Serotonin function in affective disorders

Serotonin Dysfunction, Aggressive Behavior, and Mental Illness: Exploring the Link Using a Dimensional Approach

Aggressive individuals have higher rates of mental illness compared to non-aggressive individuals. Multiple factors, including psychosocial, genetic, and neurobiological determinants modulate the liability to both aggressive behavior and mental illness. Concerning the latter factors, multiple lines of evidence have shown a dysfunction in the serotonin (5-HT) system occurring in aggressive and in mentally ill individuals. In particular, reduced 5-HT activity has been associated with depression as well as with aggressive behavior, especially with impulsive aggression. Consistently, psychopharmacological interventions aimed at boosting the 5-HT system (e.g., with selective serotonin reuptake inhibitors) have demonstrated therapeutic efficacy in a high percentage of patients with either or both pathological conditions. Current knowledge does not yet allow to clearly disentangle whether 5-HT dysfunction, most often a 5-HT deficiency, is the cause or the consequence of the aggressive/violent behavior, of the underlying mental disease/s, or the expression of the comorbidity. Future studies are thus needed to clarify the association between changes in 5-HT levels, altered activity of 5-HT receptors and their intracellular signaling cascades, and modifications of 5-HT genes, and in particular the neurobiological link between the altered 5-HT machinery and aggressive behavior in the context or in the absence of mental illness. In this Review, we employ a dimensional approach to discuss the trivariate relationship among the 5-HT system, aggressive behavior, and mental illness, focusing our attention on 5-HT levels, 5-HT receptors, metabolic enzymes, and their genes. Emphasis is given to controversial findings, still unanswered questions, and future perspectives.

Stress-hyperresponsive WKY rats demonstrate depressed dorsal raphe neuronal excitability and dysregulated CRF-mediated responses

Major depression is a debilitating psychiatric disease that may be precipitated by a dysregulation of stress neurocircuitry caused by chronic or severe stress exposure. Moreover, hyperresponsivity to stressors correlates with depressed mood and may contribute to the etiology of major depression. The serotonergic dorsal raphe nucleus (DRN) is an important site in the neurocircuitry underlying behavioral responses to stressors, and is tightly regulated, in part, by a combination of intrinsic cell properties, autoinhibition, and GABAergic synaptic transmission. The stress-related neurotransmitter corticotropin-releasing factor (CRF) modulates DRN neuronal excitability and subsequent 5-HT release in the forebrain. Wistar Kyoto (WKY) rats exhibit exaggerated behavioral responses to stressors, that is, stress hyperresponsivity, and are considered an animal model of depression. To better understand the neurobiological basis of the stress hyperresponsivity, we used a combination of mRNA analysis and whole-cell electrophysiological techniques to measure differences in intrinsic activity and receptor response, in 5-HT- and non-5-HT-containing neurons of the DRN in WKY rats compared with Sprague-Dawley controls. In the WKY rat, there was a decrease in the neuronal excitability of 5-HT neurons coupled with decreased TPH2 production. Additionally, we found that CRF did not increase GABAergic activity in 5-HT neurons as is normally seen in 5-HT neurons of Sprague-Dawley controls. The CRF modulation of 5-HT DRN neurotransmission at the single-cell level is selectively disrupted in the WKY animal model of depression and may be one of the cellular correlates underlying depression.

Chronic treatment with antidepressants decreases intraoperative core hypothermia

We investigated temperature regulation during anesthesia and postoperative shivering in chronically depressed patients given antidepressant drugs. We studied 35 depressed patients and 35 control patients who underwent orthopedic surgery. Tympanic membrane temperatures 60, 75, and 90 min after induction in the depression group were significantly (P < 0.05) higher than those of the control group. There were no significant differences in mean skin temperature between the depression and the control groups. Eight of 35 patients in the depression group and 2 of 35 patients in the control group developed postanesthetic shivering. The incidence of shivering in the depression group was significantly more frequent than that in the control group (P = 0.04). The tympanic membrane temperature of the patients treated with clomipramine tended to be higher than that of the patients treated with maprotiline. In conclusion, intraoperative core hypothermia in chronically depressed patients was decreased. However, the incidence of shivering in depressed patients was significantly more frequent.

IMPLICATIONS

Thermoregulation in chronically depressed patients is often altered. The alteration of body temperature is affected by depression itself and by antidepressants. General anesthesia has an influence on thermoregulatory control. However, temperature regulation during anesthesia in chronically depressed patients remains unclear.

Long-term effects on serotonin transporter mRNA expression of chronic neonatal exposure to a serotonin reuptake inhibitor

Chronic administration of clomipramine or other serotonin (5-hydroxytryptamine, 5-HT) reuptake inhibitors to neonatal rats produces behaviours that resemble a depressive state in the adult animal, and this model is therefore regarded as a putative animal model of depression. Alterations in the activity of the central 5-HT system are important in understanding the pathophysiology of depression, and therefore, we examined whether this model was associated with changes in the expression of 5-HT1A receptor, 5-HT1B receptor, and 5-HT transporter mRNA in the dorsal raphe nucleus and the hippocampus. Wistar rats were injected twice daily with the serotonin reuptake inhibitors clomipramine and 5-chloro-1-[3-(dimethylamino)propyl]-1-(4-fluoro-phenyl)-1,3-dihydroi so-benzofurane, hydrochloride (code Lu 10-134-C) at doses of 15 mg kg(-1) or vehicle i.p. from postnatal day 8 for 14 days. Groups of rats (n = 10) were either killed the day after the last injection or left undisturbed for 69 days before they were killed. The expression of 5-HT transporter, 5-HT1A receptor, and 5-HT1B receptor mRNA was examined in the dorsal raphe nucleus and in the CA1 of the hippocampus by means of quantitative in situ hybridisation histochemistry. Both compounds resulted in an increase in 5-HT transporter mRNA expression (40% more than vehicle) in the dorsal raphe nucleus the day after the last injection (postnatal day 22). A small but significant increase in 5-HT1B receptor mRNA expression in the CA1 was seen after clomipramine, but not after Lu 10-134-C, probably reflecting clomipramine's affinity for both the 5-HT and noradrenaline transporters as well as for a number of monoamine receptor sites. Levels of 5-HT1A receptor mRNA were unchanged. In contrast, 5-HT transporter mRNA expression in the dorsal raphe nucleus was significantly decreased in the adult after neonatal treatment with either of the two drugs compared to vehicle. No changes in 5-HT1A receptor and 5-HT1B receptor mRNA expression were observed in any of the regions examined in these animals. The results show that the persistent depressive behaviour previously shown in this model is also associated with changes in the expression of 5-HT transporter mRNA. This long-term alteration in gene expression may result from disturbances in 5-HT neurotransmission in the brain of the neonatal animals.

Citalopram: differential sleep/wake and EEG power spectrum effects after single dose and chronic administration

The sleep/wake effects of the selective serotonin re-uptake inhibitor citalopram were studied in both a single-dose study with three dose levels (0.5, 2.0 and 5.0 mg/kg), and a 5-week chronic administration study (15 mg/kg/24 h). Single doses of citalopram resulted in a dose-dependent inhibition of rapid eye movement (REM) sleep. After chronic citalopram treatment there was a sustained REM sleep inhibition. Single doses of citalopram resulted in only minor changes in non-REM (NREM) sleep as well as in NREM EEG power spectral density. Chronic administration resulted in a major shift from SWS-2 to SWS-1. The observed corresponding changes in EEG power density were regional. A 30 to 40 percent reduction of power density in the 0.5-15 Hz range in the fronto-parietal EEG derivation was seen for the whole 8-h registration period. In the fronto-frontal EEG derivation only minor changes were seen. A decreasing trend in NREM sleep power density between 0.5 and 7 Hz, usually seen during the course of the light period, was not observed in the chronic condition, but was seen in control and single-dose condition, suggesting altered diurnal distribution of slow wave activity in the chronic condition. The data indicate that acute and chronic administration of citalopram shows clear differences in sleep effect, which may be caused by alteration of serotonergic transmission, and may be related to the antidepressant effect.

Effect of chronic ipsapirone treatment on the density of 5-HT1A receptors and 5-HT1A receptor mRNA in discrete regions of the rat brain

There is increasing evidence that the 5-hydroxytryptamine (HT)1A partial agonist ipsapirone is an effective anxiolytic/antidepressant agent, although its mechanism of action is not clear. In this study, we investigated the effects of chronic ipsapirone treatment (5 or 10 mg/kg; twice daily, 3 weeks) on 5-HT1A receptor density 8-hydroxy-2-(di-n-propyl amino) tetralin (8O H-DPAT) binding and the level of its mRNA (in situ hybridization) in various regions of the rat (male Wistar 250 g) brain. Receptor density was reduced in the frontal cortex, but did not change significantly in the hippocampus and dorsal raphe nucleus. The level of receptor mRNA was unchanged in each of these brain regions. The present results suggest that the clinical anxiolytic effects of ipsapirone may be mediated partly by postsynaptic action on serotonergic transmission in the frontal cortex, but not in the hippocampus or dorsal raphe nuclei.

Comparative localization of serotonin1A, 1C, and 2 receptor subtype mRNAs in rat brain

Serotonin (5-HT) mediates its effects on neurons in the central nervous system through a number of different receptor types. To gain better insight as to the localization of 5-HT responsive cells, the distribution of cells expressing mRNAs encoding the three 5-HT receptor subtypes 1A, 1C, and 2 was examined in rat brain with in situ hybridization using cRNA probes. 5-HT1A receptor mRNA labeling was most pronounced in the olfactory bulb, anterior hippocampal rudiment, septum, hippocampus (dentate gyrus and layers CA1-3), entorhinal cortex, interpeduncular nucleus, and medullary raphe nuclei. 5-HT1C receptor mRNA labeling was the most abundant and widespread of the three 5-HT receptor subtypes examined. Hybridization signal was densest in the choroid plexus, anterior olfactory nucleus, olfactory tubercle, piriform cortex, septum, subiculum, entorhinal cortex, claustrum, accumbens nucleus, striatum, lateral amygdala, paratenial and paracentral thalamic nuclei, subthalamic nucleus, substantia nigra, and reticular cell groups. 5-HT2 receptor mRNA was localized to the olfactory bulb, anterior hippocampal rudiment, frontal cortex, piriform cortex, entorhinal cortex, claustrum, pontine nuclei, and cranial nerve motor nuclei including the oculomotor, trigeminal motor, facial, dorsal motor nucleus of the vagus, and hypoglossal nuclei. The distributions of mRNAs for the three different 5-HT receptor subtypes overlap with regions that bind various 5-HT receptor-selective ligands and are present in nearly all areas known to receive serotonergic innervation. The results of this study demonstrate that neurons which express these 5-HT receptor subtypes are very widespread in the central nervous system, yet possess unique distributions within the rat brain. Moreover, previously unreported regions of 5-HT receptor subtype expression were observed, particularly with the 5-HT2 receptor riboprobe in the brainstem. Finally, several brain areas contain multiple 5-HT receptor subtype mRNAs, which leads to the possibility that individual cells may express more than one 5-HT receptor subtype.

Stress and/or tranylcypromine treatment affects serotonergic measures in blood and brain in rats

Since stress can alter serotonin (5-hydroxytryptamine, 5-HT) turnover in the brain and the periphery, the effects of different types of acute stress on serotonin and related substances in the whole blood and various brain areas in rats pretreated with tranylcypromine (TCP) were studied. TCP administered alone caused a rise in 5-HT, a fall in its metabolite (5-hydroxyindoleacetic acid, 5-HIAA) in the whole blood and in every part of the brain analyzed relative to controls. In rats given TCP and subjected to footshock or water-immersion restraint stress similar changes, but to a different extent, were observed. 5-HT level remained essentially constant except in the blood and the limbic system, whereas 5-HIAA level was found to be increased in the blood and the brain, mainly in the limbic system and the brainstem following footshock. Water-immersion restraint stress caused an increase in 5-HT only in the limbic system without any changes in 5-HT and 5-HIAA in the blood. Relative to controls, an increase in total tryptophan concentration in the whole blood and in every part of the brain was found only after footshock application with or without pretreatment with TCP. In conclusion, responses to stress in rats may depend upon the type of stimulus applied as well as of a concurrent administration of TCP. Some regional differences may account for an altered in vivo efficacy of this drug.

Effects of glucocorticoids on 5-HT1A presynaptic function in the mouse

8-OH-DPAT, a selective 5-HT1A agonist, produced a hypothermic response in mice at a dosage of 0.5 mg/kg. Administration of corticosterone-21-acetate (0.5, 5 and 50 mg/kg, daily for 3 and 10 days) produced a dose-dependent attenuation of this hypothermic response in mice. When all controls and corticosterone treated mice were retested, 14 days after initial testing, they did not differ in the hypothermic responses induced by 8-OH-DPAT. Mice treated with aldosterone (50 mg/kg), dexamethasone (50 mg/kg) and the specific type 2 corticosteroid receptor agonist, 11b,17b-dihydroxy-21-methyl-17a-pregna-1,4,6-trien-20-yn+ ++-3-one (RU26988, 30 mg/kg) for 10 days, did not differ from vehicle treated controls in the hypothermic response to 8-OH-DPAT. Mice administered corticosterone-21-acetate (30 mg/kg, daily) for 10 days displayed a motor behavioural syndrome, which was not seen in controls, when injected with 5-hydroxytryptophan (5-HTP, 100 mg/kg) 15 min after the injection of carbidopa (25 mg/kg). This was significantly decreased by pretreatment with the 5-HT1A receptor antagonist 1-(2-methoxyphenyl)-4-(4-phthalimidobutyl)-piperazine (NAN-190 5 mg/kg, 30 min prior to administration of carbidopa). Taken together, this evidence is compatible with a specific corticosterone induced facilitation of 5-HT release due to attenuation of inhibitory 5-HT1A autoreceptor function.

The effect of chronic imipramine administration on the densities of 5-HT1A and 5-HT2 receptors and the abundances of 5-HT receptor and transporter mRNA in the cortex, hippocampus and dorsal raphe of three strains of rat

We have recently demonstrated that the LEW/N rat contains lower concentrations of cortical and hippocampal 5-HT1A receptors compared with the F344/N and out-bred HSD rats. To further characterize these strains, we investigated the effect of chronic (8 wk) imipramine administration (5 mg/kg/day) on 5-HT1A and 5-HT2 receptor densities and mRNA in the cortex and hippocampus and 5-HT transporter mRNA in the dorsal raphe of LEW/N, HSD, and F344/N rats, using quantitative autoradiography and in situ hybridization histochemistry. After imipramine treatment, a significant increase in the levels of hippocampal 5-HT1A receptors, but not mRNA, was observed in LEW/N rats while the abundance of hippocampal 5-HT1A receptor mRNA, but not 5-HT1A receptor densities, decreased in F344/N rats. Cortical and hippocampal 5-HT2 receptor densities, but not mRNA, significantly decreased after imipramine administration in all three strains. Finally, 5-HT1A receptor densities and the abundance of mRNAs encoding the 5-HT1A receptor and 5-HT transporter in the dorsal raphe remained unaltered after imipramine administration in all three strains. The effects of imipramine on the levels of cortical and hippocampal 5-HT1A and 5-HT2 receptors and their transcripts, therefore, appear to be strain-dependent. The implications of these findings are discussed.

Monoamine oxidase inhibitors: effects on tryptophan concentrations in rat brain

It has been suggested that inhibition of tryptophan (Trp) pyrrolase and a subsequent elevation of brain Trp may contribute to the actions of antidepressant drugs. In our laboratories, we have conducted a series of experiments measuring brain Trp levels in the rat after both acute and chronic administration of several monoamine oxidase (MAO) inhibitors. The drugs studied during the course of the long-term (28 day) experiments were phenelzine, N2-acetylphenelzine, tranylcypromine, 4-fluorotranylcypromine, 4-methoxytranylcypromine and (-)-deprenyl. High-pressure liquid chromatography with electrochemical detection was employed to measure Trp levels in brains of both MAO inhibitor- and vehicle-treated animals. No significant increases in brain Trp levels were observed as a consequence of MAO inhibitor treatment. Acute time-response (up to 24 h) and dose-response studies were conducted following the administration of phenelzine and tranylcypromine. Only after administration of high doses of these drugs was an elevation in brain Trp observed and the increase was relatively short-lived. These results suggest that elevation of brain Trp may be an important factor in the actions of MAO inhibitors only at high doses of these drugs.

Significant correlation between 14C-5-HT uptake by and 3H-paroxetine binding to platelets from healthy volunteers

The lack of correlation between 5-HT uptake by platelets and the ability of platelet membrane to bind antidepressant drugs, particularly imipramine, has been reported. However, more recently it has been suggested that 3H-paroxetine could be a better drug with which to study the platelet 5-HT uptake mechanism in disease states. We have therefore compared the ability of platelet-rich plasma (PRP) from normal individuals to take up 5-HT with the ability of platelet membranes to bind paroxetine. A significant correlation was apparent between the Vmax of 14C-5-HT uptake by PRP and the Bmax of 3H-paroxetine binding to platelet membrane from 30 individuals (r = 0.6468, p = 0.0001). Furthermore, this correlation was highly significant in the 20 female (r = 0.7768, p = 0.00006) but not in the 10 male volunteers. There was also a significant association between Vmax and Bmax and the month of blood sampling but this did not totally account for the correlation between Vmax and Bmax. The simultaneous measurement of 5-HT uptake by PRP and paroxetine binding to platelet membranes from both depressed patients and matched controls should be carried out to confirm and extend these findings.

Chronic, combined treatment with desipramine and mianserin: enhanced 5-HT1A receptor function and altered 5-HT1A/5-HT2 receptor interaction in rats

A greater percentage of depressed patients respond to combined treatment with a tricyclic antidepressant and the tetracyclic antidepressant mianserin than to treatment with these drugs given alone. The functional sensitivity of the 5-hydroxytryptamine (5-HT)1A receptor, and the functional interaction between the 5-HT1A and the 5-HT2 receptors were investigated after treatment with desipramine and mianserin either alone or combined for 21-28 days. Pretreatment with desipramine and mianserin in combination induced the most intense 5-HT syndrome and the greatest fall in colonic temperature after injection of the 5-HT1A agonist 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT). The rats pretreated with desipramine alone had the largest elevation of the response temperature in the increasing temperature hot-plate test after injection of 8-OH-DPAT. After the combined pretreatment with desipramine and mianserin, no enhanced functional response in these tests was found when the 5-HT1A and the 5-HT2 receptors were stimulated simultaneously using 8-OH-DPAT and the 5-HT2 agonist, (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane HCl (DOI), contrasting the findings for desipramine or mianserin treatments given alone, where an increased functional response was found for the colonic temperature and the response temperature in the increasing temperature hot-plate test. In vitro receptor binding using [3H]-8-OH-DPAT as ligand revealed an increase in Kd and Bmax in the spinal cord after chronic treatment with the combination of desipramine and mianserin. In the hippocampus and the frontal cortex the changes were small.(ABSTRACT TRUNCATED AT 250 WORDS)

A comparison of fluoxetine and imipramine in the treatment of outpatients with major depressive disorder

A double-blind clinical trial was undertaken to evaluate the clinical efficacy and safety of fluoxetine compared with imipramine in the treatment of 59 outpatients suffering from major depressive disorder. The mean scores of all depression rating scales showed that the drugs had comparable efficacy. The side effect profile of imipramine was found to be mainly anticholinergic, which was not the case for fluoxetine, where it was mainly found to be gastrointestinal, such as nausea and diarrhoea. In both groups the total number of adverse events reported were the same. Fluoxetine treatment resulted in weight loss, whereas imipramine treatment resulted in a slight but significant weight increase.

The effects of corticosterone on 5-HT receptor function in rodents

In the mouse, administration of corticosterone-21-acetate (30 mg/kg, s.c. daily) for 3 and 10 days produced an attenuation of the hypothermic response to the 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), which was not present after administration for 1 day. A similar effect was observed in the rat after administration of corticosterone-21-acetate (30 mg/kg, s.c. daily) for 10 days. Mice which had been given corticosterone for 10 days displayed the serotonin syndrome when injected with 5-hydroxytryptophan (5-HTP, 100 mg/kg, s.c.), 15 min after injection of carbidopa (25 mg/kg, i.p.). This was not seen in control animals. The serotonin syndrome was also induced in mice using 8-OH-DPAT; this increased in a dose-dependent manner and could be significantly decreased by pre-treatment with 1-(2-methoxyphenyl)-4-(4-phthalimidobutyl)-piperazine (NAN-190 5 mg/kg, i.p., 30 min prior to administration of 8-OH-DPAT), a 5-HT1A receptor antagonist. Administration of corticosterone (30 mg/kg, s.c. daily) did not significantly alter the serotonin syndrome induced in treated mice, compared with controls. Mice pre-treated for 3 or 10 days with corticosterone did not differ from controls in the number of head-twitches induced by 5-HTP and carbidopa or 5-methoxy-N,N-dimethyltryptamine, nor did they differ from controls in their response to the putative 5-HT1B agonist 5-methoxy-3 (1,2,3,6-tetrahydropyridin-4-yl)1-H indole (RU 24969, 3 mg/kg, i.p.).(ABSTRACT TRUNCATED AT 250 WORDS)

Desipramine, administered chronically, influences 5-hydroxytryptamine1A-receptors, as measured by behavioral tests and receptor binding in rats

The 5-hydroxytryptamine1A (5-HT1A) receptor subtype seems to be of importance in the pathogenesis of depression and in the mode of action of antidepressants. In this study, behavioural experiments were performed in rats after oral administration of desipramine for 18-20 days, followed by an acute injection of the selective 5-HT1A receptor agonist, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), either systemically or intrathecally. Chronic administration of desipramine prolonged the behavioural 5-HT syndrome in the animals injected systemically with 8-OH-DPAT. Treatment with desipramine was also found to potentiate and prolong the antinociceptive effect of an acute injection, systemically or intrathecally, of 8-OH-DPAT in the increasing temperature hot plate test. After systemic administration of 8-OH-DPAT, the colonic temperature was lowered similarly in the desipramine-treated group and in controls, whereas an intrathecal injection of 8-OH-DPAT resulted in a fall in the colonic temperature in the desipramine-treated group only. In vitro receptor binding studies, using [3H]8-OH-DPAT as the ligand, showed a statistically significant reduction of Kd and Bmax in the frontal cortex and of Kd in the spinal cord, after treatment with desipramine. No changes of Kd and Bmax were found in the hippocampus after this treatment. Thus, desipramine, administered chronically, resulted in a functional up-regulation of the 5-HT1A-receptors, both spinally and supraspinally, whereas in the in vitro receptor binding, a slight down-regulation or no change was found. It seems therefore that the results of in vitro receptor binding studies do not necessarily reflect the functional state of the neuronal system.(ABSTRACT TRUNCATED AT 250 WORDS)

Clinical and biochemical aspects of depressive disorders: II. Transmitter/receptor theories

The present document is the second of three parts in a review that focuses on recent data from clinical and animal research concerning the biochemical bases of depressive disorders, diagnosis, and treatment. Various receptor/transmitter theories of depressive disorders are discussed in this section. Specifically, data supporting noradrenergic, serotonergic, cholinergic, dopaminergic, GABAergic, and peptidergic theories, as well as interactions between noradrenergic and serotonergic, or cholinergic and catecholaminergic systems are presented. Problems with the data and future directions for research are also discussed. A previous publication, Part I of this review, dealt with the classification of depressive disorders and research techniques for studying the biochemical mechanisms of these disorders. A future publication, Part III of this review, discusses treatments for depression and some of the controversies in this field.

No influence of the antidepressant paroxetine on carbamazepine, valproate and phenytoin

A single-blind, placebo-controlled, cross-over trial investigating possible interactions between paroxetine, a serotonin re-uptake inhibitor, and carbamazepine (CBZ), valproate (VPA) and phenytoin (PHT) was carried out in 20 outpatients with epilepsy. Patients on long-term treatment with CBZ, VPA, or PHT were given a 7-day placebo treatment, followed by paroxetine co-treatment for 16 days. Side effects were infrequent and mild. Paroxetine caused no changes in the plasma concentrations and all values were within the recommended ranges. No changes in protein binding were found. Plasma concentrations of paroxetine at steady state (8-147 ng/ml) were in the normal range for a 30-mg daily dosing regimen. None of the patients experienced epileptic seizures during the study.

Comparative anatomical distribution of 5-HT1A receptor mRNA and 5-HT1A binding in rat brain--a combined in situ hybridisation/in vitro receptor autoradiographic study

The present study examined the comparative distribution of 5-HT1A receptor mRNA and 5-HT1A receptors in rat brain using a combination of in situ hybridisation histochemistry and in vitro receptor autoradiography. 5-HT1A mRNA was visualized using a 910 bp cRNA probe synthesised from a BalI-PvuII fragment of the rat 5-HT1A reetor gene, while 5-HT1A receptors were labelled with the 5-HT1A-selective ligand 8-OH-DPAT. In general terms, there was a complementary distribution of cells expressing 5-HT1A receptor mRNA and 5-HT1A receptor sites. High levels of both 5-HT1A mRNA and 5-HT1A receptors were evident in the hippocampal formation (CA1, CA3, dentate gyrus), entorhinal cortex, and raphe nuclei and lower levels in neocortex and thalamus. Although 5-HT1A mRNA was not expressed in any regions which did not also exhibit 5-HT1A receptors, within both the diagonal band and the medial septal nucleus mRNA levels were proportionately higher than 5-HT1A receptor levels, possibly reflecting receptor transport or a heterogeneity in 5-HT1A receptor turnover mechanisms. 5-HT1A receptor mRNA and 5-HT1A binding sites were undetectable in caudate/putamen and cerebellar regions. The present data indicate the synthesis of 5-HT1A receptors both in raphe serotonergic cells and anatomically specific serotonergic projection areas, further supporting both a presynaptic autoregulatory and postsynaptic modulatory role for this receptor in serotonergic transmission.

Low brain uptake of L-[11C]5-hydroxytryptophan in major depression: a positron emission tomography study on patients and healthy volunteers

The precursor of serotonin, L-5-hydroxytryptophan (L-5-HTP), was radiolabelled with 11C in the beta-position, yielding [beta-11C]serotonin after decarboxylation, allowing positron emission tomography studies of L-5-HTP uptake across the blood-brain barrier. We studied 8 healthy volunteers and 6 patients with histories of DSM-III major depression, 2 with repeated examinations after clinically successful treatment. We report a significantly lower uptake of [11C]5-HTP across the blood-brain barrier in depressed patients, irrespective of phase of illness. The findings emphasize that serotonin is involved in depressive pathophysiology and support earlier suggestions that the transport of 5-HTP across the blood-brain barrier is compromised in major depression.

Subsensitivity of the 5-hydroxytryptamine1A (5-HT1A) receptor-mediated hypothermic response to ipsapirone in unipolar depression

The selective 5-HT1A receptor ligand ipsapirone (IPS) induces hypothermia in humans. To explore 5-HT1A receptor-mediated thermoregulation in depression, 24 subjects (12 patients with unipolar depression and 12 individually matched controls) received 0.3 mg/kg IPS or placebo in random order. Compared with controls, the depressed patients exhibited significantly attenuated hypothermic responses to IPS. The impaired hypothermic response following 5-HT1A receptor activation in unipolar depression could have resulted from subsensitivity of the (presynaptic) 5-HT1A receptor and/or related effector mechanisms, thus supporting the hypothesis that altered serotonergic activity may be present in affective disorders. Future studies of the hypothermic response to direct-acting 5-HT1A ligands, such as IPS should facilitate the assessment of 5-HT receptor function in various affective disorders and its involvement in psychotropic drug effects.