Extinction of spatial memory alters CREB phosphorylation in hippocampal CA1.

Although the importance of cAMP-response element binding protein (CREB) phosphorylation in long-term memory formation is well documented for hippocampus-dependent tasks, little is known about the changes in phosphorylated CREB (pCREB) that occur during the process of extinction. The purpose of this study was to characterize the temporal patterns of pCREB in the CA1 and the amygdala after the extinction of previously acquired spatial information in the water maze. Mice were trained to find a hidden platform located at a fixed position and then were given extinction sessions in which the platform was either absent (NoPF) or relocated every day (RandomPF). We show that water maze spatial training evoked a biphasic response of pCREB in the CA1, with two different peaks occurring 15 min and 8 h postacquisition. The extinction of the original spatial preference significantly reduced the two peaks of CA1 pCREB in both RandomPF and NoPF groups whereas CA1 pCREB at 60 min post-training remained unaffected. Moreover, the early and late phases of extinction training produced regionally dissociable effects on pCREB in the CA1 and the lateral nucleus of the amygdala. These findings provide new insights on the molecular dynamics and anatomical dissociations underlying spatial memory and extinction learning.

Effects of age and spatial learning on adenylyl cyclase mRNA expression in the mouse hippocampus.

Adenylyl cyclase (AC) subtypes have been implicated in memory processes and synaptic plasticity. In the present study, the effects of aging and learning on Ca2+/calmodulin-stimulable AC1, Ca2+-insensitive AC2 and Ca2+/calcineurin-inhibited AC9 mRNA level were compared in the dorsal hippocampus of young-adult and aged C57BL/6 mice using in situ hybridization. Both AC1 and AC9 mRNA expression were downregulated in aged hippocampus, whereas AC2 mRNA remained unchanged, suggesting differential sensitivities to the aging process. We next examined AC mRNA expression in the hippocampus after spatial learning in the Morris water maze. Acquisition of the spatial task was associated with an increase of AC1 and AC9 mRNA levels in both young-adult and aged groups, suggesting that Ca2+-sensitive ACs are oppositely regulated by aging and learning. However, aged-trained mice had reduced AC1 and AC9, but greater AC2, mRNA levels relative to young-trained mice and age-related learning impairments were correlated with reduced AC1 expression in area CA1. We suggest that reduced levels of hippocampal AC1 mRNA may greatly contribute to age-related defects in spatial memory.

Serotonin 1B knockout mice exhibit a task-dependent selective learning facilitation.

Serotonin 1B knockout (KO) mice exhibit better spatial reference memory performance in the Morris water maze than their wild type (WT) controls. The present study was aimed at dissecting the underlying cognitive bases of this facilitation using a stepwise water maze paradigm. The performance of KO mice did not differ from WT in a single start-goal task, nor when using two opposite starts. However, KO mice exhibited better performance in stages requiring cognitive flexibility or the higher level of spatial navigation planning (standard version). In a short-term memory version of the task, no such genotype effect was observed, confirming our previous findings. These results suggest that the serotonin 1B receptor gene deletion selectively enhances learning performance when the cognitive requirement of the task is elevated.

Region-specific decrease in 5-HT1A and 5-HT1B binding sites after intra-hippocampal ibotenic acid injections in the rat.

The cellular location of 5-HT1A and 5-HT1B binding sites in the hippocampus was investigated using a stereotaxic unilateral lesion of the CA1 field by ibotenic acid, followed by autoradiography on brain sections with the specific ligands [3H]8-OH-DPAT and S-CM-G[125I]TNH2, respectively. As compared to the contralateral side of the lesion, the ipsilateral side exhibited a decrease in the density of 5-HT1A binding sites in the strata oriens and radiatum of CA1, and of 5-HT1B binding sites in the dorsal subiculum and stratum oriens of CA1. The results demonstrate that 5-HT1A binding sites are located on dendrites whereas 5-HT1B binding sites are located on axon terminals of CA1 pyramidal cells.

Exploration, anxiety, and spatial memory in transgenic anophthalmic mice.

Contradictory results are found in the literature concerning the role of vision in the perception of space or in spatial navigation, in part because of the lack of murine models of total blindness used so far. The authors evaluated the spatial abilities of anophthalmic transgenic mice. These mice did not differ qualitatively from their wild-type littermates in general locomotor activity, spontaneous alternation, object exploration, or anxiety, but their level of exploratory activity was generally lower. In the spatial version of the water maze, they displayed persistent thigmotaxic behavior and showed severe spatial learning impairments. However, their performances improved with training, suggesting that they may have acquired a rough representation of the platform position. These results suggest that modalities other than vision enable some degree of spatial processing in proximal and structured spaces but that vision is critical for accurate spatial navigation.

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.

Anxiety, motor activation, and maternal-infant interactions in 5HT1B knockout mice.

This study describes the development of anxiety and motor activation in mice lacking the serotonin (5HT) 1B receptor and in wild type controls and characterizes their early mother-infant interactions. In the isolation-induced ultrasonic vocalization paradigm, 5HT1B knockout pups vocalized less and were hyperactive, rearing, jumping, and rolling more often than wild type pups. One week postpartum, 5HT1B knockout mothers spent 20% more of their time outside the nest and were also hyperactive, rearing and climbing to the edge of the cage more often than the wild type mothers. There were no genotype effects on pup retrieval. Knockout adults were less anxious in the elevated plus-maze, defecated less, and head-dipped more, although none of the standard measures of anxiety (time and entries in the open arms) were different. 5HT1B knockout mice of both sexes were hyperactive during both the light and the dark phases of the 24-hr cycle. Thus, 5HT1B knockout mice show reduced anxiety and are hyperactive throughout their life.

5-HT1B receptor knock-out mice exhibit increased exploratory activity and enhanced spatial memory performance in the Morris water maze.

In an attempt to characterize the contribution of the 5-HT1B receptor to behavior, 5-HT1B knock-out (KO) mice were subjected to a battery of behavioral paradigms aimed at differentiating various components of cognitive and emotional behaviors. In an object exploration task, wild-type (WT) and 5-HT1B KO mice did not differ in locomotor activity. 5-HT1B KO mice, however, displayed lower thigmotaxis (an index of anxiety) associated with a higher level of object exploratory activity, but no genotype differences were observed in the elevated plus maze. 5-HT1B KO mice also displayed a lack of exploratory habituation. In the spatial version of the Morris water maze, 5-HT1B KO mice showed higher performances in acquisition and transfer test, which was not observed in the visual version of the task. No genotype differences were found in contextual fear conditioning, because both WT and 5-HT1B KO mice were able to remember the context where they had received the aversive stimulus. The deletion of the 5-HT1B receptor, associated with appropriate behavioral paradigms, thus allowed us to dissociate anxiety from response to novelty, and perseverative behavior (lack of habituation) from adaptive behavioral inhibition underlying cognitive flexibility (transfer stage in the water maze). The deletion of the 5-HT1B receptor did not result in significant developmental plasticities for other major 5-HT receptor types but may have influenced other neurotransmission systems. The 5-HT1B receptor may be a key target for serotonin in the modulation of cognitive behavior, particularly in situations involving a high cognitive demand.

Serotonin receptors and cognitive behaviour--an update.

The serotonergic system appears to play a role in behaviors that involve a high cognitive demand and in memory improvement or recovery from an impaired cognitive performance. This is made evident after administration of 5-HT(2A/2C), and 5-HT4 receptor agonists, or 5-HT1A, 5-HT3 and probably 5-HT1B receptor antagonists. These serotonin receptor subtypes are localized on 'cognitive' pathways, with the hippocampus and frontal cortex as the main target structures. A better understanding of the role played in cognition by these and other serotonin receptor subtypes is likely to result from the recent availability of highly specific ligands such as 5-HT1A, 5-HT1B, and 5-HT2A receptor antagonists, and new molecular tools such as gene knock-out mice, especially inducible mice for which the genetic alteration can be restricted both temporally and anatomically.

Selective increases in serotonin 5-HT1B/1D and 5-HT2A/2C binding sites in adult rat basal ganglia following lesions of serotonergic neurons.

Quantitative autoradiography was used to examine possible adaptive changes in serotonin 5-HT1B/1D and 5-HT2A/2C receptor binding sites in adult rat basal ganglia, after partial or severe lesions of serotonergic neurons produced by intraraphe injections of variable amounts of 5,7-dihydroxytryptamine. In controls, the 5-HT1B/1D sites labeled with S-CM-G[125I]TNH2 were evenly distributed in the core and the shell of the nucleus accumbens. The density of 5-HT1B/1D sites was higher in the ventral than dorsal part of the striatum and no regional differences were detected along the rostrocaudal axis of the structure. The 5-HT2A/2C sites labeled with [125I]DOI were preferentially distributed in the mediodorsal striatum and higher densities were detected in the shell than core of the nucleus accumbens. Following 5,7-dihydroxytryptamine injections, there were no changes in binding of either receptor subtype after partial lesions entailing 80-90% 5-HT depletions. After severe 5-HT depletions (over 95%), large increases in 5-HT1B/1D binding were observed in the substantia nigra (78%), but no changes took place in the globus pallidus. Increases in 5-HT1B/1D binding were also detected in the shell of the nucleus accumbens (27%). Similar sized increases in 5-HT2A/2C binding (22%) were restricted to the medial striatum. The present results suggest a preferential association between 5-HT1B/1D receptors and the striatonigral neurons containing substance P, as indicated by the striatal distribution of these receptors and their selective increases in the substantia nigra after severe 5-HT deprivation. We recently proposed a similar relationship between the 5-HT4 receptors and the striatopallidal neurons containing met-enkephalin. Moreover, the increases in 5-HT1B/1D binding in the substantia nigra and in the shell of the nucleus accumbens reinforce the view of an implication of this receptor subtype in motor functions. In contrast, the prominent increases in 5-HT2A/2C binding after severe 5-HT deprivation as restricted to the medial region of the striatum and suggest up-regulation of most probably 5-HT2C receptors in a region implicated in cognitive functions.

Differential effects of serotonin (5-HT) lesions and synthesis blockade on neuropeptide-Y immunoreactivity and 5-HT1A, 5-HT1B/1D and 5-HT2A/2C receptor binding sites in the rat cerebral cortex.

The present study was aimed at comparing the effects of serotonin (5-HT) synthesis blockade using chronic administration of p-chlorophenylalanine (PCPA) and 5,7-dihydroxytryptamine injections of variable volume (3 vs. 6 microl) on the density of NPY immunoreactive (Ir) neurons and binding of [3H]8-OH-DPAT, S-CM-G[125I]TNH2 and [125I]DOI to 5-HT1A, 5-HT1B/1D, and 5-HT2A/2C receptors in rat cortical regions. Three weeks after large but partial (89% depletion in 5-HT tissue concentration) lesions of 5-HT neurons no changes in neither NPY immunoreactivity nor 5-HT receptor binding were detected. The complete 5,7-DHT lesions produced increases in the number of NPY-Ir neurons in the upper regions of the cingular (134%), frontal (140%) and parietal cortex (48%) and corresponding decreases in 5-HT2A/2C binding (16-26%). No changes in 5-HT1A and 5-HT1B/1D binding were observed after lesions of this kind. After PCPA treatment, decreases in NPY-Ir neurons density (22-40%) and increases in 5-HT1A and 5-HT1B/1D receptor binding sites (20-50%) were distributed in both upper and deeper cortical regions. The lack of effect of the partial lesion suggests that spared 5-HT neurons may exert compensatory mechanisms up to a large extent. The changes in NPY immunoreactivity and 5-HT2A/2C binding detected in the upper regions of the cortex after complete 5-HT lesions probably result from local cellular rearrangements, whereas blocking 5-HT synthesis has more widespread influence on NPY neurons and on 5-HT1A and 5-HT1B/1D receptor subtypes. Moreover, decreases in DOPAC concentrations detected only after complete lesions suggest that the involvement of catecholaminergic transmission may also differentiate 5,7-DHT and PCPA treatments. Altogether, these data suggest that different receptor subtypes might be involved in 5-HT-NPY relationships.

Serotonin receptors in cognitive behaviors.

The serotonergic system appears to play a role in behaviors that involve a high cognitive demand and in memory improvement or recovery from impaired cognitive performance, as made evident after administration of serotonin 5-HT2A/5-HT2C or 5-HT4 receptor agonists or 5-HT1A or 5-HT3 receptor antagonists. These serotonin receptor subtypes are localized on 'cognitive' pathways, with the hippocampus and frontal cortex as the main target structures. A better understanding of the role played by these and other serotonin receptor subtypes in cognition is likely to result from the recent availability of new specific ligands and new molecular tools, such as gene knock-out and transgenic mice.

Effortful information processing in a spontaneous spatial situation by rats with medial prefrontal lesions.

Previous research has suggested that the rat prefrontal cortex might play a role in spatial information processing and in divided attention. More recent work showed that the effect of prefrontal lesions is more important when the task involves response selection in complex situations. The first aim of the present study was to test the effect of lesions of the prelimbic area of the rat prefrontal cortex in spatial exploration, a situation involving the processing of spatial and non-spatial information, but requiring no response selection. The second aim was to manipulate the degree of cognitive effort required by the task. The latter effect was tested by manipulating the number of items to explore. Rats explored either a simple (3 objects) or a complex (6 objects) situation. We reasoned that acquiring spatial information so as to react adequately to spatial or non spatial changes involved more effortful processing in the complex situation than in the simpler one. The results suggest that the medial prefrontal cortex is not crucially involved in effortful processing when the task requires no response selection.

Effects of limitations on the use of some visual and kinaesthetic information in spatial mapping during exploration in the rat.

The purpose of this experiment was to study the effects of limiting visual and/or locomotor access to a part of the environment in the building up of a spatial representation of the whole space. During five sessions, rats were allowed to explore separately and successively the two halves (subspaces) of a circular open field containing four objects. During exploration of each half, continuous or discontinuous locomotor and/or visual access to the other half was provided by using opaque or transparent partitions, with or without doors. Once habituation was complete, the partition was removed for some subjects but remained for others. The locomotor and exploratory reactions to this removal were recorded. Whatever their locomotor experience (continuous or discontinuous), rats that had a discontinuous visual experience between the subspaces displayed a renewal of exploratory activity, whereas the rats that had received a continuous visual experience did not re-explore the objects. This result suggests that continuous visual access to the whole space is necessary for the construction of an overall representation. Furthermore, continuous locomotor activity does not seem to compensate for the discontinuity of visual information.

5-HT1B receptor knock out--behavioral consequences.

Serotonin is a neuromodulator that is involved in a number of mood disorders such as depression, anxiety and impulsive violence. In an attempt to dissect the contribution of individual 5-HT receptor subtypes to behavior, we have generated by homologous recombination, mutant mice lacking the 5-HT1B receptor. These mice did not exhibit any obvious developmental or behavioral defect. However, the hyperlocomotor effect of the 5-HT1A/1B agonist, RU 24969 was completely absent in mutant mice, indicating that this effect is mediated by 5-HT1B receptors. Moreover, when confronted with an intruder, isolated mutant mice attacked the intruder faster and more intensely than wild-type mice, suggesting an involvement of 5-HT1B receptors in the modulation of aggressive behavior. These data might be related to the fact that a class of 5-HT1 agonists, termed serenics, have anti-aggressive properties, and with the findings that certain impulsive aggressive behaviors are associated with deficits in central serotonin.

Spatial memory deficits following stimulation of hippocampal 5-HT1B receptors in the rat.

In this study we examined a possible contribution of serotonin (5-hydroxytryptamine, 5-HT) to spatial memory performance in the rat. Rats were trained to run in a radial maze in a manner that involved two kinds of memory function, i.e. working memory and reference memory. They received intrahippocampal microinjections of a 5-HT1A [8-hydroxy-2-(di-n-propylamino)tetralin or 8-OH-DPAT], or a 5-HT1B [3-(1,2,5,6-tetrahydropyrid-4-yl)pyrrolo[3,2-b]pyrid-5-one or CP-93,129] receptor agonist, and a muscarinic receptor antagonist (scopolamine). 8-OH-DPAT (5 micrograms/microliters), like injections of saline, induced no change in performance levels. In contrast, rats suffered an impairment in both reference and working memory following injection of scopolamine (10 micrograms/microliters). CP-93,129 induced a higher frequency of reference memory errors than of working memory errors at the intermediate (10 micrograms/microliters) and higher doses (16 micrograms/microliters). Thus, the stimulation of 5-HT1B receptors in the CA1 field of the dorsal hippocampus impairs the performance of rats in a spatial learning task.

Distractibility and locomotor activity in rat following intra-collicular injection of a serotonin 1B-1D agonist.

The superior colliculus (SC) is thought to be the decision center for reactions to novel and/or moving stimuli in the peripheral visual field. Serotonin 1B (5-HT1B) receptors were previously demonstrated to be located on collicular axon terminals of retinal ganglion cells and their activation might depress afferent inputs from the retina. The effects of intra-collicular injections of 5-HT1 drugs on distractibility were studied in hooded rats trained to run toward illuminated targets for a food reward in a 2-choice runway. 8-hydroxy-2-(di-n-propylamino)tetraline (8-OH-DPAT), a 5-HT1A receptor agonist, RU 24969, a mixed 5-HT1A and 5-HT1B agonist, serotonin-O-carboxymethylglycyltyrosinamide (S-CM-GTNH2), a mixed 5-HT1B and 5-HT1D receptor agonist and saline (control) were alternately injected. Following the S-CM-GTNH2 treatment alone, animals exhibited an erratic running style, involving side-to-side movements of the head, without change in the overall accuracy of their locomotor trajectories, but with substantial decrease in their running speed. When distracting peripheral lights were introduced at the mid-points of the animals' run, in the weaker distracting condition (unilateral distractor) only, distraction indexes were found lower following the S-CM-GTNH2 treatment than following the other drug or saline treatments. It is concluded that serotonin, via 5-HT1B-1D receptors, may induce an elevation of the visual distractibility threshold by modulating directly the transmission of the primary visual signal.

Changes in exploratory activity following stimulation of hippocampal 5-HT1A and 5-HT1B receptors in the rat.

The object exploration task allows the measure of changes in locomotor and exploratory activities, habituation, and reaction to a spatial change and to novelty. The effects of intrahippocampal (dorsal CA1 field) microinjections of serotonin 1 receptor (5-HT1) agonists on these behavioral components were evaluated in the rat. 8-Hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT, 5 micrograms/microliters) was used as a 5-HT1A agonist, 3-(1,2,5,6-tetrahydropyrid-4-yl)pyrrolo[3,2-b]pyrid-5-one (CP 93,129,16 micrograms/microliters) as a 5-HT1B agonist, and scopolamine (10 micrograms/microliters) as a muscarinic cholinergic antagonist. Scopolamine induced a long-lasting increase in locomotor activity and a lack of reaction to spatial change; both these results are in agreement with the known crucial influence of the septo-hippocampal cholinergic system in hippocampal functioning. Stimulation of 5-HT1A and 5-HT1B receptors induced a decrease in object exploration and habituation without affecting the retrieval of spatial information. But stimulation of hippocampal 5-HT1B receptors induced a selective change in the animal's emotional state, i.e., an initial decrease in locomotor activity and a neophobic reaction in response to a new object; such effects did not occur following stimulation of 5HT1A receptors. These results have to be considered in the light of the anxiogenic property of 5-HT1B agonists. On the whole, they support the hypothesis of the involvement of the serotonergic system, via 5HT1A and 5-HT1B receptors, in the modulation of hippocampal functions.

Serotonin 1B receptor regulation after dorsal subiculum deafferentation.

The subiculum may be the key structure in the transfer of relevant processed information from the hippocampal formation to cortical areas. We investigated the location of the serotonin 1B receptor (5-HT1B) in the hippocampus with the specific ligand serotonin-O-carboxymethyl-glycyl[125I]tyrosinamide in rat brain sections using in vitro autoradiography. A high density of 5-HT1B binding sites was found in the dorsal subiculum (DS), in the lacunosum moleculare, and in the most dorsal layer of the stratum oriens of the CA1 field. CA1 pyramidal neurons that contain 5-HT1B mRNA project primarily to the DS. We interrupted the pyramidal CA1 axons unilaterally by a stereotaxic knife cut. Histological analysis showed that the lesion was restricted to a trial of cells lost between CA1 and DS. Specific 5-HT1B binding site density was decreased in the DS on the ipsilateral side of the lesion compared to the contralateral side. We conclude that 5-HT1B receptors are located on CA1 pyramidal axon terminals in the DS. Serotonin, acting on these receptors, should inhibit CA1 neurotransmitter release and, in this way, modulate subicular functions.