5-HT1A receptors and 5-HT1A-mediated responses: effect of treatments that modify serotonergic neurotransmission

Tryptophan depletion affects compulsive behaviour in rats: strain dependent effects and associated neuromechanisms

RATIONALE

Compulsive behaviour, present in different psychiatric disorders, such as obsessive-compulsive disorder, schizophrenia and drug abuse, is associated with altered levels of monoamines, particularly serotonin (5-hydroxytryptamine) and its receptor system.

OBJECTIVES

The present study investigated whether 5-HT manipulation, through a tryptophan (TRP) depletion by diet in Wistar and Lister Hooded rats, modulates compulsive drinking in schedule-induced polydipsia (SIP) and locomotor activity in the open-field test. The levels of dopamine, noradrenaline, serotonin and its metabolite were evaluated, as well as the 5-HT_2A and 5-HT_1A receptor binding, in different brain regions.

METHODS

Wistar rats were selected as high (HD) or low (LD) drinkers according to their SIP behaviour, while Lister hooded rats did not show SIP acquisition. Both strains were fed for 14 days with either a TRP-free diet (T-) or a TRP-supplemented diet (T+) RESULTS: The TRP depletion diet effectively reduced 5-HT levels in the frontal cortex, amygdala and hippocampus in both strains of rats. The TRP-depleted HD Wistar rats were more sensitive to 5-HT manipulation, exhibiting more licks on SIP than did the non-depleted HD Wistar rats, while the LD Wistar and the Lister Hooded rats did not exhibit differences in SIP. In contrast, the TRP-depleted Lister Hooded rats increased locomotor activity compared to the non-depleted rats, while no differences were found in the Wistar rats. Serotonin 2A receptor binding in the striatum was significantly reduced in the TRP-depleted HD Wistar rats.

CONCLUSIONS

These results suggest that alterations of the serotonergic system could be involved in compulsive behaviour in vulnerable populations.

Influence of Intensity and Duration of Yoga on Anxiety and Depression Scores Associated with Chronic Illness

Background:

Chronic illness is commonly associated with anxiety and depression. Both anxiety and depression respond to yoga. However, there is no report on the association between the intensity and duration of yoga practice with the benefits seen.

Aim:

The present study was intended to determine whether the daily duration of yoga practice and the duration of experience in months would predict anxiety and depression, associated with chronic illness.

Subjects and Methods:

Seven hundred and sixty-three volunteers with ages between 14 and 86 years (group mean age standard deviation, 50.2 [14.2]) who attended a 7 day residential yoga camp in the north of India were included in this cross-sectional study. All participants had chronic illnesses, which were under control with treatment, and which were categorized and are detailed. Participants were assessed for state anxiety scores using State-Trait Anxiety Inventory and for anxiety with hospital anxiety and depression scale (HADS-A), and depression was assessed using HADS-D scores of the HADS. Linear multiple regression analyses were performed using PASW SPSS version 18.0 (Armonk, New York, U.S.) to determine how the daily and monthly duration of yoga practice could influence state anxiety, hospital anxiety and depression of the participants.

Results:

Yoga practice in months and the time spent practicing yoga each day significantly predict the level of state anxiety (P < 0.001, P = 0.03) and HAD-A (P < 0.01, P < 0.01). The duration of yoga practice in months alone was a significant predictor of the HAD-D (P < 0.01).

Conclusions:

The results suggest that the duration of yoga practice in months and daily practice in minutes predict anxiety associated with chronic illness. In contrast the duration of yoga practice in months alone, predicted depression scores.

Development of brainstem 5-HT1A receptor-binding sites in serotonin-deficient mice

The sudden infant death syndrome is associated with a reduction in brainstem serotonin 5-hydroxytryptamine (5-HT) and 5-HT(1A) receptor binding, yet it is unknown if and how these findings are linked. In this study, we used quantitative tissue autoradiography to determine if post-natal development of brainstem 5-HT(1A) receptors is altered in two mouse models where the development of 5-HT neurons is defective, the Lmx1b(f/f/p) , and the Pet-1⁻/⁻ mouse. 5-HT(1A) receptor agonist-binding sites were examined in both 5-HT-source nuclei (autoreceptors) and in sites that receive 5-HT innervation (heteroreceptors). In control mice between post-natal day (P) 3 and 10, 5-HT(1A) receptor binding increased in several brainstem sites; by P25, there were region-specific increases and decreases, refining the overall binding pattern. In the Lmx1b(f/f/p) and Pet-1⁻/⁻ mice, 5-HT(1A)-autoreceptor binding was significantly lower than in control mice at P3, and remained low at P10 and P25. In contrast, 5-HT(1A) heteroreceptor levels were comparable between control and 5-HT-deficient mice. These data define the post-natal development of 5-HT(1A)-receptor binding in the mouse brainstem. Furthermore, the data suggest that 5-HT(1A)-heteroreceptor deficits detected in sudden infant death syndrome are not a direct consequence of a 5-HT neuron dysfunction nor reduced brain 5-HT levels. To elucidate the developmental relationship between serotonin (5-HT) levels and 5-HT(1A) receptors in the brainstem, we examined 5-HT(1A) binding in two 5-HT-deficient mouse models. In nuclei containing 5-HT neurons, 5-HT(1A) binding was decreased (autoreceptors), while binding was maintained in projection sites (heteroreceptors). Thus, brainstem 5-HT(1A)-heteroreceptor-binding sites do not appear developmentally sensitive to reduced brain 5-HT levels.

Recent progress in understanding the pathophysiology of post-traumatic stress disorder: implications for targeted pharmacological treatment

Post-traumatic stress disorder (PTSD) is a common and chronic anxiety disorder that can result after exposure to a traumatic event. Though our understanding of the aetiology of PTSD is incomplete, several neurobiological systems have been implicated in the pathophysiology and vulnerability towards developing PTSD after trauma exposure. We aimed to provide a concise review of benchmark findings in important neurobiological systems related to the aetiology and maintenance of PTSD symptomology. Specifically, we discuss functional aetiologies in the noradrenergic, serotonergic, endogenous cannabinoid and opioid systems as well as the hypothalamic-pituitary adrenal (HPA) axis. This article provides a succinct framework to appreciate the current understanding of neurobiological mechanisms related to the pathophysiology of PTSD and how these findings may impact the development of future, targeted pharmacological treatments for this debilitating disorder.

Noradrenergic and serotonergic mechanisms in the neurobiology of posttraumatic stress disorder and resilience

Posttraumatic stress disorder (PTSD) is characterized mainly by symptoms of re-experiencing, avoidance and hyperarousal as a consequence of catastrophic and traumatic events that are distinguished from ordinary stressful life events. Although extensive research has already been done, the etiology of PTSD remains unclear. Research on the impact of trauma on neurobiological systems can be expected to inform the development of treatments that are directed specifically to symptoms of PTSD. During the past 25 years there has been a dramatic increase in the knowledge about noradrenergic and serotonergic mechanisms in stress response, PTSD and more recently in resilience and this knowledge has justified the use of antidepressants with monoaminergic mechanisms of action for patients with PTSD. Nevertheless, available treatments of PTSD are only to some extent effective and enhanced understanding of the neurobiology of PTSD may lead to the development of improved treatments for these patients. In the present review, we aim to close existing gaps between basic research in psychopathology, neurobiology and treatment development with the ultimate goal to translate basic research into clinically relevant findings which may directly benefit patients with PTSD.

Serotonin-1A receptor imaging in recurrent depression: replication and literature review

INTRODUCTION

Serotonin-1A receptor (5-HT1AR) function appears to be decreased in major depressive disorder (MDD) based on physiological responses to 5-HT1AR agonists in vivo and to 5-HT1AR binding in brain tissues postmortem or antemortem. We have previously assessed 5-HT1AR binding potential (BP) in depression using positron emission tomography (PET) and [carbonyl-(11)C]WAY-100635, and we have demonstrated reduced 5-HT1AR BP in the mesiotemporal cortex (MTC) and raphe in depressives with primary recurrent familial mood disorders (n=12) versus controls (n=8) [Drevets WC, Frank E, Price JC, Kupfer DJ, Holt D, Greer PJ, Huang Y, Gautier C, Mathis C. PET imaging of serotonin 1A receptor binding in depression. Biol Psychiatry 1999;46(10):1375-87]. These findings were replicated by some, but not other, studies performed in depressed samples that were more generally selected using criteria for MDD. In the current study, we attempted to replicate our previous findings in an independent sample of subjects selected according to the criteria for primary recurrent depression applied in our prior study.

METHODS

Using PET and [carbonyl-(11)C]WAY-100635, 5-HT1AR BP was assessed in 16 depressed subjects and 8 healthy controls.

RESULTS

Mean 5-HT1AR BP was reduced by 26% in the MTC (P<.005) and by 43% in the raphe (P<.001) in depressives versus controls.

CONCLUSIONS

These data replicate our original findings, which showed that BP was reduced by 27% in the MTC (P<.025) and by 42% in the raphe (P<.02) in depression. The magnitudes of these reductions in 5-HT1AR binding were similar to those found postmortem in 5-HT1AR mRNA concentrations in the hippocampus in MDD [López JF, Chalmers DT, Little KY, Watson SJ. Regulation of serotonin 1A, glucocorticoid, and mineralocorticoid receptor in rat and human hippocampus: implications for neurobiology of depression. Biol Psychiatry 1998;43:547-73] and in 5-HT1AR-binding capacity in the raphe in depressed suicide victims [Arango V, Underwood MD, Boldrini M, Tamir H, Kassir SA, Hsiung S, Chen JJ, Mann JJ. Serotonin 1A receptors, serotonin transporter binding and serotonin transporter mRNA expression in the brainstem of depressed suicide victims. Neuropsychopharmacology 2001;25(6):892-903]. There exists disagreement within the literature, however, regarding the presence and direction of 5-HT1AR-binding abnormalities in depression, which may be explained in some cases by differences in anatomical location (e.g., [Stockmeier CA, Shapiro LA, Dilley GE, Kolli TN, Friedman L, Rajkowska G. Increase in serotonin-1A autoreceptors in the midbrain of suicide victims with major depression--postmortem evidence for decreased serotonin activity. J Neurosci 1998;18(18):7394-401]) and in other cases by pathophysiological heterogeneity within MDD (e.g., some depressives hypersecrete cortisol, which would be expected to down-regulate 5-HT1AR expression [López JF, Chalmers DT, Little KY, Watson SJ. Regulation of serotonin 1A, glucocorticoid, and mineralocorticoid receptor in rat and human hippocampus: implications for neurobiology of depression. Biol Psychiatry 1998;43:547-73]). Antidepressant drug treatment does not alter these abnormalities in 5-HT1AR binding [Sargent PA, Kjaer KH, Bench CJ, Rabiner EA, Messa C, Meyer J, Gunn RN, Grasby PM, Cowen PJ. Brain serotonin1A receptor binding measured by positron emission tomography with [11C]WAY-100635: effects of depression and antidepressant treatment. Arch Gen Psychiatry 2000;57(2):174-80; Moses-Kolko EL, Price JC, Thase ME, Meltzer CC, Kupfer DJ, Mathis CA, Bogers WD, Berman SR, Houck PR, Schneider TN, Drevets WC. Measurement of 5-HT1A receptor binding in depressed adults before and after antidepressant drug treatment using positron emission tomography and [11C]WAY-100635. Synapse 2007;61(7):523-30] but may compensate for blunted 5-HT1AR function by increasing post-synaptic 5-HT1AR transmission [Chaput Y, de Montigny C, Blier P. 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;5(4):219-29].

Measurement of 5-HT1A receptor binding in depressed adults before and after antidepressant drug treatment using positron emission tomography and [11C]WAY-100635

OBJECTIVE

To assess effects of chronic antidepressant drug treatment on serotonin type-1A receptor (5-HT(1A)R) binding potential (BP) in major depressive disorder.

METHODS

Depressed subjects (n = 27) were imaged using PET and [(11)C]WAY-100635 at baseline and following a median of 9.4 weeks of treatment with selective serotonin reuptake inhibitor or dual reuptake inhibitor antidepressant agents. Fifteen subjects had complete pre- and post-treatment scan data. The 5-HT(1A)R BP was derived from the tissue time-radioactivity concentrations from regions-of-interest defined a priori, using a simplified reference tissue model (SRTM), and in a subset of subjects, compartmental modeling (CMOD).

RESULTS

Chronic treatment had no effect on pre- or post-synaptic 5-HT(1A)R BP, as confirmed by both the SRTM and CMOD analyses. These results were unaffected by treatment response status and were consistent across brain regions. Among the 22 subjects for whom the clinical response-to-treatment was established, the treatment nonresponders (n = 7) had higher baseline BP values in the left (P = 0.01) and right orbital cortex (P = 0.02) than the responders (n = 15).

CONCLUSIONS

Chronic antidepressant drug treatment did not significantly change cerebral 5-HT(1A)R binding, consistent with preclinical evidence that the alterations in serotonergic function associated with antidepressant drug administration are not accompanied by changes in 5-HT(1A)R density. Higher baseline 5-HT(1A)R binding was associated with poorer response to treatment.

5-HT2 receptor activation facilitates a persistent sodium current and repetitive firing in spinal motoneurons of rats with and without chronic spinal cord injury

We examined the modulation of persistent inward currents (PICs) by serotonin (5-HT) in spinal motoneurons of normal and chronic spinal rats. PICs are composed of both a TTX-sensitive persistent sodium current (Na PIC) and a nimodipine-sensitive persistent calcium current (Ca PIC), and we focused on quantifying the Na PIC (and its action on the total PIC), which is known to be critical in enabling repetitive firing. Intracellular recordings were made from motoneurons of the whole sacrocaudal spinal cord of normal adult rats after the cord was acutely transected at the S2 spinal level (acute spinal rat condition), removed from the animal, and then maintained in vitro. In vitro motoneuron recordings were likewise made from rats that had a sacral spinal transection 2 mo previously (chronic spinal rats). In motoneurons from acute spinal rats, moderately high doses of 5-HT (> or = 10 microM), or the 5-HT2 receptor agonist DOI (> or = 30 microM), significantly increased the total PIC, hyperpolarized the PIC onset voltage, and hyperpolarized the spike threshold, whereas lower doses had no effect. Both 5-HT and DOI specifically increased the Na PIC portion of the total PIC (tested with nimodipine blocking the Ca PIC). Additionally, 5-HT, but not DOI, depolarized the resting membrane potential (Vm) and increased the input resistance (Rm) in a dose-dependent manner. Therefore 5-HT2 receptor activation facilitated the Na PIC, whereas other 5-HT receptors modulated Vm and Rm. Motoneurons of chronic spinal rats responded to 5-HT and DOI in the same way, but with larger responses and at much lower doses (0.3-1 microM), thus exhibiting a 30-fold supersensitivity to 5-HT. Specifically the Na PIC was supersensitive to 5-HT2 receptor activation with DOI. Also, Rm and Vm were supersensitive to 5-HT. Consistent with the known critical role of the Na PIC in repetitive firing, enhancement of the Na PIC by DOI or 5-HT facilitated the repetitive firing evoked by steady current injection and enabled repetitive firing in a subpopulation of motoneurons of acute spinal rats that were initially unable to produce sustained repetitive firing. We suggest that after spinal transection, residual endogenous spinal sources of 5-HT help facilitate the Na PIC and repetitive firing. With chronic injury, the developed 5-HT supersensitivity more than compensates for lost brain stem 5-HT, so that the Na PIC is large and motoneurons are very excitable, thus contributing to spasticity.

Serotonin function in panic disorder: important, but why?

The essential role of serotonin (5-hydroxytryptamine (5-HT)) system in the neurobiology and pharmacotherapy of panic disorder (PD) continues to be a topic of intensive interdisciplinary research. Interest in the involvement of 5-HT in PD has been fuelled by clinical studies demonstrating that medications increasing the synaptic availability of 5-HT, such as selective 5-HT re-uptake inhibitors, are effective in the treatment of PD. Rival theories of 5-HT deficiency vs excess have attempted to explain the impact of 5-HT function in PD. In the past decade, knowledge of the role of 5-HT in the neurobiology of PD has expanded dramatically due to much new research including experimental, treatment, brain-imaging, and genetic studies. The current review attempts to summarize the new data and their implications. The challenge and treatment studies generally confirm the specific inhibitory influence of 5-HT on panicogenesis. The brain-imaging studies in PD patients demonstrate functional and clinically relevant alterations in various elements of 5-HT system affecting the neurocircuitry of panic. The findings of genetic association studies suggest that certain 5-HT-related genes may contribute to the susceptibility to PD; however, these data are rather limited and inconsistent. It appears that, even if not the primary etiological factor in PD, the 5-HT function conveys important vulnerability, as well as adaptive factors. A better understanding of these processes may be critical in achieving progress in the treatment of patients suffering from PD.

Characterization of 5-HT(₁A,B) and 5-HT(₂A,C) serotonin receptor binding

This unit describes assays for measuring the binding of radioligands to two major types of receptors for 5-hydroxytryptamine (5-HT or serotonin), 5-HT₁ and 5-HT₂ receptors, in homogenates of brain tissue or cloned into cells in culture. The specific receptor subtypes covered are 5-HT(₁A), 5-HT(₁B), 5-HT(₂A), and 5-HT(₂C). In addition, methodology for using quantitative autoradiography to measure radioligand binding to serotonin receptors in brain slices is described. Protocols are provided for characterization of both saturation and competition binding assays, and instructions for data analysis of these assays is also described. In addition, methodology is provided for the quantification (image analysis) of radioligand binding in brain tissue sections to determine receptor density, preparation of rat brain sections for quantitative autoradiography, and thionin staining of thaw-mounted tissue sections to define certain brain regions.

Serotonin type-1A receptor imaging in depression

Regional 5-hydroxytryptamine(1A) (5-HT(1A)) receptor binding potential (BP) of depressed subjects with primary, recurrent, familial mood disorders was compared to that of healthy controls by using positron emission tomography and [carbonyl-(11)C]WAY-100635 ¿[(11)C]N-(2-(4-(2-methoxyphenyl)-1-piperazin-1-yl)ethyl)-N-(2- pyridy l)cyclohexanecarboxamide¿. The mean 5-HT(1A) receptor BP was reduced 42% in the midbrain raphe and 25-33% in limbic and neocortical areas in the mesiotemporal, occipital, and parietal cortex. The magnitude of these abnormalities was most prominent in bipolar depressives and unipolar depressives who had bipolar relatives. These abnormal reductions in 5-HT(1A) receptor BP are consistent with in vivo evidence that 5-HT(1A) receptor sensitivity is reduced in major depressive disorder and postmortem data showing a widespread deficit of 5-HT(1A) receptor expression in primary mood disorders.

Effect of a long-term low tryptophan diet on the prolactin responses to the 5-HT1A and 5-HT2C agonists, 8-OH-DPAT and mCPP in the male rat

The study was undertaken to assess the long term effects of tryptophan (TRP) depletion through diet on the prolactin (PRL) responses to the serotonin (5-hydroxytryptophan, 5-HT) agonists m-chlorophenyl-piperazine (mCPP) and 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) in the male rat. The low TRP diet caused significant reductions in both plasma total TRP and brain cortical 5-HT content together with a significant increase in the PRL responses to mCPP. In contrast the PRL responses to 8-OH-DPAT in animals on the low TRP diet for 1 week and 6 weeks were similar to control rats. However, a small but significant increase in PRL was observed at 2 min after dosing in the 1-week group. At the same time the 3H-8-OH-DPAT binding parameters, Kd and Bmax, were similar in both control and TRP depleted animals. The results confirm that long-term TRP depletion causes a deficiency of brain TRP and a subsequent reduction of brain 5-HT. This is associated with an enhanced PRL response to mCPP probably resulting from a functional up-regulation of post-synaptic 5-HT2C receptors. The small or transient changes brought about by long-term TRP depletion on post-synaptic 5-HT1A receptors, suggests that these receptors may be less susceptible to 5-HT depleting effects than the 5-HT2C subtype.

Agonist- and antagonist-induced plasticity of rat 5-HT1A receptor in hippocampal cell culture

We examined the response and regulation of 5-HT1A receptor on hippocampal cultured fetal neurons grown in the absence of serotonin and steroids using three experimental designs: 1) functional response using an antibody against phosphorylated cyclic adenosine monophosphate response element binding protein (pCREB); 2) transcriptional regulation using in situ hybridization; and 3) translational expression using antipeptide 5-HT1A receptor antibody. Pretreatment of cultured hippocampal cells with the agonist 8-hydroxy-2-(di-N-propylamino)-tetralin (8-OH-DPAT) (10(-8) M) or ipsapirone (IPS) (10(-9) M) for 10 min blocked the forskolin-stimulated increase in pCREB immunoreactivity. In situ hybridization radioautography revealed that IPS (10(-9) M) decreased the 5-HT1A receptor mRNA expression (-33%) after a 24-h treatment. The decrease in 5-HT1A receptor mRNAwas accompanied by a change in protein immunoreactivity using a 5-HT1A receptor antipeptide antibody. Computer-assisted morphometric analyses showed a reduction in the 5-HT1A receptor immunoreactive (IR) intensity as compared to control 24 h after treatment with 8-OH-DPAT (10(-7)-10(-12) M) and IPS (10(-9) M). Thus, fetal hippocampal neurons have a functional 5-HT1A receptor that is downregulated at both the transcription and translation levels. In addition, we found increased 5-HT1A receptor-IR intensity (+17% approximately +39%) 24 h after treatment with the antagonist N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl) cyclohexane carboxamide (WAY 100635) (10(-7)-10(-12) M). Our results indicate that the 5-HT1A receptor is sensitive to both agonists (downregulation) and antagonists (upregulation) in hippocampal fetal neurons grown in the absence of serotonin and steroids.

(+)-S-20499 -- a potential antidepressant? A behavioural and neurochemical investigation in the olfactory bulbectomised rat

The study was designed to assess the potential antidepressant properties of the 5-HT1A receptor agonist, (+)-S-20499 (10 mg kg(-1) i.p.) in the olfactory bulbectomised (OB) rat. Following 2 weeks of treatment, the rats were tested in the elevated plus maze and the "open field". A characteristic hyperactive response was evident in the OB animals in the "open field" which was reversed following chronic treatment with (+)-S-20499. In the elevated plus maze an increase in the number of open arm entries and the time spent on the open arms was observed, although this failed to reach significance. A significant decrease in beta1 receptor affinity was evident following olfactory bulbectomy which was normalised by (+)-S-20499. (+)-S-20499 also significantly reduced the density of 5-HT2 receptors in the sham operated (SO) animals. These studies demonstrate the usefulness of the OB rat as a screening test for compounds with novel putative mechanisms of action, and highlights the potential antidepressant properties of (+)-S-20499.

Salivary cortisol in women with and without perimenstrual mood changes

An increase in the activity of the hypothalamic-pituitary-adrenal axis (HPA axis) is frequently associated with major depression. During the premenstrual phase of their reproductive cycle some women experience depressive mood changes that are proposed to be of similar intensity to that experienced during periods of major depression. This study examined the secretion of cortisol, the end-product in the HPA axis, at different stages of the menstrual cycle in women with and without premenstrual depression. Women who experienced only mild physical and emotional changes in the premenstrual phase of their cycle had a significantly higher cortisol secretion on a premenstrual day (measured hourly) compared to a postmenstrual day. Those who were significantly more depressed premenstrually showed the opposite pattern of cortisol secretion with significantly lower levels on the premenstrual day compared with the postmenstrual day. Across the menstrual cycle, women who were significantly more depressed premenstrually also had lower evening cortisol levels in their premenstrual phase. The results of this study indicate that, unlike major depression where the underlying neurological changes are manifest as overactivity of the HPA axis, premenstrual depressive changes are associated with reduced HPA axis activity. Premenstrual depression may, therefore, be similar neurologically to seasonal affective disorder, which is associated with underactivity of the HPA axis.

Neurochemical effects of the enantiomers of mirtazapine in normal rats

The present study was designed to examine the neurochemical effects of (+/-)-mirtazapine (10 mg kg(-1) i.p.) and its enantiomers in rats. Male Sprague-Dawley rats received either (+)-mirtazapine, (-)-mirtazapine, (+/-)-mirtazapine or vehicle, by intraperitoneal injection for two weeks. Maximum change in temperature from baseline, following a single dose of the 5-HT1A receptor agonist 8-Hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) (0.15 mg kg(-1) s.c.), was used to assess the function of the 5-HT1A receptors. Chronic drug treatment potentiated this response, with (+/-)-mirtazapine > (-)-mirtazapine > (+)-mirtazapine. Receptor changes were also observed with a slight decrease in beta1-adrenoceptor density, although this failed to reach significance. A significant decrease in beta1-adrenoceptor affinity was observed following (-)-mirtazapine treatment. All drugs tested significantly reduced the density of the 5-HT2 receptors. Results of the present study suggest that in so far as alterations in these receptor populations are important for the therapeutic action of antidepressants, neither of the enantiomers appear to be more active than the racemic mixture.

NCS-MPP (4-(2'-methoxy-phenyl)-1-[2'-(N-2"-pyridyl)-p-isothiocyanobenz amido]-ethyl-piperazine): a high affinity and irreversible 5-HT1A receptor ligand

A novel irreversible 5-HT1A receptor binding ligand, NCS-MPP (4-(2'- methoxy-phenyl)-1-[2'-(N-2"-pyridyl)-p-isothiocyanobenzamido]- ethyl-piperazine), based on the new 5-HT1A receptor antagonist p-MPPI (4-(2'-methoxy-phenyl)-1-[2'-(N-2"-pyridyl)-p-iodobenzamido]-ethyl -piperazine ), was synthesized, and its binding characteristics were evaluated using in vitro homogenate binding with rat hippocampal membranes. The Ki value of NCS-MPP was estimated to be 1.8 +_ 0.2 nM using analysis of concentration-dependent inhibition for the binding of [125I]p-MPPI to 5-HT1A receptors. NovaScreen of NCS-MPP showed low to moderate binding affinities to alpha-1, alpha-2-adrenergic and 5-HT2 receptors, with Ki values of 350, 420, and 103 nM, respectively. These data strongly suggest that the ligand bound to 5-HT1A receptors with high affinity and high selectivity. Irreversible inhibition of [125I]p-MPPI binding by NCS-MPP following a 5 min incubation at room temperature was concentration dependent; the inhibition increased to 50% at a concentration less than 10 nM, and became more pronounced (90%) at 400 nM. Under similar assay conditions, NCS-MPP was significantly less efficient in irreversibly inhibiting agonist ligand [125I]8-OH-PIPAT binding to 5-HT1A receptors at lower concentrations (<10nM). After pretreatment of membranes with a low concentration of NCS-MPP (2nM), there was an apparent loss of [125I]p-MPPI binding sites, as expected, but no change in the binding affinity (Kd) was observed. However, the significant increase in Kd at a higher concentration of NCS-MPP (50 nM) indicated that there may be a secondary alkylation site, which may not be directly involved in p-MPPI binding to receptors; nevertheless, it would lead to an increased Kd value. The availability of an irreversible ligand, NCS-MPP, may provide a useful tool for studies of 5-HT1A receptors in the central nervous system.

Multiple serotonin receptors: too many, not enough, or just the right number?

In this manuscript, current knowledge about central nervous system serotonin (5-HT) receptors is discussed with an emphasis toward describing the functional significance of the multiple 5-HT receptors. Five characteristics of 5-HT receptors, which are hypothesized to contribute to this functional significance, are discussed: (a) 5-HT has varying affinity and potency for the different receptor subtypes; (b) multiple transduction pathways are used by the different receptor subtypes; (c) receptor subtypes differ in their susceptibility to agonist-mediated desensitization/downregulation; (d) receptor subtypes interact in mediating cellular responses to the neurotransmitter; and (e) receptor subtypes respond differently to changes in the physiological environment. It is hypothesized that these characteristics of the multiple neurotransmitter receptors provide the nervous system with a capacity for coding and decoding of 5-HT-mediated neuronal transmission that could not take place with a single neurotransmitter receptor. Serotonergic regulation of female reproduction and regulation of glucocorticoid release are used to illustrate the integrative potential deriving from the existence of multiple 5-HT receptors.

5-HT1A receptor binding sites in post-mortem brain samples from depressed suicides and controls

5-HT1A receptor binding sites were measured, by saturation binding with [3H]8-OH-DPAT, in frontal and occipital cortex, hippocampus and amygdala obtained at post-mortem examination from suicide victims with a firm retrospective diagnosis of depression, and matched controls. The number of 5-HT1A binding sites did not differ significantly between suicides and controls, either in the total sample or when the suicides were divided on the basis of violence of death or recent antidepressant treatment.

Effect of corticosterone on serotonin and catecholamine receptors and uptake sites in rat frontal cortex

The effects of corticosterone (1 mg/kg per day for 7 days) on serotonin 5-HT1A, 5-HT2A, 5-HT uptake sites, and alpha 2-adrenergic receptor sites were measured. Corticosterone treatment significantly decreased the number of 5-HT1A receptor sites (Bmax = 108 +/- 8.20 fmol/mg protein and 152.31 +/- 13.36 fmol/mg protein in corticosterone- and vehicle-treated rats, respectively). No significant differences were found in other measures. It is possible that corticosteroids exert some of their behavioral effects via regulation of 5-HT1A sites in frontal cortex.

Time-course of recovery of 5-HT1A receptors and changes in 5-HT1A receptor mRNA after irreversible inactivation with EEDQ

In this study, the relationship between the expression of 5-HT1A receptors and level of receptor mRNA in discrete regions of rat brain was examined by inactivation of 5-HT1A receptors with the alkylating agent N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ; i.p., 10 mg/kg) and measurement of the time-course of receptor recovery and changes in receptor mRNA levels. Inactivation of 5-HT1A receptors ranged from 84% in the dorsal raphe to 97% in the cortex 12 h after administration of EEDQ. Receptor levels returned to 62-100% of control levels by day 7 and the rate of recovery was uniform across all regions examined. The rate of recovery of 5-HT1A receptors labeled by the agonist [3H]8-hydroxy-2-(di-n-propylamino)tetralin ([3H]8-OH-DPAT) and by the putative antagonist [125I]4-(2'-methoxy)phenyl-1-[2'-(N-2"-pyridinyl)-p-iodobenzamido] ethylpiperazine ([125I]p-MPPI) did not differ across regions, suggesting that the ratio of high versus low affinity states of the 5-HT1A receptor remains relatively constant during receptor recovery. However, there did appear to be a short lag in the recovery of sites labeled with the agonist. Significant increases in 5-HT1A receptor mRNA levels were observed as early as 12 h after treatment in all regions but the magnitude of these increases varied. The time-courses of recovery of 5-HT1A receptors and changes in mRNA levels were not parallel in individual regions. Moreover, inactivation of low (8-26%) to moderate (29-57%) levels of 5-HT1A receptors produced no change in mRNA levels, whereas inactivation of greater than 90% elicited a robust increase in mRNA levels. Thus, changes in 5-HT1A receptor expression are not mediated exclusively by changes in mRNA levels and extensive receptor inactivation is required to trigger transcriptional regulation.

Increased 5-HT1A receptor immunoreactivity in the rat hippocampus following 5,7-dihydroxytryptamine lesions in the cingulum bundle and fimbria-fornix

Serotonin (5-HT) projections from the ascending raphe nuclei reach the dorsal hippocampus via the cingulum bundle (CB) and fimbria-fornix (FF). Microinjection of the serotonergic neurotoxin 5,7-dihydroxytryptamine (5,7-DHT) into the CB and FF produces a significant decrease in the density of 5-HT immunoreactive fibers in the hippocampus as early as 3 days postlesion (Zhou, F.C. and Azmitia, E.C. (1983) Brain Res. Bull., 373, 337-348). In the present study we used an anti-peptide antibody against the second extracellular loop of the 5-HT1A receptor and employed immunocytochemistry to examine changes in the expression and distribution of the 5-HT1A receptor in the hippocampus 14 days following administration of 5,7-DHT into the CB and FF. The density of 5-HT immunoreactive fibers was greatly reduced 14 days following the lesions. 5-HT1A immunoreactivity (IR) was localized to the proximal axon near the axon hillock of cells in the pyramidal cell layer of the cornu Ammonus and in the granule cell layer of the dentate gyrus. The intensity of 5-HT1A-IR was increased in the CA1 and dentate gyrus following 5,7-DHT lesions. Intensity in the CA3 also increased but not to a significant level. These findings demonstrate that 5-HT denervation in the hippocampus is followed by increased expression of the 5-HT1A receptor protein. These changes in receptor expression 14 days postlesion may represent adaptive changes by postsynaptic cells following reduced 5-HT innervation and may be the molecular basis for 5-HT1A receptor supersensitivity.

Localized alterations in pre- and postsynaptic serotonin binding sites in the ventrolateral prefrontal cortex of suicide victims

Altered serotonin indices have been reported in the brain of suicide victims. We sought to localize the changes in presynaptic and postsynaptic serotonin receptors and identify an area of prefrontal cortex that may influence suicide risk. Quantitative autoradiography was performed in coronal sections of prefrontal cortex to determine whether serotonin 5-HT1A receptor (postsynaptic in cortex) and serotonin transporter (presynaptic) binding are different in suicide victims compared to matched controls. 5-HT1A receptor binding was higher in 85 of the 103 sampled areas in the suicide group (n = 18 pairs; P < 0.0001). The increase ranged from 17 to 30%. The increase was more pronounced in the ventrolateral prefrontal cortex. Serotonin transporter binding was found to be lower in the suicide group in all but one of the 43 sampled regions (n = 22 pairs; P < 0.0001). The reduction in binding was most pronounced in the ventrolateral prefrontal cortex, where the difference between suicides and controls ranged between 15 and 27%. Serotonin transporter and 5-HT1A binding were negatively correlated (r = -0.35 to -0.44, P = 0.04 to 0.007) within the same brain areas, suggesting common regulatory factors with opposite effects on binding to the two receptors. We conclude that suicide victims have an abnormality in the serotonin system involving predominantly the ventrolateral prefrontal cortex, and hypothesize that the serotonergic dysfunction in this brain region contributes to the risk for suicidal behavior.

Synthesis of (+)-(R)- and (-)-(S)-trans-8-hydroxy-2-[N-n-propyl-N-(3'-iodo-2'-propenyl)] aminotetralin: new 5-HT1A receptor ligands

(R,S)-trans-8-Hydroxy-2-[N-n-propyl-N-(3'-iodo-2'- propenyl)amino]tetralin 7, a new radioiodinated ligand based on 8-OH-DPAT, was reported as a potential ligand for 5-HT1A receptors. The optically active (+)-(R)- and (-)-(S)-7 were prepared to investigate the stereoselectivity of (R,S)-7. Racemic intermediate 8-methoxy-2-N-n-propyltetralin was reacted with the acyl chloride of (-)-(R)-O-methylmandelic acid to form a mixture of (S,R)- and (R,R)-diastereoisomers, which were separated by flash column chromatography. After removing the N-acyl group from the diastereoisomers, the desired (+)-(R)- or (-)-(S)-7 was obtained by adding an N-iodopropenyl group. In vitro homogenate binding studies showed the stereoselectivity of this new compound for 5-HT1A receptors. (+)-(R)-7 isomer displayed 100-fold higher affinity than the (-)-(S)-7 isomer. Biochemical study indicated that (+)-(R)-7 potently inhibited forskolin-stimulated adenylyl cyclase activity in hippocampal membranes (Emax and EC50 were 24.5% and 5.4 nM, respectively), while (-)-(S)-7 showed no effect at 1 microM. The radioiodinated (+)-(R)- and (-)-(S)-[125I]7 were confirmed by coelution with the resolved unlabeled compound on HPLC (reverse phase column PRP-1, acetonitrile/pH 7.0 buffer, 80/20). The active isomer, (+)-(R)-[125I]7, displayed high binding affinity to 5-HT1A receptors (Kd = 0.09 +/- 0.02 nM). In contrast, the (-)-(S)-7 isomer displayed a significantly lower affinity to the 5-HT1A receptor (Kd > 10 nM).(ABSTRACT TRUNCATED AT 250 WORDS)

Behavioural and neuroendocrine responses to D-fenfluramine in rats treated with neurotoxic amphetamines

The amphetamine derivatives p-chloroamphetamine (pCA), 3,4-methylenedioxymethamphetamine (MDMA, 'Ecstasy') and D-fenfluramine can, if given repeatedly in high doses to rats, produce a degeneration of serotonergic nerve terminals which we have previously shown to result in a reduction in D-fenfluramine-evoked release of 5-HT in vivo. It is therefore possible that fenfluramine-evoked responses may have value as a probe of 5-HT neurodegeneration in man. The present study examined the effect of pre-treatment with these three agents (pCA 12 mg/kg×2; MDMA 20 mg/kg×8; D-fenfluramine 12.5 mg/kg×8, 14 days prior to testing) on behavioural (5-HT syndrome) and neuroendocrine [prolactin and adrenocorticotrophin (ACTH)] responses in rats to acute administration of D-fenfluramine and other serotonergic agonists. All three pre-treatments attenuated the D-fenfluramine-evoked behavioural syndrome, but did not affect the prolactin or ACTH responses to acute challenge with D-fenfluramine (apart from a small effect of pre-treatment with pCA on the ACTH response to D-fenfluramine). For comparison, the effect of pCA pre-treatment on the behavioural responses to acute administration of pCA and the 5-HT(1A) and 5-HT(2) receptor agonists 8-hydroxy-2-(di- n- propylamino)tetralin (8-OH-DPAT) and 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI), respectively, were also examined. pCA pre-treatment attenuated all components of the behavioural response to pCA but had little or no effect on the behavioural responses to 8-OH-DPAT or DOI, suggesting that there was no alteration in post-synaptic 5-HT(1A) or 5-HT(2) receptor function. While the loss of behavioural effect of D-fenfluramine on rats pre-treated with neurotoxic amphetamines can be understood in terms of the loss of D-fenfluramine's 5-HT-releasing action following 5-HT neurodegeneration, the lack of change in the neuroendocrine responses to D-fenfluramine is not easily explicable in this way. These results emphasise the need for further research into the actions of D-fenfluramine before carrying it forward as a probe of neurodegeneration in man.

Heterogeneity of cortical and hippocampal 5-HT1A receptors: a reappraisal of homogenate binding with 8-[3H]hydroxydipropylaminotetralin

The selective serotonin (5-HT) agonist 8-hydroxydipropylaminotetralin (8-OH-DPAT) has been extensively used to characterize the physiological, biochemical, and behavioral features of the 5-HT1A receptor. A further characterization of this receptor subtype was conducted with membrane preparations from rat cerebral cortex and hippocampus. The saturation binding isotherms of [3H]8-OH-DPAT (free ligand from 200 pM to 160 nM) revealed high-affinity 5-HT1A receptors (KH = 0.7-0.8 nM) and low-affinity (KL = 22-36 nM) binding sites. The kinetics of [3H]8-OH-DPAT binding were examined at two ligand concentrations, i.e., 1 and 10 nM, and in each case revealed two dissociation rate constants supporting the existence of high- and low-affinity binding sites. When the high-affinity sites were labeled with a 1 nM concentration of [3H]8-OH-DPAT, the competition curves of agonist and antagonist drugs were best fit to a two-site model, indicating the presence of two different 5-HT1A binding sites or, alternatively, two affinity states, tentatively designated as 5-HT1AHIGH and 5-HT1ALOW. However, the low correlation between the affinities of various drugs for these sites indicates the existence of different and independent binding sites. To determine whether 5-HT1A sites are modulated by 5'-guanylylimidodiphosphate, inhibition experiments with 5-HT were performed in the presence or in the absence of 100 microM 5'-guanylylimidodiphosphate. The binding of 1 nM [3H]8-OH-DPAT to the 5-HT1AHIGH site was dramatically (80%) reduced by 5'-guanylylimidodiphosphate; in contrast, the low-affinity site, or 5-HT1ALOW, was seemingly insensitive to the guanine nucleotide. The findings suggest that the high-affinity 5-HT1AHIGH site corresponds to the classic 5-HT1A receptor, whereas the novel 5-HT1ALOW binding site, labeled by 1 nM [3H]8-OH-DPAT and having a micromolar affinity for 5-HT, may not belong to the G protein family of receptors. To further investigate the relationship of 5-HT1A sites and the 5-HT innervation, rats were treated with p-chlorophenylalanine or with the neurotoxin p-chloroamphetamine. The inhibition of 5-HT synthesis by p-chlorophenylalanine did not alter either of the two 5-HT1A sites, but deafferentation by p-chloroamphetamine caused a loss of the low-affinity [3H]8-OH-DPAT binding sites, indicating that these novel binding sites may be located presynaptically on 5-HT fibers and/or nerve terminals.

A potential 5-HT1A receptor antagonist: p-MPPI

A potential 5-HT1A receptor antagonist, p-MPPI, 4-(2'-methoxy-)phenyl-1-[2'-(n-2"-pyridinyl)-p-iodobenzamido-]ethy l- piperazine, was developed. The [125I]p-MPPI demonstrated high affinity and selectivity toward 5-HT1A receptors; Kd = 0.36 nM and Bmax = 264 fmol/mg of protein in rat hippocampal membrane homogenates. The binding is not sensitive to GTP (300 microM) or Gpp(NH)p (100 microM). In forskolin-stimulated adenylyl cyclase assay using rat hippocampus, p-MPPI (up to 10 microM) showed no agonist activity as compared to that of (+/-)-8-OH-DPAT. At 100 nM it completely antagonized the inhibition of forskolin-stimulated adenylyl cyclase activity produced by 100 nM of (+/-)-8-OH-DPAT. This potential 5-HT1A antagonist may provide a powerful tool for studies of the pharmacology of the 5-HT1A receptor system.

Time course of the effects of adrenalectomy and corticosterone replacement on 5-HT1A receptors and 5-HT uptake sites in the hippocampus and dorsal raphe nucleus of the rat brain: an autoradiographic analysis

Previous studies have shown that adrenalectomy (ADX) increases the binding of 3H-DPAT to 5-HT1A receptors in the hippocampus (HIP) and this effect is partially overcome by corticosterone (CORT) replacement. The present study investigated the time course of the effects of ADX with or without CORT replacement on serotonin (5-HT) pre- and postsynaptic systems in the HIP and dorsal raphe nucleus (DR) by quantitative autoradiography. In the HIP, ADX for 7, 10 or 14 days caused a significant increase in 3H-DPAT binding in the CA1 region (pyramidal layer), CA2,3 region (molecular and pyramidal layers) and in the dentate gyrus (molecular and granular layers) which returned to control levels when measurements were made 35 days post-ADX. A decrease in 3H-DPAT binding was observed 14 days after ADX in the DR but not in the median raphe nucleus (MR). Although replacement with CORT did not lead to a reversal in 3H-DPAT binding early time points, binding was restored to control levels 7-28 days after CORT replacement in all regions of the HIP. In the DR, CORT did not cause a reversal in 3H-DPAT binding at any of the time points examined. In contrast to the effects seen on the 5-HT1A receptor subtype, no significant change was noted on the binding of 3H-CN-IMI to uptake sites for 5-HT in the HIP or DR after ADX or CORT replacement. The results of this study indicate that long-term alterations in the HPA axis lead to changes in the 5-HT1A receptor system that are both region-specific and time-dependent.

[11C]WAY 100635: a radioligand for imaging 5-HT1A receptors with positron emission tomography

The potent and selective 5-HT1A antagonist WAY 100635 (N-[2-]4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2- pyridinyl)cyclohexanecarboxamide) was radiolabeled with 11C in high specific activity, and the in vivo properties of this radioligand were assessed in the brains of rats and monkeys. Following i.v. tail vein injection in rats, [11C]WAY 100635 rapidly penetrated into brain tissue and was retained over a 30-90 min time period in a manner consistent with the known distribution of 5-HT1A receptors. Pretreatment of rats with the selective 5-HT1A agonist (+/-)-8-OH-DPAT effectively blocked the retention of radioactivity in brain regions known to contain high densities of 5-HT1A receptors. The hippocampus-to-cerebellum radioactivity concentration ratio reached a maximum of 16:1 at 60 min post injection. Following i.v. injection of [11C]WAY 100635 in rhesus monkeys, the concentrations of radioactivity in brain regions were consistent with the reported distribution of 5-HT1A receptors in primates, and the frontal cortex-to-cerebellum ratio reached 5.5:1 at 80 min post injection. Pretreatment of the monkeys with (+/-)-8-OH-DPAT reduced this ratio to 1.4:1, and injection of (+/-)-8-OH-DPAT 20 min after the injection of [11C]WAY 100635 significantly displaced frontal cortex binding. The in vivo properties of [11C]WAY 100635 in rats and monkeys strongly support the future utility of this radioligand for imaging 5-HT1A receptors using positron emission tomography (PET).

Stress and antidepressant effects on hippocampal and cortical 5-HT1A and 5-HT2 receptors and transport sites for serotonin

The interactions between 14 days of repeated restraint stress and daily administration of imipramine or tianeptine (2 h before the beginning of stress) were investigated in rats to assess responses of 5-HT2 and 5-HT1A receptors and serotonin transporter sites labelled by [3H]paroxetine in the cerebral cortex and hippocampus, two brain regions in which adrenal steroid effects on serotonin receptor-binding have been reported. 5-HT2 sites, labelled by [125I]7-amino-8-iodo ketanserin, were decreased in parietal cerebral cortex layers 3 and 5 by imipramine treatment, but not by tianeptine treatment and not by daily restraint stress. Stress, but not antidepressant, depressed 5-HT1A sites labelled with [3H]8-hydroxy-DPAT in hippocampal fields CA3, CA4 and dentate gyrus. [3H]paroxetine-binding to serotonin transporter sites was decreased by tianeptine treatment as well as by imipramine in both hippocampus and cerebral cortex, with some overlap of the fields that were significantly affected, whereas there were no effects of stress per se and no evidence of a stress x drug interaction. These results are discussed in relation to similarities and differences in the effects of different antidepressant drugs on the serotonergic system of the rat brain. Whereas the actions of imipramine and tianeptine on 5-HT2 and 5-HT1A receptors are specific to each drug, the surprising finding of a similar effect of both drugs to reduce serotonin transporter sites labelled by [3H]paroxetine suggest the possibility of a common action for these two drugs in spite of their opposite effects on serotonin re-uptake.

A review and reevaluation of the role of serotonin in the modulation of lordosis behavior in the female rat

The role of serotonin (5-HT) in the modulation of sexual receptivity (lordosis) in the female rat is reviewed and reevaluated. The effects on lordosis of drug treatments that decrease or increase the activity and availability of central 5-HT are first discussed, and this is followed by an evaluation of the effects of drugs that act directly at 5-HT receptors. In order to shed light on the physiological significance of effects of serotonergic drugs on lordosis, there is also a review of what is known of changes in levels of serotonergic activity and densities of 5-HT receptors in the female rat brain that take place through the estrous cycle and in response to administration of behaviorally effective doses of gonadal steroids. Serotonin has generally been thought to have a tonic, inhibitory effect on lordosis. However, it is concluded that 5-HT can either inhibit or facilitate lordosis depending on which subtypes of central 5-HT receptors become activated. Because of a lack of consistent or compelling evidence of effects of ovarian hormones on serotonergic activity or 5-HT receptors in critical areas of the brain, it is stated that there is at present no basis to conclude that the effects of pharmacological manipulations of serotonergic activity on lordosis reflect an important, physiological role of 5-HT in the modulation of lordosis behavior in the female rat.

Sex difference for tolerance of 5-HT1A receptor-mediated temperature and corticosterone responses in mice

Repeated treatment with 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) resulted in significant attenuation of 8-OH-DPAT-induced hypothermia and adrenocorticol effect in mice of both sexes, while it did not affect the 8-OH-DPAT-induced decrease in 5-hydroxyindoleacetic acid in the hypothalamus in either sex. The attenuated responses developed more rapidly in female than in male mice, indicating sex differences in the adaptive regulation of the 5-HT1A receptor-mediated responses.

Effect of the selective lesion of serotoninergic neurons on the regional distribution of 5-HT1A receptor mRNA in the rat brain

The effects of the selective lesion of serotoninergic neurons by an intra-raphe administration of 5,7-dihydroxytryptamine on the 5-HT1A receptor protein and the 5-HT1A receptor mRNA were examined in various regions of the rat brain using specific antibodies and an antisense riboprobe, respectively. Twenty one days after the treatment, the 5-HT1A receptor protein was no longer detected within the dorsal raphe nucleus but was still present in the hippocampus and entorhinal cortex. Quantitative in situ hybridization showed an 85% decrease in the levels of 5-HT1A receptor mRNA within the dorsal raphe nucleus, but no significant change in the hippocampus, interpeduncular nucleus and entorhinal cortex of 5,7-dihydroxytryptamine-treated rats. These data demonstrate that 5-HT1A receptors are synthesized by serotoninergic neurons in the dorsal raphe nucleus, and by neurons located postsynaptically with regard to serotoninergic projections in other areas. The unchanged levels of 5-HT1A receptor mRNA in the hippocampus, interpeduncular nucleus and entorhinal cortex three weeks after the extensive lesion of serotoninergic neurons are consistent with the absence of 5-HT1A receptor up regulation already reported under this condition.

Effect of repeated electroconvulsive shocks on serotonergic neurons

The hypothermia induced by the serotonin (5-HT)1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) was attenuated in rats that had received a course of six electroconvulsive shocks (ECS) over a two-week period. The firing activity of dorsal raphe 5-HT neurons, as well as their responsiveness to microiontophoretic applications of 5-HT and 8-OH-DPAT, was unaltered in ECS-treated rats. The electrically evoked overflow of [3H]5-HT from preloaded slices of guinea pig hypothalamus was unchanged after the same ECS treatment. The concentration-effect curves of the 5-HT autoreceptor agonist 5-carboxyamidotryptamine (0.1-100 nM) were similar in slices prepared from control and ECS-treated guinea pigs. In addition, the reduction in the evoked [3H]5-HT overflow obtained by increasing the stimulation frequency from 1 to 5 Hz, which is due to a greater activation of terminal 5-HT autoreceptors at the higher frequency, was not altered by the ECS treatment. The enhancing effects of the 5-HT autoreceptor antagonist methiothepin (0.1-1 microM) and of the 5-HT3 agonist 2-methyl-5-HT (0.1-1 microM) on the evoked [3H]5-HT overflow were unaltered by the ECS treatment. These results thus indicate that repeated ECS attenuates the 8-OH-DPAT-induced hypothermia in rats, as previously reported, but does not affect the firing activity of 5-HT neurons and the sensitivity of their somatodendritic 5-HT1A autoreceptors in the dorsal raphe. The function of 5-HT terminals in the guinea pig hypothalamus was also unaffected by repeated ECS. In conclusion, repeated ECS does not affect the function of 5-HT neurons at the cell body and nerve terminal.

Long-term fluoxetine treatment decreases 5-HT1A receptor responsivity in obsessive-compulsive disorder

Fluoxetine (FLX) is a selective serotonin (5-HT) reuptake inhibitor with therapeutic benefit in patients with obsessive-compulsive disorder (OCD). To evaluate the effect of chronic FLX treatment on 5-HT1A receptor responsivity, hypothermic, neuroendocrine, and behavioral responses to the selective 5-HT1A receptor ligand ipsapirone (IPS) were examined in patients with primary OCD. A single dose of 0.3 mg/kg of IPS or placebo were given under double-blind, random-assignment conditions to ten patients before and during FLX treatment. The ability of IPS to induce hypothermia and ACTH/cortisol release was significantly attenuated during chronic FLX as compared to the pretreatment IPS challenge. The behavioral effects of IPS, though minimal, were less pronounced during FLX treatment. While FLX was effective in reducing the severity of OC symptoms, no significant correlation between attenuation of 5-HT1A receptor-mediated functional measures and FLX-induced improvement in OC symptoms was detected. These findings are consistent with the development of adaptive hyporesponsivity of the 5-HT1A receptor-effector system complex possibly involving subsensitivity of the 5-HT1A receptor itself and/or decreased functional activity of the postreceptor signal transduction. Modulation of 5-HT1A receptor-effector system function may be critical to the antidepressant/anti-OC efficacy of 5-HT reuptake inhibitors.