The subcommissural organ of the frog Rana perezi is innervated by nerve fibres containing GABA

Cloning of the GABAB Receptor Subunits B1 and B2 and their Expression in the Central Nervous System of the Adult Sea Lamprey

In vertebrates, γ-aminobutyric acid (GABA) is the main inhibitory transmitter in the central nervous system (CNS) acting through ionotropic (GABA_A) and metabotropic (GABA_B) receptors. The GABA_B receptor produces a slow inhibition since it activates second messenger systems through the binding and activation of guanine nucleotide-binding proteins [G-protein-coupled receptors (GPCRs)]. Lampreys are a key reference to understand molecular evolution in vertebrates. The importance of the GABA_B receptor for the modulation of the circuits controlling locomotion and other behaviors has been shown in pharmacological/physiological studies in lampreys. However, there is no data about the sequence of the GABA_B subunits or their expression in the CNS of lampreys. Our aim was to identify the sea lamprey GABA_B1 and GABA_B2 transcripts and study their expression in the CNS of adults. We cloned two partial sequences corresponding to the GABA_B1 and GABA_B2 cDNAs of the sea lamprey as confirmed by sequence analysis and comparison with known GABA_B sequences of other vertebrates. In phylogenetic analyses, the sea lamprey GABA_B sequences clustered together with GABA_Bs sequences of vertebrates and emerged as an outgroup to all gnathostome sequences. We observed a broad and overlapping expression of both transcripts in the entire CNS. Expression was mainly observed in neuronal somas of the periventricular regions including the identified reticulospinal cells. No expression was observed in identifiable fibers. Comparison of our results with those reported in other vertebrates indicates that a broad and overlapping expression of the GABA_B subunits in the CNS is a conserved character shared by agnathans and gnathostomes.

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.