Diencephalic progenitors contribute to the posterior septum through rostral migration along the hippocampal axonal pathway.

Septal nuclei are telencephalic structures associated with a variety of brain functions as part of the limbic system. The two posterior septal nuclei, the triangular septal nucleus (TS) and the bed nuclei of the anterior commissure (BAC), are involved in fear and anxiety through their projections to the medial habenular nucleus. However, the development of both the TS and BAC remains unclear. Here, we found a novel caudal origin and putative migratory stream of mouse posterior septal neurons arising from the thalamic eminence (TE), a transient developmental structure at the rostral end of the rodent diencephalon. TE-derived cells, which have glutamatergic identity, migrated rostrally and entered the telencephalic territory by passing beneath the third ventricle. Subsequently, they turned dorsally toward the posterior septum. We also observed that TS and BAC neurons in the postnatal septum were labeled with GFP by in utero electroporation into the TE, suggesting a shared origin. Furthermore, TE-derived septal neurons migrated along the fornix, an efferent pathway from the hippocampus. These results demonstrate that posterior septal neurons have a distinct extratelencephalic origin from other septal nuclei. This heterogeneous origin may contribute to neuronal diversity of the septal nuclear complex.

Bottom-Up and Top-Down Mechanisms of General Anesthetics Modulate Different Dimensions of Consciousness.

There has been controversy regarding the precise mechanisms of anesthetic-induced unconsciousness, with two salient approaches that have emerged within systems neuroscience. One prominent approach is the "bottom up" paradigm, which argues that anesthetics suppress consciousness by modulating sleep-wake nuclei and neural circuits in the brainstem and diencephalon that have evolved to control arousal states. Another approach is the "top-down" paradigm, which argues that anesthetics suppress consciousness by modulating the cortical and thalamocortical circuits involved in the integration of neural information. In this article, we synthesize these approaches by mapping bottom-up and top-down mechanisms of general anesthetics to two distinct but inter-related dimensions of consciousness: level and content. We show how this explains certain empirical observations regarding the diversity of anesthetic drug effects. We conclude with a more nuanced discussion of how levels and contents of consciousness interact to generate subjective experience and what this implies for the mechanisms of anesthetic-induced unconsciousness.

Gonadotropin-inhibitory hormone promoter-driven enhanced green fluorescent protein expression decreases during aging in female rats.

Gonadotropin-inhibitory hormone (GnIH) neurons project to GnRH neurons to negatively regulate reproductive function. To fully explore the projections of the GnIH neurons, we created transgenic rats carrying an enhanced green fluorescent protein (EGFP) tagged to the GnIH promoter. With these animals, we show that EGFP-GnIH neurons are localized mainly in the dorsomedial hypothalamic nucleus (DMN) and project to the hypothalamus, telencephalon, and diencephalic thalamus, which parallels and confirms immunocytochemical and gene expression studies. We observed an age-related reduction in c-Fos-positive GnIH cell numbers in female rats. Furthermore, GnIH fiber appositions to GnRH neurons in the preoptic area were lessened in middle-aged females (70 weeks old) compared with their younger counterparts (9-12 weeks old). The fiber density in other brain areas was also reduced in middle-aged female rats. The expression of estrogen and progesterone receptors mRNA in subsets of EGFP-GnIH neurons was shown in laser-dissected single EGFP-GnIH neurons. We then examined estradiol-17ß and progesterone regulation of GnIH neurons, using c-Fos presence as a marker. Estradiol-17ß treatment reduced c-Fos labeling in EGFP-GnIH neurons in the DMN of young ovariectomized adult females but had no effect in middle-aged females. Progesterone had no effect on the number of GnIH cells positive for c-Fos. We conclude that there is an age-related decline in GnIH neuron number and GnIH inputs to GnRH neurons. We also conclude that the response of GnIH neurons to estrogen diminishes with reproductive aging.

Transcriptional regulatory mechanisms underlying the GABAergic neuron fate in different diencephalic prosomeres.

Diverse mechanisms regulate development of GABAergic neurons in different regions of the central nervous system. We have addressed the roles of a proneural gene, Ascl1, and a postmitotic selector gene, Gata2, in the differentiation of GABAergic neuron subpopulations in three diencephalic prosomeres: prethalamus (P3), thalamus (P2) and pretectum (P1). Although the different proliferative progenitor populations of GABAergic neurons commonly express Ascl1, they have distinct requirements for it in promotion of cell-cycle exit and GABAergic neuron identity. Subsequently, Gata2 is activated as postmitotic GABAergic precursors are born. In P1, Gata2 regulates the neurotransmitter identity by promoting GABAergic and inhibiting glutamatergic neuron differentiation. Interestingly, Gata2 defines instead the subtype of GABAergic neurons in the rostral thalamus (pTh-R), which is a subpopulation of P2. Without Gata2, the GABAergic precursors born in the pTh-R fail to activate subtype-specific markers, but start to express genes typical of GABAergic precursors in the neighbouring P3 domain. Thus, our results demonstrate diverse mechanisms regulating differentiation of GABAergic neuron subpopulations and suggest a role for Gata2 as a selector gene of both GABAergic neuron neurotransmitter and prosomere subtype identities in the developing diencephalon. Our results demonstrate for the first time that neuronal identities between distinct prosomeres can still be transformed in postmitotic neuronal precursors.

Effect of tooth pulp and periaqueductal central gray stimulation on the expression of genes encoding the selected neuropeptides and opioid receptors in the mesencephalon, hypothalamus and thalamus in rats.

Nociceptive stimulation has been considered to affect the expression of genes encoding endogenous neuropeptides and their receptors. The effect of electric stimulation of the tooth pulp and/or periaqueductal gray (PAG) in rats on mRNA levels of the selected neuropeptides and opioid receptors (ORs) was investigated in comparison with control group, without stimulation. The levels of mRNA for the selected neuropeptides: galanin (GAL), vasopressin (AVP), oxytocin (OT), substance P (SP), somatostatin (SOM), vasoactive intestinal peptide (VIP), endomorphin-2 (EM-2), and opioid receptors: MOR, DOR and KOR in mesencephalic, hypothalamic and thalamic tissues were determined by real-time PCR. It was demonstrated that in the control group expression of the tested neuropeptides was at a very low level in the mesencephalon and thalamus, but at the higher level in the hypothalamus. The highest expression of ORs was observed in the mesencephalon. Nociceptive tooth pulp stimulation had the strongest effect in the hypothalamus, elevating mRNA levels of all tested neuropeptides except SOM. Electric stimulation of PAG either did not change or down-regulated mRNA levels of the neuropeptides in the cerebral structures. Simultaneous stimulation of PAG and tooth pulp either did not affect mRNA levels of the investigated neuropeptides or caused their slight decrease versus tooth pulp stimulation. The noxious stimulation of tooth pulp increased also the levels of OR mRNAs, while stimulation of PAG had the opposite effect. The above results demonstrated that tooth pulp stimulation significantly up-regulated the mRNA levels for a number of neuropeptides and all three types of ORs in the rat brain, which would result in more potent antinociception. In contrast, PAG stimulation down-regulated the mRNA levels of several neuropeptides and ORs in the cerebral tissues, which would cause decreased synthesis of ORs. The obtained results represent a new insight into the mechanism of orofacial pain.

Spatial and temporal requirements for sonic hedgehog in the regulation of thalamic interneuron identity.

In caudal regions of the diencephalon, sonic hedgehog (Shh) is expressed in the ventral midline of prosomeres 1-3 (p1-p3), which underlie the pretectum, thalamus and prethalamus, respectively. Shh is also expressed in the zona limitans intrathalamica (zli), a dorsally projecting spike that forms at the p2-p3 boundary. The presence of two Shh signaling centers in the thalamus has made it difficult to determine the specific roles of either one in regional patterning and neuronal fate specification. To investigate the requirement of Shh from a focal source of expression in the ventral midline of the diencephalon, we used a newly generated mouse line carrying a targeted deletion of the 525 bp intronic sequence mediating Shh brain enhancer-1 (SBE1) activity. In SBE1 mutant mice, Shh transcription was initiated but not maintained in the ventral midline of the rostral midbrain and caudal diencephalon, yet expression in the zli was unaffected. In the absence of ventral midline Shh, rostral thalamic progenitors (pTH-R) adopted the molecular profile of a more caudal thalamic subtype (pTH-C). Surprisingly, despite their early mis-specification, neurons derived from the pTH-R domain continued to migrate to their proper thalamic nucleus, extended axons along their normal trajectory and expressed some, but not all, of their terminal differentiation markers. Our results, and those of others, suggest a model whereby Shh signaling from distinct spatial and temporal domains in the diencephalon exhibits unique and overlapping functions in the development of discrete classes of thalamic interneurons.

Morphology and distribution of neurons expressing serotonin 5-HT1A receptors in the rat hypothalamus and the surrounding diencephalic and telencephalic areas.

Disorders of serotonergic neurotransmission are involved in disturbances of numerous hypothalamic functions including circadian rhythm, mood, neuroendocrine functions, sleep and feeding. Among the serotonin receptors currently recognized, 5-HT(1A) receptors have received considerable attention due to their importance in the etiology of mood disorders. While previous studies have shown the presence of 5-HT(1A) receptors in several regions of the rat brain, there is no detailed map of the cellular distribution of 5-HT(1A) receptors in the rat diencephalon. In order to characterize the distribution and morphology of the neurons containing 5-HT(1A) receptors in the diencephalon and the adjacent telencephalic areas, single label immunohistochemistry was utilized. Large, multipolar, 5-HT(1A)-immunoreactive (IR) neurons were mainly detected in the magnocellular preoptic nucleus and in the nucleus of diagonal band of Broca, while the supraoptic nucleus contained mainly fusiform neurons. Medium-sized 5-HT(1A)-IR neurons with triangular or round-shaped somata were widely distributed in the diencephalon, populating the zona incerta, lateral hypothalamic area, anterior hypothalamic nucleus, substantia innominata, dorsomedial and premamillary nuclei, paraventricular nucleus and bed nucleus of stria terminalis. The present study provides schematic mapping of 5-HT(1A)-IR neurons in the rat diencephalon. In addition, the morphology of the detected 5-HT(1A)-IR neural elements is also described. Since rat is a widely used laboratory animal in pharmacological models of altered serotoninergic neurotransmission, detailed mapping of 5-HT(1A)-IR structures is pivotal for the neurochemical characterization of the neurons containing 5-HT(1A) receptors.

Expression of the paralogous tyrosine hydroxylase encoding genes th1 and th2 reveals the full complement of dopaminergic and noradrenergic neurons in zebrafish larval and juvenile brain.

The development of dopaminergic and noradrenergic neurons has received much attention based on their modulatory effect on many behavioral circuits and their involvement in neurodegenerative diseases. The zebrafish (Danio rerio) has emerged as a new model organism with which to study development and function of catecholaminergic systems. Tyrosine hydroxylase is the entry enzyme into catecholamine biosynthesis and is frequently used as a marker for catecholaminergic neurons. A genome duplication at the base of teleost evolution resulted in two paralogous zebrafish tyrosine hydroxylase-encoding genes, th1 and th2, the expression of which has been described previously only for th1. Here we investigate the expression of th2 in the brain of embryonic and juvenile zebrafish. We optimized whole-mount in situ hybridization protocols to detect gene expression in the anatomical three-dimensional context of whole juvenile brains. To confirm whether th2-expressing cells may indeed use dopamine as a neurotransmitter, we also included expression of dopamine beta hydroxylase, dopa decarboxylase, and dopamine transporter in our analysis. Our data provide the first complete account of catecholaminergic neurons in the zebrafish embryonic and juvenile brain. We identified four major th2-expressing neuronal groups that likely use dopamine as transmitter in the zebrafish diencephalon, including neurons of the posterior preoptic nucleus, the paraventricular organ, and the nuclei of the lateral and posterior recesses in the caudal hypothalamus. th2 expression in the latter two groups resolves a previously reported discrepancy, in which strong dopamine but little tyrosine hydroxylase immunoreactivity had been detected in the caudal hypothalamus. Our data also confirm that there are no mesencephalic DA neurons in zebrafish.

The role of Fgf8 in telencephalic and diencephalic patterning.

Correct patterning of the developing brain is crucial importance for accurate wiring and function. Although the adult brain contains many complex structures, it begins with a simple structure-the neural tube. As it develops, the neural tube is divided into several regions, including the telencephalon, diencephalon, midbrain, and hindbrain. In each of these regions, signaling molecules are secreted from discrete zones, which establish positional information and regulate regional growth. There are many mechanistic questions that remain to be resolved about the action of these growth and differentiation factors. The cellular factors mediating patterning in response to these factors are largely unknown. Furthermore, identical differentiation factors are expressed in different regions of the brain and yet control significantly different patterning mechanisms, and the factors that control region-specific responses to these factors are mostly obscure. Furthermore, differentiation factors also show dramatically different expression patterns in different vertebrate species that may underlie changes in brain structure, but the mechanisms by which these changes in gene expression occur poorly understood. To address these issues, we discuss the role of Fgf8, which controls anterior/posterior patterning in different regions of the developing brain. We also discuss how modifications of Fgf8 expression in the diencephalon controlled by retrotransposons can change the shape and function of the brain in various species.

Transcription factor Gbx2 acts cell-nonautonomously to regulate the formation of lineage-restriction boundaries of the thalamus.

Relatively little is known about the development of the thalamus, especially its differentiation into distinct nuclei. We demonstrate here that Gbx2-expressing cells in mouse diencephalon contribute to the entire thalamic nuclear complex. However, the neuronal precursors for different thalamic nuclei display temporally distinct Gbx2 expression patterns. Gbx2-expressing cells and their descendents form sharp lineage-restriction boundaries delineating the thalamus from the pretectum, epithalamus and prethalamus, revealing multiple compartmental boundaries within the mouse diencephalon. Without Gbx2, cells originating from the thalamus abnormally contribute to the epithalamus and pretectum. This abnormality does not result from an overt defect in patterning or cell-fate specification in Gbx2 mutants. Chimeric and genetic mosaic analysis demonstrate that Gbx2 plays a cell-nonautonomous role in controlling segregation of postmitotic thalamic neurons from the neighboring brain structures that do not express Gbx2. We propose that, within the developing thalamus, the dynamic and differential expression of Gbx2 may be involved in the specific segregation of thalamic neurons, leading to partition of the thalamus into different nuclei.

Tissue interactions in the developing chick diencephalon.

BACKGROUND: The developing vertebrate brain is patterned first by global signalling gradients that define crude anteroposterior and dorsoventral coordinates, and subsequently by local signalling centres (organisers) that refine cell fate assignment within pre-patterned regions. The interface between the prethalamus and the thalamus, the zona limitans intrathalamica (ZLI), is one such local signalling centre that is essential for the establishment of these major diencephalic subdivisions by secreting the signalling factor Sonic hedgehog. Various models for ZLI formation have been proposed, but a thorough understanding of how this important local organiser is established is lacking. RESULTS: Here, we describe tissue explant experiments in chick embryos aimed at characterising the roles of different forebrain areas in ZLI formation. We found that: the ZLI becomes specified unexpectedly early; flanking regions are required for its characteristic morphogenesis; ZLI induction can occur independently from ventral tissues; interaction between any prechordal and epichordal neuroepithelial tissue anterior to the midbrain-hindbrain boundary is able to generate a ZLI; and signals from the dorsal diencephalon antagonise ZLI formation. We further show that a localised source of retinoic acid in the dorsal diencephalon is a likely candidate to mediate this inhibitory signal. CONCLUSION: Our results are consistent with a model where planar, rather than vertical, signals position the ZLI at early stages of neural development and they implicate retinoic acid as a novel molecular cue that determines its dorsoventral extent.

Sonic hedgehog from the basal plate and the zona limitans intrathalamica exhibits differential activity on diencephalic molecular regionalization and nuclear structure.

The diencephalon is the most complex area of the vertebrate brain, being particularly complex in amniotes. It has been suggested that diencephalic regionalization partially depends on local signaling mediated by sonic hedgehog (Shh). However, since the Shh gene is expressed in both the diencephalic basal plate and the zona limitans intrathalamica (ZLI), it is still unclear which of these tissues exerts morphogenetic influence on thalamic regionalization. In the present study using chick and quail embryos, we have found that although Shh from the ZLI and the basal plate induces ectopic expression of diencephalic genes in the posterior prosencephalic alar plate, only Shh originating from the ZLI can induce ectopic gene expression in the anterior alar plate, indicating that the ZLI exerts specific activity in the anterior epithelium. By introducing microbarriers between the diencephalic alar neuroepithelium and either the ZLI or the basal plate, we generated local loss of Shh expression in the ZLI, leading to alterations in molecular regionalization and subsequently, in the nuclear organization of the alar diencephalic derivatives on both sides of the ZLI. We thus demonstrate in vivo that basal signals are required to induce Shh expression in the ZLI and that Shh from the ZLI plays a pivotal role in regionalizing the alar diencephalon. The structural phenotype of Shh abolition in the ZLI consisted of a progressive pattern of alterations in diencephalic organization which were associated with the observed gradient effects in the molecular regionalization of the diencephalon. We conclude that the ZLI is a secondary organizer which exerts its morphogenetic activity through Shh.

Hedgehog signalling from the zona limitans intrathalamica orchestrates patterning of the zebrafish diencephalon.

Midway between the anterior neural border and the midbrain-hindbrain boundary, two well-known local signalling centres in the early developing brain, is a further transverse boundary with putative signalling properties -- the zona limitans intrathalamica (ZLI). Here, we describe formation of the ZLI in zebrafish in relation to expression of sonic hedgehog (shh) and tiggy-winkle hedgehog (twhh), and to development of the forebrain regions that flank the ZLI: the prethalamus and thalamus. We find that enhanced Hh signalling increases the size of prethalamic and thalamic gene expression domains, whereas lack of Hh signalling leads to absence of these domains. In addition, we show that shh and twhh display both unique and redundant functions during diencephalic patterning. Genetic ablation of the basal plate shows that Hh expression in the ZLI alone is sufficient for diencephalic differentiation. Furthermore, acquisition of correct prethalamic and thalamic gene expression is dependent on direct Hh signalling. We conclude that proper maturation of the diencephalon requires ZLI-derived Hh signalling.

Hypothalamic GnRH-I and its precursor during photorefractoriness onset in free-living male Dark-eyed Juncos (Junco hyemalis) of different year classes.

The termination of seasonal breeding in most photoperiodic passerines is heralded by spontaneous gonadal regression as a result of long day exposure. This phenomenon is termed absolute photorefractoriness and this insensitivity to long days may only be dissipated by short photoperiod exposure. The timing of the transition to a photorefractory stage in adult free-living male Dark-eyed Juncos is age-dependent. Second-year males (SY, entering their first breeding season) molt earlier, suggesting that they become refractory earlier, than older males (ASY). We determined whether the earlier onset of photorefractoriness in SY males concurs with decreased expression of hypothalamic GnRH-I or its precursor, pro-GnRH-GAP. Male SY and ASY juncos were caught at the end of the breeding season either during, or following, testicular regression. Immunoreactivity for GnRH-I and pro-GnRH-GAP was found in perikarya in the preoptic area, and fibers in the median eminence. The number and size of cells immunoreactive for GnRH-I and pro-GnRH-GAP did not significantly differ with age or time of year. In ASY juncos, there was a significant decrease in both GnRH-I and pro-GnRH-GAP staining density at the median eminence following testicular regression at a time of photorefractoriness onset. Density of pro-GnRH-GAP at the median eminence was also reduced following testicular regression in SY males but GnRH-I staining density did not change. Thus, age-class differences in the transition to photorefractoriness do not appear to result from differences in GnRH-I synthesis. Instead, the regulation of GnRH-I secretion by non-photoperiodic factors may determine photorefractoriness onset.

Connections of the lateral and medial divisions of the goldfish telencephalic pallium.

Biotinylated dextran amine and fluorescent carbocyanine dye (DiI) were used to examine connections of the lateral (Dl) and medial (Dm) divisions of the goldfish pallium. Besides numerous intrinsic telencephalic connections to Dl and Dm, major ascending projections to these pallial divisions arise in the preglomerular complex of the posterior tuberculum, rather than in the dorsal thalamus. The rostral subnucleus of the lateral preglomerular nucleus receives auditory input via the medial pretoral nucleus, lateral line input via the ventrolateral toral nucleus, and visual input via the optic tectum, and it projects to both Dl and Dm. The anterior preglomerular nucleus and caudal subnucleus of the lateral preglomerular nucleus receive auditory input via the central toral nucleus and project to Dm. This pallial division also receives chemosensory information via the medial preglomerular nucleus. The central posterior (CP) nucleus, which receives both auditory and visual inputs, also projects to Dm and is the only dorsal thalamic nucleus projecting to the pallium. Thus, both Dl and Dm clearly receive multisensory inputs. Major projections of CP and projections of all other dorsal thalamic nuclei are to the subpallium, however. Descending projections of Dl are primarily to the preoptic area and the caudal hypothalamus, whereas descending projections of Dm are more extensive and particularly heavy to the anterior tuber and nucleus diffusus of the hypothalamus. The topography and connections of Dl are remarkably similar to those of the hippocampus of tetrapods, whereas the topography and connections of Dm are similar to those of the amygdala.

Thalamic development induced by Shh in the chick embryo.

Patterning of the early neural tube is achieved in part by the inductive signals, which arise from neuroepithelial signaling centers. The zona limitans intrathalamica (ZLI) is a neuroepithelial domain in the alar plate of the diencephalon which separates the prethalamus from the thalamus. The ZLI has recently been considered to be a possible secondary organizer, effecting its inductions via sonic hedgehog (Shh), a signaling molecule which drives morphogenetic information for the thalamus. Using experimental embryological techniques involving the generation of chimeric embryos, we show that the formation of the ZLI in the diencephalic alar plate is due to an interaction between the prechordal and epichordal plate neuroepithelia. We also provide evidence that Shh expression in the ZLI underlies the morphogenetic activity of this putative diencephalic organizer. Ectopic Shh led to the auto-induction of its own gene expression in host cells, as well as to the expression of other genes involved in diencephalic regionalization and histogenesis. Analysis of long-term surviving embryos after Shh ectopic expression demonstrated that Shh was able to induce thalamic structures and local overgrowth. Overall, these results indicate that Shh expressed in the ZLI plays an important role in diencephalic growth and in the development of the thalamus.

Evidence for serotonin influencing the thalamic infiltration of mast cells in rat.

Serotonin (5-HT) is involved in neuroimmunomodulation. We analyzed the effects of sumatriptan, a 5-HT(1B/1D) receptor agonist, and ondansetron, a 5-HT(3) receptor antagonist, on thalamic mast cell (TMC) population, the only immunocytes known to infiltrate the brain in physiological conditions. Only sumatriptan was effective, significantly increasing TMC numbers versus controls, and especially those containing 5-HT. 5-HT(1B) receptors are concentrated in the median eminence on non-serotonergic axonal endings, probably hypothalamic terminal fibers, involved in hypothalamic-pituitary neuroendocrine modulating processes. TMC variations could reflect serotonergic actions on these fibers. TMCs would thus be cellular interfaces mediating immune action in the nervous system in relation with the hormonal status of the organism.

Topographic representation of vestibular and somatosensory signals in the anuran thalamus.

In the isolated brain of the fire-bellied toad, Bombina orientalis, the spatial distribution of vestibular and somatosensory responses in thalamic nuclei was studied following electrical activation of the Vth nerve, the ramus anterior of the VIIIth nerve and of the dorsal roots of spinal nerves 3 and 8. Responses were systematically mapped in frontal planes through the diencephalon at four rostro-caudal levels. The calculated activity maps were superimposed on the outlines of diencephalic nuclei, and those nuclei that received particularly large inputs from the stimulated sensory nerve roots were indicated. Maximal response amplitudes coincided with ventral, central, and posterior thalamic areas and exhibited a topography that differed for each sensory nerve root. Maximal responses evoked from the Vth nerve were largely separated from those from spinal dorsal roots 3 and 8, whereas maximal vestibular responses partly overlapped with those from the other somatosensory nerve roots. Our findings indicate that within the amphibian thalamus sensory signals originating from different nerve roots are largely represented in separate areas as is the case in the thalamus of amniotes. However, the anterior dorsal thalamus which is the only origin of ascending pathways to the medial and dorsal pallium (assumed homologues of the mammalian hippocampus and neocortex, respectively) receives only minor vestibular and somatosensory input. This corroborates the view that amphibians lack a direct sensory thalamo-cortical, or "lemnothalamic," pathway typical of mammals and birds.

Distribution of muscimol, QNB, and 5HT binding in the vertebrate diencephalon: a comparative study of eight mammals and three non-mammals.

The distribution of muscimol, quinuclidinyl benzilate (QNB), and serotonin (5HT)-bound receptors in the diencephalon was examined by conventional receptor-binding methods in 11 species of amniotes including 2 reptiles, 1 bird, and 8 mammals, selected mostly on the basis of their differing last common ancestor with Anthropoids. We found that receptor binding can help define major subdivisions of the forebrain. The results show that in each of these species, the distribution of muscimol and QNB binding across the four major subdivisions of the diencephalon was consistent; densest in the dorsal thalamus, with hypothalamus and then either ventral thalamus or epithalamus with successively lesser amounts. However, the binding of serotonin (5HT) was most prevalent in the hypothalamus with equivalent amounts in the other diencephalic subdivisions. Myelin- and cell-stained materials showed that the pattern of high-density binding probably is not the secondary result of non-neurochemical factors such as differences in cell or neuropil density or in total available membrane. Perhaps more importantly, the receptor distributions suggest functional roles for major subdivisions across taxa. Results show that GABA-A and muscaranic Ach receptors are common in the dorsal diencephalon across vertebrate species and, therefore, are probably responsible for the gating of information to the cortex. Results show that serotonin is predominant in the hypothalamus. The lack of it in the dorsal thalamus indicates that it is probably not responsible for gating of information to the cortex. Results also show that in nonmammals the amount of GABA-A and muscaranic Ach differs from that found in mammals. For muscaranic Ach, the labeling in marsupials differs from that in placentals. Primates differ from other species (nonmammals and mammals combined) in the amount of 5HT found in the ventral diencephalon and the hypothalamus.

Topography and associations of leu-enkephalin and luteinizing hormone-releasing hormone neuronal systems in the human diencephalon.

Although several studies indicated that leu-enkephalin controls gonadal function, the morphological substrate of this modulation is unknown. To reveal potential interaction sites between leu-enkephalin and LH-releasing hormone (LHRH) in the hypothalamus, the distribution and connections of leu-enkephalin-immunoreactive (IR) and LHRH-IR systems were examined in the human diencephalon using double-label immunohistochemistry. First the leu-enkephalin-IR and LHRH-IR neural elements were mapped, then the maps of the two different neurotransmitter systems were superimposed unveiling the overlapping areas. The putative juxtapositions between leu-enkephalin-IR and LHRH-IR structures were revealed with double label immunocytochemistry. Close contacts were detected in the medial preoptic area and in the infundibulum/median eminence. In these areas, diaminobenzidine-silver-intensified, black leu-enkephalin-IR fibers abutted fusiform, brown, diaminobenzidine-labeled LHRH neurons often forming multiple contacts. Examination of semithin sections of these close associations with the aid of oil immersion revealed no cleft between the contacting LHRH-IR and leu-enkephalin-IR elements. Our findings indicate that the juxtapositions between LHRH-IR and leu enkephalin-IR neurons may be functional synapses forming the morphological substrate of the leu-enkephalin-modulated LHRH secretion in the human diencephalon. Moreover, the wide distribution of leu-enkephalin-IR elements suggests leu-enkephalin control of other diencephalic functions as well.