Skip to main content
PMC home page
NIHPA Author Manuscripts logoLink to NIHPA Author Manuscripts
. Author manuscript; available in PMC: 2012 Nov 1.
Published in final edited form as: Eur Neuropsychopharmacol. 2011 Mar 12;21(11):835–840. doi: 10.1016/j.euroneuro.2011.02.002

Sequential and opposing alterations of 5-HT1A receptor function during withdrawal from chronic morphine

Pierre-Eric Lutz 1, Amynah A Pradhan 1,2, Celia Goeldner 1,3, Brigitte L Kieffer 1
PMCID: PMC3149735  NIHMSID: NIHMS277486  PMID: 21402471

Abstract

Addiction is a brain chronic relapsing disorder associated with emotional distress. The serotonergic system and especially the 5-HT1A receptor crucially regulate emotional behaviors both in humans and rodents. Using [35S]GTPγS autoradiography in mice, we show that 5-HT1A receptor function is enhanced by chronic morphine treatment in the medial prefrontal cortex, and decreased in dorsal raphe nucleus one week later, two regions involved in emotional processing. These molecular adaptations could contribute to the development of emotional disorders experienced by former opiate addicts.

Keywords: addiction, 5-HT1A receptor, autoradiography, mouse, dorsal raphe nucleus, medial prefrontal cortex

1. Introduction

Serotonergic neurons, mainly located in the dorsal raphe nucleus (DRN), send axonal projections throughout the brain where serotonin (5-HT) release activates several 5-HT receptors. The 5-HT1A receptor (5-HT1AR), a seven transmembrane domain receptor coupled to Gi/o proteins (Pucadyil et al., 2005), has been identified as a key player in emotions. Functional imaging reveals its implication in the pathophysiology of depression in humans (Parsey et al., 2006), and buspirone, a classical 5-HT1AR agonist, is used as an anxiolytic (Goodman, 2004). This receptor is strongly expressed both in the DRN and the medial prefrontal cortex (mPFC), two limbic regions implicated in emotional responses. Selective serotonin reuptake inhibitors (SSRI) are recognized as first-line antidepressants, and their effects have been correlated with 5-HT1AR desensitization in DRN (Hensler, 2003; Savitz et al., 2009). In mice, 5-HT1AR gene knock-out (Ramboz et al., 1998) or reduced 5-HT1AR expression in the DRN (Richardson-Jones et al., 2010) decrease behavioural despair.

Opiate addiction is associated with emotional distress. Acute withdrawal, defined when access to the drug is prevented, produces physical symptoms and a negative affect (Koob and Volkow, 2010). After prolonged periods of withdrawal, opiate addicts further show increased prevalence of anxiety and major depressive episodes (Grant et al., 2004). In rodents, SSRI treatment alleviates acute withdrawal symptoms following chronic exposure to morphine, a prototypical opiate (Gray, 2002). Further, SSRI prevent both heightened anxiety (Harris and Aston-Jones, 2001) and despair (Goeldner et al., 2011) during protracted withdrawal from chronic morphine.

Together therefore, data from both human and rodent studies suggest that the serotonergic regulation of emotional behaviors is altered during morphine withdrawal, and could implicate 5-HT1AR dysfunction. In this study, we investigated the kinetics of 5-HT1AR function following chronic morphine exposure. We treated mice using a morphine regimen known to induce dependence (Matthes et al., 1996) and focused on three time points (2 hours, 1 and 4 weeks after the last morphine injection) to match our previous study on behavioral adaptations to chronic morphine (Goeldner et al., 2011). We then performed [35S]GTPγS autoradiography, stimulated by the specific 5-HT1AR agonist 8-OH-DPAT (Meneses and Terron, 2001), to evaluate functional coupling of the receptor to G-proteins in DRN and mPFC. Our data show regional-specific and sequential modifications of 5-HT1AR function.

2. Experimental Procedures

2. 1. Animals

Eight-week-old male C57Bl/6J mice (Charles River Laboratories, St-Germain-sur-l'Arbresle, France) were housed 4/cage (12h light/dark cycle, food and water ad libitum). All experiments followed ethical guidelines (European Community Guidelines 86/609/EEC) and were approved by the local ethical committee (CREMEAS, 2003-10-08-[1]-58).

2. 2. General Procedure

Experiment 1

4 naïve mice were used to determine the potency (defined as -logEC50) and maximal effect (Emax) of (R)-(+)-8-Hydroxy-2-dipropylamino-tetralin hydrobromide (8-OHDPAT, Sigma) for stimulating [35S]GTPγS binding across 4 different brain regions: DRN, mPFC, dorsal and ventral hippocampus (dHIPP, vHIPP, respectively).

Experiment 2

48 mice (consisting in 2 equivalent cohorts processed independently) were injected intraperitoneally with daily escalating doses of morphine sulfate (20, 40, 60, 80, 100 mg/kg; Francopia, Gentilly, France) or saline (0.9% sodium chloride) as control, twice daily for five days and received a single 100 mg/kg injection on day 6. Mice were then sacrificed (i) 2 hours after the last injection (chronic treatment; saline and morphine, n=8/group), (ii) 1 week after the last injection (1-week withdrawal; saline and morphine, n=8/group), (iii) 4 weeks after the last injection (4-week withdrawal; saline, n=8; morphine, n=7).

2. 3. Tissue preparation

Mice were cervically dislocated, brains rapidly removed and frozen in 2-methylpentane. Coronal sections (20 μm) were obtained at −20°C using a cryostat microtome (Leica CM 3050) at the level of the mPFC (+2.0 to +2.4 mm from bregma), dHIPP (−2.0 to −2.4 mm), vHIPP (−3.1 to −3.5) and DRN (−4.4 to −4.8 mm) according to the mouse brain atlas (Paxinos and Franklin, 1997). Sections were thaw-mounted onto gelatin-coated slides, air-dried (room temperature, 10 minutes) and stored at −80°C.

2. 4. [35S]GTPγS autoradiography

Sections were thawed at room temperature and rehydrated for 20 minutes in plastic slide mailers in assay buffer containing 50 mM Tris-HCl, 5 mM MgCl2-6H2O, 100 mM NaCl, and 10 mM EDTA (pH 7.4). Sections were preincubated for 1 hour with assay buffer plus 2 mM GDP (Sigma) and 1 μM 8-cyclopentyl-1,3-dipropylxanthine (DPCPX, Tocris bioscience). The sections were then incubated for 1.5 hours with preincubation buffer plus 1 mM dithiothreitol (DTT) and 80 pM guanosine 5'(γ-35S-thio)-triphosphate ([35S]GTPγS; 1,250 Ci/mmol, Perkin Elmer). Mailers were allocated to three incubation conditions: basal (no agonist present), agonist-stimulated and nonspecific (no agonist and 1 μM unlabeled GTPγS, Sigma) bindings. We used 5 (0.001, 0.01, 0.1, 1, 10 μM) and 2 (0.1 and 10 μM) increasing concentrations of 8-OH-DPAT in experiments 1 and 2, respectively. Sections were then rinsed and exposed to Kodak Biomax MR films for 24 hours to generate autoradiograms.

2. 5. Image analysis

Autoradiograms were analyzed with an image processor (MCID Elite 7.0 software Imaging Research Inc., St Catharines, ON, Canada). Regions of interest (ROI, see figure 1) were drawn on images from basal binding sections and reported on images from non-specific and agonist-stimulated binding sections. Densitometric values in ROI were transformed into relative radioactive counts by calibration with simultaneously exposed [14C] standards (ARC-146; American Radiochemicals) of known tissue equivalent values (nCi/g) (Miller, 1991). Non-specific binding was subtracted from both basal and agonist-stimulated binding.

Figure 1.

Figure 1

Open in a new tab

Representative images (artificial colors, middle) and dose-response curves (right) for 8-OH-DPAT-stimulated [35S]GTPγS binding in dorsal raphe nucleus (DRN) (A), medial prefrontal cortex (mPFC) (B), dorsal (dHIPP) (C) and ventral hippocampus (vHIPP) (D) of naive mice. Dashed lines, regions of interest for densitometry measurements according to the mouse brain atlas. Data are mean ± s.e.m. n=4 mice.

2. 6. Data analysis

All data are expressed as mean±sem. For experiment 1, pEC50 and Emax of 8-OH-DPAT-stimulated [35S]GTPγS binding were calculated for each mouse by non-linear regression analysis. One-way analysis of variance was then used to compare Emax and pEC50 across 4 brain regions. For experiment 2, we used two-way analysis of variance with treatment and duration of withdrawal as factors, followed by Fischer's post-hoc analysis when relevant.

3. Results

In experiment 1 we quantified [35S]GTPγS binding stimulated by 8-OH-DPAT in 4 brain regions of naïve mice. Fig. 1 shows dose-response activation curves for 5 agonist concentrations and images at 3 representative concentrations. pEC50 and Emax values for each region are shown in Table 1. Analysis of variance indicates that 8-OH-DPAT is equally potent in stimulating [35S]GTPγS binding in the 4 brain areas [F(3,12)=0.52, p=0.67]. In contrast and as expected, the Emax greatly varied [F(3,12)=34.7, p<0.0001], ranging from 69.2% in the DRN to 523% in the dHIPP, reflecting variations across brain regions in both 5-HT1AR density and basal G-protein coupled receptors activity. Post-hoc comparisons of Emax values between regions were all significant (p<0.001), except for vHIPP and mPFC (p=0.82). This pilot experiment indicates that 8-OH-DPAT potency is comparable across brain areas, as was previously shown in rat [35S]GTPγS autoradiography experiments (Meller et al., 2000).

Table 1.

Maximal effect (Emax) and potency (pEC50) of [35S]GTPγS binding stimulated with 5 increasing concentrations of the 5-HT1A receptor agonist 8-OH-DPAT, in dorsal raphe nuclei (DRN), medial prefrontal cortex (mPFC), dorsal (dHIPP) and ventral (vHIPP) hippocampus of naïve mice (n=4). Data are mean ± s.e.m.

Region pEC50 Emax (% over basal binding)
DRN 6.92±0.22 68.4±5.8
mPFC 6.90±0.15 238±15
dHIPP 7.11±0.17 521±32
vHIPP 6.91±0.20 221±18
Open in a new tab

In experiment 2, we analyzed the effects of escalating doses of morphine on 5-HT1AR function. Because of the large sample number required for quantification, we did not perform full dose-response experiments. Based on experiment 1, we selected two critical agonist concentrations that reliably reflect modifications of agonist potency and efficacy in the two brain regions, i.e. a low (0.1 μM) and a high (10 μM) concentrations corresponding to EC50 value and maximal activation in naïve mice. In a first animal cohort, we found significant effects of morphine exposure on [35S]GTPγS binding in mPFC and DRN, but not in vHIPP or dHIPP (data not shown). Thus only mPFC and DRN were analyzed in a second cohort and data pooled with those of the first cohort. Basal binding was not significantly modified by chronic injections or duration of abstinence in any of the brain regions examined (Table 2). Binding values were analyzed separately for each 8-OH-DPAT concentration and brain region and results are shown in Fig. 2.

Table 2.

Basal binding (nCi/g) of [35S]GTPγS in DRN and mPFC is not modified either by chronic injections or duration of withdrawal. Data are mean ± s.e.m. n=7–8 mice for each treatment at each time point.

Chronic
 1-Week
 4-Week
Region Saline Morphine Saline Morphine Saline Morphine
DRN 2356±148 2302±96 2348±77 2495±71 2378±55 2371±51
mPFC 483±31 413±30 414±40 428±19 437±41 334±61
Open in a new tab

Figure 2.

Figure 2

Open in a new tab

5-HT1A receptor function, measured by 8-OH-DPAT-stimulated [35S]GTPγS binding, is increased after chronic morphine treatment (A, D) in mPFC, decreased after 1-week withdrawal (B, E) in DRN and unchanged after 4 weeks (C, F). Data are mean ± s.e.m. n=7–8 mice for each treatment at each time point. *p<0.05, **p<0.01, saline versus morphine-treated mice, two-way ANOVA with Fischer's post-hoc analysis.

In the mPFC (Fig. 2A–C) and for 10 μM of 8-OH-DPAT, two-way analysis of variance revealed a main significant effect of morphine treatment [F(1,41)=4.64, p=0.037]. In the chronic group, post-hoc analysis showed a significant increase of [35S]GTPγS binding in morphine-treated mice as compared to saline controls (p=0.017, Fig. 2A). At this concentration, the duration of withdrawal had no effect [F(2,41)=1.90, p=0.16] and there was no interaction between factors [F(2,41)=1.19, p=0.31]. For 0.1 μM 8-OH-DPAT, there was no effect of treatment [F(1,41)=1.518, p=0.22] or duration of withdrawal [F(2,41)=0.077, p=0.93] and no interaction between factors [F(2,41)=1.96, p=0.15]. Hence, response to 8-OH-DPAT is increased in mPFC at the end of the chronic treatment, and this effect is no more detected after 1- and 4-week withdrawal. Chronic morphine thus transiently sensitizes 5-HT1AR, specifically at the level of mPFC.

In the DRN (Fig. 2D–F) and for 0.1 μM 8-OH-DPAT, there was a significant main effect of treatment [F(1,42)=6.22, p=0.0167]. In the 1-week withdrawal group, post-hoc comparison revealed a significant decreased agonist-stimulated binding in morphine-treated mice (p=0.0091, Fig. 2E). There was no effect of withdrawal duration [F(2,42)=1.25, p=0.30] and no interaction between factors [F(2,42)=1.27, p=0.30]. At 10 μM 8-OH-DPAT, we observed no effect of treatment [F(1,42)=2.66, p=0.11] or time [F(2,42)=0.936, p=0.40] and no interaction [F(2,42)=0.57, p=0.57]. Therefore, 8-OH-DPAT response is decreased in the DRN after 1-week withdrawal, an effect that is not detectable either at the end of the chronic regimen or after 4 weeks. Chronic morphine leads to delayed and transient 5-HT1AR desensitization, specifically in the DRN.

4. Discussion

Our data show that 5-HT1AR function is enhanced in mPFC immediately after repeated morphine exposure, and decreased in DRN after 1-week withdrawal. These successive and opposing adaptations of 5-HT1AR coupling to G-proteins, which develop either at the level of receptor density or receptor/G-protein interaction (Sovago et al., 2001), do not persist since receptor coupling is restored in both brain areas 4 weeks after treatment.

After chronic morphine, 5-HT1AR function is enhanced in mPFC, but not in DRN. While acute morphine disinhibits serotonergic neurons and activates 5-HT1ARs in both DRN and projection areas (Tao and Auerbach, 1995; Fadda et al., 2005), chronic morphine was shown to decrease 5-HT neurons firing rate (Jolas et al., 2000) and sensitize 5-HT1AR responsivity to systemic 8-OH-DPAT (Sastre-Coll et al., 2002). Negative feedback mechanisms may operate at local (DRN) or distant (mPFC) sites (Stamford et al., 2000; Celada et al., 2001). Our result suggests that chronic morphine sensitizes 5-HT1AR-mediated control over serotonergic neurons mainly at the cortical level.

Most 5-HT1AR studies have focused on immediate effects of pharmacological treatment or behavioral manipulation. Here we examined delayed consequences of morphine exposure, and show for the first time that reduced 5-HT1AR function is detected only after a drug-free period. Notably, this 5-HT1AR desensitization is detected in DRN, as was found in previous rodent studies using chronic mild stress-induced models of depression (Lanfumey et al., 1999; Froger et al., 2004; Bambico et al., 2009), indicating that DRN-specific adaptations also occur in response to chronic opiates. The time course of DRN 5-HT1AR desensitization is intriguing. We recently showed (Goeldner et al., 2011) that emotional-like deficits following chronic morphine are undetectable at 1-week but significant at 4-week withdrawal. Despair behavior may in fact be related to the 5-HT1AR dysfunction observed here. Chronic morphine could disturb 5-HT1ARs activity transiently, and this primary event may trigger further adaptations within 5-HT circuits which may ultimately contribute to the incubation of behavioral deficits expressed at a later time point.

In conclusion our data reveal dynamic modifications of 5-HT1ARs, which may contribute to homeostatic dysregulation of 5-HT system in morphine abstinence and have implications towards understanding emotional distress in former opiate addicts.

Acknowledgements

The authors gratefully acknowledge A. Ferrandon, Dr A. Nehlig and Pr J.M. Danion for the use of the MCID system; Dr D.Massotte for her G-protein coupled receptors expertise and Dr K.Befort for critical reading of the manuscript.

Footnotes

Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

References

  1. Bambico FR, Nguyen NT, Gobbi G. Decline in serotonergic firing activity and desensitization of 5-HT1A autoreceptors after chronic unpredictable stress. Eur Neuropsychopharmacol. 2009;19:215–228. doi: 10.1016/j.euroneuro.2008.11.005. [DOI] [PubMed] [Google Scholar]
  2. Celada P, Puig MV, Casanovas JM, Guillazo G, Artigas F. Control of dorsal raphe serotonergic neurons by the medial prefrontal cortex: Involvement of serotonin-1A, GABA(A), and glutamate receptors. J Neurosci. 2001;21:9917–9929. doi: 10.1523/JNEUROSCI.21-24-09917.2001. [DOI] [PMC free article] [PubMed] [Google Scholar]
  3. Fadda P, Scherma M, Fresu A, Collu M, Fratta W. Dopamine and serotonin release in dorsal striatum and nucleus accumbens is differentially modulated by morphine in DBA/2J and C57BL/6J mice. Synapse. 2005;56:29–38. doi: 10.1002/syn.20122. [DOI] [PubMed] [Google Scholar]
  4. Froger N, Palazzo E, Boni C, Hanoun N, Saurini F, Joubert C, Dutriez-Casteloot I, Enache M, Maccari S, Barden N, Cohen-Salmon C, Hamon M, Lanfumey L. Neurochemical and behavioral alterations in glucocorticoid receptor-impaired transgenic mice after chronic mild stress. J Neurosci. 2004;24:2787–2796. doi: 10.1523/JNEUROSCI.4132-03.2004. [DOI] [PMC free article] [PubMed] [Google Scholar]
  5. Goeldner C, Lutz PE, Darcq E, Halter T, Clesse D, Ouagazzal AM, Kieffer BL. Impaired emotional-like behavior and serotonergic function during protracted abstinence from chronic morphine. Biol Psychiatry. 2011;69:236–244. doi: 10.1016/j.biopsych.2010.08.021. [DOI] [PMC free article] [PubMed] [Google Scholar]
  6. Goodman WK. Selecting pharmacotherapy for generalized anxiety disorder. J Clin Psychiatry. 2004;65(Suppl 13):8–13. [PubMed] [Google Scholar]
  7. Grant BF, Stinson FS, Dawson DA, Chou SP, Dufour MC, Compton W, Pickering RP, Kaplan K. Prevalence and co-occurrence of substance use disorders and independent mood and anxiety disorders: results from the National Epidemiologic Survey on Alcohol and Related Conditions. Arch Gen Psychiatry. 2004;61:807–816. doi: 10.1001/archpsyc.61.8.807. [DOI] [PubMed] [Google Scholar]
  8. Gray AM. The effect of fluvoxamine and sertraline on the opioid withdrawal syndrome: a combined in vivo cerebral microdialysis and behavioural study. Eur Neuropsychopharmacol. 2002;12:245–254. doi: 10.1016/s0924-977x(02)00028-7. [DOI] [PubMed] [Google Scholar]
  9. Harris GC, Aston-Jones G. Augmented accumbal serotonin levels decrease the preference for a morphine associated environment during withdrawal. Neuropsychopharmacology. 2001;24:75–85. doi: 10.1016/S0893-133X(00)00184-6. [DOI] [PubMed] [Google Scholar]
  10. Hensler JG. Regulation of 5-HT1A receptor function in brain following agonist or antidepressant administration. Life Sci. 2003;72:1665–1682. doi: 10.1016/s0024-3205(02)02482-7. [DOI] [PubMed] [Google Scholar]
  11. Jolas T, Nestler EJ, Aghajanian GK. Chronic morphine increases GABA tone on serotonergic neurons of the dorsal raphe nucleus: association with an up-regulation of the cyclic AMP pathway. Neuroscience. 2000;95:433–443. doi: 10.1016/s0306-4522(99)00436-4. [DOI] [PubMed] [Google Scholar]
  12. Koob GF, Volkow ND. Neurocircuitry of addiction. Neuropsychopharmacology. 2010;35:217–238. doi: 10.1038/npp.2009.110. [DOI] [PMC free article] [PubMed] [Google Scholar]
  13. Lanfumey L, Pardon MC, Laaris N, Joubert C, Hanoun N, Hamon M, Cohen-Salmon C. 5-HT1A autoreceptor desensitization by chronic ultramild stress in mice. Neuroreport. 1999;10:3369–3374. doi: 10.1097/00001756-199911080-00021. [DOI] [PubMed] [Google Scholar]
  14. Matthes HW, Maldonado R, Simonin F, Valverde O, Slowe S, Kitchen I, Befort K, Dierich A, Le Meur M, Dolle P, Tzavara E, Hanoune J, Roques BP, Kieffer BL. Loss of morphine-induced analgesia, reward effect and withdrawal symptoms in mice lacking the mu-opioid-receptor gene. Nature. 1996;383:819–823. doi: 10.1038/383819a0. [DOI] [PubMed] [Google Scholar]
  15. Meller E, Li H, Carr KD, Hiller JM. 5-Hydroxytryptamine(1A) receptor-stimulated [(35)S]GTPgammaS binding in rat brain: absence of regional differences in coupling efficiency. J Pharmacol Exp Ther. 2000;292:684–691. [PubMed] [Google Scholar]
  16. Meneses A, Terron JA. Role of 5-HT(1A) and 5-HT(7) receptors in the facilitatory response induced by 8-OH-DPAT on learning consolidation. Behav Brain Res. 2001;121:21–28. doi: 10.1016/s0166-4328(00)00378-8. [DOI] [PubMed] [Google Scholar]
  17. Miller JA. The calibration of 35S or 32P with 14C-labeled brain paste or 14C-plastic standards for quantitative autoradiography using LKB Ultrofilm or Amersham Hyperfilm. Neurosci Lett. 1991;121:211–214. doi: 10.1016/0304-3940(91)90687-o. [DOI] [PubMed] [Google Scholar]
  18. Parsey RV, Oquendo MA, Ogden RT, Olvet DM, Simpson N, Huang YY, Van Heertum RL, Arango V, Mann JJ. Altered serotonin 1A binding in major depression: a [carbonyl-C-11]WAY100635 positron emission tomography study. Biol Psychiatry. 2006;59:106–113. doi: 10.1016/j.biopsych.2005.06.016. [DOI] [PubMed] [Google Scholar]
  19. Paxinos G, Franklin KBJ. The Mouse Brain in Stereotaxic Coordinates. 2nd ed Academic Press; New York: 1997. [Google Scholar]
  20. Pucadyil TJ, Kalipatnapu S, Chattopadhyay A. The serotonin1A receptor: a representative member of the serotonin receptor family. Cell Mol Neurobiol. 2005;25:553–580. doi: 10.1007/s10571-005-3969-3. [DOI] [PubMed] [Google Scholar]
  21. Ramboz S, Oosting R, Amara DA, Kung HF, Blier P, Mendelsohn M, Mann JJ, Brunner D, Hen R. Serotonin receptor 1A knockout: an animal model of anxiety-related disorder. Proc Natl Acad Sci U S A. 1998;95:14476–14481. doi: 10.1073/pnas.95.24.14476. [DOI] [PMC free article] [PubMed] [Google Scholar]
  22. Richardson-Jones JW, Craige CP, Guiard BP, Stephen A, Metzger KL, Kung HF, Gardier AM, Dranovsky A, David DJ, Beck SG, Hen R, Leonardo ED. 5-HT1A autoreceptor levels determine vulnerability to stress and response to antidepressants. Neuron. 2010;65:40–52. doi: 10.1016/j.neuron.2009.12.003. [DOI] [PMC free article] [PubMed] [Google Scholar]
  23. Sastre-Coll A, Esteban S, Garcia-Sevilla JA. Supersensitivity of 5-HT1A autoreceptors and alpha2-adrenoceptors regulating monoamine synthesis in the brain of morphine-dependent rats. Naunyn Schmiedebergs Arch Pharmacol. 2002;365:210–219. doi: 10.1007/s00210-001-0508-8. [DOI] [PubMed] [Google Scholar]
  24. Savitz J, Lucki I, Drevets WC. 5-HT(1A) receptor function in major depressive disorder. Prog Neurobiol. 2009;88:17–31. doi: 10.1016/j.pneurobio.2009.01.009. [DOI] [PMC free article] [PubMed] [Google Scholar]
  25. Sovago J, Dupuis DS, Gulyas B, Hall H. An overview on functional receptor autoradiography using [35S]GTPgammaS. Brain Res Brain Res Rev. 2001;38:149–164. doi: 10.1016/s0165-0173(01)00106-0. [DOI] [PubMed] [Google Scholar]
  26. Stamford JA, Davidson C, McLaughlin DP, Hopwood SE. Control of dorsal raphe 5-HT function by multiple 5-HT(1) autoreceptors: parallel purposes or pointless plurality? Trends Neurosci. 2000;23:459–465. doi: 10.1016/s0166-2236(00)01631-3. [DOI] [PubMed] [Google Scholar]
  27. Tao R, Auerbach SB. Involvement of the dorsal raphe but not median raphe nucleus in morphine-induced increases in serotonin release in the rat forebrain. Neuroscience. 1995;68:553–561. doi: 10.1016/0306-4522(95)00154-b. [DOI] [PubMed] [Google Scholar]

RESOURCES