Electrophysiological effects of fluoxetine and duloxetine in the dorsal raphe nucleus and hippocampus

Duloxetine in the treatment of stress urinary incontinence

This manuscript reviews the pharmacodynamics and pharmacokinetics of duloxetine and its efficacy and safety in women with stress urinary incontinence. Duloxetine is a selective inhibitor of neuronal serotonin and norepinephrine uptake which increases urethral striated muscle activity and bladder capacity. Duloxetine is readily absorbed and extensively metabolized; cytochrome P450 1A2 (CYP1A2) inhibiting drugs can markedly increase duloxetine exposure. The clinical efficacy of duloxetine has consistently been demonstrated in several randomized, double-blind studies in women with moderate-to-severe stress urinary incontinence, but the additional benefit relative to placebo was moderate. Duloxetine treatment is frequently associated with adverse events such as nausea, dry mouth, fatigue, insomnia and constipation, but serious adverse events are rare. Therefore, duloxetine appears suitable for the treatment of stress urinary incontinence.

Duloxetine blocks cloned Kv4.3 potassium channels

The effects of duloxetine were examined on cloned Kv4.3 channels stably expressed in CHO cells using the whole-cell patch-clamp technique. Duloxetine decreased the peak amplitude of Kv4.3 currents with an acceleration of the decay rate of current inactivation in a concentration-dependent manner. The IC(50) values required for the blocking effects of duloxetine on the peak amplitude and the integral of currents were 8.4 and 2.1μM, respectively. Duloxetine accelerated the rate of inactivation of Kv4.3 currents and thereby decreased the time-to-peak in a concentration-dependent manner. Analysis of the time dependence of the drug block produced estimates of 21.9μM(-1)s(-1) and 165.9s(-1), for the respective association (k(+1)) and dissociation (k(-1)) rate constants. The K(d) value (k(-1)/k(+1)) yielded 7.5μM, which approximates the experimental IC(50) value obtained from the concentration-response curve. The block of Kv4.3 by duloxetine was voltage-dependent at a membrane potential coinciding with the activation of the channels. At a more positive potential, however, the block was relieved. Duloxetine produced a hyperpolarizing shift in the voltage dependence of the steady-state inactivation of Kv4.3, and accelerated the closed-state inactivation of Kv4.3 in the subthreshold voltage range. Duloxetine induced a significant use-dependent block at frequencies of 1 and 2Hz. In the presence of duloxetine, the recovery from inactivation was slower than under control conditions. These results demonstrate that duloxetine exerts a concentration-dependent block of Kv4.3 by binding to the channels in the open and inactivated states and these actions may contribute to its analgesic effect in neuropathic pain.

Characterization of the electrophysiological properties of triple reuptake inhibitors on monoaminergic neurons

Triple reuptake inhibitors represent a potential new class of antidepressant drugs that block norepinephrine (NE), dopamine (DA) and serotonin [5-hydroxytryptamine (5-HT)] transporters. The present in-vivo electrophysiological study was undertaken to determine the effects of the triple reuptake inhibitors SEP-225289 and DOV216303 on the neuronal activities of locus coeruleus (LC) NE, ventral tegmental area (VTA) DA and dorsal raphe (DR) 5-HT neurons. Administered acutely, SEP-225289 and DOV216303 dose-dependently decreased the spontaneous firing rate of LC NE, VTA DA and DR 5-HT neurons through the activation of α₂, D₂ and 5-HT(₁A) autoreceptors, respectively. Both compounds predominantly inhibited the firing rate of LC NE neurons while producing only a partial decrease in VTA DA and DR 5-HT neuronal discharge. SEP-225289 was equipotent at inhibiting 5-HT and NE transporters since it prolonged to the same extent the time required for a 50% recovery (RT₅₀) of the firing activity of dorsal hippocampus CA3 pyramidal neurons from the inhibition induced by microiontophoretic application of 5-HT and NE. Finally, in the presence of WAY100635, a 5-HT(₁A) receptor antagonist, SEP-225289 activated 5-HT neurons at doses that normally did not inhibit them. Taken together, the present results indicate that reciprocal interactions among NE, DA and 5-HT inputs need to be considered to anticipate the net effect of triple reuptake inhibitors on the enhancement of brain monoamine transmission. The results also suggest that the therapeutic action of triple reuptake inhibitors may be potentiated by antagonizing the cell body 5-HT(₁A) autoreceptors.

Serotonin transporter and memory

The serotonin transporter (SERT) has been associated to diverse functions and diseases, though seldom to memory. Therefore, we made an attempt to summarize and discuss the available publications implicating the involvement of the SERT in memory, amnesia and anti-amnesic effects. Evidence indicates that Alzheimer's disease and drugs of abuse like d-methamphetamine (METH) and (+/-)3,4-methylenedioxymethamphetamine (MDMA, "ecstasy") have been associated to decrements in the SERT expression and memory deficits. Several reports have indicated that memory formation and amnesia affected the SERT expression. The SERT expression seems to be a reliable neural marker related to memory mechanisms, its alterations and potential treatment. The pharmacological, neural and molecular mechanisms associated to these changes are of great importance for investigation.

Monoaminergic Antidepressants in the Relief of Pain: Potential Therapeutic Utility of Triple Reuptake Inhibitors (TRIs)

Over 75% of depressed patients suffer from painful symptoms predicting a greater severity and a less favorable outcome of depression. Imaging, anatomical and functional studies have demonstrated the existence of common brain structures, neuronal pathways and neurotransmitters in depression and pain. In particular, the ascending serotonergic and noradrenergic pathways originating from the raphe nuclei and the locus coeruleus; respectively, send projections to the limbic system. Such pathways control many of the psychological functions that are disturbed in depression and in the perception of pain. On the other hand, the descending pathways, from monoaminergic nuclei to the spinal cord, are specifically implicated in the inhibition of nociception providing rationale for the use of serotonin (5-HT) and/or norepinephrine (NE) reuptake inhibitors (SSRIs, NRIs, SNRIs), in the relief of pain. Compelling evidence suggests that dopamine (DA) is also involved in the pathophysiology and treatment of depression. Indeed, recent insights have demonstrated a central role for DA in analgesia through an action at both the spinal and suprasinal levels including brain regions such as the periaqueductal grey (PAG), the thalamus, the basal ganglia and the limbic system. In this context, dopaminergic antidepressants (i.e., containing dopaminergic activity), such as bupropion, nomifensine and more recently triple reuptake inhibitors (TRIs), might represent new promising therapeutic tools in the treatment of painful symptoms with depression. Nevertheless, whether the addition of the dopaminergic component produces more robust effects than single- or dual-acting agents, has yet to be demonstrated. This article reviews the main pathways regulating pain transmission in relation with the monoaminergic systems. It then focuses on the current knowledge regarding the in vivo pharmacological properties and mechanism of action of monoaminergic antidepressants including SSRIs, NRIs, SNRIs and TRIs. Finally, a synthesis of the preclinical studies supporting the efficacy of these antidepressants in analgesia is also addressed in order to highlight the relative contribution of 5-HT, NE and DA to nociception.

Targeted screening for biogenic amine transporters: Potential applications for natural products

The biogenic amine transporters (BATs) are integral membrane proteins that terminate the actions of dopamine (DA), serotonin (5-HT) and norepinephrine (NE) by pumping these substrates from the extracellular space back into the nerve terminal. Numerous drugs and medications target BATs, acting as inhibitors or substrates. This paper will review some of the methods used to measure the activity of test drugs at the BATs. These methods include traditional uptake inhibition assays and transporter binding assays, as well as methods developed in our lab to determine if a test agent is a BAT substrate or inhibitor. Newer methods, developed in our lab, are used to determine the potency of test drugs as BAT substrates in a relatively high throughput manner. The potential application of these methods to characterizing natural products will be discussed in reference to results obtained with "purified" natural products, such as ephedrine stereoisomers.

Duloxetine for management of stress urinary incontinence

OBJECTIVE

The aim of this article was to review data regarding the efficacy and tolerability of duloxetine, a selective serotonin (5-HT)-norepinephrine (NE) reuptake inhibitor that has received US Food and Drug Administration marketing approval for the treatment of major depressive disorder and painful diabetic neuropathy, and that has been investigated as a treatment for stress urinary incontinence.

METHODS

A MEDLINE/PubMed search was conducted to identify English-language study reports. In addition, proceedings of meetings of the International Continence Society, European Association of Urology, American Urological Association, and American College of Obstetrics and Gynecology were reviewed for relevant abstracts (search terms included duloxetine, thiophenes, serotonin uptake inhibitors, adrenergic uptake inhibitors, and stress urinary incontinence). Additional references were obtained from the bibliographies of these sources. Data for the period from 1986 through January 2005 were reviewed.

RESULTS

All in vitro and in vivo studies of duloxetine were included. Because both 5-HT and NE are involved in the maintenance of urinary continence, duloxetine may have a role in the treatment of urinary incontinence. Duloxetine is primarily eliminated via metabolism, with < 1% of the parent compound excreted via urine. Duloxetine QD or BID has been found to be significantly superior to placebo in reducing incontinence episode frequency (P < 0.001 to P < 0.05), increasing the interval between micturitions (P < 0.001 to P = 0.004), and improving the condition as measured by patient self-report (P < 0.001 to P = 0.028) and incontinence quality-of-life scores (P = 0.002 to P = 0.03). The most problematic adverse events are nausea, dry mouth, constipation, dizziness, and insomnia.

CONCLUSIONS

Although statistically superior to placebo in efficacy trials, the clinical effects of duloxetine therapy on incontinence are small, suggesting that any benefits to the patient would be modest and must be weighed against the drug's adverse event profile. No comparative efficacy/tolerability data with alpha-receptor agonists (eg, pseudoephedrine) are available. On the basis of available data, duloxetine is a modest, but welcome, advance in the pharmacotherapeutic management of stress urinary incontinence.

Duloxetine: A Dual Serotonin-Norepinephrine Reuptake Inhibitor for Treatment of Major Depressive Disorder

The burden of mental illness has been underestimated worldwide. Depression was the fourth leading cause of disease burden in the world in 1990 and is projected to be the second leading cause of disability by 2020. It is a leading cause of morbidity and mortality in the United States, costing billions of dollars annually in direct and indirect medical costs and losses in productivity. Patients with major depressive disorder (MDD) may experience both psychological and medical complaints, including somatic sensations or pain. Some antidepressants have been shown to treat chronic pain syndromes, but despite the variety of antidepressants available in the United States, only 65-70% of patients respond to initial antidepressant treatment. Treatments are limited by delayed onset of antidepressant effects, side effects, partial response, and treatment resistance. Duloxetine, approved by the U.S. Food and Drug Administration for the treatment of MDD, is a reuptake inhibitor at serotonergic and noradrenergic neurons and appears to have low affinity for other neurotransmitter systems. In clinical trials, duloxetine was effective for the treatment of MDD and was well tolerated. Further study is needed to compare its efficacy with that of other antidepressants, to clarify effects on somatic symptoms, and to assess potential adverse cardiovascular and sexual side effects. Duloxetine is also approved for the management of diabetic peripheral neuropathic pain and is under investigation for the treatment of stress urinary incontinence in women.

Fluoxetine inhibits A-type potassium currents in primary cultured rat hippocampal neurons

The effects of fluoxetine (Prozac) on the transient A-currents (IA) in primary cultured hippocampal neurons were examined using the whole-cell patch clamp technique. Fluoxetine did not significantly decrease the peak amplitude of whole-cell K+ currents, but it accelerated the decay rate of inactivation, and thus decreased the current amplitude at the end of the pulse. For further analysis, IA and delayed rectifier K+ currents (IDR) were isolated from total K+ currents. Fluoxetine decreased IA (the integral of the outward current) in a concentration-dependent manner with an IC50 of 5.54 microM. Norfluoxetine, the major active metabolite of fluoxetine, was a more potent inhibitor of IA than was fluoxetine, with an IC50 of 0.90 microM. Fluoxetine (3 microM) inhibited IA in a voltage-dependent manner over the whole range of membrane potentials tested. Analysis of the time dependence of inhibition gave estimates of 34.72 microM(-1) s(-1) and 116.39 s(-1) for the rate constants of association and dissociation, respectively. The resulting apparent Kd was 3.35 microM, similar to the IC50 value obtained from the concentration-response curve. In current clamp configuration, fluoxetine (3 microM) induced depolarization of resting membrane potential and reduced the rate of action potential. Our results indicate that fluoxetine produces a concentration- and voltage-dependent inhibition of IA, and that this effect could affect the excitability of hippocampal neurons.

Differential acute effects of fluoxetine on frontal and auditory cortex networks in vitro

Primary cultures of neuronal networks grown on microelectrode arrays were used to quantify acute effects of fluoxetine (Prozac) on spontaneous spike and burst activity. For frontal cortex cultures, fluoxetine showed consistent inhibitory effects and terminated activity at 10-16 microM. IC(50) mean+/-S.E. for spike rates was 5.4+/-0.7 microM (n=15). For auditory cortex cultures, fluoxetine caused excitation at 1-10 microM, initial inhibition at 15 microM, and activity cessation at 20-25 microM. The spike rate IC(50) was 15.9+/-1.0 microM (n=11). Fluoxetine did not change the action potential waveform shape. However, at high concentrations, it caused total cessation of spike activity on all channels. The inhibition caused by fluoxetine was reversible for both tissues. Based on the results, we conclude that cultures showed repeatable, concentration-dependent sensitivities to fluoxetine but demonstrated tissue-specific responses for frontal and auditory cortex networks. These responses may not be due to the interference with serotonin reuptake, but may be due to a secondary effect on ionic channels.

Therapeutic and adverse actions of serotonin transporter substrates

A variety of drugs release serotonin (5-HT, 5-hydroxytryptamine) from neurons by acting as substrates for 5-HT transporter (SERT) proteins. This review summarizes the neurochemical, therapeutic, and adverse actions of substrate-type 5-HT-releasing agents. The appetite suppressant (+/-)-fenfluramine is composed of (+) and (-) isomers, which are N-de-ethylated in the liver to yield the metabolites (+)- and (-)-norfenfluramine. Fenfluramines and norfenfluramines are potent 5-HT releasers. (+/-)-3,4-Methylenedioxymethamphetamine ((+/-)-MDMA, "ecstasy") and m-chlorophenylpiperazine (mCPP) are substrate-type 5-HT releasers. Fenfluramines, (+/-)-MDMA, and mCPP release neuronal 5-HT by a common non-exocytotic diffusion-exchange mechanism involving SERTs. (+)-Norfenfluramine is a potent 5-HT(2B) and 5-HT(2C) receptor agonist. The former activity may increase the risk of valvular heart disease, whereas the latter activity is implicated in the anorexic effect of systemic fenfluramine. Appetite suppressants that increase the risk for developing primary pulmonary hypertension (PPH) are all SERT substrates, but these drugs vary considerably in their propensity to increase this risk. For example, fenfluramine and aminorex are clearly linked to the occurrence of PPH, whereas other anorectics are not. Similarly, some SERT substrates deplete brain tissue 5-HT in animals (e.g., fenfluramine), while others do not (e.g., mCPP). In addition to the established indication of obesity, 5-HT releasers may help treat psychiatric disorders, such as drug and alcohol dependence, depression, and premenstrual syndrome. Viewed collectively, we believe new medications can be developed that selectively release 5-HT without increasing the risk for adverse effects of valvular heart disease, PPH, and neurotoxicity. Such agents may be useful for treating a variety of psychiatric disorders.

Serotonin releasing agents. Neurochemical, therapeutic and adverse effects

This review summarizes the neurochemical, therapeutic and adverse effects of serotonin (5-HT) releasing agents. The 5-HT releaser (plus minus)-fenfluramine is composed of two stereoisomers, (+)-fenfluramine and (minus sign)-fenfluramine, which are N-de-ethylated to yield the metabolites, (+)-norfenfluramine and (minus sign)-norfenfluramine. Fenfluramines and norfenfluramines are 5-HT transporter substrates and potent 5-HT releasers. Other 5-HT releasing agents include m-chlorophenylpiperazine (mCPP), a major metabolite of the antidepressant drug trazodone. Findings from in vitro and in vivo studies support the hypothesis that fenfluramines and mCPP release neuronal 5-HT via a non-exocytotic carrier-mediated exchange mechanism involving 5-HT transporters. (+)-Norfenfluramine is a potent 5-HT(2B) and 5-HT(2C) receptor agonist. The former activity may increase the risk of developing valvular heart disease (VHD), whereas the latter activity is implicated in the anorectic effect of systemic fenfluramine. Anorectic agents that increase the risk of developing primary pulmonary hypertension (PPH) share the common property of being 5-HT transporter substrates. However, these drugs vary considerably in their propensity to increase the risk of PPH. In this regard, neither trazodone nor mCPP is associated with PPH. Similarly, although some 5-HT substrates can deplete brain 5-HT (fenfluramine), others do not (mCPP). In addition to the established indication of obesity, 5-HT releasers may be helpful in treating psychiatric problems such as drug and alcohol dependence, depression and premenstrual syndrome. Viewed collectively, it seems possible to develop new medications that selectively release 5-HT without the adverse effects of PPH, VHD or neurotoxicity. Such agents may have utility in treating a variety of psychiatric disorders.

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.

Venlafaxine and its interaction with WAY 100635: effects on serotonergic unit activity and behavior in cats

The therapeutic efficacy of antidepressant drugs that inhibit the reuptake of serotonin (5-hydroxytryptamine, 5-HT) may be enhanced by blocking their indirect activation of 5-HT(1A) autoreceptors, which mediate feedback inhibition of serotonergic neuronal activity. In this study, we examined the effects of venlafaxine, a dual 5-HT/noradrenaline reuptake inhibitor, alone and in combination with the selective 5-HT(1A) receptor antagonist N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl) cyclohexanecarboxamide (WAY 100635), on the single-unit activity of serotonergic dorsal raphe neurons and concurrent behavior in freely moving cats. Systemic administration of venlafaxine (0.05-1.0 mg/kg, i.v.) produced a dose-dependent decrease in firing rate (ED(50)=0.19 mg/kg), with virtually complete inhibition of neuronal discharge at the highest dose tested. The subsequent administration of WAY 100635 (0.1 mg/kg, i.v.) rapidly reversed the neuronal suppression produced by venlafaxine and significantly elevated the firing rate above baseline levels. The overshoot in neuronal activity was associated with the onset of an adverse behavioral reaction resembling the 5-HT syndrome resulting from excessive levels of brain 5-HT. The intensity of this reaction paralleled the degree of neuronal restoration induced by WAY 100635, suggesting a causal relationship. Such behavioral responses were either not observed previously, or of a low intensity, when WAY 100635 was combined with selective 5-HT reuptake inhibitors. Overall, these results suggest that the risk of inducing adverse effects, such as the 5-HT syndrome, may be higher with dual 5-HT/noradrenaline reuptake inhibitors than with selective 5-HT reuptake inhibitors, when these agents are combined with a potent 5-HT(1A) autoreceptor antagonist. Possible mechanisms that might account for these differences in drug interaction are discussed.

The augmentation hypothesis for improvement of antidepressant therapy: is pindolol a suitable candidate for testing the ability of 5HT1A receptor antagonists to enhance SSRI efficacy and onset latency?

The development of selective serotonin reuptake inhibitors (SSRIs) provided a major advancement in the treatment of depression. However, these drugs suffer from a variety of drawbacks, most notably a delay in the onset of efficacy. One hypothesis suggests that this delay in efficacy is due to a paradoxical decrease in serotonergic (5-HT) neuronal impulse flow and release, following activation of inhibitory presynaptic 5-HT1A autoreceptors, following acute administration of SSRIs. According to the hypothesis, efficacy is seen only when this impulse flow is restored following desensitization of 5-HT1A autoreceptors and coincident increases in postsynaptic 5-HT levels are achieved. Clinical proof of this principal has been suggested in studies that found a significant augmenting effect when the beta-adrenergic/5-HT1A receptor antagonist, pindolol, was coadministered with SSRI treatment. In this article, we review preclinical electrophysiological and microdialysis studies that have examined this desensitization hypothesis. We further discuss clinical studies that utilized pindolol as a test of this hypothesis in depressed patients and examine preclinical studies that challenge the notion that the beneficial effect of pindolol is due to functional antagonism of the 5-HT1A autoreceptors.

Modulatory effect of p-chlorophenylalanine microinjected into the dorsal and median raphe nuclei on cocaine-induced behaviour in the rat

The present study examined whether a potentiation of cocaine-induced behaviour in rats following peripheral pretreatment with the 5-hydroxytryptamine (5-HT) biosynthesis inhibitor p-chlorophenylalanine may be due to depletion of 5-HT in the dorsal raphe nucleus and/or median raphe nucleus. Following peripheral pretreatment with p-chlorophenylalanine (100 mg/kg, i.p.) for 3 consecutive days, a potentiation of cocaine-induced locomotor activity and rears was observed. To investigate a possible involvement of serotonergic neurones arising in the midbrain raphe nuclei in the observed potentiation, p-chlorophenylalanine (0.5 microg) was microinjected in either the dorsal raphe nucleus or median raphe nucleus followed by behavioural testing 48 h later. Application of p-chlorophenylalanine in the dorsal raphe nucleus resulted in an enhancement of cocaine-induced locomotor activity and head bobs. In contrast, the stimulant effect of cocaine on behaviour was not altered by microinjection of p-chlorophenylalanine in the median raphe nucleus. Peripheral and central administration of p-chlorophenylalanine did not consistently alter the baseline behaviour of saline-treated animals. Biochemical results indicated only a moderate depletion of 5-HT in the midbrain raphe nuclei following peripheral p-chlorophenylalanine administration. Surprisingly, the central application of p-chlorophenylalanine in the dorsal raphe nucleus and median raphe nucleus did not alter the 5-HT levels in the midbrain raphe nucleus investigated. In addition, peripheral and central administration of p-chlorophenylalanine did not alter the 5-HT levels in the nucleus accumbens. In conclusion, the behavioural results suggest that the potentiation of cocaine-induced behaviour following peripheral p-chlorophenylalanine administration may be attributed to the dorsal raphe nucleus but not the median raphe nucleus suggesting that, serotonergic dorsal raphe nucleus neurones may normally mediate a tonic inhibitory effect on cocaine-induced behaviour. Furthermore, the biochemical data may indicate the existence of neurochemical resistance of the midbrain raphe nuclei to the 5-HT depleting effects of p-chlorophenylalanine.

Cellular electrophysiological effects of chronic fluoxetine and duloxetine administration on serotonergic responses in the aging hippocampus

The pharmacological and physiological effects of chronic administration of the selective serotonin (5-hydroxytryptamine, 5-HT) reuptake inhibitor (SSRI) fluoxetine and the dual 5-HT/norepinephrine (NE) reuptake inhibitor duloxetine were compared on 5-HT-mediated electrophysiological responses recorded in the hippocampus of young (3-5 months) and old (17-20 months) female Fischer 344 rats. Fluoxetine, duloxetine, or vehicle (saline) was administered once daily for 14 days (10 mg/kg, i.p.) and extracellular recordings of spontaneously firing CA1 and CA3 pyramidal neurons were conducted 24 h following the last injection using microiontophoretic drug application techniques in a chloral hydrate anesthetized preparation. The recovery times (RT50 values; sec) following 5-HT application on pyramidal neurons were significantly increased in the young and old chronic fluoxetine (FLX) treated groups (73% and 104%, respectively; P < 0.05), but not chronic duloxetine- (DLX) or vehicle- (VEH) treated groups. Following prolonged application of duloxetine (5-10 min), the 5-HT RT50 values were significantly increased in the young FLX groups as compared to the age-matched DLX- and VEH-treated groups. In contrast, a significant decline in the time to recovery produced by 5-HT (52%) was observed in the old vs. young FLX-treated group following the second co-application of 5-HT with duloxetine. Within each drug treatment and age group, co-application of duloxetine and 5-HT did not alter the inhibitory responses (IT50 values; nC) produced by the application of 5-HT alone. These results demonstrate cellular adaptive changes in serotonergic neuronal function occur following repeated exposure to 5-HT reuptake inhibitors in an age-dependent manner.