Subcommissural organ-Reissner's fiber complex of the teleost Clarias batrachus responds to GABA treatment

Subcommissural organ (SCO) is a highly specialized ependymal gland located in the roof of the third ventricle. The secretory products of the SCO, whic h condense to form Reissner's fiber (RF), were recently found to cross-reac t with the anti-calcitonin antibody. To understand the mechanisms regulatin g the formation of the RF and the possible function of these discrete struc tures, we studied the response of the SCO-RF complex to intracranially admi nistered GABA, using immunocytochemical labeling with anti-calcitonin antib ody. Although the SCO-RF complex of control fish was intensely immunostaine d, 1 h after GABA treatment, the ependymal cells revealed partial loss of i mmunoreactivity; the RF showed occasional loss of immunoreactivity with its diameter increased by about 56% of the control value. Following 2 h of GAB A treatment, the SCO revealed dramatic loss of calcitonin-like immunoreacti vity from the ependymal cells. The RF showed a dual response in this group, while in some segments the RF appeared conspicuously thick, elsewhere it a ppeared thin. The mean diameter was, however, not significantly different f rom the normal. Following 4 h of GABA treatment, while calcitonin-like immu noreactive material made its reappearance in the SCO, the RF diameter was u niformly 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 pre treatment with bicuculline, a GABA(A) receptor antagonist. The results sugg est that GABA, acting via GABA(A) receptors, may trigger the release of sec retory material from the SCO and induce histomorphological changes in the R F indicative of discharge of stored material. (C) 2000 Elsevier Science B.V . All rights reserved.