Feedback Regulation Of Transmitter Release Research Articles

β‐adrenoceptor‐mediated facilitation of [3H]‐noradrenaline release from the intramural nerves of bovine mesenteric lymphatic vessels

1. Isolated segments of bovine mesenteric lymphatic vessels were loaded with [3H]-noradrenaline and its efflux in response to field stimulation at 2 Hz (0.3 ms pulses, 1 min train) examined. 2. Isoprenaline (10(-6) M) increased evoked fractional 3H efflux (3H released as a percentage of total tissue 3H at the onset of stimulation) from its control value of 0.67 +/- 0.09 to 1.2 +/- 0.18% (s.e.mean; n = 5). 3. Propranolol (10(-6) M) alone had no effect on evoked fractional 3H efflux but blocked the increase in transmitter overflow induced by isoprenaline (10(-6) M). 4. In the presence of the alpha-adrenoceptor antagonist phentolamine (10(-6) M) the facilitatory effect of isoprenaline (10(-6) M) on transmitter efflux was enhanced. In 5 experiments isoprenaline increased evoked fractional 3H efflux from its control value of 1.07 +/- 0.17 to 2.5 +/- 0.37% when phentolamine was present. 5. Adrenaline (10(-8) M) increased evoked fractional 3H efflux from its control value of 0.76 +/- 0.09 to 1.13 +/- 0.17% (n = 7). 6. These findings are consistent with the presence of presynaptic beta-adrenoceptors in this preparation. There is no evidence that these receptors play any role in feedback regulation of transmitter release.

A study of the actions of P1‐purinoceptor agonists and antagonists in the mouse vas deferens in vitro

1. We have examined the effects of purinoceptor agonists and antagonists on the mechanical 'twitch' response, excitatory junction potential (e.j.p.) amplitude and [3H]-noradrenaline overflow in the mouse vas deferens. 2. The agonist profile for inhibition of the mechanical response was N6-([R]-2-phenylisopropyl)adenosine (L-PIA) congruent to N6-cyclohexyladenosine (CHA) greater than 5' N-ethylcarboxamido-adenosine (NECA) greater than 2-chloroadenosine (2ClA) congruent N6-([S]-2-phenylisopropyl)adenosine (D-PIA). 3. The P1-purinoceptor agonists inhibited the e.j.p. with an agonist profile of CHA greater than L-PIA congruent to NECA greater than 2ClA. 4. 2ClA inhibited [3H]-noradrenaline overflow with an EC50 of 1.2 microM which was not significantly different from the values for inhibition of the e.j.p. and the mechanical response. 5. The inhibitory action of 2ClA on the mechanical response was antagonized by 5 microM 8-phenyltheophylline (8-PT). However, neither blockade of P1-purinoceptors by 8-PT nor increasing the rate of degradation of endogenous adenosine by addition of adenosine deaminase had any effect on the mechanical response per se. 8-PT (5 microM) also failed to alter the e.j.p. amplitude or [3H]-noradrenaline overflow. 6. These results indicate that there are P1-purinoceptors present on sympathetic nerve terminals of the mouse vas deferens which are more like A1- than A2-receptors, but may be better classified as being of the A3-subtype (Ribeiro & Sebastiao, 1986). These receptors are not normally involved in the feedback regulation of transmitter release in this tissue.

Cardiac α-adrenoceptors: postjunctional and prejunctional

Postjunctional alpha-adrenoceptors subserve positive inotropic and chronotropic responses. When beta-adrenoceptors are blocked, agonists that act on alpha 1-adrenoceptors evoke positive chronotropic responses in the pithed rat and rat isolated atria. The rank order of potency for this effect is adrenaline greater than noradrenaline greater than phenylephrine greater than methoxamine. The order of potency for antagonists to block the responses is prazosin greater than phentolamine greater than yohimbine. Thus, the postjunctional alpha-adrenoceptors are of the alpha 1-subtype. The positive chronotropic responses elicited by activating alpha 1-adrenoceptors have a slower time course than those elicited by activation of beta-adrenoceptors. When beta-adrenoceptors are blocked by propranolol, the positive chronotropic response to phenylephrine is enhanced by increasing the calcium concentration or by the calcium channel activator Bay K 8644 (0.1 microM), whereas the response is decreased by lowering the calcium concentration or by calcium antagonists (verapamil, nifedipine, nicardipine and diltiazem). Therefore, the positive chronotropic response to alpha 1-adrenoceptor activation involves an increased influx of calcium through calcium channels. Prejunctional alpha 1-adrenoceptors are involved in autoinhibitory feedback regulation of transmitter release from noradrenergic neurones. In rat atria, the release of noradrenaline induced by sympathetic nerve stimulation is inhibited by both clonidine and methoxamine, and is enhanced (by disruption of noradrenaline-mediated autoinhibition) by both idazoxan and prazosin. Thus, the prejunctional alpha-adrenoceptors are of both alpha 1- and alpha 2-subtypes. Drugs which produce blockade of postjunctional alpha 1-adrenoceptors could also produce an increase in neurogenic release of noradrenaline due to blockade of prejunctional alpha 1-adrenoceptors, and this might result in more complex effects than would be anticipated.

Evidence for the involvement of alpha 1-adrenoceptors in negative feedback regulation of noradrenergic transmitter release in rat atria.

Experiments were undertaken to determine if prejunctional alpha 1-adrenoceptors are involved in autoinhibitory regulation of transmitter noradrenaline release in rat atria. The noradrenergic transmitter stores in rat isolated atria were radiolabelled with [3H]noradrenaline and transmitter release was deduced from the efflux of the radiolabel evoked by field stimulation of the atrial intramural sympathetic nerves. Phentolamine (3 mumol/l) and prazosin (0.1 and 3 mumol/l) enhanced the release of radiolabel evoked by stimulation with trains of 4, 8 and 16 pulses, whereas idazoxan (10 mumol/l) enhanced release evoked by stimulation with 8 and 16 pulses. Idazoxan and prazosin were about equi-effective in enhancing the evoked release but phentolamine produced much greater enhancement than either idazoxan or prazosin. Combination of idazoxan (10 mumol/l) and prazosin enhanced the stimulation-evoked release with 4, 8 and 16 pulses to the same extent as phentolamine (3 mumol/l). The selective alpha 1-adrenoceptor agonist methoxamine (10 mumol/l) inhibited stimulation-evoked release and this effect was blocked by the alpha 1-adrenoceptor antagonist prazosin (0.1 mumol/l). The findings indicate that alpha 1-adrenoceptors may contribute to autoinhibitory feedback regulation of transmitter release in rat atria.

A hypothesis to explain the presynaptic effects of adrenoceptor antagonists.

The hypothesis of negative feedback regulation of transmitter release was examined in a range of tissues obtained from three species. Tissues were transmurally stimulated with 100 pulses at 2 Hz with pulse durations from 50 microseconds to 5,000 microseconds, and the efflux of [3H]-noradrenaline determined. The stimulation-induced efflux of tritium increased with increasing pulse duration, but yohimbine, a prototypal alpha 2-antagonist had an effect which was consistently contrary to expectations for a negative feedback system. Enhancement of efflux by the antagonist, supposedly correlated directly with the extent of ongoing auto-inhibition, became smaller rather than larger as the stimulation-induced efflux rose with increases in pulse duration, with all other parameters of stimulation maintained constant. Similar findings were obtained in rat spleen with the haloalkylamine antagonist, phenoxybenzamine. It is concluded that the presynaptic effects of adrenoceptor antagonists do not involve a negative feedback function nor do they relate, in any detectable way, to the extracellular concentration of transmitter. The effects on stimulation-induced tritium efflux of yohimbine, phenoxybenzamine and enlargment of the pulse duration, in a variety of tissues, support the previously described hypothesis of a common action to enhance efflux. The antagonists increased efflux to approximately the same value between 50 and 1,000 microseconds pulse durations and that value was equivalent to that obtained in each given tissue with pulses of 1,000-2,000 microseconds in the absence of the antagonist. Tetraethylammonium, an inhibitor of stimulation-induced potassium efflux from nerves had an effect on transmitter efflux in rat spleen essentially like that of the adrenoceptor antagonists. These findings provide further support for an alternative to the hypothesis of negative feedback. Yohimbine and other presynaptic antagonists may prolong the period of potassium efflux from nerve varicosities, and by this means prolong depolarization and the associated period of transmitter release, rather than act by disrupting an ongoing system sensing and responding to fluctuations in extracellular transmitter levels.

The effects of yohimbine on presynaptic and postsynaptic events during sympathetic nerve activation in cattle iris: a critique of presynaptic receptor theory.

1 The effects of presynaptic alpha-adrenoceptor blockade on both the efflux of 3H-transmitter and on the magnitude of the effector response were measured simultaneously in a smooth muscle preparation which responds to field stimulation with noradrenergic beta-receptor-mediated relaxation. 2 In the presence of atropine, the circular muscle of cattle iris relaxes in response to noradrenaline and to field stimulation at 2 Hz with 10, 20, 50 and 100 pulses. 3 Yohimbine (3 x 10(-6) M), a potent presynaptic alpha-adrenoceptor antagonist, increased the stimulation-induced efflux of tritium to about 2.0 times control values and, contrary to theory, did so to a similar extent regardless of pulse number and with apparent indifference to the synaptic concentration of transmitter, as confirmed by the varying size of the postsynaptic response. 4 In most cases, yohimbine had no significant effect on the magnitude of the relaxations to nerve stimulation. 5 It is concluded that negative feedback regulation of transmitter release, if it functions at all, and this itself seems doubtful, would not have a substantial impact on the size of the effector response.

P-214 - Negative feedback regulation of transmitter release through the presynaptic α2-adrenoceptor in vascular smooth muscle

P-214 - Negative feedback regulation of transmitter release through the presynaptic α2-adrenoceptor in vascular smooth muscle

Conditions required for the inhibitory feedback loop in noradrenergic transmission.

One of the most striking advances in the study of neurotransmission during the past decade has been the recognition that nerve terminals have receptor sites which, when activated, modulate transmitter release1–4. The most extensively studied have been the α-adrenoceptors of noradrenergic sympathetic terminals, activation of which inhibits transmitter release. It is widely believed that the prejunctional α-adrenoceptors are activated by transmitter noradrenaline, which results in inhibition of subsequent transmitter release1–6, thereby forming an inhibitory feedback system. However, this hypothesis has recently been challenged by Angus and Korner7 on the basis of observations made with short trains of stimulation applied to the intramural sympathetic nerves of isolated guinea pig atria. We have, therefore, re-examined the question using the same tissue and applying not only short, but also longer, trains of stimulation. We report here a time delay before the inhibitory feedback effect through prejunctional α-adrenoceptors is manifested; thus the effect will not be observed unless a sufficiently long train of stimulation is applied. Furthermore, there is no pulse-to-pulse inhibitory feedback regulation of transmitter release for trains of stimulation in which the pulse interval is extended to 8 s.

Effects of neuroleptics on release of 3H-dopamine from slices of rat corpus striatum.

The characteristics of 3H-DA release from striatal slices by electrical stimulation were analyzed and the effects of a number of neuroleptics thereon were examined under different experimental conditions. The butyrophenones, haloperidol and spiroperidol, already at low concentrations (0.1 - 1 micronM) increased basal tritium efflux in a dose-dependent manner. The phenothiazines, chlorpromazine and fluphenazine, were much less effective in this respect. The butyrophenones strongly inhibited the electrically stimulated overflow of both 3H-DA and 14C-GABA, while the phenothiazines again had little effect. The action of 1 micronM haloperidol on 3H-DA release could be blocked by 10 micronM cocaine, but not with 1 micronM apomorphine. Apomorphine itself had no significant effect on 3H-DA release. Our data do not support the suggestion that presynaptic DA receptors on dopaminergic nerve terminals may modulate the release of newly taken-up 3H-DA. Some neuroleptics, particularly the butyrophenones may have presynaptic effects not related to interaction with DA receptors. It is suggested that different mechanisms may be involved in the local presynaptic receptor-mediated feedback regulation of transmitter release in noradrenergic and dopaminergic systems in the CNS.