Topographical relationships between catecholamine- and neuropeptide-containing fibers in the median eminence of the newt, Triturus alpestris. An ultrastructural immunocytochemical study

Dopaminergic inhibition of gonadotropin-releasing hormone neurons in the cichlid fish Astatotilapia burtoni

Dopamine regulates reproduction in part by modulating neuronal activity within the hypothalamic-pituitary-gonadal (HPG) axis. Previous studies suggested numerous mechanisms by which dopamine exerts inhibitory control over the HPG axis, ultimately changing the levels of sex steroids that regulate reproductive behaviors. However, it is not known whether these mechanisms are conserved across vertebrate species. In particular, it is unknown whether mechanisms underlying dopaminergic control of reproduction are shared between mammals and teleost fish. In mammals, dopamine directly inhibits gonadotropin-releasing hormone (GnRH1) hypothalamic neurons, the gatekeepers for activation of the HPG axis. Here, we demonstrate, for the first time in teleost fish, dopaminergic control of GnRH1 neurons via direct dopamine type-2-like receptor (D2R)-mediated inhibition within the hypothalamus. These results suggest that direct dopaminergic control of GnRH1 neurons via interactions in the hypothalamus is not exclusive to tetrapod reproductive control, but is likely conserved across vertebrate species.

Distribution of somatostatin-like immunoreactivity in the brain of the caecilian Dermophis mexicanus (Amphibia: Gymnophiona): comparative aspects in amphibians

The organization of the somatostatin-like-immunoreactive (SOM-ir) structures in the brain of anuran and urodele amphibians has been well documented, and significant differences were noted between the two amphibian orders. However, comparable data are not available for the third order of amphibians, the gymnophionans (caecilians). In the present study, we analyzed the anatomical distribution of SOM-ir cells and fibers in the brain of the gymnophionan Dermophis mexicanus. In addition, because of its known relationship with catecholamines in other vertebrates, double immunostaining for SOM and tyrosine hydroxylase was used to investigate this situation in the gymnophionan. Abundant SOM-ir cell bodies and fibers were widely distributed throughout the brain. In the telencephalon, pallial and subpallial cells were labeled, being most numerous in the medial pallium and amygdaloid region. Most of the SOM-ir neurons were found in the preoptic area and hypothalamus and showed a clear projection to the median eminence. Less conspicuously, SOM-ir structures were found in the thalamus, tectum, tegmentum, and reticular formation. Both SOM-ir cells and fibers were demonstrated in the spinal cord. The double-immunohistofluorescence technique revealed that catecholaminergic neurons and SOM-ir cells are largely intermingled in many brain regions but form totally separated populations. Many differences were found between the distribution of SOM-ir structures in Dermophis and that in anurans or urodeles. Some features were shared only with anurans, such as the abundant pallial SOM-ir cells, whereas others were common only to urodeles, such as the organization of the hypothalamohypophysial SOM-ir system. In addition, some characteristics were found only in Dermophis, such as the localization of the SOM-ir spinal cells and the lack of colocalization of catecholamines and SOM throughout the brain. Therefore, any conclusions concerning the SOM system in amphibians are incomplete without considering evidence for gymnophionans.

Lipopolysaccharide induces preproenkephalin transcription in hypophysiotropic neurons of the rat paraventricular hypothalamic nucleus suggesting a neuroendocrine role for enkephalins during immune stress

Opioids have impact on stress responses and possess immune modulatory functions. We have previously shown that immune stress elevates the levels of preproenkephalin transcript in a variety of autonomic structures in the rat brain, including the paraventricular hypothalamic nucleus. By using in situ hybridization with an intronic probe recognizing the preproenkephalin heteronuclear RNA combined with retrograde tract tracing, we examined the efferent target of the enkephalinergic neurons in the paraventricular hypothalamic nucleus that display induced transcriptional activity during immune challenge. Rats were first given i.p. injections of the tracer substance Fluoro-Gold, which following this route of administration is taken up only by nerve terminals residing outside the blood-brain barrier, and were then given an i.v. injection of lipopolysaccharide. Neuronal cell bodies retrogradely labeled with Fluoro-Gold were detected by immunohistochemistry, and-using a dual-labeling approach-the same cells were then examined for their expression of preproenkephalin heteronuclear RNA. We found that over 90% of the neurons that expressed preproenkephalin heteronuclear RNA also contained Fluoro-Gold. In addition, approximately 40% of the neurons expressing preproenkephalin heteronuclear RNA co-expressed mRNA for corticotropin-releasing hormone, the main adrenocorticotropic hormone secretagogue. These data show that the paraventricular hypothalamic neurons that display induced preproenkephalin transcription following immune challenge are almost exclusively hypophysiotropic neurons, indicating a role for enkephalin in the hypothalamic control of hormone release during infectious and inflammatory conditions.

Three-dimensional representation of the neurotransmitter systems of the human hypothalamus: inputs of the gonadotrophin hormone-releasing hormone neuronal system

The gonadotrophin-releasing hormone (GnRH) represents the final common pathway of a neuronal network that integrates multiple external and internal factors to control fertility. Among the many inputs GnRH neurones receive, oestrogens play the most important role. In females, oestrogen, in addition to the negative feedback, also exhibits a positive feedback influence upon the activity and output of GnRH neurones to generate the preovulatory luteinising hormone surge and ovulation. Until recently, the belief has been that the GnRH neurones do not contain oestrogen receptors and that the action of oestrogen upon GnRH neurones is indirect, involving several, oestrogen-sensitive neurotransmitter and neuromodulator systems that trans-synaptically regulate the activity of the GnRH neurones. Although this concept still holds for humans, recent studies indicate that oestrogen receptor-beta is expressed in GnRH neurones of the rat. This review provides three dimensional stereoscopic images of GnRH-immunoreactive (IR) and some peptidergic (neuropeptide Y-, substance P-, beta-endorphin-, leu-enkaphalin-, corticotrophin hormone-releasing- and galanin-IR) and catecholaminergic neurones and the communication of these potential oestrogen-sensitive neuronal systems with GnRH neurones in the human hypothalamus. Because the post-mortem human tissue does not allow the electron microscopic identification of synapses on GnRH neurones, the data presented here are based on light microscopic immunocytochemical experiments using high magnification with oil immersion, semithin sections or confocal microscopy.

Co-localization of mesotocin and opsin immunoreactivity in the hypothalamic preoptic nucleus of Xenopus laevis

The purpose of the present investigation was to provide a detailed description of the encephalic photoreceptors of Xenopus laevis at the light microscopic level and to determine their relationship with the neurosecretory cells of the hypothalamus in order to further our understanding of photoperiodic regulation of seasonal rhythms. Numerous opsin-positive neurons were found in the hypothalamic magnocellular preoptic nucleus and their axonal processes were seen to run laterally towards the basal regions of the brain, reaching the neural lobe of the hypophysis. Analysis of labelling with different antisera in adjacent sections, as well as double-immunolabelling carried out on the same section, revealed that mesotocin immunoreactivity was present in most of the opsin-positive neurons; however, no evidence for opsin and vasotocin coexpression was found in any of the sections analysed. The close localization of LHRH and opsin/mesotocin fibers in some regions of the brain, such as the median eminence, suggests that some interaction between these two systems might exist. In conclusion, in this study we provide the first strong evidence that the hypothalamic mesotocinergic neurons, which have been proved to be connected to the GnRH system in other species, are directly involved in photoreception in Xenopus laevis. These findings represent a novel contribution to our understanding of how light influences the seasonal reproductive cycles in lower vertebrates.

Somatostatin-like immunoreactivity in the brain of the urodele amphibian Pleurodeles waltl. Colocalization with catecholamines and nitric oxide

The neuronal structures with somatostatin-like immunoreactivity have been studied in the brain of the urodele amphibian Pleurodeles waltl. Intense immunoreactivity was observed in neurons and fibers distributed throughout the brain. Within the telencephalon, the subpallial regions were densely labeled containing both cells and fibers, primarily in the striatum and amygdala. The majority of the somatostatin immunoreactive neurons were located in the preoptic area and hypothalamus, although less numerous cells were also found in the thalamus. A conspicuous innervation of the median eminence was revealed, which arises from the hypothalamic cell populations. In the brainstem, intense fiber labeling was present in the tectum and tegmentum, whereas cell bodies were located only in the tegmentum of the mesencephalon and in the interpeduncular, raphe and reticular nuclei of the rhombencephalon. Longitudinal fiber tracts throughout the brainstem were observed and they continued into the spinal cord in the laterodorsal funiculus. The localization of somatostatin in catecholaminergic and nitrergic neurons was studied by double labeling techniques with antisera against tyrosine hydroxylase and nitric oxide synthase. Catecholamines and somatostatin only colocalized in a cell population in the ventral preoptic area. In turn, the striatum and amygdala contained neurons with somatostatin and nitric oxide synthase. Our results demonstrated that the somatostatin neuronal system in the brain of Pleurodeles waltl is consistent with that observed in anuran amphibians and shares many characteristics with those of amniotes. Colocalization of somatostatin with catecholamines and nitric oxide is very restricted in the urodele brain, but in places that can be easily compared to those reported for mammals, suggesting that interactions between these neurotransmitter systems are a primitive feature shared by tetrapod vertebrates.

Noradrenergic and adrenergic systems in the brain of the urodele amphibian, Pleurodeles waltlii, as revealed by immunohistochemical methods

The distribution of noradrenaline and adrenaline in the brain of the urodele amphibian Pleurodeles waltlii has been studied with antibodies raised against noradrenaline and the enzymes dopamine-beta-hydroxylase and phenylethanolamine-N-methyltransferase. Noradrenaline-containing cell bodies were found in the anterior preoptic area, the hypothalamic nucleus of the periventricular organ, the locus coeruleus and in the solitary tract/area postrema complex at the level of the obex. Noradrenergic fibers are widely distributed throughout the brain innervating particularly the ventrolateral forebrain, the medial amygdala, the lateral part of the posterior tubercle, the parabrachial region and the ventrolateral rhombencephalic tegmentum. Putative adrenergic cell bodies were found immediately rostral to the obex, ventral to the solitary tract. Whereas the cell bodies and their dendrites were Golgi-like stained, axons were more difficult to trace. Nevertheless, some weakly immunoreactive fibers could be traced to the basal forebrain. A comparison of these results with data previously obtained in anurans reveals not only several general features, but also some remarkable species differences.

Distribution of catecholaminergic and serotoninergic systems in forebrain and midbrain of the newt, Triturus alpestris (Urodela)

Mapping of monoaminergic systems in the brain of the newt Triturus alpestris was achieved with antisera against (1) thyrosine hydroxylase (TH), (2) formaldehyde-conjugated dopamine (DA), and (3) formaldehyde-conjugated serotonin (5-HT). In the telencephalon, the striatum was densely innervated by a large number of 5-HT-, DA- and TH-immunoreactive (IR) fibers; IR fibers were more scattered in the amygdala, the medial and lateral forebrain bundles, and the anterior commissure. In the anterior and medial diencephalon, TH-IR perikarya contacting the cerebrospinal fluid (CSF-C perikarya) were located in the preoptic recess organ (PRO), the organum vasculosum laminae terminalis and the suprachiasmatic nucleus. Numerous TH-IR perikarya, not contacting the CSF, were present in the posterior preoptic nucleus and the ventral thalamus. At this level, DA-IR CSF-C neurons were only located in the PRO. In the posterior diencephalon, large populations of 5-HT-IR and DA-IR CSF-C perikarya were found in the paraventricular organ (PVO) and the nucleus infundibularis dorsalis (NID); the dorsal part of the NID additionally presented TH-IR CSF-C perikarya. Most regions of the diencephalon showed an intense monoaminergic innervation. In addition, numerous TH-IR, DA-IR and 5-HT-IR fibers, originating from the anterior and posterior hypothalamic nuclei, extended ventrally and reached the median eminence and the pars intermedia of the pituitary gland. In the midbrain, TH-IR perikarya were located dorsally in the pretectal area. Ventrally, a large group of TH-IR cell bodies and some weakly stained DA-IR and 5-HT-IR neurons were observed in the posterior tuberculum. No dopaminergic system equivalent to the substantia nigra was revealed. The possible significance of the differences in the distribution of TH-IR and DA-IR neurons is discussed, with special reference to the CSF-C neurons.

Application of cryosubstitution in neurohormone- and neurotransmitter-immunocytochemistry

Pituitaries of the African catfish, Clarias gariepinus, were prefixed in aldehyde fixatives, frozen in liquid propane and submitted to a cryosubstitution procedure. Ultrathin sections of the Lowicryl HM20-embedded tissue were treated with primary antisera raised in rabbits to gonadotropin releasing hormone (GnRH), vasopressin or gamma amino butyric acid (GABA) respectively. Binding of the primary antisera was visualized with goat anti-rabbit (GAR) labeled with gold. The general morphology of the tissue components in the cryosubstituted pituitaries matches with that obtained after routine embedding procedures. In addition, a strong labeling intensity of the neuropeptides/neurotransmitters investigated in the present study was demonstrated. Due to these qualities cryosubstitution provides optimal conditions for studying co-localization of neurosecretory products, using double-immunostaining procedures. In the pars distalis of the catfish pituitary several types of hypothalamus-derived nerve fibers are present between or synapting on the secretory cells. It is demonstrated that the two known catfish GnRHs are co-localized in the same nerve fiber and within these nerve fibers even co-exist in the same neurosecretory granules. GABA and vasopressin-immunolabeling each occurred in different nerve fibers. The present data demonstrate that cryosubstitution and low temperature-embedding results in an excellent morphological preservation compared to ultracryotomy and a better preserved immunoreactivity of small antigenic molecules in comparison to conventional fixation and embedding techniques.