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

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.

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.

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)

Nonsynaptic diffusion neurotransmission (NDN) in the brain

The synapse has dominated the conceptual model of neurotransmission; other mechanisms, such as neuromodulation, have been considered to support and complement synaptic transmission. In this commentary, the conceptual framework considers synaptic transmission as one of several mechanisms of neurotransmission. One of these is nonsynaptic diffusion neurotransmission (NDN), which includes both the diffusion of neurotransmitters and other neuroactive substances through the extracellular fluid to reach extrasynaptic receptors, and the diffusion of substances such as nitric oxide through both the extracellular fluid and cellular membranes to act within the cell. The possible roles of NDN in mass, sustained functions such as mood, sleep and brain "tone", as well as in various other functions, such as in long term potentiation, at the retinal, lateral geniculate nucleus and visual cortex levels of the visual system, in recovery from brain damage and in neuropharmacology, are explored.

The innervation of salivary glands as revealed by morphological methods

Salivary secretion is nerve mediated. The salivary glands are supplied by parasympathetic and sympathetic efferent nerves which travel to the glands by separate routes. Once in the glands the axons from each type of nerve intermingle and travel together in association with Schwann cells, forming Schwann-axon bundles. Two types of neuro-effector relationships exist with salivary parenchymal and myoepithelial cells: epilemmal (outside the parenchymal basement membrane) and hypolemmal (within the parenchymal basement membrane). Their relative frequencies with either type of nerve differ greatly between glands and species. Salivary blood vessels receive epilemmal innervations by both sympathetic and parasympathetic axons. The classical transmitters--acetylcholine in parasympathetic and noradrenaline in sympathetic axons--are stored in small vesicles. A variety of non-conventional neuropeptide transmitters have also been found in salivary nerves by immunohistochemistry, and they occur in large dense-cored vesicles. Prolonged high frequency stimulation has been found to cause depletion of large dense-cored vesicles from glandular nerves. In recent years afferent nerves have started to be identified and are found in greatest numbers around the main salivary ducts, where they may form a hypolemmal association with the epithelial cells. Functional studies demonstrate complex interactions between parasympathetic and sympathetic nerves. Morphological assessments of changes in the parenchymal cells after nerve stimulations or denervations add greatly to our understanding of the nerve functions. At least four types of influence can be exerted on salivary parenchymal cells by the nerves: hydrokinetic (water mobilizing), proteokinetic (protein secreting), synthetic (inducing synthesis), and trophic (maintaining normal functional size and state). In respect to each role, wide glandular and species differences exist between the relative contributions made by each type of nerve.

Epinephrine and the genesis of hypertension

Several lines of evidence suggest a psychophysiological link between stress, adrenomedullary activation, and the genesis of hypertension. Experimental data support four important concepts: 1) epinephrine stimulates prejunctional beta 2-adrenergic receptors that facilitate norepinephrine release from sympathetic nerve endings; 2) epinephrine can be converted into a cotransmitter by neuronal uptake and on subsequent release augment the simultaneous discharge of norepinephrine; 3) exogenous epinephrine can induce sustained hypertension in rats; and 4) there is a period of critical sensitivity to endogenous epinephrine in a genetic model of rat hypertension. Plasma epinephrine concentrations are elevated in many young subjects with borderline or mild hypertension. The hypothesis that intermittent surges in epinephrine could initiate or promote the development of primary hypertension by amplifying peripheral neurotransmission, both directly (facilitative effect) and indirectly (cotransmitter action), is supported by reports that hemodynamic and noradrenergic responses to sympathetic activation can be augmented by increases in endogenous epinephrine or by its local or systemic (up to 30 ng/kg/min) infusion. Such responses have been documented in both normotensive and hypertensive subjects and can be blocked by propranolol. Although the weight of evidence (mostly indirect) indicates that epinephrine can augment norepinephrine release in humans, the epinephrine hypothesis, itself, remains unproven. Expression of hypertension by this mechanism may be restricted to a specific epinephrine-sensitive subset of individuals with a genetic predisposition to high blood pressure.

Presynaptic modulation of release of noradrenaline from the sympathetic nerve terminals in the rat spleen

Strips of rat spleen, loaded with [3H]NA, were superfused with Krebs' stimulated electrically at different frequencies (2, 8 and 32 Hz) with the same number of pulses and the effect of different receptor agonists and antagonists was studied on the S2/S1 ratio. Evidence has been obtained that the sympathetic nerve terminals in the spleen of the rat are equipped with presynaptic alpha 2B-adrenoreceptors, M-muscarinic, N-nicotinic and P1-purinoreceptors. Clonidine, an alpha 2-adrenoreceptor agonist, did not change the release of NA at low frequency but increased it at a high frequency of stimulation. It is suggested that clonidine is a partial agonist and reveals its antagonistic property when the biophase concentration of the full agonist, NA, released endogenously large. A novel, potent and highly selective competitive antagonist of the alpha 2-adrenoreceptor, CH-38083, enhanced significantly the release of NA, removing presynaptic negative feedback control of NA. It is suggested that the release of NA is subject to presynaptic modulation, through different presynaptic receptors and that it is involved in the communication between the central nervous system and a secondary lymphoid organ, the spleen.

An electrophysiological study of the actions of angiotensin II at the sympathetic neuroeffector junction in the guinea-pig vas deferens

1. The effects of angiotensin II on sympathetic neuroeffector transmission in the guinea-pig vas deferens have been investigated by the use of intracellular and focal extracellular recording techniques to measure indirectly, the release of adenosine 5'-triphosphate (ATP). 2. Angiotensin II (10-100 nM) did not alter the amplitude of the first excitatory junction potential (e.j.p.) in a train but increased the amplitude of subsequent e.j.ps. There was a corresponding increase in the probability of occurrence of extracellularly recorded evoked excitatory junction currents (e.j.cs). Spontaneous quantal transmitter release was unaffected by angiotensin II. 3. The enhancement of transmitter release produced by angiotensin II was prevented by the angiotensin receptor antagonist, saralasin. 4. The increase in transmitter release produced by angiotensin II was due to an increase in the probability of transmitter release from individual varicosities and not due to any detectable change in the configuration of the nerve terminal impulse or to the induction of repetitive firing. 5. There was no overall enhancement of e.j.ps or e.j.cs by angiotensin II in reserpinized tissues. Surprisingly, the predominant effect of angiotensin II in reserpinized vasa deferentia was to inhibit evoked transmitter release, an effect reversed by indomethacin (3 microM). 6. The results show that angiotensin II increases the release of sympathetic transmitter by activating prejunctional angiotensin II receptors. However, when the co-transmitter noradrenaline was depleted, angiotensin II now inhibited transmitter release indirectly, presumably by stimulating prostaglandin formation in the smooth muscle cells which then inhibited release transjunctionally.

K+ and Ca2+ channel blockers may enhance or depress sympathetic transmitter release via a Ca(2+)-dependent mechanism "upstream" of the release site

Extracellular recording of the pre- and postjunctional electrical activity in guinea-pig or mouse vas deferens or rat tail artery was employed to study the mechanisms by which the K+ channel blockers, tetraethylammonium and 4-aminopyridine and the Ca2+ channel blockers, Cd2+, Mn2+ or nifedipine influence the nerve stimulation-induced release of adenosine 5'-triphosphate as a sympathetic co-transmitter. The K+ and Ca2+ channel blocking agents examined had no effect on the spontaneous quantal release of adenosine 5'-triphosphate. However, addition of tetraethylammonium and 4-aminopyridine inside the recording electrode broadened the nerve terminal action potential and caused it to become more resistant to local application of tetrodotoxin, and dramatically increased the magnitude and tetrodotoxin resistance of adenosine 5'-triphosphate release within the patch. Surprisingly, tetraethylammonium and 4-aminopyridine were equally effective when added outside the recording electrode; now they did not increase the duration of the nerve terminal action potential inside the patch but increased its resistance to locally applied tetrodotoxin and dramatically increased the magnitude as well as the tetrodotoxin resistance of adenosine 5'-triphosphate release from sites inside the patch. Both tetraethylammonium and 4-aminopyridine contributed to these effects, with a strong potentiating interaction. Nifedipine was without effect, but application of 1-100 microM Cd2+ or 1-5 mM Mn2+ either inside or outside the recording electrode blocked adenosine 5'-triphosphate release inside the patch. The results indicate: (i) that the nerve terminal action potential is generated by activation of voltage-gated, regenerative Na+ channels but also has a small component carried by influx of Ca2+ and that it is "normally" terminated by activation of voltage- as well as Ca(2+)-dependent K+ channels; (ii) that the release probability is tonically depressed by the resting K+ efflux, and promoted by the resting Ca2+ influx, "upstream" of the release sites; and (iii) that the upstream control of the release probability may involve both changes in properties of ionic channels in the nerve terminal membrane, and effects on the cytoskeleton leading to changes in the availability of releasable quanta in varicosities within the patch.