8-OH-DPAT facilitated memory consolidation and increased hippocampal and cortical cAMP production

5-HT6 Receptors Control GABAergic Transmission and CA1 Pyramidal Cell Output of Dorsal Hippocampus

The blockade of 5-HT6 receptors represents an experimental approach that might ameliorate the memory deficits associated with brain disorders, including Alzheimer's disease and schizophrenia. However, the synaptic mechanism by which 5-HT6 receptors control the GABAergic and glutamatergic synaptic transmission is barely understood. In this study, we demonstrate that pharmacological manipulation of 5-HT6 receptors with the specific agonist EMD 386088 (7.4 nM) or the antagonist SB-399885 (300 nM) modulates the field inhibitory postsynaptic potentials of the dorsal hippocampus and controls the strength of the population spike of pyramidal cells. Likewise, pharmacological modulation of 5-HT6 controls the magnitude of paired-pulse inhibition, a phenomenon mediated by GABAergic interneurons acting via GABAA receptors of pyramidal cells. The effects of pharmacological manipulation of the 5-HT6 receptor were limited to GABAergic transmission and did not affect the strength of field excitatory postsynaptic potentials mediated by the Schaffer collaterals axons. Lastly, in a modified version of the Pavlovian autoshaping task that requires the activation of the hippocampal formation, we demonstrated that the anti-amnesic effect induced by the blockade of the 5-HT6 receptor is prevented when the GAT1 transporter is blocked, suggesting that modulation of GABAergic transmission is required for the anti-amnesic properties of 5-HT6 receptor antagonists.

Biased agonism in drug discovery: Is there a future for biased 5-HT1A receptor agonists in the treatment of neuropsychiatric diseases?

Serotonin (5-HT) is one of the fundamental neurotransmitters that contribute to the information essential for an organism's normal, physiological function. Serotonin acts centrally and systemically. The 5-HT_1A receptor is the most widespread serotonin receptor, and participates in many brain-related disorders, including anxiety, depression, and cognitive impairments. The 5-HT_1A receptor can activate several different biochemical pathways and signals through both G protein-dependent and G protein-independent pathways. Preclinical experiments indicate that distinct signaling pathways in specific brain regions may be crucial for antidepressant-like, anxiolytic-like, and procognitive responses. Therefore, the development of new ligands that selectively target a particular signaling pathway(s) could open new possibilities for more effective and safer pharmacotherapy. This review discusses the current state of preclinical studies focusing on the concept of functional selectivity (biased agonism) regarding the 5-HT_1A receptor and its role in antidepressant-like, anxiolytic-like, and procognitive regulation. Such work highlights not only the differential effects of targeted autoreceptors, vs. heteroreceptors, but also the importance of targeting specific downstream intracellular signaling processes, thereby enhancing favorable over unfavorable signaling activation.

Sub-Acute Treatment of Curcumin Derivative J147 Ameliorates Depression-Like Behavior Through 5-HT1A-Mediated cAMP Signaling

Background

Major depressive disorder (MDD) is a severe mental disorder related to the deficiency of monoamine neurotransmitters, particularly to abnormalities of 5-HT (5-hydroxytryptamine, serotonin) and its receptors. Our previous study suggested that acute treatment with a novel curcumin derivative J147 exhibited antidepressant-like effects by increasing brain derived neurotrophic factor (BDNF) level in the hippocampus of mice. The present study expanded upon our previous findings and investigated the antidepressant-like effects of sub-acute treatment of J147 for 3 days in male ICR mice and its possible relevancy to 5-HT_1A and 5-HT_1B receptors and downstream cAMP-BDNF signaling.

Methods

J147 at doses of 1, 3, and 9 mg/kg (via gavage) was administered for 3 days, and the anti-immobility time in the forced swimming and tail suspension tests (FST and TST) was recorded. The radioligand binding assay was used to determine the affinity of J147 to 5-HT_1A and 5-HT_1B receptor. Moreover, 5-HT_1A or 5-HT_1B agonist or its antagonist was used to determine which 5-HT receptor subtype is involved in the antidepressant-like effects of J147. The downstream signaling molecules such as cAMP, PKA, pCREB, and BDNF were also measured to determine the mechanism of action.

Results

The results demonstrated that sub-acute treatment of J147 remarkably decreased the immobility time in both the FST and TST in a dose-dependent manner. J147 displayed high affinity in vitro to 5-HT_1A receptor prepared from mice cortical tissue and was less potent at 5-HT_1B receptor. These effects of J147 were blocked by pretreatment with a 5-HT_1A antagonist NAD-299 and enhanced by a 5-HT_1A agonist 8-OH-DPAT. However, 5-HT_1B receptor antagonist NAS-181 did not appreciably alter the effects of J147 on depression-like behaviors. Moreover, pretreatment with NAD-299 blocked J147-induced increases in cAMP, PKA, pCREB, and BDNF expression in the hippocampus, while 8-OH-DPAT enhanced the effects of J147 on these proteins’ expression.

Conclusion

The results suggest that J147 induces rapid antidepressant-like effects during a 3-day treatment period without inducing drug tolerance. These effects might be mediated by 5-HT_1A-dependent cAMP/PKA/pCREB/BDNF signaling.

Intraperitoneal 8-OH-DPAT reduces competition from contextual but not discrete conditioning cues

The effects of the serotonergic (5-hydroxytryptamine, 5-HT) agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT; 0.2 and 0.4 mg/kg i.p.) were examined in trace conditioning (Experiment 1) and overshadowing (Experiment 2) procedures. Both experiments used a fear conditioning procedure conducted off-the-baseline in water deprived male Wistar rats. 8-OH-DPAT was administered during conditioning and its effects were examined drug free as the suppression of an established licking response, both upon re-exposure to the cues provided by the conditioning chambers and upon presentation of experimental stimuli. There were no statistically significant effects of 8-OH-DPAT on conditioning to the discrete cue provided by a 5 s conditioned stimulus (CS), irrespective of the length of the trace interval used in Experiment 1, and irrespective of whether the CS took the form of a light alone, or a noise plus light compound in the Experiment 2 overshadowing procedure. The successful demonstration of overshadowing required the use of a second conditioning session which allowed further evaluation of the effects of 8-OH-DPAT in that neither a weak nor a strong overshadowing effect was modulated by either drug dose. Nonetheless conditioning to contextual cues was attenuated by treatment with 8-OH-DPAT at the 30 s trace interval. We therefore conclude that 8-OH-DPAT reduces competition from contextual but not discrete conditioning cues. This pattern of results lends further support to the view that contextual cue conditioning and discrete cue conditioning are modulated by different neuropharmacological mechanisms.

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8-OH-DPAT (0.2 and 0.4 mg/kg i.p.) was tested in two fear conditioning procedures.

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8-OH-DPAT reduced conditioning to contextual cues at a 30 s trace.

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However, overshadowing produced by presentation of a compound cue was unaffected by 8-OH-DPAT.

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8-OH-DPAT reduced competition from contextual but not discrete conditioning cues.

Potential learning and memory disruptors and enhancers in a simple, 1-day operant task in mice

The objective of this study was to develop a rapid, 1-day learning and memory assay in mice that is sensitive to the effects of compounds that could impair or enhance acquisition and retrieval. Swiss-Webster, male mice were placed in experimental chambers for a 1-h acquisition session with an intermittent, audible tone. If a nose-poke response occurred during the tone, an Ensure water solution was presented. After 1 h, the mice returned to the chambers for 2 h. Drugs were injected before or after sessions to determine the effects on acquisition and/or retrieval. Mice injected with saline learned a nose-poke response as measured by decreased latencies to earn 10 reinforcers, increased reinforced response rates, and decreased nonreinforced response rates. Scopolamine and acetazolamide impaired retrieval of the nose-poke response, whereas ketamine only modestly impaired retrieval. Doses of 8-OH-DPAT or the novel carbonic anhydrase activator, MAI27, either had no effect or impaired some measures of responding. Neither 8-OH-DPAT nor MAI27 were able to prevent the modest impairments produced by ketamine. The simple, 1-day operant task is a rapid assay that can be used as an initial screen to test the effects of learning and memory disruptors and potentially enhancers.

Systemic administration of two different anxiolytic drugs decreases local field potential theta frequency in the medial entorhinal cortex without affecting grid cell firing fields

Neurons coding spatial location (grid cells) are found in medial entorhinal cortex (MEC) and demonstrate increasing size of firing fields and spacing between fields (grid scale) along the dorsoventral axis. This change in grid scale correlates with differences in theta frequency, a 6-10Hz rhythm in the local field potential (LFP) and rhythmic firing of cells. A relationship between theta frequency and grid scale can be found when examining grid cells recorded in different locations along the dorsoventral axis of MEC. When describing the relationship between theta frequency and grid scale, it is important to account for the strong positive correlation between theta frequency and running speed. Plotting LFP theta frequency across running speeds dissociates two components of this relationship: slope and intercept of the linear fit. Change in theta frequency through a change in the slope component has been modeled and shown experimentally to affect grid scale, but the prediction that change in the intercept component would not affect grid scale has not been tested experimentally. This prediction about the relationship of intercept to grid scale is the primary hypothesis tested in the experiments presented here. All known anxiolytic drugs decrease hippocampal theta frequency despite their differing mechanisms of action. Specifically, anxiolytics decrease the intercept of the theta frequency-running speed relationship in the hippocampus. Here we demonstrate that anxiolytics decrease the intercept of the theta frequency-running speed relationship in the MEC, similar to hippocampus, and the decrease in frequency through this change in intercept does not affect grid scale.

Serotonin 5-HT7 receptor agonist, LP-211, exacerbates Na(+), K(+)-ATPase/Mg(2+)-ATPase imbalances in spinal cord-injured male rats

BACKGROUND

The observed controversy that N-(4-cyanophenylmethyl)-4-(2-diphenyl)-1-piperazinehexanamide (LP-211), a selective serotonin (5-HT7) receptor agonist, may either modify or exacerbate imbalances in serum electrolyte concentrations and renal tissue of spinal cord trauma cases has not been reported yet. The aim of this study was to better understand the effects of a new 5-HT7 receptor agonist, LP-211, on serum electrolyte changes in spinal cord injured- (SCI) rats.

METHODS

Sixty male rats were assigned to the following groups: A) Intact (saline as vehicle, 1 ml/kg, i.p.), B) Intact [LP-211, (0.003-0.3 mg/kg, i.p.)], C) Sham-operated [laminectomy + vehicle (1 ml/kg, i.p.)], D) Sham-operated [laminectomy + LP-211 (0.003-0.3 mg/kg, i.p.)], E) Treatment [laminectomy + spinal trauma (SCI) + vehicle (1 ml/kg, i.p.)], F) Treatment [laminectomy + spinal trauma + LP-211 (0.003-0.3 mg/kg, i.p.)]. SCI was performed by placing an aneurysm clip, extradurally at the level of T10. After two weeks, LP-211 was administered cumulatively and each dose was injected (i.p.) with 20 min interval. At the end of the experiment, blood samples were collected for biochemical evaluations of the electrolytes employing standard commercial kits.

RESULTS

The present results indicate elevated serum levels of Na(+), K(+), and Mg(2+) in SCI rats and significant differences demonstrated between the groups [P < 0.001, F(5, 35) = 23.92], [P < 0.001, F(5, 35) = 67.63], [P < 0.001, F(5, 35) = 71.144], respectively. So that, in groups B, D and F, there was a significant increase in K(+) and Mg(2+) serum levels compared to the groups A, C, and E (P < 0.001). Furthermore, Na(+) serum levels in SCI (LP-211), laminectomy (LP-211), and intact (LP-211) groups tended to be statistically lower than SCI (saline), laminectomy (saline) and intact (saline) groups. Infact, hyponatremia, hyperkalemia and hypermagnesemia was obtained in group F. Nevertheless, in the remaining measured serum electrolytes such as calcium (Ca(2+)), iron (Fe(2+)) and phosphorus (P(3-)), chlorine (Cl(-)), copper (Cu(+)), and zinc (Zu(+)), no significant changes were observed.

CONCLUSION

It was shown that acute additive LP-211 treatments in the SCI group led to hyponatremia, hyperkalemia and hypermagnesemia, it may be stated that LP-211 treatment as a promising candidate for treating SCI complications in some systems especially urinary tract might take into consideration and further studies would be needed to clarify its benefits or drawbacks. The observed discrepancies, nevertheless; will also pose new questions. Altogether, this will ultimately contribute to further understanding the pathophysiological role regarding 5-HT7 receptor activation.

Assessing cognitive function in bipolar disorder: challenges and recommendations for clinical trial design

OBJECTIVE

Neurocognitive impairment in schizophrenia has been recognized for more than a century. In contrast, only recently have significant neurocognitive deficits been recognized in bipolar disorder. Converging data suggest the importance of cognitive problems in relation to quality of life in bipolar disorder, highlighting the need for treatment and prevention efforts targeting cognition in bipolar patients. Future treatment trials targeting cognitive deficits will be met with methodological challenges due to the inherent complexity and heterogeneity of the disorder, including significant diagnostic comorbidities, the episodic nature of the illness, frequent use of polypharmacy, cognitive heterogeneity, and a lack of consensus regarding measurement of cognition and outcome in bipolar patients. Guidelines for use in designing future trials are needed.

PARTICIPANTS

The members of the consensus panel (each of the bylined authors) were selected based upon their expertise in bipolar disorder. Dr Burdick is a neuropsychologist who has studied cognition in this illness for 15 years; Drs Ketter, Calabrese, and Goldberg each bring considerable expertise in the treatment of bipolar disorder, both within and outside of controlled clinical trials. This consensus statement was derived from work together at scientific meetings (eg, symposium presentation at the 2014 Annual Meeting of the American Society of Clinical Psychopharmacology, among others) and ongoing discussions by conference call. With the exception of the public presentations on this topic, these meetings were closed to outside participants.

EVIDENCE

A literature review was undertaken by the authors to identify illness-specific challenges relevant to the design and conduct of treatment trials targeting neurocognition in bipolar disorder. Expert opinion from each of the authors guided the consensus recommendations.

CONSENSUS PROCESS

Consensus recommendations, reached by unanimous opinion of the authors, are provided here as a preliminary guide for future trial design. Recommendations comprise exclusion of certain syndromal-level comorbid diagnoses and current affective instability, restrictions on numbers and types of medications, and use of prescreening assessment to ensure enrollment of subjects with adequate objective evidence of baseline cognitive impairment.

CONCLUSIONS

Clinical trials to address cognitive deficits in bipolar disorder face distinctive design challenges. As such trials move from proof-of-concept to confirmation of clinical efficacy, it will be important to incorporate distinctive design modifications to adequately address these challenges and increase the likelihood of demonstrating cognitive remediation effects. The field is now primed to address these challenges, and a comprehensive effort to formalize best practice guidelines will be a critically important next step.

5-HT7 receptors as modulators of neuronal excitability, synaptic transmission and plasticity: physiological role and possible implications in autism spectrum disorders

Serotonin type 7 receptors (5-HT7) are expressed in several brain areas, regulate brain development, synaptic transmission and plasticity, and therefore are involved in various brain functions such as learning and memory. A number of studies suggest that 5-HT7 receptors could be potential pharmacotherapeutic target for cognitive disorders. Several abnormalities of serotonergic system have been described in patients with autism spectrum disorder (ASD), including abnormal activity of 5-HT transporter, altered blood and brain 5-HT levels, reduced 5-HT synthesis and altered expression of 5-HT receptors in the brain. A specific role for 5-HT7 receptors in ASD has not yet been demonstrated but some evidence implicates their possible involvement. We have recently shown that 5-HT7 receptor activation rescues hippocampal synaptic plasticity in a mouse model of Fragile X Syndrome, a monogenic cause of autism. Several other studies have shown that 5-HT7 receptors modulate behavioral flexibility, exploratory behavior, mood disorders and epilepsy, which include core and co-morbid symptoms of ASD. These findings further suggest an involvement of 5-HT7 receptors in ASD. Here, we review the physiological roles of 5-HT7 receptors and their implications in Fragile X Syndrome and other ASD.

Prepuberal stimulation of 5-HT7-R by LP-211 in a rat model of hyper-activity and attention-deficit: permanent effects on attention, brain amino acids and synaptic markers in the fronto-striatal interface

The cross-talk at the prefronto-striatal interface involves excitatory amino acids, different receptors, transducers and modulators. We investigated long-term effects of a prepuberal, subchronic 5-HT7-R agonist (LP-211) on adult behaviour, amino acids and synaptic markers in a model for Attention-Deficit/Hyperactivity Disorder (ADHD). Naples High Excitability rats (NHE) and their Random Bred controls (NRB) were daily treated with LP-211 in the 5th and 6th postnatal week. One month after treatment, these rats were tested for indices of activity, non selective (NSA), selective spatial attention (SSA) and emotionality. The quantity of L-Glutamate (L-Glu), L-Aspartate (L-Asp) and L-Leucine (L-Leu), dopamine transporter (DAT), NMDAR1 subunit and CAMKIIα, were assessed in prefrontal cortex (PFC), dorsal (DS) and ventral striatum (VS), for their role in synaptic transmission, neural plasticity and information processing. Prepuberal LP-211 (at lower dose) reduced horizontal activity and (at higher dose) increased SSA, only for NHE but not in NRB rats. Prepuberal LP-211 increased, in NHE rats, L-Glu in the PFC and L-Asp in the VS (at 0.250 mg/kg dose), whereas (at 0.125 mg/kg dose) it decreased L-Glu and L-Asp in the DS. The L-Glu was decreased, at 0.125 mg/kg, only in the VS of NRB rats. The DAT levels were decreased with the 0.125 mg/kg dose (in the PFC), and increased with the 0.250 mg/kg dose (in the VS), significantly for NHE rats. The basal NMDAR1 level was higher in the PFC of NHE than NRB rats; LP-211 treatment (at 0.125 mg/kg dose) decreased NMDAR1 in the VS of NRB rats. This study represents a starting point about the impact of developmental 5-HT7-R activation on neuro-physiology of attentive processes, executive functions and their neural substrates.

The role of serotonin in memory: interactions with neurotransmitters and downstream signaling

Serotonin, or 5-hydroxytryptamine (5-HT), is found to be involved in many physiological or pathophysiological processes including cognitive function. Seven distinct receptors (5-HT1-7), each with several subpopulations, have been identified for serotonin, which are different in terms of localization and downstream signaling. Because of the development of selective agonists and antagonists for these receptors as well as transgenic animal models of cognitive disorders, our understanding of the role of serotonergic transmission in learning and memory has improved in recent years. A large body of evidence indicates the interplay between serotonergic transmission and other neurotransmitters including acetylcholine, dopamine, γ-aminobutyric acid (GABA) and glutamate, in the neurobiological control of learning and memory. In addition, there has been an alteration in the density of serotonergic receptors in aging and Alzheimer's disease, and serotonin modulators are found to alter the process of amyloidogenesis and exert cognitive-enhancing properties. Here, we discuss the serotonin-induced modulation of various systems involved in mnesic function including cholinergic, dopaminergic, GABAergic, glutamatergic transmissions as well as amyloidogenesis and intracellular pathways.

Modulation of 5-HT7 receptor: effect on object recognition performances in mice

OBJECTIVE

Recent data suggest that 5-HT7 receptors (5-HT7R) are involved in memory processes and, particularly, those related to novelty-induced arousal, even though this remains so far speculative and controversial. In order to assess the role of 5-HT7R in episodic-like memory, mice were administered 5-carboxamidotryptamine (5-CT, a 5-HT1A/1B/1D/7R agonist) and/or SB-269970 (a selective 5-HT7R antagonist) immediately after the acquisition session of the novel object recognition test.

MATERIALS AND METHODS

The object recognition test was performed in order to assess the effects of modulation of 5-HT7R during consolidation phase on episodic-like memory performances in mice. A protocol including 3 days of familiarisation to the apparatus has been realised in order to decrease the effect of novelty-induced arousal.

RESULTS

With a 2-h delay, SB-269970 (3 and 10 mg/kg, administered subcutaneously) impaired the discrimination of the novel object. With a 4-h delay, while control mice were not able to discriminate the novel object, mice treated with 5-CT (1 mg/kg) showed a significant discrimination. This promnesic effect with a long delay is effectively mediated by 5-HT7R activation since it was blocked by SB-269970 (10 mg/kg), but not by WAY-100135 (10 mg/kg) or by GR-127935 (10 mg/kg).

CONCLUSION

These data suggest that 5-HT7R tonically modulates cognitive processes involved in consolidation performances in object recognition. Therefore, 5-HT7R could be a promising target to treat memory dysfunctions (especially episodically related deficits) related to normal or pathological ageing.

Behavioral effects of SQSTM1/p62 overexpression in mice: support for a mitochondrial role in depression and anxiety

Affective spectrum and anxiety disorders have come to be recognized as the most prevalently diagnosed psychiatric disorders. Among a suite of potential causes, changes in mitochondrial energy metabolism and function have been associated with such disorders. Thus, proteins that specifically change mitochondrial functionality could be identified as molecular targets for drugs related to treatment for affective spectrum disorders. Here, we report generation of transgenic mice overexpressing the scaffolding and mitophagy related protein Sequestosome1 (SQSTM1/p62) or a single point mutant (P392L) in the UBA domain of SQSTM1/p62. We show that overexpression of SQSTM1/p62 increases mitochondrial energy output and improves transcription factor import into the mitochondrial matrix. These elevated levels of mitochondrial functionality correlate directly with discernible improvements in mouse behaviors related to affective spectrum and anxiety disorders. We also describe how overexpression of SQSTM1/p62 improves spatial learning and long term memory formation in these transgenic mice. These results suggest that SQSTM1/p62 provides an attractive target for therapeutic agents potentially suitable for the treatment of anxiety and affective spectrum disorders.

Human fear reconsolidation and allelic differences in serotonergic and dopaminergic genes

Fear memory persistence, central for the development and maintenance of anxiety disorders, is partially genetically controlled. Recently, consolidation and reconsolidation processes have been reported to affect fear memory stability and integrity. This study explored the impact of reconsolidation processes and genetic make-up on fear reacquisition by manipulating reconsolidation, using extinction performed outside or inside a reconsolidation interval. Reacquisition measured by skin conductance responses was stronger in individuals that extinguished outside (6 h) than inside (10 min) the reconsolidation interval. However, the effect was predominantly present in val/val homozygotes of the functional val158met polymorphism of the catechol O-methyltransferase (COMT) enzyme and in short-allele carriers of the serotonin-transporter length 5-HTTLPR polymorphism. These results demonstrate that reconsolidation of human fear memory is influenced by dopamine and serotonin-related genes.

Chronic administration of tibolone modulates anxiety-like behavior and enhances cognitive performance in ovariectomized rats

Hormone replacement therapy (HRT) may be prescribed to prevent the symptoms of menopause. This therapy may include estrogenic and/or progestin components and may increase the incidence of endometrial and breast cancers. Tibolone (TIB), which is also made up of estrogen and progestin components, is often used to reduce the impact of HRT. However, the effect of TIB on the processes of learning, memory and anxiety has yet to be fully elucidated. The aim of this study was to evaluate the long-term effect on learning, memory processes and anxiety in ovariectomized rats caused by different doses of TIB (0 mg/kg, 0.01 mg/kg, 0.1 mg/kg 1.0 mg/kg and 10 mg/kg, administered daily via the oral route for 18 weeks). Two behavioral animal models, the autoshaping and T maze models were employed. The concentrations of acetyl choline transferase (ChAT) and tryptophan hydroxylase (TPH) in the hippocampus were directly measured by Western blot. No significant changes were observed in the autoshaping model and spontaneous activity test. In the T maze, increased latency was observed with TIB doses of 1 and 10 mg/kg compared to the vehicle. We observed that the ChAT content decreased with increasing doses of TIB, whereas TPH content increased with doses of 1 and 10 mg/kg of TIB. These data indicate that high doses of TIB improved emotional learning, which may be related to the modulation of the cholinergic and serotonergic systems by TIB.

The 5-HT(7) receptor in learning and memory

The 5-HT(7) receptor is a more recently discovered G-protein-coupled receptor for serotonin. The functions and possible clinical relevance of this receptor are not yet fully understood. The present paper reviews to what extent the use of animal models of learning and memory and other techniques have implicated the 5-HT(7) receptor in such processes. The studies have used a combination of pharmacological and genetic tools targeting the receptor to evaluate effects on behavior and cellular mechanisms. In tests such as the Barnes maze, contextual fear conditioning and novel location recognition that involve spatial learning and memory there is a considerable amount of evidence supporting an involvement of the 5-HT(7) receptor. Supporting evidence has also been obtained in studies of mRNA expression and cellular signaling as well as in electrophysiological experiments. Especially interesting are the subtle but distinct effects observed in hippocampus-dependent models of place learning where impairments have been described in mice lacking the 5-HT(7) receptor or after administration of a selective antagonist. While more work is required, it appears that 5-HT(7) receptors are particularly important in allocentric representation processes. In instrumental learning tasks both procognitive effects and impairments in memory have been observed using pharmacological tools targeting the 5-HT(7) receptor. In conclusion, the use of pharmacological and genetic tools in animal studies of learning and memory suggest a potentially important role for the 5-HT(7) receptor in cognitive processes.

Role of the 5-HT7 receptor in the central nervous system: from current status to future perspectives

Pharmacological and genetic tools targeting the 5-hydroxytryptamine (5-HT)7 receptor in preclinical animal models have implicated this receptor in diverse (patho)physiological processes of the central nervous system (CNS). Some data obtained with 5-HT7 receptor knockout mice, selective antagonists, and, to a lesser extent, agonists, however, are quite contradictory. In this review, we not only discuss in detail the role of the 5-HT7 receptor in the CNS but also propose some hypothetical models, which could explain the observed inconsistencies. These models are based on two novel concepts within the field of G protein-coupled receptors (GPCR), namely biphasic signaling and G protein-independent signaling, which both have been shown to be mediated by GPCR dimerization. This led us to suggest that the 5-HT7 receptor could reside in different dimeric contexts and initiate different signaling pathways, depending on the neuronal circuitry and/or brain region. In conclusion, we highlight GPCR dimerization and G protein-independent signaling as two promising future directions in 5-HT7 receptor research, which ultimately might lead to the development of more efficient dimer- and/or pathway-specific therapeutics.

Hyperfunction of muscarinic receptor maintains long-term memory in 5-HT4 receptor knock-out mice

Patients suffering from dementia of Alzheimer's type express less serotonin 4 receptors (5-HTR₄), but whether an absence of these receptors modifies learning and memory is unexplored. In the spatial version of the Morris water maze, we show that 5-HTR₄ knock-out (KO) and wild-type (WT) mice performed similarly for spatial learning, short- and long-term retention. Since 5-HTR₄ control mnesic abilities, we tested whether cholinergic system had circumvented the absence of 5-HTR₄. Inactivating muscarinic receptor with scopolamine, at an ineffective dose (0.8 mg/kg) to alter memory in WT mice, decreased long-term but not short-term memory of 5-HTR₄ KO mice. Other changes included decreases in the activity of choline acetyltransferase (ChAT), the required enzyme for acetylcholine synthesis, in the septum and the dorsal hippocampus in 5-HTR₄ KO under baseline conditions. Training- and scopolamine-induced increase and decrease, respectively in ChAT activity in the septum in WT mice were not detected in the 5-HTR₄ KO animals. Findings suggest that adaptive changes in cholinergic systems may circumvent the absence of 5-HTR₄ to maintain long-term memory under baseline conditions. In contrast, despite adaptive mechanisms, the absence of 5-HTR₄ aggravates scopolamine-induced memory impairments. The mechanisms whereby 5-HTR₄ mediate a tonic influence on ChAT activity and muscarinic receptors remain to be determined.

Antipsychotic drug development

Schizophrenia typically manifests itself with a wide array of symptoms--positive, negative, cognitive, and affective--and may also involve neurodevelopmental and neurodegenerative aspects. Each of these symptom dimensions may be derived from pathology at one or more receptor types, localized in different regions of the brain. The absence of a single therapeutic target for schizophrenia has therefore prompted the de-emphasis of selective "magic bullets" and a critical re-examination of the intramolecular polypharmacy afforded by antipsychotics. In this chapter, we present a review of some of the receptor targets that are currently thought to mediate symptoms of schizophrenia, and discuss their possible implications for future antipsychotic drug development. Therapeutic strategies for schizophrenia that successfully exploit the multifunctionality of antipsychotics will take into account the entire receptor activity "portfolio" of the agent and provide a total therapeutic response that, like the elephant of the Buddhist parable, is greater than the sum of its parts.

Building a better antipsychotic: receptor targets for the treatment of multiple symptom dimensions of schizophrenia

Attempts to develop selective ("magic bullet") drugs for the treatment of schizophrenia have been frustrated by the complex etiology of the disease. The symptomatology of schizophrenia does not appear to arise from a single neurobiological entity, but rather may be derived from pathology at one or more receptor types. This has prompted multifactorial approaches to the development of new therapeutics, as embodied by polypharmacy and an alternative (or augmentative) strategy known as "intramolecular polypharmacy," in which a single drug possesses the capacity to affect multiple receptor types. Atypical antipsychotics are a well-known example of this approach; each atypical possesses a unique portfolio of activities at receptors that may contribute to therapeutic effects (as well as side effects). In this article we present a discussion of some of the receptor targets that are currently thought to mediate symptoms of schizophrenia, as well as their possible implications for the design of future multifunctional antipsychotics.

Ex vivo study of 5-HT(1A) and 5-HT(7) receptor agonists and antagonists on cAMP accumulation during memory formation and amnesia

The cyclic adenosine monophosphate (cAMP) is a second messenger and a central component of intracellular signaling pathways that regulate a wide range of biological functions, including memory. Hence, in this work, firstly the time-course of memory formation was determined in an autoshaping learning task, which had allowed the identification of testing times for increases or decreases in performance. Next, untrained, trained and overtrained groups were compared in cAMP production. Moreover, selective stimulation and antagonism of 5-HT(1A) and 5-HT(7) receptors during memory formation and cAMP production were determined. Finally, since there is scarce information about how pharmacological models of amnesia affect cAMP production, the cholinergic or glutamatergic antagonists, scopolamine and dizocilpine, were tested. The major findings of this work showed that when the time-course was determined inasmuch as training and testing sessions occurred, memory performance was graduate and progressive. Notably, for the fourth to seventh (i.e., 48-120 h following autoshaping training session) testing session performance was significantly higher from the previous ones. When animals received 5-HT(1A) and 5-HT(7) receptor agonists and antagonists or amnesic drugs significant increases or decrements in memory performance were observed at 24 and 48 h. Moreover, when ex vivo cAMP production from trained and overtrained groups were compared to untrained ones, significant differences were observed among groups and brain areas. Trained animals treated with 8-OHDPAT, AS19, 8-OHDPAT plus AS19, WAY100635, SB-269970, scopolamine or dizocilpine were compared to similar untrained groups, and eightfold-reduced cAMP production was evident, showing the importance of cAMP production in the signaling case in mammalian memory formation.

The role of 5-HT(1A) receptors in learning and memory

The ascending serotonin (5-HT) neurons innervate the cerebral cortex, hippocampus, septum and amygdala, all representing brain regions associated with various domains of cognition. The 5-HT innervation is diffuse and extensively arborized with few synaptic contacts, which indicates that 5-HT can affect a large number of neurons in a paracrine mode. Serotonin signaling is mediated by 14 receptor subtypes with different functional and transductional properties. The 5-HT(1A) subtype is of particular interest, since it is one of the main mediators of the action of 5-HT. Moreover, the 5-HT(1A) receptor regulates the activity of 5-HT neurons via autoreceptors, and it regulates the function of several neurotransmitter systems via postsynaptic receptors (heteroreceptors). This review assesses the pharmacological and genetic evidence that implicates the 5-HT(1A) receptor in learning and memory. The 5-HT(1A) receptors are in the position to influence the activity of glutamatergic, cholinergic and possibly GABAergic neurons in the cerebral cortex, hippocampus and in the septohippocampal projection, thereby affecting declarative and non-declarative memory functions. Moreover, the 5-HT(1A) receptor regulates several transduction mechanisms such as kinases and immediate early genes implicated in memory formation. Based on studies in rodents the stimulation of 5-HT(1A) receptors generally produces learning impairments by interfering with memory-encoding mechanisms. In contrast, antagonists of 5-HT(1A) receptors facilitate certain types of memory by enhancing hippocampal/cortical cholinergic and/or glutamatergic neurotransmission. Some data also support a potential role for the 5-HT(1A) receptor in memory consolidation. Available results also implicate the 5-HT(1A) receptor in the retrieval of aversive or emotional memories, supporting an involvement in reconsolidation. The contribution of 5-HT(1A) receptors in cognitive impairments in various psychiatric disorders is still unclear. However, there is evidence that 5-HT(1A) receptors may play differential roles in normal brain function and in psychopathological states. Taken together, the evidence indicates that the 5-HT(1A) receptor is a target for novel therapeutic advances in several neuropsychiatric disorders characterized by various cognitive deficits.

Cognitive dysfunction in neuropsychiatric disorders: selected serotonin receptor subtypes as therapeutic targets

The indolamine, serotonin (5-hydroxytryptamine-5-HT) was identified and initially characterized around the middle of the twentieth century and it is now known to participate in multiple physiologic processes in mammalians. As a neurotransmitter, 5-HT is well documented to play a significant role in the pathophysiology and treatment of a variety of psychiatric disorders including anxiety, depression, and schizophrenia. In addition, there is also some evidence to suggest that 5-HT function in the brain may be important (particularly in the behavioral disturbances) in various forms of dementia including Alzheimer's disease. While 5-HT is undoubtedly involved in cognitive function, its role in specific domains of cognition (attention, learning, and memory, etc.) is poorly understood. This understanding has been impeded to some extent by the many complex interactions between 5-HT neurons and other neuronal phenotypes, 5-HT receptor heterogeneity, and the conflicting results of some behavioral experiments in animals conducted to date. Through the combined use of modern molecular biology, transgenic animal models, and other more traditional research methods such as medicinal chemistry and classical pharmacology, a clearer picture of the role of serotonin and its receptor subtypes in mnemonic processes is beginning to emerge, however. Considerable data now support the argument that selective ligands at specific 5-HT receptor subtypes can serve as therapeutic agents designed to enhance cognitive function in psychiatric disorders such as schizophrenia as well as age-related neurodegenerative illnesses such as Alzheimer's disease. The purpose of this review is to provide a brief overview of these therapeutic targets within the 5-HT system and the pharmacologic approaches (including the most recently developed compounds) designed to enhance memory function.

The 5-HT7 receptor: Role in novel object discrimination and relation to novelty-seeking behavior

Despite showing high affinity for neuroleptics and hallucinogens, the function of the 5-HT7 receptor in cognition remains largely speculative. This study tests the hypothesis that 5-HT7 participates in gauging salience of novel visual stimuli as a function of the animal's initial tendency for novelty-seeking. Novelty-seeking behavior in the rat is thought to model some aspects of sensation-seeking in humans, a personality trait closely associated to drug abuse. We analyzed the effects of the 5-HT7 receptor antagonist SB269.970 (3 mg kg(-1) or 15 mg kg(-1) i.p.) on object-recognition tasks using rats that differed in exploration of novel environments, namely high (HR) and low (LR) responders. The task involved a first encounter with an object ("old"), which after a delay of 3 h had to be discriminated from a different object ("new"). The antagonist was injected into HR and LR rats immediately after the first encounter with the objects and its effects on recall of objects were evaluated. In the absence of drug, LR but not HR rats were able to discriminate the familiarity of previously encountered objects. A low dose (3 mg kg(-1)) of SB269.970 was ineffective in altering object discrimination. A higher dose (15 mg kg(-1)) inhibited novel-object exploration in LR animals thus curtailing differences in object recognition, a finding that was replicated. In order to validate our studies, the effects of the cholinergic muscarinic antagonist scopolamine (0.2 mg kg(-1), i.p.) on object recognition were also evaluated in one of the cohorts 2 weeks after the first NOD experiment. In the Choice phase, all vehicle-treated rats succeeded in recognizing the new object. Scopolamine inhibited object discrimination in HR rats more efficiently than it did in LR rats. Taken together, these results suggest that 5-HT7 may mediate attentional and memory processes relevant to novelty-induced arousal.

Molecular targets for treating cognitive dysfunction in schizophrenia

Cognitive impairment is a core feature of schizophrenia as deficits are present in the majority of patients, frequently precede the onset of other positive symptoms, persist even with successful treatment of positive symptoms, and account for a significant portion of functional impairment in schizophrenia. While the atypical antipsychotics have produced incremental improvements in the cognitive function of patients with schizophrenia, overall treatment remains inadequate. In recent years, there has been an increased interest in developing novel strategies for treating the cognitive deficits in schizophrenia, focusing on ameliorating impairments in working memory, attention, and social cognition. Here we review various molecular targets that are actively being explored for potential drug discovery efforts in schizophrenia and cognition. These molecular targets include dopamine receptors in the prefrontal cortex, nicotinic and muscarinic acetylcholine receptors, the glutamatergic excitatory synapse, various serotonin receptors, and the gamma-aminobutyric acid (GABA) system.

Do serotonin1–7 receptors modulate short and long-term memory?

Evidence from invertebrates to human studies indicates that serotonin (5-hydroxytryptamine; 5-HT) system modulates short- (STM) and long-term memory (LTM). This work is primarily focused on analyzing the contribution of 5-HT, cholinergic and glutamatergic receptors as well as protein synthesis to STM and LTM of an autoshaping learning task. It was observed that the inhibition of hippocampal protein synthesis or new mRNA did not produce a significant effect on autoshaping STM performance but it did impair LTM. Both non-contingent protein inhibition and 5-HT depletion showed no effects. It was basically the non-selective 5-HT receptor antagonist cyproheptadine, which facilitated STM. However, the blockade of glutamatergic and cholinergic transmission impaired STM. In contrast, the selective 5-HT(1B) receptor antagonist SB-224289 facilitated both STM and LTM. Selective receptor antagonists for the 5-HT(1A) (WAY100635), 5-HT(1D) (GR127935), 5-HT(2A) (MDL100907), 5-HT(2C/2B) (SB-200646), 5-HT(3) (ondansetron) or 5-HT(4) (GR125487), 5-HT(6) (Ro 04-6790, SB-399885 and SB-35713) or 5-HT(7) (SB-269970) did not impact STM. Nevertheless, WAY100635, MDL100907, SB-200646, GR125487, Ro 04-6790, SB-399885 or SB-357134 facilitated LTM. Notably, some of these changes shown to be independent of food-intake. Concomitantly, these data indicate that '5-HT tone via 5-HT(1B) receptors' might function in a serial manner from STM to LTM, whereas working in parallel using 5-HT(1A), 5-HT(2A), 5-HT(2B/2C), 5-HT(4), or 5-HT(6) receptors.

5-HT1A receptors and memory

The study of 5-hydroxytryptamine (5-HT) systems has benefited from the identification, classification and cloning of multiple 5-HT receptors (5-HT(1)-5-HT(7)). Increasing evidence suggests that 5-HT pathways, reuptake site/transporter complex and 5-HT receptors represent a strategic distribution for learning and memory. A key question still remaining is whether 5-HT markers (e.g., receptors) are directly or indirectly contributing to the physiological and pharmacological basis of memory and its pathogenesis or, rather, if they represent protective or adaptable mechanisms (at least in initial stages). In the current paper, the major aim is to revise recent advances regarding mammalian 5-HT(1A) receptors in light of their physiological, pathophysiological and therapeutic implications in memory. An attempt is made to identify and discuss sources of discrepancies by employing an analytic approach to examine the nature and degree of difficulty of behavioral tasks used, as well as implicating other factors (for example, brain areas, training time or duration, and drug administration) which might offer new insights into the understanding and interpretation of these data. In this context, 8-OH-DPAT deserves special attention since for many years it has been the more selective 5-HT drug and, hence, more frequently used. As 5-HT(1A) receptors are key components of serotonergic signaling, investigation of their memory mechanisms and action sites and the conditions under which they might operate, could yield valuable insights. Moreover, selective drugs with agonists, neutral antagonists or inverse agonist properties for 5-HT(1A) (and 5-HT(7)) receptors may constitute a new therapeutic opportunity for learning and memory disorders.

An mRNA expression analysis of stimulation and blockade of 5-HT7 receptors during memory consolidation

Despite the compelling support for 5-hydroxytryptamine (5-HT) receptors participation in learning and memory in mammal species, the molecular basis had been largely absent from any discussion of its mechanistic underpinnings. Here, we report that reverse transcriptase-polymerase chain reaction (RT-PCR) analysis revealed that there was a higher level of expression of the investigated 5-HT receptor mRNAs in autoshaping-trained relative to untrained groups. Actually, pharmacological naïve untrained and autoshaping-trained rats showed significant differences, the latter groups expressing, in decreasing order, 5-HT1A < 5-HT6 < 5-HT4 < or = 5-HT7 receptors mRNA in prefrontal cortex and hippocampus. In order to determine more precisely mRNA expression and memory consolidation, we combined selective 5-HT7 receptors stimulation or blockade in the same animals, and brain areas individually analyzed. 5-HT7 receptors were strongly expressed in all the three brain areas of vehicle-trained rats relative to untrained group. The potential selective 5-HT7 receptor agonist AS 19 enhanced memory consolidation, attenuated mRNA receptors expression, and the facilitatory memory effect was reversed by SB-269970. Finally, pharmacological stimulation of 5-HT7 receptors reversed scopolamine- or dizocilpine-induced amnesia and receptor down-regulation.

5-Hydroxytryptamine 1A receptor blockade facilitates aversive learning in mice: interactions with cholinergic and glutamatergic mechanisms

The effects of 5-hydroxytryptamine 1A (5-HT(1A)) receptor ligands on aversive learning were examined in the passive avoidance (PA) task in mice. Anxiety and autonomic functions were investigated using the elevated plus-maze and heart rate measurements. The main findings from this study are as follows. 1) Pretraining administration of the 5-HT(1A) receptor agonist 8-OH-DPAT [8-hydroxy-2-(di-n-propylamino)tetralin hydrobromide] facilitated PA retention at low doses (0.01 and 0.03 mg/kg) but impaired PA retention at higher doses (0.1-1.0 mg/kg), consistent with previous findings in the rat. 2) Similar to the acetylcholinesterase inhibitor physostigmine, pretraining administration of the 5-HT(1A) receptor antagonists [(R)-3-N,N-dicyclobutylamino-8 fluoro-3,4-dihydro-3H-1-benzopyran-5-carboxamide hydrogen(2R,3R)-tartrate monohydrate] NAD-299 (0.1-2 mg/kg) and [N-2-4-(2-methoxyphenyl)-1-piperazinylethyl-N-(2-pyridinyl)cyclohexane carboxamide trihydrochloride] WAY-100635 (0.3-3 mg/kg) enhanced PA retention. 3) The impairment (1 mg/kg) but not the facilitation (0.03 mg/kg) induced by 8-OH-DPAT was fully blocked by NAD-299 (0.3 mg/kg). 4) 5-HT(1A) receptor ligands given immediate post-training failed to alter PA retention. 5) NAD-299 (0.3-1 mg/kg) blocked the impairment of PA retention caused by a) the nonselective muscarinic receptor antagonist scopolamine and b) the non-competitive N-methyl-D-aspartate receptor antagonist MK-801 [(5R,10S)-(+)-5-methyl-10,11-dihydro-5H-dibenzo(a,d)cyclohepten-5,10-imine hydrogen maleate]. 6) A subthreshold dose of scopolamine completely blocked the facilitatory effect of NAD-299 on PA retention. 7) Anxiety-related behaviors and autonomic function were unchanged by NAD-299. 8) In situ hybridization showed that septal neurons expressing 5-HT(1A) receptor mRNA were codistributed with markers for cholinergic, GABAergic, and glutamatergic neurons. These results indicate that systemic administration of 5-HT(1A) receptor antagonists can facilitate cognitive performance, most likely by enhancing hippocampal/cortical cholinergic and glutamatergic neurotransmissions. Selective 5-HT(1A) receptor antagonists may be useful in the treatment of cognitive deficits such as Alzheimer's disease.

5-HT1A receptor expression during memory formation

RATIONALE

It has been reported that 5-HT(1A) receptors modulate learning and memory and diverse pharmacological and genetic evidence supports this notion. Nevertheless, there are few works about expression of these receptors during memory formation.

OBJECTIVE

We aimed to determine 5-HT(1A) receptor expression in brain areas of untrained, passive, and autoshaping trained groups of rats.

METHODS

Ex vivo receptor autoradiography using the ligand agonist [(3)H]8-hydroxy-2-[di-n-propylamino]tetralin] (8-OH-DPAT) was used.

RESULTS

The trained group relative to untrained animals showed increases of 5-HT(1A) receptor expression in 14 brain areas, decrements in 7, and no changes in 12. Thus, in contrast to untrained rats, 5-HT(1A) receptor expression of autoshaping trained rats was augmented in the tubercule olfactory, septal nucleus, nucleus accumbens, caudate putamen, globus pallidus, striate, and parietal (1 and 2), temporal cortex (1 and 3), granular retrosplenial cortex (1), amygdala, and median and dorsal raphe nuclei. In contrast, in the latter group, receptors were decreased in the CA1 area, hypothalamus dorsal, frontal cortex (1 and 3), occipital cortex, cingulate cortex (1 and 2), and cuneiform nucleus. There were significant differences between passive vs trained groups, but not regarding untrained rats, in the lateral olfactory tract, dentate gyrus, CA3 area, ventromedial hypothalamic, lateral hypothalamus, preoptic medial, frontal cortex (2), granular retrosplenial cortex (2), entorhinal cortex (1 and 2), piriform cortex, and substantia nigra.

CONCLUSIONS

These data suggest that upregulated, downregulated, and "silence" of 5-HT(1A) receptors in brain areas form part of neural circuits engaged in memory formation by demonstrating a high degree of specificity and memory mapping.

Effects of the potential 5-HT7 receptor agonist AS 19 in an autoshaping learning task

This work aimed to evaluate further the role of 5-HT7 receptors during memory formation in an autoshaping Pavlovian/instrumental learning task. Post-training administration of the potential 5-HT7 receptor agonist AS 19 or antagonist SB-269970 enhanced memory formation or had no effect, respectively. The AS 19 facilitatory effect was reversed by SB-269970, but not by the selective 5-HT1A antagonist WAY100635. Amnesia induced by scopolamine (cholinergic antagonist) or dizocilpine (NMDA antagonist) was also reversed by AS 19. Certainly, reservations regarding the selectivity of AS 19 for 5-HT7 and other 5-HT receptors in vivo are noteworthy and, therefore, its validity for use in animal models as a pharmacological tool. Having mentioned that, it should be noticed that together these data are providing further support to the notion of the 5-HT7 receptors role in memory formation. Importantly, this 5-HT7 receptor agonist AS 19 appears to represent a step forward respect to the notion that potent and selective 5-HT7 receptor agonists can be useful in the treatment of dysfunctional memory in aged-related decline and Alzheimer's disease.

Modifications of 5-HT4 receptor expression in rat brain during memory consolidation

Pharmacological evidence indicates a specific role of 5-HT(4) receptors on memory function. These receptors are members of G-protein-coupled 7-transmembrane domain receptor superfamily, are positively coupled to adenylyl cyclase, and are heterogeneously located in some structures important for memory, such as the hippocampus and cortical regions. To further clarify 5-HT(4) receptors' role in memory, the expression of these receptors in passive (P3) untrained and autoshaping (A3) trained (3 sessions) adult (3 months) and old (P9 or A9; 9 months) male rats was determined by autoradiography. Adult trained (A3) rats showed a better memory respect to old trained (A9). Using [(3)H] GR113808 as ligand (0.2 nM specific activity 81 Ci/mmol) for 5-HT(4) receptor expression, 29 brain areas were analyzed, 16 areas of A3 and 17 of A9 animals displayed significant changes. The medial mammillary nucleus of A3 group showed diminished 5-HT(4) receptor expression, and in other 15 brain areas of A3 or 10 of A9 animals, 5-HT(4) receptors were increased. Thus, for A3 rats, 5-HT(4) receptors were augmented in olfactory lobule, caudate putamen, fundus striatum, CA2, retrosplenial, frontal, temporal, occipital, and cingulate cortex. Also, 5-HT(4) receptors were increased in olfactory tubercule, hippocampal CA1, parietal, piriform, and cingulate cortex of A9. However, hippocampal CA2 and CA3 areas, and frontal, parietal, and temporal cortex of A9 rats, expressed less 5-HT(4) receptors. These findings suggest that serotonergic activity, via 5-HT(4) receptors in hippocampal, striatum, and cortical areas, mediates memory function and provides further evidence for a complex and regionally specific regulation over 5-HT receptor expression during memory formation.