Serotonergic innervation of the lateral geniculate nucleus of the rat during postnatal development: a light and electron microscopic immunocytochemical analysis

Anatomical and neurochemical organization of the serotonergic system in the mammalian brain and in particular the involvement of the dorsal raphe nucleus in relation to neurological diseases

The brainstem is a neglected brain area in neurodegenerative diseases, including Alzheimer's and Parkinson's disease, frontotemporal lobar degeneration and autonomic dysfunction. In Depression, several observations have been made in relation to changes in one particular the Dorsal Raphe Nucleus (DRN) which also points toward as key area in various age-related and neurodevelopmental diseases. The DRN is further thought to be related to stress regulated processes and cognitive events. It is involved in neurodegeneration, e.g., amyloid plaques, neurofibrillary tangles, and impaired synaptic transmission in Alzheimer's disease as shown in our autopsy findings. The DRN is a phylogenetically old brain area, with projections that reach out to a large number of regions and nuclei of the central nervous system, particularly in the forebrain. These ascending projections contain multiple neurotransmitters. One of the main reasons for the past and current interest in the DRN is its involvement in depression, and its main transmitter serotonin. The DRN also points toward the increased importance and focus of the brainstem as key area in various age-related and neurodevelopmental diseases. This review describes the morphology, ascending projections and the complex neurotransmitter nature of the DRN, stressing its role as a key research target into the neural bases of depression.

Non-additive modulation of synaptic transmission by serotonin, adenosine, and cholinergic modulators in the sensory thalamus

The thalamus relays sensory information to the cortex. Oscillatory activities of the thalamocortical network are modulated by monoamines, acetylcholine, and adenosine, and could be the key features characteristic of different vigilance states. Although the thalamus is almost always subject to the actions of more than just one neuromodulators, reports on the modulatory effect of coexisting neuromodulators on thalamic synaptic transmission are unexpectedly scarce. We found that, if present alone, monoamine or adenosine decreases retinothalamic synaptic strength and short-term depression, whereas cholinergic modulators generally enhance postsynaptic response to presynaptic activity. However, coexistence of different modulators tends to produce non-additive effect, not predictable based on the action of individual modulators. Acetylcholine, acting via nicotinic receptors, can interact with either serotonin or adenosine to abolish most short-term synaptic depression. Moreover, the coexistence of adenosine and monoamine, with or without acetylcholine, results in robustly decreased synaptic strength and transforms short-term synaptic depression to facilitation. These findings are consistent with a view that acetylcholine is essential for an "enriched" sensory flow through the thalamus, and the flow is trimmed down by concomitant monoamine or adenosine (presumably for the wakefulness and rapid-eye movement, or REM, sleep states, respectively). In contrast, concomitant adenosine and monoamine would lead to a markedly "deprived" (and high-pass filtered) sensory flow, and thus the dramatic decrease of monoamine may constitute the basic demarcation between non-REM and REM sleep. The collective actions of different neuromodulators on thalamic synaptic transmission thus could be indispensable for the understanding of network responsiveness in different vigilance states.

Profiles of serotonin receptors in the developing human thalamus

The critical importance of the thalamus and its serotonergic innervation with respect to neuropsychiatric syndromes is increasingly recognized. This study investigates the localization of serotonin (5-hydroxytryptamine; 5-HT) receptors by immunohistochemistry in the thalamic nuclei of human fetuses aged 21 to 32 weeks of gestation. Results indicate that, already at 21 weeks of gestation, two 5-HT receptors are present in the dorsomedial nucleus of the developing thalamus: 5-HT2A receptors are localized in neurons and 5-HT2C receptors in fibers. By 31 and 32 weeks of gestation, 5-HT1A and 5-HT4 receptors are also detected in neuronal fibers of the same nucleus. At this later developmental stage, the percentage of 5-HT2A labeled neurons has significantly increased in the dorsomedial nucleus, and 5-HT2C positive neurons are observed in the centromedian and lateroventral thalamic nuclei as well. In contrast, neither neuronal cells nor fibers display any immunoreactivity for 5-HT3 or 5-HT6 receptors at any of the ages examined. Our observation that 5-HT1A, 5-HT2A, 5-HT2C and 5-HT4 receptors are present in the human thalamus prenatally indicates that 5-HT may play a role during fetal development. Disrupted development of the thalamic serotonergic system during this gestational period may contribute to the pathophysiology of neuropsychiatric disorders.

The dorsal raphe nucleus—From silver stainings to a role in depression

Over a hundred years ago, Santiago Ramón y Cajal used a new staining method developed by Camillo Golgi to visualize, among many other structures, what we today call the dorsal raphe nucleus (DRN) of the midbrain. Over the years, the DRN has emerged as a multifunctional and multitransmitter nucleus, which modulates or influences many CNS processes. It is a phylogenetically old brain area, whose projections reach out to a large number of regions and nuclei of the CNS, particularly in the forebrain. Several DRN-related discoveries are tightly connected with important events in the history of neuroscience, for example the invention of new histological methods, the discovery of new neurotransmitter systems and the link between neurotransmitter function and mood disorders. One of the main reasons for the wide current interest in the DRN is the nucleus' involvement in depression. This involvement is particularly attributable to the main transmitter of the DRN, serotonin. Starting with a historical perspective, this essay describes the morphology, ascending projections and multitransmitter nature of the DRN, and stresses its role as a key target for depression research.

Frequency-dependent modulation of retinogeniculate transmission by serotonin

The relay of visual information converging in the lateral geniculate nucleus (LGN) en route to the visual cortex is modulated by projections from brainstem nuclei. The release of serotonin, one mediator of these effects, has been shown to act at a presynaptic site to inhibit neurotransmitter release at the retinogeniculate synapse, the connection between retinal ganglion cells and thalamocortical relay neurons in the LGN. To understand how serotonergic inhibition of synaptic transmission influences the transfer of information at this synapse, we examined the EPSCs and firing responses of relay neurons to 5-carboxytryptamine (5-CT), a 5-HT1 receptor agonist that preferentially activates the presynaptic over postsynaptic modulatory effects of serotonin. Bath application of 5-CT inhibits synaptic strength, relieves synaptic depression, and reduces the total synaptic charge transferred at the retinogeniculate synapse in mouse LGN brain slices. In contrast, 5-CT does not significantly alter the membrane potential response of relay neurons to trains of intracellular current injections. Here we show that presynaptic serotonergic modulation results in a frequency-dependent inhibition of relay neuron firing. At low-frequency stimulation, 5-CT markedly reduces charge transfer at the retinogeniculate synapse, thus inhibiting relay neuron firing. However, inhibition of firing by 5-CT is diminished during high-frequency stimulation, because relief from synaptic depression partially offsets the reduction in charge transfer. Thus, presynaptic serotonergic inhibition plays a powerful role in modulating the frequency range of visual information transmitted via the retinogeniculate synapse such that high-frequency inputs are more reliably transmitted than low-frequency inputs.

Adolescence: Booze, Brains, and Behavior

This article represents the proceedings of a symposium at the 2004 Research Society on Alcoholism meeting in Vancouver, British Columbia, Canada, organized and chaired by Peter M. Monti and Fulton T. Crews. The presentations and presenters were (1) Introduction, by Peter M. Monti; (2) Adolescent Binge Drinking Causes Life-Long Changes in Brain, by Fulton T. Crews and Kim Nixon; (3) Functional Neuroimaging Studies in Human Adolescent Drinkers, by Susan F. Tapert; (4) Abnormal Emotional Reactivity as a Risk Factor for Alcoholism, by Robert Miranda, Jr.; (5) Alcohol-Induced Memory Impairments, Including Blackouts, and the Changing Adolescent Brain, by Aaron M. White and H. Scott Swartzwelder; and (6) Discussion, by Kenneth Sher.

Comparison of hypocretin/orexin and melanin-concentrating hormone neurons and axonal projections in the embryonic and postnatal rat brain

Hypocretin/orexin (H/O) and melanin-concentrating hormone (MCH) are peptide neuromodulators found in separate populations of neurons located within the lateral and perifornical hypothalamic regions. H/O has been linked to sleep-wakefulness regulation and to the sleep disorder narcolepsy, and both systems have been implicated in energy homeostasis, including the regulation of food intake. In the present study we compared the development of H/O and MCH-expressing neuronal populations with in situ hybridization and immunohistochemistry on adjacent sections in the embryonic and postnatal rat brain. We found that MCH mRNA and protein were present in developing neurons of the hypothalamus by embryonic day 16 (E16), whereas H/O mRNA and protein were not detected until E18. We also identified previously undescribed populations of MCH-immunoreactive cells in the lateral septum, paraventricular hypothalamic nucleus, lateral zona incerta, and ventral lateral geniculate nucleus that may play a specific role in the development of these regions. MCH immunoreactive axonal processes were also evident earlier than H/O stained fibers and at the time H/O immunoreactive processes were first identified in the hypothalamus at E20, extensive MCH axonal fiber systems were already present in many brain regions. Interestingly, however, the density of axonal fibers immunoreactive for H/O in the locus coeruleus reached peak levels at the same developmental age (P21) as MCH immunoreactive axons in the diagonal band of Broca (DBB). The peak of axon density coincided with the developmental stage at which adult patterns of feeding and sleep-waking activity become established. The present results demonstrate developmental differences and similarities between the MCH and H/O systems that may relate to their respective roles in feeding and sleep regulation.

Pretectal and tectal afferents to the dorsal lateral geniculate nucleus of the turtle: An electron microscopic axon tracing and γ-aminobutyric acid immunocytochemical study

The pretectal and tectal projections to the dorsal lateral geniculate nucleus (GLd) of two species of turtle (Emys orbicularis and Testudo horsfieldi) were examined under the electron microscope by using axonal tracing techniques (horseradish peroxidase or biotinylated dextran amine) and postembedding gamma-aminobutyric acid (GABA) immunocytochemistry. After injection of tracer into the pretectum, two types of axon terminals were identified as those of pretectogeniculate pathways. Both contained pleomorphic synaptic vesicles and were more numerous in the inner part of the nucleus. They could be distinguished on the bases of size and shape of their synaptic vesicles, type of synaptic contact, and level of GABA immunoreactivity. One type had a higher density of immunolabeling and established symmetric synaptic contacts, whereas the other, less densely immunolabeled, made asymmetric synaptic contacts. In both cases, synaptic contacts were mainly with relay cells and occasionally with interneurons. We suggest that these two types of pretectogeniculate terminals originate in two separate pretectal nuclei. After injection of tracer into the optic tectum, a single population of GABA-immunonegative tracer-labeled terminals was identified as belonging to the tectogeniculate pathway. These were small, had smooth contours, contained very small round synaptic vesicles, and established asymmetric synaptic contacts with long active zones, predominantly with relay cells and less frequently with interneurons, in the inner part of the nucleus. In addition, a population of GABA-negative and occasionally GABA-positive terminals, labeled by tracer injected into either the pretectum or the tectum, was identified as retinal terminals; these were presumably labeled by the retrograde transport of tracer in collateral branches of visual fibers innervating both the GLd and the pretectum or tectum. Comparison of the present ultrastructural findings in turtles with those previously reported in mammals shows that the cytological features, synaptic morphology, and immunochemical properties of the pretectogeniculate and tectogeniculate terminals of both groups share many similarities. Nevertheless, the postsynaptic targets of these two categories of terminals display some pronounced differences between the two groups, which are discussed in terms of their possible functional significance.

Postnatal development of the noradrenergic system in the dorsal lateral geniculate nucleus of the rat

The noradrenergic innervation of the developing dorsal lateral geniculate nucleus of the rat was examined with light and electron microscopic immunocytochemistry. At birth, few, relatively thick, noradrenergic fibers innervated the nucleus. Their density was steadily increased and they became thinner, tortuous, and varicose with the progression of age. Only a minority (11-15%) of labeled varicosities made synaptic contacts. Most of these synapses were symmetrical and on dendritic shafts. The present findings demonstrate the establishment of the anatomical relationships between noradrenergic afferents and neurons of the dorsal lateral geniculate nucleus during development and may help to understand the role of noradrenaline in the processing of visual information.

Noradrenergic innervation of the developing and mature septal area of the rat

The noradrenergic innervation of the developing and mature septal area of the rat was examined with light and electron microscopic immunocytochemistry using an antibody against dopamine-beta-hydroxylase. At birth, a small number of relatively thick noradrenergic fibers were found to innervate the lateral septum (mainly its intermediate part) and the nuclei of the vertical and horizontal limbs of the diagonal band of Broca. By postnatal day 7, a substantial increase in their density was observed. At this age some labeled fibers left the medial forebrain bundle and invaded the nucleus of the horizontal limb of the diagonal band. These fibers then ran in a ventrodorsal direction and innervated the nucleus of the vertical limb before entering the medial septum. Immunoreactive fibers were finer and more varicose than at birth. In the subsequent 2 weeks, the density of labeled fibers in the septal area was further increased. By postnatal day 21, the distribution pattern and density of the noradrenergic innervation appeared similar to the adult. In the adult, noradrenergic fibers exhibited more varicosities than in younger rats. Electron microscopic analysis revealed a low proportion (peaked at P7) of noradrenergic varicosities engaged in synaptic contacts throughout development. The overwhelming majority of these synapses were symmetrical, predominantly with small or medium-sized dendrites. The present findings provide the morphological basis for the functional interactions between noradrenergic afferents and neuronal elements in the septal area. The low proportion of synaptic contacts found in this study suggests that noradrenaline may exert its action in the septal area mainly through transmission by diffusion (volume transmission), as has been suggested for other areas of the developing and adult brain.

Lack of 5-HT(1B) receptor and of serotonin transporter have different effects on the segregation of retinal axons in the lateral geniculate nucleus compared to the superior colliculus

We have shown previously that raised levels of serotonin (5-hydroxytryptamine or 5-HT) during development prevent retinal ganglion cell axons from segregating into eye-specific regions in their principal targets: the superior colliculus and the dorsal lateral geniculate nucleus. Possible mediators of 5-HT in this system include its plasma membrane transporter, which is transiently expressed by a sub-population of retinal ganglion cells, and the presynaptic 5-HT(1B) receptor carried on retinal ganglion cell axons. We analysed the retinal projections of 5-HT(1B) knockout (n=15), serotonin transporter knockout (n=14), serotonin transporter/5-HT(1B) double knockout (n=4) and monoamine oxidase A/5-HT(1B) double knockout (n=3) mice. In all four different knockout mice, the ipsilateral retinal projection to the superior colliculus was more diffuse and lost its characteristic patchy distribution. The alterations were most severe in the serotonin transporter knockout mice, where the ipsilateral retinal fibres covered the entire rostrocaudal and mediolateral extent of the superior colliculus, whereas in the 5-HT(1B) and double knockout mice, fibres retracted from the caudal and lateral superior colliculus. Abnormalities in the 5-HT(1B) knockout mice appeared only after postnatal day (P) 4. Treatment with parachlorophenylalanine (at P1-P12) to decrease serotonin levels caused an exuberance of the ipsilateral retinal fibres throughout the superior colliculus (n=9). In the dorsal lateral geniculate nucleus in contrast, the distribution and size of the ipsilateral retinal projection was normal in all four knockout mice. In the serotonin transporter knockout mice however, the contralateral retinal fibres failed to retract from the mediodorsal dorsal lateral geniculate nucleus, an abnormality that was reversed by early treatment with parachlorophenylalanine and in the serotonin transporter/5-HT(1B) double knockout. OUR OBSERVATIONS INDICATE: (1) that the lack of 5-HT transporter and the associated changes in 5-HT levels impair the segregation of retinal axons in both the superior colliculus and the dorsal lateral geniculate nucleus; (2) that 5-HT and 5-HT(1B) receptors are necessary for the normal refinement of the ipsilateral retinal fibres in the superior colliculus, but are not essential for the establishment of eye-specific segregation in the thalamus. Thus, both an excess and a lack of 5-HT affect the refinement of the superior colliculus retinal projection, while the establishment of eye-specific patterns in the dorsal lateral geniculate nucleus appears not to be sensitive to the lack of 5-HT or 5-HT(1B) receptors.

Noradrenergic innervation of the developing and mature visual and motor cortex of the rat brain: a light and electron microscopic immunocytochemical analysis

The noradrenergic (NA) innervation of the developing and adult visual and motor cortex of the rat was examined with light and electron microscopic immunocytochemistry by using antibodies against dopamine-beta-hydroxylase. At birth, NA fibers were present in both cortical areas, appearing as two tangential streams, one above and the other below the cortical plate. During the subsequent weeks, these two streams arborized gradually innervating all cortical layers. The adult pattern of distribution was attained by postnatal day 14, but the density of innervation, which was higher in the motor than in the visual cortex, appeared similar to the adult by the end of the third postnatal week. Electron microscopic analysis revealed that a low proportion of NA varicosities (the highest value was 12% in the adult motor cortex in single sections) were engaged in synaptic contact, throughout development, in both areas examined. The overwhelming majority of these synapses were symmetrical, involving predominantly small or medium dendrites. This evidence suggests that transmission by diffusion is the major mode of NA action in the developing and adult cerebral cortex. Noradrenaline released in the rare synaptic junctions may act mainly to reduce the activity of its cortical targets. The results altogether provide morphologic evidence for an involvement of noradrenaline in the development of the neocortex and, along with earlier data on the serotonergic system, indicate that the monoaminergic systems are endowed with a specific anatomic organization in various areas of the brain.

Sculpting the visual map: the distribution and function of serotonin-1A and serotonin-1B receptors in the optic tectum of the frog

Agonists of serotonin (5-HT)-1 receptors modulate the synaptic strength of the connection between retinal ganglion cells and neurons of the frog optic tectum in brain slices (Brain Res. 1998;781:167-181). We have now used autoradiographic receptor binding techniques to determine the location of 5-HT1A and 5-HT1B binding sites in the laminated optic tectum. 5-HT1A binding sites, as labeled with [3H]8-hydroxy-dipropylaminotetralin (8-OH-DPAT), were highest in the superficial, retinorecipient layers of the tectum, intermediate in layers 6 and 7 and low in the remaining layers. Binding densities in all of these layers were unaffected by optic nerve lesion. 5-HT1B binding sites were visualized using [125I]iodocyanopindolol (ICYP). Binding densities were highest in the plexiform layers 5 and 7 and intermediate in layers 6 and 8. Binding sites were present at low levels in layer 9; however, optic nerve lesion resulted in a strong upregulation of these sites in this layer. Pharmacological manipulation of receptor activation resulted in changes in the activity-dependent visual map that is created at the tectum by retinal ganglion cell terminals. Chronic treatment of the tectum with SB-224289, a selective antagonist of 5-HT1B receptors, disrupted the topographic map. In contrast, exposure to WAY-100635, a selective antagonist of 5-HT1A receptors, refined it. We conclude that both 5-HT1A and 5-HT1B receptors are present in the adult frog tectum and that changes in their activation levels can produce changes in retinotectal transmission levels that drive visual plasticity in opposite directions.

Excess of serotonin (5-HT) alters the segregation of ispilateral and contralateral retinal projections in monoamine oxidase A knock-out mice: possible role of 5-HT uptake in retinal ganglion cells during development

Retinal ganglion cell (RGCs) project to the ipsilateral and contralateral sides of the brain in the dorsal lateral geniculate nucleus (dLGN) and the superior colliculus (SC). Projections from both eyes are initially intermingled until postnatal day 3 (P3) but segregate into eye-specific layers by P8. We report that this segregation does not occur in monoamine oxidase A knock-out mice (MAOA-KO) that have elevated brain levels of serotonin (5-HT) and noradrenaline. The abnormal development of retinal projections can be reversed by inhibiting 5-HT synthesis from P0 to P15. We found that in MAOA-KO mice, 5-HT accumulates in a subpopulation of RGCs and axons during embryonic and early postnatal development. The RGCs do not synthesize 5-HT but reuptake the amine from the extracellular space. In both MAOA-KO and normal mice, high-affinity uptake of 5-HT and serotonin transporter (SERT) immunoreactivity are observed in retinal axons from the optic cup to retinal terminal fields in the SC and dLGN. In the dLGN, transient SERT labeling corresponds predominantly to the ipsilateral retinal projection fields. We show that, in addition to SERT, developing RGCs also transiently express the vesicular monoamine transporter gene VMAT2: thus, retinal axons could store 5-HT in synaptic vesicles and possibly use it as a borrowed neurotransmitter. Finally we show that the 5-HT-1B receptor gene is expressed by RGCs throughout the retina from E15 until adult life. Activation of this receptor is known, from previous studies, to reduce retinotectal activity; thus 5-HT in excess could inhibit activity-dependent segregation mechanisms. A hypothesis is proposed whereby, during normal development, localized SERT expression could confer specific neurotransmission properties on a subset of RGCs and could be important in the fine-tuning of retinal projections.

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.

Transient expression of the vesicular monoamine transporter during development in the rat thalamus and cortex

The postnatal developmental pattern of the central vesicular monoamine transporter-2 (VMAT2) was analyzed in the rat brain by means of quantitative autoradiography with a specific and high affinity ligand [3H]dihydrotetrabenazine ([3H]TBZOH). We show a dense expression of VMAT2 in the cortex (especially area 17) and thalamus (particularly the dorsal lateral geniculate nucleus) at postnatal days 1 and 8. This pattern of VMAT2 distribution was transient since it was no longer observed at day 20 or in the adult rat brain where VMAT2 density was weak and uniform in these regions. These data suggest that monoamine vesicular storage participates in the early postnatal maturation of thalamus and cortex.

A role for serotonin in the circadian system revealed by the distribution of serotonin transporter and light-induced Fos immunoreactivity in the suprachiasmatic nucleus and intergeniculate leaflet

Components of the circadian system, the suprachiasmatic nucleus and the intergeniculate leaflet receive serotonin input from the raphe nuclei. Manipulations of serotonin neurotransmission disrupt cellular, electrophysiological, and behavioural responses of the circadian system to light, suggesting that serotonin plays a modulatory role in photic regulation of circadian rhythms. To study the relation between serotonin afferents and light-activated cells in the suprachiasmatic nucleus and intergeniculate leaflet, we used immunostaining for the serotonin transporter and for the transcription factor, Fos. Serotonin transporter, a plasma membrane protein located on serotonin neurons, regulates the amount of serotonin available for neurotransmission by re-accumulating released serotonin into presynaptic neurons; expression of Fos in the suprachiasmatic nucleus identifies light-activated cells involved in photic resetting of circadian clock phase. In the suprachiasmatic nucleus, immunostaining for serotonin transporter revealed a dense plexus of fibres concentrated primarily in the ventrolateral region. In the intergeniculate leaflet, serotonin transporter immunostaining identified vertically-oriented columns of fibres. Serotonin transporter immunostaining was abolished by pretreatment with the serotonin neurotoxin, 5,7-dihydroxytryptamine. Exposure to light for 30 min during the dark phase of the light cycle induced Fos expression in the ventrolateral suprachiasmatic nucleus and intergeniculate leaflet regions. In both structures the Fos-expressing cells were encircled by serotonin transporter-immunoreactive fibres often in close apposition to these cells. These results support the idea that serotonin activity plays a modulatory role in processing of photic information within the circadian system.

Synchronizing circadian rhythms in early infancy

Entrainment to the 24-hour light-dark cycle is of adaptive significance to mammals. Human infants are no exception, but some postnatal care habits prevalent in developed countries can interfere with the physiological mechanisms underlying circadian synchronization. We describe the physiological mechanisms of entrainment to the light-dark cycle in fetuses and newborns, and some common parental care behaviors which subject the developing circadian system of the newborn to conflicting temporal cues. Improvements in parental care are proposed which may improve the circadian synchronization of newborns, and their parents or caregivers.

The serotonin innervation of the basal forebrain shows a transient phase during development

The serotonergic innervation of the adult and developing basal forebrain nuclei of the rat was studied with immunocytochemical techniques at the light and electron microscopic levels. A substantial number of relatively thick serotonergic fibers with few varicosities and random orientation were observed at the time of birth. During the subsequent weeks, the serotonergic fibers increased in number and became thinner with many varicosities. They were also re-oriented, and around the end of the third postnatal week they exhibited the pattern of distribution and density seen in the adult. Electron microscopic analysis revealed that serotonin varicosities formed symmetrical or asymmetrical synapses mainly with dendritic shafts throughout postnatal life. Stereological extrapolation from single sections to the whole volume of varicosities showed that the percentage of serotonin varicosities engaged in synaptic junctions varied according to age. The proportion of labelled varicosities forming synapses increased from birth (21.3%) to the end of the second postnatal week (42.5%), then declined markedly in the following week (17.1%) before increasing again to an adult value of 46%. These findings suggest that the formation of synaptic connections by serotonin axons in the basal forebrain shows two distinct phases in postnatal development: exuberant synapses present in the first two weeks of life may be related to the involvement of serotonin in the maturation of this area, whereas synapses formed later in development may affect the functional state of basal forebrain projections to the neocortex and hippocampus. Thus, at these late stages of development and in the adult, serotonin may influence the activity of these forebrain structures both directly and indirectly.