Relation between the amount of smooth muscle of venous tissue and the degree of supersensitivity to isoprenaline caused by inhibition of catechol-O-methyl transferase

Summary The relation between the smooth muscle cell mass of dog saphenous vein strips and the degree of supersensitivity to isoprenaline caused by U-0521 (3,4-dihydroxy-2-methyl propiophenone), an inhibitor of the catechol-O-methyl transferase (COMT), was studied. For the quantitative determination of smooth muscle mass, the thickness of the muscle layer as determined by light microscopy and the maximal shortening induced by supramaximal concentration of phenylephrine were used.After the strips had been contracted by 3×10^–6 M phenylephrine, a concentration which was able to produce an about 90% maximal contraction, dose-response curves to the relaxant effect of isoprenaline were determined in the absence and in the presence of U-0521 (10^–4 M).It was observed that U-0521 caused marked supersensitivity to the relaxant effect of isoprenaline (varying between 3 and 81 times), as well as an increase of the maximal relaxation caused by this amine (varying between 7 and 120%). The correlation between these data and the smooth muscle cell mass shows that there was a direct proportionality between these parameters.Oxytetracycline (10^–4 M), an inhibitor of binding of catecholamines to collagen, did not produce any enhancement of the effects of isoprenaline.It is concluded that COMT is related to smooth muscle cells in this tissue.This study was supported by a grant from Instituto de Alta Cultura (PMC-2).Preliminary results of this study were presented to the Joint Meeting of German, Hungarian, Portuguese and Yugoslav Pharmacological Societies (Graz, 2–5 September, 1974).     

The role played by the extraneuronal system in the disposition of noradrenaline and adrenaline in vessels

Summary The role played by extraneuronal sites in the disposition of noradrenaline and adrenaline was studied in the saphenous vein and in the mesenteric artery of the dog, taking as parameters the influence of blockade of these sites on the sensitivity to and on the time for half-relaxation (t ₅₀) (both in oil and in Krebs solution) of these agonists. Preliminary experiments have shown that the t ₅₀ values are not significantly changed by the changes in the height of the contraction provided the contraction is caused by the same concentration of the agonist.The results obtained permit us to conclude that in both vessels the removal of amines depends on the concentrations used. In low (0.023 and 0.23 M) or in moderately high (2.3 M) concentrations, adrenaline is removed preferentially by extraneuronal sites, whereas noradrenaline preferred neuronal sites.The selectivity of adrenaline for extraneuronal sites was present for such low concentrations that a possible physiological role of these sites in the inactivation of circulating adrenaline must be considered.The results obtained by studying the relaxation in oil or in Krebs solution and by using cortexone (60 M) or U-0521 (dihydroxy-2-methyl propiophenone; 0.1 mM) support the view that, at least in the vein, adrenaline may accumulate in extraneuronal cells and diffuse back into the biophase during the relaxation, thereby slowing the latter.Both in the veins and in the arteries noradrenaline was inactivated more rapidly than adrenaline. The difference in the rate of inactivation of these amines, already observed in controls (when all inactivation pathways are operative) became more marked when both neuronal and extraneuronal sites were blocked. The existence of an important pathway not blocked by cocaine + cortexone + iproniazid which may preferentially inactivate noradrenaline cannot be ruled out.This study was supported by a grant from the Instituto de Alta Cultura (PMC-2)Preliminary results were presented at the VI International Congress of Pharmacology (Helsinki, July 1975)     

Two different biophases for adrenaline released by electrical stimulation or tyramine from the sympathetic nerve endings of the dog saphenous vein

Summary To study the distribution of -and -adrenoceptors dog saphenous vein strips were electrically stimulated (2 ms, 30 V, 0.25–20 Hz). The strips either had spontaneous tone (contraction experiments) or were contracted by 0.28 M prostaglandin F2 in the presence of 7 M phentolamine (relaxation experiments). In strips without preloading or in strips preloaded with (–)-noradrenaline -adrenoceptor-mediated excitatory responses were readily evoked (contraction experiments) but not -adrenoceptor-mediated inhibitory responses (relaxation experiments). In strips preloaded with (–)-adrenaline both -(contraction experiments) and -effects (relaxation experiments) were readily elicited by electrical stimulation and by tyramine. Thus, strips preloaded with (–)-adrenaline were used to compare -with -effects. In these strips the latency between the beginning of the electrical stimulation and the onset of the response was longer for -than for -responses. The same applies to responses to exogenous (–)-adrenaline. However, the ratio latency for -/latency for -response was 3.6±0.2 (n=8) for responses to electrical stimulation and 1.8±0.1 (n=12) for responses to (–)-adrenaline (P<0.001).Cocaine (12M) enhanced the -effect elicited by electrical stimulation 2.8±0.2 (n=7) times but did not change the -effect, whereas U-0521 (50 M) enhanced the -effect 3.4±0.2 (n=8) times without changing the -effect.In strips preloaded with (–)-adrenaline also tyramine caused concentration-dependent -responses (relaxation experiments).The concentration of phentolamine and prazosin required to inhibit contractions caused by electrical stimulation were about 5–7 times higher than those required to inhibit contractions caused by exogenous adrenaline or noradrenaline, whereas propranolol was equipotent in reducing -responses to adrenaline released by electrical stimulation and to exogenous adrenaline. Our results strongly support the view that -adrenoceptors are in close contact with the nerve endings and -adrenoceptors are in close proximity of COMT in a vessel with the nerve endings evenly distributed throughout the media.     

Further study of the adrenoceptors of the saphenous vein of the dog: Influence of factors which interfere with the concentrations of agonists at the receptor level

In the present study the affinities of some sympathomimetic amines for α- and β-adrenoceptors of the dog saphenous vein tissue were determined after all known factors interfering with the concentration of these agonists at the receptor level had been assessed and excluded. It was observed that in control experiments the relative potencies of sympathomimetic agonists for inducing contractions were: adrenaline (1.6) > noradrenaline (1.0) > phenylephrine (0.38) > isoprenaline (0.009).The elimination of neuronal uptake by cocaine, 4 × 10^?6M, enhanced predominantly the effects of noradrenaline (by a factor of 7.5), whereas block of catechol-O-methyl transferase (COMT) by U-0521, 10^?4 M, only enhanced those of adrenaline (by a factor of 2.6) and block of β-adrenoceptors by propranolol, 5 × 10^?7 M, enhanced those of isoprenaline (by a factor of 3) and adrenaline (by a factor of 1.8).Block of COMT enhanced the effects of adrenaline approximately as much as did the blockade of neuronal uptake; this seems to indicate that the affinity of adrenaline for extraneuronal and neuronal uptake processes is approximately the same.Regarding the relaxation-inducing capacity of sympathomimetic agents it was observed that isoprenaline, adrenaline and noradrenaline are full agonists, whereas phenylephrine was not able to produce relaxation amounting to more than 5% of the maximum. Denervation did not modify the relaxant effects of isoprenaline. After elimination of all known factors interfering with the concentration of the sympathomimetic agonists in the biophase, the ratios between the ED₅₀'s of each amine for α- and β-adrenoceptors were: adrenaline = 34, noradrenaline = 109 and isoprenaline = 0.0041.     

The saturability of a site of loss and the degree of supersensitivity to agonists which are substrates of this site of loss

Summary The present investigation was undertaken to test the hypothesis that the experimentally determined degree of supersensitivity to an agonist caused by inhibition of a site of loss depends on the ratio K _m of the site of loss/ED₅₀ of the agonist.The influence of inhibition of neuronal uptake by cocaine on the -adrenoceptor-mediated effect of noradrenaline was studied on the dog saphenous vein; the influence of inhibition of COMT by U-0521 on the -adrenoceptor-mediated effect of isoprenaline was studied on the dog saphenous vein and on the guinea-pig trachea; the influence of inhibition of acetylcholinesterase by physostigmine on the effect of acetylcholine was studied on the guinea-pig ileum. To further extend the range of values of the ratio K _m/ED₅₀, several concentrations of phentolamine, propranolol or atropine were used to increase the ED₅₀ of the respective agonist. It was thus possible to determine the degree of supersensitivity caused by inhibition of a site of loss over a range of K _m/ED₅₀ values of 0.02 to 2,307.In seven situations the ratio K _m/ED₅₀ was higher than 10. In all of these cases the supersensitivity caused by inhibition of the site of loss was maximal. In eleven situations the ratio K _m/ED₅₀ was less than 10 and higher than 0.1 and the supersensitivity obtained was sub-maximal but was closer to the maximum, the closer the ratio was to 10. In two situations the ratio was less than 0.1 and no supersensitivity was obtained.The results confirm the hypothesis.     

The actions of 5-hydroxytryptamine receptor agonists and antagonists at pre- and postjunctional level on the canine saphenous vein

Summary The prejunctional and postjunctional 5-HT receptors of the canine saphenous vein were studied. The release of ³H-noradrenaline (³H-NA) from incubated saphenous vein strips was inhibited by 5-hydroxytryptamine (5-HT) in a concentration-dependent way (5-HT concentrations: 0.01, 0.1 and 1.0 mol · l^–1), but not by the selective 5-HT_1A agonist 8-hydroxy-dipropylaminotetralin (8-OH-DPAT; 1 and 10 mol · l^–1). The inhibitory effect of 5-HT was antagonized by metitepine and methysergide, but not by yohimbine, (–)-pindolol or ketanserin. In strips preincubated with 5-HT (1.2 mol · l^–1), the fractional release of ³H-NA was slightly reduced (paired experiments). 5-HT and 8-OH-DPAT caused concentration-dependent contractions of the saphenous smooth muscle. A parallel shift of the concentration-response curve for 8-OH-DPAT to the right was caused by metitepine and yohimbine, but not by ketanserin. The contractions caused by 5-HT were antagonized by metitepine and yohimbine (parallel displacement of the curves to the right), as well as by ketanserin and methysergide (with a depression of the upper part of the curve). Blockade of -adrenoceptors (due to prazosin plus a low concentration of yohimbine) also resulted in a weak antagonistic effect. Ketanserin and metitepine displaced the noradrenaline concentration-response curve to the right. We conclude that the saphenous vein of the dog is endowed with prejunctional receptors of the 5-HT₁ type which can not be classified as belonging either to the 1 A or 1 B subtype; and that at the postjunctional level 5-HT₁ (possibly of the 1D subtype) and 5-HT₂ receptors are present. 5-HT is able to activate all these receptors.Supported by INIC (Centro de Farmacologia e Biopatologia Química da Universidade do Porto) Send offprint requests to M. Q. Paiva at the above address     

泡沫硬化剂注射与静脉缝扎在大隐静脉曲张手术中的效果比较

目的：本文探讨在大隐静脉曲张手术中处理分支的曲张静脉时,静脉缝扎与泡沫硬化剂注射两种方法的效果差别。方法本研究采用前瞻式随机对照分组研究方法,收集本院普外一科在2008年1月~2010年12月共计3年间完成的大隐静脉曲张手术150例患者（患肢205条）的临床资料进行分析,比较手术时间、住院费用、止痛药用量、并发症及随访1年、2年、3年患者的复发率。结果使用泡沫硬化剂组手术时间,止痛药明显少于静脉缝扎组。住院费用、并发症及复发率两者差异无统计学意义。结论大隐静脉曲张手术中处理小腿的分支静脉时泡沫硬化剂注射术较分支静脉缝扎有明显优势。     

The effects of drugs and denervation on removal and accumulation of noradrenaline in the perfused hind-limb of the dog

Summary The removal of noradrenaline by the autoperfused hind-limb of dogs anaesthetized with pentobarbital, as well as the accumulation of noradrenaline in the saphenous vein were studied. Sensitivity of the perfused vascular area was determined by the response of the perfusion pressure to infusions of noradrenaline.The removal of noradrenaline declined very slowly during infusions lasting for up to 2 h, but edema of the perfused limb occurred after 45 to 60 min; therefore, the duration of infusion was limited to 30 min. During this period, noradrenaline was infused in rates increasing by a factor of 2 and ranging from 0.5 to 16 g/kg per minute. Accumulation capacity was saturated at 1 g/kg · min^–1, but the amount removed increased until a four-to eightfold rate was reached and then levelled off.At a rate of 1 g/kg · min^–1 the influence of drugs and of surgical denervation was investigated in other experimental series. Cocaine, nialamide, phenoxybenzamine and pretreatment with reserpine reduced removal (by 50, 45, 40 and 35%, respectively). Cortexone had no detectable influence on removal with this rate of infusion, but blocked it effectively when 4 g/kg · min^–1 were infused. Accumulation of noradrenaline in the vein was prevented by cocaine or reserpine, slightly reduced by phenoxybenzamine and enhanced by nialamide. The effects of nialamide plus cocaine did not differ significantly from those of cocaine alone, but cortexone plus cocaine completely blocked removal and accumulation. Surgical denervation reduced removal by about 70% and abolished accumulation; reserpine plus nialamide had similar effects. In the case of nialamide, removal progressively diminished during the infusion period and this time dependence of effects was accompanied by a prolongation of noradrenaline washout.Cocaine, reserpine and denervation caused supersensitivity of the perfused vessels to noradrenaline, whereas nialamide and cortexone had no such effect and phenoxybenzamine caused subsensitivity.The pronounced ability of the perfused vessels of the hind-limb to remove noradrenaline from the circulating blood is attributed primarily to neuronal uptake and intraneuronal oxidative deamination; extraneuronal uptake and inactivation seem to play an important role when neuronal mechanisms are saturated (infusion of higher noradrenaline doses) or impaired (after cocaine or denervation).Supported by Instituto de Alta Cultura (Research Project PMC/2). Part of this work was presented at the Fifth International Congress on Pharmacology (S.Francisco, July 23–28, 1972).     

The part played by catechol-O-methyltransferase in the plasma kinetics of 3,4-dihydroxyphenylglycol and 3,4-dihydroxyphenylalanine in the anaesthetized rabbit

Summary The present study, carried out in anaesthetized rabbits, aimed at determining the effects of catechol-O-methytransferase (COMT) inhibition on the plasma kinetics of infused 3,4-dihydroxyphenylglycol (DOPEG) and 3,4-dihydroxyphenylalanine (DOPA) as well as on endogenous plasma noradrenaline, DOPEG, DOPA and 3-methoxy-4-hydroxyphenylglycol (MOPEG). The plasma kinetics of infused MOPEG were also evaluated. To block the function of COMT, 3,4-dihydroxy-4-methyl-5-nitrobenzophenone (Ro 40-7592) was given intravenously. Dose-finding, experiments, in which the drug-induced fall in endogenous plasma MOPEG was used to quantify COMT inhibition, indicated that a Ro 40-7592 dose of 3 mg/kg followed by 1.5 mg/kg every 30 min was sufficient to obtain a virtually complete inhibition of COMT.More than 150 min of COMT inhibition were required for endogenous MOPEG to disappear from plasma, since the plasma half-life of MOPEG was 54 min. COMT inhibition produced marked increases in the plasma levels of endogenous DOPA (1.7-fold) and DOPEG (3.9-fold) and did not alter endogenous plasma noradrenaline. The results concerning the effect of COMT inhibition on the plasma kinetics of infused DOPA and DOPEG were as follows: the plasma clearance of DOPA was not altered, whereas that of DOPEG fell by 41%; the plasma half-life of DOPA increased from 4.9 to 13.0 min and that of DOPEG from 4.8 to 31.0 min; there was an increase in the volume of distribution of DOPA (2 to 3-fold) and DOPEG (4 to 5-fold).Hence, COMT inhibition was much more effective in increasing endogenous plasma DOPA and DOPEG than in increasing the plasma concentrations of infused DOPA and DOPEG, suggesting that endogenously formed DOPA and DOPEG are more extensively metabolized by COMT than infused DOPA and DOPEG. Moreover, as the increase in the plasma half-lives of DOPA and DOPEG induced by COMT inhibition was mainly due to an increase in the volume of distribution, it can be concluded that the action of COMT limits the distribution of infused DOPA and DOPEG within the body.This study was supported by the Deutsche Forschungsgemeinschaft (GR 490/5). A preliminary account of the results was presented to the German Society for Pharmacology and Toxicology (Friedgen 1992)Correspondence to K.-H. Graefe at the above address     

The contribution by monoamine oxidase and catechol-O-methyltransferase to the total-body and pulmonary plasma clearance of catecholamines

To study the effects of inhibition of catechol-O-methyltransferase (COMT) and monoamine oxidase (MAO) on the removal of circulating catecholamines, anaesthetized rabbits were infused for 120 min with ³H-labelled noradrenaline, adrenaline and dopamine. Total-body plasma clearances (Cl_tot) and pulmonary fractional extractions (ER_P) of the infused amines and the cardiac output of plasma (CO_P) were determined under steady-state conditions at the end of each of two consecutive 60-min treatment periods. MAO and COMT were inhibited by treatment with pargyline (40 mg/kg)and tolcapone (3 mg/kg followed by 1.5 mg/kg given every 30 min), respectively. Two groups of animals were studied. Group I involved animals treated with tolcapone throughout and given pargyline at the beginning of the second treatment period. In group II, pargyline was given at the beginning of the first, and the treatment with tolcapone was started at the beginning of the second treatment period. As previous experiments had shown that COMT inhibition alone is without any effect on Cl_tot, of the three catecholamines considered here, the results obtained in the first treatment period of group I can be taken to reflect control results.At the end of the first treatment period, Cl_tot of noradrenaline, adrenaline and dopamine (expressed as a percentage of CO_P) was 88%, 85% and 142%, respectively, in group I (COMT inhibition) and 67%, 77% and 115%, respectively, in group II (MAO inhibition; P < 0.05 for the group difference regarding Cl_tot of noradrenaline and dopamine). MAO inhibition on top of COMT inhibition (group I) lowered Cl_tot of noradrenaline, adrenaline and dopamine by 23%, 12% and 26%, respectively, and COMT inhibition on top of MAO inhibition (group II) reduced Cl_tot of these catecholamines by 13%, 20% and 17%, respectively. At the end of the first treatment period, the pulmonary plasma clearance (Cl_p = ER_P x CO_P) of noradrenaline and dopamine was 13 and 25 ml kg^–1 min ^–, respectively, in group I and 12 and 28 ml kg^–1 min^–, respectively, in group II. Cl_P of adrenaline did not differ from zero in either group. Cl_P of noradrenaline and dopamine was reduced by 74% and 70%, respectively, when both enzymes were inhibited in group I and by 70% and 67%, respectively, when both enzymes were inhibited in group II.Hence, inhibition of either MAO or COMT alone had little, if any, effect on the removal of noradrenaline, adrenaline and dopamine on passage through the systemic and pulmonary circulation. Combined inhibition of both MAO and COMT was highly effective in reducing the pulmonary clearance of noradrenaline and dopamine, but produced only minor decreases in the total-body clearance of all three catecholamines.     

Simultaneous measurement of [3H]noradrenaline release and neurogenic contraction under identical conditions, to determine the prejunctional inhibitory effects of SKF 99101H and BRL 56905 in dog saphenous vein

Using a tissue bath system which allowed the simultaneous measurement of electrically-induced [³H]nor-adrenaline release and neurogenic contraction under identical conditions, we investigated the prejunctional inhibitory activity of the selective 5-HT_1D/1B receptor agonists BRL 56905 ((±)-3-amino-6-carboxamido-1,2,3,4-tetrahydrocarbazole) and SKF 99101H (3-(2-dimethylaminoethyl)-4-chloro-5-propoxyindole hemifumarate), compared to sumatriptan and 5-HT. Transmural electrical stimulation (2 Hz) of dog saphenous vein induced consistent increases in [³H]noradrena- line release as well as reproducible contractile responses (<10% decrease over four stimulation periods). BRL 56905, SKF 99101H, sumatriptan and 5-HT (60 nM – 6 μM) inhibited electrically-evoked [³H]noradrenaline release and neurogenic contractile responses in dog saphenous vein. However, despite being measured under identical conditions, the inhibition of [³H]noradrenaline release was consistently greater than the inhibition of neurogenic contraction induced by a particular concentration of agonist, suggesting that neurogenic contractile responses in dog saphenous vein result from the combined release of noradrenaline and other non-noradrenergic neurotransmitters. Under the present assay conditions, since the agonists produced only small (BRL 56905, sumatriptan and 5-HT) or marginal (SKF 99101H) contractile responses, it is unlikely that this is the cause of the discrepancy observed between inhibition of release and inhibition of contraction. The inhibitory effects of BRL 56905, sumatriptan and 5-HT were blocked by the 5-HT_1D/1B receptor antagonist methiothepin, consistent with the involvement of canine ca-5-HT_1D/1B receptors in inhibiting neurotransmitter release and subsequent smooth muscle contraction in dog saphenous vein. The present results show that the novel 5-HT_1D/1B receptor agonists BRL 56905 and SKF 99101H are at least as potent as sumatriptan and 5-HT, at activating prejunctional inhibitory ca-5-HT_1D/1B heteroreceptors on sympathetic axon terminals in dog saphenous vein. In addition, when measured simultaneously in the same tissue preparation, [³H]noradrenaline release was inhibited to a much greater extent than neurogenic contraction by any particular agonist. Received: 7 October 1996 / Accepted: 23 December 1996     

Comparison of the kinetic characteristics of inhibitory effects exerted by biguanides and H2-blockers on human and rat organic cation transporter-mediated transport: Insight into the development of drug candidates

In this study, the comparison of the transport of substrates (1-methyl-4-phenylpydinium (MPP) and tetraethyl ammonium (TEA)) and the inhibition potency of the inhibitors (biguanides and H₂-blockers) for human and rat organic cation transporters (hOCTs and rOcts), and the inhibition type of inhibitors for these transporters were investigated using HEK293 cells that stably express hOCT/rOct. The concentration-dependent uptake of [³H]-MPP and [¹⁴C]-TEA by hOCT1-3/rOct1-3 had K_m values similar to those in the literature. It was also deduced that MPP and TEA are competitive inhibitors for hOCT1-2/rOct1-2. The K_i values for phenformin inhibition of [³H]-MPP and [¹⁴C]-TEA uptake by hOCT1-3/rOct1-3 were lower than that for metformin. The [³H]-MPP uptake by hOCT1/rOct1 and hOCT3/rOct3 was inhibited by famotidine and ranitidine whereas that by hOCT2/rOct2 was not. The inhibitory potency of cimetidine for hOCT1-2 was very weak. In most cases, the differences in the V_max/K_m values of substrates and the K_i values of inhibitors between hOCT and rOct were minor. The acquisition of information on OCT/Oct mediated-transport and/or inhibition such as that presented in this report is very useful for further understanding of certain aspects of uptake, distribution, and excretion for drug candidates.     

The changes of antioxidant defense system caused by quercetin administration do not lead to DNA damage and apoptosis in the spleen and bone marrow cells of rats.

Quercetin may have the opposite effect, namely anti- as well as pro-oxidant. The aim of this study was to assess the results of quercetin anti- and/or pro-oxidant activity in the bone marrow and spleen cells of rats. The experimental rats were treated daily, with quercetin in a dose of 8 or 80mg/kg b.w. by gavage for 40 days. The intracellular redox state in cells were assessed by measuring the ferric ion reducing antioxidant power (FRAP) level and malonodialdehyde concentration. HO-1 mRNA expression was examined with real-time PCR. The extent of DNA damage was determined by the alkaline-labile comet assay. A potential pro-apoptotic quercetin action was determined using the FITC-Annexin V kit. The quercetin and isorhamnetin concentrations in serum were analyzed by HPLC-ECD. MDA concentration and FRAP values, were significantly decreased in the spleen and bone marrow cells of rats treated with quercetin, in a dose of 80mg/kg b.w. in comparison with the control rats; no significant changes were observed after quercetin was administered in a dose ten times as low. Treatment with quercetin dose-dependently upregulated the expression of HO-1 mRNA in the bone marrow cells. Quercetin administration to the rats did not induce either DNA damage or apoptosis in the examined cells. The results of our study prove that changes in the antioxidant state, caused by quercetin, do not lead to DNA damage or exert any pro-apoptotic activity in vivo.     

The inhibition of release by mGlu7 receptors is independent of the Ca2+ channel type but associated to GABAB and adenosine A1 receptors

Neurotransmitter release is inhibited by G-protein coupled receptors (GPCRs) through signalling pathways that are negatively coupled to Ca(2+) channels and adenylyl cyclase. Through Ca(2+) imaging and immunocytochemistry, we have recently shown that adenosine A(1), GABA(B) and the metabotropic glutamate type 7 receptors coexist in a subset of cerebrocortical nerve terminals. As these receptors inhibit glutamate release through common intracellular signalling pathways, their co-activation occluded each other responses. Here we have addressed whether the occlusion of receptor responses is restricted to the glutamate release mediated by N-type Ca(2+) channels by analysing this process in nerve terminals from mice lacking the alpha(1B) subunit (Ca(v) 2.2) of these channels. We found that glutamate release from cerebrocortical nerve terminals without these channels, in which release relies exclusively on P/Q type Ca(2+) channels, is not modulated by mGlu7 receptors. Furthermore, there is no occlusion of the release inhibition by GABA(B) and adenosine A(1). Hence, in the cerebrocortical preparation, these three receptors only appear to coexist in N-type channel containing nerve terminals. In contrast, in hippocampal nerve terminals lacking this subunit, where mGlu7 receptors modulate glutamate release via P/Q type channels, the occlusion of inhibitory responses by co-stimulation of adenosine A(1), GABA(B) and mGlu7 receptors was observed. Thus, occlusion of the responses by the three GPCRs is independent of the Ca(2+) channel type but rather, it is associated to functional mGlu7 receptors.