Nature of the central serotonin receptors mediating hypotension

Further evidence that the discriminative stimulus properties of indorenate are mediated by 5-HT 1A/1B/2C receptors

Indorenate (5-methoxytryptamine beta-methylcarboxylate, INDO) is a serotonin (5-hydroxytryptamine, 5-HT) agonist that has affinity for 5-HT(1A/1B/2C) receptors. Unlike other anxiolytics such as 5-HT receptor agonists, INDO may not share tolerance or dependency with the benzodiazepine anxiolytics. It has been reported that the discriminative stimulus properties of 5-HT(1A/1B/2C) agonists, but not those of 5-HT(3/4) agonists, generalize to INDO. Therefore, the aim of the present study was to obtain further evidence on the differential involvement of 5-HT(1A/1B/2C) receptors in the discriminative stimulus properties of INDO by evaluating its interactions with antagonists of the 5-HT(1A), 5-HT(1B), 5-HT(2C), and 5-HT(3/4) receptor subtypes. Rats were trained to discriminate INDO from saline in a conditioned taste aversion paradigm. For Group D(+)S(-), administration of INDO signalled that saccharin flavour was followed by LiCl, while injection of vehicle signalled safe consumption of saccharin solution. Group D(-)S(+) had the contingencies reversed. After this training, rats had generalization tests where INDO administration was preceded by different doses of the following antagonists: WAY100635 (5-HT(1A)), NAN190 (5-HT(1A)), methiothepin (5-HT(1A/1B/2C)), GR127935 (5-HT(1B/1D)), ketanserin (5-HT(2A/2C)), ritanserin (5-HT(2C/2A)), mesulergine (5-HT(2C/2A)), metergoline (5-HT(2C/2A)), SB206553 (5-HT(2B/2C)), and tropisetron (5-HT(3/4)). In Group D(+)S(-), the order of potency to block the discriminative stimulus properties of INDO was WAY100635>ketanserin>ritanserin>GR127935>mesulergine congruent with SB206553>metergoline>methiothepin>NAN190, while in Group D(-)S(+), the order was WAY100635>GR127935>ketanserin>ritanserin>mesulergine congruent with SB206553>metergoline>methiothepin>NAN190. Tropisetron did not produce any alteration of the discriminative control by INDO. These results suggest that the discriminative signal of INDO is mediated by 5-HT(1A/2C/1B) receptors and that blockade of any of its components produces a degradation of its discriminative effects.

Full substitution of the discriminative cue of a 5-HT(1A/1B/2C) agonist with the combined administration of a 5-HT(1B/2C) and a 5-HT(1A) agonist

The present study examined whether animals attend to the individual components of the cue produced by a drug that stimulates different 5-hydroxytryptamine (5-HT) receptor populations, using a drug discrimination task based on the conditioned taste aversion (CTA) procedure. The training drug was indorenate (5-methoxytryptamine beta-methylcarboxylate) (INDO) that has been described as a 5-HT(1A/2C/1B) agonist able to exert discriminative control in both operant and CTA procedures. The principal objective was to examine generalization with the combined administration of agonists for the different receptor sites that may mimic the mechanism of action of the training drug. Male Wistar rats, deprived of water, were trained to discriminate INDO from saline; during the drug trials, the administration of INDO preceded saccharin-LiCl pairings, while, during the saline trials, the administration of saline preceded the saccharin-saline pairings. In generalization tests, INDO, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT, a 5-HT(1A) agonist), 1-(3-trifluoromethylphenyl)piperazine (TFMPP, a 5-HT(1B) agonist), alpha-methyl-5-HT (a 5-HT(2C) agonist) or 2-methyl-5-HT (a 5-HT(3) agonist), were administered alone or in combination. The results showed that 8-OH-DPAT, TFMPP and alpha-methyl-5-HT produced dose-dependent generalization, up to 88% in the case of 8-OH-DPAT. The combined administration of the following pairs of drugs, 8-OH-DPAT+TFMPP or 8-OH-DPAT+ alpha-methyl-5-HT, at doses that produced only 15-55% generalization when administered alone, produced greater than 80% generalization to INDO. However, the single administration of 2-methyl-5-HT produced only saline-like responding and its combined administration with 8-OH-DPAT did not modify the generalization produced by the single administration of 8-OH-DPAT. These results suggest that animals attend to the individual components of the drug cue; in the case of INDO, which has three elements, each mediated by a different receptor subpopulation (5-HT(1A), 5-HT(1B) and 5-HT(2C) ), the separate stimulation of at least two receptor subpopulations was 'interpreted' by the subject as the presence of the training drug.

Discriminative stimulus properties of indorenate in a conditioned taste aversion paradigm

Indorenate (5-methoxytryptamine beta-methylcarboxylate, INDO) is a serotonin (5-hydroxytryptamine, 5-HT) agonist that has affinity for 5-HT(1A/1B/2C) receptors. It possesses anxiolytic and antihypertensive actions mediated by 5-HT(1A) receptors and anorectic activity mediated by 5-HT(2C/1B) receptors. This study examined whether INDO may exert discriminative control using a conditioned taste aversion (CTA) paradigm, and whether differential participation of 5-HT receptor subtypes may be involved in its cue. Male Wistar rats trained to drink their daily water in a 30-min period were trained to discriminate INDO from saline. One group received the intraperitoneal administration of INDO (10.0 mg/kg) before saccharin-LiCl pairings; on alternate days, rats received saline before the saccharin-saline pairings (Group D(+)S(-)). The other group had the contingencies reversed (i.e., the administration of INDO preceded saccharin-saline pairings: Group D(-)S(+)). In two-bottle generalization tests (one bottle containing saccharin, the other plain water), the preference for saccharin was evaluated after different doses of INDO, [3H]-8-hydroxy-2-(di-N-propylamino)tetralin (8-OH-DPAT) (5-HT(1A)), buspirone (5-HT(1A)), RU24969 (5-HT(1A/1B)), TFMPP (5-HT(1B/2C)), MK212 (5-HT(2C)), alpha-Me-5-HT (5-HT(2C/2A)), 2-Me-5-HT (5-HT(3)) and cisapride (5-HT(4)). The results showed that INDO, RU24969, TFMPP, alpha-Me-5-HT and MK 212 produced a dose-dependent generalization; 8-OH-DPAT and buspirone produced only partial generalization, while 2-Me-5-HT and cisapride did not produce generalization. The results indicate that INDO administration may exert discriminative control over saccharin preference mediated mainly by 5-HT(1B/2C) receptors, but with an important contribution of 5-HT(1A) receptors.

Respiratory and circulatory effects of centrally administered phencyclidine in anesthetized dogs

1. Anesthetized dogs were given phencyclidine HCl (PCP) by intracerebroventricular (i.c.v.) injection. 2. Physiological parameters were monitored after consecutive doses of 0.5, 1.0 and 2.0 mg of PCP. 3. Dose-related changes seen, including bradycardia, hypotension and bradypnea, were opposite to those produced by i.v. doses. 4. Single doses of 1.0 or 2.0 mg of PCP confirmed the prior observations, and the latter provided the baseline for further observations on dogs receiving PCP before various i.c.v. pretreatments--atropine, haloperidol, phentolamine or propranolol--in efforts to characterize the central neurotransmitter system(s) involved in the PCP effects.

Interaction of thyrotropin-releasing hormone (TRH) and serotonin in cardiovascular control

Intravenous injection of the synthetic TRH analog, MK-771, to anesthetised cats raised the blood pressure by a central mechanism, i.e. by activating the outflow of sympathetic stimuli from the CNS to the periphery and raising the plasma concentration of adrenaline and noradrenaline. In contrast, noradrenaline in the CSF was lowered by intravenously injected MK-771. The concentration of 5-HT in the CSF was increased and the pressor action of the peptide was correspondingly enhanced by 5-HT antagonists. These findings suggest that the release of 5-HT into the CSF lowers blood pressure and thus reduces the hypertensive effect of MK-771. Methysergide acts synergistically with MK-771 to combat the sudden fall in pressure induced by acute hemorrhage in the anesthetised cat. Combination of the two drugs enhanced and accelerated recovery from this hypotensive state.

The central site of the sympatho-inhibitory action of 5-hydroxytryptamine in the cat

The aim of this study was to determine the site in the CNS at which 5-hydroxytryptamine (5-HT) inhibits efferent sympathetic nerve activity in the cat. 5-Hydroxytryptamine (3 and 10 micrograms/kg), given into the lateral cerebral ventricle, produced immediate non dose related increases in mean blood pressure (MBP), heart rate (HR) and renal nerve activity (RNA). Larger doses (30 and 100 micrograms/kg i.c.v.) produced gradual decreases in blood pressure, heart rate and renal nerve activity, which did not occur when access of the drug to the fourth ventricle was prevented. Administration of 5-HT (10 and 30 micrograms/kg) into the fourth ventricle produced only decreases in blood pressure, heart rate and renal nerve activity after 15-40 min, which were accompanied by decreases in cardiac output and renal vascular resistance, but little or no change in total peripheral resistance. Application of 5-HT onto the ventral surface of the medulla, into the subarachnoid space at various levels along the spinal cord or into various parts of the nucleus tractus solitarius produced no effect on blood pressure heart rate or renal nerve activity. However, application of a cotton wool pledget soaked in a 5-HT solution (3 mg/ml) over the entire obex/NTS region produced immediate decreases in blood pressure, heart rate and renal nerve activity. These studies suggest that the sympatho-inhibitory effect of 5-HT is due to an action at a site near the caudal end of the dorsal surface of the medulla.

Cardiovascular and sympathetic responses to intracarotid and intravenous injections of serotonin in rats

The possible central regulation of cardiovascular system by serotoninergic neurones was investigated in rats by using a cross-circulated head preparation. Intracarotid injections of serotonin (5-HT, 0.1-10 micrograms) consistently produced neurally mediated vasodepression in the recipient body. Although intravenous injections of 5-HT (0.1-1 micrograms) caused similar depressor responses, larger doses (5-10 micrograms) caused biphasic responses, consisting of vasodepression followed by a sustained pressor response. Despite the biphasic blood pressure responses, sympathetic nerve activity was consistently reduced by the intravenous 5-HT. Bilateral vagotomy abolished the vasodepressor responses to intravenous injections of 5-HT (10 micrograms) and made the responses purely pressor, but the vasodepressor responses to intracarotid injections of 5-HT (10 micrograms) were largely unaffected. Following destruction of central noradrenergic neurones by intracerebroventricular injections of 6-hydroxydopamine, the vasodepressor responses to intracarotid 5-HT (1-10 micrograms), but not to the intravenous 5-HT, were significantly reduced. These results suggest that intravenous injections of 5-HT reduce nerve activity by affecting the reflex mechanism via the vagus nerves. On the other hand, the vasodepression and reduction of the nerve activity by the intracarotid injections appears to be of central origin, and the vasodepression could be mediated via noradrenergic mechanisms in the brain.

Effects of LM 5008, a selective inhibitor of 5-hydroxytryptamine uptake, on blood pressure and responses to sympathomimetic amines

LM 5008 (4-[2-(3-indolyl)ethyl]piperidine) (10, 20 and 50 mg kg-1) had no significant effect on pressor responses to noradrenaline or tyramine in rats anaesthetized with urethane. Desmethylimipramine (1 mg kg-1) blocked the response to tyramine but chlorimipramine (5 mg kg-1) had no significant effect on responses to noradrenaline or tyramine. In the rabbit, anaesthetized with chloralose, LM 5008 (5 mg kg-1) had no effect on pressor responses to noradrenaline, tyramine or angiotensin II, while desmethylimipramine (0.25 mg kg-1) inhibited responses to tyramine and potentiated those to noradrenaline. LM 5008 (10 mg kg-1) had no effect on resting blood pressure of conscious normotensive or DOCA-saline hypertensive rats. Tranylcypromine (5 mg kg-1) produced a fall in blood pressure in conscious normotensive and in DOCA hypertensive rats. Treatment with a combination of LM 5008 (10 mg kg-1) and tranylcypromine (5 mg kg-1) resulted in the appearance of a behavioural hyperactivity syndrome, but blood pressure was not different from that of animals treated with tranylcypromine alone. These results further demonstrate the selectivity of LM 5008 for 5-hydroxytryptamine as opposed to catecholamine uptake.

Alpha-Adrenergic and 5-hydroxytryptaminergic receptor stimulants as new antihypertensive drugs, with observations on involvement of opiate receptors

Cardiovascular actions of central alpha-adrenergic and 5-hydroxytryptaminergic stimulants are discussed in terms of overall effects, correlation of various activities, receptor activation and side-effects. The involvement of opiate receptors in their actions is also examined.

Modification of the vasomotor actions of methysergide in the femoral arterial bed of the anaesthetized dog by changes in sympathetic nerve activity

1 Methysergide has been shown to have a remarkably selective vasoconstrictor action in the carotid arterial bed of the anaesthetized dog following intravenous administration. However we have now shown that under conditions which produce sympathetic blockade methysergide will also constrict the femoral arterial bed and the mechanism involved has been investigated. 2 Methysergide (10.100 microgram/kg i.v.) produced small but variable effects on femoral arterial blood flow in the anaesthetized dog. However following ganglion blockade (mecamylamine 5 mg/kg i.v.), section of the lumbar sympathetic chain between L4-L5 or catecholamine depletion with syrosingopine, methysergide consistently caused dose-related decreases in femoral arterial flow which were associated with increases in femoral arterial vascular resistance. 3 Intravenous infusion of methysergide (10 microgram/kg/min) or 5-hydroxytryptamine (5-HT, 10 microgram/kg/min) inhibited the increases in femoral arterial vascular resistance produced by stimulation of the lumbar sympathetic chain by 70% and 44% respectively whilst increases in vascular resistance produced by close intra-arterially administered noradrenaline were potentiated by 25% and 11% respectively 4 Our results show that the vasomotor actions of methysergide in the dog femoral arterial bed are dependent on the degree of sympathetic activity. This suggests that in the dog the post-junctional vasoconstrictor action of methysergide can be masked by a pre-junctional inhibitory effect on sympathetic nerves which may be mediated through stimulation of a specific pre-junctional receptor for 5-HT.