Modification of 8-OH-DPAT effects on learning by manipulation of the assay conditions

Measuring reinforcement learning and motivation constructs in experimental animals: relevance to the negative symptoms of schizophrenia

The present review article summarizes and expands upon the discussions that were initiated during a meeting of the Cognitive Neuroscience Treatment Research to Improve Cognition in Schizophrenia (CNTRICS; http://cntrics.ucdavis.edu) meeting. A major goal of the CNTRICS meeting was to identify experimental procedures and measures that can be used in laboratory animals to assess psychological constructs that are related to the psychopathology of schizophrenia. The issues discussed in this review reflect the deliberations of the Motivation Working Group of the CNTRICS meeting, which included most of the authors of this article as well as additional participants. After receiving task nominations from the general research community, this working group was asked to identify experimental procedures in laboratory animals that can assess aspects of reinforcement learning and motivation that may be relevant for research on the negative symptoms of schizophrenia, as well as other disorders characterized by deficits in reinforcement learning and motivation. The tasks described here that assess reinforcement learning are the Autoshaping Task, Probabilistic Reward Learning Tasks, and the Response Bias Probabilistic Reward Task. The tasks described here that assess motivation are Outcome Devaluation and Contingency Degradation Tasks and Effort-Based Tasks. In addition to describing such methods and procedures, the present article provides a working vocabulary for research and theory in this field, as well as an industry perspective about how such tasks may be used in drug discovery. It is hoped that this review can aid investigators who are conducting research in this complex area, promote translational studies by highlighting shared research goals and fostering a common vocabulary across basic and clinical fields, and facilitate the development of medications for the treatment of symptoms mediated by reinforcement learning and motivational deficits.

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.

Serotonin and psychostimulant addiction: Focus on 5-HT1A-receptors

Serotonin(1A)-receptors (5-HT(1A)-Rs) are important components of the 5-HT system in the brain. As somatodendritic autoreceptors they control the activity of 5-HT neurons, and, as postsynaptic receptors, the activity in terminal areas. Cocaine (COC), amphetamine (AMPH), methamphetamine (METH) and 3,4-methylenedioxymethamphetamine ("Ecstasy", MDMA) are psychostimulant drugs that can lead to addiction-related behavior in humans and in animals. At the neurochemical level, these psychostimulant drugs interact with monoamine transporters and increase extracellular 5-HT, dopamine and noradrenalin activity in the brain. The increase in 5-HT, which, in addition to dopamine, is a core mechanism of action for drug addiction, hyperactivates 5-HT(1A)-Rs. Here, we first review the role of the various 5-HT(1A)-R populations in spontaneous behavior to provide a background to elucidate the contribution of the 5-HT(1A)-Rs to the organization of psychostimulant-induced addiction behavior. The progress achieved in this field shows the fundamental contribution of brain 5-HT(1A)-Rs to virtually all behaviors associated with psychostimulant addiction. Importantly, the contribution of pre- and postsynaptic 5-HT(1A)-Rs can be dissociated and frequently act in opposite directions. We conclude that 5-HT(1A)-autoreceptors mainly facilitate psychostimulant addiction-related behaviors by a limitation of the 5-HT response in terminal areas. Postsynaptic 5-HT(1A)-Rs, in contrast, predominantly inhibit the expression of various addiction-related behaviors directly. In addition, they may also influence the local 5-HT response by feedback mechanisms. The reviewed findings do not only show a crucial role of 5-HT(1A)-Rs in the control of brain 5-HT activity and spontaneous behavior, but also their complex role in the regulation of the psychostimulant-induced 5-HT response and subsequent addiction-related behaviors.

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.

Role of the serotonin 5-HT(2A) receptor in learning

This study reviews the role of the serotonin 5-HT2A receptor in learning as measured by the acquisition of the rabbit's classically conditioning nictitating membrane response, a component of the eyeblink response. Agonists at the 5-HT2A receptor including LSD (d-lysergic acid diethylamide) enhanced associative learning at doses that produce cognitive effects in humans. Some antagonists such as BOL (d-bromolysergic acid diethylamide), LY53,857, and ketanserin acted as neutral antagonists in that they had no effect on learning, whereas others (MDL11,939, ritanserin, and mianserin) acted as inverse agonists in that they retarded learning through an action at the 5-HT2A receptor. These results were placed in the context of what is known concerning the anatomical distribution and electrophysiological effects of 5-HT2A receptor activation in frontal cortex and hippocampus, as well as the role of cortical 5-HT2A receptors in schizophrenia. It was concluded that the 5-HT2A receptor demonstrates constitutive activity, and that variations in this activity can produce profound alterations in cognitive states.

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.

Regional infusions of serotonin into the striatum and memory consolidation

Lesions, temporal inactivation, electrical stimulation and administration of drugs that antagonize synaptic activity of the striatum lead to significant deficits of memory. Also, it has been shown that interruption of dopaminergic, GABAergic, or cholinergic activity in discrete areas of this structure is sufficient to disrupt cognitive functions. In spite of the known interactions among dopamine, GABA, acetylcholine, and serotonin, there is a notable scarcity of data germane to the participation of striatal serotonin in learning and memory. It was important, therefore, to investigate the possible involvement of serotonin in cognition. In light of the differential distribution of serotonergic elements within the striatum, a prediction was made that focal injections of serotonin into distinctive regions would produce dissimilar effects on memory. Rats were trained in a one-trial step-through inhibitory avoidance task and a retention test was carried out 24 h later. Posttraining injections of serotonin into the dorsal and ventral aspects of the posterior region produced strong amnesia compared to similar injections into the dorsal and ventral aspects of the anterior region. The present findings support the hypothesis that striatal serotonergic activity is involved in memory functions and also provide further evidence of neurochemical heterogeneity within the striatum regarding memory consolidation.

Effects of pretraining intrastriatal administration of p-chloroamphetamine on inhibitory avoidance

Pretraining systemic administration of p-chloroamphetamine (PCA) consistently produces retention deficits of inhibitory avoidance. This drug causes a widespread acute release of serotonin from cerebral neuronal terminals, but it is not known where in the brain PCA exerts its disruptive cognitive effects. The present experiment was aimed at determining whether the striatum is a site of action of this drug. PCA (5 microg) was infused, bilaterally, into the striatum of rats at 30, 15, or 5 min before training of inhibitory avoidance, and retention of the task was measured 24 h later. An inversely related time-dependent deficit was found. Together with the results from appropriate control groups, this result strongly suggest that systemic PCA produces its reported cognitive effects, at least in part, through its pharmacological action within the striatum.

Enhanced inhibitory avoidance training protects against the amnesic effect of p-chloroamphetamine

The contribution of acetylcholine (ACh) to memory processing is well documented, but it has been proposed that it is not necessary for memory consolidation after an enhanced learning experience. It has been suggested that serotonin (5-HT) interacts with ACh during memory consolidation, although the nature of this interaction is unknown in the case of strong learning. As an initial approach to the study of these interactions, we determined whether training of inhibitory avoidance using relatively high aversive stimulation protects against the typical retention deficits produced by pre-training administration of the 5-HT releaser p-chloroamphetamine (PCA). Rats were trained after intraperitoneal administration of PCA or isotonic saline, using 2.0, 2.5, 3.0 or 3.5 mA and retention of the task was measured 24 h later. A significant amnesic state was observed only in the PCA groups that had been trained with the two lower intensities. These results indicate that 5-HT systems behave similarly to ACh systems, in the sense that the amnesic effect produced by interference with their physiological activity may be cancelled when animals are submitted to an intense learning situation.

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.

Simultaneous modulation of retrieval by dopaminergic D(1), beta-noradrenergic, serotonergic-1A and cholinergic muscarinic receptors in cortical structures of the rat

Retrieval of inhibitory avoidance has been recently shown to require intact glutamate receptors, protein kinases A and C and mitogen-activated protein kinase in the CA1 region of the rat hippocampus and in the entorhinal, posterior parietal and anterior cingulate cortex. These enzymatic activities are known to be modulated by dopamine D(1), beta-noradrenergic, 5HT1A and cholinergic muscarinic receptors. Here we study the effect on retrieval of this task of well-known agonists and antagonists of these receptors infused in the same brain cortical regions and into the basolateral amygdala, in rats. The drugs used were SKF38393 (D(1) agonist), noradrenaline, 8-HO-DPAT (5HT1A agonist), oxotremorine (muscarinic agonist), SCH23390 (D(1) antagonist), timolol (beta antagonist), NAN-190 (5HT1A antagonist) and scopolamine (muscarinic antagonist). All were studied at two different dose levels. The localised infusion of SKF38393, noradrenaline, NAN-190 and oxotremorine into any of the cortical structures mentioned 10 min prior to a 24-h retention test session of one-trial step-down inhibitory avoidance enhanced retention test performance. SCH2330, timolol, 8-HO-DPAT and scopolamine hindered retention test performance. In the basolateral amygdala only an enhancing effect of noradrenaline and an inhibitory effect of timolol were seen. Three hours after the infusions, retention test performance returned to normal in all cases. None of the treatments affected locomotion or rearing in an open field or behaviour in the elevated plus maze. Therefore, their effects on retention testing can be attributed to an influence on retrieval. In conclusion, memory retrieval of this apparently simple task requires the participation of CA1, entorhinal, posterior parietal and anterior cingulate cortex, and is strongly modulated by, dopaminergic D(1), beta-noradrenergic, muscarinic cholinergic and 5HT1A receptors in the four areas. The first three types of receptor enhance, and the latter inhibits, retrieval. Only beta-adrenoceptors appears to be involved in the modulation of retrieval of this task by the amygdala. The results bear on the well-known influence of emotion and mood on retrieval, and indicate that this involves many areas of the brain simultaneously. In addition, the results point to similarities and differences between the modulatory mechanisms that affect retrieval and those involved in the consolidation of the same task.

Could the 5-HT1B receptor inverse agonism affect learning consolidation?

Diverse evidence indicates that, the 5-HT system might play a role in learning and memory, since it occurs in brain areas mediating such processes and 5-HT drugs modulate them. Hence in this work, in order to explore further 5-HT involvement on learning and memory 5-HT1B receptors' role is investigated. Evidence indicates that SB-224289 (a 5-HT1B receptor inverse agonist) post-training injection facilitated learning consolidation in an associative autoshaping learning task, this effect was partially reversed by GR 127935 (a 5-HT1B/1D receptor antagonist), but unaffected by MDL 100907 (a 5-HT2A receptor antagonist) or ketanserin (a 5-HT1D/2A/7 receptor antagonist) at low doses. Moreover, SB-224289 antagonized the learning deficit produced by TFMPP (a 5-HT1A/1B/1D/2A/2C receptor agonist), GR 46611 (a 5-HT1A/1B/1D receptor agonist), mCPP (a 5-HT2A/2C/3/7 receptor agonist/antagonist) or GR 127935 (at low dose). SB-224289 did not alter the 8-OH-DPAT (a 5-HT1A/7 receptor agonist) learning facilitatory effect. SB-224289 eliminated the deficit learning produced by the anticholinergic muscarinic scopolamine or the glutamatergic antagonist dizocilpine. Administration of both, GR 127935 (5mg/kg) plus ketanserin (0.01 mg/kg) did not modify learning consolidation; nevertheless, when ketanserin dose was increased (0.1-1.0mg/kg) and SB-224289 dose was maintained constant, a learning facilitation effect was observed. Notably, SB-224289 at 1.0mg/kg potentiated a subeffective dose of the 5-HT1B/1D receptor agonist/antagonist mixed GR 127935, which facilitated learning consolidation and this effect was abolished by ketanserin at a higher dose. Collectively, the data confirm and extend the earlier findings with GR 127935 and the effects of non-selective 5-HT(1B) receptor agonists. Clearly 5-HT1B agonists induced a learning deficit which can be reversed with SB-224289. Perhaps more importantly, SB-224289 enhances learning consolidation when given alone and can reverse the deficits induced by both cholinergic and glutamatergic antagonist. Hence, 5-HT1B receptor inverse agonists or antagonists could represent drugs for the treatment of learning and memory dysfunctions.

Role of 5-HT1A receptors in a mouse passive avoidance paradigm

The effect on memory processes of modulation of 5-HT1A receptor subtype was investigated in the mouse passive avoidance test. The administration of 5-HT1A-receptor antagonists NAN-190 (1-(2-methoxyphenyl)-4-[4-2-phthalimmido)butyl]piperazine) and WAY-100635 (N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinyl-cyclohexanecarboxamide) produced a dose-dependent amnesic effect comparable to that obtained with the well-known amnesic agents scopolamine and dicyclomine. Pretreatment with the 5-HT1A-receptor agonists 8-OH-DPAT ((+/-)-8-hydroxy-dipropylaminotetralin) and 5-CT (5-carboxamidotryptamine) dose-dependently prevented the amnesia induced by 5-HT1A antagonists, scopolamine, dicyclomine and exposure to an hypoxic environment. The antiamnesic effect exerted by 5-HT1A-receptor agonists was comparable to that produced by the nootropic drug piracetam and cholinesterase inhibitor physostigmine. At effective doses, neither 5-HT1A-receptor agonists nor 5-HT1A-receptor antagonists produced any impairment of mouse motor coordination (rota-rod test), spontaneous motility (Animex apparatus) and inspection activity (hole board). These results indicate that modulation of 5-HT1A-receptors appears to play an important role in the regulation of cognitive processes.

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.

5-HT1A receptors modulate the consolidation of learning in normal and cognitively impaired rats

Attempts were made to further analyze the role of 5-HT1A receptors in consolidation of learning by evaluating the role of these receptors in cognitively normal and impaired animals. The effects of post-training administration of 8-OH-DPAT and 5-HT1A receptor antagonists, WAY 100135, WAY 100635, and S-UH-301, plus the cholinergic and glutamatergic antagonists, scopolamine and dizolcipine, respectively, were determined using an autoshaping learning task. The results showed that 8-OH-DPAT increased the number of conditioned responses, whereas WAY100135, WAY100635, and S-UH-301, and the 5-HT depleter, p-chloroamphetamine (PCA), had no effect. PCA did not change the silent properties of the 5-HT1A receptor antagonists. PCA, WAY100635, and S-UH-301, but not GR127935 (a 5-HT1B/1D-receptor antagonist) or MDL100907 (a 5-HT2A receptor antagonist), reversed the effect to 8-OH-DPAT. Ketanserin (a 5-HT2A/2C receptor antagonist) and ondansetron (a 5-HT3 receptor antagonist), at a dose that increased the conditioned responses by itself, reversed the effect of 8-OH-DPAT. Moreover, 8-OH-DPAT or S-UH-301 reversed the learning deficit induced by scopolamine and dizocilpine whereas WAY100635 reversed the effect of scopolamine only. These data confirm a role for presynaptic 5-HT1A receptors during the consolidation of learning and support the hypothesis that serotonergic, cholinergic, and glutamatergic systems interact in cognitively impaired animals.

Effects of the 5-HT receptor antagonists GR127935 (5-HT1B/1D) and MDL100907 (5-HT2A) in the consolidation of learning

We have previously reported that 5-HT1B/1D and 5-HT2A/2B/2C receptors play a role in learning and memory. The present investigation was devoted to analyze further in the autoshaping learning task: (1) the effects of the 5-HT1A/1B/1D receptor agonist, GR46611, the 5-HT1B/1D receptor antagonist, GR127935, and the selective 5-HT2A receptor antagonist, MDL100907. Consistent with a role of 5-HT1B/1D receptors in learning, the post-training injection of GR46611 (1-10 mg/kg) decreased the consolidation of learning whereas GR127935 (10 mg/kg) increased it; the effects of both drugs were reversed by PCA pretreatment. GR127935 abolished the decrease induced by GR46611, TFMPP and mCPP, whereas MDL100907 (0.1-3.0 mg/kg) had no effect by itself but abolished the effects of DOI, ketanserin and TFMPP and moderately inhibited the effects elicited by mCPP, 1-NP and mesulergine. Neither did GR127935 nor MDL100907 significantly modify the increase in the consolidation of learning induced by 8-OH-DPAT. Thus, the present findings suggest that stimulation of presynaptic 5-HT1B/1D receptors impairs the consolidation of learning whilst stimulation of 5-HT2A/2C receptors enhances it; the blockade of 5-HT2A receptors has no effects. In addition, 5-HT2 receptors seem to modulate this cognitive stage.

Effects of 5-HT4 receptor agonists and antagonists in learning

In the present work, the effects of pre- or post-training (ip) injection of BIMU1 and BIMU8 (5-HT4 agonists) were figured out in the autoshaping learning task. Furthermore, the post-training effects of these agonists after treatment with SDZ 205-557 and GR 125487D (5-HT4 antagonists) or p-Chloroamphetamine (PCA) were also explored. Animals were individually trained in a lever-press response on the autoshaping task and 24 hours later were tested. The results showed that pre-training injection of BIMU1 (5 20 mg/Kg) or BIMU8 (20 mg/Kg) increased the CR; in contrast, the post-training administration of BIMU1 (10-20 mg/Kg) or BIMU8 (5 and 20 mg/Kg) decreased it. Further experiments revealed that the post-training injections of SDZ 205-557 (1.0-10.0 mg/Kg) or GR 125487D (0.39-1.56 mg/Kg) by themselves did not alter the CR. When BIMU1 or BIMU8 was administered to rats pretreated with SDZ 205-557 (10 mg/Kg) or GR 125487D (0.78 mg/Kg), the decrement induced by 5-HT4 the agonists was reversed; in contrast, the administration of PCA failed to modify the CR or the agonist-induced responses. The findings showed that the pre-training stimulation of 5-HT4 receptors enhanced the acquisition of CR, while, post-training activation of 5-HT4 receptors, impaired the consolidation of learning. The latter effect was not altered by PCA pretreatment. The data show that 5-HT4 receptors are involved in the acquisition and consolidation of learning. It seems that postsynaptic 5-HT4 receptors are involved in the latter effect.

A pharmacological analysis of serotonergic receptors: effects of their activation of blockade in learning

1. The authors have tested several 5-HT selective agonists and antagonists (5-HT1A/1B, 5-HT2A/2B/2C, 5-HT3 or 5-HT4), an uptake inhibitor and 5-HT depletors in the autoshaping learning task. 2. The present work deals with the receptors whose stimulation increases or decreases learning. 3. Impaired consolidation of learning was observed after the presynaptic activation of 5-HT1B, 5-HT3 or 5-HT4 or the blockade of postsynaptic 5-HT2C/2B receptors. 4. In contrast, an improvement occurred after the presynaptic activation of 5-HT1A, 5-HT2C, and the blockade of presynaptic 5-HT2A, 5-HT2C and 5-HT3 receptors. 5. The blockade of postsynaptic 5-HT1A, 5-HT1B, 5-HT3 or 5-HT4 receptors and 5-HT inhibition of synthesis and its depletion did no alter learning by themselves. 6. The present data suggest that multiple pre- and postsynaptic serotonergic receptors are involved in the consolidation of learning. 7. Stimulation of most 5-HT receptors increases learning, however, some of 5-HT subtypes seem to limit the data storage. 8. Furthermore, the role of 5-HT receptors in learning seem to require an interaction with glutamatergic, GABAergic and cholinergic neurotransmission systems.

Serotonergic regulation of associative learning

This paper presents a review of studies dealing with the effects of 5-HT agonists and antagonists on learning as measured by classical conditioning of the rabbit's nictitating membrane response or the conditioned avoidance response in the rat. These studies indicate that the 5-HT2A/2C receptors are importantly involved in learning. In these behavioral paradigms, enhancement of learning is only produced by drugs that are agonists at the 5-HT2A/2C receptors, and this enhancement is only blocked by drugs that are antagonists at these receptors. In addition, evidence is presented for the existence of two classes of 5-HT2A/2C antagonists consisting of negative antagonists that retard learning when given alone (ritanserin, MDL-11,939, pizotifen and cyproheptadine) and those that are neutral antagonists in that they have no effect on learning (ketanserin, mianserin, BOL and LY-53,857). However, both the neutral and negative antagonists are equally capable of blocking the enhancement of learning produced by 5-HT2A/2C agonists. It was concluded that 5-HT2A and/or 5-HT2C agonists may provide a new approach to the treatment of learning disorders in aging or Alzheimer's disease.

Effect of fluoxetine on learning and memory involves multiple 5-HT systems

Diverse evidence suggests that 5-HT uptake blockers enhance learning and memory. However, there is no information about the mechanisms of action involved in such effects. The aim of the present work was to investigate the nature of the receptors involved in the effects of fluoxetine on learning. Therefore, a dose-response curve of posttraining injection (intraperitoneal) of fluoxetine was carried out in an associative learning task (auto-shaping). Fluoxetine or the vehicle was injected 10 min after 5-HT antagonists: (+/-)-pindolol, (+/-)-propanolol, NAN-190, ketanserin, ritanserin, mesulergine, MDL 72222, or SDZ 205-557. Presynaptic activity was eliminated by means of chloroamphetamine pretreatment. Scopolamine (an anticholinergic) and dizocilpine (a noncompetitive NMDA receptor antagonist) were also used. Results showed that fluoxetine enhanced learning of the conditioned response (CR) in a dose-dependent fashion. All 5-HT antagonists had no effects by themselves but inhibited the effects of fluoxetine at different degrees. Decrement of CR produced by scopolamine was reversed by fluoxetine. Dizocilpine did not affect CR but prevented the effects of fluoxetine. The present findings suggest that the actions of fluoxetine on learning are due to an interaction with multiple receptors of postsynaptic nature.

Mechanism of action of 8-OH-DPAT on learning and memory

It has been previously demonstrated that pretraining injection of 8-hydroxy-2-(di-n-propilamino)tetralin (8-OH-DPAT, a 5-HT1A agonist) impairs conditioned response (CR) in an autoshaping learning task. Therefore, in the present work we intended to determine whether such an effect could be prevented by pretraining, and whether pre- or postsynaptic 5-HT receptors are involved. Groups of rats received or did not receive food magazine training. On the next day, all groups in both conditions, pre- or posttraining, were treated with 8-OH-DPAT (0, 0.062, or 0.250 mg/kg). Posttraining groups were tested on a second session of autoshaping 24 h later. In a second experiment, naive rats received para-chlorophenylalanine (PCPA) (300 mg/kg x 3 days) before pre- or posttraining injection of 8-OH-DPAT. Results showed that in those groups trained to food magazine and treated 24 h later with 8-OH-DPAT, CR was not affected or enhanced. PCPA injection had no effect by itself, but blocked or attenuated the effect of a post- or pretraining injection of 8-OH-DPAT. The present data suggest that a) the pretraining effect of 8-OH-DPAT eliciting a decrease in CR can be eliminated by a food magazine training session; and b) presynaptic 5-HT1A receptors are involved in the effect of 8-OH-DPAT on the acquisition and consolidation of learning.