Modulation of adrenergic neurotransmission in the rat tail artery by dietary lipids

The effects of dietary lipids on prejunctional alpha 2-adrenoceptor function were investigated in perfused/superfused caudal arteries from adult rats. The investigation was designed to study the effects of diet supplemented with saturated fatty acids (coconut oil diet) or unsaturated fatty acids (sunflower oil diet) on alpha 2-adrenoceptor neuronal function in the proximal rat tail artery. Pregnant rats were fed Purina Rodent Chow (reference diet) or a semisynthetic diet containing 16% (wt/wt) of either sunflower oil or coconut oil. Neonatal pups were exposed to the diet via maternal milk and weaned rats were maintained on the same diet throughout adulthood. Artery segments (5-6 cm) were prelabeled with [3H]norepinephrine and perfused/superfused with Krebs-bicarbonate solution at 37 degrees C. The release of endogenous norepinephrine, total 3H, and [3H]norepinephrine was measured during field stimulation (supramaximal voltage, 5 Hz, 1 ms duration, for a total of 1,200 pulses). Both test diets caused a significant increase in norepinephrine content when compared with the reference diet (p less than 0.01). The results show that field stimulation-evoked release of norepinephrine from arteries obtained from rats fed coconut oil diet was significantly less than that exhibited by either the sunflower oil group or the reference group (p less than 0.05). Phentolamine (3 microM) caused a significant increase in percent release of endogenous norepinephrine and total 3H (p less than 0.05) in all groups. However, the increase above control values for the sunflower oil group was higher than the coconut oil group which suggested that dietary manipulations altered alpha2-adrenoceptor sensitivity.(ABSTRACT TRUNCATED AT 250 WORDS)     

Sympathetic neurotransmission in the tail artery of aging rats.

1. Age-related changes in noradrenergic neurotransmission in the tail arteries of three rat strains: outbred Wistar (WI/Ico), inbred Wistar (WAG/Rij) and inbred Fischer (F344) have been compared in the present study. 2. The arterial noradrenaline content varied from 5 to 10 ng mg-1 wet weight amongst young (3 to 6-month old) representatives of each strain, but did not change with age. As protein content increased in senescent rats (24-month old) by 30-40%, arterial tissue growth would not appear to receive a concomitant increase in sympathetic growth leading to relative, age-related, structural sympathectomy in all strains. 3. The vasoconstrictor response to transmural electrical stimulation was diminished in adult and senescent rats of all strains. 4. As far as could be judged from the increase in noradrenaline release following perfusion with the alpha-adrenoceptor antagonist, phentolamine (1 microM), the presynaptic alpha 2-adrenoceptor-mediated inhibition of noradrenaline release was intact in old representatives of all strains. 5. With blockade of the two main systems which control noradrenaline release in the rat tail artery, viz, neuronal reuptake with cocaine (4 microM) and presynaptic alpha 2-adrenoceptors with phentolamine (1 microM), stimulation-evoked release of noradrenaline was similar at all ages and in all strains. This suggests that in the rat tail artery the basic mechanism of neuronal release of noradrenaline is not functionally modified by aging.(ABSTRACT TRUNCATED AT 250 WORDS)     

Palytoxin-induced contraction and release of endogenous noradrenaline in rat tail artery.

1. The mechanism of the contractile effect of a potent marine toxin, palytoxin (PTX) on the rat isolated tail artery was examined. 2. PTX (10(-7) M) induced a contraction in the tail artery which was dependent on external Ca2+. This contraction was inhibited (by 75% or more) by 10(-6) M prazosin, 2.4 x 10(-5) M bretylium and 10(-4) M 6-hydroxydopamine (6-OHDA), and partially (by 40%) by 10(-5) M indomethacin. However, this contraction was not affected by 10(-6) M tetrodotoxin (TTX), 10(-6) M nifedipine or reserpine treatment. The PTX-induced contraction in reserpine-treated artery was partially inhibited by nifedipine and indomethacin but not by prazosin. 3. Transmural electrical stimulation induced a transient contraction which was dependent on external Ca2+. The contraction induced by electrical stimulation was inhibited by TTX, prazosin, bretylium, reserpine treatment and 6-OHDA but not by nifedipine or indomethacin. 4. PTX increased the release of noradrenaline from this artery. However PTX did not release noradrenaline from reserpine-treated arteries. PTX-induced noradrenaline release was only partially inhibited by TTX or by Ca2+-free solution. 5. These results suggest that PTX has pre- and postsynaptic effects in the rat tail artery. PTX may stimulate adrenergic nerves and release noradrenaline mainly by a TTX-insensitive and Ca2+-independent mechanism and partially by a TTX-sensitive and Ca2+-dependent mechanism. Further, PTX may also release prostaglandins and depolarize smooth muscle cell membrane to induce a contraction.     

The role of endogenous norepinephrine release in potassium-evoked vasoconstriction of the rat tail artery.

Potassium-containing solutions are often used to study the sequence of events leading from excitation to vasoconstriction. In densely innervated vessels, such as the rat tail artery, potassium-induced vasoconstriction may be mediated via smooth muscle depolarization and release of endogeneous norepinephrine. The relative contribution of these two mechanisms--a 'direct' depolarization of the vascular smooth muscle cell membrane, and an 'indirect' sympathomimetic action--to the vasoconstrictor response was studied in the present paper. Perfusion/superfusion of the rat tail artery in vitro with potassium-containing solutions had different effects depending on the concentration used. A change in potassium concentration from 4.7 to 20 mM had no effect on either perfusion pressure or norepinephrine overflow. From 30 to 70 mM, potassium produced increasing amounts of norepinephrine overflow. Experiments with phentolamine and reserpine showed that this norepinephrine overflow contributed for up to half of the vasoconstrictor response observed. A second norepinephrine-independent mechanism was also involved but the latter appeared to be incapable of producing sustained contraction. At concentrations of potassium above 50-70 mM, the results of experiments with (+/-)-propranolol suggest that the norepinephrine released by potassium has a beta-adrenoceptor-mediated vasorelaxant effect.     

Basal inhibitory action of endogenous endothelin on the sympathetic contraction in the isolated rat tail artery

In order to test whether endogenous endothelin modulates the sympathetic vasoconstriction, arterial segments, 2 mm long, from rat tail artery were mounted in organ baths for isometric tension recording. Electrical field stimulation (2-8 Hz, 0.2 ms, 70 V during 1 s) produced frequency-dependent arterial contraction (maximal contraction 770+/-49 mg) that was nearly abolished (over 95% reduction) by tetrodotoxin (10(-6) M) or phentolamine (10(-6) M). This contraction was increased by pretreatment with the antagonist of endothelin ET(B) receptors N-(N-(N-(2, 6-dimethyl-1-piperidinyl)carbonyl)-4-methyl-L-leucyl)-1-(methoxycarbo nyl)-D-tryptophyl)D-norleucine (BQ-788, 10(-7)-3x10(-6) M), and was not modified either by the antagonist of endothelin ET(A) receptors cyclo(D-alpha-aspartyl-L-prolyl-D-valyl-L-leucyl-D-tryptophyl) (BQ-123, 10(-7)-3x10(-6) M) or the agonist of endothelin ET(B) receptors endothelin-1 (8-21), N-Suc-(Glu(9), Ala(11,15)) (IRL-1620, 10(-8)-10(-7) M). The potentiating effect of BQ-788 was not modified in arterial segments without endothelium or pretreated with the inhibitor of nitric oxide synthesis N(W)-nitro-L-arginine (L-NA, 10(-4) M) or with the inhibitor of endothelin converting enzyme N-(alpha-rhamnopyranosyloxy-hydroxyphosphinyl)-leu-trp (phosphoramidon, 10(-4) M). Exogenous noradrenaline (10(-9)-10(-4) M) produced concentration-dependent arterial contractions that were not modified by BQ-788 (3x10(-6) M), BQ-123 (3x10(-6) M) or IRL-1620 (10(-7) M). Therefore, an inhibitory action of endogenous endothelin on sympathetic vasoconstriction may be present under basal conditions. This inhibition could be produced by endothelin through activation of prejunctional endothelin ET(B) receptors, which may inhibit noradrenaline release from perivascular sympathetic nerves.     

Effect of desipramine and amphetamine on noradrenergic neurotransmission: electrophysiological studies in the rat brain.

The present electrophysiological experiments were undertaken to investigate the effect of desipramine and d-amphetamine on noradrenergic neurotransmission in the rat central nervous system. The effectiveness of electrical stimulation of the locus coeruleus and of microiontophoretic application of norepinephrine (NE) in suppressing the firing activity of CA3 pyramidal neurons was studied in the dorsal hippocampus. Desipramine (0.5 and 5 mg/kg i.v.) and d-amphetamine (0.25 and 5 mg/kg i.v.) decreased the effectiveness of locus coeruleus stimulation and prolonged the effect of microiontophoretically applied NE on the same pyramidal neurons. Subsequent i.v. administration of idazoxan, an alpha 2-adrenoceptor antagonist, reversed the effects of desipramine and d-amphetamine on the effectiveness of locus coeruleus stimulation and decreased that of microiontophoretically applied NE. In addition, idazoxan prevented the effect of subsequent administration of desipramine (5 mg/kg i.v.) on the effectiveness of locus coeruleus stimulation. High doses of d-amphetamine (5 and 10 mg/kg i.v.) decreased the firing activity of hippocampus pyramidal neurons by 70 and 98%, respectively, whereas low doses of desipramine (0.5 mg/kg i.v.) or of d-amphetamine (0.25 mg/kg i.v.) were without effect. After lesioning of NE projections with 6-hydroxydopamine, the effect of the 5 mg/kg dose of d-amphetamine on the firing activity of hippocampus pyramidal neurons was markedly reduced, whereas the cumulative 10 mg/kg dose of d-amphetamine completely suppressed, as in control rats, the firing activity of these neurons. This effect of d-amphetamine in 6-hydroxydopamine-pretreated rats was reversed by the administration of the 5-HT1A receptor antagonist BMY 7378. These data provide evidence that acute administration of desipramine and d-amphetamine decreases the effectiveness of locus coeruleus stimulation by increasing the activation of terminal alpha 2-adrenoceptor autoreceptors. In addition, acute administration of high doses of d-amphetamine decreases the firing rate of hippocampus pyramidal neurons by increasing NE and serotonin release.     

Effects of atrial natriuretic factor on noradrenergic neurotransmission in vivo in the rat mesentery

The effects of synthetic atrial natriuretic factor (ANF) on noradrenergic neurotransmission were assessed in the in situ blood-perfused rat mesentery. Synthetic ANF was infused (.003-1 micrograms/min) directly into the mesenteric artery, and vascular responses to periarterial (i.e., sympathetic) nerve stimulation (PNS) and exogenous norepinephrine (NE) were elicited. Synthetic ANF did not significantly alter vascular responses to PNS and only slightly suppressed responses to exogenous NE at blood levels greater than 10 ng/ml. In contrast, under similar conditions, low doses of adenosine attenuated vascular responses to PNS without reducing responses to exogenous NE, and high doses of adenosine nearly abolished responses to both PNS and exogenous NE. We conclude that synthetic ANF exerts very little influence on noradrenergic neurotransmission in the rat mesenteric vascular bed.     

Enhancement of serotonergic and noradrenergic neurotransmission in the rat hippocampus by sustained administration of bupropion

Rationale  

Previous studies reported that bupropion, an effective antidepressant, exerts modulatory actions on serotonin (5-HT) and norepinephrine (NE) neurons.     

Role of the increased noradrenergic neurotransmission in drug self-administration

Psychostimulants increase extracellular monoamine concentrations in the CNS. While the contributions of dopamine (DA) and serotonin (5-HT) to the reinforcing effect of psychostimulants have been examined, less is known about the involvement of norepinephrine (NE). In the present study, cocaine, desipramine (DMI) and JZ-III-84 were made available to rhesus monkeys (n=4) responding under a progressive-ratio (PR) schedule. These compounds vary in their in vitro selectivities for blocking NE uptake relative to DA from high (DMI) to modest (JZ-III-84) to non-selective (cocaine). Additionally, cocaine mixed with DMI in mg/kg dose-ratios of 1:1 to 1:3 was made available for self-administration. NE uptake inhibition by the mixture of cocaine and DMI at a ratio of 1:3 was evaluated in an ex vivo uptake assay. Cocaine (0.01-0.1 mg/(kg injection)) and JZ-III-84 (0.001-0.1 mg/(kg injection)) functioned as positive reinforcers with sigmoidal or biphasic dose-response functions, whereas DMI failed to do so. The addition of DMI to cocaine did not systemically alter self-administration of cocaine. In the ex vivo uptake assay, the addition of DMI to cocaine significantly increased the NE uptake inhibition compared to cocaine. These results support the conclusion that CNS NE is not involved in the reinforcing mechanism of psychostimulants.     

Modulation by the endothelium of sympathetic vasoconstriction in an in vitro preparation of the rat tail artery.

1. The influence of the endothelium on transmural electrical stimulation was investigated in isolated and perfused segments of the rat tail artery. Noradrenaline release (NA, quantified by h.p.l.c.-electrochemical detection) and changes in perfusion pressure (PP, measured at constant flow rate) were simultaneously recorded in unstimulated and stimulated arterial segments, in the absence and in the presence of drugs. The ratio PP/NA release (mmHg pg-1) was taken as an index of the noradrenergic effectiveness. 2. Removal of the endothelium produced an increase in NA release and PP, in unstimulated and stimulated arteries. This can be taken as evidence of an endothelium-derived inhibitory factor (EDIF) acting at the prejunctional level, inhibiting NA release. Furthermore, in unstimulated arteries, the ratio PP/NA release decreased suggesting the existence of an endothelium-derived contracting factor (EDCF). 3. Perfusion of arteries with N omega-nitro-L-arginine methyl ester (L-NAME, 10 microM) or methylene blue (MeB, 0.5 microM) had no effect on PP or NA release in unstimulated arteries. In stimulated arteries, both drugs potentiated the increase in PP without changing NA release and therefore, led to an increase in noradrenergic effectiveness. After removal of the endothelium, neither L-NAME nor MeB affected the increases in PP and NA release following electrical stimulation. 4. Carbachol (1 microM) attenuated both NA release and the increase in PP during electrical stimulation, and increased the ratio PP/NA release. L-NAME and MeB did not modify the inhibitory effect of carbachol on NA release, or the facilitatory effect of carbachol on the noradrenergic effectiveness.(ABSTRACT TRUNCATED AT 250 WORDS)     

Enhanced sympathetic neurotransmission in the tail artery of 1,3-dipropyl-8-sulphophenylxanthine (DPSPX)-treated rats.

1. Sympathetic neurotransmission and noradrenaline content of the tail artery of Wistar rats treated for 7 days with the adenosine antagonist, 1,3-dipropyl-8-sulphophenylxanthine (DPSPX), were examined. 2. Systolic blood pressure of the DPSPX-treated rats (164.0 +/- 2.9 mmHg; n = 6) was significantly greater than saline-treated controls (140.0 +/- 2.8 mmHg; n = 5) after 7 days treatment. 3. The pressor responses of the arterial rings to transmural nerve stimulation (65 V, 0.1 ms, 4-64 Hz, for 1 s) were markedly enhanced in the DPSPX-treated compared with the saline-treated animals. Both noradrenergic and purinergic components of perivascular sympathetic neurotransmission were enhanced during DPSPX-induced hypertension. 4. Vasoconstrictor responses to exogenous noradrenaline (0.1-300 microM) and adenosine 5'-triphosphate (0.01-3 mM) were unaffected after DPSPX treatment, indicating prejunctional alteration of sympathetic cotransmission during DPSPX-induced hypertension. 5. Acute exposure to DPSPX (10 microM) did not modify vasoconstrictor responses to transmural nerve stimulation, thus supporting the claim that the enhancement of sympathetic neurotransmission only results from long-term DPSPX treatment. 6. The noradrenaline content of the tail arteries of DPSPX-treated (4.498 +/- 0.26 ng cm-1; n = 4) was significantly greater than saline-treated (3.440 +/- 0.30 ng cm-1; n = 5) animals. 7. These findings show that chronic inhibition of the actions of endogenous adenosine by DPSPX results in an elevation of systolic blood pressure accompanied by enhanced sympathetic cotransmission and enhanced noradrenaline content of the rat tail artery.     

Endothelium-independent potentiating effects of neuropeptide Y in the rat tail artery.

The role of the endothelium in the potentiating action of neuropeptide Y (NPY) to contraction induced by KCl, alpha, beta-methylene ATP (mATP), and noradrenaline (NA) was tested on rat tail arteries. Endothelium-intact and denuded ring segments and freshly isolated single smooth muscle cells were used in the study. Contraction responses to KCl and mATP were potentiated by NPY (50 nM) in both intact and denuded arteries. Contraction to NA was potentiated by NPY at 500 nM but not at 50 nM. The potentiation effect of NPY was antagonized by nifedipine. Similarly, the shortening of single smooth muscle cells in response to KCl and mATP was potentiated by NPY (50 nM). The noradrenaline response was potentiated by NPY at 500 nM but not at 50 nM. Our results suggest that the potentiating effect of NPY is more specific to contraction mediated by nifedipine-sensitive calcium channels and is not dependent on the presence of an intact endothelium.     

Protons modulate perivascular axo-axonal neurotransmission in the rat mesenteric artery

Background and Purpose

Previous studies have demonstrated that nicotine releases protons from adrenergic nerves via stimulation of nicotinic ACh receptors and activates transient receptor potential vanilloid-1 (TRPV1) receptors located on calcitonin gene-related peptide (CGRP)-containing (CGRPergic) vasodilator nerves, resulting in vasodilatation. The present study investigated whether perivascular nerves release protons, which modulate axon-axonal neurotransmission.

Experiment Approach

Perfusion pressure and pH levels of perfusate in rat-perfused mesenteric vascular beds without endothelium were measured with a pressure transducer and a pH meter respectively.

Key Results

Periarterial nerve stimulation (PNS) initially induced vasoconstriction, which was followed by long-lasting vasodilatation and decreased pH levels in the perfusate. Cold-storage denervation of the preparation abolished the decreased pH and vascular responses to PNS. The adrenergic neuron blocker guanethidine inhibited PNS-induced vasoconstriction and effects on pH, but not PNS-induced vasodilatation. Capsaicin (CGRP depletor), capsazepine and ruthenium red (TRPV1 inhibitors) attenuated the PNS-induced decrease in pH and vasodilatation. In denuded preparations, ACh caused long-lasting vasodilatation and lowered pH; these effects were inhibited by capsaicin pretreatment and atropine, but not by guanethidine or mecamylamine. Capsaicin injection induced vasodilatation and a reduction in pH, which were abolished by ruthenium red. The use of a fluorescent pH indicator demonstrated that application of nicotine, ACh and capsaicin outside small mesenteric arteries reduced perivascular pH levels and these effects were abolished in a Ca²⁺-free medium.

Conclusion and Implication

These results suggest that protons are released from perivascular adrenergic and CGRPergic nerves upon PNS and these protons modulate transmission in CGRPergic nerves.Tables of Links

Effects of urotensin I on the isolated rat tail artery.

Urotensin I (UI) was found to elicit dose-related relaxation responses in isolated helical strips of the rat tail artery. The responses were not prevented by adrenergic, cholingergic or histaminergic blocking agents. Competitive and non-competitive components of antagonism were observed to noradrenaline-, 5-hydroxytryptamine-, and arginine vasopressin-induced contractions. Atropine caused a direct relaxation of the isolated vascular tissues, as well as a significant potentiation of UI responses.     

Angiotensin facilitation of noradrenergic neurotransmission in central tissues of the rat: effects of sodium restriction

This study investigated the ability of angiotensin II (Ang II) to facilitate the stimulation-induced release of [3H]norepinephrine [( 3H]NE) from two cardiovascular regulatory areas in normal and sodium-restricted rats. Ang II (10(-7) M) facilitated the field-stimulation-induced release of [3H]NE from the A2 area of the nucleus tractus solitarius but not from the anterior hypothalamus of Sprague-Dawley and Wistar rats. Placement of rats on a sodium-restricted diet abolished the facilitation of [3H]NE release due to Ang II. Captopril given during sodium restriction partially restored the facilitory effects of Ang II. In an effort to determine the interaction of Ang II and sodium reduction, the effects of chronic Ang II were studied. Seven-day intravenous Ang II infusions blocked the facilitory effect of Ang II on [3H]NE release in a manner similar to that seen with sodium restriction. These results suggest that low sodium diets may alter the facilitation of [3H]NE release by Ang II by interactions with the renin-angiotensin system.     

Pharmacological evidence for a functional role of the prejunctional alpha-adrenoreceptor in noradrenergic neurotransmission in the conscious rat

Summary The functional role of the prejunctional alpha-adrenoreceptor was studied in normal, adrenalectomized and desmethylimipramine pretreated conscious rats. Alpha-adrenoreceptor antagonists with differing in vitro selectivities for the prejunctional alpha-adrenoreceptor were administered, and their effects on arterial blood pressure, heart rate and plasma noradrenaline concentration determined. Following prazosin, an alpha-receptor antagonist with selectivity for the postjunctional alpha-adrenoreceptor, plasma noradrenaline concentration increased to a level slightly above that attributable merely to reflex activation of the sympathetic nervous system by the direct-acting vasodilator, hydralazine. In contrast, administration of equipotent hypotensive doses of phenoxybenzamine and phentolamine, alpha-receptor antagonists which block both pre- and postjunctional alpha-adrenoreceptors, resulted in marked and sustained increases in plasma noradrenaline concentration. The increases in heart rate following these agents were proportional to the changes in noradrenaline. Yohimbine, an alpha-receptor antagonist with selectivity for the prejunctional alpha-adrenoreceptor, resulted in a modest and transient reduction in arterial blood pressure but a sustained increase in heart rate and a three-fold increase in plasma noradrenaline concentration. In addition, dose-response studies revealed that for a given reduction in arterial pressure, prazosin caused a lesser increment in plasma noradrenaline concentration and heart rate, than phentolamine. Studies in adrenalectomized rats and rats pretreated with desmethylimipramine excluded a direct effect on the adrenal medulla and blockade of the neuronal uptake process, respectively, as the mechanism for the observed increments in plasma noradrenaline concentration. As the spectrum of changes in plasma noradrenaline concentration and heart rate is consistent with the in vitro selectivities of these agents to block the prejunctional alpha-receptor, these findings provide evidence for a functional role of this receptor in noradrenergic neurotransmission in the rat.This study was presented in part at the 62nd Annual Meeting of the Federation of the American Societies for Experimental Biology and published as an abstract (Federation Proc. 37, 308, 1978)     

Inhibition of noradrenaline release by omega-conotoxin GVIA in the rat tail artery.

1. The perivascular nerves of isolated tail arteries from Wistar rats were stimulated with field pulses (1 Hz, 2 pulses, every 2 min). omega-Conotoxin 10 nmol l-1 depressed neurogenically mediated contractions, but did not influence the contractions to noradrenaline 0.1-0.3 mumol l-1. 2. The inhibitory effect of omega-conotoxin was concentration-dependent (IC50 = 3.8 nmol l-1). It did not reach a steady-state during 30 min incubation and could not be reversed upon subsequent washout for another 60 min. 3. A gradual increase in the Ca2+ concentration of the medium from 1.25 mmol l-1 to 10 mmol l-1 enhanced vasoconstriction and attenuated the action of omega-conotoxin 10 nmol l-1. When a low stimulation intensity (120 mA) was used at high external Ca2+ (10 mmol l-1), similar contractile responses were obtained as under normal conditions (200 mA current, 2.5 mmol l-1 Ca2+). However, the inverse relationship between the effect of the toxin and external Ca2+ remained unchanged. 4. The time-course and degree of the inhibition by omega-conotoxin 3 nmol l-1 was identical in tail arteries of spontaneously hypertensive rats (SHR) and their normotensive controls (WKY). 5. When tail arteries of Wistar rats were preincubated with [3H]-noradrenaline, field stimulation (0.4 Hz, 24 pulses, every 16 min) evoked tritium overflow and vasoconstriction. omega-Conotoxin 30 nmol l-1 inhibited both responses to a similar extent. 6. Our results suggest that omega-conotoxin selectively blocks Ca2+ channels in the terminals of perivascular nerves and thereby reduces the release, but not the contractile effect of the sympathetic transmitter.