Guadalupe Manrique-Maldonado

The 5-HT1 receptors inhibiting the rat vasodepressor sensory CGRPergic outflow: Further involvement of 5-HT1F, but not 5-HT1A or 5-HT1D, subtypes

We have previously shown that 5-HT(1B) receptors inhibit prejunctionally the rat vasodepressor CGRPergic sensory outflow. Since 5-HT(1) receptors comprise 5-HT(1A), 5-HT(1B), 5-HT(1D) and 5-HT(1F) functional subtypes, this study has further investigated the role of 5-HT(1A), 5-HT(1D) and 5-HT(1F) receptor subtypes in the inhibition of the above vasodepressor sensory outflow. Pithed rats were pretreated with i.v. continuous infusions of hexamethonium and methoxamine, followed by 5-HT(1) receptor agonists. Then electrical spinal stimulation (T(9)-T(12)) or i.v. bolus injections of exogenous α-CGRP produced frequency-dependent or dose-dependent vasodepressor responses. The electrically-induced vasodepressor responses remained unchanged during infusions of the 5-HT(1A) receptor agonists 8-OH-DPAT and NN-DP-5-CT. In contrast, these responses were inhibited by the agonists sumatriptan (5-HT(1A/1B/1D/1F)), indorenate (5-HT(1A)), PNU-142633 (5-HT(1D)) or LY344864 (5-HT(1F)), which did not affect the vasodepressor responses to exogenous CGRP (implying a prejunctional sensory-inhibition). When analysing the effects of antagonists: (i) 310 μg/kg (but not 100 μg/kg) GR127935 (5-HT(1A/1B/1D/1F)) abolished the inhibition to sumatriptan, indorenate, PNU-142633 or LY344864; (ii) 310 μg/kg SB224289 (5-HT(1B)) or BRL15572 (5-HT(1D)) failed to block the inhibition to sumatriptan or PNU-142633, whereas SB224289+BRL15572 partly blocked the inhibition to sumatriptan; and (iii) 10 μg/kg WAY100635 (5-HT(1A)) failed to block the inhibition to indorenate. These results suggest that 5-HT(1F), but not 5-HT(1A) or 5-HT(1D), receptor subtypes inhibit the vasodepressor sensory CGRPergic outflow although, admittedly, no selective 5-HT(1F) receptor agonist is available yet. The pharmacological profile of these receptors resembles that shown in rat dorsal root ganglia by molecular biology techniques.

Pharmacological identification of the α2-adrenoceptor subtypes mediating the vasopressor responses to B-HT 933 in pithed rats

It has been shown that α(2)-adrenoceptors mediate vasopressor responses in pithed rats. However, the corresponding α(2)-adrenoceptor subtypes have not been pharmacologically identified. Thus, this study set out to identify the specific subtypes (α(2A), α(2B) and α(2C)) mediating the vasopressor responses to the α(2)-adrenoceptor agonist, B-HT 933, by using the antagonists prazosin (α(1A/1B/1D)), rauwolscine (α(2A/2B/2C)), BRL44408 (α(2A)), imiloxan (α(2B)) and/or JP-1302 (α(2C)). In pithed rats, consecutive i.v. bolus injections of B-HT 933 produced dose-dependent increases in diastolic blood pressure, without affecting heart rate. The vasopressor responses to B-HT 933: (1) remained unaltered after, i.v., bolus injections of vehicles (1 ml/kg) or prazosin (10, 30, 100 and 300 μg/kg); (2) were dose-dependently blocked by rauwolscine (100 and 300 μg/kg), BRL44408 (100 and 300 μg/kg), imiloxan (1000 and 3000 μg/kg) and/or JP-1302 (10, 30, 100, and 300 μg/kg); and (3) were abolished by the combination BRL44408 (300 μg/kg)+imiloxan (1000 μg/kg)+JP-1302 (300 μg/kg). The above results support our contention that the α(2)-adrenoceptors mediating the vasopressor responses to B-HT 933 in pithed rats pharmacologically correlate with the α(2A), α(2B) and α(2C)-adrenoceptor subtypes.

Analysis of anandamide- and lysophosphatidylinositol-induced inhibition of the vasopressor responses produced by sympathetic stimulation or noradrenaline in pithed rats.

The endocannabinoid system exhibits multiple functions in cardiovascular regulation mainly by cannabinoid (CB1 and CB2) receptors, vanilloid TRPV1 receptors and, probably, by the orphan G protein-coupled receptor 55 (GPR55). Hence, the role of these receptors was investigated in Wistar pithed rats on anandamide- and lysophosphatidylinositol (LPI)-induced inhibition of the vasopressor responses induced by preganglionic (T7-T9) stimulation of the vasopressor sympathetic outflow or i.v. bolus injections of noradrenaline. The corresponding frequency- and dose-dependent vasopressor responses were analyzed before and during i.v. continuous infusions of anandamide (CB1, CB2, TRPV1 and GPR55), JWH-015 (CB2) and LPI (GPR55) in animals receiving (i.v.) the antagonists NIDA41020 (CB1), AM630 (CB2), capsazepine (TRPV1) and/or cannabidiol (GPR55). Anandamide (0.1-3.1 μg/kg min) inhibited the vasopressor responses by electrical stimulation, but not those by noradrenaline; while LPI (5.6-10 μg/kg min) inhibited both responses. In contrast, JWH-015 (5.6-10 μg/kg min) failed to induce sympatho-inhibition. Anandamide-induced sympatho-inhibition was: (i) dose-dependently blocked by 31 and 100 μg/kg NIDA41020; (ii) slightly blocked by 310 μg/kg AM630 or 31 μg/kg cannabidiol; and (iii) unaffected by 310 μg/kg capsazepine. Moreover, LPI-induced inhibition of both vasopressor responses was blocked and abolished by 10 and 31 μg/kg cannabidiol, respectively, and weakly blocked by 100 μg/kg NIDA41020. Thus, the sympatho-inhibition by anandamide is primarily mediated by cannabinoid CB1 and, minimally, by cannabidiol-sensitive receptors. In contrast, LPI-induced inhibition of both responses seems to be mainly mediated by postjunctional cannabidiol-sensitive (presumably endothelial GPR55) receptors.

The role of dopamine D2, but not D3 or D4, receptor subtypes, in quinpirole‐induced inhibition of the cardioaccelerator sympathetic outflow in pithed rats

Quinpirole (a dopamine D2‐like receptor agonist) inhibits the cardioaccelerator sympathetic outflow in pithed rats by sympathoinhibitory D2‐like receptors. The present study was designed to identify pharmacologically the specific D2‐like receptor subtypes (i.e. D2, D3 and D4) involved in this sympathoinhibition by quinpirole.

The role of pre-junctional D2 -like receptors mediating quinpirole-induced inhibition of the vasodepressor sensory CGRPergic out-flow in pithed rats.

Calcitonin gene‐related peptide (CGRP) released from perivascular sensory nerves plays a role in the regulation of vascular tone. Indeed, electrical stimulation of the perivascular sensory out‐flow in pithed rats produces vasodepressor responses, which are mainly mediated by CGRP release. This study investigated the potential role of dopamine D1‐like and D2‐like receptors in the inhibition of these vasodepressor responses. For this purpose, male Wistar pithed rats (pre‐treated i.v. with 25 mg/kg gallamine and 2 mg/kg min. hexamethonium) received i.v. continuous infusions of methoxamine (20 μg/kg min.) followed by physiological saline (0.02 ml/min.), the D1‐like receptor agonist SKF‐38393 (0.1–1 μg/kg min.) or the D2‐like receptor agonist quinpirole (0.03–10 μg/kg min.). Under these conditions, electrical stimulation (0.56–5.6 Hz; 50 V and 2 ms) of the thoracic spinal cord (T9–T12) resulted in frequency‐dependent vasodepressor responses which were (i) unchanged during the infusions of saline or SKF‐38393 and (ii) inhibited during the infusions of quinpirole (except at 0.03 μg/kg min.). Moreover, the inhibition induced by 0.1 μg/kg min. quinpirole (which failed to inhibit the vasodepressor responses elicited by i.v. bolus injections of exogenous α‐CGRP; 0.1–1 μg/kg) was (i) unaltered after i.v. treatment with 1 ml/kg of either saline or 5% ascorbic acid and (ii) abolished after 300 μg/kg (i.v.) of the D2‐like receptor antagonists haloperidol or raclopride. These doses of antagonists (enough to completely block D2‐like receptors) essentially failed to modify per se the electrically induced vasodepressor responses. In conclusion, our results suggest that quinpirole‐induced inhibition of the vasodepressor sensory CGRPergic out‐flow is mainly mediated by pre‐junctional D2‐like receptors.

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. α(2A), α(2B) and/or α(2C)-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 (α(2A/2B/2C)), 2-[(4,5-dihydro-1H-imidazol-2-yl)methyl]-2,3-dihydro-1-methyl-1H-isoindole maleate (BRL44408; α(2A)), imiloxan (α(2B)) or acridin-9-yl-[4-(4-methylpiperazin-1-yl)-phenyl]amine (JP-1302; α(2C)). Infusions of capsaicin, α-CGRP and acetylcholine dose-dependently increased the external carotid conductance. Intrathecal B-HT 933 (1000 and 3100 μg) inhibited the vasodilator responses to capsaicin, but not those to α-CGRP or acetylcholine. This inhibition, abolished by rauwolscine (310 μg), was: (i) unaffected by 3,100 μg imiloxan; (ii) partially blocked by 310 μg of BRL44408 or 100 μg of JP-1302; and (iii) abolished by 1,000 μg of BRL44408 or 310 μg 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 α(2B)-adrenoceptor subtype, resembles the pharmacological profile of α(2C)-adrenoceptors and, to a lesser extent, α(2A)-adrenoceptors.

The Dopamine Receptors Mediating Inhibition of the Sympathetic Vasopressor Outflow in Pithed Rats: Pharmacological Correlation with the D2‐like Type

This study investigated in pithed rats whether dopamine can inhibit the sympathetic vasopressor outflow and analysed the pharmacological profile of the receptors involved. Male Wistar pithed rats were pre-treated with intravenous (i.v.) bolus injections of gallamine (25 mg/kg) and desipramine (50 μg/kg). The vasopressor responses to electrical stimulation of the sympathetic vasopressor outflow (0.03-3 Hz; 50 V and 2 msec.) were analysed before and during i.v. continuous infusions of the agonists dopamine (endogenous ligand), SKF-38393 (D(1) -like) or quinpirole (D(2) -like). If inhibition was produced by any agonist, then its capability to inhibit the vasopressor responses to i.v. bolus injections of exogenous noradrenaline (0.03-3 μg/kg) was also investigated. Dopamine (3-100 μg/kg min.) inhibited the vasopressor responses to both electrical stimulation and noradrenaline. In contrast, SKF-38393 (10-100 μg/kg min.) failed to inhibit the vasopressor responses to electrical stimulation; whereas quinpirole (0.1-30 μg/kg min.) inhibited the vasopressor responses to electrical stimulation but not those to noradrenaline. The sympatho-inhibition by quinpirole (1 μg/kg min.) remained unaltered after i.v. SCH 23390 (300 and 1000 μg/kg; D(1) -like receptor antagonist), but was abolished after i.v. raclopride (1000 μg/kg; D(2) -like receptor antagonist). These doses of antagonists did not modify per se the sympathetically-induced vasopressor responses. In conclusion, quinpirole-induced inhibition of the sympathetic vasopressor outflow is primarily mediated by activation of dopamine D(2) -like receptors.

Pharmacological evidence that histamine H3 receptors inhibit the vasodepressor responses by selective stimulation of the rat perivascular sensory CGRPergic outflow

This study has investigated whether pharmacological activation of Gi/o coupled histamine H3/H4 receptors inhibits the rat vasodepressor sensory outflow. For this purpose, 100 male Wistar rats were pithed, artificially ventilated and pretreated (i.v.) with: 25mg/kg gallamine, 2mg/kg/min hexamethonium and 20μg/kg/min methoxamine, followed by i.v. continuous infusions of physiological saline (0.02ml/min) or immepip (3.1, 10 or 31μg/kg/min; a histamine H3/H4 receptor agonist). Under these conditions, electrical stimulation (0.56-5.6Hz; 50V and 2ms) of the spinal cord (T9-T12) resulted in frequency-dependent vasodepressor responses, which were: (i) unchanged during the infusions of saline or immepip (3.1μg/kg/min); and (ii) significantly but, surprisingly, not dose-dependently inhibited by 10 and 31μg/kg/min immepip. Moreover, the sensory-inhibition by 10μg/kg/min immepip (which failed to inhibit the vasodepressor responses by i.v. bolus injections of α-CGRP; 0.1-1µg/kg) was: (i) essentially unaltered after i.v. administration of saline (1ml/kg) or blocking doses of the antagonists ketotifen (100μg/kg; H1), ranitidine (1000μg/kg; H2) or JNJ7777120 (310μg/kg; H4); and (ii) abolished after i.v. thioperamide (310µg/kg; H3). In conclusion, our results suggest that immepip-induced inhibition of the vasodepressor sensory outflow is mainly mediated by prejunctional activation of histamine H3 receptors.

Olcegepant blocks neurogenic and non‐neurogenic CGRPergic vasodepressor responses and facilitates noradrenergic vasopressor responses in pithed rats

Olcegepant (BIBN4096BS) is a selective non‐peptide CGRP receptor antagonist with acute antimigraine properties. Since systemic vascular tone is modulated by perivascular (primary sensory CGRPergic and sympathetic) nerves, this randomized study investigated in pithed rats the effect of acute i.v. treatment with olcegepant on the neurogenic and non‐neurogenic: (i) CGRPergic vasodepressor responses; and (ii) noradrenergic vasopressor responses. The pithed rat is an experimental model predictive of systemic (cardio) vascular side effects.

Role of pre-junctional CB1, but not CB2 , TRPV1 or GPR55 receptors in anandamide-induced inhibition of the vasodepressor sensory CGRPergic outflow in pithed rats.

Stimulation of the perivascular sensory outflow in pithed rats produces vasodepressor responses mediated by CGRP release. Interestingly, endocannabinoids such as anandamide (which interacts with CB1, CB2, TRPV1 and GPR55 receptors) can regulate the activity of perivascular sensory nerves in dural blood vessels by modulating CGRP release. Yet, as no publication has reported whether this mechanism is operative in the healthy systemic vasculature, this study has specifically analysed the receptors mediating the potential inhibitory effects of the cannabinoid (CB) receptor agonists anandamide (non‐selective), JWH‐015 (CB2) and lysophosphatidylinositol (GPR55) on the rat vasodepressor sensory CGRPergic outflow (an index of systemic vasodilatation). Healthy pithed rats were pre‐treated with consecutive i.v. continuous infusions of hexamethonium, methoxamine and the above agonists. Electrical spinal (T9–T12) stimulation of the vasodepressor sensory CGRPergic outflow or i.v. injections of α–CGRP produced frequency‐dependent or dose‐dependent vasodepressor responses. The infusions of anandamide in a dose‐dependent manner inhibited the vasodepressor responses by electrical stimulation (remaining unaffected by JWH‐015 or lysophosphatidylinositol), but not those by α–CGRP. After i.v. administration of antagonists, the inhibition by 3.1 μg/kg min anandamide was: (i) potently blocked by 31–100 μg/kg NIDA41020 (CB1), (ii) unaffected by 180 μg/kg AM630 (CB2), 31 μg/kg cannabidiol (GPR55) or 31–100 μg/kg capsazepine (TRPV1) and (iii) slightly blocked by 310 μg/kg AM630. The above doses of antagonists were enough to block their respective receptors. These results suggest that anandamide‐induced inhibition of the vasodepressor sensory CGRPergic outflow is mainly mediated by pre‐junctional activation of CB1 receptors, with no pharmacological evidence for the role of CB2, TRPV1 or GPR55 receptors.