Proliferating cell nuclear antigen (PCNA) immunoreactivity and development of the pineal complex and habenula of the sea lamprey

The development of the pineal complex and the habenula of the sea lamprey was studied with proliferating cell nuclear antigen (PCNA) immunocytochemistry. The pineal organ and the habenula primordia appeared in late embryos, and neuron differentiation began in prolarvae, as indicated by the presence of PCNA-negative cells. The parapineal primordium could not be distinguished until early prolarval stages, and cell differentiation was delayed to the larval period. Although the number of cycling (PCNA-immunoreactive) cells gradually decreased during the larval period in the three organs studied, their patterns of differentiation were different. We conclude that the unusual developmental pattern observed is related with the complex life cycle of lampreys.

Early development of the retina and pineal complex in the sea lamprey: comparative immunocytochemical study

Lampreys have a complex life cycle, with largely differentiated larval and adult periods. Despite the considerable interest of lampreys for understanding vertebrate evolution, knowledge of the early development of their eye and pineal complex is very scarce. Here, the early immunocytochemical organization of the pineal complex and retina of the sea lamprey was studied by use of antibodies against proliferating cell nuclear antigen (PCNA), opsin, serotonin, and gamma-aminobutyric acid (GABA). Cell differentiation in the retina, pineal organ, and habenula begins in prolarvae, as shown by the appearance of PCNA-negative cells, whereas differentiation of the parapineal vesicle was delayed until the larval period. In medium-sized to large larvae, PCNA-immunoreactive (-ir) cells were numerous in regions of the lateral retina near the differentiated part of the larval retina (central retina). A late-proliferating region was observed in the right habenula. Opsin immunoreactivity appears in the pineal vesicle of early prolarvae and 3 or 4 days later in the retina. In the parapineal organ, opsin immunoreactivity was observed only in large larvae. In the pineal organ, serotonin immunoreactivity was first observed in late prolarvae in photoreceptive (photoneuroendocrine) cells, whereas only a few of these cells appeared in the parapineal organ of large larvae. No serotonin immunoreactivity was observed in the larval retina. GABA immunoreactivity appeared earlier in the retina than in the pineal complex. No GABA-ir perikaryon was observed in the retina of larval lampreys, although a few GABA-ir centrifugal fibers innervate the inner retina in late prolarvae. First GABA-ir ganglion cells occur in the pineal organ of 15-17 mm larvae, and their number increases during the larval period. The only GABA-ir structures observed in the parapineal ganglion of larvae were afferent fibers, which appeared rather late in development. The time sequence of development in these photoreceptive structures is rather different from that observed in teleosts and other vertebrates. This suggests that the unusual development of the three photoreceptive organs in lampreys reflects specialization for their different functions during the larval and adult periods.

The transcription factor Pax6 is required for development of the diencephalic dorsal midline secretory radial glia that form the subcommissural organ

During brain development, Pax6 is expressed in specific regions of the diencephalon including secretory cells of the subcommissural organ (SCO), a circumventricular organ at the forebrain-midbrain boundary that originates from the pretectal dorsal midline neuroepithelial cells beneath the posterior commissure (PC). Homozygous small eye (Sey/Sey) mice lack functional Pax6 protein and fail to develop the SCO, a normal PC and the pineal gland. Small eye heterozygotes (Sey/+) show defective development of the SCO's basal processes which normally penetrate the PC, indicating that normal development of the gland requires normal Pax6 gene-dosage. A correlation between the defects of SCO formation and altered R- and OB-cadherin expression patterns in the SCO is observed in mutants suggesting a role for cadherins in SCO development.

Cloning and developmental expression of STAT5 in Xenopus laevis

Transcription factors of the signal transducer and activator of transcription (STAT) family are required for cellular responses to multiple signalling molecules. After ligand binding-induced activation of cognate receptors, STAT proteins are phosphorylated, hetero- or homodimerize, and enter the nucleus. STAT dimers bind to specific DNA elements and alter the transcriptional activity of the signal-responsive genes. We report the cloning and developmental pattern of expression of XSTAT5, a Xenopus laevis member of the STAT family, closely related to the mammalian STAT5A and STAT5B. XSTAT5 is expressed maternally and zygotically. With the onset of neurulation, XSTAT5 RNA are clearly localized in the anterior neural plate and subsequently in the neural structures of the developing eye, the pineal gland and the cement gland anlage. At late tailbud stages, a faint expression is detected in a ventral location that might correspond to the ventral blood islands.

SPACRCAN in the developing retina and pineal gland of the rat: spatial and temporal pattern of gene expression and protein synthesis

SPACRCAN is a hyaluronan-binding proteoglycan that is present in the pineal gland and interphotoreceptor matrix of the retina. Here, we evaluate the pattern of SPACRCAN gene expression and protein appearance during retinal and pineal gland development in the rat. In situ hybridization histochemistry with SPACRCAN riboprobes indicates that hybridization signals are first evident in the retina over developing photoreceptor cells at embryonic day 16 (E16) and in the pineal gland at E21. Immunocytochemistry using a SPACRCAN antibody shows localization of SPACRCAN protein in the developing interphotoreceptor matrix by Postnatal day 5 (P5) and in the pineal gland by P6. These studies suggest that SPACRCAN mRNA expression may occur substantially earlier than the time when SPACRCAN protein is detectable in both the retina and the pineal gland. The period of retinal histogenesis when SPACRCAN is detected first is coincident with the time photoreceptors begin to extend from the outer retinal surface, suggesting that SPACRCAN may participate in the maturation and maintenance of the light-sensitive photoreceptor outer segment.

Developmental potency of cultured pineal cells: an approach to pineal developmental biology

The pineal organ is still an enigma in regard to its developmental and phylogenetic origin. Little is known of the mechanism involved in determination and differentiation of pineal cells and virtually no studies have been done on the induction and tissue interactions during pinealogenesis. Interest is also centered on the evolutional transformation in structure and function, which may be related to the developmental alterations in pineal morphogenesis between the lower and higher vertebrate species. For developmental studies, avian embryos have great advantages for various experimental manipulations, such as cell and organ culture, surgical operation, and in situ transfection of developmental genes. The present review describes our cell culture studies, which have been done on developing rat and quail pineal organs, in order to elucidate the developmental potency of pineal cells and the regulatory mechanism involved in the phenotypic expression of cell properties. A number of phenotypes including numerous neuron-specific substances are shown immunohistochemically to be expressed only under culture conditions, and not observed in the mature pineal organ. As development proceeds, some of the potencies for cell differentiation are lost; hence, in the mature pineal organs most neuronal phenotypes are not expressed. Numerous factors were discovered which affect phenotypic expression of cultured pineal cells in a cell-type-specific manner. These findings, together with immunohistochemical observations on developing pineal organs, reveal that the developing pineal organ is a unique and useful model system for developmental neurobiology and that cell culture techniques offer a powerful tool for the understanding of development and cell differentiation of this particular organ.

Rhythms of the pineal N-acetyltransferase mRNA and melatonin concentrations during embryonic and post-embryonic development in chicken

Development of a daily rhythmicity in transcription of a gene encoding a rate-limiting enzyme of melatonin biosynthesis, the arylalkylamine-N-acetyltransferase (AA-NAT) was studied by northern blot analysis in pineal glands of 16 and 19-day-old embryos and 1, 4, 8, 11, and 14-day-old chicks. In a parallel experiment, melatonin content in pineal glands and plasma was measured. A significant rhythm of AA-NAT expression was found at embryonic day (ED) 16, the earliest day assayed in this experiment. Expression was low during the daytime and a clear signal was found in the middle of the darktime. The intensity of the signal was increasing during the ontogeny. The nocturnal pineal melatonin concentrations were increasing over the studied period (from ED 19 until post-embryonic day 21). Midnight plasma melatonin concentrations increased from ED19 to PD 3 and oscillated around this value afterwards. Data show that rhythmic expression of AA-NAT mRNA starts very early in development of chicken and plays a major role in melatonin rhythm generation during embryonic development.

Asymmetric nodal signaling in the zebrafish diencephalon positions the pineal organ

The vertebrate brain develops from a bilaterally symmetric neural tube but later displays profound anatomical and functional asymmetries. Despite considerable progress in deciphering mechanisms of visceral organ laterality, the genetic pathways regulating brain asymmetries are unknown. In zebrafish, genes implicated in laterality of the viscera (cyclops/nodal, antivin/lefty and pitx2) are coexpressed on the left side of the embryonic dorsal diencephalon, within a region corresponding to the presumptive epiphysis or pineal organ. Asymmetric gene expression in the brain requires an intact midline and Nodal-related factors. RNA-mediated rescue of mutants defective in Nodal signaling corrects tissue patterning at gastrulation, but fails to restore left-sided gene expression in the diencephalon. Such embryos develop into viable adults with seemingly normal brain morphology. However, the pineal organ, which typically emanates at a left-to-medial site from the dorsal diencephalic roof, becomes displaced in position. Thus, a conserved signaling pathway regulating visceral laterality also underlies an anatomical asymmetry of the zebrafish forebrain.

A nodal signaling pathway regulates the laterality of neuroanatomical asymmetries in the zebrafish forebrain

Animals show behavioral asymmetries that are mediated by differences between the left and right sides of the brain. We report that the laterality of asymmetric development of the diencephalic habenular nuclei and the photoreceptive pineal complex is regulated by the Nodal signaling pathway and by midline tissue. Analysis of zebrafish embryos with compromised Nodal signaling reveals an early role for this pathway in the repression of asymmetrically expressed genes in the diencephalon. Later signaling mediated by the EGF-CFC protein One-eyed pinhead and the forkhead transcription factor Schmalspur is required to overcome this repression. When expression of Nodal pathway genes is either absent or symmetrical, neuroanatomical asymmetries are still established but are randomized. This indicates that Nodal signaling is not required for asymmetric development per se but is essential to determine the laterality of the asymmetry.

Ectopic engrailed 1 expression in the dorsal midline causes cell death, abnormal differentiation of circumventricular organs and errors in axonal pathfinding

A series of gain- or loss-of-function experiments performed in different vertebrate species have demonstrated that the Engrailed genes play multiple roles during brain development. In particular, they have been implicated in the determination of the mid/hindbrain domain, in cell proliferation and survival, in neurite formation, tissue polarization and axonal pathfinding. We have analyzed the consequences of a local gain of En function within or adjacent to the endogenous expression domain in mouse and chick embryos. In WEXPZ.En1 transgenic mice (Danielian, P. S. and McMahon, A. P. (1996) Nature 383, 332–334) several genes are induced as a consequence of ectopic expression of En1 in the diencephalic roof (but in a pattern inconsistent with a local di- to mes-encephalon fate change). The development of several structures with secretory function, generated from the dorsal neuroepithelium, is severely compromised. The choroid plexus, subcommissural organ and pineal gland either fail to form or are atrophic. These defects are preceded by an increase in cell death at the dorsal midline. Comparison with the phenotype of Wnt1(sw/sw) (swaying) mutants suggests that subcommissural organ failure is the main cause of prenatal hydrocephalus observed in both strains. The formation of the posterior commissure is also delayed, and errors in axonal pathfinding are frequent. In chick, ectopic expression of En by in ovo electroporation, affects growth and differentiation of the choroid plexus.

Expression of synaptic vesicle trafficking proteins in the developing rat pineal gland

Adult mammalian pinealocytes contain several synaptic membrane proteins that are probably involved in the regulation of targeting and exocytosis of synaptic-like microvesicles (SLMVs). Immunohistochemical techniques have now demonstrated the spatiotemporal expression pattern of some of these proteins during rat pineal ontogenesis. Various synaptic vesicle trafficking proteins are detectable in proliferating epithelial cells of the pineal anlage even at embryonic day 17.5 (E 17.5), with the exception of syntaxin I (weakly expressed from E 19.5) and dynamin I (whose levels increase markedly during the first postnatal week). Numerous cells exhibiting strong immunoreactivity for synaptobrevin II, SNAP-25, synaptophysin, and munc-18-1 are distributed throughout the increasingly compact gland at E 19.5 and E 20.5; however, their number declines toward the proximal deep part of the organ. Groups of postmitotic cells situated at the surface of the developing gland exhibit marked immunoreactivity for the aforementioned proteins and lie close to the laminin-immunoreactive outer limiting basement membrane or to its remnants in regions of basement membrane dissolution. We also show that synthesis of vimentin and S-antigen seems to begin earlier during pineal development than previously recognized. Thus, synaptic vesicle trafficking proteins are the earliest molecular markers of pinealocyte differentiation known to date, being expressed well before the onset of rhythmic hormone secretion in the pineal gland, where they may play a role in morphogenetic events. Components of the extracellular matrix such as laminin may be critically involved in the upregulation of synaptic membrane protein expression. The dynamin immunostaining pattern indicates that SLMVs of pinealocytes begin to undergo regulated cycles of exo/endocytosis during postnatal week 1.

Chick sox10, a transcription factor expressed in both early neural crest cells and central nervous system

Human SOX10 and mouse Sox10 have been cloned and shown to be expressed in the neural crest derivatives that contribute to formation of the peripheral nervous system during embryogenesis. Mutations in Sox10 have been identified as a cause of the Dominant megacolon mouse and Waardenburg-Shah syndrome in human, both of which include defects in the enteric nervous system and pigmentation (and in the latter, sometimes hearing). We have cloned a chick Sox10 ortholog (cSox10) in order to study its role in neural crest cell development. This cDNA reveals a 1383 bp open reading frame encoding 461 amino acids which is highly conserved with human SOX10 and mouse Sox10. In situ hybridization showed cSox10 is expressed in migrating neural crest cells just after the zinc finger transcription factor Slug, but is lost as cells undergo neuronal differentiation in ganglia of the peripheral nervous system. In addition, cSox10 is expressed in the developing otic vesicle, the developing central nervous system and pineal gland.

The pineal gland

The pineal gland is located posterior to the midbrain and is the site of melatonin production. Research on pineal gland function in neonates is very limited. This article will discuss pineal gland development and the possible relationship between melatonin production and sudden infant death syndrome. Further research on pineal gland function is needed in order to establish its significance for the neonate.

Ontogeny of circadian and light regulation of melatonin release in Xenopus laevis embryos

The retinal photoreceptors of Xenopus laevis contain a circadian clock that controls the synthesis and release of melatonin, resulting in high levels during the night and low levels during the day. Light is also an important regulator of melatonin synthesis and acts directly to acutely suppress melatonin synthesis during the day and indirectly to entrain the circadian clock. We examined the development of circadian and light regulation of melatonin release in Xenopus retinas and pineal glands. Pineal glands are capable of making measurable melatonin in culture soon after they evaginate from the diencephalon at stage 26. In cyclic light, the melatonin rhythms are robust, with higher overall levels and greater amplitudes than in constant darkness. However, the rhythm of melatonin release damps strongly and quickly toward baseline in constant darkness. Similar results are observed in older (stage 47) embryos, indicating that cyclic light has a positive effect on melatonin synthesis in this tissue. Optic vesicles dissected at stage 26 do not release melatonin in culture until the second or third day. It is weakly rhythmic in cyclic light, but in constant dark it is released at constitutively high levels throughout the day. By stage 41, the eyes release melatonin rhythmically in both cyclic light and constant darkness with similar amplitude. Our results show that Xenopus embryos develop a functional, photoresponsive circadian clock in the eye within the first few days of life and that rhythmic melatonin release from the pineal gland at comparable stages is highly dependent on a light-dark cycle.

Effects of static electromagnetic fields on chick embryo pineal gland development

The effects of static electromagnetic fields on the development of the chick embryo pineal gland were studied. A total of 144 fertilized White Leghorn eggs were sacrificed after 5, 10 and 15 days of incubation. The stage of development was determined in all embryos using the Hamburger and Hamilton method [J Morphol 49: 88-92, 1951]. The various morphometric parameters (diameter and distance of the pineal gland and its lumen) were measured on serial 7-micron-thick sections. The data were obtained in a morphometer and processed statistically. The intensities of the static electromagnetic fields were 18 and 36 mT. Control and exposed embryos were equally distributed and randomly assigned. After 5 days of incubation, 25% of embryos exposed to a static electromagnetic field of 18 mT had a more advanced stage of development than controls and embryos exposed to 36 mT. On the 10th and 15th day, embryos exposed to either 18 or 36 mT tended to be more developed than controls. In the morphometric study, results were similar for the controls and exposed embryos after 5 and 10 days of incubation. However, the values of the 15-day-old embryos exposed to static magnetic fields were lower than the values of the controls (p > 0.01). These differences were more pronounced in the embryos exposed to 36 mT. These results seem to indicate that static electromagnetic fields affect the development and growth of embryos unequally, and that their action can depend not only on the intensity of the static electromagnetic field, but also on the length of exposure and the organ which is developing. It may be interesting to use these data in ultrastructural and physiological studies.

Zebrafish serotonin N-acetyltransferase-2: marker for development of pineal photoreceptors and circadian clock function

Serotonin N-acetyltransferase (AANAT), the penultimate enzyme in melatonin synthesis, is typically found only at significant levels in the pineal gland and retina. Large changes in the activity of this enzyme drive the circadian rhythm in circulating melatonin seen in all vertebrates. In this study, we examined the utility of using AANAT messenger RNA (mRNA) as a marker to monitor the very early development of pineal photoreceptors and circadian clock function in zebrafish. Zebrafish AANAT-2 (zfAANAT-2) cDNA was isolated and used for in situ hybridization. In the adult, zfAANAT-2 mRNA is expressed exclusively in pineal cells and retinal photoreceptors. Developmental analysis, using whole mount in situ hybridization, indicated that pineal zfAANAT-2 mRNA expression is first detected at 22 h post fertilization. Retinal zfAANAT-2 mRNA was first detected on day 3 post fertilization and appears to be associated with development of the retinal photoreceptors. Time-of-day analysis of 2- to 5-day-old zebrafish larvae indicated that zfAANAT-2 mRNA abundance exhibits a dramatic 24-h rhythm in a 14-h light, 10-h dark cycle, with high levels at night. This rhythm persists in constant darkness, indicating that the zfAANAT-2 mRNA rhythm is driven by a circadian clock at this stage. The techniques described in this report were also used to determine that zfAANAT-2 expression is altered in two well characterized genetic mutants, mindbomb and floating head. The observations described here suggest that zfAANAT-2 mRNA may be a useful marker to study development of the pineal gland and of circadian clock mechanisms in zebrafish.

Differentiation of pinopsin-immunoreactive cells in the developing quail pineal organ: an in-vivo and in-vitro immunohistochemical study

The avian pineal organ contains several types of photoreceptors with different photopigments: rhodopsin, iodopsin, and pinopsin. We have previously examined the differentiation of both rhodopsin-like and iodopsin-like immunoreactive cells during pineal development in quail embryos to determine the onset of synthesis of specific proteins and their cellular localization. In the present study, we have performed pinopsin immunohistochemistry on in-vivo developing and in-vitro cultured pineal organs of quail embryos. The results were compared with those obtained with rhodopsin and iodopsin immunohistochemistry. In the developing pineal organs, pinopsin immunoreactivity was detected at embryonic day 8, i.e. five days earlier than rhodopsin-like and iodopsin-like immunoreactivities. It was localized exclusively in the protrusions extending into the lumen throughout development, whereas rhodopsin-like and iodopsin-like immunoreactivities were usually found both in cell bodies and processes. These differences were also observed under two different types of culture conditions (dissociated cell culture and organ culture) indicating that, in the avian pineal organ, the expression pattern of the pinopsin gene is basically different from those of the other two pineal photopigments. The present study suggests that pineal cells have a mechanism for the polarized transport of pinopsin molecules.

Embryonic development of pineal gland vesicles: a morphological and morphometrical study in chick embryos

Pineal cell aggregates in 5, 10 and 15 day-old chick embryos have been studied. Cell aggregates were classified into rosettes or vesicles and spheroid and ellipsoid vesicles distinguished. The number of pineal vesicles per unit of surface (vesicle density) was determined in three pineal portions: apical, anterior and posterior. By day 5, only cellular rosettes were found, mainly in the apical portion. After 10 and 15 days, the presence of rosettes was occasional. The posterior wall showed only small spheroid vesicles, while in the apical and anterior areas ellipsoid vesicles were also observed. Moreover, the spheroid/ellipsoid vesicle ratio increased from the 10th to the 15th day of incubation. The vesicle density decreased between the 10th and 15th day because of the increase in both vesicle and pineal size, without changes in the total number of vesicles. The results suggest that changes in vesicle morphology and density could be related to the functional activity of the pineal gland during embryonic development.

Perinatal development of circadian melatonin production in domestic chicks

In contrast to the situation in mammals, in which circadian melatonin production by the pineal gland does not begin until some time after birth, the development of pineal gland rhythmicity is an embryonic event in the precocial domestic fowl. A distinct melatonin rhythm was found in 19-d-old chick embryos maintained under light:dark (LD) 16:8. No significant variation in melatonin levels was detected in embryos exposed to LD 8:16. The melatonin rhythm in the pineal gland and plasma of chick embryos incubated for 18 d in LD 12:12 persisted for 2 d in constant darkness indicating that melatonin production is under circadian control at least from the end of embryonic life. A 1-d exposure to a LD cycle during the first postembryonic day was sufficient to entrain the melatonin rhythm, and previous embryonic exposure to either LD or constant darkness (DD) neither modified this rapid synchronization nor did it affect the melatonin pattern during the two subsequent days in DD. It is suggested that, in contrast to the situation in mammals, the avian embryo has evolved its own early circadian melatonin-producing system because, as a consequence of its extrauterine development, it cannot use the system of its mother.

Structure of the ovine pineal gland during prenatal development

The structure of the pineal gland of 32 clinically healthy ovine embryos at different stages of development was studied. Embryos were arranged in four age groups, each containing eight embryos (four males and four females), defined in terms of the most relevant histological features: group 1 (27 to 69 days of prenatal development), group 2 (70 to 97 days), group 3 (98 to 116 days), and group 4 (117 to 150 days). At around 30 days of prenatal life, according to topographic criteria, the pineal outline begins to differentiate into a dorsal evagination of the diencephalic medium line, close to the anterior and posterior commissures. The growth of the pineal is biphasic. The ontogenic-proliferative phase begins at 30 days and includes the invasion of ependymal cells and the proliferation of the pineal parenchyma cells. The hypertrophic-differentiation phase includes the volume increment of the pinealoblasts and their differentiation into pinealocytes; this occurs at around 118 days. At around 98 days, the gland acquires its definitive compact appearance due to 1) glandular growth in constant volume and 2) the obliteration of pineal recess. The glandular structure displays a parenchyma made up of pinealoblasts, interstitial cells, and cells containing pigment. The pineal stroma is structured in pseudolobes formed by reticular and collagen fiber septae, which constitute together the interstitial cell prolongation net, which is the support structure of the whole glandular cytology. Capillaries are detected all over the glandular surface, being more abundant in the medullary zone. At around 98 days of prenatal development, VIP (Vasoactive Intestinal Peptide) positive fibers, distributed around blood vessels and among pinealoblasts were detected.

Regulation of rhythmic melatonin production in pineal cells of chick embryo by cyclic AMP

The pineal cells of chick embryos incubated in vitro exhibited a daily rhythm of melatonin synthesis under a 12:12 light:dark (LD) cycle at the embryonic days 16 and 19. In order to elucidate whether cyclic adenosine monophosphate (cAMP)--a component of the melatonin generating system--is already at work in the embryonic period, we measured the effects of forskolin and isobuthylmethylxantine (IBMX) on melatonin production, cAMP efflux and accumulation. Forskolin (after 10, 20, 30, 45, 60 and 90 min of administration) and IBMX (6 h), when applied during the light phase of LD cycle, stimulated melatonin production and cAMP efflux and accumulation during the embryonic period (at days 16 and 19 fo development). Our results suggest that the biochemical pathway involving cAMP, which controls melatonin production in the postnatal period, is developed before hatching and already on embryonic day 19 works in a way similar to that in post-hatched chicks. Differences in response to cAMP stimulation between 16- and 19-day-old pinealocytes seem to be mostly quantitative.

Short note: The fetal origins hypothesis: linking pineal gland hypoplasia with coronary heart disease and stroke

Pineal gland hypoplasia secondary to intrauterine malnutrition is suggested as a fetal contribution to sudden infant death syndrome, coronary artery disease and ischemic stroke. The loss of melatonin's antioxidant activity is proposed as an additional factor to consider in atherosclerotic vascular disease.

Serotonin transporter messenger RNA in the developing rat brain: early expression in serotonergic neurons and transient expression in non-serotonergic neurons

Serotonin has been shown to affect the development of the mammalian nervous system. The serotonin transporter is a major factor in regulating extracellular serotonin levels. Using in situ hybridization histochemistry the rat serotonin transporter messenger RNA was localized during embryogenesis, the first four weeks postnatally and adulthood. Three general classes of serotonin transporter messenger RNA expression patterns were observed: (i) early detection with continued expression through adult age, (ii) transient expression colocalized with vesicular monoamine transporter 2 messenger RNA but with no detectable tryptophan hydroxylase immunoreactivity, and (iii) transient expression in the apparent absence of both vesicular monoamine transporter 2 messenger RNA and tryptophan hydroxylase immunoreactivity. For example, hybridization for serotonin transporter messenger RNA was strong in serotonin cell body-containing areas beginning early in gestation, and remained intense through adulthood. Immunoreactivity for tryptophan hydroxylase, the rate-limiting enzyme in serotonin synthesis, was completely overlapping with the presence of serotonin transporter messenger RNA in raphe nuclei postnatally. Sensory relay systems including the ventrobasal nucleus (somatosensory), lateral and medial geniculate nuclei (visual and auditory, respectively) as well as trigeminal, cochlear and solitary nuclei were representative of the second class of observations. In general, the limbic system expressed serotonin transporter messenger RNA in the third pattern with various limbic structures differing in the timing of expression. Septum, olfactory areas and the developing hippocampus contained serotonin transporter messenger RNA early in the developing brain. Other regions such as cingulate and frontopolar cortex exhibited hybridization peri- and postnatally, respectively. Several hypothalamic nuclei and pituitary transiently expressed serotonin transporter messenger RNA either postnatally or perinatally, respectively. If the observed patterns correlate with functional protein expression, distinct classes of serotonin transporter messenger RNA expression may reflect different functional roles for the serotonin transporter and serotonin, itself. Since the serotonin transporter is a target for a number of addictive substances including cocaine and amphetamine derivatives as well as antidepressants, transient expression of the serotonin transporter might suggest a window of vulnerability of associated cells to fetal drug exposure. Re-uptake, storage and re-release from non-serotonergic neurons might serve as a feedback mechanism from target neurons to serotonergic neurons. Alternatively, the transient expression of serotonin transporter messenger RNA may reflect critical periods important for tight regulation of extracellular serotonin in several brain regions, and may indicate previously unappreciated roles for serotonin as a developmental cue.