Potentiation of Ca2+-dependent K+ activation by theophylline is independent of cyclic nucleotide elevation

New concepts of molecular communication among neurons

Recently a number of complex electrophysiological responses to neurotransmitters have been observed that cannot be described as simple excitation or inhibition. These responses are often characterized as modulatory, although there is no consensus on what defines modulation. Morphological studies reveal certain neurotransmitters stored in what might be release sites without synaptic contact. There is no direct evidence for nonsynaptic release from CNS sites, although such release does occur in the periphery and in invertebrates. Nonsynaptic release might provide a basis for diffuse one-cell-to-many communication, but it might also simply be a means of sending the transmitter to a broader area of a single neuron than occurs in typical synapses. Several kinds of macromolecules have been found to be transported in a retrograde direction – and in some cases transsynaptically. There have been suggestions that some neurons may release more than one type of transmitter. Particularly intriguing is the possibility of release of substances that modulate actions of a primary transmitter. Taken together this range of evidence suggests that neurons may use a variety of forms of molecular communication in addition to traditionally described synaptic transmission.

Several authors have suggested modes of communication distinct from classical synaptic transmission and have classified released substances using terms such as neurohumor, neurohormone, neuroregulator, and modulator. These suggestions have the heuristic value of drawing together diverse kinds of data, but it remains to be established that the pieces fit together in that fashion – for example, that complex electrophysiological effects are associated with substances released nonsynaptically. In order to reduce confusion, a flexible, generic approach to nomenclature for substances released from neurons and for hypothetical modes of communication is recommended. Some behavioral implications of nonconventional transmission are considered.

Methods for studying neurotransmitter transduction mechanisms

This review describes the methodologies used to study the transduction mechanisms that are activated in excitable cells by G-protein-coupled agonists. In view of the complexity of second-messenger systems, it is no longer relevant to ask, "What is the transduction mechanism involved in the action of a given neuromodulator?" because, in many cases, a variety of transduction mechanisms and physiological responses are invoked following receptor activation. This means that a single aspect of the physiological response must be selected for study in order to address the question of transduction mechanism. This review is therefore concerned with a description the use of patch- and voltage-clamp procedures to study transduction mechanism because they are designed to isolate one aspect of the physiological response: the change in activity of a single type of membrane ion channel.

Elevation of intracellular cyclic AMP concentration fails to inhibit adrenaline-induced hyperpolarization in amphibian sympathetic neurons

1. The effect of drugs on the adenosine 3':5'-cyclic monophosphate (cyclic AMP) content of desmethylimipramine (DMI)-treated bullfrog paravertebral sympathetic ganglia was studied by radioimmunoassay. The adrenaline-induced hyperpolarization (Adh) in the tissue was recorded by means of the sucrose-gap technique. 2. In the presence of propranolol (1 microM) and DMI (0.5 microM), adrenaline (1 microM) significantly reduced the concentration of cyclic AMP in forskolin-treated ganglia. This effect was prevented by pertussis toxin (5 micrograms ml-1). 3. The relative potency for drugs which increased ganglionic cyclic AMP content was: 50 microM forskolin much greater than 5 mM fluoride greater than 2 mM fluoride greater than 2 mM isobutylmethylxanthine (IBMX) greater than 5 mM caffeine. In contrast, their relative potency for inhibition of the Adh was: 2 mM IBMX greater than 5 mM fluoride greater than 5 mM caffeine much greater than 2 mM fluoride greater than 50 microM forskolin. The Adh was unaffected by pertussis toxin (5 micrograms ml-1). 4. Although the Adh was slightly reduced by the extracellular application of 8-bromo (8-Br) cyclic AMP, the majority of the data suggest that the transduction mechanism underlying the Adh is independent of the intracellular cyclic AMP concentration and provide an example of an alpha 2-adrenoceptor-mediated response that occurs independently of inhibition of adenylate cyclase.

Effect of theophylline on respiratory muscle function

Theophylline improves diaphragmatic contractility of the respiratory muscles both in isolated muscle preparations, as well as in animals and normal human beings. Furthermore, theophylline restores diaphragmatic fatigue and prevents fatigue of the diaphragm when given prophylactically. Finally, it was recently shown that theophylline improves diaphragmatic function in COPD patients, all of whom were CO2 retainers (PaCO2 53 +/- 3 mm Hg) and hypoxemic (PaO2 57 +/- 8 mm Hg). Patients improved transdiaphragmatic pressure and were less susceptible to fatigue. Presently the mechanisms of action of theophylline regarding its effects on diaphragmatic function are not fully elucidated. Experimental evidence, however, suggests that theophylline may have an effect on transmembrane calcium movements by blocking adenosine receptors.

Effects of cyclic AMP on fluid absorption and ion transport across frog retinal pigment epithelium. Measurements in the open-circuit state

A modified version of a capacitance probe technique has been used to measure fluid transport across the isolated retinal pigment epithelium (RPE)-choroid of the bullfrog. The accuracy of this measurement is 0.5-1.0 nl/min. Experiments carried out in the absence of external osmotic or hydrostatic gradients show that the RPE-choroid transports fluid from the retinal to the choroid side of the tissue at a rate of approximately 10 nl/min (4-6 microliters/cm2 X h). Net fluid absorption (Jv) was abolished within 10 min by the mitochondrial uncoupler 2,4-dinitrophenol. It was also inhibited (70%) by the removal of bicarbonate from the bulk solutions bathing the tissue. Ouabain caused a slow decrease in Jv (no effect at 10 min, 70% at 3 h), which indicates that RPE fluid transport is not directly coupled to the activity of the Na-K pump located at the apical membrane of this epithelium. In contrast to ouabain, cyclic AMP (cAMP) produced a quick decrease in Jv (84% within 5 min). Radioisotope experiments in the open circuit show that cAMP stimulated secretory fluxes of Na and Cl, which accounted for the observed cAMP-induced decrease in Jv. The direction of net fluid absorption, the magnitudes of the net ionic fluxes in the open circuit, and the dependence of Jv on external bicarbonate concentration strongly suggest that fluid absorption is generated primarily by the active absorption of bicarbonate.

Effects of dantrolene and methylxanthines on the sensory nerve terminal of the frog muscle spindle

The application of 1.5-4 microM dantrolene decreased the threshold and the current sensitivity of the rhythmic hyperpolarizations that occur during depolarization of the sensory nerve terminal in the frog muscle spindle. The higher concentration provoked spontaneous rhythmic changes even without depolarization. Methylxanthines (5 mM caffeine, theophylline or pentylene-tetrazole) increased the threshold and the sensitivity. Electron microscopic observations of the dantrolene-treated spindles revealed numerous electron-dense deposits associated with the cytoplasmic membrane of the sensory terminals and with mitochondrial membranes. The deposits were found to contain K+ and Ca2+ by energy dispersive X-ray microanalysis. Electron-dense deposits containing Ca2+ were usually observed in the inner capsular space and in the mitochondria of the sensory terminals perfused by normal or high Ca2+ Ringer solutions. They were reduced in number following incubation with methylxanthines. The amplitudes of afferent spikes and the spindle potential were increased by methylxanthines in much the same way as by K+ channel blockers, suggesting that GK of the terminal membrane may be reduced by methylxanthines. We suggest that methylxanthines may modulate the terminal responses both as a K+ channel blocker and by enhancing the release of Ca2+ from a storage site, perhaps in the inner capsular space, whereas dantrolene has the opposite effect.

Some methodological issues in the conduct of caffeine research

The caffeine literature of the past 10 years shows the frequent recurrence of several weaknesses--the use of weak or erroneous hypotheses in experimental design, design flaws (e.g. the use of excessively high doses) that prevent the derivation of any meaningful implication for humans, the violation of ordinary rules of logic in extrapolating experimental results to human situations (e.g. the application of findings in naive animals to caffeine-tolerant human subjects) and the biased selection of literature citations to support a hypothesis and ignore opposing evidence. Workers who review the literature dispassionately, who use dosages relevant to human caffeine consumption when they wish to extrapolate from animal studies to man, who distinguish carefully between acute and chronic effects and who include proper controls in feeding studies to prevent confusion of the effects of dietary deprivation with the pharmacological effects should succeed in advancing our understanding of caffeine's effects in the human body.

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.

The ionic basis of adenosine receptor actions on post-ganglionic neurones in the rat

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

Divergent effects of methylxanthines and adenylate cyclase agonists on monocyte cytotoxicity and cyclic AMP levels

The effects of theophylline, isobutylmethylxanthine (IBMX), prostaglandin E1 (PGE1), and isoproterenol on monocyte antibody-dependent cytotoxicity (ADCC) were compared with their effects on monocyte cyclic adenosine 3':5'-monophosphate (cAMP) levels. Theophylline (2 mmol/l) halved ADCC and gave a 2-fold increase in cAMP levels. At concentrations not elevating cAMP theophylline inhibited ADCC significantly. In comparison, incubation of monocytes with IBMX, PGE1 and isoproterenol ADCC was only modestly inhibited while these agents gave larger increments (3- to 8-fold) in cAMP levels than theophylline did. Low concentrations of IBMX (50 mumol/l) elevated cAMP without affecting monocyte ADCC whereas PGE1 and isoproterenol inhibited ADCC dose-dependently comparable to increases in cAMP. However, in doses giving similar inhibition of ADCC addition of PGE1 resulted in larger cAMP increments than isoproterenol. The effects of IBMX, PGE1 and isoproterenol was dependent on target cell to effector cell ratio and increased during preincubation with the agents. The inhibition of ADCC by the agents was accompanied by a depressed monocyte lysozyme release and depressed activation of hexose monophosphate shunt. However, only theophylline affected monocyte attachment to sensitized target cells. These results argue against the general inverse relationship between cAMP content and inhibition of monocyte ADCC and demonstrate that theophylline independent on increases in cAMP inhibits ADCC probably by abrogation of monocyte binding activity.

The effect of theophylline on macrophage elastase secretion

Theophylline increased the elastolytic activity in conditioned media from murine peritoneal macrophages maintained in a monolayer system. Continuing accumulation of enzyme activity with time required the continuing presence of the drug. Theophylline was active in the presence of a maximally stimulatory concentration of colchicine, a known enhancer of elastase secretion. The effect of theophylline was not due to interference with any elastase inhibitory activity in macrophage conditioned medium. The elastolytic activity from theophylline-stimulated cells exhibited an inhibitor profile similar to the profiles of colchicine-stimulated and unstimulated cells. The results demonstrate that theophylline regulates the amount of extracellular elastolytic activity from murine peritoneal macrophages differently from granulocytic elastase.

Cyclic AMP, 5-HT, and the modulation of transmitter release at the crayfish neuromuscular junction

In this study it was found that several agents which elevate cAMP levels in cells also increase dramatically the quantity of transmitter released from crayfish excitatory nerve terminals in response to a stimulus. With respect to time course and magnitude, the increase produced by one of these agents, the cyclic nucleotide phosphodiesterase inhibitor Squibb 20,009 (SQ 20,009), is unlike any reported for such a drug at a synapse. Additionally, SQ 20,009 potentiated the facilitation of transmitter release produced by serotonin (5-HT) at this synapse. These results establish a possible role for cAMP in the control and modulation of transmitter release at the crayfish neuromuscular junction (NMJ). They further suggest that 5-HT functions here by activation of a presynaptically located adenylate cyclase.

Caffeine blocks the delayed K+ outward current of molluscan neurons

Extracellular application of caffeine inhibits the delayed K+ outward current of Aplysia neurons in a dose dependent manner without changing the kinetics. Half-maximum blockade is produced with a concentration of 16.0 +/- 0.7 mM (S.E.M.) caffeine after 1-2 min. Intracellular injection of caffeine has an almost immediate blocking effect. The evidence suggests that the blocking site is at or close to the inner surface of the cellular membrane.