Pharmacological identification of α1- and α2-adrenoceptor subtypes involved in the vasopressor responses induced by ergotamine in pithed rats

Blocking properties of terguride at the 5-HT2 receptor subtypes mediating cardiovascular responses in the rat

In vitro studies have suggested that terguride blocks the contractile and relaxant responses produced by 5-hydroxytryptamine (5-HT) via 5-HT_2A/2B receptors. This study has now investigated terguride's blocking properties on central/peripheral 5-HT₂ receptors in anaesthetized or pithed rats. Male Wistar anaesthetized/pithed rats were cannulated for recording blood pressure and heart rate and for i.v. administration of several compounds. In both groups of rats, i.v. bolus injections of 5-HT or (±)-DOI (a 5-HT₂ receptor agonist; 1-1000 μg/kg) produced dose-dependent increases in diastolic blood pressure and heart rate. These responses were dose-dependently antagonized by terguride (10-3000 μg/kg). In anaesthetized rats, i.v. bolus injections of BW723C86 (a 5-HT_2B receptor agonist; 1-1000 μg/kg) produced dose-dependent increases in diastolic blood pressure and not dose-dependent increases in heart rate, while in pithed rats, these responses were attenuated. The vasopressor responses elicited by BW723C86 in anaesthetized rats were dose-dependently blocked by terguride (10-300 μg/kg), whereas its the tachycardic responses were dose-independently blocked. These results, taken together, suggest that terguride behaved as an antagonist at the 5-HT₂ receptors located in the central nervous system and (or) the systemic vasculature. This is the first evidence demonstrating that terguride can block central/peripheral 5-HT₂ receptors mediating cardiovascular responses in anaesthetized or pithed rats.

The role of α1- and α2-adrenoceptor subtypes in the vasopressor responses induced by dihydroergotamine in ritanserin-pretreated pithed rats

Background

Dihydroergotamine (DHE) is an acute antimigraine agent that displays affinity for dopamine D₂-like receptors, serotonin 5-HT_1/2 receptors and α₁/α₂-adrenoceptors. Since activation of vascular α₁/α₂-adrenoceptors results in systemic vasopressor responses, the purpose of this study was to investigate the specific role of α₁- and α₂-adrenoceptors mediating DHE-induced vasopressor responses using several antagonists for these receptors.

Methods

For this purpose, 135 male Wistar rats were pithed and divided into 35 control and 100 pretreated i.v. with ritanserin (100 μg/kg; to exclude the 5-HT₂ receptor-mediated systemic vasoconstriction). Then, the vasopressor responses to i.v. DHE (1–3100 μg/kg, given cumulatively) were determined after i.v. administration of some α₁/α₂-adrenoceptor antagonists.

Results

In control animals (without ritanserin pretreatment), the vasopressor responses to DHE were: (i) unaffected after prazosin (α₁; 30 μg/kg); (ii) slightly, but significantly, blocked after rauwolscine (α₂; 300 μg/kg); and (iii) markedly blocked after prazosin (30 μg/kg) plus rauwolscine (300 μg/kg). In contrast, after pretreatment with ritanserin, the vasopressor responses to DHE were: (i) attenuated after prazosin (α₁; 10 and 30 μg/kg) or rauwolscine (α₂; 100 and 300 μg/kg); (ii) markedly blocked after prazosin (30 μg/kg) plus rauwolscine (300 μg/kg); (iii) attenuated after 5-methylurapidil (α_1A; 30–100 μg/kg), L-765,314 (α_1B; 100 μg/kg), BMY 7378 (α_1D; 30–100 μg/kg), BRL44408 (α_2A; 100–300 μg/kg), imiloxan (α_2B; 1000–3000 μg/kg) or JP-1302 (α_2C; 1000 μg/kg); and (iv) unaffected after the corresponding vehicles (1 ml/kg).

Conclusion

These results suggest that the DHE-induced vasopressor responses in ritanserin-pretreated pithed rats are mediated by α₁- (probably α_1A, α_1B and α_1D) and α₂- (probably α_2A, α_2B and α_2C) adrenoceptors. These findings could shed light on the pharmacological profile of the vascular side effects (i.e. systemic vasoconstriction) produced by DHE and may lead to the development of more selective antimigraine drugs devoid vascular side effects.

Further analysis of the inhibition by agmatine on the cardiac sympathetic outflow: Role of the α2-adrenoceptor subtypes

This study has investigated the role of the α₂-adrenoceptor subtypes involved in the inhibition of the cardiac sympathetic outflow induced by intravenous (i.v) infusions of agmatine. Therefore, we analysed the effect of an i.v. bolus injections of the selective antagonists BRL 44408 (300μg/kg; α_2A), imiloxan (3000μg/kg; α_2B), and JP-1302 (300μg/kg; α_2C) given separately, and their combinations: BRL 44408 plus Imiloxan, JP 1302 plus imiloxan, BRL 44408 plus JP-1302, BRL 44408 plus imiloxan plus JP-1302 on the cardiac sympatho-inhibition of agmatine. Also, the effect of the combination BRL 44408 plus JP-1302 plus AGN 192403 (3000μg/kg; I₁ antagonist) was evaluated. In this way, i.v. infusions of 1000μg/kg min of agmatine, but not 300, inhibited the tachycardic response induced by electrical stimulation. Furthermore, the antagonists used or their combinations had no effect on the electrically-induced tachycardic response. On the other hand, the inhibitory response of agmatine was: (1) partially antagonized by BRL 44408 or JP-1302 given separately, a similar response was observed when we administered their combination with imiloxan, but not by imiloxan alone, (2) antagonized in greater magnitude by the combination BRL 44408 plus JP-1302 or the combination BRL 44408 plus imiloxan plus JP-1302, and (3) abolished by the combination BRL 44408 plus JP-1302 plus AGN 192403. Taken together, these results demonstrate that the α_2A- and α_2C-adrenoceptor subtypes and I₁-imidazoline receptors are involved in the inhibition of the cardiac sympathetic outflow induced by agmatine.

Pharmacological evidence that 5-HT1A/1B/1D, α2-adrenoceptors and D2-like receptors mediate ergotamine-induced inhibition of the vasopressor sympathetic outflow in pithed rats

The sympathetic nervous system that innervates the peripheral circulation is regulated by several mechanisms/receptors. It has been reported that prejunctional 5-HT1A, 5-HT1B, 5-HT1D, D2-like receptors and α2-adrenoceptors mediate the inhibition of the vasopressor sympathetic outflow in pithed rats. In addition, ergotamine, an antimigraine drug, displays affinity at the above receptors and may explain some of its adverse/therapeutic effects. Thus, the aims of this study were to investigate in pithed rats: (i) whether ergotamine produces inhibition of the vasopressor sympathetic outflow; and (ii) the major receptors involved in this effect. For this purpose, male Wistar pithed rats were pre-treated with gallamine (25 mg/kg; i.v.) and desipramine (50 µg/kg) and prepared to stimulate the vasopressor sympathetic outflow (T7-T9; 0.03-3 Hz) or to receive i.v. bolus of exogenous noradrenaline (0.03-3 µg/kg). I.v. continuous infusions of ergotamine (1 and 1.8 μg/kgmin) dose-dependently inhibited the vasopressor responses to sympathetic stimulation but not those to exogenous noradrenaline. The sympatho-inhibition elicited by 1.8 μg/kg min ergotamine was (i) unaffected by saline (1 ml/kg); (ii) partially antagonised by WAY 100635 (5-HT1A; 30 μg/kg) and rauwolscine (α2-adrenoceptor; 300 μg/kg), and (iii) dose-dependently blocked by GR 127935 (5-HT1B/1D; 100 and 300 μg/kg) or raclopride (D2-like; 300 and 1000 μg/kg), The above doses of antagonists did not modify per se the sympathetically-induced vasopressor responses. The above results suggest that ergotamine induces inhibition of the vasopressor sympathetic outflow by activation of prejunctional 5-HT1A, 5-HT1B/1D, α2-adrenoceptors and D2-like receptors in pithed rats.