Effects of Maternal Ethanol Consumption and Buspirone Treatment on 5-HT1A and 5-HT2A Receptors in Offspring

In utero ethanol exposure results in a decreased concentration of serotonin (5-HT) in brain regions containing the cell bodies of 5-HT neurons and their cortical projections. The concentration of 5-HT reuptake sites is also reduced in several brain areas. The present study extended prior work by evaluating the effects of chronic maternal ethanol consumption and maternal buspirone treatment on 5-HT_1A and 5-HT_2A receptors in multiple brain areas of offspring. Receptors were quantitated early in postnatal development and at an age when the 5-HT networks are normally well-established. Because fetal 5-HT functions as an essential neurotrophic factor, these studies also determined whether treatment of pregnant rats with buspirone, a 5-HT_1A agonist, could overcome the effects of the fetal 5-HT deficit and prevent ethanol-associated receptor abnormalities. The results demonstrated that in utero ethanol exposure significantly alters the binding of 0.1 nM [³H]-8-hydroxy-dipropylaminotetralin to 5-HT_1A receptors in developing animals. Ethanol impaired the development of 5-HT_1A receptors in the frontal cortex, parietal cortex, and lateral septum; these receptors did not undergo the normal developmental increase between postnatal days 19 and 35. The dentate gyms was also sensitive to the effects of in utero ethanol exposure. 5-HT_1A receptors were increased in this region at 19 days. Maternal buspirone treatment prevented the ethanol-associated abnormalities in 5-HT_1A receptors in the dentate gyms, frontal cortex, and lateral septum. Neither maternal ethanol consumption nor buspirone treatment altered the binding of 2 nM [³H]ketanserin to 5-HT_2A receptors in the ventral dentate gyrus, dorsal raphe, parietal and frontal cortexes, striatum, substantia nigra, or nucleus accumbens.     

Impaired hippocampal-dependent learning and functional abnormalities in the hippocampus in mice lacking serotonin(1A) receptors

The hippocampus is a major limbic target of the brainstem serotonergic neurons that modulate fear, anxiety, and learning through postsynaptic serotonin(1A) receptors (5-HT(1A) receptors). Because chronic stress selectively down-regulates the 5-HT(1A) receptors in the hippocampus, we hypothesized that mice lacking these receptors may exhibit abnormalities reminiscent of symptoms of stress-related psychiatric disorders. In particular, a hippocampal deficit in the 5-HT(1A) receptor could contribute to the cognitive abnormalities often seen in these disorders. To test whether a deficit in 5-HT(1A) receptors impairs hippocampus-related functions, we studied hippocampal-dependent learning and memory, synaptic plasticity in the hippocampus, and limbic neuronal excitability in 5-HT(1A)-knockout (KO) mice. 5-HT(1A)-KO animals showed a deficit in hippocampal-dependent learning and memory tests, such as the hidden platform (spatial) version of the Morris water maze and the delayed version of the Y maze. The performance of KO mice was not impaired in nonhippocampal memory tasks such as the visible platform (nonspatial) version of the Morris water maze, the immediate version of the Y maze, and the spontaneous-alternation test of working memory. Furthermore, paired-pulse facilitation in the dentate gyrus of the hippocampus was impaired in 5-HT(1A)-KO mice. Finally, 5-HT(1A)-KO mice, as compared with wild-type animals, displayed higher limbic excitability manifested as lower seizure threshold and higher lethality in response to kainic acid administration. These results demonstrate that 5-HT(1A) receptors are required for maintaining normal hippocampal functions and implicate a role for the 5-HT(1A) receptor in hippocampal-related symptoms, such as cognitive disturbances, in stress-related disorders.     

Running enhances neurogenesis, learning, and long-term potentiation in mice

Running increases neurogenesis in the dentate gyrus of the hippocampus, a brain structure that is important for memory function. Consequently, spatial learning and long-term potentiation (LTP) were tested in groups of mice housed either with a running wheel (runners) or under standard conditions (controls). Mice were injected with bromodeoxyuridine to label dividing cells and trained in the Morris water maze. LTP was studied in the dentate gyrus and area CA1 in hippocampal slices from these mice. Running improved water maze performance, increased bromodeoxyuridine-positive cell numbers, and selectively enhanced dentate gyrus LTP. Our results indicate that physical activity can regulate hippocampal neurogenesis, synaptic plasticity, and learning.     

5-HT4 and 5-HT2 receptors antagonistically influence gap junctional coupling between rat auricular myocytes

5-hydroxytryptamine-4 (5-HT₄) receptors have been proposed to contribute to the generation of atrial fibrillation in human atrial myocytes, but it is unclear if these receptors are present in the hearts of small laboratory animals (e.g. rat). In this study, we examined presence and functionality of 5-HT₄ receptors in auricular myocytes of newborn rats and their possible involvement in regulation of gap junctional intercellular communication (GJIC, responsible for the cell-to-cell propagation of the cardiac excitation). Western-blotting assays showed that 5-HT₄ receptors were present and real-time RT-PCR analysis revealed that 5-HT_4b was the predominant isoform. Serotonin (1 μM) significantly reduced cAMP concentration unless a selective 5-HT₄ inhibitor (GR113808 or ML10375, both 1 μM) was present. Serotonin also reduced the amplitude of L-type calcium currents and influenced the strength of GJIC without modifying the phosphorylation profiles of the different channel-forming proteins or connexins (Cxs), namely Cx40, Cx43 and Cx45. GJIC was markedly increased when serotonin exposure occurred in presence of a 5-HT₄ inhibitor but strongly reduced when 5-HT_2A and 5-HT_2B receptors were inhibited, showing that activation of these receptors antagonistically regulated GJIC. The serotoninergic response was completely abolished when 5-HT₄, 5-HT_2A and 5-HT_2B were simultaneously inhibited. A 24 h serotonin exposure strongly reduced Cx40 expression whereas Cx45 was less affected and Cx43 still less. In conclusion, this study revealed that 5-HT₄ (mainly 5-HT_4b), 5-HT_2A and 5-HT_2B receptors coexisted in auricular myocytes of newborn rat, that 5-HT₄ activation reduced cAMP concentration, I_CaL and intercellular coupling whereas 5-HT_2A or 5-HT_2B activation conversely enhanced GJIC.     

Ablation of hippocampal neurogenesis impairs contextual fear conditioning and synaptic plasticity in the dentate gyrus

Although hippocampal neurogenesis has been described in many adult mammals, the functional impact of this process on physiology and behavior remains unclear. In the present study, we used two independent methods to ablate hippocampal neurogenesis and found that each procedure caused a limited behavioral deficit and a loss of synaptic plasticity within the dentate gyrus. Specifically, focal X irradiation of the hippocampus or genetic ablation of glial fibrillary acidic protein-positive neural progenitor cells impaired contextual fear conditioning but not cued conditioning. Hippocampal-dependent spatial learning tasks such as the Morris water maze and Y maze were unaffected. These findings show that adult-born neurons make a distinct contribution to some but not all hippocampal functions. In a parallel set of experiments, we show that long-term potentiation elicited in the dentate gyrus in the absence of GABA blockers requires the presence of new neurons, as it is eliminated by each of our ablation procedures. These data show that new hippocampal neurons can be preferentially recruited over mature granule cells in vitro and may provide a framework for how this small cell population can influence behavior.     

Identification of a serotonin receptor coupled to adenylyl cyclase involved in learning-related heterosynaptic facilitation in Aplysia

Serotonin (5-HT) plays a critical role in modulating synaptic plasticity in the marine mollusc Aplysia and in the mammalian nervous system. In Aplysia sensory neurons, 5-HT can activate several signal cascades, including PKA and PKC, presumably via distinct types of G protein-coupled receptors. However, the molecular identities of these receptors have not yet been identified. We here report the cloning and functional characterization of a 5-HT receptor that is positively coupled to adenylyl cyclase in Aplysia neurons. The cloned receptor, 5-HT_apAC1, stimulates the production of cAMP in HEK293T cells and in Xenopus oocytes. Moreover, the knockdown of 5-HT_apAC1 expression by RNA interference blocked 5-HT-induced cAMP production in Aplysia sensory neurons and blocked synaptic facilitation in nondepressed or partially depressed sensory-to-motor neuron synapses. These data implicate 5-HT_apAC1 as a major modulator of learning related synaptic facilitation in the direct sensory to motor neuron pathway of the gill withdrawal reflex.     

5-HT7 receptors mediate dilation of rat cremaster muscle arterioles in vivo

Objective

Serotonin (5-HT) infusion in vivo causes hypotension and a fall in total peripheral resistance. However, the vascular segment and the receptors that mediate this response remain in question. We hypothesized that 5-HT₇ receptors mediate arteriolar dilation to 5-HT in skeletal muscle microcirculation.

Methods

Cremaster muscles of isoflurane-anesthetized male Sprague-Dawley rats were prepared for in vivo microscopy of third- and fourth-order arterioles and superfused with physiological salt solution at 34°C. Quantitative real-time PCR (RT-PCR) was applied to pooled samples of first- to third-order cremaster arterioles (2–4 rats/sample) to evaluate 5-HT₇ receptor expression.

Results

Topical 5-HT (1–10 nmols) or the 5-HT_1/7 receptor agonist, 5-carboxamidotryptamine (10–30 nM), dilated third- and fourth-order arterioles, responses that were abolished by 1 μM SB269970, a selective 5-HT₇ receptor antagonist. In contrast, dilation induced by the muscarinic agonist, methacholine (100 nmols) was not inhibited by SB269970. Serotonin (10 nmols) failed to dilate cremaster arterioles in 5-HT₇ receptor knockout rats whereas arterioles in wild-type litter mates dilated to 1 nmol 5-HT, a response blocked by 1 μM SB269970. Quantitative RT-PCR revealed that cremaster arterioles expressed mRNA for 5-HT₇ receptors.

Conclusions

5-HT₇ receptors mediate dilation of small arterioles in skeletal muscle and likely contribute to 5-HT-induced hypotension, in vivo.     

Serotonin Agonists and Antagonists in Obstructive Sleep Apnea

Obstructive sleep apnea hypopnea syndrome (OSAHS) is a prevalent disorder associated with substantial cardiovascular and neurobehavioral morbidity. Yet this is a disorder for which there are no widely effective pharmacotherapies. The pathophysiology of obstructive sleep apnea namely, normal respiration in waking with disordered breathing only in sleep, suggests that this disorder should be readily amenable to drug therapy. Over the past 10 years, we have gained tremendous insight into the neurochemical mechanisms involved in state-dependent control of respiration. It is apparent from this work that there are many potential avenues for pharmacotherapies, including several seemingly conflicting directions for serotonergic therapies.Serotonin delivery is reduced to upper airway dilator motor neurons in sleep, and this contributes, at least in part, to sleep-related reductions in dilator muscle activity and upper airway obstruction. The dilator motor neuron post-synaptic serotonin receptors are 5-HT_2A and 5-HT_2C subtypes, and in adults the presynaptic 5-HT receptor in motor nuclei is 5-HT_1B, an inhibitory receptor. Serotonin receptors are also found within central respiratory neuronal groups, and these receptor subtypes include 5-HT_1A (inhibitory) and 5-HT₂ receptors. Peripherally, stimulation of 5-HT_2A, 5-HT_2C and 5-HT₃ receptor subtypes have an inhibitory effect on respiration via action at the nodose ganglion. Many of these receptor subtypes and their signal transduction pathways may be affected by oxidative stress in obstructive sleep apnea. These alterations will make finding drug therapies for sleep apnea more challenging, but not insurmountable. Future directions are suggested for elucidating safe, well-tolerated serotonergic drugs for this disorder.Tryptophan was one of the first serotonergic drugs tested for OSAHS. This drug was withdrawn from the market as a result of reports linking tryptophan use with eosinophilic myalgia syndrome and life-threatening pulmonary hypertension. Newer drugs with serotonergic activity tested in persons with sleep-disordered breathing include buspirone, fluoxetine and paroxetine. Trials are presently being conducted to evaluate the effects of 5-HT_2A and 5-HT₃ antagonists on OSAHS. Many of the drugs tested have not shown significant improvement in sleep apnea. However, with continued effort to elucidate the pharmacology of neurochemical control of state-dependent changes in respiratory control, the availability of pharmacological therapy for this disorder is not too far away.     

Serotonin receptor subtypes influence prolactin secretion in the turkey

Serotonin (5-HT) stimulation of prolactin (PRL) secretion is mediated through the dopaminergic (DAergic) system, with 5-HT ligands having no direct effect on pituitary PRL release. Infusion of 5-HT into the third ventricle (ICV) or electrical stimulation (ES) of the medial preoptic area (POM) or the ventromedial nucleus (VMN) induces an increase in circulating PRL in the turkey. These increases in PRL do not occur when a selective antagonist blocks the D₁ dopamine (DA) receptors in the infundibular area (INF). In this study, the ICV infusion of (R)(−)-DOI hydrochloride (DOI), a selective 5-HT_2A eceptor agonist, caused PRL to increase. Pretreatment with Ketanserin tartrate salt (KETAN), a selective 5-HT_2A receptor antagonist, blocked DOI-induced PRL secretion, attesting to the specificity of the response. DOI-induced PRL secretion was prevented when the D₁ DA receptors in the INF were blocked by the D₁ DA receptor antagonist, R(+)-SCH-23390 hydrochloride microinjection, suggesting that the DAergic activation of the vasoactive intestinal peptide (VIP)/PRL system is mediated by a stimulatory 5-HT_2A receptor subtype. The DOI-induced PRL increase did not occur when (±)-8-OH-DPAT (DPAT) was concurrently infused. DPAT is a 5-T_1A receptor agonist which appears to mediate the inhibitory influence of 5-HT on PRL secretion. When DPAT was microinjected directly into the VMN, it blocked the PRL release affected by ES in the POM. These data suggested that when 5-HT_2A receptors are activated, they influence the release of DA to the INF. When 5-HT_1A receptors are stimulated, they somehow inhibit the PRL-releasing actions of 5-HT_2A receptors. This inhibition could take place centrally, or it could occur postsynaptically at the pituitary level. It is known that D₂ DA receptors in the pituitary antagonize PRL-releasing effect of VIP. A release of DA to the pituitary, initiated by 5-HT_1A receptors, could effectively inhibit PRL secretion.     

Dynamics and Patterning of 5-Hydroxytryptamine 2 Subtype Receptors in JC Polyomavirus Entry

The organization and dynamics of plasma membrane receptors are a critical link in virus-receptor interactions, which finetune signaling efficiency and determine cellular responses during infection. Characterizing the mechanisms responsible for the active rearrangement and clustering of receptors may aid in developing novel strategies for the therapeutic treatment of viruses. Virus-receptor interactions are poorly understood at the nanoscale, yet they present an attractive target for the design of drugs and for the illumination of viral infection and pathogenesis. This study utilizes super-resolution microscopy and related techniques, which surpass traditional microscopy resolution limitations, to provide both a spatial and temporal assessment of the interactions of human JC polyomavirus (JCPyV) with 5-hydroxytrypamine 2 receptors (5-HT₂Rs) subtypes during viral entry. JCPyV causes asymptomatic kidney infection in the majority of the population and can cause fatal brain disease, and progressive multifocal leukoencephalopathy (PML), in immunocompromised individuals. Using Fluorescence Photoactivation Localization Microscopy (FPALM), the colocalization of JCPyV with 5-HT₂ receptor subtypes (5-HT_2A, 5-HT_2B, and 5-HT_2C) during viral attachment and viral entry was analyzed. JCPyV was found to significantly enhance the clustering of 5-HT₂ receptors during entry. Cluster analysis of infected cells reveals changes in 5-HT₂ receptor cluster attributes, and radial distribution function (RDF) analyses suggest a significant increase in the aggregation of JCPyV particles colocalized with 5-HT₂ receptor clusters in JCPyV-infected samples. These findings provide novel insights into receptor patterning during viral entry and highlight improved technologies for the future development of therapies for JCPyV infection as well as therapies for diseases involving 5-HT₂ receptors.     

Dentate gyrus-specific manipulation of beta-Ca2+/calmodulin-dependent kinase II disrupts memory consolidation

Although the functions of alpha-Ca(2+)/calmodulin-dependent kinase II (CaMKII) have been studied extensively, the role of betaCaMKII, a coconstituent of the CaMKII holoenzyme in synaptic plasticity, learning, and memory has not been examined in vivo. Here we produce a transgenic mouse line in which the inducible and reversible manipulation of betaCaMKII activity is restricted to the hippocampal dentate gyrus, the region where long-term potentiation was originally discovered. We demonstrate that betaCaMKII activity in the dentate gyrus selectively impaired long-term potentiation in the dentate perforant path, but not in the CA1 Schaffer collateral pathway. Although the transgenic mice showed normal 1-day memories, they were severely impaired in 10-day contextual fear memory. Systematic manipulations of dentate betaCaMKII activity during various distinct memory stages further reveal the initial day within the postlearning consolidation period as a critical time window that is highly sensitive to changes in betaCaMKII activity. This study provides evidence not only for the functional role of betaCaMKII in the dentate gyrus plasticity and hippocampal memory, but also for the notion that the mismatch between the actual learning pattern and reactivation patterns in the dentate gyrus circuit can underlie long-term memory consolidation.     

Gq/5-HT2c receptor signals activate a local GABAergic inhibitory feedback circuit to modulate serotonergic firing and anxiety in mice

Serotonin 2c receptors (5-HT_2c-Rs) are drug targets for certain mental disorders, including schizophrenia, depression, and anxiety. 5-HT_2c-Rs are expressed throughout the brain, making it difficult to link behavioral changes to circuit specific receptor expression. Various 5-HT-Rs, including 5-HT_2c-Rs, are found in the dorsal raphe nucleus (DRN); however, the function of 5-HT_2c-Rs and their influence on the serotonergic signals mediating mood disorders remain unclear. To investigate the role of 5-HT_2c-Rs in the DRN in mice, we developed a melanopsin-based optogenetic probe for activation of Gq signals in cellular domains, where 5-HT_2c-Rs are localized. Our results demonstrate that precise temporal control of Gq signals in 5-HT_2c-R domains in GABAergic neurons upstream of 5-HT neurons provides negative feedback regulation of serotonergic firing to modulate anxiety-like behavior in mice.Serotonin (5-hydroxytryptamine, or 5-HT) is an important modulator of anxiety circuits (1). The diverse effects of serotonin are mediated through various 5-HT receptors (5-HT-Rs), including 5-HT_1–7-Rs (2). Recent pharmacologic and genetic studies have highlighted an important role of 5-HT_2c-Rs in anxiety disorders; however, the interpretation of physiological and behavioral data remains difficult owing to a lack of selective pharmacologic ligands (3).5-HT_2c-Rs are expressed in various cell types and brain regions of the anxiety circuit, including the amygdala and the dorsal raphe nucleus (DRN), a midbrain region containing high concentrations of 5-HT neurons. It has been suggested that 5-HT_2c-Rs are expressed in GABAergic neurons, and that 5-HT_2c-R activation may contribute to an inhibitory feedback control of 5-HT cell firing (4). The functional and behavioral consequences of such a possible inhibitory feedback mechanism for 5-HT firing have not yet been investigated, however.Unfortunately, current techniques for identifying the functions of 5-HT_2c-Rs in vertebrate brains are of limited value. For example, agonists and antagonists of 5-HT_2c-Rs are often unspecific, and their action is not restricted to a specific cell type. Complete and conditional knockouts of the receptor gene have limited control of developmental and compensation effects by other G-protein–coupled receptors (GPCRs), and none of the current techniques allows for the physiological control of the 5-HT_2c-R activation on a millisecond to second time scale.To overcome the limitations of pharmacologic and genetic approaches, we have developed a new light-activated GPCR based on vertebrate melanopsin (vMo). Both 5-HT_2c-Rs and vMo couple to the Gq signaling pathway (5, 6). To investigate the functional consequence of Gq signal activation in the cell types and cellular structures where 5-HT_2c-Rs are located, we virally expressed vMo carrying the C terminus (CT) of the 5-HT_2c-R in GABAergic neurons in the DRN. We found that light activation of vMo-CT_5-HT2c decreases the firing of 5-HT neurons and modulates anxiety behaviors in mice. Our results demonstrate a previously unidentified, autoregulatory negative feedback mechanism for the firing of serotonergic neurons to control anxiety in mice.     

Behavioral pharmacogenetic analysis on the role of the α4 GABA(A) receptor subunit in the ethanol-mediated impairment of hippocampus-dependent contextual learning

Background: A major effect of low‐dose ethanol is impairment of hippocampus‐dependent cognitive function. α4/δ ‐containing GABA_ARs are highly expressed within the dentate gyrus region of the hippocampus where they mediate a tonic inhibitory current that is sensitive to the enhancement by low ethanol concentrations. These receptors are also powerful modulators of learning and memory, suggesting that they could play an important role in ethanol’s cognitive impairing effects. The goal of this study was to develop a high‐throughput cognitive ethanol assay, amenable to use in genetically modified mice that could be used to test this hypothesis. Methods: We developed a procedure where preexposure to a conditioning chamber is used to rescue the “immediate shock deficit.” Using this task, ethanol can be specifically targeted at the hippocampus‐dependent process of contextual learning without interfering with pain sensitivity or behavioral performance. Results: Validation of this task in C57BL/6 mice indicated that 1.0 g/kg ethanol and 10 mg/kg allopregnanolone disrupt contextual learning. Ro15‐4513 reversed the effects of ethanol but not allopregnanolone, whereas it produced an impairment when given alone. The high‐throughput nature of this task allowed for its application in a large cohort of α4 GABA_AR KO mice. Loss of the α4 GABA_AR subunit produced an enhanced sensitivity to the cognitive impairing effects of ethanol. This is consistent with the enhanced ethanol sensitivity of synaptic GABA_ARs that has been previously observed in the dentate gyrus in these mice, but inconsistent with the reduced ethanol sensitivity of extrasynaptic GABA_ARs observed in the same cells. Conclusions: Overall, these findings are consistent with our hypothesis that ethanol acts directly at GABA_A receptors to impair hippocampus‐dependent cognitive function. Furthermore, validation of this high‐throughput assay will allow for future studies to use anatomically and temporally restricted genetic manipulations to probe more deeply into the neural mechanisms of ethanol action on learning and memory circuits.     

Serotonin responsiveness through 5-HT2A and 5-HT4 receptors is differentially regulated in hypertrophic and failing rat cardiac ventricle

Cardiac ventricular responsiveness to serotonin appears in rat postinfarction congestive heart failure (CHF), mainly mediated by 5-HT₄ receptors in chronic dilated CHF and 5-HT_2A receptors in acute CHF. To differentiate between the effects of left ventricular (LV) hypertrophy and failure on 5-HT_2A- and 5-HT₄-mediated inotropic serotonin response, male Wistar rats with increasing LV hypertrophy (AB1-3) and failure (ABHF) 6 weeks after banding of the ascending aorta were screened for contractile function in vivo (echocardiography) and ex vivo in LV papillary muscles, and mRNA expression level determined by RT-PCR. Both AB1-3 and ABHF displayed LV hypertrophy and remodelling. In ABHF, systolic LV and left atrial diameter increased and cardiac output decreased compared to AB3. Serotonin induced a positive inotropic response (PIR) in papillary muscles correlated with the degree of hypertrophy reaching a maximum in ABHF. Both 5-HT_2A and 5-HT₄ receptors contributed to the PIR. The 5-HT_2A contribution increased with increasing hypertrophy, and the 5-HT₄ contribution increased upon transition to heart failure. No 5-HT_2B-mediated PIR was observed, consistent with increased 5-HT_2B mRNA only in non-cardiomyocytes. The 5-HT_2A, 5-HT_2B and 5-HT₄ mRNA levels increased in AB1-3 and increased further in ABHF compared to AB3, but did not correlate with degree of hypertrophy. 5-HT_2A mRNA was also increased in LV of terminally failing human hearts. In conclusion, functional 5-HT_2A and 5-HT₄ receptors are differentially induced in LV hypertrophy and failure. While the 5-HT_2A-mediated PIR is linearly correlated with the degree of hypertrophy, the 5-HT₄-mediated PIR seems to increase with LV dilatation, as also seen in postinfarction CHF.     

Compensatory network changes in the dentate gyrus restore long-term potentiation following ablation of neurogenesis in young-adult mice

It is now well established that neurogenesis in the rodent subgranular zone of the hippocampal dentate gyrus continues throughout adulthood. Neuroblasts born in the dentate subgranular zone migrate into the granule cell layer, where they differentiate into neurons known as dentate granule cells. Suppression of neurogenesis by irradiation or genetic ablation has been shown to disrupt synaptic plasticity in the dentate gyrus and impair some forms of hippocampus-dependent learning and memory. Using a recently developed transgenic mouse model for suppressing neurogenesis, we sought to determine the long-term impact of ablating neurogenesis on synaptic plasticity in young-adult mice. Consistent with previous reports, we found that ablation of neurogenesis resulted in significant deficits in dentate gyrus long-term potentiation (LTP) when examined at a time proximal to the ablation. However, the observed deficits in LTP were not permanent. LTP in the dentate gyrus was restored within 6 wk and this recovery occurred in the complete absence of neurogenesis. The recovery in LTP was accompanied by prominent changes within the dentate gyrus, including an increase in the survival rate of newborn cells that were proliferating just before the ablation and a reduction in inhibitory input to the granule cells of the dentate gyrus. These findings suggest that prolonged suppression of neurogenesis in young-adult mice results in wide-ranging compensatory changes in the structure and dynamics of the dentate gyrus that function to restore plasticity.     

Autoradiographic characterization of [3H]-5-HT-moduline binding sites in rodent brain and their relationship to 5-HT1B receptors

5-HT-moduline is an endogenous tetrapeptide [Leu-Ser-Ala-Leu (LSAL)] that was first isolated from bovine brain tissue. To understand the physiological role of this tetrapeptide, we studied the localization of 5-HT-moduline binding sites in rat and mouse brains. Quantitative data obtained with a gaseous detector of β-particles (β-imager) indicated that [³H]-5-HT-moduline bound specifically to rat brain sections with high affinity (K_d = 0.77 nM and B_max = 0.26 dpm/mm²). Using film autoradiography in parallel, we found that 5-HT-moduline binding sites were expressed in a variety of rat and mouse brain structures. In 5-HT_1B receptor knock-out mice, the specific binding of [³H]-5-HT-moduline was not different from background labeling, indicating that 5-HT-moduline targets are exclusively located on the 5-HT_1B receptors. Although the distribution of 5-HT-moduline binding sites was similar to that of 5-HT_1B receptors, they did not overlap totally. Differences in distribution patterns were found in regions containing either high levels of 5-HT_1B receptors such as globus pallidus and subiculum that were poorly labeled or in other regions such as dentate gyrus of hippocampus and cortex where the relative density of 5-HT-moduline binding sites was higher than that of 5-HT_1B receptors. In conclusion, our data, based on autoradiographic localization, indicate that 5-HT-moduline targets are located on 5-HT_1B receptors present both on 5-HT afferents and postsynaptic neurons. By interacting specifically with 5-HT_1B receptors, this tetrapeptide may play a pivotal role in pathological states such as stress that involves the dysfunction of 5-HT neurotransmission.     

Reversal of hippocampal neuronal maturation by serotonergic antidepressants

Serotonergic antidepressant drugs have been commonly used to treat mood and anxiety disorders, and increasing evidence suggests potential use of these drugs beyond current antidepressant therapeutics. Facilitation of adult neurogenesis in the hippocampal dentate gyrus has been suggested to be a candidate mechanism of action of antidepressant drugs, but this mechanism may be only one of the broad effects of antidepressants. Here we show a distinct unique action of the serotonergic antidepressant fluoxetine in transforming the phenotype of mature dentate granule cells. Chronic treatments of adult mice with fluoxetine strongly reduced expression of the mature granule cell marker calbindin. The fluoxetine treatment induced active somatic membrane properties resembling immature granule cells and markedly reduced synaptic facilitation that characterizes the mature dentate-to-CA3 signal transmission. These changes cannot be explained simply by an increase in newly generated immature neurons, but best characterized as “dematuration” of mature granule cells. This granule cell dematuration developed along with increases in the efficacy of serotonin in 5-HT₄ receptor-dependent neuromodulation and was attenuated in mice lacking the 5-HT₄ receptor. Our results suggest that serotonergic antidepressants can reverse the established state of neuronal maturation in the adult hippocampus, and up-regulation of 5-HT₄ receptor-mediated signaling may play a critical role in this distinct action of antidepressants. Such reversal of neuronal maturation could affect proper functioning of the mature hippocampal circuit, but may also cause some beneficial effects by reinstating neuronal functions that are lost during development.     

Serotonin and the GI tract

Serotonin (5-hydroxytryptamine, 5-HT) participates in several functions of the gastrointestinal tract. Receptors in seven families (5-HT₁-5-HT₇) were identified, many of which are present on enterocytes, intrinsic and extrinsic neurons, interstitial cells, and gut myocytes. Most 5-HT is released from enterochromaffin cells in response to physiologic and pathologic stimuli. Roles of 5-HT in health include control of normal gut motor activity, secretion, and sensation, and regulation of food intake and cell growth. Abnormalities of serotonergic function contribute to symptom genesis in functional bowel disorders, inflammatory and infectious diseases of the gut, emetic responses to varied stimuli, obesity, and dysregulation of cell growth. Therapies acting as agonists or antagonists of 5-HT receptors or that modulate 5-HT reuptake play prominent roles in managing these conditions, although use of many agents is hampered by cardiopulmonary complications. Novel agents are in testing, which may exhibit efficacy without significant toxicity.     

Long-term potentiation increases tyrosine phosphorylation of the N-methyl-D-aspartate receptor subunit 2B in rat dentate gyrus in vivo.

Long-term potentiation (LTP) is a form of synaptic memory that may subserve developmental and behavioral plasticity. An intensively investigated form of LTP is dependent upon N-methyl-D-aspartate (NMDA) receptors and can be elicited in the dentate gyrus and hippocampal CA1. Induction of this type of LTP is triggered by influx of Ca2+ through activated NMDA receptors, but the downstream mechanisms of induction, and even more so of LTP maintenance, remain controversial. It has been reported that the function of NMDA receptor channel can be regulated by protein tyrosine kinases and protein phosphatases and that inhibition of protein tyrosine kinases impairs induction of LTP. Herein we report that LTP in the dentate gyrus is specifically correlated with tyrosine phosphorylation of the NMDA receptor subunit 2B in an NMDA receptor-dependent manner. The effect is observed with a delay of several minutes after LTP induction and persists in vivo for several hours. The potential relevance of this post-translational modification to mechanisms of LTP and circuit plasticity is discussed.     

Promiscuous coassembly of serotonin 5-HT3 and nicotinic α4 receptor subunits into Ca2+-permeable ion channels

Serotonin (5-hydroxytryptamine) type 3 receptors (5-HT₃R) and nicotinic acetylcholine receptors are structurally and functionally related proteins, yet distinct members of the family of ligand-gated ion channels. For most members of this family a diversity of heteromeric receptors is known at present. In contrast, known 5-HT₃R subunits are all homologs of the same 5-HT₃R-A subunit and form homopentameric receptors. Here we show, by heterologous expression followed by immunoprecipitation, that 5-HT₃R and nicotinic acetylcholine receptor α4 subunits coassemble into a novel type of heteromeric ligand-gated ion channel, which is activated by 5-HT. The Ca²⁺ permeability of this heteromeric ion channel is enhanced as compared with that of the homomeric 5-HT₃R channel. Heteromeric 5-HT₃/α4 and homomeric 5-HT₃Rs have similar pharmacological profiles, but distinct sensitivities to block by the antagonist d-tubocurarine. Coassembly of subunits beyond the boundaries of ligand-gated ion channel families may constitute an important mechanism contributing to the diverse properties and functions of native neurotransmitter receptors.