Wout Wouters

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

Flesinoxan, a new phenylpiperazine derivative has been shown to lower blood pressure in different species after both oral and i.v. administration. The present study shows that the hypotensive potency of flesinoxan in anaesthetised cats increased 35 times after administration via the vertebral arteries compared to i.v. administration. These results, which were confirmed by intracisternal administration, point strongly to a central site of action. Haemodynamic studies indicated that the blood pressure reduction in anaesthetised cats was mainly due to a reduction in the total peripheral resistance and only to some extent to a reduced cardiac output. Flesinoxan seems not to affect sympathetic function by a peripheral mechanism. Its cardiovascular profile can be explained by a centrally mediated reduction of sympathetic tone and increase in vagal tone. Receptor binding studies indicated that flesinoxan is a very potent and selective 5-HT1A ligand. The decreases in blood pressure and heart rate induced by centrally administered flesinoxan and 8-OH-DPAT, could be antagonized effectively by the putative 5-HT1A antagonist pindolol. This suggests a relationship between blood pressure reduction and central 5-HT1A receptors.

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.

Presynaptic action of the pyrethroid insecticide allethrin in the frog motor end plate

Abstract • The action of pyrethroid insecticide allethrin on synaptic transmission in the frog motor end plate was studied by means of intracellular microelectrodes. Allethrin in concentrations as low as 10−7 M produced pronounced repetitive activity in the motor end plate, and a single nerve stimulus could evoke a train of up to 13 end plate potentials. This repetitive activity was presynaptic in origin and was due to repetitive firing of the motor nerve Terminal. The repetitive activity in the nerve terminal was, however, not accompanied by repetitive firing in the more proximal part of the motor fibres. By recording nervous activity along the whole length of the nerve innervating the muscle, from the spinal roots down to the smallest nerve branches in the muscle, it was demonstrated that repetitive firing did not start till the motor nerve fibres reached the vicinity of the muscle. From that point on their tendency to fire repetitively after treatment with allethrin increased in the distal direction till ultimately all motor nerve terminals showed pronounced repetitive activity. This difference in response to allethrin between proximal and distal parts of the motor nerve fibres is probably not due to a different pharmacological action of allethrin, but may be explained by a shift in membrane conductance parameters, specially the rate of recovery from sodium inactivation, as the nerve fibre approaches the nerve terminal. • The allethrin-induced repetitive activity in the nerve terminals was highly dependent on temperature and became much more pronounced as the temperature was lowered, thus showing a truly negative temperature coefficient. Apart from repetitive activity of the presynaptic nerve membrane, no genuine synaptic effect of allethrin was observed, either pre- or post-synaptic, nor was there any significant effect of allethrin on the muscle fibre membrane.

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.

Pressor effects following microinjection of 5‐HT1A receptor agonists into the raphe obscurus of the anaesthetized rat

1 The effects of electrical stimulation and microinjections (90 nl) of the 5‐HT1A receptor agonists, flesinoxan and 8‐hydroxy‐2‐(di‐n‐propylamino) tetralin (8‐OH‐DPAT), and glutamate into the raphe obscurus on blood pressure, heart rate and phrenic nerve activity (central inspiratory drive) were investigated in rats anaesthetized with α‐chloralose. 2 Electrical stimulation of the raphe obscurus caused a rise in blood pressure which was associated with bradycardia, while glutamate (2.7 nmol) caused only a rise in blood pressure. 3 Flesinoxan (1.3 nmol) and 8‐OH‐DPAT (0.7 nmol) increased blood pressure by 9 ± 1 and 14 ± 2 mmHg, respectively and did not affect heart rate. For both agonists the effect on blood pressure was shown to be dose‐dependent; again no effect on the heart rate was observed over the dose‐ranges chosen. 4 Microinjections of the non‐selective 5‐HT1A receptor antagonists, (±)‐pindolol (2.7 nmol) or methiothepin (5.2 nmol), into the raphe obscurus prevented the increase in blood pressure caused by microinjection of flesinoxan. However, (±)‐pindolol caused a sustained rise in blood pressure of 15 ± 1 mmHg while methiothepin caused a transient rise in blood pressure. Neither drugs affected heart rate. The ability of methiothepin to attenuate the pressor effect of flesinoxan was found to be partially reversed after 30 min. 5 It is suggested that activation of 5‐HT1A receptors within the raphe obscurus can cause sympathoexcitation.

Effects of met-enkephalin on slow synaptic inhibition in frog sympathetic ganglion

Abstract The influence of met-enkephalin on slow inhibitory postsynaptic potentials (IPSP) in the frog sympathetic ganglion was studied with the help of a sucrose gap technique. Application of 1 μM met-enkephalin caused a hyperpolarization of the ganglionic neurones, probably via a postsynaptic mechanism. In addition, met-enkephalin depressed the amplitude of the slow IPSP by 40%. Both effects were completely antagonized by 1 μM naloxone. d -ala-met-enkephalinamide (1 μM) and morphine (5 μM) also produced a hyperpolarization together with a depression of the slow IPSP. By varying the external potassium concentration it was shown that the depression of the slow IPSP was not due to the hyperpolarization of the ganglionic neurones by met-enkephalin. Further experimentation revealed that the sensitivity of the ganglion to exogenous dopamine, the putative transmitter for the slow IPSP, was only slightly suppressed by met-enkephalin. It is concluded that the depression of the slow IPSP by met-enkephalin is presynaptic in origin and may be the result of a decrease in the amount of transmitter released from the nerve terminals. The possibility that the reduction in transmitter release is brought about by a hyperpolarization of the nerve terminals is discussed. A hyperpolarization of the preganglionic nerve terminals was actually observed after application of 5 μM morphine.

Effects of ACTH4–10 on synaptic transmission in frog sympathetic ganglion

The influence of ACTH4-10, a behaviourally active fragment of adrenocorticotropic hormone (ACTH) devoid of endocrine activity, on synaptic transmission in the paravertebral sympathetic ganglion of the frog was investigated. Postsynaptic potentials evoked by electrical stimulation of pregnanglionic nerves were recorded using a sucrose gap method. Fast excitatory postsynaptic potentials (EPSPs), which are mediated via nicotinic cholinergic synapses, were not affected by 10(-6) M ACTH4-10. Application of ACTH4-10 in a concentration as low as 10(-8) M for 60 min caused a marked augmentation of the amplitude of slow inhibitory postsynaptic potentials (IPSPs) which are mediated via dopaminergic synapses. The increase in amplitude developed gradually after a latency of 60--90 min and outlasted the application of the peptide. In addition, ACTH4-10 at 10(-6) M increased the hyperpolarising response of the ganglion to exogenous dopamine, as studied by a micro-application method. There was no significant effect of ACTH4-10 on the muscarinic cholinergic depolarising response of the ganglion towards exogenous acetylcholine. The behaviourally active vasopressin fragment DG-LVP (10(-6) M) had no effect on slow IPSPs. The results demonstrate that ACTH4-10 specifically affects slow synaptic inhibition in frog sympathetic ganglion, probably by acting upon the postsynaptic membrane. The possibility is discussed that ACTH4-10 affects one of the intermediate steps between dopaminergic receptor interaction and generation of the slow IPSP.