Conditions required for the inhibitory feedback loop in noradrenergic transmission

Adaptative machine vision with microsecond-level accurate perception beyond human retina

Visual adaptive devices have potential to simplify circuits and algorithms in machine vision systems to adapt and perceive images with varying brightness levels, which is however limited by sluggish adaptation process. Here, the avalanche tuning as feedforward inhibition in bionic two-dimensional (2D) transistor is proposed for fast and high-frequency visual adaptation behavior with microsecond-level accurate perception, the adaptation speed is over 104 times faster than that of human retina and reported bionic sensors. As light intensity changes, the bionic transistor spontaneously switches between avalanche and photoconductive effect, varying responsivity in both magnitude and sign (from 7.6 × 104 to -1 × 103 A/W), thereby achieving ultra-fast scotopic and photopic adaptation process of 108 and 268 μs, respectively. By further combining convolutional neural networks with avalanche-tuned bionic transistor, an adaptative machine vision is achieved with remarkable microsecond-level rapid adaptation capabilities and robust image recognition with over 98% precision in both dim and bright conditions.

Novel attempts to optimize vagus nerve stimulation parameters on serotonin neuronal firing activity in the rat brain

BACKGROUND

Vagus nerve stimulation (VNS) is indicated for treatment-resistant epilepsy and depression. Electrophysiologic recordings in the rat brain have shown that VNS promptly increases the firing rate of NE neurons and subsequently that of 5-HT neurons. Thus far, it appears that the standard stimulation parameters currently used in depressed patients produce an optimal activation of 5-HT neurons.

OBJECTIVE/HYPOTHESIS

This study was therefore aimed at investigating additional alterations of stimulation parameters to optimize VNS efficacy to further increase 5-HT neuronal activity.

METHODS

Rats were implanted with a VNS device and stimulated for 14 days using standard (0.25 mA/20 Hz/500 microseconds/30 seconds ON-5 minutes OFF, continuously) or various stimulation parameters: extension of the OFF period (30 seconds ON every 10 to 30 minutes), the OFF and ON periods, discontinuous stimulation (12 hours per day using standard parameters), and burst stimulation modes. Rat dorsal raphe 5-HT neurons were recorded under chloral hydrate anesthesia.

RESULTS

Both 12-hour stimulation periods for 14 days, and the 30-second stimulation every 10 or 15 minutes significantly increased the firing activity of 5-HT neurons to the same extent as standard parameters while the 30-minute intervals were ineffective. Stimulations in a burst mode and the pseudo-one-pulse stimulations also significantly increased 5-HT neuronal activity.

CONCLUSIONS

These results indicate that less stimulation is sufficient to achieve the same VNS efficacy on 5-HT neuronal firing. These data may be relevant for patients using VNS because these new parameters could minimize or prevent side effects and increase battery life of the stimulator.

Studies on the mechanism of enhancement of purinergic transmission by caffeine in the guinea-pig isolated vas deferens

1. Purinergic transmission from sympathetic nerves in the guinea-pig vas deferens was monitored using intracellular recording techniques. Stimulation of the hypogastric nerve with trains of 15 pulses at 1 Hz evoked excitatory junction potentials (EJPs) which increased in amplitude from the first pulse and reached a maximum after 6-8 pulses. 2. Caffeine (3 and 10 mm), depolarized cells by 5-10 mV and increased the amplitude of the first few EJPs in each train but reduced the maximum amplitude of EJPs late in the train. 3. The adenosine receptor antagonist 8-p-sulphophenyl-theophylline (8-SPT; 30 microm) had no effect on either the resting membrane potential or the EJP amplitude; however, at 100 microm it reduced the amplitude of all EJPs by 5-10%. 4. Adenosine (10 and 30 microm) reduced the amplitude of EJPs in a concentration-dependent manner. The inhibitory effect of adenosine on EJP amplitude was prevented by pretreatment with either caffeine (3 mm) or 8-SPT (30 microm). 5. Ryanodine (30 microm) did not alter EJP amplitude and did not inhibit the enhancement of the first EJP by caffeine (3 mm). Incubation of the tissue with the cell permeable calcium chelator 1-2-bis(o-aminophenoxy)ethane-N,N-N',N'-tetraacetic acid (BAPT-AM) resulted in a depression of EJP amplitude and a longer time to reach maximum amplitude. In cells that had been exposed to BAPT-AM, caffeine 3 mm still increased amplitude of EJP early in the train. 6. The phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine (IBMX; 500 microm), hyperpolarized cells and increased the amplitude of EJP throughout the train of stimulation. In the presence of IBMX, caffeine 3 mm still depolarized the cells and enhanced the EJP early in the train of stimulation. 7. The findings in this study confirm that caffeine and 8-SPT are effective inhibitors of the actions of adenosine. However, caffeine has an additional action to enhance EJP early during a train of stimulation, which cannot be attributed to blockade of adenosine receptors, but which may be related to inhibition of phosphodiesterase.

Individual sympathetic varicosities possess different sensitivities to alpha 2 and P2 receptor agonists and antagonists in mouse vas deferens

1. The diversity of alpha(2) and purinergic autoreceptor actions on action potential evoked calcium transients in single varicosities has been investigated using the calcium indicator Oregon Green 488 BAPTA-1. 2. During long trains of impulses (10 Hz for 30 s), the change in calcium concentration in varicosities from its resting level (Delta[Ca(2+)](v)) increased in many varicosities during the first 3 s of stimulation before reaching a plateau. 3. The alpha(2) adrenoceptor agonist clonidine (1 microM) decreased Delta[Ca(2+)](v) by over 40% during short trains (five impulses at 5 Hz) in most varicosities, although some were unaffected. The alpha(2) adrenoceptor antagonist idazoxan (2 microM) increased the Delta[Ca(2+)](v) plateau following long trains in most varicosities. Hence, most varicosities possess alpha(2) adrenoceptors which are activated when noradrenaline accumulates extracellularly. 4. During long trains of impulses, the P(2y)-purinergic receptor agonist 2-methyl-thio-ATP (100 microM) decreased Delta[Ca(2+)](v) plateau by about 50% in most varicosities; alpha,beta-methylene ATP (100 microM) decreased it by about 50% in a minority of varicosities; adenosine (200 microM) had no significant effect. Suramin (100 microM) increased the Delta[Ca(2+)](v) during all stimulus protocols in most varicosities, suggesting that ambient ATP modulates Delta[Ca(2+)](v) responses. The P(2y) receptor antagonist reactive blue (100 microM) affected a minority of varicosities. Given that most varicosities respond to suramin, other P(2) receptor subtypes are probably present. 5. The ATP ectoenzyme antagonist ARL67157 (50 microM) decreased the plateau Delta[Ca(2+)](v) during long trains in complete strings of varicosities but not in others. 6. The present technique indicates that varicosities have diverse autoreceptor utilization.

One hundred years of adrenaline: the discovery of autoreceptors

The active principle of suprenal extract that produces its pressor effects was isolated by the joint research of John Abel in 1899 and Jokichi Takamine in 1901. Within three years Elliott, working in Langley's laboratory, suggested that this active principle, referred to by British physiologists as "adrenaline" and named "Adrenalin" by Takamine, was released from sympathetic nerve terminals to act on smooth muscle cells. However, it was not until 1946 that von Euler showed that demythelated adrenaline (noradrenaline) rather than adrenaline is a sympathetic transmitter. The possibility that this sympathetic transmitter could also act on nerve terminals was not developed until 1971. Research on autoreceptors culminated in the identification of adrenergic receptors on nerve terminals different to those on muscle cells. This paper assesses the contributions that established the idea of the adrenergic autoreceptor, 100 years after the discovery of adrenaline.

Omega-conotoxin GVIA-resistant neurotransmitter release from postganglionic sympathetic nerves in the guinea-pig vas deferens and its modulation by presynaptic receptors

1 Intracellular recording techniques were used to study neurotransmitter release mechanisms in postganglionic sympathetic nerve terminals in the guinea-pig isolated vas deferens. 2 Recently, a component of action potential-evoked release which is insensitive to high concentrations of the selective N-type calcium channel blocker omega-conotoxin GVIA termed 'residual release' has been described. Under these conditions, release of the neurotransmitter ATP evoked by trains of low frequency stimuli is abolished, but at higher frequencies a substantial component of release is revealed. 3 'Residual release' was studied with trains of 5 or 10 stimuli at stimulation frequencies of 10, 20 and 50 Hz. The alpha2-adrenoceptor agonist clonidine (30-100 nM) inhibited 'residual release', the degree of inhibition being most marked at the beginning of a train. 4 The alpha2-adrenoceptor antagonist yohimbine (1 microM) induced a marked increase in 'residual release' which was dependent on both the frequency of stimulation and the number of stimuli in a train. 5 Prostaglandin E2 (30 nM) and neuropeptide Y (100 nM) caused a rapid inhibition of 'residual release' at all stimulation frequencies examined. 6 4-Aminopyridine (100 microM) induced a powerful potential of 'residual release' and could reverse the inhibition of omega-conotoxin GVIA. 7 'Residual release' was modulated through presynaptic alpha2-adrenoceptors suggesting that (i) residual release of ATP is subject to alpha-autoinhibition through the co-release of noradrenaline, (ii) noradrenaline release can be triggered by calcium channels other than the N-type and (iii) when presynaptic receptors are activated, inhibition of transmitter release can occur by mechanisms other than modulation of calcium-entry through N-type calcium channels in postganglionic sympathetic nerves. Prostaglandin E2 and neuropeptide Y also modulated neurotransmitter release.

P2-receptor modulation of noradrenergic neurotransmission in rat kidney

1. ATP has previously been shown to act as a sympathetic cotransmitter in the rat kidney. The present study analyses the question of whether postganglionic sympathetic nerve endings in the kidney possess P2-receptors which modulate noradrenaline release. Rat kidneys were perfused with Krebs-Henseleit solution containing the noradrenaline uptake blockers cocaine and corticosterone and the alpha2-adrenoceptor antagonist rauwolscine. The renal nerves were electrically stimulated, in most experiments by 30 pulses applied at 1 Hz. The outflow of endogenous noradrenaline (or, in some experiments, of ATP and lactate dehydrogenase) as well as the perfusion pressure were measured simultaneously. 2. The P2-receptor agonist adenosine-5'-O-(3-thiotriphosphate) (ATPgammaS, 3-30 microM) reduced the renal nerve stimulation (RNS)-induced outflow of noradrenaline (estimated EC50 =8 microM). The P2-receptor antagonist cibacron blue 3GA (30 microM) shifted the concentration-inhibition curve for ATPgammaS to the right (apparent pKB value 4.7). 3. Cibacron blue 3GA (3-30 microM) and its isomer reactive blue 2 (3-30 microM) significantly increased RNS-induced outflow of noradrenaline in the presence of the P1-receptor antagonist 8-(p-sulphophenyl)theophylline (8-SPT, 100 microM) by about 70% and 90%, respectively. The P2-receptor antagonist suramin (30-300 microM) only tended to enhance RNS-induced outflow of noradrenaline. When the nerves were stimulated by short pulse trains consisting of 6 pulses applied at 100 Hz (conditions under which autoinhibition is inoperative), reactive blue 2 did not affect the RNS-induced outflow of noradrenaline. 4. RNS (120 pulses applied at 4 Hz) induced the outflow of ATP but not of the cytoplasmatic enzyme lactate dehydrogenase. 5. ATPgammaS (3-30 microM) concentration-dependently reduced pressor responses to RNS at 1 Hz. Cibacron blue 3GA, reactive blue 2 as well as suramin also reduced pressor responses to RNS (maximally by 50 to 70%). 6. This study in rat isolated kidney, in which the release of endogenous noradrenaline was measured, demonstrates that renal sympathetic nerves possess prejunctional P2-receptors that mediate inhibition of transmitter release. These prejunctional P2-receptors are activated by endogenous ligands, most likely ATP, released upon nerve activity. Both, P2-receptor agonists and P2-receptor antagonists reduced pressor responses to RNS either by inhibiting transmitter release or by blocking postjunctional vasoconstrictor P2-receptors.

Prominent sympathetic purinergic vasoconstriction in the rabbit splenic artery: potentiation by 2,2'-pyridylisatogen tosylate

1. Vasoconstrictions induced by transmural electrical field stimulation were frequency-dependent from 2 to 32 Hz in the rabbit isolated splenic artery. All contractions were abolished in the presence of tetrodotoxin 1 microM or guanethidine 100 microM. Stimulation at a frequency of more than 32 Hz induced both neurogenic and myogenic responses. 2. Prazosin (1 microM) did not significantly affect vascular contractions to electrical stimulation. Desensitization of P2X-purinoceptors with alpha, beta-methylene ATP (alpha, beta-meATP, 3 microM) abolished the contractions to stimulation at 2-8 Hz and inhibited more than 80% of the vascular response at 16 Hz, but it did not significantly change the responses at 32 Hz. Contractile responses at 32 Hz were inhibited by a combination of prazosin and alpha, beta-meATP. Effects of pyridoxal-phosphate-6-azophenyl-2', 4'-disulphonic acid tetrasodium salt (a selective P2X-purinoceptor antagonist) and suramin (a competitive P2-purinoceptor antagonist) on the neurogenic responses were investigated in this study. 3. 2,2'-Pyridylisatogen tosylate (PIT, 0.3-3 microM) significantly potentiated the vasoconstrictions to electrical stimulation at 2-32 Hz in a concentration-dependent manner. Potentiated responses were restored to the control level 30 min after washing. Concentration-dependent response curves for noradrenaline (NA) or alpha, beta-meATP were not significantly changed by 3 microM PIT, and vasoconstriction by adenosine 5'-triphosphate (ATP, 300 microM) was unaffected by PIT. Coomassie brilliant blue-G (1 microM), which shares the potentiating effect on a recombinant P2Y-purinoceptor with PIT (King et al., 1996), did not inhibit or potentiate the purinergically-mediated component of the response to sympathetic nerve stimulation. The selective alpha 2-adrenoceptor antagonist yohimbine (1 microM) also potentiated the vascular responses to electrical stimulation. 4. The present results indicate that ATP evokes postjunctional contractile responses at low and high frequency electrical stimulation of sympathetic nerves supplying the rabbit splenic artery. PIT potentiates the responses to sympathetic (purinergic) nerve stimulation; this appears to be mainly via prejunctional rather than postjunctional actions.

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.

Effect of morphine on the nerve terminal impulse and transmitter release from sympathetic varicosities innervating the mouse vas deferens

1 The effect of morphine on both the propagation of the nerve terminal impulse along the sympathetic varicose axons as well as the evoked and spontaneous transmitter release has been evaluated. 2 Morphine (1 microM) did not significantly change the shape or the regularity by which the nerve terminal impulse was recorded while evoked transmitter release was greatly reduced. 3 Morphine induced a uniform decrease in evoked transmitter release irrespective of the release probability of individual varicosities of their position along terminal branches. 4 Procedures which are thought to increase intracellular calcium concentration such as increasing the extracellular calcium concentration, stimulation of the nerve with trains of impulses and increasing the duration of the action potential with 4-aminopyridine reduced the ability of morphine to decrease evoked transmitter release. 5 Morphine had to act directly on the varicosities to induce a decrease in evoked transmitter release. 6 The decrease in evoked quantal release does not involve an affect on the nerve terminal impulse or the vesicle release process and morphine may affect the dependence of the secretory process on calcium.

Caffeine enhances sympathetic purinergic and noradrenergic transmission in the guinea-pig isolated vas deferens

Intracellular recording techniques were used to monitor the resting membrane potential of smooth muscle cells and the excitatory junction potentials (EJPs) evoked by stimulation of the hypogastric nerve. Stimulation with trains of 15 pulses at 1 Hz or 0.33 Hz evoked individual EJPs which increased in amplitude from the first pulse and reached a plateau after 6-8 pulses. Stimulation at 1 Hz resulted in EJPs facilitating to a plateau level of approximately 25 mV, whereas with stimulation at 0.33 Hz the EJPs only facilitated to a plateau level of about 12 mV. With stimulation at 1 Hz, caffeine (3 mM and 10 mM), increased the amplitude of the first few EJPs in each train and decreased the extent of facilitation and reduced the amplitude of fully facilitated EJPs. In comparison, the amplitude of all EJPs evoked by stimulation at 0.33 Hz was increased by caffeine (3 mM and 10 mM). With 0.33 Hz stimulation, facilitation of the first few EJPs was observed in the presence of 3 mM caffeine but not in the presence of 10 mM caffeine. In the presence of the alpha 2-adrenoceptor antagonist idazoxan, caffeine (3 mM and 10 mM) still enhanced the amplitude of EJPs early in trains of stimulation but there was no depression of EJPs later in the trains. Similarly, in reserpine-treated vasa deferentia, caffeine (3 mM) enhanced EJPs early in the train of stimulation at 1 Hz and there was no depression of EJPs at the end of the train. In addition to electrophysiological experiments, the effect of caffeine (0.1-30 mM) on the resting and stimulation-induced (S-I) efflux of radioactivity was investigated in guinea-pig isolated vasa deferentia previously incubated with [3H]-noradrenaline. Caffeine (10 mM) did not affect the resting efflux of [3H]-noradrenaline but significantly enhanced the S-I efflux by 150-160%. The present findings suggest that caffeine enhances sympathetic purinergic and noradrenergic transmission at the sympathetic neuroeffector junction in the guinea-pig vas deferens. Moreover, the increased release of transmitter noradrenaline can modulate purinergic transmission by activation of alpha 2-adrenoceptors located at sympathetic neuroeffector sites.

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.

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.

Alpha-adrenoceptors

Major advances have been made in our understanding of the molecular structure and function of the alpha-adrenoceptors. Many new subtypes of the alpha-adrenoceptor have been identified recently through biochemical and pharmacological techniques and several of these receptors have been cloned and expressed in a variety of vector systems. Currently, at least seven subtypes of the alpha-adrenoceptor have been identified and the molecular structure and biochemical functions of these subtypes are beginning to be understood. The alpha-adrenoceptors belong to the super family of receptors that are coupled to guanine nucleotide regulatory proteins (G-proteins). A variety of G-proteins are involved in the coupling of the various alpha-adrenoceptor subtypes to intracellular second messenger systems, which ultimately produce the end-organ response. The mechanisms by which the alpha-adrenoceptor subtypes recognize different G-proteins, as well as the molecular interactions between receptors and G-proteins, are the topics of current research. Furthermore, the physiological and pathophysiological role that alpha-adrenoceptors play in homeostasis and in a variety of disease states is also being elucidated. These major advances made in alpha-adrenoceptor classification, molecular structure, physiologic function, second messenger systems and therapeutic relevance are the subject of this review.

Effects of the venom of the theridiid spider, Steatoda capensis Hann, on autonomic transmission in rat isolated atria and caudal artery

The possibility of alpha-latrotoxin-like activity in the crude venom gland extract (VGE) of a related Theridiid spider, Steatoda capensis Hann, was investigated. The VGE from female S. capensis Hann spiders produced vasoconstriction in isolated segments of rat caudal artery but was without effect in artery segments obtained from rats that had been pretreated with reserpine (2.5 mg/kg) 24 hr prior to experimentation, indicating that the vasoconstriction was due to the release of noradrenaline from periarterial sympathetic nerves. Steatoda capensis Hann VGE also increased the rate of beating of rat isolated atrial preparations. The positive chronotropic action of the VGE was partly due to the release of noradrenaline from atrial sympathetic nerves since it was reduced by the beta-adrenoceptor antagonist propranolol, and smaller increases in rate were observed in atria taken from rats pretreated with reserpine. The positive chronotropic effect of the VGE was enhanced by atropine, suggesting that the VGE also releases acetylcholine from atrial parasympathetic nerves. The VGE evoked release of radioactivity from rat atria in which the transmitter stores of the atrial intramural noradrenergic nerves had been labelled with [3H]noradrenaline. There appeared to be two components of the release, one involving omega-conotoxin GVIA-sensitive Ca2+ channels, and the other independent of extracellular Ca2+.

Electrically induced release of endogenous noradrenaline and dopamine from brain slices: pseudo-one-pulse stimulation utilized to study presynaptic autoinhibition

Slices of rat hypothalamus (noradrenaline experiments) or rabbit caudate nucleus (dopamine experiments) were prepared, superfused, and field-stimulated using series of monophasic rectangular pulses. Noradrenaline, dopamine and the main dopamine metabolite, dihydroxyphenylacetic acetic acid (DOPAC), were determined using HPLC with electrochemical detection. Electrical stimulation was performed using the following protocols: 1) 4 pulses delivered at 100 Hz; this type of stimulation is referred to as pseudo-one-pulse stimulation (POP); its short duration of only 32 ms does not allow the development of autoinhibition; 2) 2 bursts of 4 pulses at 100 Hz, delivered 1 s apart (2-POP-stimulation); 3) 8 pulses at 1 Hz (dopamine experiments only); 4) 36 pulses at 3 Hz. Noradrenaline experiments. The alpha 2-adrenoceptor antagonist yohimbine (1 mumol/l) did not enhance noradrenaline overflow following POP stimulation, but enhanced the overflow following 2-POP-stimulation by about 50% and that following 36-pulse-stimulation by almost 100%. Dopamine experiments. The D2-dopamine receptor antagonist sulpiride (3 mumol/l) facilitated the overflow of dopamine elicited with 2-POP-stimulation (66%), 8 pulses/1 Hz (92%), and 36 pulses/3 Hz (140%). It did not significantly facilitate the overflow of dopamine following POP-stimulation (19%). The overflow of DOPAC was not, or only slightly, increased by electrical stimulation, and its spontaneous outflow was more than three times higher than that of dopamine. Furthermore, the electrically induced overflow of dopamine did not exceed the outflow of DOPAC at any of the stimulation conditions employed. The results of the present study bear out important claims of the autoreceptor theory and confirm the data obtained in previous experiments using labelled transmitters.

Effects of nifedipine and ryanodine on adrenergic neurogenic contractions of rat vas deferens: evidence for a pulse-to-pulse change in Ca2+ sources

1. The effects of nifedipine and ryanodine on the adrenergic component of neurogenic contractions of the rat isolated vas deferens were studied in an attempt to identify the sources of Ca2+ mediating the contraction. The tissue was electrically stimulated by single pulses or pairs of widely spaced pulses. The purinergic component of contraction was suppressed by the presence of 300 microM suramin. 2. In Mg(2+)-free medium, nifedipine (0.01-10 microM) reduced the first and, to a greater extent, the second twitch elicited by two pulses 3 s apart. This pattern of inhibition was observed both in the absence of rauwolscine (when twitch 2 was smaller than twitch 1) and in the presence of 0.1 microM rauwolscine (when, due to interruption of prejunctional alpha 2-adrenoceptor-mediated autoinhibition, twitch 2 was of similar height to twitch 1). Nifedipine reduced only twitch 2 but not twitch 1 in medium containing 1.2 mM Mg2+. 3. Single pulses of increasing current strength elicited increasing contraction. Nifedipine reduced contractions by about the same absolute extent at all current strengths, so that the relative contribution of the nifedipine-sensitive component decreased with increasing current strength. 4. When the pulse interval in a pair was increased from 5 to 60 s, the inhibition by nifedipine of the second twitch was most marked at an interval of 5 s and declined as the interval increased. 5. In contrast to nifedipine, 20 microM ryanodine reduced the first twitch of a pair to a greater extent than a second twitch 5 s later, so that twitch 2 became greater than twitch 1. The inhibition by ryanodine of twitch 2 increased with increasing pulse interval.6. In vasa deferentia preincubated with [3H]-noradrenaline, I microM nifedipine and 20 microM ryanodine did not change the electrically evoked overflow of tritium, whereas 10 microM nifedipine increased it.7. It is concluded that, when the sympathetic axons of the vas deferens are stimulated by a single pulse(or the first pulse of a pair) in Mg2+-free medium, both Ca2+ mobilization inside the smooth muscle cells and Ca2+ entry contribute to the ensuing adrenergic contraction. The relative contribution of Ca2+ entry is small at maximal stimulus strength but increases with decreasing stimulus strength. When a second pulse follows the first after an appropriate interval, a switch of Ca2+ sources occurs: intracellular Ca2+mobilization is decreased during twitch 2, whereas Ca2+ entry is increased.

Alpha 2-autoreceptor subclassification in rat isolated kidney by use of short trains of electrical stimulation

1 Rat kidneys were perfused with Krebs-Henseleit solution and incubated with [3H]-noradrenaline. The renal nerves were electrically stimulated at either 1 Hz for 30 s or 100 Hz for 0.06 s. The stimulation induced (S-I) outflow of radioactivity was taken as an index of endogenous noradrenaline release. 2 At a frequency of 1 Hz for 30 s the alpha-adrenoceptor antagonists BRL 44408 (0.01, 0.1 microM) and imiloxan (0.1, 1.0 microM) enhanced S-I outflow of radioactivity. However, at a frequency of 100 Hz for 0.06 s the alpha-adrenoceptor antagonists, idazoxan (0.1, 1.0 microM), imiloxan (0.1, 1.0 microM), BRL 44408 (0.1, 1.0 microM), BRL 41992 (0.1, 1.0 microM) and prazosin (0.01 microM) failed to enhance S-I outflow of radioactivity. 3 Thus, the rat isolated kidney stimulated at 100 Hz for 0.06 s, avoids autoinhibition by endogenous noradrenaline and alpha-adrenoceptor antagonist affinities (pKB) at the prejunctional alpha-autoreceptor were estimated without disturbance by the endogenous activator. 4 The alpha 2-adrenoceptor agonist, clonidine, inhibited the S-I outflow of radioactivity with a maximum of 90% and an EC50 of 7.2 nM. 5 All alpha-adrenoceptor antagonists used caused parallel shifts of the concentration-response curve for clonidine to the right. The rank order of potencies was: rauwolscine (alpha 2A/B) > idazoxan (alpha 2A/B) > phentolamine (alpha 2A/B) > WB 4101 (alpha 2A) > BRL 44408 (alpha 2A) > BRL 41992 (alpha 2B) > prazosin (alpha 2B) = imiloxan (alpha 2B).(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of presynaptic α-adrenoceptors and neuronal reuptake on noradrenaline overflow and cardiac response

Using an in situ perfused, innervated rat heart model, we studied the effects of presynaptic alpha-adrenergic and neuronal reuptake inhibition on evoked noradrenaline (NA) overflow and the postsynaptic response by sympathetic ganglion stimulation. NA overflow was significantly increased by neuronal reuptake inhibitors (desipramine and (+)-oxaprotiline) or by alpha-adrenoceptor antagonists (phentolamine and yohimbine), but the inotropic response was augmented only by alpha-antagonists. In the presence of desipramine, nerve stimulation induced a frequency-dependent increase in NA overflow and postsynaptic response, both were enhanced by yohimbine. In the absence of desipramine, however, postsynaptic response was potentiated by yohimbine despite an unchanged (at 2 and 5 Hz) or even reduced NA overflow (at 10 Hz). Thus, this study suggests that NA release and cardiac response are modulated by presynaptic alpha-adrenoceptors, and that the neuronal reuptake modifies the amount of NA overflow but has little effect on the postsynaptic response.

Rapid kinetics of potassium-evoked release of acetylcholine from rat brain synaptosomes: analysis by rapid superfusion

The rapid kinetics of spontaneous and evoked [3H]acetylcholine efflux from synaptosomes was investigated using the technique of rapid superfusion. Synaptosomes were isolated from whole rat brain and the intraterminal pool of acetylcholine was radiolabeled by preincubation with [3H]choline. Synaptosomes were retained within the superfusion system on filter disks and superfused with Krebs-bicarbonate buffer, pH 7.4, at flow rates of 0.3-0.5 ml/s. These experimental conditions provided a mixing half-life of 119 ms and efficiency of superfusion of greater than 85%. The kinetics of tritium efflux was followed on the second and subsecond time scales by collection of serial 4.8-s and 50-ms samples for a total of 67.2 and 1.0 s, respectively. Superfusion for 48 s with isoosmotic Krebs buffer containing 10, 20, 30, 50, 75, and 100 mM potassium ion stimulated concentration-dependent tritium release. All of potassium-evoked release, but only 17% of spontaneous release, was calcium-dependent. Kinetic analysis of net (total minus spontaneous) potassium-stimulated release revealed a single calcium-dependent component of release that fit a single exponential function with a half-life of 12.7 s. Analysis of the area under the tritium efflux curves observed on the millisecond time scale revealed that 0.111, 0.550, and 0.614% net tritium release was evoked by superfusion for 750 ms with isoosmotic buffer containing 20, 50, and 100 mM KCl, respectively. Consistent with the results observed on the second time scale, a small fraction of spontaneous release and all of potassium-evoked release observed on the millisecond time scale were calcium-dependent. These data indicate that the technique of rapid superfusion can be utilized for the direct investigation of spontaneous and evoked [3H]acetylcholine release, as well as the factors that regulate this release from brain synaptosomes on the second and millisecond time scales.

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.

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)

Presynaptic alpha 2-autoinhibition in a vascular neuroeffector junction where ATP and noradrenaline act as co-transmitters

1. alpha 2-Autoinhibition of transmitter release was investigated in the largest rami caecales of the rabbit ileocolic artery. Vasoconstriction, elicited by electrical field stimulation or by exogenous agonists, was measured as an increase in perfusion pressure. 2. Short periods of electrical stimulation elicited monophasic vasoconstriction, whereas longer periods (greater than 10 s) produced biphasic vasoconstriction. Prazosin had no significant effect on the first component of the biphasic vasoconstriction elicited by electrical stimulation, but did reduce the second component at higher frequencies. alpha, beta-Methylene ATP significantly attenuated the first component whilst the second component was relatively resistant. 3. The alpha 2-adrenoceptor antagonist yohimbine did not change responses evoked by very short pulse trains (less than 2 s) but enhanced responses to longer pulse trains. When vasoconstriction was biphasic, both phases were potentiated by yohimbine. 4. The results indicate that the vasoconstriction elicited by brief trains of sympathetic nerve impulses is mainly or exclusively mediated by ATP, whereas at longer pulse trains a noradrenergic component comes into play. The potentiation produced by yohimbine is due to interruption of presynaptic alpha 2-adrenoceptor-mediated autoinhibition of transmitter release. The autoinhibition affects both purinergic and adrenergic components of sympathetic neurotransmission.

Presynaptic alpha 2-adrenoreceptors affecting terminal synaptic transmission by the nervi cardiaci accelerantes in the rat

1. Changes in heart rate were monitored in response to supramaximal electrical stimulation of the spinal sympathetic cardiac nerves in the pithed rat. 2. Submaximal positive chronotropic responses of the heart to stimulation (0.1-1.0 Hz) were inhibited after intravenous injection of the alpha 2-adrenoreceptor agonists B-HT 920 and xylazine, but not after the alpha 1-adrenoreceptor agonist methoxamine. 3. The alpha 2-adrenoreceptor antagonist yohimbine or idazoxan but not the alpha 1-adrenoreceptor antagonist prazosin, given intravenously, reduced the inhibitory effect of B-HT 920 on positive chronotropic responses to cardiac sympathetic nerve stimulation. Neither yohimbine, idazoxan nor prazosin had any effect on positive chronotropic responses to nerve stimulation. 4. Desmethylimipramine (DMI) potentiated the increased heart rate during continuous stimulation of the cardiac nerves at 0.2 Hz. Responses to stimulation, at this frequency, were unaffected by yohimbine, either in the absence or presence of the neuronal uptake inhibitor, DMI. 5. Intravenous infusion of the angiotensin antagonist saralasin or injection of the angiotensin converting enzyme inhibitor captopril had no significant effect, either on positive chronotropic responses to stimulation or on the inhibitory effects of B-HT 920. Positive chronotropic responses to stimulation were also unaffected by yohimbine in the presence of captopril, indomethacin, or indomethacin plus captopril. 6. Although presynaptic effects of alpha 2-agonists and their antagonists on cardiac nerve function were demonstrated, no evidence was obtained that prejunctional alpha 2-adrenoreceptors modulate transmission during continuous supramaximal firing at constant frequency.(ABSTRACT TRUNCATED AT 250 WORDS)

Some pharmacological applications of an extracellular recording method to study secretion of a sympathetic co-transmitter, presumably ATP

Extracellular recording in guinea-pig or mouse vas deferens or rat tail artery was used to study the effects of some pharmacological agents on the nerve terminal spike (NTS) and the secretion of a sympathetic co-transmitter (presumably ATP), as reflected in the excitatory junction current (EJC). A negative-going EJCi (i for inside) was assumed to reflect release from sites inside, and a positive-going EJCo (o for outside) release from sites outside the recording electrode. Passage into or out of the electrode seemed to be slow. Tetrodotoxin (TTX) in the outer medium blocked the NTS and ECJo as well as EJCi; TTX in the pipette blocked stimulus-evoked but not spontaneous EJCi. The dihydropyridine Ca2+ channel blocking agent, nifedipine, was without effect, but Cd2+ in the external medium blocked EJCo and also, by an effect apparently 'upstream' of varicosities, inhibited EJCi (i.e. release within the patch) but not the NTS. When present in the outer medium the alpha 2-adrenoceptor agonists, clonidine and xylazine, blocked both EJCo and EJCi, but not the NTS. The effects of clonidine were blocked by yohimbine, which in itself increased the EJCo by about 50%. Neuropeptide Y and met-enkephalin in the outer medium blocked EJCo; the effect of met-enkephalin was blocked by naloxone. The K+ channel blocking agents, tetraethylammonium and 4-aminopyridine, inside or outside the electrode, increased dramatically the size of EJCi or EJCo, respectively.

Transmitter release patterns of noradrenergic, dopaminergic and cholinergic axons in rabbit brain slices during short pulse trains, and the operation of presynaptic autoreceptors

Slices of rabbit brain were field-stimulated either by single electrical pulses or by trains of 4 or 8 pulses at 1 or 100 Hz in order to study transmitter release patterns and the autoinhibition of transmitter release. The slices were preincubated with 3H-noradrenaline (cortex), 3H-dopamine (caudate nucleus) or 3H-choline (caudate nucleus). Slices preincubated with 3H-noradrenaline were superfused with medium containing desipramine 1 mumol/l. The overflow of tritium elicited by single pulses amounted to 0.19% of the tritium content of the tissue. The overflow elicited by 4 pulses/1 Hz was similar, whereas that elicited by 4 pulses/100 Hz was 5.1-fold higher. Yohimbine 10-1000 nmol/l increased up to 2.5-fold the overflow evoked by 4 pulses/1 Hz but did not change the overflow evoked by single pulses or 4 pulses/100 Hz. - Slices preincubated with 3H-dopamine were superfused with medium containing nomifensine 1 mumol/l. The overflow of tritium elicited by single pulses was 0.39% of the tritium content of the tissue. The overflow elicited by 4 pulses/1 Hz was 1.3-fold and the overflow elicited by 4 pulses/100 Hz 1.4-fold higher. Domperidone 1-100 nmol/l and sulpiride 10-1000 nmol/l increased up to 2.4-fold the overflow evoked by 4 pulses/1 Hz but increased only slightly the overflow evoked by single pulses or 4 pulses/100 Hz. - Slices preincubated with 3H-choline were superfused either with physostigmine-free medium or with medium containing physostigmine 1 mumol/l. In physostigmine-free medium, atropine did not increase the evoked overflow of tritium at any stimulation condition. In physostigmine-containing medium, the overflow elicited by single pulses was 0.18% of the tritium content of the tissue. The overflow elicited by 8 pulses/1 Hz was 2.0-fold and the overflow elicited by 8 pulses/100 Hz 2.2-fold higher. Atropine 2-200 nmol/l increased up to 2.4-fold the overflow evoked by 8 pulses/1 Hz but increased only slightly the overflow evoked by single pulses or 8 pulses/100 Hz. In physostigmine-free medium, sulpiride 10-1000 nmol/l did not change the single-pulse-evoked overflow of tritium in the absence but increased it in the presence of nomifensine 1 mumol/l. Single pulses elicit a large release of 3H-noradrenaline, 3H-dopamine and 3H-acetylcholine under the conditions of these experiments. Release elicited by single pulses is not subject to autoinhibition except for a small inhibition by spontaneously released transmitter in the case of dopaminergic and cholinergic axons.(ABSTRACT TRUNCATED AT 400 WORDS)

Frequency-dependence of 3H-noradrenaline release from rabbit pulmonary artery: effect of alpha-adrenoceptor antagonists and inhibitors of transmitter inactivation

The aim of this study was to examine the modulating role of presynaptic alpha 2-adrenoceptors on transmitter release from vascular sympathetic neurones. This was done by examining the influence of removal of inactivation pathways on the effect of alpha-adrenoceptor antagonists on the release of transmitter from noradrenergic neurones. The rabbit main pulmonary artery preloaded with 3H-noradrenaline (3H-NA) was used. The artery was stimulated with 300 pulses at various frequencies (1, 3, 10 and 30 Hz). Pargyline (3 x 10(-4) M) increased the stimulation-evoked 3H-overflow at 1 and 3 Hz and decreased it at 30 Hz. U-0521 (3',4'-dihydroxy-2-methylpropiophenone; 3 x 10(-6) M) enhanced the overflow at 1 Hz and had no effect at 3-30 Hz. Corticosterone (4 x 10(-5) M) did not alter the stimulation-evoked 3H-overflow at 1-30 Hz. Cocaine (3 x 10(-6) M) enhanced the 3H-overflow slightly at 1-30 Hz. At 3 x 10(-5) M, cocaine enhanced 3H-overflow at 1 Hz and reduced it at 30 Hz. Neither corticosterone (4 x 10(-5) M) nor propranolol (10(-7) M) modified this effect of cocaine. Propranolol (10(-7) M) alone decreased the 3H-overflow at 30 Hz and had no effect at 1-10 Hz. Phenoxybenzamine (10(-6) M) and chlorpromazine (3 x 10(-6) M) potentiated the stimulation-evoked 3H-overflow at 1-30 Hz.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparative assay of neuronal uptake and autoinhibitory feedback in guinea-pig and rat atria

1. The roles of neuronal uptake and autoinhibitory feedback were studied in guinea-pig and rat isolated right atria. Tachycardia was used as a measure of noradrenaline concentration at the post-junctional beta-adrenoreceptors in response to electrical field stimulation, 1-16 pulses at 1 Hz, or to exogenous noradrenaline. 2. The EC50 values for noradrenaline concentration-response curves were similar (7.3, -log M) in the two species when neuronal uptake was inhibited by desipramine (DMI, 1 microM). In the absence of DMI, however, the EC50 values were different for the rat (7.0) and guinea-pig (6.3). 3. DMI (0.01-1 microM) caused a substantial increase in half-response time (t 1/2), an integrated measure of tachycardia, in the rat during field stimulation but only caused modest increases in the guinea-pig atria. Following pretreatment with the irreversible alpha 2-adrenoceptor antagonist benextramine, the t 1/2 values were substantially raised in guinea-pig atria with no further change to t 1/2 in the rat atria. 4. The existence of presynaptic inhibitory alpha 2-adrenoceptors in the rat was established using clonidine which caused parallel rightward shifts of field stimulation-tachycardia curves. 5. These results suggest that in the guinea-pig atria blockade of both autoinhibitory feedback and neuronal uptake cause a very large increase in tachycardia compared with blockade of either system alone. In rat atria the most important modulation is from neuronal uptake, which suggests that in this species autoinhibitory feedback is of little consequence.

Involvement of alpha 1-adrenoceptors in chronotropic responses to endogenously released amines in the pithed rat

1. In pithed rats, yohimbine (1 mg/kg i.v.) enhanced the positive chronotropic responses to spinal stimulation of cardiac sympathetic nerves with eight pulses delivered at 2 or 4 Hz, indicating that auto-inhibition was operating, but did not increase responses to shorter lengths of trains of 8 pulses at 8, 16 or 32 Hz which did not allow sufficient time for auto-inhibition to come into effect. 2. The positive chronotropic response to cardiac sympathetic nerve stimulation with eight pulses at 8 Hz of about 60 beats/min was not affected by prazosin (1 mg/kg) or diltiazem (0.2 mg/kg), but was reduced to about 20% of the control value by propranolol (1 mg/kg). 3. In the presence of propranolol, the residual positive chronotropic responses to cardiac sympathetic nerve stimulation were virtually abolished by prazosin (1 mg/kg) or diltiazem (0.2 mg/kg). 4. The positive chronotropic response to tyramine (0.1 mg/kg i.v.) was reduced from 100 to 12 beats/min by propranolol (1 mg/kg), and the residual response was abolished by prazosin. 5. The findings indicate that noradrenaline released from cardiac sympathetic terminals by nerve stimulation or by tyramine acts on alpha 1-adrenoceptors to produce a positive chronotropic response that is revealed when beta-adrenoceptors are blocked.

Evidence against a functional link between noradrenaline uptake mechanisms and presynaptic alpha-2 adrenoceptors

Slices prepared from rat cerebral cortex were labelled with 3H-noradrenaline and superfused. Electrical field stimulation was carried out 15 min (S1) and 45 min (S2) after the start of collection of 5-min samples using 4 pulses delivered at 100 Hz. Drugs acting at alpha 2-adrenoceptors were added 20 min before S2, and their effects were evaluated using the S2/S1-ratio. The alpha 2-adrenoceptor antagonists idazoxan (1 mumol/l) and rauwolscine (1 mumol/l) failed to increase stimulation-evoked overflow of radioactivity in the absence or presence of the noradrenaline reuptake inhibitor desipramine (1 mumol/l). This indicates that the duration of electrical stimulation was too short to allow development of alpha 2-adrenoceptor-mediated autoinhibition by released noradrenaline. The effect of clonidine (3-1000 nmol/l) on stimulation-evoked overflow of radioactivity was tested in the absence and presence of three different reuptake inhibitors (desipramine, 1 mumol/l; maprotiline, 1 mumol/l; cocaine, 10 mumol/l). The analysis yielded identical concentration-response curves under all conditions. These results argue against an action of inhibitors of neuronal reuptake of noradrenaline at the presynaptic alpha 2-adrenoceptor and against the concept of a functional link between uptake site and receptor.

Effects of blockade of alpha 1- and alpha 2-adrenoceptors on vasoconstrictor responses to single and twin pulse stimulation in rat tail artery

Perfused/superfused proximal segments of Sprague-Dawley rat tail artery were stimulated at supramaximal voltage with two 1 ms square wave pulses. The pulse interval was either 10 or 20 s. With either interval the response to each pulse was similar, amounting to about 20 mm Hg increase in perfusion pressure. Prazosin (0.1 nM) approximately halved the response to both pulses whereas idazoxan (30 nM) was without effect. With an interval of 10 s, cocaine (4 microM) greatly increased the response to the first but not to the second pulse; in the presence of cocaine, prazosin (1 nM) again reduced both responses whereas idazoxan (30 nM) reduced the response to the first pulse but increased the response to the second. With an interval of 20 s, cocaine increased the responses to both pulses to a similar degree; in the presence of cocaine, idazoxan reduced the responses to both pulses. The results suggest that in rat tail artery, inhibition of the neuronal uptake process is required if noradrenaline released after stimulation with a single pulse is to reach smooth muscle alpha 2-adrenoceptors, and if feedback inhibition is to persist for more than 10 s.

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.

Noradrenaline and adenosine triphosphate as co-transmitters of neurogenic vasoconstriction in rabbit mesenteric artery

The largest rami caecales of the ileocolic artery, which is a branch of the mesenteric artery, were perfused at a constant rate of flow. Either vasoconstriction (as an increase in perfusion pressure) or the release of previously incorporated [3H]-noradrenaline was measured. Noradrenaline and ATP, but not carbachol, serotonin, adenosine, Arg-vasopressin and neuropeptide Y, caused marked vasoconstriction. When the sympathetic vasoconstrictor axons in the arterial wall were stimulated by electrical field pulses (either 5 pulses at 10 Hz or 100 pulses at 5 Hz; 0.3 ms pulse width, 200 mA current strength), the ensuing vasoconstriction was at best slightly reduced by phentolamine, prazosin and phenoxybenzamine. The response to 100 pulses, 5 Hz was even enhanced by phentolamine and yohimbine. All antagonists except yohimbine blocked the effect of exogenous noradrenaline. Prazosin did not change the effect of exogenous ATP. alpha,beta-Methylene-ATP (3-15 mumol/l) elicited transient vasoconstriction. Subsequently, responses to ATP as well as to electrical stimulation were reduced and recovered slowly. The response to noradrenaline was not changed. That part of the electrically induced vasoconstriction that remained after alpha,beta-methylene-ATP was almost abolished by phentolamine or prazosin. Pre-treatment of the animals with reserpine decreased but did not prevent the electrically evoked contraction of their arteries. The reserpine-resistant response was not changed by prazosin but was abolished by alpha,beta-methylene-ATP. The vasoconstriction elicited by electrical pulses was not affected by atropine or methysergide but was entirely blocked by tetrodotoxin, guanethidine or exposure to 6-hydroxydopamine. In arteries pre-incubated with [3H]-noradrenaline, electrical stimulation (100 pulses at 5 Hz) increased the outflow of tritium. The evoked overflow was blocked by tetrodotoxin, not changed by alpha,beta-methylene-ATP (9 mumol/l) or prazosin, and enhanced by phentolamine, phenoxybenzamine and yohimbine. We conclude that, in the branch of the mesenteric artery examined, both noradrenaline and ATP or a closely related compound transmit information from sympathetic neurones to smooth muscle. An alpha-adrenoceptor antagonist can reduce neurogenic vasoconstriction by blockade of post-junctional alpha-(probably alpha 1) receptors, reserpine by selective depletion of noradrenaline, and alpha,beta-methylene-ATP by desensitization of the post-junctional ATP (probably P2) receptor mechanism. Noradrenaline and ATP appear to be released from the same neurone. In addition, prejunctional alpha 2-adrenergic autoinhibition of transmitter release operates in the artery. alp

Influence of neuronal uptake on the presynaptic alpha-adrenergic modulation of noradrenaline release

Conditions required for the inhibitory feedback modulation of noradrenergic neurotransmission were studied in isolated atria of the rat. The alpha adrenergic antagonist, yohimbine, 0.8 microM, or phentolamine, 1 microM, did not affect the chronotropic response to 4 or 8 shocks at 0.8 Hz but increased it when a higher number of shocks was applied. When neuronal uptake was inhibited by cocaine, 2.9 microM, or desipramine, 0.1 microM, the enhancement of neurotransmission by yohimbine or phentolamine was higher than that observed in the presence of alpha-adrenergic antagonists alone. In atria preincubated with 3H-noradrenaline, the effect of the drugs on the 3H-overflow evoked by 240 shocks at 2.0 Hz was studied. Cocaine 2.9 microM, did not increase the evoked overflow but yohimbine, 0.8 microM, did. The 3H-overflow obtained in the group of yohimbine plus cocaine was significantly higher than was expected from the effects of both drugs alone. It is concluded that yohimbine or phentolamine enhance the chronotropic response in rat atria only when the concentration of noradrenaline in the biophase is sufficiently high to activate presynaptic receptors. In this tissue, the efficiency of the neuronal uptake influences the degree of alpha-adrenergic autoinhibition.

Effects of locally generated angiotensin II on noradrenergic transmission in guinea-pig isolated atria

The extent to which cardiac neuroeffector function may be modulated by angiotensin II (AII) generated locally from angiotensin I (AI) was investigated in guinea-pig spontaneously beating isolated atria radiolabelled with [3H]noradrenaline. AI and AII were equipotent in increasing the rate and force of atrial contractions with a threshold concentration between 0.1 and 1.0 nM. In contrast, AI was approximately twenty times less potent than AII in enhancing the stimulation-induced (S-I) efflux of radioactivity. The actions of AI and AII on rate and force of atrial contractions, as well as those on S-I efflux of radioactivity, were substantially blocked by the AII receptor antagonist saralasin. On the other hand, the converting enzyme inhibitors captopril and MK-422 selectively blocked the actions of AI without affecting those of AII. These results suggest that significant local conversion of AI to AII occurs in guinea-pig isolated atria. This locally generated AII can act on myocardial AII receptors to increase rate and force of atrial contractions, and on AII receptors located on sympathetic nerve terminals to facilitate sympathetic neurotransmission. The lower potency of AI compared to AII in modulating transmitter release, in contrast to the equal potency of the peptides in mediating chronotropic and inotropic responses, may be due to conversion of AI to AII at sites remote from the neuroeffector junction.

Transmitter release modulated by alpha-adrenoceptor antagonists in the rabbit mesenteric artery: a comparison between noradrenaline outflow and electrical activity

Effects of alpha-adrenoceptor blockers (prazosin, yohimbine, phentolamine and phenoxybenzamine) on the outflow of noradrenaline (NA) and 3,4-dihydroxyphenylglycol (DOPEG) during perivascular nerve stimulation were observed in relation to electrical events in the rabbit mesenteric artery. Cocaine or imipramine increased the NA outflow and reduced the DOPEG outflow induced by nerve stimulation. In the absence of stimulation, cocaine and imipramine did not significantly modify the NA and DOPEG outflows. The alpha-adrenoceptor blockers we used enhanced the NA and DOPEG outflow during nerve stimulation, in a dose-dependent manner; the potency of the enhancement was higher for phentolamine and phenoxybenzamine than for prazosin and yohimbine. Higher concentrations (10(-5) M) of yohimbine reduced the NA and DOPEG outflows induced by nerve stimulation. Prazosin increased the DOPEG outflow in the absence of stimulation, and this effect was not inhibited by pretreatment with cocaine. Guanethidine increased the NA and DOPEG outflow in the absence of stimulation, and the NA outflow was reduced during nerve stimulation. These effects of guanethidine were prevented by pretreatment with cocaine or imipramine. Perivascular nerve stimulation evoked excitatory junction potentials (e.j.ps) and with high frequency stimulation, slow depolarization and spike potentials. Application of phentolamine, phenoxybenzamine or yohimbine enhanced, and of prazosin had no effect, on the amplitude of the e.j.p. Spike potentials were not affected by these alpha-adrenoceptor blockers. Slow depolarization ceased in the presence of prazosin, phentolamine or phenoxybenzamine, and was slightly inhibited by yohimbine. Guanethidine blocked all of these electrical responses induced by perivascular nerve stimulation. Application of prazosin, phentolamine or phenoxybenzamine did not alter the resting membrane potential of the smooth muscle cells. Depolarizations of smooth muscle membrane produced by exogenously applied NA were inhibited by prazosin, phentolamine or phenoxybenzamine. Yohimbine itself depolarized the membrane and the inhibitory effects on the NA-induced depolarization were weaker. We conclude that the smooth muscle membrane of the rabbit mesenteric artery possesses alpha 1-adrenoceptors. Increase in NA outflow by alpha-adrenoceptor antagonists during nerve stimulation was not always consistent with increase in e.j.p. amplitude, presumably due to involvement of actions other than alpha-adrenoceptor blockade with each of these antagonists.

Differences between the prejunctional effects of phenylephrine and clonidine in guinea-pig isolated atria

The prejunctional effects of clonidine and phenylephrine were studied in guinea-pig isolated atria by means of field stimulation of the sympathetic nerve terminals during the cardiac refractory period, in the presence of 1 microM atropine. Clonidine (10-100 nM) produced a dose-dependent decrease in the stimulus-inotropic response curve; the IC50 for clonidine was increased about 70 times by the pretreatment of the preparations with 1 microM yohimbine. The effect of clonidine was not modified by 0.5 microM prazosin. Unlike clonidine, phenylephrine (1-10 microM) induced a statistically insignificant increase in the contractile force of preparations stimulated at 4 Hz. The inhibitory effect of phenylephrine (1-10 microM) was partially prevented by either 1 microM yohimbine or 0.5 microM prazosin. However, it was antagonized, to about the same degree as that observed with clonidine, by the pretreatment of the preparations with both 1 microM yohimbine and 0.5 microM prazosin. The results seem to indicate that one component of the prejunctional effects of phenylephrine may be mediated by presynaptic alpha-adrenoceptors belonging to the alpha 1-subtype.

Further study of prerequisites for the enhancement by alpha-adrenoceptor antagonists of the release of noradrenaline

Segments of the rabbit ear artery were preincubated with (-)-3H-noradrenaline and then perfused/superfused and stimulated by transmural electrical pulses. The outflow of 3H-noradrenaline and total tritium was determined. In the first series of experiments, stimulation periods of approximately constant length (50 s) were used (cocaine 5 microM present). Thirteen pulses (0.25 Hz) elicited an overflow of 3H-noradrenaline of 0.024% of tissue tritium; 26 pulses (0.5 Hz) elicited an overflow of 0.059%, and 52 pulses (1 Hz) of 0.166%. Rauwolscine 1 microM did not change the overflow evoked by 13 pulses, increased that evoked by 26 pulses and increased most markedly that evoked by 52 pulses. Phentolamine 1 microM decreased the overflow at 13, did not change the overflow at 26, and increased the overflow at 52 pulses. Corynanthine 1 microM decreased the overflow at 13, and did not change the overflow at 26 and 52 pulses. The effect of tetraethylammonium (TEA) 100 microM was opposite to that of rauwolscine; it increased most markedly the overflow evoked by 13 pulses, increased less that evoked by 26 pulses, and least the overflow at 52 pulses. In the second series of experiments, the frequency of stimulation was kept constant (2 Hz). In the absence of cocaine, 10 pulses elicited an overflow of 3H-noradrenaline of 0.023% of tissue tritium; 20 pulses elicited an overflow of 0.043%, and 40 pulses of 0.089%. Phentolamine 1 microM did not change the overflow evoked by 10 pulses, increased that evoked by 20 pulses, and increased most markedly that evoked by 40 pulses.(ABSTRACT TRUNCATED AT 250 WORDS)

Autoinhibition of noradrenaline release from the rat heart as a function of the biophase concentration. Effects of exogenous alpha-adrenoceptor agonists, cocaine, and perfusion rate

Rat isolated perfused hearts with the right sympathetic nerves intact were loaded with 3H-(-)-noradrenaline. The nerves were stimulated with trains of 180 pulses at 3 Hz and at 10 min intervals. The overflow of 3H-noradrenaline and 3H-metabolites was determined by liquid scintillation spectrometry. Clonidine (IC50 17 nM), oxymetazoline (IC50 63 nM), and alpha-methylnoradrenaline (apparent IC50 35 nM, determined in the presence of cocaine and propranolol) decreased the stimulation-evoked overflow of 3H-noradrenaline by 26, 49, and 78%, respectively, but not methoxamine up to 100 microM (propranolol present). Oxymetazoline and alpha-methyl-noradrenaline did not cause desensitization of the presynaptic adrenoceptors when present at their IC80 for 33 min. At a perfusion rate of 7 ml/min, yohimbine 1 microM enhanced the stimulation-evoked 3H-noradrenaline overflow by 26% in the absence, and by 58% in the presence of cocaine. Phentolamine 1 microM increased it by 69% when the neuronal reuptake was blocked. The increase by the antagonists faded with successive period of nerve stimulations, and was positively correlated with the biophase concentration of noradrenaline as reflected by the amount of 3H-noradrenaline released into the perfusate per nerve stimulation. At a perfusion rate of 1.8 ml/min (neuronal reuptake blocked), yohimbine 1 microM increased the overflow by 127%. The results indicate that the alpha 2-adrenoceptor-mediated autoinhibition in the rat perfused heart depends on the clearance of transmitter from the biophase via neuronal reuptake and diffusion into the vascular space. Reduction of either elimination pathway enhances the biophase concentration of noradrenaline, thus increasing the autoinhibition of release.

Role of auto-inhibitory feed-back in cardiac sympathetic transmission assessed by simultaneous measurements of changes in 3H-efflux and atrial rate in guinea-pig atrium

Guinea-pig right atria were labelled with [3H]-noradrenaline or [3H]-dopamine before superfusion in a flow-cell. Choice of label did not significantly alter either the relationship between 3H-efflux and number of electrical field pulses or the inhomogeneity of labelling. The relationship between 3H-efflux and frequency of 4 field pulses (0.125-2 Hz) was hyperbolic and similar to the tachycardia-frequency relationship measured simultaneously. No evidence was found for a U shaped 3H-efflux-frequency relationship (Story, McCulloch, Rand & Standford-Starr, 1981). Phentolamine (1 microM) did not alter the 3H-efflux or atrial rate responses to 4 field pulses at stimulus levels that gave 50-60% of the maximum rate response. In the presence of neuronal uptake inhibition (desipramine, DMI 0.1 microM), rate and 3H-efflux responses to 4 field pulses were enhanced at all frequencies and were further increased by phentolamine. In the absence of DMI, prolonged trains of field pulses (8 and 12 pulses) at low frequency (0.25 Hz) were not sufficient to activate auto-inhibitory feed-back. At 2 Hz phentolamine enhanced both 3H-efflux and rate responses at 12 field pulses. We conclude that in guinea-pig right atrium auto-inhibitory feed-back plays little role in the modulation of transmitter release at levels of stimulation that cause 50-60% of maximum tissue response. This is because neuronal uptake normally prevents synaptic concentrations of noradrenaline from activating prejunctional alpha 2-adrenoceptors. Stimulation sufficient to induce a near-maximal response or the presence of neuronal uptake inhibition are necessary to evoke autoinhibitory feed-back.