Acceleration of the effect of selected antidepressant drugs in major depression by 5-HT1A antagonists

Serotonergic autoreceptor blockade in the reduction of antidepressant latency: personality variables and response to paroxetine and pindolol

No antidepressant currently in use exerts a significant antidepressant effect for at least two to three weeks after the patient starts taking it. Open studies suggest that, for selective serotonergic re-uptake inhibitor (SSRI) antidepressants, this latency may be reduced when the drug is taken with the 5HT1A receptor blocker pindolol. We have undertaken a randomised, placebo controlled, double blind trial of augmentation of the selective SSRI antidepressant paroxetine in combination with pindolol. All our patients (n = 54; mean age 36 [range 19-65]) met criteria for major depression and received a standard dose (20 mg o.d.) of paroxetine plus, randomly, either pindolol (2.5 mg t.d.s.) or placebo for six weeks. We examined personality variables in 48 consecutive subjects according to a short version (TCI-125) of Cloninger et al's self-rated Temperament and Character Inventory (Cloninger et al., 1994) and correlated the results with clinical responses in the trial. The results suggest that personality can influence clinical outcome. After the double blind period patients were offered paroxetine 20 mg or 40 mg for up to 6 months. Twenty-six patients took this up. The results suggest that high scores in the temperament dimension of Reward Dependence and low scores in the temperament dimension of Harm Avoidance had a better outcome at 6 weeks. Patients who had received paroxetine and pindolol during the trial and who reported high Novelty Seeking and low Harm Avoidance scores had a better outcome at 6 weeks and 6 months. We suggest that temperament factors may influence outcome of antidepressant treatment.

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.

Human sleep EEG following the 5-HT1A antagonist pindolol: possible disinhibition of raphe neuron activity

The sleep electroencephalogram (EEG) was used to assay central effects of pindolol (10 and 30 mg p.o.), a mixed beta(1/2)-adrenoceptor/5-hydroxytryptamine (5-HT)(1A/1B) receptor blocker, in humans. Compared to placebo, pindolol produced a dose-related suppression of rapid-eye-movement (REM) sleep, including a prolongation of REM latency, and a decrease of REM time and REM density. At the higher dose, it also reduced EEG spectral power during non-REM sleep in portions of the delta, theta, and alpha frequencies (1.125-5.125 Hz, 7.125-9.625 Hz). By contrast, betaxolol (20 mg p.o.), a selective beta1-antagonist devoid of serotonergic affinity, affected neither REM sleep nor EEG power. REM sleep is, in part, under the inhibitory control of serotonergic neurons projecting from the dorsal raphe nucleus to pontine cholinergic/cholinoceptive cells. The EEG power spectrum induced by pindolol tended to be opposite to what has previously been reported for ipsapirone, a 5-HT1A agonist. Therefore, the present data, tentatively, are consistent with the contention that pindolol inhibits, possibly selectively, somatodendritic 5-HT1A autoreceptors in humans and may antagonize self-inhibition of midbrain raphe nuclei 5-HT neurons.

Selective decline of 5-HT1A receptor binding sites in rat cortex, hippocampus and cholinergic basal forebrain nuclei during aging

The effect of aging on 5-HT1A receptor binding in several forebrain areas associated with the basal forebrain cholinergic system was investigated in rats of 3-, 24- and 30-months-old by receptor autoradiography and biochemical binding assay using [3H]8-OH-DPAT as a ligand. Autoradiographic measurements demonstrated a marked region-specific decline of ligand binding in: (i) regions of the basal forebrain cholinergic cell groups, i.e. the medial septum, diagonal band nuclei and magnocellular nucleus basalis, (ii) the frontal and parietal neocortex and (iii) the dentate gyrus of the hippocampus. No change or only a slight decrease of the 5-HT1A receptor density was found in other areas investigated: the CA1 and CA3 sectors of hippocampus, the cingular and perirhinal cerebral cortex and the lateral septum. The autoradiographic findings were substantiated by the biochemical binding assay, which revealed a comparable loss of 5-HT1A receptor in the hippocampus and neocortex at the age of 30 months. The results clearly show that with increasing age the decrement of 5-HT1A receptor binding in the rat forebrain is remarkably region-selective and particularly affects the cholinergic cell groups that innervate cortex and hippocampus. This phenomenon appears to be especially significant in relation to the neuronal substrates underlying the age-related alterations of mood and cognition.

Autoreceptor antagonists enhance the effect of the reuptake inhibitor citalopram on extracellular 5-HT: this effect persists after repeated citalopram treatment

The effect of repeated administration of the reuptake inhibitor citalopram (10 mg/kg s.c., b.i.d. for 14 days) or saline on extracellular 5-hydroxytryptamine (5-HT) and autoreceptor sensitivity was assessed using microdialysis in the frontal cortex (FCx) and dorsal hippocampus (DH) of unanesthetized rats. Acute citalopram (5 mg/kg s.c.) challenge produced significant increases in DH and FCx 5-HT. The nonselective 5-HT1A/1B receptor antagonist (-)+penbutolol (8 mg/kg s.c.), administered 2 hr after citalopram challenge, significantly enhanced 5-HT in FCx and DH of both the chronic citalopram and saline pretreatment groups. Administration of the selective 5-HT1A receptor antagonist WAY 100635 (0.3 mg/kg s.c.) after citalopram challenge significantly enhanced 5-HT in FCx but not DH of both pretreatment groups. This suggests that there may be differences between DH and FCx in regulation of 5-HT release. Nevertheless, these results provide evidence that 5-HT autoreceptors are still active in restraining 5-HT release. Nevertheless, these results provide evidence that 5-HT autoreceptors are still active in restraining 5-HT release even after repeated administration of an antidepressant drug.

Effect of 5-HT1A receptor antagonists on citalopram-induced increase in extracellular serotonin in the frontal cortex, striatum and dorsal hippocampus

The aim of the present study was to compare the effects of citalopram, either alone or combined with 5-HT1A receptor antagonists, on extracellular serotonin levels in brain regions innervated by the dorsal or median raphe nuclei. Using intracerebral microdialysis in awake rats with separate probes in the frontal cortex or dorsal hippocampus, we studied the ability of 8 mg/kg s.c. (-)penbutolol, a beta-adrenoceptor antagonist with antagonist action at 5-HT1A and 5-HT1B receptors, and 0.3 mg/kg s.c. WAY-100635, a selective 5-HT1A receptor blocker, to modify the effect of 1 and 10 mg/kg i.p. citalopram on extracellular serotonin. Both doses of citalopram had more effect on extracellular serotonin levels in the dorsal hippocampus than in the frontal cortex. The effect of 1 mg/kg citalopram was significantly potentiated by (-)penbutolol in the frontal cortex only, but a clear-cut potentiation of the effect of citalopram was seen in both regions at a dose of 10 mg/kg. The effect of 10 mg/kg citalopram was potentiated by WAY-100635 in the frontal cortex but not in the dorsal hippocampus. In a second set of experiments, the combined effect of WAY-100635 and citalopram was studied in the same rat implanted with vertical probes in the striatum and dorsal hippocampus. Citalopram (1 and 10 mg/kg i.p.) raised extracellular serotonin to a similar extent in both regions. However, 0.3 mg/kg s.c. WAY-100635 potentiated the effect of 10 mg/kg citalopram in the striatum but not in the dorsal hippocampus. The results suggest that only a combined blockade of 5-HT1A and 5-HT1B receptors potentiates the effect of citalopram on extracellular concentrations of serotonin in the dorsal hippocampus. The findings may be relevant in designing clinical trials aimed at enhancing the antidepressant action of selective serotonin re-uptake inhibitors by combining them with serotonin receptor antagonists.

Potentiation of fluoxetine-induced penile erections by combined blockade of 5-HT1A and 5-HT1B receptors

The serotonin reuptake inhibitor, fluoxetine (10.0 mg/kg, s.c.), elicited penile erections in rats. Selective blockade of 5-HT1A autoreceptors with WAY 100,635 ((N-(2-[4-(2-methoxyphenyl)-1- piperazinyl]ethyl)-N-(2-pyridinyl) cyclo-hexanecarboxamide) (0.16 mg/kg, s.c.), or of 5-HT1B autoreceptors with GR 127,935 (N-[4-methoxy-3-(4-methylpiperazin-1-yl)phenyl]-2'-methyl-4'-(5-me thyl- 1,2,4-oxadiazol-3-yl)biphenyl-4-carboxamide) (2.5 mg/kg, s.c.), slightly (1.5- to 2-fold) increased fluoxetine-induced penile erections. However, conjoint administration of WAY 100,635 and GR 127,935 markedly (5-fold) potentiated induction of penile erections by fluoxetine. Penile erections were abolished by the novel 5-HT2C receptor antagonist, SB 206,553 (5 methyl-1-(3-pyridil-carbamoyl)-1,2,3,5- tetrahydropyrrolo[2,3-f]indole). These data provide functional evidence for redundancy in autoreceptor control of 5-HT release. Combined blockade of 5-HT1A and 5-HT1B autoreceptors markedly enhances the actions of serotonin reuptake inhibitors.

Polarized expression of the antidepressant-sensitive serotonin transporter in epinephrine-synthesizing chromaffin cells of the rat adrenal gland

Antidepressant-sensitive serotonin (5-hydroxytryptamine, 5HT) transporters (SERTs) clear the amine from extracellular spaces in the CNS and periphery as a mechanism for transmitter inactivation and recycling. Although it is known that SERTs are preferentially expressed on basolateral domains in transfected epithelial cells, details of the transporter's membrane localization in vivo are lacking. 5HT and 5HT receptors have been identified in the rodent adrenal gland. Using SERT antagonist autoradiography, we establish the presence of antidepressant-sensitive transport sites in the rat adrenal medulla. Immunofluorescence experiments using antibodies specific for the SERT COOH and NH2 termini, for 5HT, or for catecholamine biosynthetic enzymes suggest that SERT mediates intra-cellular 5HT accumulation by epinephrine-secreting chromaffin cells. Using confocal microscopy, we establish that SERT expression is nonuniformly distributed along the plasma membrane of chromaffin cells. Notably, SERT immunoreactivity is largely absent from plasma membranes bordering smooth muscle that surrounds vascular sinusoids. Rather, SERT is highly expressed in membranes adjoining other chromaffin cells, consistent with a role for 5HT and SERT in autocrine or paracrine control of chromaffin cell physiology. SNAP-25, a t-SNARE protein implicated in neurotransmitter release, was found to colocalize with SERT. In contrast, Na,K ATPase and NCAM are uniformly distributed along the entire perimeter of chromaffin cell membranes. These findings underscore a role for 5HT and SERT in adrenal physiology, reveal unrecognized polarity of chromaffin cell plasma membranes, and warrant a consideration of common targeting mechanisms localizing amine transporters near release sites.

The 5-HT1A antagonist WAY-100635 selectively potentiates the presynaptic effects of serotonergic antidepressants in rat brain

The increases in extracellular serotonin (5-hydroxytryptamine; 5-HT) produced by some antidepressent drugs in forebrain are attenuated by the activation of somatodendritic 5-HT1A autoreceptors by the excess 5-HT induced by these agents in the midbrain raphe. Using microdialysis, we have examined the effects of the selective 5-HT1A antagonist WAY-100635 in rats pretreated with the selective 5-HT reuptake inhibitors (SSRIs) citalopram, fluoxetine, fluvoxamine, the tricyclic antidepressants clomipramine and desipramine and the monoamine oxidase inhibitor phenelzine. WAY-100635 markedly potentiated the increases in 5-HT produced by the SSRIs, clomipramine and phenelzine but it did not alter that produced by desipramine. These results indicate that the effects of serotonergic antidepressant drugs (but not those of desipramine, which mainly blocks noradrenaline reuptake) can be potentiated by 5-HT1A autoreceptor blockade.