Comparative localization of corticotropin and corticotropin releasing factor-like peptides in the brain and hypophysis of a primitive vertebrate, the sturgeon Acipenser ruthenus L

Distribution of gamma-aminobutyric acid immunoreactivity in the brain of the Siberian sturgeon (Acipenser baeri): Comparison with other fishes

Gamma-aminobutyric acid (GABA) is the main inhibitory neurotransmitter in the central nervous system (CNS) of vertebrates. Immunohistochemical techniques with specific antibodies against GABA or against its synthesizing enzyme, glutamic acid decarboxylase (GAD) allowed characterizing GABAergic neurons and fibers in the CNS. However, studies on the CNS distribution of GABAergic neurons and fibers of bony fishes are scant and were done in teleost species. With the aim of understanding the early evolution of this system in bony vertebrates, we analyzed the distribution of GABA-immunoreactive (-ir) and GAD-ir neurons and fibers in the CNS of a basal ray-finned fish, the Siberian sturgeon (Chondrostei, Acipenseriformes), using immunohistochemical techniques. Our results revealed the presence and distribution of GABA/GAD-ir cells in different regions of the CNS such as olfactory bulbs, pallium and subpallium, hypothalamus, thalamus, pretectum, optic tectum, tegmentum, cerebellum, central grey, octavolateralis area, vagal lobe, rhombencephalic reticular areas, and the spinal cord. Abundant GABAergic innervation was observed in most brain regions, and GABAergic fibers were very abundant in the hypothalamic floor along the hypothalamo-hypophyseal tract and neurohypophysis. In addition, GABA-ir cerebrospinal fluid-contacting cells were observed in the alar and basal hypothalamus, saccus vasculosus, and spinal cord central canal. The distribution of GABAergic systems in the sturgeon brain shows numerous similarities to that observed in lampreys, but also to those of teleosts and tetrapods.

New insights into the neuroanatomical distribution and phylogeny of opioids and POMC-derived peptides in fish

This review re-evaluates the use of immunological probes to map enkephalinergic, dynorphinergic, and endorphinergic circuits in the CNS of lobe-finned fishes, ray-finned fishes, and cartilaginous fishes in light of the characterization of proenkephalin, prodynorphin, and POMC sequences from representatives of these groups of fish over the past 20 years. The use of α-MSH specific antisera is a reliable method for detecting POMC immunopositive cell bodies and fibers. Since α-MSH and β-endorphin are co-localized in the same neurons, these studies also reveal the distribution of endorphinergic networks. Met-enkephalin specific antisera can be used to detect enkephalinergic circuits in the CNS of gnathostomes because of the ubiquitous presence of this pentapeptide in the proenkephalin sequences of gnathostomes. However, the use of leu-enkephalin specific antisera to detect enkephalinergic networks is more problematic. While this immunological probe is appropriate for analyzing enkephalinergic networks in mammals and perhaps teleosts, for the lungfishes and cartilaginous fishes this probe is more likely able to detect dynorphinergic circuits. In this regard, there is a need to re-examine dynorphinergic networks in non-mammalian gnathostomes by using species specific antisera directed against dynorphin end-products.

Distribution of somatostatin immunoreactive neurons and fibres in the central nervous system of a chondrostean, the Siberian sturgeon (Acipenser baeri)

Somatostatin (SOM) is a neuropeptide that is widely distributed in the central nervous system of vertebrates. Two isoforms of somatostatin (SS1 and SS2) have been characterized in sturgeon and in situ hybridisation studies in the sturgeon brain have demonstrated that mRNAs of the two somatostatin precursors (PSS1 and PSS2) are differentially expressed in neurons [Trabucchi, M., Tostivint, H., Lihrmann, I., Sollars, C., Vallarino, M., Dores, R.M., Vaudry, H., 2002. Polygenic expression of somatostatin in the sturgeon Acipenser transmontanus: molecular cloning and distribution of the mRNAs encoding two somatostatin precursors. J. Comp. Neurol. 443, 332-345.]. However, neither the morphology of somatostatinergic neurons nor the patterns of innervation have yet been characterized. To gain further insight into the evolution of this system in primitive bony fishes, we studied the distribution of somatostatin-immunoreactive (SOM-ir) cells and fibres in the brain of the Siberian sturgeon (Acipenser baeri). Most SOM-ir cells were found in the preoptic area and hypothalamus and abundant SOM-ir fibres coursed along the hypothalamic floor towards the median eminence, suggesting a hypophysiotrophic role for SOM in sturgeon. In addition, SOM-ir cells and fibres were observed in extrahypothalamic regions such as the telencephalon thalamus, rhombencephalon and spinal cord, which also suggests neuromodulatory and/or neurotransmitter functions for this peptide. Overall there was a good correlation between the distribution of SOM-ir neurons throughout the brain of A. baeri and that of PSS1 mRNA in Acipenser transmontanus. Comparative analysis of the results with those obtained in other groups of fishes and tetrapods indicates that widespread distribution of this peptide in the brain is shared by early vertebrate lines and that the general organization of the somatostatinergic systems has been well-conserved during evolution.

Plasticity of nonapeptidergic neurosecretory cells in fish hypothalamus and neurohypophysis

The structure and function of nonapeptidergic neurosecretory cells (NP-NSC) are considered in terms of comparative morphology. Among NSC of different ergicity for NP-NSC the most characteristic involve massive accumulation and storage of neurohormonal products. Only in NP-NSC are the secretory cycles of functioning clearly expressed. Their highest reactivity is established during experimental and physiological stresses. In contrast, liberinergic, statinergic, and monoaminergic NSC, unlike NP-NSC, are characterized even in the "norm" by a constantly high level of extrusion processes. As signs of maximum NP-NSC plasticity, we consider the largest size of elementary neurosecretory granules, the diversity of secretion forms, and the maximum development of Herring bodies-clear manifestations of secretory cycles of functioning. In particular, phases of massive storage of neurosecretory granules in the extrusion cycle of NP-NSC neurosecretory terminals express accumulation of neurosecretory products. It is concluded that a particularly high degree of plasticity of NP-NSC is provided by their capability for functional reversion. This reversion is manifested first in the form of the restoration of the initial moderate level of functioning and especially in the accumulation of neurosecretory products. The reversion is considered an important mechanism providing a high degree of NSC plasticity. This degree turns out to be sufficient for participation of NP-NSC in the integration of fish reproduction. It is shown that NP-NSC are organized by the principle of a triad of the balanced system. This system consists of two alternative states: accumulation and release of neurosecretory products and the center of control of dynamics of their interrelations, the self-regulating center. In the latter, the key role is probably played by the Golgi complex.

Distribution of galanin-like immunoreactivity in the brain of the Siberian sturgeon (Acipenser baeri)

Galanin is a 29-amino acid peptide widely distributed in the central nervous system of vertebrates. The organization of galaninergic systems is well known in teleosts, the most advanced actinopterygians, but no data are available on primitive bony fish. To extend the evolutionary analysis of galaninergic systems we studied the distribution of galanin-like immunoreactive (GAL-ir) cells and fibers in the sturgeon brain, since chondrosteans are among the most primitive extant actinopterygians. Double-immunolabeling experiments were performed to compare the distribution of galanin with that of neurophysin, tyrosine hydroxylase, and serotonin. Numerous GAL-ir cells of cerebrospinal fluid-contacting (CSF-C) type were found in the ventral telencephalon, preoptic area, and in the tuberal and caudal hypothalamus. The distribution of GAL-ir elements in the sturgeon brain shows many similarities to that observed in other vertebrates, but also important differences, such as the abundance of GAL-ir CSF-C cells, which appear to be a primitive characteristic. GAL-ir neurons observed in the sturgeon telencephalic hemispheres perhaps represent the basic organization of common ancestors of bony fishes and tetrapods. In the preoptic-hypophyseal system, GAL-ir cells appeared to be related not only with neurophysin-expressing neurons (in the tuberal hypothalamus) but also with serotoninergic and catecholamines-synthesizing neurons (in preoptic and tuberal nuclei). Numerous GAL-ir fibers were observed in the median eminence and neural lobe of the hypophysis, indicating that galanin may play a role in the modulation of hypophyseal secretion. GAL-ir neurons were absent from the sturgeon brainstem, suggesting that their presence in other vertebrates could represent an evolutionary recent acquisition.

Hypothalamic and extrahypothalamic sauvagine-like immunoreactivity in the bullfrog (Rana catesbeiana) central nervous system

In the present study, immunocytochemistry and radioimmunoassay were used to investigate the presence of sauvagine in both hypothalamic and extrahypothalamic areas of the central nervous system (CNS) of the bullfrog (Rana catesbeiana) using a specific antiserum raised against synthetic non-conjugated sauvagine (SVG), a frog (Phyllomedusa sauvagei) skin peptide of the corticotropin-releasing factor (CRF) family. Sauvagine-immunoreactive (SVG-ir) bipolar neurons were found in the nucleus of the fasciculus longitudinalis medialis located in the rostral mesencephalic tegmentum. In the tectal mesencephalon, beaded SVG-ir fibres were present in the optic tectum, and in the torus semicircularis. Abundant SVG-ir varicose fibres were seen in the granulosa layer of the cerebellum, the nucleus isthmi, and the obex of the spinal cord. SVG-ir fibres were also seen by the alar plate of the rombencephalon. In the diencephalon, the antiserum stained parvocellular neurons of the preoptic nucleus (PON) which extended their dendrites into the cerebro-spinal fluid (CSF) of the third ventricle and projected their ependymofugal fibres to the zona externa (ZE) of the median eminence. Immunopositive fibres were also present in the medial forebrain bundle at the chiasmatic field, the posterior thalamus, the pretectal gray, and the ventrocaudal hypothalamus. In the telencephalon (forebrain), SVG-ir fibres were seen in the medial septum, the lateral septum, and the amygdala. The SVG immunoreactivity could not be detected after using the SVG antiserum previously immunoabsorbed with synthetic SVG (0.1 microM), but immunoblock of the antiserum with sucker (Catostomus commersoni) urotensin I (sUI), sole (Hippoglossoides elassodon) urotensin I, sucker CRF, rat/human CRF, or ovine CRF (0.1-10 microM) did not eliminate visualization of the immunoreactivity. In radioimmunoassay, the SVG antiserum did not crossreact with sUI, or the SVG fragments SVG1-16, SVG16-27, and SVG26-34, but it recognized the C-terminal fragment SVG35-40. Crossreaction with mammalian ovine CRF and rat/human CRF was negligible. Both hypothalamic and mesencephalic extracts gave parallel displacement curves to SVG. The results suggest the presence in the bullfrog brain of a SVG-like neuropeptide, i.e., a peptide of the CRF family, that either is SVG or shares high homology with the C-terminus of that peptide. The function of this neuropeptide in amphibians is not known at this time, but based on its anatomical distribution to the ZE it could affect the release of adrenocorticotropin (ACTH) or other substances from the amphibian pars distalis. Involvement of the SVG-like peptide in behavioural (forebrain), visual (thalamus-tegmentum mesencephali-pretectal gray-optic tectum), motor coordination (cerebellum), and autonomic (spinal) functions, as well as an undefined interaction with the CSF in the bullfrog, seems likely.