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

Dissociation of inhibitory effects of guanethidine on adrenergic and on purinergic transmission in isolated canine splenic artery

The aim of this study was both to investigate the effects of progressive inhibition of adrenergic neurons by increasing concentrations of guanethidine (0.1-10 microM) on the double-peaked vasoconstrictor responses to electrical periarterial nerve stimulation in the isolated and perfused canine splenic artery, and to clarify whether release of noradrenaline is presynaptically separate from release of adenosine 5'-triphosphate (ATP). Double-peaked vasoconstrictions (biphases of vasoconstrictions) were consistently observed under the conditions of 30-s trains of pulses at 1-10 Hz frequencies. Guanethidine, at a lower concentration (0.1 microM) did not modify the first (1st) phase vasoconstriction at low frequencies (1-2 Hz), but markedly inhibited the second (2nd) responses. On the other hand, it slightly but significantly inhibited the double-peaked vasoconstrictor responses at high frequencies (6-10 Hz). Furthermore, a 10-fold increase of concentration of guanethidine (1 microM) almost completely inhibited the 2nd phase responses at any frequencies used but did not completely inhibit the 1st phase response. A further increased concentration of guanethidine (10 microM) failed to enhance the 1 microM guanethidine-induced inhibition. The 1 microM guanethidine-resistant 1st phase responses at any frequencies used (1-10 Hz) were sensitive to tetrodotoxin (30 nM). Treatment with 0.1 microM prazosin did not modify the 1st phase response at any frequencies used in the 1 microM guanethidine-treated preparation. The responses remaining after 1 microM guanethidine and 0.1 microM prazosin were completely suppressed by a subsequent application of 1 microM alpha,beta-methylene ATP at any frequencies used. The results indicated that guanethidine, an adrenergic neuron blocker, may exert a dominant inhibitory effect on adrenergic rather than on purinergic components of sympathetic nerve co-transmission, indicating that guanethidine-sensitive mechanisms may mainly contribute to determine noradrenaline secretion from neurosecretory vesicles rather than ATP secretion.

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.

Presynaptic alpha 2-autoreceptors in brain cortex: alpha 2D in the rat and alpha 2A in the rabbit

Presynaptic alpha 2-autoreceptors in rat and rabbit brain cortex were compared by means of antagonists and agonists. Brain cortex slices were preincubated with [3H]-noradrenaline and then superfused and stimulated by 3 (rat) or 4 (rabbit) pulses at a frequency of 100 Hz. The alpha 2-adrenoceptor agonist bromoxidine (UK 14304) reduced the electrically evoked overflow of tritium with EC50 values of 4.5 nmol/l in the rat and 0.7 nmol/l in the rabbit. The antagonists phentolamine, 2-[2H-(1-methyl-1,3-dihydroisoindole)methyl]-4,5-dihydroimidazo le (BRL 44408), rauwolscine, 1,2-dimethyl-2,3,9,13b-tetrahydro-1H-dibenzo(c,f)imidazo(1,5-a)aze pine (BRL 41992), 2-(2,6-dimethoxyphenoxyethyl)aminomethyl-1,4-benzodioxane (WB 4101), 6-chloro-9-[(3-methyl-2-butenyl)oxy]-3-methyl-1H-2,3,4, 5-tetrahydro-3-benzazepine (SKF 104078), imiloxan, prazosin and corynanthine did not per se increase the evoked overflow of tritium but shifted the concentration-inhibition curve of bromoxidine to the right in a manner compatible with competitive antagonism. Up to 4 concentrations of each antagonist were used to determine its dissociation constant KD. The KD values correlated only weakly between the rat and the rabbit. Dissociation constants KA of bromoxidine were calculated from equieffective concentrations in unpretreated brain slices and slices in which part of the alpha 2-adrenoceptors had been irreversibly blocked by phenoxybenzamine. The KA value was 123 nmol/l in the rat and 7.2 nmol/l in the rabbit. The results confirm the species difference between rat and rabbit brain presynaptic alpha 2-autoreceptors. Comparison with data from the literature indicates that the rat brain autoreceptors can be equated with the alpha 2D subtype as defined by radioligand binding, whereas the rabbit brain autoreceptors conform to the alpha 2A subtype. For example, the antagonist affinities for the rat autoreceptors correlate with their binding affinities for the gene product of alpha 2-RG20, the putative rat alpha 2D-adrenoceptor gene (r = 0.97; P < 0.01), but not with their binding affinities for the gene product of alpha 2-C10, the putative human alpha 2A-adrenoceptor gene. Conversely, the rabbit autoreceptors correlate with the alpha 2-C10 (r = 0.98; P < 0.001) but not with the alpha 2-RG20 gene product. Since presynaptic alpha 2-autoreceptors are also alpha 2D in rat submaxillary gland and perhaps vas deferens and alpha 2A in rabbit pulmonary artery, the possibility arises that the majority of alpha 2-autoreceptors generally are alpha 2D in the rat and alpha 2A in the rabbit. Moreover, receptors of the alpha 2A/D group generally may be the main mammalian alpha 2-autoreceptors.

Accumulation and release of adrenaline, and the modulation by adrenaline of noradrenaline release from rabbit blood vessels in vitro

The accumulation of (-)-3H-adrenaline (3H-A) by rabbit isolated aorta was studied. In all experiments, monoamine oxidase and catechol-O-methyltransferase were inhibited by treatment with pargyline and 3',4'-dihydroxy-2-methyl-propiophenone, respectively. The relationship between the accumulation of 3H derived from 3H-A and the duration of incubation was linear. The 3H-accumulation after 3 h incubation was 22.5 ml/g. In reserpine-treated tissue, the 3H-accumulation levelled off after 30 min and was 8.5 ml/g after 3 h. The concentration of 3H-A or (-)-3H-noradrenaline (3H-NA) and the 3H-accumulation (ml/g) were inversely related. At 10(-8) M, the 1-hour accumulation of 3H derived from 3H-A and 3H-NA was 7.8 and 15.2 ml/g, respectively. With increasing concentrations the accumulation values approached each other. The accumulation of 3H derived from 3H-A by reserpine-treated tissue also showed an inverse relationship with concentration. The accumulation of 3H derived from 3H-A was dependent on the bath temperature. Storage of tissue (0-5 days in salt solution without equilibration with 95% O2/5% CO2; 4 degrees C) did not affect the accumulation of 3H derived from 3H-A. Thereafter (7-14 days), the accumulation decreased. The inhibitory potency (IC50; -log M) of desipramine, cocaine, propranolol, isoprenaline, and normetanephrine on accumulation of 3H derived from 3H-A was found to be 8.26; 6.50; 5.48; 4.88, and 4.02, respectively. The maximal degree of inhibition was almost the same for these drugs, while that of clonidine and corticosterone was 50 and 20%, respectively. In the presence of desipramine, either clonidine, corticosterone or isoprenaline reduces the accumulation of 3H derived from 3H-A. Ouabain and iodoacetic acid, but not sodium cyanide and 2,4-dinitrophenol, reduced the accumulation of 3H derived from 3H-A. Anoxia (95% N2/5% CO2; 37 degrees C; 1-24 h) did not alter the accumulation of 3H derived from 3H-A. Glucose deprivation alone or combined with anoxia markedly reduced the 3H-accumulation. The release of 3H-A from rabbit isolated aorta was studied. This release was compared with that of 3H-NA. The stimulation-evoked 3H-overflow from aorta preloaded with 3H-A decreased with repeated stimulation. In contrast, prestimulation enhanced subsequent stimulation-evoked 3H-overflows. For both 3H-amines, the 3H-overflow increased concomitantly to the same degree with the number of pulses. The time course of 3H-overflows with either 3H-A or 3H-NA was compared.(ABSTRACT TRUNCATED AT 400 WORDS)

Release of 3H-noradrenaline from rabbit isolated ear artery

The stimulation-evoked 3H-overflow from rabbit isolated ear arteries preincubated with 3H-noradrenaline was studied. Three strips were derived from each central artery. The strips were incubated (30 min.) with 3H-noradrenaline (10(-7) M) and the spontaneous 3H-outflow and stimulation-evoked 3H-overflow were followed by fractional collection. After a wash-out period (75 min.), the strips were stimulated (225 mA; 150 monophasic pulses; 3 Hz; 0.5 msec.) several times. The initial stimulation-evoked 3H-overflow (S1) was about 5-fold higher than the subsequent five 3H-overflows (S2-S6) which remained almost constant. Bretylium (10(-5) M), tetrodotoxin (10(-6) M), and omission of Ca2+ from the physiological salt solution reduced the stimulation-evoked 3H-overflow by maximally 52%, 77% and 62%, respectively. An increase in stimulation current from 50 to 225 mA caused a continuous rise in stimulation-evoked 3H-overflow, which tended to be Ca2(+)-sensitive. The stimulation-evoked 3H-overflow was frequency-dependent: at 1-4 Hz, the 3H-overflows were the same; at 8 and 16 Hz, they increased. Cocaine (3 x 10(-5) M) plus corticosterone (4 x 10(-5) M) enhanced the stimulation-evoked 3H-overflow at 1-8 Hz, while it had no effect at 16 Hz. Propranolol (3 x 10(-7) M) did not antagonize this enhancement. An increase in number of pulses from 10 to 1000 in the stimulus caused a corresponding rise in the evoked 3H-overflow. This was also the case when cocaine plus corticosterone were present.(ABSTRACT TRUNCATED AT 250 WORDS)

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 dopamine DA2-receptors in rabbit jejunal arteries. An electrophysiological study

Excitatory junction potentials (e.j.ps) evoked by nerve stimulation with 15 pulses at 1 Hz were recorded from muscle cells of rabbit isolated jejunal arteries. LY 171555 1 mumol/l, SKF 38393 10 mumol/l, dopamine 10 mumol/l and clonidine 0.1 mumol/l depressed all e.j.ps in the train. The percentage inhibition was inversely related to the number of pulses. S- and R-sulpiride, 10 mumol/l, domperidone 1 mumol/l, SCH 23390 1 mumol/l and rauwolscine 1 mumol/l did not change, or even depressed the first e.j.ps. Of these compounds only S- and R-sulpiride, 10 mumol/l and rauwolscine 1 mumol/l facilitated the late e.j.ps. The percentage facilitation increased with the number of pulses until a maximum was reached; rauwolscine 1 mumol/l had the largest effect. S- and R-sulpiride, 10 mumol/l, as well as domperidone 1 mumol/l antagonized the action of LY 171555 1 mumol/l. S-Sulpiride was more potent than its R-isomer. SCH 23390 1 mumol/l and rauwolscine 1 mumol/l blunted the effect of SKF 38393 10 mumol/l. Rauwolscine 1 mumol/l slightly reduced the inhibition by dopamine 10 mumol/l; S-sulpiride 10 mumol/l was antagonistic only in the presence of rauwolscine 1 mumol/l. When rauwolscine 1 mumol/l, prazosin 0.1 mumol/l, propranolol 1 mumol/l and cocaine 10 mumol/l was added to the medium, dopamine 10 mumol/l continued to produce the same depression of e.j.ps, as in the absence of these compounds. Under such conditions S-sulpiride 10 mumol/l also counteracted dopamine 10 mumol/l. Rauwolscine 1 mumol/l prevented the effect of clonidine 0.1 mumol/l. The antagonists were not absolutely selective against only one type of agonist. We suggest that both presynaptic DA2- and postsynaptic DA1-receptors are present in rabbit jejunal arteries. The activation of either receptor-type may depress the e.j.ps. Dopamine interferes with neuroeffector transmission due to alpha 2-adrenoceptor agonist properties; its DA2-effect is unmasked only after alpha 2-adrenoceptor blockade. There was no evidence for a co-transmitter function of dopamine.

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)

Inhibitory effects of noradrenaline and dopamine on calcium influx and neurotransmitter glutamate release in mammalian brain slices

Noradrenaline and dopamine (0.1-100 microM) inhibited 45Ca2+ uptake and glutamate release induced by veratrine (25 microM) in cortical and striatal slices but were without effect when added alone. Each parameter was inhibited in a dose-dependent manner by noradrenaline in cortical slices (IC50 = 0.05 microM) and by dopamine in striatal slices (IC50 = 0.08 microM). Noradrenaline (0.01-100 microM) was without influence on veratrine-induced 45Ca2+ influx or glutamate release in the striatal preparation, and likewise dopamine was inactive in cortex slices. The use of adrenoceptor antagonists suggests that the action of noradrenaline is mediated by the alpha 2-receptor which is thought to be adenylate cyclase linked. Dopamine appeared to be acting through the D-2 receptor.

Electrophysiological evidence for an alpha 2-adrenergic inhibitory control of transmitter release in the rabbit mesenteric artery

Excitatory junction potentials (e.j.p.s) evoked by nerve stimulation with 15 pulses at 1 Hz were recorded from muscle cells of the rabbit isolated mesenteric artery. Clonidine and B-HT 933 depressed all e.j.p.s in the train. The percentage inhibition was inversely related to the number of pulses. Yohimbine, rauwolscine and tolazoline reduced the early e.j.p. amplitudes but enhanced the later ones. The percentage facilitation of e.j.p.s increased with the number of pulses until a maximum was reached. Prazosin and corynanthine did not influence the first few e.j.p.s but potentiated the subsequent ones; their effects were less pronounced than those of yohimbine and rauwolscine. All the drugs antagonized the inhibition by clonidine but the effects of yohimbine and rauwolscine were more marked than those of prazosin and corynanthine. Phenylephrine, St 587 and noradrenaline depressed the e.j.p.s. Yohimbine diminished the effects of these substances and was a stronger antagonist of phenylephrine than prazosin. We suggest that, in the rabbit mesenteric artery, noradrenaline and the neuroeffector transmitter (probably ATP) are co-released from the terminals of postganglionic sympathetic nerves. Noradrenaline activates presynaptic alpha 2-adrenoceptors and thereby depresses transmitter release. The degree of presynaptic inhibition depends on the number of pulses applied, i.e. on the biophase concentration of noradrenaline.

Prejunctional alpha 2-adrenoceptors modulate stimulation-evoked norepinephrine release in rabbit lateral saphenous vein

Segments of rabbit lateral saphenous vein prelabelled with [3H]noradrenaline were perfused and superfused with physiological salt solution. Tritium overflow evoked by transmural nerve stimulation (3 Hz for 2 min) was abolished by tetrodotoxin (1 microM). The selective alpha 2-adrenoceptor agonist UK 14,304 inhibited stimulation-evoked 3H-overflow in a concentration-dependent manner, with an IC50 of 71 nM. In contrast, the alpha 2-adrenoceptor agonist B-HT 933 had no effect on 3H-overflow in concentrations up to 10 microM. The selective alpha 2-adrenoceptor antagonists idazoxan and SKF 86466, as well as the non-selective alpha-antagonist phentolamine, facilitated the nerve stimulation evoked 3H-overflow, with an order of potency of idazoxan greater than or equal to phentolamine greater than SK&F 86466. Prazosin (100 nM) had little effect on 3H-overflow. These findings suggest that stimulation-evoked neurotransmitter release is modulated via prejunctional alpha 2-adrenoceptors.

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.

Blockade of alpha 2-adrenoceptors permits the operation of otherwise silent opioid kappa-receptors at the sympathetic axons of rabbit jejunal arteries

We sought for presynaptic, release-inhibiting opioid receptors at the postganglionic sympathetic axons innervating the jejunal arteries of rabbits. Evoked excitatory junction potentials (e.j.p.s; trains of 15 pulses at 1 Hz) as well as evoked overflow of tritium after preincubation with [3H]-noradrenaline (trains of 120 pulses at 1 Hz) were used to estimate transmitter release. In otherwise untreated tissues ethylketocyclazocine reduced neither the e.j.p. amplitudes nor the evoked overflow of tritium; [Met5]-enkephalin depressed the evoked overflow of tritium. Ethylketocyclazocine reduced e.j.p. amplitudes, however, in tissues exposed to either yohimbine, tolazoline or phentolamine, but not in tissues exposed to prazosin. Ethylketocyclazocine also depressed the evoked overflow of tritium when yohimbine was present. The inhibition produced by ethylketocyclazocine in the presence of yohimbine was antagonized by (-)-3-furylmethyl)-alpha-noretazocine (MR 2266) but not by N,N-diallyl-Tyr-alpha-aminoisobutyric acid-alpha-aminoisobutyric acid-Phe-Leu-OH (ICI 174864). It is concluded that the sympathetic neurones of rabbit jejunal arteries possess presynaptic kappa-receptors in addition to the previously identified delta-receptors. The kappa-receptors become operative only when presynaptic alpha 2-adrenoceptors have been blocked.

Possible involvement of presynaptic alpha 1-adrenoceptors in the effects of idazoxan and prazosin on 3H-noradrenaline release from tail arteries of SHR

The effects of several alpha-adrenoceptor antagonists have been examined on tritium release elicited by electrical stimulation from isolated perfused SHR tail artery preparations prelabelled with 3H-noradrenaline (3H-NA). Phentolamine and yohimbine potently facilitated the stimulation evoked release of tritium at low frequencies of stimulation, but the alpha 2-adrenoceptor antagonist idazoxan was only weakly active at 1 mumol/l, despite antagonising the clonidine-evoked inhibition of 3H-release at a lower concentration of 0.1 mumol/l. The alpha 1-adrenoceptor antagonists prazosin and corynanthine also increased stimulation evoked tritium release in this preparation, suggesting the presence of prejunctional alpha 1-adrenoceptors. Furthermore, the alpha 1-adrenoceptor agonist methoxamine (3 mumol/l) caused a significant inhibition of tritium-evoked release, an effect which was blocked by prazosin (10 nmol/l). When alpha 1-adrenoceptors were blocked in the presence of prazosin, idazoxan (0.1 mumol/l) produced a significant facilitatory effect on the electrically-evoked release of 3H-transmitter. On the other hand, when alpha 2-adrenoceptors were blocked in the presence of yohimbine, exposure to idazoxan (0.1 mumol/l) reduced significantly the stimulation-evoked release of tritium elicited by electrical stimulation. The results indicate that in the SHR tail arteries, idazoxan has a partial agonist inhibitory activity on transmitter release, which can mask the facilitatory effects due to blockade of presynaptic alpha 2-adrenoceptors. The inhibitory effects of idazoxan appear to involve presynaptic alpha 1-adrenoceptors, which when stimulated, reduce 3H-NA release in SHR tail arteries.

Modulation of [3H]-dopamine released by different frequencies of stimulation from rabbit retina

Rabbit retinal pieces were incubated with [3H]-dopamine and superfused with Krebs solution. Calcium-dependent tritium release was elicited twice within each experiment by electrical field stimulation (360 pulses, 2 ms, 20 mA) at frequencies of 1 Hz, 3 Hz, and 6 Hz. The evoked release of [3H]-dopamine for the first period of stimulation (S1) was 2.09 +/- 0.23% (n = 5) at 1 Hz and was of similar magnitude at all other frequencies of stimulation employed. The D2-dopamine receptor agonist, LY 171555 (quinpirole HCl; 0.01-1 microM) added to the superfusion medium before the second period of stimulation, inhibited the calcium-dependent release of [3H]-dopamine in a concentration-dependent manner, and was more potent the lower the stimulation frequency. The isomer of quinpirole, LY 181990 (0.01-1 microM) did not inhibit the stimulation-evoked overflow of tritium, regardless of concentration or stimulation frequency. The stereoisomers of the D2-dopamine receptor antagonist sulpiride (0.01-3 microM) increased the calcium-dependent release of [3H]-dopamine in a concentration-dependent manner, being more potent the higher the frequency of stimulation. S-sulpiride was more potent than R-sulpiride at all stimulation frequencies. The inhibitory effect of quinpirole was stereoselectively antagonized by sulpiride, but not by LY 181990. The alpha-adrenoceptor antagonist phentolamine (1 microM) did not modify the quinpirole-induced inhibition of [3H]-dopamine release. When the synaptic concentration of dopamine was increased by the presence of the dopamine uptake inhibitor nomifensine (3 microM), the potency of the agonist quinpirole in inhibiting the release of [3H]-dopamine elicited by field stimulation at 1 Hz (180 pulses) was decreased. In contrast, the potency of S-sulpiride in enhancing the evoked release of [3H]-dopamine was increased when nomifensine was present in the superfusion medium. Picomolar concentrations of the hormone melatonin (0.1-10 nM) inhibited the calcium-dependent release of [3H]-dopamine from rabbit retina with the same potency regardless of the frequency of stimulation applied (1 Hz, 3 Hz or 6 Hz). The potency of D2-dopamine receptor agonists and antagonists in modifying [3H]-dopamine release from rabbit retina appears to depend on the synaptic concentration of dopamine which is altered by the frequency of stimulation, and by the dopamine uptake inhibitor nomifensine.(ABSTRACT TRUNCATED AT 400 WORDS)

Is there feedback regulation of neurotransmitter release by autoreceptors?

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

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