Presynaptic α-block and inhibition of noradrenaline and 5-hydroxytryptamine reuptake by a series of compounds related to mianserin

A structure-activity relationship study was undertaken for a variety of structural analogues of the tetracyclic antidepressant mianserin. Presynaptic α-blocking activity in vitro was evaluated measuring the potentiation of depolarization-induced noradrenaline (NA) release from rat cerebral cortex slices. Inhibition of NA and 5-hydroxytryptamine reuptake was measured in rat hypothalamic or striatal synaptosomes, respectively. Presynaptic α-blockade was only found in molecules with an overall bent shape. Flat rigid molecules or flexible ones were not active. Six-membered, chair-formed D-rings (containing the -NCH₃ moiety) appeared better than 5- or 7-membered ones. Heteroatom substitution, but not hydroxylation or methylation, of the bridge between the two aromatic rings left presynaptic α-blockade unaffected. N-Demethylation and aromatic methyl- or chlorine-subsitution reduced presynaptic α-blockade. In pyridine ring-substituted analogues the localization of the heteroatom appeared to be crucial. 5-Hydroxytryptamine reuptake inhibitory activity was only found in desmethylmianserin. NA reuptake inhibition was found in many mianserin analogues, especially those with an exocyclic -N(CH₃)₂ moiety. Structure activity relationships for NA reuptake inhibition differed from those for presynaptic α-blockade and were generally less stringent. For both properties simple additivity relationships appeared to be absent.     

Specific inhibition of hypothalamic self-stimulation by selective reuptake blockade of either 5-hydroxytryptamine or noradrenaline

The effects of two new phthalane-derived bicyclic thymoleptics on hypothalamic self-stimulation were investigated in rats. The drugs, LU 10-171 and LU 5-003 are potent and highly selective reuptake blockers of 5-hydroxytryptamine (5-HT) and noradrenaline (NA), respectively. The use of a two-way shuttle-box permitted the differentiation of specific reward modulation effects from the variety of nonspecific performance changes that these drugs may produce. Selective reuptake blockade of either 5HT or NA produced a dose-dependent reduction in reward that could be clearly dissociated from any nonspecific performance decrements. Besides providing direct evidence for a significant role for 5HT in the mediation of hypothalamic self-stimulation, these data show that symply increasing transmitter availability is not a sufficient condition to enhance self-stimulation reward. It is suggested that self-stimulation is dependent on response-contingent transmitter release and that any operation that increases reward-transmitter availability in a response-independent manner should attenuate self-stimulation.     

Evidence for inhibition of the reuptake of 5-hydroxytryptamine and noradrenaline by tetrahydronaphthylamine in rat brain

1. The concentrations of 5-hydroxytryptamine (5-HT), 5-hydroxyindolacetic acid (5-HIAA) and noradrenaline (NA) in homogenized rat brains were determined after intraperitoneal injection of 1,2,3,4-tetrahydro-2-naphthylamine (THN).2. THN caused a decrease in the concentration of brain 5-HIAA without altering its 5-HT content, but the percentage of ;free 5-HT' in the supernatant increased. The decrease in 5-HIAA and the increase in free 5-HT were negatively correlated, suggesting inhibition of the reuptake of 5-HT.3. THN decreased brain NA content without changing free NA. The fact that no increase in free NA occurred is ascribed to the action of catechol-O-methyl-transferase.4. Inhibition of the reuptake of NA and of 5-HT was further studied by using the compounds 5-methyl-alpha-ethyl-meta-tyramine (H 75/12) and 4,alpha-dimethyl-meta-tyramine (H 77/77). The results of these studies also suggested inhibition by THN of the reuptake of 5-HT as well as of NA.5. The action of THN is explained by inhibition of the reuptake of NA and of 5-HT and by release of NA from its stores. However, the possibility is not excluded that, instead of releasing NA from its stores, THN inhibits the enzyme dopamine-beta-hydroxylase.     

Presynaptic alpha 2-adrenoceptor modulation of 5-hydroxytryptamine and noradrenaline release from vascular adrenergic nerves

Modulation of the stimulation-evoked release of 5-hydroxytryptamine (5-HT) and noradrenaline (NA) by presynaptic alpha 2-adrenoceptors was characterized in the perfused mesenteric vascular bed of the rat. The vasoconstrictor response to periarterial nerve stimulation (PNS; 8 Hz), previously abolished in the presence of 30 nM prazosin, was restored after 15 min treatment with 10 microM 5-HT, without a significant effect on the pressor response to 1 nmol of infused NA, which was previously abolished with prazosin. The restored pressor response to PNS was abolished by 100 nM tetrodotoxin and 100 nM ketanserin. Clonidine (1-10 microM) in the presence of prazosin induced a dose-dependent potentiation of the restored pressor response to PNS after 5-HT treatment while BHT 920 (10 nM-1 microM) and 100 nM clonidine inhibited the restored response. In the presence of 100 nM phentolamine, the restored pressor response to PNS was not altered by clonidine, but was inhibited by BHT 920. The PNS (8 Hz)-evoked tritium release in a preparation labeled with [3H]5-HT was facilitated by clonidine (100 nM-10 microM) while BHT 920 (10 nM-1 microM) and cocaine (1-10 microM) reduced the release. Yohimbine (1 microM) antagonized the effects of clonidine and cocaine but not of BHT 920 on the PNS-evoked tritium release. In the preparation labeled with [3H]NA, clonidine did not alter the PNS-evoked tritium release while BHT 920 inhibited it and cocaine facilitated it. Yohimbine did not antagonize the effect of BHT 920.(ABSTRACT TRUNCATED AT 250 WORDS)     

Hypertension and tachycardia produced by inhibition of reuptake of 5-hydroxytryptamine by fluoxetine in the rat

In rats anaesthetized with urethane, increasing the activity of 5-hydroxytryptamine receptors or the level of cuntional serotonin in the brain with the inhibitors of the reuptake of serotonin, fluoxetine, produced both hypertension and tachycardia. The hypertension induced by fluoxetine was significantly inhibited by pretreatment of the animals with ketanserin (a serotonin receptor antagonist), by bilateral vagotomy, spinal transection or bilateral adrenalectomy. On the other hand, the tachycardia induced by fluoxetine was significantly inhibited by pretreatment with ketanserin or bilateral vagotomy, but not by spinal transection or adrenalectomy. The data indicate that fluoxetine acts through serotonin receptors in the central nervous system by influencing autonomic outflow to induce both hypertension and tachycardia.     

Oxidation of 5-hydroxytryptamine and related compounds by mytilus gill plates

Homogenates of gill plates of Mytilus edulis L. used oxygen when 5-hydroxytryptamine was added. The oxidation of 5-hydroxytryptamine was not due to the presence of an amine oxidase, but to that of an enzyme that catalysed the oxidation of other 5-hydroxyindoles (5-hydroxytryptophan, bufotenine). The oxidation was cyanide-sensitive, but was not inhibited by iproniazid. In the reaction a yellowish-brown substance was formed. The occurrence of an amine oxidase in the anterior retractor muscle of the byssus and in the digestive gland was confirmed.     

Effect of a selective 5-hydroxytryptamine reuptake inhibitor on brain extracellular noradrenaline: microdialysis studies using paroxetine

The clinical efficacy of selective serotonin (5-hydroxytryptamine, 5-HT) reuptake inhibitors (SSRIs) is normally attributed to their ability to increase brain 5-HT function although recent preclinical findings indicate that their selectivity for 5-HT over noradrenaline may be less evident in vivo. The present study investigated the effects of the SSRI, paroxetine, on extracellular levels of noradrenaline.Microdialysis was carried out in the hippocampus of the awake rat. In rats treated twice daily for 14 days with paroxetine (5 mg/kg s.c.), dialysate levels of noradrenaline showed a maintained two-fold increase compared to saline-injected controls. Paroxetine (5 mg/kg s.c.) administered once daily for 14 days did not cause a sustained increase in noradrenaline but levels showed a moderate (+58%) increase in response to a paroxetine challenge. Acute injection of paroxetine (5 mg/kg s.c.) did not elevate noradrenaline levels. Paroxetine (5 mg/kg s.c.) elevated dialysate 5-HT after both acute and repeated (twice daily for 14 days) treatment. The paroxetine-induced increase in noradrenaline (and 5-HT) was positively correlated with plasma concentrations of the drug, which were around the therapeutic range. In comparison to paroxetine, desipramine (10 mg/kg s.c.) caused a four-fold increase in dialysate noradrenaline (but did not change 5-HT) following repeated (once daily for 14 days) treatment and a two-fold increase at for acute treatment.In summary, despite its selectivity as a 5-HT reuptake inhibitor, paroxetine increased extracellular levels of noradrenaline in rat hippocampus following repeated administration. We discuss the possibility that a facilitation of noradrenaline function might be involved in the antidepressant effect of paroxetine, and possibly other SSRIs.     

The role of noradrenaline and selective noradrenaline reuptake inhibition in depression.

Depression is a common disorder that impacts on all aspects of a person's life. For the past 10 years, clinicians have focused on serotonin in their treatment of depression. This is largely due to the growing acceptance of the efficacy and safety of the selective serotonin reuptake inhibitors (SSRIs) in comparison with older tricyclic antidepressants (TCAs). However, evidence for a role of noradrenaline in depression has been accumulating for some time, beginning with the discovery that drugs which either caused or alleviated depression acted to alter noradrenaline metabolism. Until recently, the role of noradrenaline in depression was predicted from clinical experience with noradrenergic TCAs (desipramine, nortriptyline and protriptyline) and selective serotonin and noradrenaline reuptake inhibitors (venlafaxine, milnacipran). The licensing of reboxetine, a selective noradrenaline reuptake inhibitor now allows the role of noradrenaline in depression to be investigated directly. This review presents key data from the literature that support a role for noradrenaline in depression taking into account neurophysiology, psychopharmacology and clinical trial data.     

Presynaptic regulation of the release of acetylcholine by 5-hydroxytryptamine.

1 The effect of 5-hydroxytryptamine (5-HT) on the release of acetylcholine (ACh) from bullfrog sympathetic preganglionic nerve terminals and frog sciatic nerve terminals was studied with intra-cellular microelectrodes. 2 The change in transmitter release was measured from the mean quantal content calculated by the variance method from evoked fast e.p.s.ps or e.p.ps in low Ca2+-high Mg2+ Ringer solution. 3 5-HT facilitated the release of ACh in low concentrations and depressed it in relatively high concentrations at both preganglionic and motor nerve terminals. 4 These results suggest the possibility that 5-HT may play a role in regulating cholinergic transmission in general.     

Inhibition of 5-hydroxytryptamine reuptake by amitriptyline and zimelidine and its relationship to their therapeutic action

Rats were rendered tolerant to ethanol by daily gavage of 4–5 g/kg. The degree of motor impairment on the moving belt test and of hypothermia after i.p. test doses of ethanol was measured prior to and at various times during the chronic treatment, to assess the rates of tolerance development. l-Tryptophan (75 mg/kg twice daily) was administered chronically to elevate brain serotonin level. This treatment did not alter the motor impairment or hypothermia produced by the initial test doses of ethanol (2.0 and 2.5 g/kg respectively). However, the development of tolerance to both the motor impairment and hypothermia effects of ethanol was accelerated in the tryptophan-treated rats. This finding complements our earlier observations that depletion of 5-HT with p-CPA slows down tolerance. Blood ethanol measurements at 20 min (motor impairment) or 90 min (hypothermia) after the administration of the test dose reveal no significant difference between the control and tryptophan-treated rats, suggesting that tryptophan did not influence the metabolism of ethanol. This finding supports the hypothesis that brain serotonin modulates the development of tolerance to ethanol.     

Differential inhibition of nucleoside transport systems in mammalian cells by a new series of compounds related to lidoflazine and mioflazine

The sensitivity of facilitated-diffusion and Na(+)-dependent nucleoside transporters to inhibition by a series of novel compounds related to lidoflazine and mioflazine was investigated. Uridine transport by rabbit erythrocytes, which proceeds solely by the nitrobenzylthioinosine (NBMPR)-sensitive facilitated-diffusion system, was inhibited with apparent Ki values of less than 10 nM by lidoflazine, mioflazine, soluflazine and R73-335. These compounds also blocked site-specific [3H]NBMPR binding to rabbit erthrocyte membranes in a competitive fashion. The NBMPR-sensitive system in rat erythrocytes was also inhibited by lidoflazine, mioflazine, soluflazine and R73-335 but was two to three orders of magnitude less sensitive to inhibition than the system in rabbit erythrocytes (apparent Ki 7.3, 2.4, 5.7 and 0.1 microM, respectively). Lidoflazine, mioflazine and R73-335 exhibited a similar potency for the NBMPR-sensitive and -insensitive nucleoside transporters in rat erythrocytes. In contrast, soluflazine was 20- to 100-fold more potent as an inhibitor of the NBMPR-insensitive nucleoside transport component in rat erythrocytes (IC50 of 0.08-0.2 microM) compared to the NBMPR-sensitive nucleoside carrier in these cells (IC50 approximately 10 microM). None of the test compounds were potent inhibits of Na(+)-dependent uridine transport in bovine renal brush-border membrane vesicles. These results indicate that lidoflazine, mioflazine, soluflazine and R73-335 are selective inhibitors of nucleoside transport in animal cells and that the potency of these compounds as nucleoside transport inhibitors is species dependent.     

Modulation of the extracellular 5-hydroxytryptamine brain concentrations by the serotonin and noradrenaline reuptake inhibitor, milnacipran. Microdialysis studies in rats.

We examined the effects of the administration of milnacipran, a dual inhibitor of serotonin (5-hydroxytryptamine, 5-HT) and noradrenaline uptake on the 5-HT output in rat brain. Local milnacipran administration increased the 5-HT output in frontal cortex and the midbrain raphe nuclei 7- and 10-fold by a Ca(2+)- and tetrodotoxin-dependent mechanism. However, the subcutaneous administration of milnacipran (1-60 mg/kg s.c.) elevated the 5-HT output much less in these areas (200-230% of baseline at 60 mg/kg). In hypothalamus, 10 mg/kg s.c. raised 5-HT levels to 170%. The 5-HT1A antagonist WAY-100635 caused a small potentiation of the effects of milnacipran. The baseline 5-HT output was unaffected by 2-week treatments with milnacipran (30 and 60 mg/kg.day). The distinct regional profile and the lack of enhancement of its effects by WAY-100635 and prolonged treatment suggest that milnacipran does not exert its antidepressant action through an enhancement of the serotonergic function.