The effects of chronic treatment with amitriptyline and MDL 72394 on the control of 5-HT release in vivo

Role of central serotonin and noradrenaline interactions in the antidepressants' action: Electrophysiological and neurochemical evidence

The development of antidepressant drugs, in the last 6 decades, has been associated with theories based on a deficiency of serotonin (5-HT) and/or noradrenaline (NA) systems. Although the pathophysiology of major depression (MD) is not fully understood, numerous investigations have suggested that treatments with various classes of antidepressant drugs may lead to an enhanced 5-HT and/or adapted NA neurotransmissions. In this review, particular morpho-physiological aspects of these systems are first considered. Second, principal features of central 5-HT/NA interactions are examined. In this regard, the effects of the acute and sustained antidepressant administrations on these systems are discussed. Finally, future directions including novel therapeutic strategies are proposed.

Long-term adaptive changes induced by serotonergic antidepressant drugs

The development of conventional antidepressants has been largely based on the hypothesis of monoaminergic dysfunctions and focuses particularly on the serotonin 5-hydroxytryptamine (5-HT) system. Hence, various classes of antidepressant treatments enhance 5-HT neurotransmission with a time course consistent with their delayed therapeutic effect. This delayed onset appears to be associated with the gradual development of specific adaptive changes of functional 5-HT receptors. However, recent theories suggest that major depressive disorders may be associated with impairments of functional plasticity and cellular flexibility. This review discusses several physiological mechanisms by which 5-HT function and hippocampal neuroplasticity are regulated. Knowledge of these long-term adaptations will increase not only our understanding of pathological processes underlying affective disorders, but could also lead to the development of new strategies to treat these devastating illnesses.

Effects of anpirtoline on regional serotonin synthesis in the rat brain: an autoradiographic study

Anpirtoline has been described as an agonist at 5-HT1B receptors with a relatively high potency. It also acts as an agonist at 5-HT1A receptors, but has a lower potency than at the 5-HT1B sites. There is very little known about the mechanism by which anpirtoline influences regional 5-HT synthesis. The aim of the present study was to investigate the effects of acutely and chronically administered anpirtoline on 5-HT synthesis in the rat brain using the autoradiographic alpha-[14C]methyl-L-tryptophan method. In the acute study, anpirtoline (2.0 mg/kg) was administered intraperitoneally 30 min before the tracer injection. The control rats were injected with the same volume of saline. In the chronic study, anpirtoline (2 mg/kg per day) was injected subcutaneously in saline once a day for 10 days. There were no significant differences between the plasma-free and total tryptophan concentrations between the anpirtoline treatment and the respective control groups. In the acute experiment, 5-HT synthesis rates in all of the brain areas investigated were significantly decreased by anpirtoline when compared to the saline-treated group. In the chronic anpirtoline experiment, 5-HT synthesis rates of almost all of the projection areas, as well as the raphe nuclei, were normalized or had a tendency to be normalized. These results suggest that it is likely that the terminal 5-HT1B receptors are involved in the regulation of 5-HT synthesis in the projection areas and that 5-HT synthesis, in the raphe, is likely influenced by anpirtoline's 5-HT1A and/or 5-HT1B agonistic properties.

Assessing substrates underlying the behavioral effects of antidepressants using the modified rat forced swimming test

Selective serotonin reuptake inhibitors (SSRIs) are the most widely prescribed antidepressant class today and exert their antidepressant-like effects by increasing synaptic concentrations of serotonin (5-HT). The rat forced swim test (FST) is the most widely used animal test predictive of antidepressant action. Procedural modifications recently introduced by our laboratory have enabled SSRI-induced behavioral responses to be measured in the modified FST. The use of this model to understand the pharmacological and physiological mechanisms underlying the role of 5-HT in the behavioral effects of antidepressant drugs is reviewed. Although all antidepressants reduced behavioral immobility, those antidepressants that increase serotonergic neurotransmission predominantly increase swimming behavior whereas those that increase catacholaminergic neurotransmission increase climbing behavior. The 5-HT(1A), 5-HT(1B/1D) and 5-HT(2C) receptors are the 5-HT receptors most important to the therapeutic effects of SSRIs, based on extensive evaluation of agonists and antagonists of individual 5-HT receptor subtypes. Studies involving chronic administration have shown that the effects of antidepressants are augmented following chronic treatment. Other studies have demonstrated strain differences in the response to serotonergic compounds. Finally, a physiological model of performance in the rat FST has been proposed involving the regulation of 5-HT transmission by corticotropin releasing factor (CRF).

Selective 5-HT1B receptor agonist reduces serotonin synthesis following acute, and not chronic, drug administration: results of an autoradiographic study

The effects of acute and chronic administration of the serotonin (5-HT)1B agonist CP-93,129, on 5-HT synthesis rates were evaluated using the alpha-[14C]methyl-L-tryptophan (alpha-MTrp) autoradiographic method. In the acute treatment study, CP-93,129 (7 mg/kg) was injected intraperitoneally 30 min before the alpha-MTrp injection (30 microCi over 2 min). A single dose of CP-93,129 caused a significant increase in the synthesis in the median raphe nucleus (MR) without a significant influence on the dorsal raphe nucleus (DR). There was a reduction in 5-HT synthesis in almost all of the projection areas. In the chronic treatment study, CP-93,129 was administered continuously (7 mg/kg/day) for 14 days using an osmotic minipump implanted subcutaneously. The chronic treatment with CP-93,129 did not produce a significant change in 5-HT synthesis in the raphe nuclei nor in the nerve terminal structures, except for the medial frontal bundle and the visual and sensory-motor cortices. The unaltered 5-HT synthesis rates in the chronic treatment study probably reflect a normalization of the synthesis as a result of the desensitization of 5-HT1B autoreceptors and/or heteroreceptors.

Duloxetine pharmacology: profile of a dual monoamine modulator

Dysregulation within central monoaminergic systems is believed to underlie the pathology of depression. Drugs that selectively inhibit the reuptake of central monoamines have been used clinically to alleviate symptoms of depressive illnesses. Duloxetine, a novel compound currently under investigation for the treatment of depression, binds selectively with high affinity to both norepinephrine (NE) and serotonin (5-HT) transporters and lacks affinity for monoamine receptors within the central nervous system. It has been suggested that dual inhibition of monoamine reuptake processes may offer advantages over other antidepressants currently in use. In preclinical studies, duloxetine mimics many physiologic effects of antidepressants. Consistent with other antidepressants, duloxetine, by acute administration, elevates extracellular monoamine levels, while by chronic administration it does not alter basal monoamine levels. Like the selective serotonin reuptake inhibitor, fluoxetine, by microiontophoretic application, duloxetine inhibits neuronal cell firing. However, in comparison with fluoxetine, duloxetine is a more potent serotonin reuptake inhibitor. Furthermore, in behavioral experiments, duloxetine attenuates immobility in forced swim tests in animal models of depression to a greater extent than several other commonly used antidepressants. In a six-week open label uncontrolled study, duloxetine was evaluated in patients with a history of depression. Duloxetine was effective in treating depression as determined by marked reduction in Hamilton Depression Rating scores. Adverse effects reported during duloxetine treatment were minor and similar to those of other antidepressants. In an eight-week multicenter, double-blind, placebo-controlled study in patients with a major depressive disorder, duloxetine was effective as an antidepressant, particularly in patients with greater symptom severity. Only limited data are available regarding the pharmacokinetic profile of duloxetine in humans, although a half-life of 10 to 15 h has been reported. Studies conducted in healthy human subjects confirm the preclinical profile of duloxetine as an inhibitor of 5-HT and NE reuptake. Taken together, existing data suggest that duloxetine is a novel and effective antidepressant.

Neuropharmacology of venlafaxine

Venlafaxine (Effexor) is an effective antidepressant and has also been approved for the treatment of generalized anxiety disorder. Venlafaxine was initially characterized as an inhibitor of both serotonin (5HT) and norepinephrine (NE) uptake and was therefore termed a "dual uptake inhibitor." This chapter reviews data from both in vitro and in vivo studies regarding its effects on 5HT and NE neurotransmission. In addition, the effects of venlafaxine on other systems that may play a role in its therapeutic efficacy effects are described. The data indicate that venlafaxine is a relatively weak inhibitor of NE transport in vitro. In vivo studies indicate that venlafaxine selectively inhibits 5HT uptake at low therapeutic doses and inhibits both 5HT and NE uptake at higher therapeutic doses. This chapter concludes with a discussion of the effects of venlafaxine on various aspects of physiology.

The possible role of 5-HT(1B/D) receptors in psychiatric disorders and their potential as a target for therapy

Serotonin (5-hydroxytryptamine, 5-HT) is implicated in several psychiatric diseases. Is this also true for 5-HT(1B/D) receptors? These receptors are found in high density in substantia nigra, globus pallidus, striatum and basal ganglia and in other brain regions. This ubiquity makes 5-HT(1B/D) receptors responsible for many physiological and behavioural functions. This review focuses on the role of 5-HT(1B) receptors in the regulation of 5-HT release and synthesis. Microdialysis experiments performed on freely moving animals are an interesting in vivo model to study the function of the terminal 5-HT(1B) autoreceptor. Synthesis of 5-HT, estimated by the measurement of the accumulation of 5-hydroxytryptophan (5-HTP) ex vivo or in vitro, is modulated by the 5-HT(1B) autoreceptors. Many reports have shown that chronic administration with selective serotonin reuptake inhibitors leads to the desensitisation of the terminal 5-HT(1B) autoreceptors. With the help of some animal models of depression and anxiety and with some data from clinical studies it has been hypothesised that 5-HT(1B) receptors may be supersensitive in depression, anxiety and obsessive compulsive disorder. Thus, since the dysfunction of 5-HT(1B) receptors may be involved in some pathological states, particularly in the psychiatric field, these receptors represent important potential targets for drugs to treat mental diseases.

5-HT1A and beyond: the role of serotonin and its receptors in depression and the antidepressant response

Since its discovery 50 years ago, the role of the indoleamine 5-HT (5-hydroxytryptamine; serotonin) in the pathogenesis of depression and in the mechanism of action of antidepressant drugs has been the subject of considerable research. Advances in molecular biology and radioligand techniques have led to the functional characterisation of at least 14 serotonin receptor subtypes. This classification has led to the development of selective compounds that have aided in the efforts of dissecting the complex role of 5-HT in depression and in mediating the antidepressant response. This review focuses largely on novel strategies of targeting specific 5-HT receptors subtypes, especially the presynaptic 5-HT(1A) and 5-HT(1B/1D) receptors. These subtypes are of primary importance in that they control the firing of the 5-HT neuron and the release of 5-HT. In addition, a number of postsynaptic 5-HT receptors have been shown to be dysfunctional in depression and are also potential targets for a number of antidepressants. We conclude that selective targeting of 5-HT receptors may lead to a faster acting and more efficient antidepressant response. Copyright 2000 John Wiley & Sons, Ltd.

Venlafaxine: acute and chronic effects on 5-hydroxytryptamine levels in rat brain in vivo

Venlafaxine is a dual serotonin (5-hydroxytryptamine, 5-HT) and noradrenaline uptake inhibitor which has been claimed to have an onset of antidepressant action which is faster than for other comparable drugs. The effects of venlafaxine on brain 5-HT levels in vivo have not yet been examined. Acute administration of venlafaxine to rats by i.p. injection resulted in dose-dependent increases in cortical and hippocampal 5-HT levels, as measured by in vivo microdialysis, over the range 5-20 mg/kg. The effect of venlafaxine (10 mg/kg i.p.) was potentiated by prior administration of pindolol (10 mg/kg s.c.) in hippocampus but not in frontal cortex. Daily administration of venlafaxine (5 mg/kg i.p.) for 4 weeks did not change basal 5-HT levels in either brain area. The effect of 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT, 0.2 mg/kg s.c.) to reduce 5-HT levels was unaffected by chronic venlafaxine at this dose, indicating that there was no change in sensitivity of presynaptic 5-HT1A autoreceptors.

Diurnal variation in 5-HT1B autoreceptor function in the anterior hypothalamus in vivo: effect of chronic antidepressant drug treatment

1. Intracerebral microdialysis was used to examine the function of the terminal 5-hydroxytryptamine (5-HT) autoreceptor in the anterior hypothalamus of anaesthetized rats at two points in the light phase of the light-dark cycle. 2. Infusion of the 5-HT1A/1B agonist 5-methoxy-3-(1,2,3,6-tetrahydro-4-pyridyl)-1H-indole (RU24969) 0.1, 1.0 and 10 microM through the microdialysis probe led to a concentration-dependent decrease (49, 56 and 65% respectively) in 5-HT output. The effect of RU24969 (1 and 5 microM) was prevented by concurrent infusion of methiothepin (1 and 10 microM) into the anterior hypothalamus via the microdialysis probe. Infusion of methiothepin alone (1.0 and 10 microM) increased (15 and 142% respectively) 5-HT output. 3. Infusion of RU24969 (5 microM) through the probe at mid-light and end-light resulted in a quantitatively greater decrease in 5-HT output at end-light compared with mid-light. 4. Following treatment with either paroxetine hydrochloride (10 mg kg(-1) i.p.) or desipramine hydrochloride (10 mg kg)(-1) i.p.) for 21 days the function of the terminal 5-HT1B autoreceptor was more markedly attenuated at end-light. 5. The data show that, as defined by the response to RU24969, the function of the 5-HT1B receptors that control 5-HT output in the anterior hypothalamus is attenuated following chronic desipramine or paroxetine treatment in a time-of-day-dependent manner.

The combined effects of chronic ethanol/desipramine treatment on beta-adrenoceptor density and coupling efficiency in rat brain

Both ethanol and desipramine influence beta-adrenoceptor regulation. We reported previously that ethanol partially counteracted desipramine's effects on beta-adrenoceptor. Previous studies utilized beta-adrenoceptor radioligands that also bind to 5-HT1B receptors, thus, changes in 5-HT1B receptors could have confounded the results. The effects of chronic ethanol, desipramine and ethanol/desipramine treatment on beta-adrenoceptor coupling efficiency to Gs protein in rat brain were examined using 125I-iodocyanopindolol after blocking binding to 5-HT1B receptors. In the frontal cortex, ethanol uncoupled beta-adrenoceptor from GS. Desipramine decreased beta-adrenoceptor density, particularly in the high-conformational state, with no effect on coupling. In combined treatment, desipramine prevented ethanol-induced uncoupling. In the hippocampus, desipramine enhanced beta-adrenoceptor coupling, but ethanol had no effect. In combination with desipramine, ethanol enhanced desipramine-induced decrease in beta-adrenoceptor density in the high-conformational state, but uncoupled beta-adrenoceptors, an effect not observed with ethanol alone. These results suggest a complex interplay between ethanol and antidepressants in modulating beta-adrenoceptor function.

Chronic electroconvulsive shock and 5-HT autoreceptor activity in rat brain: an in vivo microdialysis study

In vivo microdialysis was used to determine the effects of chronic electroconvulsive shock (ECS), given daily for 10 days, on basal 5-HT levels in rat frontal cortex and hippocampus and on the effect of systemic administration of the 5-HT-la receptor agonist, 8-OH-DPAT (0.2 mg/kg), to reduce 5-HT levels in these areas by activation of somatodendritic autoreceptors. Neither basal 5-HT levels nor the effects of 8-OH-DPAT on 5-HT levels were altered after chronic ECS. The effect of systemic administration of the 5-HT1A and 5-HT1B antagonist, (+/-)-pindolol (10 mg/kg), to increase 5-HT levels in hippocampus, was also not affected by chronic ECS.

Functional interaction between serotonin and other neuronal systems: focus on in vivo microdialysis studies

In this review, the functional interactions between serotonin (5-HT) and other neuronal systems are discussed with the focus on microdialysis studies in the mammalian brain (mainly rats). 5-HT release is negatively regulated not only by somatodendritic 5-HT1A and terminal 5-HT1B (5-HT1D) autoreceptors but also by alpha 2-adrenergic and mu-opioid heteroreceptors that are located on serotonergic nerve terminals. 5-HT by itself is involved in the inhibitory effects of noradrenaline release and the facilitatory regulation of dopamine release via multiple 5-HT receptors. Acetylcholine release appears to be regulated by inhibitory 5-HT1B heteroreceptors located on cholinergic nerve terminals. Long-term treatment with 5-HT-uptake inhibitors and noradrenaline-uptake inhibitor produces desensitization of 5-HT1A autoreceptors and alpha 2-heteroreceptors, respectively, which may be related therapeutically to the delayed onset of the effects of antidepressants. Some microdialysis studies have predicted that the combination of a 5-HT-uptake inhibitor and 5-HT1A-autoreceptor antagonist might produce much greater availability of 5-HT in the synaptic cleft in terms of much faster induction of subsensitivity of 5-HT1A autoreceptors. Clinical trials based on this hypothesis have revealed that combination therapy with a 5-HT-uptake inhibitor and 5-HT1A-autoreceptor antagonist ameliorated the therapeutic efficacy in depressive patients. Taken together, neurochemical approaches using microdialysis can contribute not only to clarification of the physiological role of the serotonergic neuronal systems but also might be a powerful pharmacological approach for the development of therapeutic strategies.

Effects of acute and repeated administration of citalopram on extracellular levels of serotonin in rat brain

The effects of acute (2 days) and repeated (21 days) administration (50 mg/kg in the diet) of the selective serotonin (5-HT, 5-hydroxytryptamine) reuptake inhibitor, citalopram, on extracellular levels of 5-HT and their modulation by terminal autoreceptors in the hypothalamus of freely moving rats were compared in vivo by microdialysis. When studied without washout, extracellular levels of 5-HT were increased by both acute and repeated citalopram administration. In rats treated repeatedly, extracellular 5-HT levels were 43% (but not significantly) greater than in those treated acutely. Extracellular levels of 5-HT in control and citalopram-treated rats were similar when measured after 24 h washout. The enhancing effect of non-selective serotonergic autoreceptor antagonists, methiothepin (100 microM) or 1-(1-naphthyl)piperazine (NP) (10 microM), administered through the microdialysis probe, after 24 h washout, was similar in both control and chronically treated groups. These results suggest that repeated administration of citalopram followed by a washout of 24 h does not lead to desensitization of the terminal autoreceptor as measured in vivo in contrast to the effects we have shown previously in vitro. In rats treated chronically with citalopram without washout, methiothepin had a greater maximal effect on 5-HT outflow in comparison to rats receiving acute citalopram treatment. This finding suggests that a 5-HT autoreceptor antagonist or a combination of such a drug with a 5-HT uptake inhibitor would produce a greater increase of extracellular levels of 5-HT in hyposerotonergic states such as depression.

Effects of single and repeated oral administration of fluvoxamine on extracellular serotonin in the median raphe nucleus and dorsal hippocampus of the rat

The delay in clinical effects of selective serotonin reuptake inhibitors (SSRIs) suggest the existence of adaptive phenomena, such as receptor sensitivity changes. To examine the effects of repeated administration of SSRIs on serotonin neurotransmission, we investigated the effects of acute and chronic administration of the SSRI fluvoxamine on the extracellular levels of 5-HT in the median raphe nucleus and dorsal hippocampus of conscious rats by means of brain microdialysis. A single oral dose of fluvoxamine (30 mg/kg) augmented extracellular 5-HT in the median raphe and dorsal hippocampus to 270 and 191% of baseline level, respectively. Administration of fluvoxamine (30 mg/kg) or vehicle for 14 days did not affect 5-HT baseline levels. Moreover, the increase in extracellular 5-HT in the median raphe nucleus and dorsal hippocampus after an oral dose of fluvoxamine (30 mg/kg) in rats chronically treated with fluvoxamine was not different from rats treated with vehicle. Using RU 24969 as a probe for the sensitivity of the 5-HT1B autoreceptors in the dorsal hippocampus, no change in receptor sensitivity could be observed. These results demonstrate that repeated oral treatment with fluvoxamine does not affect extracellular 5-HT in the median raphe and dorsal hippocampus, suggesting that presynaptic functional changes of 5-HT in the brain areas tested are not implicated in the observed delayed onset of action of this SSRI in humans.

Chronic treatment with fluvoxamine by osmotic minipumps fails to induce persistent functional changes in central 5-HT1A and 5-HT1B receptors, as measured by in vivo microdialysis in dorsal hippocampus of conscious rats

This study investigated the alterations of the 5-HT1A and 5-HT1B autoreceptor function following chronic treatment with fluvoxamine using osmotic minipumps. The 5-HT1A and 5-HT1B autoreceptor function were studied using microdialysis in the dorsal hippocampus. The effect of the 5-HT1A receptor agonist 8-OH-DPAT (0.3 mg/kg, SC) and the 5-HT1B receptor agonist RU-24969 (100 nM through the dialysis probe for 30 min) on 5-HT release was compared with rats chronically treated with saline. 8-OH-DPAT decreased 5-HT release to 55% and 60% of baseline, while RU-24969 decreased 5-HT release to 66% and 70% of baseline value in the saline and fluvoxamine group, respectively. In both cases, differences between the saline and fluvoxamine groups were not statistically significant. Plasma levels of fluvoxamine after 21 days of treatment ranged from 3 to 5 ng/ml. Fluvoxamine concentration in rat brain during treatment was estimated between 100 and 200 nM, which approximates to the IC50 value of fluvoxamine on the 5-HT transporter in synaptosomes and is 50 times higher than the Kd value for the 5-HT reuptake site. In conclusion, no evidence was found for changes in 5-HT1A,B receptor function using 8-OH-DPAT and RU-24969 as probes after continuous treatment with fluvoxamine by means of osmotic minipumps.

Effects of single and chronic treatment with tranylcypromine on extracellular serotonin in rat brain

We have examined the effects of tranylcypromine, a monoamine oxidase inhibitor used as antidepressant, on the tissue and extracellular concentration of serotonin (5-hydroxytryptamine, 5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) in frontal cortex and dorsal raphe nucleus using microdialysis in conscious rats. Single treatment with tranylcypromine sulphate (0.5, 3 and 15 mg/kg, i.p.) dose dependently elevated dialysate 5-HT in both areas but more markedly in the DRN. Extracellular and tissue 5-HT concentrations were affected by the drug in a different manner. The former increased sharply when tissue 5-HT reached a plateau. This may have reflected saturation of intracellular stores and overflow of the amine. In contrast, tissue and extracellular 5-HIAA concentrations--that indicate metabolic effects of tranylcypromine--were affected similarly. A 2-week treatment with 0.5 mg/kg.day of tranylcypromine sulphate increased basal extracellular 5-HT in frontal cortex and dorsal raphe nucleus (ca. 220%) whereas a further injection of 0.5 mg/kg was without effect in both areas. Thus, chronic, but not acute, treatment with low doses of tranylcypromine increases extracellular 5-HT concentration, suggesting that clinical effects of this monoamine oxidase inhibitor are related to its capacity to enhance serotonergic transmission.

Neuropharmacology of 5-hydroxytryptamine1B/D receptor ligands

In spite of a lack of compounds acting selectively at the 5-hydroxytryptamine (5-HT)1B and 5-HT1D receptor subtypes, by cross-relating the available data, this review attempts to tentatively assign behavioural and other in vivo correlates of these receptor subtypes. In addition, a summary of data from microdialysis studies is included to develop an integrated view. Finally, a suggestion is made as to the possible pathophysiological consequences of 5-HT1D receptor dysfunction in man.

Effect of chronic antidepressant treatment on 5-HT1B presynaptic heteroreceptors inhibiting acetylcholine release

The effect of long term treatment with two tricyclic antidepressants on the sensitivity of 5-HT1B presynaptic heteroreceptors inhibiting acetylcholine (ACh) release was investigated. Groups of male rats received during 14 days either saline, citalopram (20 mg/kg), a serotonin (5-HT) uptake blocker, or tianeptine (2 x 10 mg/kg), an antidepressant that enhances 5-HT uptake. The efficacy of the 5-HT1B selective agonist 7-trifluoromethyl-4-(4-1-piperazinyl)-pyrrolo[1,2-a]quinoxaline (CGS 12066B) in reducing K(+)-evoked [3H]acetylcholine release from hippocampal synaptosomes was determined 24 hr after the last administration. The chronic treatment with citalopram or tianeptine modified neither the basal nor the K(+)-evoked release of [3H]acetylcholine. In contrast, these treatments significantly reduced the efficacy of CGS 12066B to inhibit the release of [3H]acetylcholine induced by K+ depolarization. These data suggest that chronic antidepressant treatment desensitizes 5-HT1B presynaptic heteroreceptors through a mechanism which seems to be independent of the synaptic availability of 5-HT.

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.

Flerobuterol, a beta-adrenoceptor agonist, enhances serotonergic neurotransmission: an electrophysiological study in the rat brain

The two beta-adrenoceptor agonists salbutamol and clenbuterol have been shown to be effective antidepressant drugs. Flerobuterol, a new beta-adrenoceptor agonist, exhibits antidepressant activity in animal models. Given the long-standing notion that the serotonergic (5-HT) system might be involved in the etiology and/or the therapeutics of affective disorders and that this class of adrenergic agents can alter factors regulating 5-HT transmission, the effects of acute and repeated administrations of flerobuterol on the 5-HT system were studied. Acute administration of flerobuterol (up to 2 mg/kg, IV) did not modify the firing rate of dorsal raphe 5-HT neurons. However, the sustained administration of flerobuterol for two days (0.5 mg/kg/day, SC. delivered by an osmotic minipump) produced a marked decrease of the firing rate of 5-HT neurons. The reversal of this effect of flerobuterol by the somatodendritic 5-HT autoreceptor antagonist spiperone suggests that this decrease in the firing activity of 5-HT neurons in rats treated for 2 days with flerobuterol resulted from an enhanced synaptic availability of 5-HT. This initial decrease in firing activity of 5-HT neurons was followed by a progressive recovery to normal after 14 days of treatment with flerobuterol. At this point in time, the effect of intravenous lysergic acid diethylamide on the firing of 5-HT neurons was attenuated, indicating that the somatodendritic 5-HT autoreceptors had desensitized. The effectiveness of the electrical stimulation of the ascending 5-HT pathway in suppressing the firing activity of dorsal hippocampus pyramidal neurons was markedly enhanced in rats treated with flerobuterol for 14 days.(ABSTRACT TRUNCATED AT 250 WORDS)