Enhancement Of Transmitter Release Research Articles

Neuromodulators Enhance Transmitter Release by Two Separate Mechanisms at the Inhibitor of Crayfish Opener Muscle

A presynaptic voltage control method has been used to investigate the modulatory effects of serotonin (5-HT) and okadaic acid (OA) on the inhibitory junction of the crayfish opener muscle. Instead of using action potentials, we used 20 msec pulses depolarized to 0 mV to activate transmitter release. This approach allowed us to monitor two separate physiological parameters related to the release process. The first parameter, transmitter release kinetics, is characterized as the delay when inhibitory postsynaptic conductance reaches its half-maximum (IPSG50). The second parameter, the total area of IPSG (IPSGarea), estimates total transmitter output. We have reported previously that the F2 component of synaptic facilitation is associated with a decrease in IPSG50 but without a change in IPSGarea. These results raised the possibility that IPSG50 and IPSGarea could be mediated by separate mechanisms that were modulated independently. To explore this possibility, we investigated the effects of 5-HT (100-200 nM) and OA (2.5 microM) on the two parameters. 5-HT and OA enhanced IPSG neither by changing the sensitivity of postsynaptic receptors, as tested by iontophoretically ejected GABA, nor by elevating resting and action potential-activated presynaptic free calcium, as monitored by fura-2 imaging. 5-HT and OA decreased IPSG50 by 3.0 +/- 1.4 and 3.6 +/- 1.1 msec, respectively, and increased IPSGarea by 50 +/- 21 and 37 +/- 6%, respectively. The ability of F2 facilitation to accelerate release kinetics was reduced in the presence of the modulators, suggesting that the mechanism underlying the accelerated release kinetics was shared by the two modes of synaptic enhancement. This report demonstrates that the acceleration in release kinetics and the increase in total release are two separate mechanisms for enhancing transmitter output and that these two mechanisms can be activated without changes in presynaptic calcium dynamics.

Protein kinase C: a physiological mediator of enhanced transmitter output

Protein kinase C (PKC), activated by either diacylglycerol and/or arachidonic acid, through the activation of presynaptic receptors or nerve depolarization is involved in the enhancement of transmitter release from many neural types. This facilitation is most likely mediated by the phosphorylation of proteins involved in vesicle dynamics although a role for ion channels cannot be ruled out. PKC is not fundamental to the release process but rather has a modulatory role. In some systems the role of PKC is to help maintain transmitter output during prolonged or elevated levels of activation and this seems to parallel suggestions that PKC is involved in the movement of reserve pools of vesicles into release-ready sites. Presynaptic facilitatory actions mediated by PKC are also involved in integrated modulatory functions such as long term potentiation, again where it elevates or maintains transmitter output. Although studies have tried to identify specific roles for the various PKC isoforms, the actions of phorbol esters in elevating transmitter release do not fit with known potencies on individual isoforms and it is suggested that PKC may be located at an intraneuronal location which is difficult to access for lipophilic phorbol esters and further work is required in this area.

Metabotropic Glutamate Receptor–Mediated Control of Neurotransmitter Release

Presynaptic metabotropic glutamate receptors (mGluRs) modulate the release of transmitter from most central synapses. However, difficulties in recording from presynaptic structures has lead to an incomplete understanding of the mechanisms underlying these fundamental processes. By recording directly from presynaptic reticulospinal axons and postsynaptic motoneurons of the lamprey spinal cord, we have obtained electrophysiological and optical evidence that vertebrate presynaptic metabotropic glutamate receptors modulate neurotransmitter release at this synapse through two distinct mechanisms: (1) mGluR activation in the presynaptic terminal depresses transmitter release by activating a presynaptic K + current, and (2) mGluR activation enhances transmitter release by amplifying the action potential–evoked presynaptic Ca 2+ signal by rapidly releasing Ca 2+ from intracellular stores in a Ca 2+-dependent manner. Furthermore, this effect is mediated by physiological release of glutamate from the presynaptic terminals. These autoreceptor–mediated processes are likely to generate complex effects on transmitter release evoked by repetitive stimulation.

Evidence that nerve growth factor plays a role in long-term potentiation in the rat dentate gyrus

An inbred strain of Wistar rat (GH), which is deficient in nerve growth factor (NGF), was used to assess the possible role of NGF in the generation of long-term potentiation in perforant path-granule cell synapses. The data show that NGF was significantly decreased in the dentate gyrus of GH rats, that this deficit was accompanied by an impairment in long-term potentiation (LTP) and that intraventricular injection of NGF substantially reversed this impairment. Analysis of depolarization-induced glutamate release in synaptosomes prepared from dentate gyrus of control rats revealed that NGF alone was without effect, but in combination with the metabotropic glutamate receptor agonist, aminocyclopentane-1,3-dicarboxylic acid (ACPD), NGF induced a significant increase in release. This effect was occluded by prior induction of LTP, suggesting that the interaction between these agents may be required to enhance transmitter release which accompanies LTP in dentate gyrus. In contrast to the effect of NGF and ACPD on glutamate release in control rats, the combination of these agents had no effect on release in synaptosomes prepared from GH rats, which might be explained by the marked decrease in trk receptors in dentate gyrus of GH rats. It was concluded that the impaired ability of GH rats to sustain LTP is associated with a reduction in NGF concentration, a reduction in stimulated release of NGF and a decrease in trk receptors in dentate gyrus. It is proposed that these data indicate a role for NGF in the generation of long-term potentiation in perforant path-granule cell synapses.

Enhanced neurotransmitter release is associated with reduction of neuronal branching in a Drosophila mutant overexpressing frequenin.

Frequenin is a Drosophila Ca2+ binding protein whose overexpression causes a chronic facilitation of transmitter release at the larval neuromuscular junction and multiple firing of action potentials. These functional abnormalities are similar to those found in other hyperexcitable mutants (Shaker, ether-a-gogo, Hyperkinetic) which, in turn, exhibit increased branching at the motor nerve endings. We report here that mutants which overexpress frequenin have motor nerve terminals with reduced number and length of branches as well as number of synaptic boutons. Similar defects are observed in transgenic flies which have additional copies of the frequenin gene indicating that the phenotype can be adscribed to the overexpression of the protein. The ultrastructure of boutons, however, appears indistinguishable from wild type. In addition, we show here that frequenin overexpression leads also to a down regulation of Shaker proteins expression. The contrast between the observations in frequenin and the other hyperexcitable mutants indicates that nerve terminal morphology and enhanced transmitter release do not have a direct causal relationship.

Late developmental changes in the ability of adenosine A1 receptors to regulate synaptic transmission in the hippocampus

Paired-pulse facilitation (PPF) of CA3–CA1 excitatory postsynaptic potentials (EPSP) was compared in hippocampal slices from juvenile (postnatal day (P) 15–21) and young adult rats (P28–P35) following application of adenosine. Relative to juveniles, young adults expressed an increase in baseline synaptic strength that was accompanied by a decrease in PPF suggesting a developmental increase in transmitter release. While adenosine depressed the EPSP slope to a similar extent in juveniles and young adults, PPF increased during adenosine application only for young adults. The differential effect of adenosine on PPF was not due to differences in receptor function or in extracellular ligand levels, since the A 1 antagonist cyclopentyltheophylline (CPT) did not differentially affect PPF across age. Adenosine could increase PPF in juvenile slices under conditions of enhanced transmitter release, through an increase in the bath Ca 2+ concentration, or addition of forskolin to the bath. These data indicate that the ability to modify synaptic transmission through presynaptic adenosine A 1 receptors increases across postnatal development with the maturation of release mechanisms.

Protein kinase C and transmitter release.

1. Protein kinase C (PKC) is an important second messenger-activated enzyme. In noradrenergic nerves it appears to be tonically activated by diacylglycerol (DAG) to facilitate transmitter release and the steps in this involve activation of phospholipase C, generation of DAG and activation of PKC. It is suggested that the subsequent facilitation of transmitter release is due to the phosphorylation of proteins involved in the release process distal to Ca2+ entry, presumably those involved in vesicle dynamics. 2. There are differences between central noradrenergic neurons and sympathetic nerves. In central neurons PKC appears to be tonically active and its inhibition results in a decrease in noradrenaline release under most, if not all, conditions. 3. In sympathetic nerves PKC inhibitors only decrease transmitter release during high-frequency stimulation and not during low-frequency stimulation. At high frequency there is a gradual increase in the effect of PKC inhibitors on transmitter release during the first 15 s of a stimulation train. It is suggested that this is due to a progressive rise in intracellular Ca2+ and a consequent activation of PKC. 4. Activation of PKC by phorbol esters produces a large enhancement in action potential-evoked noradrenaline release in both the central nervous system and in peripheral tissues. The structural requirements of the phorbol esters for maximal effect suggest that the phorbol esters must access the interior of the nerve terminal to activate PKC and the neural membrane acts as a barrier for highly lipophilic phorbol esters, thereby reducing their activity. Activation of PKC represents one of the most powerful ways to enhance transmitter release and may have therapeutic potential.

Enhanced Fast Synaptic Transmission and a Delayed Depolarization Induced by Transient Potassium Current Blockade in Rat Hippocampal Slice as Studied by Optical Recording

In hippocampal neurons, a slowly inactivating aminopyridine-sensitive transient potassium current, D-current, influences the time course of action potential repolarization and therefore activity-dependent Ca2+ entry. We used high-speed optical recording techniques to study the effects of selectively inhibiting D-current with 4-AP (40 microM) on transmission at the Schaffer collateral (CA3)-CA1 synapse in rat hippocampal slices stained with the voltage-sensitive dye RH-155. We observed that addition of 4-AP to the bathing solution resulted in (1) augmentation of a fast component of the optical signal corresponding to the postsynaptic EPSP and action potential, and (2) the appearance of a delayed depolarization of CA1 neurons and other adjacent cells. 4-AP appeared to alter the presynaptic action potential and the dynamics of synaptic transmission to both reduce the sensitivity of the postsynaptic EPSP and action potential to omega-toxin calcium channel blockers (omega-conotoxin GVIA and omega-agatoxin IVA) and the Ca(2+)-dependent potassium channel blocker charybdotoxin, and to increase sensitivity to the dihydropyridine nifedipine, the NMDA receptor blocker aminophosphonopentanoic acid, and the intracellular Ca2+ release inhibitor thapsigargin. The delayed depolarization induced by 4-AP was inhibited in hyperosmotic extracellular solution, suggesting that enhanced transmitter release resulted in increased accumulation of K+ in the extracellular space. Because 4-AP is a convulsant at concentrations similar to those used here, we suggest that the 4-AP-targeted channel(s) carrying D-current may contribute to the hyperexcitability associated with epilepsy.

Induction of transmitter release at the neuromuscular junction prevents motoneuron death after axotomy in neonatal rats.

Motoneurons to rat hindleg muscles die after neonatal nerve injury. Here we show that increasing transmitter release of motor nerve terminals by treatment with 4-aminopyridine, prior to nerve injury at three days, reduces the extent of motoneuron death. Retrograde labelling of soleus motoneurons was carried out in 10-week-old animals that had their sciatic nerve crushed on one side when they were three days old. Only 20% (+/- 4.2 S.E.M.) of the motoneurons survived the nerve injury. A group of animals similarly injured at three days had their calf muscles treated with 4-aminopyridine at birth, prior to nerve injury. In these animals a significantly higher percentage (51 +/- 6.6% S.E.M.) of soleus motoneurons survived. In order to assess the proportion of surviving alpha-motoneurons only, the number of motor units in both the soleus and extensor digitorum longus muscles was established by following the stepwise increments of twitch tension in response to increasing intensity of stimulation of the respective motor nerve. After nerve injury at three days only 18% (+/- 4.1% S.E.M.) of motor units to soleus and 28.5% (+/- 4.9% S.E.M.) to extensor digitorum longus survived and were able to reinnervate their respective muscles. If the nerve injury was preceded by local application of 4-aminopyridine, then the number of motor units present in the reinnervated muscles was significantly greater, so that in soleus 52.7% (+/- 5.4% S.E.M.) and in extensor digitorum longus 52.1% (+/- 2.4% S.E.M.) of motor units were present. This increase of motoneuron survival was reflected in a smaller weight loss and in a better restoration of force production by the pretreated as compared to untreated muscles on reinnervation after nerve injury. It is suggested that enhancing transmitter release from nerve endings in neonatal animals induces the motoneuron to become more resistant to nerve injury.

Examination of the role of cGMP in long-term potentiation in the CA1 region of the hippocampus.

The mechanisms underlying the generation of NMDA receptor-dependent LTP in the CA1 region of the hippocampus continue to receive a great deal of attention because of the postulated importance of LTP as a synaptic mechanism for learning and memory. It is well accepted that the initial induction of LTP occurs in the postsynaptic cell, but the site of expression remains controversial. One prominent hypothesis is that LTP involves the release of one or more retrograde messengers that act on the presynaptic terminal to enhance transmitter release. Recently, evidence has been presented that retrograde messengers function to activate presynaptic guanylyl cyclase and that the resulting rise in presynaptic cGMP levels, when accompanied by presynaptic activity, is responsible for generating an early component of LTP. We have tested this hypothesis by examining whether synaptic strength is increased by coupling tetanic stimulation with application of a membrane-permeable analog of cGMP. The experiments were done in the presence of an NMDA receptor antagonist to block postsynaptic induction mechanisms. Under a variety of experimental conditions, this manipulation failed to generate LTP, suggesting that an increase in cGMP levels accompanied by presynaptic activity is not sufficient to generate LTP in the CA1 region of the hippocampus.

Long-term potentiation in mice lacking synapsins

Synapsin I and synapsin II are widely expressed synaptic vesicle phosphoproteins that have been proposed to play an important role in synaptic transmission and synaptic plasticity. To gain further insight into the functional significance of the phosphorylation sites on the synapsins, we have examined a number of synaptic processes thought to be mediated by protein kinases in knockout mice lacking both forms of synapsin (Rosahl et al., 1995). Long-term potentiation (LTP) at both the mossy fiber (MF)-CA3 pyramidal cell synapse and the Schaffer collateral-CA1 pyramidal cell synapse appears normal in hippocampal slices prepared from mice lacking synapsins. Moreover, the effects on synaptic transmission of forskolin at MF synapses and H-7 at synapses on CA1 cells are also normal in the mutant mice. These results indicate that the synapsins are not necessary for: (1) the induction or expression of two different forms of LTP in the hippocampus, (2) the enhancement in transmitter release elicited by activation of the cAMP-dependent protein kinase (PKA) and (3) the depression of synaptic transmission caused by H-7. Although disappointing, these results are important in that they exclude the most abundant family of synaptic phosphoproteins as an essential component of long-term synaptic plasticity.

Neurotrophins and neuronal plasticity.

There is increasing evidence that neurotrophins (NTs) are involved in processes of neuronal plasticity besides their well-established actions in regulating the survival, differentiation, and maintenance of functions of specific populations of neurons. Nerve growth factor, brain-derived neurotrophic factor, NT-4/5, and corresponding antibodies dramatically modify the development of the visual cortex. Although the neuronal elements involved have not yet been identified, complementary studies of other systems have demonstrated that NT synthesis is rapidly regulated by neuronal activity and that NTs are released in an activity-dependent manner from neuronal dendrites. These data, together with the observation that NTs enhance transmitter release from neurons that express the corresponding signal-transducing Trk receptors, suggest a role for NTs as selective retrograde messengers that regulate synaptic efficacy.

Intrinsic neuromodulation in the Tritonia swim CPG: the serotonergic dorsal swim interneurons act presynaptically to enhance transmitter release from interneuron C2

Heterosynaptic enhancement of transmitter release is potentially very important for neuronal computation, yet, to our knowledge, no prior study has shown that stimulation of one neuron directly enhances release from an interneuron. Here, we demonstrate that in the marine mollusk Tritonia diomedea, the serotonergic dorsal swim interneurons (DSIs) heterosynaptically increase the amount of transmitter released from another interneuron, C2. Stimulation of a single DSI at physiological firing frequencies increases the size of synaptic potentials evoked by C2. This increase in synaptic efficacy is correlated with an increase in homosynaptic paired-pulse facilitation by C2. Thus, it is likely to be due to an enhancement of transmitter release from C2, rather than a postsynaptic action on the followers of C2. This is further supported by the fact that DSI stimulation enhances the strengths of all chemical synapses made by C2 within the swim network, regardless of their sign. Furthermore, DSI enhances the amplitude of C2 synaptic potentials recorded in neurons that DSI itself does not synapse with. Finally, DSI differentially modulates different synaptic inputs to the same postsynaptic target; while increasing C2-evoked EPSPs it simultaneously decreases the size of EPSPs evoked by other DSIs. The heterosynaptic facilitation of C2 synaptic potentials by DSI is not caused by a simple depolarization of C2, but may be a direct action on the transmitter release mechanism. This neuromodulatory effect, which is intrinsic to the circuitry of the central pattern generator for escape swimming in Tritonia, may be important for self-reconfiguration of the swim motor network.

Effect of thiocyanate on AMPA receptor mediated responses in excised patches and hippocampal slices.

The binding affinity of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors for [3H]AMPA is increased 10-30-fold by the chaotropic anion thiocyanate. The present experiments tested if thiocyanate alters AMPA receptor mediated current fluxes and if any such effects are reflected in the waveform of synaptic responses. Currents were measured after a step application of glutamate or AMPA to patches excised from pyramidal cells of hippocampal slice cultures. Application of 1 mM AMPA produced responses with an average peak amplitude of 86 pA at -50 mV and a 10-90% rise time of 1.7 +/- 0.1 ms; the responses desensitized to a steady-state level below 10% of the peak current with a time constant of 11.1 +/- 0.7 ms. Glutamate in presence of D-amino-phosphonopentanoate produced similar responses which were inhibited by 6-cyano-7-nitro-quinoxaline-dione and enhanced by aniracetam or cyclothiazide and thus are characteristic for AMPA receptors. Thiocyanate accelerated the decay of AMPA responses two-fold and reduced the peak current by 30-50% with an EC50 of 3.2 mM which is comparable to its EC50 for enhancing binding. Effects on the desensitization of glutamate induced responses were much smaller and only evident at the highest thiocyanate concentration; no effect was seen on response amplitude. Binding and physiological effects can be adequately explained by assuming that thiocyanate enhances conversion from the sensitive to the desensitized state of the receptor and reduces ligand dissociation from the desensitized state. Synaptic responses were measured in disinhibited hippocampal slices. Perfusion with 20 mM sodium thiocyanate increased the slope of the field excitatory postsynaptic potential by 44.9 +/- 4.2% and reduced its decay time by 10.4 +/- 4.3%. The former effect appears to result at least in part from an increase in transmitter release since it was accompanied by a decrease in paired-pulse facilitation and was reduced in magnitude after enhancing transmitter release. The decrease in the decay time constant points to an effect of thiocyanate on AMPA receptors in situ which is similar to that seen in excised patches. These results demonstrate that an increase in binding affinity may be indicative of reduced rather than enhanced current flow through AMPA receptors. In addition, the results provide further evidence that the kinetics of the AMPA receptor channel contribute significantly to at least the decay phase of fast excitatory synaptic responses.

Presynaptic influence on the time course of fast excitatory synaptic currents in cultured hippocampal cells

Since the lifetime of synaptically released glutamate is thought to be very brief, reflecting diffusion and glutamate uptake, the decay of synaptic currents is thought to represent the average elementary lifetime of a receptor channel bound only once by transmitter molecules. We show here that the decay of evoked non-NMDA synaptic currents can reflect presynaptic factors, in particular, the prolonged action of transmitter at postsynaptic receptors under conditions of enhanced transmitter release. We show that diffusion, high-affinity glutamate uptake, and non-NMDA receptor desensitization are insufficiently rapid to limit the decays of evoked synaptic currents to those of miniature synaptic currents in microcultures of rat hippocampal cells. Our results are consistent with recent studies suggesting that during evoked release, multiple glutamate quanta can interact with overlapping postsynaptic receptor domains.

Inhibition of neutral endopeptidase increases airway responsiveness to ACh in nonsensitized normal rats.

The effects of sensory neuropeptides on the airway responsiveness to acetylcholine (ACh) were investigated in normal nonsensitized rats. The airway responsiveness to inhaled ACh was significantly increased after treatment with neurokinin A (NKA; 0.001%) or substance P (SP; 0.01%) aerosol in the presence of the neutral endopeptidase (NEP) inhibitor. NKA had a more potent effect than SP. Interestingly, the intravenous treatment with NEP inhibitor alone also induced airway hyperresponsiveness (AHR) to inhaled ACh. This AHR was significantly attenuated by pretreatment with a nonselective NK-receptor antagonist, [D-Pro2,D-Trp7,9]SP, systemic capsaicin, or bilateral cervical vagotomy, indicating that decreased NEP activity results in accumulation of endogenous sensory neuropeptide(s) and enhancement of vagal reflex to cause AHR. The airway responsiveness to ACh of isolated left main bronchus was also increased after treatment with 10(-6) M NKA, but not SP, together with 10(-6) M phosphoramidon. This in vitro AHR to ACh induced by phosphoramidon plus NKA was significantly attenuated by pretreatment with 10(-6) M tetrodotoxin. These findings suggest that overaccumulated sensory neuropeptides, especially NKA, may enhance the probability of transmitter release, probably via NK2 receptors, and that the enhanced transmitter release might be involved in AHR in rats.

A role for calcium/calmodulin-dependent protein kinase in mediating synaptic modulation by a neuropeptide

The crayfish neuropeptide DRNFLRFamide (DF 2) has previously been shown to increase the amplitude of excitatory post-synaptic potentials (EPSPs) in crayfish muscles by enhancing transmitter release from synaptic terminals [18]. This effect involves at least two types of kinase enzyme: one or more kinases which mediate the initial rise in EPSP amplitude, and protein kinase C (PKC) which prolongs the elevation in EPSP amplitude for several minutes [6,7]. The present investigation was aimed at identifying kinases that participate in the initial response. KN-62, an inhibitor of Ca 2+/calmodulin-dependent protein kinase, delayed the rise in EPSP amplitude induced by DF 2. The maximal response to the peptide occurred in about 40 min when KN-62 was present and in 15 min when the inhibitor was absent. KN-62 did not significantly alter EPSP amplitude by itself. KN-04, a structural analog of KN-62 which does not block Ca 2+/CaM kinase activity, did not alter EPSP amplitude or the response to the neuropeptide. These data strongly suggest that Ca 2+/CaM kinase participates in the initial increase in transmitter release.

SNAP-mediated protein-protein interactions essential for neurotransmitter release.

The constitutive fusion of transport vesicles with intracellular membranes requires soluble proteins called SNAPs. Certain presynaptic proteins implicated in synaptic vesicle exocytosis also bind SNAPs, suggesting that SNAPs participate in the calcium-regulated membrane fusion events mediating neurotransmitter release. Here we show that injection of recombinant SNAPs into the giant synapse of squid enhances transmitter release. Conversely, injection of peptides designed to mimic the sites at which SNAP interacts with its binding partners inhibits transmitter release downstream of synaptic vesicle docking. A SNAP-dependent protein complex must therefore mediate transmitter release, showing that transmitter release shares a common molecular mechanism with constitutive membrane fusion.

Effects of pyrocatechol on neuromuscular transmission

Effects of pyrocatechol on neuromuscular transmission were studied both in the frog pectoral-cutaneous muscle and in the mouse phrenic-diaphragmatic preparation by means of extracellular microelectrode recording of synaptic signals. Pyrocatechol applied in a concentration of 0.05 mM increased the frequency of miniature end-plate currents (MEPC) and the amplitude of end-plate current (EPC) by increasing its quantum content. Pyrocatechol also increased the duration of presynaptic response. When voltage-dependent potassium channels had been blocked, pyrocatechol affected neither the EPC quantum content nor the duration of presynaptic response. It is suggested that the pyrocatechol-induced enhancement of transmitter release results from modulatory effects of pyrocatechol on voltage-dependent potassium current in the membrane of a nerve terminal.

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

1. 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. 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. 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. 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. 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. 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. 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.