Autoinhibitory Feedback Research Articles

Prejunctional α2-Adrenoceptors and Norepinephrine Release in the Forearm of Normal Humans

To investigate the prejunctional alpha 2-adrenoceptor-mediated autoinhibitory feedback for norepinephrine (NE) release, we measured the response of forearm blood flow (FAF, mercury in silastic strain gauge plethysmography) and changes of the forearm venous-arterial NE difference (delta VA-NE, deep forearm vein, brachial artery norepinephrine concentration) induced by six intrabrachial artery 5-min infusions of clonidine in 13 normal volunteers. NE spillover into the forearm circulation was estimated as the product of delta VA-NE and FAF after three clonidine infusions. Clonidine caused dose-dependent decreases in FAF with a maximum reduction of FAF after the highest dose of clonidine of 32 +/- 8%, (N = 7, p less than 0.01) indicating postjunctional alpha-adrenergic stimulation. delta VA-NE increased slightly yet calculated norepinephrine spillover remained statistically unchanged after clonidine. The results suggest that auto-inhibitory feedback control of neuronal transmitter release via prejunctional alpha 2-adrenoceptors may not to an important degree modulate norepinephrine release in the forearm vasculature of healthy subjects.

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.

Role of Autoinhibitory Feedback in Cardiac Sympathetic Transmission

The relationship between two indices of transmitter release measured simultaneously and the frequency of 4 field pulses (0.125-2 Hz) were obtained from superfused guinea pig right atria after labelling with 3H-noradrenaline. The relationships between 3H-efflux or rate responses and frequency were hyperbolic. Autoinhibitory feedback did not play a role since phentolamine (1 microM) did not alter the 3H-efflux or rate responses to 4 field pulses that gave 50-60% of the maximum rate response. In the presence of neuronal uptake block (desipramine (0.1 microM) phentolamine enhanced 3H-efflux and rate responses to 4 field pulses at all frequencies. In the absence of desipramine prolonged trains of field pulses (8-12 pulses) at low frequency (0.25 Hz) were not sufficient to activate autoinhibitory feedback. At 2 Hz phentolamine enhanced both responses at 12 field pulses. We conclude that in the right atrium autoinhibitory feedback plays little role in the modulation of transmitter release at levels of stimulation that cause 50-60% of maximum tissue response. The presence of neuronal uptake inhibition or high stimulus strengths are necessary to evoke autoinhibitory feedback.

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.

Sympathetic nervous system in essential hypertension and antihypertensive response to alpha 2-adrenoceptor stimulation.

In patients with essential hypertension, the vasodilator response in the forearm circulation to postjunctional alpha 1-adrenoceptor blockade with prazosin is enhanced. Besides the effects at classical postjunctional alpha 1-adrenoceptor, postjunctional alpha 2-adrenoceptor-mediated effects contribute to vascular tone, and these may be comparable in magnitude to those observed with prazosin. Owing to the extrajunctional localization of alpha 2-adrenoceptors, they may be preferential targets for circulating catecholamines and adrenaline in particular. No evidence of autoinhibitory noradrenaline feedback system via prejunctional alpha 2-adrenoceptors was found by measuring noradrenaline release from the forearm. Therefore increased sympathetic nerve activity, as demonstrated by increased plasma adrenaline concentrations, plays an important role in the pathophysiology of essential hypertension by contributing to increased vascular resistance. Central prejunctional alpha 2-adrenoceptor-stimulating drugs lower sympathetic nerve activity and thereby peripheral resistance and blood pressure, and this effect seems to override peripheral (pre-) and postjunctional alpha 2-adrenoceptor-stimulating effects.

Effects of mianserin on noradrenergic mechanisms

Mianserin increased stimulation-induced release of noradrenaline from guinea-pig atria and brain cortex slices. The increase still occured when neuronal reuptake of noradrenaline was blocked by cocaine. Mianserin blocked the neuronal uptake of 3H-noradrenaline only in high concentrations (1 μ M in atria produced 40% block; 10 μ M in cortex produced 57% block). Lower concentrations (0.01 μ M in atria; 1 μ M in cortex) significantly increased stimulation-induced release of noradrenaline. It is concluded that increases in transmitter release from noradrenergic nerves produced by mianserin can be attributed to blockade of the α 2-adrenoceptors involved in autoinhibitory feedback mechanism at nerve terminals, rather than to blockade of neuronal reuptake of noradrenaline. This effect on noradrenergic transmission in the central nervous system could explain the antidepressant actions of mainserin in accordance with the hypothesis of Schildkraut.

Characterization of the Clonidine Receptor Site

At the cardiac sympathetic nerve terminal the alpha 2-adrenoceptor is presynaptic and appears to be located at an extrasynaptic site. This is suggested by (1) absence of evidence of autoinhibitory feedback at physiologic stimulus levels up to about 50 percent of the maximum chronotropic response in the isolated guinea pig right atrium, and (2) absence of significant competition between clonidine and synaptically released noradrenaline (NA) for the presynaptic site. In the central nervous system (CNS) cardiovascular alpha 2-receptors are probably located at a postsynaptic site in bulbospinal regions of the brain, since they produce effects identical to those of synaptic release of NA. Experiments with the clonidine analog alinidine (ST 567) suggest that there are differences in central receptor type subserving clonidine-mediated baroreflex heart rate and blood pressure changes.

Characterization of the Clonidine Receptor Site

At the cardiac sympathetic nerve terminal the α2-adrenoceptor is presynaptic and appears to be located at an extrasynaptic site. This is suggested by (1) absence of evidence of autoinhibitory feedback at physiologic stimulus levels up to about 50 percent of the maximum chronotropic response in the isolated guinea pig right atrium, and (2) absence of significant competition between clonidine and synaptically released noradrenaline (NA) for the presynaptic site. In the central nervous system (CNS) cardiovascular α2-receptors are probably located at a postsynaptic site in bulbospinal regions of the brain, since they produce effects identical to those of synaptic release of NA. Experiments with the clonidine analog alinidine (ST 567) suggest that there are differences in central receptor type subserving clonidine-mediated baroreflex heart rate and blood pressure changes. At the cardiac sympathetic nerve terminal the α2-adrenoceptor is presynaptic and appears to be located at an extrasynaptic site. This is suggested by (1) absence of evidence of autoinhibitory feedback at physiologic stimulus levels up to about 50 percent of the maximum chronotropic response in the isolated guinea pig right atrium, and (2) absence of significant competition between clonidine and synaptically released noradrenaline (NA) for the presynaptic site. In the central nervous system (CNS) cardiovascular α2-receptors are probably located at a postsynaptic site in bulbospinal regions of the brain, since they produce effects identical to those of synaptic release of NA. Experiments with the clonidine analog alinidine (ST 567) suggest that there are differences in central receptor type subserving clonidine-mediated baroreflex heart rate and blood pressure changes.

Prejunctional alpha-adrenoreceptors subserve a physiological role in cardiac noradrenergic transmission.

1 The question whether prejunctional alpha-adrenoreceptors subserve a physiological role in a noradrenaline-mediated autoinhibitory feedback loop has been studied in guinea-pig isolated atria by stimulating the accelerans nerve and measuring chronotropic responses and the release of radioactivity after labelling transmitter stores with 3H-noradrenaline. 2 Phentolamine (0.3 micrometer) significantly enhanced chronotropic responses when stimulation was with 0.5 Hz for 30 s, but the increase in the release of radioactivity was too small to be measured reliably. When the frequency of stimulation was increased to 4 Hz for 30 s, phentolamine significantly increased the release of radioactivity but the chronotropic response to stimulation was near maximal and phentolamine had no significant effect on it. 3 With prolonged stimulation (12 min) at the lower frequency (0.5 Hz), both the release of radioactivity and the chronotropic response to stimulation were significantly enhanced by phentolamine (3 micrometers). 4 The results support a physiological role for prejunctional alpha-adrenoreceptors in guinea-pig isolated atria.

Inhibition by dopamine of the stimulation-induced efflux of [3H]noradrenaline in renal arteries: limitations of the unitary hypothesis of presynaptic regulation of transmitter release.

The supposition was tested that a pattern of agonist inhibition of 3H-labelled transmitter efflux which shows a decline in intensity as the frequency of stimulation rises provides major evidence for the existence of a functional autoinhibitory feedback loop mediated by released transmitter and located on adrenergic nerve terminals. For these experiments dopamine was used, as it is not released in significant quantities in the ordinary course of sympathetic nerve activation and it appears to act presynaptically in several tested tissues at a locus discrete from that acted on by noradrenaline. Dopamine (3 X 10(-7) and 3 X 10(-6) M) inhibited the stimulation-induced efflux of [3H]noradrenaline from cattle renal arteries and did so to a declining extent with increasing frequency (1-15 Hz). Known antagonists of dopamine action, pimozide and metoclopromide, antagonized this effect of dopamine but did not by themselves enhance stimulation-induced transmitter efflux or block the inhibiting effects of exogenous noradrenaline on efflux, establishing the specificity of dopamine action in renal artery and indicating the absence of an operative negative feedback loop mediated by dopamine. This interpretation was substantiated by the finding that although dopamine reduced the magnitude of contractile responses to nerve stimulation neither pimozide nor metoclopromide enhanced the amplitude of nerve-induced contractions. It thus appears that a pattern of agonist effect on transmitter efflux which manifests a diminution in intensity as the frequency of stimulation climbs is not derived from the operation of an ongoing autoinhibitory feedback system regulating transmitter release but by a yet to be established factor.