Role of 5-HT1AReceptors in Acquisition, Consolidation and Retrieval of Learning

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.

Dose-response effects of chronic lithium regimens on spatial memory in the black molly fish

Lithium is widely used in the management of bipolar disorder, yet memory impairment is a serious side effect. To assess the effects of lithium on spatial working and reference memories, we have employed a plus maze utilizing spontaneous alternation (SA) and place-learning paradigms in two experiments with the black molly fish. Four treatment groups were gavaged with 20 microl of a 10, 100, or 1000 mM lithium chloride (LiCl) solution or ddH(2)O vehicle every 12 h for 22 to 24 days. On Day 15, subjects began an 8-day SA task or a 10-day place-learning task. Results indicate that there is a significant difference in SA performance among the treatment groups for Days 1, 2, and 3. Results of the place-learning task indicate that the 1 M dose group needed significantly more trials to reach criterion and made significantly fewer correct first choices than the other dose groups. Capillary ion analysis determinations of plasma and brain lithium levels illustrate linear dose-response relationships to doses administered. Regression analyses indicate that there is a relationship between SA performance and plasma/brain lithium levels during the initial part of testing. Collectively, the results indicate that chronic lithium administration impairs spatial working and reference memories.

Role of 5-HT(1A) and 5-HT(7) receptors in the facilitatory response induced by 8-OH-DPAT on learning consolidation

The present study further explored the mechanisms involved in the facilitatory effect induced by (+/-)-8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) on learning consolidation. For this purpose, we analyzed in parallel the effects of LY215840 and ritanserin, two 5-HT(2) receptor antagonists with high affinity for the 5-HT(7) receptor, and WAY100635, a selective 5-HT(1A) receptor antagonist, on the facilitatory effect induced by 8-OH-DPAT on learning consolidation. We also determined whether LY215840 and/or ritanserin could be beneficial in restoring a deficient learning condition. Using the model of autoshaping task, post-training injection of LY215840 or WAY100635 had no effect on learning consolidation. However, both drugs abolished the enhancing effect of 8-OH-DPAT, with LY215840 being slightly more effective than WAY100635 in this respect. Ritanserin produced an increase in performance by itself and also abolished the effect of 8-OH-DPAT. Remarkably, selective blockade of 5-HT(2A) and 5-HT(2B/2C) receptors with MDL100907 and SB200646, respectively, failed to alter the 8-OH-DPAT effect. LY215840 and ritanserin, at the doses that inhibited the 8-OH-DPAT-induced response, reversed the learning deficits induced by scopolamine and dizocilpine. The present results suggest that the enhancing effect produced by 8-OH-DPAT on learning consolidation involves activation of 5-HT(1A) receptors and an additional mechanism, probably related to the 5-HT(7) receptor. Blockade of 5-HT(2) receptors, and perhaps of 5-HT(7) receptors as well, may provide some benefit in reversing learning deficits associated with decreased cholinergic and/or glutamatergic neurotransmission.

5-HT(1A) receptor-mediated inhibition and 5-HT(2) as well as 5-HT(3) receptor-mediated excitation in different subdivisions of the rat amygdala

The techniques of extracellular single cell recording and microiontophoresis were used to study the effects of serotonin (5-HT) and of 5-HT(1A), 5-HT(2A/2C) and 5-HT(3) receptor agonists on the spontaneous activity of amygdaloid neurons in rats anesthetized with urethane. The background discharge rate was modified by 5-HT as well as by 5-HT agonists in about two-thirds of neurons tested in different nuclei of the amygdaloid complex. Whereas the 5-HT(2) and 5-HT(3) agonists significantly increased the neuronal discharge rate in nearly all subdivisions of the amygdala, the 5-HT(1A) agonist significantly inhibited the firing rate. Co-administration of bicuculline and 5-HT receptor agonists prevented the 8-OH-DPAT-induced increases in the firing rate in most cases tested, as well as the inhibitory effects of DOI or 2-methyl-5HT. Therefore, GABAergic interneurons seem to be involved in the mediation of serotonergic effects. The action of 5-HT agonists on the neuronal discharge rate was blocked by different receptor-specific antagonists. The results support the hypothesis that 5-HT exerts control throughout the amygdala by acting at least on 5-HT(1A), 5-HT(2A/2C) and 5-HT(3) receptors seemingly located both on projection and interneurons.

Tissue distribution, autoradiography, and metabolism of 4-(2'-methoxyphenyl)-1-[2' -[N-2"-pyridinyl)-p-[(18)F]fluorobenzamido]ethyl]piperazine (p-[(18)F]MPPF), a new serotonin 5-HT(1A) antagonist for positron emission tomography: An In vivo study in rats

The in vivo behavior of 4-(2'-methoxyphenyl)-1-[2'-[N-(2"-pyridinyl)-p-[(18)F]fluorobenzamido ]ethyl]-piperazine (p-[(18)F]MPPF), a new serotonin 5-HT(1A) antagonist, was studied in awake, freely moving rats. Biodistribution studies showed that the carbon-fluorine bond was stable in vivo, that this compound was able to cross the blood-brain barrier, and that a general diffusion equilibrium could account for the availability of the tracer. The great quantity of highly polar metabolites found in plasma did not contribute to the small amounts of metabolites found in hippocampus, frontal cortex, and cerebellum. Exvivo p-[(18)F]MPPF and in vitro 8-hydroxy-2-(di-n-[(3)H]propylamino)tetralin autoradiography were compared both qualitatively and quantitatively. Qualitative evaluation proved that the same brain regions were labeled and that the p-[(18)F]MPPF labeling is (a) in total agreement with the known distribution of 5-HT(1A) receptors in rats and (b) characterized by very low nonspecific binding. Quantitative comparison demonstrated that the in vivo labeling pattern obtained with p-[(18)F]MPPF cannot be explained by differences in regional blood flow, capillary density, or permeability. The 5-HT(1A) specificity of p-[(18)F]MPPF and binding reversibility were confirmed in vivo with displacement experiments. Thus, this compound can be used to evaluate parameters characterizing 5-HT(1A) binding sites in the brain.

[(18)F]p-MPPF: aA radiolabeled antagonist for the study of 5-HT(1A) receptors with PET

This paper summarizes the present status of the researches conducted with [(18)F]4-(2'-methoxyphenyl)-1-[2'-[N-(2"-pyridinyl)-p-fluorobenzamido ]ethyl]-piperazine known as [(18)F]p-MPPF, a new 5-HT(1A) antagonist for the study of the serotonergic neurotransmission with positron emission tomography (PET). This includes chemistry, radiochemistry, animal data (rats, cats, and monkeys) with autoradiography and PET, human data with PET, toxicity, and metabolism.

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.

Facilitation by 8-OH-DPAT of passive avoidance performance in rats after inactivation of 5-HT(1A) receptors

1. Pretraining administration of 8-hydroxy-2-di-n-propylamino-tetralin (8-OH-DPAT 0.1 mg kg(-1)), a 5-HT(1A) receptor agonist, or buspirone (1 mg kg(-1)), a 5-HT(1A) receptor partial agonist, markedly impaired passive avoidance retention in rats 24 h later. The effect of 8-OH-DPAT was prevented by the 5-HT(1A) receptor antagonists, NAN-190 and WAY-100635, at doses without any intrinsic effect. 2. N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ 10 mg kg(-1)), an alkylating agent that inactivates different G-protein coupled receptors, impaired retention performance when given 48 h pretraining. The disruptive effect of EEDQ was reversed by 8-OH-DPAT or buspirone, given 30 min before training. 3. Non-specific actions did not account for 8-OH-DPAT-induced reversal of the EEDQ effect since no significant difference in locomotor activity or in pain threshold was found between rats receiving EEDQ or EEDQ+8-OH-DPAT. 4. When NAN-190 (1 mg kg(-1)) or WAY-100635 (0.5 mg kg(-1)) were given before 8-OH-DPAT to EEDQ-pretreated animals, the reversal by 8-OH-DPAT of EEDQ-induced retention impairment was still more pronounced. However, no EEDQ reversal by 8-OH-DPAT was found when 5-HT(1A) receptors were protected by WAY-100635 (10 mg kg(-1)) 30 min before EEDQ. 5. In the hippocampus of EEDQ-treated rats, 5-HT(7) receptors were less inactivated than 5-HT(1A) receptors and significant increases were found in 5-HT(1A) but not in 5-HT(7) receptor mRNA levels. Ritanserin and methiothepin (10 mg kg(-1) each), antagonists with higher affinity at 5-HT(7) than at 5-HT(1A) receptors, prevented the retention impairment induced by EEDQ but did not significantly protect against 5-HT(7) receptor inactivation. 6. The results indicate that the facilitatory effect of 8-OH-DPAT is not mediated through 5-HT(1A) receptors and suggest that other 8-OH-DPAT-sensitive receptors could be involved in the dual effect of 8-OH-DPAT on passive avoidance performance in rats.

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.

Genetic selection of alcohol preference can be countered by conditioning processes

Twenty-four P rats and 24 NP rats were conditioned to consume 10% w/v alcohol in daily 0.5-h limited access periods using a modified version of Samson's sucrose fading procedure. Both P and NP rats demonstrated a strikingly similar day-to-day pattern of alcohol intakes. For the most part, P rats drank more than NP rats, but by the middle of the fourth month of drinking, P and NP rats were drinking equivalent amounts of alcohol. Both P and NP rats consumed alcohol in amounts similar to two outbred strains of rats (Wistar and Sprague-Dawley) previously conditioned to drink alcohol in this laboratory.

Spontaneously hypertensive rats: a potential model to identify drugs for treatment of learning disorders

Spontaneously hypertensive rats (SHR) of 3 to 12 months of age learned and retrieved less information than normotensive Wistar-Kyoto rats (WKY), although no difference was found with animals from 18 and 24 months of age. The combined influence of hypertension and aging had an additive detrimental effect on cognitive functions. Notwithstanding these deficiencies in learning and memory, SHR have seldom been used as a model in the screening of drugs with therapeutic potential for treatment of disorders of cognitive processes. Moreover, the calcium channel blocker nimodipine has beneficial effects on learning in both aged and hypertensive animals and humans. However, no attempt has been made to investigate whether nimodipine can reverse the additive deleterious effects of aging and hypertension in the same subject. We recently reported that deteriorated animals (middle-aged and/or hypertensive) chronically treated with nimodipine (via osmotic minipumps) exhibit higher learning scores. This information indicates that nimodipine can reverse the impairing effects of either aging or hypertension on learning; the presence of the two conditions, however, produces a severe impairment that can be partially reversed by this drug. Therefore, we propose that mature and middle-aged SHR represent a model for the screening of potentially useful drugs in the treatment of learning disorders, probably associated with hypertension and/or aging. Nevertheless, it must be remembered that the SHR is a genetic model and the appearance of neural disturbances could be a parallel genetic phenomenon and not necessarily or exclusively related to hypertension per se.

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.

Role of 5-HT1B, 5-HT2A and 5-HT2C receptors in learning

The effects of post-training (i.p.) injection of TFMPP, mCPP, DOI or 1-NP in the autoshaping learning task was explored. Furthermore, the post-training effects of these agonists after treatment with the antagonists (+/-)-pindolol, (+/-)-propranolol, NAN-190, ketanserin, ritanserin, mesulergine, MDL-72222 or p-chloroamphetamine (5-HT depleter) were studied. Rats were individually trained with a lever-press response (conditioned response; CR) on the autoshaping task and tested 24 h later. The results showed that the injection of TFMPP (1-10 mg/kg), mCPP (1-10 mg/kg), 1-NP (0.1-1.0 mg/kg) or mesulergine (0.4 mg/kg) decreased the rate of CR, while DOI (0.01-0.1 mg/kg) and ritanserin (0.5 mg/kg) and ketanserin (0.001-0.1 mg/kg) increased it. However, the effect induced by TFMPP was reversed by (+/-)-pindolol, ketanserin, ritanserin and PCA; the mCPP-induced effect was antagonized by (+/-)-propranolol, ketanserin, ritanserin and MDL-72222; and the effect produced by 1-NP was reversed by ketanserin, ritanserin and PCA. In addition, the increment in CR provoked by DOI was enhanced by ketanserin, and reversed by ritanserin, mesulergine and PCA. These findings suggest that TFMPP, 1-NP and DOI exerted their effects via stimulation of presynaptic 5-HT receptors. The effects of mCPP most probably reflect activation of postsynaptic receptors. The present data suggest that both 5-HT1B and 5-HT2A-2C receptors play a significant role in the consolidation of learning.