Elsevier

Neuropharmacology

Volume 42, Issue 8, June 2002, Pages 1031-1038
Neuropharmacology

Repeated treatment with antidepressants differentially alters 5-HT1A agonist-stimulated [35S]GTPγS binding in rat brain regions

https://doi.org/10.1016/S0028-3908(02)00064-3Get rights and content

Abstract

Electrophysiological studies have led to the proposal that the neurobiological mechanism(s) underlying drug therapy of anxiety and depression involve(s) regionally specific adaptations in 5-HT1A receptor sensitivity. Depending on the drug utilized, a decrease in sensitivity of inhibitory somatodendritic autoreceptors, an increase in sensitivity of postsynaptic receptors, or both alterations, occur after several weeks of treatment. This hypothesis was tested using N,N-dipropyl-5-carboxamidotryptamine-stimulated guanosine-5′-O-(3-thio)triphosphate ([35S]GTPγS) binding assessed by autoradiography. Rats were treated for 21 days with one of four different anxiolytic/antidepressant drugs (in mg/kg): fluoxetine (10), imipramine (10), clorgyline (1), ipsapirone (2×20) or saline. Three brain regions rich in 5-HT1A receptors were examined: the dorsal raphe (somatodendritic), the dorsal hippocampus (postsynaptic) and the lateral septum (postsynaptic). Only imipramine (+17%) and fluoxetine (+54%) significantly increased agonist-stimulated binding in the dorsal hippocampus; all drugs except imipramine significantly decreased binding in the dorsal raphe (−19 to −41%). These results generally support the concept of a net enhancement of hippocampal 5-HT neurotransmission via one or more 5-HT receptor subtypes. The most consistent effect, however, was a significant decrease in stimulated [35S]GTPγS binding in the lateral septum after all four treatments (−14 to −23%), suggesting that this may be a heretofore unrecognized common outcome of antidepressant treatment deserving further study.

Introduction

Two opposing theories have been advanced regarding alterations in 5-HT neurotransmission occurring on treatment with anxiolytics/antidepressants. Because 5-HT1A agonists (buspirone, gepirone, ipsapirone) display anxiolytic/antidepressive activity in various animal models (De Vry et al., 1991) and humans (Cott et al., 1988, Robinson, 1991), inhibit 5-HT neuronal firing (Sprouse and Aghajanian, 1988), and decrease transmitter synthesis (Meller et al., 1990) and release (Hjorth and Sharp, 1991) in forebrain areas, one hypothesis suggested that a decrease in 5-HT transmission underlies their therapeutic effects (De Vry et al., 1991; Sommermeyer et al., 1993). However, it is well known that several weeks of treatment with anxiolytics/antidepressants is required before the therapeutic effects become apparent. Thus, the acute reduction in 5-HT neurotransmission elicited by these agents is unlikely to be related to their therapeutic effects. The opposite hypothesis, put forth principally by De Montigny and colleagues (Blier and De Montigny, 1994, Haddjeri et al., 1998), proposes that an enhancement of 5-HT1A receptor-mediated neurotransmission in forebrain targets underlies therapeutic effects, and that this occurs only after repeated anxiolytic/antidepressant treatment. Utilizing electrophysiological approaches, these investigators showed that repeated treatment with 5-HT1A agonists (Blier and De Montigny, 1990, Blier and De Montigny, 1994, Dong et al., 1997), selective serotonin reuptake inhibitors (SSRIs; e.g. fluoxetine) (Chaput et al., 1986, De Montigny et al., 1990, Blier and De Montigny, 1994), or monoamine oxidase inhibitors (MAOIs; e.g. clorgyline) (Blier and De Montigny, 1985) desensitized the inhibitory somatodendritic 5-HT1A autoreceptor in the raphe nuclei; the sensitivity of postsynaptic 5-HT1A receptors in the hippocampus was not altered (except by clorgyline, which desensitized hippocampal postsynaptic receptors) (Blier et al., 1988, Blier et al., 1990). Thus, after repeated treatment, autoreceptor-mediated neuronal inhibition was reduced or abolished; coupled with (generally) unaltered sensitivity at postsynaptic receptors, the results supported the idea of a net increase in 5-HT neurotransmission in forebrain areas (specifically in the hippocampus), via 5-HT1A or other (Martin et al., 1998) 5-HT receptor subtypes. In contrast, tricyclic antidepressants (TCAs; e.g. imipramine) increased postsynaptic 5-HT1A receptor sensitivity without altering autoreceptor sensitivity (De Montigny and Aghajanian, 1978, Chaput et al., 1991), at least potentially yielding the same (5-HT1A receptor-mediated) net effect. This hypothesis is consistent with clinical evidence that treatments that presumably decrease 5-HT neurotransmission worsen depression (Shopsin et al., 1976, Delgado et al., 1990) and that repeated SSRI treatment increases basal extracellular 5-HT levels in forebrain areas (Rutter et al., 1994, Kreiss and Lucki, 1995); perhaps most convincingly, an electrophysiological study recently showed that repeated treatment with all four drug classes increased tonic activation of postsynaptic 5-HT1A receptors in the hippocampus (Haddjeri et al., 1998).

The effects of repeated treatment with these drugs in other models of pre- and postsynaptic 5-HT1A receptor activity are less clear. After chronic 5-HT1A agonist or SSRI treatment both decreases and no changes have been reported in 5-HT1A receptor density in the dorsal raphe (Schechter et al., 1990, Fanelli and McMonagle-Strucko, 1992), autoreceptor-mediated 5-HT synthesis inhibition (Schechter et al., 1990, Bohmaker et al., 1993), autoreceptor-mediated inhibition of 5-HT release in forebrain areas (Hjorth and Auerbach, 1994, Kreiss and Lucki, 1995), and forskolin-stimulated adenylyl cyclase activity at 5-HT1A receptors in the hippocampus (Varrault et al., 1991, Newman et al., 1992).

The objective of the present study was to test the above hypothesis using a different functional assay, 5-HT1A receptor activation of G proteins, visualized autoradiographically as agonist-stimulated binding of [35S]GTPγS in specific brain regions (Sim et al., 1997, Waeber and Moskowitz, 1997, Dupuis et al., 1998, Meller et al., 2000). This method directly measures the initial, activation step of receptor/G protein coupling and can be used to assess changes in receptor sensitivity after repeated drug treatments (Sim et al., 1996a, Sim et al., 1996b). Representative drugs from each of the four classes discussed previously were utilized in a chronic treatment paradigm, and three brain regions were analyzed: dorsal hippocampus, dorsal raphe, and lateral septum. The lateral septum was included in the analysis because it is rich in postsynaptic 5-HT1A receptors and yields a robust agonist-stimulated [35S]GTPγS binding signal (Meller et al., 2000). While this study was underway, it was reported that chronic treatment with buspirone elicits a desensitization of 5-HT1A-stimulated [35S]GTPγS binding in the dorsal raphe without altering receptor sensitivity in the hippocampus and several forebrain cortical regions, consistent with a net increase in 5-HT neurotransmission as predicted by the above hypothesis (Sim-Selley et al., 2000). However, a significant desensitization of the response at postsynaptic 5-HT1A receptors in the lateral septum was also found (Sim-Selley et al., 2000). The present study also generally supports the hypothesis (Blier and De Montigny, 1994; Haddjeri et al., 1998) that an enhancement of 5-HT1A receptor-mediated neurotransmission in the forebrain occurs after repeated anxiolytic/antidepressant treatment and shows that the effect in the lateral septum generalizes to all four drug classes.

Section snippets

Animals

Male Sprague–Dawley rats (200–250 g; Taconic Farms, Germantown, NY) were used and were maintained on a 12 h light/dark cycle and housed four per cage with food and water ad libitum. All experimental procedures were carried out in accordance with the NIH Guide for the Care and Use of Laboratory Animals.

Materials

N,N-dipropyl-5-carboxamidotryptamine (N,N-DP-5-CT) maleate, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), fluoxetine hydrochloride, clorgyline hydrochloride, imipramine hydrochloride, adenosine

Chronic fluoxetine treatment

Representative autoradiograms comparing the effects of chronic treatment with vehicle or the antidepressant SSRI fluoxetine on N,N-DP-5-CT-stimulated binding of [35S]GTPγS in the three brain regions examined are shown in Fig. 1. It is readily apparent that fluoxetine treatment for 21 days reduced stimulated binding in the dorsal raphe (Fig. 1, top) and the lateral septum (Fig. 1, middle), but increased binding in the dorsal hippocampus (Fig. 1, bottom). The meaned data (Fig. 2) revealed that

Discussion

Using primarily electrophysiological means, De Montigny and colleagues have demonstrated quite convincingly that all the major anxiolytic/antidepressant therapies, administered as chronic treatment paradigms, produce a net increase in the tonic activation of 5-HT neurotransmission at hippocampal pyramidal neurons via 5-HT1A receptors. However, the different treatment modalities do not yield this common effect via a common mechanism. SSRIs, for example, desensitize somatodendritic 5-HT1A

Acknowledgements

The authors would like to thank Drs Jack Hiller and Kenneth Carr for assistance with various aspects of the project. This study was supported by grant MH 60592 from the National Institute of Mental Health, U.S. Public Health Service.

References (46)

  • L.J Sim-Selley et al.

    Region-specific changes in 5-HT1A receptor-activated G-proteins in rat brain following chronic buspirone

    European Journal of Pharmacology

    (2000)
  • L.J Sim et al.

    In vitro autoradiographic localization of 5-HT1A receptor-activated G-proteins in the rat brain

    Brain Research Bulletin

    (1997)
  • H Sommermeyer et al.

    Anxiolytic effects of the 5-HT1A receptor agonist ipsapirone in the rat: neurobiological correlates

    European Journal of Pharmacology

    (1993)
  • J.S Sprouse et al.

    Responses of hippocampal pyramidal cells to putative serotonin 5-HT1A and 5-HT1B agonists: a comparative study with dorsal raphe neurons

    Neuropharmacology

    (1988)
  • S.G Beck et al.

    Fluoxetine selectively alters 5-hydroxytryptamine1A and gamma-aminobutyric acidB receptor-mediated hyperpolarization in area CA1, but not area CA3, hippocampal pyramidal cells

    Journal of Pharmacology and Experimental Therapeutics

    (1997)
  • P Blier et al.

    Electrophysiological investigation of the adaptive response of the 5-HT system to the administration of 5-HT1A receptor agonists

    Journal of Cardiovascular Pharmacology

    (1990)
  • P Blier et al.

    Long-term 5-HT reuptake blockade, but not monoamine oxidase inhibition, decreases the function of terminal 5-HT autoreceptors: an electrophysiological study in the rat brain

    Naunyn-Schmiedebergs Archives of Pharmacology

    (1988)
  • P Blier et al.

    A role for the serotonin system in the mechanism of action of antidepressant treatments: preclinical evidence

    Journal of Clinical Psychiatry

    (1990)
  • Y Chaput et al.

    Effects of a selective 5-HT reuptake blocker, citalopram, on the sensitivity of 5-HT autoreceptors: electrophysiological studies in the rat brain

    Naunyn-Schmiedebergs Archives of Pharmacology

    (1986)
  • Y Chaput et al.

    Presynaptic and postsynaptic modifications of the serotonin system by long-term administration of antidepressant treatments. An in vivo electrophysiologic study in the rat

    Neuropsychopharmacology

    (1991)
  • J.M Cott et al.

    A 5-HT1A ligand with both antidepressant and anxiolytic properties

    Psychopharmacology Bulletin

    (1988)
  • R.F Cox et al.

    Electrophysiological evidence for a large receptor reserve for inhibition of dorsal raphe neuronal firing by 5-HT1A agonists

    Synapse

    (1993)
  • C De Montigny et al.

    Tricyclic antidepressants: long-term treatment increases responsivity of rat forebrain neurons to serotonin

    Science

    (1978)
  • Cited by (79)

    • G-protein activation revealed by [<sup>35</sup>S]-GTPγS binding assay is involved on the antidepressant-like effect of Hypericum caprifoliatum and Hypericum polyanthemum cyclohexane extracts

      2015, Revista Brasileira de Farmacognosia
      Citation Excerpt :

      These results are in agreement with Ozawa and Rasenick (1989) and Chen and Rasenick (1995) who observed that antidepressant drugs did not have direct effect on adenylyl cyclase, a protein modulated by Gα subunit. In addition, antidepressants that acutely reduce immobility time in the FST, normally show their effects on GPCR only after 14–21 days of treatment (Hensler, 2002; Pejchal et al., 2002; Shen et al., 2002; Castro et al., 2003). Thus, it is reasonable to consider that the effects of acute and the 5 day treatments could be a consequence of monoamines increase in the synaptic cleft following the uptake inhibition induced by the extracts, HC1 and uliginosin B (Viana et al., 2005; Stein et al., 2012).

    View all citing articles on Scopus
    View full text