Noradrenergic Nerve Terminals Research Articles

Presence of a noradrenaline uptake system on a ligated cat sympathetic nerve.

[3H]noradrenaline [( 3H]NA) uptake studies were carried out in cat hypogastric nerves ligated in vivo, 2 cm distal to the inferior mesenteric ganglion, for different time periods. Atria from the same animals served as controls to determine the uptake of the amine by the U1 uptake system present in noradrenergic nerve terminals. The net uptake of [3H]NA by hypogastric nerves increased with time of ligation, reaching a maximum 24 h after ligation in the segment of nerve immediately proximal to the ligature (P1 segment, neurosome). No further increase in uptake was observed at 48 or 72 h. Segments distal (D1) to the ligature also retained significant amounts of [3H]NA. In both cases the uptake was blocked by cocaine (3 microM). Reserpine pre-treatment (2 mg/kg I.M.) markedly decreased the endogenous NA content to 1-2% of untreated cats and the net uptake of [3H]NA was lowered to 25% both in cat hypogastric nerve and atria. The uptake was further decreased in the presence of cocaine (3 microM). 6-Hydroxydopamine (100 microM) did not modify the [3H]NA uptake by ligated cat hypogastric nerves but almost abolished the [3H]NA net uptake by right atria from the same animals. After collagenase pre-treatment (0.05% for 15 min) the net uptake of [3H]NA was not altered in the atrium. However, collagenase-pre-treated ligated nerves, took up almost twice as much [3H]NA; under these conditions, 6-hydroxydopamine produced a marked decrease in [3H]NA net uptake. These data suggest the presence in the perineurium of a diffusion barrier for very polar substances, including 6-hydroxydopamine. In conclusion, our data demonstrate that the cat hypogastric nerve ligated in vivo has a cocaine-sensitive system for NA uptake which resembles the NA uptake mechanism (U1) present in noradrenergic nerve terminals. Our data further support the view that the ligated cat hypogastric nerve (neurosome) could be considered as a model of noradrenergic nerve terminal free of effector cell.

The acute effects of 6-hydroxydopamine treatment on noradrenergic function in the rat hippocampus in vitro

The electrophysiological consequences of in vitro treatment with 6-hydroxydopamine (6-OHDA) were examined in the CA1 region of the rat hippocampal slice. In control slices, norepinephrine (NE) increased the amplitude of the population spike response elicited by synaptic stimulation of hippocampal pyramidal neurons with a threshold of approximately 5 μM. When hippocampal slices were pretreated with 500 μM 6-OHDA for 10 min, perfusion with a subthreshold concentration of NE (0.5 μM) produced responses similar to those observed with a 10-fold higher concentration of NE in untreated slices. Baseline electrophysiological responses were unchanged following the 6-OHDA exposure. The potentiation of the response to NE by in vitro pretreatment with 6-OHDA was accompanied by a > 40% decreased in NE content and > 90% decreased in [ 3H]NE accumulation. In vivo treatment with 6-OHDA or N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine hydrochloride (DSP4) also potentiated the electrophysiological response to NE in a manner similar to that observed with acute in vitro 6-OHDA pretreatment. This action does not appear to be due to development of β-adrenergic receptor supersensitivity, because the apparent potency of isoproterenol in increasing the population spike amplitude was unaffected. These data suggest that the increase in the potency of NE in slices pretreated with 6-OHDA is due to the rapid disruption of the high-affinity NE uptake mechanism characteristic of noradrenergic nerve terminals.

Selective inhibition of monoamine oxidase by p-aminosubstituted phenylalkylamines in catecholaminergic neurones

The in vivo inhibition of monoamine oxidase (MAO) inside and outside noradrenergic and dopaminergic nerve terminals in the hypothalamus and striatum, respectively, was examined in the rat after oral administration of a series of substituted p-aminophenethylamines and some related compounds. This was achieved by measuring their ability to protect MAO from irreversible inhibition by phenelzine, determined by the deaminating activity of synaptosomal preparations in the absence and presence of maprotiline, a selective inhibitor of the uptake of noradrenaline, or of amfonelic acid, a potent inhibitor of the uptake of dopamine, with small (0.25 μM) concentrations of [ 14C]noradrenaline or [ 14C]dopamine as substrate. It was found that several of these compounds were much more potent in protecting MAO within the noradrenergic neurones than MAO in other cells. Since the inhibitors of the uptake of noradrenaline, desipramine and CPP 199 antagonized this preference for noradrenergic MAO it is concluded that these MAO inhibitors are accumulated in the noradrenergic neurones by the membranal uptake carrier. Hence the selectivity for MAO within noradrenergic neurones seems to reflect the ability of the compounds to be transported by this carrier. The structure-activity relationship obtained showed the greatest selectivity for the unsubstituted p-dimethylamino-(FLA 289), p-methylamino-(FLA 727) and p-amino-(FLA 334)-amphetamines, whereas the 2-fluoro compound (FLA 558) had the greatest potency. N, N-didesmethylamiflamine [FLA668(+)] had an almost specific effect in the noradrenergic nerve terminals. The primary p-amino derivatives, FLA 334 and FLA 668, produced a marked selective protection of MAO in dopaminergic nerve terminals, whereas the tertiary and secondary derivatives had much less preference for dopaminergic MAO. The inhibitors of the uptake of dopamine, amfonelic acid and nomifensin antagonized the preferred inhibition by FLA 334 in the dopaminergic neurones, which indicates that this compound is accumulated in these neurones via the uptake mechanism for dopamine. It is concluded from these findings that it is possible to develop inhibitors of MAO with selective action in a specific monoaminergic neurone system.

Chapter 1 Chemical transmission and Dale's principle

Publisher Summary A tremendous effort has been made to establish the multiplicity of synaptic transmitter substances for a single neuron and its synaptic endings. Ionotropic transmitter action occurs at special receptor sites on the postsynaptic membrane and rapidly opens ionic channels, so that there is a fast and large change in postsynaptic membrane conductance. In contrast, metabotropic transmitter action is effective postsynaptically by stimulating adenylate cyclase to give increased production of cyclic nucleotides with the consequent metabolic change in the postsynaptic cell by the second messenger system. There is overwhelming evidence that the emission of transmitter from the presynaptic terminals is quantal for the classical transmitters and that this quantal emission is due to the exocytosis of synaptic vesicles. The chapter illustrates a simple presynaptic feedback, noradrenalin inhibiting vesicular emission, and the varieties of transmitter interaction. Presynaptic feedbacks are postulated to be negative controls of transmitter release, especially the adenosine triphosphate (ATP) control of the purinergic, noradrenergic, and cholinergic nerve terminals.

Evidence for a presynaptic adenylate cyclase system facilitating [3H]norepinephrine release from rat brain neocortex slices and synaptosomes

The effects of drugs known to enhance intracellular cyclic AMP levels on depolarization-induced [3H]norepinephrine release from superfused rat neocortical slices and synaptosomes were investigated. The adenylate cyclase activator forskolin, the membrane-permeating cyclic AMP analogues 8-bromo-cyclic AMP and dibutyryl cyclic AMP, as well as the phosphodiesterase inhibitors isobutylmethylxanthine and 4-(3-cyclopentyloxy-4-methoxyphenyl)-2-pyrolidone (ZK 62771) enhanced the electrically evoked release of [3H]norepinephrine from superfused rat brain neocortex slices. 8-Bromo-cyclic GMP was without effect on the electrically evoked release. When [3H]norepinephrine release was enhanced by prolonging the electrical pulse duration from 2 msec to 10 msec, the relative inhibitory effect of the Ca2+ channel blocker Cd2+ and the relative facilitatory effect of the K+ channel blocker 4-aminopyridine remained unaffected. In striking contrast, the relative facilitatory effects of forskolin and 8-bromo-cyclic AMP were strongly reduced, whereas the effect of ZK 62771 was almost doubled. When veratrine-induced release of [3H]norepinephrine from cortex synaptosomes was examined, the facilitatory effects of forskolin, 8-bromo-cyclic AMP, and ZK 62771 were even more pronounced than in brain slices. The data strongly support the hypothesis that a presynaptic adenylate cyclase system plays a facilitatory role in the stimulus-secretion coupling process in central noradrenergic nerve terminals.

Compensatory changes in contralateral sympathetic neurons of the superior cervical ganglion and in their terminals in the pineal gland following unilateral ganglionectomy

The sympathetic noradrenergic neurons of the rat superior cervical ganglia (SCGs) provide the major source of innervation to the pineal gland. The present study sought to determine if this sympathetic innervation can undergo collateral sprouting following partial denervation of the pineal by unilateral removal of the SCG (ganglionectomy), and whether such growth of axon terminals is associated with biochemical changes in the contralateral SCG. In the pineal gland following partial denervation, residual noradrenergic terminals underwent compensatory changes indicative of collateral sprouting, as evidenced by: a rapid reduction in tyrosine hydroxylase (TH) activity and in [3H]norepinephrine (NE) uptake, to about 50% of control by 2 days, which was followed by a gradual but sustained increase to levels of approximately 80% of control by 10 days and a reduction in the intensity and density but not in the distribution of fibers containing NE-induced fluorescence by 2 days, which was followed by a sustained increase. In the contralateral SCG, choline acetyltransferase (CAT) activity, a marker of cholinergic preganglionic terminals, was transiently increased to about 115% of control by 4 days and returned to control levels by 14 days after unilateral ganglionectomy; later, TH activity in noradrenergic cell bodies was gradually increased to about 140% of control by 10 days where it remained for up to 52 days. Unilteral ganglionectomy combined with decentralization of the contralateral SCG by preganglionic nerve cut prevented the compensatory changes in noradrenergic nerve terminals within the pineal.(ABSTRACT TRUNCATED AT 250 WORDS)

Bromocriptine reduces plasma noradrenaline and 3,4-dihydroxyphenylacetic acid in normal and hypotensive subjects.

The effect of a single oral dose of bromocriptine 2.5 mg was evaluated in 11 normotensive and 6 hypertensive volunteers. 150 min after drug administration, a significant decrease in plasma noradrenaline concentration from 202 to 124 pg/ml in normotensive and from 197 to 119 pg/ml in hypertensive patients was observed. Plasma 3,4-dihydroxyphenylacetic acid, a major metabolite of dopamine, fell from 1132 to 956 pg/ml in normal subjects and from 1242 to 807 pg/ml in hypertensives. No change in plasma adrenaline was found. At the same time, mean arterial pressure showed a significant decrease from 90 to 81 and from 132 to 111 mmHg in normotensive and hypertensive subjects, respectively. Bromocriptine also inhibited the increase in noradrenaline level that occurred when the subjects changed from the supine to the standing position. The inhibition was more evident in hypertensive subjects. It is suggested that the hypotensive effect of bromocriptine is mediated by the inhibition of noradrenaline release due to the stimulation of dopamine receptors on noradrenergic nerve terminals.

Neurochemical investigations in vitro with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) in preparations of rat brain

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) in concentrations ranging from 10−8 M to 10−6 M induced a decrease, and at 10−5 M an increase of both basal and electrically evoked tritium outflow from [3H]dopamine-pre-labeled rat striatal slices. These effects of MPTP were almost abolished in the presence of nomifensine. Chromatographic separation of the released tritium compounds revealed that the decrease of tritium outflow was mostly due to a decrease in the outflow of the dopamine metabolite [3H]3,4-dihydroxyphenylacetic acid (DOPAC) and the increase of tritium outflow due to a massive release of [3H]dopamine. MPTP inhibited oxidative deamination of [3H]dopamine non-competitively in a crude mitochondrial preparation of rat brain, with an apparent Ki value of 4.5 μM. No relevant effect of MPTP on adenylate cyclase activity in homogenates and on basal and electrically evoked tritium outflow from [3H]choline-prelabeled slices of rat striatum could be detected. In contrast, MPTP facilitated both basal and electrically evoked tritium outflow from [3H]noradrenaline-prelabeled rat cerebral occipital cortex slices. Furthermore, MPTP counteracted the inhibitory effect of clonidine on evoked tritium outflow from rat cerebral occipital cortex slices. Moreover, in the presence of cocaine, the effect on basal, but not that on electrically evoked tritium outflow was attenuated. These results are compatible with the view that MPTP has no affinity to dopamine receptors but is preferentially taken up into dopaminergic nerve terminals by the nomifensine-sensitive uptake system where it reaches a concentration sufficient to inhibit intraneuronal monoamine oxidase (MAO). In contrast, the facilitatory effect of MPTP on evoked tritium outflow from [3H]noradrenaline prelabeled rat cerebral occipital cortex slices appears to result from antagonistic effects at presynaptic α-adrenoceptors. The observation that MPTP at lower concentrations (10−8 to 10−7 M) inhibits basal tritium outflow from rat cerebral occipital cortex slices suggests that this compound inhibits also intraneuronal deamination of noradrenaline by MAO in noradrenergic nerve terminals.

The complex binding of tricyclic antidepressants to rat brain: The case of nortriptyline

[ 3H]Nortriptyline (NT) binding to rat brain sections was characterized using saturation and competition experiments, quatitative autoradiographic localization and monoaminergic lesions. [ 3H]NT binding exhibits a saturable component in the nM range (high affinity binding). Final saturation is reached only in micromolar concentrations (low affinity binding). The high affinity binding sites of [ 3H]NT appear in areas rich in noradrenergic and serotonergic terminals, and are decreased by 6-OHDA and 5,7-DHT lesions, to an extent depending on the brain region and the relative density of noradrenergic and serotonergic terminals there. We conclude that [ 3H]NT binds with high affinity to both noradrenergic nerve terminals, and with low affinity to other yet uncharacterized entities. This complex binding pattern is compatible with the multiplicity of biochemical, physiological and behavioral effects of NT and tricyclic antidepressants in general.

Reserpine and the role of axonal transport in the independent regulation of pre- and postsynaptic β-adrenoreceptors

The response of pre- and postsynaptic β-adrenoreceptors to depletion of brain norepinephrine (NE) with reserpine in the rat was characterized by studying the anterograde and retrograde axonal transport of presynaptic receptors and the receptor binding changes induced in postsynaptic frontal cortex cells. Anterograde transport was shown to occur by the linear accumulation of [ 3H]dihydroalprenolol ([ 3H]DHA) binding sites (by in vitro binding assay) proximal to a 6-hydroxydopamine (6-OHDA) lesion placed in the ascending pathway of the locus coeruleus and was blocked by more proximal lesions in the pathway. Retrograde transport was demonstrated by the accumulation of [ 125I]iodocyanopindolol binding distal to similar lesions. Autoradiograms from sections of 6-OHDA injected brains were produced with [ 3H]DHA binding in the presence of the β 2-agonist, zinterol, and suggested that the antegrade accumulation of binding sites was primarily of the β 1-subtype. A single injection of reserpine (5 mg/kg, i.p.) produced a long lasting (6–8 weeks), biphasic decrease in cortical NE levels with nadirs and 4 and 28 days (10% and 45% of control, respectively). Frontal cortex binding of [ 3H]DHA increased to a maximum at 7–14 days and again at 28 days post-reserpine (230% and 167% of control, respectively). These increases were not prevented by the destruction of presynaptic noradrenergic nerve terminals with intraventricular administration of 6-OHDA 1 day prior to sacrifice and therefore appeared to take place solely in postsynaptic cells. Presynaptic, anterograde axonal transport of β-receptors was completely blocked from 4–14 days post-reserpine, increased to 323% of control at 21 days, was blocked again at 6 weeks and returned to control by 8 weeks. Retrograde transport of β-receptors followed a similar pattern suggesting that the presynaptic alterations in β-receptors in noradrenergic neurons of the locus coeruleus take place independently from those in postsynaptic cortical β-receptors as a response to NE depletion by reserpine.

Synthesis of catecholamines in the developing spinal cord of the rat.

Using mass fragmentographic techniques, the synthesis of dopamine (DA) and norepinephrine (NE) was studied in five functional regions in the spinal cord of the developing rat. Norepinephrine was first detectable in the whole cord on fetal day (FD) 16. Thereafter, there was a rapid increase in synthesis. The fastest rate occurred in the ventral horn. The peak concentration was recorded in the latter half of neonatal life, followed by a decline back to the levels found in the adult cord. The capacity of the developing cord to synthesize NE from injected L-dihydroxyphenylalanine (L-DOPA) increased in concert with its normal synthetic capacity. There was no clear consistent pattern in the development of DA synthesis, with two exceptions: (1) in the cervical dorsal horn (CDH), cervical ventral horn ( CVH ), and thoracic zona intermedia ( TZI ), there was a peak of DA on the day of birth; (2) in all regions there was a peak of DA on neonatal day (ND) 20. At FD 16 (the earliest time studied) the developing cord was capable of considerable synthesis (6.0 nmol/g) of DA from injected L-DOPA. This synthetic capacity developed rapidly, peaked at ND 4, and declined back to the adult levels by ND 20. Control experiments indicate that only about 10% (at best) of the DA synthesized from injected L-DOPA occurs in monoaminergic nerve terminals. Norepinephrine is synthesized exclusively in noradrenergic nerve terminals. The newly synthesized DA and NE are both extensively metabolized in the cord. The results are discussed in relationship to current attempts to understand the functional importance of spinal monoaminergic nerves.(ABSTRACT TRUNCATED AT 250 WORDS)

Retrograde transport of dopamine β-hydroxylase antibodies in sympathetic neurons: Effects of drugs modifying noradrenergic transmission

Antibodies to dopamine β-hydroxylase (anti-DβH) were taken up by noradrenergic nerve terminals in the iris following attachment to DβH, and were transported back to, and accumulated in, the superior cervical ganglion (SCG). Concurrent, or prior destruction of noradrenergic terminals with 6-hydroxydopamine, injected intraocularly, blocked the retrograde transport of anti-DβH. However, recovery was rapid, reaching 50% of control values within 1 day. Such transport was characterized by a shorter time period before accumulation could be detected in the SCG and by a slower rate of accumulation. These results suggest that noradrenergic neurons recover their ability to turn over synaptic vesicles by exocytosis and transport these back to the ganglion early during the period of axonal regeneration when the axonal length is shorter than normal. The uptake and transport of anti-DβH was regulated by α-adrenergic agents administered locally in the vicinity of noradrenergic nerve terminals. Thus intraocular injection of phentolamine resulted in an increased accumulation of anti-DβH in the SCG, while amphetamine and the postsynaptic α-receptor antagonist, phenylephrine, decreased accumulation. Clonidine and desipramine, which have a predominant presynaptic action, failed to influence the transport of anti-DβH. These results suggest that in vivo the uptake of anti-DβH can be increased more by local postsynaptic reflex actions than by a mechanism depending on the inhibition of presynaptic α-receptors.

Preferential uptake of norepinephrine into dopaminergic terminals of a synaptosomal preparation from rat cerebral cortex

In a synaptosomal preparation from male rat cerebral cortex only 34% of total norepinephrine (NE) uptake could be inhibited by nanomolar concentrations of desmethylimipramine (DMI) with an apparent IC 50 value of 0.37 nM. The residual uptake was efficiently inhibited by micromolar concentrations of DMI (IC 50 = 4.0 μM). In synaptosomes from the hypothalamus, 74% of total NE uptake could be blocked by DMI with an IC 50 of 0.1 nM whereas in synaptosomes from the striatum the IC 50 for DMI inhibition was 3.8 μM. It is concluded that in synaptosomes from rat cerebral cortex only 34%, and in synaptosomes from the hypothalamus 74% of total NE are taken up by noradrenergic nerve terminals whereas the residual NE uptake occurs in dopaminergic nerve endings.

Effect of destruction of central noradrenergic and serotonergic nerve terminals by systemic neurotoxins on the long-term effects of antidepressants on beta-adrenoceptors and 5-HT2 binding sites in the rat cerebral cortex.

The dependence of intact noradrenergic and serotonergic nerve terminals for the decrease in the number of beta-adrenoceptors and 5-HT2 binding sites in the cerebral cortex produced by long-term treatment of rats with antidepressant drugs was examined. Noradrenergic nerve terminals were destroyed with the selective noradrenaline neurotoxin DSP4, and serotonergic nerve terminals were destroyed with p-chloroamphetamine (PCA). It was found that lesioning of the noradrenergic nerve terminals abolished the decrease in beta-adrenoceptors produced by desipramine, mianserin and zimeldine and partially antagonized that of the beta-adrenoceptor agonist clenbuterol. PCA pretreatment did not antagonize the long-term effects on the beta-adrenoceptor produced by these compounds. Lesioning of serotonergic nerve terminals affected the down-regulation of 5-HT2 binding sites produced by long-term treatment with mianserin, desipramine and amiflamine. DSP4 pretreatment partially abolished the down-regulation of 5-HT2 binding sites produced by long-term treatment with desipramine, while the effects of mianserin and amiflamine were unaffected by pretreatment with DSP4.

Effect of s-adenosyl- L-homocysteine upon sleep inp-chlorophenylalanine pretreated rats

S-Adenosyl- L-homocysteine (7 mg/kg), administered to PCPA-pretreated rats, was able to restore normal SWS and PS quantities, as well as circadian rhythms. This effect was at its maximum when SAH was injected 48 h after PCPA (400 mg/kg). These results are discussed in terms of post-synaptic synergetic control by SAH of serotonergic and noradrenergic nerve terminals via the periventricular system and caudal medulla in relation to environment input.

Electrophysiological effects of cocaine in the rat hippocampus in vitro

The effects of cocaine on electrophysiological activity were examined in the CA1 region of the in vitro hippocampal slice. Low concentrations of cocaine (0.25–2.5 μM) had no direct effect on evoked potentials, but potentiated responses to threshold concentrations of norepinephrine (NE; 0.5 μM); this effect was also seen with other inhibitors of catecholamine uptake such as desipramine. The ability of low doses of cocaine to augment the changes in population spike amplitude elicited by NE appeared to be related to the ability of cocaine to block the uptake of NE into the noradrenergic nerve terminals in the rat hippocampus. Higher concentrations of cocaine (5–200 μM) elicited responses consistent with local anesthetic actions on neuronal membranes.

Alpha-adrenoreceptor subtypes in blood vessels: physiology and pharmacology.

A significant advance in the field of neurotransmission was made with the discovery of presynaptic release-modulating alpha-adrenoreceptors on noradrenergic nerve terminals. The concept of presynaptic modulation of noradrenaline release developed in parallel with the pharmacological evidence for two subtypes of alpha-adrenoreceptors as defined by a different profile of affinity and relative order of potencies for agonists and for antagonists. The alpha 1-adrenoreceptor is stimulated preferentially by methoxamine and cirazoline and blocked selectively by prazosin or corynanthine. The alpha 2-adrenoreceptor is stimulated preferentially by agonists such as clonidine, TL-99, GHT-933, and UK-14,304, and the responses mediated by these agonists are selectively blocked by the alpha 2-adrenoreceptor antagonist idazoxan. In blood vessels, both the alpha 1- and the alpha 2-adrenoreceptor subtypes are present postsynaptically, where they mediate vasoconstriction, although the alpha 1-adrenoreceptor is the predominant receptor in vascular smooth muscle. Presynaptically on noradrenergic nerve terminals, the stimulation of inhibitory alpha 2-adrenoreceptors reduces the depolarization-evoked release of the transmitter. In most vascular beds, the alpha 1-adrenoreceptor is also the preferentially innervated subtype. In spontaneously hypertensive rats, smooth-muscle alpha 2-adrenoreceptors mediate vasoconstrictor responses to exogenous noradrenaline and to sympathetic nerve stimulation to a greater extent than in the corresponding normotensive Wystar-Kyoto rats, which may point to a pathophysiological role of these alpha 2-adrenoreceptors in hypertension.

S1-5) - Purinergic control of the release of noradrenaline from nor-adrenergic nerve terminals in vascular tissues

S1-5) - Purinergic control of the release of noradrenaline from nor-adrenergic nerve terminals in vascular tissues

3H]RX 781094: A new antagonist ligand labels α2-adrenoceptors in the rat brain cortex

[ 3H]RX 781094 [(imidazolinyl-2)-2 benzodioxane-1,4 [ 3H]chlorhydrate], a specific α 2-adrenoceptor antagonist radioligand, has been used to characterize α 2-adrenoceptors in rat cortical membranes. [ 3H]RX 781094 binding is reversible, saturable and stereospecific. It labels with high affinity a single population of non-interacting sites. The K D value was 3.9 ± 0.4 nM and the B max 189.0 ± 12.4 fmol/mg protein. Competition curves with different α-adrenoceptor agonists and antagonists showed that the binding sites labelled with [ 3H]RX 781094 had the pharmacological characteristics of α 2-adrenoceptors. Pretreatment with reserpine (2.5 mg/kg s.c., 24 h before the experiment) did not affect the K D or B max values of [ 3H]RX 781094 binding. Chemical destruction of noradrenergic pathways by systemic injection of DSP4 or intracerebral injection of 6-hydroxydopamine did not modify the K D or the B max of [ 3H]RX 781094 binding. It is concluded that the major proportion of α 2-adrenoceptors labelled with [ 3H]RX 781094 are not localised to noradrenergic nerve terminals.

Exocytosis from large and small dense cored vesicles in noradrenergic nerve terminals

The noradrenergic nerve terminals and their vesicle populations have been subject to much ultrastructural research which has served to support and extend data from biochemical analysis and pharmacological-physiological experiments. Although a great deal of information has been collected, there are still many problems which need further investigation before the various aspects of large and small dense-cored vesicle function and their possible relationship to the various types of clear vesicles and vacuoles in the terminals can be fully explained. In particular, the ontogeny of microvesicles (30-40 nm), intermediate clear vesicles (45-55 nm) and large vacuoles or cisternae (150-200 nm) needs clarification before they can be fruitfully compared to similar structures assigned various roles in other types of terminals. From the data presently available the following conclusions can be drawn (Fig. 32): (1) both large and small dense-cored vesicles participate in exocytosis. (2) The various steps of the exocytotic process from vesicle fusion to vesicle membrane retrieval can be captured by ultrastructural methods but the mode(s) of membrane recapture remains to be resolved. (3) A morphologically heterogeneous population of clear vesicles probably reflects organelles of different ontogeny some being formed via terminal membrane endocytosis and some representing smooth endoplasmic reticulum or small "dense-cored" vesicles devoid of noradrenaline. (4) The small dense-cored vesicles may be formed in the cell body and transported to the terminals as this type of vesicle can be seen in the axons (where they probably are not undergoing retrograde transport as suggested for some of the small clear vesicles). Some small dense-cored vesicles may also be formed from the permanganate and dichromate-positive tubular structures common in rodent terminals. (5) There is no morphological evidence for the presence of large protein molecules such as dopamine beta-hydroxylase in the dense core of small vesicles. Their staining properties appear to reflect mainly noradrenaline and small molecules that can leak out from the vesicles in parallel with the transmitter. (6) Dopamine beta-hydroxylase and opioid peptides secreted from the noradrenergic terminals most probably originate from the large vesicles. (7) The hypothesis that only the large dense-cored vesicles contain and secrete dopamine beta-hydroxylase and opioids can help to explain the species differences in the concentrations of these substances observed in certain tissues.