Canine External Carotid Research Articles

Inhibitory effect of chronic oral treatment with fluoxetine on capsaicin-induced external carotid vasodilatation in anaesthetised dogs.

During migraine, capsaicin-sensitive trigeminal sensory nerves release calcitonin gene-related peptide (CGRP), resulting in cranial vasodilatation and central nociception. Moreover, 5-HT is involved in the pathophysiology of migraine and depression. Interestingly, some limited lines of evidence suggest that fluoxetine may be effective in migraine prophylaxis, but the underlying mechanisms are uncertain. Hence, this study investigated the canine external carotid vasodilator responses to capsaicin, α-CGRP and acetylcholine before and after acute and chronic oral treatment with fluoxetine. Forty-eight vagosympathectomised male mongrel dogs were prepared to measure blood pressure, heart rate and external carotid blood flow. The thyroid artery was cannulated for infusions of agonists. In 16 of these dogs, a spinal cannula was inserted (C1-C3) for infusions of 5-HT. The external carotid vasodilator responses to capsaicin, α-CGRP and acetylcholine remained unaffected after intracarotid or i.v. fluoxetine. In contrast, the vasodilator responses to capsaicin, but not those to α-CGRP or acetylcholine, were inhibited after chronic oral treatment with fluoxetine (300 µg/kg; for 90 days) or intrathecal 5-HT. Chronic oral fluoxetine inhibited capsaicin-induced external carotid vasodilatation, and this inhibition could partly explain its potential prophylactic antimigraine action.

Pharmacological evidence that spinal α2C- and, to a lesser extent, α2A-adrenoceptors inhibit capsaicin-induced vasodilatation in the canine external carotid circulation

During a migraine attack capsaicin-sensitive trigeminal sensory nerves release calcitonin gene-related peptide (CGRP), producing cranial vasodilatation and central nociception; hence, trigeminal inhibition may prevent this vasodilatation and abort migraine headache. This study investigated the role of spinal α2-adrenoceptors and their subtypes (i.e. α2A, α2B and/or α2C-adrenoceptors) in the inhibition of the canine external carotid vasodilator responses to capsaicin. Anaesthetized vagosympathectomized dogs were prepared to measure arterial blood pressure, heart rate and external carotid conductance. The thyroid artery was cannulated for one-min intracarotid infusions of capsaicin, α-CGRP and acetylcholine. A cannula was inserted intrathecally for spinal (C1–C3) administration of 2-amino-6-ethyl-4,5,7,8-tetrahydro-6H-oxazolo-[5,4-d]-azepin-dihydrochloride (B-HT 933; a selective α2-adrenoceptor agonist) and/or the α2-adrenoceptor antagonists rauwolscine (α2A/2B/2C), 2-[(4,5-dihydro-1H-imidazol-2-yl)methyl]-2,3-dihydro-1-methyl-1H-isoindole maleate (BRL44408; α2A), imiloxan (α2B) or acridin-9-yl-[4-(4-methylpiperazin-1-yl)-phenyl]amine (JP-1302; α2C). Infusions of capsaicin, α-CGRP and acetylcholine dose-dependently increased the external carotid conductance. Intrathecal B-HT 933 (1000 and 3100μg) inhibited the vasodilator responses to capsaicin, but not those to α-CGRP or acetylcholine. This inhibition, abolished by rauwolscine (310μg), was: (i) unaffected by 3100μg imiloxan; (ii) partially blocked by 310μg of BRL44408 or 100μg of JP-1302; and (iii) abolished by 1000μg of BRL44408 or 310μg of JP-1302. Thus, intrathecal B-HT 933 inhibited the external carotid vasodilator responses to capsaicin. This response, mediated by spinal α2-adrenoceptors unrelated to the α2B-adrenoceptor subtype, resembles the pharmacological profile of α2C-adrenoceptors and, to a lesser extent, α2A-adrenoceptors.

The Trevo device: preclinical data of a novel stroke thrombectomy device in two different animal models of arterial thrombo-occlusive disease

BackgroundThe currently available mechanical devices fail to achieve recanalization in as many as 20–40% of proximal arterial occlusion strokes.ObjectiveThe preclinical evaluation of the safety and efficacy of a novel thrombectomy...

O-025 The Trevo device: preclinical data of a novel stroke thrombectomy device in two different animal models of arterial thrombo-occlusive disease

IntroductionThe currently available mechanical devices fail to achieve recanalization in as many as 20–40% of proximal arterial occlusion strokes. We now report on the preclinical evaluation of the safety and...

Spinal sumatriptan inhibits capsaicin-induced canine external carotid vasodilatation via 5-HT 1B rather than 5-HT 1D receptors

Migraine is a neurovascular disorder associated with trigeminal activation, vasodilatation and trigeminal release of calcitonin gene-related peptide (CGRP). The antimigraine properties of triptans may be due to: i) vasoconstriction of the carotid arterial bed via 5-HT 1B receptors; and ii) inhibition of CGRP release from trigeminal nerves, via 5-HT 1B/1D receptors. This study investigated the effects of intrathecally administered sumatriptan (a 5-HT 1B/1D receptor agonist) and PNU-142633 (a 5-HT 1D receptor agonist) on the canine external carotid vasodilator responses to capsaicin, α-CGRP and acetylcholine. For this purpose, 42 mongrel dogs were anaesthetised with sodium pentobarbitone and, subsequently, vagosympathectomized. The animals were prepared to measure arterial blood pressure, heart rate and external carotid blood flow; the thyroid artery was cannulated for infusion of agonists. 1-min intracarotid (i.c.) continuous infusions of capsaicin, α-CGRP and acetylcholine produced dose-dependent increases in external carotid blood flow without affecting arterial blood pressure or heart rate. These vasodilator responses remained unaffected after intrathecal (i.t.) administration of physiological saline (0.5 ml) or PNU-142633 (300–1000 μg); in contrast, i.t. sumatriptan (300–1000 μg) significantly inhibited the vasodilator responses to capsaicin, but not those to α-CGRP or acetylcholine. Furthermore, i.t. administration of SB224289 (a 5-HT 1B receptor antagonist), but not of BRL15572 (a 5-HT 1D receptor antagonist), abolished the above inhibition by sumatriptan. These results suggest that sumatriptan-induced inhibition of the external carotid vasodilatation to capsaicin involves a central mechanism mainly mediated by 5-HT 1B receptors.

Donitriptan, but not sumatriptan, inhibits capsaicin‐induced canine external carotid vasodilatation via 5‐HT1B rather than 5‐HT1D receptors

It has been suggested that during a migraine attack capsaicin-sensitive trigeminal sensory nerves release calcitonin gene-related peptide (CGRP), resulting in cranial vasodilatation and central nociception; hence, trigeminal inhibition may prevent this vasodilatation and abort migraine headache. This study investigated the effects of the agonists sumatriptan (5-HT(1B/1D) water-soluble), donitriptan (5-HT(1B/1D) lipid-soluble), PNU-142633 (5-HT(1D) water-soluble) and PNU-109291 (5-HT(1D) lipid-soluble) on vasodilator responses to capsaicin, alpha-CGRP and acetylcholine in dog external carotid artery. 59 vagosympathectomized dogs were anaesthetized with sodium pentobarbitone. Blood pressure and heart rate were recorded with a pressure transducer, connected to a cannula inserted into a femoral artery. A precalibrated flow probe was placed around the common carotid artery, with ligation of the internal carotid and occipital branches, and connected to an ultrasonic flowmeter. The thyroid artery was cannulated for infusion of agonists. Intracarotid infusions of capsaicin, alpha-CGRP and acetylcholine dose-dependently increased blood flow through the carotid artery. These responses remained unaffected after intravenous (i.v.) infusions of sumatriptan, PNU-142633, PNU-109291 or physiological saline; in contrast, donitriptan significantly attenuated the vasodilator responses to capsaicin, but not those to alpha-CGRP or acetylcholine. Only sumatriptan and donitriptan dose-dependently decreased the carotid blood flow. Interestingly, i.v. administration of the antagonist, SB224289 (5-HT(1B)), but not of BRL15572 (5-HT(1D)), abolished the inhibition by donitriptan. Our results suggest that the inhibition produced by donitriptan of capsaicin-induced external carotid vasodilatation is mainly mediated by 5-HT(1B), rather than 5-HT(1D), receptors, probably by a central mechanism.

Pharmacological profile of the mechanisms involved in the external carotid vascular effects of the antimigraine agent isometheptene in anaesthetised dogs.

The present study set out to investigate the external carotid vascular effects of isometheptene in vagosympathectomised dogs, anaesthetised with pentobarbital. One-minute intracarotid (intra-arterial; i.a.) infusions of isometheptene (10, 30, 100 and 300 microg/min) produced dose-dependent decreases in external carotid blood flow, without affecting blood pressure or heart rate. The vasoconstrictor responses to 100 microg/min and 300 microg/min of isometheptene were clearly attenuated in animals pretreated with reserpine (5,000 microg/kg). Moreover, after prazosin (an alpha1-adrenoceptor antagonist; 100 microg/kg), the responses to isometheptene remained unaltered in untreated as well as reserpine-pretreated dogs. In contrast, the responses to isometheptene were attenuated by rauwolscine (an alpha2-adrenoceptor antagonist; 300 microg/kg) in untreated animals, and were practically abolished in reserpine-pretreated dogs. Further investigation into the specific alpha2-adrenoceptor subtypes, using selective antagonists, showed that BRL44408 (alpha2A) and MK912 (alpha2C) markedly attenuated this response, while imiloxan (alpha2B) was ineffective. The involvement of 5-HT1B and 5-HT1D receptors seems highly unlikely since antagonists at 5-HT1B (SB224289) and 5-HT1D (BRL15572) receptors (both at 300 microg/kg) were ineffective. On this basis, it is concluded that isometheptene-induced canine external carotid vasoconstriction is mediated by both indirect (a tyramine-like action) and direct (acting at receptors) mechanisms, which mainly involve alphaA- and alpha2C-adrenoceptors, while the involvement of alpha1-adrenoceptors seems rather limited.

Pharmacological identification of the major subtypes of adrenoceptors involved in the canine external carotid vasoconstrictor effects of adrenaline and noradrenaline

This study investigated the potential effects of adrenaline and noradrenaline on the external carotid blood flow of vagosympathectomised dogs and the receptor mechanisms involved. One minute (1 min) intracarotid infusions of adrenaline and noradrenaline produced dose-dependent decreases in external carotid blood flow without changes in blood pressure or heart rate. These responses, which remained unaffected after saline, were: ( i) mimicked by the adrenoceptor agonists, phenylephrine (α 1) and BHT933 (6-Ethyl- 5,6,7,8- tetrahydro- 4H- oxazolo [4,5-d] azepin-2-amine dihydrochloride; α 2); ( ii) abolished after phentolamine (2000 μg/kg) unmasking a vasodilator component (subsequently blocked by propranolol; 1000 μg/kg); and ( iii) partly blocked by rauwolscine (30 and 100 μg/kg), and subsequently abolished by prazosin (100 μg/kg). Accordingly, rauwolscine (100 and 300 μg/kg) markedly blocked the responses to BHT933 without affecting those to phenylephrine; likewise, prazosin (100 μg/kg) markedly blocked the responses to phenylephrine without affecting those to BHT933. These results show that both α 1- and α 2-adrenoceptors mediate vasoconstriction within the canine external carotid circulation. Moreover, after blockade of α 1/α 2-adrenoceptors, both adrenaline and noradrenaline exhibit a β-adrenoceptor-mediated vasodilator component.

The role of several alpha(1)- and alpha(2)-adrenoceptor subtypes mediating vasoconstriction in the canine external carotid circulation.

1. It has recently been shown that both alpha(1)- and alpha(2)-adrenoceptors mediate vasoconstriction in the canine external carotid circulation. The present study set out to identify the specific subtypes (alpha(1A), alpha(1B) and alpha(1D) as well as alpha(2A), alpha(2B) and alpha(2C)) mediating the above response. 2. Consecutive 1 min intracarotid infusions of phenylephrine (alpha(1)-adrenoceptor agonist) and BHT933 (alpha(2)-adrenoceptor agonist) produced dose-dependent decreases in external carotid blood flow, without affecting mean arterial blood pressure or heart rate. 3. The responses to phenylephrine were selectively antagonized by the antagonists, 5-methylurapidil (alpha(1A)) or BMY7378 (alpha(1D)), but not by L-765,314 (alpha(1B)), BRL44408 (alpha(2A)), imiloxan (alpha(2B)) or MK912 (alpha(2C)). In contrast, only BRL44408 or MK912 affected the responses to BHT933. 4. The above results support our contention that mainly the alpha(1A), alpha(1D), alpha(2A) and alpha(2C)-adrenoceptor subtypes mediate vasoconstriction in the canine external carotid circulation.

Unravelling the pharmacological profile of the canine external carotid vasodilator '5-HT1-like' receptors: coexistence of sympatho-inhibitory 5-HT1B and postjunctional 5-HT7 receptors.

It has been suggested that the external carotid vasodilatation produced by serotonin (5-hydroxytryptamine; 5-HT) in anaesthetised dogs with intact vagosympathetic trunks is mediated by sympatho-inhibitory '5-HT1D' receptors and musculotropic '5-HT1-like' receptors. The present study has re-analysed this suggestion with regard to the classification schemes recently proposed by the NC-IUPHAR subcommittee on 5-HT receptors. In pentobarbital-anaesthetised dogs with intact vagosympathetic trunks, 1-min intracarotid (i.c.) infusions of 5-carboxamidotryptamine (5-CT; 0.01-0.3 microg/min), 5-HT (0.3-30 microg/ min), 5-methoxytryptamine (5-MeO-T; 1-100 microg/min) or sumatriptan (1-100 microg/min) dose-dependently increased the external carotid blood flow without affecting blood pressure or heart rate. The selective 5-HT1D receptor agonist, PNU-142633 (1-1000 microg/min), was essentially inactive. After mesulergine (300 microg/kg, i.v.), an antagonist at cardiovascular 5-HT7 receptors, the above responses to 5-HT, 5-CT and 5-MeO-T were blocked, whilst those to sumatriptan remained unaffected. In contrast, after the 5-HT1B/1D receptor antagonist, GR127935 (10 microg/kg, i.v.), the responses to 5-HT, 5-CT and 5-MeO-T were not affected, but those to sumatriptan were abolished. Furthermore, after the selective 5-HT1B receptor antagonist, SB224289 (300 microg/kg, i.v.), the responses to 5-HT, 5-CT and 5-MeO-T were significantly enhanced, whereas those to sumatriptan were abolished. Interestingly, the responses to all these agonists remained unmodified after the selective 5-HT1D receptor antagonist, BRL15572 (300 microg/kg, i.v.). The above results suggest that the '5-HT1-like' receptors, which mediate canine external carotid vasodilatation, display the pharmacological profile of sympatho-inhibitory 5-HT1B receptors and musculotropic 5-HT7 receptors, and confirm the existence of vasoconstrictor 5-HT1B receptors.

Further pharmacological analysis of the orphan 5-HT receptors mediating feline vasodepressor responses: close resemblance to the 5-HT7, receptor.

It has been suggested that the late hypotensive response to serotonin (5-hydroxytryptamine; 5-HT) in vagosympathectomised cats, being potently mimicked by 5-carboxamidotryptamine (5-CT), not modified by ketanserin and blocked by methiothepin or methysergide, is mediated by '5-HT1-like' receptors. Nevertheless, current guidelines for 5-HT receptor classification refer to this receptor as an orphan receptor. Thus, the present study set out to reanalyse the above suggestion in terms of the classification schemes proposed in 1994 and 1998 by the NC-IUPHAR subcommittee on the classification of 5-HT receptors. Intravenous (i.v.) bolus injections of 5-CT (0.003-0.3 microg/kg), 5-HT (1-100 microg/kg) and 5-methoxytryptamine (5-MeO-T; 1-100 microg/kg) produced dose-dependent vasodepressor responses with a rank order of agonist potency of 5-CT >> 5-HT = 5-MeO-T with sumatriptan (10-300 microg/kg) virtually inactive. The vasodepressor responses to 5-HT, 5-CT and 5-MeO-T were not attenuated following i.v. administration of the antagonists GR127935 (5-HT(IB/ID); 30 microg/kg), tropisetron (5-HT3/4; 3000 microg/kg), (+/-)-pindolol (beta-adrenergic and 5-HT1A; 4000 microg/kg) or equivalent volumes of physiological saline. In contrast, the above vasodepressor responses were markedly and specifically antagonised by i.v. methiothepin (100 microg/kg), lisuride (30 microg/kg and 100 microg/kg), mesulergine (300 microg/kg and 1000 microg/kg) or LY215840 (300 microg/kg and 1000 microg/kg). The above lines of evidence, therefore, indicate that the orphan receptors mediating the vasodepressor responses to 5-HT in vagosympathectomised cats are pharmacologically similar to other 5-HT7 receptors mediating vascular and non-vascular responses (e.g. relaxation of the canine external carotid artery and guinea-pig ileum as well as feline tachycardia).

The canine external carotid vasoconstrictor 5-HT 1 receptor: blockade by 5-HT 1B (SB224289), but not by 5-HT 1D (BRL15572) receptor antagonists

In vagosympathectomised dogs pre-treated intravenously (i.v.) with mesulergine (300 μg/kg), 1-min intracarotid (i.c.) infusions of 5-hydroxytryptamine (5-HT; 0.3–30 μg/min) and sumatriptan (1–30 μg/min) dose-dependently decreased external carotid blood flow, without affecting mean blood pressure or heart rate. Treatment with the selective 5-HT 1B receptor antagonist SB224289 (2,3,6,7-tetrahydro-1′-methyl-5-[2′-methyl-4′(5-methyl-1,2,4-oxadiazol-3-yl) biphenyl-4-carbonyl]furo[2,3 f]indole-3-spiro-4′-piperidine hydrochloride; 30–300 μg/kg, i.v.) produced a potent, specific and dose-dependent blockade of this response, whereas the selective 5-HT 1D receptor antagonist BRL15572 (1-(3-chlorophenyl)-4-[3,3-diphenyl(2-( S, R) hydroxypropanyl)piperazine]hydrochloride; 30–300 μg/kg, i.v.) was ineffective. It is concluded that mainly 5-HT 1B, but not 5-HT 1D receptors mediate the canine external carotid vasoconstriction by 5-HT and sumatriptan.

Characterization of putative 5-HT7 receptors mediating tachycardia in the cat.

1. It has been suggested that the tachycardic response to 5-hydroxytryptamine (5-HT) in the spinal-transected cat is mediated by '5-HT1-like' receptors since this effect, being mimicked by 5-carboxamidotryptamine (5-CT), is not modified by ketanserin or MDL 72222, but it is blocked by methiothepin, methysergide or mesulergine. The present study was set out to reanalyse this suggestion in terms of the IUPHAR 5-HT receptor classification schemes proposed in 1994 and 1996. 2. Intravenous (i.v.) bolus injections of the tryptamine derivatives, 5-CT (0.01, 0.03, 0.1, 0.3, 1, 3, 10 and 30 microg kg(-1)), 5-HT (3, 10 and 30 microg kg(-1)) and 5-methoxytryptamine (3, 10 and 30 microg kg(-1)) as well as the atypical antipsychotic drug, clozapine (1000 and 3000 microg kg(-1)) resulted in dose-dependent increases in heart rate, with a rank order of agonist potency of 5-CT >> 5-HT > 5-methoxytryptamine >> clozapine. 3. The tachycardic effects of 5-HT and 5-methoxytryptamine were dose-dependently antagonized by i.v. administration of lisuride (30 and 100 microg kg(-1)), ergotamine (100 and 300 microg kg(-1)) or mesulergine (100, 300 and 1000 microg kg(-1)); the highest doses of these antagonists used also blocked the tachycardic effects of 5-CT. Clozapine (1000 and 3000 microg kg(-1)) did not affect the 5-HT-induced tachycardia, but attenuated, with its highest dose, the responses to 5-methoxytryptamine and 5-CT. However, these doses of clozapine as well as the high doses of ergotamine (300 microg kg(-1)) and mesulergine (300 and 1000 microg kg(-1)) also attenuated the tachycardic effects of isoprenaline. In contrast, 5-HT-, 5-methoxytryptamine- and 5-CT-induced tachycardia were not significantly modified after i.v. administration of physiological saline (0.1 and 0.3 ml kg(-1)), the 5-HT(1B/1D) receptor antagonist, GR127935 (500 microg kg(-1)) or the 5-HT(3/4) receptor antagonist, tropisetron (3000 microg kg(-1)). 4. Intravenous injections of the 5-HT1 receptor agonists, sumatriptan (30, 100 and 300 microg kg(-1)) and indorenate (300 and 1000 microg kg(-1)) or the 5-HT4 receptor (partial) agonist cisapride (300 and 1000 microg kg(-1)) were devoid of effects on feline heart rate per se and failed to modify significantly 5-HT-induced tachycardic responses. 5. Based upon the above rank order of agonist potency, the failure of sumatriptan, indorenate or cisapride to produce cardioacceleration and the blockade by a series of drugs showing high affinity for the cloned 5-ht7 receptor, the present results indicate that the 5-HT receptor mediating tachycardia in the cat is operationally similar to other putative 5-HT7 receptors mediating vascular and non-vascular responses (e.g. relaxation of the rabbit femoral vein, canine external carotid and coronary arteries, rat systemic vasculature and guinea-pig ileum). Since these responses represent functional correlates of the 5-ht7 gene product, the 5-HT7 receptor appellation is reinforced. Therefore, the present experimental model, which is not complicated by the presence of other 5-HT receptors, can be utilized to characterize and develop new drugs with potential agonist and antagonist properties at functional 5-HT7 receptors.

Operational characteristics of the 5-HT1-like receptors mediating external carotid vasoconstriction in vagosympathectomized dogs. Close resemblance to the 5-HT1D receptor subtype.

It has recently been shown that the external carotid vasoconstrictor response to 5-HT in the dog is primarily mediated by sumatriptan-sensitive 5-HT1-like receptors; however, the fact that these receptors are not blocked by metergoline, a 5-HT1D ligand, raises questions about their possible correlation with the 5-HT1D receptor subtype. Since a number of drugs display high affinity for the 5-HT1D (GR127935) and 5-HT1F (e.g. methysergide and oxymetazoline) receptor subtypes, in this study we have used these drugs to determine whether the above vasoconstrictor 5-HT1-like receptors correlate with the 5-HT1D and/or 5-HT1F receptor subtypes. One-minute intracarotid infusions of 5-HT (0.3-30 micrograms/min), sumatriptan (1-30 micrograms/min), oxymetazoline (0.03-3 micrograms/min) and noradrenaline (0.3-3 micrograms/min) resulted in dose-dependent decreases in external carotid blood flow without changes in arterial blood pressure or heart rate. These vasoconstrictor responses remained unaltered after i.v. administration of physiological saline (0.015, 0.05 and 0.15 ml/kg; n = 4) or ritanserin (1 mg/kg; n = 5). In contrast, GR127935 (1, 3 and 10 micrograms/kg, n = 6) potently blocked the responses to 5-HT (unmasking a dose-dependent vasodilator component) and sumatriptan without affecting those to oxymetazoline or noradrenaline. Interestingly, methysergide (10, 30 and 100 micrograms/kg, n = 5) also blocked the vasoconstrictor responses to 5-HT and sumatriptan, but unlike GR127935, did not revert the vasoconstrictor response to 5-HT; the responses to oxymetazoline remained unaffected, but those to noradrenaline were apparently attenuated by the highest dose. Taken together, the above findings suggest that the sumatriptan-sensitive 5-HT1-like receptors mediating canine external carotid vasoconstriction resemble 5-HT1D receptors, probably of the 5-HT1D beta subtype on the basis of the resistance to blockade by ritanserin. The pharmacological profile of these receptors could be similar (bovine and human cerebral arteries, porcine carotid arteriovenous anastomoses and human coronary arteries) to other putative 5-HT1D receptors mediating vascular responses.

Role of α 1-adrenoceptors in the reduction of external carotid blood flow induced by buspirone and ipsapirone in the dog

The effects of the 5-HT 1A receptor agonists with anxiolytic properties, buspirone and ipsapirone, in the external carotid bed of anaesthetized dogs were analyzed. Since these agonists produce several vascular effects via activation of both 5-HT receptors and α 1adrenoceptors, their effects were compared with those elicited by the 5-HT agonist, quipazine, and the α 1-adrenoceptor agonist, methoxamine. 1-Min intracarotid (i.e.) infusions of buspirone (300 μg/min), ipsapirone (40 μg/min), quipazine (300 μg/min) and methoxamine (15 μg/min) produced consistent decreases in external carotid blood flow (ECBF); since these changes in blood flow were not accompanied by modifications in systemic blood pressure, the agonists produced parallel increases in external carotid resistance. After interruption of the sympathetic tone by bilateral cervical vagosympathectomy, the vasoconstrictor responses to all the agonists remained unaffected. The intravenous (i.v.) administration of the nonselective 5-HT 1-like receptor antagonist, methiothepin (1–100 μg/kg), potently and dose-dependently antagonized buspirone-, ipsapirone- and quipazine-induced vasoconstriction; methiothepin similarly antagonized the vasoconstrictor responses to methoxamine. Interestingly, the aradrenoceptor antagonist, prazosin (1–100 μg/kg, i.v.), also antagonized the vasoconstrictor responses to buspirone, ipsapirone and methoxamine in a dose-dependent manner. Finally, buspirone (300 μg/min, i.c.) and ipsapirone (40 μg/min, i.e.) did not modify the responses to noradrenaline (10 μg/min, i.e.) or tyramine (100 μg/min, i.e.). It is concluded that canine external carotid vasoconstriction induced by buspirone and ipsapirone is mainly mediated by activation of α 1-adrenoceptors located in vascular smooth muscle. These data further highlight the ability of the above anxiolytics to produce significant vascular effects under in vivo conditions.

Pharmacological profile of the receptors that mediate external carotid vasoconstriction by 5‐HT in vagosympathectomized dogs

1. 5-Hydroxytryptamine (5-HT) can produce vasodilatation or vasoconstriction of the canine external carotid bed depending upon the degree of carotid sympathetic tone. Hence, external carotid vasodilatation to 5-HT in dogs with intact sympathetic tone is primarily mediated by prejunctional 5-HT1-like receptors similar to the 5-HT1D subtype, which inhibit the carotid sympathetic outflow. The present investigation is devoted to the pharmacological analysis of the receptors mediating external carotid vasoconstriction by 5-HT in vagosympathectomized dogs. 2. Intracarotid (i.c.) infusions for 1 min of 5-HT (0.3, 1, 3, 10, 30 and 100 micrograms) resulted in dose-dependent decreases in both external carotid blood flow and the corresponding conductance; both mean arterial blood pressure and heart rate remained unchanged during the infusions of 5-HT. These responses to 5-HT were resistant to blockade by antagonists at 5-HT2 (ritanserin) and 5-HT3/5-HT4 (tropisetron) receptors, but were partly blocked by the 5-HT1-like and 5-HT2 receptor antagonist, methiothepin (0.3 mg kg-1); higher doses of methiothepin (1 and 3 mg kg-1) caused little, if any, further blockade. These methiothepin (3 mg kg-1)-resistant responses to 5-HT were not significantly antagonized by MDL 72222 (0.3 mg kg-1) or tropisetron (3 mg kg-1). 3. The external carotid vasoconstrictor effects of 5-HT were mimicked by the selective 5-HT1-like receptor agonist, sumatriptan (3, 10, 30 and 100 micrograms during 1 min, i.c.), which produced dose-dependent decreases in external carotid blood flow and the corresponding conductance; these effects of sumatriptan were dose-dependently antagonized by methiothepin (0.3, 1 and 3 mg kg-1), but not by 5-HT1D-like receptor blocking doses of metergoline (0.1 mg kg-1). 4. The above vasoconstrictor effects of 5-HT remained unaltered after administration of phentolamine, propranolol, atropine, hexamethonium, brompheniramine, cimetidine and haloperidol, thus excluding the involvement of alpha- and beta-adrenoceptors, muscarinic, nicotinic, histamine and dopamine receptors. Likewise, inhibition of either 5-HT-uptake (with fluoxetine) or cyclo-oxygenase (with indomethacin), depletion of biogenic amines (with reserpine) or blockade of calcium channels (with verapamil) did not modify the effects of 5-HT. 5. Taken together, the above results support our contention that the external carotid vasoconstrictor responses to 5-HT in vagosympathectomized dogs are mainly mediated by activation of sumatriptan-sensitive 5-HT1-like receptors. It must be emphasized, notwithstanding, that other mechanisms of 5-HT, including an interaction with a novel 5-HT receptor (sub)type and/or an indirect action that may lead to the release of a known (or even unknown) neurotransmitter substance cannot be categorically excluded.

Inhibition of serotonin‐induced increase in canine external carotid blood flow by drugs that decrease the sympathetic outflow

1. The present study was designed to analyse the effect of the centrally-acting sympatho-inhibitory drugs, prazosin and ketanserin, on the increase in external carotid blood flow (external CBF) produced by 5-hydroxytryptamine (5-HT) in pentobarbital-anaesthetized dogs. 2. Intracarotid (i.c.) infusions of 5-HT (10 micrograms min-1 during 1 min) produced an increase in external CBF without changes in mean arterial blood pressure or heart rate. This response to 5-HT was dose-dependently blocked by intravenous (i.v.) administration of prazosin (1, 3, 10, 30 and 100 micrograms kg-1) or ketanserin (10, 30, 100 and 300 micrograms kg-1). 3. Furthermore, 5-HT-induced increase in external CBF was inhibited by either the ganglionic blocking agent, mecamylamine (0.03, 0.1, 0.3, 1, 3 and 10 mg kg-1), the mixed 5-HT1-like and 5-HT2 receptor antagonist, methiothepin (3, 10 and 30 micrograms kg-1) or the 5-HT1A ligand, spiroxatrine (10, 30, 100 and 300 micrograms kg-1). In contrast, the selective 5-HT2 and 5-HT1C receptor antagonist, ritanserin (30 and 100 micrograms kg-1, i.v.), was unable to block the above response to 5-HT. 4. It is concluded that the inhibition of 5-HT-induced increase in external CBF by prazosin, ketanserin, mecamylamine and spiroxatrine is due to a reduction in the sympathetic tone and not to a blockade of 5-HT receptors.

Role of 5-HT 1-like receptors in the increase in external carotid blood flow induced by 5-hydroxytryptamine in the dog

This study investigated the receptor involved in the 5-hydroxytryptamine (5-HT)-induced increase in external carotid blood flow in pentobarbital-anaesthetized dogs. One-minute intracarotid (i.c.) infusions of 5-HT (0.3, 1, 3 and 10 μg) and 5-carboxamidotryptamine (5-CT; 0.01, 0.03, 0.1 and 0.3 μg) produced dose-dependent increases in external carotid blood flow without changes in mean arterial blood pressure or heart rate. After vagosympathectomy, the above vasodilator responses to 5-HT and 5-CT were abolished and remained so even after restoration of carotid vascular tone with noradrenaline. Furthermore, the 5-HT- and 5-CT-induced increases in external carotid blood flow were not modified by the 5-HT 2 receptor antagonist, ritanserin (100 μg/kg i.v.), nor the 5-HT 3 receptor antagonist, 1αH,3α,5αH-tropan-3yl-3,5-dichlorobenzoate (MDL 72222; 140 μg/kg i.v.), but were potently and dose dependently antagonized by the mixed 5-HT 1-like and 5-HT 2 receptor blocker, methiothepin (3, 10 and 30 μg/kg i.v.). Interestingly, the 5-HT 1A and 5-HT 1B receptor antagonist, cyanopindolol (100, 300 and 1000 μg/kg i.v.), blocked the effects of 5-HT, but the block was not elicited in a dose-dependent manner, with only the response induced by 0.3 μg/min 5-CT being significantly antagonized by the highest dose of cyanopindolol; however, this blockade was not selective. Unlike 5-HT and 5-CT, 1 min i.c. infusions of either the 5-HT 1C/5-HT 2 receptor agonist, 1-(2,5-dimethoxy-4-iodophenyl)-aminopropane (DOI; 30–300 μg), or the 5-HT 3 receptor agonist, 2-methyl-5-HT (10–300 μg), were devoid of effects on the canine external carotid blood flow. It is concluded that the 5-HT-induced increase in external carotid blood flow is mediated by 5-HT 1-like receptors probably located on carotid sympathetic nerves. These receptors, however, do not seem to correspond to either the 5-HT 1A, 5-HT 1B or 5-HT 1C receptor subtypes.

Time-dependent potentiation of contractile response to norepinephrine in canine isolated cerebral arteries.

The time course of contractile responses to alpha-adrenoceptor agonists was investigated using various arteries isolated from dogs and monkeys. The contractile response to norepinephrine was increased during the time course of the experiment in canine basilar and internal carotid arteries, whereas the response of isolated canine external carotid arteries and monkey internal carotid arteries did not change significantly. Treatment with 10(-7) M propranolol, 5 x 10(-6) M cocaine plus 10(-5) M hydrocortisone, or 5 x 10(-5) M acetylsalicylic acid did not significantly affect the time-dependent potentiation of the norepinephrine-induced contraction in canine internal carotid arteries. The time-dependent enhancement in the response to norepinephrine was also observed in the arterial preparations from which the endothelial cells were removed. The contractile response of canine internal carotid arteries to phenylephrine did not alter significantly throughout the experiments. On the other hand, the responses to clonidine and xylazine were markedly enhanced with time. Significant potentiation of the norepinephrine-induced contraction was observed in canine internal carotid arteries treated with 10(-8) M prazosin, whereas 10(-8) M yohimbine attenuated the time-dependent potentiation. These results suggest that the contractile responses of isolated canine basilar and internal carotid arteries to norepinephrine are potentiated during the course of the experiment, which is likely to be related, in part, to an enhancement in alpha 2-adrenoceptor mediated contraction.

Actions of bradykinin on isolated cerebral and peripheral arteries.

The addition of bradykinin (BK) caused a dose-related contraction in helical strips of canine cerebral, internal carotid, external carotid, and femoral arteries, while the peptide elicited a relaxation in canine coronary, renal, and mesenteric arteries contracted with 5+ or prostaglandin F2alpha. In contrast to canine cerebral arteries, human cerebral arteries contracted with K+ or prostaglandin relaxed with BK. Contractile responses of canine cerebral arteries to BK were not influenced by phentolamine, diphenhydramine, and methysergide, but were attenuated by aspirin and indomethacin. Contractions induced by K+ were not or only slightly inhibited by these anti-inflammatory agents. Polyphloretin phosphate failed to reduce BK-induced contractions. Relaxing effects of BK on canine coronary arterial strips were not altered by atropine, propranolol, metiamide, and aminophylline, but were inhibited by aspirin and indomethacin. Adenosine-induced relaxation was unaffected by the latter two agents. It may be concluded that adrenergic, cholinergic, histaminergic, and adenosine-related mechanisms are not involved in the genesis of BK-induced contraction and relaxation. Contractile responses of canine cerebral arteries to BK do not appear to derive from prostaglandins released, but rather from a direct action on vascular smooth muscle cells.