Flesinoxan lowers blood pressure and heart rate in cats via 5-HT1A receptors

The concept of selectivity in 5-HT receptor research

Since the demonstration that serotonin (5-hydroxytryptamine, 5-HT) interacts with different (sub)types of membrane receptors, several compounds have been proposed as potent and selective ligands for one of these 5-HT subtypes. Unfortunately, specific and highly selective ligands (selectivity ratios greater than or equal to 1000) for the majority of 5-HT subtypes are still lacking. A few compounds are selective (ratios greater than or equal to 100), but most of the reputed 'selective' tools display affinities for other 5-HT subtypes and/or other (neuro-) transmitter receptors. Mainly due to different interpretations of the concept of selectivity, many of these nonselective compounds are still used to characterize 5-HT receptors. In this paper, we present the affinities (obtained by radioligand binding studies) of the most selective tools known today for each of the 5-HT subtypes and discuss the structure-activity relationships of some interesting series. The potential use of several of these selective ligands as pharmacological tools and therapeutics will be briefly reviewed.

Role of the adrenal gland in the metabolic and cardiovascular effects of 8-OH-DPAT in the rat

In the present study, the role of the adrenal gland in the hypoinsulinaemic and hyperglycaemic effects of 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) vis-à-vis the cardiovascular effects were examined using adrenalectomized and sham-operated (with intact adrenals) Sprague-Dawley rats. Intravenous administration of 8-OH-DPAT (150 micrograms/kg) into conscious sham-operated animals caused significant increases in basal plasma glucose levels without any change in basal plasma insulin levels whereas glucose-stimulated plasma insulin levels were markedly inhibited. These changes were associated with significant and sustained falls in blood pressure (BP) and heart rate (HR) preceded by transient (less than 5 min) increases only in BP. In adrenalectomized animals, 8-OH-DPAT-mediated initial vasoconstriction, hyperglycaemia and inhibition of evoked plasma insulin levels were abolished. However, in these animals, 8-OH-DPAT produced falls in BP and HR that were comparable to those observed in sham-operated animals. Plasma corticosterone was increased 15 min after 8-OH-DPAT in sham-operated animals whereas it was undetectable in adrenalectomized animals. The data indicate that unlike the effects of 8-OH-DPAT to decrease BP and HR, the initial increase in BP and its effects on the metabolic parameters are clearly dependent on intact adrenals.

Comparison of the cardiovascular effects of the 5-HT1A receptor agonist flesinoxan with that of 8-OH-DPAT in the rat

The cardiovascular response to flesinoxan and 8-OH-DPAT (8-hydroxy-2-(di-N-propylamino)tetralin), 5-HT1A receptor agonists, has been investigated in anaesthetized Wistar rats and spontaneously hypertensive rats (SHR) and in conscious SHR. Flesinoxan and 8-OH-DPAT potently lowered blood pressure and heart rate in these models. In conscious SHR, atropine reversed the bradycardia induced by flesinoxan partially and that induced by 8-OH-DPAT completely. In pithed rats with vasopressin-raised blood pressure, neither flesinoxan nor 8-OH-DPAT lowered blood pressure or heart rate. Intracisternal administration of either flesinoxan or 8-OH-DPAT was less efficacious than intravenous administration. The cardiovascular responses to flesinoxan and 8-OH-DPAT in the anaesthetized Wistar were inhibited by the putative 5-HT1A antagonists methiothepin, buspirone, spiroxatrine and 8-MeO-C1EPAT (8-methoxy-2-(N-2-cholroethyl-N-n-propylamino)tetralin). 8-MeO-C1EPAT appeared to be the most suitable antagonist in this model. The 5-HT1C, antagonist ritanserin or the 5-HT3 antagonist GR 38032F had no effect on the responses to flesinoxan or 8-OH-DPAT. In conscious SHR however, 8-MeO-C1EPAT did not antagonize these cardiovascular responses. This study confirms the involvement of central 5-HT1A receptors in the cardiovascular effects of flesinoxan and 8-OH-DPAT.

Systemic and regional haemodynamic responses to microinjection of 5-HT agonists in the rostral ventrolateral medulla in the rat

In anaesthetised rats bilateral microinjection of 2 nmol 5-HT or the agonists 5-carboxyamidotryptamine (5-CT), 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT) into nucleus paragigantocellularis lateralis produced a fall in blood pressure and heart rate, an increase in hindlimb vascular conductance but no significant change in renal conductance. In contrast, alpha-methyl-5-HT injected into the same region had no such effects. It is suggested that these cardiovascular effects are mediated by activation of 5-HT1A receptors. The possibility that there may be differences between species with respect to central 5-HT receptor-mediated cardiovascular effects is also discussed.

Evidence that different regional sympathetic outflows vary in their sensitivity to the sympathoinhibitory actions of putative 5-HT1A and alpha 2-adrenoceptor agonists in anaesthetized cats

1. An investigation was carried out to determine whether the centrally acting hypotensive drugs whose mechanisms of action are due either to activation of 5-HT1A receptors (flesinoxan, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) and urapidil--also an alpha 1-adrenoceptor antagonist) or to activation of alpha 2-adrenoceptors (clonidine and moxonidine) cause differential sympathoinhibition. 2. Cats were anaesthetized with alpha-chloralose and simultaneous recordings were made of whole cardiac, splanchnic and renal nerve activity, blood pressure and heart rate. Cumulative dose-response (i.v.) curves were constructed in separate experiments for the above hypotensive agents on these parameters. 3. Renal nerve activity was found to be more sensitive to the sympathoinhibitory action of flesinoxan and 8-OH-DPAT when compared with cardiac nerve activity, whereas the reverse was observed for clonidine and moxonidine, cardiac being more sensitive than renal nerve activity. Splanchnic nerve activity was similarly affected by all drugs. Furthermore at the highest dose, all drugs tended to cause complete inhibition in all regional sympathetic nerve outflows. 4. Urapidil differed from all the above hypotensive drugs in that it caused a similar degree of sympathoinhibition in all sympathetic outflows at all doses. It is suggested that this may be due to the ability of urapidil to block central alpha 1-adrenoceptors in addition to stimulation of 5-HT1A receptors.

Characterization of DOI, a putative 5-HT2 receptor agonist in the rat

DOI (1-100 micrograms/kg i.v.) induced an increase in mean blood pressure in the anaesthetized rat. Similarly, in the pithed rat, DOI (1-100 micrograms/kg i.v.) induced a dose-dependent increase in mean blood pressure, as did 5-HT. However, in contrast to 5-HT, DOI did not change the heart rate in either intact or pithed rats. In the pithed rat, the dose-pressor response curves to both 5-HT and DOI were unaffected by MDL 72222 (5-HT3 receptor antagonist), spiroxatrine or (+/-)-pindolol (5-HT1A receptor antagonists), idazoxan (alpha 2-adrenoceptor blocking agent) and AR-C 239 (alpha 1-adrenoceptor blocking agent). Only the selective 5-HT2 receptor antagonist. LY 53857, significantly and dose dependently shifted to the right the dose-response curves to both 5-HT and DOI. These results indicated that DOI possesses 5-HT2 agonistic properties and that the pressor response induced by DOI in the pithed rat is mediated via 5-HT2 receptors.

Cardiovascular effects from stimulation of 5-hydroxytryptamine receptors

The cardiovascular effects of 5-hydroxytryptamine (5-HT), consisting of bradycardia or tachycardia, hypotension or hypertension, and vasodilatation or vasoconstriction, are mediated by three main types of receptors called 5-HT1-like, 5-HT2, and 5-HT3. In intact animals 5-HT elicits a short-lasting bradycardia, accompanied by hypotension, via stimulation of 5-HT3 receptors located on sensory vagal nerve endings in the heart (Bezold-Jarisch reflex). The nature of 5-HT receptors mediating tachycardiac responses is species-dependent. Myocardial 5-HT1-like and 5-HT2 receptors subserve tachycardia in the cat and rat, respectively. Tachycardia in the dog and rabbit is due to a release of catecholamines effected via the 5-HT2 receptors on the adrenal medulla and the 5-HT3 receptors on postganglionic cardiac sympathetic nerve fibres, respectively. The receptors mediating tachycardia in the pig are unique as they do not resemble any of the three 5-HT receptors characterized so far. The blood pressure response to 5-HT is usually triphasic: initial short-lasting hypotension due to reflex bradycardia (via 5-HT3 receptors), a middle pressor phase (via 5-HT2 receptors), and a longer-lasting hypotension (via 5-HT1-like receptors). Vascular contraction by 5-HT is generally mediated by 5-HT2 receptors (located primarily on the large conducting vessels), though in some instances (e.g., dog saphenous vein, dog and human basilar artery, and porcine arteriovenous anastomoses) the contractile response is (also) mediated via 5-HT1-like receptors. Venous dilatation and arteriolar dilatation (leading to increased capillary ['nutrient'] blood flow) occur via 5-HT1-like receptors located mainly on the vascular smooth muscles but also on the endothelium; the smooth muscle and endothelial 5-HT1-like receptors seem to be heterogeneous. In addition, 5-HT can elicit vasodilatation and hypotension as a result of decreased sympathetic nervous tone by acting within the central nervous system and by inhibiting noradrenaline release by a presynaptic action. Both these effects also involve 5-HT1-like receptors that do not appear to be identical. Last, knowledge of the cardiovascular effects of 5-HT and the nature of the receptors involved should be helpful in developing 5-HT-related compounds that may be useful in the treatment of hypertension, migraine, and peripheral vascular diseases.

8-OH-DPAT, flesinoxan and guanfacine: systemic and regional haemodynamic effects of centrally acting antihypertensive agents in anaesthetized rabbits

1. 8-Hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) and flesinoxan, agents which show high affinity and selectivity for 5-HT1A receptors, were administered intravenously in doses of 0.003 to 0.1 and 0.01 to 0.3 mg kg-1 respectively to 5 rabbits each. Their effects were compared with those of the centrally acting agent and alpha 2-adrenoceptor agonist, guanfacine, 0.01-0.3 mg kg-1, administered to a group of 5 rabbits. Five further rabbits were used as controls and treated with the vehicle of the active agents. 2. Both flesinoxan and 8-OH-DPAT induced similar systemic and regional haemodynamic changes. Both lowered mean arterial pressure and heart rate. The principal blood pressure lowering mechanism was vasodilatation; cardiac output changed minimally despite the falls in heart rate and myocardial contractile force. 3. With guanfacine the maximal fall of blood pressure was comparable to that obtained with the 5-HT1A receptor ligands; however, in contrast to the latter, the dose-response curve was U-shaped, the highest dose eliciting a pressor effect with reversal of the vasodilatation. 4. Widespread peripheral vasodilatation was found with all the agents in the splanchnic circulation and also in the brain and skeletal muscle. A weak tendency towards vasodilatation was found in the kidneys where the dose-response curve was bell-shaped for guanfacine. 5. This spectrum of activity is different from that of peripheral vasodilators, such as calcium antagonists, potassium channel activating agents or hydralazine; it is, however, consistent with the putative mechanism of action of these compounds to reduce peripheral sympathetic tone by a central mechanism of action.

Anticonflict effects of the 5-HT1A compound flesinoxan

The new phenylpiperazine derivative flesinoxan, a potent and selective serotonin(1A) (5-HT(1A)) agonist, was examined under a procedure that has proved to be a reliable and sensitive method for detecting novel anxiolytic drugs believed to produce their effects at the 5-HT( 1A) receptor subtype. Key pecking by pigeons was maintained by the presentation of food following every 30th response in the presence of a white keylight; during an alternate component, correlated with a red keylight, every 30th response produced food and electric shock which suppressed responding (punishment). Flesinoxan doses from 0.001 to 0.3 mg/ kg, intramuscularly, produced significant increases in punished responding at doses that did not affect unpunished responding. Doses of flesinoxan between 1.0 and 3.0 mg/kg also increased punished responding but produced decreases in responding that was not punished. In a second study flesinoxan substituted for the 5-HT(1A) anxiolytic buspirone under a drug discrimination procedure, providing further evidence that the behavioural effects offlesinoxan are mediated by 5-HT(1A) mechanisms. Based on these findings, it would appear that flesinoxan should be a useful compound in the clinical management of anxiety.

Evidence that the novel antihypertensive agent, flesinoxan, causes differential sympathoinhibition and also increases vagal tone by a central action

The effects of flesinoxan were studied on thoracic preganglionic, splanchnic and renal sympathetic nerve activity, carotid sinus nerve activity, blood pressure and heart rate in anaesthetised cats. In some experiments femoral or renal arterial conductance was also recorded. Flesinoxan (3-300 micrograms kg-1) caused a dose-related fall in blood pressure and heart rate and also caused sympathoinhibition. This fall in blood pressure was not associated with changes in femoral arterial conductance but was with a large increase in renal arterial conductance. In this respect flesinoxan had a greater sympathoinhibitory action on the renal nerve compared with the other sympathetic outflows. The bradycardia was unaffected by the 5-HT3 antagonist, MDL 72222, but was reversed by atropine and was abolished in bi-vagotomised cats. Flesinoxan also caused sympathoinhibition in bi-vagotomised cats and decreased carotid sinus nerve activity and blood pressure. It is concluded that flesinoxan acts centrally to cause sympathoinhibition and an increase in vagal tone.