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

PET/CT study of dopamine transporter (DAT) binding with the triple reuptake inhibitor toludesvenlafaxine in rats and humans

PURPOSE

Toludesvenlafaxine is a recently developed antidepressant that belongs to the triple reuptake inhibitor class. Despite the in vitro evidence that toludesvenlafaxine inhibits the reuptake of serotonin (5-HT), norepinephrine (NE) and dopamine (DA), there is no in vivo evidence that toludesvenlafaxine binds to DAT and increases DA level, a mechanism thought to contribute to its favorable clinical performance.

METHODS

Positron emission tomography/computed tomography (PET/CT) was used to examine the DAT binding capacity in healthy rats and human subjects and microdialysis was used to examine the striatal DA level in rats. [18F]FECNT and [11C]CFT were used as PET/CT radioactive tracer for rat and human studies, respectively.

RESULTS

In rats, 9 mg/kg of toludesvenlafaxine hydrochloride (i.v.) followed by an infusion of 3 mg/kg via minipump led to the binding rate to striatum DAT at 3.7 - 32.41% and to hypothalamus DAT at 5.91 - 17.52% during the 45 min scanning period. 32 mg/kg oral administration with toludesvenlafaxine hydrochloride significantly increased the striatal DA level with the AUC0 - 180 min increased by 63.9%. In healthy volunteers, 160 mg daily toludesvenlafaxine hydrochloride sustained-release tablets for 4 days led to an average occupancy rates of DAT at 8.04% ± 7.75% and 8.09% ± 7.00%, respectively, in basal ganglion 6 h and 10 h postdose.

CONCLUSION

These results represent the first to confirm the binding of toludesvenlafaxine to DAT in both rats and humans using PET/CT, and its elevation of brain DA level, which may help understand the unique pharmacological and functional effects of triple reuptake inhibitors such as toludesvenlafaxine.

CLINICALTRIALS

GOV IDENTIFIERS

NCT05905120. Registered 14 June 2023. (retrospectively registered).

A multicenter, double-blind, placebo-controlled, randomized, Phase 1b crossover trial comparing two doses of ulotaront with placebo in the treatment of narcolepsy-cataplexy

BACKGROUND

Ulotaront (SEP-363856) is a novel agonist at trace amine-associated receptor 1 and serotonin 5-HT_1A receptors in clinical development for the treatment of schizophrenia. Previous studies demonstrated ulotaront suppresses rapid eye movement (REM) sleep in both rodents and healthy volunteers. We assessed acute and sustained treatments of ulotaront on REM sleep and symptoms of cataplexy and alertness in subjects with narcolepsy-cataplexy.

METHODS

In a multicenter, double-blind, placebo-controlled, randomized, 3-way crossover study, ulotaront was evaluated in 16 adults with narcolepsy-cataplexy. Two oral doses of ulotaront (25 mg and 50 mg) were administered daily for 2 weeks and compared with matching placebo (6-treatment sequence, 3-period, 3-treatment).

RESULTS

Acute treatment with both 25 mg and 50 mg of ulotaront reduced minutes spent in nighttime REM compared to placebo. A sustained 2-week administration of both doses of ulotaront reduced the mean number of short-onset REM periods (SOREMPs) during daytime multiple sleep latency test (MSLT) compared to placebo. Although cataplexy events decreased from the overall mean baseline during the 2-week treatment period, neither dose of ulotaront statistically separated from placebo (p = 0.76, 25 mg; p = 0.82, 50 mg), and no significant improvement in patient and clinician measures of sleepiness from baseline to end of the 2-week treatment period occurred in any treatment group.

CONCLUSIONS

Acute and sustained treatment with ulotaront reduced nighttime REM duration and daytime SOREMPs, respectively. The effect of ulotaront on suppression of REM did not demonstrate a statistical or clinically meaningful effect in narcolepsy-cataplexy.

REGISTRATION

ClinicalTrials.gov identifier: NCT05015673.

In vitro and in vivo pharmacological characterization of dasotraline, a dual dopamine and norepinephrine transporter inhibitor in vivo

Inhibitors of dopamine transporters (DAT), norepinephrine transporters (NET) and serotonin transporters (SERT) are effective treatments for neuropsychiatric diseases. Dasotraline [(1R,4 S)- 4-(3,4-dichlorophenyl)- 1,2,3,4-tetrahydro-1-naphthalenamine, also known as SEP-225289) was evaluated for its inhibitory potency at DAT, NET and SERT using in vitro and in vivo assays. In vitro radiometric functional uptake studies showed preferential inhibition by dasotraline of hDAT (IC₅₀ =3 nM) and hNET (IC₅₀ =4 nM relative to hSERT(IC₅₀ =15 nM). In mouse ex vivo occupancy studies, dasotraline demonstrated total plasma concentration-dependent occupancy at DAT, NET and SERT. Determination of the TO₅₀ (50% transporter occupancy) were 32, 109 and 276 ng/ml, respectively. In SPECT imaging studies in baboons, dasotraline (0.2 mg/kg iv) displaced radiotracer binding to DAT by 87% but only 20% at NET and SERT. Rat microdialysis studies were performed in prefrontal cortex and striatum. Dasotraline produced sustained (>4 h) increases in dopamine and norepinephrine concentrations. Dasotraline was also more potent at increasing synaptic dopamine in the striatum, and norepinephrine in the prefrontal cortex than serotonin in these regions. In summary, dasotraline preferentially inhibits DAT and NET relative to SERT. Together, the occupancy and neurochemical profile of dasotraline provide a mechanistic basis for the treatment of diseases that have an underlying causality involving dopamine and norepinephrine dysfunction.

Triple reuptake inhibition of serotonin, norepinephrine, and dopamine increases the tonic activation of α2-adrenoceptors in the rat hippocampus and dopamine levels in the nucleus accumbens

Clinical studies have shown the therapeutic efficacy of an increase in dopamine (DA) transmission in treatment of major depressive disorder (MDD). In the present study, we investigated whether blockade of DA transporters in addition to serotonin (5-HT) and norepinephrine (NE) produced additional adaptations of monoaminergic systems. In vivo electrophysiological recordings were carried out in male anesthetized rats. Vehicle, the 5-HT reuptake inhibitor escitalopram, the NE/DA reuptake blocker nomifensine and their combination (triple reuptake inhibition; TRI) were delivered for 2 or 14 days. Firing activity of NE, 5-HT and DA neurons was assessed. Tonic activation of 5-HT_1A receptors and α₁- and α₂-adrenoceptors was determined in the hippocampus and extracellular DA levels in the nucleus accumbens (NAc). Unlike escitalopram, nomifensine and TRI administration increased the tonic activation of α₂-adrenoceptors in the hippocampus despite decreasing NE neuronal firing activity after 2 and 14 days of administration. The firing activity of 5-HT neurons was increased after prolonged nomifensine and TRI regimens, while addition of nomifensine to escitalopram prevented the early 2-day suppression of firing by 5-HT reuptake inhibition. The tonic activation of 5-HT_1A receptors was enhanced only with escitalopram. Whereas escitalopram and nomifensine decreased firing activity of DA neurons after a 2-day administration, their combination normalized it to baseline level after 14 days; this was accompanied by a robust increase in extracellular DA levels in the NAc. In summary, these results indicate that TRI increases NE and DA but not 5-HT transmission, suggesting a differential efficacy profile in MDD patients.

Mechanism of Action of Atypical Antipsychotic Drugs in Mood Disorders

Atypical antipsychotic drugs were introduced in the early 1990s. Unlike typical antipsychotics, which are effective only against positive symptoms of schizophrenia, atypical antipsychotics are effective against negative and cognitive symptoms as well. Furthermore, they are effective not only in psychotic but also in affective disorders, on their own or as adjuncts to antidepressant drugs. This review presents the neural mechanisms of currently existing atypical antipsychotics and putative antipsychotics currently being investigated in preclinical and clinical studies and how these relate to their effectiveness in mood disorders such as depression, anxiety, and post-traumatic stress disorder (PTSD). Typical antipsychotics act almost exclusively on the dopamine system. Atypical drugs, however, modulate serotonin (5-HT), norepinephrine, and/or histamine neurotransmission as well. This multimodal mechanism of action putatively underlies the beneficial effect of atypical antipsychotics in mood and anxiety disorders. Interestingly, novel experimental drugs having dual antipsychotic and antidepressant therapeutic potential, such as histamine, adenosine, and trace amine-associated receptors (TAAR) ligand, are also characterized by a multimodal stimulatory effect on central 5-HT, norepinephrine, and/or histamine transmission. The multimodal stimulatory effect on central monoamine neurotransmission may be thus primarily responsible for the combined antidepressant and antipsychotic therapeutic potential of certain central nervous system (CNS) drugs.

New Medications for Neuropsychiatric Disorders

This article seeks to summarize the mechanisms of action, clinical trials, and FDA approval status of several psychiatric medications that are either newly available or in the FDA approval process. This article highlights medications that demonstrate novel mechanisms of action, examines nonpsychiatric medications that are being used to augment existing psychiatric treatments, and elucidates treatments for illnesses that have not previously received FDA indications.

Antidepressant efficacy of a selective organic cation transporter blocker in a mouse model of depression

Current antidepressants act principally by blocking monoamine reuptake by high-affinity transporters in the brain. However, these antidepressants show important shortcomings such as slow action onset and limited efficacy in nearly a third of patients with major depression disorder. Here, we report the development of a prodrug targeting organic cation transporters (OCT), atypical monoamine transporters recently implicated in the regulation of mood. Using molecular modeling, we designed a selective OCT2 blocker, which was modified to increase brain penetration. This compound, H2-cyanome, was tested in a rodent model of chronic depression induced by 7-week corticosterone exposure. In male mice, prolonged administration of H2-cyanome induced positive effects on several behaviors mimicking symptoms of depression, including anhedonia, anxiety, social withdrawal, and memory impairment. Importantly, in this validated model, H2-cyanome compared favorably with the classical antidepressant fluoxetine, with a faster action on anhedonia and better anxiolytic effects. Integrated Z-scoring across these depression-like variables revealed a lower depression score for mice treated with H2-cyanome than for mice treated with fluoxetine for 3 weeks. Repeated H2-cyanome administration increased ventral tegmental area dopaminergic neuron firing, which may underlie its rapid action on anhedonia. H2-cyanome, like fluoxetine, also modulated several intracellular signaling pathways previously involved in antidepressant response. Our findings provide proof-of-concept of antidepressant efficacy of an OCT blocker, and a mechanistic framework for the development of new classes of antidepressants and therapeutic alternatives for resistant depression and other psychiatric disturbances such as anxiety.

Electrophysiology and Behavioral Assessment of the New Molecule SMe1EC2M3 as a Representative of the Future Class of Triple Reuptake Inhibitors

SMe1EC2M3 is a pyridoindole derivative related to the neuroleptic drug carbidine. Based on the structural similarities of SMe1EC2M3 and known serotonin (5-HT), norepinephrine, and dopamine reuptake inhibitors, we hypothesized that this compound may also have triple reuptake inhibition efficacy and an antidepressant-like effect. PreADMET and Dragon software was used for in silico prediction of pharmacokinetics and pharmacodynamics of SMe1EC2M3. Forced swim test was used to evaluate its antidepressant-like effects. Extracellular in vivo electrophysiology was used to assess 5-HT, norepinephrine, and dopamine reuptake inhibition efficacy of SMe1EC2M3. PreADMET predicted reasonable intestinal absorption, plasma protein binding, and blood-brain permeability for SMe1EC2M3. Dragon forecasted its efficiency as an antidepressant. Using behavioral measurements, it was found that SMe1EC2M3 decreased immobility time and increase swimming time during the forced swim test (FST). Electrophysiological investigations showed that SMe1EC2M3 dose-dependently suppressed the excitability of 5-HT neurons of the dorsal raphe nucleus (DRN), norepinephrine neurons of the locus coeruleus (LC), and dopamine neurons of the ventral tegmental area (VTA). The SMe1EC2M3-induced suppression of 5-HT, norepinephrine, and dopamine neurons was reversed by the antagonists of serotonin-1A (5-HT_1A; WAY100135), α-2 adrenergic (α₂, yohimbine), and dopamine-2 receptors (D₂, haloperidol), respectively. We conclude that SMe1EC2M3 is prospective triple 5-HT, norepinephrine, and dopamine reuptake inhibitor with antidepressant-like properties, however future studies should be performed to complete the pharmacological profiling of this compound.

Lack of correlation between the activity of the mesolimbic dopaminergic system and the rewarding properties of pregabalin in mouse

RATIONALE

Pregabalin is a psychoactive drug indicated in the treatment of epilepsy, neuropathic pain, and generalized anxiety disorders. Pregabalin acts on different neurotransmission systems by inactivating the alpha2-delta subunit of voltage-gated calcium channels. In light of this pharmacological property, the hypothesis has been raised that pregabalin may regulate the mesolimbic dopamine pathway and thereby display a potential for misuse or abuse as recently observed in humans. Although some preclinical data support this possibility, the rewarding properties of gabapentinoid are still a matter for debate.

OBJECTIVE

The aim of this work was to evaluate the rewarding properties of pregabalin and to determine its putative mechanism of action in healthy mice.

RESULTS

Pregabalin alone (60 mg/kg; s.c.) produced a rewarding effect in the conditioned place preference (CPP) test albeit to a lower extent than cocaine (30 mg/kg; s.c.). Interestingly, when assessing locomotor activity in the CPP, the PGB60 group, similarly to the cocaine group, showed an increased locomotor activity. In vivo single unit extracellular recording showed that pregabalin had mixed effects on dopamine (DA) neuronal activity in the ventral tegmental area since it decreased the activity of 50% of neurons and increased 28.5% of them. In contrast, cocaine decreased 75% of VTA DA neuronal activity whereas none of the neurons were activated. Intracerebal microdialysis was then conducted in awake freely mice to determine to what extent such electrophysiological parameters influence the extracellular DA concentrations ([DA]ext) in the nucleus accumbens. Although pregabalin failed to modify this parameter, cocaine produced a robust increase (800%) in [DA]ext.

CONCLUSIONS

Collectively, these electrophysiological and neurochemical experiments suggest that the rewarding properties of pregabalin result from a different mode of action than that observed with cocaine. Further experiments are warranted to determine whether such undesirable effects can be potentiated under pathological conditions such as neuropathic pain, mood disorders, or addiction and to identify the key neurotransmitter system involved.

In vivo electrophysiological recordings of the effects of antidepressant drugs

Antidepressant drugs are a standard biological treatment for various neuropsychiatric disorders, yet relatively little is known about their electrophysiologic and synaptic effects on mood systems that set moment-to-moment emotional tone. In vivo electrical recording of local field potentials (LFPs) and single neuron spiking has been crucial for elucidating important details of neural processing and control in many other systems, and yet electrical approaches have not been broadly applied to the actions of antidepressants on mood-related circuits. Here we review the literature encompassing electrophysiologic effects of antidepressants in animals, including studies that examine older drugs, and extending to more recently synthesized novel compounds, as well as rapidly acting antidepressants. The existing studies on neuromodulator-based drugs have focused on recording in the brainstem nuclei, with much less known about their effects on prefrontal or sensory cortex. Studies on neuromodulatory drugs have moreover focused on single unit firing patterns with less emphasis on LFPs, whereas the rapidly acting antidepressant literature shows the opposite trend. In a synthesis of this information, we hypothesize that all classes of antidepressants could have common final effects on limbic circuitry. Whereas NMDA receptor blockade may induce a high powered gamma oscillatory state via direct and fast alteration of glutamatergic systems in mood-related circuits, neuromodulatory antidepressants may induce similar effects over slower timescales, corresponding with the timecourse of response in patients, while resetting synaptic excitatory versus inhibitory signaling to a normal level. Thus, gamma signaling may provide a biomarker (or “neural readout”) of the therapeutic effects of all classes of antidepressants.

Altered brainstem responses to modafinil in schizophrenia: implications for adjunctive treatment of cognition

Candidate pro-cognitive drugs for schizophrenia targeting several neurochemical systems have consistently failed to demonstrate robust efficacy. It remains untested whether concurrent antipsychotic medications exert pharmacodynamic interactions that mitigate pro-cognitive action in patients. We used functional MRI (fMRI) in a randomized, double-blind, placebo-controlled within-subject crossover test of single-dose modafinil effects in 27 medicated schizophrenia patients, interrogating brainstem regions where catecholamine systems arise to innervate the cortex, to link cellular and systems-level models of cognitive control. Modafinil effects were evaluated both within this patient group and compared to a healthy subject group. Modafinil modulated activity in the locus coeruleus (LC) and ventral tegmental area (VTA) in the patient group. However, compared to the healthy comparison group, these effects were altered as a function of task demands: the control-independent drug effect on deactivation was relatively attenuated (shallower) in the LC and exaggerated (deeper) in the VTA; in contrast, again compared to the comparison group, the control-related drug effects on positive activation were attenuated in LC, VTA and the cortical cognitive control network. These altered effects in the LC and VTA were significantly and specifically associated with the degree of antagonism of alpha-2 adrenergic and dopamine-2 receptors, respectively, by concurrently prescribed antipsychotics. These sources of evidence suggest interacting effects on catecholamine neurons of chronic antipsychotic treatment, which respectively increase and decrease sustained neuronal activity in LC and VTA. This is the first direct evidence in a clinical population to suggest that antipsychotic medications alter catecholamine neuronal activity to mitigate pro-cognitive drug action on cortical circuits.

Computational modeling of spike generation in serotonergic neurons of the dorsal raphe nucleus

Serotonergic neurons of the dorsal raphe nucleus, with their extensive innervation of limbic and higher brain regions and interactions with the endocrine system have important modulatory or regulatory effects on many cognitive, emotional and physiological processes. They have been strongly implicated in responses to stress and in the occurrence of major depressive disorder and other psychiatric disorders. In order to quantify some of these effects, detailed mathematical models of the activity of such cells are required which describe their complex neurochemistry and neurophysiology. We consider here a single-compartment model of these neurons which is capable of describing many of the known features of spike generation, particularly the slow rhythmic pacemaking activity often observed in these cells in a variety of species. Included in the model are 11 kinds of ion channels: a fast sodium current INa, a delayed rectifier potassium current IKDR, a transient potassium current IA, a slow non-inactivating potassium current IM, a low-threshold calcium current IT, two high threshold calcium currents IL and IN, small and large conductance potassium currents ISK and IBK, a hyperpolarization-activated cation current IH and a leak current ILeak. In Sections 3-8, each current type is considered in detail and parameters estimated from voltage clamp data where possible. Three kinds of model are considered for the BK current and two for the leak current. Intracellular calcium ion concentration Cai is an additional component and calcium dynamics along with buffering and pumping is discussed in Section 9. The remainder of the article contains descriptions of computed solutions which reveal both spontaneous and driven spiking with several parameter sets. Attention is focused on the properties usually associated with these neurons, particularly long duration of action potential, steep upslope on the leading edge of spikes, pacemaker-like spiking, long-lasting afterhyperpolarization and the ramp-like return to threshold after a spike. In some cases the membrane potential trajectories display doublets or have humps or notches as have been reported in some experimental studies. The computed time courses of IA and IT during the interspike interval support the generally held view of a competition between them in influencing the frequency of spiking. Spontaneous activity was facilitated by the presence of IH which has been found in these neurons by some investigators. For reasonable sets of parameters spike frequencies between about 0.6Hz and 1.2Hz are obtained, but frequencies as high as 6Hz could be obtained with special parameter choices. Topics investigated and compared with experiment include shoulders, notches, anodal break phenomena, the effects of noradrenergic input, frequency versus current curves, depolarization block, effects of cell size and the effects of IM. The inhibitory effects of activating 5-HT1A autoreceptors are also investigated. There is a considerable discussion of in vitro versus in vivo firing behavior, with focus on the roles of noradrenergic input, corticotropin-releasing factor and orexinergic inputs. Location of cells within the nucleus is probably a major factor, along with the state of the animal.

The effects of LPM570065, a novel triple reuptake inhibitor, on extracellular serotonin, dopamine and norepinephrine levels in rats

Triple reuptake inhibitors (TRIs) are currently being developed as a new class of promising antidepressants that block serotonin (5-HT), dopamine (DA) and norepinephrine (NE) transporters, thereby increasing extracellular monoamine concentrations. The purpose of this study was to investigate the effects of LPM570065, a novel TRI and a desvenlafaxine prodrug, on extracellular 5-HT, DA and NE levels in the rat striatum after acute and chronic administration relative to desvenlafaxine, using High Performance Liquid Chromatography (HPLC) and microdialysis. Acute administration was performed by providing rodents with oral solutions (0.06 mmol·kg(-1) p.o.), oral suspensions (0.06 mmol·kg(-1) p.o.) and intravenous solutions (0.04 mmol·kg(-1) i.v.) of LPM570065 and desvenlafaxine. Oral suspensions (0.06 mmol·kg(-1)·day(-1)) of the two drugs were also administered for a 14-day chronic period. HPLC analysis revealed that LPM570065 rapidly penetrated the rat striatum, converted into desvenlafaxine and exhibited larger total exposure compared with the administration of desvenlafaxine. Microdialysis revealed that acute and chronic administration of oral suspension of LPM570065 increased the 5-HT, DA and NE levels more than the relative administration of desvenlafaxine. Unlike desvenlafaxine, acute administration of an intravenous LPM570065 solution did not induce the undesirable 90% decrease in extracellular 5-HT levels. In contrast to the fully dose-dependent elevation of 5-HT induced by desvenlafaxine, the acute administration of LPM570065 showed a capped increase in extracellular 5-HT levels when combined with WAY-100635. Additionally, forced swim test demonstrated that acute and chronic administration of LPM570065 reduced the immobility time more than the relative administration of desvenlafaxine. These data suggest that LPM570065 may have greater efficacy and/or a more rapid onset of antidepressant action than desvenlafaxine and also counterbalance the harmful effects of desvenlafaxine on 5-HT neurotransmission related to 5-HT1A autoreceptors. Thus, this new class of drugs, TRIs has the potential to provide a new therapeutic mechanism for treating depression.

Monoamine neurocircuitry in depression and strategies for new treatments

Extensive studies showed that monoaminergic neurotransmission that involves serotonin (5-HT), norepinephrine (NE) and dopamine (DA) exerts major influence on brain circuits concerned by the regulation of mood, reactivity to psychological stress, self-control, motivation, drive, and cognitive performance. Antidepressants targeting monoamines directly affect the functional tone of these circuits, notably in limbic and frontocortical areas, and evidence has been provided that this action plays a key role in their therapeutic efficacy. Indeed, at least some of functional changes detected by functional magnetic resonance imaging in emotion- and cognitive-related circuits such as the one involving limbic-cortical-striatal-pallidal-thalamic connections in depressed patients can be reversed by monoamine-targeted antidepressants. However, antidepressants acting selectively on only one monoamine, such as selective inhibitors of 5-HT or NE reuptake, alleviate depression symptoms in a limited percentage of patients, and are poorly effective to prevent recurrence. Thorough investigations for the last 30 years allowed the demonstration of the existence of functional interactions between 5-HT, NE and DA systems, and the identification of the specific receptors involved. In particular, 5-HT systems were shown to exert negative influence on NE and DA systems through 5-HT2A and 5-HT2C receptor- mediated mechanisms, respectively. On the other hand, complex positive and negative influences of NE system on 5-HT neurotransmission are mediated through α1- and α2-adrenergic receptors, respectively. These data provided a rationale for the design of new, multimodal, therapeutic strategies involving drugs acting not only at the "historical" targets such as the 5-HT and/or the NE transporter, but also at other molecular targets to improve their efficacy and their tolerability.

Behavioral and dorsal raphe neuronal activity following acute and chronic methylphenidate in freely behaving rats

Concomitant behavioral and dorsal raphe (DR) neuronal activity were recorded following acute and chronic dose response of methylphenidate (MPD) in freely moving rats previously implanted with permanent semi-microelectrodes using telemetric (wireless) technology. On experimental day (ED) 1, the neuronal and locomotor activity were recorded after saline (baseline) and MPD (0.6, 2.5 or 10.0mg/kg) injection (i.p.). Animals were injected daily with a single dose of MPD for five consecutive days (ED 2-6) to elicit behavioral sensitization or tolerance. After three washout days, the neuronal and locomotor activity recording was resumed on ED 10 followed by saline and MPD rechallenge injection. The main findings were: (1) the same dose of chronic MPD administration elicited behavioral sensitization in some animals and behavioral tolerance in others. (2) 46%, 56% and 73% of DR units responded to acute 0.6, 2.5 and 10.0mg/kg MPD respectively. (3) 89%, 70% and 86% of DR units changed their baseline activity on ED 10 compared to that on ED 1 in the 0.6, 2.5 and 10.0mg/kg MPD groups respectively. (4) A significant difference in ED 10 baseline activity was observed in the DR neuronal population recording from animals expressing behavioral sensitization compared to that of animals expressing behavioral tolerance. (5) 89%, 78% and 88% of DR units responded to chronic 0.6, 2.5 and 10.0mg/kg MPD respectively. (6) The DR neuronal population recording following acute MPD on ED 1 and rechallenge MPD on ED 10 from animals expressing behavioral sensitization was significantly different from the neuronal population recorded from animals exhibited behavioral tolerance. The correlation between the DR neuronal activity and animal's behavior following chronic MPD exposure suggested that the DR neuronal activity may play an important role in the expression of behavioral sensitization and tolerance induced by chronic MPD administration.

Effects of swim stress and fluoxetine on 5-HT1A receptor gene expression and monoamine metabolism in the rat brain regions

Changes in gene expression of the brain serotonin (5-HT) 1A receptors may be important for the development and ameliorating depression, however identification of specific stimuli that activate or reduce the receptor transcriptional activity is far from complete. In the present study, the forced swim test (FST) exposure, the first stress session of which is already sufficient to induce behavioral despair in rats, significantly increased 5-HT1A receptor mRNA levels in the brainstem, frontal cortex, and hippocampus at 24 h. In the brainstem and frontal cortex, the elevation in the receptor gene expression after the second forced swim session was not affected following chronic administration of fluoxetine, while in the cortex, both control and FST values were significantly reduced in fluoxetine-treated rats. In contrast to untreated rats, no increase in hippocampal 5-HT1A receptor mRNA was observed in response to FST in rats chronically treated with fluoxetine. Metabolism of 5-HT (5-HIAA/5-HT) in the brainstem was significantly decreased by fluoxetine and further reduced by swim stress, showing a certain degree of independence of these changes on 5-HT1A receptor gene expression that was increased in this brain region only after the FST, but not after fluoxetine. FST exposure also decreased the brainstem dopamine metabolism, which was unexpectedly positively correlated with 5-HT1A receptor mRNA levels in the frontal cortex. Together, these data suggest that the effects of the forced swim stress as well as fluoxetine involve brain region-dependent alterations in 5-HT1A receptor gene transcription, some of which may be interrelated with concomitant changes in catecholamine metabolism.

Relevance of norepinephrine-dopamine interactions in the treatment of major depressive disorder

Central dopaminergic and noradrenergic systems play essential roles in controlling several forebrain functions. Consequently, perturbations of these neurotransmissions may contribute to the pathophysiology of neuropsychiatric disorders. For many years, there was a focus on the serotonin (5‐HT) system because of the efficacy of selective serotonin reuptake inhibitors (SSRIs), the most prescribed antidepressants in the treatment of major depressive disorder (MDD). Given the interconnectivity within the monoaminergic network, any action on one system may reverberate in the other systems. Analysis of this network and its dysfunctions suggests that drugs with selective or multiple modes of action on dopamine (DA) and norepinephrine (NE) may have robust therapeutic effects. This review focuses on NE‐DA interactions as demonstrated in electrophysiological and neurochemical studies, as well as on the mechanisms of action of agents with either selective or dual actions on DA and NE. Understanding the mode of action of drugs targeting these catecholaminergic neurotransmitters can improve their utilization in monotherapy and in combination with other compounds particularly the SSRIs. The elucidation of such relationships can help design new treatment strategies for MDD, especially treatment‐resistant depression.