On the geometry, kinetics and plasticity of sympathetic neuromuscular transmission

Multiple oscillators, dynamic synchronization and sympathetic control

1. Intermittent bursts of activity are a robust feature of the discharges of sympathetic nerves. There are at least two major mechanisms producing such discharges: (i) phasic inputs influencing sympathetic circuits; and (ii) oscillators embedded within sympathetic networks. The functional significance of patterned and synchronized activity underlying bursts of population activity may reside in their influence on information transfer between excitable cells. At the level of the single neuron, firing pattern appears to be an important determinant of synaptic/neuroeffector function (e.g. the probability of transmitter release, the types of transmitter released, the types of receptor activated and plasticity). Synchronization of inputs at a target favours summation and, therefore, may influence response (short term and long term). 2. In the present paper, I review the work from my laboratory that has focused on furthering understanding of the potential functional importance of pattern and synchrony coding in sympathetic nervous control of cardiovascular function. Because the rat tail artery has been used extensively as a model for studying neuroeffector transmission, in our investigations we have recorded from its sympathetic innervation. 3. In the anaesthetized preparation, under steady state conditions, we have established that the discharges of these sympathetic neurons have a distinct rhythm (frequency approximately 0.8 Hz). This can be detected both at single neuron and population levels. 4. A family of oscillators appears to control their discharge such that under some conditions all neurons do not have the same frequency of rhythmical activity. However, these weakly coupled or uncoupled oscillators can be synchronized dynamically by various inputs, such as central respiratory drive, lung inflation cycle-related inputs and inputs arising from visceral and somatic afferents. 5. The potential functional significance of dynamic synchronization of sympathetic oscillators in relation to sympathetic pattern generation and neuroeffector transmission is discussed.

Physiological features of visceral smooth muscle cells, with special reference to receptors and ion channels

Visceral smooth muscle cells (VSMC) play an essential role, through changes in their contraction-relaxation cycle, in the maintenance of homeostasis in biological systems. The features of these cells differ markedly by tissue and by species; moreover, there are often regional differences within a given tissue. The biophysical features used to investigate ion channels in VSMC have progressed from the original extracellular recording methods (large electrode, single or double sucrose gap methods), to the intracellular (microelectrode) recording method, and then to methods for recording from membrane fractions (patch-clamp, including cell-attached patch-clamp, methods). Remarkable advances are now being made thanks to the application of these more modern biophysical procedures and to the development of techniques in molecular biology. Even so, we still have much to learn about the physiological features of these channels and about their contribution to the activity of both cell and tissue. In this review, we take a detailed look at ion channels in VSMC and at receptor-operated ion channels in particular; we look at their interaction with the contraction-relaxation cycle in individual VSMC and especially at the way in which their activity is related to Ca2+ movements and Ca2+ homeostasis in the cell. In sections II and III, we discuss research findings mainly derived from the use of the microelectrode, although we also introduce work done using the patch-clamp procedure. These sections cover work on the electrical activity of VSMC membranes (sect. II) and on neuromuscular transmission (sect. III). In sections IV and V, we discuss work done, using the patch-clamp procedure, on individual ion channels (Na+, Ca2+, K+, and Cl-; sect. IV) and on various types of receptor-operated ion channels (with or without coupled GTP-binding proteins and voltage dependent and independent; sect. V). In sect. VI, we look at work done on the role of Ca2+ in VSMC using the patch-clamp procedure, biochemical procedures, measurements of Ca2+ transients, and Ca2+ sensitivity of contractile proteins of VSMC. We discuss the way in which Ca2+ mobilization occurs after membrane activation (Ca2+ influx and efflux through the surface membrane, Ca2+ release from and uptake into the sarcoplasmic reticulum, and dynamic changes in Ca2+ within the cytosol). In this article, we make only limited reference to vascular smooth muscle research, since we reviewed the features of ion channels in vascular tissues only recently.

Differential presynaptic modulation of noradrenaline release in human atrial tissue in normoxia and anoxia

1. Presynaptic modulation of noradrenaline release in human atrial tissue specimens was investigated under normoxic and anoxic conditions. 2. Noradrenaline release was induced by electrical stimulation and release during experimental intervention (S2) was compared with release during a preceding control stimulation (S1). The results were expressed as the geometric means and 95% confidence intervals of the S2/S1 ratio. 3. The alpha 2-adrenoceptor agonist, UK 14304 (0.1 mumol-1) significantly inhibited noradrenaline release, resulting in a S2/S1 ratio of 0.49 (0.40-0.59), and the a 2-adrenoceptor antagonist, yohimbine (1 mumol l-1) increased noradrenaline release (S2/S1 1.83 [1.43-2.35]) during normoxia. Both compounds were ineffective during anoxia. 4. Adenosine (30 mumol-1) inhibited noradrenaline release with a S2/S1 ratio of 0.54 (0.42-0.66). The adenosine antagonist, 8-phenyltheophylline, alone had no effect during normoxia. During anoxia, neither adenosine nor 8-phenyltheophylline altered noradrenaline release. 5. The beta 2-adrenoceptor agonist, terbutaline (1 mumol l-1) increased (1.53 [1.14-2.01]) and the beta-adrenoceptor antagonist, pindolol (1 mumol l-1) suppressed noradrenaline release (0.62 [0.49-0.79]) under normoxic conditions. During anoxia, pindolol significantly inhibited noradrenaline release with a S2/S1 ratio of 0.66 (0.51-0.85), whereas terbutaline did not influence noradrenaline release. 6. Angiotensin II (0.1 mumol l-1 enhanced noradrenaline release resulting in a S2/S1 ratio of 1.44 (1.34-1.54), while the angiotensin II antagonist, losartan (1 mumol l-1) had no effect on noradrenaline release during normoxia. Conversely, angiotensin II did not increase noradrenaline release and losartan significantly inhibited noradrenaline release to a S2/S1 ratio of 0.60 (0.46-0.77) during anoxia. 7. In conclusion, human cardiac tissue possesses presynaptic inhibitory alpha 2-adrenoceptors and adenosine receptors, as well as facilitatory beta 2-adrenoceptors and angiotensin II receptors regulating noradrenaline release under normoxic conditions. During anoxia the responses to alpha 2-adrenoceptors and adenosine receptor stimulation are lost, whereas facilitatory responses to beta 2-adrenoceptors and adenosine II receptor stimulation are maintained and these receptors appear to be maximally stimulated. This differential presynaptic modulation in anoxia may contribute to enhanced sympathetic activity in ischaemia.

Geometry, kinetics and plasticity of release and clearance of ATP and noradrenaline as sympathetic cotransmitters: roles for the neurogenic contraction

The paper compares the microphysiology of sympathetic neuromuscular transmission in three model preparations: the guinea-pig and mouse vas deferens and rat tail artery. The first section describes the quantal release of ATP and noradrenaline from individual sites. The data are proposed to support a string model in which: (i) most sites (> or = 99%) ignore the nerve impulse and a few (< or = 1%) release a single quantum of ATP and noradrenaline; (ii) the probability of monoquantal release is extremely non-uniform; (iii) high probability varicosities form 'active' strings; and (iv) an impulse train causes repeated quantal release from these sites. Analogy with molecular mechanisms regulating transmitter exocytosis in other systems is proposed to imply that coincidence of at least two factors at the active zone, Ca2+ and specific cytosolic protein(s), may be required to remove a 'fusion clamp', form a 'fusion complex' and trigger exocytosis of a sympathetic transmitter quantum, and that the availability of these proteins may regulate the release probability. The second section shows that clearance of noradrenaline in rat tail artery is basically > or = 30-fold slower than of co-released ATP, and that saturation of local reuptake and binding to local buffering sites maintain the noradrenaline concentration at the receptors, in spite of a profound decline in per pulse release during high frequency trains. The third section describes differences in the strategies by which mouse vas deferens and rat tail artery use ATP and noradrenaline to trigger and maintain the neurogenic contraction.

Nerve activity-dependent variations in clearance of released noradrenaline: regulatory roles for sympathetic neuromuscular transmission in rat tail artery

The aim of this study was to find out if clearance of noradrenaline released from sympathetic nerve terminals in rat isolated tail artery is a physiological variable and if so, to determine its role for the noradrenaline-mediated neurogenic contraction. The per pulse release of noradrenaline induced by electrical nerve stimulation and the fluctuations of the level of noradrenaline at the receptors driving the contractions were assessed from the electrochemically determined noradrenaline oxidation current at a carbon fibre electrode at the surface of the artery. Both were compared with the noradrenaline-mediated neurogenic contraction. The effects on these parameters of cocaine or desipramine, or of corticosterone, were used to assess the relative roles of neuronal and extraneuronal uptake, respectively. The effects of cocaine or desipramine, which enhance the noradrenaline level at the receptors by blocking neuronal reuptake, were compared with those of yohimbine, presumed to act exclusively by enhancing the per pulse release of noradrenaline. The results seem to support the following tentative conclusions. Clearance of released noradrenaline occurs by neuronal uptake and diffusion, while extraneuronal uptake is negligible. The noradrenaline-induced neurogenic contraction is mediated via adrenoceptors on cells near the plane of the nerve plexus; the excitation spreads from these cells throughout the syncytium. The contractile response to exogenous noradrenaline may also be mediated via receptors on the innervated key cells. Reuptake of noradrenaline into the releasing varicosities, i.e. in "active junctions", is highly efficient for single quanta but rapidly saturated by repeated release, while reuptake of noradrenaline in the "surround" of active junctions is probably rarely saturated and more independent of nerve activity. Saturation of the transporter by repeated release of quanta from the same varicosity and the consequent accumulation of "residual" noradrenaline and increased diffusion out of the junction and recruitment of noradrenaline receptors in the surround may be the cause of the rapid growth of the contraction during a high frequency train. Diffusion of released noradrenaline away from the postjunctional receptors is restricted by a local nerve activity-dependent buffering mechanism which, in spite of fading of the per pulse release, helps maintain the noradrenaline concentration at the receptors and the contraction during long high-frequency trains. Reactivation of the clearance mechanisms upon cessation of nerve activity accelerates the relaxation.(ABSTRACT TRUNCATED AT 400 WORDS)

Spatiotemporal pattern of quantal release of ATP and noradrenaline from sympathetic nerves: consequences for neuromuscular transmission

The recent explosive development in research concerning the fundamental mechanisms of synaptic transmission helps put the present paper in context. It is now evident that not all transmitter vesicles in a nerve terminal, not even all those docked at its active zones, are immediately available for release (36). We watch, fascinated, the unraveling of the amazingly complex cellular mechanisms and molecular machinery that determine whether or not a vesicle is "exocytosis-competent" (77,78,39,79). Studies on quantal release in different systems show that neurons are fundamentally similar in one respect: that transmitter release from individual active zones is monoquantal (2). But they also show that active zones in different neurons differ drastically in the probability of monoquantal release and in the number of quanta immediately available for release (3). This implies that one should not extrapolate directly from transmitter release in one set of presynaptic terminals (e.g., in neuromuscular endplate or squid giant synapse) to that in other nerve terminals, especially if they have a very different morphology. As shown here, one should not even extrapolate from transmitter release in sympathetic nerves in one tissue (e.g., rat tail artery) to that in other tissues or species (e.g., mouse vas deferens). It is noteworthy that most studies of quantal release are based on electrophysiological analysis and therefore deal with release of fast, ionotropic transmitters from small synaptic vesicles at the active zones, especially in neurons in which these events may be examined with high resolution (49,48,46,33,32). Such data are useful as general models of the release of both fast and slow transmitters from small synaptic vesicles at active zones in other systems, provided that these transmitters are released in parallel, as are apparently ATP and NA in sympathetic nerves. They tell us little or nothing, however, about the release of transmitters (e.g., neuropeptides) from the large vesicles, nor about the spatiotemporal pattern of monoquantal release from small synaptic vesicles in the many neurons that have boutons-en-passent terminals. They show that the time course of effector responses to fast, rapidly inactivated transmitters such as ACh or ATP is necessarily release related. But they do not even address the possibility that the effector responses to slow transmitters such as NA, co-released from the same terminals, may obey completely different rules and perhaps rather be clearance related (7).(ABSTRACT TRUNCATED AT 400 WORDS)

Kinetics of ATP- and noradrenaline-mediated sympathetic neuromuscular transmission in rat tail artery

Electrophysiological, electrochemical and mechanical recordings were employed to study the kinetics of the release and clearance of adenosine 5'-triphosphate (ATP) and noradrenaline (NA) as sympathetic co-transmitters and of the neurogenic and non-neurogenic contractions in rat isolated tail artery. The life-time of ATP and NA released by a single pulse or 10 pulses at 50 Hz was brief (< 100 ms, or < 3 s, respectively); the neurogenic contractile responses occurred largely after the transmitters had been removed from the extracellular space. The ATP-induced neurogenic contractile responses to a single pulse or 10 pulses at 50 Hz were similar in time-course to the responses to direct muscle stimulation at low voltage; both seemed to be caused by activation of nifedipine-sensitive voltage-gated L-type Ca2+ channels. The alpha 1- and alpha 2-adrenoceptor-mediated components of the NA-induced neurogenic contractile response to 10 pulses at 50 Hz were more delayed and prolonged and determined by properties of the post-receptor mechanisms. The per pulse release of both ATP and NA faded rapidly during long high-frequency trains. So did the ATP level at the receptors and the ATP-induced neurogenic contraction. The NA levels and the contractile responses induced via alpha 1- and alpha 2-adrenoceptors were much better maintained during ongoing stimulation at 20 Hz but relaxed rapidly afterwards, suggesting that nerve activity suppressed, and cessation of nerve activity reactivated NA clearance.

Frequency- and train length-dependent variation in the roles of postjunctional alpha 1- and alpha 2-adrenoceptors for the field stimulation-induced neurogenic contraction of rat tail artery

The present paper examines the roles of postjunctional alpha 1- and alpha 2-adrenoceptors for the noradrenaline (NA)-induced neurogenic contractile response to field stimulation mainly with 1-100 pulses at 2 or 20 Hz, in the tail artery of adult normotensive rats. Pharmacological tools were employed to isolate and characterize the alpha 1- and alpha 2-adrenoceptor-mediated components of this response. The degree to which the drugs influenced NA release or reuptake was assessed by their effects on the electrochemically determined, stimulation-induced rise in the NA concentration at the innervated outer surface of the media. This response was unaffected by alpha,beta-methylene ATP (10 microM) or suramin (500 microM), added to desensitize or block P2-purinoceptors, respectively prazosin (0.1 microM) or SK&F 104078 (6-chloro-9-[(3-methyl-2-butenyl)oxyl]-3-methyl- 1H-2,3,4,5-tetrohydro-3-benzazepine, 0.1 microM), used to block postjunctional alpha 1- and alpha 2-adrenoceptors respectively, nifedipine (10 microM), blocker of Ca2+ influx through L-type channels, and ryanodine (10 microM), which blocks mobilization of Ca2+ from intracellular stores; it was moderately enhanced by yohimbine (0.1 microM), blocker of pre- and postjunctional alpha 2-adrenoceptors, and strongly enhanced by cocaine (3 microM) or desipramine (1 microM), blockers of NA reuptake. Judging from their inhibitory effects on the contractile responses to the alpha 1- and alpha 2-adrenoceptor agonists, phenylephrine and xylazine, prazosin (0.1 microM) and SK&F 104078 (0.1 microM) could be used to selectively block alpha 1- and alpha 2-adrenoceptors respectively, while yohimbine (0.1 microM) was less selective, strongly depressing alpha 2- and slightly depressing alpha 1-adrenoceptor-mediated responses. The alpha 1-adrenoceptor-mediated component of the contractile response to short trains at 20 Hz was fast in onset, brief in duration and abolished by ryanodine; that mediated by alpha 2-adrenoceptors was more delayed, prolonged and insensitive to ryanodine. Both components were dose-dependently depressed by nifedipine (0.1-10 microM). The small contractile responses to single pulses, or up to 50 pulses at 2 Hz, or short train (< 4 pulses) at 20 Hz, were more markedly depressed by 0.1 microM yohimbine or SK&F 104078 than by 0.1 microM prazosin and, hence, mediated mainly by alpha 2-adrenoceptors. The reverse was true of the much larger response to longer trains at 20 Hz, which thus probably was mediated mainly by alpha 1-adrenoceptors.(ABSTRACT TRUNCATED AT 400 WORDS)

Characteristics of function-specific pathways in the sympathetic nervous system

The autonomic nervous system enables all of our body systems to operate in an external environment that is both physically and emotionally challenging. Despite voluntary and involuntary interventions, the composition of the internal environment is maintained. Autonomic dysfunction, particularly in aging people, reveals the importance of this efferent neural control for the wellbeing of our bodies and minds. Although the sympathetic component of this system has been widely thought to be concerned only with the body's response to stress, we discuss here how a range of neuroscientific techniques has started to reveal the specialized properties of functional pathways in the sympathetic system at molecular, cellular and integrative levels. The diversity observed is not compatible with a simple neuroendocrine role of this system.