Mechanism of negative feed-back inhibition of norepinephrine release by alpha-adrenergic agonists

Effect of chronic clonidine treatment on transmitter release from sympathetic varicosities of the guinea-pig vas deferens

1. Previous studies have demonstrated that chronic pre-synaptic inhibition of transmitter release by morphine evokes a counter-adaptive response in the sympathetic nerve terminals that manifests itself as an increase in transmitter release during acute withdrawal. In the present study we examined the possibility that other pre-synaptically acting drugs such as clonidine also evoke a counter-adaptive response in the sympathetic nerve terminals. 2. In chronically saline treated (CST) preparations, clonidine (0.5 microM) completely abolished evoked transmitter release from sympathetic varicosities bathed in an extracellular calcium concentration ([Ca(2+)](o)) of 2 mM. The inhibitory effect of clonidine was reduced by increasing [Ca(2+)](o) from 2 to 4 mM and the stimulation frequency from 0.1 to 1 Hz. 3. The nerve terminal impulse (NTI) was not affected by concentrations of clonidine that completely abolished evoked transmitter release. 4. Sympathetic varicosities developed a tolerance to clonidine (0.5 microM) following 7-9 days of chronic exposure to clonidine. 5. Acute withdrawal of preparations following chronic clonidine treatment (CCT) resulted in a significant (P < 0.005) enhancement of neurotransmitter release (3.75 times) above control levels observed in CST preparations. 6. The present findings demonstrate an enhancement of neurotransmitter release from sympathetic varicosities following acute withdrawal from chronic clonidine treatment.

Presynaptic A2-adrenoceptors and neuropeptide Y Y2 receptors inhibit [3H]noradrenaline release from rat hypothalamic synaptosomes via different mechanisms

Presynaptic receptors may reduce transmitter release with different mechanisms. Both the alpha 2-agonist, clonidine and the Y2-agonist, neuropeptide Y fragment 13-36 (NPY 13-36), induce a concentration-dependent inhibition of the 4-aminopyridine (4-AP)-evoked [3H]noradrenaline ([3H]NA) release from hypothalamic synaptosomes. Changes in alpha 2- and Y2-modulation of noradrenaline (NA) release were observed by lowering the calcium influx with the use of omega-conotoxin (omega-CgTx), a calcium-channel blocking agent. In these experimental conditions, clonidine was less active, whereas NPY 13-36 preserved its efficacy. It therefore seems possible that presynaptic alpha 2-adrenoceptors can primarily inhibit NA release by reducing calcium influx via voltage-sensitive calcium channels (VSCC), while Y2-receptors may inhibit the intracellular release process with a mechanism independent of the calcium entry.

Differential sensitivity to blockade by 4-aminopyridine of presynaptic receptors regulating [3H]acetylcholine release from rat hippocampus

The inhibitory effect of an adenosine analogue, R-N6-phenylisopropyl adenosine (R-PIA), of the cholinergic agonist carbachol, and of morphine on 3H efflux from [3H]choline-labeled field-stimulated rat hippocampal slices was compared with that produced by two inhibitors of N- and L-type Ca2+ channels, omega-conotoxin (CgTx; conotoxin GVIA) and cadmium chloride. 4-Aminopyridine (4-AP) caused a dose-dependent increase in evoked transmitter release, with a maximal effect (an almost threefold increase) at 100 microM. 4-AP (100 microM) did not affect the actions of CgTx, cadmium chloride, and R-PIA but almost abolished the effect of carbachol and morphine. The present results indicate that presynaptic muscarinic and opiate receptors reduce acetylcholine release by a mechanism that is somewhat different from that used by adenosine A1 receptors. Furthermore, the results indicate that presynaptic A1 receptors on hippocampal cholinergic neurons do not primarily regulate 4-AP-dependent potassium channels, but that they might act directly on a Ca2+ conductance.

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

The results appear to support the following tentative working hypothesis. (1) Nerve impulse-induced transmitter release from sympathetic nerve varicosities is monoquantal and highly intermittent (probability range: 0-0.03). (2) Nerve impulses invade varicosities as all-or-none, Na+ channel-dependent action potentials; invasion failure may be rare. (3) The release probability is not controlled by properties (amplitude or duration) of the invading action potential or the resulting Ca2+ current, but by the availability of an as yet unidentified permissive factor. (4) The permissive factor is actively transported intra-axonally, probably in association with organelles (LDVs?). (5) The activation and/or transport of the permissive factor are controlled "upstream" of the varicosity; they depend on Ca2+ influx through channels insensitive to nifedipine (hence, not of L-type) but blocked by Cd2+ and apparently opened by slight depolarization of the resting membrane, in this respect behaving more as T- than N-type channels. (6) A high resting K+ efflux "upstream" of the varicosity restricts the availability of the permissive factor; it is the main mechanism maintaining the (economically necessary) low release probability. (7) Prejunctional agonists do not inhibit transmitter secretion by causing a conduction block or by reducing the action potential-induced Ca2+ influx into the varicosity itself, but by depressing the Ca2(+)-dependent activation and/or transport of the permissive factor; they act at least in part via receptors "upstream" of the varicosity. (8) This hypothesis for regulation of the release probability in sympathetic nerves may apply, at least in part, to other neurons as well.

Alpha 2-adrenoceptor agonist-mediated inhibition of [3H]noradrenaline release from rat hippocampus is reduced by 4-aminopyridine, but that caused by an adenosine analogue or omega-conotoxin is not

The inhibitory effect of an adenosine analogue, R-PIA, and an alpha 2-adrenoceptor agonist, UK 14,304, on [3H]NA efflux from field-stimulated rat hippocampal slices was examined. The effect of 0.1 microM UK 14,304 was mimicked by 30 nM omega-conotoxin and by 10 microM cadmium chloride, inhibitors of N- and L-type Ca2+ channels. R-PIA (1 microM) had no effect per se, but caused a clear-cut inhibition after blockade of the pre-synaptic alpha 2-receptor by yohimbine. 4-Aminopyridine (4-AP) caused a dose-dependent increase in evoked transmitter release. At 30 microM 4-AP did not affect the actions of omega-conotoxin or cadmium chloride. The pre-synaptic effect of R-PIA was similarly unaffected by 30 microM 4-AP. The pre-synaptic effect of UK 14,304 was virtually abolished by 4-AP (30 microM). The effect of UK 14,304 (0.1 microM) could be partly restored by reducing the Ca2+ concentration during treatment with 4-AP (22% inhibition compared to 42% with normal Ca2+). The magnitude of increase in evoked [3H]NA efflux by yohimbine (1 microM) was decreased by 4-AP in a concentration-dependent manner from 142% increase in controls to 21% at 100 microM 4-AP. The present results indicate that NA release is reduced by somewhat different mechanisms by pre-synaptic alpha 2- and adenosine A1-receptors. Furthermore, the results indicate that pre-synaptic A1-receptors on hippocampal NA neurons do not primarily regulate 4-AP-dependent potassium channels, but they might act directly on a Ca2+ conductance.

Demonstration of adrenergic and dopaminergic receptors in cultured sympathetic neurons--their coupling to cAMP but not to the transmitter release process

Experiments were carried out on cultured sympathetic neurons of the chick embryo; first, to demonstrate the presence of adrenergic and dopaminergic receptors, and then to see if these receptors are involved in regulation of transmitter release. We show that alpha 2-agonists, norepinephrine, epinephrine and clonidine, had no effect on neuronal cyclic 3',5'-adenosine monophosphate content. Forskolin enhanced neuronal cyclic 3',5'-adenosine monophosphate from a control value of about 20 pmoles/mg protein to 150 pmoles/mg protein. In the presence of alpha 2-agonists and forskolin the cyclic 3,5'-adenosine monophosphate content increased between 340 and 430 pmoles/mg protein. The alpha 1-agonist, phenylephrine, had no such effect. The facilitatory effect of alpha 2-agonist on forskolin-stimulated cyclic 3',5'-adenosine monophosphate production was blocked by the alpha 2-antagonist, yohimbine, but not the alpha 1-agonist, prazosin. Dopamine did not affect neuronal cyclic 3',5'-adenosine monophosphate content, but forskolin-stimulated increase in cyclic 3',5'-adenosine monophosphate was further facilitated by dopamine, and this effect was blocked by haloperidol. Activation of neuronal alpha 2-receptors by norepinephrine, epinephrine and clonidine did not interfere with electrically induced release of tritium from [3H]-norepinephrine-loaded sympathetic neurons. However, if sympathetic neurons were co-cultured with heart cells, clonidine, norepinephrine and epinephrine markedly inhibited the stimulation-induced release. Yohimbine or phentolamine partially reversed the inhibitory effects of alpha 2-agonists. alpha 2-Agonists and -antagonists also modified stimulation-induced release of tritium from [3H]norepinephrine-loaded hearts of the chick embryo.(ABSTRACT TRUNCATED AT 250 WORDS)

Glutamate and kainate effects on the noradrenergic neurons innervating the rat vas deferens

The effects of glutamate on the noradrenergic innervation of the rat vas deferens have been investigated. Administration of a single dose of kainate (50 micrograms) to the peripheral noradrenergic ganglia innervating the vas deferens induced a time-dependent decrease of norepinephrine in the organ; after 10 days the norepinephrine concentration had fallen to 35% of control values. This effect was accompanied by a 70% decrease in the potassium-induced release of recently incorporated 3H-norepinephrine. Concomitantly, postsynaptic hypersensitivity to both norepinephrine and dopamine appeared. The finding that adrenergic ganglia possess high-affinity glutamate binding sites suggests that the effect of kainate may be ascribed to glutamate receptors present on the perikarya of the noradrenergic neurons. It is concluded that noradrenergic neurons of the vas deferens are under glutamatergic control and that this might be important in the motor control of the organ.

Method for stimulating the adrenergic system of an isolated perfused rat heart

A method is presented whereby isolated perfused rat hearts can be rapidly prepared for the stimulation of chronotropic activity by electrical pulses or exogenous noradrenaline. The mediation of the response by sympathetic nerves is demonstrated through modulation of the response by compounds with established pharmacological actions. Propranolol inhibits the increase in heart rate to both electrical stimulation and exogenous noradrenaline, whereas bretylium inhibits only electrically induced increases. Chronic pretreatment with 6-hydroxydopamine decreases the response to electrical stimulation but increases that to exogenous noradrenaline.

Effect of the dihydropyridine Bay K 8644 on the release of [3H]-noradrenaline from the rat isolated vas deferens

The effects of Bay K 8644 on the release of [3H]-noradrenaline evoked by potassium, electrical stimulation or tyramine from the rat isolated vas deferens labelled with [3H]-noradrenaline were investigated. Bay K 8644 (1 microM) by itself did not affect the spontaneous release of tritium from the rat isolated vas deferens. However, it increased the calcium-dependent release of tritium elicited by both high potassium (59 mM) and electrical field stimulation. The exposure of rat vas deferens to phentolamine (10 microM) increased the release of tritium induced by potassium (59 mM) and electrical field stimulation. Bay K 8644 (1 microM) failed to increase further the release of tritium elicited by both stimuli in preparations previously treated with phentolamine (10 microM). The calcium-independent release of [3H]-noradrenaline evoked by tyramine (10 microM) was not affected by Bay K 8644 (1 microM). The results of our study support the view that alpha2-adrenoceptors modulate noradrenaline release by restricting calcium influx into sympathetic nerve terminals through voltage-dependent channels.