DNA microarray analysis and functional profile of pituitary transcriptome under core-clock protein BMAL1 control

Although it is known to contain five cell types that synthesize and release hormones with a circadian pattern, the pituitary gland is poorly characterized as a circadian oscillator. By a differential microarray analysis, 252 genes were found to be differentially expressed in pituitaries from Bmal1(-/-) knockout versus wild-type mice. By integrative analyses of the data set with the Annotation, Visualization, and Integrated Discovery (DAVID) Bioinformatics Resources annotation analysis system, pituitary genes with altered expression in Bmal1(-/-) mice were dispatched among functional categories. Clusters of genes related to signaling and rhythmic processes as well as transcription regulators, in general, were found enriched in the data set, as were pathways such as circadian rhythm, transforming growth factor β (TGFβ) signaling, valine, leucine, and isoleucine degradation, and peroxisome proliferator-activated receptor (PPAR) signaling pathways. Gene Ontology term overrepresentation analyses revealed significant enrichment for genes involved in 10 key biological processes. To determine whether genes with altered expression in Bmal1(-/-) mice were actually circadian genes, we further characterized in the mouse pituitary gland the daily pattern of some of these genes, including core-clock genes. Core-clock genes and genes selected from three identified overrepresented biological processes, namely, hormone metabolic process, regulation of transcription from RNA polymerase II promoter, and cell adhesion, displayed a rhythmic pattern. Given the enrichment in genes dedicated to cell adhesion and their daily changes in the pituitary, it is hypothesized that cell-cell interactions could be involved in the transmission of information between endocrine cells, allowing rhythmic hormone outputs to be controlled in a temporally precise manner.

Vitamin A is a necessary factor for sympathetic-independent rhythmic activation of mitogen-activated protein kinase in the rat pineal gland

The circadian clock in the suprachiasmatic nucleus (SCN) controls day-to-day physiology and behavior by sending timing messages to multiple peripheral oscillators. In the pineal gland, a major SCN target, circadian events are believed to be driven exclusively by the rhythmic release of norepinephrine from superior cervical ganglia (SCG) neurons relaying clock messages through a polysynaptic pathway. Here we show in rat an SCN-driven daily rhythm of pineal MAPK activation that is not dependent on the SCG and whose maintenance requires vitamin A as a blood-borne factor. This finding challenges the dogma that SCG-released norepinephrine is an exclusive mediator of SCN-pineal communication and allows the assumption that humoral mechanisms are involved in pineal integration of temporal messages.

Long‐lasting effects of serotonin deficiency on differentiating peptidergic neurons in the rat suprachiasmatic nucleus

Serotonin (5-HT, 5-hydroxytryptamine) is known to be an inductor of the brain development [Whitaker-Azmitia, P.M., Druse, M., Walker, P., Lauder, J.M., 1996. Serotonin as a developmental signal. Behav. Brain Res. 73, 19-29; Ugrumov, M.V., 1997. Hypothalamic monoaminergic systems in ontogenesis: development and functional significance. Int. J. Dev. Biol. 41, 809-816]. This study was aimed to test whether it provides long-lasting effects on the differentiating vasoactive intestinal polypeptide (VIP) and vasopressin (VP) neurons of the suprachiasmatic nucleus (SCN) in rats. To this aim, 5-HT was depleted in fetal brain by daily injections of p-chlorophenylalanine (pCPA), an inhibitor of 5-HT synthesis, to pregnant rats from the 13th to the 21st day of gestation. Pregnant rats injected with saline served as controls. The offsprings (males) of pCPA-treated and control pregnant rats were maintained after birth for two months under normal laboratory conditions. Then, the SCN was processed for immunocytochemistry of VIP and VP and in situ hybridization of appropriate mRNAs. There were no differences in concentrations of VIP and VP mRNAs in the SCN in adult offsprings of the 5-HT-depleted pregnant rats compared to the controls. Moreover, 5-HT deficiency did not induce any change in size of VIP-immunoreactive (IR) and VP-IR neurons. Conversely, both the numbers of VIP- and VP-immunoreactive neurons and concentrations of the peptides in cell bodies increased significantly. It is concluded that 5-HT provides long-lasting effects on differentiating VIP and VP neurons in the SCN resulting in attenuated release rather than elevated synthesis of both peptides in adulthood.

The effects of serotonin on the differentiation of neurons producing vasoactive intestinal polypeptide in the suprachiasmatic nucleus of the rat

The morphogenetic influences of serotonin on the differentiation of neurons synthesizing vasoactive intestinal polypeptide (VIP) in the suprachiasmatic nucleus were studied in rats. This was addressed by comparative morphofunctional analysis of VIP neurons in adult rats whose brains developed prenatally in conditions of normal and deficient serotonin metabolism. Serotonin deficiency was created in fetuses by treatment of their mothers with p-chlorophenylalanine (PCPA). Pregnant females in controls were treated with 0.9% NaCl. VIP neurons in experimental and control animals were found to show no differences in VIP mRNA concentrations and, probably, in the level of VIP synthesis. However, inhibition of serotonin synthesis led to an increase in the number of VIP-immunoreactive neurons and an increase in the VIP concentration within these cells. This was not associated with any change in neuron size, which was an indicator of the absence of functional hypertrophy accompanying activation of specific synthesis. Comparison of the data obtained here showed that during prenatal ontogenesis, serotonin has an imprinting influence on the differentiation of VIP neurons and is probably involved in the formation of the mechanism of VIP secretion.

Adrenergic inducibility of AP-1 binding in the rat pineal gland depends on prior photoperiod

The main known function of the pineal gland in mammals is the temporal synchronization of physiological rhythms to seasonal changes of day length (photoperiod). In rat, the transcription factor activating protein-1 (AP-1) displays a circadian rhythm in its DNA binding in the pineal gland, which results from the rhythmic expression of Fra-2. We postulated that, if AP-1 is an important component of pineal gland functioning, then variations in photoperiodic conditions should lead to an adaptation of the AP-1 binding rhythm. Here we show that AP-1 binding patterns adapt to variations in lighting conditions, in the same way as the rhythm of arylalkylamine-N-acetyltransferase (AA-NAT) activity. This adaptation appeared to result from photoperiodic adaptation of the rhythmic fra-2 gene expression and was reflected by an adapted delay between the onset of night and the acrophase of the nocturnal peak. We further showed that photoperiodic adaptation of both the AP-1 binding and AA-NAT activity rhythms resulted from adapted changes in adrenergic inducibility of both variables at night onset. We finally provided evidence that AP-1 shared with the CREM gene encoding the transcriptional repressor protein inducible cAMP early repressor (ICER) the ability to be hypersensitive or subsensitive to adrenergic stimuli, depending on prior photoperiod.

Striatal NPY-Containing Neurons Receive GABAergic Afferents and may also Contain GABA: An Electron Microscopic Study in the Rat

Dual labelling methods were applied to localize simultaneously neuropeptide Y (NPY) and glutamate decarboxylase (GAD) immunoreactivities on ultrathin sections of the rat caudate-putamen (CP). By means of a double peroxidase-anti-peroxidase technique, using 3,3'-diaminobenzidine and benzidine dihydrochloride as chromogens in animals with no colchicine pretreatment, GAD immunoreactivity was found to be present in terminals only whereas NPY immunoreactivity was detected in neurons displaying the features of aspiny type cells and processes. With this approach, we observed numerous synaptic associations of the symmetrical type between GAD-immunoreactive (-Ir) axonal boutons and NPY-Ir cell bodies and dendrites. By combining immunoperoxidase and radioimmunocytochemical labelling in animals pretreated with colchicine, NPY was again detected in a single population of aspiny type neurons whereas GAD immunoreactivity was observed in neurons which could be classified as aspiny and spiny on the basis of their ultrastructural characteristics. All the cells of the aspiny type displaying clear-cut NPY immunoreactivity were also found to be GAD-positive. Some other neurons of both the aspiny and the spiny type were found to be immunoreactive to GAD alone. GAD/NPY dually labelled terminals were also observed and some axo-axonic appositions between GAD- and NPY-Ir terminals were also detected. All in all, these data show that NPY aspiny type neurons of the rat CP receive GABAergic afferents and provide morphological support for two hypotheses: that NPY is co-localized with GABA in some cell bodies, dendrites and axons, and that presynaptic interactions may occur between NPY and GABAergic neuronal systems.

[Effect of serotonin on differentiation of neurons producing vasoactive intestinal polypeptide in the rat suprachiasmatic nucleus]

This study was aimed to evaluate the morphogenetic influence of serotonin on the differentiating vasoactive intestinal polypeptide (VIP) neurons of the suprachiasmatic nucleus (SCN) in rats. The comparative morpho-functional analysis of VIP neurons was made in adult rats which developed under normal metabolism of serotonin or in its deficiency. The serotonin deficiency was induced in foetuses by injections of p-chlorophenilalanine to pregnant mothers. Control rats received the saline. According to our data, there was no difference in the VIP mRNA concentration and most probably in VIP synthesis in the SCN in adult rats following prenatal serotonin depletion compared to the control. However, the serotonin deficiency resulted in increase of the VIP concentration in cell bodies and of the VIP neurones number. The size of the VIP-neurones did not change in pharmacologically treated rats compared to the controls showing no functional hypertrophy of the neurones as a result of the activation of specific syntheses. From the above data, it follows that serotonin provides an imprinting influence differentiating the VIP neurones, contributing most probably to development of the VIP release mechanism.

Long-term variations of AP-1 composition after CRH stimulation: consequence on POMC gene regulation

It has been shown previously that the CRH-induced POMC gene transcription in the corticotroph cell line AtT-20 involves an increase in AP-1 DNA binding activity that remained elevated for at least 24 h, while induction of c-fos was transient. We showed here that there were dramatic changes in protein components of AP-1 including an initial recruitment of the transcriptional activators c-Fos and Jun-B then of Fra-2 and Jun-D. Changes in AP-1 composition were concomitant with a decrease in POMC mRNA. Moreover, the presence of Fra-2/Jun-D dimers suppressed the CRH-induction of c-fos mRNA expression as well as c-Fos/Jun-B recruitment in AP-1 complexes, suggesting the existence of autoregulatory loops of AP-1 composition that involve complex interactions between the different members of the Jun and Fos families. It is concluded that CRH stimulation of corticotroph cells involves successive recruitment of activators and repressors, possibly contributing to prevent over expression of POMC.

Involvement of the suprachiasmatic nucleus in diurnal ACTH and corticosterone responsiveness to stress

We explored the contribution of the suprachiasmatic nucleus (SCN) in ACTH and corticosterone (CORT) diurnal responsiveness of the rat to restraint stress applied either in the morning (AM) or in the evening (PM). Ablation of the SCN caused the diurnal rhythmicity of the CORT response to disappear but had no effects on AM vs. PM differences in the ACTH response. Stress-response curves in SCN-lesioned rats that had prestress levels of CORT either in the AM range or in the PM range, when compared with those obtained for AM and PM controls, showed that the SCN differentially regulates the stress response depending on the underlying secretory activity of the adrenal cortex. When basal CORT secretion is at its lowest, the SCN inhibits CORT responsiveness to stress by controlling pituitary corticotrophs; but when it is at its highest, it has a permissive action that will bypass the hypophysis and reach the adrenals to adjust the response of the gland to ACTH.

Is light-regulated AP-1 binding in the rat suprachiasmatic nucleus gated by the circadian clock?

In mammals, photic entrainment of circadian rhythms likely involves light- and clock-dependent expression of immediate early genes, including fos-like and jun-like genes, in the rat suprachiasmatic nucleus. Using an electrophoretic mobility shift assay, we evaluated whether the photic regulation of DNA-binding activity and composition of activating protein-1 (AP-1) complexes in the suprachiasmatic nucleus is also dependent on circadian phase. Phase-dependent light inducibility in the expression of fra-2 and c-fos genes and in immunoreactive Fra-2 and c-Fos protein expression was also evaluated, by in situ hybridization and immunocytochemistry. Light's effects on AP-1 DNA-binding differed both qualitatively and quantitatively according to the circadian phase at which light was applied. This phase dependence accounted for by both compartmentalization of proteins involved in constitutive AP-1 complexes within the nucleus or cytoplasm and control of the extent to which the expression of specific complexes was induced. It was then shown that the mechanisms by which the circadian clock gates the photic induction of AP-1 components differed according to the nature of the protein.

Circadian binding activity of AP-1, a regulator of the arylalkylamine N-acetyltransferase gene in the rat pineal gland, depends on circadian Fra-2, c-Jun, and Jun-D expression and is regulated by the clock's zeitgebers

The daily rhythm in circulating melatonin is driven by a circadian rhythm in the expression of the arylalkylamine N:-acetyltransferase gene in the rat pineal gland. Turning off expression of this gene at the end of night is believed to involve inhibitory transcription factors, among which Fos-related antigen 2 (Fra-2) appears as a good candidate. Circadian rhythms in the expression of three proteins of activating protein-1 (AP-1) complexes, namely, Fra-2, c-Jun, and Jun-D, are shown here to account for circadian variations in AP-1 binding activity. Quantitative variations in the Fra-2 component over the circadian cycle were associated with qualitative variations in protein isoforms. Destruction of the suprachiasmatic nucleus resulted in decreased nocturnal AP-1 activity, showing that AP-1 circadian rhythm is driven by this nucleus. Exposure to light during subjective night and administration of a serotonin 5-HT(1A)/5-HT(7) receptor agonist during subjective day, respectively, induced a 50% decrease and a 50% increase in both AP-1 and Fra-2 expression. These effects were impaired by suprachiasmatic nucleus lesions. These data show that pineal AP-1 binding activity, which results from Fra-2 expression, can be modulated by light and serotonin through the suprachiasmatic nucleus according to a "phase dependence" that is characteristic of the rhythm of clock sensitivity to both zeitgebers.

Glucocorticoids up-regulate the expression of glial fibrillary acidic protein in the rat suprachiasmatic nucleus

Immunoreactivity against glial fibrillary acidic protein (GFAP) was used as a dynamic index in adrenalectomized rats subjected or not to corticosterone replacement to investigate whether glucocorticoids may interact with astrocytes in the suprachiasmatic nucleus (SCN), the master component of the central circadian clock. GFAP staining in the SCN was significantly higher in rats having received implants that restored physiological plasma levels of corticosterone within diurnal or nocturnal limits than in non-normalized rats. The effects of corticosterone were similar in the parvocellular portion of the paraventricular nucleus but were opposite in the hippocampus, another major site of negative feed-back regulation of the hypothalamic-pituitary-adrenal axis, where a decreased GFAP staining was observed in discrete regions of the dentate gyrus. This indicates that glucocorticoids may positively or negatively regulate GFAP, depending on the target brain structure. In the SCN, that contains only few if any glucocorticoid receptors, indirect mechanisms that may involve serotoninergic neurons are probably responsible for the effects of corticosterone level. It is proposed that the corticosterone-induced increase in GFAP staining in that nucleus accounts for dynamic changes in neurone-astrocyte interactions that might occur in relation with natural fluctuations of glucocorticoids over the 24 h period.

Anatomical and functional reconstruction of the nigrostriatal system in vitro: selective innervation of the striatum by dopaminergic neurons

To study development of the nigrostriatal pathway in an in vitro model system, organotypic slices obtained from rat pups (P4) and containing the striatum and the cortex were grown together with apposed embryonic (E13.5) mesencephalic blocks according to the static slice culture method of Stoppini et al. (1991; J. Neurosci. Methods 37:173-182). Under these conditions, mesencephalic dopaminergic (DA) fibers rapidly grow through the slice, preferentially its striatal portion. This innervation provides a true synaptic innervation to the striatum, as shown by the presence of DA terminals on striatal neurons. DA fibers are able to exert a functional influence, as seen by their ability to modulate c-Fos expression in striatal neurons in the same way as in vivo. Thus, blockade, under basal conditions, of the effect of spontaneously released dopamine by the D2 receptor antagonist haloperidol leads to the activation of c-Fos expression in the striatum. Furthermore, stimulation of DA release by amphetamine induces striatal c-Fos expression in a D1 receptor-dependent manner. Next, the mechanisms of the selective striatal innervation were examined. Indeed, DA fibers innervated specifically the striatum, avoiding the cortical portion of the slice. This selectivity seems to be specific for DA neurons; no selectivity could be observed when noradrenergic neurons were substituted for DA neurons. Short-term cocultures in a collagen gel of mesencephalic blocks with striatal blocks failed to reveal any oriented outgrowth of DA fibers from the mesencephalon, suggesting that the selective innervation observed in the organotypic slices results from some contact-dependent, presumably adhesive interactions rather than from the presence of some diffusible substance orienting the growth of DA fibers towards the striatum. On the other hand, DA neurons seeded onto striatal slices did not attach selectively onto the striatal portion of the slice, indicating that the putative specific adhesive interactions governing the selective striatal innervation are not the same as those determining the adhesion of the DA neurons. These results show that cocultures of cortex-striatum and mesencephalic slices result in a system that displays a number of the morphological and functional traits of the normal nigrostriatal system and that can be relied on as a good in vitro model of in vivo development.

Vasoactive intestinal peptide neurons as synaptic targets for vasopressin neurons in the suprachiasmatic nucleus. Double-label immunocytochemical demonstration in the rat

Cellular relationships between neurons producing vasopressin or vasoactive intestinal peptide in the suprachiasmatic nucleus of the hypothalamus, the main component of the central circadian timing system in mammals, were investigated in the rat using double immunocytochemistry. Analysis of serial confocal images revealed that the vasopressin-synthesizing neurons not only are important targets for the vasoactive intestinal peptide-synthesizing neurons, as previously demonstrated, but also establish reciprocal axosomatic contacts with these neurons, which have never been reported. On average, 5.4 vasoactive intestinal peptide contacts per vasopressin perikaryon and 1.7 vasopressin contacts per vasoactive intestinal peptide perikaryon were counted. That both types of neurons are linked by reciprocal synapses was confirmed at the electron microscopic level using a combination of immunoperoxidase and immunogold-silver labeling. Existence of an anatomical substrate for a vasopressinergic control of the vasoactive intestinal peptide neurons may have important functional consequences. In view (i) of the presumed, direct or indirect, involvement of the vasopressin neurons in relaying pacemaker information within and outside the suprachiasmatic nucleus, and (ii) of the established role of the vasoactive intestinal peptide neurons as the main light-sensitive cells, it provides support for a neuronal mechanism through which the circadian clock may regulate inputs related to environmental messages. Our electron-microscopic data also extended earlier observations, pointing to the involvement of vasopressin and vasoactive intestinal peptide terminals in so-called double synapses that, conceivably, could regulate neuronal synchronization in the suprachiasmatic nucleus. A morphological basis for non-synaptic interactions that could be involved in ephaptic and/or paracrine communication between both types of peptidergic neurons is also reported.

Serotonin axons of the neostriatum show a higher affinity for striatal than for ventral mesencephalic transplants: a quantitative study in adult and immature recipient rats

We previously showed that grafts of fetal ventral mesencephalic tissue are practically not innervated by host serotonin (5-HT) axons after implantation into the striatum of rats aged more than 14 days, at variance with transplants of cortical or striatal tissue into the adult striatum, which are well innervated by these axons. Using 5-HT immunohistochemistry and in vitro [3H]5-HT uptake/autoradiography, we have examined and quantified the innervation of ventral mesencephalic versus striatal grafts several months after implantation into the striatum of neonatal (postnatal day 5 or P5), juvenile (P15), and adult rats. Ventral mesencephalic grafts implanted in P5 rats received a moderate 5-HT innervation, while similar grafts implanted in P15 or adult recipients were almost free of any 5-HT fibers (-80%, compared to P5). The density of 5-HT innervation showed a tendency toward higher values in striatal than in ventral mesencephalic grafts (1.6-2 times higher in P5 and adult recipients; 4 times higher in P15 recipients). The difference was more striking, and significant, when only the true striatal portions of the striatal grafts were considered, i.e., DARPP-32-immunopositive areas (4-5 times higher in P5 and adult recipients; 10 times higher in P15 recipients). Accordingly, these DARPP-32-positive areas were also more densely innervated than the DARPP-32-negative zones of the same grafts (3 times higher at any age). The 5-HT innervation density also decreased with increasing age of the recipients in DARPP-32-positive, as well as DARPP-32-negative compartments of the striatal grafts (-75% in adults), but this decrease appeared more gradual (-50% in juveniles) than with mesencephalic grafts. It is concluded that the 5-HT axons innervating the neostriatum have a better affinity for striatal grafts than for ventral mesencephalic grafts or the nonstriatal portions of striatal grafts. In adulthood, the relative affinity of these axons for the different types of grafts is maintained, even though their growth capacity decreases irrespective of the target tissue considered. This experimental model may prove useful for the identification of the receptors and ligands that are responsible for target recognition by 5-HT axons and to test the possibility that the progressive decrease of axonal growth capacity from neonatal age to adulthood be related to a downregulation of such molecules.

Post-lesion up-regulation of 5-HT1B binding sites in the suprachiasmatic nucleus may be reversed after spontaneous or graft-induced serotonin reinnervation

We have previously reported that selective axotomy of serotoninergic neurons produced by an intraventricular injection of 5, 7-dihydroxytryptamine is followed by an increase in 5-HT1B binding sites in the suprachiasmatic nucleus of the hypothalamus. This post-lesion up-regulation is shown here to be spontaneously reversed after long-term survival in spite of an incomplete reinnervation of the nucleus. Recovery may be accelerated by fetal raphe transplants that produce more rapid reinnervation.

Quantitative and morphometric data indicate precise cellular interactions between serotonin terminals and postsynaptic targets in rat substantia nigra

We have quantified the density of serotonin axonal varicosities, their synaptic incidence and their distribution among potential targets in the pars reticulata and pars compacta of the rat substantia nigra. Serotonin axonal varicosities, counted at the light microscopic level following in vitro [3H]serotonin uptake and autoradiography, amounted to 9 x 10(6)/mm3 in the pars reticulata and 6 x 10(6)/mm3 in the pars compacta, among the densest serotonin innervations in brain. As determined at the electron microscopic level following immunolabelling for serotonin, virtually all serotonin varicosities in the pars reticulata and 50% of those in the pars compacta formed a synapse, essentially with dendrites. The combination of serotonin immunocytochemistry with tyrosine hydroxylase immunolabelling of dopamine neurons reveals that 20% of the serotonin synaptic contacts in the pars reticulata are on dopamine dendrites and 6% are on a type of unlabelled dendrite characterized by its peculiarly high cytoplasmic content of microtubules. The comparison of the diameter of the dendritic profiles that were in synaptic contact with serotonin-immunoreactive varicosities with the diameter of all other dendritic profiles of the same type suggests that serotoninergic varicosities innervate dopamine dendrites uniformly along their length, whereas they tend to contact microtubule-filled dendrites in more proximal regions and the other, unidentified dendrites in more distal regions. Furthermore, the size of the serotonin-immunoreactive varicosities and of their synaptic junctions is significantly smaller on dopamine dendrites and larger on microtubule-filled dendrites than on other, unidentified dendrites, indicating that the nature of the postsynaptic target is an important determinant of synaptic dimensions. These data should help to clarify the role of serotonin in the nigral control of motor functions. They indicate that this dense serotonin input to the substantia nigra is very precisely organized, acting through both "non-junctional" and "junctional" modes of neurotransmission in the pars compacta, which projects to the neostriatum and the limbic system, whereas the predominant mode of serotonin transmission appears to be of the "junctional" type in the pars reticulata, where serotonin can finely control the motor output of the basal ganglia by acting on the GABA projection neurons either directly or through the local release of dopamine by dopaminergic dendrites. The data also raise the possibility that the postsynaptic targets have trophic retrograde influences on serotoninergic terminals.

Dual character, asynaptic and synaptic, of the dopamine innervation in adult rat neostriatum: a quantitative autoradiographic and immunocytochemical analysis

Dopamine (DA) axon terminals (varicosities) in the neostriatum of adult rats were examined for shape, size, content, synaptic incidence, type of junction, synaptic targets, and microenvironment after electron microscopic identification either by [3H]DA uptake autoradiography or by immunocytochemistry with monoclonal antibodies against DA-glutaraldehyde-protein conjugate. Both approaches yielded comparable results. Whether they were from the paraventricular or the mediodorsal neostriatum, respectively, the [3H]DA-labeled and DA-immunostained varicosities were generally oblong and relatively small; more than 60% contained one or more mitochondria. Sixty to seventy percent were asynaptic, and 30-40% were endowed with a synaptic membrane differentiation (junctional complex), as inferred by stereological extrapolation from single thin sections (both approaches) or observed directly in long, uninterrupted series of thin sections (immunocytochemistry). The synaptic DA varicosities always displayed symmetrical junctions: 67% with dendritic branches, 30% with dendritic spines, and 2-3% with neuronal cell bodies. DA varicosities juxtaposed to one another were frequent. Other axonal varicosities were more numerous in the immediate vicinity of DA varicosities than around randomly selected, unlabeled terminals. The respective microenvironments of DA and unlabeled varicosities also showed enrichment in the preferred synaptic targets of both groups of varicosities, with dendritic branches for DA and dendritic spines for the unlabeled ones. These data suggest a dual mode of operation that is diffuse as well as synaptic for the nigrostriatal DA system. In such a densely DA-innervated brain region, they also lead to the hypothesis that a basal level of extracellular DA might be maintained permanently around every tissue constituent and, thus, contribute to the mechanisms of action, properties, and functions (or dysfunctions) of DA within the neostriatum itself and as part of the basal ganglia circuitry.

Intrinsic organization and monoaminergic innervation of the suprachiasmatic nucleus transplanted to adult rats. A light- and electron-microscopic study

Light- and electron-microscopic immunocytochemistry was used to investigate grafts of foetal hypothalamic tissue implanted close to the site of the suprachiasmatic nucleus in adult rats with bilateral surgical ablation of this nucleus. The transplants contained vasoactive intestinal peptide and vasopressin cell clusters, which have previously been shown to characterize functional suprachiasmatic nucleus grafts. Vasoactive intestinal peptide and vasopressin neurons presented synaptic features that have not been described in the native suprachiasmatic nucleus. More specifically, their terminals within the graft were involved in 'double' synapses with separate unlabelled dendrites. Moreover, in dually stained sections, an unexpected synaptic investment of vasoactive intestinal peptide neurons by vasopressin endings was detected, which revealed reversed vasoactive intestinal peptide/vasopressin interactions compared to those described in the native nucleus. These observations could reflect some immature features of the grafted neurons. Ultrastructural relationships of monoaminergic fibres arising from host and/or intragraft neurons were also examined. Within the engrafted suprachiasmatic nucleus, tyrosine hydroxylase-labelled fibres, which probably belonged to cografted dopaminergic neurons, showed normal patterns of distribution and synaptic connections, with no preferential relationships with vasoactive intestinal peptide or vasopressin neurons. Serotoninergic axons arborized within transplants but, in agreement with previous data showing an inhibitory influence of the suprachiasmatic nucleus on ingrowing serotoninergic fibres, they had no predilection for the area corresponding to that nucleus. In spite of their relative scarcity, serotoninergic fibres within the engrafted suprachiasmatic nucleus showed an almost normal synaptic incidence, but synapses were not predominantly shared with the vasoactive intestinal peptide neurons, known to be their major targets in the native nucleus. This may contribute not only to the failure of functional grafts to synchronize with environmental conditions, but also to the inability of transplants to restore hormonal rhythms such as estrous cyclicity.

Efficient immunodetection of various protein antigens in glutaraldehyde-fixed brain tissue

Optimal ultrastructural preservation of brain tissue for electron microscopy is best achieved with fixatives containing high concentrations of glutaraldehyde, which is generally considered detrimental to the immunogenicity of most protein antigens. We tested seventeen mono- or polyclonal antibodies against peptide or protein antigens, including a majority for which immunoreactivity had previously been reported to be sensitive to glutaraldehyde fixation. Forebrain sections of rats or mice fixed by perfusion with 3.5% glutaraldehyde were processed for pre-embedding immunocytochemistry by the avidin-biotin method. The resulting immunostaining was in most cases at least similar to that obtained in sections fixed with paraformaldehyde. Immunoreactivity against the mouse or human neurofilament protein NF-L was even improved, being similar to that previously reported for unfixed brain tissue. Of all antigens tested, only choline acetyltransferase, phenylethanolamine-N-methyl transferase, and neuropeptide Y were detected with lower sensitivity than after paraformaldehyde fixation, which was attributed to a rather restricted penetration of the primary antibody into glutaraldehyde-fixed tissue sections. These results indicate that glutaraldehyde may be envisaged as a possible fixative for optimal immunocytochemical detection of any tissue antigen at the electron microscopic level, including antigens which, on the basis of results obtained after fixation with paraformaldehyde-glutaraldehyde mixtures, were considered highly sensitive to glutaraldehyde fixation.

Catecholaminergic innervation of the suprachiasmatic nucleus in the adult rat: ultrastructural relationships with neurons containing vasoactive intestinal peptide or vasopressin

Catecholaminergic fibers in the suprachiasmatic nucleus of adult rats were investigated by use of light- and electron-microscopic immunocytochemistry. The suprachiasmatic nucleus receives a modest density of tyrosine hydroxylase-containing axons, homogeneously distributed in the nucleus and forming varicosities throughout its entire rostro-caudal extension. Immunolabeling with antibodies against dopamine showed that this catecholamine input comprises a dopaminergic component. Many tyrosine hydroxylase-positive cells were localized at the immediate periphery of the suprachiasmatic nucleus. With electron-microscopic examination, dendrites of these neurons were found within the limits of the nucleus as well as at a border zone between the suprachiasmatic nucleus proper and the optic tract where they received unlabeled synapses, providing a morphological support for a possible role of dopaminergic neurons in the integration and/or transfer of light-related signals. More than 91% of catecholaminergic axonal varicosities were found to establish morphologically defined synapses with dendrites. To investigate whether these synapses might be shared with neurons of one or both of the two main peptidergic populations of the nucleus, namely vasoactive intestinal peptide- and vasopressin-containing neurons, we carried out double-labelling experiments combining immunoperoxidase and immunogold-silver labeling. Results showed only a few cases of direct association of the catecholaminergic terminals with these peptidergic categories. In both types of dually stained sections, catecholaminergic synapses were preferentially made with unlabeled dendrites. The homogeneous distribution of tyrosine hydroxylase-immunoreactive fibers in the suprachiasmatic nucleus could therefore reflect a lack of significant catecholaminergic innervation of both vasoactive intestinal peptide- and vasopressin-synthesizing neurons.

Synaptic connectivity of serotonin graft efferents in the suprachiasmatic and supraoptic nuclei of the hypothalamus

We have previously reported that a cell suspension from the rostral part of the embryonic raphe grafted to the basal hypothalamus of 5,7-dihydroxytryptamine-denervated rats produced incomplete serotonin (5-HT) re-innervation of the suprachiasmatic nucleus (SCN) as opposed to hyper-innervation of the supraoptic nucleus (SON). We took advantage of this experimental model to investigate whether the graft-derived, 5-HT fibres retained normal ultrastructural features, and, particularly, a normal density of synaptic junctions, irrespective of the extent of target re-innervation. The intrinsic features of immunostained, graft-derived 5-HT axonal varicosities in both the SCN (ventral portion) and the SON were essentially similar to those exhibited by the respective endogenous innervation. Analysis of well-preserved varicosities in uninterrupted series of thin sections allowed us to evaluate directly the proportions of junctional to non-junctional 5-HT varicosities in both regions. Synaptic incidences were also remarkably conserved after grafting (45.5% in the SCN versus 38.5% in the SON; 48% and 38% in normal rats, respectively). Synapses were primarily reestablished on dendritic shafts, which also were identified as the major post-synaptic targets of the normal 5-HT innervations. We noted, however, a tendency toward increased numbers of symmetrical versus asymmetrical synapses in both the SCN and SON of grafted rats. Thus, irrespective of whether hypo- or hyper-innervation patterns developed post-grafting, the transplanted 5-HT neurons essentially retained normal ultrastructural features in their target territories, with a normal incidence of synaptic junctions. The data provide further support to the hypothesis that the innervation territory is the major determinant of the frequency with which ingrowing 5-HT fibres make synaptic junctions.

Catecholaminergic projections from the solitary tract nucleus to the perifornical hypothalamus

The source of adrenergic and other catecholaminergic fibers innervating the perifornical lateral hypothalamus was localized in the medulla after combination of Fluoro-Gold retrograde tracing and immunohistochemistry for either tyrosine-hydroxylase or phenylethanolamine-N-methyltransferase. Following perifornical injections, Fluoro-Gold-labeled neurons were observed mainly in regions including the noradrenergic and adrenergic cell groups. In the caudal solitary tract nucleus, two kinds of doubly labeled neurons were found: a) numerous noradrenergic neurons in the A2 group at the level of, or caudal to the area postrema; b) some adrenergic neurons in the C2 group at a level immediately rostral to the area postrema. These catecholaminergic neurons connecting the caudal solitary tract nucleus to the perifornical hypothalamus might convey feeding relevant information such as glycemic level or satiety signals.

Effect of changes in the intrahippocampal vasopressin on memory retrieval and relearning

Previous results have indicated the involvement of the hippocampus in the behavioral effect of vasopressin, with a better effect when the peptide was injected in the ventral part rather than in the dorsal part of this structure. The purpose of the present study was to determine, in mice, whether the injection of vasopressin or vasopressin antisera into the ventral hippocampus has an effect on retrieval and relearning of a Go-No Go visual discrimination task and, if so, to what extent this involvement of the vasopressin system depends on the integrity of the medial amygdaloid nucleus, the main source of vasopressin innervation in the ventral hippocampus in rats. In the first experiment, we showed that pretest microinjection of Arg8-vasopressin (25 pg per animal) in the ventral hippocampus alleviated forgetting observed after a prolonged interval of 24 days between the acquisition of information and its retrieval. This enhancing effect was characterized by better retrieval and relearning in vasopressin-treated mice than those in control mice. Conversely, an immunoneutralization of endogenous vasopressin in the ventral hippocampus by the microinjection of vasopressin antisera (1/10 dilution) resulted in the drastic impairment of retrieval and relearning. Since the lack of an observable change in a locomotor activity test might explain these results, we postulated that the vasopressin system in the ventral hippocampus is involved in retrieval processes. Moreover, the effects of these treatments in a nonassociative context suggest that the effect of vasopressin could be dependent on the contextual paradigm used. In the second experiment, we localized vasopressin immunoreactive fibers in the CA1-CA2 ventral hippocampal fields and CA4-gyrus dentatus region, and vasopressin perikarya in the medial amygdaloid nucleus. Then, the projection of vasopressin cells from the medial amygdaloid nucleus to the ventral hippocampus was evaluated by studying changes in vasopressin immunoreactive fiber density in the ventral hippocampus after a lesion of the medial amygdaloid nucleus. The results showed the almost complete disappearance of vasopressin fibers in the CA1-CA2 hippocampal fields after the medial amygdaloid lesion. In contrast, vasopressin fibers in the CA4 and gyrus dentatus region remain unchanged. On the basis of our immunohistochemical results, our third experiment tested the repercussions of the change in vasopressin innervation in the ventral hippocampus, due to the medial amygdaloid lesion, on the effects of exogenously administered vasopressin on both retrieval and relearning processes. The medial amygdaloid lesion induced a deleterious effect on retrieval without really affecting the ability to relearn. No observable change in locomotor activity could explain this impairment.(ABSTRACT TRUNCATED AT 400 WORDS)

Convergent serotonin and GABA innervation of VIP neurons in the suprachiasmatic nucleus demonstrated by triple labeling in the rat

By means of a combination of serotonin (5-HT) uptake radioautography and dual immunocytochemistry, vasoactive intestinal polypeptide (VIP) neurons in the suprachiasmatic nucleus are demonstrated to simultaneously receive both 5-HT and GABA afferents at their somatic and dendritic levels. These data constitute a further step towards the improved characterization of the morphological substrate of the integrative function of these neurons, which are known to play an important role in the delivery of light-mediated rhythmic signals to other parts of the brain.

Long-term serotonin reinnervation of the suprachiasmatic nucleus after 5,7-dihydroxytryptamine axotomy in the adult rat

We present here immunohistochemical evidence that serotonin (5-HT) reinnervation in the suprachiasmatic nucleus (SCN) of adult rats, after intraventricular administration of 5,7-dihydroxytryptamine, is more extensive than formerly believed. The first fibers appeared at 4 months and developed until more than 7 months. However, comparison with age-matched controls showed that, even 24 months post-lesion, the density of 5-HT innervation remained abnormally low. At the electron microscopic level, many synaptic contacts were found to have been re-established. The cellular mechanisms possibly responsible for incomplete 5-HT reinnervation in the SCN are discussed.

Co-localization of tyrosine hydroxylase, GABA and neuropeptide Y within axon terminals innervating the intermediate lobe of the frog Rana ridibunda

Possible co-existence of gamma-aminobutyric acid (GABA), catecholamines, and neuropeptide Y (NPY) in the same nerve terminals of the frog intermediate lobe was investigated by immunocytochemistry at the electron microscopic level. Co-localization of GABA and tyrosine hydroxylase (TH) was studied by using a double immunogold labeling procedure. Co-localization of glutamate decarboxylase (GAD) and NPY was studied by combining, respectively, the peroxidase-antiperoxidase method and a radioimmunocytochemical labeling procedure. Catecholamines and GABA were systematically co-localized in nerve endings of the pars intermedia. Most of the NPY-immunoreactive fibers also contained GAD-like immunoreactivity. However, a few NPY-positive nerve terminals were not immunoreactive for GAD. These data provide evidence for co-existence of a regulatory peptide (NPY) and several neurotransmitters (i.e., GABA and catecholamines) within the same axon terminals in the intermediate lobe. Since GABA, dopamine, and NPY have all been shown to inhibit the activity of frog melanotrope cells, the present findings suggest that these neuroendocrine factors may interact either at the pre-synaptic or post-synaptic level.

Association of neurotensin receptors with VIP-containing neurons and serotonin-containing axons in the suprachiasmatic nucleus of the rat

The aim of the present study was to identify cellular elements bearing high-affinity neurotensin (NT) binding sites in the suprachiasmatic nucleus (SCN) of the rat hypothalamus. Because the distribution of these binding sites had previously been reported to conform to that of both vasoactive intestinal peptide (VIP)-containing nerve cell bodies and serotonin (5-HT)-containing axons, the following experimental approaches were used: (1) the overlap between autoradiographically labeled NT binding sites and immunocytochemically labeled VIP neurons was examined in adjacent 5-microns-thick sections taken across the entire rostrocaudal extent of the SCN; and (2) the density of NT binding sites was examined by quantitative autoradiography following cytotoxic lesioning of 5-HT afferents. Double-labeling studies demonstrated precise overlap between 125I-NT binding and VIP immunostaining throughout the SCN. Moreover, at high magnification intensely VIP-immunoreactive neurons were found in direct register with 125I-NT-labeled cells visualized in adjacent sections. Densitometric autoradiographic studies demonstrated a significant reduction in specific 125I-NT binding within the SCN following intracerebroventricular injection of the neurotoxin, 5,7-dihydroxytryptamine. Taken together, these results indicate that within the SCN, NT receptors are present both presynaptically on serotonin axons and postsynaptically on the perikarya and dendrites of VIP-containing neurons.

Dual immunocytochemistry using 125I-labeled protein A: a new electron microscopic technique applied to the investigation of chemical connectivity and axonal transmitter co-localization in the brain

We have developed a double labeling immunocytochemical method utilizing peroxidase conjugated Fab fragments and 125I-labeled protein A to localize two neuronal markers on the same light or electron microscopic section with primary antibodies raised in the same animal species. The technique is applicable to the study of chemical connectivity in the brain, as illustrated by data obtained in the hypothalamus using rabbit polyclonal antisera against tyrosine hydroxylase (TH), phenylethanolamine-N-methyltransferase (PNMT), neuropeptide Y (NPY), and vasoactive intestinal peptide (VIP). Moreover, due to a high level of sensitivity and resolution, the technique offers considerable advantages over many previously developed dual labeling immunocytochemical methods for the demonstration of transmitter axonal co-localizations. Utilizing the peroxidase Fab/[125I]protein A method, we present here the first direct evidence that PNMT is present in many endings also containing NPY in the thalamic and hypothalamic paraventricular nuclei and in the arcuate nucleus. The method also may be combined as required with other labeling methods for localizing more than two neurochemical markers on one and the same electron microscopic section.

GABA neurons in the rat suprachiasmatic nucleus: involvement in chemospecific synaptic circuitry and evidence for GAD-peptide colocalization

Dual labelling methods were employed for the electron microscopic detection of glutamate decarboxylase (GAD) immunoreactivity, together with vasoactive intestinal peptide (VIP) or neuropeptide Y (NPY) immunoreactivity in the suprachiasmatic nucleus (SCN) of colchicine pretreated and untreated rats. These methods involved the combined use of diaminobenzidine and benzidine dihydrochloride as distinct chromogens to visualize peroxidase-anti-peroxidase (PAP) immunostaining, and a combination of the PAP procedure with a radioimmunocytochemical method employing 125I-labelled secondary antisera. We were thereby able to demonstrate that gamma-aminobutyric acid (GABA) terminals provide an important afferent synaptic input to VIP neurons. Some of these VIP-immunoreactive neurons also exhibited GAD immunoreactivity. Examples of direct appositions between GABA and NPY terminals, and of a convergence of the two types of terminals on to the same postsynaptic targets, were frequently encountered. NPY/GAD colocalization within a few axonal varicosities was also demonstrated. These data provide additional information concerning chemospecific neuronal interactions that could be of functional importance in the regulation of circadian rhythmicity at the level of the SCN.

Serotonin reinnervation of the rat organum vasculosum laminae terminalis (OVLT) after 5,7-dihydroxytryptamine deafferentation

The time course and pattern of serotonin (5-HT) reinnervation in the rat organum vasculosum laminae terminalis (OVLT) following intracerebroventricular administration of 5,7-dihydroxytryptamine were examined by means of [3H]5-HT uptake radioautography. 5-HT axonal varicosities reappeared in the OVLT within 3 months post-lesion. Six months later, they were found to have preferentially reinvested the neurohemal contact area (juxtavascular zone) of the organ. Regenerated terminals further increased in number. At the end of a 16-month survival time, they displayed normal-looking distributional patterns, similar to those of age-matched control animals, and formed new synaptic junctions in the juxtaventricular zone. The cellular mechanisms possibly responsible for 5-HT recovery in the OVLT are discussed.

Adrenergic innervation of noradrenergic locus coeruleus neurons. A dual labeling immunocytochemical study in the rat

By means of dual immunocytochemistry, synaptic associations between adrenergic terminals and noradrenergic neurons were directly demonstrated in the rat locus ceruleus (LC). It could be estimated that every adrenergic afferent contacts at least one noradrenergic dendrite in the nucleus. An adrenergic innervation of non-noradrenergic targets was also evidenced. These data add to our knowledge on the synaptic circuitry by which activation of the adrenergic input could affect central mechanisms known to be influenced by LC neurons.

Are adrenergic neurons subject to a serotoninergic influence in the nucleus tractus solitarii? A morphological and biochemical study in the rat

The possible relationships between adrenaline-synthesizing neurons and serotoninergic afferent fibers in the nucleus tractus solitarii of the rat were investigated both morphologically and biochemically. Adrenergic elements (cell bodies, dendrites and nerve endings) were detected simultaneously with serotoninergic axonal varicosities in the same electron-microscopic sections by means of combined phenylethanolamine-N-methyltransferase immunocytochemistry and [3H]serotonin-uptake radioautography. Among some 500 serotoninergic varicosities scanned in the areas of significant overlap between the 2 types of labeling, only 3 were directly apposed to an adrenergic process, identified as a dendrite in each case. No synaptic membrane differentiations were seen at these occasional sites of contact. Destruction of the serotonin input by 5,7-dihydroxytryptamine had no significant effect on the tyrosine hydroxylase dopamine-beta-hydroxylase and phenylethanolamine-N-methyltransferase enzymatic activities in the C2 adrenergic region, but induced 22% and 38% increases of tyrosine hydroxylase and dopamine-beta-hydroxylase activities, respectively, in the neighboring A2 noradrenergic area. Taken together, these results suggest that serotoninergic and adrenergic neurons do not significantly interact in the nucleus tractus solitarii; this implies that the possible catecholaminergic relays for the action of serotonin in autonomic regulation at this level could consist of noradrenergic neurons rather than of their adrenergic counterparts.

Host afferents into intrastriatal transplants of fetal ventral mesencephalon

Host afferents into fetal ventral mesencephalic tissue grafted to the neostriatum of adult rats have been studied by using anterograde transport of Phaseolus vulgaris leucoagglutinin (PHA-L) and immunocytochemistry for serotonin (5-HT), Substance P (SP), and dopamine-adenosine 3':5'-monophosphate-regulated-phosphoprotein-32 (DARPP-32). Numerous fibers of cortical origin were detected in the transplants following multiple (11-15) iontophoretic injections of PHA-L into the frontal and anterior cingulate cortex. The labeled fibers occurred with an apparently random distribution throughout the graft tissue. Their overall density was lower than that of the surrounding striatum but similar to that found in the host nigra-ventral tegmental area. The majority of the PHA-L-labeled fibers in the grafts were thin and tortuous with varicosities or lateral clubs with terminal boutons. Dual labeling showed frequent close appositions between PHA-L-labeled terminals and dopamine-immunoreactive cell bodies. In parallel electron microscopy, synaptic contacts were observed between PHA-L-labeled terminals and unlabeled neuronal profiles in the graft. Other labeled fibers in the grafts were thick and smooth, corresponding probably to labeled myelinated axons observed in the electron microscope. These thick fibers were often seen to give off collaterals of the thin type. The virtual absence of such thick fibers in the normal striatal neuropil suggests that at least some of the cortical afferents to the grafts may have sprouted from axons normally projecting to diencephalic or brain stem regions. Serotonin fibers occurred in patches or as scattered single fibers in both deep and superficial portions of the nigral transplants. In the electron microscope some of these terminals were seen to establish synaptic contacts with nonimmunoreactive elements in the graft. These fibers were present also when the graft tissue had been pretreated with 5,7-dihydroxytryptamine at the time of transplantation. This treatment eliminated all 5-HT-containing neurons from the grafts without any noticeable adverse effect on the survival of the dopaminergic neurons. The serotonin fibers in the grafts were thus most likely of host origin. SP-positive fibers formed a dense plexus inside the grafts. Since many SP-positive cell bodies were visualized inside the transplant after colchicine pretreatment, it is unclear, however, whether any of these fibers were of host origin. Intrastriatal injections of PHA-L or DARPP-32 immunocytochemistry indicated that the deep portions of the nigral grafts were entirely devoid of host striatal afferents.(ABSTRACT TRUNCATED AT 400 WORDS)

Ultrastructural correlates of functional relationships between nigral dopaminergic or cortical afferent fibers and neuropeptide Y-containing neurons in the rat striatum

This study examines the ultrastructural relationships established by the nigrostriatal dopaminergic and the corticostriatal afferent fibers with neuropeptide Y (NPY)-containing neurons in the rat striatum. By means of dual immunolabeling procedures using peroxidase conjugated F(ab) fragments and 125I-labeled protein A, direct appositions and morphologically defined synaptic contacts of the symmetrical type were visualized between tyrosine hydroxylase-labeled nerve terminals and NPY-labeled neurons. After deafferentation of the striatum from its cortical input direct appositions and asymmetrical synaptic contacts were evidenced between characteristic degenerative boutons and NPY-positive neurons in the striatum. These results suggest that striatal NPY interneurons undergo direct influence from both nigrostriatal dopaminergic and corticostriatal neuronal systems.

Adrenergic neurons in the nucleus tractus solitarius receive GABAergic synapses. Demonstration by dual immunocytochemistry in the rat

By means of a dual immunocytochemical labeling for phenylethanolamine-N-methyltransferase and glutamate decarboxylase, synaptic associations between adrenaline-synthesizing neurons and GABAergic terminals are demonstrated in the medial nucleus tractus solitarius of the rat. These relationships could constitute an anatomical substrate for the presumed role of GABA in modulation of baroreceptor reflexes at this level.

Ultrastructural relationships of serotonin and GABA terminals in the rat suprachiasmatic nucleus. Evidence for a close interconnection between the two afferent systems

Serotonin (5-HT) and gamma-aminobutyric acid (GABA) nerve endings were identified in the rat suprachiasmatic nucleus (SCN) by combined [3H]5-HT uptake radioautography and glutamate decarboxylase (GAD) immunocytochemistry at the electron microscope level. In areas of good overlap between radioautographic and immunocytochemical labellings, there were no axonal varicosities exhibiting both labellings, indicating that 5-HT and GABA are not co-localized in the SCN. The systematic survey in these areas of all profiles that had accumulated [3H]5-HT and of all GAD-immunoreactive varicosities allowed the analysis of 247 of the former and 896, i.e. an almost four-fold greater number, of the latter. This seems concordant with the view that GABA endings would be the most numerous of all classes of nerve terminals so far identified in the SCN. More than 22% of the [3H]5-HT labelled profiles showed the membrane specialization typically associated with synaptic junctions. Thereby, it was possible to evaluate that about 45% of the 5-HT terminals actually form a synapse in the SCN. Some 37% of the GAD-positive varicose profiles which could be formally interpreted also showed well differentiated synaptic contacts, suggesting that the GABAergic innervation of the SCN could be entirely junctional. Whereas 5-HT terminals usually innervated only one dendritic or somatic element, a convergence of several GABAergic terminals onto the same postsynaptic target also receiving a 5-HT input was frequently observed.(ABSTRACT TRUNCATED AT 250 WORDS)

Central serotonergic projections to the nucleus tractus solitarii: evidence from a double labeling study in the rat

Projections from several brainstem serotonergic nuclei to the nucleus tractus solitarii were investigated in the rat. Experiments were performed using a double labeling method combining retrograde radioautographic tracing and serotonin immunohistochemistry. After injection of the radioactive tracer ([3H] wheat germ agglutinin) into the lateral nucleus tractus solitarii, nerve cell bodies exhibiting both radioautographic labeling and immunostaining were detected in all the serotonergic nuclei investigated, namely the nucleus raphe magnus, the ventromedial paragigantocellular nucleus, the nuclei raphe pontis, medianus and dorsalis, the medial lemniscus and the reticulotegmental nucleus of the pons. Most of the double labeled perikarya observed were in the nucleus raphe magnus, the adjacent part of the paragigantocellular nucleus and the nucleus raphe dorsalis. Nerve cell bodies retrogradely labeled but devoid of immunostaining were also observed, together with the double labeled perikarya, within serotonergic nuclei. These results provide direct evidence that brainstem serotonergic neurons contribute to the innervation of the nucleus tractus solitarii. They indicate that the nucleus raphe magnus and the nucleus raphe dorsalis constitute two major sources of central serotonergic projections to the nucleus tractus solitarii.

Monoamine innervation of the organum vasculosum laminae terminalis (OVLT): a high resolution radioautographic study in the rat

The monoamine innervation of the organum vasculosum laminae terminalis (OVLT) was examined in the adult rat by light and electron microscope radioautography after intraventricular administration of tritiated serotonin [( 3H]5-HT) or dopamine [( 3H]DA). Radioautographic and biochemical controls after 5,7-dihydroxytryptamine or 6-hydroxydopamine lesioning established the respective serotonin (5-HT) and catecholamine (CA) identities of the axonal varicosities labeled under the conditions of the present experiments. For descriptive purposes, the OVLT was subdivided in three parts: two parenchymal zones, one juxtaventricular, the other juxtavascular, and the vascular core. Almost 10% of all axonal varicosities in the OVLT were found to be labeled with [3H]5-HT. This 5-HT innervation was most prominent in the rostrocaudal and ventrodorsal portions of the juxtaventricular zone and the dorsal aspect of the juxtavascular zone; there was none in the vascular core. [3H]DA-labeled varicosities were much less abundant and yet more numerous than earlier histofluorescent and immunohistochemical studies would have predicted. They predominated in the juxtavascular zone, where a majority presumably had a dopamine (DA) rather than a noradrenaline identity. Some were also found in the vascular core, where they most likely corresponded to peripheral autonomic noradrenaline endings. In the juxtaventricular zone of the OVLT, a significant proportion of the [3H]5-HT-labeled varicosity profiles could be observed to form axodendritic synapses, but in the juxtavascular zone no 5-HT or any [3H]DA-labeled ones were ever seen in synaptic junction. In the juxtavascular zone, the 5-HT and the presumed DA endings established close relationships with neurosecretory axons, and with astrocytic or tanycytic processes on which they occasionally formed "synaptoid contacts." A few endings of either type were also seen to about directly on the outer basement membrane of the perivascular space. It therefore appears probable that in OVLT monoamines influence neural and nonneural elements. At a proximal level of regulation (juxtaventricular zone), 5-HT could act both synaptically and nonsynaptically as an interneuronal transmitter or modulator. In contrast, distally (juxtavascular zone), both DA and 5-HT could be released as neurohormones in addition to modulating neurosecretion. 5-HT and DA varicosities in the OVLT could also behave as sensors for circulating factors that do not cross the blood-brain barrier.

Putative neurotransmitters in the red nucleus and their involvement in postlesion adaptive mechanisms

A variety of putative neurotransmitters has been described in the red nucleus (RN). Measurement of neurotransmitter biochemical markers and study of their specific localizations using morphological techniques in lesion and deafferentation of the RN indicate the participation of glutamate (Glu) in corticorubral transmission and the presence of GABA in RN intrinsic neurones. The cerebellorubral projection may contain at least two populations of fibres, the one using acetylcholine and the other Glu as neurotransmitter. The presence of a serotoninergic input was also demonstrated. Selective deafferentations of the RN, particularly from its cerebellar input, result in biochemical and immunohistochemical responses indicative of increased corticorubral glutamatergic and local GABAergic transmission. These adaptive changes of neuronal transmission as well as the previously described sprouting of corticorubral nerve terminals may contribute to functional recovery after cerebellectomy in adult animals.

Serotonin innervation of neuropeptide Y-containing neurons in the rat arcuate nucleus

Structural non-synaptic appositions between serotonin (5-HT) nerve endings and neuropeptide Y (NPY)-containing neurons were demonstrated in the rat arcuate nucleus by means of a combined radioautographic and immunocytochemical detection of [3H]5-HT uptake sites and NPY-immunoreactivity. Such cellular relationships are proposed to constitute a morphological substrate for putative 5-HT/NPY interactions in neuroendocrine hypothalamus.

Morphological correlates of serotonin-neuropeptide Y interactions in the rat suprachiasmatic nucleus: combined radioautographic and immunocytochemical data

The morphological substrate of putative serotonin (5-HT)/neuropeptide Y (NPY) interactions in the suprachiasmatic nucleus (SCN) was investigated by combined radioautography and immunocytochemistry after intraventricular administration of (3H)5-HT in the rat. In the ventral portion of the SCN, the distribution of (3H)5-HT uptake sites overlapped closely the NPY-immunoreactive terminals. Previous investigations have shown that the dense 5-HT and NPY innervations of the SCN originate in different structures, i.e., the midbrain raphe nuclei and the ventral lateral geniculate nucleus, respectively. Accordingly, in the present study, destruction of 5-HT afferents by 5,7-dihydroxytryptamine was not found to induce any modification in NPY staining and, in ultrastructural immuno-radioautographic preparations, two distinct pools of axonal varicosities could be identified. Both 5-HT and NPY terminals established morphologically defined synaptic junctions, sometimes on the same neuronal target. Some cases of direct axo-axonic appositions between the two types of terminals were also encountered. These data constitute additional criteria for characterizing the cytological basis of the multiple transmitter interactions presumably involved in the function of the SCN as a central regulator of circadian biological rhythms.

GABA nerve endings in the rat red nucleus combined detection with serotonin terminals using dual immunocytochemistry

Immunocytochemical methods were used to examine the ultrastructural features and cellular interrelationships of GABA and serotonin afferent fibers to the rat red nucleus. GABAergic nerve endings were identified in two ways, either using a pre-embedding immunoperoxidase procedure with an antibody against glutamate decarboxylase, the GABA-synthesizing enzyme, or after post-embedding immunogold labelling with an anti-GABA antibody. With the latter approach, it was possible to simultaneously visualize the GABAergic and serotoninergic innervation of the red nucleus (magnocellular part) in electron microscope preparations. This procedure involved GABA labelling of ultrathin sections obtained from specimens previously immunostained for serotonin using the pre-embedding peroxidase-antiperoxidase technique. The doubly stained sections showed gold and peroxidase markers to be present in two distinct populations of axonal varicosities. Unlike the GABAergic nerve endings, which were found to be profusely distributed throughout the nucleus, the serotonin nerve endings were relatively scarce. They contacted dendrites of large-sized neurons usually endowed with several GABA-gold labelled terminals. Not uncommonly, direct appositions between serotonin and GABA-positive terminals were also encountered. These data provide morphological evidence that red nucleus outputs may be dually regulated by GABAergic and serotoninergic afferents, while suggesting that presynaptic GABA/serotonin interactions might also play a significant part in red nucleus functions.

Electron-microscopic characterization of adrenergic axon terminals in the diencephalon of the rat

The fine structure of adrenergic axon terminals was examined in the paraventricular nucleus of the thalamus (PNT) and in the hypothalamic arcuate nucleus-median eminence (ARC-ME) complex by use of phenylethanolamine-N-methyl transferase (PNMT) immunocytochemistry. In the PNT, immunoreactive terminals formed a dense and well-circumscribed plexus. In the ARC, labeled varicosities were less numerous and more evenly distributed. In the ME, they were scarce and confined to the inner zone. In all these areas, the diameter of immunoreactive varicosities ranged between 0.2 and 1.3 micron; in the ME and in the transitional zone between the ARC and the ME, a population of larger boutons (greater than 2 microns) was also visible. All immunoreactive varicosities exhibited densely packed small, clear vesicles associated with a few large granular vesicles. In the PNT and the ARC, but not in the ME, they formed synaptic contacts with dendritic elements and were occasionally apposed to neuronal cell bodies. These axo-somatic appositions showed no junctional specializations. In the ME and transitional zone, immunoreactive terminals were frequently juxtaposed to, and occasionally established differentiated synaptic contacts with, tanycytes. These data support a transmitter role for adrenaline in the diencephalon and suggest that adrenaline plays a role in hypothalamo-hypophysiotropic regulation through interactions with neural and glial elements.

Immunohistochemical evidence of plasticity of gamma-aminobutyric acid neurons in the red nucleus and adjacent reticular formation after contralateral cerebellectomy in the adult cat

A comparative mapping of gamma-aminobutyric acid neurons identified by means of glutamic acid decarboxylase (GAD) immunohistochemistry was performed in the red nucleus (RN) in both intact and hemicerebellectomized adult cats (21 days postoperative). In the deafferented RN (contralateral to the lesion) as well as in the adjacent dorsolateral reticular formation, a marked increase in the number of GAD-positive perikarya was observed. In this mesencephalic area, some neurons may therefore increase their endogenous levels of immunodetectable GAD, as a response to cerebellar deafferentation. This can be viewed as one of the events contributing to functional recovery.

Striatal neuropeptide Y neurones are under the influence of the nigrostriatal dopaminergic pathway: immunohistochemical evidence

Selective unilateral 6-hydroxydopamine lesion of the nigrostriatal dopaminergic neurones in rats resulted, 12 to 90 days later, in a marked increase in the number of neuropeptide Y (NPY)-immunoreactive perikarya in the ipsilateral deafferented striatum compared to the contralateral intact side in the same animal and to both sides in control animals. The staining intensity of most positive cells also appeared to be enhanced in the deafferented striatum. These effects which can be interpreted as resulting from an increase in the intraneuronal levels of NPY may be accounted for by an increased production or a decreased metabolization of the peptide in the striatum deafferented from its dopaminergic input. The results therefore provide morphological evidence that the NPY-containing neuronal system of the striatum may undergo tonic influence from nigrostriatal dopaminergic afferents.

VIP neurons as prime synaptic targets for serotonin afferents in rat suprachiasmatic nucleus: a combined radioautographic and immunocytochemical study

Cellular relationships between serotonin (5-HT) axon terminals and neurons containing vasoactive intestinal peptide (VIP) were characterized by combined radioautography and immunocytochemistry in rat suprachiasmatic nucleus (SCN). Light microscopic immunoradioautographs showed significant overlap between (3H)5-HT uptake sites and VIP-immunoreactive elements in the ventral half of the SCN. Of the 255 (3H)5-HT-labelled axonal profiles detected in a systematic electron microscopic survey of single thin sections from this area, 75 (30%) were directly apposed to VIP-immunoreactive nerve cell bodies and/or dendrites. Radioautographically labelled 5-HT varicosities often showed well-differentiated, symmetrical or asymmetrical synaptic junctions, 60% of which were established on VIP-immunoreactive nerve cell bodies or dendrites. In a separate sampling of 198(3H)5-HT-labelled terminals seen in apposition with VIP-immunoreactive elements, 50 showed a junctional complex at the site of contact. Postsynaptic immunoreactive elements were mostly dendrites but also included nerve cell bodies. Despite the methodological limitations inherent to the present double labelling approach, these data strongly support the view that VIP neurons are prime synaptic targets for 5-HT afferents in the SCN. VIP/5-HT interactions are thus likely to play an important functional role in this nucleus and may in particular subserve the 5-HT mediated regulation of certain circadian rhythms, including that of pituitary hormone secretion.

Glutamate decarboxylase-immunoreactive boutons in synaptic contacts with hypothalamic dopaminergic cells: a light and electron microscopy study combining immunocytochemistry and radioautography

Double post-embedding immunolabeling of both tyrosine hydroxylase and glutamate decarboxylase on 1-micron semi-thin sections allowed the visualization of numerous endings that use gamma-aminobutyrate as a transmitter apposed to dopaminergic cell bodies in the periventricular-arcuate hypothalamic complex. Up to fifteen glutamate decarboxylase-positive contacts per tyrosine hydroxylase-positive cell profile could be observed. In some favourable planes of section glutamate decarboxylase-positive endings were also seen in close apposition to proximal dopaminergic dendrites. About 250 tyrosine hydroxylase-positive cell profiles, whose diameter approached the maximum diameter of the dopaminergic cells, were surveyed. An average of 7.4 glutamate decarboxylase-positive contacts were counted on these profiles. From these figures it was estimated that a dopaminergic cell body was contacted on average by 75-175 terminals that use gamma-aminobutyrate as a transmitter. At the electron-microscopic level, the nature of these contacts was investigated by a method combining radioautographic detection of cell bodies having taken up tritiated dopamine and pre-embedding immunostaining of glutamate decarboxylase containing endings. Glutamate decarboxylase-positive axon terminals were seen apposed to somatic and dendritic elements. On some favorable planes of section, they were found to be engaged in morphologically defined synaptic complexes of the symmetrical or asymmetrical type. A number of the postsynaptic perikarya were labelled by tritiated dopamine and, in agreement with the light microscopic observations, they were frequently seen in contact with more than one immunopositive ending. The present findings provide a morphological substratum for a direct gamma-aminobutyrate control of the tuberoinfundibular dopaminergic neurons. Such a control could account more particularly for the central, stimulatory effects of gamma-aminobutyrate on prolactin secretion.

GABA neurons in the cat red nucleus: a biochemical and immunohistochemical demonstration

Following unilateral kainic acid lesioning of neuronal cell bodies in the cat red nucleus (RN), a large decrease in glutamic acid decarboxylase (GAD) activity was detected in the injected RN, compared to the RN from control, non-injected animals. Using GAD immunohistochemistry, reactive perikarya were visualized dorsolaterally to the rostral part of the nucleus as well as within the RN proper. Taken together, these results point to the existence of an intrinsic GABAergic innervation in the RN area of the cat. The GABAergic neurons characterized here might thus correspond to the inhibitory interneurons previously detected electrophysiologically as a putative source of GABA for large-sized neurons of the RN.