The subcommissural organ of the rat secretes Reissner's fiber glycoproteins and CSF-soluble proteins reaching the internal and external CSF compartments.

BACKGROUND: The subcommissural organ (SCO) is a highly conserved brain gland present throughout the vertebrate phylum; it secretes glycoproteins into the cerebrospinal fluid (CSF), where they aggregate to form Reissner's fiber (RF). SCO-spondin is the major constituent protein of RF. Evidence exists that the SCO also secretes proteins that remain soluble in the CSF. The aims of the present investigation were: (i) to identify and partially characterize the SCO-secretory compounds present in the SCO gland itself and in the RF of the Sprague-Dawley rat and non-hydrocephalic hyh mouse, and in the CSF of rat; (ii) to make a comparative analysis of the proteins present in these three compartments; (iii) to identify the proteins secreted by the SCO into the CSF at different developmental periods. METHODS: The proteins of the SCO secreted into the CSF were studied (i) by injecting specific antibodies into ventricular CSF in vivo; (ii) by immunoblots of SCO, RF and CSF samples, using specific antibodies against the SCO secretory proteins (AFRU and anti-P15). In addition, the glycosylated nature of SCO-compounds was analysed by concanavalin A and wheat germ agglutinin binding. To analyse RF-glycoproteins, RF was extracted from the central canal of juvenile rats and mice; to investigate the CSF-soluble proteins secreted by the SCO, CSF samples were collected from the cisterna magna of rats at different stages of development (from E18 to PN30). RESULTS: Five glycoproteins were identified in the rat SCO with apparent molecular weights of 630, 450, 390, 320 and 200 kDa. With the exception of the 200-kDa compound, all other compounds present in the rat SCO were also present in the mouse SCO. The 630 and 390 kDa compounds of the rat SCO have affinity for concanavalin A but not for wheat germ agglutinin, suggesting that they correspond to precursor forms. Four of the AFRU-immunoreactive compounds present in the SCO (630, 450, 390, 320 kDa) were absent from the RF and CSF. These may be precursor and/or partially processed forms. Two other compounds (200, 63 kDa) were present in SCO, RF and CSF and may be processed forms. The presence of these proteins in both, RF and CSF suggests a steady-state RF/CSF equilibrium for these compounds. Eight AFRU-immunoreactive bands were consistently found in CSF samples from rats at E18, E20 and PN1. Only four of these compounds were detected in the cisternal CSF of PN30 rats. The 200 kDa compound appears to be a key compound in rats since it was consistently found in all samples of SCO, RF and embryonic and juvenile CSF. CONCLUSION: It is concluded that (i) during the late embryonic life, the rat SCO secretes compounds that remain soluble in the CSF and reach the subarachnoid space; (ii) during postnatal life, there is a reduction in the number and concentration of CSF-soluble proteins secreted by the SCO. The molecular structure and functional significance of these proteins remain to be elucidated. The possibility they are involved in brain development has been discussed.

The destination of the aged, nonreleasable neurohypophyseal peptides stored in the neural lobe is associated to the remodeling of the neurosecretory axon.

The present investigation was designed to investigate the fate of the large pool of neurohypophyseal hormones that is never released into the blood. Normal Sprague-Dawley and taiep mutant rats were investigated under normal water balance, after dehydration and after dehydration-rehydration. Lectin histochemistry and light- and electron-microscopic immunocytochemistry using antibodies against vasopressin, oxytocin, and neurophysins used at low (1:1,000) and high (1:15,000) dilutions allowed to distinguish (1) recently packed immature granules, as those located in the perikaryon; (2) mature; and (3) aged granules. The distribution of these granules within the different domains of the neurosecretory axons located in the neural lobe, namely, undilated segments, swellings, terminals, and Herring bodies, and the response of these compartments to dehydration and dehydration-rehydration allowed to roughly follow the routing of the granules through such axonal domains. It is suggested that granules may move backward and forward between the terminals and the swellings. At variance, aged granules located in Herring body are retained in this compartment and would finally become degraded. Herring bodies displayed distinct lectin binding and immunocytochemical properties, allowing to distinguish them from axonal swellings. After a dehydration-rehydration cycle, immunocytochemistry and electron microscopy revealed that Herring bodies were no longer present in the neural lobe and that several terminals had degenerated. It is concluded that (1) the neurophysin axons may undergo remodeling under appropriate stimuli and (2) Herring bodies are a specialized and plastic domain of the magnocellular neurosecretory neuron involved in the disposal of aged neurosecretory granules. No differences were detected at the neural lobe level between normal and mutant rats subjected to the same experimental conditions.

Transcription of SCO-spondin in the subcommissural organ: evidence for down-regulation mediated by serotonin.

The subcommissural organ (SCO) is a brain gland located in the roof of the third ventricle that releases glycoproteins into the cerebrospinal fluid, where they form a structure known as Reissner's fiber (RF). On the basis of SCO-spondin sequence (the major RF glycoprotein) and experimental findings, the SCO has been implicated in central nervous system development; however, its function(s) after birth remain unclear. There is evidence suggesting that SCO activity in adult animals may be regulated by serotonin (5HT). The use of an anti-5HT serum showed that the bovine SCO is heterogeneously innervated with most part being poorly innervated, whereas the rat SCO is richly innervated throughout. Antibodies against serotonin receptor subtype 2A rendered a strong immunoreaction at the ventricular cell pole of the bovine SCO cells and revealed the expected polypeptides in blots of fresh and organ-cultured bovine SCO. Analyses of organ-cultured bovine SCO treated with 5HT revealed a twofold decrease of both SCO-spondin mRNA level and immunoreactive RF glycoproteins, whereas no effect on release of RF glycoproteins into the culture medium was detected. Rats subjected to pharmacological depletion of 5HT exhibited an SCO-spondin mRNA level twofold higher than untreated rats. These results indicate that 5HT down-regulates SCO-spondin biosynthesis but apparently not its release, and suggest that 5HT may exert the effect on the SCO via the cerebrospinal fluid.