Search

The subcommissural organ (SCO) is an ependymal brain gland that secretes into the cerebrospinal fluid glycoproteins that polymerize, forming Reissner's fiber (RF). The SCO-RF complex seems to be involved in vertebrate nervous system development, although its role in adults is unknown. Furthermore, its physiology is still greatly undetermined, and little is known about the release control of SCO secretion and the underlying intracellular mechanisms. In this report, we show that up to 90% of 3-5-day-old in vitro SCO cells from both intact and partially-dispersed SCO explants displayed spontaneous cytosolic Ca2+ oscillations. The putative role of these spontaneous calcium oscillations in SCO secretory activity is discussed taking into consideration several previous findings. Two distinct subpopulations of SCO cells were detected, each one containing cells with synchronized calcium oscillations. A possible existence of different functional domains in SCO is therefore discussed. Oscillations persisted in the absence of extracellular Ca2+, indicating the major involvement of Ca2+ released from internal stores. Depolarization failed to induce intracellular calcium increases, although it disturbed the oscillation frequency, suggesting a putative modulator role of depolarizing agonists on the calcium oscillating pattern through voltage-gated calcium channels. Carbachol, a cholinergic agonist, evoked a switch in Ca2+ signaling from a calcium oscillating mode to a sustained and increased intracellular Ca2+ mode in 30% of measured cells, suggesting the involvement of acetylcholine in SCO activity, via a calcium-mediated response.

Subcommissural organ (SCO) is a highly specialized ependymal gland located in the roof of the third ventricle. The secretory products of the SCO, which condense to form Reissner's fiber (RF), were recently found to cross-react with the anti-calcitonin antibody. To understand the mechanisms regulating the formation of the RF and the possible function of these discrete structures, we studied the response of the SCO-RF complex to intracranially administered GABA, using immunocytochemical labeling with anti-calcitonin antibody. Although the SCO-RF complex of control fish was intensely immunostained, 1 h after GABA treatment, the ependymal cells revealed partial loss of immunoreactivity; the RF showed occasional loss of immunoreactivity with its diameter increased by about 56% of the control value. Following 2 h of GABA treatment, the SCO revealed dramatic loss of calcitonin-like immunoreactivity from the ependymal cells. The RF showed a dual response in this group, while in some segments the RF appeared conspicuously thick, elsewhere it appeared thin. The mean diameter was, however, not significantly different from the normal. Following 4 h of GABA treatment, while calcitonin-like immunoreactive material made its reappearance in the SCO, the RF diameter was uniformly reduced to about 35% of the control value. The responses by the RF as well as the SCO to intracranially administered GABA were blocked by pretreatment with bicuculline, a GABA(A) receptor antagonist. The results suggest that GABA, acting via GABA(A) receptors, may trigger the release of secretory material from the SCO and induce histomorphological changes in the RF indicative of discharge of stored material.

The ability of axonal outgrowth of serotonin neurons in the implanted brain tissue of subcommissural organ (SCO) was immunohistochemically studied. The serotonin neuron system of the experimental rats was completely destroyed by the intraventricular injection of 5,6-dihydroxytryptamine. The raphe region of normal fetal rats was implanted into the caudal part of the third ventricle of the neurotoxic drug pretreated rats. The host brain was examined 3 months after transplantation. The numerous serotonin fibers were distributed in the SCO and periventricular region of the third ventricle of the host brain. The outgrowing serotonin fibers from the raphe transplant seemed to innervate the SCO with the target specificity.