Dendritic colocalisation of serotonin1B receptors and the glutamate NMDA receptor subunit NR1 within the hippocampal dentate gyrus: An ultrastructural study

Serotonin modulation of hippocampal functions: From anatomy to neurotherapeutics

The hippocampal region receives a dense serotoninergic innervation originating from both medial and dorsal raphe nuclei. This innervation regulates hippocampal activity through the activation of distinct receptor families that are expressed in excitatory and inhibitory neurons, terminals of several afferent neurotransmitter systems, and glial cells. Preclinical and clinical studies indicate that hippocampal dysfunctions are involved in learning and memory deficits, dementia, Alzheimer's disease, epilepsy and mood disorders such as anxiety, depression and post-traumatic syndrome disorder, whereas the hippocampus participates also in the therapeutic mechanisms of numerous medicines. Not surprisingly, several drugs acting via 5-HT mechanisms are efficacious to some extent in some diseases and the link between 5-HT and the hippocampus although clear remains difficult to untangle. For this reason, we review reported data concerning the distribution and the functional roles of the 5-HT receptors in the hippocampal region in health and disease. The impact of the 5-HT systems on the hippocampal function is such that the research of new 5-HT mechanisms and drugs is still very active. It concerns notably drugs acting at the 5-HT_1A,_2A,_2C,_4,6 receptor subtypes, in addition to the already existing drugs including the selective serotonin reuptake inhibitors.

Modulation of NMDA Receptors by G-protein-coupled receptors: Role in Synaptic Transmission, Plasticity and Beyond

NMDA receptors (NMDARs) play a critical role in excitatory synaptic transmission, plasticity and in several forms of learning and memory. In addition, NMDAR dysfunction is believed to underlie a number of neuropsychiatric conditions. Growing evidence has demonstrated that NMDARs are tightly regulated by several G-protein-coupled receptors (GPCRs). Ligands that bind to GPCRs, such as neurotransmitters and neuromodulators, activate intracellular pathways that modulate NMDAR expression, subcellular localization and/or functional properties in a short- or a long-term manner across many synapses throughout the central nervous system. In this review article we summarize current knowledge on the molecular and cellular mechanisms underlying NMDAR modulation by GPCRs, and we discuss the implications of this modulation spanning from synaptic transmission and plasticity to circuit function and brain disease.

Modulation of Kalirin-7 expression by hippocampal CA1 5-HT1B receptors in spatial memory consolidation

Serotonin 5-HT1B receptors (5-HT1BRs) are distributed in hippocampal CA1 and play a pivotal role in cognitive function. Activation of 5-HT1BRs regulates synaptic plasticity at the excitatory synapses in the hippocampus. However, the role and its underlying mechanism of 5-HT1BR activation-mediated glutamatergic synaptic plasticity in spatial memory are not fully understood. In this study, spatial memory of Sprague-Dawley (SD) rats was assessed in a Morris water maze after bilateral dorsal hippocampal CA1 infusion of the 5-HT1BR antagonist GR55562 (25 μg/μL) or agonist CP93129 (25 μg/μL). GR55562 did not affect the spatial memory acquisition but significantly increased the target quadrant preference during the memory consolidation probe performed 14 d after the training session, while CP93129 impaired the memory consolidation process. Moreover, GR55562 significantly increased, while CP93129 significantly decreased, the density of dendritic spines on the distal apical dendrites of CA1 pyramidal neurons. Furthermore, western blot experiments indicated that GR55562 significantly increased, but CP93129 significantly reduced, the expression of Kalirin-7 (Kal-7), PSD95, and GluA2/3 subunits of AMPA receptors. Our results suggest that Kal-7 and Kal-7-mediatedalteration of AMPA receptor subtype expression may play crucial roles in the impact of hippocampal CA1 5-HT1BR activation on spatial memory consolidation.

S-Ketamine Mediates Its Acute and Sustained Antidepressant-Like Activity through a 5-HT1B Receptor Dependent Mechanism in a Genetic Rat Model of Depression

Rationale: The mechanisms responsible for the unique antidepressant properties of ketamine have only been partly resolved. Recent preclinical reports implicate the neurotransmitter serotonin [5-hydroxytryptamine (5-HT)] in the antidepressant-like response of ketamine, and modulation of 5-HT_1B receptors has been hypothesized to attain an important role.

Objectives: To evaluate the role of endogenous stimulation of 5-HT_1B heteroreceptors in the antidepressant-like activity of S-ketamine.

Method: Flinders sensitive line (FSL) rats, a genetic model of depression, were depleted of endogenous 5-HT by 4-chloro-DL-phenylalanine methyl ester HCl administration (pCPA; 86 mg/kg/day for 3 days). In pCPA-pretreated and control FSL rats, the acute and sustained effects of a single dose of S-ketamine (15 mg/kg) and the selective 5-HT_1B receptor agonist CP94253 (1–6 mg/kg) alone and in combination with S-ketamine were studied in the forced swim test (FST), a commonly used assay that detects antidepressant activity.

Results: pCPA pretreatment decreased cortical 5-HT levels to ∼6% but did not affect the baseline behavioral phenotype of FSL rats. S-ketamine demonstrated acute and sustained antidepressant-like activity, both of which were abolished by 5-HT depletion. Combining S-ketamine with a sub-effective dose of CP94253 (1 mg/kg) rescued S-ketamine’s acute and sustained antidepressant-like effects, when CP94253 was administered 2 h prior to the FST. Co-administration of S-ketamine and CP94253 did not affect the plasma level of either compound, suggesting that the observed behavioral interaction could not be ascribed to a kinetic drug-drug interaction.

Conclusion: 5-HT_1B receptor activation during testing appears to be critical for S-ketamine’s antidepressant-like potentials in this model.

[Pharmacological properties of vortioxetine and its pre-clinical consequences]

Selective Serotonin Reuptake Inhibitors (SSRIs) are extensively used for the treatment of major depressive disorder (MDD). SSRIs are defined as indirect receptor agonists since the activation of postsynaptic receptors is a consequence of an increase in extracellular concentrations of serotonin (5-HT) mediated by the blockade of serotonin transporter. The activation of some serotoninergic receptors (5-HT1A, post-synaptic, 5-HT1B post-synaptic, 5-HT2B, and 5-HT4), but not all (5-HT1A, pre-synaptic, 5-HT1B pre-synaptic, 5-HT2A, 5-HT2C, 5-HT3, and probably 5-HT6), induces anxiolytic/antidepressive - like effects. Targetting specifically some of them could potentially improve the onset of action and/or efficacy and/or prevent MD relapse. Vortioxetine (Brintellix, 1- [2-(2,4-dimethylphenyl-sulfanyl)-phenyl]-piperazine) is a novel multi-target antidepressant drug approved by the Food and Drug Administration (FDA) and by European Medicines Agency. Its properties are markedly different from the extensively prescribed SSRIs. Compared to the SSRIs, vortioxetine is defined as a multimodal antidepressant drug since it is not only a serotonin reuptake inhibitor, but also a 5-HT1D, 5-HT3, 5-HT7 receptor antagonist, 5-HT1B receptor partial agonist and 5-HT1A receptor agonist. This specific pharmacological profile enables vortioxetine to affect not only the serotoninergic and noradrenergic systems, but also the histaminergic, cholinergic, gamma-butyric acid (GABA) ergic and glutamatergic ones. Thus, vortioxetine not only induces antidepressant-like or anxiolytic-like activity but also improves cognitive parameters in several animal models. Indeed, vortioxetine was shown to improve working memory, episodic memory, cognitive flexibility and spatial memory in young adult rodents and also in old animal models. These specific effects of the vortioxetine are of interest considering that cognitive dysfunction is a common comorbidity to MDD. Altogether, even though this molecule still needs to be investigated further, especially in the insufficient-response to antidepressant drugs, vortioxetine is already an innovative therapeutic option for the treatment of major depression.

Regional distribution of serotonergic receptors: a systems neuroscience perspective on the downstream effects of the multimodal-acting antidepressant vortioxetine on excitatory and inhibitory neurotransmission

Previous work from this laboratory hypothesized that the multimodal antidepressant vortioxetine enhances cognitive function through a complex mechanism, using serotonergic (5-hydroxytryptamine, 5-HT) receptor actions to modulate gamma-butyric acid (GABA) and glutamate neurotransmission in key brain regions like the prefrontal cortex (PFC) and hippocampus. However, serotonergic receptors have circumscribed expression patterns, and therefore vortioxetine's effects on GABA and glutamate neurotransmission will probably be regionally selective. In this article, we attempt to develop a conceptual framework in which the effects of 5-HT, selective serotonin reuptake inhibitors (SSRIs), and vortioxetine on GABA and glutamate neurotransmission can be understood in the PFC and striatum-2 regions with roles in cognition and substantially different 5-HT receptor expression patterns. Thus, we review the anatomy of the neuronal microcircuitry in the PFC and striatum, anatomical data on 5-HT receptor expression within these microcircuits, and electrophysiological evidence on the effects of 5-HT on the behavior of each cell type. This analysis suggests that 5-HT and SSRIs will have markedly different effects within the PFC, where they will induce mixed effects on GABA and glutamate neurotransmission, compared to the striatum, where they will enhance GABAergic interneuron activity and drive down the activity of medium spiny neurons. Vortioxetine is expected to reduce GABAergic interneuron activity in the PFC and concomitantly increase cortical pyramidal neuron firing. However in the striatum, vortioxetine is expected to increase activity at GABAergic interneurons and have mixed excitatory and inhibitory effects in medium spiny neurons. Thus the conceptual framework developed here suggests that vortioxetine will have regionally selective effects on GABA and glutamate neurotransmission.

Modes and nodes explain the mechanism of action of vortioxetine, a multimodal agent (MMA): modifying serotonin's downstream effects on glutamate and GABA (gamma amino butyric acid) release

Vortioxetine is an antidepressant with multiple pharmacologic modes of action at targets where serotonin neurons connect with other neurons. These actions modify the release of both glutamate and GABA (gamma amino butyric acid) within various brain circuits.

Serotonergic Regulation of Prefrontal Cortical Circuitries Involved in Cognitive Processing: A Review of Individual 5-HT Receptor Mechanisms and Concerted Effects of 5-HT Receptors Exemplified by the Multimodal Antidepressant Vortioxetine

It has been known for several decades that serotonergic neurotransmission is a key regulator of cognitive function, mood, and sleep. Yet with the relatively recent discoveries of novel serotonin (5-HT) receptor subtypes, as well as an expanding knowledge of their expression level in certain brain regions and localization on certain cell types, their involvement in cognitive processes is still emerging. Of particular interest are cognitive processes impacted in neuropsychiatric and neurodegenerative disorders. The prefrontal cortex (PFC) is critical to normal cognitive processes, including attention, impulsivity, planning, decision-making, working memory, and learning or recall of learned memories. Furthermore, serotonergic dysregulation within the PFC is implicated in many neuropsychiatric disorders associated with prominent symptoms of cognitive dysfunction. Thus, it is important to better understand the overall makeup of serotonergic receptors in the PFC and on which cell types these receptors mediate their actions. In this Review, we focus on 5-HT receptor expression patterns within the PFC and how they influence cognitive behavior and neurotransmission. We further discuss the net effects of vortioxetine, an antidepressant acting through multiple serotonergic targets given the recent findings that vortioxetine improves cognition by modulating multiple neurotransmitter systems.

Serotonergic modulation of glutamate neurotransmission as a strategy for treating depression and cognitive dysfunction

Monoamine-based treatments for depression have evolved greatly over the past several years, but shortcomings such as suboptimal efficacy, treatment lag, and residual cognitive dysfunction are still significant. Preclinical and clinical studies using compounds directly targeting glutamatergic neurotransmission present new opportunities for antidepressant treatment, with ketamine having a surprisingly rapid and sustained antidepressant effect that is presumably mediated through glutamate-dependent mechanisms. While direct modulation of glutamate transmission for antidepressant and cognition-enhancing actions may be hampered by nonspecific effects, indirect modulation through the serotonin (5-HT) system may be a viable alternative approach. Based on localization and function, 5-HT can modulate glutamate neurotransmission at least through the 5-HT1A, 5-HT1B, 5-HT3, and 5-HT7 receptors, which presents a rational pharmacological opportunity for modulating glutamatergic transmission without the direct use of glutamatergic compounds. Combining one or more of these glutamate-modulating 5-HT targets with 5-HT transporter inhibition may offer new therapeutic opportunities. The multimodal compounds vortioxetine and vilazodone are examples of this approach with diverse mechanisms, and their different clinical effects will provide valuable insights into serotonergic modulation of glutamate transmission for the potential treatment of depression and associated cognitive dysfunction.

Bidirectional regulation of emotional memory by 5-HT1B receptors involves hippocampal p11

Cognitive impairments are common in depression and involve dysfunctional serotonin neurotransmission. The 5-HT1B receptor (5-HT(1B)R) regulates serotonin transmission, via presynaptic receptors, but can also affect transmitter release at heterosynaptic sites. This study aimed at investigating the roles of the 5-HT(1B)R, and its adapter protein p11, in emotional memory and object recognition memory processes by the use of p11 knockout (p11KO) mice, a genetic model for aspects of depression-related states. 5-HT(1B)R agonist treatment induced an impairing effect on emotional memory in wild type (WT) mice. In comparison, p11KO mice displayed reduced long-term emotional memory performance. Unexpectedly, 5-HT(1B)R agonist stimulation enhanced memory in p11KO mice, and this atypical switch was reversed after hippocampal adeno-associated virus mediated gene transfer of p11. Notably, 5-HT(1B)R stimulation increased glutamatergic neurotransmission in the hippocampus in p11KO mice, but not in WT mice, as measured by both pre- and postsynaptic criteria. Magnetic resonance spectroscopy demonstrated global hippocampal reductions of inhibitory GABA, which may contribute to the memory enhancement and potentiation of pre- and post-synaptic measures of glutamate transmission by a 5-HT(1B)R agonist in p11KO mice. It is concluded that the level of hippocampal p11 determines the directionality of 5-HT(1B)R action on emotional memory processing and modulates hippocampal functionality. These results emphasize the importance of using relevant disease models when evaluating the role of serotonin neurotransmission in cognitive deficits related to psychiatric disorders.

A noncanonical postsynaptic transport route for a GPCR belonging to the serotonin receptor family

Postsynaptic receptor trafficking plays an essential role in tuning neurotransmission and signal plasticity and has emerged as a potential therapeutic target in neuropsychiatric disease. Using a novel application of fluorescence recovery after photobleaching in rat hippocampal neurons, we examined transport from the soma to dendrites of seven G-protein-coupled receptors (GPCRs) implicated in mood disorders. Most GPCRs were delivered to dendrites via lateral diffusion, but one GPCR, the serotonin 1B receptor (5-HT(1B)), was delivered to the dendrites in secretory vesicles. Within the dendrites, 5-HT(1B) were stored in a reservoir of accessible vesicles that were recruited to preferential sites in plasma membrane, as observed with superecliptic pHluorin labeling. After membrane recruitment, 5-HT(1B) transport via lateral diffusion and temporal confinement to inhibitory and excitatory synapses was monitored by single particle tracking. These results suggest an alternative mechanism for control of neuronal activity via a GPCR that has been implicated in mood regulation.

Increased hippocampal CA1 density of serotonergic terminals in a triple transgenic mouse model of Alzheimer's disease: an ultrastructural study

Alzheimer's disease (AD) is a neurodegenerative pathology that deteriorates mnesic functions and associated brain regions including the hippocampus. Serotonin (5-HT) has an important role in cognition. We recently demonstrated an increase in 5-HT transporter (SERT) fibre density in the hippocampal CA1 in an AD triple transgenic mouse model (3xTg-AD). Here, we analyse the ultrastructural localisation, distribution and numerical density (N_v) of hippocampal SERT axons (SERT-Ax) and terminals (SERT-Te) and their relationship with SERT fibre sprouting and altered synaptic N_v in 3xTg-AD compared with non-transgenic control mice. 3xTg-AD animals showed a significant increase in SERT-Te N_v in CA1 at both, 3 (95%) and 18 months of age (144%), being restricted to the CA1 stratum moleculare (S. Mol; 227% at 3 and 180% at 18 months). 3xTg-AD animals also exhibit reduced N_v of perforated axospinous synapses (PS) in CA1 S. Mol (56% at 3 and 52% at 18 months). No changes were observed in the N_v of symmetric and asymmetrical synapses or SERT-Ax. Our results suggest that concomitant SERT-Te N_v increase and PS reduction in 3xTg-AD mice may act as a compensatory mechanism maintaining synaptic efficacy as a response to the AD cognitive impairment.

Involvement of 5-HT2A receptors in MDMA reinforcement and cue-induced reinstatement of MDMA-seeking behaviour

The serotonergic system appears crucial for (±)-3,4-methylenedioxymethamphetamine (MDMA) reinforcing properties. Current evidence indicates that serotonin 5-HT2A receptors (5-HT2ARs) modulate mesolimbic dopamine (DA) activity and several behavioural responses related to the addictive properties of psychostimulants. This study evaluated the role of 5-HT2ARs in MDMA-induced reinforcement and hyperlocomotion, and the reinstatement of MDMA-seeking behaviour. Basal and MDMA-stimulated extracellular levels of DA in the nucleus accumbens (NAc) and serotonin and noradrenaline in the prefrontal cortex were also assessed. Self-administration of MDMA was blunted in 5-HT2AR knockout (KO) mice compared to wild-type (WT) littermates at both doses tested (0.125 and 0.25 mg/kg per infusion). Horizontal locomotion was increased by MDMA (10 and 20 mg/kg i.p.) to a higher extent in KO than in WT mice. DA outflow in the NAc was lower in KO compared to WT mice under basal conditions and after MDMA (20 mg/kg) challenge. In WT mice, MDMA (5 and 10 mg/kg i.p.) priming did not reinstate MDMA-seeking behaviour, while cue-induced reinstatement was prominent. This cue-induced reinstatement was blocked by administration of the selective 5-HT2AR antagonist, SR46349B (eplivanserin) at a dose of 0.5 mg/kg, but not at 0.25 mg/kg. Our results indicate that 5-HT2ARs are crucial for MDMA-induced reinforcement and cue-induced reinstatement of MDMA-seeking behaviour. These effects are probably due to the modulation of mesolimbic dopaminergic activity.

Presynaptic NMDARs in the hippocampus facilitate transmitter release at theta frequency

A rise in [Ca(2+)](i) provides the trigger for neurotransmitter release at neuronal boutons. We have used confocal microscopy and Ca(2+) sensitive dyes to directly measure the action potential-evoked [Ca(2+)](i) in the boutons of Schaffer collaterals. This reveals that the trial-by-trial amplitude of the evoked Ca(2+) transient is bimodally distributed. We demonstrate that "large" Ca(2+) transients occur when presynaptic NMDA receptors are activated following transmitter release. Presynaptic NMDA receptor activation proves critical in producing facilitation of transmission at theta frequencies. Because large Ca(2+) transients "report" transmitter release, their frequency on a trial-by-trial basis can be used to estimate the probability of release, p(r). We use this novel estimator to show that p(r) increases following the induction of long-term potentiation.

N-methyl-D-aspartate receptor-mediated modulations of the anti-allodynic effects of 5-HT1B/1D receptor stimulation in a rat model of trigeminal neuropathic pain

Previous studies showed that triptans and other 5-HT(1B/1D)-receptor agonists attenuate hyper-responsiveness to mechanical stimulation of the face in a rat model of trigeminal neuropathic pain, probably by activating 5-HT(1B/1D)-receptors on primary afferent nociceptive fibers. We now tested whether blockade of post-synaptic receptors for the excitatory amino acid glutamate released by these fibers would increase this action. We thus evaluated whether (±)1-hydroxy-3-aminopyrrolidine-2-one (HA-966), an antagonist at the glycine/D-serine site of N-methyl-D-aspartate (NMDA)-receptors, would potentiate the anti-allodynic action of dihydroergotamine and zolmitriptan in rats with chronic constriction injury to the infraorbital nerve (CCI-ION). Complementary studies were performed with other NMDA-receptor ligands and in rats with chronic constriction injury to the sciatic nerve (CCI-SN) for comparison. Injury was produced by loose ligatures of the nerves. Responsiveness to mechanical stimulation (vibrissae or hindpaw territories) with von Frey filaments was used to evaluate allodynia 2 weeks after nerve ligature. Rats received NMDA-receptor ligands or saline 20 min before dihydroergotamine (25-100 μg/kg, i.v.) or zolmitriptan (25-100 μg/kg, s.c.). HA-966 (2.5mg/kg, s.c.), inactive on its own, enhanced the anti-allodynic effects of dihydroergotamine (eightfold increase) and zolmitriptan (threefold increase) in CCI-ION rats, but these drugs exerted no effects in allodynic CCI-SN rats. NMDA-receptor blockade by memantine (5mg/kg, i.p.) also enhanced, whereas activation at glycine/NMDA site by D-cycloserine (3mg/kg, i.p.) reduced the anti-allodynic properties of zolmitriptan in CCI-ION rats. Combined administration of NMDA-receptor antagonist and 5-HT(1B/1D)-receptor agonist may be a promising approach for alleviating trigeminal neuropathic pain.

A subpopulation of serotonin 1B receptors colocalize with the AMPA receptor subunit GluR2 in the hippocampal dentate gyrus

The serotonin(1B) receptor (5-HT(1B)R) plays a role in cognitive processes that also involve glutamatergic neurotransmission via amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid (AMPA) receptors. Accumulating experimental evidence also highlights the involvement of 5-HT(1B)Rs in several neurological disorders. Consequently, the 5-HT(1B)R is increasingly implicated as a potential therapeutic target for intervention in cognitive dysfunction. Within the hippocampus, a brain region critical to cognitive processing, populations of pre- and post-synaptic 5-HT(1B)Rs have been identified. Thus, 5-HT(1B)Rs could have a role in the modulation of hippocampal pre- and post-synaptic conductance. Previously, we demonstrated colocalization of 5-HT(1B)Rs with the N-methyl-D-aspartate (NMDA) receptor subunit NR1 in a subpopulation of granule cell dendrites (Peddie et al. [53]). In this study, we have examined the cellular and subcellular distribution of 5-HT(1B)Rs with the AMPA receptor subunit GluR2. Of 5-HT(1B)R positive profiles, 28% displayed colocalization with GluR2. Of these, 87% were dendrites, corresponding to 41% and 10% of all 5-HT(1B)R labeled or GluR2 labeled dendrites, respectively. Dendritic labeling was both cytoplasmic and membranous but was not usually associated with synaptic sites. Colocalization within dendritic spines and axons was comparatively rare. These findings indicate that within the dentate gyrus molecular layer, dendritic 5-HT(1B)Rs are expressed predominantly on GluR2 negative granule cell processes. However, a subpopulation of 5-HT(1B)Rs is expressed on GluR2 positive dendrites. Here, it is suggested that activation of the 5-HT(1B)R may play a role in the modulation of AMPA receptor mediated conductance, further supporting the notion that the 5-HT(1B)R represents an interesting therapeutic target for modulation of cognitive function.

Serotonin controls the maturation of the GABA phenotype in the ventral spinal cord via 5-HT1b receptors

Serotonin (5-hydroxytryptamine or 5-HT) is a pleiotropic neurotransmitter known to play a crucial modulating role during the construction of brain circuits. Descending bulbo-spinal 5-HT fibers, coming from the caudal medullary cell groups of the raphe nuclei, progressively invade the mouse spinal cord and arrive at lumbar segments at E15.5 when the number of ventral GABA immunoreactive (GABA-ir) interneurons reaches its maximum. We thus raised the question of a possible interaction between these two neurotransmitter systems and investigated the effect of 5-HT descending inputs on the maturation of the GABA phenotype in ventral spinal interneurons. Using a quantitative anatomical study performed on acute and cultured embryonic mouse spinal cord, we found that the GABAergic neuronal population matured according to a similar rostro-caudal gradient both in utero and in organotypic culture. We showed that 5-HT delayed the maturation of the GABA phenotype in lumbar but not brachial interneurons. Using pharmacological treatments and mice lacking 5-HT(1B) or 5-HT(1A), we demonstrated that the 5-HT repressing effect on the GABAergic phenotype was specifically attributed to 5-HT(1B) receptors.

Serotonin fibre sprouting and increase in serotonin transporter immunoreactivity in the CA1 area of hippocampus in a triple transgenic mouse model of Alzheimer's disease

Alzheimer's disease (AD) is a neurodegenerative disease that deteriorates cognitive functions and associated brain regions such as the hippocampus, being the primary cause of dementia. Serotonin (5-HT) is widely present in the hippocampus, being an important neurotransmitter involved in learning and memory. Although recent evidence suggests alterations in 5-HT neurotransmission in AD, it is not clear how hippocampal 5-HT innervation is modified. Here, we studied hippocampal 5-HT innervation by analysing: (i) the expression, density and distribution of 5-HT transporter (SERT)-immunoreactive fibres; (ii) the specific morphological characteristics of serotonergic fibres and their relation to amyloid plaques; and (iii) the total number of 5-HT neurons within the raphe nuclei in triple transgenic mouse model of AD. We used quantitative light microscopy immunohistochemistry comparing transgenic and non-transgenic animals of different ages (3, 6, 9, 12 and 18 months). The transgenic animals showed a significant increase in SERT fibres in the hippocampus in a subfield-, strata- and age-specific manner. The increase in SERT fibres was specific to the CA1 stratum lacunosum-moleculare. An increase in SERT fibres in transgenic animals was observed at 3 months (by 61%) and at 18 months (by 74%). No changes, however, were found in the total number of raphe 5-HT neurons at any age. Our results indicate that triple transgenic mice display changes in the expression of SERT and increased SERT fibres sprouting, which may account for imbalanced serotonergic neurotransmission associated with (or linked to) AD cognitive impairment.

Synapse regression in depression: the role of 5-HT receptors in modulating NMDA receptor function and synaptic plasticity

Depression is accompanied by an increase in activity in the amygdala and a decrease in the rostral anterior cingulate cortex (rACC), with the former attributed to a failure of the latter to exert its normal inhibitory influence. This failure is likely due to regression of synaptic connections between the rACC and the amygdala, a process reversed in part by selective serotonin reuptake inhibitors (SSRIs). The present work presents a hypothesis as to how SSRIs might bring about this process and hence normalization of activity, at least in patients that are responsive to SSRIs. Serotonin receptors of the excitatory 5-HT(2A)R class increase N-methyl-D-aspartate receptor (NMDAR) efficacy, while those of the inhibitory 5-HT(1A)R class decrease NMDAR efficacy. A decrease of 5-HT transporter (5-HTT) efficacy, either during human development through functional polymorphisms, or in animals through 5-HTT transgenic knockouts, is accompanied by a decrease in 5-HT(1A)R and hence an increase in excitability and NMDAR efficacy which drives an increase in synaptic spines in the amygdala. As the limbic region of the brain normally possesses high levels of 5-HT(1A)R the effect of loss of these is to increase excitation in this region, as is observed. Changes in the level of extracellular 5-HT in adult animals also modulates the density of synaptic spines, with these increasing with an increase in 5-HT, possibly as a consequence of increases in 5-HT(2A)R activity over that of 5-HT(1A)R. Increasing extracellular levels of 5-HT with SSRIs would then lead to an increase in excitability and in synaptic spines for afferents in the dorsal rostral anterior cingulate cortex but not in the ventral regions such as the amygdala that have few 5-HT(2A)R. This allows dorsal regions to once more exert their inhibitory influence over ventral regions. In this way, SSRIs may exert their effect in normalizing dorsal hypometabolism and ventral hypermetabolism in those suffering from depression.

Postnatal changes in the expressions of serotonin 1A, 1B, and 2A receptors in ten brain stem nuclei of the rat: implication for a sensitive period

A critical period in respiratory network development occurs in the rat around postnatal days (P) 12-13, when abrupt neurochemical, metabolic, and physiological changes were evident. As serotonin and its receptors are involved in respiratory modulation, and serotonergic abnormality is implicated in sudden infant death syndrome, we hypothesized that 5-HT receptors are significantly downregulated during the critical period. This was documented recently for 5-HT(2A)R in several respiratory nuclei. The present study represents a comprehensive analysis of postnatal development of 5-HT(1A)R and 5-HT(1B)R in 10 brain stem nuclei and 5-HT(2A)R in six nuclei not previously examined. Optical densitometric analysis of immunohistochemically-reacted neurons from P2 to P21 indicated four developmental patterns of expression: (1) Pattern I: a high level of expression at P2-P11, an abrupt and significant reduction at P12, followed by a plateau until P21 (5-HT(1A)R and 5-HT(1B)R in raphé magnus [RM], raphé obscurus [ROb], raphé pallidus [RP], pre-Bötzinger complex [PBC], nucleus ambiguus [Amb], and hypoglossal nucleus [XII; 5-HT(1A)R only]). (2) Pattern II: a high level at P2-P9, a gradual decline from P9 to P12, followed by a plateau until P21 (5-HT(1A)R and 5-HT(1B)R in the retrotrapezoid nucleus (RTN)/parafacial respiratory group (pFRG)). (3) Pattern III: a high level at P2-P11, followed by a gradual decline until P21 (5-HT(1A)R in the ventrolateral subnucleus of solitary tract nucleus [NTS(VL)] and the non-respiratory cuneate nucleus [CN]). (4) Pattern IV: a relatively constant level maintained from P2 to P21 (5-HT(1A)R in the commissural subnucleus of solitary tract nucleus (NTS(COM)); 5-HT(1B)R in XII, NTS(VL), NTS(COM), and CN; and 5-HT(2A)R in RM, ROb, RP, RTN/pFRG, NTS(VL), and NTS(COM)). Thus, a significant reduction in the expression of 5-HT(1A)R, 5-HT(1B)R, and 5-HT(2A)R in multiple respiratory-related nuclei at P12 is consistent with reduced serotonergic transmission during the critical period, thereby rendering the animals less able to respond adequately to ventilatory distress.