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

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.

Autoreceptors do not regulate routinely neurotransmitter release: focus on adrenergic systems

The theory that neurotransmitter release is regulated locally at the individual terminals of neurons has achieved a rapid and seemingly secure status in our understanding of neuronal function both in the periphery and in the central nervous system. This concept of negative feedback control through the monitoring of the perineuronal concentration of previously released transmitter has been extended to a multiplicity of transmitters and utilized to explain the mechanisms of action of diverse classes of drugs, ranging from antihypertensives to antidepressants. It is my view that negative feedback by terminal and by somadendritic receptors cannot account for the existing body of experimental work. Analyses of the profiles of action of agonists and antagonists, and of the per pulse release of transmitter in the absence of drugs in a variety if peripheral organ systems, as well as in superfused brain slices, demonstrates the need for alternate interpretations of the available data. Evidence is provided that the actions of agonists to inhibit transmitter release and that of antagonists to enhance release occur at different cellular loci and that the purported unitary action of these two classes that is so central to the validity of presynaptic theory is unsupportable.

Regarding the unitary theory of agonist and antagonist action at presynaptic adrenoceptors

1. The linkage between potentiation of field stimulation-induced noradrenaline release and blockade of the presynaptic inhibitory effect of exogenous noradrenaline by a presynaptic antagonist was examined in superfused rabbit aorta preparations. 2. Rauwolscine clearly potentiated the release of noradrenaline in response to 100 pulses at 2 Hz but reduced the capacity of noradrenaline to inhibit transmitter release to a questionable extent, and then only when comparisons were made with untreated, rather then to rauwolscine-treated, controls. 3. Aortic preparations exposed for 60 min to rauwolscine followed by superfusion with antagonist-free Krebs for 60 min retained the potentiation of stimulation-induced transmitter release but no antagonism of the noradrenaline-induced inhibition could be detected at either of two noradrenaline concentrations when comparisons were made with rauwolscine treated controls. 4. Comparisons of the inhibitory effect of exogenous noradrenaline (1.8 x 10-6 M) on transmitter efflux in the presence and absence of rauwolscine pretreatment revealed that the antagonist enhanced rather than antagonized the presynaptic inhibition by noradrenaline. 5 It is concluded that the unitary hypothesis that asserts that antagonist enhancement of transmitter release and its blockade of noradrenaline induced inhibition are manifestations of a unitary event are not supportable.

Autoregulation of neurotransmitter release at autonomic nerve terminals: a questionable theory

1. The evidence for feedback regulation of neurotransmitter release by means of autoreceptors is questioned. 2. Autoreceptor function must meet the expectations for feedback loops. However, this concordance has not been observed in most neuroeffector systems. 3. The characteristics of per pulse transmitter release with changes in the parameters of stimulation in several autonomic systems do not support the ongoing operation of negative feedback loop mediated by locally released transmitter. Also, the effects of antagonists and agonists often do not comply with feedback expectations. 4. Evidence is provided that agonists and antagonists act at different loci to achieve their inhibiting and potentiating effects on transmitter release. 5. Future efforts should be directed to exploring the mechanism(s) of antagonist action and to a system-by-system analysis of the evidence for and against autoreceptor operation.

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.

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.

A novel electrophysiological approach to monitor pulse by pulse the concentration of released noradrenaline at the presynaptic alpha 2-adrenoceptors of sympathetic nerves in rat tail artery

The excitatory junction current (EJC) evoked by electrical stimulation of postganglionic sympathetic nerves of rat tail artery with 100 pulses at 2 Hz, at 1.3 mmol/l external Ca2+, was used as a measure of the per pulse release of ATP. In controls the EJCs were initially facilitated, then gradually depressed during the stimulus train. The first EJC was slightly depressed by the alpha 2-adrenoceptor antagonist yohimbine, but starting from the 4th pulse the EJCs were enhanced. Yohimbine increased the early facilitation without markedly modifying the subsequent depression. The yohimbine-induced enhancement of EJCs caused by pulses 11-100 was, thus, constant. The noradrenaline reuptake blocker cocaine depressed the EJCs, abolished the early facilitation and slightly enhanced the depression. These effects of cocaine were reversed by further addition of yohimbine. The alpha 2-adrenoceptor agonist xylazine (1 and 10 mumol/l) dose dependently depressed the EJCs starting from the first pulse. The inhibitory effect of 1 mumol/l xylazine, but not that of 10 mumol/l xylazine, declined with train length. The inhibition of individual EJCs caused by activation of presynaptic alpha 2-adrenoceptors was used to monitor the concentration of released noradrenaline at these receptors. The ratio of individual EJCs in the presence and absence of yohimbine was assumed to reflect, pulse by pulse, the relative concentration of released noradrenaline at the presynaptic alpha 2-adrenoceptors, and hence termed [NA]alpha 2. For comparison, the concentration of endogenous noradrenaline was monitored electrochemically by differential pulse amperometry with a carbon fibre microelectrode; this signal is termed [NA]CF. [NA]alpha 2 and [NA]CF grew during the first 7-10 or 14-16 pulses, respectively, and then remained relatively constant throughout the stimulus train. Cocaine caused [NA]alpha 2 and [NA]CF to continue to grow during the first 35 and 50 pulses, and enhanced their peak levels by 180% and 320%, respectively. For comparison with the effects on the EJCs mediated via presynaptic alpha 2-adrenoceptors, those caused by varying external Ca2+ level were examined. At 0.65 mmol/l Ca2+ the amplitude of the first EJC was smaller than that at 1.3 mmol/l Ca2+, but the facilitation of later EJCs was enhanced and the subsequent depression reduced. An increase in external Ca2+ to 2.6 mmol/l had the opposite effects. All effects on EJCs caused by changes in external Ca2+ were maximal for the first EJC and then declined with the train length.(ABSTRACT TRUNCATED AT 400 WORDS)

The problem with autoreceptors

There are numerous problems with the concept that antagonists enhance transmitter release by blockade of feedback. It was shown that antagonist enhancement of transmitter release does not correlate satisfactorily with the intensity of stimulation or with other indices of biophase transmitter concentration. Wide variations were shown to exist between antagonists in the amount of enhancement of release they induce. Also, antagonists enhance transmitter release or the effector response with a single stimulation pulse, a condition under which no feedback is possible. A study of agonist/antagonist relationships indicates different sites of action, and it was determined that the antagonist effect has negligible or minimal latency and that enhancement by antagonists is maximal under minimal condition of stimulation. Antagonists were shown to enhance release by a direct action, not by passive occupancy of agonist sites. Experiments were described in which acetylcholine and cold selectively antagonized antagonist but not agonist effects. Further, experiments with pulse duration shifts and with veratridine pointed to a direct action of antagonists on Na+ (also Ca++?) channel gating mechanisms, which results in a shift in the voltage dependence of activation. If antagonists, in some particular instances, enhance release by blockade of sites involved in negative feedback this is likely lost or mired in their more prominent direct actions on neurosecretion--and these must be sorted out. The acceptance of the fact that antagonists act directly to alter transmitter release (and not only as passive occupiers of presynaptic receptors), as the present study shows, both in the central nervous system and in the periphery, opens a new area for future investigation, and may be exploitable for therapeutic purposes and to gain an enriched understanding of the mechanism of neurosecretion.

Properties of excitatory junction potentials and currents in smooth muscle cells of the mouse vas deferens

Intracellular recordings made from superficial smooth muscle cells of the mouse vas deferens confirmed the presence of two populations of cells, distinguishable by their membrane potential and measured input resistance. In this study, we have concentrated on cells with high input resistance (50-500 M omega) and membrane potentials of -45 to -65 mV. These cells fired action potentials when depolarized by intracellular current injection and appeared to be electrically isolated from adjacent cells. Stimulation of the intramural nerves evoked excitatory junction potentials (e.j.p.s.), which fluctuated in amplitude, with the largest firing action potentials. The e.j.p. amplitude was increased by hyperpolarizing and decreased by depolarizing the cell membrane. Under voltage-clamp, nerve stimulation evoked excitatory junction currents (e.j.c.s.) which reached a peak in 5 ms, and declined exponentially with a time-constant of 28 ms. The e.j.c. amplitude was linearly related to membrane potential with a reversal potential near -10 mV.

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.

Relative pre- and postjunctional roles of noradrenaline and adenosine 5'-triphosphate as neurotransmitters of the sympathetic nerves of guinea-pig and mouse vas deferens

Experiments were designed to examine the relative roles of noradrenaline and adenosine 5'-triphosphate (ATP) as mediators of the contractile responses of the guinea-pig and the mouse vas deferens to electrical nerve stimulation. To study possible prejunctional actions of the agents used, in some experiments their effects on the secretion of [3H]noradrenaline were determined. The contractile responses were recorded with force displacement transducers. Pharmacological techniques were employed to examine the pre- and/or postjunctional effects mediated by noradrenaline and ATP, respectively. Noradrenaline-mediated components were "removed" by depleting the neuronal stores of noradrenaline (by pretreatment with reserpine), or by addition of adrenoceptor-blocking agents. ATP-mediated components were "removed" by desensitizing ATP receptors (with the stable analogue alpha, beta-methylene ATP). The results permit three major conclusions: (1) In both species noradrenaline and ATP "auto-inhibit" mechanisms responsible for transmitter secretion; the prejunctional effects of ATP are less marked in the mouse vas deferens, and in both species much weaker than those mediated by noradrenaline, acting via alpha 2-adrenoceptors. (2) In these species, both noradrenaline and ATP participate in the generation of both phases of the contractile responses to nerve stimulation. The relative roles of each vary with the frequency and train length of stimulation, and also with the species. The main transmitter of "phase I" contractions in guinea-pig vas deferens is ATP, and in the mouse vas deferens, noradrenaline. "Phase II" contractions are triggered mainly by noradrenaline, in both species. Sympathetic neuroeffector transmission in these tissues can be accounted for almost entirely in terms of dynamic interplay between pre- and postjunctional actions of noradrenaline and ATP. (3) The results are compatible with the hypothesis that ATP is a co-transmitter with noradrenaline in these sympathetic nerves.