Louis Segu

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 septohippocampal 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 5HT3 and 5-HT1B receptor antagonists prevents memory impairment and facilitates learning in situations involving a high cognitive demand. In contrast, antagonists for 5-HTZ2A/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.

The mouse 5-hydroxytryptamine 1B receptor is localized predominantly on axon terminals

The 5-hydroxytryptamine1B receptor is a serotonin receptor subtype which is expressed predominantly in the basal ganglia. It has been suggested to play a role in movement and appetite control as well as in certain pathological states such as migraine. The recent cloning of the 5-hydroxytryptamine1B gene as well as the discovery of a radioligand that labels in rodents 5-hydroxytryptamine1B and possibly 5-hydroxytryptamine1D alpha receptors (S-CM-G[125I]TNH2) allowed us to compare the distribution of the messenger RNA and of the protein in mouse brain sections. A high 5-hydroxytryptamine1B messenger RNA level is found in the caudate-putamen in medium spiny neurons that project to the globus pallidus and the substantia nigra. In contrast, no messenger RNA is expressed in the globus pallidus and substantia nigra although these structures reveal the highest level of 5-hydroxytryptamine1B binding sites. In the hippocampus, 5-hydroxytryptamine1B messenger RNA is localized in the cell bodies of pyramidal cells of the CA1 field while the protein is found predominantly in the dorsal subiculum, a projection zone for the CA1 pyramidal neurons. In the cerebellum, 5-hydroxytryptamine1B messenger RNA is expressed in the Purkinje cells, which display no receptor binding sites. Conversely, moderate binding is found in the deep nuclei of the cerebellum, the main projection zone of the Purkinje cells. 5-Hydroxytryptamine1B sites are also detected in the superficial gray layer of the superior colliculus and the lateral geniculate nucleus, brain regions containing the terminals of retinal ganglion cells. The soma of these ganglion cells express high levels of 5-hydroxytryptamine1B messenger RNA while no 5-hydroxytryptamine1B binding sites were found in the retina. This study demonstrates that the main brain regions, expressing 5-hydroxytrypamine1B messenger RNA contain low densities of 5-hydroxytryptamine1B binding sites. Conversely, the major projection areas of these anatomical structures do not express detectable levels of 5-hydroxytryptamine1B messenger RNA, but present a high density of binding sites. In addition, our data suggest that the distribution of the 5-hydroxytryptamine1D alpha binding sites is different from that of the 5-hydroxytryptamine1D alpha messenger RNA. These results together with previous lesion studies, indicate that the 5-hydroxytryptamine1B and possibly the 5-hydroxytryptamine1D alpha receptors are localized predominantly on axon terminals, while their expression is low or absent at the somatodendritic level. The 5-hydroxytryptamine1D alpha proteins might therefore contain an addressing signal allowing their transport toward nerve endings.(ABSTRACT TRUNCATED AT 400 WORDS)

Serotonin transporters are located on the axons beyond the synaptic junctions : Anatomical and functional evidence

The serotonin (5-HT) transporter (5-HTT) is known to play a role in depression and many 5-HT related diseases, and is the target site for drugs of abuse, such as cocaine, MDMA, and methamphetamine. The major role of the 5-HTT has long been considered to be to inactivate serotonin transmission through the elimination of serotonin at release sites. However, immunocytochemistry using an antibody against the N-terminal of the 5-HTT at the light microscopic (LM) level indicates that the 5-HTT is associated not only with 5-HT varicosities but also with axons. Electron microscopy (EM) reveals that the majority of the 5-HTTs exist on the axolemma outside the synaptic junctions. In studying whether axonal 5-HTTs are involved in the uptake of 5-HT, we found with autoradiography that [3H]citalopram bound to all major 5-HT fibers, not only in the terminal regions, but also in 5-HT axonal bundles such as the cingulum bundle and medial forebrain bundle. Furthermore, voltammetry recordings indicated that serotonin axonal bundles were actively engaged in high affinity serotonin uptake. The evidence indicates that 5-HTTs on 5-HT axons away from the synapse are likely to be functional in a manner similar to the terminal 5-HTT for serotonin uptake. It also suggests that the role of the 5-HTT may not only be for the termination of synaptic transmission, but also for the regulation of 5-HT through extrasynaptic (volume) transmission. Our findings may also impact the understanding of the sites of action of selective serotonin reuptake inhibitors and drug entry into serotonin neurons via the numerous axonal sites.

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.

Gating of Sema3E/PlexinD1 signaling by neuropilin-1 switches axonal repulsion to attraction during brain development.

The establishment of functional neural circuits requires the guidance of axons in response to the actions of secreted and cell-surface molecules such as the semaphorins. Semaphorin 3E and its receptor PlexinD1 are expressed in the brain, but their functions are unknown. Here, we show that Sema3E/PlexinD1 signaling plays an important role in initial development of descending axon tracts in the forebrain. Early errors in axonal projections are reflected in behavioral deficits in Sema3E null mutant mice. Two distinct signaling mechanisms can be distinguished downstream of Sema3E. On corticofugal and striatonigral neurons expressing PlexinD1 but not Neuropilin-1, Sema3E acts as a repellent. In contrast, on subiculo-mammillary neurons coexpressing PlexinD1 and Neuropilin-1, Sema3E acts as an attractant. The extracellular domain of Neuropilin-1 is sufficient to convert repulsive signaling by PlexinD1 to attraction. Our data therefore reveal a gating function of neuropilins in semaphorin-plexin signaling during the assembly of forebrain neuronal circuits.

Increase of central 5-HT1B binding sites following 5,7-dihydroxytryptamine axotomy in the adult rat

The effects of selective axotomy of serotoninergic neurons produced by an intracerebroventricular injection of 5,7-dihydroxytryptamine (200 micrograms free base) on 5-HT1B binding sites labeled with S-CM-G-[125I]TNH2 were investigated by quantitative autoradiography in the rat brain. Results show, 21 days after surgery, an upregulation of 5-HT1B receptors in the entorhinal cortex and the dorsomedial and suprachiasmatic nuclei of the hypothalamus. The cellular localization of those 5-HT1B receptors exhibiting post-lesion plastic properties is discussed.

High-affinity serotonin binding sites: Autoradiographic evidence for their location on retinal afferents in the rat superior colliculus

High affinity 5-HT binding sites (5-HT1) were labeled in vitro on mounted rat brain slices using [3H]5-HT as a radioligand. In the first stage of experimentation, the bound radioactivity was measured on slices by liquid scintillation count in order to define the biochemical characteristics of the binding. Saturation curves were drawn, as well as association and elution curves for a 2 nM radioligand concentration. The mean affinity constant of the specific binding (Kd) was found to be 2.9 nM. In the second stage, the experimental parameters giving optimum binding were applied to the frozen slices prepared exactly as for the biochemical approach in order to investigate the effects of degeneration of retinal axon terminals on the distribution of 5-HT1 sites in the visual upper layers of the superior colliculus. The optical densities directly measured from tritium-sensitive film clearly indicate that the ablation of one eye causes a progressive reduction in the binding in the contralateral, largely deafferented, stratum griseum superficiale (SGS); with a 24-day survival period, the reduction was about 35-40%. In the homologous region of the ipsilateral colliculus, the binding decreased by about 10-15%. It is concluded that at least two populations of 5-HT1 binding sites coexist in the visual collicular layers, one of which is probably located on the axon terminals of retinal afferents. The present results confirm a previous hypothesis based on iontophoretic data, according to which this monoamine is involved in retino-collicular transmission. As far as the retinofugal terminal binding sites are concerned, 5-HT seems to exert a presynaptic control on visual inputs.

Impairment of serotoninergic transmission is followed by adaptive changes in 5HT1B binding sites in the rat suprachiasmatic nucleus

Serotonin1B (5-HT1B) receptor binding in the suprachiasmatic nucleus (SCN) following impairment of serotoninergic transmission was studied by quantitative autoradiography. Serotonin (5-HT) denervation with 5,7-dihydroxytryptamine (5,7-DHT) caused a significant increase in the density of 5-HT1B receptors in both the ventral (62%) and dorsal (53%) parts of the SCN as early as 3 days after axotomy. The magnitude of this increase did not differ 3, 15 or 21 days post-lesion. An up-regulation of 5-HT1B receptors with similar magnitude was obtained in the two parts of the SCN after inhibition of 5-HT synthesis by chronic parachlorophenylalanine treatment. In this case, up-regulation was shown to be reversible after restoration of 5-HT synthesis with L-5-hydroxytryptophan. These results indicate that 5-HT1B receptor density in the SCN was inversely correlated with 5-HT levels. These plastic properties exhibited by 5-HT1B receptors in the SCN are discussed in relation to the mode of 5-HT transmission and possible localization of the receptors onto the main chemically defined cell populations of the nucleus.

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.

Effects of retinal deafferentation on serotonin receptor types in the superficial grey layer of the superior colliculus of the rat

The effects of retinal axon terminal degeneration on the serotonin-1A, -1B, -2, nuerokinin-1 and gamma-amionobutyric acid-A high affinity binding sites in the superficial grey layer of the superior colliculus were tested with quantitative autoradiography on rat brain sections. The binding to serotonin-2, neurokinin-1 and gamma-aminobutyric acid-A high affinity receptors was not changed in the deafferented superficial grey layer of the superior colliculus after unilateral enucleation. By contrast, we demonstrate that the previously described 21% decrease in the binding of [3H]serotonin to serotonin-1 receptors observed in the deafferented superficial grey layer of the superior colliculus after enucleation, was not due to a decrease in the affinity of the serotonin-1 receptors for the radioligand, but to a decrease in the number of binding sites. Of the different serotonin-1 receptor subtypes, only the serotonin-1B was lost. This signifies that these receptors are probably located on the optic fibre terminals. Visual cortex lesion caused no apparent regulation of the serotonin-1 binding sites in the superficial grey layer of the superior colliculus. A bilateral enucleation produced a smaller decrease in serotonin-1 receptor density than that observed after unilateral enucleation, suggesting the existence of a compensatory mechanism.