Developmental neuron-glia interaction: role of the serotonin innervation upon the onset of GABA uptake into the ependymocytes of the rat subcommissural organ

Cell lineage of the subcommissural organ secretory ependymocytes: differentiating role of the environment

SCO-ependymocytes have a secretory activity and a neural innervation relating them to neurosecretory nerve cells. To elucidate the cell lineage of the SCO-ependymocytes and emphasize the role of the neural innervation in their differentiation, in particular 5-HT innervation, we analyzed the developmental pattern of expression of several glial and neuronal markers: (1) in the SCO of mammals possessing (rat, cat) or devoid (mouse, rabbit) of 5-HT innervation, (2) in rat 5-HT deafferented SCO, and (3) in rat SCO transplanted in a foreign environment, the fourth ventricle. The ability of SCO-ependymocytes to transiently express GFAP during development and express the glial alpha alpha-enolase confirms the glial lineage of the SCO-ependymocytes. Synthesis of vimentin by SCO-ependymocytes relates them to the classical ependymocytes. The ability of mature SCO-ependymocytes to take up GABA only when they are innervated by 5-HT terminal underlines the role of the neural environment on the differentiation of these ependymocytes and suggests that differential maturation of the SCO according to its innervation, may lead to specific functional specialization of this organ in different species.

Neural input and neural control of the subcommissural organ

The neural control of the subcommissural organ (SCO) has been partially characterized. The best known input is an important serotonergic innervation in the SCO of several mammals. In the rat, this innervation comes from raphe nuclei and appears to exert an inhibitory effect on the SCO activity. A GABAergic innervation has also been shown in the SCO of the rat and frog Rana perezi. In the rat, GABA and the enzyme glutamate decarboxylase are involved in the SCO innervation. GABA is taken up by some secretory ependymocytes and nerve terminals, coexisting with serotonin in a population of synaptic terminals. Dopamine, noradrenaline, and different neuropeptides such as LH-RH, vasopressin, vasotocin, oxytocin, mesotocin, substance P, alpha-neoendorphin, and galanin are also involved in SCO innervation. In the bovine SCO, an important number of fibers containing tyrosine hydroxylase are present, indicating that in this species dopamine and/or noradrenaline-containing fibers are an important neural input. In Rana perezi, a GABAergic innervation of pineal origin could explain the influence of light on the SCO secretory activity in frogs. A general conclusion is that the SCO cells receive neural inputs from different neurotransmitter systems. In addition, the possibility that neurotransmitters and neuropeptides present in the cerebrospinal fluid may also affect the SCO activity, is discussed.

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, 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.

Tropism of serotonergic neurons towards glial targets in the rat ependyma

During development, recognition mechanisms between neurons and their targets are necessary for the formation of the neuronal network. Neural connections are synaptic or non-junctional. Both types of communication can be found between neurons and glial elements in the periventricular walls. Serotonergic fibers form synaptic contacts on the specialized ependymocytes of the subcommissural organ, a structure which forms the roof of the third ventricle at its junction with the aqueduct. A network of non-junctional fibers containing both GABA and serotonin spread between the cilia of the classical ependymocytes in the ventricles. These anatomical, morphological and biochemical features suggest a tropism and specific recognition mechanisms between glial elements and serotonergic neurons. This hypothesis can be tested by the study of the innervation of the subcommissural organ and the classical ependyma by grafted embryonic neurons after a chemical destruction of the serotonergic endogenous innervation. Solid implants or cell suspensions prepared from embryonic metencephalon were transplanted to either the third ventricle or the periventricular gray matter in 5,7-dihydroxytryptamine denervated rats. Grafted serotonergic neurons were able to reinnervate the classical ependyma and the subcommissural organ. The fibers forming the supraependymal plexus were non-junctional and contained both serotonin and GABA while those innervating the subcommissural organ formed synaptic contacts and contained only serotonin. The signals capable of inducing the ependymal innervation were specific for serotonergic neurons since catecholaminergic neurons present in the grafts were unable to innervate either classical or specialized ependymocytes. These results demonstrate that glial cells are targets for serotonergic neurons and that the morphological and biochemical characteristics of the serotonergic innervation are closely related to the target cell phenotype.

Developmental neuron-glia interactions: role of serotonin innervation upon the differentiation of the ependymocytes of the rat subcommissural organ

The rat subcommissural organ (SCO), which forms the roof of the third ventricle is an adequate model to study certain mechanisms of neuron-glia interactions in vivo. The ependymocytes, the main component of the SCO, have a glial origin. They possess particular phenotypic characteristics: they accumulate [3H]GABA by a specific uptake mechanism, contain transitory GFAP during ontogenesis and do not express PS100; on the other hand they receive a 5HT input which forms typical synaptic contacts. This innervation is of particular interest to approach neuron-glia interactions during the differentiation. Studies of GABA uptake carriers during ontogenesis in SCO ependymocytes show a correlation between the onset of the 5HT innervation and the advent of the GABA uptake. Moreover, destruction of the 5HT innervation by a neurotoxin (5-7-dihydroxytryptamine), before its arrival at the SCO in newborn rat, inhibits the formation of the GABA uptake system and causes the expression of PS100 in adult SCO cells. On the other hand, the SCO of newborn rats transplanted to the fourth ventricle of an adult host rat had no capacity to take up GABA and expressed PS100 3 months after its transplantation. Finally, the SCO ependymocytes of species devoid of 5HT innervation (rabbit, mice) were unable to take up GABA and contain PS100. These data suggest that neuron-glia interactions are necessary for the advent of GABA uptake carriers and can control the expression of glial markers during ontogenesis in SCO ependymocytes.

Developmental expression of glial markers in ependymocytes of the rat subcommissural organ: role of the environment

The rat subcommissural organ (SCO), principally composed of modified ependymocytes (a type of glial cell), is a suitable model for the in vivo study of glial differentiation. An immunohistochemical study of the ontogenesis of rat SCO-ependymocytes from embryonic day 13 to postnatal day 10 shows that these cells express transitory glial fibrillary acidic protein (GFAP) from embryonic day 19 until postnatal day 3. However, S100 protein (S100) is never expressed in the SCO-cells, contrasting with the ventricle-lining cells of the third ventricle, which contain S100 as early as embryonic day 17. Environmental factors could be responsible for the repression of GFAP and S100 in adult rats, because GFAP and S100 are observed in ependymocytes of SCO 3 months after being grafted from newborn rat into the fourth ventricle of an adult rat. Neuronal factors might be involved in the control of the expression of S100, since after the destruction of serotonin innervation by neurotoxin at birth, S100 can be observed in some SCO-ependymocytes of adult rats. On the other hand, GFAP expression is apparently not affected by serotonin denervation, suggesting the existence of several factors involved in the differentiation of SCO-cells.