The 5-HT1A receptor and recognition memory

Involvement of the GABAergic and the serotonergic systems in the anxiolytic effects expressed by the nitric oxide (NO) donor sodium nitroprusside (SNP) in the male rat

RATIONALE

Anxiety is a chronic severe psychiatric disorder. In a series of studies, the implication of the gaseous molecule nitric oxide (NO) in anxiety has been evidenced. Further, the outcome of preclinical research suggests that different NO donors, including sodium nitroprusside (SNP), have expressed an anxiolytic profile revealed in animal models of anxiety. Regardless of this, it is not yet clarified the mechanism(s) of action by which SNP induces its beneficial effects on anxiety. In this context, it has been hypothesized that these effects might be attributed to a potential interaction of this NO donor with the GABA type A and the 5-HT1A serotonergic receptors.

OBJECTIVES

The current study was designed to investigate this issue in the male rat.

METHODS

To this end, the light/dark box and the open field tests were utilized.

RESULTS

SNP (1 mg/kg, i.p.) applied acutely induced an anti-anxiety-like effect evidenced either in the light/dark box or in the open field test. Either the GABAA receptor antagonist flumazenil (10 mg/kg, i.p.) or the 5-HT1A serotonin receptor agonist 8-OH-DPAT (0.25 mg/kg, i.p.) suppressed the above reported anxiolytic effects of SNP.

CONCLUSIONS

The results here reported propose a functional interaction between SNP with the GABAergic and the serotonergic systems on anxiety and thus, might offer a plausible explanation for SNP's anxiolytic effects.

5-HT1A and 5-HT2A receptor effects on recognition memory, motor/exploratory behaviors, emotionality and regional dopamine transporter binding in the rat

Both dopamine (DA) and serotonin (5-HT) play key roles in numerous functions including motor control, stress response and learning. So far, there is scarce or conflicting evidence about the effects of 5-HT1A and 5-HT2A receptor (R) agonists and antagonists on recognition memory in the rat. This also holds for their effect on cerebral DA as well as 5-HT release. In the present study, we assessed the effects of the 5-HT1AR agonist 8-OH-DPAT and antagonist WAY100,635 and the 5-HT2AR agonist DOI and antagonist altanserin (ALT) on rat behaviors. Moreover, we investigated their impact on monoamine efflux by measuring monoamine transporter binding in various regions of the rat brain. After injection of either 8-OH-DPAT (3 mg/kg), WAY100,635 (0.4 mg/kg), DOI (0.1 mg/kg), ALT (1 mg/kg) or the respective vehicle (saline, DMSO), rats underwent an object and place recognition memory test in the open field. Upon the assessment of object exploration, motor/exploratory parameters and feces excretion, rats were administered the monoamine transporter radioligand N-o-fluoropropyl-2b-carbomethoxy-3b-(4-[123I]iodophenyl)-nortropane ([123I]-FP-CIT; 8.9 ± 2.6 MBq) into the tail vein. Regional radioactivity accumulations in the rat brain were determined post mortem. Compared vehicle, administration of 8-OH-DPAT impaired memory for place, decreased rearing behavior, and increased ambulation as well as head-shoulder movements. DOI administration led to a reduction in rearing behavior but an increase in head-shoulder motility relative to vehicle. Feces excretion was diminished after ALT relative to vehicle. Dopamine transporter (DAT) binding was increased in the caudateputamen (CP), but decreased in the nucleus accumbens (NAC) after 8-OH-DPAT relative to vehicle. Moreover, DAT binding was decreased in the NAC after ALT relative to vehicle. Findings indicate that 5-HT1AR inhibition and 5-HT2AR activation may impair memory for place. Furthermore, results imply associations not only between recognition memory, motor/exploratory behavior and emotionality but also between the respective parameters and the levels of available DA in CP and NAC.

The 5-HT1A receptor agonist 8-OH-DPAT modulates motor/exploratory activity, recognition memory and dopamine transporter binding in the dorsal and ventral striatum

In the present studies, we assessed the effect of the 5-HT1A receptor (R) agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) on motor and exploratory behaviors, object and place recognition and dopamine transporter (DAT) and serotonin transporter (SERT) binding in the rat brain. In Experiment I, motor/exploratory behaviors were assessed in an open field after injection of either 8-OH-DPAT (0.1 and 3 mg/kg) or vehicle for 30 min without previous habituation to the open field. In Experiment II, rats underwent a 5-min exploration trial in an open field with two identical objects. After injection of either 8-OH-DPAT (0.1 and 3 mg/kg) or vehicle, rats underwent a 5-min test trial with one of the objects replaced by a novel one and the other object transferred to a novel place. Subsequently, N-o-fluoropropyl-2b-carbomethoxy-3b-(4-[123I]iodophenyl)-nortropane ([123I]FP-CIT; 11 ± 4 MBq) was injected into the tail vein. Regional radioactivity accumulations were determined post mortem with a well counter. In both experiments, 8-OH-DPAT dose-dependently increased ambulation and exploratory head-shoulder motility, whereas rearing was dose-dependently decreased. In the test rial of Experiment II, there were no effects of 8-OH-DPAT on overall activity, sitting and grooming. 8-OH-DPAT dose-dependently impaired recognition of object and place. 8-OH-DPAT (3 mg/kg) increased DAT binding in the dorsal striatum relative to both vehicle and 0.1 mg/kg 8-OH-DPAT. Furthermore, in the ventral striatum, DAT binding was decreased after 3 mg/kg 8-OH-DPAT relative to vehicle. Findings indicate that motor/exploratory behaviors, memory for object and place and regional dopamine function may be modulated by the 5-HT1AR. Since, after 8-OH-DPAT, rats exhibited more horizontal and less (exploratory) vertical motor activity, while overall activity was not different between groups, it may be inferred, that the observed impairment of object recognition was not related to a decrease of motor activity as such, but to a decrease of intrinsic motivation, attention and/or awareness, which are relevant accessories of learning. Furthermore, the present findings on 8-OH-DPAT action indicate associations not only between motor/exploratory behavior and the recognition of object and place but also between the respective parameters and the levels of available DA in dorsal and ventral striatum.

Alterations of learning and memory are accompanied by alterations in the expression of 5-HT receptors, glucocorticoid receptor and brain-derived neurotrophic factor in different brain regions of an animal model of depression generated by neonatally male treatment with clomipramine in male rats

Depressive illness has been associated with impaired cognitive processes accompanied by reduced neurotrophin levels, especially brain-derived neurotrophic factor (BDNF), and dysfunctions in the hypothalamic-pituitary-adrenal (HPA) axis. In addition, depression is characterized by a decreased functioning of the serotonergic system due to changes in the activity or expression of its receptors including, most significantly, 5-HT1A, 5-HT2A, and 5-HT3 in brain regions that regulate mood, emotions, and memory, such as the prefrontal cortex, hippocampus, and amygdala. In this regard, rats treated with clomipramine (CMI) in the neonatal stage show depression-like behaviors that persist into adulthood; hence, this constitutes an adequate model of depression for exploring various molecular aspects associated with the etiology of this disorder. This, study, then, was designed to analyze the long-term effects of early postnatal exposure to CMI on the expression of 5-HT1A, 5-HT2A, and 5-HT3 receptors, as well as BDNF and GR in the following brain regions: PFC, amygdala, hippocampus, and hypothalamus, which could be related to alterations in memory and learning, as evaluated using the novel object recognition (NOR) and Morris water maze (MWM). Expression of the 5-HT1A, 5-HT2A, and 5-HT3 receptors, BDNF, and the glucocorticoid receptor (GR) was assessed by RT-qPCR in the four aforementioned brain regions, all of which play important roles in the control of memory and mood. Findings show that neonatal treatment with CMI causes alterations in memory and learning, as indicated by alterations in the results of the MWM and NOR tests. Expression of the 5-HT1A receptor increased in the hippocampus, amygdala, and hypothalamus, but decreased in the PFC, while the 5-HT2A and BDNF receptors decreased their expression in the PFC, amygdala, and hippocampus. There was no change in the expression of the 5-HT3 receptor. In addition, expression of GR in the hippocampus and PFC was low, but increased in the hypothalamus. Taken together, these data show that neonatal CMI treatment produces permanent molecular changes in brain regions related to learning and memory that could contribute to explaining the behavioral alterations observed in this model.

Ghrelin Is Effective on Passive Avoidance Memory by Altering the Expression of NMDAR and HTR1a Genes in the Hippocampus of Male Wistar Rats

Background

Memory-dependent psychological behaviors have an important role in life. Memory strengthening in adulthood to prevent its defects in aging is a significant issue. The ghrelin endogenous hormone improves memory by targeting glutamatergic and serotonergic circuits. Also, citicoline, a memory strengthening drug in aging, is not recommended to adults due to its side effects. The current study aims to test that ghrelin treatment, like citicoline, would improve passive avoidance memory via expression of the genes encoding the N-methyl-D-aspartate receptor (NMDAR1) and the serotonin receptor 1A (HTR1α) involved in this process.

Methods

Five groups of adult male rats received (1) saline (as control), (2) 0.5 mg/kg citicoline, or (3-5) 0.3, 1.5, and 3 nmol/μl ghrelin). The rats received the drugs via intra-hippocampal injection. Passive avoidance memory was determined using a shuttle box device. The latency to enter the dark chamber before (IL) and after (RL) injection and the total duration of the animal's presence in the light compartment (TLC) were evaluated. Then, the gene expression rates of NMDAR1 and HTR1α were measured by the Real-Time PCR.

Results

Ghrelin and citicoline had some similar and significant effects on passive avoidance memory, and both increased NMDAR1 and decreased HTR1α expression.

Conclusion

Ghrelin, like citicoline, improves passive avoidance learning by altering the NMDAR1 and HTR1α expression in the hippocampus.

5-HT1AR alleviates Aβ-induced cognitive decline and neuroinflammation through crosstalk with NF-κB pathway in mice

The 5-hydroxytryptamine (5-HT) receptor is significant for the regulation of mood and memory. However, the role of 5-HT1AR in β-Amyloid protein (Aβ)-induced cognitive decline, neuroinflammation and the possible mechanism remains elusive. Thus, we aimed to evaluate the effects of 5-HT1AR on Aβ-induced learning and memory decline and neuroinflammation in mice. Novel object recognition and Morris water maze tests were performed to observe learning and memory behavior in mice. Protein levels of Iba1, GFAP, MAP2, TNF-α, Tβ4, C-fos, IKK-β, IKB-α, NF-κBp65, phospho-NF-κBp65 in the hippocampus were examined by immunostaining or western blotting. Aβ1-42-treatment inducing learning and memory decline was shown in novel object recognition and Morris water maze tests; neuroinflammation shown in immunostaining. Our study found out that 5-HT1AR inhibitor WAY100635 showed significant improvement in Aβ-induced learning and memory decline. Moreover, WAY100635 decreases levels of Iba1, GFAP, and TNF-α in the hippocampus, which were related to neuroinflammation. While treatment with 5-HT1AR agonist 8-OH-DPAT or ERK inhibitor U0126 exerted no effects or even aggravated Aβ-induced learning and memory decline. In addition, WAY100635 could downregulate phospho-NF-κB in the hippocampus of Aβ1-42-injected mice. These results provide new insight into the mechanism, for 5-HT1AR in Aβ-induced cognitive impairments through crosstalk with the NF-κB signaling pathway. Our data indicated that WAY100635 was involved in the protective effects against neuroinflammation and improvement of learning and memory in Alzheimer's disease.

The effect of NAD-299 and TCB-2 on learning and memory, hippocampal BDNF levels and amyloid plaques in Streptozotocin-induced memory deficits in male rats

RATIONALE

Alzheimer's disease (AD) is the most common form of dementia characterized by a progressive decline in cognitive function. The serotonergic system via the 5-HT1A receptor and 5-HT2A receptor is proposed to affect the cognitive process.

OBJECTIVE

In the present study, the effects of NAD-299 (5-HT1AR antagonist) and TCB-2 (5-HT2AR agonist) on learning and memory processes, hippocampal brain-derived neurotrophic factor (BDNF) levels, neuronal necrosis, and Aβ plaque production have been investigated on the intracerebroventricular (icv) injection of streptozotocin (STZ)-induced memory deficits in rats.

METHODS

Fifty-four adult male Wistar rats (250-300 g) were divided into six groups (n = 9 in each group): control, sham-operated, AD (icv-STZ (3 mg/kg, 10 μl)), AD+NAD-299 (5 μg/1 μl icv for 30 days), AD+TCB-2 (5 μg/1 μl icv for 30 days), and AD+NAD-299 + TCB-2 (NAD-299 (5 μg/0.5 μl icv) and TCB-2 (5 μg/0.5 μl icv) for 30 days). Following the treatment period, rats were subjected to behavioral tests of learning and memory. Then, hippocampal BDNF, amyloid-beta (Aβ) plaque, and neuronal loss were determined by ELISA Kit, Congo red staining, and Nissl staining, respectively.

RESULTS

The results of behavioral tests showed that icv-STZ injection decreased the discrimination index in the novel object recognition (NOR) test. In the passive avoidance learning (PAL) task, icv-STZ injection significantly decreased step-through latency (STLr) and increased time spent in dark compartment (TDC). Treatment with NAD-299, TCB-2, and NAD-299 + TCB-2 attenuated the STZ-induced memory impairment in both NOR and PAL tasks. icv-STZ induced a decrease in hippocampal BDNF levels and increased Aβ plaques production in the brain, whereas treatment with NAD-299, TCB-2, and NAD-299 + TCB-2 reduced Aβ plaques in the brain and increased the hippocampal BDNF level. Results of Nissl staining showed that icv-STZ injection increased neuronal loss in the hippocampus, while treatment with NAD-299, TCB-2, and NAD-299 + TCB-2 reduced hippocampal neurodegeneration.

CONCLUSION

These findings suggest that 5-HT1AR blockade by NAD-299 and 5-HT2AR activation by TCB-2 improve cognitive dysfunction in icv-STZ-treated rats, and these drugs may potentially prevent the progression of AD.

Potential functional and pathological side effects related to off-target pharmacological activity

Most pharmaceutical companies test their discovery-stage proprietary molecules in a battery of in vitro pharmacology assays to try to determine off-target interactions. During all phases of drug discovery and development, various questions arise regarding potential side effects associated with such off-target pharmacological activity. Here we present a scientific literature curation effort undertaken to determine and summarize the most likely functional and pathological outcomes associated with interactions at 70 receptors, enzymes, ion channels and transporters with established links to adverse effects. To that end, the scientific literature was reviewed using an on-line database, and the most commonly reported effects were summarized in tabular format. The resultant table should serve as a practical guide for research scientists and clinical investigators for the prediction and interpretation of adverse side effects associated with molecules interacting with components of this screening battery.

A critical evaluation of the activity-regulated cytoskeleton-associated protein (Arc/Arg3.1)'s putative role in regulating dendritic plasticity, cognitive processes, and mood in animal models of depression

Major depressive disorder (MDD) is primarily conceptualized as a mood disorder but cognitive dysfunction is also prevalent, and may limit the daily function of MDD patients. Current theories on MDD highlight disturbances in dendritic plasticity in its pathophysiology, which could conceivably play a role in the production of both MDD-related mood and cognitive symptoms. This paper attempts to review the accumulated knowledge on the basic biology of the activity-regulated cytoskeleton-associated protein (Arc or Arg3.1), its effects on neural plasticity, and how these may be related to mood or cognitive dysfunction in animal models of MDD. On a cellular level, Arc plays an important role in modulating dendritic spine density and remodeling. Arc also has a close, bidirectional relationship with postsynaptic glutamate neurotransmission, since it is stimulated by multiple glutamatergic receptor mechanisms but also modulates α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor internalization. The effects on AMPA receptor trafficking are likely related to Arc's ability to modulate phenomena such as long-term potentiation, long-term depression, and synaptic scaling, each of which are important for maintaining proper cognitive function. Chronic stress models of MDD in animals show suppressed Arc expression in the frontal cortex but elevation in the amygdala. Interestingly, cognitive tasks depending on the frontal cortex are generally impaired by chronic stress, while those depending on the amygdala are enhanced, and antidepressant treatments stimulate cortical Arc expression with a timeline that is reminiscent of the treatment efficacy lag observed in the clinic or in preclinical models. However, pharmacological treatments that stimulate regional Arc expression do not universally improve relevant cognitive functions, and this highlights a need to further refine our understanding of Arc on a subcellular and network level.

Predicting targets of compounds against neurological diseases using cheminformatic methodology

Recently developed multi-targeted ligands are novel drug candidates able to interact with monoamine oxidase A and B; acetylcholinesterase and butyrylcholinesterase; or with histamine N-methyltransferase and histamine H3-receptor (H3R). These proteins are drug targets in the treatment of depression, Alzheimer's disease, obsessive disorders, and Parkinson's disease. A probabilistic method, the Parzen-Rosenblatt window approach, was used to build a "predictor" model using data collected from the ChEMBL database. The model can be used to predict both the primary pharmaceutical target and off-targets of a compound based on its structure. Molecular structures were represented based on the circular fingerprint methodology. The same approach was used to build a "predictor" model from the DrugBank dataset to determine the main pharmacological groups of the compound. The study of off-target interactions is now recognised as crucial to the understanding of both drug action and toxicology. Primary pharmaceutical targets and off-targets for the novel multi-target ligands were examined by use of the developed cheminformatic method. Several multi-target ligands were selected for further study, as compounds with possible additional beneficial pharmacological activities. The cheminformatic targets identifications were in agreement with four 3D-QSAR (H3R/D1R/D2R/5-HT2aR) models and by in vitro assays for serotonin 5-HT1a and 5-HT2a receptor binding of the most promising ligand (71/MBA-VEG8).

Spinal nNOS regulates phrenic motor facilitation by a 5-HT2B receptor- and NADPH oxidase-dependent mechanism

Acute intermittent hypoxia (AIH) induces phrenic long-term facilitation (pLTF) by a mechanism that requires spinal serotonin (5-HT) receptor activation and NADPH oxidase (NOX) activity. Here, we investigated whether: (1) spinal nitric oxide synthase (NOS) activity is necessary for AIH-induced pLTF; (2) episodic exogenous nitric oxide (NO) is sufficient to elicit phrenic motor facilitation (pMF) without AIH (i.e. pharmacologically); and (3) NO-induced pMF requires spinal 5-HT2B receptor and NOX activation. In anesthetized, mechanically ventilated adult male rats, AIH (3 × 5-min episodes; 10% O2; 5 min) elicited a progressive increase in the amplitude of integrated phrenic nerve bursts (i.e. pLTF), which lasted 60 min post-AIH (45.1 ± 8.6% baseline). Pre-treatment with intrathecal (i.t.) injections of a neuronal NOS inhibitor (nNOS-inhibitor-1) near the phrenic motor nucleus attenuated pLTF (14.7 ± 2.5%), whereas an inducible NOS (iNOS) inhibitor (1400 W) had no effect (56.3 ± 8.0%). Episodic i.t. injections (3 × 5μl volume; 5 min) of a NO donor (sodium nitroprusside; SNP) elicited pMF similar in time-course and magnitude (40.4 ± 6.0%, 60 min post-injection) to AIH-induced pLTF. SNP-induced pMF was blocked by a 5-HT2B receptor antagonist (SB206553), a superoxide dismutase mimetic (MnTMPyP), and two NOX inhibitors (apocynin and DPI). Neither pLTF nor pMF was affected by pre-treatment with a protein kinase G (PKG) inhibitor (KT-5823). Thus, spinal nNOS activity is necessary for AIH-induced pLTF, and episodic spinal NO is sufficient to elicit pMF by a mechanism that requires 5-HT2B receptor activation and NOX-derived ROS formation, which indicates AIH (and NO) elicits spinal respiratory plasticity by a nitrergic-serotonergic mechanism.

Vortioxetine dose-dependently reverses 5-HT depletion-induced deficits in spatial working and object recognition memory: a potential role for 5-HT1A receptor agonism and 5-HT3 receptor antagonism

We previously reported that the investigational multimodal antidepressant, vortioxetine, reversed 5-HT depletion-induced memory deficits while escitalopram and duloxetine did not. The present report studied the effects of vortioxetine and the potential impact of its 5-HT1A receptor agonist and 5-HT3 receptor antagonist properties on 5-HT depletion-induced memory deficits. Recognition and spatial working memory were assessed in the object recognition (OR) and Y-maze spontaneous alternation (SA) tests, respectively. 5-HT depletion was induced in female Long-Evans rats using 4-cholro-DL-phenylalanine methyl ester HCl (PCPA) and receptor occupancies were determined by ex vivo autoradiography. Rats were acutely dosed with vortioxetine, ondansetron (5-HT3 receptor antagonist) or flesinoxan (5-HT1A receptor agonist). The effects of chronic vortioxetine administration on 5-HT depletion-induced memory deficits were also assessed. 5-HT depletion reliably impaired memory performance in both the tests. Vortioxetine reversed PCPA-induced memory deficits dose-dependently with a minimal effective dose (MED) ≤0.1mg/kg (∼80% 5-HT3 receptor occupancy; OR) and ≤3.0mg/kg (5-HT1A, 5-HT1B, 5-HT3 receptor occupancy: ∼15%, 60%, 95%) in SA. Ondansetron exhibited a MED ≤3.0μg/kg (∼25% 5-HT3 receptor occupancy; OR), but was inactive in the SA test. Flesinoxan had a MED ≤1.0mg/kg (∼25% 5-HT1A receptor occupancy; SA); only 1.0mg/kg ameliorated deficits in the NOR. Chronic p.o. vortioxetine administration significantly improved memory performance in OR and occupied 95%, 66%, and 9.5% of 5-HT3, 5-HT1B, and 5-HT1A receptors, respectively. Vortioxetine's effects on SA performance may involve 5-HT1A receptor agonism, but not 5-HT3 receptor antagonism, whereas the effects on OR performance may involve 5-HT3 receptor antagonism and 5-HT1A receptor agonism.

Repeated mirtazapine nullifies the maintenance of previously established methamphetamine-induced conditioned place preference in rats

The atypical antidepressant mirtazapine enhances monoaminergic transmission; thus, mirtazapine therapy may counter the hypo-activation of monoamine systems associated with withdrawal from methamphetamine abuse. Human addiction therapy will likely require chronic administration that is given after brain and behavioral maladaptations are established. To emulate this scenario in rats, we ascertained if acute or repeated mirtazapine treatments could antagonize previously established consequences of repeated methamphetamine. Methamphetamine-induced conditioned place preference (CPP) was used, wherein methamphetamine (1mg/kg, i.p.) was administered in a unique environmental context once-daily for three days interposed by saline injections in an alternate context. Subsequently, mirtazapine (5mg/kg, i.p.) was administered in the home cage either as 10 once-daily injections or a single injection. The expression of CPP was determined in drug-free rats three days after the last mirtazapine injection. Expression of methamphetamine-induced CPP was inhibited by 10 home cage administrations of mirtazapine but not by a single injection of mirtazapine. These findings reveal that mirtazapine can inhibit the maintenance of methamphetamine-induced CPP and that treatment duration and/or treatment timing contributes to this effect of mirtazapine.

Alpha-tocopherol protects against memory impairment caused by L-NAME and modulates the injury marker and blood coagulant parameters

Cerebrovascular disease studies have shown similarity between humans and spontaneously hypertensive rats stroke-prone rats in the development of spontaneous stroke and transitory ischemic attacks (TIA). In addition, nitric oxide (NO) suppression by L-arginine methyl ester (L-NAME) can precipitate several vascular diseases including TIA and strokes. On the other hand, alpha-tocopherol (AT) has been associated with beneficial effects on vascular disorders. Four groups were tested to evaluate AT effects on NO inhibition: AT, control (C), AT + L-NAME, and L-NAME. During 4 weeks, all groups had their physiologic parameters evaluated and were submitted to neurological tests. After the sacrifice of the animals, total L-lactate dehydrogenase, fibrinogen levels, and platelet counts were measured. Our results demonstrated improvement in memory function and sensory-motor function of the rats treated with AT. The AT treatment also demonstrated a significant difference on the injury identifier, fibrinogen levels, and platelet count between the treated groups and the L-NAME group. In conclusion, AT reversed damaging L-NAME neurological effects and could be considered as a possible protective agent in neurological diseases.

Drug targets for cognitive enhancement in neuropsychiatric disorders

The investigation of novel drug targets for treating cognitive impairments associated with neurological and psychiatric disorders remains a primary focus of study in central nervous system (CNS) research. Many promising new therapies are progressing through preclinical and clinical development, and offer the potential of improved treatment options for neurodegenerative diseases such as Alzheimer's disease (AD) as well as other disorders that have not been particularly well treated to date like the cognitive impairments associated with schizophrenia (CIAS). Among targets under investigation, cholinergic receptors have received much attention with several nicotinic agonists (α7 and α4β2) actively in clinical trials for the treatment of AD, CIAS and attention deficit hyperactivity disorder (ADHD). Both glutamatergic and serotonergic (5-HT) agonists and antagonists have profound effects on neurotransmission and improve cognitive function in preclinical experiments with animals; some of these compounds are now in proof-of-concept studies in humans. Several histamine H3 receptor antagonists are in clinical development not only for cognitive enhancement, but also for the treatment of narcolepsy and cognitive deficits due to sleep deprivation because of their expression in brain sleep centers. Compounds that dampen inhibitory tone (e.g., GABA(A) α5 inverse agonists) or elevate excitatory tone (e.g., glycine transporter inhibitors) offer novel approaches for treating diseases such as schizophrenia, AD and Down syndrome. In addition to cell surface receptors, intracellular drug targets such as the phosphodiesterases (PDEs) are known to impact signaling pathways that affect long-term memory formation and working memory. Overall, there is a genuine need to treat cognitive deficits associated with many neuropsychiatric conditions as well as an increasingly aging population.

E-6801, a 5-HT6 receptor agonist, improves recognition memory by combined modulation of cholinergic and glutamatergic neurotransmission in the rat

RATIONALE AND OBJECTIVES

In rats, 5-hydroxytryptamine(6) (5-HT(6)) receptor antagonists improve learning and memory, but the effects of agonists are poorly defined. This study investigated the effects of 5-HT(6) receptor agonists and antagonists on a rodent model of recognition memory.

METHODS

Selective 5-HT(6) receptor agonists and antagonists were administered either alone, after a scopolamine-induced impairment, or combined with sub-effective doses of the acetylcholinesterase inhibitor, donepezil, or the glutamate NMDA receptor antagonist, memantine, in a novel object discrimination paradigm in adult rats.

RESULTS

After a 4-h inter-trial delay to induce natural forgetting, vehicle-treated rats spent an equivalent time exploring novel and familiar objects during the choice trial. The 5-HT(6) receptor agonists, E-6801 (1.25-10 mg/kg i.p.) and EMD-386088 (5-10 mg/kg i.p.), and antagonists, SB-271046 and Ro 04-6790 (5 and 10 mg/kg), along with donepezil (0.1-3 mg/kg) and memantine (5-20 mg/kg) all produced significant and mostly dose-dependent increases in novel object exploration, indicative of memory enhancement. Furthermore, sub-effective doses of E-6801 (1 mg/kg) when co-administered with either SB-271046 (3 mg/kg), donepezil (0.1 mg/kg) or memantine (5 mg/kg), and EMD-386088 (2 mg/kg) co-administered with SB-271046 (3 mg/kg) also significantly enhanced object-recognition memory. Additionally, using a 1-min inter-trial delay, E-6801 (2.5 and 5 mg/kg) was as effective as donepezil (0.3 and 1 mg/kg) in reversing a scopolamine-induced (0.5 mg/kg) impairment in object recognition.

CONCLUSIONS

This is the first study to demonstrate that E-6801, a potent 5-HT(6) receptor agonist, improves recognition memory by combined modulation of cholinergic and glutamatergic neurotransmission.

Serotonergic mechanisms in addiction-related memories

Drug-associated memories are a hallmark of addiction and a contributing factor in the continued use and relapse to drugs of abuse. Repeated association of drugs of abuse with conditioned stimuli leads to long-lasting behavioral responses that reflect reward-controlled learning and participate in the establishment of addiction. A greater understanding of the mechanisms underlying the formation and retrieval of drug-associated memories may shed light on potential therapeutic approaches to effectively intervene with drug use-associated memory. There is evidence to support the involvement of serotonin (5-HT) neurotransmission in learning and memory formation through the families of the 5-HT(1) receptor (5-HT(1)R) and 5-HT(2)R which have also been shown to play a modulatory role in the behavioral effects induced by many psychostimulants. While there is a paucity of studies examining the effects of selective 5-HT(1A)R ligands, the available dataset suggests that 5-HT(1B)R agonists may inhibit retrieval of cocaine-associated memories. The 5-HT(2A)R and 5-HT(2C)R appear to be integral in the strong conditioned associations made between cocaine and environmental cues with 5-HT(2A)R antagonists and 5-HT(2C)R agonists possessing potency in blocking retrieval of cocaine-associated memories following cocaine self-administration procedures. The complex anatomical connectivity between 5-HT neurons and other neuronal phenotypes in limbic-corticostriatal brain structures, the heterogeneity of 5-HT receptors (5-HT(X)R) and the conflicting results of behavioral experiments which employ non-specific 5-HT(X)R ligands contribute to the complexity of interpreting the involvement of 5-HT systems in addictive-related memory processes. This review briefly traces the history of 5-HT involvement in retrieval of drug-cue associations and future targets of serotonergic manipulation that may reduce the impact that drug cues have on addictive behavior and relapse.

Selective serotonin reuptake inhibitor (fluoxetine) decreases the effects of ghrelin on memory retention and food intake

Ghrelin (Ghr) is an appetite stimulating hormone that is produced peripherally, by the stomach, and centrally as well. Previous investigations show that Ghr increases food intake and memory retention in rats, and that extra-hypothalamic structures, such as the hippocampus, participate in these effects. In the present work we analyzed the effect on food intake and memory retention induced by Ghr after serotonin (5-HT) availability modification at the serotoninergic synapses. Animals only treated with a selective serotonin reuptake inhibitor (SSRI), fluoxetine (FLU) 5 mg/kg or clomipramine (CLO) 2.5 and 5 mg/kg, showed a significant reduction in both food intake and memory retention. On the contrary, Ghr administration induces a significant increase in food intake and a dose-dependent increase in short and long term memory retention. When the animals were treated with FLU prior to Ghr injection, the food intake induced, as well as the expression of short and long term memory retention, was decreased. In conclusion, evidence presented in this paper suggests that the effects of Ghr on both feeding and memory retention in extra-hypothalamic structures such as the hippocampus, could depend on the availability of 5-HT.

The pharmacology, neuroanatomy and neurogenetics of one-trial object recognition in rodents

Rats and mice are attracted by novel objects. They readily approach novel objects and explore them with their vibrissae, nose and forepaws. It is assumed that such a single explorative episode leaves a lasting and complex memory trace, which includes information about the features of the object explored, as well as where and even when the object was encountered. Indeed, it has been shown that rodents are able to discriminate a novel from a familiar object (one-trial object recognition), can detect a mismatch between the past and present location of a familiar object (one-trial object-place recognition), and can discriminate different objects in terms of their relative recency (temporal order memory), i.e., which one of two objects has been encountered earlier. Since the novelty-preference paradigm is very versatile and has some advantages compared to several other memory tasks, such as the water maze, it has become a powerful tool in current neurophamacological, neuroanatomical and neurogenetical memory research using both rats and mice. This review is intended to provide a comprehensive summary on key findings delineating the brain structures, neurotransmitters, molecular mechanisms and genes involved in encoding, consolidation, storage and retrieval of different forms of one-trial object memory in rats and mice.

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.

Modulation of serotonergic neurotransmission by nitric oxide

Nitric oxide (NO) and serotonin (5-HT) are two neurotransmitters with important roles in neuromodulation and synaptic plasticity. There is substantial evidence for a morphological and functional overlap between these two neurotransmitter systems, in particular the modulation of 5-HT function by NO. Here we demonstrate for the first time the modulation of an identified serotonergic synapse by NO using the synapse between the cerebral giant cell (CGC) and the B4 neuron within the feeding network of the pond snail Lymnaea stagnalis as a model system. Simultaneous electrophysiological recordings from the pre- and postsynaptic neurons show that blocking endogenous NO production in the intact nervous system significantly reduces the B4 response to CGC activity. The blocking effect is frequency dependent and is strongest at low CGC frequencies. Conversely, bath application of the NO donor DEA/NONOate significantly enhances the CGC-B4 synapse. The modulation of the CGC-B4 synapse is mediated by the soluble guanylate cyclase (sGC)/cGMP pathway as demonstrated by the effects of the sGC antagonist 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ). NO modulation of the CGC-B4 synapse can be mimicked in cell culture, where application of 5-HT puffs to isolated B4 neurons simulates synaptic 5-HT release. Bath application of diethylamine NONOate (DEA/NONOate) enhances the 5-HT induced response in the isolated B4 neuron. However, the cell culture experiment provided no evidence for endogenous NO production in either the CGC or B4 neuron suggesting that NO is produced by an alternative source. Thus we conclude that NO modulates the serotonergic CGC-B4 synapse by enhancing the postsynaptic 5-HT response.

Neonatal 3,4-methylenedioxymethamphetamine (MDMA) exposure alters neuronal protein kinase A activity, serotonin and dopamine content, and [35S]GTPgammaS binding in adult rats

Recreational use of methylenedioxymethamphetamine (MDMA) has dramatically increased among juveniles and young adults of child-bearing age, and the potential for fetal exposure has increased. For this reason, it is surprising that comparatively few studies have assessed the long-term impact of early MDMA exposure on serotonin (5-HT) and dopamine (DA) neurotransmitter systems. The purpose of this study was to determine whether repeated exposure to MDMA during the preweanling period would cause long-term changes in 5-HT and DA functioning. Rats were treated with saline or 20 mg/kg MDMA (two injections per day) from postnatal day (PD) 11-20. At PD 90, rats were killed, and their dorsal striatum, prefrontal cortex, and hippocampus were removed. 5-HT and DA content, as well as their metabolites, were measured using HPLC. In addition, cAMP-dependent protein kinase A (PKA) activity and agonist-stimulated [35S]GTPgammaS binding was assayed using tissue homogenates from each brain region. Results indicated that early MDMA exposure caused a decrease in PKA activity and 5-HT content in the prefrontal cortex and hippocampus while increasing the efficacy of 5-HT1A receptors as measured by agonist-stimulated [35S]GTPgammaS binding. Additionally, DA content was reduced in the dorsal striatum and prefrontal cortex. These data indicate that early MDMA exposure has long-term effects on the 5-HT and DA neurotransmitter systems that may be mediated, at least partially, by changes in 5-HT1A receptor sensitivity.