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.     

Pretraining infusion of DSP-4 into the amygdala impaired retention in the inhibitory avoidance task: involvement of norepinephrine but not serotonin in memory facilitation

The present study investigated the involvement of amygdala noradrenergic (NE) and serotonergic (5-HT) systems in memory storage processing. Rats bearing chronic cannulae in the amygdala were trained on a one-trial inhibitory avoidance task and tested for retention 24 hrs later. Five days prior to training, rats received intra-amygdala infusion of vehicle or various doses of N-2-chloroethyl-N-ethyl-2-bromobenzylamine (DSP-4)-a NE-specific neurotoxin when given peripherally. Results showed that pretraining intra-amygdala infusion of 10.0 micrograms or 30.0 micrograms of DSP-4 impaired retention. Further, 30.0 micrograms of DSP-4 also abolished the memory enhancing effect of epinephrine (E) injected peripherally. However, local infusion of DSP-4 depleted not only NE but also 5-HT and DA substantially. Subsequent experiments found that the retention deficit induced by 30.0 micrograms of DSP-4 could be ameliorated by 0.2 microgram NE but not by 5-HT at a wide range of doses infused into the amygdala shortly after training, which ascribed the deficit to depletion of NE. After protecting the 5-HT terminals by a pretreatment of fluoxetine (15.0 mg/kg), pretraining intra-amygdala infusion of 30.0 micrograms DSP-4 shifted the memory-enhancing dose of E from 0.1 mg/kg to 1.0 mg/kg. In contrast, pretraining intra-amygdala infusion of 15.0 micrograms 5,7-dihydroxytryptamine (5,7-DHT) or DSP-4 with a pretreatment of desipramine (DMI, 25.0 mg/kgx2) to protect NE terminals failed to impair retention or attenuate the memory enhancing effect of 0.1 mg/kg E injected peripherally. These findings, taken together, suggest that the memory modulatory effect of peripheral E involved, at least partially, the amygdala NE system.     

Role of serotonin receptors in the amygdaloid complex and central gray substance in conditioned passive avoidance reaction in rats

The functional role of 5-HT1 receptors in the memory trace retrieval was investigated in amygdala (AM), central gray substance of midbrain (CGS) and frontal cortex. There is used the passive avoidance response in the rat. The decrease of 5-HT1 binding sites in AM and CGS was revealed for the rats with retention of the passive avoidance response. The binding of 3H-5-HT in AM was found two sets of binding sites. It was concluded, that 5-HT1 receptors of AM and CGS are involved in learning processes either in the moment of the memory trace retrieval or immediately after it.     

Over-reinforcement protects against memory deficits induced by muscarinic blockade of the striatum

It has been shown that blockade of muscarinic receptors of the anterior striatum (AS) induces significant impairments in the retrieval of stored information of a passive avoidance task, trained with conventional parameters of footshock, and that the same blockade is ineffective in altering short-term memory of this task. The results of the present experimental series showed that in conditions of over-reinforcement, microinjections of scopolamine into the AS shortly after training or before retention testing of passive avoidance, do not produce memory deficits when retention is assessed 30 min, 24 h or 48 h after training. It is suggested that after an enhanced learning experience (over-reinforcement) striatal cholinergic activity is not involved in short- and long-term memory functions.     

Deltorphin, a naturally occurring peptide with high selectivity for delta opioid receptors, improves memory consolidation in two inbred strains of mice

Deltorphin, a heptapeptide that selectively binds to delta receptors, was intracerebroventricularly administered to mice belonging to the strains C57BL/6 and DBA/2 immediately after training in a one-trial inhibitory avoidance task. The retention performance of both strains was improved by the peptide administration, with DBA mice being more sensitive to this effect. The results show different effects of delta and mu receptor agonists on memory consolidation and of strain differences in number and/or distribution of delta opioid receptors in the brain.     

Dexamethasone reverses the memory impairment induced by antagonism of hippocampal gastrin-releasing peptide receptors

Storage of emotionally influenced memory is regulated by activation of glucocorticoid receptors (GRs) as well as of gastrin-releasing peptide receptors (GRPRs) in the dorsal hippocampus. In the present study, male Wistar rats were given a bilateral infusion of saline or the GRPR antagonist (D-Tpi6, Leu13 psi[CH2NH]-Leu14) bombesin (6-14) (RC-3095) (1.0 microg/side) into the dorsal hippocampus 10 min before training on an inhibitory avoidance task, followed by an immediate post-training i.p. injection of vehicle or the GR agonist dexamethasone (0.3 mg/kg). A retention test trial, carried out 24 h after training, indicated that intrahippocampal infusion of RC-3095 impaired inhibitory avoidance retention. Post-training administration of dexamethasone induced an enhancement of retention regardless of whether the animals had received saline or RC-3095 into the hippocampus before training. The findings indicate that hippocampal GRPR blockade does not prevent memory enhancement induced by dexamethasone. Together with previous results, these findings suggest that endogenous activation of GRPRs in the hippocampus modulates the consolidation of emotional memory, but is not a critical receptor system mediating memory formation.     

Effects of Milnacipran in Animal Models of Anxiety and Memory

Serotonin (5-HT) and noradrenaline (NA) are involved in both pathogenesis and recovery from depression and anxiety. We examined the effects of acute and chronic treatment with milnacipran, a serotonin/noradrenaline reuptake inhibitors (SNRIs) antidepressant, on anxiety and memory retention in rats. Male Wistar rats received acute or chronic administration of milnacipran (12.5, 25 or 50 mg/kg) or saline (control group). The animals were separately submitted to elevated plus-maze, inhibitory avoidance and open-field tasks 1 h after injection, in the acute group, or 23 h after last injection, in the chronic group. Our results showed an anxiolytic-like effect after chronic administration of milnacipran at doses of 25 and 50 mg/kg. The treatment does not interfere in memory retention and habituation to a novel environment at any doses studied. These findings support that milnacipran, an established SNRIs antidepressant, can also be useful in the treatment of anxiety disorders.     

Target size analysis of serotonin 5-HT1 and 5-HT2 receptors in bovine brain membranes

Freeze-dried crude synaptic membranes prepared from bovine cerebral cortex and striatum were exposed to high energy gamma ray from the source of 60Co. The size of serotonin 5-HT1 receptors labeled by [3H]serotonin and that of 5-HT2 receptors labeled by [3H]spiperone or [3H]ketanserin was determined by target size analyses. The values were 57,000 daltons, 145,000 daltons and 152,000 daltons for the cerebral cortex and 56,000 daltons, 141,000 daltons and 150,000 daltons for the striatum, respectively. The estimated sizes were deduced by reference to enzyme standards with known molecular masses and which were irradiated in parallel. Our results demonstrate that the molecular entities in situ for 5-HT1 receptors are distinct from those for 5-HT2 receptors, thus supporting data on the existence of two distinct populations of serotonin receptors, hitherto evidenced physiopharmacologically.     

Posttraining infusion of norepinephrine and corticotropin releasing factor into the bed nucleus of the stria terminalis enhanced retention in an inhibitory avoidance task

The present study investigated whether the bed nucleus of the stria terminalis (BNST) is involved in formation and retrieval of affective memory. Male Wistar rats with cannulae bilaterally implanted into the BNST were trained on a one-trial step-through inhibitory avoidance task. Shortly after training they received bilateral intra-BNST infusion of lidocaine, various noradrenergic drugs, or corticotropin releasing factor (CRF). Results showed that posttraining intra-BNST infusion of lidocaine impaired retention. Posttraining intra-BNST infusion of norepinephrine or the alpha1 antagonist prazosin induced a dose- and time-dependent retention enhancement or deficit, respectively. The enhancing effect of norepinephrine was mimicked by the alpha1 agonist phenylephrine, and antagonized by prazosin at a non-impairing dose. Posttraining intra-BNST infusion of the alpha2 antagonist idazoxan or the beta antagonist propranolol failed to affect retention. Posttraining intra-BNST infusion of CRF also enhanced retention in a dose-dependent manner. Various drugs infused shortly before testing did not significantly influence locomotor activity and retention. These findings, taken together, suggest that the BNST is involved in memory formation processes for affective experience and norepinephrine released in the BNST acting via alpha1 receptors plays a critical role in this function.     

Naloxone blocks the effect of 5-hydroxy-tryptophan on passive avoidance learning in rats: Implication of endogenous opioid peptides

The synthetic opioid peptide D-Ala²-met-enkephalinamide (DALA) impairs retention performance of rats on a one-trial passive avoidance task. The same effect is observed after administration of 5-hydroxy-tryptophan (5-HTP). Both drugs appear to impair learning by disrupting memory consolidation since similar effects are observed when they are given either peripherally before the acquisition test or intracerebroventricularly immediately after. A moderate dose of naloxone prevents the amnesia produced by DALA or 5-HTP. However, the opiate antagonist does not block the behavioral syndrome induced by clorgyline and 5-HTP. Moreover, bacitracin, a peptidase inhibitor, potentiates the effect of 5-HTP on the passive avoidance task. The results suggest that endogenous opioid peptides are involved in the amnesic effect of 5-HTP.     

Rapid and Transient Learning-Associated Increase in NMDA NR1 Subunit in the Rat Hippocampus

Several lines of evidence indicate that glutamate NMDA receptors are critically involved in long-term potentiation (LTP) and in certain forms of learning. It was previously demonstrated that memory formation of an inhibitory avoidance task in chick is specifically associated with an increase in the density of NMDA receptor in selected brain regions. Here we report on the effect of a one trial inhibitory avoidance training in rats, a hippocampal-dependent learning task, on the levels of different subunits of the glutamate NMDA receptor in synaptic plasma membranes (SPM) isolated from the hippocampus. Training rats on a one trial inhibitory avoidance task results in a rapid, transient and selective increase (+33 %, p < 0.05) in NMDA NR1 subunit expression in hippocampal SPM of rats sacrificed 30 min posttraining. No changes were observed at 0 or 120 min after training or in shocked animals in comparison to naive control rats. In addition, no training-associated increase in the levels of NMDA NR2A and NR2B or AMPA GluR 2/3 subunits was observed at any timepoint tested. In conclusion, the present findings support the hypothesis that alterations in expression of synaptic NMDA NR1 subunits in the hippocampus are specifically associated with memory formation of an inhibitory avoidance task and strongly suggest that hippocampal NMDA receptors are crucially involved in the neural mechanisms underlying certain forms of learning.These authors contributed equally to this work     

Involvement of striatum serotonergic system in the expression of genetically defined catalepsy

The activity of the rate-limiting enzyme of serotonin biosynthesis, tryptophan hydroxylase, and specific binding of [3H]ketanserin to 5-HT2A receptors and [3H]8-OH-DPAT to 5-HT1A receptors in the striatum of genetically predisposed to catalepsy rats and mice have been studied. The activity of tryptophan hydroxylase in the striatum of rats bred for many generations for predisposition to catalepsy was higher than in nonselected rats. Mice of highly susceptible to pinch-induced catalepsy CBA strain also differed from noncataleptic AKR and C57BL mouse strains by higher activity of tryptophan hydroxylase in striatum. Inhibition of tryptophan hydroxylase with p-chlorophenylalanine or p-chloromethamphetamine significantly decreased immobility time in genetically predisposed to catalepsy rats and mice. A decrease in the [3H]ketanserin specific binding in the striatum of cataleptic rats and CBA mice was found indicating a decrease in 5-HT2A receptor density. A decrease in [3H]8-OH-DPAT binding in striatum of cataleptic rats but not in CBA mice was shown. These results indicate that serotonergic system of striatum is involved in the expression of hereditary catalepsy and suggest that hereditary catalepsy may result from genetic changes in the regulation of serotonin metabolism and reception in striatum.     

The role of group I metabotropic glutamate receptors in memory consolidation and reconsolidation in the passive avoidance task in 1-day-old chicks

Although reconsolidation of memory after reminder does not seem to be the simple reiteration of the sequential stages occurring during memory consolidation, both phenomena probably employ similar mechanisms including activation of glutamate receptors and protein synthesis. It is known that group I metabotropic glutamate receptors (mGluRs) are involved in memory consolidation and modulation of protein synthesis. The aim of present study was to investigate the role of mGluR5 in memory consolidation and reconsolidation and to determine whether inhibition of these receptors may affect protein synthesis in these processes. The one-trial passive avoidance task on chicks was used as the experimental model of learning. Injection of the mGluR5 antagonist MPEP into a specific chick brain region IMM resulted in amnesia, provided the injection was made either shortly before or after training, or approximately 4 h after training. This amnesia was permanent, resembling the effects of protein synthesis inhibitors. MPEP injection immediately after reminder resulted in only a transient amnesia revealed 1h later. Increased expression of Zif/268 and c-Fos proteins 2 h after initial training was abolished bilaterally in chicks injected with MPEP. Injection of MPEP immediately after reminder did not inhibit c-Fos and Zif/268 expression, on the contrary, their expression was increased, specifically in left IMM and was similar to that observed after initial training. These results show that at least in the chick model mGluR5 play an important role in both consolidation and reconsolidation of memory but the mechanisms triggered by their activation in these processes differ. It is suggested that Ca(2+) signal derived from mGluR5 stimulation is necessary for complete memory consolidation, whereas during reconsolidation other mGluR5 triggered mechanisms of protein synthesis activation and regulation may be involved.     

Role of Somatodendritic and Postsynaptic 5-HT1A Receptors on Learning and Memory Functions in Rats

Memory impairment is a major problem afflicting mankind. The association between memory functions and neurotransmitter functions is of great interest for understanding brain function. Serotonergic pathways play an important role in the modulation of memory functions but the importance of its receptor types and subtypes on memory functions is still unclear. Activation and blockade of various serotonin (5-HT) receptors has been reported to alter cognitive processes and 5-HT receptor antagonism could be beneficial in the treatment of cognitive diseases. The role of 5-HT on memory functions is complicated. Among the 5-HT receptors subtypes, 5-HT(1A) receptors are of special interest because these receptors are present in the brain areas involved in learning and memory functions such as hippocampus and cortex. The present study was therefore designed to investigate the effect of activation and blockade of somatodendritic and/or postsynaptic 5-HT(1A) receptor on learning and memory functions in rats using modified version of water maze. In this study, 8-OH-DPAT (8-hydroxy-2-(di-N-propylamino) tetralin) at 0.3?mg/kg significantly enhanced learning acquisition (LA), short-term memory (STM) and long term memory (LTM) of rats pre-injected with saline suggesting that the activation of pre-synaptic 5-HT(1A) receptors by its agonist enhanced the memory functions of rats. Conversely, rats injected with 8-OH-DPAT at 1.0?mg/kg exhibited impaired LA and STM and had no effect on LTM. It was also shown in this study that blockade of 5-HT(1A) receptors by spiperone enhanced LA, had no effect on STM but impaired the LTM, which showed that the blockade of 5-HT(1A) receptors by its antagonist exerts different effect on different types of memory. This study suggests that 5-HT(1A) receptor could be used as a significant pharmacological target for the treatment of CNS diseases. Unraveling the role of serotonin in cognition and memory disorders could provide better therapy and it may lead to new insights in our understandings of learning and memory.     

5-HT3 receptors in selective animal models of cognition

Role of 5-HT3 receptors in cholinergic hypofunctional models of cognitive impairment in the elevated plus maze model and a passive avoidance model is studied. Cognitive impairment was caused by scopolamine (1 mg/kg, ip) in mice and 5-HT3 ligands mCPBG (1 and 5 mg/kg, ip) and ondansetron (0.5 and 5 mg/kg, ip) were administered before the pre-learning phase to study the effects on acquisition, while post-learning administration was used to determine the effects on consolidation. Ondansetron improved acquisition and retention in cholinergic hypofunctional models while mCPBG potentiated selected impaired cognitive indices. The results indicate the role of 5-HT3 receptors in cognition and that an ideal evaluation of 5-HT3 ligands in cognition should distinguish true cognitive effects from locomotor, motivational and emotional effects.     

Repeated administration of lead decreases brain 5-HT metabolism and produces memory deficits in rats

Long-term exposure to low levels of lead (Pb2+) has been shown to produce learning and memory deficits in rodents and humans. These deficits are thought to be associated with altered brain monoamine neurotransmission. Increased brain 5-HT (5-hydroxytryptamine; serotonin) activity is thought to be a prerequisite for maintaining control over the cognitive information process, and is said to have a role in learning and memory. This study was designed to investigate the effects of Pb2+ administration on brain 5-HT metabolism and memory function in rats. Rats were injected daily for three weeks with Pb2+-acetate at a dose of 100 mg/kg body weight. The assessment of memory was done using the Radial arm maze (RAM) and Passive avoidance tests. The results showed spatial working memory (SWM) deficits as well as decreased brain 5-HT metabolism. Increased serotonin activity is considered to be an indication of improved cognitive performance. The results are discussed in the context of lead-induced decreases in 5-HT metabolism playing a role in the impairment of memory.     

Foetal amygdalar transplantation facilitates recovery of retention deficit in CeA lesioned rats

Neural tissue transplant has come off age as a valuable technique for studying normal development and regeneration. Bilateral lesions of the central nucleus of amygdala (CeA) produce complete retention and acquisition deficit in inhibitory avoidance paradigms. The present study reports recovery of retention deficit in active avoidance task (AA) after amygdalar tissue transplantation in CeA lesioned rats. In a group of adult wistar rats, bilateral lesions of the CeA were produced electrolytically. In a separate group of rats foetal amygdalar tissue was transplanted at the CeA lesioned site 2 days after producing lesion. All the rats were trained on AA task before and after 5 days of lesion. In bilaterally CeA lesioned rats, the percentage of avoidance (% avoidance) decreased significantly (P < 0.05) from 85 +/- 18% prelesion to 15.5 +/- 35% postlesion. However, no change in the % avoidance was observed after amygdalar tissue transplantation. The results indicate that the transplanted rats are capable of retaining the learnt information in contrast to the lesion alone group of rats.     

Expression of serotonin receptors in bone

The 5-hydroxytryptamine (5-HT) receptors 5-HT(2A), 5-HT(2B), and 5-HT(2C) belong to a subfamily of serotonin receptors. Amino acid and mRNA sequences of these receptors have been published for several species including man. The 5-HT(2) receptors have been reported to act on nervous, muscle, and endothelial tissues. Here we report the presence of 5-HT(2B) receptor in fetal chicken bone cells. 5-HT(2B) receptor mRNA expression was demonstrated in osteocytes, osteoblasts, and periosteal fibroblasts, a population containing osteoblast precursor cells. Pharmacological studies using several agonists and antagonists showed that occupancy of the 5-HT(2B) receptor stimulates the proliferation of periosteal fibroblasts. Activity of the 5-HT(2A) receptor could however not be excluded. mRNA for both receptors was shown to be equally present in adult mouse osteoblasts. Osteocytes, which showed the highest expression of 5-HT(2B) receptor mRNA in chicken, and to a lesser extent osteoblasts, are considered to be mechanosensor cells involved in the adaptation of bone to its mechanical usage. Nitric oxide is one of the signaling molecules that is released upon mechanical stimulation of osteocytes and osteoblasts. The serotonin analog alpha-methyl-5-HT, which preferentially binds to 5-HT(2) receptors, decreased nitric oxide release by mechanically stimulated mouse osteoblasts. These results demonstrate that serotonin is involved in bone metabolism and its mechanoregulation.     

Aversive Learning under Different Training Conditions: Effects of NMDA Receptor Blockade in Area CA1 of the Hippocampus

Adult male Wistar rats were given either a single training trial or one training trial per day during 3 days followed by a retention test trial in an inhibitory avoidance (IA) task. In animals given a single training trial, pretraining, but not pretest bilateral infusion of the NMDA glutamate receptor antagonist d,l-2-amino-5-phosphonopentanoic acid (AP5) (5.0 μg) into the CA1 hippocampal area blocked IA retention. In animals given three training trials, infusions of AP5 given prior to each of the three training trials severely impaired, but did not block retention. The results indicate that NMDA receptors in the hippocampus are involved in the formation, but not in expression, of aversive memory. In addition, rats given repeated training were able to show a mild improvement of performance across training trials, possibly through mechanisms that do not depend on NMDA receptor activation in the dorsal hippocampus.     

Role of Serotonin2A and Serotonin2B/2C Receptor Subtypes in the Control of Accumbal and Striatal Dopamine Release Elicited In Vivo by Dorsal Raphe Nucleus Electrical Stimulation

This study investigates, using in vivo microdialysis, the role of serotonin2A (5-HT2A) and 5-HT(2B/2C) receptors in the effect of dorsal raphe nucleus (DRN) electrical stimulation on dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC), and 5-hydroxyindoleacetic acid (5-HIAA) extracellular levels monitored in the nucleus accumbens (NAC) and the striatum of halothane-anesthetized rats. Following DRN stimulation (300 microA, 1 ms, 20 Hz, 15 min) DA release was enhanced in the NAC and reduced in the striatum. The 5-HT2A antagonist SR 46349B (0.5 mg/kg) and the mixed 5-HT(2A/2B/2C) antagonist ritanserin (0.63 mg/kg) significantly reduced the effect of DRN stimulation on DA release in the NAC but not in the striatum. DA responses to DRN stimulation were not affected by the 5-HT(2B/2C) antagonist SB 206553 (5 mg/kg) in either region. None of these compounds was able to modify the enhancement of DOPAC and 5-HIAA outflow induced by DRN stimulation in either the NAC or the striatum. Finally, in both brain regions basal DA release was significantly increased only by SB 206553. These results indicate that 5-HT2A but not 5-HT(2B/2C) receptors participate in the facilitatory control exerted by endogenous 5-HT on accumbal DA release. Conversely, 5-HT(2B/2C) receptors tonically inhibit basal DA release in both brain regions.