Development of putative GABAergic neurons in the appendicularian urochordateOikopleura dioica

Abstract

Studying the developing brain of urochordates can increase our understanding of brain evolution in the chordate lineage. To begin addressing regional patterns of neuronal differentiation in appendicularian urochordates, we examined the development of putative GABAergic neurons in Oikopleura dioica using GABA immunohistochemistry and in situ hybridization for the GABA-synthesizing enzyme GAD. First, we assessed the developmental dynamics of neuron number and organization in the cerebral and caudal ganglia. We then identified and mapped the positions of putative GABAergic neurons using confocal microscopy. We found GAD mRNA-positive and GABA-immunopositive neurons in the first brain nerves and the cerebral and caudal ganglia, but not in the caudal nerve cord. In both ganglia GAD mRNA-positive and GABA-immunopositive neurons are found in the same characteristic intraganglionic locations. The differentiation of these GABAergic markers occurs first in the first brain nerves and the cerebral ganglion and then with a several-hour delay in the caudal ganglion. In all three structures GAD mRNA expression appears 2-3 hours prior to GABA expression. In general, GABA is expressed by the same number of neurons as express GAD. Several discrepancies suggest differential regulation of the GABAergic phenotype in different neurons, however. Our results show that the GABAergic phenotype has a stereotyped pattern of expression along the anteroposterior axis of the CNS. Given recent genome sequencing and developmental patterning gene studies in this species, the GABAergic neurons in O. dioica provide a good model for assessing, at the invertebrate-vertebrate transition, the molecular mechanisms that specify the GABAergic phenotype.

GABA: A Pioneer Transmitter That Excites Immature Neurons and Generates Primitive Oscillations

“…Hence, GABA exerts a cation-mediated excitatory action. GABAergic neurons that are also GAD positive are present in the first brain nerves and cerebral and caudal ganglia of the urochordate Oikopleura dioica (575). Therefore, although GABA has a major metabolic role at a very early stage of evolution, its multiple facets have served many functions including early signaling actions notably as a transmitter.…”

Development of the γ-amino butyric acid (GABA)-ergic signaling system and its role in larval swimming in sea urchin

“…In the urochordate Oikopleura, transcription of the GAD gene occurs 2-3h earlier than GABA synthesis (Søviknes et al, 2005). It remains to be clarified whether this chronological sequence occurs in sea urchin larvae.…”

Developmental Characterization of Tail Movements in the Appendicularian Urochordate <b><i>Oikopleura dioica</i></b>

“…Ten pairs of cholinergic motoneurons are distributed from the caudal ganglion to the distal part of the cord, roughly in register with the ten pairs of tail muscle cells. A small number of putative GABAergic interneurons are located within the caudal ganglion [Søviknes et al, 2005]. Putative glutamatergic and glycinergic interneurons are also present in the caudal ganglion and the caudal nerve cord [Trentani and Glover, unpubl.…”

“…It is organized into the so-called cerebral ganglion, which contains in the order of 75-90 neurons [Søviknes et al, 2005;Trentani and Glover, unpubl. observations], the caudal ganglion at the base of the tail that contains about 25 neurons, and the caudal nerve cord that extends along the length of the tail and contains about 30 neurons .…”