Local regulation of transmitter release from rodent sympathetic nerve terminals?

Neuroeffector Ca2+ transients for the direct measurement of purine release and indirect measurement of cotransmitters in rodents

Determining whether ATP and noradrenaline are released from the same vesicle at mature autonomic neuroeffector junctions is challenging because of the difficulty of simultaneously detecting the packeted release of these neurotransmitters. Contraction, overflow and electrophysiology experiments all show that both ATP and noradrenaline are released following field stimulation (although the ratio might vary) from autonomic nerves in tissues including the vas deferens, rat tail artery and mesenteric artery. The occurrence of purinergic neuroeffector Ca(2+) transients (NCTs) has been used to detect the packeted release of the neurotransmitter ATP acting on postjunctional P2X receptors to cause Ca(2+) influx. Neuroeffector Ca(2+) transients can also be used to detect the local effects of noradrenaline through its alpha(2)-adrenoceptor-mediated prejunctional autoinhibitory effects on nerve terminal Ca(2+) concentration and the probability of exocytosis (measured by counting NCTs). Evidence is presented that exocytosis from sympathetic varicosities does not occur in a manner independent of the history of that varicosity, but rather that the release of a packet of ATP transiently suppresses (or predicts the transient suppression of) subsequent release. This could arise by autoinhibition (by the prejunctional action of noradrenaline or purines) or due to a transient shortage of vesicles readily available for release. In summary, two high-resolution approaches are proposed to measure the intermittent release of packets of neurotransmitter: (1) local transient suppression of nerve terminal Ca(2+) transients; and (2) the local and transient inhibition of NCTs to infer local autoinhibition, hence transmitter release. Such approaches may allow the packeted corelease of ATP and noradrenaline to be investigated without the need to measure both neurotransmitters directly.

Electrical and optical study of nerve impulse-evoked ATP-induced, P2X-receptor-mediated sympathetic neurotransmission at single smooth muscle cells in mouse isolated VAS deferens

Simultaneous electrophysiology and confocal microscopy were used to investigate purinergic neurotransmission at single smooth muscle cells (SMCs) in mouse isolated vas deferens, and to explore the relationship between two high-resolution P2X-receptor-mediated measures of per pulse ATP release: transient peaks in the first time derivative of the rising phase of excitatory junction potentials (EJPs) recorded in single SMCs (‘discrete events’; DEs) and neuroeffector Ca²⁺ transients (NCTs) in the impaled SMCs.

This study shows that discrete events represent neurotransmitter release onto the impaled cell. First, the median amplitude of the first derivative of the EJP was larger when there was a coincident NCT in the impaled cell, compared with instances when no coincident NCT occurred. Second, the time-to-peak amplitude of the first derivative was shorter if there was a coincident NCT in the impaled cell, compared with when no coincident NCT was observed within the field.

Surprisingly, first derivative amplitude increased with the distance (of the corresponding NCT) from the microelectrode. The microelectrode did not locally inhibit the functional quantal size as there was no effect of distance on the normalized NCT amplitude. When the significant effect of distance (between the microelectrode and NCTs) on the first derivative amplitude was removed, there was no correlation between the unstandardized residual (of distance vs. first derivative amplitude) and NCT amplitude.

The absence of a correlation between DE and NCT amplitudes suggests that the NCT amplitude is a poor measure of quantal size. The usefulness of NCTs hence lies primarily in locating neurotransmitter release and measuring changes in local release probability.

Non-synaptic transmission at autonomic neuroeffector junctions

Non-synaptic transmission is characteristic of autonomic neuroeffector junctions. The structure of the autonomic neuromuscular junction is described. The essential features are that: the terminal portions of autonomic nerve fibers are varicose and mobile, transmitters being released 'en passage' from varying distances from the effector cells; while there is no structural post-junctional specialization on effector cells, receptors for neurotransmitters accumulate on cell membranes at close junctions; muscle effectors are bundles rather than single smooth muscle cells, that are connected by gap junctions which allow electrotonic spread of activity between cells. A multiplicity of transmitters are utilized by autonomic nerves, and cotransmission occurs often involving synergistic actions of the cotransmitters, although pre- and post-junctional neuromodulation of neurotransmitter release also take place. It is suggested that autonomic neural control of immune, epithelial and endothelial cells also involves non-synaptic transmission.

Selective modulation of noradrenaline release by alpha 2-adrenoceptor blockade in the rat-tail artery in vitro

The effects of blocking alpha(2)-adrenoceptors on noradrenaline (NA) and adenosine 5'-triphosphate (ATP) release from postganglionic sympathetic nerves have been investigated in rat-tail artery in vitro. Continuous amperometry was used to measure NA release and intracellularly recorded excitatory junction potentials (e.j.p.'s) were used to measure ATP release. Application of the alpha(2)-adrenoceptor antagonist, idazoxan (1 microm), increased the amplitude of NA-induced oxidation currents evoked by trains of 10 stimuli at 1 and 10 Hz. In cells deep in the media, idazoxan (1 microm) had no effect on the amplitude of e.j.p.'s evoked by trains of 10 stimuli at 1 and 10 Hz. In cells close to the adventitial - medial border, idazoxan produced a small increase in the amplitude of e.j.p.'s evoked at the end of trains of 10 stimuli at 1 Hz. In tissues pretreated with the neuronal NA uptake inhibitor, desmethylimpramine (0.3 microm), idazoxan (1 microm) markedly increased the amplitude of e.j.p.'s in cells deep in the media. The alpha(2)-adrenoceptor agonist, clonidine (0.5 microm), produced similar reductions in the amplitudes of both NA-induced oxidation currents and e.j.p.'s evoked by 10 stimuli at 1 Hz. These effects of clonidine were reversed by the subsequent addition of idazoxan (1 microm). The release of both NA and ATP is inhibited to a similar extent by activation of prejunctional alpha(2)-adrenoceptors by clonidine. In contrast, endogenously released NA more markedly inhibits NA release. These findings provide further support for the differential modulation of NA and ATP release.

Perspectives on the integrative functions of the 'sympatho-adrenomedullary system'

An historic survey is given of the gradual change of views and concepts concerning how the sympatho-adrenomedullary system is organized and operates: While it for nearly a century was considered to merely exhibit more or less generalized activation-inhibition responses, experimental studies during the last 50 years have revealed how it instead constitutes a highly sophisticated instrument for control, engaged in a variety of differentiated response patterns by which the brain controls events in major organ systems, down to include their cellular-molecular levels of organisation.

Autonomic neuromuscular transmission at a varicosity

This review attempts to clarify the definition of what constitutes an autonomic neuromuscular function formed by a varicosity. Ultrastructural studies of serial sections through varicosities, partly or wholly bare of Schwann cell covering, show that areas of close apposition occur between varicosities and muscle cell membrane that vary between 20 and 150 nm, depending on the muscle considered. Consideration of the diffusion of purine transmitters and their receptor kinetics after secretion in a packet show that the number of purinergic receptor channels opened at a site of 150 nm apposition by a varicosity is about 15% of that at a site of 50 nm apposition. These results, together with the analysis of the stochastic fast component and the deterministic slow components of the rising phase of the EJP suggest that the stochastic fast component is due to varicosities that form especially close appositions (20-50 nm), whereas the deterministic slow component is due to the large number of varicosities at distances up to about 150 nm. Varicosities forming appositions of 20-150 nm with muscle cells several hundred micrometers long possess junctional receptor types distinct from extrajunctional receptors. According to this argument, then, there are two different classes of varicosities: one that gives rise to a relatively large junctional current and another that is responsible for a very small junctional current. Present evidence suggests that two subclasses of varicosities can be discerned amongst the varicosities that generate large junctional currents. One of these subclasses of varicosity possesses relatively few post-junctional receptors compared with the amount of transmitter reaching the receptors from the varicosity, so that the junctional current generated is determined by the size of the receptor population; in this case, the size of the transmitter packages released from these varicosities is unknown and the size of the junctional current is relatively constant. The other subclass of varicosity possesses large receptor patches, sufficient to accommodate the largest amounts of transmitter released from the varicosities: in this case, the size of the transmitter packages is shown to be highly non-uniform. These speculations await confirmation by direct labelling of the receptor patches beneath varicosities, a possibility that is likely to be realized in the near future.

Quantal components of spontaneous excitatory junction potentials at visualised varicosities

The electrical signs of spontaneous transmitter release were recorded with an extracellular electrode from single visualized sympathetic varicosities on the mouse vas deferens. Ultrastructural examination of these varicosities with the electron microscope showed that they formed close-contacts with smooth muscle cells. Amplitude-frequency histograms of the spontaneous excitatory junction potentials (SEJPs) were constructed in order to determine the statistical nature of spontaneous transmitter release from a varicosity. SEJP histograms often possessed several peaks. Statistical tests showed that these were separate modes in the histograms indicating that the SEJPs were composed of subunits. The SEJP histograms were described by a mixture of distributions in which the components were identified as quanta and there was a Poisson release of quanta. The second mode in the SEJP histograms was sometimes twice that of the first mode but generally greater, suggesting a potentiation of the effects of one quantum released nearly simultaneously with another quantum. The components in the SEJP histograms were well fitted by either a Gaussian or a gamma distribution indicating that the quantum of transmitter could be described as a Gaussian or gamma variate.

Statistics of quantal secretion during long trains of sympathetic nerve impulses in mouse vas deferens

1. A statistical analysis has been made of the occurrence of excitatory junctional currents (EJCs) of similar amplitude recorded with an extracellular electrode during long trains of nerve impulses to the mouse vas deferens. 2. The number of EJCs of similar amplitude that occurred in consecutive impulses during trains of 500-1000 impulses at 0.5-2.0 Hz increased with the number of EJCs evoked during the train. 3. There was no evidence of significant dependence between consecutive EJCs of similar amplitude in sixteen out of eighteen trains in eighteen preparations. 4. The time course of clusters of EJCs of similar amplitude was examined by determining the standard deviation of different groups of EJCs within a cluster throughout their time course. Most EJCs within a cluster could be grouped with a coefficient of variation < 0.1 throughout their time course. 5. The observations on EJCs of similar amplitude leave open the possibility that secretion from single varicosities is, in general, multiquantal.

Local application of drugs to sympathetic nerve terminals: an electrophysiological analysis of the role of prejunctional alpha-adrenoceptors in the guinea-pig vas deferens

1. Focal extracellular recording techniques were used to study the effects of clonidine, yohimbine and tyramine on the intermittent transmitter release mechanism in the guinea-pig vas deferens in vitro. Drugs were applied locally to the varicosities located within the recording electrode. Statistical methods were used to determine whether noradrenaline (NA) acts locally to inhibit secretion from the same or a closely related release site (local regulation) on an impulse-to-impulse basis. 2. The alpha-adrenoceptor agonist, clonidine, inhibited transmitter release, an effect reversed by the alpha-adrenoceptor antagonist, yohimbine. Yohimbine alone increased action potential-evoked transmitter release, findings consistent with the idea that transmitter release is regulated through prejunctional alpha-adrenoceptors. 3. The indirectly acting sympathomimetic, tyramine, powerfully inhibited evoked transmitter release, an effect reversed by both yohimbine and phentolamine. The inhibitory effects of tyramine were greatly reduced in tissues taken from animals pretreated with reserpine. Clonidine powerfully inhibited transmitter release in reserpinized tissues showing that prejunctional alpha-adrenoceptors were functionally intact. The inhibitory effects of tyramine on transmitter release are therefore mediated indirectly through the release of endogenous NA. 4. Paradoxically, when transmitter release from a small population of variscosities on a single nerve fibre was studied in the absence of alpha-adrenoceptor antagonists, no evidence was found for local regulation of transmitter release. 5. The intermittent character of the transmitter release process makes it difficult to envisage how impulse-to-impulse regulation could occur. Furthermore, it is unlikely that NA will accumulate to any appreciable extent in the vicinity of the secreting varicosity. 6. The pharmacological evidence clearly supports the view that NA released from sympathetic nerve terminals by nerve impulses modulates subsequent transmitter release. However, the evidence does not support the view that released NA acts locally to inhibit secretion from recently activated varicosities.

Impulse conduction in sympathetic nerve terminals in the guinea-pig vas deferens and the role of the pelvic ganglia

Focal extracellular recording techniques were used to study nerve impulse propagation and the intermittent transmitter release mechanism in sympathetic nerve terminals of the guinea-pig vas deferens in vitro. In particular, the nature of impulse propagation in postganglionic nerve fibres was characterized following pre- or postganglionic stimulation. Conventional intracellular recording techniques were also used to study directly ganglionic transmission in cell bodies in the anterior pelvic ganglia. When brief electrical stimuli were applied to the hypogastric nerve trunk close to the prostatic end of the vas deferens, the nerve terminal impulses recorded extracellularly could be evoked either directly by stimulation of the parent axon (i.e. postganglionically) or indirectly by stimulation of the preganglionic nerve fibre. In 364 separate recordings, nerve terminal impulse conduction failure was not observed during trains of stimuli at 1 Hz. However, apparent "intermittent conduction" of nerve impulses was noted on 16 occasions. In these fibres, the degree of intermittent conduction decreased as the frequency of stimulation was increased. Conduction in these intermittent fibres was reversibly interrupted by removing calcium from the Krebs' solution or by the addition of the ganglion blocker, hexamethonium (30-100 microM). Thus, the cause of intermittent conduction is failure of the transmission of excitation in the sympathetic ganglia. Impulses evoked by postganglionic stimulation never failed to propagate into the nerve terminals, and changes in the shape or amplitude of the nerve terminal impulse during trains of stimuli were not detected. One effect of stimulation was a frequency-dependent increase in the latency of the nerve terminal impulse which developed during the train of stimuli. Thus, intermittence of transmitter release from individual varicosities cannot be attributed to failure of impulse propagation in sympathetic nerve terminals. Transmission in the anterior pelvic ganglia was investigated directly by making intracellular recordings from cell bodies whose terminals projected to the vas deferens. Many cell bodies received a strong synaptic input which generated an action potential in the postganglionic cell body on a one-to-one basis. However, in some cell bodies there was a low safety factor for the generation of the action potential by the excitatory postsynaptic potential. The safety factor for generating an action potential in the postganglionic cell body was raised by increasing the frequency of stimulation. These findings suggest that peripheral ganglia are not simple one-to-one relay stations, but may well play an important role in controlling the patterns of nerve impulse traffic in postganglionic sympathetic neurons.

Presynaptic receptors in hypertension

The physiological status of presynaptic receptors, regulating sympathetic neurotransmitter release, remains subject to debate. Nevertheless, pharmacological techniques have shown presynaptic alpha-adrenoceptors, mediating a negative feedback inhibition of neuronal noradrenaline (NA) release, and presynaptic beta-adrenoceptors mediating a positive feedback facilitation. Decreased presynaptic alpha- or increased beta-adrenoceptor responsiveness might be expected to result in enhanced per-pulse release of NA and may contribute to hypertension development and maintenance. A potential role in hypertension development, but not its maintenance, has been established for presynaptic beta-adrenoceptors. Attempts to identify altered presynaptic adrenoceptor responsiveness in hypertension have, however, been inconclusive.

Alpha 2-adrenoceptor-mediated autoinhibition of sympathetic transmitter release in guinea-pig vas deferens studied by intracellular and focal extracellular recording of junction potentials and currents

Excitatory junction potentials (e.j.ps; intracellular electrodes) and excitatory junction currents (e.j.cs; extracellular electrodes) elicited by stimulation (20 pulses at 1 Hz every minute) of the hypogastric nerve trunk were recorded from guinea-pig isolated vas deferens. Intracellular recording. At a variety of stimulation intensities, bath-applied yohimbine (0.1-1 mumol/l) did not change the first one to three e.j.ps in a train but increased the amplitude of subsequent e.j.ps. The effect of yohimbine was abolished in tissues from reserpine-pretreated guinea pigs. Bath-applied desipramine (0.1 mumol/l) diminished the amplitude of all but the first one to three e.j.ps in a train.--Extracellular recording. Yohimbine (0.1-1 mumol/l), when applied locally through the recording suction electrode, increased the number of e.j.cs per given number of stimuli, i.e., enhanced the probability of occurrence of e.j.cs. When desipramine (0.1 mumol/l) was present both in the bath and in the recording electrode, the probability of the occurrence of e.j.cs was decreased. In the presence of desipramine, yohimbine (0.1-1 mumol/l) increased the number of e.j.cs even more markedly. Neither the nerve terminal impulse nor the number of spontaneous e.j.cs was changed by yohimbine. A mixture of tetraethylammonium (2 mmol/l) and 4-aminopyridine (0.2 mmol/l), when applied locally, both increased the number of e.j.cs and changed markedly the shape of the nerve terminal impulse.(ABSTRACT TRUNCATED AT 250 WORDS)

"Upstream" regulation of the release probability in sympathetic nerve varicosities

The results appear to support the following tentative working hypothesis. (1) Nerve impulse-induced transmitter release from sympathetic nerve varicosities is monoquantal and highly intermittent (probability range: 0-0.03). (2) Nerve impulses invade varicosities as all-or-none, Na+ channel-dependent action potentials; invasion failure may be rare. (3) The release probability is not controlled by properties (amplitude or duration) of the invading action potential or the resulting Ca2+ current, but by the availability of an as yet unidentified permissive factor. (4) The permissive factor is actively transported intra-axonally, probably in association with organelles (LDVs?). (5) The activation and/or transport of the permissive factor are controlled "upstream" of the varicosity; they depend on Ca2+ influx through channels insensitive to nifedipine (hence, not of L-type) but blocked by Cd2+ and apparently opened by slight depolarization of the resting membrane, in this respect behaving more as T- than N-type channels. (6) A high resting K+ efflux "upstream" of the varicosity restricts the availability of the permissive factor; it is the main mechanism maintaining the (economically necessary) low release probability. (7) Prejunctional agonists do not inhibit transmitter secretion by causing a conduction block or by reducing the action potential-induced Ca2+ influx into the varicosity itself, but by depressing the Ca2(+)-dependent activation and/or transport of the permissive factor; they act at least in part via receptors "upstream" of the varicosity. (8) This hypothesis for regulation of the release probability in sympathetic nerves may apply, at least in part, to other neurons as well.

The topographical basis of cholinergic transmission in guinea-pig ileum myenteric plexus

Myenteric plexus-longitudinal muscle strips were used to study nerve action potential propagation and transmission and their differences between the proximal and the distal regions of cholinergic terminals. Neurogenic twitches of a portion of the strip were evoked by focal electrical stimulation. Twitches mediated by the distal regions of cholinergic nerve terminals were more influenced by drugs affecting Ca2+ "utilization" (Bay K 8644, kappa opiate ligand ethylketocyclazocine, changes in extracellular Ca2+ or Co2+ concentration) in contrast to twitches mediated by proximal regions of these terminals which were more influenced by drugs affecting sodium-potassium spike (tetrodotoxin, dendrotoxin, 4-aminopyridine, tetraethylammonium). Post-tetanic potentiation of twitches was prominent with that portion of the strip where the distal regions of nerve terminals were involved. Drugs interfering with Na+/K+ spikes indiscriminately influenced both the twitch height and post-tetanic potentiation whereas changes in extracellular Ca2+ concentration affected selectively only post-tetanic potentiation. Release of [3H]acetylcholine from pre-labelled strips evoked by 1 Hz continuous stimulation or by train stimulation at 30 Hz was measured selectively from portions containing either proximal and distal or only distal regions of nerve terminals. The release from portions containing the distal regions was relatively higher when evoked by 30 Hz than by 1 Hz. The distal regions of nerve terminals might be thus recruited to participate in transmission by a frequency-dependent process. Nerve impulses were recorded from strands of nerve fibres in the myenteric plexus. At 1 and 5 mm distance from the stimulation focus nerve impulses were completely suppressed by tetrodotoxin. At 5 mm, in some strands the amplitude of nerve impulses was also subject to the effect of drugs affecting Ca2+ "utilization"; facilitation of nerve impulse amplitude during 30 Hz train stimulation was always influenced by drugs affecting Ca2+ "utilization". Propagation of nerve impulses in the distal region of cholinergic nerve terminals was found to be Ca-sensitive and frequency-dependent; this might form the basis for facilitation and post-tetanic potentiation of muscarinic transmission.

Interactions between the effects of yohimbine, clonidine and [Ca]o on the electrical response of the mouse vas deferens

Excitatory junction potentials (e.j.ps) were recorded from mouse vas deferens and resolved into families of 'discrete events' (d.es) reflecting intermittent release of packets of transmitter from one or a few sites. Within families d.es vary in amplitude between a few preferred values unaffected by any treatments used in these experiments. As [Ca]o is raised from 1.1 to 4.0 mM there is a rise in d.e. amplitude due to an increase in the frequency of large events and a decrease in that of small. At all [Ca]o clonidine reduces d.e. amplitude by increasing failures and small events and decreasing large events. Yohimbine has opposite effects. Both drug effects are concentration-dependent in the range 5 X 10(-9) - 10(-6)M. As [Ca]o is raised from 1.1 to 4.0 mM, and therefore more natural agonist is released, clonidine becomes more effective at altering d.e. amplitude whereas yohimbine becomes less so. With very low frequency stimulation yohimbine elevates e.j.p. amplitude only if [Ca]o is below 1.6 mM. These results are not easily compatible with the notion that yohimbine breaks a 'negative feedback' control of transmitter release.

Differential effects of alpha-beta-methylene ATP on responses to nerve stimulation in SHR and WKY tail arteries

The effects of alpha,beta-,methylene-adenosine triphosphate, (alpha,beta-methylene ATP, a P2-receptor desensitising agent) have been evaluated on vasoconstrictor responses elicited by exogenous agonists or electrical field stimulation in isolated perfused SHR or WKY tail arteries and on tritium release elicited by electrical field stimulation in SHR-tail arteries pre-labeled with 3H-noradrenaline. Exposure to alpha,beta-methylene ATP (0.1 mumol/l) significantly inhibited vasoconstrictor responses to electrical field stimulation in SHR tail arteries. These inhibitory effects were not further increased at a higher concentration of alpha,beta-methylene ATP (1 mumol/l). In WKY tail arteries, alpha,beta-methylene ATP (1 mumol/l) failed to significantly inhibit vasoconstrictor responses to electrical stimulation. In SHR tail arteries prelabelled with 3H-noradrenaline, alpha,beta-methylene ATP (1 mumol/l) did not inhibit the stimulation evoked release of tritium. However, at this concentration, alpha,beta-methylene ATP significantly antagonized the vasoconstrictor responses of SHR tail arteries induced by exogenous ATP (1 mumol/l), beta,gamma-methylene ATP (30 mumol/l), a stable agonist at P2-receptors, or 60 mmol/l KCl. These effects of alpha,beta-methylene ATP on contractile responses to KCl were not observed in WKY-tail arteries. In tail arteries obtained from reserpine pretreated SHR, despite a 85-95% decrease in endogenous noradrenaline tissue content, the vasoconstrictor responses induced by periarterial field stimulation were greatly diminished, but not abolished. These residual responses to periarterial field stimulation were not antagonized by prazosin (0.1 mumol/l), but were practically abolished by the addition of alpha,beta-methylene ATP (1 mumol/l). In tail arteries from WKY rats pretreated with reserpine, exposure to prazosin (0.1 mumol/l) further reduced the residual responses elicited by electrical field stimulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Is there feedback regulation of neurotransmitter release by autoreceptors?

Neurotransmitter release does not seem to be regulated by neuronal receptors mediating feedback and the mechanism of action of presynaptically active agents is still uncertain. In a recent set of papers [27, 82], experiments were described in which major modifications were made to the amount of neurotransmitter released per impulse, with all other parameters of field stimulation, such as pulse number, voltage and frequency, fully controlled. These studies done with a number of sympathetically innervated tissues give some insight into an antagonist action presynaptically which is independent of the ambient concentration of extracellular transmitter. It appears to involve, instead, the gating mechanisms which control neuronal membrane depolarization and repolarization. It was found that the effects of yohimbine and also of phenoxybenzamine on stimulation-induced efflux appeared to be essentially "all or none". That is, the absolute total release of tritiated transmitter with 100 pulses was elevated to roughly the same dpm value by the presynaptic antagonist at each of the pulse durations between 50 and 1000 microsec, in a variety of test tissues. The declining percentage effect of the antagonist on tritium efflux, as the pulse duration was enlarged between 50 and 1000 microsec, referred to earlier (Fig. 3), was due to rising values for transmitter release in the controls not matched by proportionally similar increases in the antagonist-treated tissues. Values for the amount of transmitter released during stimulation in the presence of yohimbine, at pulse lengths between 50 and 1000 microsec, were all in the range of values achieved in the absence of yohimbine with long pulse lengths (1000-2000 microsec). In other words, prolongation of the pulse duration from 50 to 1000 microsec and the exposure of tissues to a presynaptic antagonist, such as yohimbine or phenoxybenzamine, may involve a common mechanism, and the effects of these two procedures are not additive. In fact, with much prolonged pulse durations (2000-5000 microsec), the presynaptic antagonists are virtually ineffective. It is known that the release of transmitter from sympathetic nerves is directly related to the duration of the action potential. If it is prolonged, the calcium channels stay open longer leading to greater entry of calcium and to an increased release of transmitter [45, 46]. Yohimbine and phenoxybenzamine may prolong the duration of depolarization by indirect modification of the calcium gating mechanism.(ABSTRACT TRUNCATED AT 400 WORDS)

Antidromic activation of dorsal raphe neurons from neostriatum: physiological characterization and effects of terminal autoreceptor activation

Three types of neurons, distinguished on the basis of their spontaneous firing rates and patterns, extracellularly recorded waveforms and responses to neostriatal stimulation, were observed in the dorsal raphe nucleus in urethane-anesthetized rats. Type 1 neurons (presumed to be serotonergic) fired spontaneously from 0.1 to 3 spikes/s in a regular pattern, with initial positive-going bi- or triphasic action potentials. Type 1 cells exhibited long-latency antidromic responses to neostriatal stimulation (mean +/- S.E.M. 24.9 +/- 0.3 ms) that sometimes occurred at discrete multiple latencies, and supernormal periods persisting up to 100 ms following spontaneous spikes. Type 2 cells fired spontaneously in an irregular, somewhat bursty pattern from 0 to 2 spikes/s with initial negative-going biphasic spikes, and were antidromically activated from neostriatal stimulation at shorter latencies than Type 1 cells (21.8 +/- 0.9 ms). Type 3 cells were characterized by initial positive-going biphasic waveforms and displayed a higher discharge rate (5-30 spikes/s) than Type 1 or Type 2 cells. Type 3 cells could not be antidromically activated from neostriatal stimulation. The relatively long conduction time to neostriatum of the Type 1 presumed serotonergic neuron is discussed with respect to previous interpretations of the synaptic action of serotonin in the neostriatum. In conjunction with these antidromic activation studies, the neurophysiological consequences of serotonergic terminal autoreceptor activation were examined by measuring changes in the excitability of serotonergic terminal fields in the neostriatum following administration of the serotonin autoreceptor agonist, 5-methoxy-N,N-dimethyltryptamine (5-MeODMT). The excitability of serotonergic terminal fields was decreased by intravenous injection of 40 micrograms/kg 5-MeODMT, and by infusion of 10-50 microM 5-MeODMT directly into the neostriatum. These results are interpreted from the perspective of mechanisms underlying autoreceptor-mediated regulation of serotonin release.

Mechanism of alpha 2-adrenergic inhibition of neuroeffector transmission in the mouse vas deferens

The process by which the activation of presynaptic alpha 2-adrenoceptors inhibits the release of noradrenaline from terminals of postganglionic sympathetic nerves was studied in the mouse isolated vas deferens. Clonidine was used as a prototypic agonist. Field stimulation-evoked excitatory junction potentials (e.j.p.s) were recorded from individual muscle cells. The e.j.p. amplitudes were taken as a measure of transmitter release. Changes in the external Ca2+ concentration from 2.5 to 1.25 or 5 mM caused corresponding changes in the size of e.j.p.s. When the normal Ca2+ concentration of the medium (2.5 mM) was substituted by equimolar quantities of Ba2+ or Sr2+, the e.j.p. amplitudes decreased considerably. Clonidine (0.3-30 nM) inhibited the nerve stimulation-evoked e.j.p. amplitudes in a concentration-dependent manner, without altering appreciably the frequency of spontaneous e.j.p.s. Procedures known to enhance Ca2+ entry into nerve terminals, like a high Ca2+ medium (Ca2+ 5 mM) or 4-aminopyridine 30 microM reduced the effect of clonidine. Repetitive nerve stimulation at 3 Hz, which is supposed to lead to an accumulation of free Ca2+ inside nerve terminals, similarly counteracted the effect of clonidine 10 nM. Whereas the alpha 2-adrenergic inhibition of the first e.j.p. in a train was unaffected, the inhibition of all successive e.j.p.s was gradually decreased. At 5 mM Ca2+ only the time-course of facilitation became faster, the decrease in alpha 2-adrenergic inhibition proceeded with the same pulse-dependent rate as at a normal external Ca2+ concentration, although from a lower initial level.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of age on the adrenergic cardiac neuroeffector junction

Although it is clear that adrenergic nervous system control of cardiac function decreases with age and that the effector organ fails to adjust to this decreased control, it is not completely evident which of the many mechanisms operant at the adrenergic-cardiac neuroeffector junction contribute to this state. Prejunctionally, it appears that norepinephrine content decreases with age and that adrenergic axonal degeneration occurs. Also, evidence is available to suggest that modulation by prejunctional alpha adrenergic receptors of norepinephrine release is altered with increasing age, as is neuronal uptake of norepinephrine. Postjunctionally, it appears that beta-adrenergic receptor sensitivity to agonists undergoes age-related alterations, and possibly post receptor mechanisms involved in receptor-response coupling. Other mechanisms, such as those involved in transmitter uptake into extraneuronal sites, adrenergic neuronal responsiveness to stimulation, transmitter release and turnover, calcium and prejunctional receptor modulation of transmitter release, postjunctional receptor development of supersensitivity or subsensitivity, need further elucidation in order to have an understanding of the factors that contribute to the breakdown of homeostatic mechanisms that regulate the heart.

Transmitter secretion from individual varicosities of guinea-pig and mouse vas deferens: highly intermittent and monoquantal

A modification of "classical" electrophysiological techniques was used to characterise the secretory activity of individual release sites of the sympathetic nerves of guinea-pig and mouse vas deferens. The rising phases of the intracellularly recorded excitatory junction potentials of a smooth muscle cell, were electrically differentiated, and fluctuations in the rate of rise recorded as phasic peaks (in the dV/dt of excitatory junction potentials), termed "discrete events". Experimental factors which may influence discrete events were examined systematically, and criteria established to recognize a discrete event as the "image" of transmitter secreted from a particular release site. To determine the quantal content of a stimulus-evoked discrete event, it was compared with discrete events occurring spontaneously in the same cell. Furthermore, the frequency dependence of the probability of occurrence of discrete events was compared with that of the evoked fractional secretion of tritium-labelled noradrenaline, to find out if release sites from which noradrenaline is secreted share the characteristics of those secreting the (still unknown) transmitter causing discrete events. The results obtained permit the following tentative conclusions: Both in guinea-pig and mouse vas deferens stimulus-induced secretion of transmitter from a single varicosity of the sympathetic nerves is highly "intermittent". Transmitter secretion from a varicosity is basically monoquantal. Spontaneous secretion of transmitter quanta occurs from "random" sites, but a nerve impulse causes secretion of a quantum from a "preferred site" of a varicosity: during a stimulus train quanta are secreted in "complementary pairs". Secretion of the first quantum in a "pair" does not lead to "autoinhibition" of the site from which it was released, but induces a short-lasting facilitation. (5) Some characteristic features of the secretion of the neurotransmitter causing discrete events, seem to apply to the secretion of noradrenaline from noradrenergic nerves also.

A hypothesis to explain the presynaptic effects of adrenoceptor antagonists

The hypothesis of negative feedback regulation of transmitter release was examined in a range of tissues obtained from three species. Tissues were transmurally stimulated with 100 pulses at 2 Hz with pulse durations from 50 microseconds to 5,000 microseconds, and the efflux of [3H]-noradrenaline determined. The stimulation-induced efflux of tritium increased with increasing pulse duration, but yohimbine, a prototypal alpha 2-antagonist had an effect which was consistently contrary to expectations for a negative feedback system. Enhancement of efflux by the antagonist, supposedly correlated directly with the extent of ongoing auto-inhibition, became smaller rather than larger as the stimulation-induced efflux rose with increases in pulse duration, with all other parameters of stimulation maintained constant. Similar findings were obtained in rat spleen with the haloalkylamine antagonist, phenoxybenzamine. It is concluded that the presynaptic effects of adrenoceptor antagonists do not involve a negative feedback function nor do they relate, in any detectable way, to the extracellular concentration of transmitter. The effects on stimulation-induced tritium efflux of yohimbine, phenoxybenzamine and enlargment of the pulse duration, in a variety of tissues, support the previously described hypothesis of a common action to enhance efflux. The antagonists increased efflux to approximately the same value between 50 and 1,000 microseconds pulse durations and that value was equivalent to that obtained in each given tissue with pulses of 1,000-2,000 microseconds in the absence of the antagonist. Tetraethylammonium, an inhibitor of stimulation-induced potassium efflux from nerves had an effect on transmitter efflux in rat spleen essentially like that of the adrenoceptor antagonists. These findings provide further support for an alternative to the hypothesis of negative feedback. Yohimbine and other presynaptic antagonists may prolong the period of potassium efflux from nerve varicosities, and by this means prolong depolarization and the associated period of transmitter release, rather than act by disrupting an ongoing system sensing and responding to fluctuations in extracellular transmitter levels.

Facilitation at single release sites of a sympathetic neuroeffector junction in the mouse

Electrophysiological techniques were used to observe the release of transmitter from one or a few release sites of the sympathetic neuroeffector junction of the mouse vas deferens. Release produces transient accelerations of the depolarizing phase of the excitatory junction potential, known as 'discrete events'. Discrete events associate into families at a constant latency and peak time, but vary in amplitude between a few preferred values. As facilitation develops there is a decrease in the frequency of small members of families and an increase in the frequency of large members, a change in apparent 'quantal content'. A similar change in amplitude distribution occurs when the [Ca]o is raised. The alpha-adrenoceptor antagonist yohimbine increases quantal content when facilitation has developed, but has no significant effect on unfacilitated discrete event amplitude unless the [Ca]o is below 2.0 mM. The alpha-adrenoceptor agonist clonidine reduces facilitated and unfacilitated quantal content under all conditions examined.

Alpha-adrenoceptors and facilitation at a sympathetic neuroeffector junction

Excitatory junction potentials were recorded from the mouse vas deferens following 5 stimuli at 1Hz and a single stimulus at times from 100 msec to 9 sec later. E.j.ps facilitate and facilitation decayed over a biexponential time course, with time constants of 120 msec and 4.3 sec. Clonidine (10(-8)-10(-7) M) depressed e.j.p. amplitude, and also accelerated both phases of the decay of facilitation. Yohimbine (10(-7) M) and piperoxan (10(-7)-10(-6) M) increased the amplitude of all but the first e.j.p. in a train, and slowed both phases of the decay of facilitation. One way in which prejunctional modulation by alpha-adrenoceptors may occur is therefore by modifying the carry-over of facilitation from one stimulus to the next.

Adrenergic transmission in the dog mesenteric vein and its modulation by alpha-adrenoceptor antagonists

Adrenergic transmission was investigated in the dog mesenteric vein by recording electrical responses of single smooth muscle cells to perivascular nerve stimulation. Perivascular nerve stimulation generated an excitatory junction potential (e.j.p.) and a slow depolarization of the membrane. The amplitude of the e.j.p. was increased by increasing the stimulus intensity, and at high intensity, a spike potential was generated. Repetitive stimulation of the nerves showed facilitation of e.j.ps and enhanced the amplitude of slow depolarization. A linear relationship was observed between the amplitude of the e.j.p. and of slow depolarization. The slow depolarization was inhibited by application of yohimbine or phentolamine, but not by prazosin. The amplitude of e.j.p. was increased by prazosin and was decreased by yohimbine. Both e.j.p. and slow depolarization were inhibited by guanethidine or tetrodotoxin. Exogenously applied noradrenaline depolarized the muscle membrane and, in high concentrations (greater than 10(-7)M), generated slow waves. These effects of noradrenaline were blocked by yohimbine. High concentrations of prazosin (greater than 10(-6)M) showed weak inhibitory effects on the noradrenaline actions. The amplitude of e.j.p. was decreased by exogenously applied noradrenaline in a dose-dependent manner. The inhibitory effect of noradrenaline on the e.j.p. was suppressed by yohimbine, but not by prazosin or phentolamine. Phentolamine, but not prazosin, enhanced the facilitation process of e.j.ps. This effect was not suppressed by exogenously applied noradrenaline. Application of neostigmine but not of atropine, reduced the e.j.p. amplitude without affecting the slow depolarization. It was concluded that, in the dog mesenteric vein, perivascular nerve stimulation produced three types of electrical responses of the smooth muscle membrane, i.e., e.j.p., slow depolarization and spike potential. The slow depolarization was generated by activation of alpha 2-adrenoceptors. Exogenously applied noradrenaline reduced the e.j.p. amplitude through activation of prejunctional alpha 2-adrenoceptors, but the reduction may not involve alpha-autoinhibitory mechanisms.

Yohimbine and prolongation of stimulation pulse duration alter similarly 3H-transmitter efflux in heart: an alternative to the negative feedback hypothesis

The hypothesis of negative feedback regulation of noradrenaline release was studied in guinea-pig left atrial halves mounted in vitro. Tissues were transmurally stimulated with 30, 100 or 300 pulses at 2 Hz with pulse durations ranging from 50 mus to 2,000 mus, and the efflux of 3H-transmitter determined. The efflux of tritium increased with increasing pulse duration as was anticipated, but the effects of supposed presynaptic antagonism by yohimbine were opposite to expectations for a negative feedback system. The magnification of efflux by yohimbine, compared to untreated controls was less rather than more as stimulation-induced transmitter efflux climbed with increases in pulse duration, and with all other parameters of stimulation held constant. It is concluded that the neuronal effect of yohimbine is not linked to negative feedback or to any other system sensing the perineuronal concentration of previously released transmitter. Analysis of the effects on tritium efflux of yohimbine and of prolongation of the stimulation pulse duration, reveals a similarity in the way that they promote transmitter release. Yohimbine increased efflux to approximately the same value at all pulse durations between 50 and 1,000 mus and the value reached was equivalent to that obtained in untreated atria during stimulation with very long pulses (2,000 mus duration). It is suggested that yohimbine prolongs the outward current attributable to the efflux of potassium from axon terminals, and by this means prolongs depolarization and the period of transmitter release. Tetraethylammonium (TEA), a quaternary ion known to plug potassium efflux channels, had an effect on transmitter efflux that was, in some ways, similar to that of yohimbine but of greater magnitude. The present findings provide, for the first time, an alternative to the hypothesis of negative feedback, that might explain the presynaptic effects of adrenoceptor antagonists and possibly other compounds.

Modulation of noradrenergic transmission in the guinea-pig mesenteric artery: an electrophysiological study

We carried out electrophysiological experiments on guinea-pig mesenteric arteries in an attempt to clarify the modification of noradrenaline (NA) release from noradrenergic nerve terminals by the action of prejunctional adrenoceptors. NA (10(-7)-10(-5) M) suppressed the amplitude of the first excitatory junction potential (e.j.p.(f], the facilitation process, and the e.j.p. after facilitation was completed (e.j.p.(s]with no change in the post-junctional membrane properties of smooth muscles. These actions of NA on e.j.p.s were antagonized by high concentrations of extracellular Ca, [Ca]o, but not in a simple competitive manner. NA (10(-7) M) suppressed the appearance but not the amplitude of the miniature e.j.p.s. These effects of NA on transmission indicate that NA acts on prejunctional nerve terminals and suppresses the release of NA from nerve terminals rather than producing a desensitization of post-junctional adrenoceptors. Prazosin and phentolamine (10(-6) M) did suppress the NA-induced contraction (greater than 10(-6) M) but did not suppress the contraction evoked by perivascular nerve stimulation, below a frequency of 1.0 Hz. At a dose of 10(-7) M, yohimbine, clonidine, prazosin and phentolamine had no effect on the muscle membrane potential and resistance. Yet on the e.j.p.(f), yohimbine and clonidine caused suppression, phentolamine enhancement and prazosin had no effect. On the e.j.p.(s), yohimbine and phentolamine caused enhancement, clonidine suppression and prazosin had no effect. These results indicate that at least three different adrenoceptors are distributed on the neuromuscular junction in this tissue, i.e. alpha 1-extrajunctional, alpha 2-prejunctional, and an unknown subtype of intrajunctional adrenoceptors. Furthermore, the feedback mechanism on NA release is mediated by suppression of the influx of Ca. Nonselective and non-specific actions of alpha-adrenoceptor agonists and antagonists were also elucidated.

The effects of yohimbine on presynaptic and postsynaptic events during sympathetic nerve activation in cattle iris: a critique of presynaptic receptor theory

1 The effects of presynaptic alpha-adrenoceptor blockade on both the efflux of 3H-transmitter and on the magnitude of the effector response were measured simultaneously in a smooth muscle preparation which responds to field stimulation with noradrenergic beta-receptor-mediated relaxation. 2 In the presence of atropine, the circular muscle of cattle iris relaxes in response to noradrenaline and to field stimulation at 2 Hz with 10, 20, 50 and 100 pulses. 3 Yohimbine (3 x 10(-6) M), a potent presynaptic alpha-adrenoceptor antagonist, increased the stimulation-induced efflux of tritium to about 2.0 times control values and, contrary to theory, did so to a similar extent regardless of pulse number and with apparent indifference to the synaptic concentration of transmitter, as confirmed by the varying size of the postsynaptic response. 4 In most cases, yohimbine had no significant effect on the magnitude of the relaxations to nerve stimulation. 5 It is concluded that negative feedback regulation of transmitter release, if it functions at all, and this itself seems doubtful, would not have a substantial impact on the size of the effector response.