation of the B-type preganglionic nerve at 20 Hz for 4 min consistently enhanced the amplitude and quantal content of fast EPSP for > 2 h, but only sometimes enhanced the quantal size. The LTP of quantal content produced by the conditioning tetanus was blocked by thapsigargin, a blocker of Ca(2+) pumps at Ca(2+) stores, applied before or after the conditioning tetanus, and by Xestospongin C, a blocker of inositoltrisphosphate (IP(3)) receptors, applied before the tetanus. It was not, however, blocked by ryanodine, a blocker and/or activator of ryanodine receptors, or by propranolol, a blocker of beta-adrenergic receptors. Thus the long-lasting activity of the preganglionic nerve at a high frequency causes the LTP of impulse-evoked transmitter release by the activation of CICR from thapsigargin-sensitive Ca(2+) stores in the nerve terminals. It is likely that a large Ca(2+) entry into the nerve terminals during tetanic activity primes ryanodine-insensitive Ca(2+) release channels for activation.

Haloperidol catalepsy consolidation in the rat as a model of neuromodulatory integration

Haloperidol, a non-selective D(2) dopamine antagonist, both in vitro (1 microM) and in vivo (2.5 mg/kg i.p.), induced a long-term potentiation of K(+)-induced Ca(2+)-dependent release of endogenous noradrenaline and dopamine in rat brain cortical slices, by increasing the content of noradrenaline and dopamine known to be controlled by dopamine auto- and heteroreceptors. Haloperidol administration (2.5 mg/kg i.p.) evoked catalepsy and increased the content of noradrenaline and dopamine in the same structures of the brain. Haloperidol catalepsy consolidated without any additional learning and could be retrieved up to two weeks later by placing the animals in the test box. The catalepsy is disordered and retrieved only in the test box. The catalepsy was more intense on day 14 than on day 7. Injection of haloperidol immediately after conditioning evened the reflex retrieval on the following days. Moreover, learning increased the intensity of catalepsy in animals tested on the day of injection. Repeated testing of the reflex on the following days led to specific modifications of catalepsy retrieval. Pre-conditioned rats exhibited maximal catalepsy when tested immediately after being placed in the test box. These results suggest that both the processes of long-term potentiation and catalepsy consolidation are mediated by the same type of receptors, long-term modulation-inducing receptors. Endogenous neuromodulators, acting non-specifically or diffusely via their respective long-term modulation-inducing receptors, can initiate and consolidate generalized states which form the basis for emotional and motivational states.

Role of adrenergic receptors in veratridine-stimulated amylase secretion from rabbit pancreatic lobules

Sympathetic inhibition of pancreatic enzyme secretion has been attributed to vasoconstriction and direct inhibition of acinar cells. We observed both adrenergic inhibition and facilitation of cholinergic transmission in rabbit pancreatic ganglia, which innervate acini. Here we used pancreatic lobules to determine whether adrenergic receptors also regulate synaptic transmission between pancreatic nerves and acini. Stimulation of pancreatic nerve terminals with veratridine (Ver), an activator of voltage-dependent Na+ channels, resulted in a 102% increase in amylase secretion, which was unaffected by alpha and beta receptor antagonists but inhibited 65% by atropine. At a concentration of 10 microM, norepinephrine inhibited (38%) and epinephrine potentiated (40%) Ver-stimulated secretion. At the same concentration, the alpha2 agonist clonidine (Clon) inhibited (39%), whereas the nonselective beta agonist isoproterenol (Iso) and the selective beta3 agonist BRL 37344 potentiated (71 and 67%, respectively) nerve-stimulated secretion. The effects of Clon and Iso and BRL 37344 were antagonized by yohimbine and propranolol, respectively. Phenylephrine, dobutamine, and terbutaline had no effect. Neither basal, bethanechol-stimulated, nor noncholinergic nerve-stimulated secretion was significantly altered by Clon or Iso. Thus, cholinergic nerve terminals innervating pancreatic acini exhibit both inhibitory alpha2 and atypical facilitatory beta adrenergic receptors. The apparent lack of adrenergic innervation suggests that adrenergic receptors on the nerve terminals of cholinergic pancreatic neurons are under hormonal control by circulating catecholamines. These results provide further evidence that intrinsic pancreatic neurons, which supply most, if not all, of the cholinergic innervation of acini, are important sites of sympathetic regulation of pancreatic exocrine secretion.

Involvement of cGMP-dependent protein kinase in adrenergic potentiation of transmitter release from the calyx-type presynaptic terminal

I have previously reported that norepinephrine (NE) induces a sustained potentiation of transmitter release in the chick ciliary ganglion through a mechanism pharmacologically distinct from any known adrenergic receptors. Here I report that the adrenergic potentiation of transmitter release was enhanced by a phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine (IBMX) and by zaprinast, an inhibitor of cGMP-selective phosphodiesterase. Exogenous application of the membrane-permeable cGMP, 8-bromo-cGMP (8Br-cGMP), potentiated the quantal transmitter release, and after potentiation, the addition of NE was no longer effective. On the other hand, 8Br-cAMP neither potentiated the transmitter release nor occluded the NE-induced potentiation. The NE-induced potentiation was blocked by neither nitric oxide (NO) synthase inhibitor nor NO scavenger. The quantal transmitter release was not potentiated by NO donors, e.g., sodium nitroprusside. The NE-induced potentiation and its enhancement by IBMX was antagonized by two inhibitors of protein kinase G (PKG), Rp isomer of 8-(4-chlorophenylthio) guanosine-3', 5'-cyclic monophosphorothioate and KT5823. As with NE-induced potentiation, the effects of 8Br-cGMP on both the resting intraterminal [Ca2+] ([Ca2+]i) and the action potential-dependent increment of [Ca2+]i (DeltaCa) in the presynaptic terminal were negligible. The reduction of the paired pulse ratio of EPSC is consistent with the notion that the NE- and cGMP-dependent potentiation of transmitter release was attributable mainly to an increase of the exocytotic fusion probability. These results indicate that NE binds to a novel adrenergic receptor that activates guanylyl cyclase and that accumulation of cGMP activates PKG, which may phosphorylate a target protein involved in the exocytosis of synaptic vesicles.

Long-term use-dependent enhancement of impulse-induced exocytosis by adrenaline at frog motor nerve terminals

Adrenaline (5-20 microM) use-dependently increased end-plate potentials (EPPs) in normal Ringer solution (containing d-tubocurarine to partially block acetylcholine receptors) and a low Ca2+, high Mg2+ solution for more than several hours and decreased the coefficient of variation of EPP amplitude in the latter solution in frog neuromuscular junctions. The amplitude and frequency of miniature EPPs and impulse-induced increases in intraterminal Ca2+ concentration were unaffected. Adrenaline thus causes sustained enhancement of impulse-induced exocytosis by acting at a mechanism of exocytosis downstream to Ca2+ entry.

Noradrenaline modulates transmitter release by enhancing the Ca2+ sensitivity of exocytosis in the chick ciliary presynaptic terminal

1. The giant presynaptic terminal of chick ciliary ganglion was used to examine how noradrenaline (NA) modulates neurotransmitter release. The cholinergic excitatory postsynaptic currents (EPSCs) were recorded under whole-cell voltage clamp of the postsynaptic neuron. 2. Although the EPSC was potentiated by NA, the current directly activated by acetylcholine (IACh) was unaffected. NA also increased the quantal contents without changing the quantal size. 3. The NA-dependent potentiation was antagonized by neither phentolamine nor propranolol. The EPSC was also potentiated by adrenaline and dopamine but not by normetanephrine, phenylephrine or isoprenaline. The EPSC was attenuated by clonidine. Therefore, NA potentiated the transmitter release through a receptor pharmacologically different from both alpha- and beta-adrenergic receptors. 4. The Ca2+ increment produced by an action potential (delta[Ca2+]pre) was reduced by NA through an alpha 2-adrenergic receptor. However, when alpha 2-adrenergic receptors were blocked, neither delta[Ca2+]pre nor resting Ca2+ were changed by NA. 5. The [Ca2+]o-EPSC relation was shifted by NA, decreasing the half-saturating [Ca2+]o, without changing the maximum. 6. It is concluded that NA-dependent potentiation of transmitter release was due to an increase in the Ca2+ sensitivity of the exocytotic process. The enhancement of the fusion probability is suggested.

L-DOPA induces concentration-dependent facilitation and inhibition of presynaptic acetylcholine release in the guinea-pig submucous plexus

Neurotransmitter- or neuromodulator-like actions of L-DOPA were investigated with intracellular recordings from submucous plexus neurons of the guinea-pig caecum. L-DOPA at 30 nM augmented the amplitude of fast EPSPs, but did not affect depolarizations elicited by puff application of acetylcholine (ACh). The augmenting effect of L-DOPA on the fast EPSPs was counteracted by L-DOPA methyl ester. The fast EPSPs were depressed by 10 microM L-DOPA, but transiently augmented after rinsing the drug. L-DOPA methyl ester did not affect the inhibitory action of L-DOPA on the fast EPSPs, but antagonized the potentiation following the inhibition. The depolarization elicited by exogenously applied ACh was inhibited by 10 microM L-DOPA. Intracellular Ca2+ concentrations ([Ca2+]i) of the neuronal soma were measured with fura-2 microfluorophotometry. The transient increase in the [Ca2+]i evoked by the somatic action potential (delta[Ca2+]AP) was facilitated by 30 nM L-DOPA, but decreased by the drug at 10 microM. It is concluded that L-DOPA at low concentrations enhances the delta[Ca2+]AP, increasing the neurotransmitter release, but at high dose diminishes the delta[Ca2+]AP, inhibiting the neurotransmission.

Nitric oxide release and long term potentiation at synapses in autonomic ganglia

1. Long-term potentiation (LTP) of synaptic transmission in autonomic ganglia is reviewed, together with the possible role of nitric oxide (NO) in this process. 2. Calcium levels in preganglionic nerve terminals are elevated during at least the induction phase of LTP following a tetanus as well as during LTP induced by transmitter substances acting on the nerve terminals. Of the large number of calcium-dependent processes in the nerve terminal that might affect transmitter release, only calcium-calmodulin has been shown to be important in both the induction and maintenance of LTP. 3. The possibility that there is a decrease in the open time of nerve-terminal potassium channels following a tetanus, leading to an increase in duration of the terminal action potential and hence an increase in calcium influx and transmitter release is considered. There is little evidence for such an effect as yet for preganglionic nerve terminals. 4. Phosphorylation of potassium channels by cAMP-dependent protein kinase can lead to their inactivation with consequent action potential broadening in some systems. Exogenous cAMP enhances synaptic efficacy at preganglionic nerve terminals. Whether this occurs through an inactivation of potassium channels is not known. 5. Nitric oxide (NO) synthase is present in both sympathetic ganglia and the ciliary ganglia. NO increases synaptic efficacy in both ganglia. In at least the case of ciliary ganglion this is due to elevation of quantal secretion. 6. NO can in some conditions increase the terminal action potential duration in ciliary ganglia, probably through decrease in the Ic potassium current. There is evidence that this happens through cGMP modulating cAMP phosphodiesterases, thereby affecting cAMP phosphorylation of the Ic channel. 7. Blocking NO synthase markedly decreases LTP following a tetanus in the ciliary ganglion. The possibility is considered that NO is released from the terminal during a tetanus and through altering cAMP phosphorylation of Ic enhances transmitter release.

Long-term increases in neurotransmitter release from neuronal cells expressing a constitutively active adenylate cyclase from a herpes simplex virus type 1 vector

Signal-transduction pathways mediate a wide range of short-term changes in the physiology of neuronal systems from invertebrates to mammals. However, examples of long-term changes in neuronal physiology mediated by these pathways have been limited to invertebrate systems. In this report, long-term changes in the physiology of mammalian neurons were studied by using genetic intervention to cause a long-lasting activation of the cAMP pathway. The catalytic domain of yeast adenylate cyclase (cyr), encoding a constitutive enzyme activity, was expressed in neuronal cells infected with a defective herpes simplex virus vector (pHSVcyr). In PC-12 cells infected with pHSVcyr, increases were seen in cAMP levels, protein kinase A activity, protein phosphorylation, phosphorylation of the tyrosine hydroxylase protein kinase A site (Ser40), and catecholamine release. Infection of sympathetic neurons with pHSVcyr increased cAMP levels, protein phosphorylation, and catecholamine release. Yeast adenylate cyclase immunoreactivity and elevated cAMP levels were localized to the cell bodies of sympathetic neurons. The increase in neurotransmitter release was both Ca(2+)- and activity-dependent and persisted for at least 1 week after infection of the sympathetic neurons, suggesting that sustained physiological activation of the cAMP pathway may mediate long-term changes in the neuronal physiology of mammalian systems.

Accumulation and release of adrenaline, and the modulation by adrenaline of noradrenaline release from rabbit blood vessels in vitro

The accumulation of (-)-3H-adrenaline (3H-A) by rabbit isolated aorta was studied. In all experiments, monoamine oxidase and catechol-O-methyltransferase were inhibited by treatment with pargyline and 3',4'-dihydroxy-2-methyl-propiophenone, respectively. The relationship between the accumulation of 3H derived from 3H-A and the duration of incubation was linear. The 3H-accumulation after 3 h incubation was 22.5 ml/g. In reserpine-treated tissue, the 3H-accumulation levelled off after 30 min and was 8.5 ml/g after 3 h. The concentration of 3H-A or (-)-3H-noradrenaline (3H-NA) and the 3H-accumulation (ml/g) were inversely related. At 10(-8) M, the 1-hour accumulation of 3H derived from 3H-A and 3H-NA was 7.8 and 15.2 ml/g, respectively. With increasing concentrations the accumulation values approached each other. The accumulation of 3H derived from 3H-A by reserpine-treated tissue also showed an inverse relationship with concentration. The accumulation of 3H derived from 3H-A was dependent on the bath temperature. Storage of tissue (0-5 days in salt solution without equilibration with 95% O2/5% CO2; 4 degrees C) did not affect the accumulation of 3H derived from 3H-A. Thereafter (7-14 days), the accumulation decreased. The inhibitory potency (IC50; -log M) of desipramine, cocaine, propranolol, isoprenaline, and normetanephrine on accumulation of 3H derived from 3H-A was found to be 8.26; 6.50; 5.48; 4.88, and 4.02, respectively. The maximal degree of inhibition was almost the same for these drugs, while that of clonidine and corticosterone was 50 and 20%, respectively. In the presence of desipramine, either clonidine, corticosterone or isoprenaline reduces the accumulation of 3H derived from 3H-A. Ouabain and iodoacetic acid, but not sodium cyanide and 2,4-dinitrophenol, reduced the accumulation of 3H derived from 3H-A. Anoxia (95% N2/5% CO2; 37 degrees C; 1-24 h) did not alter the accumulation of 3H derived from 3H-A. Glucose deprivation alone or combined with anoxia markedly reduced the 3H-accumulation. The release of 3H-A from rabbit isolated aorta was studied. This release was compared with that of 3H-NA. The stimulation-evoked 3H-overflow from aorta preloaded with 3H-A decreased with repeated stimulation. In contrast, prestimulation enhanced subsequent stimulation-evoked 3H-overflows. For both 3H-amines, the 3H-overflow increased concomitantly to the same degree with the number of pulses. The time course of 3H-overflows with either 3H-A or 3H-NA was compared.(ABSTRACT TRUNCATED AT 400 WORDS)

Long-term potentiation induced by a sustained rise in the intraterminal Ca2+ in bull-frog sympathetic ganglia

1. The mechanism of a long-term potentiation of transmitter release (pre-LTP) induced by a tetanic stimulation (33 Hz for 1-30 s) applied to the preganglionic nerve was examined by intracellularly recording the fast excitatory postsynaptic potentials (fast EPSPs) in bull-frog sympathetic ganglia. 2. Short-term facilitation induced by paired pulses was decreased during the course of pre-LTP; the extent of reduction paralleled with the magnitude of pre-LTP. 3. The frequency of miniature EPSPs increased after tetanic stimulation that produced the pre-LTP. 4. The Ca2+ ionophore, A23187, increased both the amplitude and quantal content of fast EPSPs and frequency of miniature EPSPs while it decreased short-term facilitation. 5. A Ca2+ chelating agent, Quin-2, loaded as acetoxymethyl ester, reduced the amplitude and quantal content of fast EPSPs and short-term facilitation, and blocked the generation of pre-LTP. 6. Activators of protein kinase C, phorbol 12,13-dibutyrate and 1-oleoyl-2-acetyl-rac-glycerol, and its inhibitors, H-7 and staurosporine, did not block the generation of pre-LTP, while the activators enhanced transmitter release. 7. Inhibitors of calmodulin, trifluoperazine and W-7, blocked the generation of pre-LTP, whereas the amplitude and quantal content of fast EPSPs were not influenced. 8. These results suggest that the pre-LTP results from a sustained rise in the basal level of intraterminal Ca2+ and an activation of the Ca(2+)-calmodulin-dependent process in the preganglionic nerve terminals.

The role of cyclic AMP in chemoreception in the rabbit carotid body

The present study identified physiological factors which influence the generation (and degradation) of cyclic AMP (cAMP) in the arterial chemoreceptor tissue of the mammalian carotid body. Experiments established a 3-way correlation between cAMP generation, neurotransmitter release from chemoreceptor cells, and carotid sinus nerve (CSN) activity. Incubation of carotid bodies in vitro for 10 min in media equilibrated with different low O2 ('hypoxic') gas mixtures (5% O2 or 10% O2, balance N2) elevated basal cAMP levels (100% O2 media) in proportion to the stimulus intensity. Similar experiments using nodose sensory ganglia showed that low O2 stimulation did not alter cAMP levels in this non-chemosensory tissue. However, the adenylate cyclase (AC) activator, forskolin (10 microM), evoked large increases in the cyclic nucleotide content in both carotid bodies and nodose ganglia. After chronic (10 days) CSN denervation or sympathectomy, the basal levels of cAMP in the carotid body were elevated; the cAMP response to low O2 media (stimulus minus control) was increased after CSN denervation but remained unaltered after sympathectomy. The effects of zero Ca2+ media on cAMP generation was examined in order to assess whether feedback from released neurotransmitters acting on known (presynaptic) type I cell receptors could have contributed to the observed changes in cAMP. Basal levels of cAMP were increased 2.8-fold, and the response to hypoxic stimulation was elevated 5-fold, in the absence of extracellular Ca2+. Forskolin (10 microM) did not alter basal release of [3H]-catecholamines ([3H]CA; synthesized from [3H]tyrosine), or resting CSN discharge; however, stimulus-evoked [3H]CA release and CSN discharge were potentiated in the presence of forskolin.(ABSTRACT TRUNCATED AT 250 WORDS)

The role of cyclic AMP and its protein kinase in mediating acetylcholine release and the action of adenosine at frog motor nerve endings

1. The importance of adenosine 3':5'-cyclic monophosphate (cyclic AMP) and its protein kinase (protein kinase A, PKA) in promoting acetylcholine (ACh) release was studied at frog motor nerve endings. The effects of cyclic AMP-dependent protein phosphorylation on the action of adenosine receptor agonists were also investigated. 2. Cyclic AMP was delivered to a local region of the cytoplasm just beneath the plasma membrane of motor nerve endings using phospholipid vesicles (liposomes) as a vehicle. Cyclic AMP in liposomes produced a parallel reduction in the mean level of evoked ACh release (m) and spontaneous ACh release (miniature endplate potential frequency; m.e.p.p.f) in most experiments. These inhibitory effects of cyclic AMP on quantal ACh release resemble the action of adenosine. 3. The effects of global increases in cytoplasmic cyclic AMP concentrations using lipophilic cyclic AMP analogues were generally different from those observed with cyclic AMP. 8-(4-Chlorophenylthio) cyclic AMP (CPT cyclic AMP) produced approximately two fold increases in m and m.e.p.p.f. Dibutyryl cyclic AMP (db cyclic AMP) also increased m and m.e.p.p.f, with the effect on m being smaller and more variable. 4. All three cyclic AMP analogues reduced the effects of adenosine receptor agonists on spontaneous and evoked ACh release. 5. The roles of protein phosphorylation in mediating ACh release and the inhibitory effects of adenosine were studied with the protein kinase inhibitor H7. H7 (30-100 microM) produced no consistent effect on evoked or spontaneous ACh release. At these concentrations, however, H7 exerted an unfortunate inhibitory action on the nicotinic ACh receptor/ion channel. 6. H7 prevented the increases in spontaneous ACh release produced by CPT cyclic AMP (250 microM). Thus H7 is likely to inhibit PK A in frog motor nerve endings. 7. H7 did not alter the inhibitory effect of adenosine on evoked and spontaneous ACh release. 8. The results suggest: (i) that the adenylyl cyclase-cyclic AMP-PK A system is compartmentalized within the motor nerve terminal, (ii) that phosphorylation does not play a major role in ACh release and (iii) the cyclic AMP-PK A system modulates rather than mediates the inhibitory effects of adenosine.

Inhibition of a cAMP-dependent Ca-activated K conductance by forskolin prolongs Ca action potential duration in lamprey sensory neurons

Intracellular recordings from primary mechanosensory neurons (dorsal cells) of the lamprey spinal cord were made to test the membrane effects of forskolin, an activator of adenylate cyclase in these cells. At a concentration of 50 microM, forskolin was found to have a pronounced broadening effect on calcium action potentials (Ca APs) produced in the presence of voltage-activated K channel blockers (TEA, 3,4-DAP). Forskolin had no effect on passive membrane properties of the cells, such as resting potential or input resistance. Nor did it affect delayed rectification or Na APs and thus appeared not to block voltage-activated K channels. Forskolin's effect was evident only when a significant Ca component to the AP was present. It appeared not to increase the conductance of the Ca channel since its action was accompanied by a decrease in membrane conductance during the Ca AP, indicating instead an inhibition of a repolarizing Ca-activated conductance, other than a Ca-activated Cl conductance. The prolongation of Ca APs by forskolin, barium or the neurotransmitter GABA were all correlated in voltage-clamp with a decrease in outward current. Under the conductions used here, the major outward conductance in dorsal cells is a Ca-activated K conductance (gK(Ca]28, and it is concluded that the most probable mode of action for forskolin is via a cyclic AMP-mediated inhibition of this conductance. GABA has also been shown to prolong Ca APs in lamprey dorsal cells by inhibiting a repolarizing gK(Ca)28. Thus, the action of this transmitter may be mediated by an increase in intracellular cyclic AMP.

Adrenaline inhibits muscarinic transmission in bullfrog sympathetic ganglia

The effect of adrenaline (Ad) on muscarinic transmission was examined in B neurones of bullfrog sympathetic ganglia by using intracellular and voltage-clamp recording methods. Bath-application of Ad (5-500 microM) caused a depression of the slow excitatory postsynaptic potential (EPSP) elicited by repetitive stimulations of preganglionic nerve fibres in the presence of curare (30 microM). Ad also depressed the 'muscarinic' ACh potential induced by ionophoretic application of ACh directly to curarized sympathetic neurones in a concentration-dependent manner. Isoprenaline mimicked the effect of Ad in producing the inhibition of the 'muscarinic' ACh potential. Propranolol antagonized the inhibitory action of Ad. Dibutyryl adenosine 3',5'-monophosphate had no significant effect on the 'muscarinic' ACh potential. Under voltage-clamp conditions, Ad caused an inward current associated with inhibition of the M-current (Brown and Adams 1980). Ad depressed the amplitude of slow postsynaptic currents produced by applications of ACh and muscarinic. At a concentration of 100 microM, Ad produced a 68 +/- 8% (n = 12) depression of the amplitude of the muscarinic ACh current. The inhibition of muscarinic transmission induced by Ad is due to a direct suppression of the muscarinic current at the postsynaptic membrane in bullfrog sympathetic ganglia.

Muscarinic receptors modulating acetylcholine release from insect synaptosomes

1. Cholinergic synapses in the central nervous system of insects contain inhibitory muscarinic receptors whose stimulation by agonists leads to a diminished output of acetylcholine; antagonists, like atropine, facilitate acetylcholine release. 2. The receptors involved appear to be of the M2-subtype. Upon activation of presynaptic receptors a significant reduction of the intrasynaptosomal cyclic AMP level as well as a significantly increased membrane potential was observed. 3. The observed membrane hyperpolarization is apparently not a consequence of a lower cyclic AMP level, thus both effects may offer alternative or synergistical mechanisms for modulating transmitter release.

Effects of oxotremorine and RMI 12330 A on [3H]acetylcholine release and adenylate cyclase activity in guinea pig superior cervical ganglion

There is considerable evidence that adenosine 3',5'-cyclic monophosphate (cAMP) is involved in the modulation of synaptic transmission in the guinea pig superior cervical ganglion (SCG). Presynaptic muscarinic receptors are known to attenuate, when activated, acetylcholine (ACh) release in the periphery as well as in the brain. Thus, the possible relationship between ganglionic adenylate cyclase activity and the output of ACh from electrically stimulated ganglia, preloaded with [3H]choline, was investigated. The muscarinic agonist oxotremorine significantly reduced in a dose-dependent manner the electrically evoked neurotransmitter release. The adenylate cyclase inhibitor N-(cis-2-phenylcyclopentyl)azacyclotridecan-2-imine hydrochloride (RMI 12330 A) also decreased ACh output. The inhibitory effects of these two drugs were additive. In crude ganglion membrane fractions oxotremorine significantly inhibited adenylate cyclase activity. The results indicate that drugs capable of inhibiting adenylate cyclase, significantly decrease ACh output from preganglionic nerve terminals in guinea pig SCG.

Methionine enkephalin presynaptically facilitates and inhibits bullfrog sympathetic ganglionic transmission

The effects of [Met5]enkephalin (ME) on the fast excitatory postsynaptic potential (EPSP) in bullfrog sympathetic ganglion cells were studied with intracellular recording in vitro. The variance and failure methods were used to calculate quantal content and quantal size of the fast EPSP in a low Ca2+/high Mg2+ solution. High concentrations of ME (1 and 10 microM) decreased the amplitude and the mean quantal content of the fast EPSP, whereas low concentrations (10 pM to 10 nM) of the peptide increased EPSP amplitude and quantal content. The mean quantal size of the EPSP was not changed by ME. The inhibitory effect of ME at high concentration (10 microM) was reversibly antagonized by the same concentration of naloxone; the facilitatory effect of ME at low concentration (1 nM) was not affected by 10 times higher concentration of naloxone (10 nM), but inhibited by 10 microM naloxone. A low concentration of naloxone (100 pM) itself increased the amplitude and the mean quantal content of the fast EPSP without changing the mean quantal size. The other concentrations of naloxone used in this study (1 pM to 10 microM) caused no significant change in the fast EPSP. ME (100 fM to 10 microM) and naloxone (1 pM to 10 microM) did not change the resting membrane potential or input resistance, the amplitude and duration of action potentials, and the sensitivity to the acetylcholine applied by iontophoresis. These results indicate that ME may act on preganglionic nerve terminals either to facilitate or to depress transmitter release; the inhibitory action is naloxone sensitive, whereas the facilitatory action is less sensitive to naloxone.

Mechanisms regulating the adrenaline-induced long-term potentiation in bullfrog sympathetic ganglia

Two regulatory mechanisms on the long-term potentiation of transmitter release induced by adrenaline (adr.-l.t.p.) in bullfrog sympathetic ganglia were studied by recording intracellularly the fast excitatory postsynaptic potentials. An increase in exposure time to adrenaline from 10 min to 60 min did not enhance the magnitude of adr.-l.t.p. However, increasing an exposure time to dibutyryl cyclic AMP (1 mM) up to 60 min progressively enhanced the magnitude of the nucleotide-induced potentiation, indicating the desensitization of the beta-adrenoceptor. The desensitization remained at 20 min after the removal of adrenaline in all the five cells but disappeared at 60-90 min in four cells out of eight. Under the latter condition, the second l.t.p. was summated on the first one. Dibutyryl cyclic GMP (100 microM) blocked the generation of the l.t.p. induced by dibutyryl cyclic AMP (1 mM) as well as that of adr.-l.t.p. Muscarine (10 microM) or adenosine (1 mM), a possible candidate for raising intraterminal cyclic GMP, did not significantly affect adr.-l.t.p. These results suggest that adr.-l.t.p. is regulated by the desensitization of beta-adrenoceptor and a process which involves endogenous cyclic GMP acting on a step subsequent to the cyclic AMP production.

Effects of synthetic omega-conotoxin on synaptic transmission

The effects of chemically synthesized omega-conotoxin GVIA (a neurotoxic peptide from Conus geographus) on synaptic transmission at the bullfrog sympathetic ganglion, frog neuromuscular junction and electric organ of the ray, Narke japonica, were studied. The synthetic toxin irreversibly suppressed synaptic transmission at these synapses by arresting the release of transmission from the nerve terminals without showing postsynaptic effects. This action of the toxin was effectively antagonized by high concentrations of extracellular Ca2+. The synthetic toxin irreversibly blocked the Ca2+-dependent action potential of bullfrog sympathetic ganglion cells. These results suggest that omega-conotoxin GVIA blocks synaptic transmission by interfering with the Ca2+ influx through the voltage-sensitive Ca2+ channel of the nerve terminal. These results indicate that the chemically synthesized omega-conotoxin GVIA acts exactly like the natural omega-conotoxin GVIA. Thus, the synthetic toxin can be used in place of the natural toxin as a useful probe for the voltage-sensitive Ca2+ channel in the nervous system.

Cyclic nucleotide potentiation of muscarinic responses in neuroblastoma cells

The role of cyclic nucleotides in modulating acetylcholine-induced and dopamine-induced responses was examined with cultured neuroblastoma N1E-115 cells by means of intracellular recording techniques. Acetylcholine-induced muscarinic hyperpolarization and muscarinic depolarization were potentiated by bath application of a dibutyryl analog of adenosine 3',5'-phosphate (cyclic AMP) or phosphodiesterase inhibitors, 3-isobutyl-1-methylxanthine and 4-(3-butoxy-4-methoxybenzyl)-2-imidazolidinone. Dibutyryl cyclic AMP did not affect the resting membrane potential and membrane resistance. Acetylcholine-induced nicotinic depolarization was unaffected by dibutyryl cyclic AMP or phosphodiesterase inhibitors. Intracellular pressure injection of cyclic AMP caused a potentiation of muscarinic hyperpolarization and muscarinic depolarization without marked change in the resting membrane potential. Nicotinic depolarization and dopamine depolarization were not affected by cyclic AMP injection. Among the possible metabolites of cyclic AMP, injection of adenosine potentiated muscarinic hyperpolarization, but did not change nicotinic depolarization and dopamine depolarization. Injection of guanosine 3',5'-phosphate (cyclic GMP) potentiated muscarinic hyperpolarization and muscarinic depolarization without effect on nicotinic depolarization and dopamine depolarization. We conclude that cyclic AMP and cyclic GMP enhance muscarinic responses in neuroblastoma cells. It is suggested that synaptic transmission in the nervous system may be modulated postsynaptically by changes in intracellular cyclic nucleotide levels.

Actions of vasoactive intestinal polypeptide in superior cervical ganglion of the cat

Effects of vasoactive intestinal polypeptide (VIP) in the superior cervical ganglion (SCG) of the cat were investigated in situ. Vasoactive intestinal polypeptide (0.075-1.5 nmol) induced a dose-dependent depolarization of the superior cervical ganglion, characterized by a long-latency, slow onset, small amplitude and long duration. Initial short-lasting hyperpolarization of the superior cervical ganglion was observed in 5 out of 8 experiments after administration of VIP in a dose of 0.75 nmol. Vasoactive intestinal polypeptide (0.075-1.5 nmol) unmasked the muscarinic slow negative potential, following the compound action potential and unmasked or enhanced the stimulus-bound decremental oscillatory potentials induced by the muscarinic agonist McN-A-343 in the superior cervical ganglion. Vasoactive intestinal peptide induced a slight, but significant increase in the amplitude of the postganglionic action potentials evoked by preganglionic stimulation. The data mentioned above suggest that VIP may act as a neuromodulator of cholinergic transmission at the ganglionic synapse.

The effects of an adenylate cyclase inhibitor on the electrophysiological correlates of neuromuscular transmission in the frog

The presynaptic and postsynaptic effects of MDL 12,330A, an adenylate cyclase inhibitor in several biological tissues, were studied at motor endplates in frog cutaneous pectoris nerve-muscle preparations. This agent increased both spontaneous quantal acetylcholine (ACh) release and neurally-evoked ACh release approximately twofold during the first 20-40 min of application. The increased ACh release was accompanied by a profound irreversible depression in the amplitudes of the miniature endplate potentials (m.e.p.ps) and endplate potentials (e.p.ps). The response to iontophoretically-applied ACh was reduced in parallel with the amplitude of the spontaneous m.e.p.ps, indicating that the depression of synaptic transmission was postsynaptic in origin. Endplates were voltage-clamped to study the postsynaptic depression in more detail. It was observed that the peak endplate current (e.p.c.) was depressed without concomitant changes in: the kinetics of e.p.c. decay, the relationship between peak e.p.c. and membrane potential, the ACh equilibrium potential or the voltage sensitivity of the e.p.c. decay. This suggests that MDL 12,330A reduces the postsynaptic sensitivity to ACh by a voltage-dependent block of the cholinoceptor. The presynaptic enhancement and the postsynaptic depression of junctional transmission produced by MDL 12,330A are discussed in conjunction with current theories of the role of adenylate cyclase and cyclic nucleotides at nicotinic cholinergic synapses.

Presynaptic effects of cholecystokinin octapeptide on neuromuscular transmission in the frog

Intracellular recordings were obtained from the frog sartorius muscle end-plate to investigate the effects of cholecystokinin octapeptide (CCK-8) on cholinergic transmission at the neuromuscular junction. A brief bath-application of CCK-8 (1 microM) produced a depression, followed by a long-lasting facilitation, of the amplitude and the quantal content of the end-plate potential (epp). CCK-8 had a biphasic effect, an initial depression followed by an augmentation of the frequency of the miniature epps. CCK-8 did not affect the sensitivity of the nicotinic receptor at the end-plate. These results suggest a significant role for CCK-8 in cholinergic transmission, possibly as a modulator of the evoked release of acetylcholine from motor nerve terminals.

Mechanism of long-term potentiation of transmitter release induced by adrenaline in bullfrog sympathetic ganglia

A mechanism of the long-term potentiation of transmitter release induced by adrenaline (ALTP) was studied by recording intracellularly the fast excitatory postsynaptic potentials (fast EPSPs). The ALTP was produced during the blockade of K+ channels at the presynaptic terminals by tetraethylammonium (TEA). The synaptic delay, possibly reflecting a relative change in the duration of an action potential at the presynaptic terminal, was not changed during the course of the ALTP. By contrast, it was significantly lengthened by TEA and other K+ channel inhibitors (4-aminopyridine and Cs+) that markedly enhanced the evoked release of transmitter. The magnitude of facilitation of the fast EPSP, induced by a conditional stimulus to the preganglionic nerve, was decreased during the generation of the ALTP, but was unchanged during the potentiation of transmitter release caused by TEA. These results, together with theoretical considerations applying the residual Ca2+ hypothesis to the facilitation, suggest that the enhancement of transmitter release during the ALTP is not caused by an increased Ca2+ influx during a presynaptic impulse owing to the blockade of K+ channel or the modulation of Ca2+ channel, but presumably is induced by a rise in the basal level of free Ca2+ in the presynaptic terminal.

Long-term potentiation of transmitter release induced by adrenaline in bull-frog sympathetic ganglia

Long-term potentiation (l.t.p.) of transmitter release induced by adrenaline in bull-frog sympathetic ganglia was studied using intracellular recording techniques. The quantal content of the fast excitatory post-synaptic potentials (fast e.p.s.p.s: evoked by the nicotinic action of acetylcholine) was potentiated for more than several hours after treatment with adrenaline (1-100 microM). A similar l.t.p. of quantal content was produced consistently by isoprenaline (10 microM) and only in a certain fraction of cells by dopamine (10 microM). The l.t.p. induced by adrenaline (10 microM) was blocked by a beta-antagonist, propranolol (1 microM), but not by an alpha-antagonist, phenoxybenzamine (1 microM). Dibutyryl adenosine 3',5'-phosphate (dibutyryl cyclic AMP) (0.8-1.0 mM), adenosine 3',5'-phosphate (cyclic AMP) (4 mM), 3-isobutyl-1-methylxanthine (10 microM), caffeine (1-2 mM), and cholera toxin (2 micrograms ml-1) applied for 20-30 min, all caused the l.t.p. of quantal content. By contrast, adenosine 5'-phosphate (AMP) (4 mM) and adenosine (4 mM) had no potentiating action. Treatment of the ganglion with adrenaline (2.5-160 microM) or dibutyryl cyclic AMP (4 mM) for 15-30 min resulted in the l.t.p. of the frequency of miniature e.p.s.p.s. The l.t.p. of quantal content induced by adrenaline was markedly suppressed by lowering temperature from 20-25 degrees C to 11-13 degrees C, and blocked by dibutyryl guanosine 3',5'-phosphate (dibutyryl cyclic GMP) (100 microM) consistently when applied together, but inconsistently when given after adrenaline. The post-synaptic sensitivity to acetylcholine was unchanged for at least 1 h after exposure to adrenaline (2.5-160 microM) or dibutyryl cyclic AMP (0.8-4 mM). It can be concluded that adrenaline produces l.t.p. of transmitter release by activating a cyclic-AMP-dependent metabolic process through the activation of beta-adrenoceptors, and that this mechanism is presumably regulated by a process involving endogenous guanosine 3',5'-phosphate (cyclic GMP).

Presynaptic beta-adrenoceptors

The existence of facilitatory presynaptic beta-adrenoceptors has been shown in approximately 30 tissues of 6 different species including human. A positive feed back loop for further release of the transmitter appears to be activated by an endogenous agonist, epinephrine, taken up and released as a cotransmitter with norepinephrine rather than norepinephrine itself released from peripheral noradrenergic nerve terminals. Presynaptic beta-adrenoceptors are mainly of a beta 2-subtype. Some beta 1-subtype receptors are also suggested. There coexist presynaptic beta 1- and beta 2-adrenoceptors in cat and rat hypothalamus. Higher sensitivity of peripheral presynaptic beta-adrenoceptors to isoproterenol may be implicated in the early development of hypertension in SHR. Epinephrine taken up and released initiates the development of hypertension in rats via activation of these receptors. Increased activation of these receptors by epinephrine may play a role in the development of essential hypertension. The antihypertensive action of beta-antagonists may be in part due to blockade of these facilitatory presynaptic beta-adrenoceptors.

Opposite actions of forskolin at pre- and postsynaptic sites in rat sympathetic ganglia

In isolated rat superior cervical ganglia, forskolin, a powerful activator of adenylate cyclase, augmented the amplitude of fast excitatory postsynaptic potentials. Quantal analysis showed that forskolin acts presynaptically to facilitate the release of the transmitter. The time course of the presynaptic action of forskolin paralleled that of the increase in cyclic AMP level in the ganglia. In addition, forskolin exerted a postsynaptic action on the nicotinic acetylcholine receptor so that the acetylcholine-induced depolarization was depressed. The action of forskolin on the nicotinic acetylcholine receptor seems to be unrelated to the cyclic AMP system.

Effect of the endogenous analgesic dipeptide, kyotorphin, on transmitter release in sympathetic ganglia

The effects of kyotorphin and the synthetic analogue, D-kyotorphin, on cholinergic fast excitatory postsynaptic potentials (fast-e.p.s.ps) were studied using intracellular recordings from bullfrog sympathetic ganglion cells. Kyotorphin and D-kyotorphin (1-100 microM) increased the amplitude and the mean quantal content of the fast-e.p.s.p. without changing the mean quantal size in a low Ca2+/high Mg2+ medium. Kyotorphin and D-kyotorphin (1-100 microM) did not change the resting membrane potential, input membrane resistance, the amplitude and duration of action potentials and the sensitivity to the transmitter, acetylcholine (ACh), of the ganglion cells. The facilitatory effect of D-kyotorphin on the fast-e.p.s.p. was reversibly inhibited by naloxone (10 microM). These results indicate that kyotorphin may increase transmitter release from preganglionic nerve terminals. The possible mechanisms underlying this action of kyotorphin are discussed.

Selective facilitatory effect of vasoactive intestinal polypeptide (VIP) on muscarinic firing in vesical ganglia of the cat

VIP immunoreactivity was identified in nerve fibers and in 10-13% of the neurons in pelvic and bladder ganglia of the cat. Ninety percent of the VIP positive neurons contained acetylcholinesterase. VIP immunoreactivity was not altered in decentralized ganglia 1 week to 8 months after transection of the pelvic and hypogastric nerves indicating that VIP fibers arose from neurons within the peripheral nervous system. The intra-arterial administration of VIP (1-50 micrograms/kg) enhanced the postganglionic discharge elicited by the muscarinic agonist, acetyl-beta-methylcholine, but did not alter the postganglionic firing elicited by the nicotinic agonist, tetramethylammonium or by electrical stimulation of preganglionic axons in the pelvic nerve. VIP did not elicit a postganglionic discharge in untreated ganglia, but did evoke a prolonged discharge in ganglia treated with an irreversible anticholinesterase agent, 217AO. This discharge was not affected by hexamethonium but was blocked by atropine. VIP suppressed the muscarinic inhibition of ganglionic transmission produced by acetyl-beta-methylcholine without altering the response to other inhibitory agents (norepinephrine, leucine-enkephalin and gamma-aminobutyric acid (GABA). VIP (0.1-0.3 micrograms/kg) also had a direct inhibitory effect on bladder smooth muscle. These findings raise the possibility that intraganglionic pathways containing VIP may exert a selective modulatory influence on muscarinic transmission in vesical parasympathetic ganglia.

A quantitative study of structural features, synapses and nearest-neighbour relationships of small, granule-containing cells in the rat superior cervical sympathetic ganglion at various adult stages

Groups and sub-groups (clusters) of small granule-containing cells ("small cells") were analysed at 3 and 6 micron intervals and in serial sections, in rats aged 2-13 months. Fully intraganglionic clusters of small cells were all found to receive an incoming ("afferent") innervation, of the order of 3-6 afferent terminals per cell, derived from axons of preganglionic type via multifocal, symmetrical, mainly axosomatic synapses. No evidence was obtained of sharing of preganglionic inputs between small cells and principal neurones. Intraganglionic clusters also regularly gave outgoing ("efferent") synapses of the asymmetrical type, of the order of 2-6 per cell, to intraganglionic nerve elements; 30-50% of these synapses were given from somata, 50-70% from processes of the small cells. Whenever the postsynaptic structure was identifiable these synapses were all found to be given to postganglionic neurones or their dendrites, principally to spine-like processes or slender twigs. In some ganglia a few efferent synapses to other small cells were observed; these were of the symmetrical type. Efferent synapses to nerve profiles resembling chemosensory axon terminals, also of the symmetrical type, were extremely infrequent (fewer than 1% of all efferent synapses) in intraganglionic small cell groups and appeared virtually restricted to glomus-like clusters of small cell, which lay intracapsularly, or in and near the bases of nerves entering or leaving the ganglion. Almost all groups and clusters of small cells were located near to fenestrated capillary vessels, which are not found elsewhere in the ganglion. The implications of possible non-synaptic release of material from small cells via membrane regions not covered by satellite cell cytoplasm, were explored in a nearest-neighbour analysis. These "exposed" regions comprised 1-3% of the small cell surface, a proportion comparable with those engaged in receiving afferent synapses or in giving efferent synapses. The majority of such regions faced toward other nerve profiles (axons and dendrites) ensheathed in satellite cytoplasm (mean 30%), intraganglionic tissue spaces wider than 3 micron (mean, 30%) or other small cells (mean, 14%); 25% faced toward blood vessels, but of these vascularly directed regions, only one fifth (or 5% of the total) on average faced directly toward fenestrated endothelium, the rest being non-fenestrated and/or separated by pericyte processes from the exposed regions of small cell membrane. Thirty-three percent of the small cells in a sample of 242 lay within 2 micron of the nearest blood vessel.(ABSTRACT TRUNCATED AT 400 WORDS)

Long-term potentiation of transmitter release induced by repetitive presynaptic activities in bull-frog sympathetic ganglia

Long-lasting potentiation of transmitter release induced by repetitive presynaptic activities in bull-frog sympathetic ganglia was studied by recording intracellularly fast excitatory post-synaptic potentials (fast e.p.s.p.s.). Following a brief period of post-tetanic potentiation or depression (less than 10 min), the amplitude of the fast e.p.s.p. was potentiated for a period between several tens of minutes and more than 2 h in response to tetanic stimulation of the preganglionic nerve in twenty-one out of twenty-eight cells. Quantal analysis revealed that this long-term potentiation of the fast e.p.s.p. (l.t.p.) was accompanied by an increase in quantal content m (in nine out of twenty-one cells), quantal size (four cells) or both (eight cells). The increased quantal content (presynaptic l.t.p.) declined exponentially (ten cells) or decayed gradually to a certain enhanced level which lasted several hours. In contrast, the increased quantal size grew with a relatively long latency (10-25 min) and remained relatively constant for at least 2 h. The magnitude of presynaptic l.t.p. increased with increased duration of the presynaptic tetanus (33 Hz) from 2 to 5 s. No l.t.p. was elicited by a 1-s tetanus, whereas the time course appears to be independent of the tetanus duration and the magnitude of l.t.p. There was a positive correlation between the magnitude of presynaptic l.t.p. and the pre-tetanic quantal content up to m = 3, but the former deviated from linear regression when the value of the latter exceeded 3. No l.t.p. occurred when quantal content was less than 0.5. A tetanus (33 Hz, 10 s) applied in Ca2+-free solution elicited no presynaptic l.t.p., while the same tetanus in normal Ringer solution produced a large presynaptic l.t.p. Presynaptic l.t.p. was enhanced in magnitude at low temperature (8-10 degrees C). These results demonstrate the existence of a use-dependent, long-term potentiation of transmitter release in bull-frog sympathetic ganglia. Several possible mechanisms are discussed in terms of Ca2+-buffering mechanisms of the presynaptic nerve terminals.

Neurophysiology and pharmacology of long-term potentiation in the rat sympathetic ganglion

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 mechanism of the inhibitory action of adrenaline on transmitter release in bullfrog sympathetic ganglia: independence of cyclic AMP and calcium ions

The effects of adrenaline and dibutyryl adenosine 3':5' - cyclic monophosphate (db cyclic AMP) on nicotinic transmission in bullfrog sympathetic ganglia were compared by use of an intracellular recording technique. The evoked release of transmitter, acetylcholine (ACh), was decreased in the presence of adrenaline (10-100 microM), while the postsynaptic sensitivity to ACh was unchanged (10 microM adrenaline) or slightly reduced (100 microM). Transmitter release was similarly inhibited by dopamine (10 microM), but not by isoprenaline (10 microM). The inhibitory action of adrenaline on transmitter release was blocked by phenoxybenzamine but not by propranolol. The inhibition of transmitter release was independent of the external calcium concentration. The evoked release of transmitter and the electrical properties of the postsynaptic membrane were unchanged during exposure to db cyclic AMP (1-4 mM), while the postsynaptic sensitivity to ACh was slightly but significantly depressed. The spontaneous release of transmitter in a high K+ (10 mM) solution was decreased in the presence of adrenaline (100-300 microM), but unchanged with db cyclic AMP (4 mM). In contrast to the effects during exposure, both the evoked and spontaneous release of transmitter were enhanced after the removal of adrenaline or db cyclic AMP. Neither adrenaline (100 microM) nor db cyclic AMP (4 mM) affected the presynaptic spike and synaptic delay. It is concluded that adrenaline mainly inhibits the release of ACh from the presynaptic terminals through its alpha-action, while db cyclic AMP reduces slightly the postsynaptic sensitivity to ACh and that both agents facilitate transmitter release when they are removed from the presynaptic terminals. It is further suggested that the inhibitory action of adrenaline is independent of endogenous cyclic AMP and calcium ions.

Long-term facilitation of peptidergic transmission by catecholamines in guinea-pig inferior mesenteric ganglia

Intracellular recordings were obtained from neurones of isolated guinea-pig inferior mesenteric ganglia. Repetitive stimulation (10-20 Hz for 1-2 s) of the hypogastric nerves evoked, in addition to the fast excitatory post-synaptic potential (e.p.s.p.), a non-cholinergic e.p.s.p. the mediator of which has previously been suggested to be substance P or a related peptide. When applied to the ganglia in the concentrations of 1-100 microM for 3-5 min, adrenaline, isoprenaline and noradrenaline produced an initial, short-lasting depression which was followed by a marked augmentation of the non-cholinergic e.p.s.p. lasting from minutes to over hours. Employed in comparable concentrations dopamine caused a slight depression that was not followed by a detectable increase of the non-cholinergic e.p.s.p. The catecholamine-induced depression and subsequent enhancement of the non-cholinergic e.p.s.p. was prevented by alpha-adrenergic antagonists (dihydroergotamine and phenoxybenzamine, 1-10 microM) and beta-adrenergic antagonists (propranolol and dichlorisoprenaline, 5-10 microM), respectively. The membrane depolarization induced by the putative transmitter substance P (1 microM) was augmented by isoprenaline; the enhancement which could be blocked by beta-antagonists was not preceded by a depression. Application of dibutyryl cyclic AMP (10 microM-1 mM) by either superfusion or intracellular ionophoresis mimicked the enhancing effect of catecholamines. It is concluded that catecholamines, with the noticeable exception of dopamine, exerted a biphasic effect on the non-cholinergic e.p.s.p. of the inferior mesenteric ganglion cells: an initial depression that was mediated by alpha-adrenergic receptors and probably reflected a presynaptic inhibitory effect of catecholamines and, on the other hand, an enduring facilitation mediated by beta-adrenergic receptors which appeared to be linked to activation of post-ganglionic cyclic AMP.

Histamine as an endogenous antagonist of nicotinic ACh-receptor

The mechanism of an inhibitory effect of histamine on the sensitivity of frog skeletal muscle endplate was analyzed by studying the dose-response relation between the quantity of ACh applied iontophoretically and the ACh-induced postsynaptic current (ACh current), and also the interaction between histamine and erabutoxin-b (ETX-b). The results obtained show that histamine, like curare, decreased the sensitivity of ACh-receptor in a competitive manner.

Serotonin and Retzius cell depress the hyperpolarization following impulses of leech touch cell

Intracellular recordings from T mechanosensory cells of Hirudo medicinalis showed, as previously demonstrated, that repetitive firing is followed by a long-lasting hyperpolarization. Serotonin application at two concentrations (1 microM and 50 microM) depressed this hyperpolarization by up to 2/3; the effect was dose-dependent, long-lasting and reversible. Intracellular stimulation of giant serotonergic neurons (Retzius cells, Rz) mimicked serotonin perfusion: the effect was proportional to the number of spikes fired by Retzius cells. The combined use of intracellular iontophoretic injection of horseradish peroxidase and lucifer yellow indicated the possible sites of contact between Rz and T cells. The effect of serotonin, released by Rz cells, is discussed with respect to its possible physiological significance.

Cyclic AMP-mediated potentiation of muscarinic hyperpolarization in neuroblastoma cells

Adenosine, 2-chloroadenosine and prostaglandin E1 which are known to increase cyclic AMP in neuroblastoma cells potentiated the acetylcholine-induced muscarinic hyperpolarization of the cells without changing the resting membrane potential. The potentiation caused by 2-chloroadenosine was further augmented by Ro 20-1724, a phosphodiesterase inhibitor. A direct intracellular pressure application of cyclic AMP potentiated the muscarinic hyperpolarization without changing the resting membrane potential. Morphine which inhibits adenylate cyclase antagonized 2-chloroadenosine-induced potentiation of the muscarinic hyperpolarization. These results suggest that changes in cyclic AMP level modulate the muscarinic response of neuroblastoma cells.

Modulation by dopamine of population responses and cell membrane properties of hippocampal CA1 neurons in vitro

Dopamine (DA) was applied to rat hippocampal slices maintained in vitro. Extracellular and intracellular recording techniques were used to study the effect of DA on population responses, membrane potentials, and membrane responses to hyperpolarizing current pulses in CA1 pyramidal cells. Temporary exposure of hippocampal slices to DA has a dual effect. The initial action of DA is to produce a suppression of the extra-cellularly recorded population responses. In individual neurons, this initial effect is seen as a membrane hyperpolarization accompanied by a decrease in the amplitude of responses to hyperpolarizing current pulses. The frequency of occurrence of spontaneous depolarizations and spikes is reduced. The early action of DA is followed by a profound potentiation of the population responses that can last for hours. This long-lasting potentiation of the population response, induced by DA, is depressed by spiroperidol, a DA antagonist. In individual neurons, the late effect of DA is a long-lasting membrane depolarization associated with an increase in the amplitude of responses to hyperpolarizing current pulses. During this late phase, spontaneous activity is increased, as are single cell responses to stimulation of afferents. The evidence presented here indicates that DA is able to induce a long-lasting modification of the excitability of CA1 hippocampal neurons. This modulation of excitability by DA may be similar in nature to previously described DA-modulatory actions in the peripheral nervous system.

An adenosine agonist inhibits and a cyclic AMP analogue enhances the release of glutamate but not GABA from slices of rat dentate gyrus

The adenosine agonist 2-chloroadenosine inhibited the K+-induced release of endogenously synthesized [3H]glutamate but not [3H]GABA from slices of rat dentate gyrus. In contrast, the K+-stimulated release of [3H]glutamate was augmented by the adenosine antagonist theophylline and was further enhanced by the cyclic AMP analogue 8-bromo-cyclic AMP in the presence of theophylline.

Heterosynaptic facilitation and behavioral sensitization are inhibited by lowering endogenous cAMP in Aplysia

An adenylate cyclase inhibitor, RMI 12330A, is able to depress cAMP synthesis stimulated by serotonin in the abdominal ganglion of Aplysia depilans and punctata. This substance reversibly blocked the heterosynaptic facilitation, induced by activation of serotonergic pathways, of the EPSP recorded from L7 motoneuron in abdominal ganglion after electrical stimulation of the siphon nerve. RMI 12330A, injected into whole unrestrained animals, inhibited the short-term dishabituation of the siphon withdrawal reflex. These findings demonstrate that the increase of endogenous cAMP in the sensory neurons mediating the gill and siphon withdrawal reflex is an essential step in the mechanism of potentiation of the transmitter output underlying heterosynaptic facilitation and short-term behavioral sensitization.

Protein phosphorylation in the brain

Protein phosphorylation represents an approach, sometimes the only approach available, to study the molecular basis for a wide variety of neurophysiological phenomena. The injection of protein kinases or protein kinase inhibitors into neurones has provided direct evidence that activation of protein kinases has an obligatory role in the mechanisms by which numerous extracellular signals produce specific physiological responses in neurones. A diversity of substrate proteins for the kinases have already been found. In several instances, the identity and functional role of these substrate proteins have been established.