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