Prolonged treatment with (+/-) propranolol induces supersensitivity to (L)noradrenaline in mesenteric arteries in the rat

In the isolated perfused mesenteric arteries of the rat, neither (+/-) propranolol (0.1 microM) nor (+/-)isoproterenol (0.05 microM) modified the overflow of DL-[3H]noradrenaline (DL-[3H]NA) induced by sympathetic nerve stimulation at either 5 or 10 Hz. The blockade of alpha presynaptic receptors with phentolamine (4.7 microM) increased the 3H-transmitter overflow at 5 and 10 Hz. (+/-)Propranolol (0.1 microM) failed to modify this effect. Vasoconstrictor responses to exogenous NA or sympathetic nerve stimulation were not modified by (+/-)propranolol (0.1 microM). Prolonged treatment with (+/-)propranolol (7 mg/kg) for 15 days potentiates responses to both exogenous NA and sympathetic nerve stimulation; however, the fractional release per pulse of DL-[3H]NA was not modified at either 5 or 10 Hz. These results provide no evidence to support the hypothesis that the release of NA is regulated by presynaptic beta-adrenoreceptors in the mesenteric arteries of the rat. The enhancement of vascular responses after prolonged treatment with propranolol could be caused by postsynaptic supersensitivity.     

Augmented inositol phosphate production in mesenteric arteries from diabetic rats.

The effects of noradrenaline (NA) on contraction and phosphoinositide metabolism were compared in mesenteric arteries from rats with chronic streptozotocin-induced diabetes and from age-matched control rats. Maximum contractile responses of mesenteric arteries from diabetic rats to NA (30 microM) were significantly greater than control in both the presence and absence of extracellular Ca2+. Basal incorporation of [3H]myoinositol into total [3H]inositol phosphates was greater in diabetic than control mesenteric arteries. NA (30 microM) resulted in a time-dependent increase in total [3H]inositol phosphate production, which was also significantly greater in diabetic than in control preparations. The increase in total [3H]inositol phosphates produced by NA in both control and diabetic arteries was blocked by the alpha 1-adrenoceptor antagonist, prazosin. Absolute levels of inositol 1,4,5-trisphosphate (I(1,4,5)P3), measured by protein binding assay, were also increased in response to 30 microM NA in both control and diabetic arteries. Although basal I(1,4,5)P3 levels were not significantly different, NA-induced increases in I(1,4,5)P3 were significantly greater in diabetic than in control arteries at each time-point measured. These data indicate that phosphoinositide metabolism is enhanced in mesenteric arteries from rats with chronic streptozotocin-induced diabetes in response to a maximum concentration of NA. Augmented production of the second messengers I(1,4,5)P3 and, presumably, 1,2-diacylglycerol may contribute to the enhanced maximum contractile responses of the diabetic arteries to NA.     

Role of adenylate cyclase in modulatory effect of neuropeptide Y on [3H]noradrenaline release in guinea-pig vas deferens

1. The effect of neuropeptide Y (NPY) on [3H]noradrenaline ([3H]NA) release-evoked by 5-Hz electrical stimulation or 5 microns calcium ionophore A23187 was studied in vitro in guinea-pig vas deferens. 2. The evoked tritium overflow (which reflected [3H]NA release) was determined by liquid scintillation spectrometry. 3. NPY, 1 microM, reduced electrically-evoked tritium overflow. NPY reduction was more pronounced upon 20-sec, 3 msec continuous stimulation (73.2 +/- 4.4%) and upon 5-min, 1 msec intermittent stimulation (47.8 +/- 2.4%) as compared to the reduction upon 5-min, 1 msec continuous stimulation (24.3 +/- 3.8%). Forskolin (0.1-1 microM) and theophylline (0.65-1.25 mM) dose-dependently diminished this NPY reducing effect. 4. NPY, 1 microM, reduced A23187-evoked tritium overflow by 52.3 +/- 7.1%. Forskolin (5 microM) and theophylline (1.25 mM) significantly decreased the effect of NPY. 5. It is concluded that in guinea-pig vas deferens NPY reduces [3H]NA release through affecting adenylate cyclase and the processes responsible for calcium mobilization.     

Relationship between accumulation, storage and overflow of noradrenaline in the rat salivary gland after chronic treatment with guanethidine.

1 The effect of guanethidine on the endogenous noradrenaline (NA) content, accumulation and overflow of [3H]-noradrenaline ([3H]-NA) in the rat salivary gland was examined at various times after drug administration. 2 Twenty-four h after a single injection of guanethidine (1 or 10 mg/kg s.c.), the respective values for the endogenous NA content and for the accumulation and overflow of [3H]-NA were approximately 55, 85, 30%, and 15, 55, 10% of the controls. 3 Although [3H]-NA accumulation had returned to control levels within 48 h after the dose of 10 mg/kg guanethidine, the overflow of [3H]-NA evoked by electrical stimulation or excess potassium (K+) remained depressed. 4 After the low or the high dose of guanethidine, the NA content of the salivary gland was restored to about 50% of the normal value between 4 to 24 and 48 to 72 h, respectively. 5 The accumulation of [3H]-NA was inhibited by about 75% by cocaine. The same degree of inhibition was obtained 4 h after 10 mg/kg guanethidine. In these experiments phenoxybenzamine did not reduce the residual (25%) uptake. 6 The reasons for differential rates of recovery of the endogenous NA content and the storage of [3H]-NA after guanethidine are discussed.     

Does lithium in vitro and ex vivo alter the release of [3H]noradrenaline from brain tissue and the sensitivity of presynaptic autoreceptors?

The effect of lithium on the release of noradrenaline (NA) was investigated in slices of the cerebral cortex and hippocampus from the rat in vitro and ex vivo. In vitro, small concentrations of lithium chloride (1 and 2 mM) failed to alter the electrically stimulated tritiated overflow from slices preincubated with [3H]NA. Larger concentrations of lithium chloride (5 and 10 mM) significantly increased the electrically evoked overflow of [3H]NA by 18-40% as well as the basal 3H efflux. The alpha 2-adrenoceptor agonist clonidine inhibited, whereas the alpha 2-adrenoceptor antagonist rauwolscine facilitated the stimulated overflow of [3H]NA. These effects were attenuated by 10 mM lithium chloride but not by 2 mM. In slices of brain obtained from rats treated for 5 weeks with lithium chloride, the electrically evoked release of [3H]NA, as well as the inhibition of release of [3H]NA, induced by the alpha 2-adrenoceptor agonist clonidine were unaltered. It is concluded that therapeutically relevant concentrations of lithium do not influence the release of NA and that the function of presynaptic alpha 2-autoreceptors is not affected by chronic treatment with lithium. The increase in release of [3H]NA by larger concentrations of lithium may be relevant to its toxic effects.     

Effects of urapidil on catecholamine turnover and release in the central nervous system of the rat

The actions of urapidil, prazosin, idazoxan, and haloperidol on the turnover of noradrenaline in the hypothalamus and dopamine in the nucleus accumbens of the rat were investigated using changes in the ratios of 3-methoxy-4-hydroxyphenyl-glycol/noradrenaline (MHPG/NA) and 3,4-dihydroxyphenylacetic acid/dopamine (DOPAC/DA), respectively, as measures for drug-induced effects. Urapidil (2.5-30 mg kg-1 i.v.) increased the ratios of MHPG/NA and DOPAC/DA. Its effects on NA turnover were maximal at 60 min (160% of control at 30 mg kg-1), and on DA turnover at 30 min (138% of control at 30 mg kg-1). Prazosin (0.5-2.5 mg kg-1 i.v.) had no effect, but the high dose of 5 mg kg-1 i.v. significantly increased the ratio of MHPG/NA in the hypothalamus. Idazoxan (2-50 mg kg-1 i.v.) and haloperidol (0.02-0.5 mg kg-1 i.v.) selectively enhanced turnover of NA and DA, respectively. In experiments on field-stimulated overflow of tritium from slices of hypothalamus and nucleus accumbens labelled with [3H]NA or [3H]DA, respectively, urapidil (1 mumol L-1) facilitated the evoked responses in both regions. Prazosin (0.1 mumol L-1) had no effect in either of the two areas. Idazoxan (0.1 mumol L-1) increased stimulated overflow of [3H]NA from the hypothalamus but not of [3H]DA from the nucleus accumbens. Conversely, haloperidol (0.1 mumol L-1) greatly enhanced evoked overflow of [3H]DA but not of [3H]NA. From the present results it is concluded that urapidil has an antagonistic effect at central alpha 2-adrenoceptors and also a weak antagonistic action at central dopamine D2-receptors.     

Augmented inositol phosphate production in mesenteric arteries from diabetic rats

The effects of noradrenaline (NA) on contraction and phosphoinositide metabolism were compared in mesenteric arteries from rats with chronic streptozotocin-induced diabetes and from age-matched control rats. Maximum contractile responses of mesenteric arteries from diabetic rats to NA (30 μM) were significantly greater than control in both the presence and absence of extracellular Ca²⁺. Basal incorporation of [³H]myoinositol into total [³H]inositol phosphates was greater in diabetic than control mesenteric arteries. NA (30 μM) resulted in a time-dependent increase in total [³H]inositol phosphate production, which was also significantly greater in diabetic than in control preparations. The increase in total [³H]inositol phosphates produced by NA in both control and diabetic arterie was blocked by the α₁-adrenoceptor antagonists, prazosin. Absolute levels of inositol 1,4,5-trisphosphate (I(1,4,5)P₃), measured by protein binding assay, were also increased in response to 30 μM NA in both control and diabetic arteries. Although basal I(1,4,5)P₃ levels were not significantly different, NA-induced increases in I(1,4,5)P₃ were significantly greater in diabetic than in control arteries at each time-point measured. These data indicate that phosphoinositide metabolism is enhanced in mesenteric arteries from rats with chronic streptozotocin-induced diabetes in response to a maximum concentration of NA. Augmented production of the second messengers I(1,4,5)P₃ and presumably, 1,2-diacylgylcerol may contribute to the enhanced maximum contractile responses of the diabetic arteries to NA.     

Altered cardiac adrenergic neurotransmission in streptozotocin-induced diabetic rats.

1. Functional alterations of the sympathetic neuroeffector junction of the left atria were studied in rats with streptozotocin-induced diabetes. 2. Eight to 12 weeks of diabetes resulted in a marked decrease in the positive inotropic response of left atria to electrical field stimulation (EFS). 3. The overflow of [3H]-noradrenaline from diabetic left atria caused by EFS was much less than that from control preparations. 4. The concentration-response curves showed no change in sensitivities of the left atria to exogenous noradrenaline and tyramine in diabetic rats. The maximum positive inotropic response to these agents were similar in diabetic and control animals. 5. The left atrial content of noradrenaline was not significantly changed in diabetic rats. The cocaine-sensitive uptake of [3H]-noradrenaline was also unaltered. 6. Atropine enhanced the positive inotropic response and [3H]-noradrenaline overflow induced by EFS in control left atria. Similarly, yohimbine caused an enhancement of EFS-evoked inotropic response in control atria. However, these effects of the antagonists were not observed in diabetic left atria. 7. It is concluded that the decrease in the positive inotropic response of the left atria to EFS in diabetic rats is caused by an impairment of noradrenaline release from the sympathetic nerve terminals through a calcium-dependent exocytotic mechanism. The present results also indicate that presynaptic alpha 2-adrenoceptors and muscarinic receptors that are linked to inhibition of the noradrenaline release during nerve stimulation may be functionally impaired in diabetic animals.     

Age-dependent modulation of neuropeptide Y on adrenergic transmission of guinea pig vas deferens.

1. The effect of neuropeptide (NPY) on [3H]noradrenaline ([3H]NA) release- and on contractions evoked by field electrical stimulation (FES) was studied in vitro in vas deferens from mature and immature guinea pigs. 2. The evoked tritium overflow (which reflected [3H]NA release) was determined by liquid scintillation spectrometry. 3. Field electrical stimulation of 5 Hz (trains of 50 pulses in 20 sec intervals) evoked guanethidine-sensitive contractions. 4. NPY (0.01-1 microM) dose-dependently inhibited the evoked contractions in both groups of animals. NPY, 1 microM, almost completely inhibited the evoked contractions in mature animals, while those in immature guinea pigs were inhibited but only by 80.4 +/- 3.6%. 5. The amount of tritium overflow evoked by 5 Hz stimulation (300 pulses: 15 trains of 20 pulses in 20 sec intervals) was higher in immature guinea pigs (0.46 +/- 0.03%) compared with the amount of the evoked tritium overflow in mature guinea pigs (0.39 +/- 0.02%). 6. NPY, 1 microM, inhibited the evoked tritium overflow. The NPY inhibition was more pronounced in vas deferens of mature (45.3 +/- 2.0%) than in immature (25.1 +/- 3.5%) guinea pigs. 7. The results suggest that NPY modulation of adrenergic transmission at the prejunctional level increases with the maturity.     

Inhibitory effect of adenosine and adenine nucleotides on potassium-evoked efflux of [3H]-noradrenaline from the rat isolated heart: lack of relationship to prostaglandins.

1 In the isolated heart of the rat prelabelled with [3H]-noradrenaline (NA) and perfused with Krebs solution, administration of potassium (K+ 60 mumol) increased the efflux of total radioactivity and of [3H]-NA in the perfusate. 2 Adenosine triphosphate (ATP), adenosine diphosphate (ADP) and adenosine, but not inosine, dibutyryl cyclic adenosine 3',5'-monophosphate (db cyclic AMP) or db cyclic GMP reduced the K+/-evoked overflow of total radioactivity and of intact [3H]-NA, in concentrations too low to cause release of prostaglandins. ATP, ADP and adenosine did not affect tyramine-evoked overflow of tritium. 3 Blockade of prostaglandin synthesis with indomethacin did not alter the inhibitory effect of either ATP, ADP or adenosine on K+/-induced overflow of tritium, thereby indicating that these nucleotides inhibit adrenergic transmission by a mechanism unrelated to stimulation of prostaglandin synthesis. 4 Theophylline which increases entry of calcium (Ca2+) across the cell membrane and reduces its binding in the cell, enhanced K+/-evoked overflow of tritium and diminished the inhibitory effect of ATP, ADP and adenosine on K+/-evoked overflow of tritium from the heart, presumably by interfering with transneuronal Ca2+ metabolism.     

Clonidine and morphine increase [3H]-noradrenaline overflow in mouse vas deferens.

1. Field stimulation of mouse isolated vas deferens produced a biphasic contraction that consisted of an initial brief non-adrenergic, non-cholinergic (NANC) twitch, followed by a more prolonged noradrenergic component. 2. Field stimulation of vasa, previously loaded with [3H]-noradrenaline ([3H]-NA), increased the amount of radioactivity in the Krebs bathing solution; 77% of this radioactivity was derived from [3H]-NA. 3. Tetrodotoxin (3 x 10(-6) M) abolished the biphasic motor response to field stimulation and the accompanying increased overflow of [3H]-NA. 4. Morphine (10(-7)-10(-5) M) inhibited the initial NANC component but potentiated the secondary noradrenergic component of the motor response to field stimulation. Morphine also increased the field stimulation-induced overflow of radioactivity. Naloxone (10(-6) M) antagonized the effects of morphine on the motor response and also on the overflow of radioactivity. 5. Clonidine (10(-9)-10(-7) M) inhibited the initial NANC component but potentiated the secondary noradrenergic component of the motor response to field stimulation. Clonidine also increased the field stimulation-induced overflow of radioactivity. 6. The ability of morphine (10(-7) M) and of clonidine (10(-9) M) to potentiate the field stimulation-induced overflow of radioactivity persisted in the presence of a combination of tranylcypromine (10(-5) M), desmethylimipramine (10(-5) M) and 17-beta-oestradiol (10(-5) M). 7. The inhibition of the initial NANC component of the motor response to field stimulation produced by morphine and clonidine may be related to the ability of these drugs to potentiate both the secondary noradrenergic component and the overflow of radioactivity, if the NANC transmitter is involved in regulating NA release.     

The inhibitory action of pgf2α on release of [3H]noradrenaline enhanced by α2-adrenoceptor blockade,sodium-pump inhibition and 4-aminopyridine in the main pulmonary artery of the rabbit

Large concentrations of prostaglandin PGF_2α inhibited the stimulation (2 Hz) evoked release of [³H]noradrenaline from the isolated main pulmonary artery of the rabbit (the inhibition caused by 3 × 10^?5 M PGF_2α was 62%). Furthermore, PGF_2α inhibited the release evoked by stimulation when it was enhanced by different procedures. During blockade of presynaptic α₂-adrenoceptors by 3 × 10^?7 M yohimbine, which by itself enhanced the overflow of [³H]NA in response to stimulation, the inhibitory action of PGF_2α was more pronounced (78.2%). In tissue in which the Na⁺-pump was inhibited (K⁺-free treatment) where the overflow of ³H was markedly increased, PGF_2α exerted nearly equal inhibition of transmitter release to that observed in control experiments (64.3%). The inhibitory effect of PGF_2α on the stimulation-evoked release of [³H]NA was less pronounced (32.1%) in the presence of 10^?4M 4-aminopyridine (a blocker of K⁺ -channels).     

Regulation of noradrenaline overflow in rat cerebral cortex by prostaglandin E2.

1 The effects of prostaglandin E2 (PGE2), PGF2 alpha and PGI2 and of inhibitors of prostaglandin synthesis and action, on the K+-evoked [3H]-noradrenaline ([3H]-NA) overflow from rat cerebral cortex slices have been investigated. 2 PGE2 reduced, while indomethacin (a prostaglandin synthesis inhibitor) or SC 19220 (a prostaglandin receptor antagonist) increased, the evoked overflow compared with controls. 3 The inhibition of [3H]-NA overflow by PGE2 was dose-dependently antagonized by SC 19220. 4 The results indicate that PGE2 modulates NA release in rat cerebral cortex in vitro.     

Cannabinoid receptor-mediated inhibition of dopamine release in the retina

The possible occurrence of cannabinoid (CB) receptors was studied on superfused guinea-pig retinal discs preincubated with [3H]dopamine or [³H]noradrenaline. Tritium overflow was evoked either electrically (3 Hz) or by re-introduction of Ca²⁺, 1.3 mM after superfusion with Ca²⁺-free medium containing K⁺ 30 rnM. The accumulation of [³H]dopamine ([³H]DA) and [³H]noradrenaline ([³H]NA) was inhibited by the selective inhibitor of the neuronal dopamine transporter GBR-12909 (pIC_50% 7.29 and 7.41, respectively) but not by the selective inhibitor of the neuronal noradrenaline transporter desipramine (1 M). The electrically or Ca²⁺-evoked tritium overflow in retinal discs preincubated with [³H]DA or [³H]NA was reduced by the CB receptor agonists CP-55,940 and WIN 55,212-2 (pIC_50% in discs preincubated with [³H]NA, electrical stimulation: 7.03 and 6.70, respectively) but not affected by the inactive S(–)enantiomer of the latter, WIN 55,212-3 (up to 10 M). The concentration-response curve of WIN 55,212-2 was shifted to the right by the CB₁ receptor antagonist SR 141716 (apparent pA₂: 8.29) which, by itself, increased the evoked overflow. The facilitatory effect of SR 141716 was not affected by GBR-12909 and the dopamine receptor antagonist haloperidol. In conclusion, the dopaminergic neurones of the guinea-pig retina can be labelled by both [³H]DA and [³H]NA. Transmitter release from the dopaminergic neurones is inhibited by activation of cannabinoid receptors of the CB₁ type, which appear to be tonically activated by an endogenous CB receptor ligand.     

Ovarial steroids and methionine-enkephalin modulation of adrenergic transmission in rabbit oviduct

• 1.1. The effect of methionine-enkephalin on the [³H]-noradrenaline ([³H]-NA) overflow and on contractions evoked by field electrical stimulation (FES) and by exogenous NA were studied in vitro in the isthmic part of oviduct of rabbits (untreated and treated with estradiol or progesterone).
• 2.2. The evoked tritium overflow (which reflected [³H]-NA overflow) was determined by liquid scintillation spectrometry.
• 3.3. Field electrical stimulation of 4 Hz (trains of 40 pulses, 0.3 msec) evoked guanethidine-sensitive contractions.
• 4.4. In all groups of animals methionine-enkephalin dose dependently decreased FES-evoked contractions but not those evoked by exogenous NA.
• 5.5. The amount of tritium overflow evoked by 4 Hz stimulation (600 pulses, 1 msec) was significantly lower in tissues from estradiol treated (1.16 ± 0.19%) compared with those obtained in tissues from untreated (1.82 ± 0.22%) and progesterone treated (2.07 ± 0.21%) rabbits. Methionine-enkephalin, 1 μM, decreased the evoked tritium overflow in the isthmus from untreated rabbits by 36.1 ± 3.6%, in estradiol treated by 22.8 ± 2.9% and in progesterone treated by 52.3 ± 4.5%.
• 6.6. The results suggest that the methionine-enkephaline effect on FES-evoked contractions could be due to a prejunctional effect on the adrenergic terminals and that there is a hormonal dependence of the opioid effect on [³H]-NA overflow.

     

Evidence that different regional sympathetic outflows vary in their sensitivity to the sympathoinhibitory actions of putative 5-HT1A and alpha 2-adrenoceptor agonists in anaesthetized cats.

1. The effects of angiotensin II (AII) and angiotensin III (AIII) on the isolated vas deferens of the guinea-pig were studied via three parameters: the overflow of adenosine 5'-triphosphate (ATP) and tritiated noradrenaline (NA), the mechanical response to field stimulation and the mechanical response to exogenous NA and alpha, beta-methylene ATP (alpha, beta-mATP). 2. At 2 Hz, AII enhanced the overflow of ATP and NA, whereas at 20 Hz, AII enhanced the overflow of NA but was without significant effect on ATP overflow. AIII, at 2 Hz, inhibited the overflow of ATP, but enhanced NA overflow, whereas at 20 Hz ATP overflow was unaffected, but NA overflow was still enhanced. 3. At 2 Hz, AII enhanced both phases of the response to field stimulation and at 20 Hz the overall response. AIII at 2 Hz enhanced the adrenergic response, but was without effect on the purinergic response to field stimulation. At 20 Hz, AIII was without effect on the overall response. 4. AII enhanced responses to exogenous NA and alpha, beta-mATP, whereas AIII was without effect. 5. These results provide evidence that both ATP and NA release are capable of being modulated by angiotensins. Furthermore, modulation of ATP release is frequency-dependent, whereas [3H]-NA release is not. These results raise questions about the mechanisms of storage and release of the sympathetic co-transmitters NA and ATP; they also show that angiotensin receptors in the guinea-pig vas deferens are not a homogeneous population.     

3,4-Diaminopyridine-evoked noradrenaline release in rat hippocampus: role of Na+ entry on Ca2+ pools and of protein kinase C.

Slices of rat hippocampus, preincubated with [3H]noradrenaline [(3H]NA), were superfused continuously and stimulated by addition of 3,4-diaminopyridine (3,4-DAP; 100 microM) for 10 min to the superfusion medium. An overflow of 3H evoked by 3,4-DAP (representing [3H]NA release) was measurable not only in the presence but also in the absence of extracellular Ca2+. Both the protein kinase C (PKC) activator 4 beta-phorbol 12,13-dibutyrate (4 beta-PDB) and the PKC inhibitor polymyxin B, affected mainly the evoked release in the absence of extracellular Ca2+ in a facilitatory or inhibitory manner, respectively. Moreover, in the absence of extracellular Ca2+, both the 3,4-DAP-evoked [3H]NA release and the facilitatory effect of 4 beta-PDB were abolished in the presence of tetrodotoxin or in the absence of Na+ in the superfusion medium. Ruthenium red, a blocker of mitochondrial Ca2+ reuptake, potently increased 3,4-DAP-evoked [3H]NA release in Ca(2+)-free EGTA-containing medium. The facilitatory effects of ruthenium red and 4 beta-PDB were additive. From these and earlier observations we conclude (1) that the mechanism of 3,4-DAP-evoked [3H]NA release involves both Ca2+ influx into the nerve terminals and mobilization of intraneuronal Ca2+ pools. Most probably Ca2+ release from cytoplasmic Ca2+ stores (e.g. endoplasmic reticular pools or mitochondria) is induced by Na+ ions entering the nerve endings during 3,4-DAP-evoked repetitive action potentials. (2) The facilitatory effect of phorbol ester on 3,4-DAP-evoked NA release appears to be mediated not by changes in Ca2+ influx, but by enhancement of intraneuronal events distal to Na+ ion entry and increased intracellular Ca2+ availability.     

Glycine stimulates [3H]noradrenaline release by activating a strychnine-sensitive receptor present in rat hippocampus

Rat hippocampus slices were prelabeled with [3H]noradrenaline ([3H]NA) and depolarized by superfusion with KCl. The release evoked by 12 mM K+ was totally calcium-dependent and more than 90% tetrodotoxin (TTX)-sensitive. Glycine (0.1-1 mM) increased the K(+)-evoked [3H]NA overflow in a concentration-dependent manner. The effect of 1 mM glycine reached 300%. Strychnine (0.3 microM) shifted to the right the concentration-response curve for glycine. The effect of glycine (0.1 or 1 mM) was totally abolished by 3 microM strychnine but was unaffected by the GABAA receptor antagonist, bicuculline (10 microM), or by 100 microM of 1-hydroxy-3-aminopyrrolidone-2 (HA-966), a proposed antagonist of glycine at the strychnine-insensitive site located on the N-methyl-D-aspartate (NMDA) receptor. The effect of glycine was mimicked by L-serine, although less potently; the release of [3H]NA was enhanced by 200% in presence of 3 mM L-serine. At this concentration D-serine was ineffective. Strychnine shifted to the right the concentration-response curve for L-serine. Glycine (1 mM) had only a minor effect (less than 20% potentiation) on the release of [3H]NA evoked by 12 mM KCl in hippocampal synaptosomes. While the effect of glycine in slices was increased by decreasing the depolarizing concentration of K+ (about 500% potentiation at 9 mM K+), the response of synaptosomes remained minimal, even in presence of 9 mM KCl. Hippocampal synaptosomes prelabeled with [3H]glycine released the radiolabeled amino acid when exposed to superfusion with 12 mM KCl. The release of [3H]glycine was more than 75% calcium-dependent. The results suggest that the release of NA in rat hippocampus may be enhanced by glycine through the activation of a strychnine-sensitive receptor. This receptor does not seem to be located on noradrenergic terminals.     

Pentobarbital differentially inhibits N-methyl-d-aspartate and kainate-stimulated [3H]noradrenaline overflow in rat cortical slices

• 1.1. This study examined the ability of pentobarbital to inhibit NMDA and kainate-stimulated [³H]noradrenaline ([³H]NA) overflow in rat brain cortical slices.
• 2.2. Pentobarbital inhibited NMDA-evoked [³H]NA overflow at 100 μM and greater and inhibited kainate-evoked [³H]NA overflow at 10 μM and greater.
• 3.3. The ability of pentobarbital to inhibit concentration-response curves for NMDA and kainate-evoked overflow of [³H]NA were also examined. Pentobarbital (300 μM) caused a 20% reduction in NMDA and a 50% reduction in kainate-induced maximal responses.

     

Differential effect of omega-conotoxin on release of the adrenergic transmitter and the vasoconstrictor response to noradrenaline in the rat isolated kidney.

1. The effects of the Ca2+ channel blockers omega-conotoxin (omega-CgTx), nifedipine and diltiazem, on the increase in tritium overflow and perfusion pressure elicited by renal nerve stimulation (RNS), veratridine (Vt) and potassium chloride (KCl), and on the vasoconstrictor response produced by noradrenaline (NA) were investigated in the isolated kidney of the rat perfused with Tyrode solution and prelabelled with [3H]-noradrenaline ([3H]-NA). 2. RNS (1-4 Hz), Vt (15-90 nmol) and KCl (150-500 mumol) produced renal vasoconstriction and enhanced the tritium overflow in a frequency- and concentration-dependent manner, respectively. 3. Administration of omega-CgTx (50 nM) inhibited RNS-, Vt- and KCl-induced overflow of tritium; the associated renal vasoconstriction produced by RNS or Vt but not by KCl was inhibited. In contrast, omega-CgTx failed to alter the vasoconstrictor response elicited by exogenous NA. 4. Infusion of nifedipine (10 microM) enhanced the tritium overflow elicited by RNS and KCl but not by Vt; a low dose of nifedipine (1.4 microM) enhanced the tritium overflow elicited by all these stimuli. Low doses of diltiazem (6 microM) failed to alter the tritium overflow produced by these stimuli. However, higher doses of diltiazem (60 microM) reduced the tritium overflow elicited by RNS or Vt but enhanced that caused by KCl. The renal vasoconstriction produced by RNS, Vt and KCl as well as by exogenous NA was inhibited by low and high doses of nifedipine and diltiazem.(ABSTRACT TRUNCATED AT 250 WORDS)