Canine external carotid vasoconstriction to methysergide, ergotamine and dihydroergotamine: role of 5-HT1B/1D receptors and alpha2-adrenoceptors.
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The antimigraine drugs methysergide, ergotamine and dihydroergotamine (DHE) produce selective vasoconstriction in the external carotid bed of vagosympathectomized dogs anaesthetized with pentobarbital and artificially respired, but the receptors involved have not yet been completely characterized. Since the above drugs display affinity for several binding sites, including alpha-adrenoceptors and several 5-HT1 and 5-HT2 receptor subtypes, this study has analysed the mechanisms involved in the above responses. Intracarotid (i.c.) infusions during 1 min of methysergide (31-310 microg min(-1)), ergotamine (0.56-5.6 microg min(-1)) or DHE (5.6-31 microg min(-1)) dose-dependently reduced external carotid blood flow (ECBF) by up to 46+/-4, 37+/-4 and 49+/-5%, respectively. Blood pressure and heart rate remained unchanged. The reductions in ECBF by methysergide were abolished and even reversed to increases in animals pre-treated with GR127935 (10 microg kg(-1), i.v.). The reductions in ECBF by ergotamine and DHE remained unchanged in animals pre-treated (i.v.) with prazosin (300 microg kg(-1)), but were partly antagonized in animals pre-treated with either GR127935 (10 or 30 microg kg(-1)) or yohimbine (1000 microg kg(-1)). Pre-treatment with a combination of GR127935 (30 microg kg(-1)) and yohimbine (1000 microg kg(-1)) abolished the responses to both ergotamine and DHE. The above doses of antagonists were shown to produce selective antagonism at their respective receptors. These results suggest that the external carotid vasoconstrictor responses to methysergide primarily involve 5-HT1B/1D receptors, whereas those to ergotamine and DHE are mediated by 5-HT1B/1D receptors as well as alpha2-adrenoceptors. (+info)
G-protein activation by putative antagonists at mutant Thr373Lys alpha2A adrenergic receptors.
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1. Replacement of a threonine by a lysine at position 373 in the C-terminal portion of the third intracellular loop of the human alpha2A-adrenergic receptor (alpha2A AR) has been reported to generate a constitutively active mutant receptor in analogy with similar mutations in the alpha1B and beta2 AR (Ren et al., 1993). In the present study, the mutant Thr373Lys alpha2A AR receptor was investigated by measuring the formation of inositol phosphates in either the absence or presence of mouse G(alpha)15 protein in Cos-7 cells. 2. Increased affinity, potency and/or efficacy for the agonists [(-)-adrenaline, UK 14304, clonidine, guanabenz and oxymetazoline] was observed, consistent with a precoupled mutant alpha2A AR: G-protein state. The basal inositol phosphates response was similar at the wild-type (wt) and mutant alpha2A AR, but was enhanced at the mutant alpha2A AR upon co-expression with the mouse G(alpha)15 protein. This enhanced response could not be attenuated in the presence of any of the tested alpha2 AR antagonists (10 microM), suggesting that inverse agonist activity did not occur at the mutant alpha2A AR. 3. Ligands that so far have been identified as antagonists at the wt alpha2A AR demonstrated either no intrinsic activity (MK 912, WB 4101, RS 15385, RX 811059 and RX 821002) or positive efficacy [Emax, % vs. 1 microM UK 14304: dexefaroxan (27+/-7), idazoxan (34+/-9), atipamezole (27+/-4), BRL 44408 (59+/-5) and SKF 86466 (54+/-9)] at the mutant alpha2A AR, but only in the presence of the mouse G(alpha)15 protein. The ligand potencies corresponded with their respective pKi values at the mutant alpha2A AR receptor. 4. The partial agonist effect of SKF 86466 was resistant to pertussis toxin treatment (100 ng ml(-1)) and not affected by co-expression of the rat G(alpha)i1 protein. It was virtually absent in the presence of 10 microM RS 15385. SKF 86466 was without intrinsic activity upon co-expression of the mouse G(alpha)q protein. 5. Some putative alpha2 AR antagonists exerted a partial agonist activity that was highly dependent on the presence of specific G-protein alpha-subunits, suggesting that these ligands cause selective G-protein activation at the mutant alpha2A AR. (+info)
Clonidine evokes vasodepressor responses via alpha2-adrenergic receptors in gigantocellular reticular formation.
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The gigantocellular depressor area (GiDA) is a functionally defined subdivision of the medullary gigantocellular reticular formation where vasodepressor responses are evoked by glutamate nanoinjections. The GiDA also contains reticulospinal neurons that contain the alpha2A-adrenergic receptor (alpha2A-AR). In the present study, we sought to determine whether nanoinjections of the alpha2-AR agonist clonidine into the GiDA evoke cardiovascular responses and whether these responses can be attributed to the alpha2-AR. We found that nanoinjections of clonidine into the GiDA evoke dose-dependent decreases in arterial pressure and heart rate. These responses were equivalent in magnitude to responses produced by clonidine nanoinjections into the sympathoexcitatory region of the rostral ventrolateral medulla. Furthermore, the vasodepressor and bradycardic responses produced by clonidine injections into the GiDA were blocked in a dose-dependent fashion by the highly selective alpha2-AR antagonist 2-methoxyidazoxan, but not by prazosin, which is an antagonist at both the alpha1-AR and the 2B subtype of the alpha-AR. The antagonism by 2-methoxyidazoxan was site specific because injections of the antagonist into the rostral ventrolateral medulla failed to block the responses evoked by clonidine injections into the GiDA. These findings support the notion that clonidine produces sympathoinhibition through multiple sites within the medullary reticular formation, which is consistent with the wide distribution of the alpha2A-AR in reticulospinal neurons. These data also suggest that clonidine may have multiple mechanisms of action because it evokes a cardiovascular depressive response from regions containing neurons that have been determined to be both sympathoinhibitory and sympathoexcitatory. (+info)
Effects of GABA on noradrenaline release and vasoconstriction induced by renal nerve stimulation in isolated perfused rat kidney.
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We examined effects of gamma-aminobutyric acid (GABA) on vasoconstriction and noradrenaline (NA) release induced by electrical renal nerve stimulation (RNS) in the isolated pump-perfused rat kidney. RNS (1 and 2 Hz for 2.5 min each, 0.5-ms duration, supramaximal voltage) increased renal perfusion pressure (PP) and renal NA efflux. GABA (3, 10 and 100 microM) attenuated the RNS-induced increases in PP by 10-40% (P<0.01) and NA efflux by 10-30% (P<0.01). GABA did not affect exogenous NA (40 and 60 nM)-induced increases in PP. The selective GABA(B) agonist baclofen (3, 10 and 100 microM) also attenuated the RNS-induced increases in PP and NA efflux, whereas the RNS-induced responses were relatively resistant to the selective GABA(A) agonist muscimol (3, 10 and 100 microM). The selective GABA(B) antagonist 2-hydroxysaclofen (50 microM), but not the selective GABA(A) antagonist bicuculline (50 microM), abolished the inhibitory effects of GABA (10 microM) on the RNS-induced responses. The selective alpha2-adrenoceptor antagonist rauwolscine (10 nM) enhanced the RNS-induced responses. GABA (3, 10 and 100 microM) potently attenuated the RNS-induced increases in PP by 40-60% (P<0.01) and NA efflux by 20-50% (P<0.01) in the presence of rauwolscine. Prazosin (10 and 30 nM) suppressed the RNS-induced increases in PP by about 70-80%. Neither rauwolscine (10 nM) nor GABA (10 microM) suppressed the residual prazosin-resistant PP response. These results suggest that GABA suppresses sympathetic neurotransmitter release via presynaptic GABA(B) receptors, and thereby attenuates adrenergically induced vasoconstriction in the rat kidney. (+info)
Blocking effects of phentolamine on L-type calcium current and ATP-sensitive potassium current in guinea pig ventricular myocytes.
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AIM: To study the effect of phentolamine on L-type calcium currents (ICa) and ATP-sensitive K+ currents (IK,ATP) in ventricular myocytes. METHODS: ICa and IK,ATP were observed using patch clamp techniques in whole-cell recording configuration. RESULTS: Phentolamine reduced ICa of ventricular myocytes in concentration-dependent and voltage-independent manners. Phentolamine 5, 25, and 100 mumol.L-1 decreased ICa from 370 +/- 99 nA to 310 +/- 95 nA (17% block, n = 6, P < 0.01), from 230 +/- 98 nA to 180 +/- 73 nA (23% block, n = 5, P < 0.05), and from 293 +/- 66 nA to 206 +/- 44 nA (30% block, n = 5, P < 0.01), respectively, without affecting the current-voltage relationship. Prazosin 100 mumol.L-1 and yohimbine 100 mumol.L-1, which were specific blockers of alpha 1 and alpha 2 adrenoceptors respectively, did not show the inhibitory effect on ICa. Phentolamine 100 mumol.L-1 also inhibited the IK,ATP induced by 2, 4-dinitrophenol (DNP) at 0 mV from 3.2 +/- 0.6 nA to 0.8 +/- 0.5 nA (75% block, n = 4, P < 0.01). CONCLUSION: Phentolamine directly inhibits ICa and IK,ATP in guinea pig ventricular myocytes. (+info)
Differential cotransmission in sympathetic nerves: role of frequency of stimulation and prejunctional autoreceptors.
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Recent reports have suggested that sympathetic nerves may store separately and release independently the cotransmitters ATP and norepinephrine (NE). It is conceivable therefore that the quantity of each neurotransmitter that is released from the nerves is not fixed but rather may vary, possibly with the frequency of stimulation. To test this hypothesis we studied the concomitant release at various frequencies and cooperative postjunctional actions of ATP and NE during the first 10 s of electrical field stimulation of the guinea pig vas deferens. We found that at lower frequencies (8 Hz), prejunctional inhibition of the release of NE, which occurs via alpha2-adrenoceptors, modulates the ultimate composition of the cocktail of cotransmitters by limiting the amount of NE that is coreleased with ATP. As the frequency of stimulation increases (above 8 Hz), the autoinhibition of the release of NE is overridden and the amount of NE relative to ATP increases. The smooth muscle of the guinea pig vas deferens reacts to changes in composition of the sympathetic neurochemical messages by increasing the amplitude of its contractions due to the enhancement by NE of the contractile responses triggered by ATP. This evidence suggests that the prejunctional alpha2-adrenoceptor may function as a sensor that "reads" the frequency of action potentials produced during a burst of neuronal activity and converts that information into discrete neurochemical messages with varying proportions of cotransmitters. The mechanism for decoding the informational content of these messages is based on the cooperative postjunctional interactions of the participating cotransmitters. (+info)
Electrochemical and electrophysiological characterization of neurotransmitter release from sympathetic nerves supplying rat mesenteric arteries.
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1. Characteristic features of noradrenaline (NA) and adenosine 5'-triphosphate (ATP) release from postganglionic sympathetic nerves in rat small mesenteric arteries in vitro have been investigated on an impulse-by-impulse basis. NA release was measured using continuous amperometry and ATP release was monitored by intracellular recording of excitatory junction potentials (e.j.ps). 2. Electrical stimuli evoked transient increases in oxidation current. During trains of ten stimuli at 0.5 - 4 Hz there was a depression in the amplitude of oxidation currents evoked following the first stimulus in the train. 3. The neuronal NA uptake inhibitor, desmethylimipramine (1 microM), increased the amplitude of the summed oxidation current evoked by ten stimuli at 1 Hz and slowed the decay of oxidation currents evoked by trains of ten stimuli at 1 and 10 Hz. 4. The alpha2-adrenoceptor antagonist, idazoxan (1 microM), increased the amplitudes of the oxidation currents evoked during trains of ten stimuli at 0.5 - 10 Hz but had no effect on the oxidation currents evoked by the first stimulus in the train. 5. Idazoxan (1 microM) increased the amplitude of all e.j.ps evoked during trains of stimuli at 0.5 and 1 Hz. In addition, the facilitatory effect of idazoxan on e.j.ps was significantly greater than that on oxidation currents. 6. The findings indicate that NA release from sympathetic nerves supplying small mesenteric arteries is regulated by activation of presynaptic alpha2-adrenoceptors and that clearance of released NA in this tissue depends, in part, upon neuronal uptake. The different effects of idazoxan on the oxidation currents and e.j.ps may indicate that the release of NA and ATP is differentially modulated. (+info)
Enhanced cortical dopamine output and antipsychotic-like effects of raclopride by alpha2 adrenoceptor blockade.
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Clozapine exerts superior clinical efficacy and markedly enhances cortical dopamine output compared with classical antipsychotic drugs. Here the alpha2 adrenoceptor antagonist idazoxan was administered to rats alone or in combination with the D2/3 dopamine receptor antagonist raclopride. Dopamine efflux in the medial prefrontal cortex and conditioned avoidance responding were analyzed. Idazoxan selectively potentiated the cortical output of dopamine and augmented the suppression of conditioned avoidance responding induced by raclopride. These results challenge basic assumptions underlying the dopamine hypothesis of schizophrenia and provide insight into clozapine's mode of action. (+info)