Enzymatic organization of the subcommissural organ

Immunocytochemical localization of 3-methylcrotonyl-CoA carboxylase in cultured ependymal, microglial and oligodendroglial cells

To evaluate the ability of ependymal, microglial and oligodendroglial cells to degrade leucine, the presence of 3-methylcrotonyl-CoA carboxylase (MCC) was investigated in cultures of these cells. MCC is a biotin-containing heterodimeric enzyme that is specific for the irreversible part of the leucine catabolic pathway. It has been reported previously that in cell culture MCC is expressed in astrocytes and a subpopulation of neurones. In the present study ependymal, microglial and oligodendroglial cell cultures, derived from the brains of newborn rats, were examined for the expression of MCC by RT-PCR, western blotting and immunocytochemistry. The results of RT-PCR and western blotting showed the presence of mRNA as well as protein of both subunits of MCC in ependymal, microglial and oligodendroglial cell cultures. Immunocytochemical investigation of the cellular and subcellular distribution of MCC demonstrated a mitochondrial location of MCC in all neuroglial cell types investigated. The ubiquitous expression of MCC in glial cells demonstrates the ability of the cells to engage in the catabolism of leucine transported into the brain, mainly for the generation of energy.

Neuronal control of [3H]GABA uptake in the ependymocytes of the subcommissural organ: an in vivo model of neuron-glia interaction

The rat subcommissural organ (SCO) is a particular but adequate paradigm for the approach, in vivo, to some aspects of neuron-glia interaction in gamma-aminobutyric acid (GABA) uptake. The rat SCO ependymocytes (the main component of this structure lying at the junction of the aqueduct and the third ventricle) accumulate [3H]GABA by a highly specific uptake mechanism and receive a serotoninergic input forming typical synaptic contacts. It seems that there is a correlation between the capacity of the rat SCO ependymocytes to take up [3H]GABA and the presence of a serotonin (5-HT) innervation. Indeed, in the newborn rat, no uptake of [3H]GABA was observed before the onset of this innervation and the increased [3H]GABA accumulation in the SCO was correlated with the appearance of the 5-HT terminals in the SCO. Moreover, in the mouse, whose SCO is devoid of a 5-HT innervation, no accumulation of [3H]GABA was observed in the SCO ependymocytes. Thus, the 5-HT innervation could be involved directly or indirectly in the onset of the GABA uptake carriers. On the other hand, in adult rats parachlorophenylalanine (pCPA) treatment decreased the 5-HT content of the SCO, and increased [3H]GABA accumulation; such an augmentation was not observed when rats were treated with pCPA plus 5-hydroxytryptophan to restore the 5-HT content. However, an increase in 5-HT content of the SCO by pargyline treatment appeared to have no effect on [3H]GABA uptake. Control of GABA uptake activity by 5-HT in the SCO ependymocytes could be an interesting model for the study of a possible interaction between amino-acids and other neurotransmitters by terminating their action in the extracellular space.

Radioautographic evidence for an innervation of the subcommissural organ by GABA-containing nerve fibres

Light-and electron-microscopic radioautographic studies were carried out in rats after intraventricular injection of [3H]GABAergic elements in the subcommissural organ (SCO). This amino acid is specifically accumulated in SCO ependymocytes and numerous nerve terminals and fibres. Some labelled terminals contain clear round vesicles and large granular vesicles and sometimes display synaptic contacts on the SCO ependymocytes. They disappear after 5,7-dihydroxytryptamine (5,7-DHT) treatment or lesions of mesencephalic raphé nuclei. They are morphologically and pharmacologically similar to the serotonin-containing terminals of the basal SCO. The other labelled terminals contain exclusively clear round vesicles, are dispersed throughout the SCO and survive after the 5,7-DHT treatment or raphé nuclei lesions; some of them are in synaptic contact with neuronal elements of the basal SCO. In both SCO ependymocytes and fibres or terminals, [3H]GABA is accumulated according to the pharmacological criteria of a GABA neuronal uptake. Under the same experimental conditions [3H[glutamine and beta[3H]alanine fail to label the SCO: [3H] glutamate produces a very light labelling and [3H] serotonin is accumulated only in nerve profiles of the basal SCO. These results, and the presence of a glutamate decarboxylase activity in the SCO, suggest the existence of GABA-synthesizing elements in the SCO and could indicate the possible involvement of GABA in the secretory activity of the SCO.