Streptozotocin-induced diabetes in rats modifies the role D2 , D3 and D4 dopamine receptors play in cardiac sympathetic inhibition.

BACKGROUND: Diabetes mellitus is associated with abnormalities in peripheral/central catecholaminergic systems, including changes in catecholamine levels and receptor expression. OBJECTIVE: Since quinpirole-induced cardiac sympathetic inhibition is greater in diabetic than in normoglycemic rats, this study pharmacologically investigated the dopamine D2 -like receptor subtypes that mediate cardiac sympathetic inhibition in diabetic (streptozotocin [STZ]-pretreated) pithed rats. METHODS: Fifty male Wistar rats were pretreated with STZ, pithed and conditioned for spinal stimulation (C7 -T1 ) of the tachycardic sympathetic tone. The resulting increases in heart rate were evaluated following i.v. blocking doses of antagonists at D2 , D3 and D4 receptors during a continuous i.v. infusion of quinpirole (an agonist at D2 -like receptors) or saline (vehicle). RESULTS: With this experimental approach, the cardiac sympathetic inhibition produced by quinpirole in diabetic rats was: (i) unchanged after administration of vehicles and; (ii) abolished by the antagonists L-741,626 (D2 ), SB-277011-A (D3 ) or L-745,870 (D4 ). CONCLUSION: These findings in diabetic pithed rats imply that: (i) the cardiac sympathetic inhibition by quinpirole involves activation of D2/3/4 dopamine receptors; and (ii) there is a differential stimulation of these receptors compared to normoglycemic rats. These D2/3/4 receptor subtypes could be a novel drug target for the therapy of typical cardiac complications of diabetes.

Prospective role of α2A/2B/2C-adrenoceptor subtypes in the modulation of cardioaccelerator sympathetic tone in an experimental model of diabetes.

Abnormalities in the cardiac sympathetic innervation and tone, as well as in the noradrenergic system are associated, among other peripheral complications, with diabetes mellitus. Furthermore, B-HT 933, an agonist at α2-adrenoceptors, induces a greater cardiac sympathetic inhibition in diabetic rats than in normoglycaemic rats. Accordingly, this pharmacological study analysed the specific involvement of the α2A/2B/2C adrenoceptor subtypes mediating inhibition of the cardioaccelerator sympathetic tone (i.e. cardiac sympathetic inhibition) in an experimental model of diabetes induced by streptozotocin (STZ). Fifty male Wistar rats were consecutively: injected i.p. with STZ to cause diabetes; pithed after four weeks; and primed for electrical preganglionic stimulation (spinal C7-T1 segment) to selectively produce increases in heart rate. These responses were evaluated after i.v. bolus injections of relatively selective α2A-, α2B- and α2C-adrenoceptor antagonists (or vehicles) during an i.v. continuous infusion of B-HT 933 or vehicle. B-HT 933 produced a significant inhibition of the sympathetic tachycardic responses, which was: (i) unaffected after the vehicles saline or dimethyl sulfoxide (DMSO); (ii) partially attenuated after blocking doses of the antagonists BRL44408 (α2A) or JP-1302 (α2C); and (iii) abolished after blocking doses of the antagonist imiloxan (α2B). These findings in diabetic pithed rats indicate that the modulation of the cardioaccelerator sympathetic tone predominantly involves the α2B-adrenoceptor subtype and, to a lesser degree, the α2A- and α2C-adrenoceptor subtypes. Hence, blockade of the sympatho-inhibitory α2B-adrenoceptor subtype (absent in normoglycaemic rats) could denote a novel pharmacological strategy for the therapy of some cardiac disorders during the early stages of diabetes.

Chronic intermittent hypoxia transiently increases hippocampal network activity in the gamma frequency band and 4-Aminopyridine-induced hyperexcitability in vitro.

Chronic intermittent hypoxia (CIH) is the most distinct feature of obstructive sleep apnea (OSA), a common breathing and sleep disorder that leads to several neuropathological consequences, including alterations in the hippocampal network and in seizure susceptibility. However, it is currently unknown whether these alterations are permanent or remit upon normal oxygenation. Here, we investigated the effects of CIH on hippocampal spontaneous network activity and hyperexcitability in vitro and explored whether these alterations endure or fade after normal oxygenation. Results showed that applying CIH for 21 days to adult rats increases gamma-band hippocampal network activity and aggravates 4-Aminopyridine-induced epileptiform activity in vitro. Interestingly, these CIH-induced alterations remit after 30 days of normal oxygenation. Our findings indicate that hippocampal network alterations and increased seizure susceptibility induced by CIH are not permanent and can be spontaneously reverted, suggesting that therapeutic interventions against OSA in patients with epilepsy, such as surgery or continuous positive airway pressure (CPAP), could be favorable for seizure control.

Blockade of 5-HT2 receptors with sarpogrelate uncovers 5-HT7 receptors inhibiting the tachycardic sympathetic drive in pithed rats.

Our group has previously shown in pithed rats that the cardiac sympathetic drive, which produces tachycardic responses, is inhibited by 5-HT via the activation of prejunctional 5-HT1B/1D/5 receptors. Interestingly, when 5-HT2 receptors are chronically blocked with sarpogrelate, the additional role of cardiac sympatho-inhibitory 5-HT1F receptors is unmasked. Although 5-HT2 receptors mediate tachycardia in rats, and the chronic blockade of 5-HT2 receptors unmasked 5-HT7 receptors mediating cardiac vagal inhibition, the role of 5-HT7 receptors in the modulation of the cardiac sympathetic tone remains virtually unexplored. On this basis, male Wistar rats were pretreated during 14 days with sarpogrelate (a 5-HT2 receptor antagonist) in drinking water (30 mg/kg/day; sarpogrelate-pretreated group) or equivalent volumes of drinking water (control group). Subsequently, the rats were pithed to produce increases in heart rate by either electrical preganglionic spinal (C7 -T1 ) stimulation of the cardiac sympathetic drive or iv administration of exogenous noradrenaline. The iv continuous infusion of AS-19 (a 5-HT7 receptor agonist; 10 µg/kg/min) (i) inhibited the tachycardic responses to sympathetic stimulation, but not those to exogenous noradrenaline only in sarpogrelate-pretreated rats. This inhibition was completely reversed by SB258719 (a selective 5-HT7 receptor antagonist; 1 mg/kg, iv) or glibenclamide (an ATP-sensitive K+ channel blocker; 20 mg/kg, iv). These results suggest that chronic 5-HT2 receptor blockade uncovers a cardiac sympatho-inhibitory mechanism mediated by 5-HT7 receptors, involving a hyperpolarization due to the opening of ATP-sensitive K+ channels. Thus, these findings support the role of 5-HT7 receptors in the modulation of the cardiac sympathetic neurotransmission.

Activation of Dopamine D3 Receptor Subtypes Inhibits the Neurogenic Systemic Vasodilation Induced by Stimulation of the Perivascular CGRPergic Discharge.

The sensory nervous system controls cardiovascular homeostasis via capsaicin-sensitive neurons that release calcitonin gene-related peptide (CGRP), which subsequently activates CGRP receptors. How this perivascular CGRPergic discharge is modulated, nevertheless, remains unclear. In pithed rats, systemic vasodilation induced by CGRPergic discharge stimulation results in diastolic blood pressure (BP) decrements that are inhibited by the dopamine D2-like receptor agonist quinpirole. Since this inhibition is mediated by raclopride- or haloperidol-sensitive D2-like receptors (comprising the D2, D3, and D4 subtypes), the present study pharmacologically investigated the specific contribution of these subtypes to the modulation of the systemic CGRPergic vasodilation, using highly specific antagonists. To that end, 55 male Wistar rats were pithed for thoracic (T9-T12) spinal stimulation of the perivascular CGRPergic discharge. The resulting frequency-dependent decrements in diastolic BP were inhibited by quinpirole, and this sensory-inhibition was (a) unchanged after i.v. injections of the antagonists L-741,626 (D2) or L-745,870 (D4) and (b) completely blocked by SB-277011-A (D3). Accordingly, we suggest the main role of the D3 receptor subtypes in the inhibition by quinpirole of the neurogenic CGRPergic systemic vasodilation. These findings contribute to a better understanding of the dopaminergic modulation of the rat perivascular CGRPergic discharge producing systemic vasodilation.

Differential cardiac sympatho-inhibitory responses produced by the agonists B-HT 933, quinpirole and immepip in normoglycaemic and diabetic pithed rats.

This study compared the cardiac sympatho-inhibitory responses produced by agonists at α2 -adrenergic (B-HT 933), dopamine D2 -like (quinpirole) and histamine H3 /H4 (immepip) receptors between normoglycaemic and streptozotocin-pretreated (diabetic) pithed rats. Intravenous (i.v.) continuous infusions of B-HT 933, quinpirole or immepip were used in normoglycaemic and diabetic pithed rats to analyse their sympatho-inhibitory effects on the electrically-stimulated cardioaccelerator sympathetic outflow. Both in normoglycaemic and diabetic animals, B-HT 933 (until 100 µg/kg per minute) and quinpirole (until 10 µg/kg per minute) inhibited the tachycardic responses to electrical sympathetic stimulation, but not those to i.v. bolus of exogenous noradrenaline. These sympatho-inhibitory responses were more pronounced in diabetic than in normoglycaemic animals. Accordingly, the areas under the curve for 100 µg/kg per minute B-HT 933 and 10 µg/kg per minute quinpirole in diabetic rats (1065 ± 70 and 920 ± 35, respectively) were significantly smaller (P < .05) than those in normoglycaemic rats (1220 ± 45 and 1360 ± 42, respectively). In contrast, immepip infusions produced cardiac sympatho-inhibition in normoglycaemic (until 10 µg/kg per minute), but not in diabetic (until 100 µg/kg per minute) animals. Our results suggest that in diabetic pithed rats: (i) the more pronounced cardiac sympatho-inhibition to B-HT 933 and quinpirole may be probably due to up-regulation of α2 -adrenergic and dopamine D2 -like receptors, respectively; (ii) the histamine H3 /H4 receptors do not seem to play a sympatho-inhibitory role; and (iii) there is a differential participation of α2 -adrenergic and dopamine D2 -like receptors, which may certainly represent therapeutic targets for the treatment of diabetic complications such as cardiovascular autonomic neuropathy.

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-HT1/2 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 (α1A; 30-100 µg/kg), L-765,314 (α1B; 100 µg/kg), BMY 7378 (α1D; 30-100 µg/kg), BRL44408 (α2A; 100-300 µg/kg), imiloxan (α2B; 1000-3000 µg/kg) or JP-1302 (α2C; 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 α1A, α1B and α1D) and α2- (probably α2A, α2B and α2C) 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.

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.

Specific role of α2A - and α2B -, but not α2C -, adrenoceptor subtypes in the inhibition of the vasopressor sympathetic out-flow in diabetic pithed rats.

Several lines of evidence have shown an association of diabetes with a catecholamines' aberrant homeostasis involving a drastic change in the expression of adrenoceptors. This homeostatic alteration includes, among other things, atypical actions of α2 -adrenoceptor agonists within central and peripheral α2 -adrenoceptors (e.g. profound antinociceptive effects in diabetic subjects). Hence, this study investigated the pharmacological profile of the α2 -adrenoceptor subtypes that inhibit the vasopressor sympathetic out-flow in streptozotocin-pre-treated (diabetic) pithed rats. For this purpose, B-HT 933 (up to 30 µg/kg min) was used as a selective α2 -adrenoceptor agonist and rauwolscine as a non-selective α2A/2B/2C -adrenoceptor antagonist; in addition, BRL 44408, imiloxan and JP-1302 were used as subtype-selective α2A -, α2B - and α2C -adrenoceptor antagonists, respectively (all given i.v.). I.v. continuous infusions of B-HT 933 inhibited the vasopressor responses induced by electrical sympathetic stimulation without affecting those by i.v. bolus injections of noradrenaline in both normoglycaemic and diabetic rats. Interestingly, the ED50 for B-HT 933 in diabetic rats (25 µg/kg min) was almost 1-log unit greater than that in normoglycaemic rats (3 µg/kg.min). Moreover, the sympatho-inhibition induced by 10 µg/kg min B-HT 933 in diabetic rats was (i) abolished by 300 µg/kg rauwolscine or 100 and 300 µg/kg BRL 44408; (ii) partially blocked by 1000 µg/kg imiloxan; and (iii) unchanged by 1000 µg/kg JP-1302. Our findings, taken together, suggest that B-HT 933 has a less potent inhibitory effect on the sympathetic vasopressor responses in diabetic (compared to normoglycaemic) rats and that can probably be ascribed to a down-regulation of α2C -adrenoceptors.

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.

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.

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.

Predominant role of the dopamine D3 receptor subtype for mediating the quinpirole-induced inhibition of the vasopressor sympathetic outflow in pithed rats.

We have recently reported that quinpirole (a D2-like receptor agonist) inhibits the vasopressor sympathetic outflow in pithed rats via sympatho-inhibitory D2-like receptors. Since D2-like receptors consist of D2, D3 and D4 receptor subtypes, this study investigated whether these subtypes are involved in the above quinpirole-induced sympatho-inhibition by using antagonists of these receptor subtypes. One hundred fifty-six male Wistar rats were pithed and prepared for preganglionic spinal (T7-T9) stimulation of the vasopressor sympathetic outflow. This approach resulted in frequency-dependent vasopressor responses which were analysed before and during i.v. continuous infusions of either saline (0.02 ml/min) or quinpirole (1 µg/kg.min) in animals receiving i.v. bolus injections of vehicle [saline or dimethyl sulfoxide (DMSO)] or the antagonists L-741,626 (D2), nafadotride or SB-277011-A (both D3) as well as L-745,870 (D4). Quinpirole inhibited the sympathetically-induced vasopressor responses. This sympatho-inhibition was (a) unaltered after 1 ml/kg saline, DMSO or 100 and 300 µg/kg L-741,626; (b) markedly blocked and abolished by, respectively, 30 and 100 µg/kg nafadotride or 100 and 300 µg/kg SB-277011-A and (c) slightly blocked after 30 and 100 µg/kg L-745,870, but 300 µg/kg L-745,870 produced no blockade whatsoever. Except for 300 µg/kg L-741,626 or 300 µg/kg L-745,870, the doses of the above compounds failed to modify per se the sympathetic vasopressor responses. The inhibition of the vasopressor sympathetic outflow induced by 1 µg/kg.min quinpirole in pithed rats is predominantly mediated by dopamine D3 and, to a lesser extent, by D4 receptor subtypes, with no evidence for the involvement of the D2 subtype.

Intrathecal dihydroergotamine inhibits capsaicin-induced vasodilatation in the canine external carotid circulation via GR127935- and rauwolscine-sensitive receptors.

It has been suggested that during a migraine attack trigeminal nerves release calcitonin gene-related peptide (CGRP), producing central nociception and vasodilatation of cranial arteries, including the extracranial branches of the external carotid artery. Since trigeminal inhibition may prevent this vasodilatation, the present study has investigated the effects of intrathecal dihydroergotamine on the external carotid vasodilatation to capsaicin, α-CGRP and acetylcholine. Anaesthetized vagosympathectomized dogs were prepared to measure blood pressure, heart rate and external carotid conductance. A catheter was inserted into the right common carotid artery for the continuous infusion of phenylephrine (to restore the carotid vascular tone), whereas the corresponding thyroid artery was cannulated for one-min intracarotid infusions of capsaicin, α-CGRP and acetylcholine (which dose-dependently increased the external carotid conductance). Another cannula was inserted intrathecally (C(1)-C(3)) for the administration of dihydroergotamine, the α(2)-adrenoceptor antagonist rauwolscine or the serotonin 5-HT(1B/1D) receptor antagonist GR127935 (N-[4-methoxy-3-(4-methyl-1-piperazinyl) phenyl]-2'-methyl-4'-(5-methyl-1,2,4-oxadiazol-3-yl)[1,1-biphenyl]-4-carboxamide hydrochloride monohydrate). Intrathecal dihydroergotamine (10, 31 and 100µg) inhibited the vasodilatation to capsaicin, but not that to α-CGRP or acetylcholine. This inhibition was: (i) unaffected by 10µg GR127935 or 100µg rauwolscine, but abolished by 31µg GR127935 or 310µg rauwolscine at 10µg dihydroergotamine; and (ii) abolished by the combination 10µg GR127935+100µg rauwolscine at 100µg dihydroergotamine. Thus, intrathecal (C(1)-C(3)) dihydroergotamine seems to inhibit the external carotid vasodilatation to capsaicin by spinal activation of serotonin 5-HT(1B/1D) (probably 5-HT(1B)) receptors and α(2) (probably α(2A/2C))-adrenoceptors.

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.

Pharmacological evidence that spinal α(2C)- and, to a lesser extent, α(2A)-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 D(2) -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.

The 5-HT(1) receptors inhibiting the rat vasodepressor sensory CGRPergic outflow: further involvement of 5-HT(1F), but not 5-HT(1A) or 5-HT(1D), 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.