Antidepressant drugs inhibit a glial 5-hydroxytryptamine transporter in rat brain

Age-Related Changes in Central Nervous System 5-Hydroxytryptamine Signalling and Its Potential Effects on the Regulation of Lifespan

Serotonin or 5-hydroxytryptamine (5-HT) is an important neurotransmitter in the central nervous system and the periphery. Most 5-HT (~99%) is found in the periphery where it regulates the function of the gastrointestinal (GI) tract and is an important regulator of platelet aggregation. However, the remaining 1% that is found in the central nervous system (CNS) can regulate a range of physiological processes such as learning and memory formation, mood, food intake, sleep, temperature and pain perception. More recent work on the CNS of invertebrate model systems has shown that 5-HT can directly regulate lifespan.This chapter will focus on detailing how CNS 5-HT signalling is altered with increasing age and the potential consequences this has on its ability to regulate lifespan.

Lesion of Serotonergic Afferents to the Retrotrapezoid Nucleus Impairs the Tachypneic Response to Hypercapnia in Unanesthetized Animals

Hypercapnia promotes an increase in pulmonary ventilation due to the stimulation of brainstem chemosensory cells that are connected to the respiratory network. Among these cells are the raphe serotonergic neurons which widely send projections to distinct central respiratory compartments. Nevertheless, the physiological role of specific raphe serotonergic projections to other chemosensitive sites on the emergence of hypercapnia ventilatory response in vivo still remains to be elucidated. Here we investigated whether the ventilatory response to hypercapnia requires serotonergic inputs to the chemosensitive cells of the retrotrapezoid nucleus (RTN) in the ventrolateral medulla. To test this, pulmonary ventilation was evaluated under baseline conditions and during hypercapnia (7% CO₂) in unanesthetized juvenile Holtzman rats (60-90 g) that received bilateral microinjections of either vehicle (control) or anti-SERT-SAP (0.1 mM, 10 pmol/100 nl) toxin in the RTN to retrogradely destroy serotonergic afferents to this region. Fifteen days after microinjections, baseline ventilation was not different between anti-SERT-SAP (n = 8) and control animals (n = 9). In contrast, the ablation of RTN-projecting serotonergic neurons markedly attenuated the hypercapnia-induced increase in respiratory frequency which was correlated with reduced numbers of serotonergic neurons in the raphe obscurus and magnus, but not in the raphe pallidus. The increase in tidal volume during hypercapnia was not significantly affected by anti-SERT-SAP microinjections in the RTN. Our data indicate that serotoninergic neurons that send projections to the RTN region are required for the processing of ventilatory reflex response during exposure to high CO₂ in unanesthetized conditions.

An Association of Serotonin with Pain Disorders and Its Modulation by Estrogens

Ovarian hormones play an important role in pain perception, and are responsible, at least in part, for the pain threshold differences between the sexes. Modulation of pain and its perception are mediated by neurochemical changes in several pathways, affecting both the central and peripheral nervous systems. One of the most studied neurotransmitters related to pain disorders is serotonin. Estrogen can modify serotonin synthesis and metabolism, promoting a general increase in its tonic effects. Studies evaluating the relationship between serotonin and disorders such as irritable bowel syndrome, fibromyalgia, migraine, and other types of headache suggest a clear impact of this neurotransmitter, thereby increasing the interest in serotonin as a possible future therapeutic target. This literature review describes the importance of substances such as serotonin and ovarian hormones in pain perception and illustrates the relationship between those two, and their direct influence on the presentation of the aforementioned pain-related conditions. Additionally, we review the pathways and receptors implicated in each disorder. Finally, the objective was to stimulate future pharmacological research to experimentally evaluate the potential of serotonin modulators and ovarian hormones as therapeutic agents to regulate pain in specific subpopulations.

Glial Activation and Expression of the Serotonin Transporter in Chronic Fatigue Syndrome

Fatigue is commonly reported in a variety of illnesses and has major impact on quality of life. Chronic fatigue syndrome (CFS) is a debilitating syndrome of unknown etiology. The clinical symptoms include problems in neuroendocrine, autonomic, and immune systems. It is becoming clear that the brain is the central regulator of CFS. For example, neuroinflammation, especially induced by activation of microglia and astrocytes, may play a prominent role in the development of CFS, though little is known about molecular mechanisms. Many possible causes of CFS have been proposed. However, in this mini-review, we summarize evidence for a role for microglia and astrocytes in the onset and the maintenance of immunologically induced CFS. In a model using virus mimicking synthetic double-stranded RNA, infection causes sequential signaling such as increased blood brain barrier (BBB) permeability, microglia/macrophage activation through Toll-like receptor 3 (TLR3) signaling, secretion of IL-1β, upregulation of the serotonin transporter (5-HTT) in astrocytes, reducing extracellular serotonin (5-HT) levels and hence reduced activation of 5-HT_1A receptor subtype. Hopefully, drug discovery targeting these pathways may be effective for CFS therapy.

Regulatory Pathways of Monoamine Oxidase A during Social Stress

Social stress has a high impact on many biological systems in the brain, including serotonergic (5-HT) system-a major drug target in the current treatment for depression. Hyperactivity of hypothalamic-pituitary-adrenal (HPA) axis and monoamine oxidase A (MAO-A) are well-known stress responses, which are involved in the central 5-HT system. Although, many MAO-A inhibitors have been developed and used in the therapeutics of depression, effective management of depression by modulating the activity of MAO-A has not been achieved. Identifying the molecular pathways that regulate the activity of MAO-A in the brain is crucial for developing new drug targets for precise control of MAO-A activity. Over the last few decades, several regulatory pathways of MAO-A consisting of Kruppel like factor 11 (KLF11), Sirtuin1, Ring finger protein in neural stem cells (RINES), and Cell division cycle associated 7-like protein (R1) have been identified, and the influence of social stress on these regulatory factors evaluated. This review explores various aspects of these pathways to expand our understanding of the roles of the HPA axis and MAO-A regulatory pathways during social stress. The first part of this review introduces some components of the HPA axis, explains how stress affects them and how they interact with the 5-HT system in the brain. The second part summarizes the novel regulatory pathways of MAO-A, which have high potential as novel therapeutic targets for depression.

Induction of interleukin-1β by activated microglia is a prerequisite for immunologically induced fatigue

We previously reported that an intraperitoneal (i.p.) injection of synthetic double-stranded RNA, polyriboinosinic:polyribocytidylic acid (poly-I:C), produced prolonged fatigue in rats, which might serve as a model for chronic fatigue syndrome. The poly-I:C-induced fatigue was associated with serotonin transporter (5-HTT) overexpression in the prefrontal cortex (PFC), a brain region that has been suggested to be critical for fatigue sensation. In the present study, we demonstrated that microglial activation in the PFC was important for poly-I:C-induced fatigue in rats, as pretreatment with minocycline, an inhibitor of microglial activation, prevented the decrease in running wheel activity. Poly-I:C injection increased the microglial interleukin (IL)-1β expression in the PFC. An intracerebroventricular (i.c.v.) injection of IL-1β neutralising antibody limited the poly-I:C-induced decrease in activity, whereas IL-1β (i.c.v.) reduced the activity in a dose-dependent manner. 5-HTT expression was enhanced by IL-1β in primary cultured astrocytes but not in microglia. Poly-I:C injection (i.p.) caused an increase in 5-HTT expression in astrocytes in the PFC of the rat, which was inhibited by pretreatment with minocycline (i.p.) and rat recombinant IL-1 receptor antagonist (i.c.v.). Poly-I:C injection (i.p.) led to a breakdown of the blood-brain barrier and enhanced Toll-like receptor 3 signaling in the brain. Furthermore, direct application of poly-I:C enhanced IL-1β expression in primary microglia. We therefore propose that poly-I:C-induced microglial activation, which may be at least partly caused by a direct action of poly-I:C, enhances IL-1β expression. Then, IL-1β induces 5-HTT expression in astrocytes, resulting in the immunologically induced fatigue.

Astrocyte pathology in major depressive disorder: insights from human postmortem brain tissue

The present paper reviews astrocyte pathology in major depressive disorder (MDD) and proposes that reductions in astrocytes and related markers are key features in the pathology of MDD. Astrocytes are the most numerous and versatile of all types of glial cells. They are crucial to the neuronal microenvironment by regulating glucose metabolism, neurotransmitter uptake (particularly for glutamate), synaptic development and maturation and the blood brain barrier. Pathology of astrocytes has been consistently noted in MDD as well as in rodent models of depressive-like behavior. This review summarizes evidence from human postmortem tissue showing alterations in the expression of protein and mRNA for astrocyte markers such as glial fibrillary acidic protein (GFAP), gap junction proteins (connexin 40 and 43), the water channel aquaporin-4 (AQP4), a calcium-binding protein S100B and glutamatergic markers including the excitatory amino acid transporters 1 and 2 (EAAT1, EAAT2) and glutamine synthetase. Moreover, preclinical studies are presented that demonstrate the involvement of GFAP and astrocytes in animal models of stress and depressive-like behavior and the influence of different classes of antidepressant medications on astrocytes. In light of the various astrocyte deficits noted in MDD, astrocytes may be novel targets for the action of antidepressant medications. Possible functional consequences of altered expression of astrocytic markers in MDD are also discussed. Finally, the unique pattern of cell pathology in MDD, characterized by prominent reductions in the density of astrocytes and in the expression of their markers without obvious neuronal loss, is contrasted with that found in other neuropsychiatric and neurodegenerative disorders.

Antidepressants act directly on astrocytes: evidences and functional consequences

Post-mortem histopathological studies report on reduced glial cell numbers in various frontolimbic areas of depressed patients implying that glial loss together with abnormal functioning could contribute to the pathophysiology of mood disorders. Astrocytes are regarded as the most abundant cell type in the brain and known for their housekeeping functions, but as recent developments suggest, they are also dynamic regulators of synaptogenesis, synaptic strength and stability and they control adult hippocampal neurogenesis. The primary aim of this review was to summarize the abundant experimental evidences demonstrating that antidepressant therapies have profound effect on astrocytes. Antidepressants modify astroglial physiology, morphology and by affecting gliogenesis they probably even regulate glial cell numbers. Antidepressants affect intracellular signaling pathways and gene expression of astrocytes, as well as the expression of receptors and the release of various trophic factors. We also assess the potential functional consequences of these changes on glutamate and glucose homeostasis and on synaptic communication between the neurons. We propose here a hypothesis that antidepressant treatment not only affects neurons, but also activates astrocytes, triggering them to carry out specific functions that result in the reactivation of cortical plasticity and can lead to the readjustment of abnormal neuronal networks. We argue here that these astrocyte specific changes are likely to contribute to the therapeutic effectiveness of the currently available antidepressant treatments and the better understanding of these cellular and molecular processes could help us to identify novel targets for the development of antidepressant drugs.

The pro-inflammatory cytokine TNF-α regulates the activity and expression of the serotonin transporter (SERT) in astrocytes

Pro-inflammatory cytokines have been implicated in the precipitation of depression and related disorders, and the antidepressant sensitive serotonin transporter (SERT) may be a major target for immune regulation in these disorders. Here, we focus on astrocytes, a major class of immune competent cells in the brain, to examine the effects of pro-longed treatment with tumor necrosis factor-alpha (TNF-α) on SERT activity. We first established that high-affinity serotonin uptake into C6 glioma cells occurs through a SERT-dependent mechanism. Functional SERT expression is also confirmed for primary astrocytes. In both cell types, exposure to TNF-α resulted in a dose- and time-dependent increase in SERT-mediated 5-HT uptake, which was sustained for at least 48 h post-stimulation. Further analysis in primary astrocytes revealed that TNF-α enhanced the transport capacity (Vmax) of SERT-specific 5-HT uptake, suggesting enhanced transporter expression, consistent with our observation of an increase in SERT mRNA levels. We confirmed that in both, primary astrocytes and C6 glioma cells, treatment with TNF-α activates the p38 mitogen-activated protein kinase (MAPK) signaling pathway. Pre-treatment with the p38 MAPK inhibitor SB203580 attenuated the TNF-α mediated stimulation of 5-HT transport in both, C6 glioma and primary astrocytes. In summary, we show that SERT gene expression and activity in astrocytes is subject to regulation by TNF-α, an effect that is at least in part dependent on p38 MAPK activation.

Disruption of the serotonergic system after neonatal hypoxia-ischemia in a rodent model

Identifying which specific neuronal phenotypes are vulnerable to neonatal hypoxia-ischemia, where in the brain they are damaged, and the mechanisms that produce neuronal losses are critical to determine the anatomical substrates responsible for neurological impairments in hypoxic-ischemic brain-injured neonates. Here we describe our current work investigating how the serotonergic network in the brain is disrupted in a rodent model of preterm hypoxia-ischemia. One week after postnatal day 3 hypoxia-ischemia, losses of serotonergic raphé neurons, reductions in serotonin levels in the brain, and reduced serotonin transporter expression are evident. These changes can be prevented using two anti-inflammatory interventions; the postinsult administration of minocycline or ibuprofen. However, each drug has its own limitations and benefits for use in neonates to stem damage to the serotonergic network after hypoxia-ischemia. By understanding the fundamental mechanisms underpinning hypoxia-ischemia-induced serotonergic damage we will hopefully move closer to developing a successful clinical intervention to treat neonatal brain injury.

Expression of monoamine transporters, nitric oxide synthase 3, and neurotrophin genes in antidepressant-stimulated astrocytes

BACKGROUND

There is increasing evidence that glial cells play a role in the pathomechanisms of mood disorders and the mode of action of antidepressant drugs.

METHODS

To examine whether there is a direct effect on the expression of different genes encoding proteins that have been implicated in the pathophysiology of affective disorders, primary astrocyte cell cultures from rats were treated with two different antidepressant drugs, imipramine and escitalopram, and the RNA expression of brain-derived neurotrophic factor (Bdnf), serotonin transporter (5Htt), dopamine transporter (Dat), and endothelial nitric oxide synthase (Nos3) was examined.

RESULTS

Stimulation of astroglial cell culture with imipramine, a tricyclic antidepressant, led to a significant increase of the Bdnf RNA level whereas treatment with escitalopram did not. In contrast, 5Htt was not differentially expressed after antidepressant treatment. Finally, neither Dat nor Nos3 RNA expression was detected in cultured astrocytes.

CONCLUSION

These data provide further evidence for a role of astroglial cells in the molecular mechanisms of action of antidepressants.

Selective p38α MAPK deletion in serotonergic neurons produces stress resilience in models of depression and addiction

Maladaptive responses to stress adversely affect human behavior, yet the signaling mechanisms underlying stress-responsive behaviors remain poorly understood. Using a conditional gene knockout approach, the α isoform of p38 mitogen-activated protein kinase (MAPK) was selectively inactivated by AAV1-Cre-recombinase infection in specific brain regions or by promoter-driven excision of p38α MAPK in serotonergic neurons (by Slc6a4-Cre or ePet1-Cre) or astrocytes (by Gfap-CreERT2). Social defeat stress produced social avoidance (a model of depression-like behaviors) and reinstatement of cocaine preference (a measure of addiction risk) in wild-type mice, but not in mice having p38α MAPK selectively deleted in serotonin-producing neurons of the dorsal raphe nucleus. Stress-induced activation of p38α MAPK translocated the serotonin transporter to the plasma membrane and increased the rate of transmitter uptake at serotonergic nerve terminals. These findings suggest that stress initiates a cascade of molecular and cellular events in which p38α MAPK induces a hyposerotonergic state underlying depression-like and drug-seeking behaviors.

Hippocampal astrocytes are necessary for antidepressant treatment of learned helplessness rats

The astrocyte is a major component of the neural network and plays a role in brain function. Previous studies demonstrated changes in the number of astrocytes in depression. In this study, we examined alterations in the number of astrocytes in the learned helplessness (LH) rat, an animal model of depression. The numbers of activated and nonactivated astrocytes in the dentate gyrus (molecular layer, subgranular zone, and hilus), and CA1 and CA3 regions of the hippocampus were significantly increased 2 and 8 days after attainment of LH. Subchronic treatment with imipramine showed a tendency (although not statistically significant) to decrease the LH-induced increment of activated astrocytes in the CA3 region and dentate gyrus. Furthermore, subchronic treatment of naïve rats with imipramine did not alter the numbers of activated and nonactivated astrocytes. However, the antidepressant-like effects of imipramine in the LH paradigm were blocked when fluorocitrate (a reversible inhibitor of astrocyte function) was injected into the dentate gyrus or CA3 region. Injection of fluorocitrate into naive rats failed to induce behavioral deficits in the conditioned avoidance test. These results indicate that astrocytes are responsive to the antidepressant-like effect of imipramine in the dentate gyrus and CA3 region of the hippocampus.

Gliogenesis and glial pathology in depression

Recent research has changed the perception of glia from being no more than silent supportive cells of neurons to being dynamic partners participating in brain metabolism and communication between neurons. This discovery of new glial functions coincides with growing evidence of the involvement of glia in the neuropathology of mood disorders. Unanticipated reductions in the density and number of glial cells are reported in fronto-limbic brain regions in major depression and bipolar illness. Moreover, age-dependent decreases in the density of glial fibrillary acidic protein (GFAP) - immunoreactive astrocytes and levels of GFAP protein are observed in the prefrontal cortex of younger depressed subjects. Since astrocytes participate in the uptake, metabolism and recycling of glutamate, we hypothesize that an astrocytic deficit may account for the alterations in glutamate/GABA neurotransmission in depression. Reductions in the density and ultrastructure of oligodendrocytes are also detected in the prefrontal cortex and amygdala in depression. Pathological changes in oligodendrocytes may be relevant to the disruption of white matter tracts in mood disorders reported by diffusion tensor imaging. Factors such as stress, excess of glucocorticoids, altered gene expression of neurotrophic factors and glial transporters, and changes in extracellular levels of neurotransmitters released by neurons may modify glial cell number and affect the neurophysiology of depression. Therefore, we will explore the role of these events in the possible alteration of glial number and activity, and the capacity of glia as a promising new target for therapeutic medications. Finally, we will consider the temporal relationship between glial and neuronal cell pathology in depression.

Brain cytokines and the 5-HT system during poly I:C-induced fatigue

Fatigue is evoked not only by peripheral factors, such as muscle fatigue, but also by the central nervous system (CNS). For example, it is generally known that the feeling of fatigue is greatly influenced by psychological aspects, such as motivation. However, little is known about the central mechanisms of fatigue. The clinical symptoms of chronic fatigue syndrome (CFS) are shown to include disorders in neuroendocrine, autonomic, and immune systems. On the other hand, it has been demonstrated that cytokines produced in the brain play significant roles in neural-immune interactions through their various central actions, including hypothalamo-pituitary and sympathetic activation, as well as immunosuppression. In this article, using the immunologically induced fatigue model, which was achieved by intraperitoneal (i.p.) injection of synthetic double-stranded RNAs, polyriboinosinic: polyribocytidylic acid (poly I:C) in rats, we show an involvement of brain interferon-alpha (IFN-alpha) and serotonin (5-HT) transporter (5-HTT) in the central mechanisms of fatigue. In the poly I:C-induced fatigue rats, expression of IFN-alpha and 5-HTT increased, while extracellular concentration of 5-HT in the medial prefrontal cortex decreased, probably on account of the enhanced expression of 5-HTT. Since the poly I:C-induced reduction of the running wheel activity was attenuated by a 5-HT(1A) receptor agonist, but not by 5-HT(2), 5-HT(3), or dopamine D(3) receptor agonists, it is suggested that the decrease in 5-HT actions on 5-HT(1A) receptors may at least partly contribute to the poly I:C-induced fatigue.

Sertraline and clomipramine inhibit nucleotide catabolism in rat brain synaptosomes

The effects of sertraline, a selective serotonin reuptake inhibitor, and clomipramine, a tricyclic antidepressant, were tested on ecto-nucleotidases from synaptosomes of cerebral cortex and hippocampus of rats. Sertraline and clomipramine (100-500 microM) inhibited NTPDase, but not ecto-5'-nucleotidase activity in both cerebral cortex and hippocampus. In cortical synaptosomes, sertraline inhibited both ATP and ADP hydrolysis in the concentrations tested. The inhibitory effect varied from 21% to 83% for ATP hydrolysis and 48% to 75% for ADP hydrolysis. The inhibition promoted by sertraline in hippocampal synaptosomes varied from 38% to 89% for ATP hydrolysis and 45% to 77% for ADP hydrolysis. A significant inhibition of cortical NTPDase activity by clomipramine was observed in the all concentrations tested (35-72% and 36-87% for ATP and ADP hydrolysis, respectively). Similar effects were observed in hippocampus (29-91% and 48-83% for ATP and ADP hydrolysis, respectively). There was no inhibitory effect of sertraline and clomipramine on AMP hydrolysis in cerebral cortex and hippocampus. Our results have shown that classical antidepressants inhibit the extracellular catabolism of ATP. Therefore, it is possible to suggest that changes induced by antidepressants on bilayer membrane could affect NTPDase activities and consequently, modulating ATP and adenosine levels in the synaptic cleft.

Enhanced expression of brain interferon-alpha and serotonin transporter in immunologically induced fatigue in rats

Immunologically induced fatigue was induced in rats by intraperitoneal injection of a synthetic double-stranded RNA, polyriboinosinic : polyribocytidylic acid (poly I:C). An injection of poly I:C (3 mg/kg) decreased the daily amounts of spontaneous running wheel activity to approximately 60% of the preinjection level until day 8. Quantitative analysis of mRNA levels demonstrated that interferon-alpha (IFN-alpha) and p38 mitogen-activated protein kinase mRNAs increased in the medial preoptic, paraventricular and ventromedial hypothalamic nuclei and in cortex on both days 1 and 8, while interleukin-1beta and an inhibitor of nuclear factor kappaB (IkappaB)-beta mRNAs increased on day 1, but recovered within a week. Serotonin transporter (5-HTT) mRNA also increased on days 1 and 8 after poly I:C injection in the same brain regions where IFN-alpha mRNA increased. The increased 5-HTT had a functional significance, because in vivo brain microdialysis revealed that an i.p. injection of poly I:C induced a decrease in the extracellular concentration of 5-HT in the prefrontal cortex; the decrease was blocked by local perfusion with a nonselective 5-HT reuptake inhibitor, imipramine. Finally, the poly I:C-induced fatigue was attenuated by a 5-HT1A receptor agonist but not by 5-HT2, 5-HT3 or dopamine D3 agonists. These findings, taken together, suggest that disorders in brain IFN-alpha and 5-HTT expression may be involved in the neuronal mechanisms of the poly I:C-induced fatigue.

Local inhibition of organic cation transporters increases extracellular serotonin in the medial hypothalamus

In the rat dorsomedial hypothalamus (DMH), serotonin (5-HT) concentrations are altered rapidly in response to acute stressors. The mechanism for rapid changes in 5-HT concentrations in the DMH is not clear. We hypothesize that the mechanism involves corticosteroid-induced alterations in the uptake of 5-HT from extracellular fluid through the action of corticosterone-sensitive organic cation transporters (OCTs). To determine if OCTs affect the clearance of 5-HT from the extracellular fluid compartment within the medial hypothalamus (MH), the OCT blocker, decynium 22 (0, 10, 30, or 100 microM), was perfused into the MH via a microdialysis probe, and dialysate 5-HT concentrations were measured at 20 min intervals. In addition, home cage behavior was measured both before and after drug administration. Inhibition of OCTs in the MH resulted in a reversible dose-dependent increase in extracellular 5-HT concentration. Increases in extracellular 5-HT concentrations were associated with increases in grooming behavior in rats treated with the highest concentration of decynium 22. No other behavioral responses were observed following administration of any concentration of decynium 22. These findings are consistent with the hypothesis that OCTs in the MH play an important role in the regulation of serotonergic neurotransmission and specific behavioral responses. Because the MH plays an important role in the neuroendocrine, autonomic, and behavioral responses to stress-related stimuli, these data lead to new questions regarding the role of interactions between corticosterone and corticosterone-sensitive OCTs in stress-induced 5-HT accumulation within the MH as well as the physiological and behavioral consequences of these interactions.

Uptake of serotonin by adult rat corpus callosum is partially reduced by common antidepressants

The corpus callosum (CC) is the main white matter tract involved in interhemispheric brain communication. We establish that uptake of [3H]5-hydroxytryptamine (5-HT) in CC is partially inhibited by some antidepressants. Slices of the adult rat CC had a high-affinity uptake of 5-HT. About 80% of this uptake was Na+ dependent, with a Michaelis-Menten constant, Km, of 420 +/- 80 nM and a rate of 5-HT uptake, Vmax, of 9.5 +/- 0.8 pmol/mg protein/min. The 5-HT uptake was reduced approximately 60% at pH 5 compared with that at pH 7. Fluoxetine (Prozac) inhibited only 43% of 5-HT uptake in a concentration-dependent manner, with an affinity constant, Ki, of 44.7 +/- 10.0 nM. We also studied the effects of other monoamine uptake inhibitors, all at 10 microM, and found that zimelidine, imipramine, and clomipramine inhibited 5-HT uptake in the CC by approximately 30-40%. The fluoxetine-insensitive 5-HT uptake was not altered by high concentrations of dopamine plus norepinephrine. The present data show that Na(+)-dependent 5-HT uptake occurs in the CC and optic nerve and that this uptake is partially sensitive to antidepressants and probably mediated by the serotonin transporter, which may be relevant during depression.

GABAB receptor mRNA in the raphe nuclei: co-expression with serotonin transporter and glutamic acid decarboxylase

We have used double-label in situ hybridization techniques to examine the cellular localization of GABAB receptor mRNA in relation to serotonin transporter mRNA and glutamic acid decarboxylase mRNA in the rat dorsal raphe, median raphe and raphe magnus nuclei. The degree of cellular co-localization of these markers notably varied among the different nuclei. In the dorsal raphe, cell bodies showing GABAB receptor mRNA were very abundant, the 85% being also labelled for serotonin transporter mRNA, and a low proportion (5%) showing glutamic acid decarboxylase mRNA. In the median raphe, the level of co-expression of GABAB receptor mRNA with serotonin transporter mRNA was significantly lower. Some cells were also identified that contained GABAB receptor mRNA in the absence of either one of the other mRNA species studied. Our results support the presence of GABAB receptors in serotonergic as well as GABAergic neurones in the dorsal and median raphe, providing the anatomical basis for the reported dual inhibitory/disinhibitory effect of the GABAB agonist baclofen on serotonergic function.

Functional expression of the norepinephrine transporter in cultured rat astrocytes

We assessed the functional expression of the norepinephrine (NE) transporter (NET) in cultured rat cortical astrocytes. Specific [3H]NE uptake increased in a time-dependent manner, and this uptake involves temperature- and Na+-sensitive mechanisms. The Na+-dependent [3H]NE uptake was saturable, and the Km for the process was 539.3 +/- 55.4 nm and the Vmax was 1.41 +/- 0.03 pmol/mg protein/min. Ouabain, a Na+-K+ ATPase inhibitor, significantly inhibited Na+-dependent [3H]NE uptake. The selective NE uptake inhibitor nisoxetine, the tricyclic antidepressants desipramine and imipramine, and the serotonin and NE reuptake inhibitor (SNRI) milnacipran very potently inhibited Na+-dependent [3H]NE uptake. On the other hand, GBR-12935 (a selective dopamine uptake inhibitor), fluvoxamine (a selective serotonin reuptake inhibitor), venlafaxine (a SNRI) and cocaine had weaker inhibitory activities. RT-PCR demonstrated that astrocytes expressed mRNA for the cloned NET protein, which was characterized as neuronal NET. Western blots indicated that anti-NET polyclonal antibody recognized a major band of 80 kDa in astrocytes. These data indicate that the neuronal NET is functionally expressed in cultured rat astrocytes. Glial cells may exert significant control of noradrenergic activity by inactivating NE that escapes neuronal re-uptake in sites distant from terminals, and are thus cellular targets for antidepressant drugs that inhibit NE uptake.

Selective serotonin reuptake inhibitor paroxetine modulates motor behavior through practice. A double-blind, placebo-controlled, multi-dose study in healthy subjects

We hypothesized that selective serotonin reuptake inhibitors (SSRIs) could modulate motor activity in healthy subjects in a dose-dependent manner. The effects of a single dose of paroxetine were tested in a double-blind, placebo-controlled study. Six randomized and counterbalanced subjects performed behavioral tests in three sessions 1 week apart (E1, E2 and E3) at peak plasma concentration (5 h after drug intake). Each subject was given 20 mg or 60 mg of the drug, or a placebo. Tasks were the Nine Peg Hole test (three trials), Moede dexteritymeter (two trials), and compatible and incompatible reaction time tasks. The results show that at the first trials, performance did not differ after placebo or paroxetine intake. However, 20 and 60 mg of paroxetine improved performance significantly at the third trial of the Nine Peg Hole test and subjects receiving the drug performed 7% faster than those under placebo. An amount of 20 mg, but not 60 mg, of paroxetine improved dexterity significantly at the second trial of the Moede test and subjects performed 30% faster. Conversely, the drug did not affect reaction time for the compatible task and subjects were 11% slower under 20 mg with the incompatible task. Thus, paroxetine decreased the ability to inhibit automatism. Thus, it was concluded that a single dose of paroxetine improved motor performance through practice. But negative effects occurred on tasks including the inhibition of an automatism. Paroxetine enhanced brain motor output (motor activity in S1M1) [NeuroImage, 15 (2002) 26]. This S1M1 hyperactivation is likely to be responsible for the better performance. The brain effect and motor improvement were dose dependent. For both, 20 mg was the optimal dose.

In vitro evidence that 5-hydroxytryptamine increases efflux of glial glutamate via 5-HT(2A) receptor activation

Recent studies have established the presence of 5-hydroxytryptamine (5-HT)(2A) receptors on glial cells in culture and in the brain in situ. Here we used cultured C6 glioma cells to investigate the possibility that 5-HT(2A) receptors on glia regulate glutamate release from the cell. The efflux of endogenous glutamate from cultured C6 glioma cells was increased by addition of 5-HT in a concentration-dependent manner (maximal effect +200%). The efflux of serine and aspartate was not altered. The effect of 5-HT was mimicked by both the nonselective 5-HT receptor agonist quipazine and the selective 5-HT(2) receptor agonist 4-iodo-2,5-dimethoxyamphetamine (DOI; both 0.01-100 microM). The 5-HT(2A) receptor antagonists ketanserin (1 microM) and spiperone (1 microM) inhibited the glutamate response to 5-HT, quipazine, and DOI, whereas the effect of 5-HT was not inhibited by the 5-HT(2B/C) receptor antagonist SB200646 (1 microM). The effect of 5-HT on glutamate was specific in that it was reduced in low-calcium medium but was not prevented by furosemide (5 mM), which prevents cell swelling-induced glutamate release. Finally, the glutamate uptake inhibitor 2,4,trans-pyrollidine dicarboxylic acid (50 microM) did not block the 5-HT-induced efflux of glutamate, making involvement of glutamate transport unlikely. In conclusion, 5-HT stimulates the efflux of glutamate from C6 glioma cells following 5-HT(2A) receptor activation and involves a calcium-dependent mechanism.

Developmental expression of monoamine oxidases A and B in the central and peripheral nervous systems of the mouse

Monoamine oxidases A (MAOA) and B (MAOB) are key players in the inactivation pathway of biogenic amines. Their cellular localization has been well established in the mature brain, but nothing is known concerning the localization of both enzymes during development. We have combined in situ hybridization and histochemistry to localize MAOA and MAOB in the developing nervous system of mice. Our observations can be summarized as five key features. (1) MAOA is tightly linked to catecholaminergic traits. MAOA is expressed in all noradrenergic and adrenergic neurons early on, and in several dopaminergic cell groups such as the substantia nigra. MAOA is also expressed in all the neurons that display a transient tyrosine hydroxylase expression in the brainstem and the amygdala and in neurons with transient dopamine-beta-hydroxylase expression in the cranial sensory ganglia. (2) MAOA and MAOB are coexpressed in the serotoninergic neurons of the raphe from E12 to P7. During postnatal life, MAOA expression declines, whereas MAOB expression remains stable. (3) MAOA is transiently expressed in the cholinergic motor nuclei of the hindbrain, and MAOB is expressed in the forebrain cholinergic neurons. (4) MAOA- and MAOB-expressing neurons are also detected in structures that do not contain aminergic neurons, such as the thalamus, hippocampus, and claustrum. (5) Starting at birth, MAOB expression is found in a variety of nonneuronal cells, the choroid plexus, the ependyma, and astrocytes. These localizations are of importance for understanding the effects of monoaminergic transmission during development.

Pharmacological characterization and visualization of the glial serotonin transporter

Astrocytes contain transport systems that are capable of removing various neurotransmitters from the synaptic cleft by transporters present in the plasma membrane. Glial serotonin transporter (SERT) plays an important role in the re-uptake of 5-hydroxytryptamine (5-HT). We examined the pharmacological characterization of 5-HT uptake into rat cortical synaptosomes and cultured rat astrocytes, and the immunodetection of glial SERT proteins using specific site-directed monoclonal antibodies (MoAb). Furthermore, using a reverse transcriptase-polymerase chain reaction (RT-PCR) method, we addressed the expression of SERT mRNA in cultured rat astrocytes. We investigated the inhibitory effects of various monoamine uptake inhibitors on the uptake of [3H]5-HT into cultured astrocytes and cortical synaptosomes. Tricyclic antidepressants (clomipramine and imipramine) as well as selective serotonin re-uptake inhibitors (fluvoxamine, fluoxetine and zimelidine) were very potent inhibitors of [3H]5-HT uptake in both preparations. In contrast, the inhibitory effects of NE uptake inhibitors (nisoxetine and desipramine) and cocaine were weaker than those of 5-HT uptake inhibitors. In addition, dopamine (DA) uptake inhibitors (nomifensine and GBR-12935) exhibited a Ki value in the low micromolar range. The inhibitory potencies were in the order 5-HT uptake inhibitors (clomipramine, fluvoxamine, fluoxetine, imipramine and zimelidine) > NE uptake inhibitors (nisoxetine and desipramine) = cocaine > DA uptake inhibitors (nomifensine and GBR-12935). There was no difference in the order of the inhibitory effects of various monoamine uptake inhibitors between the two preparations. A correlation analysis of the potencies of various monoamine uptake inhibitors in the inhibition of [3H]5-HT into cultured astrocytes and cortical synaptosomes produced a highly significant correlation coefficient of 0.9893 (P < 0.0001). Immunocytochemical staining using anti-SERT MoAb in cultured astrocytes revealed that the plasma membrane, as well as intracellular, perinuclear compartments, presumably endoplasmic reticulum or golgi membranes, showed a considerable level of immunoreactivity. Extracts of astrocytes and synaptosomes from the cortex were immunoblotted with anti-SERT MoAb. SDS-PAGE/Western blots indicate that anti-SERT MoAb recognized two bands of 120 and 73 kDa in both preparations. RT-PCR demonstrated that astrocytes in cultured expressed mRNA for the cloned SERT protein, which has been characterized as the neuronal SERT. These pharmacological experiments indicate that this uptake process takes place through glial SERT that is very similar to neuronal SERT. Furthermore, the present data also indicate that the presence of the mRNA and protein for the neuronal SERT were established in cultured rat astrocytes, and the polypeptide portion of SERT in astrocytes and frontal cortex could be the same gene product.

Regulation of serotonin transporter gene expression in human glial cells by growth factors

The aims of this study were to identify monoamine transporters expressed in human glial cells, and to examine the regulation of their expression by stress-related growth factors. The expression of serotonin transporter mRNA was detected by reverse transcriptase-polymerase chain reaction in normal human astrocytes, whereas the dopamine transporter (DAT) and the norepinephrine transporter (NET) were not detected. The cDNA sequence of the "glial" serotonin transporter in astrocytes was consistent with that reported for the "neuronal" serotonin transporter (SERT). Moreover, we also demonstrated SERT expression in glial fibrillary acidic protein-positive cells by immunocytochemical staining in normal human astrocytes. Serotonin transporter gene expression was also detected in glioma-derived cell lines (A172, KG-1-C and KGK). Addition of basic fibroblast growth factor (bFGF) or epidermal growth factor (EGF) for 2 days increased serotonin transporter gene expression in astrocytes and JAR (human choriocarcinoma cell line). Basic fibroblast growth factor, but not epidermal growth factor, increased specific [3H]serotonin uptake in astrocytes in a time (1-4 days)- and concentration (20-100 ng/ml)-dependent manner. The expression of genes for basic fibroblast growth factor and epidermal growth factor receptors was detected in astrocytes. These findings suggest that the expression of the serotonin transporter in human glial cells is positively regulated by basic fibroblast growth factor.

Methamphetamine-induced neurotoxicity is attenuated in transgenic mice with a null mutation for interleukin-6

Increasing evidence implicates apoptosis as a major mechanism of cell death in methamphetamine (METH) neurotoxicity. The involvement of a neuroimmune component in apoptotic cell death after injury or chemical damage suggests that cytokines may play a role in METH effects. In the present study, we examined if the absence of IL-6 in knockout (IL-6-/-) mice could provide protection against METH-induced neurotoxicity. Administration of METH resulted in a significant reduction of [(125)I]RTI-121-labeled dopamine transporters in the caudate-putamen (CPu) and cortex as well as depletion of dopamine in the CPu and frontal cortex of wild-type mice. However, these METH-induced effects were significantly attenuated in IL-6-/- animals. METH also caused a decrease in serotonin levels in the CPu and hippocampus of wild-type mice, but no reduction was observed in IL-6-/- animals. Moreover, METH induced decreases in [(125)I]RTI-55-labeled serotonin transporters in the hippocampal CA3 region and in the substantia nigra-reticulata but increases in serotonin transporters in the CPu and cingulate cortex in wild-type animals, all of which were attenuated in IL-6-/- mice. Additionally, METH caused increased gliosis in the CPu and cortices of wild-type mice as measured by [(3)H]PK-11195 binding; this gliotic response was almost completely inhibited in IL-6-/- animals. There was also significant protection against METH-induced DNA fragmentation, measured by the number of terminal deoxynucleotidyl transferase-mediated dUTP nick-end-labeled (TUNEL) cells in the cortices. The protective effects against METH toxicity observed in the IL-6-/- mice were not caused by differences in temperature elevation or in METH accumulation in wild-type and mutant animals. Therefore, these observations support the proposition that IL-6 may play an important role in the neurotoxicity of METH.

Role of uptake inhibition and autoreceptor activation in the control of 5-HT release in the frontal cortex and dorsal hippocampus of the rat

1. Using brain microdialysis, we compared the relative role of 5-hydroxytryptamine (5-HT; serotonin) blockade and somatodendritic 5-HT(1A) and/or terminal 5-HT(1B) autoreceptor activation in the control of 5-HT output. 2. Fluoxetine (10 mg kg(-1) i.p.) doubled the 5-HT output in frontal cortex and dorsal hippocampus. The 5-HT(1A) receptor antagonist WAY 100635, (0.3 mg kg(-1) s.c.) potentiated the effect of fluoxetine only in frontal cortex (to approximately 500 % of baseline). 3. Methiothepin (10 mg kg(-1) s.c.) further enhanced the 5-HT rise induced by fluoxetine+WAY 100635, to 835+/-179% in frontal cortex and 456+/-24% in dorsal hippocampus. Locally applied, methiothepin potentiated the fluoxetine-induced 5-HT rise more in the former area. 4. The selective 5-HT(1B) receptor antagonist SB-224289 (4 mg kg(-1) i.p.) enhanced the effect of fluoxetine (10 mg kg(-1) i.p.) in both areas. As with methiothepin, SB-224289 (4 mg kg(-1) i.p.) further enhanced the 5-HT increase produced by fluoxetine+WAY 100635 more in frontal cortex (613+/-134%) than in dorsal hippocampus (353+/-59%). 5. Locally applied, fluoxetine (10 - 300 microM; EC(50)=28 - 29 microM) and citalopram (1 - 30 microM; EC(50)=1.0 - 1.4 microM) increased the 5-HT output two to three times more in frontal cortex than in dorsal hippocampus. These data suggest that the comparable 5-HT increase produced by systemic fluoxetine in frontal cortex and dorsal hippocampus results from a greater effect of reuptake blockade in frontal cortex that is offset by a greater autoreceptor-mediated inhibition of 5-HT release. As a result, 5-HT autoreceptor antagonists preferentially potentiate the effect of fluoxetine in frontal cortex.

Serotonin metabolism in rat mesangial cells: involvement of a serotonin transporter and monoamine oxidase A

BACKGROUND

Serotonin is one of the factors regulating mesangial cell proliferation, and convergent evidence supports its involvement in the development of glomerulonephritis. In this study, we identified a serotonin transporter and the amine-degrading enzyme monoamine oxidases (MAOs) in mesangial cells, and we studied their involvement in serotonin degradation.

METHODS

MAOs were characterized in membrane preparations and intact mesangial cells by enzyme assay using [14C]5-hydroxytryptamine and [14C]beta-phenylethylamine as specific substrates for MAO-A and MAO-B, respectively, and by Western blot analysis. The expression of a serotonin transporter was determined by [14C]5-hydroxytryptamine uptake experiments and Western blot. Mesangial cell proliferation was measured by BrdU incorporation.

RESULTS

Quantitation of the MAO isoforms by enzyme assay and Western blot analysis showed that MAO-A was largely predominant in mesangial cells, accounting for approximately 90% of the total enzyme population. The MAO substrate [14C]serotonin was transported into mesangial cells by a saturable uptake system (Vmax 310 +/- 36 pmol/30 min/mg protein; Km 5.9 +/- 1.4 microM) displaying the pharmacological properties of a serotonin transporter. The expression of a serotonin transporter was confirmed by Western blot analysis. MAO activity measured in intact cells showed that after accumulation into mesangial cells, [14C]serotonin was metabolized by MAO-A. Finally, serotonin-mediated mesangial cell proliferation was significantly increased after irreversible MAO inhibition.

CONCLUSIONS

Our results suggest that serotonin concentration and function in glomeruli may be regulated in part by its transport into mesangial cells and degradation by MAO-A.

Ultrastructural localization of the serotonin transporter in limbic and motor compartments of the nucleus accumbens

Extracellular levels of serotonin [5-hydroxytryptamine (5-HT)] in the nucleus accumbens (NAc) can influence both cognitive and motor functions involving extensive connections with the frontal cortex. The 5-HT levels reflect vesicular release and plasmalemmal reuptake through the serotonin transporter (SERT). We used electron microscopic immunocytochemistry to determine the sites for SERT activation in the limbic shell and motor-associated core of the rat NAc. Of the SERT-immunoreactive profiles in each region, >90% were serotonergic axons and axon terminals; the remainder were nonserotonergic dendrites and glia. Axonal SERT immunogold labeling was seen mainly at nonsynaptic sites on plasma membranes and often near 5-HT-containing large dense core vesicles (DCVs). SERT-labeled axonal profiles were larger and had a higher numerical density in the shell versus the core but showed no regional differences in their content of SERT immunogold particles. In contrast, immunoreactive dendrites had a lower numerical density in the shell than in the core. SERT labeling in dendrites was localized to segments of plasma membrane near synaptic contacts from unlabeled terminals and/or dendritic appositions. Our results suggest that in the NAc (1) reuptake into serotonergic axons is most efficient after exocytotic release from DCVs, and (2) increased 5-HT release without concomitant increase in SERT expression in individual axons may contribute to higher extracellular levels of serotonin in the shell versus the core. These findings also indicate that SERT may play a minor substrate-dependent role in serotonin uptake or channel activity in selective nonserotonergic neurons and glia in the NAc.

Hypericum LI 160 inhibits uptake of serotonin and norepinephrine in astrocytes

Extracts of Hypericum perforatum, commonly known as St. John's wort, are frequently used in Germany and other European countries to treat mild to moderately severe depression, but the mechanism of antidepressant activity of Hypericum is not understood. Because known mechanisms of antidepressant activity include inhibition of serotonin and/or norepinephrine uptake, we investigated the effects of standardized extracts of Hypericum LI 160 on the transport of these monoamine neurotransmitters into astrocytes, cells which surround synaptic terminals and regulate neurotransmission by means of their uptake systems. We found that LI 160 inhibited both serotonin and norepinephrine uptake in a dose-dependent manner. The two monoamine transport systems were affected differently by LI 160: for serotonin, the main effect was a 50% decrease in the rate of maximal transport, whereas for norepinephrine, the main effect was a 4.5 fold reduction in the apparent affinity of norepinephrine for its uptake sites. Upon removal of LI 160, uptake was restored, thereby indicating that the inhibition was not due to a toxic effect of Hypericum on the cells. These findings suggest that the ability of LI 160 to inhibit serotonin and norepinephrine uptake may underlie the antidepressant activity of this Hypericum extract.

Intraregional variation in expression of serotonin transporter messenger RNA by 5-hydroxytryptamine neurons

The distribution of the messenger RNA encoding the 5-hydroxytryptamine transporter was investigated in rat brain. 5-Hydroxytryptamine transporter messenger RNA was found exclusively in the B1-B9 cell groups containing the cell bodies of 5-hydroxytryptamine neurons. Combined in situ hybridization and 5-hydroxytryptamine immunocytochemistry demonstrated 5-hydroxytryptamine transporter gene expression in the majority of and exclusively in 5-hydroxytryptamine neurons. Cells differed in their levels of expression of 5-hydroxytryptamine transporter messenger RNA and 5-hydroxytryptamine immunofluorescence, but with a tight correlation between the two parameters. Image analysis of cells from B7, the dorsal raphe nucleus, and B8, the median raphe nucleus, revealed significant differences between groups in the mean cellular level of 5-hydroxytryptamine transporter gene expression. Cells in the ventromedial subdivision of B7 displayed higher levels of expression than cells in B8 or cells in the lateral wings of B7. There was also heterogeneity in the distribution of the cellular levels of expression for two other genes expressed by 5-hydroxytryptamine neurons: l-aromatic amino acid decarboxylase messenger RNA and tryptophan hydroxylase messenger RNA. However, the relative levels of expression of these two genes within the four regions studied differed from that of 5-hydroxytryptamine transporter messenger RNA. These results indicate intraregional differences between 5-hydroxytryptamine neurons with respect to 5-hydroxytryptamine transporter messenger RNA levels. Such differences may account for the differential sensitivity of 5-hydroxytryptamine neurons to cytotoxins.

Serotonin transporters in adult rat brain astrocytes revealed by [3H]5-HT uptake into glial plasmalemmal vesicles

Cultured astrocytes derived from neonatal rat brain exhibited high affinity, Na+-dependent, paroxetine and fluoxetine sensitive [3H]5-HT uptake. Reverse transcriptase-PCR demonstrated that astrocytes in culture expressed messenger RNA for the cloned serotonin transporter protein which has been characterised as the neuronal serotonin transporter. Although the serotonin transporter in cultured astrocytes displayed a Km value approximately 10 times greater than found in adult brain synaptosomes, these observations indicated that astrocytes in vitro may express the same serotonin transporter as neurons. Reverse transcriptase-PCR demonstrated the presence of serotonin transporter mRNA in the adult rat cerebral cortex, suggesting that astrocytes in vivo may express low levels of this mRNA. To investigate whether astrocytes in the adult CNS express functional serotonin transporters, glial plasmalemmal vesicles were prepared from cerebral cortex, representing a subcellular fraction composed primarily of vesicles derived from astrocytes. These vesicles were characterised by [3H]-glutamate and [3H]-dopamine uptake and by immunoblot analysis, using glial and synaptic markers: glutamate synthase, SNAP-25 and synaptobrevin. [3H]5-HT was taken up into glial plasmalemmal vesicles in a high affinity (Km approximately 40 nM), Na+ dependent, paroxetine-sensitive manner. The [3H]5-HT uptake capacity (Vmax) in these vesicles was approximately one quarter of that observed in synaptosomes. These data indicate that astrocytes in culture and in vivo are capable of 5-HT uptake via the previously characterised 'neuronal' serotonin transporter.

Localization and dynamic regulation of biogenic amine transporters in the mammalian central nervous system

The monoamines, serotonin, dopamine, norepinephrine, epinephrine and histamine, play a critical role in the function of the hypothalamic-pituitary-adrenal axis and in the integration of information in sensory, limbic, and motor systems. The primary mechanism for termination of monoaminergic neurotransmission is through reuptake of released neurotransmitter by Na+, CI-dependent plasma membrane transporters. A second family of transporters packages monoamines into synaptic and secretory vesicles by exchange of protons. Identification of those cells which express these two families of neurotransmitter transporters is an initial step in understanding what adaptive strategies cells expressing monoamine transporters use to establish the appropriate level of transport activity and thus attain the appropriate efficiency of monoamine storage and clearance. The most recent advances in this field have yielded several surprises about their function, cellular and subcellular localization, and regulation, suggesting that these molecules are not static and most likely are the most important determinants of extracellular levels of monoamines. Here, information on the localization of mRNAs for these transporters in rodent and human brain is summarized along with immunohistochemical information at the light and electron microscopic levels. Regulation of transporters at the mRNA level by manipulation in rodents and differences in transporter site densities by tomographic techniques as an index of regulation in human disease and addictive states are also reviewed. These studies have highlighted the presence of monoamine neurotransmitter transporters in neurons but not in glia in situ. The norepinephrine transporter is present in all cells which are both tyrosine hydroxylase (TH)- and dopamine beta-hydroxylase-positive but not in those cells which are TH- and phenyl-N-methyltransferase-positive, suggesting that epinephrine cells may have their own, unique transporter. In most dopaminergic cells, dopamine transporter mRNA completely overlaps with TH mRNA-positive neurons. However, there are areas in which there is a lack of one to one correspondence. The serotonin transporter (5-HTT) mRNA is found in all raphe nuclei and in the hypothalamic dorsomedial nucleus where the 5-HTT mRNA is dramatically reduced following immobilization stress. The vesicular monoamine transporter 2 (VMAT2) is present in all monoaminergic neurons including epinephrine- and histamine-synthesizing cells. Immunohistochemistry demonstrates that the plasma membrane transporters are present along axons, soma, and dendrites. Subcellular localization of DAT by electron microscopy suggests that these transporters are not at the synaptic density but are confined to perisynaptic areas, implying that dopamine diffuses away from the synapse and that contribution of diffusion to dopamine signalling may vary between brain regions. Interestingly, the presence of VMAT2 in vesicles underlying dendrites, axons, and soma suggests that monoamines may be released at these cellular domains. An understanding of the regulation of transporter function may have important therapeutic consequences for neuroendocrine function in stress and psychiatric disorders.

Basal and stimulated extracellular serotonin concentration in the brain of rats with altered serotonin uptake

We examined the relationship between the density of serotonergic (5-hydroxytryptamine [5-HT]) uptake sites and extracellular 5-HT concentration in the rat brain using microdialysis with two different models, lesions with 5,7-dihydroxytryptamine (50 microg in the dorsal raphe nucleus (DRN) 15 days before) and sublines of rats genetically selected displaying extreme values of platelet 5-HT uptake. Compared to controls, lesioned rats had a reduced cortical concentration of 5-hydroxyindoles (45%), unchanged basal extracellular 5-HT in the DRN and ventral hippocampus (VHPC), and reduced basal 5-hydroxyindoleacetic acid (5-HIAA) concentrations (46%, DRN; 22%, VHPC). Yet the perfusion of 100 mmol/L KCl or 1 micromol/L citalopram elevated dialysate 5-HT significantly more in the DRN and VHPC of controls. In genetically selected rats, platelet 5-HT content and uptake were highly correlated (r2 = 0.9145). Baseline dialysate 5-HT (VHPC) was not different between high and low 5-HT rats and from normal Wistar rats. However, KCl or citalopram perfusion increased dialysate 5-HT significantly more in high 5-HT than in low 5-HT rats, and the former displayed a greater in vivo tissue 5-HT recovery. Significant but small differences in the same direction were noted in [3H]citalopram binding in several brain areas, as measured autoradiographically. Thus, basal extracellular 5-HT (but not 5-HIAA) concentrations are largely independent on the density of serotonergic innervation and associated changes in uptake sites. However, marked differences emerge during axonal depolarization or reuptake blockade. The significance of these findings for the treatment of mood disorders in patients with neurological disorders is discussed.