Modulation of the firing activity of rat serotonin and noradrenaline neurons by (+/-)pindolol

Role of central serotonin and noradrenaline interactions in the antidepressants' action: Electrophysiological and neurochemical evidence

The development of antidepressant drugs, in the last 6 decades, has been associated with theories based on a deficiency of serotonin (5-HT) and/or noradrenaline (NA) systems. Although the pathophysiology of major depression (MD) is not fully understood, numerous investigations have suggested that treatments with various classes of antidepressant drugs may lead to an enhanced 5-HT and/or adapted NA neurotransmissions. In this review, particular morpho-physiological aspects of these systems are first considered. Second, principal features of central 5-HT/NA interactions are examined. In this regard, the effects of the acute and sustained antidepressant administrations on these systems are discussed. Finally, future directions including novel therapeutic strategies are proposed.

Cannabidiol modulates serotonergic transmission and reverses both allodynia and anxiety-like behavior in a model of neuropathic pain

Low dose of cannabidiol ameliorates mechanical allodynia and anxious behavior and restores impaired serotonergic transmission in a neuropathic pain model in rats.

Clinical studies indicate that cannabidiol (CBD), the primary nonaddictive component of cannabis that interacts with the serotonin (5-HT)_1A receptor, may possess analgesic and anxiolytic effects. However, its effects on 5-HT neuronal activity, as well as its impact on models of neuropathic pain are unknown. First, using in vivo single-unit extracellular recordings in rats, we demonstrated that acute intravenous (i.v.) increasing doses of CBD (0.1-1.0 mg/kg) decreased the firing rate of 5-HT neurons in the dorsal raphe nucleus, which was prevented by administration of the 5-HT_1A antagonist WAY 100635 (0.3 mg/kg, i.v.) and the TRPV₁ antagonist capsazepine (1 mg/kg, i.v.) but not by the CB₁ receptor antagonist AM 251 (1 mg/kg, i.v.). Repeated treatment with CBD (5 mg/kg/day, subcutaneously [s.c.], for 7 days) increased 5-HT firing through desensitization of 5-HT_1A receptors. Rats subjected to the spared nerve injury model for 24 days showed decreased 5-HT firing activity, mechanical allodynia, and increased anxiety-like behavior in the elevated plus maze test, open-field test, and novelty-suppressed feeding test. Seven days of treatment with CBD reduced mechanical allodynia, decreased anxiety-like behavior, and normalized 5-HT activity. Antiallodynic effects of CBD were fully prevented by capsazepine (10 mg/kg/day, s.c., for 7 days) and partially prevented by WAY 100635 (2 mg/kg/day, s.c., for 7 days), whereas the anxiolytic effect was blocked only by WAY. Overall, repeated treatment with low-dose CBD induces analgesia predominantly through TRPV₁ activation, reduces anxiety through 5-HT_1A receptor activation, and rescues impaired 5-HT neurotransmission under neuropathic pain conditions.

The allosteric citalopram binding site differentially interferes with neuronal firing rate and SERT trafficking in serotonergic neurons

Citalopram is a clinically applied selective serotonin re-uptake inhibitor for antidepressant pharmacotherapy. It consists of two enantiomers, S-citalopram (escitalopram) and R-citalopram, of which escitalopram exerts the antidepressant therapeutic effect and has been shown to be one of the most efficient antidepressants, while R-citalopram antagonizes escitalopram via an unknown molecular mechanism that may depend on binding to a low-affinity allosteric binding site of the serotonin transporter. However, the precise mechanism of antidepressant regulation of the serotonin transporter by citalopram enantiomers still remains elusive. Here we investigate escitalopram׳s acute effect on (1) serotonergic neuronal firing in transgenic mice that express the human serotonin transporter without and with a mutation that disables the allosteric binding site, and (2) regulation of the serotonin transporter׳s cell surface localization in stem cell-derived serotonergic neurons. Our results demonstrate that escitalopram inhibited neuronal firing less potently in the mouse line featuring a mutation that abolishes the function of the allosteric binding site and induced serotonin transporter internalization independently of the allosteric binding site mechanism. Furthermore, citalopram enantiomers dose-dependently induced serotonin transporter internalization. In conclusion, this study provides new insight into antidepressant effects exerted by citalopram enantiomers in presence and absence of a functional allosteric binding site.

Rethinking 5-HT1A receptors: emerging modes of inhibitory feedback of relevance to emotion-related behavior

The complexities of the involvement of the serotonin transmitter system in numerous biological processes and psychiatric disorders is, to a substantial degree, attributable to the large number of serotonin receptor families and subtypes that have been identified and characterized for over four decades. Of these, the 5-HT(1A) receptor subtype, which was the first to be cloned and characterized, has received considerable attention based on its purported role in the etiology and treatment of mood and anxiety disorders. 5-HT(1A) receptors function both at presynaptic (autoreceptor) and postsynaptic (heteroreceptor) sites. Recent research has implicated distinct roles for these two populations of receptors in mediating emotion-related behavior. New concepts as to how 5-HT(1A) receptors function to control serotonergic tone throughout life were highlights of the proceedings of the 2012 Serotonin Club Meeting in Montpellier, France. Here, we review recent findings and current perspectives on functional aspects of 5-HT(1A) auto- and heteroreceptors with particular regard to their involvement in altered anxiety and mood states.

Transcriptional dysregulation of 5-HT1A autoreceptors in mental illness

The serotonin-1A (5-HT1A) receptor is among the most abundant and widely distributed 5-HT receptors in the brain, but is also expressed on serotonin neurons as an autoreceptor where it plays a critical role in regulating the activity of the entire serotonin system. Over-expression of the 5-HT1A autoreceptor has been implicated in reducing serotonergic neurotransmission, and is associated with major depression and suicide. Extensive characterization of the transcriptional regulation of the 5-HT1A gene (HTR1A) using cell culture systems has revealed a GC-rich "housekeeping" promoter that non-selectively drives its expression; this is flanked by a series of upstream repressor elements for REST, Freud-1/CC2D1A and Freud-2/CC2D1B factors that not only restrict its expression to neurons, but may also regulate the level of expression of 5-HT1A receptors in various subsets of neurons, including serotonergic neurons. A separate set of allele-specific factors, including Deaf1, Hes1 and Hes5 repress at the HTR1A C(-1019)G (rs6295) polymorphism in serotonergic neurons in culture, as well as in vivo. Pet1, an obligatory enhancer for serotonergic differentiation, has been identified as a potent activator of 5-HT1A autoreceptor expression. Taken together, these results highlight an integrated regulation of 5-HT1A autoreceptors that differs in several aspects from regulation of post-synaptic 5-HT1A receptors, and could be selectively targeted to enhance serotonergic neurotransmission.

Effects of quetiapine on monoamine, GABA, and glutamate release in rat prefrontal cortex

INTRODUCTION

The atypical antipsychotic drug, quetiapine (QTP), is effective in schizophrenia and mood disorders, but induces seizures compared to typical antipsychotics.

METHODS

To explore the mechanisms of action of QTP, we determined its effects on extracellular levels of norepinephrine, dopamine, serotonin, gamma-aminobutyric acid (GABA), and glutamate in the medial prefrontal cortex (mPFC) using microdialysis, and neuronal firing in the ventral tegmental area (VTA), locus coeruleus (LC), dorsal raphe nucleus (DRN), and mediodorsal thalamic nucleus (MTN) by telemetry in freely moving rats.

RESULTS

QTP (10 and 30 mg/kg, i.p.) activated neuronal firing in the VTA, LC, and MTN without affecting that in the DRN. QTP increased extracellular levels of norepinephrine, dopamine, and glutamate without affecting serotonin or GABA levels in the mPFC. The stimulatory effects of QTP on norepinephrine and dopamine were mediated by positive alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/glutamatergic and negative GABA-mediated NMDA/glutamatergic regulation.

DISCUSSION

The dopaminergic terminal projecting from the VTA received inhibitory GABA-mediated NMDA/glutamatergic regulation, but not stimulatory AMPA/glutamatergic regulation. However, both dopaminergic and noradrenergic terminals from the LC received stimulatory AMPA/glutamatergic regulation from the MTN, but not inhibitory GABA-mediated NMDA/glutamatergic regulation. These findings correlating neuronal activities in nuclei with neurotransmitter release suggested that the effects of QTP on neurotransmission in the mPFC depend on activated neuronal projections located outside the mPFC. Furthermore, positive interaction between LC and MTN afferents are potentially important in the pharmacological mechanisms of neurotransmitter regulation by QTP and hint at mechanisms underlying the atypical profile of this drug for treatment of schizophrenia and as a mood stabilizer and proconvulsive agent.

Electrophysiological characterization of the effects of asenapine at 5-HT(1A), 5-HT(2A), alpha(2)-adrenergic and D(2) receptors in the rat brain

Asenapine is a psychopharmacologic agent being developed for schizophrenia and bipolar disorder. This study electrophysiologically characterized the in vivo effects of asenapine at dorsal raphe nucleus (DRN) and hippocampus serotonin-1A (5-HT(1A)), ventral tegmental area D(2), locus coeruleus 5-HT(2A,) and alpha(2)-adrenergic receptors in anesthetized rats. Asenapine displayed potent antagonistic activity at alpha(2)-adrenoceptors (ED(50), 85+/-2 microg/kg), 5-HT(2A) (ED(50), 75+/-2 microg/kg) and D(2) receptors (ED(50), 40+/-2 microg/kg) as evidenced by its reversal of clonidine-, DOI-, and apomorphine-induced inhibition of norepinephrine and dopamine neurons. In contrast, asenapine acted as a partial agonist at 5-HT(1A) receptors in DRN and hippocampus, as indicated by blockade of its inhibitory effect on neuronal firing by the 5-HT(1A) antagonist WAY 100635 and the partial inhibition of the suppressant action of 5-HT when co-applied by microiontophoresis. These results confirm that asenapine displays potent antagonistic activity at 5-HT(2A), D(2), alpha(2)-adrenergic receptors and provide evidence to support its 5-HT(1A) partial agonistic activity.

Hypotensive and antiaggregative effects of eugenosedin-B with serotonin and alpha/beta-adrenoceptor antagonistic activities in rats and human platelets

Eugenosedin-B is able to block serotonin (5-HT) and alpha/beta receptors and to inhibit platelet aggregation. In Wistar rats, intravenous injections of eugenosedin-B (2.4, 7.2, 12 micromoL/kg) caused a dose-dependent decrease in blood pressure and heart rate. In contrast, intracisternal injection of eugenosedin-B (0.3, 0.03 micromoL) and an alpha2-antagonist yohimbine (0.03 micromoL) increased blood pressure and heart rate. Eugenosedin-B and yohimbine prevented hypotension induced by intracisternal injection of an alpha2-agonist clonidine (38 pmol). In in vitro experiments, eugenosedin-B (10, 10, 10 M) competitively antagonized norepinephrine-, clonidine-, and 5-HT (10 to 10 M)-induced vasocontractions in isolated rat aorta. It also competitively antagonized the isoproterenol (10 to 10 M)-induced positive inotropic effects in isolated rat atrium. These findings clearly suggest that eugenosedin-B possesses alpha1, alpha2, beta1, and 5-HT2A receptor blocking activities. In isolated rabbit ear artery sensitized with 16 mM K, eugenosedin-B antagonized 5-nonyloxytryptamine- and 5-HT-induced vasocontractions, indicating it also blocked 5-HT1B and 5-HT2A receptors. In radioligand-binding experiments, eugenosedin-B had significant binding affinities on alpha1, alpha2, beta1, 5-HT1B, and 5-HT2A receptors. In human platelets, eugenosedin-B inhibited epinephrine and 5-HT-induced aggregations. It also had competitive binding effects in human platelet with [H]yohimbine (alpha2), [H]ketanserin (5-HT2A). We conclude that hypotensive and vasorelaxant effects of eugenosedin-B can be attributed to its multiple actions on the blockade of 5-HT1B, 5-HT2A, alpha1/2 and beta1 receptors, and its ability to reduce platelet aggregation attributed to its blockade of alpha2 and 5-HT2A receptors.

In vivo electrophysiological assessment of the putative antidepressant Wf-516 in the rat raphe dorsalis, locus coeruleus and hippocampus

Wf-516 is a potential novel antidepressant. It has high affinity for serotonin (5-hydroxytryptamine; 5-HT) transporters, 5-HT(1A) and 5-HT(2A) receptors. In the present study, the pharmacologic properties of Wf-516 were thus assessed using in vivo electrophysiology in the rat dorsal raphe nucleus (DRN), locus coeruleus (LC) and hippocampus. Glass microelectrodes were lowered into the DRN, LC or hippocampus, and neurons were recorded and tested using systemic or microiontophoretic injections of drugs. In the DRN, cumulative doses of 0.5 mg/kg of Wf-516 were injected intravenously and total inhibition of 5-HT neurons firing was obtained with 2.8 +/- 0.3 mg/kg. The administration of 1 mg/kg of Wf-516, which by itself did not induce a change in the firing of 5-HT neurons, markedly attenuated the inhibitory effect of the 5-HT(1A) autoreceptor agonist LSD, indicating that Wf-516 is a 5-HT(1A) autoreceptor antagonist. In the LC, 1 mg/kg of Wf-516 dampened the inhibitory effect of the preferential 5-HT(2A) agonist DOI on norepinephrine (NE) neurons, indicating that Wf-516 is also a 5-HT(2A) receptor antagonist. In the hippocampus, cumulative intravenous doses of Wf-516 significantly increased the recovery time of firing activity of CA(3) pyramidal neurons after 5-HT applications, indicating an inhibitory effect on 5-HT reuptake. Unlike the 5-HT(1A) antagonist WAY100635, Wf-516 did not block the inhibitory effect of microiontophoretic application of 5-HT, indicating that this drug is devoid of 5-HT(1A) receptor antagonistic activity in this postsynaptic structure. These properties of WF-516 define the transporter/receptorial profile of an antidepressant with superior effectiveness.

5-HT1A receptors, gene repression, and depression: guilt by association

The serotonin system is implicated in major depression and suicide and is negatively regulated by somatodendritic 5-HT1A autoreceptors. Desensitization of 5-HT1A autoreceptors is implicated in the 2- to 3-week latency for antidepressant treatments. Alterations in 5-HT1A receptor levels are reported in depression and suicide, and gene knockout of the 5-HT1A receptor results in an anxiety phenotype, suggesting that abnormal transcriptional regulation of this receptor gene may underlie these disorders. The 5-HT1A receptor gene is negatively regulated in neurons by repressors including REST/NRSF, Freud-1, NUDR/Deaf-1, and Hes5. The association with major depression, suicide, and panic disorder of a new functional 5-HT1A polymorphism at C(-1019)G that selectively blocks repression of the 5-HT1A autoreceptor by NUDR further suggests a causative role for altered regulation of this receptor in predisposition to mental illness. The authors review evidence that altered transcription of the 5-HT1A receptor can affect the serotonin system and limbic and cortical areas, leading to predisposition to depression.

Role of 5-HT1A receptors in the mediation of acute citalopram effects: a 8-OH-DPAT challenge study

(1) The study was aimed to investigate the effects of the minimal effective doses of acute citalopram (5 mg/kg), (+/-)-8-hydroxydipropylaminotetralin HBr (8-OH-DPAT; 0.1 mg/kg), and their combined treatment on the rat open field and forced swimming behaviour and post-mortem monoamine content. (2) The animals were prospectively divided into the vehicle- and para-chlorophenylalanine (p-CPA)-pretreated (350 mg/kg) groups. (3) Acute citalopram (5 mg/kg), 8-OH-DPAT (0.1 mg/kg), or their combined treatment had no major effect on the rat open field and forced swimming behaviour. (4) The post-mortem catecholamine content in four brain regions studied was unchanged in all treatment groups. (5) The combined 8-OH-DPAT (0.1 mg/kg) and citalopram (5 mg/kg) treatment partially reversed the p-CPA-induced decrease of serotonin (5-HT) and 5-hydroxy-indolacetic acid (5-HIAA) content. (6) The present experiments demonstrate that the 5-HT1A receptors mediate some of the selective serotonin reuptake inhibitor (SSRI)-induced biochemical phenomena.

Control of serotonergic function in medial prefrontal cortex by serotonin-2A receptors through a glutamate-dependent mechanism

We examined the in vivo effects of the hallucinogen 4-iodo-2,5-dimethoxyamphetamine (DOI). DOI suppressed the firing rate of 7 of 12 dorsal raphe (DR) serotonergic (5-HT) neurons and partially inhibited the rest (ED(50) = 20 microg/kg, i.v.), an effect reversed by M100907 (5-HT(2A) antagonist) and picrotoxinin (GABA(A) antagonist). DOI (1 mg/kg, s.c.) reduced the 5-HT release in medial prefrontal cortex (mPFC) to 33 +/- 8% of baseline, an effect also antagonized by M100907. However, the local application of DOI in the mPFC increased 5-HT release (164 +/- 6% at 100 microm), an effect antagonized by tetrodotoxin, M100907, and BAY x 3702 (5-HT(1A) agonist) but not by SB 242084 (5-HT(2C) antagonist). The 5-HT increase was also reversed by NBQX (AMPA-KA antagonist) and 1S,3S-ACPD (mGluR 2/3 agonist) but not by MK-801 (NMDA antagonist). AMPA mimicked the 5-HT elevation produced by DOI. Likewise, the electrical-chemical stimulation of thalamocortical afferents and the local inhibition of glutamate uptake increased the 5-HT release through AMPA receptors. DOI application in mPFC increased the firing rate of a subgroup of 5-HT neurons (5 of 10), indicating an enhanced output of pyramidal neurons. Dual-label fluorescence confocal microscopic studies demonstrated colocalization of 5-HT(1A) and 5-HT(2A) receptors on individual cortical pyramidal neurons. Thus, DOI reduces the activity of ascending 5-HT neurons through a DR-based action and enhances serotonergic and glutamatergic transmission in mPFC through 5-HT(2A) and AMPA receptors. Because pyramidal neurons coexpress 5-HT(1A) and 5-HT(2A) receptors, DOI disrupts the balance between excitatory and inhibitory inputs and leads to an increased activity that may mediate its hallucinogenic action.

How does pindolol improve antidepressant action?

Since 1994, the beta-adrenoceptor and 5-HT(1A/1B) receptor ligand pindolol has been used to accelerate or enhance the clinical effects of antidepressant drugs, such as the selective 5-HT reuptake inhibitors (SSRIs), that act primarily on 5-HT-containing neurones. Pindolol was initially thought to act by preventing the inhibition of 5-HT release, elicited by SSRIs and other 5-HT-acting drugs, as a result of its ability to antagonize the action of 5-HT at midbrain raphe 5-HT(1A) autoreceptors that control the activity of ascending 5-HT-mediated pathways. However, the partial agonist properties of pindolol at 5-HT(1A) receptors and beta-adrenoceptors suggest that other explanations for its action are also possible. In this article, recent controversial data on the mechanism of action of pindolol, which are crucial for the development of more rapid and efficient antidepressant therapies, will be discussed.

Pindolol augmentation of antidepressant treatment: recent contributions from brain imaging studies

Preclinical studies suggest that augmentation of selective serotonin (5-HT) reuptake inhibitors by the 5-HT(1A) receptor agent pindolol might reduce the delay between initiation of treatment and antidepressant response, an effect largely mediated by blockade of 5-HT(1A) autoreceptors in the dorsal raphe nuclei. Although some controlled clinical trials suggest that pindolol might reduce latency to selective serotonin reuptake inhibitor response in acute depressive episodes, the effect is moderate and highly variable. Recent positron emission tomography studies investigating the occupancy of 5-HT(1A) receptors in humans by pindolol have shown that at the dose used most often in clinical trials the occupancy is low and variable, which might explain the inconsistent clinical results. Positron emission tomography studies also suggest that pindolol might be more potent at blocking 5-HT(1A) autoreceptors than at blocking postsynaptic receptors, a property that may be useful in this pharmacologic strategy. Thus, the positron emission tomography data support the potential of pindolol to augment the antidepressant response of selective serotonin reuptake inhibitors, but also imply that this potential has not been fully evaluated. Here we review the clinical trials, the positron emission tomography studies, and the possible mechanisms of pindolol augmentation. It is also suggested that positron emission tomography may be used to define therapeutic dosing early on in the process of clinical evaluation of new treatment strategies.

Pindolol excites dopaminergic and adrenergic neurons, and inhibits serotonergic neurons, by activation of 5-HT1A receptors

Pindolol accelerates the clinical actions of selective serotonin reuptake inhibitors (SSRIs) in man, and modulates extracellular levels of monoamines in corticolimbic structures in rats. Herein, we examined its influence upon electrical activity of serotonergic, dopaminergic and adrenergic perikarya in the dorsal raphe nucleus (DRN), ventral tegmental area (VTA) and locus coeruleus (LC) of anaesthetized rats. In analogy to the serotonin1A (5-HT1A) agonist, 8-OH-DPAT (-100%), pindolol dose-dependently (0.063- 1.0 mg/kg) decreased (-70%) the firing rate of serotonergic neurons. The inhibitory action of pindolol was abolished by the selective 5-HT1A antagonist, WAY-100,635 (0.031 mg/kg). In contrast, 8-OH-DPAT (+26%) and pindolol (0.25-4.0 mg/kg, +60%) dose-dependently increased the firing rate of dopaminergic cells. Of 57 neurons recorded (pindolol, 2.0 mg/kg), 36 (63%) were excited, 11 (19%) were unaffected and 10 (18%) were inhibited. This variable influence could be attributed to regularly firing neurons in the parabrachial subdivision, inasmuch as all neurons in the paranigral subnucleus were excited. The facilitation of firing by pindolol was accompanied by an increase in burst firing throughout the VTA. Both the increases in burst firing and in firing rate were reversed by WAY-100,635 (0.031 mg/kg). Finally, the electrical activity of adrenergic neurons was dose-dependently enhanced by 8-OH-DPAT and pindolol (+99% and +83%, respectively). WAY-100,635 reversed this excitation and, itself, inhibited the activity of adrenergic neurons. In conclusion, via engagement of 5-HT1A receptors, pindolol inhibits serotonergic, and activates dopaminergic and adrenergic, neurons in anaesthetized rats. Such actions may contribute to its influence upon mood, both alone and in association with antidepressant agents.

5-HT(1A) agonist potential of pindolol: electrophysiologic studies in the dorsal raphe nucleus and hippocampus

BACKGROUND

The ability of pindolol to block 5-HT(1A) autoreceptors on serotonin-containing neurons in the raphe nuclei is thought to underlie the clinical reports of enhanced efficacy and rate of improvement in depressed patients treated with pindolol/selective serotonin reuptake inhibitor (SSRI) combinations. Selectivity for somatodendritic 5-HT(1A) autoreceptors is a crucial requirement, as blockade of postsynaptic 5-HT(1A) sites may jeopardize the therapeutic response. Previous investigators have probed the effects of pindolol on serotonergic dorsal raphe cell firing in animal species; here we confirm their findings and extend them to include observations on postsynaptic 5-HT(1A) receptors in the hippocampus.

METHODS

Extracellular single-unit recordings were made in rats using standard electrophysiologic techniques. Firing rates of serotonin-containing neurons in the dorsal raphe nucleus and CA3 hippocampal pyramidal neurons were monitored and the effects of pindolol given alone or in combination with an SSRI (fluoxetine) or a 5-HT(1A) antagonist (WAY-100,635) were determined.

RESULTS

Pindolol inhibited the firing rates of serotonergic dorsal raphe neurons in a dose-dependent manner. Recovery to baseline firing rates was gradual, but this inhibition could be acutely reversed by WAY-100,635. A range of pindolol doses failed to block the inhibitory effects of fluoxetine on dorsal raphe cell firing. In the hippocampus, pindolol also inhibited cell firing as a function of dose, although these effects were insensitive to WAY-100,635 treatment.

CONCLUSIONS

The ability of pindolol to inhibit serotonergic dorsal raphe cell firing is indicative of its agonist potential and is consistent with previous studies. The lack of observable antagonism of the SSRI-induced slowing of raphe unit activity casts doubt on the suitability of this mechanism of action to account for the positive findings in clinical studies utilizing pindolol/SSRI combinations. The 5-HT(1A)-independent inhibition of hippocampal CA3 cell firing by pindolol suggests that this compound invokes multiple pharmacologic actions, all of which need to be assimilated into any proposed mechanism of action.

The augmentation hypothesis for improvement of antidepressant therapy: is pindolol a suitable candidate for testing the ability of 5HT1A receptor antagonists to enhance SSRI efficacy and onset latency?

The development of selective serotonin reuptake inhibitors (SSRIs) provided a major advancement in the treatment of depression. However, these drugs suffer from a variety of drawbacks, most notably a delay in the onset of efficacy. One hypothesis suggests that this delay in efficacy is due to a paradoxical decrease in serotonergic (5-HT) neuronal impulse flow and release, following activation of inhibitory presynaptic 5-HT1A autoreceptors, following acute administration of SSRIs. According to the hypothesis, efficacy is seen only when this impulse flow is restored following desensitization of 5-HT1A autoreceptors and coincident increases in postsynaptic 5-HT levels are achieved. Clinical proof of this principal has been suggested in studies that found a significant augmenting effect when the beta-adrenergic/5-HT1A receptor antagonist, pindolol, was coadministered with SSRI treatment. In this article, we review preclinical electrophysiological and microdialysis studies that have examined this desensitization hypothesis. We further discuss clinical studies that utilized pindolol as a test of this hypothesis in depressed patients and examine preclinical studies that challenge the notion that the beneficial effect of pindolol is due to functional antagonism of the 5-HT1A autoreceptors.