Abstract
In vertebrates, γ-aminobutyric acid (GABA) is the main inhibitory transmitter in the central nervous system (CNS) acting through ionotropic (GABAA) and metabotropic (GABAB) receptors. The GABAB 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 GABAB 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 GABAB subunits or their expression in the CNS of lampreys. Our aim was to identify the sea lamprey GABAB1 and GABAB2 transcripts and study their expression in the CNS of adults. We cloned two partial sequences corresponding to the GABAB1 and GABAB2 cDNAs of the sea lamprey as confirmed by sequence analysis and comparison with known GABAB sequences of other vertebrates. In phylogenetic analyses, the sea lamprey GABAB sequences clustered together with GABABs 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 GABAB subunits in the CNS is a conserved character shared by agnathans and gnathostomes.
Highlights
Γ-aminobutyric acid (GABA) is the main inhibitory transmitter in the central nervous system (CNS), and it acts via ionotropic (GABAA) and metabotropic (GABAB) GABA receptors (Pinard et al, 2010; Chalifoux and Carter, 2011)
The cloned GABAB1 and GABAB2 cDNAs sequences corresponded to the nucleotides 302–760 for GABAB1 (Figure 1A), and 729–1162 for GABAB2 (Figure 1A ) of the partial cDNA sequences of these receptors annotated in sea lamprey genome of the Ensembl database
Due to a high level of similarity between the cloned sequences and those annotated in the Ensembl database, we used the partial, but longer, GABAB1 and GABAB2 protein sequences (Figures 1B,B, respectively) deduced from the sea lamprey genome sequences ENSPMAG00000006844, which is located in the scaffold GL479777, and ENSPMAG00000004383, which is located in the scaffold GL478877, for the subsequent analyses
Summary
Γ-aminobutyric acid (GABA) is the main inhibitory transmitter in the central nervous system (CNS), and it acts via ionotropic (GABAA) and metabotropic (GABAB) GABA receptors (Pinard et al, 2010; Chalifoux and Carter, 2011). The expression of GABAB transcripts (GABAB1 and GABAB2) has been reported in a few jawed vertebrate species (rats: Bischoff et al, 1999; Fritschy et al, 1999; humans: Calver et al, 2000; Berthele et al, 2001; non-human primates: Muñoz et al, 1998, 2001; zebrafish: Tabor et al, 2008; and frogs: Kaeser et al, 2011) These studies reveal a wide distribution of this receptor in the entire CNS of invertebrate and vertebrate species. GABA accumulated in the form of halos around some of the descending axons after the injury and statistical analyses showed a correlation between the presence of this halos and a higher survival ability of the identified descending neurons (Fernández-López et al, 2014) This revealed a possible neuroprotective role of GABA following spinal cord injury in lampreys. This type of studies show the importance of increasing our knowledge on the GABAergic system of lampreys
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