Excitatory and inhibitory synaptic function in the rostral nucleus of the solitary tract in embryonic rat

Gustatory solitary tract development: a role for neuropilins

The rostral nucleus of the solitary tract (rNST) receives orosensory information from taste bud cells in the tongue and palate via cranial nerves VII and IX. These nerves enter the brainstem, form the solitary tract (ST) and synapse with neurons in the rNST, which then relay incoming sensory information to other brain areas to process external gustatory stimuli. Factors that direct or regulate the trajectory of the developing ST are largely unknown. We used 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI) to identify ST projections originating from cells in the geniculate ganglia of embryonic rats from embryonic day 14 through 18 (E14-E18). After identifying the ST fibers, immunolabeling for and protein expression analysis of the axon guidance molecules neuropilin-1 (Npn-1) and neuropilin-2 (Npn-2) and their binding partners, semaphorin-3A (Sema-3A) and semaphorin-3F (Sema-3F) were performed. The results detail the formation of ST projections into the gustatory brainstem and their relationship to developing rNST neurons. DiI-labeled ST fibers were present in the brainstem as early as E14. Npn-1 was expressed in the ST and in the trigeminal tract at E14, but levels of the protein declined through E18. The expression levels of the binding partner of Npn-1, Sema-3A, increased from E14 to E18. Npn-2 was expressed in the ST and, additionally, in radially oriented, tuft-like structures within the brainstem at E14. Expression levels of Npn-2 also declined through E18, in contrast to the expression levels of its binding partner, Sema-3F, which increased during this time period. For the first time, the time course and particular molecular components involved in development of the ST have been identified. These results indicate that the neuropilin and semaphorin families of axon guidance molecules are potential molecular participants in ST formation.

Novel functions of GABA signaling in adult neurogenesis

Neurotransmitter gamma-aminobutiric acid (GABA) through ionotropic GABA_A and metabotropic GABA_B receptors plays key roles in modulating the development, plasticity and function of neuronal networks. GABA is inhibitory in mature neurons but excitatory in immature neurons, neuroblasts and neural stem/progenitor cells (NSCs/NPCs). The switch from excitatory to inhibitory occurs following the development of glutamatergic synaptic input and results from the dynamic changes in the expression of Na⁺/K⁺/2Cl^- co-transporter NKCC1 driving Cl^- influx and neuron-specific K⁺/Cl^- co-transporter KCC2 driving Cl^- efflux. The developmental transition of KCC2 expression is regulated by Disrupted-in-Schizophrenia 1 (DISC1) and brain-derived neurotrophic factor (BDNF) signaling. The excitatory GABA signaling during early neurogenesis is important to the activity/experience-induced regulation of NSC quiescence, NPC proliferation, neuroblast migration and newborn neuronal maturation/functional integration. The inhibitory GABA signaling allows for the sparse and static functional networking essential for learning/memory development and maintenance.