Electrophysiological and neurochemical evidence that pindolol has agonist properties at the 5-HT1A autoreceptor in vivo.

1. It has been hypothesized that 5-HT1A autoreceptor antagonists may enhance the therapeutic efficacy of SSRIs and other antidepressants. Although early clinical trials with the beta-adrenoceptor/5-HT1 ligand, pindolol, were promising, the results of recent more extensive trials have been contradictory. Here we investigated the actions of pindolol at the 5-HT1A autoreceptor by measuring its effect on 5-HT neuronal activity and release in the anaesthetized rat. 2. Pindolol inhibited the electrical activity of 5-HT neurones in the dorsal raphe nucleus (DRN). This effect was observed in the majority of neurones tested (10/16), was dose-related (0.2-1.0 mg kg(-1), i.v.), and was reversed by the 5-HT1A receptor antagonist, WAY 100635 (0.1 mg kg(-1), i.v.), in 6/7 cases tested. 3. Pindolol also inhibited 5-HT neuronal activity when applied microiontophoretically into the DRN in 9/10 neurones tested. This effect of pindolol was current-dependent and blocked by co-application of WAY 100635 (3/3 neurones tested). 4. In microdialysis experiments. pindolol caused a dose-related (0.8 and 4 mg kg(-1), i.v.) fall in 5-HT levels in dialysates from the frontal cortex (under conditions where the perfusion medium contained 1 microM citalopram). In rats pretreated with WAY 100635 (0.1 mg kg(-1), i.v.), pindolol (4 mg kg(-1), i.v.) did not decrease, but rather increased 5-HT levels. 5. We conclude that, under the experimental conditions used in this study, pindolol displays agonist effects at the 5-HT1A autoreceptor. These data are relevant to previous and ongoing clinical trials of pindolol in depression which are based on the rationale that the drug is an effective 5-HT1A autoreceptor antagonist.

Effect of a selective 5-HT reuptake inhibitor in combination with 5-HT1A and 5-HT1B receptor antagonists on extracellular 5-HT in rat frontal cortex in vivo.

1. Selective 5-hydroxytryptamine (5-HT; serotonin) reuptake inhibitors (SSRIs) cause a greater increase in extracellular 5-HT in the forebrain when the somatodendritic 5-HT1A autoreceptor is blocked. Here, we investigated whether blockade of the terminal 5-HT1B autoreceptor influences a selective 5-HT reuptake inhibitor in the same way, and whether there is an additional effect of blocking both the 5-HT1A and 5-HT1B autoreceptors. 2. Extracellular 5-HT was measured in frontal cortex of the anaesthetized rat by use of brain microdialysis. In vivo extracellular recordings of 5-HT neuronal activity in the dorsal raphe nucleus (DRN) were also carried out. 3. The selective 5-HT reuptake inhibitor, paroxetine (0.8 mg kg-1, i.v.), increased extracellular 5-HT about 2 fold in rats pretreated with the 5-HT1A receptor antagonist, WAY100635. When administered alone neither paroxetine (0.8 mg kg-1, i.v.) nor WAY100635 (0.1 mg kg-1, i.v.) altered extracellular 5-HT levels. 4. Paroxetine (0.8 mg kg-1, i.v.) did not increase 5-HT in rats pretreated with the 5-HT1B/D receptor antagonist, GR127935 (1 mg kg-1, i.v.). GR127935 (1 and 5 mg kg-1, i.v.) had no effect on extracellular 5-HT when administered alone. 5. Interestingly, paroxetine (0.8 mg kg-1, i.v.) caused the greatest increase in 5-HT (up to 5 fold) when GR127935 (1 or 5 mg kg-1, i.v.) was administered in combination with WAY100635 (0.1 mg kg-1, i.v.). Administration of GR127935 (5 mg kg-1, i.v.) plus WAY100635 (0.1 mg kg-1, i.v.) without paroxetine, had no effect on extracellular 5-HT in the frontal cortex. 6. Despite the lack of effect of GR127935 on 5-HT under basal conditions, when 5-HT output was elevated about 3 fold (by adding 1 microM paroxetine to the perfusion medium), the drug caused a dose-related (1 and 5 mg kg-1, i.v.) increase in 5-HT. 7. By itself, GR127935 slightly but significantly decreased 5-HT cell firing in the DRN at higher doses (2.0-5.0 mg kg-1, i.v.), but did not prevent the inhibition of 5-HT cell firing induced by paroxetine. 8. In summary, our results suggest that selective 5-HT reuptake inhibitors may cause a large increase in 5-HT in the frontal cortex when 5-HT autoreceptors on both the somatodendrites (5-HT1A) and nerve terminals (5-HT1B) are blocked. This increase is greater than when either set of autoreceptors are blocked separately. The failure of a 5-HT1B receptor antagonist alone to enhance the effect of the selective 5-HT reuptake inhibitor in our experiments may be related to a lack of tone on the terminal 5-HT1B autoreceptor due to a continued inhibition of 5-HT cell firing. These results are discussed in relation to the use of 5-HT autoreceptor antagonists to augment the antidepressant effect of selective 5-HT reuptake inhibitors.

Inhibition of 5-HT cell firing in the DRN by non-selective 5-HT reuptake inhibitors: studies on the role of 5-HT1A autoreceptors and noradrenergic mechanisms.

Improved clinical antidepressant efficacy may result if the acute inhibition of 5-HT cell firing induced by antidepressants is prevented. Here we examined whether inhibition of 5-HT cell firing by non-selective 5-HT uptake inhibiting antidepressant drugs is reversed by a selective 5-HT1A receptor antagonist. In addition, we examined whether concomitant blockade of NA uptake offsets the inhibition of 5-HT cell firing resulting from 5-HT uptake blockade. Antidepressants which block 5-HT uptake (paroxetine, clomipramine, amitriptyline, venlafaxine), all caused dose-dependent and complete inhibition of 5-HT cell firing. Desipramine, a selective NA uptake blocker, caused a slight reduction in firing. The selective 5-HT1A receptor antagonist, WAY 100635, reversed the inhibition of 5-HT cell firing induced by clomipramine, amitriptyline, venlafaxine, and paroxetine, but not that induced by the alpha 1 adrenoceptor antagonist, prazosin. Desipramine, at a dose which increased extracellular NA in the DRN, reversed the effect of prazosin but did not alter the ability of paroxetine to inhibit 5-HT cell firing. Our data indicate that antidepressant drugs with 5-HT uptake blocking properties inhibit 5-HT cell firing via activation of 5-HT1A autoreceptors, and do so irrespective of their effects on NA uptake. These data are discussed in relation to the application of 5-HT1A receptor antagonists to enhance the clinical efficacy of antidepressant drugs.

Effects of co-administration of a monoamine oxidase inhibitor and a 5-HT1A receptor antagonist on 5-hydroxytryptamine cell firing and release.

We report the effects of the monoamine oxidase inhibitor, tranylcypromine, combined with the 5-HT1A receptor antagonist, N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl cyclohexanecarboxamide (WAY 100635), on both 5-hydroxytryptamine (5-HT) cell firing and cortical extracellular 5-HT in the rat. Tranylcypromine inhibited 5-HT cell firing in the dorsal raphe nucleus dose-dependently (ED50 5 mg/kg i.v.). In microdialysis experiments, tranylcypromine (5 mg/kg i.v.) increased extracellular 5-HT in the frontal cortex. WAY 100635 (0.1 mg/kg i.v.) both reversed the inhibition of 5-HT cell firing and facilitated the increase in extracellular 5-HT. In conclusion, WAY 100635 enhances the effect of tranylcypromine on presynaptic 5-HT function. These data are relevant to clinical evidence that co-therapy with a 5-HT1A receptor antagonist improves the antidepressant efficacy of a monoamine oxidase inhibitor.

The role of 5-HT1A autoreceptors and alpha 1-adrenoceptors in the inhibition of 5-HT release--II NAN-190 and SDZ 216-525.

Novel 5-HT1A receptor antagonists, WAY 100135 and WAY 100635, were used to test the involvement of 5-HT1A receptors in the decrease of hippocampal extracellular 5-HT induced by the 5-HT1A/alpha 1 ligands, NAN-190 and SDZ 216-525. Using microdialysis in anaesthetized rats, it was found that WAY 100135 (3 mg/kg s.c.) and WAY 100635 (0.3 mg/kg s.c.) antagonised the decrease of 5-HT induced by the 5-HT1A receptor agonist 8-OH-DPAT (0.025 mg/kg s.c.) but did not alter 5-HT when administered alone. Both NAN-190 (0.03 and 0.3 mg/kg s.c.) and SDZ 216-525 (1 mg/kg s.c.) decreased 5-HT. The effect of 0.03 mg/kg s.c. NAN-190 was antagonised by WAY 100135 (3 mg/kg s.c.) and WAY 100635 (0.3 mg/kg s.c.). The effect of SDZ 216-525 (1 mg/kg s.c.) was also blocked by WAY 100635 (0.3 mg/kg s.c.). However, the 5-HT response to a high dose of NAN-190 (0.3 mg/kg s.c.) was not antagonised by WAY 100635 (0.3 or 3 mg/kg s.c.). Our experiments using WAY 100635 and WAY 100135 provide clear evidence that NAN-190 and SDZ 216-525 act as agonists at the 5-HT1A autoreceptor, supporting our earlier studies using the non-selective 5-HT1A antagonist, pindolol. However, our data reveal that, at least in the case of NAN-190, non-5-HT1A receptor mechanisms mediate the decrease of 5-HT induced by higher doses. A lack of specificity of NAN-190 (and possibly SDZ 216-525) at high doses may explain the failure of previous studies to detect a 5-HT1A receptor agonist action.

Interaction between a selective 5-HT1A receptor antagonist and an SSRI in vivo: effects on 5-HT cell firing and extracellular 5-HT.

1. The acute inhibitory effect of selective 5-hydroxytryptamine (serotonin) reuptake inhibitors (SSRIs) on 5-HT neuronal activity may offset their ability to increase synaptic 5-HT in the forebrain. 2. Here, we determined the effects of the SSRI, paroxetine, and a novel selective 5-HT1A receptor antagonist, WAY 100635, on 5-HT cell firing in the dorsal raphé nucleus (DRN), and on extracellular 5-HT in both the DRN and the frontal cortex (FCx). Extracellular electrophysiological recording and brain microdialysis were used in parallel experiments, in anaesthetized rats. 3. Paroxetine dose-dependently inhibited the firing of 5-HT neurones in the DRN, with a maximally effective dose of approximately 0.8 mg kg-1, i.v. WAY 100635 (0.1 mg kg-1, i.v.) both reversed the inhibitory effect of paroxetine and, when used as a pretreatment, caused a pronounced shift to the right of the paroxetine dose-response curve. 4. Paroxetine (0.8 mg kg-1, i.v.), doubled extracellular 5-HT in the DRN, but did not alter extracellular 5-HT in the FCx. A higher dose of paroxetine (2.4 mg kg-1, i.v.) did increase extracellular 5-HT in the FCx, but to a lesser extent than in the DRN. Whereas 0.8 mg kg-1, i.v. paroxetine alone had no effect on extracellular 5-HT in the FCx, in rats pretreated with WAY 100635 (0.1 mg kg-1), paroxetine (0.8 mg kg-1, i.v.) markedly increased extracellular 5-HT in the FCx. 5. In conclusion, pretreatment with the selective 5-HT1A receptor antagonist, WAY 100635, blocked the inhibitory effect of paroxetine on 5-HT neuronal activity in the DRN and, at the same time, markedly enhanced the effect of paroxetine on extracellular 5-HT in the FCx. These results may be relevant to recent clinical observations that 5-HT1A receptor antagonists in combination with SSRIs have a rapid onset of antidepressant effect.