Development of the hamster serotoninergic system: cell groups and diencephalic projections

Neuroanatomical Distribution of the Serotonergic System in the Brain and Retina of Holostean Fishes, The Sister Group to Teleosts

Among actinopterygian fishes, holosteans are the phylogenetically closest group to teleosts but they have been much less studied, particularly regarding the neurochemical features of their central nervous system. The serotonergic system is one of the most important and conserved systems of neurotransmission in all vertebrates. By means of immunohistochemistry against serotonin (5-hydroxytryptamine), we have conducted a comprehensive and complete description of this system in the brain and retina of representative species of the 3 genera of holostean fishes, belonging to the only 2 extant orders, Amiiformes and Lepisosteiformes. Serotonin-immunoreactive cell groups were detected in the preoptic area, the hypothalamic paraventricular organ, the epiphysis, the pretectal region, the long and continuous column of the raphe, the spinal cord, and the inner nuclear layer of the retina. Specifically, the serotonergic cell groups in the preoptic area, the epiphysis, the pretectum, and the retina had never been identified in previous studies in this group of fishes. Widespread serotonergic innervation was observed in all main brain regions, but more abundantly in the subpallium, the hypothalamus, the habenula, the optic tectum, the so-called cerebellar nucleus, and the area postrema. The comparative analysis of these results with those in other groups of vertebrates reveals some extremely conserved features, such as the presence of serotonergic cells in the retina, the pineal organ, and the raphe column, while other characteristics, like the serotonergic populations in the preoptic area, the paraventricular organ, the pretectum, and the spinal cord are generally present in all fish groups, but have been lost in most amniotes.

Constructing the suprachiasmatic nucleus: a watchmaker's perspective on the central clockworks

The circadian system constrains an organism's palette of behaviors to portions of the solar day appropriate to its ecological niche. The central light-entrained clock in the suprachiasmatic nucleus (SCN) of the mammalian circadian system has evolved a complex network of interdependent signaling mechanisms linking multiple distinct oscillators to serve this crucial function. However, studies of the mechanisms controlling SCN development have greatly lagged behind our understanding of its physiological functions. We review advances in the understanding of adult SCN function, what has been described about SCN development to date, and the potential of both current and future studies of SCN development to yield important insights into master clock function, dysfunction, and evolution.

Prosomeric organization of the hypothalamus in an elasmobranch, the catshark Scyliorhinus canicula

The hypothalamus has been a central topic in neuroanatomy because of its important physiological functions, but its mature organization remains elusive. Deciphering its embryonic and adult organization is crucial in an evolutionary approach of the organization of the vertebrate forebrain. Here we studied the molecular organization of the hypothalamus and neighboring telencephalic domains in a cartilaginous fish, the catshark, Scyliorhinus canicula, focusing on ScFoxg1a, ScShh, ScNkx2.1, ScDlx2/5, ScOtp, and ScTbr1 expression profiles and on the identification α-acetylated-tubulin-immunoreactive (ir), TH-ir, 5-HT-ir, and GFAP-ir structures by means of immunohistochemistry. Analysis of the results within the updated prosomeric model framework support the existence of alar and basal histogenetic compartments in the hypothalamus similar to those described in the mouse, suggesting the ancestrality of these subdivisions in jawed vertebrates. These data provide new insights into hypothalamic organization in cartilaginous fishes and highlight the generality of key features of the prosomeric model in jawed vertebrates.

Resilience of Circadian Pacemaker Development in Hamsters

Disruptions of circadian rhythms have been linked to a wide range of pathologies from sleep disorders to cancer. The extent to which disruptions of circadian rhythms during development contribute to later conditions is not known. The present study tested the hypothesis that functional properties of the central circadian pacemaker, the suprachiasmatic nucleus (SCN), are affected by abnormal entrainment during development. The SCN is specialized for the generation of robust rhythms, for direct and indirect output to physiological and behavioral systems, and for entrainment to light/dark cycles via direct retinal input. It consists of thousands of neurons and glia with distinct phenotypes and has subdivisions delineated by both anatomical and functional criteria. In rodents, SCN rhythms develop within days after SCN cells are produced and before many other aspects of differentiation, such as synaptogenesis, are complete. We demonstrated that around the time of birth, the hamster SCN in vivo can undergo repeated phase shifts by a dopamine D1 receptor agonist (SKF-38393). For 2 days before and 2 days after birth, one group of hamsters received regular exposure to the drug at the same time of day, while another group was exposed at varying times to induce repeated phase shifts. Free-running and entrained activity rhythms were compared between the groups at different ages after weaning. Repeated phase shifts during SCN development had a significant effect on free-running period measured immediately after weaning. This effect was eliminated by subsequent entrainment to a light/dark cycle, indicating that the effect was not permanent. These and other results suggest that SCN development required for functional properties such as free-running period is resilient to perturbation.

The serotonergic system in fish

Neurons using serotonin (5-HT) as neurotransmitter and/or modulator have been identified in the central nervous system in representatives from all vertebrate clades, including jawless, cartilaginous and ray-finned fishes. The aim of this review is to summarize our current knowledge about the anatomical organization of the central serotonergic system in fishes. Furthermore, selected key functions of 5-HT will be described. The main focus will be the adult brain of teleosts, in particular zebrafish, which is increasingly used as a model organism. It is used to answer not only genetic and developmental biology questions, but also issues concerning physiology, behavior and the underlying neuronal networks. The many evolutionary conserved features of zebrafish combined with the ever increasing number of genetic tools and its practical advantages promise great possibilities to increase our understanding of the serotonergic system. Further, comparative studies including several vertebrate species will provide us with interesting insights into the evolution of this important neurotransmitter system.

Development of the serotonergic system in the central nervous system of the sea lamprey

Lampreys belong to the most primitive extant group of vertebrates, the Agnathans, which is considered the sister group of jawed vertebrates. Accordingly, characterization of neuronal groups and their development appears useful for understanding early evolution of the nervous system in vertebrates. Here, the development of the serotonergic system in the central nervous system of the sea lamprey, Petromyzon marinus, was investigated by immunohistochemical analysis of specimens ranging from embryos to adults. The different serotonin-immunoreactive (5-HT-ir) neuronal populations that are found in adults were observed between the embryonic and metamorphic stages. The earliest serotonergic neurons were observed in the basal plate of the isthmus region of late embryos. In prolarvae, progressive appearance of new serotonergic cell groups was observed: firstly in the spinal cord, then in the pineal organ, tuberal region, zona limitans intrathalamica, rostral isthmus, and the caudal part of the rhombencephalon. In early larvae a new group of serotonergic cells was observed in the mammillary region, whereas in the pretectal region and the parapineal organ the first serotonergic cells were seen in the middle and late larval stages, respectively. The first serotonergic fibres appeared in early prolarvae, with fibres that ascend and descend from the isthmic cell group, and the number of immunoreactive fibres increased progressively until the adult stage. The results reveal strong resemblances between lampreys and other vertebrates in the spatio-temporal pattern of development of brainstem populations. This study also reveals a shared pattern of early ascending and descending serotonergic pathways in lampreys and jawed vertebrates.

The circadian visual system, 2005

The primary mammalian circadian clock resides in the suprachiasmatic nucleus (SCN), a recipient of dense retinohypothalamic innervation. In its most basic form, the circadian rhythm system is part of the greater visual system. A secondary component of the circadian visual system is the retinorecipient intergeniculate leaflet (IGL) which has connections to many parts of the brain, including efferents converging on targets of the SCN. The IGL also provides a major input to the SCN, with a third major SCN afferent projection arriving from the median raphe nucleus. The last decade has seen a blossoming of research into the anatomy and function of the visual, geniculohypothalamic and midbrain serotonergic systems modulating circadian rhythmicity in a variety of species. There has also been a substantial and simultaneous elaboration of knowledge about the intrinsic structure of the SCN. Many of the developments have been driven by molecular biological investigation of the circadian clock and the molecular tools are enabling novel understanding of regional function within the SCN. The present discussion is an extension of the material covered by the 1994 review, "The Circadian Visual System."

Projections from the raphe nuclei to the suprachiasmatic nucleus of the rat

The presence of serotonergic afferents in the hypothalamic suprachiasmatic nucleus (SCN) is well documented and several functional roles of serotonin (5-HT) in circadian function are well established. However, there is some controversy about the precise location of the serotonergic neurones from where this input arises. Discrete injection of the tracer Cholera toxin, subunit B, (ChB) was centred in the rat SCN, and a few retrograde labelled neurones were distributed in the dorsal and median raphe nuclei (MnR) and in the rostral part of the raphe magnus (RMg), but no neurones were found in the raphe pallidus or raphe obscurus. In addition, a group of neurones was consistently found in the medial part of the pontine supra lemniscal nucleus but not including the serotonergic B(9) region. A combination of retrograde tracing with Fluoro-Gold together with 5-HT-immunolabelling, showed few double-labelled neurones in the dorsal, MnR and B(9). However, the majority of projecting neurones were not co-storing 5-HT immunoreactivity. Phaseolus vulgaris-leucoagglutinin (PHA-L) injections in the dorsal raphe resulted in faint labelling, whereas the MnR gave rise to several labelled fibres in the SCN. Individual delicate PHA-L nerve fibres were found in all compartments of the SCN both in terms of rostrocaudal, ventromedial and dorsomedial extent, without any sign of a topographical organisation of the MnR input to the SCN. PHA-L injections into RMg gave rise to labelling of a few processes within the SCN. In conclusion, the main serotonergic input to the rat SCN originates from a few neurones in the MnR.

Modifications of retinal afferent activity induce changes in astroglial plasticity in the hamster circadian clock

The circadian clock, located in the suprachiasmatic nucleus (SCN) of the hypothalamus in mammals, exhibits astroglial plasticity indicated by GFAP expression over the 24-h period. In this study, we evaluated the role of neuronal retinal input in the observed changes. Modifications of retinal input, either by rearing animals under darkness (DD) or under constant light (LL), or by suppressing afferent input (bilateral enucleation), induced drastic changes in astroglial plasticity. In enucleated animals, a dramatic decrease in GFAP expression was evident in the area of the SCN deprived of retinal projections, whereas persistence of a rhythmic variation was in those areas still exhibiting GFAP expression. By contrast, no changes in astrocytic plasticity were detected in hamsters maintained under LL. These data suggest two fundamental roles for astrocytes within the SCN: (1) to regulate and mediate glutamate released by retinal terminals throughout the neuronal network to facilitate photic signal transmission; (2) to contribute to synchronization between suprachiasmatic neurons.

The ascending serotonergic system in the hamster: comparison with projections of the dorsal and median raphe nuclei

The ascending serotonergic projections are derived largely from the midbrain median and dorsal raphe nuclei, and contribute to the regulation of many behavioral and physiological systems. Serotonergic innervation of the hamster circadian system has been shown to be substantially different from earlier results obtained with other methods and species. The present study was conducted to determine whether similar differences are observed in other brain regions. Ascending projections from the hamster dorsal or median raphe were identified using an anterograde tracer, Phaseolus vulgans leucoagglutinin, injected by iontophoresis into each nucleus. Brains were processed for tracer immunoreactivity, and drawings were made of the median raphe and dorsal raphe efferent projection patterns. The efferents were also compared to the distribution of normal serotonergic innervation of the hamster midbrain and forebrain. The results show widespread, overlapping projection patterns from both the median and dorsal raphe, with innervation generally greater from the dorsal raphe. In several brain regions, including parts of the pretectum, lateral geniculate and basal forebrain, nuclei are innervated by the dorsal, but not the median, raphe. The hypothalamic suprachiasmatic nucleus is the only site innervated exclusively by the median and not by the dorsal raphe. The pattern of normal serotonin fiber and terminal distribution is generally more robust than would be inferred from the anterograde tracer material. However, there is good qualitative similarity between the two sets of data. The oculomotor nucleus and the medial habenula are unusual to the extent that each has a moderately dense serotonin terminal plexus, although neither receives innervation from the median or dorsal raphe. In contrast, the centrolateral thalamic nucleus and lateral habenula have little serotonergic innervation, but receive substantial other neural input from the raphe nuclei. The normal serotonergic innervation of the hamster brain is similar to that in the rat, although there are exceptions. The anterograde tracing of ascending median or dorsal raphe projections reveals a high, but imperfect, degree of correspondence with the serotonin innervation data, and with data from rats derived from immunohistochemical and autoradiographic tract-tracing techniques.

Serotonin and the regulation of mammalian circadian rhythmicity

The suprachiasmatic nucleus (SCN), the site of the primary mammalian circadian clock, contains one of the densest serotonergic terminal plexes in the brain. Although this fact has been appreciated for some time, only in the last decade has there been substantial approach toward the understanding of the function of serotonin in the circadian rhythm system. The intergeniculate leaflet, which projects to the SCN via the geniculohypothalamic tract, receives serotonergic innervation from the dorsal raphe nucleus, and the SCN receives its serotonergic input from the median raphe nucleus. This separation of serotonergic origins provides the opportunity to investigate the function of the two projections. Loss of serotonergic neurones of the median raphe yields earlier onset and later offset of the nocturnal activity phase, longer duration of the activity phase, and increased sensitivity of circadian rhythm response to light. Despite the simplicity of the origins of serotonergic anatomy with respect to the circadian rhythm system, the actual involvement of serotonin in rhythm modulation is not so obvious. A variety of pharmacological studies have clearly implicated serotonin as a direct regulator of circadian rhythm phase, but others employing different methods suggest that simple elevation of SCN serotonin concentrations does not modify rhythm phase. The most convincing role of serotonin is its apparent ability to modulate sensitivity of the circadian rhythm to light. The putative method for such modulation is via a presynaptic 5-HT1B receptor on the retinohypothalamic tract, the activation of which attenuates photic input to the SCN thereby reducing phase response to light. Serotonin may modulate phase response to benzodiazepines, but does not appear to modify such response to environmentally induced locomotor activity. Current interest in serotonergic modulation of circadian rhythmicity is strong and the research is vigorous. There is an abundance of information about serotonin and circadian rhythm function that lacks a satisfactory framework for its interpretation. The next decade is likely to see the gradual evolution of this framework as the role of serotonin in circadian rhythm regulation is further elucidated.

The development of the synaptic organization of the serotonergic system differs in brain areas with different functions

The serotonergic innervation of the developing superior colliculus and ventrolateral nucleus of the thalamus of the rat were studied with light and electron microscope immunocytochemistry. We compared the pattern of innervation and synaptic organization of the serotonin (5-HT) system in the superficial and deep layers of the superior colliculus. We also compared the developmental pattern of synaptic incidence of 5-HT varicosities in the superior colliculus with that in the ventrolateral nucleus. Serotonin fibers were present in the superior colliculus at birth, concentrated mainly in the deep layers, whereas the superficial layers were only sparsely innervated. By the end of the first postnatal week the overall density of 5-HT fibers increased, but was still higher in the deep than in the superficial layers. The distribution pattern, density, and morphology of serotonergic axons acquired mature features by the end of the third postnatal week. In the adult, these axons were thin, varicose, forming a complex network which was denser in the lower part of the superficial layers and the upper part of the deep layers. Electron microscopical analysis revealed that the vast majority of 5-HT varicosities established symmetrical synapses with dendritic shafts in all layers of the superior colliculus throughout development. In the superficial layers, known to be involved in visual functions, the proportion of varicosities forming synapses increased gradually from birth to reach a peak at the end of the first postnatal week, then declined markedly in the subsequent 2 weeks before rising again at later stages. In contrast, in the deep layers and in the ventrolateral nucleus of the thalamus, areas involved in motor functions, the proportion of 5-HT varicosities engaged in synaptic contacts showed a continuous increase from birth until adulthood. Considering these results together with data from our previous studies, we speculate that the regional heterogeneity in the synaptic organization of the serotonergic system may reflect a differential role of 5-HT in the development of brain areas with different functions.

Melatonin entrains the restored circadian activity rhythms of syrian hamsters bearing fetal suprachiasmatic nucleus grafts

A circadian pacemaker consists of at least three essential features: the ability to generate circadian oscillations, an output signal, and the ability to be entrained by external signals. In rodents, ablation of the suprachiasmatic nucleus (SCN) results in the loss of circadian rhythms in activity. Rhythmicity can be restored by transplanting fetal SCN into the brain of the lesioned animal, demonstrating the first two of the essential pacemaker features within the grafts. External signals, such as the light/dark cycle, have not, however, been shown to entrain the restored rhythms. Melatonin injections are an effective entraining stimulus in fetal and neonatal Syrian hamsters of the same developmental ages used to provide donor tissue for transplantation. Therefore, melatonin was used to test the hypothesis that SCN grafts contain an entrainable pacemaker. Daily injections of melatonin were given to SCN-lesioned hosts beginning on the day after transplantation of fetal SCN. Two groups that received melatonin at different times of day 12 hr apart each showed significantly clustered phases but with average phases that differed by 8.67 hr. Thus melatonin was able to entrain the restored circadian activity rhythms. In contrast to these initial injections, injections given 6 weeks after transplantation were unable to entrain or phase shift the rhythms. The results demonstrate that SCN grafts contain an entrainable circadian pacemaker. In addition, the results also indicate that the fetal SCN is directly sensitive to melatonin and, as with intact hamsters, sensitivity to melatonin is lost during SCN development.

5-HT7 receptors mediate serotonergic effects on light-sensitive suprachiasmatic nucleus neurons

Serotonin (5-HT) has been shown to phase shift circadian rhythms in mammals and to affect responses of the circadian system to light, but it is not clear which receptors are involved in these actions. We found that drugs which act as 5-HT1A receptor agonists suppressed photic responses of hamster SCN cells, but these drugs also exhibit high affinity for the recently cloned 5-HT7 receptor. We therefore studied the effects of 5-HT agonists and antagonists with differential affinities for 5-HT7 and 5-HT1A receptors on responses of hamster SCN cells to retinal illumination. We confirmed that the 5-HT receptor agonists 5-HT, 8-OH-DPAT and 5-CT, dose-dependently reduced photic activation of SCN cells. These effects could be blocked by co-application of antagonists with high affinities for 5-HT7 receptors: ritanserin or clozapine. The 5-HT1A/B/D antagonist, cyanopindolol, which is inactive at 5-HT7 receptors, did not antagonize the actions of 8-OH-DPAT. Selective 5-HT1A antagonists, WAY100635 and p-MPPI, had weak or no antagonist effects on the responses to 8-OH-DPAT in the SCN, but they effectively antagonized the actions of 8-OH-DPAT in the hippocampus. In the cerebellar cortex where few 5-HT7 receptors are present, ritanserin failed to antagonize the effects of 8-OH-DPAT. Our results indicate that the 5-HT7 receptor subtype plays a major role in mediating the effects of 5-HT on photic responses of SCN cells in the hamster.

Transient expression of the serotonin transporter in the developing mouse thalamocortical system

The serotonin transporter was visualized in sections through the developing mouse thalamus by autoradiography of [3H]citalopram binding. In late gestation, a high density of transporter expression appeared in the ventrobasal thalamic complex and medial geniculate body. During the first postnatal week, binding in these areas decreased to low levels. A similar pattern of transient [3H]citalopram binding was observed in the somatosensory cortex, although the rise and decline of labeling occurred some days later. The density of the serotonergic innervation in the ventrobasal thalamic complex is known to be very low during the entire developmental period. Therefore, these data suggest that the serotonin transporter may be expressed transiently by thalamic neurons projecting to the cerebral cortex (as a "heterocarrier") which are capable of taking up serotonin in the somatosensory cortex. We propose that serotonin may act temporarily as a "false" transmitter in thalamocortical axons.

Distribution, quantification, and morphological characteristics of serotonin-immunoreactive cells of the supralemniscal nucleus (B9) and pontomesencephalic reticular formation in the rat

In their initial report on the rat, Dahlstrom and Fuxe ([1964] Acta Physiol. Scand. 62:1-55) identified nine brainstem serotonin-containing cell groups, which they termed B1-B9. B9 has received considerably less attention than other serotonergic nuclei (B1-B8) due in part to the fact that its precise location and extent have not been well documented in subprimates. B9 (supralemniscal nucleus; SLN) has been viewed as a minor serotonergic cell group. In addition, 5-hydroxytryptamine (5-HT)-containing cells have been shown to be only sparsely distributed throughout the pontomesencephalic reticular formation (PMRF). By using 5-HT immunohistochemical techniques, we examined the distribution and morphological characteristics of SLN and PMRF 5-HT neurons of the pontomesencephalic tegmentum. We showed that 5-HT cells of both SLN and the PMRF extend rostrocaudally from the rostral midbrain to the midpons. 5-HT SLN cells are located within or dorsal to the medial lemniscus (ML); those of the PMRF are widely distributed throughout the PMRF. The mean numbers of 5-HT containing cells in the SLN, PMRF, dorsal raphe, and median raphe nuclei were 4,571, 1,948, 15,191, and 4,114, respectively. The SLN (B9) contains more 5-HT neurons than any serotonergic group other than the dorsal raphe nucleus. The dendrites of both SLN and PMRF 5-HT cells are primarily oriented mediolaterally and generally extend for long distances (75-300 microns), running perpendicular to the fibers of the ML (SLN) or, to those coursing through the brainstem (PMRF). The present anatomical delineation of SLN and PMRF shows that they are major 5-HT-containing cell groups in the rat and provides the foundation for the further examination of their properties and functions.

Transient expression of neuropeptide Y (NPY) immunoreactivity in the developing hamster paraventricular thalamic area is due to apoptosis

1. A new population of neurons with transient expression of NPY immunoreactivity was described in the developing hamster paraventricular thalamic area. The present study was performed to discover whether this phenomenon is due to programmed cell death or apoptosis. 2. Toward this aim, immunocytochemical and electron microscopic examination of the paraventricular thalamic region, as well as DNA electrophoresis of tissue extracted from the described area, was performed on different stages of embryonic and postnatal development. 3. A sudden increase in neuropeptide Y immunoreactivity (NPY-IR) in the paraventricular thalamic area at embryonic day 14 (E14) was the first symptom of neuronal degeneration. 4. Electron microscopy revealed many neurons with large masses of condensed chromatin within nuclei and extracellular bodies. The affected cells had a convoluted shape and condensed cytoplasm. 5. DNA electrophoresis revealed a ladder of bands between 150 and 1000 bp that is specific for internucleosomal DNA fragmentation. 6. The data strongly suggest that developmental disappearance of NPY-IR neurons within the hamster dorsal thalamic area is due to apoptosis.

Persistence of nonphotic phase shifts in hamsters after serotonin depletion in the suprachiasmatic nucleus

Serotonin-containing fibres (5-HT) project from the raphe complex to the suprachiasmatic nucleus (SCN). Previous studies have suggested that this pathway may be involved in nonphotic resetting of the circadian clock. For example, 5-HT agonists are capable of phase shifting the biological clock both in vivo and in vitro, producing phase response curves (PRCs) similar in shape to those of other nonphotic stimuli. Therefore we studied the role of the serotonergic projection to the SCN in nonphotic phase shifts by bilateral injection of the selective 5-HT neurotoxin, 5,7-dihydroxytryptamine (5,7-DHT) onto the SCN of hamsters. About 50 days after the administration of the neurotoxin, the 5-HT and 5-HIAA (5-hydroxyindole acetic acid) levels were severely depleted in the SCN, as revealed by high performance liquid chromatography (HPLC), and immunocytochemistry (ICC). The average level of 5-HT depletion was 88% in Experiment 1 and 95% in Experiment 2. This treatment had no effect on the magnitude of phase shifts produced by 3 h of novelty-induced wheel-running starting at circadian time (CT) 4, the peak of the advance region of the PRC to this stimulus. The effect of 5-HT depletion on shifts produced by running at CT 22 were inconclusive because of changes in the behavior of control animals. No changes in the phase angle of entrainment of animals in a 14:10 light:dark (LD) cycle were detected in depleted animals. The results suggest that the 5-HT projection from the raphe to the SCN is not essential for activity-induced phase shifts in hamsters.

Serotonergic stimulation and nonphotic phase-shifting in hamsters

Stimuli that make hamsters active, such as dark pulses or triazolam administration, also phase shift their circadian clocks, producing phase advances during the subjective day and phase delays during the subjective night. Activity or its correlate appears to be important in producing the shifts because preventing locomotion blocks the phase shifts associated with these stimuli. The physiological basis of clock resetting induced by activity is not fully understood. The serotonergic (5-HT) projection from the raphe to the suprachiasmatic nucleus (SCN) is a possible route by which nonphotic information could reach the pacemaker. Administration of 8-HYDROXY-2-(DI-N-PROPYLAMINO) TETRALIN HYDROBROMIDE (8-OH-DPAT), a 5-HT1A and 5-HT7 receptor agonist, at circadian time (CT) 8 produces phase advances in the circadian rhythms of hamsters. Before concluding that 5-HT mediates the effect of activity on the pacemaker, it must be shown that 5-HT agonist do not produce shifts simply because they make animals more active. Therefore, we investigated the contribution of activity to 8-OH-DPAT-produced shifts. Preventing hamsters from moving around after administering 8-OH-DPAT did not abolish phase shifts. Moreover, higher doses of 8-OH-DPAT diminished activity on the day of injection but did not affect the amplitude of phase shifts. Suprisingly, quipazine (a non specific 5-HT agonist), when injected in the middle of subjective day did not phase shift the activity rhythm of hamsters, as it has been reported to do in rats.

Organization of permanent and transient neuropeptide Y-immunoreactive neuron groups and fiber systems in the developing hamster diencephalon

The development of neuropeptide Y-immunoreactive (NPY-IR) cell and fiber systems in the hamster diencephalon was studied. Eight perinatal groups of NPY-IR neurons develop into 12 distinct sets in nuclei of the adult diencephalon and mesencephalon. NPY-IR neurons of the thalamic precommissural nucleus, nucleus of the optic tract, and olivary pretectal nucleus are derived from the superior group. Those in the adult magnocellular nucleus of the posterior commissure and deep mesencephalic nucleus are from the dorsal group. An arcuate group contributes neurons to the arcuate nucleus and median eminence and a mammillary group transiently exists in the mammillary region. A medial group gives rise to two sets of neurons, one that migrates to the intergeniculate leaflet and another that develops in the medial nucleus reuniens. A very large ventral group provides NPY-IR neurons to the adult medial zona incerta and caudal reticular thalamus. Groups of NPY-IR neurons also appear in the bed nucleus of the stria terminalis and centromedian thalamic nucleus. Superior group neurons may undergo apoptosis. In several groups, neurons become fewer during development, and NPY-IR may disappear. NPY-IR neurons of several groups initially migrate away from the neuroepithelial zone with later emergence of a distinct, persistent set of NPY-IR neurons in the same neuroepithelial region. The data show that neuropeptide content can be used to identify particular sets of neurons early in development, thereby allowing migration patterns to be followed and principles of brain development to be elucidated.

Ontogeny of radial glia, astrocytes and vasoactive intestinal peptide immunoreactive neurons in hamster suprachiasmatic nucleus

Circadian rhythmicity of rodents is a property of the suprachiasmatic nucleus (SCN). Such rhythmicity can be demonstrated in the prenatal SCN, yet there is little information about the cells in which rhythmicity is generated. The present study was performed to discover the developmental relationships of SCN glial cells and a class of identifiable SCN neurons. Toward this end, vimentin- (VIM), glial fibrillary acidic protein- (GFAP) and vasoactive intestinal peptide- (VIP) immunoreactivity were investigated in SCN radial glia, astrocytes and neurons, respectively. VIP-IR first appears at embryonic day 13 (E13) and is clearly identifiable in neurons at E14. Substantial axon extension begins at E15 and the postnatal day 10 (P10) SCN is adult-like. VIM-IR radial glia fill the SCN region at E13, but by P0, most are absent. On P3, the remaining processes are beaded suggesting degeneration. The first GFAP-IR elements are visible on E15 with a few clear astrocytes present at P0. The number of astrocytes lateral to and in the SCN continues to increase during the postnatal period achieving an adult-like appearance by P21. The data do not support the view that prenatal circadian rhythmicity is derived from astrocytes. VIP-IR neurons are apparently present sufficiently early to be part of the rhythm generating mechanism. These tissues are discussed in the context of development of the SCN.

Effects of serotonergic agonists on firing rates of photically responsive cells in the hamster suprachiasmatic nucleus

Serotonergic neurons from the midbrain raphe nuclei innervate the suprachiasmatic nucleus (SCN) of the hypothalamus, which functions as the dominant pacemaker for mammalian circadian rhythms. We investigated the effects of serotonin (5-HT) on firing rates of light-activated SCN cells in urethane-anesthetized hamsters. Micro-iontophoretic application of 5-HT or 5-HT1A agonists (8-OH-DPAT and 5-CT) caused a dose-dependent inhibition of spontaneous activity and photic responses in the majority of SCN cells tested. Application of metergoline alone, a non-selective 5-HT antagonist, slightly increased firing rates during darkness and light exposure, suggesting a tonic serotonergic suppression of SCN activity. Metergoline also effectively attenuated suppression induced by the three 5-HT agonists. In addition, the effects of 8-OH-DPAT were blocked by a 5-HT1A antagonist, SDZ 216-525. However, other putative 5-HT antagonists were weak (propranolol and NAN-190) or ineffective (ketanserin) in blocking the action of 8-OH-DPAT. These results indicate that serotonin has a potent role in reducing photic effects on retinally activated SCN cells in hamsters, and that these effects are mediated by a receptor with properties similar to those of the 5-HT1A subtype.