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Biomedical subjects

E A Singer

Publications and source records attributed to E A Singer.

At least 55 records · Page 3Linked to original sources
PubMed1990

Increased serotoninergic and noradrenergic activity in hepatic encephalopathy in rats with thioacetamide-induced acute liver failure.

Functional changes of various neurotransmitter systems have been implicated in the pathogenesis of hepatic encephalopathy. In this study the role of brain monoaminergic neurotransmitter systems in hepatic encephalopathy was investigated in rats with thioacetamide-induced acute liver failure. Concentrations of serotonin, dopamine, noradrenaline and of their metabolites 5-hydroxyindoleacetic acid, dihydroxyphenylalanine (following inhibition of dihydroxyphenylalanine-decarboxylase), dihydroxyphenylacetic acid, homovanillic acid and 3-methoxy-4-hydroxyphenyl-glycol, were measured in the cerebral cortex, striatum and hippocampus by high performance liquid chromatography with electrochemical detection. In hepatic encephalopathy concentrations of 5-hydroxyindoleacetic acid were increased in all three brain areas (196%, 204% and 264% of saline-treated controls, p less than 0.01), and concentrations of serotonin were increased in the frontal cortex (121%, p less than 0.01). In the frontal cortex and hippocampus of encephalopathic rats dopamine levels were increased (157% and 289%, p less than 0.05), and levels of noradrenaline (53% and 46%, p less than 0.05) were decreased associated with increased 3-methoxy-4-hydroxyphenylglycol levels (173% and 206%, p less than 0.05). The extent of these changes correlated with the stage of hepatic encephalopathy. In hepatic encephalopathy dihydroxyphenylalanine accumulation was increased in the hippocampus and unchanged in the cerebral cortex. Dopamine, noradrenaline, dihydroxyphenylacetic acid and homovanillic acid concentrations were unchanged in the striatum. The results of this study indicate that hepatic encephalopathy in thioacetamide-induced acute liver failure in rats is associated with neurochemical changes, suggesting an increased activity of the noradrenergic and serotoninergic neurotransmitter systems.

Amino Acids↗
PubMed1990

Serotonin uptake blockers influence serotonin autoreceptors by increasing the biophase concentration of serotonin and not through a "molecular link".

The mechanism of the attenuation, by serotonin uptake blockers, of the release-inhibiting effect of exogenous serotonin autoreceptor agonists was studied in rabbit brain cortex and rat hypothalamus slices. The slices were preincubated with 3H-serotonin and then superfused and stimulated electrically. In rabbit brain slices stimulated by trains of 4 pulses at 100 Hz, 5-carboxamidotryptamine and 5-methoxytryptamine reduced the evoked overflow of tritium, and their concentration-response curves were not changed by any of three serotonin uptake inhibitors, namely citalopram, fluvoxamine and 6-nitroquipazine. In contrast, when the slices were stimulated by trains of 10 pulses at 0.033 Hz, fluvoxamine shifted the concentration-response curve of 5-methoxytryptamine to the right. Experiments with the autoreceptor antagonist metitepine indicated that little, if any, endogenous autoinhibitory tone developed in the course of trains of 4 pulses/100 Hz, irrespective of the absence or presence of uptake inhibitors, as well as during trains of 10 pulses/0.033 Hz in the absence of uptake inhibitors, whereas marked autoinhibition developed when 10 pulses/0.033 Hz were applied in the presence of fluvoxamine. In rat hypothalamic slices stimulated by trains of 4 pulses at 100 Hz, citalopram also failed to change the concentration-response curve of 5-methoxytryptamine. These results indicate that serotonin uptake blockers attenuate the effect of exogenous autoreceptor agonists by an increase in the biophase concentration of released serotonin and, hence, in endogenous autoinhibitory tone, and not by some direct "molecular link" unrelated to the biophase concentration of released serotonin.

5-Methoxytryptamine↗
PubMed1990

Hypotensive effects of 8-hydroxy-2-(di-n-propylamino)tetralin and 5-methylurapidil following stereotaxic microinjection into the ventral medulla of the rat.

1. The effects of the 5-HT1a receptor agonists 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) and 5-methylurapidil (5-MU) on blood pressure and heart rate were investigated in the anaesthetized normotensive rat after local injection into the ventral medulla (medial part of the area of the B1/B3 cell group). 2. Both 8-OH-DPAT and 5-MU, decreased arterial blood pressure and heart rate. Local injection of the alpha 1-adrenoceptor antagonist prazosin did not affect cardiovascular parameters. 3. Subcutaneous injection of the 5-HT1a receptor antagonists spiroxatrine or spiperone inhibited the cardiovascular response to 8-OH-DPAT and 5-MU. 4. Histological examination showed the injection sites which were associated with cardiovascular responses to the agonists to be contained within the medial part of the area of the B1/B3 cell group, corresponding to the region of the nucleus raphe magnus and pallidus. Injections of agonists into adjacent areas had no effect on blood pressure or heart rate. 5. The results support the hypothesis that activation of somatodendritic 5-HT1a autoreceptors located on 5-hydroxytryptaminergic neurones in the ventral medulla of the rat results in a decrease in blood pressure and heart rate. The region of the B1/B3 cell group is suggested as a possible site for the hypotensive action of 5-HT1a receptor agonists.

8-Hydroxy-2-(di-n-propylamino)tetralin↗
PubMed1990

Influence of urapidil and 8-OH-DPAT on brain 5-HT turnover and blood pressure in rats.

The effects of urapidil, of the selective 5-HT1A receptor agonist (+/-)-8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), and of the alpha 2-adrenoceptor agonist clonidine on the in vivo rate of synthesis of 5-hydroxytryptamine (5-HT) were determined in rat brain cortex and hypothalamus. Urapidil (10 mg/kg), 8-OH-DPAT (0.3 mg/kg) or clonidine (0.3 mg/kg; all drugs i.p.) caused significant reductions in 5-HT synthesis rate. Pretreatment with the selective 5-HT1A receptor antagonist spiroxatrine (SPX; 1 mg/kg s.c.) or the nonselective 5-HT1 receptor antagonist metitepine (1 mg/kg i.p.) abolished the effects of urapidil and 8-OH-DPAT, but not of clonidine. The effects of urapidil and 8-OH-DPAT on mean arterial blood pressure (MAP) and heart rate (HR) of pentobarbital-anesthetized, normotensive rats were measured following stereotaxic microinjection into the B1/B3 cell region of the ventral medulla. The mean percentage decreases induced by urapidil (3 micrograms) and 8-OH-DPAT (0.2 micrograms) amounted to (MAP/HR) -13%/-6% and -19%/-25%, respectively. The following pretreatments markedly attenuated or prevented the effects of intramedullary injections of urapidil or 8-OH-DPAT: (a) SPX (1 mg/kg s.c., 60 min): (b) intracisternal injection of the serotonergic neurotoxin 5,7-dihydroxytryptamine (5,7-DHT; 0.2 mg; 7-10 days); (c) bilateral injection of 5,7-DHT at the cervical level of the spinal cord (each side 5 micrograms; 7-10 days). The present results are compatible with an action of urapidil as agonist at central 5-HT1A receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

5,7-Dihydroxytryptamine↗
PubMed1989

Estimation of pA2 values at presynaptic alpha 2-autoreceptors in rabbit and rat brain cortex in the absence of autoinhibition.

An attempt was made to determine pA2 values of antagonists at the presynaptic, release-inhibiting alpha 2-autoreceptors of rabbit and rat brain cortex under conditions when there was very little released noradrenaline in the autoreceptor biophase and, hence, pA2 values were not distorted by endogenous autoinhibition. Cortex slices were preincubated with 3H-noradrenaline and then superfused and stimulated by trains of 4 pulses delivered at 100 Hz or, in a few cases, by trains of 36 pulses at 3 Hz. The alpha-adrenoceptor agonists clonidine, noradrenaline, and alpha-methylnoradrenaline concentration-dependently decreased the stimulation-evoked overflow of tritium. The alpha-adrenoceptor antagonists yohimbine, rauwolscine and idazoxan did not increase the overflow of tritium elicited by 4 pulses/100 Hz in rabbit brain slices and increased it only slightly in rat brain slices. In contrast, the antagonists increased markedly the overflow at 36 pulses/3 Hz. All antagonists caused parallel shifts to the right of the concentration-response curves of clonidine, noradrenaline, and alpha-methylnoradrenaline. pA2 values were calculated either from linear regression of log [agonist concentration ratio - 1] on log [antagonist concentration] or from sigmoid curve fitting. The slopes of the linear regression lines were close to unity, and the pA2 values calculated by the two methods agreed well. There was no consistent preferential antagonism of any antagonist to any agonist. pA2 values determined with stimulation by 4 pulses/100 Hz were by 0.53-0.80 log units higher than those determined with stimulation by 36 pulses/3 Hz.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1989

Evidence for beta 2-adrenoceptor-mediated facilitation of 3H-noradrenaline release from isolated guinea pig papillary muscles.

The effects of beta-adrenoceptor agonists and antagonists on field-stimulated release of radioactivity from superfused guinea pig papillary muscles preincubated with 3H-noradrenaline were studied. Stimulation-evoked overflow of tritium was abolished in the absence of Ca2+ or the presence of tetrodotoxin. Isoprenaline (1 mumol/L) caused a slight facilitation of evoked overflow, whereas phentolamine (1 mumol/L) exerted a strong facilitatory action. However, when phentolamine (1 mumol/L) was present throughout superfusion, isoprenaline and the selective beta 2-adrenoceptor agonist, zinterol, caused concentration-dependent increases (half-maximal effects at 1 nmol/L). The effects of the agonists were inversely related to stimulation frequency. Furthermore, the concentration-response curve of isoprenaline was shifted to the right by the selective beta 2-adrenoceptor antagonist, ICI 118,551, but not by the selective beta 1-adrenoceptor antagonist, ICI 89,406. Schild-plot analysis revealed competitive antagonism and a pA2 value of 9.04 for ICI 118,551. Both ICI 118,551 and ICI 89,406, as well as beta-adrenoceptor antagonists with intrinsic sympathomimetic activity (pindolol and celiprolol; 1 mumol/L), had no effect on stimulation-evoked overflow of tritium (phentolamine present). It is concluded that guinea pig papillary muscles are endowed with prejunctional beta 2 adrenoceptors facilitating impulse-evoked noradrenaline release. The facilitation is markedly promoted by blockade of prejunctional alpha adrenoceptors.

Adrenergic beta-Agonists↗
PubMed1989

Clonidine prevents transient loss of noradrenaline in response to cholinergic hypofunction induced by ethylcholine aziridinium (AF64A).

Intracerebroventricular injection of ethylcholine aziridinium (AF64A) (2 nmol/ventricle) induced a considerable decrease in the level of acetylcholine (ACh) in hippocampus (from 21.14 +/- 0.84 to 10.04 +/- 0.59 pmol/mg of tissue; p less than 0.001) 4 days after application. The reduction of cholinergic function was accompanied by a decrease in the level of noradrenaline (NA) (from 1.96 +/- 0.08 to 1.41 +/- 0.06 pmol/mg of tissue; p less than 0.001). Two days after administration of AF64A (1 or 2 nmol/ventricle), the dose-dependent decrease in NA level was associated with an increase in the level of its major metabolite, 3-methoxy-4-hydroxyphenylglycol (MHPG), resulting in a considerable increase in the MHPG/NA molar ratio (from 0.84 +/- 0.06 to 1.62 +/- 0.17; p less than 0.002). Chronic treatment of AF64A-injected rats with clonidine (0.02-0.2 mg/kg, i.p., every 8-12 h) had no significant effect on the loss of ACh content, whereas the decrease in NA content in hippocampus was completely prevented. Clonidine induced aggressive behavior in the AF64A-treated rats, in contrast to sedation in vehicle-injected rats. The response to clonidine under these experimental conditions and the increased MHPG/NA molar ratio in response to AF64A suggest that the transient loss of NA content following AF64A administration results from increased NA release. The increased noradrenergic activity in hippocampus may be linked to the reduction of tonic inhibitory cholinergic input. These results are discussed in relation to possible implications for senile dementia of the Alzheimer type.

Acetylcholine↗
PubMed1989

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.

Animals↗
PubMed1988

Inhibition of vasopressin-stimulated flank marking behavior by V1-receptor antagonists.

Flank marking, a form of olfactory communication displayed by hamsters, is dependent upon vasopressin-sensitive neurons in the anterior hypothalamus. In the present study two vasopressin type-1 (V1) receptor antagonists, d(CH2)5Tyr(Me)AVP and dPTyr(Me)AVP were tested for their ability to block flank marking stimulated by the microinjection of arginine vasopressin (AVP) into the anterior hypothalamus. Dose-response curves were established for AVP and flank marking in the presence or absence of different concentrations of each antagonist. DPTyr(Me)AVP was microinjected into the anterior hypothalamus 1 h before the microinjection of AVP while d(CH2)5Tyr(Me)AVP and AVP were prepared together and delivered as a single microinjection. This procedure was necessary because dPTyr(Me)AVP, but not d(CH2)5Tyr(Me)AVP, had agonist activity when initially injected into the anterior hypothalamus in concentrations ranging from 0.90-900 microM. The ED50 values (microM) for dPTyr(Me)AVP and AVP were 17.9 and 0.90, respectively. The initial agonist activity of dPTyr(Me)AVP was always followed by blocker activity. Both V1-receptor antagonists caused a dose-dependent decrease in AVP-stimulated flank marking. Maximal inhibition of AVP-stimulated flank marking was produced with approximately 1.0 mM of either antagonist. Both antagonists blocked AVP-stimulated flank marking behavior for over 12 h following their microinjection.

Animal Communication↗
PubMed1988

Only activated but not non-activated presynaptic alpha 2-autoreceptors interfere with neighbouring presynaptic receptor mechanisms.

Experiments were carried out in rabbit cerebrocortical slices in order to find out whether the attenuation by presynaptic alpha 2-autoreceptors of effects mediated by presynaptic opioid kappa- and adenosine A1-receptors requires activation of the alpha 2-receptors. The slices were preincubated with 3H-noradrenaline and then superfused with medium containing desipramine 1 mumol/l. They were stimulated electrically either with single pulses or with trains of 32 pulses at 1 Hz. The overflow of tritium elicited by a single pulse amounted to 0.21% of the tritium content of the tissue. It was Ca2+-dependent and tetrodotoxin-sensitive and not changed by rauwolscine 1 mumol/l or yohimbine 0.3 mumol/l. Ethylketocyclazocine (EK; 0.1-10 nmol/l) and R-(-)-N6-phenylisopropyladenosine (PIA; 1-1,000 nmol/l) potently inhibited the overflow evoked by a single pulse, and their effects were not changed by yohimbine. - The overflow of tritium elicited by trains of 32 pulses at 1 Hz amounted to 0.92% of the tritium content of the tissue and was increased approximately fourfold by yohimbine 0.3 mumol/l. EK and PIA were less potent inhibitors than in the one pulse experiments. Yohimbine greatly enhanced the effects of EK and PIA. The enhancement was even more pronounced when the Ca2+ concentration in the medium was reduced in order to obtain a control tritium overflow similar to that evoked by 32 pulses in the absence of yohimbine. The results demonstrate that there is no alpha 2-adrenergic autoinhibition when noradrenaline release is elicited by a single pulse. Under these conditions, the non-activated presynaptic alpha 2-adrenoceptor does not interfere with presynaptic opioid kappa- and adenosine A1-receptor mechanisms.(ABSTRACT TRUNCATED AT 250 WORDS)

Analgesics, Opioid↗
PubMed1988

Evidence against a functional link between noradrenaline uptake mechanisms and presynaptic alpha-2 adrenoceptors.

Slices prepared from rat cerebral cortex were labelled with 3H-noradrenaline and superfused. Electrical field stimulation was carried out 15 min (S1) and 45 min (S2) after the start of collection of 5-min samples using 4 pulses delivered at 100 Hz. Drugs acting at alpha 2-adrenoceptors were added 20 min before S2, and their effects were evaluated using the S2/S1-ratio. The alpha 2-adrenoceptor antagonists idazoxan (1 mumol/l) and rauwolscine (1 mumol/l) failed to increase stimulation-evoked overflow of radioactivity in the absence or presence of the noradrenaline reuptake inhibitor desipramine (1 mumol/l). This indicates that the duration of electrical stimulation was too short to allow development of alpha 2-adrenoceptor-mediated autoinhibition by released noradrenaline. The effect of clonidine (3-1000 nmol/l) on stimulation-evoked overflow of radioactivity was tested in the absence and presence of three different reuptake inhibitors (desipramine, 1 mumol/l; maprotiline, 1 mumol/l; cocaine, 10 mumol/l). The analysis yielded identical concentration-response curves under all conditions. These results argue against an action of inhibitors of neuronal reuptake of noradrenaline at the presynaptic alpha 2-adrenoceptor and against the concept of a functional link between uptake site and receptor.

Animals↗
PubMed1988

Presynaptic autoinhibition of central noradrenaline release in vitro: operational characteristics and effects of drugs acting at alpha-2 adrenoceptors in the presence of uptake inhibition.

Functional characteristics of autoinhibition of central noradrenaline release were studied in the presence of uptake inhibition. Slices of rat cerebral cortex were incubated with [3H]noradrenaline, superfused and field-stimulated with 1 to 16 monophasic rectangular pulses at frequencies of 0.02 to 40 Hz. 1) Substances acting at presynaptic alpha-2 adrenoceptors were identified as antagonists, agonists or partial agonists by comparing their effects on 3H-overflow evoked by a single pulse or by two consecutive pulses at 1 Hz. 2) When 1 to 16 pulses were delivered at 0.02, 0.08, 0.3 and 1 Hz to stimulate outflow of tritium, a frequency-dependent suppression of responses to the second and the following pulses was observed. In the presence of the alpha-2 adrenoceptor antagonist idazoxan (10(-6) M), comparable amounts of tritium were released by the first stimulus and each of the following stimuli at 0.02 Hz. In contrast, at 0.08, 0.3 and 1 Hz the amount of 3H-overflow evoked by the first pulse was not reached in response to the following pulses. Clonidine (10(-6) M) diminished markedly the response to the first as well as to the following stimuli, irrespective of the frequency of stimulation. 3) Using two consecutive pulses delivered with decreasing pulse intervals, an apparent reduction or complete abolition of autoinhibition was observed at intervals of less than 100 msec, indicated by reduction or loss of the facilitatory effects of alpha-2 adrenoceptor antagonists. The present results provide detailed insights in operational characteristics of alpha-2 adrenoceptor-mediated autoinhibition and the effects of drugs on this regulatory mechism.

Animals↗
PubMed1987

B-HT 920 and B-HT 958: presynaptic effects on electrically evoked 3H-dopamine release from slices of rat nucleus accumbens.

The effects of two thiazoloazepine derivatives, B-HT 920 (6-allyl-2-amino-5,6,7,8-tetrahydro-4H-thiazolo[4,5-d]azepine) and B-HT 958 (2-amino-6-(p-chloro-benzyl)-4H-5,6,7,8-tetrahydrothiazolo[5,4-d]a zepine) on electrically evoked overflow of 3H-dopamine were studied. Slices from nucleus accumbens of the rat were preincubated with 3H-dopamine and superfused at 23 degrees C or 37 degrees C. Electrical field stimulation was applied using frequencies of 0.5 or 5 Hz. At 37 degrees C/5 Hz, B-HT 920 markedly and dose-dependently (0.01-0.1 mumol/l) reduced the stimulation evoked overflow of tritium. Its dose-response curve was shifted to the right at 23 degrees C/0.5 Hz and 23 degrees C/5 Hz, respectively. A similar result was obtained with the dopamine receptor agonist, apomorphine (1 mumol/l). B-HT 958 (0.1-10 mumol/l) also reduced electrically induced overflow of tritium at 37 degrees C/5 Hz, had no effect at 23 degrees C/0.5 Hz, and facilitated tritium overflow at 23 degrees C/5 Hz. Sulpiride (10 mumol/l) completely prevented the effects of B-HT 920 (1 mumol/l) or B-HT 958 (1 mumol/l) at 37 degrees C/5 Hz, whereas phentolamine (1 mumol/l) had no effect on the actions of the two drugs under these experimental conditions. From the patterns of effects obtained under the different experimental conditions it is concluded that B-HT 920 acts as full agonist at presynaptic dopamine autoreceptors whereas B-HT 958 acts as partial agonist.

Adrenergic alpha-Agonists↗
PubMed1987

B-HT 920, B-HT 933, and B-HT 958: presynaptic effects on electrically evoked 3H-noradrenaline release from slices of rat brain cortex and hypothalamus.

The effects of the three azepine compounds, B-HT 920 (6-allyl-2-amino-5,6,7,8-tetrahydro-4H-thiazolo-[4,5-d]azepine), B-HT 933 [2-amino-6-ethyl-5,6,7,8-tetrahydro-4H-oxazolo[4,5-d]azepine; azepexole] and B-HT 958 (2-amino-6-(p-chloro-benzyl)-4H-5,6,7,8-tetrahydrothiazolo[5,4-d]a zepine) on electrically evoked overflow of 3H-noradrenaline were studied. Slices from parietal cortex (Cx) or nucleus anterior hypothalami (nah) were incubated with 3H-noradrenaline, superfused at 23 degrees C or 37 degrees C in the presence of the noradrenaline uptake inhibitor desipramine (3 mumol/l) and field stimulated at frequencies of 0.3 or 3 Hz. At 37 degrees C/0.3 Hz, B-HT 920 and B-HT 933 concentration-dependently decreased the evoked overflow of tritium in both regions studied, whereas B-HT 958 had no effect. In a second set of experiments each drug was tested under three additional experimental conditions, i.e. 37 degrees C/3 Hz, 23 degrees C/0.3 Hz and 23 degrees C/3 Hz. Increasing the stimulation frequency to 3 Hz or lowering the superfusion temperature to 23 degrees C reduced the effects of B-HT 920 (1 mumol/l) and B-HT 933 (10 mumol/l) as compared to the effects at 37 degrees C/0.3 Hz. When tested at 23 degrees C/3 Hz, both drugs did not significantly affect the evoked overflow of tritium in the Cx or the nah. In contrast, B-HT 958 (10 mumol/l), caused a facilitation of evoked overflow in both regions when the higher stimulation frequency or the lower superfusion temperature was used. Its release-enhancing action was most pronounced at 23 degrees C/3 Hz.(ABSTRACT TRUNCATED AT 250 WORDS)

Adrenergic alpha-Agonists↗
PubMed1986

Generation of immunoreactive neurotensin(s) and enkephalin(s) by pepsin-treatment of plasma.

Treatment of mammalian plasmas with pepsin yielded extraordinary quantities of immunoreactive neurotensin (iNT) and methionine5-enkephalin (iENK). The concentrations measured after pepsin-treatment (iNT, 1-5 microM and iENK, 0.1-0.5 microM) were 1-100 thousand times the normal circulating levels of these peptides. The reactions were shown to be time, temperature and pH dependent and to involve the action of pepsin on albumin-like proteins (Mr, ca, 65,000). Pepsin-generated iNT from rat plasma differed from NT since it reacted only with C-terminal directed antisera and eluted earlier than NT during HPLC on mu-Bondapak C-18. Partially purified iNT was active in two bioassays for NT, one which senses changes in vascular permeability to protein after intradermal injection into rats and another which measures release of histamine from isolated rat mast cells. Other biologic activities generated by pepsin-treating plasma included effects on systemic blood pressure in rats and on the contractility of the isolated guinea pig ileum. Some of these, however, were attributable to the formation of angiotensin- and bradykinin-related peptides. Pepsin-generated iENK gave three major peaks during HPLC, one of which (ca, 25%) co-eluted with oxidized ENK and also registered in a radioreceptor assay for opiate-related substances. In addition, this material produced ENK-like effects on the isolated guinea pig ileum and on vascular permeability in rat skin. The precursor-like protein(s) for iENK were distinguished from adrenal proenkephalins since it did not liberate iENK upon digestion with trypsin and carboxypeptidase B. Since pepsin can mimic renin these results suggest the existence of systems in blood (analogous to the renin/angiotensin system) for the generation of biologically active NT- and ENK-related peptides and they also raise the question as to whether other neuropeptides might be found circulating in precursor form(s).

Amino Acid Sequence↗
PubMed1986

Effects of clonidine on the stimulation-evoked release of 3H-noradrenaline from superfused rat brain slices as a function of the biophase concentration. Temperature dependent widening of extracellular space.

The influence of clonidine on the stimulation-evoked overflow of tritium was studied in brain slices preincubated with 3H-noradrenaline. The slices were prepared from parietal cortex (Cx), nucleus anterior hypothalami (nah) and nucleus tractus solitarii (nts). After preincubation, the tissues were superfused at 23 degrees C or 37 degrees C with a medium containing the noradrenaline uptake inhibitor desipramine. Electrical field stimulation was applied using stimulation frequencies of 0.3-10 Hz. At 23 degrees C/0.3 Hz, clonidine concentration-dependently inhibited the evoked overflow of tritium in all three brain regions. In contrast, at 23 degrees C/3 Hz the inhibitory effect of the drug in the Cx was abolished and a facilitation was observed in the nah and nts. When tested at increasing frequencies of stimulation in the nts at 23 degrees C, clonidine exerted a dual action, characterized by a reduction of electrically evoked responses at frequencies below 1 Hz and a facilitation at frequencies above 1 Hz. At 37 degrees C, clonidine concentration-dependently decreased the evoked overflow in all brain regions studied, this effect being more pronounced at 0.3 Hz than at 3 Hz. The apparent lack of an effect of clonidine on the stimulation-evoked overflow of tritium in the Cx at 23 degrees C/3 Hz was turned to a facilitation when noradrenaline (0.01 mumol/l) was included in the superfusion medium. Conversely, an inhibitory effect of clonidine was seen when the uptake blocker desipramine (as well as noradrenaline) was omitted from the superfusion medium.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗