The role of the dorsal raphe-serotonergic system and cholinergic receptors in the modulation of working memory

Imaging the Vesicular Acetylcholine Transporter in Schizophrenia: A Positron Emission Tomography Study Using [18F]-VAT

BACKGROUND

Despite longstanding interest in the central cholinergic system in schizophrenia (SCZ), cholinergic imaging studies with patients have been limited to receptors. Here, we conducted a proof-of-concept positron emission tomography study using [18F]-VAT, a new radiotracer that targets the vesicular acetylcholine transporter as a proxy measure of acetylcholine transmission capacity, in patients with SCZ and explored relationships of vesicular acetylcholine transporter with clinical symptoms and cognition.

METHODS

A total of 18 adult patients with SCZ or schizoaffective disorder (the SCZ group) and 14 healthy control participants underwent a positron emission tomography scan with [18F]-VAT. Distribution volume (VT) for [18F]-VAT was derived for each region of interest, and group differences in VT were assessed with 2-sample t tests. Functional significance was explored through correlations between VT and scores on the Positive and Negative Syndrome Scale and a computerized neurocognitive battery (PennCNB).

RESULTS

No group differences in [18F]-VAT VT were observed. However, within the SCZ group, psychosis symptom severity was positively associated with VT in multiple regions of interest, with the strongest effects in the hippocampus, thalamus, midbrain, cerebellum, and cortex. In addition, in the SCZ group, working memory performance was negatively associated with VT in the substantia innominata and several cortical regions of interest including the dorsolateral prefrontal cortex.

CONCLUSIONS

In this initial study, the severity of 2 important features of SCZ-psychosis and working memory deficit-was strongly associated with [18F]-VAT VT in several cortical and subcortical regions. These correlations provide preliminary evidence of cholinergic activity involvement in SCZ and, if replicated in larger samples, could lead to a more complete mechanistic understanding of psychosis and cognitive deficits in SCZ and the development of therapeutic targets.

A comparison of scopolamine and biperiden as a rodent model for cholinergic cognitive impairment

RATIONALE

The nonselective muscarinic antagonist scopolamine hydrobromide (SCOP) is employed as the gold standard for inducing memory impairments in healthy humans and animals. However, its use remains controversial due to the wide spectrum of behavioral effects of this drug.

OBJECTIVE

The present study investigated whether biperiden (BIP), a muscarinic m1 receptor antagonist, is to be preferred over SCOP as a pharmacological model for cholinergic memory deficits in rats. This was done by comparing the effects of SCOP and BIP using a battery of operant tasks: fixed ratio (FR5) and progressive ratio (PR10) schedules of reinforcement, an attention paradigm and delayed nonmatching to position task.

RESULTS

SCOP induced diffuse behavioral disruption, which included sensorimotor responding (FR5, 0.3 and 1 mg/kg), food motivation (PR10, 1 mg/kg), attention (0.3 mg/kg, independent of stimulus duration), and short-term memory (delayed nonmatching to position (DNMTP), 0.1 and 0.3 mg/kg, delay-dependent but also impairment at the zero second delay). BIP induced relatively more selective deficits, as it slowed sensorimotor responding (FR5, 10 mg/kg) and disrupted short-term memory (DNMTP, 3 mg/kg, delay-dependent but no impairment at the zero second delay). BIP had no effect on food motivation (PR10) or attention.

CONCLUSION

Muscarinic m1 antagonists should be considered an interesting alternative for SCOP as a pharmacological model for cholinergic mnemonic deficits in animals.

Increased expression of 5-HT(1B) receptors by Herpes simplex virus gene transfer in septal neurons: New in vitro and in vivo models to study 5-HT(1B) receptor function

Serotonergic modulation of acetylcholine (ACh) release after neuron-specific increase of the expression of 5-HT(1B) receptors by gene transfer was studied in vitro and in vivo. The increased expression of the 5-HT(1B) receptor in vitro was induced by treating rat primary fetal septal cell cultures for 3 days with a viral vector inducing the expression of green fluorescent protein (GFP) vector alone, or, in addition, of 5-HT(1B) receptors (HA1B/GFP vector). The transfection resulted in a high number of GFP-positive cells, part of which being immunopositive for choline acetyltransferase. In HA1B/GFP-cultures (vs. GFP-cultures), electrically evoked ACh release was significantly more sensitive to the inhibitory action of the 5-HT(1B) agonist CP-93,129. Increased expression of the 5-HT(1B) receptor in vivo was induced by stereotaxic injections of the vectors into the rat septal region. Three days later, electrically evoked release of ACh in hippocampal slices of HA1B/GFP-treated rats was lower than in their GFP-treated counterparts, showing a higher inhibitory efficacy of endogenous 5-HT on cholinergic terminals after transfection. Moreover, CP-93,129 had a higher inhibitory potency. In conclusion, the HA1B/GFP vector reveals a useful tool to induce a targeted increase of 5-HT(1B) heteroreceptors on cholinergic neurons in selected CNS regions, which provides interesting perspectives for functional approaches at more integrated levels.

5-HT receptor regulation of neurotransmitter release

Serotoninergic neurons in the central nervous system impinge on many other neurons and modulate their neurotransmitter release. This review focuses on 1) the function of presynaptic 5-hydroxytryptamine (5-HT) heteroreceptors on axon terminals of central cholinergic, dopaminergic, noradrenergic, or GABAergic neurons and 2) the role of GABAergic interneurons expressing 5-HT heteroreceptors in the regulation of acetylcholine, dopamine, or noradrenaline release. In vitro studies on slices or synaptosomes and in vivo microdialysis experiments have shown that 5-HT(1A), 5-HT(1B), 5-HT(2A), 5-HT(2C), 5-HT(3), and/or 5-HT(4) heteroreceptors mediate this modulation. 5-HT(1B) receptors on neocortical cholinergic, striatal dopaminergic, or hippocampal GABAergic axon terminals are examples for release-inhibiting 5-HT heteroreceptors; 5-HT(3) receptors on hippocampal GABAergic or 5-HT(4) receptors on hippocampal cholinergic axon terminals are examples for release-facilitating 5-HT heteroreceptors. GABA released from GABAergic interneurons upon activation of facilitatory 5-HT receptors, e.g., 5-HT(2A) or 5-HT(3) receptors, mediates inhibition of the release of other neurotransmitters such as prefrontal neocortical dopamine or neocortical acetylcholine release, respectively. Conversely, attenuated GABA release in response to activation of inhibitory 5-HT heteroreceptors, e.g., 5-HT(1A) or 5-HT(1B) receptors on GABAergic interneurons is involved in paradoxical facilitation of hippocampal acetylcholine and striatal dopamine release, respectively. Such 5-HT heteroreceptors are considered potential targets for appropriate 5-HT receptor ligands which, by enhancing the release of a relevant neurotransmitter, can compensate for its hypothesized deficiency in distinct brain areas. Examples for such deficiencies are the impaired release of hippocampal or neocortical acetylcholine, striatal dopamine, and hippocampal or neocortical noradrenaline in disorders such as Alzheimer's disease, Parkinson's disease, and major depression, respectively.

Brain serotonin depletion impairs short-term memory, but not long-term memory in rats

Intracerebroventricular injection of 5,7-dihydroxytryptamine (5,7-DHT) (150 microg; 4.5 microl/ventricle), a serotonergic neurotoxin, significantly decreased spontaneous alternation in Y-maze task and working memory in radial 8 arm-maze task, suggesting effects on short-term memory, without affecting long-term memory, explored by reference memory in radial 8 arm-maze task and step-through latency in multi-trial passive avoidance task. Parachlorophenylalanine (PCPA) (3 days treatment 200 microg, i.c.v.), a serotonin synthesis inhibitor, did not impair step-through-latency in multi-trial passive avoidance task, suggesting no effects on long-term memory. These results suggest that serotonin, among other neurotransmitters, play an important role in cognitive functions, especially short-term memory.

Presynaptic modulation of 5-HT release in the rat septal region

5-HT released from serotonergic axon terminals in the septal nuclei modulates the activity of septal output neurons (e.g. septohippocampal cholinergic neurons) bearing somatodendritic 5-HT receptors. Therefore, we studied the mechanisms involved in the presynaptic modulation of 5-HT release in the lateral (LS) and medial septum (MS), and the diagonal band of Broca (DB). HPLC analysis showed that tissue concentrations of noradrenaline, dopamine and 5-HT were highest in DB (DB>MS>LS). Slices prepared from LS, MS and DB regions were preincubated with [(3)H]5-HT, superfused in the presence of 6-nitro-2-(1-piperazinyl)-quinoline (6-nitroquipazine) and electrically stimulated up to three times (first electrical stimulation period (S(1)), S(2), S(3); 360 pulses, 3 Hz, 2 ms, 26-28 mA). In all septal regions the Ca(2+)-dependent and tetrodotoxin-sensitive electrically-evoked overflow of [(3)H] was inhibited by the 5-HT(1B) agonist CP-93,129 and the alpha(2)-adrenoceptor agonist 5-bromo-6-(2-imidazolin-2-ylamino)-quinoxaline tartrate (UK-14,304). Also the mu- and kappa-opioid receptor agonists (d-Ala(2), N-Me-Phe(4), glycinol(5))-enkephalin (DAMGO) and [trans-(1S,2S(-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl) cyclohexyl]-benzenacetamide hydro-chloride] (U-50,488H), respectively, acted inhibitory (although less potently), whereas the delta-opioid receptor agonist (d-Pen(2), d-Pen(5))-enkephalin (DPDPE), the dopamine D(2) receptor agonist quinpirole and the adenosine A(1) receptor agonist N(6)-cyclopentyladenosine were all ineffective; the GABA(B) receptor agonist baclofen had weak effects. All inhibitory effects of the agonists were antagonized by the corresponding antagonists (3-[3-(dimethylamino)propyl]-4-hydroxy-N-[4-(4-pyridinyl)phenyl]benzamide dihydrochloride (GR-55,562), idazoxan, naloxone, nor-binaltorphimine), which also significantly enhanced the evoked release of 5-HT at S(1). It is concluded that 5-HT release in septal nuclei of the rat is modulated by presynaptic 5-HT(1B) autoreceptors, as well as by alpha(2)-, mu- and kappa-opioid heteroreceptors. All of these receptors seem to be under a tonic inhibitory influence of the corresponding endogenous agonists and show qualitatively comparable modulatory properties along the dorso-ventral distribution of the 5-HT terminals.

Presynaptic serotonergic modulation of 5-HT and acetylcholine release in the hippocampus and the cortex of 5-HT1B-receptor knockout mice

Lesioning of serotonergic afferents increases hippocampal ACh release and attenuates memory deficits produced by cholinergic lesions. Improved memory performance described in 5-HT1B-knockout (KO) mice might thus be due to a weaker 5-HT1B-mediated inhibitory influence of 5-HT on hippocampal ACh release. The selective delay-dependent impairment of working memory observed in these KO mice suggests, however, that cortical regions also participate in task performance, possibly via indirect influences of 5-HT on ACh release. To provide neuropharmacological support for these hypotheses we measured evoked ACh and 5-HT release in hippocampal and cortical slices of wild-type (WT) and 5-HT1B KO mice. Superfused slices (preincubated with [3H]choline or [3H]5-HT) were electrically stimulated in the absence or presence of 5-HT1B receptor ligands. In hippocampus and cortex, 5-HT1B agonists decreased and antagonists increased 5-HT release in WT, but not in 5-HT1B KO mice. In 5-HT1B KO mice, 5-HT release was enhanced in both structures, while ACh release (in nCi) was reduced. ACh release was inhibited by 5-HT1B agonists in hippocampal (not cortical) slices of WT but not of 5-HT1B KO mice. Our data (i) confirm the absence of autoinhibition of 5-HT release in 5-HT1B-KO mice, (ii) demonstrate a reduced release of ACh, and the absence of 5-HT1B-receptor-mediated inhibition of ACh release, in the hippocampus and cortex of 5-HT1B-KO mice, and (iii) are compatible with an indirect role of cortical ACh in the working memory impairment observed in these KO mice.

Delay-dependent working memory impairment in young-adult and aged 5-HT1BKO mice as assessed in a radial-arm water maze

Serotonin (5-HT) plays a modulatory role in mnemonic functions, especially by interacting with the cholinergic system. The 5-HT1B receptor is a key target of this interaction. The 5-HT1B receptor knockout mice were found previously to exhibit a facilitation in hippocampal-dependent spatial reference memory learning. In the present study, we submitted mice to a delayed spatial working memory task, allowing the introduction of various delays between an exposure trial and a test trial. The 5-HT1BKO and wild-type mice learned the task in a radial-arm water maze (returning to the most recent presented arm containing the escape platform), and exhibited a high level of performance at delays of 0 and 5 min. However, at the delay of 60 min, only 5-HT1BKO mice exhibited an impairment. At a delay of 90 min, all mice were impaired. Treatment by scopolamine (0.8 mg/kg) induced the same pattern of performance in wild type as did the mutation for short (5 min, no impairment) and long (60 min, impairment) delays. The 22-month-old wild-type and knockout mice exhibited an impairment at short delays (5 and 15 min). The effect of the mutation affected both young-adult and aged mice at delays of 15, 30, and 60 min. Neurobiological data show that stimulation of the 5-HT1B receptor inhibits the release of acetylcholine in the hippocampus, but stimulates this in the frontal cortex. This dual function might, at least in part, explain the opposite effect of the mutation on reference memory (facilitation) and delay-dependent working memory (impairment). These results support the idea that cholinergic-serotonergic interactions play an important role in memory processes.

A pharmacological analysis of an associative learning task: 5-HT(1) to 5-HT(7) receptor subtypes function on a pavlovian/instrumental autoshaped memory

Recent studies using both invertebrates and mammals have revealed that endogenous serotonin (5-hydroxytryptamine [5-HT]) modulates plasticity processes, including learning and memory. However, little is currently known about the mechanisms, loci, or time window of the actions of 5-HT. The aim of this review is to discuss some recent results on the effects of systemic administration of selective agonists and antagonists of 5-HT on associative learning in a Pavlovian/instrumental autoshaping (P/I-A) task in rats. The results indicate that pharmacological manipulation of 5-HT1-7 receptors or 5-HT reuptake sites might modulate memory consolidation, which is consistent with the emerging notion that 5-HT plays a key role in memory formation.

Spatial learning in rats is impaired after degeneration of the nigrostriatal dopaminergic system

We investigated spatial learning in rats with unilateral and bilateral lesions of the nigrostriatal dopaminergic system. We used the Morris water maze paradigm, which tests spatial forms of learning and memory and allows discrimination between sensory-motor and learning disabilities. Animals were trained preoperatively to learn the location of a spatially fixed hidden platform to escape from the swimming pool (acquisition training). A visual and a probe test were used before and after the acquisition training, respectively. Our results show that animals with unilateral lesions, although displaying longer escape latencies, have normal spatial memory abilities. Animals with bilateral lesions were able to swim as fast or even faster than animals with unilateral lesion. Despite the fact that these animals had learned the spatial navigation tasks preoperatively, bilateral dopaminergic lesions led to a profound deficit in ability to find a hidden platform during an acquisition task. In general, animals with bilateral lesions persisted in swimming along the pool walls and their spatial navigation performance during a probe test was very poor. These results suggest that deficit of the nigrostriatal dopaminergic system can affect the selection and maintenance of behavioral strategies in spatial navigation.

Tianeptine: 5-HT uptake sites and 5-HT(1-7) receptors modulate memory formation in an autoshaping Pavlovian/instrumental task

Recent studies using invertebrate and mammal species have revealed that, endogenous serotonin (5-hydroxytryptamine, 5-HT) modulates cognitive processes, particularly learning and memory, though, at present, it is unclear the manner, where, and how long 5-HT systems are involved. Hence in this work, an attempt was made to study the effects of 5-HT endogenous on memory formation, using a 5-HT uptake facilitator (tianeptine) and, selective 5-HT(1-7) receptor antagonists to determine whether 5-HT uptake sites and which 5-HT receptors are involved, respectively. Results showed that post-training tianeptine injection enhanced memory consolidation in an autoshaping Pavlovian/instrumental learning task, which has been useful to detect changes on memory formation elicited by drugs or aging. On interaction experiments, ketanserin (5-HT(1D/2A/2C) antagonist) slightly enhanced tianeptine effects, while WAY 100635 (5-HT(1A) antagonist), SB-224289 (5-HT(1B) inverse agonist), SB-200646 (5-HT(2B/2C) antagonist), ondansetron (5-HT(3) antagonist), GR 127487 (5-HT(4) antagonist), Ro 04-6790 (5-HT(6) antagonist), DR 4004 (5-HT(7) antagonist), or fluoxetine (an inhibitor of 5-HT reuptake) blocked the facilitatory tianeptine effect. Notably, together tianeptine and Ro 04-6790 impaired learning consolidation. Moreover, 5-HT depletion completely reversed the tianeptine effect. Tianeptine also normalized an impaired memory elicited by scopolamine (an antimuscarinic) or dizocilpine (non-competitive glutamatergic antagonist), while partially reversed that induced by TFMPP (5-HT(1B/1D/2A-2C/7) agonist/antagonist). Finally, tianeptine-fluoxetine coadministration had no effect on learning consolidation; nevertheless, administration of an acetylcholinesterase inhibitor, phenserine, potentiated subeffective tianeptine or fluoxetine doses. Collectively, these data confirmed that endogenously 5-HT modulates, via uptake sites and 5-HT(1-7) receptors, memory consolidation, and are consistent with the emerging notion that 5-HT plays a key role on memory formation.

Effects of excitotoxic median raphe lesions on scopolamine-induced working memory deficits in inhibitory avoidance

The aim of the present study was to investigate the effects of excitotoxic damage of the serotonergic cell bodies in the median raphe nucleus (MRN) on the scopolamine-induced working memory deficits in a single-trial light/dark inhibitory avoidance task. Rats were given 1 mg/kg of scopolamine hydrobromide (intraperitonal, i.p.) or saline before the inhibitory avoidance training, in which initial preference to the dark compartment (escape latency) was used to measure nonmnemonic behaviors, and response latency to enter the dark compartment immediately after the shock was used to measure working memory. It was found that scopolamine significantly reduced escape latencies in sham-lesioned rats, whereas it had no effect in the rats with MRN lesions. Although MRN lesion per se did not alter response latency, it prevented scopolamine-induced decrease in this parameter. These results suggest that the antagonistic interactive processes between serotonergic projections of the MRN and the muscarinic cholinergic system modulate nonmnemonic attentional component of working memory formation in the inhibitory avoidance.

Nicotinic--serotonergic interactions in brain and behaviour

This review focuses on nicotinic--serotonergic interactions in the central nervous system (CNS). Nicotine increases 5-hydroxytryptamine (5-HT) release in the cortex, striatum, hippocampus, dorsal raphé nucleus (DRN), hypothalamus, and spinal cord. As yet, there is little firm evidence for nicotinic receptors on serotonergic terminals and thus nicotine's effects on 5-HT may not necessarily be directly mediated, but there is strong evidence that the 5-HT tone plays a permissive role in nicotine's effects. The effects in the cortex, hippocampus, and DRN involve stimulation of 5-HT(1A) receptors, and in the striatum, 5-HT(3) receptors. The 5-HT(1A) receptors in the DRN play a role in mediating the anxiolytic effects of nicotine and the 5-HT(1A) receptors in the dorsal hippocampus and lateral septum mediate its anxiogenic effects. The increased startle and anxiety during nicotine withdrawal is mediated by 5-HT(1A) and 5-HT(3) receptors. The locomotor stimulant effect of acute nicotine is mediated by 5-HT(1A) receptors and 5-HT(2) receptors may play a role in the expression of a sensitised response after chronic nicotine treatment. Unfortunately, the role of 5-HT(1A) receptors in mediating nicotine seeking has not yet been investigated and would seem an important area for future research. There is also evidence for nicotinic--serotonergic interactions in the acquisition of the water maze, passive avoidance, and impulsivity in the five-choice serial reaction task.

Effects of excitotoxic median raphe lesion on working memory deficits produced by the dorsal hippocampal muscarinic receptor blockade in the inhibitory avoidance in rats

The experiments investigated the interactions between median raphe nucleus (MRN) serotonergic and septo-hippocampal muscarinic cholinergic systems in the modulation of forming and storing performances of working memory. Rats with ibotenic acid-induced MRN-lesion bilaterally received scopolamine (2-4 microg/each side) infusion into the dentate gyrus of the dorsal hippocampus and were tested in a single trial step-through inhibitory avoidance. Initial preference to the dark compartment (escape latency) was taken as the measure of non-mnemonic behaviours and response latency to enter the dark compartment immediately after the foot-shock was used to measure working memory. The high-dose scopolamine infusion 10 min before the training decreased escape latencies in the sham-lesioned rats, whereas had no effect in the MRN-lesioned rats. Although MRN lesion per se did not alter response latency, it alleviated pre-training scopolamine-induced decrease, but aggravated post-training scopolamine-induced reduction in this parameter. These results suggest that the antagonistic interactive processes between MRN-serotonergic and hippocampal cholinergic systems modulate non-mnemonic component of working memory formation, whereas the storing performance of working memory is modulated by the synergistic interactions between these systems in the hippocampus, mainly in the dentate gyrus.

Alzheimer's disease: more than a 'cholinergic disorder' - evidence that cholinergic-monoaminergic interactions contribute to EEG slowing and dementia

The loss of cognitive (particularly mnemonic) abilities constitutes a prominent symptom of Alzheimer's disease (AD). These cognitive symptoms occur in close relation to the slowing of the electroencephalogram (EEG), and it is likely that the inability of cortical circuits to maintain an activated state contributes to the behavioral disorganization in AD. The 'cholinergic hypothesis' of AD suggests that many of the cognitive and EEG symptoms are related to the atrophy of basal forebrain cholinergic neurons, which innervate the neocortex and hippocampus, among others. However, data from behavioral and electrophysiological studies in rats suggest that selective reductions in cholinergic transmission result in relatively small mnemonic impairments, and only a partial reduction in EEG activation. Thus, cholinergic atrophy alone may not be sufficient to cause the marked changes in cognition and cortical activity typical of AD. Cholinergic deficits do, however, make neural circuits susceptible to additional neurodegenerative processes. In rats, lowered serotonergic or noradrenergic activity alone often produces only minor impairments in learning/memory tasks and does not block EEG activation. The same monoaminergic deficits, however, result in severe behavioral impairments, and reduce or abolish EEG activation when they occur in a brain already affected by lowered cholinergic activity. There is an abundance of evidence that monoamines are reduced in AD. These degenerative processes, when occurring in a neural environment compromised by cholinergic atrophy, may then contribute to the disturbances in cortical processing and cognition/behavior in AD. A prediction derived from this theory is that an enhancement of monoaminergic functions may have beneficial effects on behavior and cortical activity. Preliminary experiments support this idea: combined cholinergic-monoaminergic stimulation can be more effective in reversing behavioral (Morris water maze) impairments and EEG slowing in rats with multiple neurotransmitter deficiencies than cholinergic enhancement alone. Thus, a stimulation of monoaminergic activity, in conjunction with cholinergic therapies, may provide an effective treatment option for AD.

Role of serotonin in memory impairment

As a result of its presence in various structures of the central nervous system serotonin (5-HT) plays a role in a great variety of behaviours such as food intake, activity rythms, sexual behaviour and emotional states. Despite this lack of functional specialization, the serotonergic system plays a significant role in learning and memory, in particular by interacting with the cholinergic, glutamatergic, dopaminergic or GABAergic systems. Its action is mediated via specific receptors located in crucial brain structures involved in these functions, primarily the septo-hippocampal complex and the nucleus basalis magnocellularis (NBM)-frontal cortex. Converging evidence suggests that the administration of 5-HT2A/2C or 5-HT4 receptor agonists or 5-HT1A or 5-HT3 and 5-HT1B receptor antagonists prevents memory impairment and facilitates learning in situations involving a high cognitive demand. In contrast, antagonists for 5-HT2A/2C and 5-HT4, or agonists for 5-HT1A or 5-HT3 and 5-HT1B generally have opposite effects. A better understanding of the role played by these and other serotonin receptor subtypes in learning and memory is likely to result from the recent availability of highly specific ligands, such as 5-HT1A, 5-HT1B, 5-HT2A receptor antagonists, and new molecular tools, such as gene knock-out mice, especially inducible mice in which a specific genetic alteration can be restricted both temporally and anatomically.

5-HT system and cognition

The study of 5-hydroxytryptamine (5-HT) system has benefited from the identification, classification and cloning of multiple 5-HT receptors (5-HT1 to 5-HT7). Growing evidence suggests that 5-HT is important in learning and memory and all its receptors might be implicated in this. Actually, 5-HT pathways, 5-HT reuptake site/transporter complex and 5-HT receptors show regional distribution in brain areas implicated in learning and memory. Likewise, the stimulation or blockade of presynaptic 5-HT1A, 5-HT1B, 5-HT(2A/2C) and 5-HT3 receptors, postsynaptic 5-HT(2B/2C) and 5-HT4 receptors and 5-HT uptake/transporter sites modulate these processes. Available evidence strongly suggests that the 5-HT system may be important in normal function, the treatment and/or pathogenesis of cognitive disorders. Further investigation will help to specify the 5-HT system nature involvement in cognitive processes, pharmacotherapies, their mechanisms and action sites and to determine under which conditions they could operate. In this regard, it is probable that selective drugs with agonists, neutral antagonist, agonists or inverse agonist properties for 5-HT1A, 5-HT(1B/1D), 5-HT(2A/2B/2C), 5-HT4 and 5-HT7 receptors could constitute a new therapeutic opportunity for learning and memory alterations.

Serotonin-dependent maintenance of spatial performance and electroencephalography activation after cholinergic blockade: effects of serotonergic receptor antagonists

The interaction between acetylcholine (ACh) and serotonin (5-hydroxytryptamine, 5-HT) in the control of behavior such as spatial navigation has received considerable attention over the last years. Previous research indicates that while a selective reduction in cholinergic transmission often produces only mild impairments in spatial and other behavioral tests, additional serotonergic blockade results in the appearance of severe behavioral deficits. Consequently, it has been argued that 5-HT plays a role in the maintenance of behavioral capacities in the face of reduced cholinergic transmission. Here, we examined the effects of 5-HT depletion and receptor blockade, alone and in combination with cholinergic-muscarinic antagonism, on spatial navigation of rats in the Morris water maze. Further, electroencephalographic (EEG) recordings were taken to test the hypothesis that a loss of neocortical activation is related to the behavioral deficits apparent after cholinergic-serotonergic blockade. The muscarinic antagonist, scopolamine (1 mg/kg) produced a moderate impairment in navigational performance. The 5-HT depletor, p-chlorophenylalanine (PCPA; 500 mg kg(-1) day(-1)x2) did not impair performance when given alone but strongly potentiated the scopolamine-induced deficit and completely blocked the acquisition of an escape response in the water maze. This effect was mimicked by the non-selective serotonin(1-2) receptor antagonist, methiothepin (0.3 mg/kg), but not by the selective serotonin(1A) antagonist, WAY 100635 (0.1-0.5 mg/kg) or the serotonin(2) antagonist, ketanserin (2-4 mg/kg). None of the 5-HT antagonists impaired performance when given alone. Electrocorticographic recordings in rats treated with scopolamine and serotonergic receptor antagonists showed that during behavioral immobility, scopolamine (1 mg/kg) increased spectral power in all frequency bands between 0.5 and 20 Hz without significantly affecting cortical activity during movement. None of the 5-HT antagonists affected cortical activity when given alone. However, methiothepin, at the same dose that produced behavioral impairments, increased spectral power between 0.5 and 4 Hz and between 8 and 12 Hz during movement when co-administered with scopolamine. The results suggest that a concurrent blockade of multiple 5-HT receptors, but not selective blockade of serotonin(1A) or serotonin(2) receptors alone, mimics the ability of global 5-HT depletion to abolish behavioral capacities that are resistant to muscarinic receptor blockade. The behavioral deficits observed here are accompanied by a reduction in neocortical activation, suggesting that disturbances of processing in cortical networks can contribute to the behavioral disorganization apparent after cholinergic and serotonergic blockade. A focus on concurrent serotonergic-cholinergic deficits may provide a useful framework for the development of novel pharmacological treatments to counteract the behavioral disorganization and loss of EEG activation present in senile dementia and Alzheimer's disease.